Discrete wavelength tunable laser

1. 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 Demux configured to receive the output of the SOA and to produce a plurality of different 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; and

a back end mirror,

wherein the one or more tunable DBRs form the front end mirror of the optical cavity;

wherein the SOA is between the back end mirror and the Demux; 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.

2. The discrete wavelength tunable laser of claim 1 , wherein:

a single waveguide optically couples the output of the SOA to the input of the Demux;

the discrete wavelength tunable laser comprises further waveguides, each of which is optically coupled to a respective output of the Demux; and

wherein each of the further waveguides includes a respective one of the one or more tunable DBRs.

3. The discrete wavelength tunable laser of claim 1 , wherein the Demux is an Arrayed Waveguide Grating (AWG).

4. The discrete wavelength tunable laser of claim 3 , wherein:

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

wherein the discrete wavelength tunable laser includes 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.

5. The discrete wavelength tunable laser of claim 3 , wherein:

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;

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.

6. The discrete wavelength tunable laser of claim 5 , wherein the first 1×M AWG is a cyclic AWG wavelength router.

7. The discrete wavelength tunable laser of claim 5 wherein the first 1×M AWG has a channel spacing of Δf and an FSR of MΔf.

8. The discrete wavelength tunable laser of claim 3 , wherein:

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 tunable DBRs, the one or more tunable DBRs on each waveguide having a total number of M reflecting wavelength states.

9. The discrete wavelength tunable laser of claim 3 , wherein the Arrayed Waveguide Grating (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 is a plurality of AWGs including:

a coarse tuning AWG in the form of a 1×P AWG, having one input which is optically coupled to the SOA and P outputs, the 1×P AWG configured to output a range of spectral passbands to each of its P respective outputs;

a plurality of fine tuning AWGs, where each fine tuning AWG is a 1×M AWGs; 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 respective one of the one or more tunable DBRs.

10. The discrete wavelength tunable laser of claim 1 , wherein the SOA is a reflective semiconductor optical amplifier (RSOA) comprising the back end mirror.

11. The discrete wavelength tunable laser of claim 3 , further comprising additional SOAs and additional 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; and

a plurality M of output waveguides optically coupled to each 1×M AWG, each of the M output waveguides optically coupled to a respective one of the M outputs of that 1×M AWG,

wherein each of the M outputs of each 1×M AWG includes a respective one of the one or more tunable DBRs.

12. The discrete wavelength tunable laser of claim 1 , wherein each of the one or more tunable DBR(s) is a digital supermode-distributed Bragg reflector (DS-DBR).

13. The discrete wavelength tunable laser of claim 11 , wherein the plurality of SOAs are located on a single chip.

14. The discrete wavelength tunable laser of claim 11 , wherein each of the plurality of SOAs are located on a different chip.

15. The discrete wavelength tunable laser of claim 11 , wherein the plurality of SOAs are reflective semiconductor optical amplifiers (RSOAs).