WO2001035138A1

WO2001035138A1 - Wavelength controlled fbg filter

Info

Publication number: WO2001035138A1
Application number: PCT/NO2000/000377
Authority: WO
Inventors: Dag THINGBØ; Arne Berg; Jon Thomas Kringlebotn
Original assignee: Optoplan As
Priority date: 1999-11-09
Filing date: 2000-11-08
Publication date: 2001-05-17
Also published as: EP1257858A1; NO313606B1; NO995485L; NO995485D0; AU1312501A

Abstract

Controlled fiberoptic filtering system comprising at least one tunable FBG filter engraved into a first optical fibre, the FBG filter comprising at least one grating with a chosen wavelength, inscribed in a chosen length of fibre, the grating providing reflections at least two wavelengths, a filter and a sensor wavelength. The system comprises: actuator means coupled to the filter for providing a change in the reflected wavelengths reflected by the grating, e.g. by providing a strain/compression and/or a temperature change to the grating, thus altering the filter wavelength, a light source coupled to the first optic fibre for providing an optic signal at a range of wavelengths comprising the sensor wavelength of the filter, wavelength readout unit coupled to first optic fibre for measuring the wavelength of optical signals reflected from the grating within the sensor wavelength, and control means coupled to the output of said wavelength readout unit and to said actuator means for positioning the reflected wavelengths or controlling the change in the wavelengths reflected from the grating, as provided by the actuator, and thus simultaneously the position or change in the filter wavelength.

Description

WAVELENGTH CONTROLLED FBG FILTER

This invention relates to a controlled fiberoptic filtering system and use of this system, the system comprising at least one tunable FBG filter engraved into a first optical fibre, the FBG filter comprising a filter and a sensor grating with different wavelengths being inscribed in a chosen length of fibre, thus providing reflections at a first and a second wavelength, respectively.

A Fibre Bragg Grating (FBG) is a permanent, photo- induced periodic modulation of the refractive index in the core of an optical fibre, which reflects light within a narrow bandwidth centred at the Bragg wavelength, with negligible loss outside this wavelength band. The Bragg wavelength λ_B is given by:

where n_av is the average refractive index seen by the light and Λ is the pitch of the grating. By virtue of being able to change the design parameters such as the length, which is typically 1-lOOmm, the refractive index modulation, the average refractive index, and the pitch of the grating, a large variety of filters can be made. Such wavelength selective filters are very attractive components in dense wavelength division multiplexed (DWDM) fibre optic communication systems and networks, since they are true all- fibre components and enable formation of nearly ideal filter functions with extremely low cross-talk between neighbouring wavelength channels, which can have spacing down to 50GHz (0.4nm) .

In addition FBG filters can easily be wavelength tuned by changing the strain/compression or the temperature of the FBG, hence changing the average refractive index and the grating pitch [US patent 5, 007, 705, "Variable optical fibre Bragg filter arrangement" to W.W. Morey et.al.]. Such wavelength tuneable FBG filters are expected to be central components in future reconfigurable/dynamic optical add/drop modules (OADMs) at add/drop nodes in the DWMD optical network, where one or more wavelength channels can be added and dropped, which will enable flexibility and fast accommodation of shifts in the traffic load, as well as rapid provisioning of services. [Lightwave, August 1999, "Advanced optical-networking components at the add/drop node"]

The invention also relates to a wavelength controlled FBG based fibre laser array comprising at least one tunable FBG based fibre laser.

An FBG based fibre laser is a fibre laser where the laser feedback is provided by one (DFB laser) or two (DBR laser) FBGs imprinted in a section of an optical fibre doped with at least one of the rare earth materials, such as Erbium, which when optically pumped by a semiconductor laser provides gain in the rare earth doped fibre and consequently lasing at or close to the FBG Bragg wavelength. Alternatively the gratings can be imprinted in fibres without any of the rare earths which are spliced to each end of a rare earth doped fibre. Such a laser can be continuously tuned by straining/compressing or heating the whole laser cavity [G. Ball and W.W. Morey, Optics Letters, Vol. 17, pp. 420-422, 1992] .

The Bragg wavelength of a tuneable FBG filter has to be controlled with a high degree of stability, typically <20pm over the whole tuning range, which can be up to tens of nanometres. For use in OADMs it is important to accurately tune and lock the tuneable FBG filters to the predetermined wavelengths (in the so-called ITU grid which has a spacing of 50GHz) . This is putting extreme requirements on the control of the FBG strain and temperature, which will depend on the actuator, the filter packaging and the fixing/positioning of the fibre. With strain tuning the FBG can be tuned by fixing the fibre at each end of the grating and change the strain (or compression) of the FBG. This can for example be done by using piezoelectric actuators. To control the Bragg wavelength within 20pm, the strain in the fibre has to be controlled within ca. 17 μstrain (or the applied force to the fibre has to be controlled within ca. 15mN) . When using 20mm separation between the fixing points (FBG filters for DWDM applications are typically 15-20mm long) , the corresponding accuracy in the positioning of the fixing points is ca. 0.3μm. This is very demanding for any conventional actuator control.

In addition there will be temperature induced shifts in Bragg wavelength, both due to the inherent temperature sensitivity of the FBG and the temperature sensitivity of the actuator, filter packaging and fixing points. This will require some form of temperature compensation.

There will also be mechanical drift and hysteresis in the actuators and fixing points, which will cause drift in the filter wavelength.

The object of the present invention is thus to provide a means for accurately controlling the wavelength of one or more wavelength multiplexed tuneable FBG filters, independently of drift and hysteresis in the actuator, the filter packaging and the fixing point of the fibre, as well as temperature drift. A further objective is to provide a means for accurately controlling the wavelength of one or more wavelength multiplexed tunable FBG based fibre lasers, independently of drift and hysterisis in the actuators, the laser packaging and the fixing point of the fibre, as well as temperature drift.

The abovementioned objects are obtained with a system and a use as described above and is characterized as given in the independent claims.

A system is thus obtained in which the filter grating may be controlled using a sensor wavelength being different from the wavelength used for transmitting signals through the system, but which is reflected from the same FGB filter and therefore is subject to the same influences as the filter grating. The sensor wavelength does not influence the signals transmitted in the system and may easily be filtered out or removed.

The preferred embodiment of the invention relates to a system using overlaid FBG gratings, e.g. as described in Xu, M.G., et.al., Electron. Lett., Vol. 30, pp. 1085-1087, 1994, involving two gratings, reflecting the filter and the sensor wavelengths, respectively. The sensor and filter gratings may, however, also be positioned side-by-side in each tuneable FBG filter. Thus the system according to the preferred embodiment of the invention obtains a measure of the combined effect of strain, compression and temperature in the tuneable FBG filters, since the FBG sensors will see exactly the same strain and temperature as the tuneable FBG filters, and hence the Bragg wavelength of the sensor FBGs will be direct measures of the Bragg wavelength of the tuneable FBG filters, independently of drift and hysteresis in the actuator, the filter packaging and the fixing point of the fibre, as well as temperature drift. Alternatively, the second order reflection from the FBG filters (at wavelengths = λ_B/2) could be used for wavelength measurements, excluding the need for overlaid FBGs by using the wavelength of the grating as a the filter wavelength, while using the second order reflection as the sensor wavelength, as described in Kalli, K. et.al., OFS-94, Glasgow, post deadline paper.

A wavelength controlled FBG based fibre laser array can be obtained by imprinting a second FBG inside each of the fibre laser cavities, with a Bragg wavelength outside the wavelength range preferentially outside the gain bandwidth of the lasers, hence not affecting the laser operation. The sensor FBGs will see exactly the same strain and temperature as the tunable FBG based fibre lasers. Hence the laser wavelength can be determined with high accuracy by measuring the sensor FBG wavelengths. The measured sensor wavelengths can be used as input to the laser actuators to control the laser wavelengths with high accuracy. One advantage of this approach compared to measuring the laser wavelengths directly with an optical spectrum analyser is that the laser wavelengths can be controlled prior to turning on the lasers. Also the laser wavelengths can be controlled without tapping out laser light.

The system according to the invention is described below with reference to the drawings, which illustrates the invention by way of example.

Fig. 1 shows a preferred embodiment of the invention used in a reconfigurable OADM. Fig. 2 illustrates an FBG filter according to a preferred embodiment of the invention.

Fig. 3 illustrates an FBG filter according to an alternativ embodiment of the invention. Fig. 4 shows an embodiment of the invention used to control the wavelengths of a tuneable FBG-based fibre laser array.

The reconfigurable OADM shown in fig. 1 consists of a series of wavelength multiplexed, tuneable FBG filters 1 (here four) in an optical fibre 2 placed between two optical circulators 8. In this case the related optical network uses the 1550 nm wavelength band. The tuneable FBG filters 1 can be strain tuned individually by means of actuators 3, for example piezoelectric actuators, to reflect one or more (here maximum 4) of the incoming wavelength multiplexed signal wavelengths 11 from the optical network 10 to the drop port 8, or to add one or more (here maximum 4) signal wavelengths 12 to the optical network at the add port 9.

The add/drop ports 8,9 may be conventional optical circulators, e.g. as described in US patents 5.400.418 and 5.883.991, and will not be described in any detail here. To set and control the tuneable FBG filter wavelengths in the 1550nm wavelength band, FBG sensors 1 written in the 1300nm wavelength band (or alternatively in the 800nm wavelength band) in the same length of fibre as the tuneable FBG filters are used to measure the combined effect of strain and temperature in the tuneable FBG filters. The sensor gratings are overlaid or side-by-side as shown in figures 2 and 3, respectively, with the tuneable FBG filters .

The Bragg wavelength of the sensor FBGs la will be direct measures of the Bragg wavelength of the tuneable FBG filters lb, independently of drift and hysteresis in the actuator, the filter packaging and the fixing point of the fibre, as well as temperature drift. The measured Bragg wavelengths are thus used to control the filter actuators 3 and lock the tuneable FBG filters 1 to the desired wavelengths .

Coarse, conventional actuator control 3 can be used to tune the FBG filters 1 to the desired wavelength channels and the FBG sensor measurements can be used to do the fine control and locking to the exact desired wavelengths. The 1300nm wavelength band FBG sensor wavelengths are measured by means of a wavelength readout unit 4 connected to the tuneable filters 1 via a 1300/1550nm WDM coupler 13. The 1300nm light passing the FBGs can be filtered out of the OADM by means of a second 1300/1550nm WDM coupler 14 e.g. comprising a fibre 15 having a dead end.

The FBG sensors la will typically be written at different, non-overlapping wavelengths outside the wavelength band occupied by the DWDM system, not to interfere with the DWDM system. Typically the DWDM system will operate at wavelength in the 1550nm wavelength band, while the FBG sensors la can be made to reflect wavelength in the 1300nm wavelength band (or possibly in the 800nm wavelength band) . The sensor FBGs la should have much narrower reflection spectrum than the tuneable FBG filters lb to allow high resolution Bragg wavelength measurements. As described above, the second order reflection from the FBG filters 1 (at wavelengths = λ_B/2) could alternatively be used for wavelength measurements, excluding the need for sensor FBGs.

A wavelength readout unit 4 including a broadband light source covering the FBG sensor la wavelengths, will be connected to the tuneable FBG 1 via an optical wavelength multiplexer. The measured FBG sensor wavelengths are compared with the desired FBG filter wavelengths and this information is used to control the actuators tuning the FBGs such that the FBG filter wavelengths are locked to the desired wavelengths.

The readout unit 4 can in principle be any accurate spectrum analyser in combination with a broadband source (ELED) operating in the 1300nm wavelength range (or the 800nm wavelength band) , or alternatively an accurate tuneable source, covering the FBG sensor wavelengths, such as the read-out unit described in PCT application PCT/NO98/00031 to Kringlebotn et.al. Ideally, the read-out unit 4 should be a simple device with low volume and low cost, such as the device described in the Norwegian patent application No. 1999.4473. With this device single or pairs of analysing FBG filters, corresponding to the individual sensor FBGs, are used to measure the sensor-, and hence the tuneable filter-Bragg wavelengths. The analysing FBG filters should then have Bragg wavelengths close to the sensor FBG wavelengths corresponding to the desired ITU wavelengths. This will minimise the required tuning of the analysing FBG filters, and greatly enhance the speed of the wavelength locking, which is important in reconfigurable OADMs. The measured FBG sensor wavelengths are compared with the desired FBG filter wavelengths in a signal processing unit 7 and this information is used to control the actuators 2 tuning the FBGs such that the FBG filter wavelengths are locked to the desired wavelengths. The control unit is provided with information 16 regarding the system, such as the required grating wavelengths in each filter grating, the wavelengths to be dropped or added to the system, and possibly calibration data for each actuator, as well as the information regarding the reflected sensor wavelengths provided from the wavelength readout unit 4.

The tunable FBG-based fibre laser laser array shown in Fig. 4 consists of a series of wavelength multiplexed, tuneable FBG-based lasers 1' (here four) , preferentially Erbium-doped DFB fibre lasers operating in the 1.55μm range (between 1.5μm and 1.6μm) , spliced into an optical fibre 2. The lasers are here pumped by one single pump laser 18, preferentially at 1480nm, through an optical isolator 19 and wavelength division multiplexer 13 which separates the laser wavelengths (and the pump laser wavelength) and the FBG sensor wavelengths. At the output of the laser array light within the sensor wavelength range is filtered out to a fibre 15 with a dead end by means of a second wavelength division multiplexer 14. The laser and pump light is directed through an optical isolator 20 to an erbium doped fibre 21 which is pumped by the residual pump power to provide amplification of the laser light. An optical isolator 22 is placed at the output of the amplifying fibre to isolate the laser system from feedback from an external system.

The tunable fibre lasers can be strained tuned individually by means of actuators 3 to emit at the desired wavelengths. To set and control the laser wavelengths, FBG sensors V written inside (or outside) the laser cavities, such that they are strained equally to the fibre lasers, at Bragg wavelengths outside the laser wavelength range, preferentially in the 1300nm band, are used to measure the combined effect of strain and temperature in the fibre lasers. A wavelength read out unit 4 including a broadband source covering the FBG sensor wavelengths, is connected to the tunable fibre laser, including the FBG sensors, via an optical fibre 6 and a wavelength multiplexer 13. The measured FBG sensor wavelengths are compared with the desired laser wavelengths in a signal processing control unit 7 and this information is used to control the actuators 3 tuning the lasers such that the laser wavelengths are locked to the desired wavelengths. The control unit is provided with information 16 regarding the desired laser wavelengths and the information 17 regarding the measured FBG sensor wavelengths.

Claims

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SD tr 0⁾ o • Φ Hi . o H J o tn p. O o p. tn Φ H tr tr tr P- φ •

<! C <J 3 tr P- Φ t CΛ O Ϊ-J CΛ < o H 3 o < 3 tn tn tr Hi Φ O tr tυ

Φ CΛ φ TJ 3 P- < 3 P- Φ 3 T < Φ TJ rt 1-5 P- CΛ CΛ h-¹ ι-i fu 15 15 PJ rt P- ^ ft 3 H P- P-" Φ H- 15 tn -> 15 Φ Φ rt

Φ TJ Φ P- ^ P. ^ P- f Φ P- Φ t" ft φ ≤ P- o- tn Φ p> 1-5 φ tn rt p. 3

3 P, 3 H P- rt (D ! P- o φ O 3 P tr α^j tn CΛ P- y fn 15 o p. Φ tn O O Q o 15 φ tr tr ft σ 3 0- P- 1-5 U-, α Φ CΛ < H- 3 iQ rt P- rt p, rt S ft 3 3 rt <! rt cu Φ Φ P- OJ φ 3 P- ft C φ S 15 P- rt C 3 f-1 5 P- tn tr Φ rt t P- tr P 3 H o σ !-5 !-T n <--< H iQ sQ U p. Φ tn tn 3 < Φ Pi

• Q- CΛ 0⁾ o o CΛ 15 o o H *< rt υQ o p> p- Φ tn 1-r Φ • rt rt O ts 15 φ rt O

P- TJ Φ Ό H 1-5 tn t 0 ft 3 ft 15 tr O α •n C H"

Hi tr Hi rr rt o Z rt rt o rt Φ ft Φ r rt s -) H- o • Φ p. rt TJ Φ ω 3

15 Φ H- P- o P- P- t r t-f tn H P- rt Φ O CΛ r tn p. 3 o tn Φ

P- φ O 3 ft φ o cu Φ ≤ φ α o 3 rt t TJ o tn tr p. 3 Φ O 15 tr

P, 3 rt- 0- tr 3 fli o P- «-. t CΛ rt c D •< Φ φ CΛ < rt Hi φ 15 Φ P, Hi H Hi ^ PJ <! p. c Φ p. Φ p- p. 13 Hi fu p. Φ P- tf P- Φ Hi

Hi P> P- tr α> P- Φ o φ Φ ft 3 ≤ Φ t- O o Φ TJ ts t5 D. •*« h-¹ H-

P^J P- P- fn H- rt Hi φ P- s; tn CΛ φ p. φ CΛ P- CΛ P- ft t*l tr

Φ 3 fn rt P> o ft P- C Φ H- f-1 0J rt PJ α O ω rt o n 0 3 P- Φ α. Φ o CΛ 3 N Φ 3 tr N Φ 3 tn 1-5 Φ 3 t-5 i < o H P- n O < rt O N p, 15 3 P> en H rr o faι-i o o H rt ^ O P- ω H- Φ C- sQ o P- Φ o ω o

P- ι-i φ P- 3 ω φ P- rt O ft ft rt 3 rt ≤ 1-5 rt α Φ s: P. o o Hi J

O P- i 3 P- Φ 3 t H tr Φ o Φ 511 tn TJ t P- TJ tn φ p. o P- rt

3 cr α 15 n 3 *--! Φ CΛ P- fu o ft 1-5 c < φ C5 s: α < Hi P- 3 P-

CΛ Φ Λ 13 £ 1-r -! 1-5 Φ φ 15 fn Φ φ tr T rt n n φ P- CΛ o ≤ P- CΛ Hi tU p. s: P- T φ ft P- tn 15 α P- Φ Φ p> Φ fn ø 3 ^>< Hi SD p ^ P- 3 Φ a> 1-r rt Φ rt p. 5D Φ ti φ O α P- p. Hi P- I ⁾ cr P- CΛ CΛ < CΛ D- Hi < rt φ CΛ 1-5 rt tn -¹ rt 3 ft CΛ H-

3 o rr rt rt Φ rt ft ft H¹ Φ O φ o o iQ tn O rt CΛ Φ O rt 15 P- P- P- Φ fu fn H t Φ t t Φ Φ rt Φ 1-5 Hi O rt P- ft 3 t rt O 3 3 3 ft fn α> 3 Φ Φ f-^j 3 H t O Φ CΛ rt CΛ £ 1-T p. rt 3 P, 15 rt tn tr 3 15 15 Φ P'

Hi 15 rt- 3 rt C- ft !-5 tn O O O TJ tn t 15 tn rt t rt CΛ tn p. Pi

P- O P. fn Hi 15 JD s: CΛ Φ ^ P- Pi TJ O 3 Φ P- tr Φ tn 5n P- 3 tr fn rt o P- rt rt O (D rt . rt rt Φ Hi Q P CΛ rt tn O rt < 3 rt o rr t o tr t O CΛ t Ξ P- φ Hi rt ~^_^ Hi tr rt tr φ 15 CΛ

Φ CΛ P- Φ o ft Φ O Φ P- H) CΛ 0⁾ φ (1⁾ O rt H- t n p, P- φ O h-¹ P- ι-i Φ 3 H Φ H 3 p. o < tn rt tr O p. c o φ H¹ CΛ Φ CΛ

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3 Z 3 3 ≤ H- CD 3 iQ ft til ft Φ ω Hi Hi ≤ fn p. φ P. CΛ ω rt rt rt

P_ Φ H- fn 15 (D ft > 3 ft tn rt o rt T 1-5 P- P- P- tn O H¹ Φ n rt rt O φ

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CΛ tr Φ P C ιQ H 3 o H. φ O P- o O O rt Hi 0

Φ P- - O 3 ω o o CΛ p, o Φ Hi CΛ O "\ 3 1-5 1-5 tr TJ O t 15 P-

3 o Hi rjd 15 o O P^J J h-" tU TJ ^ Hi 15 ^< P. 3 s: p, Pi TJ

CΛ Hi Hi O fn rt H 1-5 P> ^< ft 13 ω p. rt ft P- tn o TJ tn o 5D CΛ P,

O Φ H- t ω P- P- P- CΛ tn φ P- 1-r cr rt 3 rt <! P, < Hi rt rt P-

P> Hi P> Hi 3 • ≤ P- 3 ft P 3 P- Hi O CΛ p. t D- tr H- P- Φ P- CΛ

P- Φ P- OJ Λ t *--! Hi Φ Φ ^_^ t-> α Φ P- CΛ H" Hi 3 O P- tr 3 P- ' h-¹ < rt Φ - O Φ Hi O P- Φ Φ P- sQ TJ 3

H rt rt Φ "- ft P, O P- Hi Hi P. t! CΛ 3 tr rt 15

Φ Φ tU 1-T ft cr o P- 15 • • 15 Pi S. P-

*_ P Φ O CΛ φ Φ H p. tn rt Φ P- O tn

Hi P- φ ft tn tr rt tn rt sQ Φ tf rt H tn

4. Fiberoptic filtering system according to claim 3, c h a r a c t e r i z e d in that the gratings are superimposed in the same length of fibre.

5. Fiberoptic filtering system according to claim 1, c h a r a c t e r i z e d in that said light source is contained in the wavelength readout unit.

6. Fiberoptic filtering system according to claim 1, c h a r a c t e r i z e d in that the filter gratings are FBG based fiber lasers pumped by a pump laser.

7. Use of a fiberoptic filtering system according to any one of the preceding claims 1-5 in a tuneable fiberoptic add/drop module for extracting or adding optical signals at chosen wavelengths in the first optic fibre, the first fibre being part of an optic network for transferring information.