Tunable external cavity laser with adjustable cavity length and mode-hop suppression
View Patent ↗Tunable external cavity lasers are used in applications such as interferometry, FM spectroscopy, and optical communications equipment testing. Mode hop free high bandwidth frequency modulation operation is desired in a tunable external cavity laser. This application describes new and novel techniques for controlling the output wavelength of a tunable external cavity laser while suppressing mode hop.
1. A system comprising:
a gain medium which emits light;
a diffraction grating spaced apart from the gain medium; and
a retroreflector located to reflect light incident on the diffraction grating,
where a distance between the gain medium and the diffraction grating is adjustable along an axis parallel to a direction of the light emitted by the gain medium, by an actuator which changes the distance only along that axis.
2. The system of claim 1 , wherein the gain medium comprises a laser diode with an antireflective coating.
3. The system of claim 1 , wherein the actuator comprises a piezoelectric actuator.
4. The system of claim 1 , wherein the actuator comprises a voice coil actuator.
5. The system of claim 1 , wherein the actuator is coupled to the gain medium.
6. The system of claim 1 , wherein the actuator is coupled to the diffraction grating.
7. The system of claim 1 , further comprising a detector located to measure one or more wavelengths of the light emitted from the gain medium, where a signal from the detector is coupled to a closed loop feedback system to control the distance between the gain medium and the diffraction grating.
8. The system of claim 7 , wherein the detector measures phase of the emitted light.
9. The system of claim 1 , further comprising a detector located to measure directionality of the light emitted from the gain medium, where a signal from the detector is coupled to a closed loop feedback system to control the distance between the gain medium and the diffraction grating.
10. The system of claim 9 , wherein the detector comprises a quadrant cell photodetector.
11. The system of claim 9 , further comprising a pick off located between the gain medium and diffraction grating.
12. The system of claim 1 , further comprising an additional actuator coupled to the retroreflector, to rotate the retroreflector relative to the diffraction grating.
13. The system of claim 12 , wherein rotation of the retroreflector is centered about a pivot positioned such that a cavity length of the system changes as the retroreflector rotates.
14. The system of claim 12 , wherein rotation of the retroreflector is centered about a pivot positioned such that a cavity length of the system does not change as the retroreflector rotates.
15. The system of claim 12 , further comprising an encoder coupled to measure a position of the retroreflector actuator.
16. The system of claim 15 , wherein a signal from the encoder is coupled to a closed loop feedback system to control the position of the retroreflector actuator.
17. The system of claim 15 , wherein a signal from the encoder is calibrated with respect to the distance between the gain medium and the diffraction grating.
18. The system of claim 17 , wherein a signal from the encoder is used to control the distance between the gain medium and the diffraction grating.
19. The system of claim 12 , wherein the actuator comprises a voice coil actuator.
20. The system of claim 19 , wherein the voice coil actuator comprises a rotary voice coil actuator.
21. The system of claim 19 , wherein the voice coil actuator comprises a toroidal coil rotary voice coil actuator.
22. A method of controlling light output from a tunable external cavity laser comprising:
providing a gain medium emitting light onto a diffraction grating, and a retroreflector to reflect light from the diffraction grating;
rotating the retroreflector relative to the diffraction grating to select a wavelength of light to amplify in the gain medium; and
adjusting a distance between the gain medium and the diffraction grating to control a cavity length of the laser, by an actuator which changes the distance only along an axis parallel to a direction of the emitted light.
23. The method of claim 22 , wherein rotating the retroreflector is accomplished by an actuator.
24. The method of claim 23 , wherein the actuator comprises a voice coil actuator.
25. The method of claim 22 , wherein the actuator comprises a piezoelectric actuator.
26. The method of claim 22 , wherein the actuator comprises a voice coil actuator.
27. The method of claim 22 , wherein the actuator is coupled to the gain medium.
28. The method of claim 22 , wherein the actuator is coupled to the diffraction grating.
29. The method of claim 22 , wherein a closed loop feedback system controls rotation of the retroreflector.
30. The method of claim 22 , wherein a closed loop feedback system coupled to the actuator controls a cavity length of the laser.
31. A tunable external cavity laser comprising:
a gain medium, comprising a laser diode with an antireflective coating;
a diffraction grating spaced apart from the gain medium;
a piezoelectric cavity length actuator adjusting a distance between the gain medium and the diffraction grating along an axis parallel to a direction of light emitted by the gain medium, the cavity length actuator coupled to the gain medium;
a retroreflector located to reflect the light incident on the diffraction grating;
a voice coil actuator coupled to the retroreflector and rotating the retroreflector relative to the diffraction grating;
an encoder measuring a position of the voice coil actuator, where a signal from the encoder is coupled to a first closed loop feedback system to control the position of the retroreflector; and
a detector measuring directionality of light emitted from the gain medium, where a signal from the detector is coupled to a second closed loop feedback system to control the distance between the gain medium and the diffraction grating.
32. A system comprising:
a gain medium which emits light;
a diffraction grating spaced apart from the gain medium;
a retroreflector located to reflect light incident on the diffraction grating; and
an actuator coupled to adjust the distance between the gain medium and the diffraction grating;
wherein the actuator is coupled to the gain medium, where a distance between the gain medium and the diffraction grating is adjustable along an axis parallel to a direction of the light emitted by the gain medium by the actuator.
33. A method of controlling light output from a tunable external cavity laser comprising:
providing a gain medium emitting light onto a diffraction grating, and a retroreflector to reflect light from the diffraction grating;
rotating the retroreflector relative to the diffraction grating to select a wavelength of light to amplify in the gain medium; and
adjusting a distance between the gain medium and the diffraction grating to control a cavity length of the laser;
wherein adjusting the distance between the gain medium and the diffraction grating is accomplished by an actuator coupled to the gain medium.