Phase control by active thermal adjustments in an external cavity laser
View Patent ↗A wavelength tuneable external-cavity laser module comprises a gain medium in thermal contact with a thermally stabilized substrate; an end mirror, and a phase element for controlling the phase of the optical beam and being positioned within the external cavity between the gain medium and the end mirror, wherein said phase element comprises a material having a refractive index that varies in response to changes in temperature and has a transmissivity substantially independent of wavelength across said predetermined wavelength range. The thermally-controllable phase element is configured so as to induce a phase variation that compensates the drop in the output power due to ageing or to external temperature variation. A heating element is placed in thermal contact to the phase element. By thermally controlling an intra-cavity phase element it is possible to vary continuously the output power as a function of the injection current.
1. A wavelength tuneable external-cavity laser module being tuneable across a predetermined wavelength range including a plurality of channels, the laser module comprising:
a thermally stabilised substrate;
a gain medium for emitting an optical beam passing through the external cavity along an optical axis, said gain medium being placed in thermal contact with the thermally stabilised substrate;
an end mirror for receiving and reflecting the optical beam within the external cavity;
a phase element for controlling the phase of the optical beam and being positioned within the external cavity, wherein said phase element comprises a material having a refractive index that varies in response to changes in temperature and has an optical transmissivity substantially independent of wavelength across said predetermined wavelength range.
2. The laser module of claim 1 , wherein said thermally controllable phase element is in thermal contact with a heating element.
3. The laser module of claim 2 , wherein the heating element is a resistor element.
4. The laser module of claim 1 , wherein the end mirror is in thermal contact with the thermally stabilised substrate.
5. The laser module of claim 1 , wherein the phase element is in thermal contact with the thermally stabilised substrate.
6. The laser module of claim 1 , wherein the phase element is placed on said thermally stabilised substrate so as to provide a heat flow path to the substrate having a first thermal resistance comprised between 80 and 180 K/W.
7. The laser module of claim 6 , wherein said first thermal resistance is comprised between 100 and 160 K/W.
8. The laser module of claim 1 , wherein the phase element is placed on a support, said support being placed on said thermally stabilised substrate.
9. The laser module of claim 1 , wherein said phase element comprises a first surface for receiving the optical beam and a second surface, opposite to said first surface, and wherein said first surface or said second surface has a reflectivity not larger than 2.5% across said predetermined wavelength range.
10. The laser module of claim 9 , wherein the reflectivity is not larger than 1%.
11. The laser module of claim 9 , wherein both the first and the second surface have a reflectivity not larger than 2.5%.
12. The laser module of claim 5 , wherein the phase element is placed on the thermally stabilised substrate oriented at an angle not smaller than 4° with respect to the optical axis.
13. The laser module of claim 1 , wherein said phase element is a silicon plate.
14. The laser module of claim 13 , wherein the silicon plate has a thickness substantially along the optical axis comprised between 1.0 and 1.3 mm.
15. The laser module of claim 13 , wherein the silicon plate has first surface for receiving the optical beam and a second surface, opposite to said first surface, said surface being treated with an anti-reflection coating.
16. The laser module of claim 13 , wherein the silicon plate has a second thermal resistance comprised between 3 and 8 K/W.
17. The laser module of claim 1 , wherein the thermally stabilised substrate comprises a thermoelectric cooler comprising a surface and being configured to stabilise the temperature of said surface and the laser module further comprises a thermally conductive platform being placed on the thermoelectric cooler and being in thermal coupling with the thermally stabilised surface.
18. The laser module of claim 17 , wherein said end mirror is placed on said thermally conductive platform so as to provide a heat flow path to the platform having a third thermal resistance not larger than 2 K/W.
19. The laser module of claim 1 , wherein said laser assembly is configured to emit output radiation and said external cavity defines a plurality of cavity modes, said laser module further comprising
a grid generator being arranged in the external cavity to define a plurality of pass bands substantially aligned with corresponding channels of a selected wavelength grid, and
a tuneable element being arranged in the external cavity to tuneably select one of said pass bands so as to select a channel to which to tune the optical beam.
20. The laser module according to claim 1 , further comprising an electronic circuit card configured to carry out the function of varying the temperature on the phase element so as to regulate the phase of the external cavity.