CA2217055C - Compact laser apparatus and method - Google Patents
Compact laser apparatus and method Download PDFInfo
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- CA2217055C CA2217055C CA002217055A CA2217055A CA2217055C CA 2217055 C CA2217055 C CA 2217055C CA 002217055 A CA002217055 A CA 002217055A CA 2217055 A CA2217055 A CA 2217055A CA 2217055 C CA2217055 C CA 2217055C
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- 238000000034 method Methods 0.000 title claims abstract description 13
- 230000003287 optical effect Effects 0.000 claims abstract description 60
- 230000001427 coherent effect Effects 0.000 claims abstract description 20
- 239000013078 crystal Substances 0.000 claims abstract description 17
- 239000000758 substrate Substances 0.000 claims abstract description 16
- 230000009977 dual effect Effects 0.000 claims abstract description 8
- JNDMLEXHDPKVFC-UHFFFAOYSA-N aluminum;oxygen(2-);yttrium(3+) Chemical compound [O-2].[O-2].[O-2].[Al+3].[Y+3] JNDMLEXHDPKVFC-UHFFFAOYSA-N 0.000 claims description 17
- 229910019901 yttrium aluminum garnet Inorganic materials 0.000 claims description 17
- 239000006096 absorbing agent Substances 0.000 claims description 6
- WYOHGPUPVHHUGO-UHFFFAOYSA-K potassium;oxygen(2-);titanium(4+);phosphate Chemical compound [O-2].[K+].[Ti+4].[O-]P([O-])([O-])=O WYOHGPUPVHHUGO-UHFFFAOYSA-K 0.000 claims description 4
- 229910001430 chromium ion Inorganic materials 0.000 claims 1
- 238000006243 chemical reaction Methods 0.000 description 9
- 230000010355 oscillation Effects 0.000 description 6
- 238000001069 Raman spectroscopy Methods 0.000 description 5
- 210000001525 retina Anatomy 0.000 description 5
- 230000006378 damage Effects 0.000 description 4
- 229910052691 Erbium Inorganic materials 0.000 description 3
- UYAHIZSMUZPPFV-UHFFFAOYSA-N erbium Chemical compound [Er] UYAHIZSMUZPPFV-UHFFFAOYSA-N 0.000 description 3
- 210000004127 vitreous body Anatomy 0.000 description 3
- 229910052779 Neodymium Inorganic materials 0.000 description 2
- 238000004891 communication Methods 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- QEFYFXOXNSNQGX-UHFFFAOYSA-N neodymium atom Chemical compound [Nd] QEFYFXOXNSNQGX-UHFFFAOYSA-N 0.000 description 2
- 230000008569 process Effects 0.000 description 2
- 238000005086 pumping Methods 0.000 description 2
- 239000007787 solid Substances 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- 208000027418 Wounds and injury Diseases 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 230000005540 biological transmission Effects 0.000 description 1
- 230000000903 blocking effect Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000011651 chromium Substances 0.000 description 1
- 210000004087 cornea Anatomy 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 239000000284 extract Substances 0.000 description 1
- 231100000040 eye damage Toxicity 0.000 description 1
- 238000010304 firing Methods 0.000 description 1
- 238000005286 illumination Methods 0.000 description 1
- 208000014674 injury Diseases 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- HIQSCMNRKRMPJT-UHFFFAOYSA-J lithium;yttrium(3+);tetrafluoride Chemical compound [Li+].[F-].[F-].[F-].[F-].[Y+3] HIQSCMNRKRMPJT-UHFFFAOYSA-J 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 238000004476 mid-IR spectroscopy Methods 0.000 description 1
- 230000002028 premature Effects 0.000 description 1
- 230000005855 radiation Effects 0.000 description 1
- 230000035807 sensation Effects 0.000 description 1
- 239000000126 substance Substances 0.000 description 1
- 230000001629 suppression Effects 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
Classifications
-
- G—PHYSICS
- G02—OPTICS
- G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
- G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
- G02F1/35—Non-linear optics
- G02F1/39—Non-linear optics for parametric generation or amplification of light, infrared or ultraviolet waves
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08059—Constructional details of the reflector, e.g. shape
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
- H01S3/082—Construction or shape of optical resonators or components thereof comprising three or more reflectors defining a plurality of resonators, e.g. for mode selection or suppression
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/106—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity
- H01S3/108—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering
- H01S3/1083—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating by controlling devices placed within the cavity using non-linear optical devices, e.g. exhibiting Brillouin or Raman scattering using parametric generation
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
- H01S3/1123—Q-switching
- H01S3/113—Q-switching using intracavity saturable absorbers
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Optics & Photonics (AREA)
- Nonlinear Science (AREA)
- Engineering & Computer Science (AREA)
- Plasma & Fusion (AREA)
- General Physics & Mathematics (AREA)
- Lasers (AREA)
Abstract
A laser system includes an apparatus and method having a primary laser resonator having a laser medium therein for producing a laser beam of a first wavelength and a second laser resonator optically connected to the primary resonator to allow a portion of the laser energy from the primary laser resonator to pass into the secondary laser resonator. An optical parametric oscillator is located intracavity of the secondary laser resonator and includes a non-linear crystal for producing a laser beam of a second wavelength therefrom. The dual resonators combine a secondary laser cavity and an optical parametric oscillator to produce a predetermined output wavelength. The compact multiple resonator laser system has a substrate mirror system having four mirror surfaces thereon positioned to form two laser resonators. A multi-pass corner cube is mounted to fold the light beams between a pair of substrate mirrored surfaces while a transfer corner cube is positioned to transfer a laser beam from one resonator to the second resonator to form a very compact pair of laser resonators. A method of producing a coherent light beam of a predetermined output wavelength uses the compact laser system apparatus.
Description
COMPACT LASER APPARATUS AND METHOD
3 The present invention relates to a laser system 4 and more particularly to a miniature pulsed laser system using coupled resonator cavities and to a 6 method of producing a beam of coherent light which is 7 eyesafe.
8 Increase in the use of lasers in recent years has 9 produced a requirement for lasers of higher power that are safe for the human eye. The greater the power of 11 the laser, the more risk there is to people who may 12 come into contact with the laser beam when a coherent 13 beam of light enters the eye cornea and either passes 14 through or is absorbed by the vitreous humor. The portion of the beam that is not absorbed by the 16 vitreous humor is focused by the eye onto the retina.
17 Under normal conditions, the light energy is converted 18 by the retina into chemical energy to stimulate 19 optical sensation. Injury can result to the eye when the focused energy laser beam-cannot be absorbed and 21 causes damage to the retina. This damage does not 22 occur when conventional sources of illumination are 23 exposed to the eye because the light is emitted in all 24 directions and produces a sizeable but not a focused image on the retina that can be safely absorbed.
26 Laser beams having wavelengths in the range of 1.5 m 27 - 2.2 m are absorbed by the vitreous humor, thereby 28 alleviating damage to the retina. Laser systems used 29 as optical radar and communication transmitters in populated locations need to be operated so as to avoid 31 eye damage.
Lasers operating in the 1.5 um - 2.2 /tm wavelength have generally been of low efficiency and of larger size. Two available eyesafe lasers are based on laser emissions in erbium-doped solid state host materials pumped by pulsed gas discharge lamps or frequency conversion of a neodymium laser using stimulated raman scatter in a molecular gas, such as methane. These devices, however, have shortcomings. The erbium lasers typically have an efficiency of less than .1% owing to the low stimulated emission coefficient of the laser transition in erbium 3+ ion at a 1.54 m output and to the low efficiency for optical pumping with a visible flashlamp. The erbium laser can only be operated in a pulsed mode. Stimulated Raman conversion requires a cell containing a high pressure flammable gas. This gas is excited by the neodymium pumped laser to emit stimulated radiation in the eyesafe region. Raman conversion therefore is not amenable to continuous wave operation and the Raman process deposits energy in the conversion medium causing thermal distortion so that the eyesafe Raman laser cannot be conveniently operated at high average power or repetition rate.
An article in Optics Communications, Volume 75, No. 3,4 of March 1, 1990, entitled Generation of Tunable Mid-IR (1.8-2.4 1-1m) Laser From Optical Parametric Oscillation in KTP by J. T.
Lin and J. T. Montgomery, describes an optical laser system in which an Nd:YAG (neodymium-doped yttrium aluminum garnet) laser is used in an optical parametric oscillator setup where the pumping beam of YAG (yttrium aluminum garnet) laser pumps an optical parametric oscillator to produce an output in an eyesafe wavelength. Similarly, in U.S. Patent No.
8 Increase in the use of lasers in recent years has 9 produced a requirement for lasers of higher power that are safe for the human eye. The greater the power of 11 the laser, the more risk there is to people who may 12 come into contact with the laser beam when a coherent 13 beam of light enters the eye cornea and either passes 14 through or is absorbed by the vitreous humor. The portion of the beam that is not absorbed by the 16 vitreous humor is focused by the eye onto the retina.
17 Under normal conditions, the light energy is converted 18 by the retina into chemical energy to stimulate 19 optical sensation. Injury can result to the eye when the focused energy laser beam-cannot be absorbed and 21 causes damage to the retina. This damage does not 22 occur when conventional sources of illumination are 23 exposed to the eye because the light is emitted in all 24 directions and produces a sizeable but not a focused image on the retina that can be safely absorbed.
26 Laser beams having wavelengths in the range of 1.5 m 27 - 2.2 m are absorbed by the vitreous humor, thereby 28 alleviating damage to the retina. Laser systems used 29 as optical radar and communication transmitters in populated locations need to be operated so as to avoid 31 eye damage.
Lasers operating in the 1.5 um - 2.2 /tm wavelength have generally been of low efficiency and of larger size. Two available eyesafe lasers are based on laser emissions in erbium-doped solid state host materials pumped by pulsed gas discharge lamps or frequency conversion of a neodymium laser using stimulated raman scatter in a molecular gas, such as methane. These devices, however, have shortcomings. The erbium lasers typically have an efficiency of less than .1% owing to the low stimulated emission coefficient of the laser transition in erbium 3+ ion at a 1.54 m output and to the low efficiency for optical pumping with a visible flashlamp. The erbium laser can only be operated in a pulsed mode. Stimulated Raman conversion requires a cell containing a high pressure flammable gas. This gas is excited by the neodymium pumped laser to emit stimulated radiation in the eyesafe region. Raman conversion therefore is not amenable to continuous wave operation and the Raman process deposits energy in the conversion medium causing thermal distortion so that the eyesafe Raman laser cannot be conveniently operated at high average power or repetition rate.
An article in Optics Communications, Volume 75, No. 3,4 of March 1, 1990, entitled Generation of Tunable Mid-IR (1.8-2.4 1-1m) Laser From Optical Parametric Oscillation in KTP by J. T.
Lin and J. T. Montgomery, describes an optical laser system in which an Nd:YAG (neodymium-doped yttrium aluminum garnet) laser is used in an optical parametric oscillator setup where the pumping beam of YAG (yttrium aluminum garnet) laser pumps an optical parametric oscillator to produce an output in an eyesafe wavelength. Similarly, in U.S. Patent No.
5,181,211, issued January 19, 1993, (Burnham et al.) for an Eye-Safe Laser System, an Nd:YAG or Nd:YLF (neodymium-doped yttrium lithium fluoride) solid state laser is 1 used to produce a polarized output beam which is 2 passed through a non-linear crystal in _an optical 3 parametric oscillator to convert the wavelength of the 4 pump laser to a wavelength that isabsorbed by the human eye.
6 An optical parametric oscillator or OPO places a 7 non-linear crystal within a resonant optical cavity in 8 which mirrors transmit the pump wavelength from a 9 laser beam through a non-linear crystal, such as potassium titanyl phosphate or KTP. The non-linear 11 crystal can be rotated to change the output 12 wavelength. The existence of a resonant optical 13 cavity makes the parametric oscillator superficially 14 similar to lasers since they also generate a coherent beam. However, since there is no stimulated emission 16 within the parametric oscillator cavity, it does not 17 act as a laser simply because the parametric 18 oscillator is in a resonant optical cavity. The 19 oscillator can be brought within the laser cavity.
The use of a short pulse (<10 ns) Nd:YAG laser to pump 21 a non-critically phased matched KTP optical parametric 22 oscillator in the eyesafe region results in 23 unacceptably low conversion efficiencies, such as less 24 than ten percent. This low efficiency apparently was due to the short pump pulses. When the OPO was placed 26 intracavity to the pump laser, the conversion 27 efficiency increased but the output consisted of 28 multiple pulses rather than a clean single pulse 29 required for many applications.
The present laser system in contrast to the prior 31 art uses coupled laser cavities to maintain the high 32 efficiency of an intracavity system while at the same 33 time achieving a single pulsed output to thereby 34 overcome the problems of an extra cavity optical parametric oscillator used in combination with an Nd:YAG laser and also overcomes the shortcomings of placing the OPO
intracavity to the pump laser. The present laser system is very compact for placement in very small packages which compactness has been accomplished using a single common substrate mirror with four separately coated regions and a single corner cube to form two primary laser resonators. A second smaller corner cube is used to couple the resonators.
A typical optical parametric oscillator apparatus in which the OPO is external of the laser may be seen in U.S. Patent No.
4,180,751, issued December 25, 1979, (Ammann)which has a laser having a laser cavity mounted adjacent a second resonant cavity of an optical parametric oscillator with the laser being directed into the optical parametric oscillator. In U.S. Patent No. 5,195,104, issued March 16, 1993, (Geiger et al.) an internally stimulated optical parametric oscillator and laser places the optical parametric oscillator within the laser cavity to form a dual optical resonator containing a single optical parametric oscillator and laser crystal intracavity. A
frequency modified laser which places a non-linear crystal within the laser cavity can also be seen in the Anthon et al.
U.S. Patent No. 4,884,277, issued November 28, 1999.
SUMMARY OF THE INVENTION
A laser system includes an apparatus and method having a primary laser resonator having a laser medium therein for producing a laser beam of a first wavelength and a second laser resonator optically connected to the primary resonator to allow a portion of the laser energy from the primary laser resonator to pass into the secondary laser resonator. An optical parametric oscillator is located intracavity of the secondary laser resonator and includes a non-linear crystal for producing a laser beam of a second wavelength therefrom. A coherent beam output is coupled to the optical parametric oscillator for 5 producing an output beam of predetermined wavelength of the second wavelength while blocking the output of the laser beam of the first wavelength so that a dual resonator combines a secondary laser cavity and an optical parametric oscillator to produce a predetermined output wavelength. The compact multiple resonator laser system has a substrate mirror system having four mirror surfaces thereon positioned to form two laser resonators. A multi-pass corner cube is mounted to fold the light beams between a pair of substrate mirrored surfaces while a transfer corner cube is positioned to transfer a laser beam from one resonator to the second resonator to form a very compact pair of laser resonators. One of the laser resonators is a dual resonator forming both the laser resonator and an optical parametric oscillator resonator. A method of producing a coherent light beam of a predetermined output wavelength uses the compact laser system apparatus.
In accordance with one aspect of the present invention, there is provided, a coupled cavity type laser system comprising a primary laser resonator having a lasing medium therein for producing a laser beam of a first wavelength; a secondary laser resonator optically coupled to said primary resonator with a folding corner cube to allow a portion of the laser energy from said primary laser resonator to pass into said secondary laser resonator; an optical parametric oscillator located intracavity of said secondary laser resonator which also serves as an optical parametric oscillator resonator, said optical parametric oscillator having 5a a nonlinear crystal for producing a laser beam of a second wavelength therefrom; and a coherent beam output coupled to the optical parametric oscillator for producing an output beam of a predetermined wavelength of the optical parametric oscillator resonator of the second wavelength, and to block the output of the laser beam of a first wavelength whereby the secondary laser resonator and the optical parametric oscillator resonator produce the predetermined output wavelength.
In accordance with another aspect of the present invention, there is provided a method of producing a coherent beam of light having a predetermined output wavelength comprising the steps of producing a laser beam of a first wavelength in an active laser resonator having a laser medium therein; directing the laser beam of a first wavelength through a mirror of the active laser resonator, which mirror is partially reflective to the first wavelength, and through a transfer corner cube from the active laser resonator to a passive laser resonator; producing a coherent light beam of a second wavelength with an optical parametric oscillator located intracavity of the passive laser resonator which also serves as a further resonator for the optical parametric oscillator and pumped by the laser beam of the first wavelength; outputting the coherent light beam of the second wavelength from the optical parametric oscillator whereby the passive and the further laser resonators having the intracavity optical parametric oscillator producing an output beam of the second wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features, and advantages of the present invention will be apparent from the written description and the drawings in which:
Figure 1 is a optical schematic of a laser system in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is for a miniature pulsed laser capable of producing 5-10 mJ at about 1.58 ,um and utilizes an intracavity optical parametric oscillator in a unique coupled cavity design to achieve high efficiency without multiple pulsing in a compact mechanically and optically stable package.
The compact package is less than 75 cubic centimeters and may have an eyesafe wavelength of 1.5 um - 1.6 ,um capable of generating 5-10 ns pulses for use in handheld rangefinders and the like. The output of a miniature 1.064 ;um Nd:YAG laser is shifted into the eyesafe region with a non-critical phased matched potassium titanyl phosphate (KTP) optical parametric oscillator (OPO). The coupled cavity design of Figure 1 maintains the high efficiency of an intracavity device while at the same time achieving a single pulsed output. This coupled cavity laser system meets the efficiency and output requirements and allows the laser transmitter to be very compact.
Referring to the schematic of Figure 1, the overall laser transmitter 10 includes a primary 1.06 ,um resonator 11 containing a Nd:YAG rod 12 pumped by a flashlamp 19 along with a pair of steering wedges 13 and a Q-switch 14, which is illustrated as a chromium doped YAG saturable absorber. A
polarizing element 15 may be a brewster plate. These elements are mounted inside a resonator formed by the 1.064m decoupling region of a common substrate mirror 16 having the substrate 17 having a 30% reflectance of 1.06 4m mirror 18 mounted on the substrate 17 and a 100% 1.064m reflecting mirror 20 mounted on the substrate to form a resonance cavity between the mirrored surfaces 20 and 18. The laser rod 12 is in the beam path, illustrated as 21, which is folded by the multi-pass corner cube 22 providing folding surfaces to fold the beam 21. Thus, the laser rod 12 is pumped by an optical flashlamp 19 to produce the lasing action within the resonant cavity between the mirrored surfaces 18 and 20. The beam passes through the 30% reflecting surface 18 where the beam is folded by the transfer corner cube 25 and passes back through the substrate 17 and through a mirrored surface portion 26 which has a zero reflectance for the 1.06 ,um.
The 1.06 ,um laser beam passes into a secondary 1.06 ,um resonator 27 where it is folded by the multi-pass corner cube 22 back to a fourth mirrored surface area 31 on the common substrate 17. Mirror 31 has an 80% reflectance of 1.06 ,um wavelength and allows 20% to pass therethrough into the 1.06 ,um dump 32. Since the mirror 31 is reflecting 80% of the 1.06 m, a portion of the energy is passed back through the secondary resonant cavity 27 and back through the mirrored surface 26 through the corner cube 25 where a portion of the energy passes through the mirrored surface 18 while a portion of it is reflected back into the secondary cavity.
The secondary cavity 27 forms a dual optical resonator which is both a secondary laser resonator and an optical parametric oscillator resonator. The optical parametric oscillator is formed by having the potassium titanyl phosphate (or KTP) crystal 33 within the beam path within the cavity 27. This OPO resonator is a 1.58 ,um resonator 7a containing the down scope 34 along with a pair of steering wedges 35 and a KTP crystal 33. The resonator is formed by the 1.58 /.tm outcoupling region of the common substrate mirror 1 26 which is 50o reflective of the 1.58 m beam but 2 transparent to the 1.06 m beam. The 1.58 m mirror 3 31 reflects 100% of the energy while reflecting only 4 80% of the 1.06 m beam. Similarl.y, the 1.58 m resonator also uses the multi-pass corner cube 22.
6 The 1.58 m output from the dual resonant cavity 7 27 passes through the mirrored portion 26 and is 8 coupled out of the system as the energy passes through 9 the transfer corner cube 25 and impinges on the dichroic beam splitter 36. The dichroic beam splitter 11 reflects the entire 1.58 m energy along the path 37 12 where it impinges against a second dichroic beam 13 splitter 38 to produce a 1.58 Am output 40. Thus, the 14 output from the 1.58 m resonator is produced from the output of the laser while the 1.06 m energy passes 16 back through the dichroic beam splitter 36 and a 17 portion of which passes through the mirrored surface 18 18 while a portion is reflected back through the 19 beam splitter 36, transfer corner cube and back into the secondary laser cavity 27.
21 The operation of the laser resonator is as 22 follows: Firing of the flashlamp causes the gain to 23 begin to buildup in the Nd:YAG rod 12. Initially, the 24 single pass loss of the saturable absorber 14 is high and this loss combined with the out coupling mirror 18 26 losses prevents the buildup of laser oscillation.
27 Eventually, the round trip gain exceeds the round trip 28 losses and the 1.06 m field begins to grow in both 29 primary 1.06 m cavity 11 and the secondary 1.06 m cavity 27. The feedback from the mirror in the 31 secondary 1.06 m resonator 27 lowers the threshold at 32 which the 1.06 m oscillation will begin. As the 1.06 33 m field grows, the absorption loss due to the 34 saturable absorber 14 begins to saturate allowing yet more growth of the 1.06 ,um field. The cycle continues until the saturable absorber transmission has increased significantly and a Q-switch 1.06 gm pulse has begun to develop. As the 1.06 ,um field in the secondary resonator 27 grows, it eventually reaches a level at which it begins to be converted by the KTP
crystal 33 into two longer wavelengths. The crystal 33 angle determines what two wavelengths are generated by the crystal 33 and the angle has been chosen in the present crystal, such as to produce wavelengths of 1.58 m and 3.26 um. In a pure intracavity optical parametric oscillator, the nonlinear conversion of the 1.06 ,um field to the longer wavelengths causes the 1.06 ,um field to be depleted and to cease oscillation before the stored energy in the Nd:YAG rod has been fully extracted. This residual stored energy can result in one or more secondary 1.06 ,um and 1.58 m pulses. By placing the OPO in a secondary 1.06 ,um cavity that does not contain the 1.06 m gain medium, the non-linear conversion process does not directly interact with the 1.06 m field that extracts the stored energy in the Nd:YAG rod 12. This allows the 1.06 ,um oscillation to continue in the primary cavity 11 even as the 1.06 m field in the secondary cavity 27 begins to be depleted.
The net result is a suppression of premature termination of the 1.06 ,um oscillation. That leads to a significantly reduced tendency for secondary pulsing and increased conversion of 1.06 m pump to eyesafe 1.58 ,um output.
The key to the compactness of the laser 10 is the use of a dual path corner cube 22 along with a transfer corner cube 25 and a single common substrate mirror 16 having the four mirrored surfaces thereon to 1 form all of the optical resonators, as shown in Figure 2 1. In actual practices, it has found that the present 3 laser transmitter can be placed in a total volume of 4 less than 75 cubic centimeters to produce an output in 5 excess of 6 mJ of output at 1.58 m. Thus, the output 6 energy of greater than 5 mJ per pulse at 1.58 m is 7 achieved with the efficiency of intracavity OPO
8 without the multiple pulsing problem experienced with 9 an intracavity laser OPO. In addition, the use of a 10 common substrate mirror 16 and the multi-pass corner 11 cube 22 along with the transfer corner cube 25 to form 12 the coupled resonator system results in an overall 13 miniaturization of a laser transmitter to a very small 14 volume of space and also allows for a high degree of alignment stability over extreme temperature and 16 vibration environments. The illustrated laser 17 transmitter of Figure 1, however, should not be 18 considered as limited to the schematic shown but 19 should be considered illustrative rather than restrictive.
The use of a short pulse (<10 ns) Nd:YAG laser to pump 21 a non-critically phased matched KTP optical parametric 22 oscillator in the eyesafe region results in 23 unacceptably low conversion efficiencies, such as less 24 than ten percent. This low efficiency apparently was due to the short pump pulses. When the OPO was placed 26 intracavity to the pump laser, the conversion 27 efficiency increased but the output consisted of 28 multiple pulses rather than a clean single pulse 29 required for many applications.
The present laser system in contrast to the prior 31 art uses coupled laser cavities to maintain the high 32 efficiency of an intracavity system while at the same 33 time achieving a single pulsed output to thereby 34 overcome the problems of an extra cavity optical parametric oscillator used in combination with an Nd:YAG laser and also overcomes the shortcomings of placing the OPO
intracavity to the pump laser. The present laser system is very compact for placement in very small packages which compactness has been accomplished using a single common substrate mirror with four separately coated regions and a single corner cube to form two primary laser resonators. A second smaller corner cube is used to couple the resonators.
A typical optical parametric oscillator apparatus in which the OPO is external of the laser may be seen in U.S. Patent No.
4,180,751, issued December 25, 1979, (Ammann)which has a laser having a laser cavity mounted adjacent a second resonant cavity of an optical parametric oscillator with the laser being directed into the optical parametric oscillator. In U.S. Patent No. 5,195,104, issued March 16, 1993, (Geiger et al.) an internally stimulated optical parametric oscillator and laser places the optical parametric oscillator within the laser cavity to form a dual optical resonator containing a single optical parametric oscillator and laser crystal intracavity. A
frequency modified laser which places a non-linear crystal within the laser cavity can also be seen in the Anthon et al.
U.S. Patent No. 4,884,277, issued November 28, 1999.
SUMMARY OF THE INVENTION
A laser system includes an apparatus and method having a primary laser resonator having a laser medium therein for producing a laser beam of a first wavelength and a second laser resonator optically connected to the primary resonator to allow a portion of the laser energy from the primary laser resonator to pass into the secondary laser resonator. An optical parametric oscillator is located intracavity of the secondary laser resonator and includes a non-linear crystal for producing a laser beam of a second wavelength therefrom. A coherent beam output is coupled to the optical parametric oscillator for 5 producing an output beam of predetermined wavelength of the second wavelength while blocking the output of the laser beam of the first wavelength so that a dual resonator combines a secondary laser cavity and an optical parametric oscillator to produce a predetermined output wavelength. The compact multiple resonator laser system has a substrate mirror system having four mirror surfaces thereon positioned to form two laser resonators. A multi-pass corner cube is mounted to fold the light beams between a pair of substrate mirrored surfaces while a transfer corner cube is positioned to transfer a laser beam from one resonator to the second resonator to form a very compact pair of laser resonators. One of the laser resonators is a dual resonator forming both the laser resonator and an optical parametric oscillator resonator. A method of producing a coherent light beam of a predetermined output wavelength uses the compact laser system apparatus.
In accordance with one aspect of the present invention, there is provided, a coupled cavity type laser system comprising a primary laser resonator having a lasing medium therein for producing a laser beam of a first wavelength; a secondary laser resonator optically coupled to said primary resonator with a folding corner cube to allow a portion of the laser energy from said primary laser resonator to pass into said secondary laser resonator; an optical parametric oscillator located intracavity of said secondary laser resonator which also serves as an optical parametric oscillator resonator, said optical parametric oscillator having 5a a nonlinear crystal for producing a laser beam of a second wavelength therefrom; and a coherent beam output coupled to the optical parametric oscillator for producing an output beam of a predetermined wavelength of the optical parametric oscillator resonator of the second wavelength, and to block the output of the laser beam of a first wavelength whereby the secondary laser resonator and the optical parametric oscillator resonator produce the predetermined output wavelength.
In accordance with another aspect of the present invention, there is provided a method of producing a coherent beam of light having a predetermined output wavelength comprising the steps of producing a laser beam of a first wavelength in an active laser resonator having a laser medium therein; directing the laser beam of a first wavelength through a mirror of the active laser resonator, which mirror is partially reflective to the first wavelength, and through a transfer corner cube from the active laser resonator to a passive laser resonator; producing a coherent light beam of a second wavelength with an optical parametric oscillator located intracavity of the passive laser resonator which also serves as a further resonator for the optical parametric oscillator and pumped by the laser beam of the first wavelength; outputting the coherent light beam of the second wavelength from the optical parametric oscillator whereby the passive and the further laser resonators having the intracavity optical parametric oscillator producing an output beam of the second wavelength.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features, and advantages of the present invention will be apparent from the written description and the drawings in which:
Figure 1 is a optical schematic of a laser system in accordance with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is for a miniature pulsed laser capable of producing 5-10 mJ at about 1.58 ,um and utilizes an intracavity optical parametric oscillator in a unique coupled cavity design to achieve high efficiency without multiple pulsing in a compact mechanically and optically stable package.
The compact package is less than 75 cubic centimeters and may have an eyesafe wavelength of 1.5 um - 1.6 ,um capable of generating 5-10 ns pulses for use in handheld rangefinders and the like. The output of a miniature 1.064 ;um Nd:YAG laser is shifted into the eyesafe region with a non-critical phased matched potassium titanyl phosphate (KTP) optical parametric oscillator (OPO). The coupled cavity design of Figure 1 maintains the high efficiency of an intracavity device while at the same time achieving a single pulsed output. This coupled cavity laser system meets the efficiency and output requirements and allows the laser transmitter to be very compact.
Referring to the schematic of Figure 1, the overall laser transmitter 10 includes a primary 1.06 ,um resonator 11 containing a Nd:YAG rod 12 pumped by a flashlamp 19 along with a pair of steering wedges 13 and a Q-switch 14, which is illustrated as a chromium doped YAG saturable absorber. A
polarizing element 15 may be a brewster plate. These elements are mounted inside a resonator formed by the 1.064m decoupling region of a common substrate mirror 16 having the substrate 17 having a 30% reflectance of 1.06 4m mirror 18 mounted on the substrate 17 and a 100% 1.064m reflecting mirror 20 mounted on the substrate to form a resonance cavity between the mirrored surfaces 20 and 18. The laser rod 12 is in the beam path, illustrated as 21, which is folded by the multi-pass corner cube 22 providing folding surfaces to fold the beam 21. Thus, the laser rod 12 is pumped by an optical flashlamp 19 to produce the lasing action within the resonant cavity between the mirrored surfaces 18 and 20. The beam passes through the 30% reflecting surface 18 where the beam is folded by the transfer corner cube 25 and passes back through the substrate 17 and through a mirrored surface portion 26 which has a zero reflectance for the 1.06 ,um.
The 1.06 ,um laser beam passes into a secondary 1.06 ,um resonator 27 where it is folded by the multi-pass corner cube 22 back to a fourth mirrored surface area 31 on the common substrate 17. Mirror 31 has an 80% reflectance of 1.06 ,um wavelength and allows 20% to pass therethrough into the 1.06 ,um dump 32. Since the mirror 31 is reflecting 80% of the 1.06 m, a portion of the energy is passed back through the secondary resonant cavity 27 and back through the mirrored surface 26 through the corner cube 25 where a portion of the energy passes through the mirrored surface 18 while a portion of it is reflected back into the secondary cavity.
The secondary cavity 27 forms a dual optical resonator which is both a secondary laser resonator and an optical parametric oscillator resonator. The optical parametric oscillator is formed by having the potassium titanyl phosphate (or KTP) crystal 33 within the beam path within the cavity 27. This OPO resonator is a 1.58 ,um resonator 7a containing the down scope 34 along with a pair of steering wedges 35 and a KTP crystal 33. The resonator is formed by the 1.58 /.tm outcoupling region of the common substrate mirror 1 26 which is 50o reflective of the 1.58 m beam but 2 transparent to the 1.06 m beam. The 1.58 m mirror 3 31 reflects 100% of the energy while reflecting only 4 80% of the 1.06 m beam. Similarl.y, the 1.58 m resonator also uses the multi-pass corner cube 22.
6 The 1.58 m output from the dual resonant cavity 7 27 passes through the mirrored portion 26 and is 8 coupled out of the system as the energy passes through 9 the transfer corner cube 25 and impinges on the dichroic beam splitter 36. The dichroic beam splitter 11 reflects the entire 1.58 m energy along the path 37 12 where it impinges against a second dichroic beam 13 splitter 38 to produce a 1.58 Am output 40. Thus, the 14 output from the 1.58 m resonator is produced from the output of the laser while the 1.06 m energy passes 16 back through the dichroic beam splitter 36 and a 17 portion of which passes through the mirrored surface 18 18 while a portion is reflected back through the 19 beam splitter 36, transfer corner cube and back into the secondary laser cavity 27.
21 The operation of the laser resonator is as 22 follows: Firing of the flashlamp causes the gain to 23 begin to buildup in the Nd:YAG rod 12. Initially, the 24 single pass loss of the saturable absorber 14 is high and this loss combined with the out coupling mirror 18 26 losses prevents the buildup of laser oscillation.
27 Eventually, the round trip gain exceeds the round trip 28 losses and the 1.06 m field begins to grow in both 29 primary 1.06 m cavity 11 and the secondary 1.06 m cavity 27. The feedback from the mirror in the 31 secondary 1.06 m resonator 27 lowers the threshold at 32 which the 1.06 m oscillation will begin. As the 1.06 33 m field grows, the absorption loss due to the 34 saturable absorber 14 begins to saturate allowing yet more growth of the 1.06 ,um field. The cycle continues until the saturable absorber transmission has increased significantly and a Q-switch 1.06 gm pulse has begun to develop. As the 1.06 ,um field in the secondary resonator 27 grows, it eventually reaches a level at which it begins to be converted by the KTP
crystal 33 into two longer wavelengths. The crystal 33 angle determines what two wavelengths are generated by the crystal 33 and the angle has been chosen in the present crystal, such as to produce wavelengths of 1.58 m and 3.26 um. In a pure intracavity optical parametric oscillator, the nonlinear conversion of the 1.06 ,um field to the longer wavelengths causes the 1.06 ,um field to be depleted and to cease oscillation before the stored energy in the Nd:YAG rod has been fully extracted. This residual stored energy can result in one or more secondary 1.06 ,um and 1.58 m pulses. By placing the OPO in a secondary 1.06 ,um cavity that does not contain the 1.06 m gain medium, the non-linear conversion process does not directly interact with the 1.06 m field that extracts the stored energy in the Nd:YAG rod 12. This allows the 1.06 ,um oscillation to continue in the primary cavity 11 even as the 1.06 m field in the secondary cavity 27 begins to be depleted.
The net result is a suppression of premature termination of the 1.06 ,um oscillation. That leads to a significantly reduced tendency for secondary pulsing and increased conversion of 1.06 m pump to eyesafe 1.58 ,um output.
The key to the compactness of the laser 10 is the use of a dual path corner cube 22 along with a transfer corner cube 25 and a single common substrate mirror 16 having the four mirrored surfaces thereon to 1 form all of the optical resonators, as shown in Figure 2 1. In actual practices, it has found that the present 3 laser transmitter can be placed in a total volume of 4 less than 75 cubic centimeters to produce an output in 5 excess of 6 mJ of output at 1.58 m. Thus, the output 6 energy of greater than 5 mJ per pulse at 1.58 m is 7 achieved with the efficiency of intracavity OPO
8 without the multiple pulsing problem experienced with 9 an intracavity laser OPO. In addition, the use of a 10 common substrate mirror 16 and the multi-pass corner 11 cube 22 along with the transfer corner cube 25 to form 12 the coupled resonator system results in an overall 13 miniaturization of a laser transmitter to a very small 14 volume of space and also allows for a high degree of alignment stability over extreme temperature and 16 vibration environments. The illustrated laser 17 transmitter of Figure 1, however, should not be 18 considered as limited to the schematic shown but 19 should be considered illustrative rather than restrictive.
Claims (17)
1. A coupled cavity type laser system comprising:
a primary laser resonator having a lasing medium therein for producing a laser beam of a first wavelength;
a secondary laser resonator optically coupled to said primary resonator with a folding corner cube to allow a portion of the laser energy from said primary laser resonator to pass into said secondary laser resonator;
an optical parametric oscillator located intracavity of said secondary laser resonator which also serves as an optical parametric oscillator resonator, said optical parametric oscillator having a nonlinear crystal for producing a laser beam of a second wavelength therefrom; and a coherent beam output coupled to said optical parametric oscillator for producing an output beam of a predetermined wavelength of said optical parametric oscillator resonator of said second wavelength, and to block the output of said laser beam of a first wavelength whereby the secondary laser resonator and the optical parametric oscillator resonator produce the predetermined output wavelength.
a primary laser resonator having a lasing medium therein for producing a laser beam of a first wavelength;
a secondary laser resonator optically coupled to said primary resonator with a folding corner cube to allow a portion of the laser energy from said primary laser resonator to pass into said secondary laser resonator;
an optical parametric oscillator located intracavity of said secondary laser resonator which also serves as an optical parametric oscillator resonator, said optical parametric oscillator having a nonlinear crystal for producing a laser beam of a second wavelength therefrom; and a coherent beam output coupled to said optical parametric oscillator for producing an output beam of a predetermined wavelength of said optical parametric oscillator resonator of said second wavelength, and to block the output of said laser beam of a first wavelength whereby the secondary laser resonator and the optical parametric oscillator resonator produce the predetermined output wavelength.
2. A laser system in accordance with claim 1 in which said laser system includes a common substrate mirror having a plurality of mirrored surfaces each having a predetermined reflectance for predetermined wavelengths.
3. A laser system in accordance with claim 1 in which said primary laser resonator includes a saturable absorber located intracavity.
4. A laser system in accordance with claim 1 in which said laser system includes a single corner cube optically forming said primary laser resonator and said secondary laser resonator.
5. A laser system in accordance with claim 1 in which said optical parametric oscillator includes a downscope.
6. A laser system in accordance with claim 1 in which said primary laser resonator includes a polarizing element.
7. A laser system in accordance with claim 1 in which said primary laser resonator includes a steering wedge pair therein for beam alignment.
8. A laser system in accordance with claim 1 in which said optical parametric oscillator includes a steering wedge pair therein for beam alignment.
9. A laser system in accordance with claim 1 in which said primary laser resonator active medium is Nd:YAG
(neodymium-doped yttrium aluminum garnet) and produces an output wavelength of 1.06 µm.
(neodymium-doped yttrium aluminum garnet) and produces an output wavelength of 1.06 µm.
10. A laser system in accordance with claim 2 in which said common substrate mirror has 1st, 2nd, 3rd and 4th mirror surfaces thereon, each mirror having a predefined reflectance to a predetermined wavelength.
11. A laser system in accordance with claim 1 in which said optical parametric oscillator includes KTP (potassium titanyl phosphate) crystal tuned for 1.58 µm wavelength output.
12. A laser system in accordance with claim 1 in which said secondary laser resonator has an extra-cavity first wavelength dump.
13. A laser system in accordance with claim 1 in which said coherent beam output includes a dichroic beam splitter which reflects said second wavelength without reflecting said first wavelength.
14. A laser system in accordance with claim 3 in which said saturable absorber is a chromium ion doped YAG (yttrium aluminum garnet).
15. A method of producing a coherent beam of light having a predetermined output wavelength comprising the steps of:
producing a laser beam of a first wavelength in an active laser resonator having a laser medium therein;
directing said laser beam of a first wavelength through a mirror of said active laser resonator, which mirror is partially reflective to said first wavelength, and through a transfer corner cube from said active laser resonator to a passive laser resonator;
producing a coherent light beam of a second wavelength with an optical parametric oscillator located intracavity of said passive laser resonator which also serves as a further resonator for the optical parametric oscillator and pumped by said laser beam of said first wavelength;
outputting said coherent light beam of said second wavelength from said optical parametric oscillator whereby the passive and the further laser resonators having the intracavity optical parametric oscillator producing an output beam of the second wavelength.
producing a laser beam of a first wavelength in an active laser resonator having a laser medium therein;
directing said laser beam of a first wavelength through a mirror of said active laser resonator, which mirror is partially reflective to said first wavelength, and through a transfer corner cube from said active laser resonator to a passive laser resonator;
producing a coherent light beam of a second wavelength with an optical parametric oscillator located intracavity of said passive laser resonator which also serves as a further resonator for the optical parametric oscillator and pumped by said laser beam of said first wavelength;
outputting said coherent light beam of said second wavelength from said optical parametric oscillator whereby the passive and the further laser resonators having the intracavity optical parametric oscillator producing an output beam of the second wavelength.
16. A method of producing a coherent beam of light having a predetermined output wavelength in accordance with claim 15 in which the step of producing said coherent light beam of a second wavelength includes producing said coherent light beam from a dual resonator having a passive laser resonator and said optical parametric oscillator resonator.
17. A method of producing a coherent beam of light having a predetermined output wavelength in accordance with claim 15 in which the step of producing said laser beam of a first wavelength produces said beam having a wavelength of 1.06 µm and the step of producing said coherent light beam of a second wavelength produces a light beam having a wavelength of 1.58 µm.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002543706A CA2543706A1 (en) | 1995-03-31 | 1996-03-29 | Compact multiple resonator laser system |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US08/414,530 US5608744A (en) | 1995-03-31 | 1995-03-31 | Compact laser apparatus and method |
| US414,530 | 1995-03-31 | ||
| PCT/US1996/004363 WO1996030975A1 (en) | 1995-03-31 | 1996-03-29 | Compact laser apparatus and method |
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| CA002543706A Division CA2543706A1 (en) | 1995-03-31 | 1996-03-29 | Compact multiple resonator laser system |
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