A tuneable laser includes a gain section ( 50 ) bounded by two mirrors ( 51 and 52 ) at least one mirror being a chirp grating. At least one of the mirrors includes a plurality of selectable electrodes ( 59 to 65 ) to enable the grating to be selectively activated to produce a selective reflection at a predetermined wavelength.
1. A monolithic tuneable laser, comprising:
a gain section bounded at one end by a first mirror and at the other end by a second mirror,
wherein each of the mirrors is in the form of a chirp grating,
wherein at least one of the chirp gratings has a substantially continuous wavelength reflection spectrum and a plurality of selectable electrodes by which one portion of the chirp grating is tuned to reflect substantially the same wavelength as a second portion of the same chirp grating, to thereby selectively adjust the wavelength reflection spectrum of the chirp grating to produce a single distinct enhanced reflection peak at a predetermined wavelength, and
wherein one of the chirp gratings is a front grating and the other chirp grating is a rear grating, each of the front grating and the rear grating having a pitch characteristic, and the pitch characteristics of the front and rear gratings being such that the optical cavity length of the laser is substantially constant at different wavelengths.
2. The laser according to claim 1 , wherein the chirp gratings are located in a material having a refractive index variable in response to the passage of a current there through, and wherein the chirp grating is activated at the predetermined wavelength by the variation in a local region of the refractive index.
3. The laser according to claim 1 , wherein a wavelength position of the reflection is altered by varying a refractive index of at least that region of the grating and a portion of the grating between said region and the gain section.
4. The laser according to claim 1 , wherein one of the chirp gratings has a plurality of selectable electrodes and the other chirp grating has a single electrode.
5. The laser according to claim 1 , wherein the selectable electrodes are located on the rear grating.
6. The laser according to claim 5 , wherein the pitch characteristics of the front and rear gratings are substantially identical.
7. The laser according to claim 1 , wherein the chirp gratings are linear.
8. The laser according to claim 1 , wherein a reflectivity of one of the first and second mirrors is greater than a reflectivity of the other mirror.
9. The laser according to claim 8 , wherein the reflectivity of one of the first and second mirrors is of the order of 50% and the reflectivity of the other mirror is of the order of 30%.
10. The laser according to claim 1 , wherein the first and second mirrors are formed by electron beam writing of the grating patterns.
11. The laser according to claim 10 , wherein a mark space ratio of one of the first and second mirrors is substantially unity and a mark space ratio of the other mirror is different than unity.
12. The laser according to claim 1 , wherein pitch spacings of one of the first and second mirrors is lowest adjacent the gain section and the pitch spacings of the other mirror is highest adjacent the gain section.
13. The laser according to claim 1 , further comprising a phase change section disposed between the gain section and one of the first and second mirrors.
14. The laser according to claim 1 , wherein a composition of the first and second mirrors is formed of Group III-V semiconductor layers of different refractive index.
15. The laser according to claim 1 , wherein both of the first and second mirrors have a plurality of electrodes, so as permit both mirrors to be selectively activated to produce a selective reflection at the predetermined wavelength.
16. The tuneable laser accordingly claim 15 wherein at least one of the plurality of electrodes comprises a tuning electrode disposed within an optical cavity, wherein the tuning electrode is employed as a phase controller to reduce or eliminate mode hopping.
17. The tuneable laser according to claim 1 , wherein light power is coupled out from both ends of the laser.
18. The tuneable laser according to claim 17 , wherein both chirp gratings are substantially identical.
19. A method of operating a laser as claimed in claim 1 , comprising the steps of
selecting a wavelength by passing current through one of the plurality of electrodes to reduce a refractive index of a portion of the chirp grating affected by the electrode, and
actuating one or more electrodes of said plurality of electrodes capable of reducing the refractive index of the portion of the chirp grating effective at a shorter wavelength to prevent the formation of a distorted reflection peak.