Method and apparatus for dual resonant laser upconversion

• US 5,297,156 A
• Filed: 12/02/1992
• Issued: 03/22/1994
• Est. Priority Date: 12/20/1990
• Status: Expired due to Term

• Current Assignee: Gemfire Corp. (Medtronic Plc)
• Original Assignee: Deacon Research
• Inventors: Deacon, David A. G.
• Primary Examiner(s): Epps, Georgia Y.
• Application Number:
  US07/984,382
• Time in Patent Office: 475 Days
• Field of Search: 372/21, 372/22, 372/31, 372/32, 372/94, 372/93, 359/326, 359/328, 359/330, 359/329
• US Class Current: 372/21
• CPC Class Codes: G02F 1/3501 Constructional details or arrangements of non-linear optical devices, e.g. shape of non-linear crystals
• CPC Class Codes: G02F 1/3534 Three-wave interaction, e.g. sum-difference frequency generation G02F1/3532 takes precedence
• CPC Class Codes: G02F 1/3542 Multipass arrangements, i.e. arrangements to make light pass multiple times through the same element, e.g. using an enhancement cavity
• CPC Class Codes: G02F 1/37 for second-harmonic generation G02F1/3532 takes precedence
• CPC Class Codes: G02F 1/39 for parametric generation or amplification of light, infrared or ultraviolet waves
• CPC Class Codes: H01S 3/082 defining a plurality of resonators, e.g. for mode selection or suppression
• CPC Class Codes: H01S 5/141 using a wavelength selective device, e.g. a grating or etalon H01S5/146 takes precedence

Abstract

A laser frequency conversion device employs a dual resonant optical cavity in which closed optical paths exist for both the pump beam and the generated beam. The optical cavity is designed so that the optical paths overlap the acceptance volume in phase space for coupling power from the pump beam into the output beam. At least one of the beams is pumped by an external laser which is aligned and mode matched to the cavity mode. The frequency of the pump laser or the cavity resonance modes is selected, generally by adjustment, to cause buildup of optical power at the pump frequency. The temperature and the angle of the nonlinear material are selected for phase match in the nonlinear interaction within the material. The frequencies of the modes near the output frequency are independently selected to cause buildup of optical power at the output frequency.

Claims

1. An apparatus for generating an optical output beam, the apparatus comprising:

a solid state laser pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a resonator cavity means coupled to receive said first optical beam from said solid state laser pump means, for supporting buildup of selected optical power in a first spectral range including said first preselected spectrum and in a second spectral range, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

first means for adjusting at least one longitudinal mode in said first spectral range of said resonator cavity means relative to said at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode;

a nonlinear optical element means disposed in said resonator cavity means, which is adjustable to phase match at least two optical beams through said nonlinear optical element means, for generating a second optical beam with a preselected frequency along a second defined axis at least partially overlapping said first defined axis; and

second means for adjusting a second longitudinal mode in said second spectral range of said resonator cavity means relative to said preselected frequency of said second optical beam in response to a second control input in order to achieve buildup of optical power at said preselected frequency for producing an optical output.

2. The apparatus according to claim 1 wherein said solid state laser pump means includes at least one diode laser pump.

3. The apparatus according to claim 1 wherein said solid state laser pump means includes at least one external-cavity stabilized diode laser pump.

4. The apparatus according to claim 1 wherein said solid state laser pump means includes at least one external-cavity stabilized diode laser pump with stabilizing grating.

5. The apparatus according to claim 1 wherein said solid state laser pump means includes at least one diode-pumped solid state laser pump.

6. The apparatus according to claim 1 wherein said preselected spectrum contains a preferred spectral peak, and said preselected frequency is equal to twice the frequency of said preferred spectral peak, such that the apparatus defines a doubler.

7. The apparatus according to claim 1 wherein said preselected spectrum contains two preferred spectral peaks, and said preselected frequency is equal to the sum of the frequencies of said two preferred spectral peaks, such that the apparatus defines a mixer.

8. The apparatus according to claim 1 wherein said resonator cavity means is a unidirectional propagation cavity defined by at least one concave reflecting surface and at least two other reflecting surfaces, none of said concave reflecting surfaces and said other reflecting surfaces being disposed to reflect an incident beam along a normal.

9. The apparatus according to claim 8 wherein said unidirectional propagation cavity is a three-reflection ring having a first planar reflecting surface and a second planar reflecting surface.

10. The apparatus according to claim 8 wherein said unidirectional propagation cavity is a bow-tie cavity having four reflecting surfaces in folded-path configuration.

11. The apparatus according to claim 1 wherein said resonator cavity means is a bidirectional propagation cavity defined by at least one concave reflecting surface and at least one other reflecting surface, at least two reflecting surfaces in said cavity being disposed to reflect an incident beam along a normal.

12. The apparatus according to claim 11 wherein said bidirectional propagation cavity is a plano-concave cavity.

13. The apparatus according to claim 11 wherein said bidirectional propagation cavity is a concave-concave cavity.

14. The apparatus according to claim 1 wherein said resonator cavity means is monolithic.

15. The apparatus according to claim 1 wherein said first control input is a first feedback signal which is responsive to a first error signal derived from measurement of the optical power at the wavelength of said at least one spectral peak in said resonator cavity means.

16. The apparatus according to claim 1 wherein said first means for adjusting includes a piezoelectric translator mounted to one of the reflectors which comprise said resonator cavity means, said piezoelectric translator for modifying round trip optical path length at the wavelength of said at least one longitudinal mode.

17. The apparatus according to claim 1 wherein said first means for adjusting includes optical feedback means wherein said first control input is a portion of the power built up in said resonator cavity means at the wavelength of said at least one longitudinal mode, which is coupled back into the output of said solid state laser pump means with an adjustable phase and an adjustable amplitude to injection lock said at least one spectral peak to a wavelength within the bandwidth of said at least one longitudinal mode.

18. The apparatus according to claim 17 wherein a portion of a third control input controls at least said adjustable phase to maintain the frequency of said at least one spectral peak within a feedback error difference of the central frequency of said at least one longitudinal mode.

19. The apparatus according to claim 18 wherein a portion of a fourth control input also controls the relative free-running wavelength of said solid state laser pump means, which is the wavelength of said at least one spectral peak in the absence of said optical feedback means, relative to the wavelength of said at least one longitudinal mode so that perturbations of both said adjustable phase and said relative free-running wavelength are compensated substantially independently.

20. The apparatus according to claim 1 wherein radius of curvature and separation of the mirrors forming said resonator cavity means are selected so that the angle formed between said first defined axis and said second defined axis in said nonlinear optical element means equals a characteristic walkoff angle within an angular deviation smaller than the acceptance angle for nonlinear conversion in said nonlinear optical element means.

21. The apparatus according to claim 20 wherein said nonlinear optical element means is adjustable so that radiation generated from the square of the electric field of said first optical beam is phase matched with said second optical beam.

22. The apparatus according to claim 20 wherein polarization of said second optical beam lies approximately in a plane formed by the optical axis of said nonlinear element and a characteristic wavevector of said at least one longitudinal mode, forming an extraordinary beam, and polarization of said first optical beam is approximately orthogonal to polarization of said second optical beam, forming an ordinary beam, such that said nonlinear element means provides Type I phase matching between the two beams.

23. The apparatus according to claim 1 wherein said second control input is a second feedback signal which is responsive to a second error signal derived from measurement of the optical power at said preselected frequency in said resonator cavity means.

24. The apparatus according to claim 1 wherein said second adjusting means includes an adjustable optical path length means in said resonator cavity means which adjusts the optical path length of said second optical beam relative to the optical path length of said first optical beam, such that buildup at said preselected frequency is supported by said resonator cavity means at the same time as buildup at said at least one longitudinal mode.

25. The apparatus according to claim 24 wherein said adjustable optical path length means is said nonlinear optical element means including means for rotation of said nonlinear optical element means about the polarization vector of said first optical beam.

26. The apparatus according to claim 24 wherein said adjustable optical path length means is a means for adjusting temperature of media at selected positions within said resonator cavity means through which pass at least said second optical beam.

27. The apparatus according to claim 26 wherein said temperature adjusting means is an oven and wherein said media is air.

28. The apparatus according to claim 1 further including means for stabilizing optical power, including means for saturating output of the second optical beam to cause intracavity conversion efficiency to saturate, the intracavity conversion efficiency saturation being achieved when conversion efficiency is comparable to the fundamental cavity total loss minus transmission of an input coupling means for said optical pump means, said input coupling means being the optical input of said resonator cavity means.

29. A method for generating an optical output beam comprising the steps of:

producing a first optical beam along a first defined axis with solid state laser pump means, said first optical beam having a first preselected spectrum having at least one spectral peak;

building up selected optical power in a first spectral range including said first preselected spectrum and in a second spectral range with resonator cavity means coupled to receive said first optical beam from said solid state laser pump means, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

adjusting at least one longitudinal mode in said first spectral range of said resonator cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode;

generating a second optical beam at a preselected frequency along a second defined axis at least partially overlapping said first defined axis using nonlinear optical element means disposed in said resonator cavity means, said nonlinear optical element being adjustable to phase match at least two optical beams through said nonlinear optical element means; and

adjusting a second longitudinal mode in said second spectral range of said resonator cavity means relative to said preselected frequency of said second optical beam in response to a second control input in order to achieve buildup of optical power at said preselected frequency for producing an optical output.

30. The method according to claim 29 wherein said first control input is a first feedback signal which is responsive to a first error signal derived from measurement of the optical power at the wavelength of said at least one spectral peak in said resonator cavity means.

31. The method according to claim 29 wherein said second control input is a second feedback signal which is responsive to a second error signal derived from measurement of the optical power at said preselected frequency in said resonator cavity means.

32. An apparatus for generating an optical output beam, the apparatus comprising:

an optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a resonator cavity means coupled to receive said first optical beam from said optical pump means, for supporting buildup of selected optical power in a first spectral range including said first preselected spectrum and in a second spectral range, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

first means for adjusting at least one longitudinal mode in said first spectral range of said resonator cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode, said first control input being optical feedback for injection locking at least one spectral peak of said optical pump means to said resonator cavity means, said optical feedback being derived from the optical power in said resonator cavity means;

a nonlinear optical element means disposed in said resonator cavity means, which is adjustable to phase match at least two optical beams through said nonlinear optical element means, for generating a second optical beam at a preselected frequency along a second defined axis at least partially overlapping said first defined axis; and

second means for adjusting a second longitudinal mode in said second spectral range of said resonator cavity means relative to said preselected frequency of said second optical beam in response to a second control input in order to achieve buildup of optical power at said preselected frequency, for producing an optical output.

33. The apparatus according to claim 32 wherein said resonator cavity means is a unidirectional propagation cavity defined by at least one concave reflecting surface and at least two other reflecting surfaces, none of said concave reflecting surfaces and said other reflecting surfaces being disposed to reflect an incident beam along a normal.

34. The apparatus according to claim 32 wherein said first means for adjusting includes optical feedback means wherein said first control input is a portion of the power built up in said resonator cavity means at the wavelength of said at least one longitudinal mode, which is coupled back into the output of said optical pump means with an adjustable phase and an adjustable amplitude to injection lock said at least one spectral peak to a wavelength within the bandwidth of said at least one longitudinal mode.

35. The apparatus according to claim 34 wherein a portion of a third control input controls at least said adjustable phase to maintain the frequency of said at least one spectral peak within a feedback error difference of the central frequency of said at least one longitudinal mode.

36. The apparatus according to claim 35 wherein a portion of a fourth control input also controls the relative free-running wavelength of said optical pump means, which is the wavelength of said at least one spectral peak in the absence of said optical feedback means, relative to the wavelength of said at least one longitudinal mode so that perturbations of both said adjustable phase and said relative free-running wavelength are compensated substantially independently.

37. The apparatus according to claim 32 wherein said second adjusting means includes an adjustable optical path length means in said resonator cavity means which adjusts the optical path length of said second optical beam relative to the optical path length of said first optical beam, such that buildup at said preselected frequency is supported by said resonator cavity means at the same time as buildup at said at least one longitudinal mode.

38. The apparatus according to claim 37 wherein said adjustable optical path length means is said nonlinear optical element means including means for rotation of said nonlinear optical element means about the polarization vector of said first optical beam.

39. The apparatus according to claim 37 wherein said adjustable optical path length means is a means for adjusting temperature of media at selected positions within said resonator cavity means through which pass at least said second optical beam.

40. The apparatus according to claim 39 wherein said temperature adjusting means is an oven and wherein said media is air.

41. An apparatus for generating an optical output beam, the apparatus comprising:

a optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a resonator cavity means coupled to receive said first optical beam from said optical pump means, for supporting buildup of selected optical power in a first spectral range including said first preselected spectrum and in a second spectral range, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

first means for adjusting at least one longitudinal mode in said first spectral range of said resonator cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode;

a nonlinear optical element means disposed in said resonator cavity means, which is adjustable to phase match the generation of a second optical beam with a preselected frequency and a third optical beam from said at least one longitudinal mode, said first optical beam, said second optical beam and said third optical beam at least partially overlapping spatially through said nonlinear optical element means; and

second means for adjusting a second longitudinal mode in said second spectral range of said resonator cavity means relative to said preselected frequency in response to a second control input in order to achieve buildup of optical power at said preselected frequency, for producing an optical output.

42. The apparatus according to claim 41 wherein said resonator cavity means is a unidirectional propagation cavity defined by at least one concave reflecting surface and at least two other reflecting surfaces, none of said concave reflecting surfaces and said other reflecting surfaces being disposed to reflect an incident beam along a normal.

43. The apparatus according to claim 41 wherein said first means for adjusting includes optical feedback means wherein said first control input is a portion of the power built up in said resonator cavity means at the wavelength of said at least one longitudinal mode, which is coupled back into the output of said optical pump means with an adjustable phase and an adjustable amplitude to injection lock said at least one spectral peak to a wavelength within the bandwidth of said at least one longitudinal mode.

44. The apparatus according to claim 43 wherein a portion of a third control input controls at least said adjustable phase to maintain the frequency of said at least one spectral peak within a feedback error difference of the central frequency of said at least one longitudinal mode.

45. The apparatus according to claim 44 wherein a portion of a fourth control input also controls the relative free-running wavelength of said optical pump means, which is the wavelength of said at least one spectral peak in the absence of said optical feedback means, relative to the wavelength of said at least one longitudinal mode so that perturbations of both said adjustable phase and said relative free-running wavelength are compensated substantially independently.

46. The apparatus according to claim 41 wherein said second adjusting means includes an adjustable optical path length means in said resonator cavity means which adjusts the optical path length of said second optical beam relative to the optical path length of said first optical beam, such that buildup at said preselected frequency is supported by said resonator cavity means at the same time as buildup at said at least one longitudinal mode.

47. The apparatus according to claim 46 wherein said adjustable optical path length means is said nonlinear optical element means including means for rotation of said nonlinear optical element means about the polarization vector of said first optical beam.

48. The apparatus according to claim 46 wherein said adjustable optical path length means is a means for adjusting temperature of media at selected positions within said resonator cavity means through which pass at least said second optical beam.

49. The apparatus according to claim 48 wherein said temperature adjusting means is an oven and wherein said media is air.

50. An apparatus for generating an optical output beam, the apparatus comprising:

an optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a resonator cavity means coupled to receive said first optical beam from said optical pump means, for supporting buildup of selected optical power in a first spectral range including said first preselected spectrum, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam; and

first means for adjusting at least one longitudinal mode in said first spectral range of said resonator cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode, wherein said optical path length adjusting means is a means for adjusting temperature of media through which passes at least said first optical beam at selected positions within said resonator cavity means.

51. The apparatus according to claim 50 wherein said temperature adjusting means is an oven and wherein said media is air.

52. An apparatus for generating an optical output beam, the apparatus comprising:

an optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a resonator cavity means coupled to receive said first optical beam from said optical pump means, for supporting buildup of selected optical power in a first spectral range including said first preselected spectrum and in a second spectral range, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam; and

first means for adjusting at least one longitudinal mode in said first spectral range of said resonator cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode;

a coherent optical source for producing a second optical beam along a second defined axis, said second optical beam having a second preselected spectrum having at least one spectral peak within said second spectral range; and

second means for adjusting a second longitudinal mode in said second spectral range of said resonator cavity means relative to at least one spectral peak of said coherent optical source in response to a second control input in order to achieve buildup of optical power at said second longitudinal mode, wherein said optical path length adjusting means is a means for adjusting temperature of media through which passes at least said second optical beam at selected positions within said resonator cavity means.

53. The apparatus according to claim 52 wherein said temperature adjusting means is an oven and wherein said media is air.

54. The apparatus according to claim 52 wherein said resonator cavity means is a unidirectional propagation cavity defined by at least one concave reflecting surface and at least two other reflecting surfaces, none of said concave reflecting surfaces and said other reflecting surfaces being disposed to reflect an incident beam along a normal.

55. The apparatus according to claim 52 wherein said first means for adjusting includes optical feedback means wherein said first control input is a portion of the power built up in said resonator cavity means at the wavelength of said at least one longitudinal mode, which is coupled back into the output of said optical pump means with an adjustable phase and an adjustable amplitude to injection lock said at least one spectral peak to a wavelength within the bandwidth of said at least one longitudinal mode.

56. The apparatus according to claim 55 wherein a portion of a third control input controls at least said adjustable phase to maintain the frequency of said at least one spectral peak within a feedback error difference of the central frequency of said at least one longitudinal mode.

57. The apparatus according to claim 56 wherein a portion of a fourth control input also controls the relative free-running wavelength of said optical pump means, which is the wavelength of said at least one spectral peak in the absence of said optical feedback means, relative to the wavelength of said at least one longitudinal mode so that perturbations of both said adjustable phase and said relative free-running wavelength are compensated substantially independently.

58. An apparatus for generating an optical output beam, the apparatus comprising:

an optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a resonator cavity means coupled to receive said first optical beam from said optical pump means, for supporting buildup of selected optical power in a first spectral range including said first preselected spectrum, said resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

optical feedback means for achieving buildup of optical power at said at least one longitudinal mode, wherein a fraction of power built up in said resonator cavity means at the wavelength of said at least one longitudinal mode is coupled back into the output of said optical pump means with an adjustable phase and an adjustable amplitude to injection lock said at least one spectral peak to said at least one longitudinal mode;

a first electrical control input for controlling said adjustable phase to maintain frequency of said at least one spectral peak within a feedback error difference of a central frequency of said at least one longitudinal mode; and

a second electrical control input for controlling free-running wavelength of said optical pump means relative to said at least one longitudinal mode of said resonator cavity means.

59. The apparatus according to claim 58 wherein said resonator cavity means is a unidirectional propagation cavity defined by at least one concave reflecting surface and at least two other reflecting surfaces, none of said concave reflecting surfaces and said other reflecting surfaces being disposed to reflect an incident beam along a normal.

60. The apparatus according to claim 58 wherein said resonator cavity means is monolithic.

61. An apparatus for generating an optical output beam, the apparatus comprising:

an optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a bidirectional propagation cavity means coupled to receive said first optical beam from said optical pump means for supporting buildup of selected optical power in a first spectral range and in a second spectral range, said bidirectional propagation cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

first means for adjusting at least one longitudinal mode in said first spectral range of said bidirectional propagation cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode;

a nonlinear optical element means disposed in said bidirectional propagation cavity means, which is adjustable to phase match at least two optical beams through said nonlinear optical element means, which is phase matched for generating a second optical beam at a preselected frequency along a second defined axis at least partially overlapping said first defined axis; and

second means for adjusting a second longitudinal mode in said second spectral range of said bidirectional propagation cavity means relative to said preselected frequency of said second optical beam in response to a second control input in order to achieve buildup of optical power at said preselected frequency for producing an optical output.

62. The apparatus according to claim 61 wherein said bidirectional propagation cavity means is a plano-concave cavity.

63. The apparatus according to claim 61 wherein said bidirectional propagation cavity means is a concave-concave cavity.

64. The apparatus according to claim 61 wherein said bidirectional propagation cavity means is monolithic.

65. An apparatus for generating an optical output beam, the apparatus comprising:

an optical pump means for producing a first optical beam along a first defined axis, said first optical beam having a first preselected spectrum having at least one spectral peak;

a ring resonator cavity means coupled to receive said first optical beam from said optical pump means for supporting buildup of selected optical power in a first spectral range and in a second spectral range, said ring resonator cavity means being disposed to have an axis coincident with said first optical beam for admitting a portion of said first optical beam;

first means for adjusting at least one longitudinal mode in said first spectral range of said ring resonator cavity means relative to at least one spectral peak of said optical pump means in response to a first control input in order to achieve buildup of optical power at said at least one longitudinal mode;

a nonlinear optical element means disposed in said ring resonator cavity means, which is adjustable to phase match at least two optical beams through said nonlinear optical element means, which is phase matched for generating a second optical beam at a preselected frequency along a second defined axis at least partially overlapping said first defined axis; and

second means for adjusting a second longitudinal mode in said second spectral range of said ring resonator cavity means relative to said preselected frequency of said second optical beam in response to a second control input in order to achieve buildup of optical power at said preselected frequency for producing an optical output.

66. The apparatus according to claim 65 wherein said unidirectional propagation cavity is a three-reflection ring having a first planar reflecting surface and a second planar reflecting surface.

67. The apparatus according to claim 65 wherein said unidirectional propagation cavity is a bow-tie cavity having four reflecting surfaces in folded-path configuration.

68. The apparatus according to claim 65 wherein said first means for adjusting includes optical feedback means wherein said first control input is a portion of the power built up in said ring resonator cavity means at the wavelength of said at least one longitudinal mode, which is coupled back into the output of said optical pump means with an adjustable phase and an adjustable amplitude to injection lock said at least one spectral peak to a wavelength within the bandwidth of said at least one longitudinal mode.

69. The apparatus according to claim 68 wherein a portion of a third control input controls at least said adjustable phase to maintain the frequency of said at least one spectral peak within a feedback error difference of the central frequency of said at least one longitudinal mode.

70. The apparatus according to claim 69 wherein a portion of a fourth control input also controls the relative free-running wavelength of said optical pump means, which is the wavelength of said at least one spectral peak in the absence of said optical feedback means, relative to the wavelength of said at least one longitudinal mode so that perturbations of both said adjustable phase and said relative free-running wavelength are compensated substantially independently.

71. The apparatus according to claim 65 wherein said second adjusting means includes an adjustable optical path length means in said ring resonator cavity means which adjusts the optical path length of said second optical beam relative to the optical path length of said first optical beam, such that buildup at said preselected frequency is supported by said ring resonator cavity means at the same time as buildup at said at least one longitudinal mode.

72. The apparatus according to claim 71 wherein said adjustable optical path length means is said nonlinear optical element means including means for rotation of said nonlinear optical element means about the polarization vector of said first optical beam.

73. The apparatus according to claim 71 wherein said adjustable optical path length means is a means for adjusting temperature of media at selected positions within said ring resonator cavity means through which pass at least said second optical beam.

74. The apparatus according to claim 73 wherein said temperature adjusting means is an oven and wherein said media is air.