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

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

• producing laser light at a selectable wavelength with a tunable laser;
• receiving at least a fraction of the laser light with a first polarization state into a polarization splitting grating coupler and sending the at least a fraction of the laser light with a first polarization state to a filter assembly;
• receiving the at least a fraction of the laser light with a first polarization state to the filter assembly;
• spectrally filtering the received at least a fraction of the laser light with the first polarization state by the filter assembly and producing spectrally filtered laser light with a second polarization state orthogonal to the first polarization state, the spectrally filtered laser light having a spectral profile determined by the filter assembly;
• receiving the filtered laser light with the second polarization state into the polarization splitting gating coupler and sending at least a portion of the filtered laser light with the second polarization state to a detector;
• detecting the at least a portion of the filtered laser light with the second polarization state with the detector;
• and
• tuning the tunable laser to maximize a signal from the detector, and thereby-adjusting the selectable wavelength of the tunable laser to match a peak wavelength thereby locking a wavelength of the laser light output to match the peak wavelength.

An optical system, comprising:

• a tunable laser configured to produce laser light at a selectable wavelength;
• a polarization splitting grating coupler that:
• receives at least a portion of the laser light with a first polarization state;
• directs laser light with the first polarization state to a filter assembly;
• receives filtered laser light with a second polarization state orthogonal to the first polarization state from the filter assembly;
• directs the filtered laser light with the second polarization state to a detector;
• the filter assembly connected to the polarization splitting grating coupler with a single mode optical fiber, the filter assembly comprising:
• a spectral filter having a temperature-insensitive spectral profile with a peak wavelength at which the spectral filter has either maximum power transmission or maximum reflection;
• and
• a polarization rotator 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 the first side of the polarization rotator after the light passes twice through the polarization rotator in opposite directions;
• control circuitry coupled to the detector and to the tunable laser configured to tune the tunable laser to maximize a signal from the detector, and thereby adjust the selectable wavelength of the tunable laser to match the peak wavelength, and thereby lock a wavelength of the laser light output to match the peak wavelength.