Optical system and method for locking a wavelength of a tunable laser
An optical system can lock a wavelength of a tunable laser to a specified wavelength of a temperature-insensitive spectral profile of a spectral filter. In some examples, the spectral filter, such as a Fabry-Perot filter, can have a temperature-insensitive peak wavelength and increasing attenuation at wavelengths away from the peak wavelength. The spectral filter can spectrally filter the laser light to form filtered laser light. A detector can detect at least a fraction of the filtered laser light. Circuitry coupled to the detector and the laser can tune the tunable laser to set a signal from the detector to a specified value corresponding to a specified wavelength in the spectral profile, and thereby adjust the selectable wavelength of the tunable laser to match the specified wavelength. In some examples, the optical system can include a polarization rotator, and can use polarization to separate incident light from return light.
1. An optical system, comprising:
a tunable laser configured to produce laser light at a selectable wavelength;
a first grating coupler that:
receives at least a portion of the laser light; and
directs laser light propagating in a first direction;
a spectral filter having a peak wavelength at which the spectral filter has either maximum power transmission or maximum reflection that:
receives the laser light propagating in the first direction at a non-normal incidence;
spectrally filters the laser light propagating in the first direction to form spectrally filter laser light; and
directs the spectrally filtered laser light away from the spectral filter in a second direction angled with respect to the first direction;
a second grating coupler that:
receives the spectrally filtered laser light; and
directs at least a portion of the spectrally filtered laser light to a detector; and
control circuitry coupled to the detector and to the tunable laser configured to tune the tunable laser to maximize a signal from the detector by adjusting the selectable wavelength of the tunable laser to match the peak wavelength based on an oscillation in the spectrally filtered laser light such that a wavelength of the laser light output from the optical system is matched the peak wavelength of the spectral filter.
2. The optical system of claim 1 , wherein the spectral filter is formed integrally with a photonic integrated circuit.
3. The optical system of claim 1 , wherein the oscillation is a periodic oscillation generated by the control circuitry.
4. The optical system of claim 1 , wherein the first grating coupler and the spectral filter are arranged in a geometry that eliminates back-reflections.
5. The optical system of claim 1 , wherein the first grating coupler and the spectral filter and the second grating coupler are arranged in a geometry that eliminates back-reflections.
6. The optical system of claim 1 wherein the first grating coupler and the second grating coupler are arranged in a geometry that isolates the tunable laser from return light.
7. The optical system of claim 1 , wherein the spectral filter is formed separate from a photonic integrated circuit containing the tunable laser.
8. A method for locking a wavelength of a tunable laser in an optical system, the method comprising:
producing laser light at a selectable wavelength with the tunable laser;
receiving at least a fraction of the laser light with a first grating coupler and sending the at least a fraction of the laser light with to a filter assembly, the filter assembly having a peak wavelength at which the filter assembly has either maximum power transmission or maximum reflection;
receiving the at least a fraction of the laser light at a first incident angle at the filter assembly;
spectrally filtering the received at least a fraction of the laser light by the filter assembly and producing spectrally filtered laser light, the spectrally filtered laser light having a spectral profile determined by the filter assembly;
sending the spectrally filtered laser light to a second grating coupler at a second incident angle such that the spectrally filtered laser light is angled with respect to the at least a fraction of the laser light
receiving the spectrally filtered laser light at the second grating coupler and sending at least a portion of the spectrally filtered laser light to a detector;
detecting the at least a portion of the spectrally filtered laser light with the detector; and
tuning the tunable laser to maximize a signal from the detector by adjusting the selectable wavelength of the tunable laser to match a peak wavelength based on an oscillation in the spectrally filtered laser light such that a wavelength of the laser light output from the optical system is matched the peak wavelength of the filter assembly.
9. The method of claim 8 , further comprising forming the filter assembly integrally with a photonic integrated circuit.
10. The method of claim 8 , wherein the optical system contains no polarization-sensitive elements to separate incident light from return light.
11. The method of claim 8 , wherein the first grating coupler and the filter assembly are arranged in a geometry that eliminates back-reflections.
12. The method of claim 8 , wherein the first grating coupler and the filter assembly and the second grating coupler are arranged in a geometry that eliminates back-reflections.
13. The method of claim 8 , wherein the first grating coupler and the second grating coupler are arranged in a geometry that isolates the tunable laser from return light.
14. The method of claim 8 , wherein the filter assembly is formed separate from a photonic integrated circuit containing the tunable laser.
15. An optical system, comprising:
a tunable laser configured to produce laser light at a selectable wavelength;
a first grating coupler optically coupled to the tunable laser;
a second grating coupler optically coupled to a detector;
a spectral filter optically coupled between the first grating coupler and second grating coupler in a manner that input light from the tunable laser is received from the first grating coupler in a first direction and spectrally filtered output light is sent to the second grating coupler in a second direction angled with respect to the first direction, the spectral filter having a peak wavelength at which the spectral filter has either maximum power transmission or maximum reflection; and
control circuitry coupled to the detector and to the tunable laser configured to tune the tunable laser to maximize a signal from the detector by adjusting the selectable wavelength of the tunable laser to match the peak wavelength based on an oscillation in the spectrally filtered output light such that a wavelength of the laser light output from the optical system is matched the peak wavelength of the spectral filter.
16. The optical system of claim 15 , wherein the spectral filter is formed integrally with a photonic integrated circuit.
17. The optical system of claim 15 , wherein the spectral filter is formed separate from a photonic integrated circuit containing the tunable laser.
18. The optical system of claim 15 , wherein the optical system contains no polarization-sensitive elements to separate incident light from return light.
19. The optical system of claim 15 , wherein the first grating coupler, the spectral filter, and the second grating coupler are arranged in a geometry that eliminates back-reflections.
20. The optical system of claim 15 , wherein the first grating coupler and the second grating coupler are arranged in a geometry that isolates the tunable laser from return light.