High precision wavelength measurement and control of a tunable laser

A tunable laser control system for controlling a laser output of a tunable laser, the tunable laser arranged to be scanned over a range of frequencies, the tunable laser control system comprising:

• an interferometer having a plurality of interferometer outputs, wherein at least two interferometer outputs of the plurality of interferometer outputs have a phase difference not equal to 0° and not equal to 180°;
• a wavelength reference having a spectral feature within the range of frequencies, the spectral feature not changing in an expected operating environment of the tunable laser;
• and
• processing circuitry to:
• use the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser;
• compare the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength to generate an error signal;
• and
• control the tunable laser using a feedback signal based on the error signal,
• wherein the processing circuitry comprises:
• a digital feedback loop arranged to digitally determine and generate the feedback signal in digital form, and
• an analog feedback loop arranged to generate the feedback signal in analog form and control the wavelength of the tunable laser.

A method for controlling a laser output, comprising:

• scanning a tunable laser over a range of frequencies;
• detecting a laser output of the tunable laser using an interferometer to generate at least two outputs with relative phases substantially other than 0° and substantially other than 180°;
• providing the laser output to a wavelength reference, wherein the wavelength reference generates a spectral feature within the range of frequencies of the tunable laser, and wherein the spectral feature does not change in an expected environment of the tunable laser;
• using the spectral feature of the wavelength reference and the at least two outputs to produce an absolute measurement of a wavelength of the tunable laser;
• comparing the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength and generating an error signal based on the comparing;
• and
• controlling the wavelength of the tunable laser using a feedback signal based on the error signal,
• wherein a digital feedback loop is used to digitally determine and generate the error signal in digital form, and wherein an analog feedback loop is used to generate the error signal in analog form and to control the wavelength of the tunable laser.