Apparatus and method for laser frequency control

• US 6,175,579 B1
• Filed: 10/27/1998
• Issued: 01/16/2001
• Est. Priority Date: 10/27/1998
• Status: Expired due to Fees

• Current Assignee: Brian R. Lenker, Precision Light L.L.C.
• Original Assignee: Precision Light L.L.C.
• Inventors: Antill, Charles W. Jr., Sandford, Stephen P.
• Primary Examiner(s): Arroyo, Teresa M.
• Assistant Examiner(s): Inzirillo, Gioacchino
• Application Number:
  US09/179,210
• Time in Patent Office: 812 Days
• US Class Current: 372/32
• CPC Class Codes: H01S 3/08036 using intracavity dispersive, polarising or birefringent elements
• CPC Class Codes: H01S 3/1303 by using a passive reference, e.g. absorption cell H01S3/139 takes precedence
• CPC Class Codes: H01S 3/2391 emitting at different wavelengths
• CPC Class Codes: H01S 5/0687 Stabilising the frequency of the laser

Abstract

An apparatus and method of controlling the frequency of laser outputs are provided. The apparatus includes an optical resonator having an optical length that supports a plurality of resonant modes dependent thereon. The optical resonator includes a modulation device for modulating the optical length in response to a modulation signal. A first and second laser having a respective first and second optical output at a respective first and second laser frequencies are independently locked to different resonant modes of the optical resonator using known locking mechanisms to produce first and second laser outputs having narrow linewidths. A beatnote generator generates a beatnote signal representative of the difference between the first and second frequencies of the first and second laser outputs. A negative feedback system having a reference signal and the beatnote signal as inputs generates the modulation signal representative of a difference between the reference and beatnote signals. The modulation signal so produced acts to correct for changes in the optical length of the optical resonator by actuating the modulation device so as to maintain a fixed optical length and thereby stabilize and control the first and second optical outputs.

Claims

1. A laser frequency control apparatus comprising:

(a) an optical resonator having an optical length and a plurality of resonant modes dependent upon said optical length;

(b) a first laser system including a first laser output at a first laser frequency and a first laser locking mechanism, said first laser locking mechanism adapted to modulate said first laser output so as to lock said first laser frequency to a first resonant mode of said optical resonator;

(c) a second laser system including a second laser output at a second laser frequency and a second laser locking mechanism, said second laser locking mechanism adapted to modulate said second laser output so as to lock said second laser frequency to a second resonant mode of said optical resonator;

(d) a beatnote generator arranged to receive said first and second laser outputs and produce a beatnote signal representative of the difference between said first and second laser frequencies;

(e) a modulation device adapted to modulate said optical length so as to maintain a specific optical length; and

(f) a negative feedback system having said beatnote signal and a reference signal as input signals and producing a modulation signal to actuate said modulation device to modulate said optical length responsive to said beatnote signal so as to lock said optical resonator to said reference signal.

2. The laser frequency control apparatus of claim 1 wherein said apparatus further comprises an optical conditioning system operatively arranged to condition at least one of said first and second laser outputs.

3. The laser frequency control apparatus of claim 1 wherein said negative feedback system comprises a phase-locked loop having an electronic oscillator adapted to produce said reference signal, said modulation signal being representative of a difference between said beatnote and said reference signals.

4. The laser frequency control apparatus of claim 1 wherein said negative feedback system comprises an integral feedback portion and a proportional feedback portion.

5. The laser frequency control apparatus of claim 1 wherein said beatnote generator comprises a photodetector.

6. The laser frequency control apparatus of claim 1 wherein said optical resonator is comprised of material having a low coefficient of thermal expansion.

7. The laser frequency control apparatus of claim 1 wherein said optical resonator is less than about 100 millimeters long.

8. The laser frequency control apparatus of claim 1 wherein said modulation device further comprises an optical length transducer operatively coupled to said optical resonator and being adapted to modulate said optical length in response to said modulation signal.

9. The laser frequency control apparatus of claim 1 wherein at least one of said first and second laser systems comprises a diode-pumped ring laser.

10. The laser frequency control apparatus of claim 1 wherein said optical resonator has a finesse greater than about 5,000.

11. The laser frequency control apparatus of claim 1 wherein at least one of said first and second laser locking mechanisms comprise a Pound-Drever-Hall locking mechanism.

12. The laser frequency control apparatus of claim 1 wherein said apparatus has a volume smaller than about 2700 cubic centimeters.

13. The laser frequency control apparatus of claim 1 wherein at least one of said first and second resonant modes is an axial mode of said optical resonator.

14. The laser frequency control apparatus of claim 1 wherein at least one of said first and second resonant modes is an off-axis mode of said optical resonator.

15. The laser frequency control apparatus of claim 1 wherein at least one of said first and second resonant modes is a degenerate off-axis mode of said optical resonator.

16. The laser frequency control apparatus of claim 2 wherein said optical conditioning system comprises at least one beam combiner adapted to coalign said first and second laser outputs.

17. The laser frequency control apparatus of claim 2 wherein said optical conditioning system comprises an optical distribution mechanism adapted to distribute at least one of said first or second laser outputs to an system removed from said laser frequency control apparatus.

18. The laser frequency control apparatus of claim 3 wherein said beatnote signal has a center frequency, said center frequency modulated by changes in said optical length.

19. The laser frequency control apparatus of claim 8 wherein said optical length transducer comprises a piezoelectric material.

20. The laser frequency control apparatus of claim 18 wherein said electronic oscillator comprises a natural oscillator and a clock scaler adapted to produce said reference signal at a frequency matched with said beatnote signal center frequency.

21. The laser frequency control apparatus of claim 18 further comprising a mixer operatively coupled between said beatnote generator and said negative feedback system and adapted to mix down said beatnote signal center frequency.

22. The laser frequency control apparatus of claim 18 further comprising a scaler operatively coupled between said beatnote generator and said negative feedback system and adapted to scale down said beatnote signal center frequency.

23. The laser frequency control apparatus of claim 18 wherein at least one of said first and second laser outputs is an input to a long-baseline interferometer adapted to measure a distance.

24. The laser frequency control apparatus of claim 18 wherein at least one of said first and second laser outputs and said beatnote signal is a reference input to a communications system.

25. The laser frequency control apparatus of claim 18 wherein said at least one of first and second laser outputs is an input to a frequency measurement system for another laser.

26. The laser frequency control apparatus of claim 20 wherein said electronic oscillator further comprises a stabilization mechanism adapted to stabilize said natural oscillator.

27. A method of laser frequency control comprising the steps of:

(a) providing a first laser output having a first laser frequency;

(b) locking said first laser frequency to a first resonant frequency of an optical resonator;

(c) providing a second laser output having a second laser frequency;

(d) locking said second laser frequency to a second resonant frequency of said optical resonator;

(e) producing a beatnote signal representative of the difference between said first and second laser frequencies;

(f) inputting said beatnote signal and a reference signal into a negative feedback system;

(g) producing a modulation signal representative of the difference between said beatnote and reference signals;

(h) locking said beatnote signal to said reference signal with said negative feedback system; and

(i) actuating modulation device in response to said modulation signal to modulate an optical length of said optical resonator so as to lock said optical resonator to said reference signal.

28. The method of laser frequency control of claim 27 further comprising the step of conditioning at least one of said first and second laser outputs.

29. The method of laser frequency control of claim 27 further comprising the step of utilizing at least one of said first and second laser outputs to measure a distance after locking said optical resonator to said reference signal.

30. The method of laser frequency control of claim 27 further comprising the step of utilizing at least one of said first and second laser outputs to establish a communication system reference after locking said optical resonator to said reference signal.

31. The method of laser frequency control of claim 27 further comprising the step of mixing at least one of said first or second laser outputs with another laser output to determine a frequency of said another laser after locking said optical resonator to said reference signal.

32. The method of laser frequency control of claim 27 wherein the step of locking said beatnote signal to said reference signal comprises the sequential steps of:

(a) achieving a quasi-steady-state lock with an integral feedback portion of said negative feedback system; and

then

(b) achieving a steady-state lock with a proportionate control portion of said negative feedback system.

33. The method of laser frequency control of claim 28 wherein the step of locking at lease one of said first and second laser frequencies comprises the step of applying a Pound-Drever-Hall locking method.

34. The method of laser frequency control of claim 28 wherein the step of conditioning at least one of said first and second laser outputs comprises the steps of:

(a) coaligning said first and second laser outputs; and

(b) directing said coaligned first and second laser outputs to the beatnote generator.

35. The method of laser frequency control of claim 28 wherein the step of conditioning at least one of said first and second laser outputs comprises the step of:

(a) coaligning said first and second laser outputs; and

(b) inputting said coaligned first and second laser outputs to an optical distribution mechanism, after locking said optical resonator to said reference signal.

36. The method of laser frequency control of claim 32 wherein the step of producing a beatnote signal comprises the step of mixing down a center frequency of said beatnote signal so as to preserve information related to changes in said optical length of said optical resonator.

37. The method of laser frequency control of claim 32 wherein the step of producing a beatnote signal comprises the step of scaling down a center frequency of said beatnote signal so as to preserve information related to changes in said optical length of said optical resonator.