Apparatus and method to enable precision and fast laser frequency tuning

1. A method, comprising:

generating a slave laser line fast tuned through at least a fast tuning port to at least a frequency near a reference laser line;

frequency dividing a detected beatnote between the slave laser line and the reference laser line;

detecting a phase difference between the divided beatnote and a reference signal to generate an error signal proportional to the phase difference;

converting the error signal into an appropriate feedback signal to phase lock the divided beatnote to the reference signal;

rapidly changing a slave laser frequency target based on a combination of a dynamically changing a frequency of the reference signal, a frequency dividing factor, and an effective polarity of the error signal;

switching to a frequency detect mode to supply the error signal to drive the loop towards frequency lock and switching to an intermediary wide closed-loop mode to quickly achieve phase lock when the intermediary closed-loop mode achieves phase lock by transition into a final closed-loop state; and

dynamically generating a feed-forward signal proportional to the phase difference for a plurality of closed-loop modes, with higher phase sensitivity for an intermediary closed loop mode before translating the error signal into two appropriate feedback signals and feeding the feedback signals to the slave laser to correct its optical frequency to accelerate frequency switching of a slave laser to a target through the fast tuning port where the feed-forward signal is selectively optimized dynamically or offline periodically.

2. The method of claim 1 , further comprising:

combining the slave laser line and the reference laser line to generate the beatnote, wherein the reference laser line comprises a fraction of a master laser or one laser line of an optical frequency comb generated from the master laser.

3. The method of claim 1 , further comprising:

generating and applying additional feed-forward signals to other tuning ports to dynamically tune the slave laser in a wider range, in conjunction with tuning through the fast tuning port; and

rapidly tuning the slave laser line from a first frequency near a first reference laser line to a second frequency near a second reference laser line, through one or more tuning ports.

4. The method of claim 1 , further comprising:

slowly adjusting a frequency of the slave laser by changing its temperature using a temperature setting signal, and an optional slow feedback signal converted from the error signal.

5. The method of claim 1 , further comprising:

detecting a frequency difference between the divided beatnote and the reference signal, and dynamically determining the error signal by a fast locking logic to drive a loop towards frequency lock with a configurable error signal when the frequency difference exceeds a predetermined threshold.

6. The method of claim 1 , wherein the frequency dividing of the beatnote comprises frequency dividing the beatnote by a factor of N to below a maximum frequency of a phase detector, where N is dynamically programmable and is a product of divider ratios N 1 , N 2 , . . . ,N N .

7. The method of claim 1 , further comprising:

changing gains in a phase locked loop in order for an open loop gain of the phase locked loop to remain optimized, when a frequency dividing factor N and a frequency tuning response of the fast tuning port are changed.

8. The method of claim 7 , further comprising:

rapidly switching the feed-forward signals, the frequency of the reference signal, the frequency dividing factor N, the effective polarity of the error signal, the gains in the other sections of the phase locked loop, and settings of the fast locking logic based on digital commands from an electronic controller.

9. The method of claim 8 , wherein the electronic controller is configured to wait on a triggering pulse to start each frequency switch of the slave laser.

10. The method of claim 8 , wherein the feed-forward signal to the fast tuning port is initially estimated from the frequency tuning response of the fast tuning port, and adaptively corrected to minimize the feedback signals and the detected frequency difference between the divided beatnote and the reference signal.