US5365539_TOT

US5365539_TOT - United States Patent[19 Mooradian...

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Unformatted text preview: United States Patent [19] Mooradian U8005365539A [11] Patent Number: 5,365,539 [45] Date of Patent: * Nov. 15,1994 [54] MICROCHIP LASER [75} Inventor: Aram Mooradian, Winchester, Mass. Massachusetts Institute of Technology, Cambridge, Mass. [73] Assignee: [ * ] Notice: The portion of the term of this patent subsequent to Aug. 22, 2006 has been disclaimed. [21] Appl. No.: 140,616 [22] Filed: Oct. 22, 1993 Related U.S. Application Data [60] Continuation of Ser. No. 13,562, Feb. 3, 1993, Pat. No. 5,265,116, which is a continuation of Ser. No. 999,577, Dec. 30, 1992, Pat. No. 5,256,164, which is a continua- tion of Ser. No. 512,981, Apr. 23, 1990, abandoned, which is a division of Ser. No. 308,251, Feb. 9, 1989, Pat. No. 4,953,166, which is a continuation—impart of Ser. No. 151,396, Feb. 2, 1988, Pat. No. 4,860,304. [51] Int. c1.5 ............................................ .. HOIS 3/091 [52] U.S. Cl. ...................................... .. 372/75; 372/70; 372/71; 372/92; 372/21; 372/32; 437/129 [58] Field of Search ..................... .. 372/75, 70, 71, 72, 372/92, 21, 32, 437/129 [56] References Cited U.S. PATENT DOCUMENTS 3,808,549 4/ 1974 Maurer .......................... .. 33 1/ 94.5 3,949,320 4/1976 Castleberry et al. .. 331/945 4,002,725 1/ 1977 Bridenbaugh et a1. .. 423/263 4,734,912 3/1988 Scerbak et a1. .............. .. 372/27 4,739,507 4/1988 Byer et a1. . . . . . . . . . . . .. 372/22 4,797,893 1/ 1989 Dixon . . . . . . . . . . . . . . . . .. 372/66 4,847,851 7/ 1989 Dixon . . . . . . . . . . . . . . . . .. 372/75 4,860,304 8/1989 Mooradian . . . . . . . . . . . .. 372/92 4,901,330 2/1990 Wolfram et al. . .... .. 372/75 4,953,166 8/1990 Mooradian . . . . . . . . . . . .. 372/21 5,022,043 6/ 1991 Jacobs ............ .. 372/75 5,062,117 10/1991 Anthon et a1. .... .. 372/75 5,265,116 11/1993 Mooradian .......................... .. 372/75 FOREIGN PATENT DOCUMENTS PCT/US90/- 00644 2/ 1990 European Pat. Off. . 1456603 10/1966 France . 61-10208] 5/1986 Japan. OTHER PUBLICATIONS Hatori, M., “Light Wavelength Conversion Module”, JP62-19094l, Patent Abstracts of Japan, 13(222):876 (1989). Kriihler, et 21., “Cw Oscillation at 1.05 and 1.32 pm of LiNd(PO3)4 Lasers in External Resonator and in Reso- . nator with Directly Applied Mirrors”, Applied Physics, (20):329—34 (1979). r Zayhowski, et a1., “Single—frequency Microchip Nd Lasers”, Optics Letters, 14(1):24—26 (1989). Toshiaki, M., “Solid Laser Device”, JP53—103395, Pa- tent Abstracts of Japan, 2(134) (E—070) (1978). Yoshiyuki, A., “Light—Waveguide Type Laser”, JP—A—63—029986, Patent Abstracts of Japan, 12(239) (E—630) (1988). (List continued on next page.) Primary Examiner—Georgia Y. Epps Attorney, Agent, or Firm—Hamilton, Brook, Smith & Reynolds [57} ABSTRACT A gain medium is disposed between two mirrors to form a resonant cavity. The cavity length is selected so that the gain bandwidth of the gain medium is less than or substantially equal to the frequency separation of the cavity modes and such that a cavity mode frequency falls within the gain bandwidth. A nonlinear optical material is disposed either inside or outside the cavity to generate new laser wavelengths. The nonlinear optical material may be contained in a cavity which is resonant at the microchip laser frequency. Alternatively, the microchip laser may be tuned, for example thermally or by the application of a longitudinal or transverse stress, to the frequency of the resonant cavity. The laser is optically pumped by any appropriate source such as a semiconductor injection laser or laser array. Suitable gain media include Nd:YAG, NdeSGG and Nd penta- phosphate, and suitable non-linear optical material in- clude MgO:LiNbO3 and KTP. 26 Claims, 7 Drawing Sheets 5,365,539 Page 2 OTHER PUBLICATIONS Damen, et al., “Picosecond Pulses from an Optically Pumped GaAs Laser”, Proceedings of the Second In- ternational Conference on Picoecond Phenomena, 18 Jun. 1980, Cape Cod, Mass. (Abstract). Draegert, D. A., “Single—Diode End—Pumped Nd:YAG Laser”, IEEE Journal of Quantum Electronics, QE—9(12) (1973). Kane, et al., “Frequency stability and offset locking of a laserwdiode—pumped Nd:YAG monolithic nonplanner ring oscillator”, Optics Letters, 12, pp. 175—177 (1987). Castleberry, et al., “A Single Mode 2.06 Um Miniature Laser”, Digest of Technical Papers, Jan. 21—24, 1974, MB7, pp. 1-4. Saruwatari, et al., “Electroluminescent Diode Pumped Miniaturized LiNd P4012 Lasers”, Review of the Electri- cal Communication Laboratories, 26(9—10)21111—1128 (Sep.—Oct. 1978). Owyoung, et al., “Stress—induced tuning of a di- ode—laser—excited monolithic Nd:YAG laser”, Optics Letters, 12(12):999—1001 (Dec. 1987). Zhou, et al., “Efficient, frequency—stable laser—di- ode—pumped Nd:YAG laser”, Optics Letters, 10(2):62—64 (Feb. 1985). Kubodera, et al., “Pure single—mode LiNdP4012 solid—state laser transmitter for 1.3 pm fiber—optic com- munications”, Applied Optics, 21(19):3466—2469. Kubodera, et al., “Efficient LiNdP4012 lasers pumped with a laser diode”, Applied Optics, 18(23):3882—3883 (Dec. 1979). Winzer, et al., “Laser Emission from Miniaturized NdA13 (BO3) 4 Crystals with Directly Applied Mir- rors”, IEEE, pp. 840-843 (1978). Owyoung, et al., “Gain switching of a monolithic sin- gle—frequency laser—diode—excited Nd:YAG laser”, Optics Letters, 10(10):484—486 (Oct. 1985). Stone, et al., “Self—Contained LED—Pumped Single— Crystal Nd:YAG Fiber Laser”, Fiber and Integrated Optics, 2(1):19—46 (1979). Moordian, “Laser Linewidth”, Physics Today, pp. 2—7 (May 1975). Fleming et al., “Spectral characteristics of exter- nal—cavity controlled semiconductor lasers”, IEEE, pp. 44—59 (1981). Svelto, “Principles of Lasers”, Plenum Press, NY, pp. 171—179 (1976). Smith, “Stabilized, Single—Frequency Output from a Long Laser Cavity”, IEEE J. Quantum Elec., QE—l (8):343—48 (Nov. 1965). . Kozlovsky, Wm. J ., et al., “Efficient Second Harmonic Generation of a Diode—Laser—Pumped CW Nd:YAG Laser Using Monolithic MgO:LiNbO3 ‘Extgmal Resonant Cavities”, IEEE J. of Quantum Electronics, 24(6):913—19 (1988). U.S. Patéht Nov. 15, 1994 Sheet 1 of 7 5,365,539 10 I6 '2 '4 18 .19 I . a US. Patent Nov. 15, 1994 Sheet 2 of 7 5,365,539 I .064 I .065 I066 WAVELENGTH sum) 39’ 3 I .063 CD .2._ 2 o. U) m o o 2 0.32nm oo. «o. <r. N O O O O (suun Momaw) AJJSNBiNl l.062 5,365,539 Sheet 3 of 7 Nov. 15, 1994 US. Patent 0.0? v mm :55 $38 min; 0.8 0.8 Ao\oOmn.m.Ov o\omNu>ozw_oEmm wQOJm >>EO._u o .0. QJOImMKIF ON Q (I) 0.0_ (MW) HBMOd indinO US. Patent Nov. 15, 1994 Sheet 4 of 7 5,365,539 LOGARITHMIC SCALE (IO db/div) FREQUENCY (500 kHz/div) RESOLUTION BANDWIDTH IOkHz 350,. 5a UNEAR SCALE (Arbitrary Units) FREQUENCY(50Hz/divu RESOLUTION BANDWIDTH IOkHz $37. 55 US. Patent Nov. 15, 1994 Sheet 5 of 7 5,365,539 US. Patent Nov. 15, 1994 Sheet 6 of 7 5,365,539 8|O US. Patent Nov. 15, 1994 Sheet 7 of 7 5,365,539 LASER GAIN FREQUENCY SOLID STATE LASER MATERIAL SOLID STATE LASER MATERIAL OUTPUT LASER MIRRORS 5,365,539 1 MICROCHIP LASER GOVERNMENT SUPPORT This invention was made with Government support under Contract Number F19628-90-C-0002 awarded by the Air Force. The Government has certain fights in the invention. This application is a continuation division of applica- tion Ser. No. 08/013,562, U.S. Pat. No. 5,265,116, filed on Feb. 03, 1993, which is a continuation of 07/999,577, filed Dec. 30, 1992, U.S. Pat. No. 5,856,164, which is a File Wrapper continuation of 07/512,981, filed Apr. 23, 1990 (abandoned) which is a division of 07/308,251, filed Feb. 09, 1989, (now U.S. Pat. No. 4,953,166 issued 15 Aug. 28, 1990) which is a continuation-in-part of 07/151,396 filed Feb. 02, 1988 (now U.S. Pat. No. 4,860,304 issued Aug. 22, 1989). BACKGROUND OF THE INVENTION This invention relates to single frequency microchip lasers. In this specification, numbers in brackets refer to the references listed at the end of the specification, the teachings of which are incorporated herein by refer- ence. The realization of practical single-frequency, di- ode-pumped, solid-state lasers has been the goal of sev- eral researchers over the past 20 years [1]. One ap- proach has been the solid-state, unidirectional, nonpla- nar, ring oscillator [2]. While this approach provides the desired laser characteristics, it suffers from a compli- cated fabrication process and optical alignment is criti- cal. A simpler approach is the miniature, linear, solid- state cavity [3—5]. Although there has been some work on multimode miniature flat-flat cavities [6], the most common design for single-mode miniature cavities uses one curved mirror to stabilize the resonator [3—5]. SUMMARY OF THE INVENTION The solid state, optically pumped microlaser includes a solid state gain medium disposed betWeen two mir- rors, the distance between the mirrors, the cavity length, selected so that the gain bandwidth of the gain medium is less than the frequency separation of the cavity modes. In a preferred embodiment the mirrors are in the form of coatings deposited directly on the gain medium. Alternatively, the mirrors may be sepa- rate elements and bonded to the gain medium in a sand- - wich configuration. Because the cavity length is selected to provide an 50 intracavity mode spacing larger than the bandwidth of the active gain medium, only a single longitudinal mode will oscillate when the device operates in a single trans— verse mode. Optical pumping may be provided by any appropriate source, such as a semiconductor injection laser or laser array. The solid-state, optically pumped microchip laser according to one aspect of the invention includes a solid-state gain medium disposed between two mirrors, the distance between the mirrors selected so that the gain bandwidth of the gain medium is substantially equal to the frequency separation of the cavity modes. In another aspect, a solid-state gain medium is disposed between two mirrors, the distance between the mirrors selected so that the gain bandwidth of the gain medium is less than or substantially equal to the frequency sepa- ration of the cavity modes. A nonlinear optical material is disposed to receive light from the gain medium, the 10 25 3O 35 45 55 60 65 2 nonlinear optical material selected to generate second or higher harmonics of the light from the gain medium. In yet another aspect of the invention, the microchip laser includes a solid-state gain medium/nonlinear opti- cal material combination disposed between two mirrors, the distance between the mirrors selected so that the gain bandwidth of the gain medium is less than or sub- stantially equal to the frequency separation of the cavity modes. The nonlinear optical material is selected to generate second or higher harmonics of the light from the gain medium. By selecting the cavity length so that the gain band- width is substantially equal to the frequency separation of the cavity modes, one is guaranteed that only one cavity frequency falls within the laser gain region and only one laser frequency will oscillate. The inclusion of nonlinear optical material provides light in the visible or ultraviolet regions useful for read and write optical disks and for projection television applications, among others. Both the laser gain element and the nonlinear crystal are dielectrically coated flat wafers. These wa- fers are bonded together with transparent optical ce- ment and diced into many small sections which greatly reduces the cost and complexity of such lasers as com- pared with devices using discreet optical components that are fabricated and assembled separately. The single frequency microchip lasers according to the invention employ a miniature, monolithic, flat-flat, solid-state cavity whose mode spacing is greater that the medium gain bandwidth. These lasers rely on gain- guiding or nonlinear optical effects to define the trans- verse dimensions of the lasing mode. As a result of the monolithic, flat-flat construction, the fabrication pro— cess for the microchip laser lends itself to mass produc- tion. The cost per laser is extremely low because of the small amount of material used for each laSer and the simple fabrication. The resulting microchip lasers are longitudinally pumped with the close-coupled, unfo- cused output of a diode laser. BRIEF DESCRIPTION OF THE DRAWING FIGS. la, and 1b, are graphs of laser gain and oscilla- tion modes versus frequency; FIGS. 20 and 2b are cross-sectional views of a micro— chip laser of the invention; FIG. 3 is a graph of output intensity versus wave- length; FIG. 4 is a graph of output power versus pump power for lasers of the invention; FIGS. 5:1 and 5b are graphs illustrating measured spectral response of the lasers of the invention; FIGS. 6a and 6b are cross-sectional views of an em- bodiment of the invention including a nonlinear optical element; FIG. 7 is a cross-sectional view of an embodiment of the invention with a nonlinear optical element incorpo- rated within the laser resonant cavity; FIG. 8:1 depicts an array of microchip lasers on a wafer in association with a wafer of diode pump lasers; while FIG. 8b depicts the array of FIG. 8a in associa- tion with a wafer of nonlinear optical material with Fabry-Perot resonantors; and FIG. 9a depicts an embodiment of the microlaser of FIGS. 6a and 6b with an apparatus for stress tuning and FIG. 9b depicts an embodiment of the microlaser of FIGS. 6a and 6b with an apparatus for thermal tuning. 5,365,539 3 FIG. 10 is a graph of gain bandwidth and cavity modes as a function of frequency; FIG. 11 is a perspective view of a microlaser accord- ing to the present invention; FIG. 12 is a cross-sectional view of an embodiment of the microlaser with the diode laser pump bonded to one of the laser mirrors. DESCRIPTION OF THE PREFERRED EMBODIMENT The theory on which the present invention is based will now be discussed in conjunction with FIG. 1. In FIG. 1a, a curve 10 is a plot of gain versus frequency for any solid-state laser gain medium such as Nd:YAG or Nd pentaphosphate. The gain bandwidth vg of the curve 10 is defined as the separation between arrows 12 and 14 wherein the gain exceeds the loss. Also shown in FIG. 1a are intracavity modes 16 and 18. The separation vc between adjacent cavity modes is given by the equation vc=c/2nl where c is the speed of light, 11 is the refrac- tive index of a gain medium and l is the length of a resonant cavity. As shown in FIG. la, a cavity length 1 has been selected so that l is less than c/2nvg resulting in the intracavity modes 16 and 18 being spaced greater than the gain bandwidth of the curve 10 and the abso- lute frequency of the cavity mode vg=mc/2nl where m is an integer such that the frequency falls outside the gain bandwidth. In the case illustrated the intracavity modes 16 and 18 straddle the gain curve 10 so that there will be no lasing of the gain medium since there is no overlap of the gain curve 10 with either of the modes 16 or 18. To insure that the gain medium will lase, it is necessary that there be at least some overlap of the gain curve 10 with one of the intracavity modes such as the mode 18 as shown in FIG. 1b. Assuring such overlap is accomplished by an appropriate choice of gain material and cavity length. With reference now to FIG. 20, a microchip laser 30 includes a solid-state gain medium 32 disposed between a pair of mirrors 34 and 36. The mirrors 34 and 36 are coated with multiple layers (20—30 layers) of dielectric material. The gain medium 32 is pumped optically by a laser 38 whose output light 40 is focused by a lens 42 onto the mirror 34. The mirror 34 transmits light from the pump laser 38 but reflects light generated within the gain medium 32. The length 1 of the gain medium 32 is selected so that léc/vag where Vg is the bandwidth of the gain medium. In this case, as pointed out above, a single mode only will oscillate within the gain medium 32 when va falls Within the gain bandwidth so that the output light 44 from the laser 30 is single frequency. The mirrors 34 and 36 may be separate elements bonded directly to the gain medium 32 or they may be multi- layer coatings deposited directly on the opposing flat surfaces of the gain medium 32. In FIG. 2b, the laser 38 is placed close to or bonded directly to the mirror 34 so that most of the light from the pump laser is absorbed in the fundamental mode region of the microchip laser. To demonstrate the feasibility of diode-pumped mi— crochip lasers, several different microchip lasers were constructed and operated CW at room temperature. These included: Nd:YAG (Nde3_xA15012) at 1.06 nm using a 730-um-long cavity; Nd:YAG at 1.3 pm using a 730-um-long cavity; Nd pentaphosphate (NdP5014) at 1.06 pm using a lOO-um-long cavity; and NdeSGG (Nded3.xScha3012) at 1.06 pm using a 625-um long cavity. In each case, single-longitudinal-mode, single- 5 10 15 20 25 30 35 45 50 55 60 65 4 spatial-mode operation was achieved with pump pow- ers many times above threshold. The performance of the 1.06 pm Nd:YAG microchip lasers will now be discussed. These lasers were con- structed from a slab of YAG with 1.1 wt. percent Nd doping. The slab was cut and polished to a thickness of 730 p.111. Dielectric cavity mirrors were deposited di- rectly onto the YAG. On other microchip lasers the mirrors were cut from lOO-um-thick wafer mirrors and then bonded to the Nd:YAG. The performance of the separate-mirror devices was very similar to the perfor- mance of the dielectrically coated Nd:YAG cavities. The output mirror 36 had a reflectivity of 99.7% at 1.06 pm and was designed to reflect the pump laser. The opposite mirror 34 had a reflectivity of 99.9% at 1.06 pm and transmitted the pump. The Nd:YAG was cut into pieces 1 mm square (or less) and bonded to a sap- phire heat sink (not shown). Damage to the dielectric coatings from cutting the wafers was confined to a distance of less than 30 pm from the edge of the chips. A Ti:A1203 laser was used as a pump source to char» acterize the microchip lasers prior to diode pumping. It was tuned to the Nd:YAG absorption peak at 0.809 um and focused onto the microchip laser, with an experi- mentally determined spot size of about 50 pm in the Nd:YAG crystal. Measurements showed that 18% of the incident pump power was reflected by the laser package and 27% was transmitted. The efficiency of the microchip lasers can be improved with better dielectric coatings. When the Nd:YAG microchip laser was properly aligned with the pump, single—longitudinal-mode, sin- gle-spatial—mode operation was observed. The output beam 22 was circularly symmetric with a divergence of about 20 mrad, determined by the spot size of the pump. Spectrometer traces (FIG. 3) showed only single—lon- gitudinal-mode operation for absorbed pump powers up to 40 mW. The lasing frequency tuned slightly as the pump spot on the microchip cavity was moved to posi- tions with a slightly different cavity length. The devices constructed with wafer mirrors were continuously tun- able over the entire gain spectrum by mechanical move- ment of the mirrors. In contrast to results reported in [7], the output polarization of the microchip laser was in the same direction as the polarization of the pump to better than 1 part in 100. A computer-controlled variable attenuator was intro- duced into the path of the pump beam to obtain the input-output power characteristics of the microchip laser. The lasing threshold was measured to be below 1 mW, and the slope quantum efficiency (determined from the output of the laser from the 99.7% reflecting mirror only) was slightly greater than 30%. The input- output curve is shown in FIG. 4. At higher pump pow- ers thermal effects led to unrepeatable results. The high- est single-mode CW output power achieved with the microchip laser was 22 mW. The linewidth of the Nd:YAG microchip lasers was measured by heterodyning two free-running devices together. Thermal tuning was used in order to get the lasers to operate at nearly the same frequency. The outputs of the lasers were stable enough to obtain het- erodyne measurements with a resolution of 10 kHz. At this resolution, the measured spectral response was instrument limited. (See FIG. 5) This gives a linewidth for the microchip lasers of less than 5 kHz, assuming equal contributions to the linewidth from each laser. The theoretical phase fluctuation linewidth is estimated 5,365,539 5 to be only a few hertz. Relaxation oscillations account for the observed sidebands 700 kHz away from the main peak. The intensity of the sidebands varied with time, but was always greater than 30 dB below the main peak. The microchip NszAG lasers have been pumped with the unfocused output of a 20-mW GaAlAs diode laser. The NszAG cavity was placed about 20 pm from the output facet of the diode laser and longitudi- nally pumped. The resulting pump spot size in the NszAG was about 50 pm in diameter. The output of 10 the microchip laser showed single-longitudinal-mode, fundamental (i.e. lowest order) single-spatial—mode op- eration at all available powers. The divergence of the laser was diffraction limited at about 20 mrad. Important embodiments of the present invention are shown are FIG. 6. In FIG. 6a a dielectrically coated flat wafer 50 of a nonlinear optical material is located to receive light from the microchip laser 30. The wafer 50 includes dielectric coatings 52 and 54. The nonlinear optical material of the wafer 50 has the property that, when exposed to monochromatic light, it generates a beam of light including harmonics of the incident beam. Suitable nonlinear Optical materials are, for example, MgO:LiNbO3 and KTP (potassium titanyl phosphate). 15 20 As in the embodiments of FIG. 2, the cavity length l of 25 the gain medium 32 satisfies the relationship léc/vag. Light from the microchip laser 30 passes into the non- linear optical element 50 which shifts the frequency to one of the harmonics of the incident beam. A particu- larly useful harmonic is the second harmonic. The opti- cal coatings 52 and 54 are chosen such that they form a Fabry-Perot cavity at the pump wavelength. Typical reflectivities of such coatings at the pump wavelength are 98%. Mirror 52 is also highly reflective at the har- monic wavelength while mirror 54 is highly transmis- sive at the harmonic wavelength. In addition to tech- niques whereby the cavity frequency of the harmonic crystal is tuned to the laser frequency, the single-fre— quency microchip laser may be tuned to be resonant with any of the harmonic crystal cavity modes and may be locked to that frequency in any number of ways including monitoring of the intensity of the harmonic output power. The microchip laser may be continu- ously tuned by a number of techniques that are well known including the application of a longitudinal or transverse stress to the crystal or by modifying the refractive index of the crystal thermally. FIG. 9a shows a microlaser 30 associated with a nonlinear optical material 50 located within a Fabry- Perot cavity made up of mirrors 52 and 54. The gain medium 32 of the laser 30 is positioned between a fixed stop 912 and a movable stop 910. Pressure is applied (shown here as being applied by an adjustable screw 914) to the gain medium 32 through the movable stop 910. By adjusting the pressure on the gain medium 32, the frequency of the laser light can be matched to the resonant frequency of the Fabry-Perdt cavity. In FIG. 90 a transverse stress is being applied to the medium. It is possible to stress tune the laser by the application of a longitudinal stress along the direction of the laser light. FIG. 9b shows a microlaser 30 associated with a nonlinear optical material 50 as in FIG. 9a. However, in this embodiment the gain material 32 is positioned within a temperature regulating jacket 918 which can be heated or cooled by the temperature regulating ele- ments 916. By adjusting the temperature of the gain medium 32 the frequency of the laser light can be ther- mally tuned to match the resonant frequency of the 30 35 45 50 55 6S 6 Fabry-Perot cavity. Because the laser frequency can be changed by thermal tuning, it should also be noted that in order to stress tune a microlaser and have it remain tuned, it may be necessary to regulate the temperature of the gain medium. The ability to continuously tune the microchip laser over its gain bandwidth without a mode jump is a signif- icant advantage in being able to precisely tune and lock to any of the Fabry—Perot cavity modes of the harmonic crystal. The harmonic crystal with its resonant cavity may be separate from the microchip laser or it may be bonded directly to the output end of the microchip laser using an optically tranSparent cement. The use of flat-flat cavities on the harmonic crystal simplify the fabrication process by using similar wafer processing technology as that for the microchip laser. However, any of the well known techniques for a resonant harmonic cavity may also be used in conjunction with the microchip laser such as the unidirectional ring resonator or spherical mirror cavity. Further, the nonlinear material may be incorporated within the laser cavity itself. FIG. 6(b) shows a configuration similar to FIG. 6(a) but with the diode placed close to or bonded to the laser medium. FIG. 7 is an embodiment of the invention in which nonlinear optical material forms a part of the laser cavity structure. A microchip laser 70 includes a flat wafer 72 of an active gain medium. A nonlinear optical element 74 is bonded to the gain medium 72. Dielectric mirrors 34 and 36 complete the microchip laser 70. The length 1 between the mirrors 34 and 36 satisfies the relationships vgé c/2(n111 + 11212) I=ll+lz Where 11, m are the length and index of refraction respectively of the gain medium and 12, n2 are the length and index of refraction of the non-linear material. It will be appreciated by those skilled in the art that by selection of the appropriate nonlinear optical mate- rial, the output from the harmonically converted micro- chip laser can be in the visible or ultraviolet region and be useful for read and write optical disks or projection television applications. It will also be appreciated that using the same fabrication techniques, an electro-optic or acousto-optic modulator can be incorporated into the composite structure with the modulator electrodes being photolithographically incorporated onto the wa- fers before being diced up. Such fabrication techniques would greatly reduce the cost and complexity of such devices over those using discrete optical components that are fabricated and assembled separately. In addition to harmonic generation by means of a suitable nonlinear material, nonlinear frequency conver- sion may be carried out in suitable nonlinear optical materials using optical parametric oscillation or amplifi- cation as well as frequency sum or difference mixing using the single frequency microchip laser. Similar cav- ity fabrication as-that described above may be used to create a microchip laser whose single frequency light is frequency converted by parametric conversion into light of two lower frequencies. In this parametric con- version microchip laser, the resonators differ from those previously described only in that the nonlinear optical material is a parametric conversion material such as 5,365,539 7 LiNbO3 or KNbO3 and the cavity coatings are chosen according to the well known art form for such devices. Fabrication of the foregoing embodiments will now be discussed. A boule of laser material is grown by conventional methods. The boule is sliced to the appro- priate thickness depending on whether or not a nonlin- ear optical element is to be incorporated into the reso- nant cavity. The resulting wafer is lapped to the desired length, parallel and flat to better than one wavelength using conventional lapping techniques. The wafer must be flat over the area being irradiated by the diode pump laser. At this point, the gain medium is coated with multiple layers of dielectric material on opposite faces. Alterna- tively, separate mirrors are bonded to the gain medium. The slice or wafer is then diced into individual micro- chip lasers which can be mounted to the pump optics. The nonlinear optical elements used for frequency shifting are lapped flat and parallel and incorporated either within or outside of the laser cavity as shown in FIGS. 6 and 7. In the case in which the nonlinear opti- cal material is outside the resonant cavity, both surfaces of the nonlinear material 'are coated with dielectric layers which are highly reflective to the frequency of light generated by the microchip laser. In addition, the surface of the nonlinear optical material nearest to the micr00hip laser is coated with dielectric layers which are highly reflective of the wavelength of light gener- ated by the nonlinear optical material. Although for ease of manufacture, the nonlinear element may be lapped with both surfaces flat, it is also possible, in the case of the optical material being outside the laser cav- ity, to have the surface away from the microchip laser be a nonflat surface. Any non-parallel resonator structure known to the art of resonators may be used with the microchip laser. In fact, the use of the microchip laser makes such reso- nator design easier, since the laser can be tuned to the cavity frequency. If a nonlinear optical element is to be part of the resonant cavity, the boule of gain medium is sliced and lapped flat and parallel. The nonlinear optical material is also lapped flat and parallel and bonded to the gain medium using transparent optical cement. The lengths of the gain material and the nonlinear optical material satisfy the relationship discussed above. After the gain medium and nonlinear optical material are bound, mir- rors are applied to the other surfaces either as multi- layer dielectric coatings on the material itself or as sepa- rate mirrors, and the wafers are diced into chips. It should be noted that one can construct a microchip laser array or a microchip laser nonlinear frequency converter array by simply not dicing the wafers. That is, by leaving the microchip laser in wafer form, and associating the microchip laser wafer with a two dimen— sional diode laser array to pump the microchip laser array, a two dimensional array of microchip lasers is immediately formed. Referring to FIG. 8a, a two di- mensional array of microchip lasers consists of an array of laser diodes 810 associated with a microchip laser wafer 812. Light from the laser didoes 814 excites the microchip laser to emit light. The two wafers can be cemented together using optically transparent cement. If the microchip laser wafer also contains the nonlinear optical material within its resonant cavity, frequency conversion using a two wafer system is possible. If, however, the nonlinear optical material is not on the microchip laser, an array can still be constructed by 10 15 20 25 30 35 45 50 55 60 65 8 simply placing the wafer with the nonlinear optical material and Fabry-Perot resonant cavity 816 in associ- ation with the other wafers. Such a configuration is shown in FIG. 8b. The resonant frequency of the non- linear optical material cavity must be matched to the microchip frequency in the manner described above. A microchip laser array is particularly useful if a directed laser beam is required. By properly modulating the phase of each microchip laser using, for example, individually addressable phase modulators placed in the output beam or within the cavity of each microchip laser, it is possible to phase steer the array and thereby direct the beam. The output of the two-dimensional array of single-frequency microchip lasers may be co- herently combined into a single beam using well known techniques of binary optics. The diode-pumped microchip lasers according to the invention exhibit low pump threshold, high efficiency, and single frequency operation. These lasers can be continuously tuned across their gain-bandwidth in a single frequency using a transversly or longitudinally applied stress to the microchip laser crystal. The micro- chip laser is applicable, for example, to frequency con- verters, modulators and Q-switches, with photolitho— graphically deposited electrodes. It results in low cost, volume-producible lasers and electro-optic devices. The incorporation of nonlinear optical material, either inside or outside of the cavity, generates other wave- lengths of light. The output from such a device, in the visible or ultraviolet region, is useful for read and write optical disks and projection television applications. The theory on which another embodiment of the present invention is based will now be dismissed in conjunction with FIG. 10. A curve 10 is a plot of gain versus frequency for a solid state gain medium such as Nd:YAG or Nd pentaphosphate. The gain bandwidth of the curve 10 is defined as the separation between the arrows 12 and 14. Also shown in FIG. 10 are intracavity modes 16—24 as a function of frequency. The separation between adjacent ones of the cavity modes 16—24 is given by the equation vc= c/lnl, where c is the speed of light, 11 is the refractive index of a gain medium and l is the length of the resonant cavity. As will be appreciated by those skilled in the art, if the spacing vc of cavity modes is greater than the gain bandwidth vg, then only a single longitudinal mode wiil oscillate when the oscil- lator operates in a single transverse mode. With reference to FIG. 11, a microlaser 30 includes a solid state gain medium 32 disposed between a pair of mirrors 34 and 36. In accordance with the discussion above in conjunction with FIG. 10, the cavity length 1 between the mirrors 34 and 36 satisfies the inequality: I<c/2nvg where vg is the bandwidth of the gain me- dium. Suitable solid state gain media are Nd:YAG and Nd pentaphosphate. The laser mirrors 34 and 36 may either be deposited as coatings directly on opposing surfaces of the laser gain medium 32 or made using thin glass or other appropriate material and subsequently bonded to the gain medium 32 to form a sandwich struc- ture. For a laser gain medium such as Nd:YAG, the cavity length would be approximately several hundred um, while for a stoichiometric compound laser material such as Nd pentaphosphate, cavity lengths will be typi- cally in the range of 10—100 pm. In operation, the microlaser 30 is excited by an appro- priate source, such as a semiconductor injection laser or laser array. A diode laser pump 38 is shown in FIG. 11. A pump beam 40 from the diode laser pump 38 is fo- 5,365,539 9 cused by an optical focusing system 42 and optically pumps the microlaser 30 such that it operates in a single spatial mode. The microlaser 30 then generates an out- put beam 44. Alternatively, the diode laser pump 38 may be bonded directly to the mirror 34 of the mi- crolaser 30, as shown in FIG. 12. In the case where the absorption length of the pump laser radiation is longer than the length of the laser material, an enhancement of the pump laser absorption can be made by placing the laser material inside a Fabry—Perot type resonator at the pump laser wave- length. For example, referring to FIG. 11, the dielectric coatings on both laser mirrors would have high reflec~ tivity at the pump laser wavelength in addition to being properly coated at the wavelength of the laser material. The microlaser 30 may be made to operate at several wavelengths including the 1.32 pm transition of Nd which would be useful for optical fiber communications and fiber sensors. Furthermore, the frequency of the microlaser 30 may be locked to a secondary frequency reference, such as an atomic or molecular absorption, to provide a well-defined carrier frequency as a communi- cations standard. A multiplexed communications sys- tem may be operated by generating side bands from the frequency locked master oscillator. Such a system will allow for any communications system to be compatible with any other system because each channel will have an absolutely identifiable frequency. The microlaser 30 may be tuned in a number of ways, such as changing the effective length of the optical cavity. Cavity length may be changed by altering the refractive index of the gain medium or of some other material contained within the laser cavity. The mecha- nism for altering refractive index may be thermal or electronic by utilizing a temperature dependent refrac- tive index effect or an electro—optic or piezoelectric effect, respectively. Alternatively, a piezoelectric or other mechanical device may be employed to move one of the laser cavity mirrors for laser tuning. The microlaser 30 can be made quite compact and stable using microfabrication techniques similar to those used in the semiconductor fabrication industry. The design of the microlaser 30 renders it able to withstand significant shocks for operation under adverse condi- tions. One application for a microlaser 30 device would be for fiber optic gyros which require narrow line width operation. The microlaser 30 may also be operated at low tem- perature (from the temperature of liquid helium at 1.5" Kelvin to just below room temperature). Such opera- tion will provide for stable, precision frequency output. The spectral gain linewidths for the active gain materi- als can become quite narrow at low temperatures (less than 100 Mhz in some cases). By locking the microlaser 30 to its own gain peak using a number of well-known techniques, an absolute frequency of operation may be obtained with an accuracy of as little as a few kilohertz. REFERENCES 1. For a complete review of diode pumped solid-state lasers see T. Y. Fan and R. L. Byer, IEEE J. Quan~ tum Electron, 6, 895 (1988). 2. T. J. Kane, A. C. Nilsson, and R. L. Byer, Opt. Lett. 12, 175 (1987). 3. B. Zhou, T. J. Kane, G. J. Dixen, and R. L. Byer, Opt. Lett 10, 62. (1985). 4. A. Owyoung, G. R. Hadley, P. Esherick, R. L. Schmit, and L. A. Rahn, Opt. Lett. 10, 484 (1985). 10 15 20 25 30 35 45 50 55 65 10 5. K. Kubodera and J. Noda, Appl. Opt. 12, 3466 (1982). 6. G. Winter, P. G. Mockel, R. Oberbacher, and L. Vité, Appl. Phys. 11, 121 (1976). 7. P. Esherick and A. Owyoung, Technical Digest, Conference on Lasers and Electrooptics (Optical Society of America, Washington, D.C., 1988), paper THB2. What is claimed is: 1. A solid state optically pumped laser comprising a solid state gain medium disposed between two mirrors characterized in that the distance between the mirrors is such that the gain bandwidth of the gain medium is less than the frequency separation of the cavity modes, and in that the laser is pumped to operate in a single trans- verse mode. 2. The laser of claim 1 in which the mirrors are in the form of coatings deposited directly on opposing sur- faces of the gain medium. 3. The laser of claim 1 in which the mirrors are sepa- rate elements bonded to opposing surfaces of the gain medium. 4. A laser of claim 1 in which the gain medium is NszAG. 5. The laser of claim 1 in which the gain medium is Nd pentaphosphate. 6. The laser of claim 4 in which the separation be- tween the mirrors is in the range of several hundred micrometers. 7. The laser of claim 5 is which the separation be- tween the mirrors is in the range of 10—100 micrometers. 8. The laser of claim 1 in combination with a semicon- ductor diode laser adjacent the laser for optically pump- ing the laser. 9. The laser of claim 8 in which the mirrors form facets on each end of the gain medium and the semicon- ductor diode laser is bonded to one of said facets. ’ 10. The laser of claim 1 in which the gain medium and the mirrors form a Fabry-Perot type resonator at the frequency at which the laser is optically pumped. 11. The laser of claim 2 in which the coatings have high reflectivity at the wavelength at which the gain medium is optically pumped. 12. The laser of claim 8 in which the mirrors include dielectric coatings having high reflectivity at the semi- conductor diode pump laser frequency. 13. The laser of claim 1 in which the laser is longitudi- nally pumped. 14. A method of forming a solid state optically pumped laser comprising the steps of: a) disposing a solid state gain medium between two mirrors wherein the distance between the mirrors is such that the gain bandwidth of the gain medium is less than the frequency separation of the cavity modes, and b) optically pumping the gain medium to cause the laser to operate in a single transverse mode. 15. The method of claim 14 in which the mirrors are formed by depositing coatings directly on opposing surfaces of the gain medium. 16. The method of claim 14 in which the mirrors are formed by bonding separate elements to opposing sur- face of the gain medium. 17. The method of claim 14 in which the gain medium is formed of NszAG. 18. The method of claim 14 in which the gain medium is formed of Nd pentaphosphate. 5,365,539 11 19. The method of claim 17 in which the separation between the mirrors is in the range of several hundred micrometers. 20. The method of claim 18 in which the separation between the mirrors is in the range of 10—100 microme- ters. 21. The method of claim 14 in which the laser is optically pumped by a semiconductor diode laser. 22. The method of claim 21 in which the mirrors form facets on each end of the gain medium and the semicon- ductor diode laser is bonded to one of said facets. 10 15 20 25 30 35 45 50 55 60 65 12 23. The method of claim 14 in which the gain medium and the mirrors form a Fabry—Perot type resonator at the frequency at which the laser is optically pumped. 24. The method of claim 15 in which the coatings have high reflectivity at the wavelength at which the gain medium is optically pumped. 25. The method of claim 21 in which the mirrors include dielectric coatings having high reflectivity at the semiconductor diode pump laser frequency. 26. The method of claim 14 in which the laser is longitudinally pumped. * =1: * * * ...
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US5365539_TOT - United States Patent[19 Mooradian...

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