Laser with absorption optimized pumping of a gain medium

• US 6,347,101 B1
• Filed: 04/16/1998
• Issued: 02/12/2002
• Est. Priority Date: 04/16/1998
• Status: Expired due to Term

• Current Assignee: 3D Systems, Inc. (3D Systems Corporation)
• Original Assignee: 3D Systems, Inc. (3D Systems Corporation)
• Inventors: Partanen, Jouni P., Hug, William F., Hemmati, Hamid, Wu, Xingkun
• Primary Examiner(s): Davie, James W.
• Assistant Examiner(s): Inzirillo, Gioacchino
• Application Number:
  US09/061,797
• Time in Patent Office: 1,398 Days
• Field of Search: 372/46, 372/75, 372/83, 372/22, 372/23, 372/10, 372/18, 372/69, 372/34
• US Class Current: 372/18
• CPC Class Codes: B33Y 30/00 Apparatus for additive manufacturing; Details thereof or accessories therefor
• CPC Class Codes: H01S 3/08072 Thermal lensing or thermally induced birefringence; Compensation thereof
• CPC Class Codes: H01S 3/0941 of a laser diode
• CPC Class Codes: H01S 3/1022 by controlling the optical pumping
• CPC Class Codes: H01S 3/109 Frequency multiplication, e.g. harmonic generation

Abstract

A solid state laser includes a high absorption coefficient solid state gain medium such as Nd:YVO₄ that is side pumped with a semiconductor laser diode array. The resonant cavity of the solid state laser is positioned so that the TEM₀₀ mode is spaced from the face of the laser through which the laser is pumped by a distance sufficient to reduce diffraction losses but sufficiently near to allow coupling of pump light into the gain mode. The gain medium, the doping level of the gain medium, and the operating temperature of the pump laser are selected to efficiently couple pump light into the gain mode. The pump laser is positioned to side pump the gain medium without collimating or focusing optics between the pump laser and the face of the gain medium. A gap between the pump laser and the gain medium is empirically selected to match the angular extent of the pump laser output light to the height of the gain mode at the position of the gain mode fixed to optimize coupling and diffraction losses.

Claims

1. A solid state laser system comprising:

a laser cavity having an optical axis passing through the laser cavity;

a solid state gain medium having a width in a transverse direction, the solid state gain medium having an absorption depth at a pumping wavelength less than the width in the transverse direction, the optical axis passing through the solid state gain medium; and

at least one semiconductor laser having an output at the pumping wavelength, the at least one semiconductor laser positioned to direct a beam of light to the solid state gain medium in a side pumping configuration,

the laser cavity defining a laser mode of the solid state laser positioned at a mode depth so that the laser mode is spaced from a surface of the solid state gain medium and a width of the laser mode is between about one-half and two times the absorption length of the solid state gain medium at a pumping wave length.

2. The laser system of claim 1, wherein the solid state gain medium has at least one characteristic excitation wavelength and the absorption depth of the solid state gain medium is less than the width in the transverse direction of the solid state gain medium at a wavelength near the characteristic excitation wavelength.

3. The laser system of claim 2, wherein the absorption depth of the solid state gain medium is less than the width in the transverse direction of the solid state gain medium at a wavelength different from the characteristic excitation wavelength.

4. The laser system of claim 2, wherein the pumping wavelength is different from a peak absorption wavelength.

5. The laser system of claim 1, wherein the solid state gain medium has a peak absorption at a characteristic excitation wavelength, and wherein the pumping wavelength is different from the characteristic excitation wavelength.

6. The laser system of claim 1, wherein the gain medium is characterized by a concentration of an optically active dopant and wherein the concentration, the pumping wavelength, and the mode depth are selected to provide a relative maximum in an output power produced by the solid state laser.

7. The laser system of claim 6, wherein at least 90% of the output power produced by the solid state laser is in the TEM₀₀ mode.

8. The laser system of claim 7, wherein the gain medium is neodymium yttrium vanadate.

9. The laser system of claim 8, wherein the neodymium concentration is between 0.5-1.5%.

10. The laser system of claim 8, wherein the neodymium concentration is between 0.6-1.0%.

11. The laser system of claim 8, wherein the neodymium concentration is approximately one percent.

12. The laser system of claim 1, wherein the semiconductor laser is characterized by a pump wavelength, and wherein the pump wavelength and the mode depth are selected to provide a relative maximum in the output power produced by the solid state laser.

13. The laser system of claim 1, wherein the gain medium is characterized by a concentration of an optically active dopant and wherein the pumping wavelength and the mode depth are selected to provide an output power produced by the solid state laser that is within fifty percent of the maximum output power.

14. The laser system of claim 1, wherein the gain medium is characterized by a concentration of an optically active dopant and wherein the pumping wavelength and the mode depth are selected to provide an output power produced by the solid state laser that is within twenty percent of the maximum output power.

15. The laser system of claim 1, wherein the gain medium is characterized by a concentration of an optically active dopant and wherein the pumping wavelength and the mode depth are selected to provide an output power produced by the solid state laser that is within ten percent of the maximum output power.

16. The laser system of claim 5, wherein an operating temperature of the semiconductor laser is selected so that the output wavelength is different from the characteristic excitation wavelength.

17. The laser system of claim 16, wherein the operating temperature is selected by monitoring, directly or indirectly, an output of the solid state laser.

18. The laser system of claim 1, wherein the pumping wavelength of the semiconductor laser is selected by monitoring, directly or indirectly, an output of the solid state laser.

19. The laser system of claim 1, wherein the pumping wavelength is selected to provide a maximum output from the laser system for a particular output intensity of the semiconductor laser.

20. The laser system of claim 1, wherein the laser mode is spaced from a side face of the gain medium by a distance that reduces diffraction losses through the side face to a sufficient level to allow at least ten percent optical-to-optical efficiency for the laser system.

21. The laser system of claim 19, wherein the laser mode is spaced from a side face of the gain medium by a distance that reduces diffraction losses through the side face to a sufficient level to allow at least ten percent optical-to-optical efficiency for the laser system.

22. The laser system of claim 19, wherein the laser mode is spaced from a side face of the gain medium by a distance that reduces diffraction losses through the side face to a sufficient level to allow at least twenty percent slope efficiency for the laser system.

23. The laser system of claim 1, wherein the gain medium has first and second end faces and wherein the optical axis passes through the first and second end faces.

24. The laser system of claim 23, wherein the optical axis is spaced from and parallel to a side face of the gain medium and wherein the laser mode is disposed symmetrically around the optical axis.

25. The laser system of claim 24, wherein a 1/e² point of the laser mode is spaced from the side face of the gain medium by a distance of at least approximately two hundred microns.

26. The laser system of claim 24, wherein a 1/e² point of the laser mode is spaced from the side face of the gain medium by a distance of between approximately two hundred and three hundred microns.

27. The laser system of claim 24, wherein the laser mode is spaced from the side face of the gain medium by a distance that reduces diffraction losses through the side face to a sufficient level to allow at least ten percent optical to optical efficiency for the laser system.

28. The laser system of claim 27, wherein the laser mode is a TEM₀₀ mode of the laser cavity.

29. The laser system of claim 28, wherein the gain medium is Nd:

YVO₄.

30. The laser system of claim 29, wherein the gain medium incorporates approximately 1% neodymium.

31. The laser system of claim 29, wherein the transverse dimension is less than two millimeters.

32. The laser system of claim 24, wherein the laser mode is spaced from the side face of the gain medium by a distance that reduces diffraction losses through the side face to a sufficient level to allow at least twenty percent slope efficiency for the laser system.

33. The laser system of claim 32, wherein the laser mode is a TEM₀₀ mode of the laser cavity.

34. The laser system of claim 33, wherein the gain medium is Nd:

YVO₄.

35. The laser system of claim 34, wherein the gain medium incorporates approximately 1% neodymium.

36. The laser system of claim 35, wherein the transverse dimension is less than two millimeters.

37. A laser system, comprising:

a lasing medium for producing a laser beam of a desired wavelength;

at least one diode laser for directing a pumping beam along a pumping optical path for side pumping the lasing medium and producing a population inversion in the lasing medium;

optical elements for forming a resonant cavity around the lasing medium defining a set of modes and for producing the laser beam; and

wherein the optical elements are disposed to position the optical set of modes within the lasing medium and wherein the at least one diode laser provides a pump wavelength different from a peak of absorption of the lasing medium, the pump wavelength being selected by adjustment of the operating temperature of the at least one diode laser.

38. The laser system of claim 37, wherein the lasing medium has a high absorption coefficient.

39. The laser system of claim 37, wherein an output power in the laser beam is a maximum for a selected power level of the pumping beam.

40. A laser system, comprising:

a lasing medium for producing a laser beam of a desired wavelength;

at least one diode laser for directing a pumping beam along a pumping optical path for side pumping the lasing medium and producing a population inversion in the lasing medium;

optical elements for forming a resonant cavity around the lasing medium defining a set of modes and for producing the laser beam; and

wherein the optical elements are disposed to position the set of modes within the lasing medium, spaced from all sides of the lasing medium and parallel to a side face of the lasing medium by a distance that reduces diffraction losses through the side face to a sufficient level to allow at least ten percent optical to optical efficiency for the laser system, the optical elements further having a width of the set of modes between about one half and two times an absorption length of the lasing medium at a pumping wavelength.

41. The laser system of claim 40, wherein the lasing medium has first and second end faces and wherein the laser beam passes through the first and second end faces.

42. The laser system of claim 40, wherein a 1/e² point of the set of modes is spaced from the side face of the lasing medium by a distance of at least approximately two hundred microns.

43. The laser system of claim 40, wherein a 1/e² point of the set of modes is spaced from the side face of the lasing medium by a distance of between approximately two hundred and three hundred microns.

44. The laser system of claim 43, wherein the set of modes consists only of a TEM₀₀ mode of the resonant cavity.

45. The laser system of claim 44, wherein the lasing medium is Nd:

YVO₄.

46. The laser system of claim 45, wherein the lasing medium incorporates approximately 1% neodymium.

47. A laser system, comprising:

a laser cavity defined by at least two optical elements and having an optical axis the optical elements defining a set of laser modes, the lowest order of the set of laser modes being a TEM₀₀ mode;

a solid state gain medium positioned so that the optical axis passes through the solid state gain medium;

a blocking element positioned adjacent the optical axis between the solid state gain medium and at least one of the optical elements, the blocking element blocking at least a portion of a first lobe of a first higher order mode when laser output is provided from the laser cavity;

at least one semiconductor laser having a pumping wavelength, the at least one semiconductor laser positioned to direct a beam of light to the solid state gain medium in a side pumping configuration;

the blocking element further being positioned to limit gain of one or more of the set of laser modes; and

wherein an absorption of the solid state gain medium at the pumping wavelenth of the at least one semiconductor laser limits a gain of a second lobe of the fist higher order mode.

48. The laser system of claim 47, wherein the gain the first higher order mode is below a threshold for sustained laser action in the solid state laser.

49. The laser system of claim 47, wherein the blocking element is a portion of a Q-switch.

50. The laser system of claim 47, wherein at least 90% of an output of the solid state laser is in the TEM₀₀ mode.

51. The laser system of claim 47, wherein substantially all output power from the solid state laser is in the TEM₀₀ mode.

52. The laser system of claim 47 wherein the blocking element only partially closes a perimeter around the optical axis.

53. A method of assembling a laser system including a solid state laser, the method comprising:

providing optics and a solid state gain medium to define a solid state laser cavity having a set of laser modes, the set of laser modes having a position within the gain medium and the gain medium having an absorption depth;

providing at least one semiconductor laser having a pumping wavelength, the semiconductor laser positioned to side pump the gain medium to produce a solid state laser output, the semiconductor providing a pump beam having a size; and

optimizing a configuration of the solid state laser beam with reference to an output power from the solid state laser by at least one of (1) varying the position of the set of laser modes within the gain medium, (2) varying the output wavelength of the semiconductor laser, and (3) varying the size of the pump.

54. The method of claim 53, wherein the step of optimizing includes both of varying the position of the set of laser modes within the gain medium and varying the output wavelength of the semiconductor laser.

55. The method of claim 54, wherein the step of optimizing proceeds by varying the position of the set of laser modes to identify a local maximum in a power level within the laser system and varying the output wavelength of the semiconductor laser to identify a local maximum in the power level within the laser system.

56. The method of claim 53, wherein the step of optimizing includes both of varying the position of the set of laser modes within the gain medium and varying the size of the pump beam.

57. The method of claim 53, wherein the step of optimizing references the power level of a frequency doubled laser beam.

58. The method of claim 53, wherein the step of optimizing includes both of varying the output wavelength of the semiconductor laser and varying the size of the pump beam.

59. The method of claim 58, wherein the step of varying the size of the pump beam is accomplished by altering a separation between the semiconductor laser and the gain medium.

60. The method of claim 53, wherein the step of optimizing includes all the steps of varying the position of the set of laser modes within the gain medium, varying the output wavelength of the semiconductor laser, and varying the size of the pump beam.

61. The method of claim 55, wherein the step of optimizing references the power level of the TEM₀₀ mode.

62. The method of claim 56, wherein the step of varying the size of the pump beam is accomplished by altering a separation between the semiconductor laser and the gain medium.

63. The method of claim 56, wherein the step of optimizing proceeds by varying the position of the set of laser modes to identify a local maximum in a power level within the laser system and varying the size of the pump beam to identify a local maximum in the power level within the laser system.

64. The method of claim 58, wherein the step of optimizing proceeds by varying the output wavelength of the semiconductor laser to identify a local maximum in the power level within the laser system and varying the size of the pump beam to identify a local maximum in the power level within the laser system.

65. The method of claim 60, wherein the step of optimizing is performed for a gain medium having a stable thermal lens formed therein.

66. The method of claim 60, wherein the step of optimizing proceeds by varying the position of the set of laser modes to identify a local maximum in a power level within the laser system, varying the output wavelength of the semiconductor laser to identify a local maximum in the power level within the laser system, and varying the size of the pump beam to identify a local maximum in the power level within the laser system.

67. The method of claim 61, wherein the step of optimizing references the power level of a frequency doubled laser beam.

68. The method of claim 63, wherein the step of optimizing references the power level of the TEM₀₀ mode.

69. The method of claim 64, wherein the step of optimizing references the power level of the TEM₀₀ mode.

70. The method of claim 64, wherein the step of optimizing references the power level of a frequency doubled laser beam.

71. The method of claim 66, wherein the step of optimizing references the power level of the TEM₀₀ mode.

72. The method of claim 66, wherein the step of optimizing references the power level of a frequency doubled laser beam.

73. A method for manufacturing a laser system, comprising the steps of:

providing a lasing medium for producing a laser beam of a desired wavelength;

providing at least one diode laser for directing a pumping beam along a pumping optical path for side pumping the lasing medium and producing a population inversion in the lasing medium;

providing optical elements for forming a resonant cavity around the lasing medium, the optical elements defining a set of modes and for producing the laser beam, wherein the width of the set of modes is between about one-half and two times an absorption length of the lasing medium of a pumping wavelength;

locating the optical elements to position the set of modes within the lasing medium;

locating the at least one diode laser to supply the pumping beam to the lasing medium, wherein a wavelength of the pumping beam produced by the diode laser varies with temperature of the diode laser;

varying the temperature of the diode laser a plurality of times and noting an output power of the laser beam for each of the plurality of temperatures; and

fixing the temperature of the diode laser at a selected value so that the output power is at least 50% of a maximum output power as determined in the varying step.

74. A method for preparing a laser system for operation, comprising:

a. providing a laser system, comprising;

i. a lasing medium for producing a laser beam of a desired wavelength

ii. at least one diode laser for directing a pumping beam along a pumping optical path for side pumping the lasing medium and producing a population inversion in the lasing medium wherein the width of the set of modes is between about one-half and two times an absorption length of the lasing medium at a pumping wavelength; and

iii. providing optical elements for forming a resonant cavity around the lasing medium, the optical elements defining a Set of modes and for producing the laser beam,

wherein the optical elements are located to position the set of modes within the lasing medium;

wherein the at least one diode laser is spaced at a distance from a face of the lasing medium to supply the pumping beam to the lasing medium;

b. varying a temperature of the diode laser so as to varying a wavelength of the pumping beam and noting an output power of the laser beam for each of the plurality of temperatures; and

c. fixing the temperature of the diode laser to a selected value so that the output power is at least 50% of a maximum output power as determined in the varying step.

75. A solid state laser system comprising:

a laser cavity having an optical axis passing through the laser cavity;

a solid state gain medium having the optical axis pass therethrough and having at least one characteristic excitation wavelength and a width in a transverse direction, the solid state gain medium having an absorption depth at a pumping wavelength less than the width in the transverse direction, the absorption depth being less than the width in the transverse direction at a wavelength different from the characteristic excitation wavelength;

at least one semiconductor laser having an output at the pumping wavelength, the at least one semiconductor laser positioned to direct a beam of light to the solid state gain medium in a side pumping configuration,

the laser cavity defining a laser mode of the solid state laser positioned at a mode depth so that the laser mode is spaced from a surface of the gain medium.

76. A solid state laser comprising:

a laser cavity having an optical axis passing through the laser cavity;

a solid state gain medium having a width in a transverse direction, the solid state gain medium having a absorption depth at a pumping wavelength less than the width in the transverse direction, the optical axis passing through the solid state gain medium; and

at least one semiconductor laser having an output at the pumping wavelength, the at least one semiconductor laser positioned to direct a beam of light to the solid state gain medium in a side pumping configuration,

the laser cavity defining a laser mode of the solid state laser positioned at a mode depth so that the laser mode is spaced form a surface of the gain medium including a side face of the gain medium by a distance that reduces diffraction losses through the side face to allow at least ten percent optical-to-optical efficiency for the laser system.