CA2051325C

CA2051325C - Optical communication system

Info

Publication number: CA2051325C
Application number: CA002051325A
Authority: CA
Inventors: Shigeki Watanabe
Original assignee: Fujitsu Ltd
Current assignee: Fujitsu Ltd
Priority date: 1990-09-14
Filing date: 1991-09-13
Publication date: 1999-06-29
Anticipated expiration: 2011-09-13
Also published as: CA2051325A1; EP0772314A3; US5896211A; EP0772312A3; EP0772314A2; EP0477699A2; EP0772313A3; EP0772312A2; EP0772313A2; EP0477699A3

Abstract

At a transmitting end, transmission signals corresponding to more than one channel are allocated different microwave frequencies and carriers of the microwave frequencies are modulated with the transmission signals in modulators. A filter is placed in the preceding or succeeding stage of a respective modulator to band-limit a corresponding transmission signal before or after modulation. The band-limited and modulated transmission signals are combined to produce a microwave frequency-division multiplexed signal. The multiplexed signal modulates an optical frequency modulator to produce an optical modulated signal. The optical modulated signal is transmitted to the receiving end. At the receiving end, the optical signal transmitted from the transmitting end is detected and converted to an electrical signal. The channel components are extracted from the electrical signal by bandpass filters and then demodulated.

Description

2~5~

Optlcal Communication System Background of the Inventlon 1. Field of the Invention The present lnvention relates to an optical communication system based on optical modulation using a microwave frequency-divislon multiplexed signal and, more particularly, to an SCM (subcarrier multiplexed) optical communication system.
The SCM optical communication system has an excellent feature in that all kinds of signals in either analog or digital form can be transmitted at the same time and in large quantities using only a single optlcal carrler. The present invention employs FM modulation based on direct modulation by a semiconductor laser, etc., as the optical modulation ln the SCM optlcal communlcation system. The present inventlon can be applied to all types of information communication networks including a conventional optical communication network, an optical CATV network, a broadband distribution network mainly handling image information, and future ISDN (integrated services digital network).
2. Description of the Related Art Heretofore, wavelength-division multiplexed transmission has been used mainly for multiplexing transmltting signals in optlcal communicatlon (such as large-capacity signal multiplexed transmisslon, in partlcular).
This ls intended for multi-channel transmission wlth optlcal waves of dlfferent wavelengths (or frequencles) used as 2~ ~13 2 5 -carriers. In the case of an lntensity modulation and direct detectlon (IM/DM) system, the wavelength-dlvlslon multlplexlng transmission requires a channel spacing of the order of several nanometers in wavelength so that channels can be distinguished from each other by optical filters. In the case of a coherent optical communicatlon system, a channel spaclng is presently required which ls of the order of ten and several times the blt rate because the ability to control crosstalk between ad~acent channels is limited. In the case of, for example, high-speed multlplexed transmlssion at a bit rate ln the order of glgablts, the channel spaclng ls about 20 GHz. A
recelver whlch can detect all of the channels slmultaneously cannot be realized. However, the number of channels to be multiplexed may be lncreased at the sendlng end. Therefore, only one channel can be recelved at a tlme.
On the other hand, the conventlonal SCM optlcal communlcatlon system, uses malnly an intenslty modulatlon system, based on the dlrect modulatlon of a semlconductor laser as lts optlcal modulatlon system and a dlrect detection system using a PIN photodlode or APD as its recelvlng system.
By way of example, a conventlonal FDM (frequency-divislon multlplexed) optlcal transmisslon system and a conventlonal TDM (tlme-dlvislon multlplexed) optlcal transmlsslon system are lllustrated in Figure 1 and Flgure 2, respectively. In both systems, coherent transmlsslon of four-channel, 622 Mb/s signals is made, and the optical transmission capaclty ls 2.5 Gb/s.

3 ~ ~

In the optical FDM system, as can be seen from Flgure 1, optlcal modulators 1-1 to 1-4, whlch correspond ln number (four in thls example) to channels and have optlcal carrlers of dlfferent frequencles Fsl to Fs4, each produce a modulated optlcal slgnal. These modulated slgnals from the optlcal modulators are mixed ln an optlcal coupler 2 to produce a frequency-dlvision multiplexed optical signal whlch, ln turn, ls transmitted through an optical flber 3. At the receivlng end, the optlcal signal transmitted through the optlcal flber ls mlxed with a local llght source (semlconductor laser) and - 4 - ~ 3 ~ ~

heterodyne detected by means of 4, an optical receiver 5 and an amplifier 6, whereby it is converted to an electric intermediate frequency signal. The resulting electric signal is then filtered by a bandpass filter 7 to allow only the signal on a desired channel to pass. The signal is then demodulated by a demodulator 8.
In the TDM optical transmission system, as can be seen from Figure 2, signals transmitted on channels are time-division multiplexed by a multiplexer (MUX ) 11 to produce a time-division multiplexed signal which, in turn, modulates an optical modulator 12.
The modulated signal is transmitted through an optical transmission fiber 13. At the receiving end, the optical signal transmitted from the transmitting end is optical-heterodyne detected by the use of a local light source (semiconductor laser) 14, an optical receiver 15 and an amplifier 16 for conversion to an electric signal. The resulting electric signal is filtered by a bandpass filter 17 to pass intermediate frequency signals of all the channels (bandwidth is about 25GHz x 2). These signals are demodulated by a demodulator 18 and then separated by a demultiplexer ( DEMUX ) 1 9 into the signals for the respective channels ~ ~ 5 'll 3 ~ 5 A problem wlth such a conventional optical frequency-division multiplex transmisslon system as shown in Figure 1 is that, since the channel spacing at the time of signal multiplexing must be large, of the order of ten and several times the blt rate, multiplexed signals cannot be detected simultaneously at the recelvlng end. However, the number of channels to be multlplexed may be increased at the transmitting end, but a limited number of channels that can be detected actually. In the case of hlgh-speed transmission of digital data (gigabit transmisslon) in partlcular, be only one channel that can be detected at a time by a recelver. Also, the number of channels whlch can be multlplexed ls llmited by the band of frequencies over whlch a semiconductor laser serving as a local llght source ls tunable.
An optical time-divlslon multlplex transmlssion system such as that shown ln Figure 2 requlres a multlplexer for time-divislon multiplexlng and a demultiplexer for separatlng the components in a time-division multiplexed signal transmitted over an optical transmission line. These circuits are very expensive. The use of these clrcults will lead to an increase of cost of the whole system. Moreover, the heterodyne detector requires a demodulatlon bandwldth whlch is wlde enough to cover all the channels that are multiplexed.
The conventlonal SCM optlcal communlcatlon system described above requlres that the llght output be llnear wlth the lntensity modulatlon of the semiconductor laser. Thls sets a limit to the bandwidth for modulation. Under the present clrcumstances, the bandwldth ls ln the order of 1 to 2 GHz at best. Thus, the wldeband transmlsslon is apt to be influenced by signal distortion, thus limltlng the capaclty of transmlsslon informatlon. This makes lt dlfficult to meet the requlrements of future large capaclty communlcatlons.
Moreover, slnce the dlrect detection system alone can be used as the detection system, lt ls dlfflcult to achleve a sufflcient receiver sensitlvlty. Therefore there are llmlts to transmlssion dlstance and the number of signals to be distributed.
If, therefore, optical angular modulation could be used as the optical modulation ln the SCM optlcal communlcatlon system, such technlcal problems would be solved.
In this case, however, to achleve a hlgh recelver sensltlvlty, the coherent optlcal transmlssion system must be used, whlch requlres an ~ ~ _ 7 _ ~ 2 ~

optical heterodyne receiver or an optical homodyne receiver, which have complex structures. It also requires countermeasures against degradation of the receiver sensitivity due to fluctuations in the state of polarization of signal light and local light.
This will make the receiver very expensive. It is difficult to u~e SUCll an expensive receiver in subscriber systems.

Summary of the Present Inventlon It is an object of the present invention to provide an optical comlllunication system which permits large-capacity frequency-division multiplexed optical transmission to be implemented easily and at a low cost and a number of channels to be received simultaneously by a single receiver.
It is another object of the present invention to provide an optical communication system which permits SCM optical communication by coherent optical transmission to be implemented at a low cost.
A feature of the present invention resides in an optical communication system characterized by allocation of transmission signals to more than one channel at different microwave frequencies, providing modulators for modulating carriers of the microwave frequencies with tlle translllissic,n signals and band-3 ~ ~

llmlting filters placed in the preceding stages or succeeding stages of sald modulators, combinlng band-limited and modulated transmlsslon signals to produce a microwave frequency-dlvlslon multlplexed slgnal, frequency modulating an optlcal frequency modulator with the multiplexed signal to produce an optical frequency modulated signal, optically transmlttlng the optical frequency modulated slgnal, detectlng the optically transmltted optlcal signal to thereby convert it to an electrlc slgnal, separatlng the electrlcal slgnal into channels by bandpass fllters and demodulating each of the signals separated into channels.
In accordance with the present lnventlon, there is provided an optlcal communicatlon system for transmltting a slgnal llght carrylng a plurallty of data signals from sending equlpment to receivlng equipment, said sending equipment comprising:
a plurality of optical frequency modulated signal generating units, each comprising:
a plurality of modulators modulatlng carrlers having different microwave frequencies with a plurality of transmlsslon slgnals and outputting first transmission signals;
a plurality of band limiting fllters band limitlng one of the data slgnals and the flrst transmlsslon signals, and outputting second transmission signals;
adding means for generatlng a microwave frequency multlplexed slgnal by addlng the second transmission slgnals;

- 8a and an optical frequency modulator generating an optlcal frequency modulated slgnal by frequency modulation with said mlcrowave frequency multlplexed slgnal; and optical mlxlng means for mlxlng each optlcal frequency modulated slgnal generated by the optlcal frequency modulator ln sald plurality of optlcal frequency modulated signal generating units and for generatlng an optical frequency multiplexed signal to be transmitted to the receiving equipment.
In accordance with another aspect of the invention, there is provlded an optical communicatlon method for transmitting signal light having a first polarization state includlng the steps of:
a) mlxlng the slgnal llght and local light havlng a second polarlzatlon state, after the flrst and second polarlzatlon states have been made colncldent wlth each other, and produclng mlxed llght;
b) optlcally transmlttlng the mixed light as ~0 transmitted optical light; and c) detectlng an intermedlate frequency element produced by a dlfference between a frequency of the slgnal light and a frequency of the local light.
In accordance wlth another aspect of the invention, there is provlded an optlcal communlcation method for transmitting more than one optical signal having optlcal carrlers of dlfferent frequencles lncludlng the steps of:

- 8b -a) mixlng each of the more than one optlcal slgnal and a correspondlng one of local llght slgnals after polarlzatlon states of one of correspondlng more than one optlcal slgnal have been made colncldent wlth each other, and produclng a multlplexed optlcal slgnal;
b~ optlcally transmlttlng the multlplexed optlcal slgnal; and c) detectlng an lntermedlate frequency element produced by a dlfference between a frequency of each of the more than one optlcal slgnal and a frequency of the correspondlng one of the local llght slgnals.
In accordance wlth another aspect of the lnventlon, there ls provlded an optlcal communlcatlon method lncludlng the steps of:
a) allocatlng transmlsslon slgnals on more than one channel havlng different mlcrowave frequencles;
b) modulatlng carrlers of the microwave frequencles with the transmlssion slgnals for produclng modulated slgnals;
c) combining the modulated transmlsslon slgnals for ~0 produclng a multlplexed slgnal;
d) modulatlng an optlcal frequency modulator wlth the multlplexed slgnal for producing a modulated optical slgnal;
e) mlxlng the modulated optlcal slgnal wlth local llght wlth thelr polarlzatlon state made colncldent wlth each other and optlcally transmittlng the mlxed llght;
f) detectlng an lntermedlate frequency element produced by a dlfference between a frequency of the modulated optlcal 2 ~
- 8c -signal and a frequency of the local llght to thereby convert the modulated optlcal slgnal to an electrlcal slgnal;
g) extracting correspondlng channel signals from the electrlcal signal by electric fllters; and h) demodulating each of the correspondlng channel signals.
In accordance wlth another aspect of the invention, there ls provlded an optlcal communication system for transmltting a signal light having a plurality of data signals from sending equipment to receiving equipment, the sending equipment comprising:
a plurality of optical modulated signal generating units, each comprising a plurality of modulators modulating carriers having different microwave frequencles with transmlsslon signals and outputtlng frequency modulated signals;
addlng means for generatlng a mlcrowave frequency multiplexed signal by adding the frequency modulated signals;
and an optical frequency modulator generating an optical frequency modulated signal by frequency modulation with said microwave frequency modulated signals;
optical mixing means for mlxlng each said optical frequency modulated signal generated by said optical frequency modulator in said plurality of optical frequency modulated signal generating units and generating an optical frequency multlplexed signal; and ' $
- 8d -an optical frequency discrlminator, connected to said optlcal mixing means, receiving and divlding the optical frequency multiplexed signal into two signals, a first one of the two signals having a time delay wlth respect to a second one of said two signals, and mixing the two signals to generate an optical frequency multiplexed slgnal to be transmltted to the receivlng equlpment.
In accordance wlth another aspect of the lnventlon, there is provlded an optical communicatlon system for transmlttlng to recelvlng equlpment a slgnal llght optlcally modulated by a mlcrowave frequency multlplexed slgnal, and optical communication system comprislng:
sendlng equipment optlcally modulating the signal light, sald sending equipment comprlslng:
a plurality of optlcal frequency modulated signal generatlng unlts, each optlcal frequency modulated signal generating unlt comprislng:
a plurallty of modulators modulatlng carrlers of different mlcrowave frequencles wlth each of transmlsslon slgnals correspondlng to the carrlers;
addlng means for generatlng a mlcrowave frequency multlplexed slgnal by addlng modulatlon slgnals output by sald plurallty of modulators; and an optlcal frequency modulator generatlng an optlcal frequency modulated signal by frequency modulation with sald frequency multlplexed slgnal;
optlcal mlxlng means for mlxlng each optlcal frequency ~ 5 ~ ~ 2 ~
- 8e -modulated slgnal generated by sald plurallty of optlcal fre~uency modulated slgnal generatlng unlts and for generatlng an optical frequency multlplexed slgnal; and an optlcal frequency dlscrlmlnator dlvldlng the optlcal multiplexed slgnal lnto a flrst slgnal and a second slgnal, the flrst slgnal being delayed by a delay tlme wlth respect to the second slgnal, and for generatlng the optical frequency multiplexed signal to be transmitted to said recelving equipment by mlxlng the first slgnal and the second slgnal.
In accordance wlth another aspect of the present invention, there ls provlded an optlcal communlcatlon system for transmitting a signal light carrying a plurality of data signals from a single transmltter to receiving equipment, said transmitter comprlslng:
a plurallty of optical frequency modulated signal generatlng unlts, each comprising:
a plurality of modulators modulatlng carrlers havlng dlfferent mlcrowave frequencies wlth a plurallty of transmlsslon slgnals and outputtlng flrst transmlsslon slgnals;
a plurality of band limiting filters band limiting one of the data slgnals and the flrst transmlsslon signals, and outputting second transmlssion slgnals;
addlng means for generatlng a mlcrowave frequency multlplexed slgnal by addlng the second transmlsslon slgnals;
and an optlcal frequency modulator generating an optlcal - 8f -frequency modulated signal by frequency modulatlon wlth sald microwave frequency multlplexed slgnal; and optical mlxing means for mlxlng each optlcal frequency modulated signal generated by said optlcal frequency modulator ln said plurality of optlcal frequency modulated slgnal generatlng unlts and generatlng an optlcal frequency multlplexed slgnal to be transmltted to the recelvlng equlpment.
In accordance wlth another aspect of the lnventlon, there ls provlded a sending device for transmlttlng a signal llght carrying a plurality of data signals to a receiving devlce ln an optlcal communlcatlon system, comprlslng:
a plurallty of optlcal frequency modulated slgnal generatlng unlts each comprislng:
a plurallty of modulators modulatlng carriers having different mlcrowave frequencles wlth a plurality of transmlsslon signals and outputting first transmission slgnals;
a plurallty of band llmltlng fllters band llmltlng one of the data slgnals and the flrst transmlsslon slgnals, and outputtlng second transmlsslon slgnals;
addlng means for generating a microwave frequency multlplexed slgnal by addlng the second transmission signals;
and an optlcal frequency modulator generatlng an optlcal frequency modulated slgnal by frequency modulation with said microwave frequency multlplexed slgnal; and optlcal mlxlng means for mlxlng each optlcal frequency - 8g -modulated slgnal generated by sald optlcal frequency modulator ln sald plurallty of optlcal frequency modulated slgnal generatlng unlts and generatlng an optlcal frequency multlplexed slgnal to be transmltted to the recelvlng devlce.
Brlef Descrlptlon of the Drawlngs Flgure 1 ls a block dlagram of a conventlonal optlcal FDM system for transmlsslon of four-channel 622-Mb/s slgnals;
Flgure 2 ls a block diagram of a conventlonal optical TDM system for transmlsslon of four-channel 622-Mb/s signals;
Figures 3 to 6 lllustrate the fundamentals of optical communicatlon systems of the present inventlon;
Figures 7A to 7C lllustrate the channel allocation ln the optical communlcatlon systems of the - 9 ~

present invention;
Figure 8 illustrates a general spectrum of a modulated signal;
Figure 9 illustrates frequency allocation at the 5 IF stage of an SCM optical heterodyne detection system of the present invention;
Figure 10 is a block diagram of an optical communication system according to a first embodiment of the present invention;
Figure 11 illustrates a specific arrangement for four-channel 622-Mb/s signal transmission according to the first embodiment;
Figure 12 illustrates an arrangement of the receiver in a second embodiment of the optical 15 communication system of the present invention;
Figure 13 illustrates an arrangement of the receiver according to a third embodiment of the optical communication system of the present invention;
Figure 14 is a diagram for use in explanation of 20 the principle of the frequency discrimination in the third embodiment;
Figure 15 is a block diagram of a first embodiment of an optical exchange system of the present invention;
Figure 16 is a block diagram of a second - 10 ~ 2 ~

embodiment of the optical exchange system of the present invention;
Figure 17 is a block diagram of a fourth embodiment of the optical communication system of the 5 present invention;
Figure 18 is a block diagram of a fifth embodiment of the optical communication system of the present invention;
Figure 19 is a block diagram of a sixth 10 embodiment of the optical communication system of the present invention;
Figure 20 is a block diagram of a seventh embodiment of the optical communication system of the present invention;
Figure 21 is a block diagram of an eighth embodiment of the optical communication system of the present invention;
Figure 22 is a block diagram of a ninth embodiment of the optical communication system of the present invention;
Figure 23 is a block diagram of a tenth embodiment of the optical communication system of the present invention; and Figure 24 is a block diagram of an eleventh embodiment of the optical communication system of the 3 ~ ~

present invention.
Detailed Description of the Preferred Embodiments The present invention is an improvement on the conventional SCM optical communication system and its basic configurations are illustrated in Figures 3 to 6.
Referring first to Figure 3, at the transmitting end, microwave carrier frequencies f1 to fk are 10 allocated to channels (the number of channels is supposed here to be k), respectively. In modulators (MOD) 31-1 to 31-k, the carriers of f1 to fk are modulated with data signals D1 to Dk on the channels.
The modulated carriers output from the modulators 31-1 15 to 31-k are filtered by bandpass filters 32-1 to 32-k, respectively. The output signals of the filters are added together to produce a microwave frequency-division multiplexed signal, which, in turn, frequency rnodulates an optical frequency modulator 22. The 20 frequency modulated signal output from the optical frequency modulator is translnitted over an optical transmission fiber 34.
At the receiving end, the optical signal transmitted over the optical fiber is detected by an 25 optical detector 35, whereby it is converted to an B

3 2 ~

electric signal. The resulting electric signal is applied to bandpass filters 36-1 to 36-k for separation into channels. The output signals of the bandpass filters 36-1 to 36-k are demodulated by 5 demodulators (DEMOD) 37-1 to 37-k, respectively.
In the other system of Figure 4, at the transmitting end, there are provided n optical frequency-modulated signal generating sections A1 to An each of which is the same in arrangement as the transmitter of Figure 3 constructed from the modulators 31-1 to 31-k, the bandpass filters 32-1 to 32-k and the optical frequency modulator 33. However, the optical frequency modulators 33 in the optical frequency-modulated signal generating sections A1 to 15 An have their respective optical carriers of different frequencies fs1 to fsn The optical frequency-modulated signal generating sections A1 to An produce their respective optical frequency-modulated signals in the same manner as in Figure 3. The optical frequency-modulated signals are mixed in the optical combiner or coupler 38 to produce an optical frequency-division multiplexed signal which, in turn, is transmitted over the optical transmission fiber 34.
At the receiving end, the optical frequency-25 division multiplexed signal is detected by the optical detector 35, whereby it is converted to an electric signal. The resulting electric signal is applied to m bandpass filters 36-1 to 36-m, which correspond in number to all the channels or part of the channels, 5 for separation into channel components. The channel components are demodulated by demodulators 37-1 to 37-m.
In the arranyements of Figures 3 and 4, the modulators 31-1 to 31-k may employ any of analog and 10 digital modulation systems which include amplitude modulation (AM), frequency modulation (~M), phase modulation (PM), intensity modulation, amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), etc. All the modulators 31-1 to 15 31-n are not required to employ the same modulation system, but more than one modulation system may be employed in combina~ion.
As the optical frequency modulator 33, use may be made of a semiconductor laser (a broadband DFB type 20 semiconductor laser, in particular). In this case, the direct modulation of its bias current permits the optical frequency modulation.
As the detection system in the optical detector 35, use may be made of an optical heterodyne or 25 homodyne detection system using local light, a direct 7 ~ ~ ~

detection system using optical filters, a detection system using an optical freyuency discriminator, etc.
In the case of the direct detection system using optical filters, the electrical bandpass filters 36-1 to 36-k become unnecessary, but as many optical filters as there are channels and demodulators succeeding the respective optical filters are needed instead. In the case of the optical heterodyne detection system, by adjusting the oscillation 10 frequency of its local light source, it becomes possible to select a channel among channels. In this case, only one set of a bandpass filter and a demodUlator has to be provlde~ to follow the optical detector 35, irrespective of the number of channels.
The optical communication system shown in Figure 3 or Figure 4 may be employed to implement an optical exchange system or an optical switching system. That is, the optical exchange system is implemented by 20 inputting the demodulated signals in the optical communication system to an electronic exchange system having as many input/output ports as there are channels and performing arbitrary exchanges of channels. In addition, the optical exchange system 25 will also be implemented by dividing the optical - 15 ~

signal transmitted in the above-described optical communication system into as many optical signals as there are transmission channels using branch lines, optical-heterodyne detecting the separate optical 5 signals using separate local light emitters and selecting an arbitrary optical signal from the optical signals.
Next, Figure 5 illustrates an application of the SCM optical communication system of the present 10 invention to an optical distribution network. As shown in the figure, in a transmitting station A, which may be a central office or a head end, different microwave carrier frequencies f1 to fk are allocated to channels (here their number is k) carrying data 15 signals D1 to Dk and the carriers of frequencies f1 to fk are modulated by the data signals D1 to Dk, respectively, in modulators 41-1 to 41-k. The modulation system in this case may be any of ASK, FSK, PSK, AM, FM, PM, etc. Subsequently, the resulting 20 modulated signals are added together in a combiner 42, such as a multiplexer, thereby producing a microwave frequency-division multiplexed signal. The multiplexed signal modulates (AM, FM, PM, etc.) an optical modulator 43. The resulting modulated optical 25 signal (SCM optical signal) is transmitted over an - 16 ~

optical transmission fiber 44.
The optical signal output from the transmitting station A is transmitted to each subscriber C via a repeater station B which is generally a local office, a hub, a remote terminal, etc. In the present invention, in the repeater station B, the optical signal transmitted from the transmitting station A and local light output from a local light source 45 which is common to all the subscribers C are mixed in an optical mixer 46 and the mixed signal branches off from an optical branch unit 47 to the subscriber stations C.
In each subscriber station C, the optical signal transmitted from the repeater station B is detected by an optical detector 48, whereby it is converted to an electric signal. The electric intermediate frequency signal is applied to electric bandpass filters 50-1 to 50-k via an amplifier 49, thereby separating the channel components in the optical signal. The channel components are demodulated by demodulators 51-1 to 51-k.
In the above arrangement, by transmitting a light signal obtained by coupling light sighals modulated with SCM signals in the optical modulators having different carrier frequencies, wavelength (frequency - 17 ~ 3 ~ ~

of light)-division multiplex transmission of coherent SCM optical signals is made possible.
To implement a bi-directional optical transmission system, the arrangement of Figure 5 may be modified such that a different microwave carrier frequency is allocated to each of subscribers, each carrier is modulated with a signal from a corresponding respective subscriber to produce a 10 modulated signal, an optical modulator is modulated with the modulated signal to produce an optical modulated signal, and the optical modulated signal is, in turn, transmitted as an up signal.
Next, Figure 6 illustrates a coherent optical 15 transmission system of the present invention which incorporates countermeasures against polarization fluctuation.
As shown ln Flg. 6, ln a transmltter T, a modulated optical signal obtained by modulating a 20 transmission liyht source 61 with a data signal D and a local optical signal output from a local light source 62 are mixed in an optical mixer 64 after their polarization states have been made to coincide with each other by a ~olariza~.ion controller 63. The 25 resultant mixed li~htwave trarIslllitted over an optical fiber 65.

t.i~

In a receiver R, the optical signal transmitted over the optical fiber is optical-heterodyne detected by an optical receiver 66 for conversion to an electrlc signal. The resultant IF signal ls sent 5 via an amplifier 67, an electric filter 68, etc., to a demodulator 69.
This arrangement, which uses an optical heterodyne detection system, may also be modified to use an optical homodyne detection system by making a transmission optical signal and a local optical signal coincident with each other in their frequency and phase.
The arrangement of Figure 6 may be modified to permit coherent wavelength (frequency of light) -division multiplexed transmission by transmitting light subjected to wavelength synthesis after polarization synthesis to a plurality of transmission light sources having different oscillation wavelengths.
The arrangement of Fiyure 6 may be applied to such an SCM optical communication system as shown in Fiqure 5. Moreover, it may be applied to a wavelength-division multiplex transmission system for coherent SCM optical signals.
In the arran~emerlts of Figures 5 and 6, as the modulation system use may be made any of analog and digital modulation systems which include amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), intensity modulation, amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), etc. All the modulators need not employ the same modulation system, but more than one modulation system may be employed in combination.
As the optical modulator, use may be made of a semiconductor laser (a broadband DFB type semiconductor laser, in particular), an LiNbO3 optical modulator, etc. In the case of the coherent optical system in particular, as the modulation system, use may be made of any of coherent modulation systems such as amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), etc.
In addition, as the optical receiver, use may be made of an optical heterodyne or homodyne receiver or a receiver using optical filters.
In the arrangements of Figure 3 and 4, carriers of different frequencies f1 to fk allocated to the respective channels are modulated with data signals D1 to Dk, respectively, and the resultant modulated signals are made to pass through bandpass filters 32-1 to 32-k and then combined (frequency-division multiplexed). The spectrum of the data signal and the frequency allocation to channels in the present case are illustrated in Figures 7A to 7C. Here, the channel spacing and the bandwidth of each of the bandpass filters are selected to avoid crosstalk between adjacent channels.
In general, the spectrum of a modulated signal has a shape such as that shown in Figure 8 which has high-order Bessel function components centered at multiples of the frequency of a data signal around the carrier frequency. The spectrum of Figure 8 corresponds to the case where a carrier having a frequency of fc is modulated with a frequency B.
Thus, the bandwidth of each bandpass filter is selected to reject components which overlap the bandwidth of an adjacent channel. In practice, in order to increase the number of channels, it is desired that bandpass filters each having as narrow a bandwidth as possible be used. For example, in the case of Figure 8, it is common to pass a component ranging from fc-B to fc+B, the so-called main lobe component. Therefore, the frequency range from fc-B
to fc+B may be selected as the bandwidth of the bandpass filter. The channel spacing in the 25 frequency-division multiplexed signal obtained by 2 1 ~ ~m~

synthesizing signals which passed through the bandpass filters 32-1 to 32-k can be put close up to twice the bit rate in the case of a digital signal. Therefore, it becomes possible to easily produce a high-density frequency-division multiplexed signal.
In the above, a description was made of the method by which a modulated signal on each channel was subjected to bandpass filtering. Instead, use may also be made of a method by which an original data signal is subjected to lowpass filtering before modulation.
That is, a data signal (a baseband signal), if it is a digital signal at a bit rate B, has such a Bessel-function-like spectrum as shown in Figure 7A.
Thus, by extracting only the main lobe component (0 <
f < B) by the use of a lowpass filter and then modulating a carrier with the main lobe component, a modulated signal which is the same as that when bandpass filtering is used will be obtained as shown 20 in Figure 7b-If such modulated signals are added together, then such a SCM signal as shown in Figure 7c will be obtained.
According to the present invention, as described 25 above, a frequency-division multiplexed signal is -produced by electric stages and then an optical carrier signal is frequency modulated with the frequency-division multiplexed signal in the optical frequency modulator 33, whereby an optical -modulated signal is produced. Hence, only one optical modulator suffices.
Since, as described above, the channel spacing can be set narrow, about twice the bit rate at the transmitting end, a wideband receiver can be used at the receiving end for receiving all the channels or more than one channel at a time, permitting a considerable increase in the number of channels that can be received.
Moreover, according to the present invention, the 15 frequency-division multiplexing can be implemented by the use of an inexpensive multiplexer such as a microwave combiner or coupler. That is, there is no need for an expensive multiplexer required of time-division multiplexing. This enables optical 20 communication systems to be constructed at a very low cost.
Next, in the arrangement of Figure 5, carriers of frequencies f1 to fk allocated to channels are modulated with data signals D1 to Dk, respectively, 25 and then synthesized (frequency-division multiplexed).

- 23 ~

The frequency allocation to channels in the frequency-division multiplexed signal in this case will also be shown as in Figures 7A to 7C. The channel spacing needs to be selected to avoid crosstalk between adjacent channels. However, by lowpass filtering only the main lobe component of a data signal (baseband signal) or by bandpass filtering only the main lobe component after modulation, it becomes possible to put the channel spacing close to about twice the data rate in the case where digital signals are handled.

Next, the optical modulator 43 is subjected to modulation with the multiplexed signal and then an optical modulated signal is transmitted optically.
The modulated light is mixed with local light common to subscribers in the repeater station B and then branches to each subscriber. Subsequently, the modulated light is heterodyne detected by the optical receiver 48 in each subscriber's station. The resultant IF signals are separated for each of channels and then demodulated. For information, in Figure 9 there is shown an example of frequency allocation at IF stage in the case where frequency modulation (FM) is produced in the optical modulator 43.

In the case of usual optical heterodyne - 24 - ~ 2 ~ ~

detection, in order to make the power input to an optical receiver as large as possible, the local light source is generally placed in the neighborhood of the optical receiver. In an optical subscriber system, 5 however, the distance between a repeater station B and a subscriber C is no more than several kilometers A loss caused by optical transmission over the distance is in the order of 1 dB tin the case where a single mode fiber is used). Thus, as in the present 10 invention, the local light source 45 can be placed in the repeater station B to permit parallel transmission to subscribers C by the use of direct fiber branching.
In order to suppress the deterioration of receiver sensitivity to the utmost, it is only required that 15 the branching loss be compensated for by an optical amplifier.
According to the present invention, as described above, a single local light source 45 placed in the repeater station B permits simultaneous optical 20 heterodyne reception by more than one subscriber C.
Therefore, cost per subscriber is reduced considerably and a low-cost coherent optical SCM network is implemented.
Moreover, since, as described above, the ch~nnel 25 spacing can be narrowed up to about twice the data 2 5 ~ ~a signal rate at the transmitting end, the receiving end is allowed to use a wideband receiver for simultaneous reception of all the channels or more than one channel. This permits a marked increase in the number 5 of channels which can be received. This possess a great advantage over the wavelength-division multiplex transmission in the conventional coherent optical communication system in which there is a limit to the number of channels that can be received at one time.
Next, in the arrangement of Figure 6, the local light source 62 is placed in the transmitting station T, transmission light and local light are mixed after their polarization states have been made coincident with each other and the resultant IF signal is 15 transmitted optically. Thus, the countermeasures against polarization fluctuation can be taken without imposing a burden of cost on the receiver R. In other words, stable reception is allowed without depending on the polarization state of transmission light.
In the case of usual optical heterodyne detection, in order to make the power input to an optical receiver as large as possible, the local light source is generally placed in the neighborhood of the optical receiver as described above. In an op~ical 25 subscriber system, however, the distance between a - 26 _ ~ 7 ~

transmitting station and a subscriber is relatively short and the loss of the local light is not large.
Thus, the arrangement of Figure 6 is possible.
Where the present invention is applied to an 5 optical distribution system, the arrangement of Figure 5 is suitable when the transmitting station is relatively distant from subscribers because it is desired that the local light source and the optical receiver be placed as close to each other as possible 10 in terms of optical loss as described above. On the other hand, where subscribers are relatively close to the transmlttlng statlon, the use of the arrangement of Flgure 6 ls sultable.
~lereinafter, preferred embodilllents of the present invention will be described with reference to the drawings .
Figure 10 is a block diagram of a first embodiment of an optical communication system according to the pre~n~ inven~ion. This embodiment uses an optical heterodyne detection system as the detection system at the receiving end.
At the transmittinc~ end, carriers of frequencies f1 to fk allocated to chdnnels are modulated with data signals D1 to Dk in modulators 31-1 to 31-k, respectively. The resultant modulated signals are filtered by bandpass filters 32-1 to 32-k. Here, as the modulation system used in the modulators, use may be made of any of frequency modulation (FM), amplitude modulation (AM), phase modulation (PM), intensity 5 modulation, amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), etc.
Depending on transmission signals, the modulation system may vary between channels. The channel spacing and the bandwidth of each of the bandpass filters are 10 selected to avoid crosstalk between adjacent channels in the multiplexed signal as shown in Figures 7C and 8. In particular, each of the bandpass filters is designed to have a narrow bandwidth of fc ~ B to fc +
B.
Instead of using the bandpass filters, lowpass filters may be used to filter original data signals prior to modulation.
Subsequently, the signals allowed to pass through the bandpass filters are synthesized in a multiplexer 20 71 to produce a frequency-division multiplexed signal.
The multiplexer has only to have a function of merely adding electrical signals together. For example, use may be made of a simple and inexpensive device such as a microwave coupler. The resultant multiplexed signal 25 is used to frequency-modulate an optical frequency - 2~

modulator 33, thereby producing modulated light. The modulated light is transmitted over an optical transmission fiber 34. A semiconductor laser is used as the optical frequency modulator 33. The direct 5 modulation of a bias current of the laser will produce frequency modulation.
Next, at the receiving end, an optical heterodyne detector 35 comprises a local light source (semiconductor laser) 72, an optical receiver 73 and 10 an amplifier 74, whereby the light signal transmitted from the transmitting end is optically heterodyned.
The frequency allocation at the IF stage after the optical heterodyne detection is the same as in Figure 9. An electrical signal resulting from the heterodyne 15 detection branches to channels and is then filtered by bandpass filters 36-1 to 36-k, thereby deriving corresponding respective channel signals. The channel signals are demodulated by demodulators 37-1 to 37-k.
The present invention is compatible with coherent 20 detection, such as optical heterodyne detection, as compared with the conventional SCM-AM (IM) modulation system, and the optical heterodyne detection is permitted as in the present embodiment. Thus, not only can a high receiver sensitivity be achieved easlly, but all of the channels falllng wlthln the bandwidth of the optical receiver 73 can be received simultaneously. This possesses a great advantage over the optical frequency-division multiplex transmission in the conventional coherent optical communication 5 system in which the number of channels that can be received at a time is limited to one. Therefore, not only can the transmission distance be increased, but also the number of channels that can be received can be increased considerably.
Here, to make the function and effect of the present embodiment definite, a comparison will be made between four-channel 622-Mb/s signal transmission shown in Figure 11 and the conventional coherent transmission shown in Figures 1 and 2. In both cases, 15 each of the number of channels, the bit rate and the optical transmission capacity (2.5 Gb/S) is the same.
In the first place, comparing the present embodiment with the optical FDM system of Figure 1, in order to transmit 2.5-Gb/s information, the present 20 embodiment needs only one optical modulator, while the optical FDM system needs as many optical modulators as there are channels. However, the present embodiment needs an optical modulator with a wider frequency response characteristic. That is in the order of 6-8 25 GHz, which may be achieved for example by a multielectrode DFB type semiconductor laser.
Therefore, the present embodiment has a great advantage that transmission can be made by using one optical modulator.
In addition, in the present embodiment, the channel spacing at the transmitting end can be as small as approximately twice the bit rate, as described above. Therefore, the use of a wideband (approximately 10 GHz) receiver permits simultaneous 10 reception of all of channels or more than one channel.
Of course, as in the optical FDM system, the selective reception of only one channel is also possible.
Next, making a comparison between the present embodiment and the TDM optical transmission system, 15 the present embodiment can implement frequency-division multiplexing with a multiplexer such as a microwave coupler, while the TDM optical transmission system needs an expensive multiplexer for time-division multiplexing. Moreover, as the demodulation 20 bandwidth of the optical heterodyne detector, the present embodiment needs only the bandwidth required of each channel, while the TDM optical transmission system needs a bandwidth which is wider than that required of each channel by a factor of the number of 25 channels. In addition, an expensive demultiplexer is required to separate the channels.
As can be seen from the foregoing, compared with the conventional multiplexing optical transmission system, the present embodiment permits a very simple, 5 inexpensive, large-capacity optical transmission system .
Figure 12 is a block diagrarn of the receiving end according to a second embodiment of the optical communication system of the present invention.
As shown ln Flgure 12, in thls embodlment, the light signal transmitted from the transmitting end shown in Figure 10 over the optical transmission fiber 34 is applied to an optical branch unit 81, so that it divides into a plurali ty of channels . O~tical filters 1 5 82-1 to 82-]c separate the optical channel components, which are, in turn, sub jected to direct detection in optical receivers 83-l to ~33-k, whereby they are converted to electric signals.
The present embodill)ent also permits an optical 20 transmission system to be implemented, which is very simple in structure, low in cost and large in capacity as compared with the conventional multiplexed optical transmission system.
Figure 13 is a bl ock diagram of the receiving end 25 according to a third ernbodiment of the optical ~ i communication system of the present invention.
In this embodiment, the light signal transmitted from the transmitting end shown in Figure 10 over the optical transmission fiber 34 branches into channels 5 through an optical branch unit 81. The optical signal components of channels are separated and direct-detected by optical frequency discriminators 91-1 to 91-k. The arrangement of the optical frequency discriminators is well known. As an example, a 10 discriminator using an optical delay circuit is described herein. That is, the input optical signal (coherent light) is divided into two channels, thereby causing two optical signals I1 and I2 to pass through optical paths 101 and 102, respectively, so as to 15 delay the optical signal I2 by a time &S with respect to the optical signal I1. Subsequently, the two optical signals I1 and I2 are mixed again in an optical mixer 103 and the mixture is converted to an electric signal by an optical receiver 104. The 20 principle of the frequency discrimination in this case will be described below with reference to Figure 14.
Let the frequency of the input optical signal I
be fs~ Then I = cos (2~ ~ s l ~ s (l)) ~(l) 25 where ~s(t) stands for the phase.

~n ~ 33 ~

Supposing the dividing ratio to be A : B, I1 and I2 are glven by I, .--J~ cos (2 1~ r s L ~ s ( l ) ) ~ ~ ('~), 1 2 '= r3 cos (2 1t r s (l - r) ~ s (l- r)) (3) Thus, photocurrent J output from the optical receiver 104 will be given by J -- C -1- 2 ~ ~ co~ 7C r s r -1- ~ 5 (l) - ~ S (L - T )) ~ ' (4) In this equation, constant and higher frequency components are neglected.
From equation (4), J varies with fs with a period of 1/ ~-' as shown in Figure 14. As can be seen from this relationship ~etween J and fs~ frequencies in the range from Y point to Z point can be discriminated with a point X (corresponding to frequency fso) set as 5 a reference pOil1t. By setting the reference point of each of the optical frequency discriminators 61-1 to 61-k to a different frequency, the frequency discrimination can be performed for each of the channels.
Figure 15 is a block diagram of a first embodiment of an optical exchange system of the present invention.
In this embodiment, an electronic exchange unit 111, which has as many input/output ports (k x k) as there are transmission channels (K), is disposed to 7 ~ J

succeed the demodulators 37-1 to 37-k at the receiving end in the optical communication system shown in Figure 10, thereby making exchanges of channels. That is, signals recovered by the demodulators can be 5 exchanged by the electronic exchange unit 111.
The mere placement of an electronic exchange unit to succeed the demodulators in the optical communication system of the present invention permits an optical exchange system to be implemented easily.
1 0 If, in the arrangement of Figure 15, an electronic switch is disposed in place of the electronic exchange unit 1 1 1, an optical switching system can be implemented which permits switching between channels.
Figure 16 illustrates a second embodiment of the 15 optical exchange system of the present invention.
In thi s embodiment, as many optical heterodyne detectors each having a separate local light source as there are channels are provided at the receiving end to thereby implement a k x k coherent optical exchange 20 system. That is, the optical signal transmitted from the transmitting end shown in Figure 10 is divided into as many optical signals as there are transmission channels ( k in number ) . The divided optical signals are heterodyne-detected by their respective optical 25 heterodyne detectors 122-1 to 1 22-k having their respective local light sources producing different frequencies fL1 to fLk and an arbitrary channel is selected to thereby make an exchange of channels.
In the arrangement of Figure 15, channel exchange 5 is implemented by using the electronic exchange unit 111. In the present embodiment, on the other hand, the channel exchange is implemented by tuning the oscillation wavelength of the local light source of each of the optical heterodyne detectors 122-1 to 122-10 k and selecting a desired channel. In the presentembodiment as well, an optical switching system can be lmplemented by switchlng between channels.
The transmitter in each of the above embodiments uses the principle of the transmitter shown in Figure 15 3. If the principle of the transmitter shown in Figure 4 is used instead, the transmission of more channels will be implemented.
Figure 17 illustrates a fourth embodiment of the optical communication system of the present invention.
20 In this embodiment, an optical amplifier 130 for compensating for brarlching loss is disposed between the optical mixer 46 and the optical branch unit 47 in the repeater station B in the arrange~ent shown in Figure 5.
In Figure 17, in the transmitter A, carriers of B

- 36 - J ~

different frequencies f1 to fk allocated to channels are modulated with data signals D1 to Dk in the modulators 1-1 to 1-k. Here, the modulation system used in the modulators 41-1 to 41-k may be any of 5 amplitude modulation (AM), frequency modulation (FM), phase modulation (PM), intensity modulation, amplitude-shift keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), etc.
_ Modulated signals obtained from the modulators 10 are combined by a combiner, such as a microwave coupler, to produce a frequency-division multiplexed signal. The channel spacing of the multiplexed signal is selected to avoid crosstalk between adjacent channels. As me~ans therefor, a lowpass filter may be 15 used to allow only the main lobe component of each data signal to pass or a bandpass filter may be used to allow only the main lobe component of each data signal to pass after modulation. Subsequently, the multiplexed signal thus obtained is used to modulate an optical modulator 43 and the resultant modulated light is transmitted over an optical fiber 44. As the optical modulator 43, use may be made of a semiconductor laser or a LiNO3 modulator. As the modulation system used in the optical modulator 43, use may be made of any of coherent modulation systems - 37 ~

including amplitude modulation, frequency modulation, phase modulation, etc.
The optical signal transmitted from the transmitting station A in that way is transmitted to 5 the repeater station B through the optical fiber 44.
In the repeater station B, the incoming signal light is mixed in the optical mixer 46 with local light emitted from a local light source (for example, a semiconductor laser) 45 which is common to 10 subscriber's stations and then the resultant light is amplified by the optical amplifier 130 consisting of a fiber type light amplifier or a semiconductor light amplifier. Subsequently, in the optical branch unit 47 the amplified light branches into subscriber's 15 stations C through optical fibers.
In each subscriber's station C, the optical signal transmitted from the repeater station B is converted to an electric signal, whereby it is heterodyne detected. Subsequently, the resultant IF
signal is amplified by the amplifier 49 and then divides into channels. The bandpass filters 50-1 to 50-k separate channel signal components in the IF
signal and then the channel signal components are demodulated by the demodulators 51-1 to 51-k.
According to the present embodiment, only one local light source 45 provided in the repeater station B permits simultaneous optical heterodyne reception for more than one subscriber. Therefore, each subscriber's station needs no local light source and 5 hence the cost per subscriber can be reduced to a large extent.
Moreover, the loss due to optical branching can easily be compensated for because the mixed light is amplified by the light amplifier 130 provided in the 10 repeater station B before optical branching.
Of course, it is also possible to compensate for branching loss by a light amplifier provided for each of transmission lines after optical branching by the optical branch unit 47 instead of providing the common 5 light amplifier 130 followed by Lhe optical branching unit. Further, the light amplification may be provided before and after branching.
Next, countermeasures ayainst polarization fluctuation for carrying out the present invention 20 will be considered. In order to carry out the optical heterodyne receptlon, the slgnal llght and the local llght must be colncldent wlth each other in the state of ~olarization. If they are not coincident with each other, for example, in an extreme 25 case, in the case of linear polarization in which they ~: i are perpendicular to each other in polarization, the receptlon ls lmposslble. As countermeasures against such problems, there will be methods of using (1) a polarization preserving fiber, (2) a 5 polarization diversity reception system, (3) a polarization active control reception system, (4) a polarization scrambling system, etc. In (1), the polarization preserving fiber is not suitable because it is expensive and fibers which have already been installed become unavailable. In (2), the polarization diversity reception is promising for usual coherent optical transmission systems, but a subscriber's receiver needs a dual configuration, increasing its cost. In the present invention, a local light source and a receiver are away from each other and thus it is difficult to feed an IF signal back to the local light source. Thus, (3) and (4) will be considered hereinafter.
Figure 1~ is a block diagrarn of the repeater station B according to a ~ifth embodiment of the optical communicatioll sysl~ of the present :invention.
This embodiment us~s tl-lt: me~hod (3), i.e., the polarization active controL receptlon system as the countermeasures against polarization.
As shown in the figure, in the repeater station B, the signal light transmitted from the transmitting station A shown in Figure 17 and local light obtained from the local light source 45 via a polarization controller 131 are mixed in a 2 x 2 optical coupler 132 and then divided into two branches. A signal on one of the branches is transmitted to subscriber's stations and the other is used as a monitor signal for polarization control.
For polarization control, the other signal from 10 the optical coupler 132 is subjected to heterodyne detection in an optical rece~iver 133. At this point, as a detected slgnal, a 0 th-order beat slgnal (maln-carrier component) or one-channel component in the SCM
signal is derived througl1 a bandpass filter 134. The 15 power of the IF signal t}lUS obtained is measured by a power meter 135 and tl-e measured value is then compared with a reference value by a comparator 136.
Based on the difference between the measured value and the reference value, the polarization controller 131 20 is controlled by a polarization control circuit 137 to match the polarization state of the local light to that of the signal light, and the oscillation frequency of the local light source 45 is controlled by an AFC circuit 138, thereby maximizing the power of the IF signal. Such feedback to the polarization .
~ j .

state of the local light and the oscillation frequency permits very good receiver sensltlvlty.
As the sequence of feedback control ln thls case, using a slgnal swltchlng unlt, the dlfference slgnal f-rom the comparator 136 ls flrst applled to the polarization control clrcuit 137 to thereby perform the polarlzation control by the polarlzation controller 131, and then the difference signal is applied to the AFC circuit 138 to thereby control the oscillation frequency of the local light source 45.
As the polarization controller 131, for example, A
/4 (quarter-wavelength) plate and A / 2 (half-wavelength) plate may be used in combination.
Figure lg is a block dlagram of a sixth embodlment of the optlcal communlcation system of the present invention.
Thls embodiment uses the above method (4), i.e., the polarlzation scrambling system as countermeasures against polarization.
As shown in the flgure, a polarlzation scrambler 139 is dlsposed to follow the optlcal modulator 43 ln the transmlttlng statlon A shown in Figure 17, thereby scrambling the polarization in the modulated optical slgnal obtained from the optical modulator prior to optlcal transmission thereof.
Although there ls some reduction in recelver sensltlvlty, such scrambling of polarization permits polarization-insensitive optical heterodyne reception.
Figure 20 illustrates a seventh embodiment of the optical transmission system of the present invention.
5 This embodiment is directed to a bi-directional optical communication network using the optical communication system shown in Figure 17.
In this embodiment, subscriber stations C1, C2,...., Cn are allocated different carrier 10 frequencies fs1, fs2,...., fsn. The carrier is modulated with a subscriber's data signal in a modulator 141, and the resultant modulated signal is used to modulate an optical modulator 142 for data transmission. The modulated optical signal from the optical modulator 142 is transmitted, as an up signal, from an optical coupler 143 to the repeater station B
over the same route as the down line. In this case, as the optical modulator 142, use may be made of an inexpensive semiconductor laser for low-speed optical modulation because the transmission distance is short, and the data signal from each subscriber is usually not of large capacity. As the modulation system, amplitude modulation will be used.
The optical signal from each subscriber station is converted to an electrical signal by an optical receiver 144 in the repeater station B. A signal of frequencies allocated to each subscriber C is detected by a bandpass filter 145. The resultant RF-signal combined signal (SCM signal) modulates an optical 5 modulator 146 for up signals. The modulated optical signal is coupled with the optical fiber 44 by an optical coupler 147 for transmission to the transmitting station A over the optical fiber.
In the transmitting station A, the transmitted 10 optical signal branches in an optical branch unit 148 and then detected by an optical receiver 149. As the optical modulation and detection system in this case, use may be made of the coherent modulation and demodulation system as in the case of the down signal 15 or the amplitude modulation - direct detection system if the speed of the up signal is low in comparison with the down signal.
According to the present embodiment, a bi-directional optical communication network can be implemented at a very low cost because the arrangement in which a single local light source 45 common to all the subscriber stations is installed in the repeater station permits bi-directional optical ~ommunication.
Of course, an up signal from each subscriber can 25 also be used as a request signal to the repeater statlon B or the transmltting station A. With the above arrangement, all of the channels can be received by each subscrlber station. Where there are too many channels or each subscriber's receiver has an lnsufflclent bandwidth to cover all the channels, the request signal can be used to tune the osclllatlon frequency of the local llght source 45 to select a desired channel. In this case, however, more than one local llght source would be required in the repeater station B.
In the above fourth to seventh embodiments, a slngle transmisslon llght source (optical modulator 43) is used. The use of two or more transmisslon llght sources with different oscillation frequencles to transmit llght composed of optlcal slgnals each modulated with an SCM signal as in the above embodiments permlts wavelength-dlvlslon multlplex transmission of coherent SCM optical signals. Optical communication of still larger capacity is made posslble.
Next, a descrlption will be made of embodlments lncorporatlng countermeasures agalnst polarlzation which permit a more inexpensive coherent SCM optical transmission system.
Figure 21 illustrates an eighth embodiment of the ~' optical communication system of the present invention.
The present embodiment is an application of the arrangement shown in Figure 6 to the SCM optical transmission system.
As shown in Figure 21, in the transmitting station A, a modulated optical signal (SCM optical signal) output from the optical modulator 43 is mixed with the local light output from the local light source 62 in the outical n~i~er 64. Before mixing, the local light is made coincident with the modulated signal in polarization in tlle polarization controller 63. The mixed light is transmitted over the optical fiber 44.
The optical signal thus transmitted optically branches in the optical branch unit 47 in the repeater station B placed in the neighborhood of subscribers for fiber transmission to eacl1 subscriber.
In each subscriber station C, as in the above elnbodimel1ts, the o~ticdl sigl1al transmitted frorn the repeater station B is cor-verted to an electric signal by the optical receiver ~8, whereby it is l~eterodyne detected. The resultant Il~ signal is separated into Cllarlllel COIIII~On~lltS by LJdll~lL~SS filters, and then the channel components are delllodulated by their respective delnodulators-t~ i - 46 ~

There is no need for an optical branch if transmission is made between one transmitter and one receiver.
According to the present embodiment, since the local light source 62 is installed in the transmitting 5 station A, the transmission light and the local light are mixed after they have been made coincident with each other in polarization, and the resultant mixed optical signal is transmitted. The countermeasures against polarization in the coherent SCM optical 10 transmission system can be implemented without irnposing a burden of cost on each subscriber C.
To counter polarization, use may be made of a polarization active control receiving system such as that shown in Figure 18 or a polarization scrambling system such as that sl-lown in Figure 19. With the former system, although it has high receiver sensitivity, there is some increase in cost because it is technically difficult to implement the polarization controller 131 which traces random variations in polarization. With the latter, it is somewhat difficult to implement the polarization scrambler 139 which can take sufficient measures against high-speed transmission. ~owever, the systerll used in the present embodiment has no such problems and can implement a polarization-independent coherent SCM optical ~ ' i .....

transmission system at a low cost corresponding to a network.
Figure 22 illustrates a ninth embodiment of the optical communication system of the present invention.
5In this embodiment, a light amplifier 150 is installed as a post amplifier in the transmitting station A.
According to this arrangement, the power of a signal from the transmitting station A is amplified, 10 thus increasing the system margin, the transmission distance and the number of branches.
Figure 23 illustrates a tenth embodiment of the optical communication system of the present invention.
In this embodiment, a light amplifier 151 is 15 installed in the preceding stage of the optical branch unit 47 in the repeater station B in the arrangement of Figure 21.
According to this arrangement, branch loss caused by the optical branch unit 47 can be compensated for 20 by the light amplifier 151, thus permitting an increase in the number of branches.
Following the branching process by the optical branch unit 47, a light amplifier may be used for each line. In this case, though an improvement is made in 25 receiver sensitivity, as many light amplifiers as - 48 - ~ ~ 5 ~

there are subscribers will be needed. In addition, light amplifiers may be provided in the preceding and succeeding stages of the optical branch unit.
Furthermore, these arrangements and the arrangement of 5 Figure 22 may be used in combination.
Figure 24 illustrates an eleventh embodiment of the optical communication system of the present invention. This embodiment is arranged such that an angle modulated optical signal is converted to an 10 intensity modulated optical signal which is subsequently transmitted.
As shown in the figure, in the transmitting station T, an SCM optical signal resulting from production of optical angle modulation (FM, PM) in the 15 optical modulator 43 is transmitted over the optical fiber after being converted to an amplitude modulated optical signal by an optical frequency discriminator 152.
In the receiver R, the optical signal transmitted 20 from the transmitting station is converted to an electrical signal by the optical receiver 66, and the resultant RF signal is applied via the amplifier 67 to an RF demodulator 153 where it is demodu~ated.
As described so far, the present invention is 25 permltted to use any of AM, FM and PM as lts - 49 ~

optical modulation system. As described in the Related Art, the amplltude modulatlon, when it ls produced by intensity modulation of a semiconductor laser, requires a linear relationship between 5 intensity modulation and optical output. Thus, there is a limit to the modulation bandwidth. Under the present conditions, the bandwidth lies in the range of 1 to 2 GHz at best. Therefore, wideband information transmission is liable to be affected by signal 10 distortion, thus limiting the capacity of information to be transmitted. Ilowever, the amplitude modulation offers an advantage in that receivers can be made simple in structure because direct detection as well as coherent detection can be employed.
The arrangement of Figure 24, which is obtained on the basis of such viewpolnts, transmits amplitude modulated light, so that receivers can be made simple in structure, and signal transmission is not affected by fluctuations in tlle ~tdte of polari~ation within the optical fiber q4.
The characteristics of the optical frequency discriminator 152 serving dS rneans of converting optical angle modulation to optical amplitude mo~ulation is tlle same dS tl-lat shown in Figure 14.
~or optical frequency discrimination, a method of using a Michelson interferometer would be considered.

To convert optical angle modulatlon to optical amplltude modulatlon, ~ot only an optlcal frequency dlscrlmlnator whlch ls used hereln may be used.

In the above eig~lth to eleventh embodiments, a single transmission light source (optical modulator 43) is used. The transmission to two or more _ transmission light sources with different oscillation 10 frequencies light which has been synthesized after polarization synt~lesis (or angle modulation to intensity modulation conversion) as in the above embodiments permits wavelength-division multiplexed transmission of coherent SCM optical signals with no 15 polarization dependence. Optical communication with still larger capacity is made possible.
According to the present invention shown in Figures 3 and 4, frequency-division multiplexing can be implemented easily ~y the use of a microwave coupler and a multiplexed signal can be modulated optically by the use of a single optical modulator.
Thus, large-capacity frequency-division multiplexed optical transmission can be implemented easily and at a low cost. Moreover, since each channel signal can be filtered by an electric (~andpass) filter in frequency-division multiplexing of channel signals, the channel spacing can be set narrow as compared with the conventional optical frequency-division multiplexing system. As a result, a single receiver 5 permits simultaneous reception of a large number of channels. Furthermore, a high-sensitivity reception system such as an optical heterodyne system can be used, permitting long-distance transmission with high receiver sensitivity or a distribution network to be 10 implemented easily.
With the invention in which incoming signal light and local light are mixed before branching as shown in Figure 5, the use of a single optical carrier permits high-density frequency-division multiplexed 15 transmission. Thus, a high-sensitivity coherent high-speed transmission network capable of long-distance, multidistribution transmission can be implemented easily and at low cost. Moreover, the use of the present invention permits the implementation of an 20 optical communication system having a wide range of applicability such as a bi-directional optical transmission network.
In addition, as described in Figure 6, the invention in which signal light and local light are 25 mixed after their polarization states have been made 'l -coincident with each other and then transmitted, or the invention in which an angle modulatéd optical signal is conver~ed to an intensity modulated optical signal for transmission permits, multi-distribution 5 transmission. Moreover, a polarization-insensitive high-sensitivity coherent optical transmission network can be implemented easily and at low cost. An optical communication network having a wide range of applicability can be bullt.

Claims

1. An optical communication system for transmitting a signal light carrying a plurality of data signals from sending equipment to receiving equipment, said sending equipment comprising:
a plurality of optical frequency modulated signal generating units, each comprising:
a plurality of modulators modulating carriers having different microwave frequencies with a plurality of transmission signals and outputting first transmission signals;
a plurality of band limiting filters band limiting one of the data signals and the first transmission signals, and outputting second transmission signals;
adding means for generating a microwave frequency multiplexed signal by adding the second transmission signals;
and an optical frequency modulator generating an optical frequency modulated signal by frequency modulation with said microwave frequency multiplexed signal; and optical mixing means for mixing each optical frequency modulated signal generated by the optical frequency modulator in said plurality of optical frequency modulated signal generating units and for generating an optical frequency multiplexed signal to be transmitted to the receiving equipment.

2. An optical communication system as claimed in claim 1, wherein said optical frequency modulator is a semiconductor laser, and the optical frequency modulated signal is generated by direct modulation of a bias current of the semiconductor laser.

3. An optical communications system according to claim 1, wherein said receiving equipment comprises:
optical detecting means for mixing the optical frequency multiplexed signal transmitted from the sending equipment and local Light to obtain an optical signal assigned an intermediate frequency band, and converting the optical signal to an electric signal of an intermediate frequency band;
a plurality of band filters each of which extracts a selected signal for each channel from each electric signal;
and a plurality of demodulators respectively connected to said plurality of band filters and demodulating each selected signal obtained by said plurality of band filters.

4. An optical communication system as claimed in claim 3, wherein said receiving equipment further comprises an electronic exchanger connected to said plurality of demodulators for optionally switching transmission channels, the electronic exchanger comprises input ports and output ports, the electronic exchanger receives a demodulation signal output from one of said plurality of demodulators at the input ports, the electronic exchanger outputs a resultant signal at the output ports, and each of numbers of the output ports and the input ports is equal to a number of the transmission channels.

5. An optical communication system as claimed in claim 1, wherein said receiving equipment comprises:
an optical branch unit branching the optical frequency multiplexed signal transmitted from the sending equipment into optical signals equal in number to a number of transmission channels;
a plurality of detecting means converting the optical signals to first electric signals each having an intermediate frequency band;
a plurality of band filters extracting second electric signals selected for each channel from the first electric signals having the intermediate frequency band; and a plurality of demodulators, each connected to one of said plurality of band filters, and demodulating one of the second electric signals.

6. An optical communication system according to claim 1, wherein the receiving equipment comprises:
mixing means for obtaining an optical signal having an intermediate frequency by mixing the optical frequency multiplexed signal transmitted from the sending equipment and local light;
an optical branch unit, connected to said mixing means, branching the optical signal having the intermediate frequency band into a plurality of branched optical signals;
a plurality of detecting means coupled to the optical branch unit, for detecting and receiving the plurality of branched optical signals, and for converting the branched signals to electric signals each having the intermediate frequency band;
a plurality of band filters, each coupled to one of the plurality of detecting means, and extracting a respective signal selected for each respective channel from the electric signals; and a plurality of demodulators, connected to said plurality of band filters, respectively, and demodulating the respective signals.

7. An optical communication system as claimed in claim 6, wherein the receiving equipment further comprises:
an amplifier, connected to said mixing means, amplifying the optical signal having an intermediate frequency band obtained by said mixing means.

8. An optical communication system as claimed in claim 6, wherein the sending equipment further comprises:

polarization control means for controlling polarization of an optical signal transmitted to the receiving equipment.

9. An optical communication method for transmitting signal light having a first polarization state including the steps of:
a) mixing the signal light and local light having a second polarization state, after the first and second polarization states have been made coincident with each other, and producing mixed light;
b) optically transmitting the mixed light as transmitted optical light; and c) detecting an intermediate frequency element produced by a difference between a frequency of the signal light and a frequency of the local light.

10. An optical communication method for transmitting more than one optical signal having optical carriers of different frequencies including the steps of:
a) mixing each of the more than one optical signal and a corresponding one of local light signals after polarization states of one of corresponding more than one optical signal have been made coincident with each other, and producing a multiplexed optical signal;
b) optically transmitting the multiplexed optical signal; and c) detecting an intermediate frequency element produced by a difference between a frequency of each of the more than one optical signal and a frequency of the corresponding one of the local light signals.

11. An optical communication method including the steps of:
a) allocating transmission signals on more than one channel having different microwave frequencies;
b) modulating carriers of the microwave frequencies with the transmission signals for producing modulated signals;
c) combining the modulated transmission signals for producing a multiplexed signal;
d) modulating an optical frequency modulator with the multiplexed signal for producing a modulated optical signal;
e) mixing the modulated optical signal with local light with their polarization state made coincident with each other and optically transmitting the mixed light;
f) detecting an intermediate frequency element produced by a difference between a frequency of the modulated optical signal and a frequency of the local light to thereby convert the modulated optical signal to an electrical signal;
g) extracting corresponding channel signals from the electrical signal by electric filters; and h) demodulating each of the corresponding channel signals.

12. An optical communication system for transmitting a signal light having a plurality of data signals from sending equipment to receiving equipment, the sending equipment comprising a plurality of optical modulated signal generating units, each comprising:
a plurality of modulators modulating carriers having different microwave frequencies with transmission signals and outputting frequency modulated signals;
adding means for generating a microwave frequency multiplexed signal by adding the frequency modulated signals;
and an optical frequency modulator generating an optical frequency modulated signal by frequency modulation with said microwave frequency modulated signals;
optical mixing means for mixing each said optical frequency modulated signal generated by said optical frequency modulator in said plurality of optical frequency modulated signal generating units and generating an optical frequency multiplexed signal; and an optical frequency discriminator, connected to said optical mixing means, receiving and dividing the optical frequency multiplexed signal into two signals, a first one of the two signals having a time delay with respect to a second one of said two signals, and mixing the two signals to generate an optical frequency multiplexed signal to be transmitted to the receiving equipment.

13. An optical communication method as claimed in claim 9, wherein the transmitted optical signal is divided by an optical circuit and then detected.

14. An optical communication method as claimed in claim 9, wherein the optical signal is amplified by a light amplifier.

15. An optical communication system for transmitting to receiving equipment a signal light optically modulated by a microwave frequency multiplexed signal, and optical communication system comprising:
sending equipment optically modulating the signal light, said sending equipment comprising:
a plurality of optical frequency modulated signal generating units, each optical frequency modulated signal generating unit comprising:
a plurality of modulators modulating carriers of different microwave frequencies with each of transmission signals corresponding to the carriers;
adding means for generating a microwave frequency multiplexed signal by adding modulation signals output by said plurality of modulators; and an optical frequency modulator generating an optical frequency modulated signal by frequency modulation with said frequency multiplexed signal;
optical mixing means for mixing each optical frequency modulated signal generated by said plurality of optical frequency modulated signal generating units and for generating an optical frequency multiplexed signal; and an optical frequency discriminator dividing the optical multiplexed signal into a first signal and a second signal, the first signal being delayed by a delay time with respect to the second signal, and for generating the optical frequency multiplexed signal to be transmitted to said receiving equipment by mixing the first signal and the second signal.

16. An optical communication system for transmitting a signal light carrying a plurality of data signals from a single transmitter to receiving equipment, said transmitter comprising:
a plurality of optical frequency modulated signal generating units, each comprising:
a plurality of modulators modulating carriers having different microwave frequencies with a plurality of transmission signals and outputting first transmission signals;
a plurality of band limiting filters band limiting one of the data signals and the first transmission signals, and outputting second transmission signals;
adding means for generating a microwave frequency multiplexed signal by adding the second transmission signals;
and an optical frequency modulator generating an optical frequency modulated signal by frequency modulation with said microwave frequency multiplexed signal; and optical mixing means for mixing each optical frequency modulated signal generated by said optical frequency modulator in said plurality of optical frequency modulated signal generating units and generating an optical frequency multiplexed signal to be transmitted to the receiving equipment.

17. A sending device for transmitting a signal light carrying a plurality of data signals to a receiving device in an optical communication system, comprising:
a plurality of optical frequency modulated signal generating units each comprising:
a plurality of modulators modulating carriers having different microwave frequencies with a plurality of transmission signals and outputting first transmission signals;
a plurality of band limiting filters band limiting one of the data signals and the first transmission signals, and outputting second transmission signals;
adding means for generating a microwave frequency multiplexed signal by adding the second transmission signals;
and an optical frequency modulator generating an optical frequency modulated signal by frequency modulation with said microwave frequency multiplexed signal; and optical mixing means for mixing each optical frequency modulated signal generated by said optical frequency modulator in said plurality of optical frequency modulated signal generating units and generating an optical frequency multiplexed signal to be transmitted to the receiving device.