Fringe locking subsystem and methods of making and using the same

1. A fringe locking subsystem for an optical sensing cavity, comprising:

one or more photo detectors that detect a reference signal and a cavity signal;

a first amplifier that generates a calculated differential between the reference signal and the cavity signal;

a lock-in amplifier that generates a modulation signal based on the calculated differential; and

a controller that adjusts a distance within the cavity based on the modulation signal.

2. The fringe locking subsystem of claim 1 , further comprising a waveform generator in communication with the controller.

3. The fringe locking subsystem of claim 2 , wherein the waveform generator generates a signal that together with the modulation signal determines the extent to which the controller adjusts the distance within the cavity.

4. The fringe locking subsystem of claim 1 , further comprising a driver in at least intermittent communication with the controller.

5. The fringe locking subsystem of claim 4 , wherein the driver is a piezoelectric-driver.

6. The fringe locking subsystem of claim 1 , wherein the controller is a proportional integrator.

7. The fringe locking subsystem of claim 1 , wherein the first and second reference signals comprise a unit of amplitude and a unit of time.

8. The fringe locking subsystem of claim 1 , wherein the first and second photo detectors comprise photodiodes.

9. The fringe locking subsystem of claim 1 , wherein the first and second photo detectors comprise spectrometers or cameras.

10. The fringe locking subsystem of claim 1 , wherein the error signal is a first harmonic signal.

11. A fringe locking subsystem for an optical sensing cavity, comprising:

one or more photo detectors that detect a reference signal and a cavity signal;

a first amplifier that generates a calculated differential between the reference signal and the cavity signal;

a lock-in amplifier that generates a modulation signal based on the calculated differential;

a controller that produces an output in response to the modulation signal; and

a driver in at least intermittent communication with the controller, wherein the driver adjusts a distance within the cavity based on the output from the controller.

12. The fringe locking subsystem of claim 11 , wherein the driver comprises one or more of an acoustic actuator, an electric actuator, magnetic actuator, capacitive actuator, piezoelectric actuator, thermal actuator, or a shape memory alloy based actuator.

13. The fringe locking subsystem of claim 11 , wherein a frequency of the modulation signal is of the order of 3 KHz.

14. A method of actively stabilizing a cavity length, comprising:

collecting a reference signal representative of the input light in a cavity and a cavity signal representative of the output light from the cavity;

calculating a differential between the reference signal and the cavity signal; converting a phase and amplitude of the differential signal to a time-varying low-frequency-voltage signal to generate an error signal;

providing an error signal to a driver to modulate the cavity length; and

adjusting a distance between cavity surfaces based on the error signal.

15. The method of claim 14 , further comprising amplifying the differential signal.

16. The method of claim 14 , further comprising combining a ramp function with the error signal.

17. The method of claim 16 , comprising proportionally integrating the error signal and the ramp function to provide an integrated signal.

18. The method of claim 16 , wherein the ramp function comprises a sinusoidal wave function.

19. The method of claim 14 , wherein the error signal is a first harmonic signal.