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US5256164_TOT - US Patent Oct 26 1993 Sheet 1 of 6...

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Unformatted text preview: US. Patent Oct. 26, 1993 Sheet 1 of 6 5,256,164 IO I6 ,2 '4 I8 $9 I . a 1mm 2.5.5 mw>>on_ @551 5,256,164 0.0V 0.0m 0 Om 0.0_ 6 .m 3 m. Ao\oomu .mdv o\omNn>ozm_UEuw macaw 3 m m m >>EO._uo..OImwmI._. US. Patent ON 0_ v (MW) UBMOd indinO O_ LO Q (I) 0.0_ US. Patent Oct. 26, 1993 Sheet 4 of 6 5,256,164 LOGARITHMIC SCALE (IO db/div) FREQUENCY (500 kHz/div) RESOLUTION BANDWIDTH lOkHz 347, 5a LINEAR SCALE (Arbi1rary Units) FREQUENCY(SOkz/div RESOLUTQON BANDWIDTH IOkHz :93. 5;, US. Patent Oct. 26, 1993 Sheet 5 of 6 5,256,164 US. Patent Oct. 26, 1993 Sheet 6 of 6 5,256,164 BIO 5,256,164 1 METHOD OF FABRICATING A MICROCHIP LASER The Government has rights in this invention pursuant to contract Number F19628-85-C-0002 awarded by the Department of the Air Force. This is a continuation of co-pending application Ser. No. 07/512,981 filed on Apr. 23, 1990 now abandoned, which is a divisional of Ser. No. 308,251, filed Feb. 9, 1989, now U.S. Pat. No. 4,953,166 which is a continua- tion-in-part of Ser. No. 151,396, filed Feb. 2, 1988, now U.S. Pat. No. 4,860,304. BACKGROUND OF THE INVENTION This application is a continuation-in—part of U.S. Ser. No. 151,396 filed Feb. 2, 1988. 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 [3-51. 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]. In allowed U.S. patent application Ser. No. 151,396, filed Feb. 2, 1988 now U.S. Pat. No. 4,86 issued Aug. 22, 1989, there is disclosed a solid-state, optically pumped microchip laser in which the cavity length is selected so that the gain bandwidth of the gain medium is less than the frequency separation of the cavity modes. This relationship guarantees that only a single longitudinal mode will oscillate when the frequency of this mode falls within the laser gain region. SUMMARY OF THE INVENTION 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 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. 10 15 20 2 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 I monolithic, flat-flat construction, the fabrication pro- 25 30 35 45 50 55 6S 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. 1a, and lb, 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. 5a 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. 80 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. 80 in association with a wafer of nonlinear optical material with Fabry- Perot resonantors; and FIG. 90 depicts an embodiment of the microlaser of FIG. 6 with an apparatus for stress tuning and FIG. 9b depicts an embodiment of the microlaser of FIG. 6 with an apparatus for thermal tuning. 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. la, a curve 10 is a plot of gain versus frequency for any solid-state laser gain medium such as NszAG 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. Ia are intracavity modes 16 and 18. The separation vc between adjacent cavity modes is given by the equation 5,256,164 3 v,=c/2nl where c is the speed of light, it 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 v1=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. lb. 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 l of the gain medium 32 is selected so that léc/vag where vs 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 ‘4 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 4 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 pm 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 Nd:GSGG (Nded3_xScha3012) at 1.06 pm using a 625—um long cavity. In each case, single-longitudinal-mode, single- 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 um. Dielectric cavity mirrors were deposited di- rectly onto the YAG. On other microchip lasers the mirrors were cut from IOO-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 NszAG 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. 10 15 20 25 30 35 40 45 50 55 6O 65 4 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 NszAG absorption peak at 0.809 pm and focused onto the microchip laser, with an experi- mentally determined spot size of about 50 pm in the NszAG 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, single 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. Spec- trometer traces (FIG. 3) showed only single-longitudi- nal 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 positions 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 NszAG 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 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 Nd2YAG was about 50 pm in diameter. The output of the microchip laser showed single-Iongitudinal-mode, fundamental (i.e. lowest order) single-spatial-tnode 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, 5,256,164 5 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 KT? (potassium titanyl phosphate). As in the embodiments of FIG. 2, the cavity length of 5 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 rcflectivities 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-Perot cavity. In FIG. 9a 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. 90. 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 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 10 15 20 25 3O 35 45 50 55 65 6 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 +n212) I=Ij+12 Where ll, m are the length and index of refraction respectively of the gain medium and 12, n; 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- Ision 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...
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