WO2004011995A1

WO2004011995A1 - Optical waveguide device

Info

Publication number: WO2004011995A1
Application number: PCT/GB2003/003149
Authority: WO
Inventors: John Michael Heaton
Original assignee: Qinetiq Limited
Priority date: 2002-07-25
Filing date: 2003-07-24
Publication date: 2004-02-05
Also published as: GB0217227D0; AU2003281726A1

Abstract

An optical waveguide device (10) comprises a core (14, 34) and a cladding layer (18, 38) adjacent to said core, wherein at least one of said core and said cladding layer comprises a plurality of layers (14a, 14b, 24, 26; 38a, 38b, 44, 46). Each layer (14a, 14b, 24, 26; 38a, 38b, 44, 46) has a substantially different refractive index from layers immediately adjacent thereto, thereby to determine signal losses of optical signals applied to said device having a first polarisation relative to those of signals having a second polarisation. The arrangement may be configured such that signal losses of optical signals having a first polarisation are substantially less than those of signals having a second polarisation, so that transmission of only the first polarisation is supported.

Description

OPTICAL WAVEGUIDE DEVICE

The present invention relates to optical waveguides and particularly, but not exclusively, to optical waveguides for transmittmg or conducting optical signals having only one polarisation. The waveguide of the invention permits effective transmission of optical signals of only one polarisation, signals of other polarisations being leaked from the waveguide.

Conventionally, optical waveguides permit the transmission of light therethrough in both transverse electric (TE) and transverse magnetic (TM) polarisation modes with substantially equal efficiency such that losses are generally similar for both modes along the length of the waveguide.

However, it has been observed that in certain electro-optic devices, inconvenient conversion between TE and TM polarisations caused by the applied electro-optic field degrades the performance of the device. It is believed that this problem could be removed if optical signals of only one polarisation were transmitted by the waveguide.

In addition, there are a number of applications for which optical waveguides which permit transmission of optical signals of only one polarisation may be particularly suited. Such applications include polarisation splitters and single- electrode intensity modulators.

It would be advantageous, therefore, to provide an optical waveguide device or the like in which optical signals of only one polarisation are permitted to be transmitted efficiently. It would be further advantageous to provide an optical waveguide device or the like in which optical signals applied to the device are split into a plurality of signals each having a single polarisation, each single polarisation signal being transmitted individually by the waveguide device.

According to one aspect of the present invention, therefore, there is provided an optical waveguide device or the like comprising:

a core; and

a cladding layer adjacent to said core;

wherein at least one of said core and said cladding layer comprises a plurality of. layers, each layer having a substantially different refractive index from layers immediately adjacent thereto, thereby to control signal losses of optical signals applied to said device having a first polarisation relative to those of signals having a second polarisation.

In a first advantageous embodiment, the arrangement is such that signal losses of optical signals having a first polarisation are substantially less than those of signals having a second polarisation.

The signal losses of the signals having the first polarisation may be between 10 and 10,000 times less than those of signals having the second polarisation.

In a second advantageous embodiment, the arrangement is such that signal losses of optical signals having a first polarisation are substantially identical to those of signals having a second polarisation. Conveniently, one of the core and the cladding may include at least twenty sets of layers, each set comprising a layer of a second material sandwiched between first and second layers of a first material.

In a preferred embodiment, the first and second layers of the first material may be group III-V materials and may, for example, be selected from any one of the following; GaAs, GaP, GaN, AlAs, InAs and InP. The second material may also be a group III-V material selected from any one of the following; GaAs, GaP, GaN, AlAs, InAs and InP.

Preferably, the first and second layers of the first material each comprise a layer of GaAs, preferably approximately 0.045 μm thick, and the layer of the second material comprises a layer of AlAs, preferably approximately 0.01 μm thick. This arrangement of layers and materials may be reversed.

Alternatively, one of the core and the cladding may include at least thirty sets of layers, each set comprising a layer of a first material and a layer of a second material. Advantageously, the layer of the first material comprises a 0.09 m thick layer of GaAs and the layer of the second material comprises a O.Olμm thick layer of AlAs. This arrangement of layers and materials maybe reversed.

Advantageously, the layered structure changes the apparent or effective refractive index of the core or the cladding respectively for one of the polarisations but does not affect the effective refractive index for the other polarisation. By selecting appropriate effective refractive index values for the layered structure, signal losses for one or both polarisations can be controlled. The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a cross-section through a first form of waveguide device according to the invention,

Figure 2 is a cross-section through a second form of waveguide device according to the invention,

Figure 3a illustrates the electric field for the TE polarised mode produced by the waveguide device of Figure 1,

Figure 3b illustrates the magnetic field for the TM polarised mode produced by the waveguide device of Figure 1,

Figure 4a illustrates a contour plot of the electric field for the TE polarised mode produced by the waveguide device of Figure 1,

Figure 4b illustrates a contour plot of the magnetic field for the TM polarised mode produced by the waveguide of Figure 1,

Figure 5 is a plot of predicted signal loss against waveguide width for the waveguide device of Figure 1,

Figure 6a illustrates a first contour plot of the electric field for the TE polarised mode produced by a 3μm waveguide device according to the invention, Figure 6b illustrates a second contour plot of the electric field for the TE polarised mode produced by a 3μm waveguide device according to the invention,

Figure 7a illustrates a contour plot of the electric field for the TE polarised mode produced by the waveguide device of Figure 2, and

Figure 7b illustrates a contour plot of the magnetic field for the TM polarised mode produced by the waveguide device of Figure 2.

In conventional waveguide devices, including optical fibres, transmission of light through the device is achieved through total internal reflection of the light within the core of the waveguide. In practice, this relies on the refractive index of the core being higher than that of the cladding adjacent to, or surrounding, the core. If the refractive index of the core is lower than that of the cladding, little or no internal reflection occurs and thus light is lost from the waveguide device through the cladding.

It is known that, while materials such as GaAs, AlAs and AlGaAs exhibit refractive indices which are substantially the same for both TE and TM mode polarisations (i.e. they are isotropic), there are different boundary conditions on the electromagnetic field for each polarisation at interfaces between different dielectric materials.

For light polarised in the plane of the interface (TE mode polarisation), the electric field component Et tangential to the interface is continuous and the derivative of Et with respect to the direction normal to the interface is also continuous. For light polarised perpendicular to the plane of the interface (TM mode polarisation), however, an extra factor of n² appears in the boundary condition on Et, where n is the refractive index of the relevant layer.

In conventional waveguide devices, these differences are negligible since there are only a small number of dielectric boundaries with very small refractive index changes.

However, it has been discovered that these differences can be increased by deliberately using a set of relatively thin layers of high refractive index contrast material to form an artificially anisotropic core which has highly different effective refractive indices for the different polarisation modes. For example, a set of very thin dielectric layers of alternating GaAs and AlAs material will have different effective refractive indices for different polarisations (i.e. the set will be anisotropic).

The effective or apparent refractive index for light polarised in the plane of the layers (n_TE) is approximately given by the index of Al_xGar_xAs with the same average mole fraction x as the GaAs / AlAs layers. In other words, the effective or apparent refractive index depends on the "mark-to-space" ratio of the layers.

The effective or apparent index for light polarised perpendicular to the layers (nxM) is lower than Π_TE by an amount An since the boundary conditions on the fields at the dielectric interfaces are different for the different polarisations.

Thus, it has been discovered that by carefully controlling the refractive index between the core and the cladding of the waveguide device, it is possible to set the rate at which light at each polarisation leaks into the substrate. The waveguide device of the present invention makes use of this effect to cause light applied to the device at a first polarisation to be leaked from the waveguide at a substantially greater rate than light at a second polarisation, thereby effectively permitting light at only the second polarisation to be transmitted therethrough.

Referring to Figure 1, this illustrates a cross section through a first form of waveguide device 10 according to the invention. The waveguide device 10 comprises a plurality of layers of material, in the manner of a conventional waveguide. The arrangement of layers, or "epitaxy", of the waveguide device 10 is described below.

The waveguide device 10 comprises an upper cladding layer 12, a core 14 adjacent to the upper cladding layer 12, a first lower cladding layer 16 adjacent to the core 14 and a second lower cladding layer 18 adjacent to the first lower cladding layer 16. The second lower cladding layer 18 is mounted on a substrate layer 20. A cap layer 22 is disposed on the upper surface of the upper cladding layer 12.

In the exemplary embodiment of Figure 1, each of the layers has the following properties:

The cap layer 22 is formed from Gallium Arsenide (GaAs).

The upper cladding layer 12 is approximately 1.1 μm in thicknesses and is formed of 30% AlGaAs having a refractive index of 3.2582. The first lower cladding layer 16 is approximately 0.3 μm in thickness and is also formed from 30% AlGaAs material having a refractive index of 3.2582.

The second lower cladding layer 18 is approximately 3μm in thickness and is formed from 14% AlGaAs material having a refractive index of 3.3392.

The substrate layer 20 is formed from GaAs material having a refractive index of 3.4094 and can be of any desired thickness, although approximately 600 microns is preferred.

In conventional waveguide devices, the core is formed from 10%_o AlGaAs material. In the device of the present invention, however, the core 14 is not uniform but is formed from a plurality of alternating layers of GaAs material and AlAs material with thicknesses of approximately 0.09μm and 0.01 μm respectively.

In one advantageous embodiment, the core 14 includes twenty "sets" 14a, 14b etc. of core layers, each set comprising a first core layer 24 which is approximately 0.045 μm in thickness and is formed from GaAs material, a second core layer 26, adjacent the first core layer 24, which is approximately 0.01 μm in thickness and is formed from AlAs material and a third core layer 28, adjacent the second core layer 26, which is approximately 0.045μm in thickness and is formed from GaAs material.

Each set of core layers is thus approximately 0.1 μm in thickness (0.045μm + 0.01 μm + 0.045 μm) such that the overall thickness of the core is approximately 2μm. The average or effective refractive index is substantially the same as 10% AlGaAs for the TE polarised light but significantly lower than this for the TM polarised light .

The above-described waveguide device is arranged to support transmission therethrough only of TE polarised light. Transmission of TM polarised light through the waveguide device is not supported. The principle by which the waveguide device of the invention operates is described below.

In the device of Figure 1, the arrangement of layers 24, 26, 28 in the core 14 is such that the effective or apparent refractive index as seen by the TE polarised light is 3.3521 while that seen by the TM polarised light is 3.3369. By setting the refractive index of the lower cladding layer 18 (which is the primary means by which light is confined in the guide) to the value of 3.3392, which is lower than the refractive index of the core 14 for TE polarised light but higher than that for TM polarised light, the total internal reflection, and hence the transmission, of TE polarised light through the core is maintained but that of the TM polarised light is not. Instead, much of the signal power of the TM polarised light is lost through the second lower cladding layer 18 and into the substrate 20.

Thus, by using the phenomenon that different polarisations of light "see" a different effective refractive index at the interface between different dielectric materials, the waveguide device of the present invention allows the relative effective refractive indices of the core 14 and the second lower cladding layer 18 to be altered for the different polarisations.

This change in the relative effective refractive indices between the core 14 and the second lower cladding layer 18 is achieved by forming the core 14 from a plurality of layers of GaAs and AlAs with a carefully selected mark-to-space ratio to determine the average or effective refractive indices seen by the different polarisations. In this embodiment, the waveguide permits efficient transmission only of TE polarised light through low signal loss and reduces or substantially prevents transmission of TM polarised light by causing significantly higher signal loss of this mode from the core into the substrate 20.

It will be understood from the foregoing that it is also possible to provide a waveguide device which permits transmission only of the TM polarised light and prevents transmission of TE polarised light. This can be achieved by means of a waveguide device having an epitaxy such as that shown in Figure 2.

In Figure 2, the waveguide device 10 has an upper cladding layer 32, a core 34, a first lower cladding layer 36, a second lower cladding layer 38 and a substrate layer 40 as in the device of Figure 1. However, in this embodiment, the core 34 is formed from uniform (isotropic) 7% AlGaAs material whilst the second lower cladding layer 38 is formed from alternating layers of GaAs and AlAs having thicknesses of approximately 0.09μm and 0.01 μm respectively. Specifically, the second lower cladding layer 38- comprises thirty sets 38a, 38b etc. of layers, each set comprising one 0.09/m thick GaAs layer 44 and one O.Olμm thick AlAs layer 46.

Thus the total thickness of the second lower cladding layer 38 is approximately 3μm. The average index of the second lower cladding layer 38 is approximately that of 10% AlGaAs for TE polarised light, but is considerably lower than that for TM polarised light. Figure 3 a illustrates the electric field E(x) for the TE polarised light plotted against distance x through the layers of the device of Figure 1. The surface of the epitaxy is at x = lμm and a schematic representation of the dielectric profile (e = n²) is shown at the top of each plot. As can be seen, the large majority of the field is contained within the core 14 of the waveguide device with very little power leaking into the substrate 20 (to the right of the peak). This indicates transmission of the TE polarised light along the waveguide device.

Figure 3b, on the other hand, illustrates the magnetic field for the TM polarised light in a similar plot for the device of Figure 1. Here, it can be seen that there is significantly greater leakage of power through the second lower cladding layer 18 and into the substrate 20. This indicates that transmission of the TM polarised light through the waveguide device is being reduced or substantially inhibited.

Figures 4a and 4b illustrate contour plots of the fundamental TE and TM electric and magnetic fields respectively for a 6μm wide, deep-etched guide having the epitaxy of Figure 1. The lateral confinement. due to the waveguide sidewalls reduces the effective refractive indices of the modes within the core 14 to 3.3502 for the TE polarised light and 3.33509 for the TM polarised light. This results in a negligible increase in signal loss for the TE mode but a much greater loss in the TM mode (to approximately 27.9 dB/cm). It is clear from Figures 4a and 4b that very much more TM polarised light leaks into the substrate 20 than TE polarised light.

Figure 5 illustrates a plot of predicted guide loss against guide width for the fundamental TE and TM modes and also for the first higher order lateral TE mode. These are the only modes with losses below 100 dB/cm for the device of Figure 1. It is clear that the TM mode suffers high signal loss particularly as the guide width decreases (which decreases the effective refractive index for this mode even further below that of the second lower cladding layer 18). As the guide width decreases, the effective refractive indices of the core for both of the TE modes also decrease and as they drop below the lower cladding indices, they also suffer from high signal loss.

For guide widths between 2 and 3.5μm only the fundamental TE mode has negligible loss.

Figures 6a and 6b illustrate the field contours, effective refractive indices and signal losses for the two TE modes within the core 14 for a 3μm wide waveguide. The effective refractive index of the first higher order lateral TE mode in the core (Figure 6b) is below that in the second lower cladding layer 18 and so this mode suffers a high signal loss (16.1 dB/cm). However, the effective refractive index of the fundamental TE mode in the core (Figure 6a) is still higher than that of the second lower cladding layer 18 and so the signal loss for this mode is still very "low (0.085 dB/cm). This waveguide device will therefore support only one mode at only one polarisation.

Figures 7a and 7b illustrate field contours for the TE and TM modes produced by the waveguide device of Figure 2. In this figure, it is seen that the TM mode is more strongly confined than the TE mode since it sees a lower effective refractive index for the second lower cladding layer 38 relative to that of the core 34 than does the TE mode. A single polarisation waveguide device is particularly useful for electro-optic guides because the electro-optic effect due to a field applied perpendicular to the epitaxy layers only affects the TE mode. It is preferable that the TM mode is not transmitted through the device rather than transmitted unaffected by the electro-optic effect. In addition, deep-etched electro-optic waveguide devices can give rise to field dependent polarisation rotation from TE to TM which has disadvantageous consequences if a high extinction Mach Zehnder interferometer switch is required. This is because some light passes through the device even in the off state because it has been converted from TE to TM which cannot be switched by the electro-optic effect and so passes through the output port.

This applies to other electro-optic devices such as spectrum analysers, switched time delays, LIDAR and direct digital synthesiser delay chips. There may be other applications in which a single polarisation guide may be used in the same manner as a polariser in free-space optics. This is perhaps most likely in LIDAR where a number of polarisation controlling components are usually used for maximum efficiency.

By using Impurity Induced Disordering, the different layers forming the anisotropic material could be smoothed out in selected areas of the wafer to make an isotropic material. This would allow certain parts of the waveguide circuit to support transmission of both TE and TM polarised light while other parts support transmission of only one polarisation.

By making use of the TE to TM polarisation conversion effect in a horizontal field electro-optic waveguide, the Impurity Inducing Disordering process could be used to create a single-guide intensity modulator with a potentially very high extinction ratio.

In a further embodiment (not shown) an array waveguide grating device (AWGD) employs a plurality of the above-described waveguide devices, modified to support both polarisations, but maintaining the large effective index difference between the TE and TM modes. This can be achieved by, for example, increasing the AlGaAs composition of the second lower cladding layer 18, shown in Figure 1, to approximately 18% such that the index of the second lower cladding layer is lower than the effective indices of both TE and TM modes. Thus both modes are confined.

Each AWGD comprises an input 1 x 101 splitter, a set of 101 waveguide delay lines stepped in length from 2 to 3 mm in 10 micron steps, a 101 x 51 output recombiner and 51 output guides. The device acts as a wavelength division multiplexer/demultiplexer such that different wavelengths emerge from different output guides. It has been noticed that the two different polarisations emerged from two different waveguides suggesting that the large difference in the waveguide core refractive index for the two different polarisations can be used to make the integrated optical version of a polarisation beam splitter. This is a very significant component which has particularly important applications for LIDAR applications amongst others.

An alternative application of layered GaAs /AlAs waveguide structures is that, rather than trying to maximise the difference between the TE and TM mode indices, the effect can be used to minimise the index difference or reduce it to zero so that components such as AWGDs can be made polarisation insensitive. It will be appreciated that, although the specific device structures referred to hereinbefore are layered GaAs /AlAs structures, alternatively the device may have an epitaxy structure based on any group III-V material, for example GaP, InAs, InP, GaN.

Claims

1. An optical waveguide device or the like comprising:

a core; and

a cladding layer adjacent to said core;

wherein at least one of said core and said cladding layer comprises a plurality of layers, each layer having a substantially different refractive index from layers immediately adjacent thereto, thereby to control signal losses of optical signals applied to said device having a first polarisation relative to those of signals having a second polarisation.

2. A waveguide device according to claim 1, wherein the arrangement is such that signal losses of optical signals having a first polarisation are substantially less than those of signals having a second polarisation.

3. A waveguide device according to claim 2, wherein the signal losses of the signals having the first polarisation are between 10 and 10,000 times less than those of signals having the second polarisation.

4. A waveguide device according to claim 1, wherein the arrangement is such that signal losses of optical signals having a first polarisation are substantially identical to those of signals having a second polarisation, but the effective indices of the two polarisations are substantially different. Λ7 -

5. A waveguide device according to any preceding claim, wherein one of the core and the cladding includes at least twenty sets of layers, each set comprising a layer of a second material sandwiched between first and second layers of a first material.

6. A waveguide device according to claim 5, wherein the first and second layers of the first material are group III-V materials.

7. A waveguide device as claimed in claim 5 or claim 6, wherein the second material is a group III-V material.

8. A waveguide device according to claim 6 or claim 7, wherein the first material comprises GaAs and the second material comprises AlAs.

9. A waveguide device according to claim 8, wherein the layers of GaAs are approximately 0.045μm thick and the layer of AlAs is approximately O.Olμm thick.

10. A waveguide device according to any of claims 1 to 4, wherein one of the core and the cladding includes at least thirty sets of layers, each set comprising a layer of a first material and a layer of a second material.

11. A waveguide device according to claim 10, wherein at least one of the first and second materials is a group III-V material.

12. A waveguide device according to claim 11, wherein the first material comprises GaAs and the second material comprises AlAs.

13. A waveguide device according to claim 12, wherein the layer of GaAs is approximately 0.09μm thick and the layer of AlAs is approximately O.Olμm thick.