Patent Publication Number: US-2020295537-A1

Title: Discrete wavelength tunable laser

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 16/820,422, filed Mar. 16, 2020, entitled “DISCRETE WAVELENGTH TUNABLE LASER”, which is a continuation of U.S. patent application Ser. No. 16/077,437, filed Aug. 10, 2018, entitled “DISCRETE WAVELENGTH TUNABLE LASER”, issued as U.S. Pat. No. 10,594,109, issued on Mar. 17, 2020, which is a U.S. National Stage application of International Patent Application number PCT/GB2017/050408, filed Feb. 17, 2017, which claims priority to GB Patent Application number 1602947.2, filed Feb. 19, 2016. 
     The present application is related to U.S. Pat. No. 9,270,078 (the “&#39;078 Patent”), which is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a discrete wavelength tunable laser, particularly to a discrete wavelength tunable laser having an optical cavity which includes a combination of a passive optical grating and one or more digital supermode-distributed Bragg reflector(s). 
     BACKGROUND 
     Continuously tunable lasers are well-established and are commonplace in telecommunications applications. Although telecommunications lasers operate to fixed grids (e.g., ITU grids), tunable lasers need to be set up for a variety of applications and some extent of wavelength tunability is desirable to allow for correction of wavelength drift as the laser ages. Unfortunately, the requirement for full range continuous tunability results in expensive and power-hungry electronic circuitry, most particularly due to the requirement for digital-analog conversion (DAC) chips. 
     Distributed Feedback (DFB) lasers in which the gratings are built into the gain medium are being replaced by Distributed Bragg Reflector (DBR) lasers, particularly where tunability is required. For a wide range of tunability a Sampled Grating (SG) DBR laser is one typical option. In such a laser, the grating (often referred to as a “comb grating”) gives rise to a comb of reflectivity peaks which can be tuned to select the required lasing wavelength. 
     In an alternative design of tunable laser, Digital Supermode DBRs (DS-DBRs) may be utilized. The DS-DBR design has the advantage over SG-DBR in that no DACs are required. Semiconductor lasers, made absent DACs for primary control, are disclosed herein. This may be achieved, for example, by devising finite state tunable devices. 
     SUMMARY 
     Accordingly, embodiments of the present invention aim to solve the above problems by providing, according to a first aspect, a discrete wavelength tunable laser having an optical cavity which comprises: a semiconductor optical amplifier (SOA); a wavelength demultiplexer (Demux) having a single input and a plurality of outputs, the AWG configured to receive the output of the SOA and to produce a plurality of fixed spectral passbands within the gain bandwidth of the SOA; one or more tunable distributed Bragg reflector(s) (DBR(s)) arranged to receive the outputs of the Demux, each tunable-DBR configured to select a reflective spectral band within the gain bandwidth of the SOA upon application of a bias current; wherein the SOA forms the back end mirror of the optical cavity; the one or more tunable-DBRs form the front end mirror of the optical cavity; and wherein the lasing channel of the discrete wavelength tunable laser is chosen by the overlap of the selected reflective spectral band of one of the one or more tunable-DBRs with a fixed spectral passband of the Demux. 
     The one or more tunable-DBRs are configured to select a given spectral passband from the Demux which corresponds to the chosen mode of the laser (i.e. to the desired output of the wavelength) by reflecting the reflective spectral band which corresponds to the desired wavelength. The optical cavity of the tunable laser is therefore formed between a back end mirror on the SOA and a front end mirror formed by the one of the tunable DBRs to which a bias current is provided. 
     A single SOA can therefore provide the light for all channels of the tunable laser. This means that the wavelength tunable laser of some embodiments achieves wavelength switching without the need for multiple SOAs. Unlike previous prior art examples where there exists a need to switch between multiple SOAs, there is no requirement in the tunable lasers described herein for complicated driving circuits to switch SOAs on and off. Instead, the entire tunable laser can be operated by a simple driving circuit which drives the one or more tunable DBRs directly. 
     Optional features will now be set out. These are applicable singly or in any combination with any aspect of embodiments of the invention. 
     The Demux may take the form of any component suitable for demultiplexing the output of the SOA according to the wavelength. This Demux may take the form of an arrayed waveguide grating (AWG). Other forms of Demux include: a Planar Concave Grating (PCG), a ring resonator array, an interleaver structure, a multimode interference device MMI, a cascaded Mach Zehnder interferometer, or, as discussed in further detail below, an echelle grating. 
     Each of the tunable DBRs may be a digital supermode distributed Bragg Reflector (DS-DBR). 
     Each of the one or more tunable DBRs may be a phase-tunable Distributed Bragg Reflector which may include a phase tuning region, the carrier density of which can be manipulated by application of a current or voltage bias. The bias may be a forward bias or a reverse bias depending on the properties of the phase tuning region chosen. Adjusting the bias will in turn adjust the phase and position in frequency space of the reflectance spectrum or transmittance spectrum of that region of the DBR. The phase tuning region may be a portion of or all of the DBR. 
     Optionally, the phase tuning region includes a p-n junction device. In this way, the p-n junction device provides an electrically tunable phase tuning region whereby the phase of the reflectance spectrum of the DBR can be adjusted via carrier depletion by application of a varying reverse bias. The p-n junction may optionally be a p+−p−n−n+, or p++−p+−p−n−n+−n++ structure. 
     The AWG may be fabricated integrally with other waveguides on a single SOI chip. Alternatively, the AWG may be fabricated as a separate chip and aligned with other waveguides on the silicon chip during assembly. 
     The discrete wavelength tunable laser may include a single waveguide which optically couples the output of the SOA to the input of the AWG; and may include further waveguides, each of which is optically coupled to a respective output of the AWG; each of the further waveguides including a respective one of the one or more tunable DBRs. 
     In this way, the AWG receives light from the SOA via a single waveguide. The AWG then transmits light corresponding to each respective fixed spectral passband via a respective waveguide, each respective waveguide comprising a tunable DBR. 
     Optionally, the AWG is a 1×N AWG which splits the spectral output of the SOA into an integer number N of respective output waveguides; and the discrete wavelength tunable laser may include a further AWG which acts as a multiplexer to multiplex the signals from the N respective output waveguides to produce a common output of the laser. 
     Optionally, the AWG is a first 1×M AWG which splits the spectral output of the SOA into an integer number M of respective output waveguides; wherein the discrete wavelength tunable laser includes a further AWG which acts as a multiplexer to multiplex the signals from the M respective output waveguides to produce a common output of the laser; and wherein each respective waveguide includes one or more tunable DBRs the one or more tunable DBRs on each waveguide having a total number of P reflecting wavelength states. 
     Optionally, the AWG is a cyclic AWG wavelength router, having at least P cycles. 
     Optionally, the 1×M AWG has a channel spacing of Δf and an FSR of MΔf. 
     Where the AWG is a 1×M cyclic router, the first output waveguide would receive spectral passbands from the AWG corresponding to wavelength values λ 1 , λ M+1 , etc. up to λ (P−1)M+1 . The tunable DBR gratings on that waveguide will therefore be configured (by way of the grating separation) to reflect wavelength states with wavelength values λ 1 , λ M+1 , etc. up to λ (P−1)M+1 . The second output waveguide would receive spectral passbands corresponding to wavelength values λ 2 , λ M+2 , etc. up to λ (P−1)M+2 . The tunable DBR gratings on that waveguide will therefore be configured (by way of the grating separation) to reflect wavelength states with wavelength values λ 2 , λ M+2 , etc. up to λ (P−1)M+2 . The final output waveguide (the M th  output) would receive spectral passbands from the AWG corresponding to wavelength values λ M , λ 2M , etc. up to λ (P−1)M+M  (i.e. λ PM ,). It can therefore be understood that the total number of discrete wavelength states that the tunable laser can be tuned to is given by M×P. 
     Optionally, the AWG is a non-cyclic 1×P AWG which splits the spectral output of the SOA into an integer number P of respective output waveguides; wherein the discrete wavelength tunable laser includes a further AWG which acts as a multiplexer to multiplex the signals from the P respective output waveguides to produce a common output of the laser; and wherein each respective waveguide includes one or more DS-DBRs the one or more tunable DBRs on each waveguide having a total number of M reflecting wavelength states. 
     Where this 1×P AWG is non-cyclic AWG, the first of the P output waveguides would receive spectral passbands from the AWG corresponding to wavelength values λ 1 , λ 2 , etc. up to λ M . The tunable DBR gratings on that waveguide will therefore be configured (by way of the grating design) to reflect wavelength states with wavelength values λ 1 , λ 2 , etc. up to λ M . The second output waveguide would receive spectral passbands corresponding to wavelength values λ M+1 , λ M+2 , etc. up to λ 2M . The tunable DBR gratings on that waveguide will therefore be configured (by way of the grating design) to reflect wavelength states with wavelength values λ M+1 , λ M+2 , etc. up to λ 2M . The final output waveguide (the P th  output) would receive spectral passbands from the AWG corresponding to wavelength values λ (P−1)M+1 , λ (P−1)M+2 , etc. up to λ (P−1)M+M  (i.e. λ PM ,). As with the cyclic example, it can therefore be understood that the total number of discrete wavelength states that the tunable laser can be tuned to is given by M×P. 
     Optionally, the wavelength demultiplexer (i.e., the wavelength demultiplexer which is configured to receive the output of the SOA and to produce a plurality of fixed spectral passbands within the gain bandwidth of the SOA) includes: a coarse tuning AWG in the form of a 1×P AWG, the coarse tuning AWG having one input which is optically coupled to the SOA and P outputs, the 1×P AWG configured to output a sub-region of the SOA spectral passband to each of its P respective outputs; a plurality of fine tuning AWGs, where each fine tuning AWG is a 1×M AWG; the input of each of the 1×M AWG being optically coupled to one of the P respective outputs of the 1×P AWG; M output waveguides respectively coupled to the M outputs of each fine tuning AWG to give a total of M×P output waveguides from the 1×M AWGs; and a further AWG which acts as a multiplexer to multiplex the signals from the M×P respective output waveguides to produce a common output of the laser; wherein each of the M output waveguides of each of the fine tuning AWGs comprises a tunable DBR grating. 
     Optionally, the discrete wavelength tunable laser of claim  1 , further comprises additional P SOAs and additional P AWGs such that the tunable laser includes: a plurality of SOAs and a plurality 1×M AWGs, the output of each SOA providing an input to a 1×M AWG; a plurality M of output waveguides optically coupled to the respective M outputs of each 1×M AWG, wherein each of the M outputs of each 1×M AWG includes a tunable DBR grating. Optionally, each 1×M AWG may have a channel spacing of Δf and an FSR of MΔf. 
     It is envisaged that the AWG could be replaced by any other passive optical component configured to produce a comb-like transmittance spectrum. For example, suitable alternatives could include a ring resonator, a waveguide Fabry-Perot filter or a Multimode Interference (MMI) device configured to act as a Fabry-Perot filter. 
     It is also envisaged that the tunable DBRs (which may take the form of DS-DBRs) could be adapted to compensate for the spectral profile of the gain medium. The gain element will have a large spectral range over which the power of light generated will depend upon the wavelength. Usually there will be less power at the two extremes of the spectral range, creating a “drop off” in power at the edges of the range. The DBRs could be adapted to compensate for such drops in gain. For example, the reflectivity of the DBR could be reduced at parts of the DBR which correspond to high-gain regions of the spectral profile of the gain medium. Alternatively, or in addition, the reflectivity of DBRs could be increased at sections configured to reflect wavelengths which correspond to spectral regions of low-gain from the gain medium. 
     Optionally, for any one of the aspects above, the discrete wavelength tunable laser may further comprise one or more phase tuner(s) for fine tuning the wavelength of the laser. 
     This phase tuner would be separate from any phase tuners that may form part of the tunable DBR(s). The fine tuning phase tuner may be used to account for drift due to temperature or other environmental factors. 
     In any of the embodiments described herein, said mirror located at the back end of the semiconductor gain medium may have a reflectivity of at least 85% and even more, the mirror has a reflectivity of at least 90%. A standard high reflectivity coating may be applied to give the desired reflectivity over the desired bandwidth. 
     In some embodiments, the SOA (Semiconductor Optical Amplifier) is an RSOA (Reflective Semiconductor Optical Amplifier). In this way, the RSOA forms the back mirror of the optical cavity. 
     According to an embodiment of the present invention, there is provided a discrete wavelength tunable laser including: a first semiconductor optical amplifier (SOA); a first wavelength demultiplexer having an input and a plurality of outputs, the input being connected to the first SOA; one or more first tunable reflectors, each connected to a respective output of the first wavelength demultiplexer; and a back end mirror, the SOA being between the back end mirror and the first wavelength demultiplexer. 
     In some embodiments: a first waveguide connects an output of the SOA to the input of the first wavelength demultiplexer; the discrete wavelength tunable laser includes a plurality of waveguides, each of which is optically coupled to a respective output of the first wavelength demultiplexer; and each of the waveguides includes a respective tunable reflector of the one or more first tunable reflectors. 
     In some embodiments, the first wavelength demultiplexer is a first echelle grating. 
     In some embodiments, the first wavelength demultiplexer is a first arrayed waveguide grating (AWG). 
     In some embodiments, the discrete wavelength tunable laser further includes a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein: the first wavelength demultiplexer is a first echelle grating, the first echelle grating being a 1×N echelle grating configured to split the spectral output of the SOA into N respective output waveguides, N being an integer greater than 1; and the multiplexer is a second echelle grating. 
     In some embodiments, the discrete wavelength tunable laser further includes a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein: the first wavelength demultiplexer is a first AWG, the first AWG being a 1×N AWG configured to split the spectral output of the SOA into N respective output waveguides, N being an integer greater than 1; and the multiplexer is a second AWG. 
     In some embodiments, the discrete wavelength tunable laser further includes a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein: the first wavelength demultiplexer is a first echelle grating, the first echelle grating being a 1×M echelle grating configured to split the spectral output of the SOA into M respective output waveguides, M being an integer greater than 1; and the multiplexer is a second echelle grating, and each waveguide includes a respective tunable reflector, of the one or more first tunable reflectors, having P reflecting wavelength states, P being an integer greater than 1. 
     In some embodiments, the 1×M echelle grating is a cyclic echelle grating wavelength router. 
     In some embodiments, the 1×M echelle grating has a channel spacing of Δf and an FSR of MΔf. 
     10. The discrete wavelength tunable laser of claim  1 , further including a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein: the first wavelength demultiplexer is a first AWG, the first AWG being a 1×M AWG configured to split the spectral output of the SOA into M respective output waveguides, M being an integer greater than 1; and the multiplexer is a second AWG, and each waveguide includes a respective tunable reflector, of the one or more first tunable reflectors, having P reflecting wavelength states, P being an integer greater than 1. 
     In some embodiments, the discrete wavelength tunable laser further includes a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein: the first wavelength demultiplexer is a non-cyclic 1×P echelle grating configured to split the spectral output of the SOA into P respective output waveguides, P being an integer greater than 1; and each waveguide includes a respective tunable reflector, of the one or more first tunable reflectors, having M reflecting wavelength states, M being an integer greater than 1. 
     In some embodiments, the discrete wavelength tunable laser further includes a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein: the first wavelength demultiplexer is a non-cyclic 1×P AWG configured to split the spectral output of the SOA into P respective output waveguides, P being an integer greater than 1; and each waveguide includes a respective tunable reflector, of the one or more first tunable reflectors, having M reflecting wavelength states, M being an integer greater than 1. 
     In some embodiments, the discrete wavelength tunable laser further includes a multiplexer having a plurality of inputs and an output, each of the inputs of the multiplexer being connected to a respective tunable reflector of the one or more first tunable reflectors, wherein the first wavelength demultiplexer includes a plurality of echelle gratings including: a 1×P echelle grating configured to perform coarse tuning, the 1×P echelle grating having an input connected to the SOA, and P outputs; and a plurality of 1×M echelle gratings configured to perform fine tuning, each of the 1×M echelle gratings having: an input connected to a respective output of the P outputs of the 1×P echelle grating, and M outputs, each connected to respective tunable reflector of the one or more first tunable reflectors. 
     In some embodiments, the SOA is a reflective semiconductor optical amplifier (RSOA) including the back end mirror. 
     In some embodiments, one of the one or more first tunable reflectors includes a plurality of digital supermode-distributed Bragg reflectors (DS-DBRs). 
     In some embodiments, one of the one or more first tunable reflectors includes a tunable distributed Bragg reflector. 
     In some embodiments, the discrete wavelength tunable laser further includes: a second SOA; a second wavelength demultiplexer having an input and a plurality of outputs, the input being connected to the second SOA; and one or more second tunable reflectors, each connected to a respective output of the second wavelength demultiplexer. 
     In some embodiments, the first SOA and the second SOA are located on a single chip. 
     In some embodiments, the first SOA is located on a first chip and the second SOA is located on a second chip, different from the first chip. 
     In some embodiments, each of the first SOA and the second SOA is a reflective semiconductor optical amplifier (RSOA). 
     Further optional features are set out below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
         FIG. 1 a    shows a schematic diagram of a discrete wavelength tunable laser; 
         FIG. 1 b    shows a schematic diagram of spectral profiles of the AWG and DBR gratings of the embodiment shown in  FIG. 1   a;    
         FIG. 2 a    shows a schematic diagram of an alternative discrete wavelength tunable laser; 
         FIG. 2 b    shows a schematic diagram of an alternative discrete wavelength tunable laser; 
         FIG. 2 c    shows a schematic diagram of spectral profiles of the AWGs and DS-DBRs of the embodiment shown in  FIG. 2   b;    
         FIG. 3  shows a schematic diagram of an alternative discrete wavelength tunable laser; 
         FIG. 4  shows a schematic diagram of an alternative discrete wavelength tunable laser; 
         FIG. 5 a    shows a tunable reflector, according to some embodiments of the present invention; 
         FIG. 5 b    shows a tunable reflector, according to some embodiments of the present invention; 
         FIG. 5 c    shows a tunable reflector, according to some embodiments of the present invention; 
         FIG. 5 d    shows a tunable reflector, according to some embodiments of the present invention; and 
         FIG. 5 e    shows a tunable reflector, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment of a discrete wavelength tunable laser  100  is described with reference to  FIGS. 1 a    and  1   b.  The discrete wavelength tunable laser  100  includes a semiconductor optical amplifier (SOA) reflective semiconductor optical amplifier (RSOA)  101  which generates light over a given gain bandwidth. The back surface of the RSOA  102  includes a high reflectivity mirror at  102  forming the back end mirror of the optical cavity of the tunable laser. 
     The tunable laser includes an arrayed waveguide grating (AWG)  105  in the form of a 1×N AWG which has a single input optically coupled to the output of the RSOA via a waveguide  104 . The AWG  105  operates as a wavelength demultiplexer. A phase tuner  103  is located at the waveguide  104  for fine tuning of the wavelength. 
     The AWG has N outputs, each of which transmits or passes a respective fixed spectral passband, each of which lies within the gain bandwidth of the RSOA. 
     A plurality N of waveguides are each optically coupled to a respective one of the N outputs of the 1×N AWG. Each output waveguide therefore corresponds to a particular wavelength channel of the AWG. 
     Each of the N output waveguides includes a tunable reflector, such as a tunable distributed Bragg reflector (DBR) grating  106  and each DBR is configured to be able to select the spectral passband of that waveguide (cause a back-reflection within the spectral passband of that waveguide) upon application of a bias current. As used herein, a “tunable reflector” is an optical element with a wavelength-dependent reflectance that is adjustable, e.g., by adjusting one or more bias signals (e.g., one or more control voltages or currents) applied to the tunable reflector. Each tunable DBR may have a reflectance peak (or “reflection peak”) centered on a wavelength that is adjustable by adjusting a bias applied to the tunable DBR. 
     The reflection peak of each tunable DBR  106  is slightly detuned from the wavelength channel for the waveguide at which it is located when no bias (or “bias signal”) is applied. When a bias is applied to the DRB grating to select that channel, the resulting phase change means that the reflection peak of the grating becomes tuned at or near to the center wavelength of that channel, and the grating becomes the front end mirror of an optical comprising that waveguide, creating a laser. The lasing channel (i.e., one of the possible lasing modes of the laser) of the discrete wavelength tunable laser is therefore chosen by the overlap of the reflection band of a given DBR with a fixed spectral pass-band of the AWG. 
     Each of tunable DBRs includes a pn junction and electrical connections including a drive circuit and drive pins which provide an electrical contact between the drive circuit and the pn junction. By applying a bias from the drive circuit, the DBR is configured such that it can be switched to its “on” wavelength by application of a single set voltage. In the “off” state the wavelength of the gratings  106  corresponds with a high-loss off-band wavelength of the AWG  105 . In the “on” state the reflection wavelength of the DBR corresponds to one of the spectral pass-bands of the AWG. The resulting overlap enables the optical cavity to lase at the selected wavelength. Only one of the gratings  106  will be on at a given time. When a DBR  106  is “selected”, it forms the front end mirror of the optical cavity. 
     The activation of a required wavelength channel (i.e. the lasing wavelength of the laser) can be seen in more detail in  FIG. 1 b    in which the required channel “I” is activated because the transmission spectrum of the ith one of the N passbands of the AWG overlaps spectrally with the reflection spectrum of the selected one of the N DBR gratings. 
     The output of the laser light is directed to a single output waveguide by connecting the channelized output waveguides to a multiplexer. In  FIGS. 1 a  and 1 b    this multiplexer takes the form of an N×1 AWG  107  although other types of multiplexers (e.g., an echelle grating) may be used. In some embodiments, the DBRs have high reflectivity, the multiplexer is absent, the mirror on the RSOA is partially transmitting, and the output is from (the left side of) the RSOA. 
     The phase tuner  103  provides fine tuning and therefore seeks to provide a mechanism to counter wavelength drift over the lifetime of the tunable laser. In the example shown in  FIG. 1 , N may be 48, the gratings  106  may have reflectivity of 10%, and the channel spacing of the AWG may be 100 GHz. An advantage of this embodiment is that one commercially available RSOA powers all channels, for example all 48 channels. In general the tolerance to aging of this design is also advantageously high. 
     The AWG of this embodiment and the AWGs of embodiments described below may be fabricated integrally with the other features (waveguides) or may be fabricated as a separate chip and aligned during assembly with the waveguides on the silicon chip. 
     The AWG  105  may be an integrated part of the optical chip ( 100 ) or may be a separate device. The optical chip  100  may optionally be fabricated in silicon such as silicon-on-insulator. 
     Two further embodiments of discrete wavelength tunable lasers  300   a  and  300   b  are described below with reference to  FIGS. 2 a    and  2   b.    
     Each discrete wavelength tunable laser  300   a,    300   b  comprises an RSOA  301  with a highly reflective back facet  302 . 
     In the embodiment  300   a  shown in  FIG. 2 a   , the RSOA is optically coupled to a 1×M AWG  305  and the M output waveguides of this AWG include DS-DBR gratings  306 ,  316 ,  326  located therewith. In  FIG. 2 a   , the AWG  305  is a cyclic AWG with at least P cycles, that has (i) a channel spacing of Δf, where Δf is the tunable laser grid channel spacing, and (ii) an FSR of M*Δf. The cyclic AWG  305  operates as a wavelength demultiplexer. The DS-DBR gratings  306 ,  316 ,  326  on each of the waveguides  1  to M are fabricated to have P reflecting wavelength states, the first waveguide having wavelength values 1, M+1, 2M+1, etc up to (P−1)M+1, the second waveguide having values 2, M+2, 2M+2, etc. up to (P−1)M+2, and the last waveguide having values M to PM (since (P−1)M+M=MP) where P is N/M, and N is the number of total wavelength states of the tunable laser. The respective set of DS-DBR gratings on each of the M waveguides is a tunable reflector. The multiplexer  307  may also be a cyclic AWG (e.g., an M×1 cyclic AWG with at least P cycles). 
     For example, if P=7, there would be 7 grating wavelengths available per waveguide, and 7 sections to each DS-DBR grating. In other words, there would be 7 grating wavelengths available to be selected on for each spectral passband of the AWG. If M is 7, then there are 49 total modes corresponding to 49 wavelength channels available for the tunable laser. When a wavelength is selected on a DS-DBR, the DS-DBR becomes reflective at that wavelength so that the optical cavity of the laser is formed between the reflector  302  of the RSOA and the selected DS-DBR. 
     The embodiment shown in  FIG. 2 b    differs from that of  2   a  in that the 1×M AWG  305  of  FIG. 2 a    is replaced by a non-cyclic 1×P AWG  305   b.  The non-cyclic 1×P AWG  305   b  operates as a wavelength demultiplexer. The non-cyclic 1×P AWG  305   b  may have P transmission bands, each corresponding to a different one of its P outputs. Each of the outputs may be connected to a respective waveguide of the P waveguides, the waveguide including M DS-DBRs  306   b,    316   b,    326   b  which together form a respective tunable reflector on the waveguide. The multiplexer  307   b  may also be a non-cyclic AWG (e.g., a P×1 non-cyclic AWG with at least P cycles). 
     A non-cyclic AWG can be advantageous in that the losses for the “worst case channel” of the AWG can be made to be lower. The transmission losses through the channels at the edges of an AWG&#39;s FSR are typically higher, and in a cyclic AWG the channels at the edges of the FSR are used. With a non-cyclic AWG, the FSR can be designed to be significantly larger than the bandwidth of the channels that are used, so that the channels that are used are in the center of the FSR and therefore have a lower loss. 
     On the other hand, the use of cyclic AWGs can be advantageous over non-cyclic AWGs because when using a non-cyclic AWG for this purpose the individual channel transmission bands may each have pass band width of M*Δf, and the P pass bands may pass all P*M channels, therefore the passbands may have transmission spectra that are close to square-shaped (thereby leading to a constant loss across all wavelengths being passed, and high isolation of all other wavelengths). For example, AWG channel  1  may pass all sub-channels  1  to M with little loss variation, and reject all other sub-channels, and AWG channel  2  may pass all sub-channels M+1 to 2M with little loss variation, and reject all other sub-channels. However such difficulties can be mitigated if the wavelength grid used by the system is allowed to have gaps between each group of M wavelength combs. 
     In the embodiment of  FIG. 2 b   , the AWG  305   b  is a P-channel AWG with a channel spacing of MΔf, and a 3-dB channel transmission spectrum bandwidth sufficient to pass M channels of Δf channel spacing. The DS-DBR gratings on waveguides  1  to P are fabricated to have M reflecting wavelength states, the first waveguide having wavelength values 1, 2, . . . M, the second having values M+1, M+2, . . . M+P, the third waveguide having wavelength values 2M+1, 2M+2, . . . 2M+P, and the last waveguide having wavelength values (P−1)M+1, (P−1)M+2, . . . (P−1)M+M, which is equal to PM. 
       FIG. 2 c    shows example transmission spectra of the P AWG channels of the AWG outputs shown in  FIG. 2 b   , each output channel of the AWG having a spectral range “W” which lies within the bandwidth of the SOA.  FIG. 2 c    also shows example reflection spectra of the M-section DS-DBR gratings on each of the P waveguides. 
     The selection of the wavelength channel M−2 is shown. This occurs when section M−2 of a DS-DBR is selected on the first waveguide by a bias voltage applied to electrode M−2 on the DS-DBR grating on the waveguide of AWG channel  1 . In this way, the laser mode M−2 is selected out of M×P discrete modes available for this discrete-mode tunable laser. In some embodiments (not shown), rather than selecting the desired lasing channel by applying a bias directly to the section of the DBR corresponding to that channel, the section may be “selected” by applying a bias of the opposite polarity to a section immediately adjacent to the desired section. 
     An alternative discrete wavelength tunable laser  400  is described below with reference to  FIG. 3 . The embodiment shown in  FIG. 3  differs from that shown in  FIGS. 1 a  and 1 b    in that the wavelength demultiplexer is made up of two separate AWG stages; a first coarse AWG stage  405  and a second fine tuning AWG stage  409 . The overall transmission function of the combination of AWGs  405  and  409  is the same or similar to the transmission function of AWG  105  in  FIG. 1 . 
     The coarse tuning AWG  405  takes the form of a 1×P AWG, having one input which is optically coupled to the RSOA on its input side and optically connected to a plurality P of output waveguides on its output side. The 1×P AWG itself passes a range of spectral passbands across each of its P respective outputs. 
     The fine tuning AWGs  409  each take the form of a 1×M AWG. The input of each of the 1×M AWG is optically coupled to one of the P respective outputs of the 1×P AWG and the M outputs are each optically coupled to an output waveguide. Each of the M output waveguides includes a tunable DBR grating. 
     Unlike the embodiments of  FIGS. 2 a  and 2 b   , each selectable wavelength channel of the AWG has its own single-section grating. The single-section DBRs are much simpler to manufacture than the DS-DBR gratings of  FIGS. 2 a  and 2 b   . The size of the 1×M and 1×P AWGs combined can be made to be smaller than a single 1×N grating. Therefore the size of the device of this embodiment can be less than that in  FIG. 1 , but will still be larger than those in  FIGS. 2 a    and  2   b.    
     In an alternative embodiment of  FIG. 3 , the location of the coarse and fine AWGs may be swapped so that AWG  405  is a cyclic AWG and gives a fine wavelength selection, in the same way as the 1×M cyclic AWG in  FIG. 2 a   , and AWGs  409  give coarse wavelength selection. 
     The discrete wavelength tunable laser  400  includes a multiplexer  407 , e.g., a further N×1 AWG  407 , which acts as a multiplexer to multiplex the signals from the M×P respective output waveguides to produce a common output of the laser. 
       FIG. 4  shows a schematic diagram of an embodiment containing multiple RSOAs coupled to multiple respective AWGs. 
     For the same discrete wavelength states for the tunable laser as in previous embodiments, the lasing cavities are divided amongst a plurality P of RSOAs, each RSOA having the same gain bandwidth in the case where the RSOAs are an array of RSOA waveguides on a single chip coming from the same wafer, or, alternatively, having different gain bandwidths optimized for its corresponding downstream AWG, in the case where the ROSA are different chips. In the embodiment shown, an RSOA  201  generates optical power for M lasing wavelengths using a downstream 1×M AWG. Each 1×M AWG  205  is designed to pass a comb with a channel spacing of M*Δf, the first AWG passing wavelength values 1, 2, . . . M, the second passing values M+1, M+2, . . . 2M, the third passing wavelength values 2M+1, 2M+2, . . . 3M, etc., and the last waveguide having wavelength values (P−1)M+1, (P−1)M+2, . . . PM. The 1×M AWGs are thus designed the same as the AWGs  409  in  FIG. 3 . Each of the 1×M AWGs  205  operates as a wavelength demultiplexer. 
     As in the previous embodiments, the M outputs from the lasers from all of the AWGs  205  are combined using an N×1 multiplexer  207 , which acts to multiplex the signals from the M×P respective output waveguides to produce a common output of the laser. 
     Compared with the embodiment in  FIG. 3 , the first 1×P AWG is replaced with RSOAs, thus the optical path length of the laser cavities is shorter and so tuning speed is faster and laser mode spacing larger, but this is achieved at the cost of a plurality of RSOAs. Additionally, if the laser is to be power efficient, only the RSOA that provides gain for the laser mode being selected should be powered on during the time that mode is selected, and all other RSOAs should be powered off. This adds additional complexity to the tuning electronics. For a 49-wavelength output laser, P could be 7 and the number of RSOAs would be 7. In this embodiment the whole device could be integrated on one chip or it could be constructed from several chips. The chip and chips could be SOI. 
     In some embodiments each tunable reflector includes a ring resonator  535 , as shown for example in  FIG. 5 a   . The tunable reflector of  FIG. 5 a    has two external ports, both being waveguide ports: a first port  505  (which may for example be connected to the wavelength demultiplexer, in one of the embodiments shown in  FIGS. 1, 2   a ,  2   b ,  3 , and  4 ), and a second port  510  (which may be connected to a multiplexer, in one of the embodiments shown in  FIGS. 1, 2   a ,  2   b ,  3 , and  4 ). Both the first port  505  and the second port  510  may be connected to a first 2×2 coupler  515  to which may also be connected a first internal waveguide  520  and a second internal waveguide  525 , each of which may be terminated with a respective optical absorber  530 . A ring resonator  535  may be coupled to each of the first internal waveguide  520  and a second internal waveguide  525 , e.g., by respective waveguide directional couplers  540  as illustrated, or, e.g., by multimode interference (MMI) couplers (as discussed in further detail below). 
     In operation, when the ring resonator is not resonant, light that is received at either the first port  505  or the second port  510  is almost entirely transmitted to a respective optical absorber  530  and absorbed. When light is received at the first port  505  at a wavelength at which the ring resonator is resonant, the light is in part transmitted (through the first 2×2 coupler  515 ) to the first internal waveguide  520 , coupled to the second internal waveguide  525  through the ring resonator  535 , and, at the first 2×2 coupler  515 , (i) coupled back into the first port  505  (contributing to the reflectance of the tunable reflector) and (ii) coupled into the second port  510  (contributing to the transmittance of the tunable reflector). The light received at the first port  505  is also in part transmitted (through the first 2×2 coupler  515 ) to the second internal waveguide  525 , coupled to the first internal waveguide  520  through the ring resonator  535 , and, at the first 2×2 coupler  515 , (i) coupled back into the first port  505  (also contributing to the reflectance of the tunable reflector) and (ii) coupled into the second port  510  (also contributing to the transmittance of the tunable reflector). As such, the reflectance of the tunable reflector is enhanced at the resonant wavelength. The resonant reflectance of the tunable reflector may depend on the coupling ratio of the first 2×2 coupler  515 . The ring resonator may be tunable, and the resonant wavelength may be tuned, for example, by adjusting one or more phase shifters in the waveguide of the ring resonator, or with an integrated heater, as discussed below. 
     In other embodiments, a tunable reflector is constructed as illustrated in  FIG. 5 b   . In operation, when the ring resonator is not resonant, light that is received at the first port  505  is nearly entirely transmitted to the second port  510 . When light is received at the first port  505  at a wavelength at which the ring resonator is resonant, the light is in part transmitted (through the first 2×2 coupler  515 ) to an internal waveguide  545 , reflected, at a first end of the internal waveguide  545  by a mirror  550 , and coupled back to the first port  505 , through the ring resonator  535 . As in the case of the embodiment of  FIG. 5 a   , light may be coupled into and out of the ring resonator  535  by waveguide directional couplers  540  as illustrated, or, e.g., by MMI couplers. A second end of the internal waveguide  545  (opposite the first end) may be terminated by an optical absorber  530  as shown. In some embodiments, as mentioned above, an MMI coupler  555  is used in place of one or both of the waveguide directional couplers  540  of the embodiments of  FIGS. 5 a  and 5 b    (as shown in  FIGS. 5 c  and 5 d   ), In some embodiments the ring resonators  535  are constructed, or coupled to waveguides, as described in the &#39;078 Patent. In some embodiments, two (or more) coupled ring resonators may be used in place of a single ring resonator  535  (as described in the &#39;078 Patent);  FIG. 5 e    shows an embodiment that may result from making this substitution (as well as substituting MMI couplers  555  for the waveguide directional couplers  540 ) in the embodiment of  FIG. 5 b   . In any embodiment using a ring resonator  535 , the ring resonator  535  may be made tunable using phase shifters (as discussed, for example, in the &#39;078 Patent) in the waveguide of the ring resonator, or with an integrated heater. In some embodiments, one or more of the MMI couplers  555  are tunable MMI couplers as discussed in the &#39;078 Patent, or Mach-Zehnder couplers (or tunable Mach-Zehnder couplers). 
     While embodiments of the invention have been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. 
     For example, in some embodiments described herein, the AWG is used to give a wavelength filter function (i.e. as a wavelength demultiplexer). However, AWGs with coarse filter functions can be difficult to design and manufacture. As an alternative, Mach-Zehnder (MZ) interferometers such as Mach-Zehnder waveguide interferometers may be used. When constructed in a cascade, with differing phase changes in the arms of each MZ in the cascade, such MZs can be made into a useful coarse filter with square-like pass-band shapers. 
     In each embodiment disclosed herein in which one or more AWGs are employed, one or more echelle gratings may be used in place of one or more of the AWGs. For example, a 1×M cyclic echelle grating may be used in place of a 1×M cyclic AWG, a 1×P non-cyclic echelle grating may be used in place of a 1×P non-cyclic AWG, and echelle gratings may be combined to form coarse and fine echelle grating stages of a wavelength demultiplexer, in a manner analogous to the manner, described above, in which AWGs may be combined to form coarse and fine AWG stages of a wavelength demultiplexer. 
     In some embodiments, a wavelength multiplexer (e.g., the wavelength multiplexer  107  of  FIG. 1 a   ) may include coarse and fine AWG stages or coarse and fine echelle grating stages, in a manner analogous to the manner in which a wavelength demultiplexer may include coarse and fine (AWG or echelle grating) stages. 
     Whilst the embodiments described herein all comprise a reflective semiconductor optical amplifier (RSOA), it is envisaged that they could all be carried out using a standard SOA (without a mirrored surface). In this case, the SOA would be a double sided structure and both sides would need to be coupled to the SOI host chip. A separate rear mirror (not part of the SOA) would need to be fabricated to be optically coupled to the back side of the SOA. 
     All references referred to above are hereby incorporated by reference.