High precision wavelength measurement and control of a tunable laser

Abstract

A tunable laser system includes a tunable laser to be scanned over a range of frequencies and an interferometer having a plurality of interferometer outputs. At least two interferometer outputs of the plurality of interferometer outputs have a phase difference. A wavelength reference has a spectral feature within the range of frequencies, and the spectral feature does not change in an expected operating environment of the tunable laser. Processing circuitry uses the spectral feature and the plurality of interferometer outputs to produce an absolute measurement of a wavelength of the tunable laser and controls the tunable laser based on a comparison of the absolute measurement of the wavelength of the tunable laser with a setpoint wavelength.

What is claimed is: 
     
         1 . A laser control system for a causal wavelength control of a laser output of a tunable laser, the laser control system comprising:
 a wavemeter to generate measurement signals for measuring a wavelength of the laser output;   a laser actuator to be driven by one or more cyclic control signals to control the wavelength of the laser output in accordance with a cyclic wavelength setpoint waveform; and   processing circuitry to:
 initialize a drive waveform of the one or more cyclic control signals based on a steady-state transfer function and the cyclic wavelength setpoint waveform, the steady-state transfer function being of the laser actuator for a repetitive control signal, and 
 iteratively modify the drive waveform based on the steady-state transfer function and an error, the error being between the cyclic wavelength setpoint waveform and an actual waveform of the wavelength of the laser output, the actual waveform of the wavelength of the laser output resulting from application of the drive waveform to the laser actuator and being inferred from the measurement signals generated by the wavemeter. 
   
     
     
         2 . The laser control system of  claim 1 , wherein the tunable laser is an external cavity laser comprising a mirror, and wherein the laser actuator comprises a mirror actuator associated with the mirror. 
     
     
         3 . The laser control system of  claim 1 , wherein the laser actuator behaves non-linearly for the repetitive control signal. 
     
     
         4 . The laser control system of  claim 1 , wherein a frequency content of the repetitive control signal is restricted to frequencies to be used to control the tunable laser. 
     
     
         5 . The laser control system of  claim 1 , wherein the one or more cyclic control signals comprise two differential control signals. 
     
     
         6 . The laser control system of  claim 1 , wherein the cyclic wavelength setpoint waveform comprises a triangular waveform with parabolic transitions at peak and trough. 
     
     
         7 . The laser control system of  claim 1 , wherein only a phase of the transfer function is used when iteratively modifying the drive waveform. 
     
     
         8 . The laser control system of  claim 1 , wherein the wavemeter is configured for absolute-wavelength measurement of the laser output. 
     
     
         9 . The laser control system of  claim 8 , wherein the wavemeter comprises an interferometer and a wavelength reference, and wherein the measurement signals comprise:
 a plurality of output signals generated by the interferometer, at least two output signals of the plurality of output signals having a phase difference not equal to 0° and not equal to 180°; and   a reference signal generated by the wavelength reference and corresponding to a spectral feature within a wavelength range of the cyclic wavelength setpoint waveform, the spectral feature not changing in an expected operating environment of the tunable laser.   
     
     
         10 . A method for a causal wavelength control of a laser output of a tunable laser, the method comprising:
 driving a laser actuator with one or more cyclic control signals to control a wavelength of the laser output in accordance with a cyclic wavelength setpoint waveform;   measuring an actual waveform of the wavelength of the laser output;   initializing a drive waveform of the one or more cyclic control signals based on the cyclic wavelength setpoint waveform and a steady-state transfer function of the laser actuator for a repetitive impulse control signal; and   iteratively modifying the drive waveform based on the transfer function and an error, the error being between the cyclic wavelength setpoint waveform and the measured actual waveform of the wavelength of the laser output.   
     
     
         11 . The method of  claim 10 , wherein the tunable laser is an external cavity laser comprising a mirror, and wherein the laser actuator comprises a mirror actuator associated with the mirror. 
     
     
         12 . The method of  claim 10 , wherein the steady-state transfer function is for a non-linear actuator response. 
     
     
         13 . The method of  claim 10 , wherein a frequency content of the repetitive impulse control signal is restricted to frequencies to be used to control the tunable laser. 
     
     
         14 . The method of  claim 10 , wherein the one or more cyclic control signals comprise two differential control signals. 
     
     
         15 . The method of  claim 10 , wherein the cyclic wavelength setpoint waveform comprises a triangular waveform with parabolic transitions at peak and trough. 
     
     
         16 . The method of  claim 10 , wherein only a phase of the transfer function is used when the drive waveform is iteratively modified. 
     
     
         17 . The method of  claim 10 , wherein measuring the actual waveform comprises absolute-wavelength measurements of the laser output. 
     
     
         18 . The method of  claim 17 , wherein the absolute-wavelength measurements comprise:
 using an interferometer to generate a plurality of output signals, at least two output signals of the plurality of output signals having a phase difference not equal to 0° and not equal to 180°; and   using a wavelength reference to generate a reference signal corresponding to a spectral feature within a wavelength range of the cyclic wavelength setpoint waveform, the spectral feature not changing in an expected operating environment of the tunable laser.   
     
     
         19 . A memory storing instructions for execution by a computer, the instructions causing the computer to perform operations for a causal wavelength control of a laser output of a tunable laser, the operations comprising:
 determining a steady-state transfer function of a laser actuator for a repetitive impulse control signal;   initializing a drive waveform with which a laser actuator is to control a wavelength of the laser output, the initializing being based on a steady-state transfer function and a cyclic wavelength setpoint waveform, the steady-state transfer function being of the laser actuator and for a repetitive impulse control signal; and   iteratively modifying the drive waveform based on the steady-state transfer function and an error, the error being between the cyclic wavelength setpoint waveform and a measured actual waveform of the wavelength of the laser output, the measured actual waveform resulting from application of the drive waveform to the laser actuator.   
     
     
         20 . The memory of  claim 19 , wherein the instructions are implemented in firmware.