WO2001067148A1

WO2001067148A1 - Waveguide array device

Info

Publication number: WO2001067148A1
Application number: PCT/GB2001/000973
Authority: WO
Inventors: Richard Ian Laming
Original assignee: Alcatel Optronics Uk Limited
Priority date: 2000-03-06
Filing date: 2001-03-05
Publication date: 2001-09-13
Also published as: GB0005391D0; GB2360098A

Abstract

A waveguide array device is provided comprising a substrate (18) having first and second couplers (14); a first plurality of waveguides (12), arrayed generally side-by-side, and connected between the first and second couplers; a second plurality of input/output waveguides (20) connected to the first coupler, and one or more further input/output waveguide(s) (20) connected to the second coupler, wherein the waveguides have cores (30, 32) surrounded by cladding (34), the refractive index of each waveguide core being greater than that of its surrounding cladding, and wherein a region (36, 36') between at least a pair of adjacent waveguides has a refractive index different to its surrounding cladding.

Description

WAVEGUIDE ARRAY DEVICE

The invention relates to a waveguide array device and more particularly ^to an

Array, or Arrayed. Waveguide Grating (AWG), for example, functionⁱng as a swⁱtch or a multiplexer/demultiplexer, for a Dense Wavelength Division Multiplexⁱng

(DWDM) system.

DWDM employs many closely spaced optical carrier wavelengths, multiplexed together onto a single waveguide such as an optical fibre. The carrier wavelengths are spaced apart by as little as 50 GHz in a spacing arrangement defined by an ITU (International Telecommunications Union) channel "grid". Each carrⁱer wavelength may be modulated to provide a respective data transmission channel. By usⁱng many channels, the data ra_te of each channel can be kept down to a manageable level^, so avoiding the need for expensive very high data rate optical transmitters, op^tical receivers and associated electronics. An Array Waveguide Grating consists of a number of arrayed channel waveguides which together act like a diffraction grating in a spectrometer^. Such gratings have a high wavelength resolution, thus attaining narrow wavelength channel spacings down to as little as 0.8nm or lower in the ITU channel allocation.

An AWG multiplexer is a device which combines optical signals of differen^t wavelengths; conversely, an AWG demultiplexer splits a multiplexed sⁱgnal ⁱn^to a plurality of signals. An AWG multiplexer typically comprises two focusⁱng slab regions, often called couplers, connected to either end of an arrayed waveguⁱde grating, a plurality of input waveguides connected to one of the slab regions, and one or more output waveguide(s) connected to the other slab region^. An AWG demultiplexer typically comprises two focusing slab regions connected to either end of an arrayed waveguide grating, one or more input waveguide(s) connected to one of the slab regions, and a plurality of output waveguides connected to the other slab regⁱon.

Since AWGs work in both directions, a multiplexer can often be used as a demultiplexer by using it in the reverse direction. The most flexible device is ob^taⁱned by employing multiple input and output waveguides. The function of the devⁱce then depends on the nature of the signal, or signals, input to the device^, that is whether the input is a multiplexed signal of many wavelengths or a plurality of single wavelength signals.

A typical AWG mux/demux, as shown in Fig. 1. comprises two focusing slab regions or couplers 14. connected to either end of an arrayed waveguide grating 10. The grating 10 consists of an array of channel waveguides 12, only some of which are shown. Input multiple WDM signals are dispersed and focused simultaneously to each prescribed output waveguide 22.

In the arrangement shown in Figure 1 a substrate 18 has formed on it first and second couplers 14. a plurality of array waveguides 12 a plurality of input/output waveguides 20 connected to the first coupler, and a plurality of input/output waveguides 22 connected to the second coupler. The array waveguides 12 and/or input/Output waveguides 20. 22 are preferably arranged generally side-by-side.

Each of the waveguides 12, 20, 22, has a core 15 with a cladding material 16 at least on either side of it. The couplers 14 are preferably star couplers, which are well known in the art. and which have curved input and output surfaces. The curve of the input/output of the slabs enhances focusing of the signals, but also enables neighbouring waveguides to be angled away from each other in the vicinity of the coupler, thereby reducing the cross talk. The waveguide array device preferably comprises an array 10 of waveguides

12 having different lengths, to provide an array of different path lengths to an input signal. Preferably the path length difference between neighbouring waveguides of the array is a constant, L where L=mλ_c/n_c and λ_c is the central wavelength of the grating, ri_c is the effective refractive index of the channel waveguides and m is an integer number. The grating acts as a phase grating of order m. Single mode channel waveguides are preferably used to enable exact phase control in the grating.

The arrangement works as follows. Light from an input waveguide expands as a 2D diverging wave inside the first slab and excites the input of the arrayed waveguide channels which start along a curve, in this example an arc ot a circle with radius r, around the slab input. After travelling through the arrayed waveguides, the light at the end face of the grating, arranged on a circle with radius r, is radiated as a 2D wave into the second slab region as a converging wavefront and converges to a focal point at the slab exit where the outgoing waveguide is located.

The signal path length difference in the array 10 results in a wavelength dependent tilt of the radiated spherical wave, converging to a shifted focal point. The pitch of the channel waveguides at the grating exit is typically around 17μm.

Referring to the device shown schematically in Figure 2, for a signal transmitted in the direction of arrow 'A', the device comprises a single input waveguide and multiple output waveguides. The device functions as a demultiplexer, splitting an input signal of many wavelengths into a plurality of output signals, each of a single wavelength. The same grating could function in reverse as a multiplexer, as shown by arrow 'B'.

W^rith ever increasing bandwidth demands, there is a need to provide a waveguide array device with as many channels as possible within a given bandwidth, and to make the transfer function of each channel as square as possible. That is. the channel spacing is desired to be as small as possible. However, the smaller the channel spacing the greater the problems caused by interference between neighbouring channels, such as cross-talk.

In the paper "Efficient Techniques for Widening the Pass Band of a Wavelength Router". Journal of Lightwave Technology. Vol. 16, No 10, pp. 1895- 1906, October 1998 by C. Dragone, which is directed to methods for widening the pass band of a wavelength router, or AWG, rather than to methods for reducing the channel spacings, it was realised that the transmission coefficient of a conventional AWG varied depending on the V-number of the waveguides. It found that the decay constant of the transmission function of a waveguide, which is the sharpness of the cut-off of the transmission function, is specified by the output waveguide V-number, with a higher V-number leading to a sharper cut-off.

The V-number of a waveguide is a dimensionless parameter, often called the "waveguide parameter", which is well understood by those skilled in the art. The V- number of a waveguide determines its modal behaviour. If V»l then the waveguide is multimoded, or '"overmoded", and more than one bound modes can propagate. However, in many applications single-moded waveguides are required. It is preferable to use single-mode waveguides in an Arrayed Waveguide Grating to enable precise control of the phase of the signals.

One aspect of the present invention addresses the problem of increasing the decay rate of the transmission curve of the waveguide channels. The invention provides a waveguide array device comprising a substrate having first and second couplers, a first plurality of waveguides, arrayed generally side-by-side, and connected between the first and second couplers, a second plurality of input/output waveguides connected to the first coupler, and one or more further input/output waveguide(s) connected to the second coupler, wherein the waveguides have cores adjacent to cladding, the refractive index of each waveguide core being greater than that of its adjacent cladding, and wherein a modifying region between at least a pair of adjacent waveguides has a refractive index different to its surrounding cladding.

In a first preferred embodiment of the invention, the modifying region is arr.anged between a pair of adjacent input/output waveguides and has a refractive index less than its surrounding cladding. By decreasing the refractive index of a region between two waveguides the effective numerical aperture of the waveguides is increased, thereby increasing their V-number in the locality of the region. This increase in V-number usually takes place only in the x/y plane as shown in Figure 3, hereinafter referred to as the "horizontal" direction. However, it is possible that the V- number of the waveguide could also be modified in the J , or vertical, direction. By appropriately positioning the modifying region near to a coupler, it is possible to increase the V-number of the waveguides near the coupler connection, whilst still maintaining a lower V-number for single mode propagation elsewhere. Thus an output signal with a sharper cutoff, and more square shape, may be produced by the waveguides.

In another embodiment of the invention, the modifying region has a higher refractive index than its surrounding cladding and is preferably arranged between at least a pair of waveguides of the array. This tends to decrease the signal loss in the coupling of the slab region to the array waveguides. The modifying region(s) may be provided in a number of different shapes, sizes and by several different methods, according to the particular application of the device.

Other methods of modifying the V-number of the waveguide may additionally be employed, such as varying the thickness, or refractive index, of the core.

A preferred feature of the invention is the provision of the modifying region in the proximity of a coupler. Preferably the region is tapered and broadens in section towards the coupler adiabatically, that is the angle of the taper is so small that there is insignificant coupling of a transmitted signal with higher order modes. The modifying region may be, for example, provided by etching a region of the cladding and then either filling it with a material of a different refractive index, or leaving it filled just with air.

In another modification, the modifying region may comprise a plurality of small holes sufficiently small and close together so as to appear as a continuous region of a given refractive index. These holes again may be filled with air only, or with another material.

The invention is described in more detail in the appended claims to which reference should now be made.

Preferred embodiments of the invention will now be described with reference to the drawings in which:

Figure 1 shows the waveguide pattern of an AWG multi/demultiplexer chip;

Figure 2 shows schematically the function of an AWG multi/demultiplexer;

Figure 3 shows a schematic perspective of an optical substrate;

Figure 4 shows schematically a section of a device according to an embodiment of the present invention;

Figure 5 shows schematically a modification of the embodiment shown in Figure 4; and

Figure 6 shows schematically a section of a device according to another embodiment of the invention. Figure 3 shows schematically how an optical waveguide is formed on a substrate. A silica waveguide is defined to consist of the following regions: • a substrate 18 of silicon, SiO₂ (silica) or the like; • a (possibly doped) silica buffer layer 19 deposited by thermal oxidation or by flame hydrolysis deposition or another method . and of course not required on a silica substrate;

• a (possibly doped) silica cladding layer 16 deposited by flame hydrolysis (FHD) or plasma enhanced chemical vapour deposition; and

• one or more (possibly doped) cores 15 surrounded by the cladding and buffer regions. The cores may be formed by laying down a layer of core glass by FHD and a consolidation step, then photolithographically masking and etching to form the core paths. The cladding and any other subsequent layers can then be established by FHD.

For the purpose of characterising an optical waveguide, the following parameters are defined: n_Substrate substrate 18 refractive index na_ffer buffer 19 refractive index n_ci_ad cladding 16 refractive index n_COr_e core 15 refractive index tsu_bstrate substrate 18 thickness tbu fer buffer 19 thickness tciad cladding 16 thickness t_core core 15 thickness

W_core core 15 width

A waveguide fabricated according to embodiments of the invention is defined to possess the following characteristics:

Refractive Index (RI) ⁿbuffer n_SUbstrate^>>nbufffer, n_ciad, n_COre (for Si substrate) n_SUbstrate < n_COre (for SiO substrate) Dimensions tsubstrate-^>'^>t_ciad^"' ^buffer tclad_> ^buffer -⁵" tςore

The present invention can be used in the context of the AWG mux/demux shown in Figure 1, and described earlier. Preferably, such an arrangement is provided as a planar silica-on-silicon integrated chip produced by FHD. However, other substrates may also be used. The present invention also finds application in many other optical devices.

Figure 4 shows an area at a junction between two waveguides 30, 32 and a star coupler 14 in accordance with a preferred embodiment. Each waveguide core 30, 32 is surrounded on either side by cladding 34. The waveguide cores each have a refractive index, n_COre, which is greater than the refractive index of the cladding, ncia - The cores are generally fabricated to have the same refractive index as each other, but this is not essential.

A region 36 in the cladding, between the pair of adjacent waveguides 30, 32 has a refractive index n_mod, different than that of its surrounding cladding, that is n_mod≠n_cι_ad. The region preferably tapers outwardly towards the coupler, that is it increases in section towards the coupler. This is preferably an adiabatic taper, the taper being arranged to adiabatically transmit the light signals, such that the mode(s) of the signals passing along the waveguides do not couple with higher order modes. The taper dimensions are typically about 500microns long and 7microns wide.

In a first embodiment the modifying region has a refractive index lower than the surrounding cladding, so that n_mod<n_Ciad. In this case the region is usually arranged between two input/ouput waveguides Looking at the theory behind how the system works, we can look to the V-number of a conventional fibre waveguide which is given by the well-known formula:

V= 2β NA λ where NA, the Numerical Aperture, is conventionally given by (n_COre" - n^~ _C|_ad) ^" where n_COre is the refractive index of the core and n_cι_ad is the refractive index of the cladding. Thus the V-number depends on the difference between the refractive index of the core and that of its cladding. By including a region in the cladding having a lower refractive index than the rest, the effective NA of the waveguide is increased, leading to an increase in the V-number of the waveguide, and consequently a sharper cutoff of the transmission function of the waveguide.

Preferably, the waveguide device is symmetric, with each input/output waveguide having a modifying region on each side of it in the proximity of a coupler. However, many modifications can be made to the arrangement without moving away from the inventive concept. For example modifying regions may be arranged between only some of the input/output waveguides, or between waveguides connected to a particular one only of the couplers.

The modifying region may be fabricated by etching a slot or recess of appropriate size and shape in the cladding using a conventional etching technique, or by laser ablation. The slot or recess can then be filled with air, or with another material, for example with molten polymer by spin coating.

Alternatively the modifying region may comprise a plurality of small holes filled with air as above which are sufficiently small and close together to present an effective continuous medium to the light waves or photonic band gap. The small holes alter the effective refractive index of the modifying region so that it differs from that of the rest of the cladding. Since the air inside the holes has a lower refractive index than the rest of the cladding, the effective refractive index of the region is lower than that of the rest of the cladding, thus producing an increase in the V-number of the waveguide as described above.

The region of holes preferably tapers outwardly towards the coupler, that is it increases in section towards the coupler, as shown in Figure 4. Again, this is preferably an adiabatic taper, such that there is no higher order coupling of the mode(s) of the signals passing along the waveguides caused by the change in refractive index of the cladding. These holes could be fabricated by etching.

In another embodiment of the invention, the modifying region 36' has a higher refractive index than its surrounding cladding, and in this case is usually arranged between at least a pair of waveguides 12 of the array, as is shown in Figure 6. By increasing the refractive index of an area between two array waveguides, the power loss of signals transmitted along the array waveguides is decreased. This is because the coupling loss between the slab and the array waveguides is decreased. Of course, the device could have one or more modifying regions having a lower refractive index than the surrounding cladding between one or more pairs of input/output or array waveguides as well as one or more modifying regions having a higher refractive index than the surrounding cladding arranged between one or more pairs of array or input/output waveguides. One or more cores of the waveguide array device may alternatively be fabricated such that the layer of core material is not etched all the way through, leaving a thin layer of core material in areas where no waveguides are provided. In this case the cladding material, provided over the surface of the remaining core material lies adjacent to the non-etched waveguide cores, substantially surrounding them on either side.

In another embodiment of the invention the waveguide array of the device may comprise a plurality of waveguides of the same length, perhaps in combination with waveguides of different lengths. Such gratings have been used as switches, with heaters being used to apply phase changes to individual waveguides of the array to select particular signals.

The embodiments of the invention thus provide an improved waveguide array device having a sharper transmission cut-off for each waveguide and/or reduced signal transmission loss.

Claims

1. A waveguide array device comprising a substrate having first and second couplers; a first plurality of waveguides, arrayed generally side-by-side, and connected between the first and second couplers; a second plurality of input/output waveguides connected to the first coupler, and one or more further input/output waveguide(s) connected to the second coupler, wherein the waveguides have cores adjacent to cladding, the refractive index of each waveguide core being greater than that of its adjacent cladding, and wherein a region between at least a pair of adjacent waveguides has a refractive index different to its surrounding cladding.

2. A device according to claim 1, wherein the modifying region is arranged between a pair of adjacent input/output waveguides.

3. A device according to claim 1 or 2, wherein the modifying region has a refractive index less than its surrounding cladding.

4. A device according to any preceding claim, wherein the modifying region is provided in the proximity of a coupler.

5. A device according to claim 4, in which the modifying region tapers outwardly towards the coupler.

6. A device according to claim 6, in which the taper is arranged to adiabatically transmit light signals.

7. A device according to any preceding claim wherein the modifying region is provided by etching an area of the cladding and filling it with a material having a different refractive index to its surrounding cladding.

8. A device according to claim 7, in which the etched region is filled with air

9. A device according to any preceding claim, in which the region comprises a plurality of small holes.

10. A device according to claim 1, in which the modifying region is pro\ ιded between a pair of arrayed waveguides.

11. A device according to claim 10 in which the modifying region has a refractive index higher than its surrounding cladding.

12. A device according to any preceding claim in which the arrayed waveguides connected between the first and second couplers have different lengths.

13. A device according to claim 12, in which the difference in length between adjacent waveguides of the array is constant.

14. A multiplexer, demultiplexer, or multiplexer/demultiplexer comprising a waveguide array device according to any preceding claim.

15. A switcher comprising a waveguide array device according to any of claims lto 13.

16. A wavelength router comprising a waveguide array device according to any of claims 1 to 13.