WO2004001470A1

WO2004001470A1 - Athermal arrayed waveguide grating

Info

Publication number: WO2004001470A1
Application number: PCT/GB2003/002615
Authority: WO
Inventors: Tsjerk Hans Hoekstra; Roland Munzner
Original assignee: Gemfire Europe Limited
Priority date: 2002-06-19
Filing date: 2003-06-18
Publication date: 2003-12-31
Also published as: GB0214085D0

Abstract

An athermal mux/demux device comprising an AWG with a Mach-Zender apparatus optically coupled to an input side thereof. The Mach-Zender apparatus has first and second sides, the first side being optically coupled to an input waveguide of the device and the second side comprising two output ports optically coupled to an input side of the first slab waveguide of the AWG. The Mach-Zender apparatus is configured so that its temperature sensitivity substantially compensates for the temperature sensitivity of the AWG whereby the device is substantially athermal. In one embodiment one arm of the Mach-Zender has a portion of its length made of a polymer material, this portion being deigned such that the rate of change of effective refractive with temperature (dneff/dT) of this polymer waveguide portion is of negative sign.

Description

ATHERMAL ARRAYED WAVEGUIDE GRATING

Description

FIELD OF THE INVENTION The present invention relates to arrayed waveguide gratings

(AWGs) and, in particular, to athermal AWGs and a new technique for making AWGs athermal.

BACKGROUND TO THE INVENTION AWGs, sometimes also known as "phasars" or "phased arrays", are now well-known components in the optical communications network industry. An AWG is a planar structure comprising an array of waveguides arranged side-by-side which together act like a diffraction grating in a spectrometer. AWGs can be used as multiplexers and as demultiplexers, and a single AWG design can commonly be used both as a multiplexer and demultiplexer. The construction and operation of such AWGs is well known in the art. See for example, "PHASAR-based WDM-Devices: Principles, Design and Applications", M K Smit, IEEE Journal of Selected Topics in Quantum Electronics Vol.2, No.2, June 1996, and US 5,002, 350 and US 5,732,171.

A typical AWG mux/demux is illustrated in Fig.l and comprises a substrate or "die" 1 having provided thereon an arrayed waveguide grating 5 consisting of an array of channel waveguides 8 (hereinafter referred to as "array waveguides"), only some of which are shown, which are optically coupled between two free space regions 3,4 each in the form of a slab waveguide. At least one input waveguide 2 is optically coupled to an input face 9 of the first slab waveguide 3 for inputting a multiplexed input signal thereto, and a plurality of output waveguides 10 (only two shown) are optically coupled to an output face 20 of the second slab waveguide 4 for outputting respective wavelength channel outputs therefrom to the edge 12 of the die 1. In generally known manner, there is a constant predetermined optical path length difference between the lengths of adjacent array waveguides 8 (typically the physical length of the waveguides increases incrementally by the same amount from one waveguide to the next) which determines the position of the different wavelength output channels on the output face of the second slab waveguide 4. Typically, the physical length of the array waveguides increases incrementally by the same amount, ΔL, from one waveguide to the next, where ΔL = mλ_c/n_c where λ_c is the central wavelength of the grating, n_c is the effective refractive index of the array waveguides, and m is an integer number. The material system most commonly used for the fabrication of such AWG mux/demux devices is silica-on-silicon. Fig.1(b) shows a cross- sectional view of a typical waveguide formed in this material system. In known manner, the array waveguides and slab waveguides are typically formed (e.g. using standard photolithographic and etching techniques) as "cores" 15 on a silicon substrate 14 (an oxide layer and/or cladding layer 17 may be provided on the substrate prior to depositing the waveguide cores) and are covered in a cladding material 16, this being done for example by Flame Hydrolysis Deposition (FHD) or Chemical Vapour Deposition (CVD) fabrication processes.

The temperature sensitivity of AWGs manufactured in silica-on- silicon is a known problem. The temperature coefficient of wavelength selective devices manufactured in silica-on-silicon is approximately 0.012nm/°C. This corresponds to a frequency shift of about 45 GHz over a temperature range of 30°C, which is often unacceptable to telecoms network designers. Various mechanisms have been tried to reduce this temperature sensitivity, so that the AWG will be substantially insensitive to temperature i.e. "athermal". One technique is to use active temperature stabilisation (i.e. a thermostatically controlled heater on the PLC), but this consumes a lot of power. Another technique overlays all or part of the AWG with a material having a negative thermo-optic coefficient, such as a polymer material, in order to counteract the change in refractive index of the array waveguides due to temperature changes. Similarly, another technique is to use a negative thermal expansion substrate in order to counteract change in optical path length of the array waveguides, as described in EP 1 072 908A. A problem with such

"counteracting" techniques is that it is difficult to exactly counteract the change in optical path length of every array waveguide: a better approach is to try to ensure that the change in optical path length with temperature is the same for all the array waveguides. One technique for achieving this is to insert one or more sections of a material having a negative thermo-optic coefficient, for example a polymer material, into the silica array waveguides and/or the slab waveguides to counteract the change in optical path length of the array waveguides. However, this has the disadvantage that it may introduce phase errors into the array waveguides and/or the slab waveguides, which in turn will degrade the AWG performance.

Yet another technique for achieving athermalisation is to use a mechanically movable input optical fibre waveguide at the input slab waveguide of the AWG in order to try to make the AWG athermal. See or example W098/13718. This solution is based on the fact that the position of the launching point for the input signal, on the input face 9 of the first slab waveguide, determines the central wavelength λ_c output from the AWG. A slight change in the position of the launching point can be used to compensate for drift in the central wavelength with temperature change. This mechanical solution has the possible disadvantage of being relatively complex to manufacture and also requires the whole device to be hermetically packaged (in order to protect the interface between the movable input fibre and the input slab waveguide face) which makes the packaging of the device complex and expensive.

It is an aim of the present invention to avoid or minimise one or more of the foregoing disadvantages.

SUMMARY OF THE INVENTION

According to the present invention there is provided an athermal multiplexer/demultiplexer device comprising: an arrayed waveguide grating (AWG) comprising first and second slab waveguides and a plurality of array waveguides optically coupled therebetween, the array waveguides having predetermined optical path length differences therebetween; a plurality of output waveguides optically coupled to the second slab waveguide of the AWG; and a Mach-Zender apparatus having first and second sides, the first side being optically coupled to at least one input waveguide and the second side comprising a plurality of output ports optically coupled to an input side of the first slab waveguide of the AWG, wherein the Mach-Zender apparatus is formed and arranged so that the temperature sensitivity of the Mach-Zender apparatus substantially compensates for the temperature sensitivity of the AWG whereby the device is substantially athermal.

The Mach-Zender apparatus preferably comprises an optical coupler coupling an output end of the at least one input waveguide to at least two waveguide arms which terminate in two said output ports respectively. The coupler may, for example, conveniently comprise a Y- branch coupler, a directional coupler, or a multi-mode interferometer (MMI) coupler.

The design of the Mach-Zender apparatus can be used to control the position, on the input face of the first slab waveguide, of a combined output signal formed by the signals output from the plurality of output ports of the Mach-Zender apparatus, in response to an input signal directed into an input end of the at least one input waveguide. Preferably, the two waveguide arms of the Mach-Zender apparatus are formed and arranged such that the change in the position (on the input face of the first slab waveguide) of the combined output signal of the two output ports of the Mach-Zender apparatus with change in temperature compensates sufficiently for the temperature sensititivity of the AWG so that the central wavelength of the mux/demux device is substantially independent of the temperature of the device. For example, one or more of the difference (ΔL) in path length between the waveguide arms, the material of the waveguide arms, and the dimensions of the waveguide arms may be chosen to achieve the desired temperature insensitivity. In one embodiment, one waveguide arm of the Mach-Zender apparatus may comprise a portion made of a material having a rate of change of refractive index with temperature of opposite sign to the rest of the waveguide arm. For example, where the waveguides of the device are made of silica (which has a positive rate of change of refractive index with temperature), one waveguide arm of the Mach-Zender apparatus may have a portion of its length made of a material having a negative rate of change of refractive index with temperature, for example a polymer material. In another embodiment, one waveguide arm of the Mach-Zender apparatus may comprise a portion made of a material having a rate of change of refractive index with temperature of different magnitude to, but with the same sign, as the rest of the waveguide arm. The output ports of the Mach-Zender apparatus may be coupled directly to the input side of the first slab waveguide. Alternatively, the output ports may be optically coupled to the first slab waveguide by another optical feature such as a multi-mode interference filter (hereinafter referred to as an MMI) as described in US 6,289,147 and which may, for example, be acting as a passband flattening feature. If desired, one or more passband flattening features may be optically coupled between the second slab waveguide and the output waveguides. In a first embodiment, the free spectral range of the Mach-Zender apparatus is preferably equal to the channel spacing of the AWG. In another embodiment, the free spectral range of the AWG is approximately equal to half the free spectral range of the Mach-Zender apparatus.

The Mach-Zender apparatus may include one or more further couplers cascaded with the first coupler, to form a lattice filter. This may allow greater design freedom in one or more other features of the device, in order to achieve a desired device performance. Preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig.1 is a plan schematic view of a prior art AWG mux/demux device;

Fig.2(a) is a plan schematic view of another prior art mux/demux device, designed to have a flat passband;

Fig.2(b) is a schematic plan view of a Mach-Zender apparatus used in the embodiment of Fig.2(a); Fig.3 is a plan schematic view of an athermal mux/demux device according to one embodiment of the invention;

Fig.4 is a plan schematic view of a modified version of the device of Fig.3; and Fig.5 is a schematic plan view of a lattice filter arrangement for use in a further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig.2 illustrates an AWG mux/demux device as described in US 5,488,680. Like parts in Figs. 1 and 2 are referenced by like reference numerals. The device comprises an AWG like that of the device of Fig.l, fabricated in silica-on-silicon technology, but with a Mach-Zender apparatus 20 coupled between the input waveguide 2 and the input slab waveguide 3. The Mach-Zender apparatus is shown in more detail in Fig.2(b) and is in the form of a Mach-Zender Interferometer (MZI) comprising a Y-branch coupler 22 connected between the input waveguide 2 of the device and two waveguide arms 23,24 of unequal length which diverge way from one another and then approach each other again in an output section 28 of the MZI. In the described device, this output section of the MZI has a length such that the two waveguide arms are strongly coupled in the output section 28. The two waveguide arms terminate in two output ports 25,26 connected to the input face 9 of the first slab waveguide 3. The length of the waveguides in the output section 28 is chosen so that half the power propagating in each of the waveguide arms prior to reaching the output ports is transferred to the other one of the waveguide arms. The two output ports 25,26 of the waveguide arms 23,24 are spaced apart by a predetermined distance d. In operation, the Y-branch coupler 22 transfers half the power of the input signal to each of the waveguide arms 23,24 of the MZI. Because there is a difference ΔL in the physical length of the two waveguide arms, there is a wavelength-dependent phase difference Δφ between the two signals which emerge from the output ports 25,26. The spacing d between the two output ports, and the difference in length ΔL of the two waveguide arms, can be chosen so as to achieve a desired phase difference Δφ between the output ports. An input signal directed into the Y-branch coupler 22 from the input waveguide 2 of the device, will produce two signal beam spots at the MZI output ports 25,26 respectively which effectively combine to produce a resultant image of the input signal, on the input face of the first slab waveguide 3. The phase difference Δφ produced between the two signals emerging from the output ports depends on the wavelength λ of the input signal. For example, if an input signal at a wavelength λo produces a phase difference Δφ which is an integer multiple of 2π, the output signal formed at the output of the MZI will be centered at a midpoint P₀ between the two output ports 25,26, since half the power of the input signal is propagating in each arm of the MZI, at the output end of the MZI. If the input signal wavelength is increased to λ₂ so that the phase difference is decreased by π/2, the output signal will be centered on the optical axis P₂ (in the output section 28) of one of the waveguide arms. Similarly, if the input signal wavelength is decreased to λi so that the phase difference is increased by π/2, the output signal will be centered on the optical axis Pi of the other one of the waveguide arms(in the output section 28). In summary, an input signal directed through the MZI provides an image on the input face 9 of the first slab waveguide 3 at a location between the two output ports 25,26 that is a periodic function of wavelength. In US5,488,680 this effect is exploited in order to flatten the passband of the channel outputs of the AWG. Fig.3 illustrates an athermal mux/demux device according to one embodiment of the present invention. This is a modified version of the device of Fig.2. Like reference numerals are used to refer to like parts in the devices of Figs. 1,2 and 3. In the device of Fig.3, a portion 30 of the length of one of the waveguide arms 23 of the MZI is made of a polymer material. The polymer material, and the length L_p of the polymer section (measured along the optical axis of the respective waveguide arm 23), are chosen so as to control the movement of the imaged input signal on the input face 9 of the first slab waveguide 3 in response to change of temperature, so as to compensate for the temperature driven angular dispersion of the AWG. In effect, the temperature driven angular dispersion of the MZI is used to compensate (automatically) for the temperature driven angular dispersion of the AWG, so that the overall device behaves athermally. No driving heaters are required to maintain temperature stability of the AWG, or to control the phase difference between the waveguide arms of the MZI. This keeps power consumption of the device to a minimum.

The temperature driven angular dispersion of an AWG is defined as the change in the central wavelength λ_c of the AWG with temperature T, namely dλ_c/dT. Variation in the central wavelength of an AWG with temperature change is highly undesirable. The change in the central wavelength is largely caused by the change in optical path lengths of the array waveguides 8 in response to temperature change. In the Fig.3 device, temperature change will also cause a change in the optical path length difference between the two waveguide arms of the MZI. In a first embodiment, the free spectral range FSR_MZI of the MZI is set equal to the channel spacing Δλ_Aw_G of the AWG. It can then be shown that the MZI design is determined by: mλ = n_effAL where

where n_effis the effective refractive index of the polymer waveguide portion of the MZI, Δλ_Aw_G is the channel spacing of the AWG, ΔL is length difference between the two waveguide arms of the MZI, λ is the wavelength of a signal input to the MZI, and m is the operating order of the MZI. By careful design of the rate of change in the effective refractive index n_{e f} of the polymer waveguide portion with temperature (i.e. dn_eff/dT), the inherent temperature driven angular dispersion of the AWG can be compensated by an inverse shift in the position of the imaged signal incident (from the MZI) on the input face 9 of the input slab waveguide, so that the central wavelength λ_c output from the overall mux/demux device (i.e. MZI +AWG) remains constant, or at least substantially constant e.g. for a temperature range from -5°C to +70°C, the temperature shift of the central wavelength λ_c is preferably lpm/K or less. This is about one order of magnitude smaller than what is observed for a non-athermalised AWG.

It can be shown that the required dn_eff/dT of the MZI can be calculated using the following equation:

dn_eff _ Δ λ dT 2πALAT

where Δφ here denotes the phase shift required at the input to the coupler section 28 in order to form the correctly positioned field distribution (i.e. the imaged input signal) at the ports 25,26 to compensate for the temperature driven angular dispersion dλ/dT of the AWG, for a signal wavelength λ over a temperature change ΔT. Ideally we have to compensate the temperature drift for all wavelength channels of the AWG, although in practice there will usually be a deviation for the outer channels and the above condition is optimised only for the central channel, wavelength λ_c. In general it will be found that for most AWG designs, a negative dn_eff/dT will be required, which can be most conveniently achieved by using a polymer material for the "different material" portion 30 of the waveguide arm 23. It will be appreciated that the polymer material, and the configuration (e.g. width and height) of the polymer waveguide section can be readily chosen and designed respectively so that the MZI has a negative dn_eff/dT of the required magnitude. For example, in one possible embodiment a "partial" polymer fill-in in the waveguide cladding is used i.e. the cladding is removed (e.g. by etching) along a portion of the length of one of the MZI arms and the resulting empty volume is partially filled with a polymer material. The dn_eff/dT of this modified waveguide portion then simply depends on the portion of the waveguide mode field which reaches into this polymer region. In effect, the positive dn_eff/dT of the silica waveguide is partially compensated by the negative dn_eff/dT of the polymer. (Etching down through the waveguide core as well as the cladding is also possible, but less desirable as there would be higher insertion losses.) An appropriate depth of polymer infill can be readily determined using a mode solver (software program) to determine the mode field, mode index, and dispersion of the waveguide mode within the modified MZI arm and, using this information, adjusting the depth of infill as required in order the resulting mode field does not introduce losses which are unacceptably high from a design perspective e.g. insertion loss, and polarisation-dependent loss (PDL).

The length L_p of the polymer section of the waveguide also influences the optical path length difference between the two arms of the MZI and so can be adjusted, in conjunction with the n_{e f} of the polymer section, so as to arrive at the calculated required optical path length difference. In our preferred process, the desired depth of polymer in-fill in the waveguide cladding is first determined and then the length L_p of the polymer section can be calculated utilising this information and the above-described equations defining mλ and m (assuming that the other parts of the two waveguides of the MZI are equivalent in length and mode index).

Nevertheless, in some AWG designs it may be possible to achieve the required dn_eff/dT without using a different material section in any of the waveguide arms of the MZI i.e. an all silica MZI solution may be possible. Furthermore, in some cases the "different material" portion 30 of the waveguide arm 23 may have a dn_eff/dT of different magnitude, but of the same sign, as the silica waveguides.

It will be appreciated that it is highly desirable for the temperature driven angular dispersion, d#₀„JdT, of the MZI to be considerably less than its wavelength driven angular dispersion , άϋ_OM fάλ, preferably such that:

where f _out is the output angle of the focussed (signal) beam on the output side of the second AWG slab coupler 4.

This does have the effect of setting a lower limit for the channel spacing Δλ of the AWG, namely:

where Δλ is the AWG channel spacing and ΔL is in this case the path length difference between adjacent array waveguides of the AWG.

In another possible embodiment the free spectral range of the AWG (FSRA_WG) is covered in approximately half of the free spectral range of the MZI (FSR_Mzι)- This can be defined as follows:

where N_AWG_channeis is the number of channels of the AWG; Δλ_AwG is the channel spacing of the AWG (in terms of wavelength); and ΔL_Mzι is the length difference between the two waveguide arms 23,24 of the MZI. This may have the effect of restricting the number of AWG channels available in the working device. In this embodiment the MZI is in principle still determined by the same equations as the first-described embodiment, namely:

mλ = n_effAL

where

where m is the operating order of the MZI. The polymer section 30 of the waveguide arm 23 may conveniently be fabricated by first etching a slot through the (upper) cladding 16 and down through the core 15 of the waveguide arm using a conventional dry etching technique such as reactive ion etching or plasma etching. The slot can then be filled with polymer solution by spin casting. Alternative fabrication techniques are also possible.

The embodiment of Fig.3 uses a directional coupler 28 at the output end of the MZI, as in the prior art device of Figs.2(a) and2(b). Fig.4 shows a modified version of the embodiment of Fig.3 in which a multi-mode interference filter 40 (hereinafter referred to as an "MMI") is coupled directly between the output ports 25,26 of the MZI, and the input face of the input slab waveguide 3 of the AWG, instead of using a directional coupler 28. Like reference numerals are used to refer to like parts, in Figs. 3 and 4. The operation of the device of Fig.4 is similar to the device described in US6,289,147 (which utilises an MMI between an MZI and an AWG, in order to broaden the passband of each output channel of the AWG), but modified so that the MZI is designed to compensate for the temperature driven angular dispersion of the AWG as described already above in relation to Fig.3. For example, the MZI may need to incorporate a section 30 of polymer or other material having a negative thermal coefficient (dn_eff/dT) in order to make the mux/demux device substantially athermal. In the Fig.4 embodiment, if one sets the free spectral range FSR_Mzι of the MZI equal to the channel spacing of the AWG (as in the first-described version of the Fig.3 embodiment), this will result in passband broadening which may be a desirable additional feature of the mux/demux device.

It will be appreciated that variations on the above-described embodiments are possible within the scope of the invention. For example, the coupler between the input waveguide 2 and the two waveguide arms 23,24 of the MZI need not always be a Y-branch coupler. A directional coupler could equally well be used, or another form of coupler e.g. an MMI coupler.

In another possible embodiment, more than one MZI may be used at the input side of the AWG. In this case, three or more couplers may effectively be cascaded, with two waveguide arms linking adjacent couplers, so as to form a lattice filter arrangement (sometimes alternatively referred to as a cascaded coupler arrangement), as illustrated in Fig.5 which shows four directional couplers spaced apart by three pairs of waveguide arms to form three cascaded MZIs 40,42,44. This design allows finer adjustment (than in the "single MZI" embodiments of Figs.3 and 4) of the temperature sensitivity of the portion of the mux/demux device prior to the AWG. This, in turn, may allow greater freedom in the design of other features of the device. Furthermore, although all the embodiments shown in Figs. 2 to 5 show MZIs having only two waveguide arms, in other possible embodiments Mach-Zender apparatus having more than two waveguide arms may be used. For example, the Mach-Zender apparatus may be formed by three waveguides 45,46,47 coupled between an MMI coupler 48 and the first slab waveguide (or an MMI coupled to the first slab waveguide). Similarly, the lattice filter of Fig.5 could be formed by several such Mach-Zender apparatus coupled end-to-end as shown in Fig.6 (i.e. MMI coupler 48 + three waveguide arms + MMI coupler 50 + three waveguide arms + etc.). As shown, there may be more than one input waveguide 2a,2b coupled to the first MMI coupler. In such embodiments it may be more convenient to use waveguide arms of different widths or other physical dimensions, and/or having portions made of different materials, instead of arms having different physical lengths, in order to control the output of the various stages of the filter. It will be appreciated that the present invention in essence utilises the same principle as the prior art mechanical approach to athermalisation (i.e. adjusting the position of the input signal spot on the input face of the input slab waveguide, in order to compensate for drift in the central wavelength λ_c of the AWG), but has the benefit of being an integrated approach in which the whole athermal device is integrally formed on the same PLC chip. No mechanical movement, or moving parts, are required in the invention and thus fabrication is easier than the prior art mechanical approach. Moreover, there is no gap between the input fibre and the slab causing reliability problems.

Claims

LA multiplexer/demultiplexer device comprising: an arrayed waveguide grating (AWG) comprising first and second slab waveguides and a plurality of array waveguides optically coupled therebetween, the array waveguides having predetermined optical path length differences therebetween; a plurality of output waveguides optically coupled to the second slab waveguide of the AWG; and a Mach-Zender apparatus having first and second sides, the first side being optically coupled to at least one input waveguide and the second side comprising a plurality of output ports optically coupled to an input side of the first slab waveguide of the AWG, wherein the Mach- Zender apparatus is formed and arranged so that the temperature sensitivity of the Mach-Zender apparatus substantially compensates for the temperature sensitivity of the AWG whereby the device is substantially athermal.

2. A multiplexer/demultiplexer device according to claim 1, wherein the Mach-Zender apparatus comprises an optical coupler coupling said at least one input waveguide to at least two waveguide arms terminating in two said output ports respectively.

3. A multiplexer/demultiplexer device according to claim 2, wherein the optical coupler comprises a Y-branch coupler.

4. A multiplexer/demultiplexer device according to claim 2, wherein the optical coupler comprises a directional coupler.

5. A multiplexer/demultiplexer device according to claim 2, wherein the optical coupler comprises an MMI coupler.

6. A multiplexer/demultiplexer device according to any of claims 2 to 5, wherein the waveguide arms of the Mach-Zender apparatus are formed and arranged such that the change in the position, on the input face of the first slab waveguide, of the combined output signal of the output ports of the Mach-Zender apparatus with change in temperature compensates sufficiently for the temperature sensititivity of the AWG so that the central wavelength of the mux/demux device is substantially independent of the temperature of the device.

7. A multiplexer/demultiplexer device according to claim 6, wherein the Mach-Zender apparatus has two said waveguide arms and there is a difference (ΔL) in length between the two waveguide arms.

8. A multiplexer/demultiplexer device according to any of claims 2 to 7, wherein at least one said waveguide arm of the Mach-Zender apparatus comprises a portion made of a material having a rate of change of refractive index with temperature of opposite sign to the rest of said waveguide arm.

9. A multiplexer/demultiplexer device according to claim 8, wherein the waveguides of the device are made of silica and said at least one waveguide arm of the Mach-Zender apparatus comprises a portion made of a material having a negative rate of change of refractive index with temperature.

10. A multiplexer/demultiplexer device according to claim 9, wherein said material having a negative rate of change of refractive index with temperature is a polymer material.

11. A multiplexer/demultiplexer device according to any preceding claim, wherein the output ports of the Mach-Zender apparatus are coupled directly to the input side of the first slab waveguide and said at least two waveguide arms are optically coupled where they terminate in said output ports.

12. A multiplexer/demultiplexer device according to any of claims 1 to 10, wherein the output ports are optically coupled to the first slab waveguide by an MMI.

13. A multiplexer/demultiplexer device according to any preceding claim, wherein the free spectral range of the Mach-Zender apparatus is equal to the channel spacing of the AWG.

14. A multiplexer/demultiplexer device according to any of claims 1 to 12, wherein the free spectral range of the AWG is approximately equal to half the free spectral range of the Mach-Zender apparatus.

15. A multiplexer/demultiplexer according to any preceding claim, wherein the Mach-Zender apparatus comprises a plurality of Mach- Zender interferometers (MZIs).

16. A multiplexer/demultiplexer device according to any preceding claim, wherein the Mach-Zender apparatus comprises a lattice filter.