Optical system and method for locking a wavelength of a tunable laser

1. An optical system, comprising:

a tunable laser configured to produce laser light at a selectable wavelength, wherein a first fraction of the laser light forms a laser light output;

a spectral filter configured to spectrally filter a second fraction of the laser light, separate from the first fraction of the laser light, to form filtered laser light, the spectral filter having a temperature-insensitive spectral profile, the spectral profile having a peak wavelength and increasing attenuation at wavelengths away from the peak wavelength;

a detector configured to detect at least a fraction of the filtered laser light; and

circuitry coupled to the detector and the tunable laser, the circuitry being configured to:

adjust either the selectable wavelength, the peak wavelength or both so that the filtered laser light varies with a periodic oscillation;

sense a polarity of the periodic oscillation;

determine which side of the peak wavelength the selectable wavelength of the tunable laser is on based on the polarity; and

adjust the selectable wavelength of the tunable laser to match the peak wavelength based on the determination, and thereby lock a wavelength of the laser light output to match the peak wavelength.

2. The optical system of claim 1 , further comprising:

a first grating coupler configured to direct at least a fraction of the laser light to the spectral filter; and

a first waveguide configured to direct the laser light from the tunable laser to the first grating coupler.

3. The optical system of claim 2 , wherein the spectral filter is a Fabry-Perot filter configured to receive the laser light propagating in a first direction, and direct the filtered laser light to the first grating coupler along a second direction, opposite the first direction.

4. The optical system of claim 3 ,

further comprising a polarization rotator positioned in an optical path between the grating coupler and the Fabry-Perot filter, the polarization rotator being configured such that a polarization state of laser light entering a first side of the polarization rotator is orthogonal to a polarization state of filtered laser light exiting from the first side of the polarization rotator after the laser light passes twice through the polarization rotator in opposite directions;

wherein the first grating coupler is a polarization-splitting grating coupler configured to:

receive the laser light with a first polarization state from the first waveguide;

direct the laser light to the Fabry-Perot filter;

receive the filtered laser light with a second polarization state, orthogonal to the first polarization state, from the Fabry-Perot filter; and

direct the filtered laser light to the detector.

5. The optical system of claim 3 ,

wherein the first grating coupler is further configured to direct at least a fraction of the filtered laser light to the first waveguide; and

further comprising a tap coupled to the first waveguide and configured to direct at least a fraction of the filtered laser light to the detector.

6. The optical system of claim 2 ,

wherein the spectral filter is a reflective Fabry-Perot filter configured to receive the laser light propagating in a first direction, spectrally filter the laser light to form filtered laser light, and direct the filtered laser light to a second grating coupler away from the Fabry-Perot filter in a second direction, angled with respect to the first direction; and

further comprising a second grating coupler positioned to receive the filtered laser light from the Fabry-Perot filter and direct at least a portion of the filtered laser light to the detector.

7. The optical system of claim 1 , wherein the tunable laser is a diode laser having an electrically-adjustable wavelength.

8. The optical system of claim 1 , wherein the spectral filter is a temperature-insensitive Fabry-Perot filter.

9. The optical system of claim 8 , wherein the Fabry-Perot filter is formed from first and second reflectors defining a cavity between the first and second reflectors, the cavity having an optical path length that remains constant over a specified range of temperatures.

10. A method for locking a wavelength of a tunable laser, the method comprising:

producing laser light at a selectable wavelength with a tunable laser;

directing a first fraction of the laser light to form a laser light output;

spectrally filtering a second fraction of the laser light, separate from the first fraction of the laser light, with a spectral filter to form filtered laser light, the spectral filter having a temperature-insensitive spectral profile, the spectral profile having a peak wavelength and increasing attenuation at wavelengths away from the peak wavelength;

detecting at least a fraction of the filtered laser light with a detector; and

adjusting either the selectable wavelength, the peak wavelength or both so that the filtered laser light varies with a periodic oscillation;

sensing a polarity of the periodic oscillation;

determining which side of the peak wavelength the selectable wavelength of the tunable laser is on based on the polarity; and

adjusting the selectable wavelength of the tunable laser to match the peak wavelength based on the determination, thereby locking a wavelength of the laser light output to match the peak wavelength.

11. The method of claim 10 , further comprising:

rotating a polarization state of the filtered laser light; and

separating the filtered laser light from the laser light with a polarization-splitting grating coupler.

12. The method of claim 10 , wherein spectrally filtering the second fraction of the laser light comprises:

propagating the laser light through a temperature-insensitive optical path length in a first direction through a Fabry-Perot cavity, the temperature-insensitive optical path length defining the peak wavelength;

reflecting the laser light from a longitudinal end of the Fabry-Perot cavity to form the filtered laser light; and

propagating the filtered laser light through the temperature-insensitive optical path length in a second direction, opposite the first direction, through the Fabry-Perot cavity.

13. The method of claim 10 , wherein the tunable laser is a diode laser having an electrically-adjustable wavelength.

14. The method of claim 10 , wherein the spectral filter is a temperature-insensitive Fabry-Perot filter.

15. The method of claim 14 , wherein the Fabry-Perot filter is formed from first and second reflectors defining a cavity between the first and second reflectors, the cavity having an optical path length that remains constant over a specified range of temperatures.

16. An optical system, comprising:

a tunable laser configured to produce laser light at a selectable wavelength, wherein a first fraction of the laser light forms a laser light output;

a spectral filter configured to spectrally filter a second fraction of the laser light, separate from the first fraction of the laser light, to form filtered laser light, the spectral filter having a temperature-insensitive spectral profile, the spectral profile having a null wavelength and decreasing attenuation at wavelengths away from the null wavelength;

a detector configured to detect at least a fraction of the filtered laser light; and

circuitry coupled to the detector and the tunable laser, the circuitry being configured to:

adjust either the selectable wavelength, the null wavelength or both so that the filtered laser light varies with a periodic oscillation;

sense a polarity of the periodic oscillation;

determine which side of the null wavelength the selectable wavelength of the tunable laser is on based on the polarity; and

adjust the selectable wavelength of the tunable laser to match the null wavelength based on the determination, and thereby lock a wavelength of the laser light output to match the null wavelength.

17. The optical system of claim 16 , wherein the tunable laser is a diode laser having an electrically-adjustable wavelength.

18. The optical system of claim 16 , wherein the spectral filter is a temperature-insensitive Fabry-Perot filter.

19. The optical system of claim 18 , wherein the Fabry-Perot filter is formed from first and second reflectors defining a cavity between the first and second reflectors, the cavity having an optical path length that remains constant over a specified range of temperatures.

20. A method for locking a wavelength of a tunable laser, the method comprising:

producing laser light at a selectable wavelength with a tunable laser;

directing a first fraction of the laser light to form a laser light output;

spectrally filtering a second fraction of the laser light, separate from the first fraction of the laser light, with a spectral filter to form filtered laser light, the spectral filter having a temperature-insensitive spectral profile, the spectral profile having a null wavelength and decreasing attenuation at wavelengths away from the null wavelength;

detecting at least a fraction of the filtered laser light with a detector; and

adjusting either the selectable wavelength, the null wavelength or both so that the filtered laser light varies with a periodic oscillation;

sensing a polarity of the periodic oscillation;

determining which side of the null wavelength the selectable wavelength of the tunable laser is on based on the polarity; and

adjusting the selectable wavelength of the tunable laser to match the null wavelength based on the determination, thereby locking a wavelength of the laser light output to match the null wavelength.

21. The method of claim 20 , wherein the tunable laser is a diode laser having an electrically-adjustable wavelength.

22. The method of claim 20 , wherein the spectral filter is a temperature-insensitive Fabry-Perot filter.

23. The method of claim 22 , wherein the Fabry-Perot filter is formed from first and second reflectors defining a cavity between the first and second reflectors, the cavity having an optical path length that remains constant over a specified range of temperatures.