US5327447_TOT - United States Patent[19 Mooradian...

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Unformatted text preview: United States Patent [19] Mooradian lllllllllllllllllllllllllllllllllllllllllHllllllIllllllHlIlllIlllllllllll Usoos327447A [11] Patent Number: 5,327,447 [45] Date of Patent: * Jul. 5, 1994 [54] WAVEGUIDE OPTICAL RESONANT CAVITY LASER [75] Inventor: Aram Mooradian, Winchester, Mass. [73] Assignee: Massachusetts Institute of Technology, Cambridge, Mass. [ " ] Notice: The portion of the term of this patent . subsequent to Jan. 22, 2009 has been disclaimed. [21] Appl. No.: 938,851 [22] Filed: ' Sep. 1, 1992 Related US. Application Data [60] Continuation of Ser. No. 712,185, Jun. 7, 1991, Pat. No. 5,150,374, which is a division of Ser. No. 341,028, Apr. 20, 1989, Pat. No. 5,050,179. [51] Int. Cl.5 ................................................ H018 3/08 [52] US. Cl. ......................................... 372/92; 372/99 [58] Field of Search ............................ 372/92, 99, 101 [56] References Cited U.S. PATENT DOCUMENTS 4,426,707 1/1984 Martin et al. ....................... 372/ 101 4,504,950 3/1985 AuYeung ............. 372/ 101 4,793,675 12/1988 Handa ............ 350/9613 4,856,871 8/1989 Van Sant .....,. .. 350/253 4,934,784 6/1990 Kapany et al. .. 372/101 5,018,831 5/1991 Wyatt et a]. .......... 372/101 5,150,374 9/1992 Mooradian ............................ 372/92 FOREIGN PATENT DOCUMENTS 56-42389 4/ 1981 Japan . 58-114338 7/1983 Japan . 61-264777 11/1986 Japan . 63-029330 8/1988 Japan . . WO 91/12544 8/1991 PCT Int’l Appl. . OTHER PUBLICATIONS “External—Cavity Semiconductor Laser with 1000 6112 {57] Continuous Piezoelectric Tuning Range”, Schremer et 31., IEEE Photonics Letters, vol. 2, No. 1, Jan. 1990, pp. 3—5. “Miniature Packaged External—Cavity Semiconductor Laser with 50 GHz Continuous Electrical Tuning Range”, Mellis et al., Electronics Letters, vol. 24, No. 16, Aug. 4, 1988, pp. 988—989. Kapany & Burke, “Optical Waveguides”, Academic Press, N.Y., pp. 312—319 (Jan. 1972). Smith, “High Pressure Waveguide Gas Lasers”, Laser Spectroscopy, Ed. Brewer & Mooradian, Plenum Press, N.Y. (Jan. 1974) p. 247. Abrams, “Wideband Waveguide C02 Lasers,” Plenum Press, N.Y., Ed. Brewer & Mooradian, pp. 263—272 (Jan. 1974). Castleberry et 21., “A Single Mode 2.06 um Miniature Laser,” Digest of Tech. Papers on Integrated Optics, by Optical Soc. of America, pp. MB71—MB72 (Jan. 1974). Degnan, John J ., “The Waveguide Laser: A Review”, Applied Physics, vol. II, pp. 1—33 (Jan. 1976). Primary Examiner—Rodney B. Bovernick Assistant Examiner—Robert E. Wise Attorney, Agent, or Firm-Hamilton, Brook, Smith & Reynolds ABSTRACT An external cavity semiconductor laser comprising a resonance cavity coupled to a diode laser. The cavity may contain a lens or lens system or may be constructed as an optical waveguide. The external cavity may also contain a nonlinear optical material to produce light of a frequency which is higher than that produced by the semiconductor laser. The use of an external cavity in- sures the single mode and/or single frequency operation of the semiconductor laser. 30 Claims, 7 Drawing Sheets 1 f—+—f—>{ f. [72 13' I4 I? '6 US. Patent July 5, 1994 Sheet 1 of 7 5,327,447 5—H f 1 .\\ m I E, .3 I4 I? ? l6 |O\ r—f—fief—fi I .7492 (b) H '3,4 l7 l6 4 AH) '13 20% 6 lo I \ l3 - '7 4 7 ’ / I 7"_I_§ 2° 3'9 W) ' l8 I .7 ¢ ¢ L————f’——~l \ IO “’1 l7 u \.:-H 22 Exam} ‘0\ I3 I “3‘4”! U) Ma 22 “1“] f 5,327,447 Sheet 2 of 7 July 5, 1994 § US. Patent US. Patent July 5, 1994 Sheet 3 of 7 _ 5,327,447 \ ll '3 7 ”=5- / ’ 2a 12 30 IS ii 19 () Waveguiding in diode IO\ 13 D O I IO\ D 32 II '3 7" M§ I 2 =..__<@:__,_______§ l9 (0) :2 7 § l6 F—f *4 IO\ :22 \ 1% \\-l_\ Sheet 4 of 7 5,327,447 July 5, 1994 US. Patent zmlwl lll|l_ «VON m /////V /!_!fl/A ,_ /O_ m. o _Al||\v_ S w} /////////ln.ulfl O. US. Patent July 5, 1994 Shéet 5 of 7 5,327,447 US. Patent July 5, 1994 Sheét 6 of 7 5,327,447 I 428 424 US. Patent July 5, 1994 Sheet 7 of 7 > 5,327,447 6|O 620 6|O 628 620 5,327,447 1 WAVEGUIDE OPTICAL RESONANT CAVITY LASER The US. Government has rights in this invention pursuant to Contract Number F19628-85-0002 awarded by the Department of the Air Force. This application is a continuation of application Ser. No. 07/712,185, filed on Jun. 7, 1991 now US. Pat. No. 5,150,374, which is a divisional of Ser. No. 07/341,028, filed on Apr. 20, 1989 now US Pat. No. 5,050,179. BACKGROUND This invention relates to the single-frequency, broadly tunable, and single spatial mode high power operation of semiconductor diode lasers. The peak output power from a diode laser is prepor- tional to the active area of the emitting facet of the device and is limited by catastrophic degradation. To increase the output power from from semiconductor diode lasers, it is necessary to increase the area of the active gain region. However, monolithic diode lasers with large emitting areas produce a broad spectral out- put because they usually operate in many spatial modes and/of filaments. In addition, monolithic diode lasers have an output wavelength which is usually centered near the gain peak and can not be easily tuned. Large active area devices are those with a Fresnel number describing the cavity given by FEDZ/M>1, where D is the width of the active gain region, 1 is the cavity length and A is the wavelength of the laser. Note that although the width of the active gain region is referred to, the height of the active gain region in most mono- lithic diode lasers is usually comparable to the wave- length of light and the wave is guided in this direction. The same Fresnel consideration applies to the height of the active region when the wave is not guided. As the width of the active region increases, there is enough total gain perpendicular to the direction of the resonant cavity that stimulated emission begins to be- come large along this direction compared to the pre- ferred direction. This turn has two effects. First, it takes away energy from the preferred direction of emission and second, it contributes to the propagation of dark line defects in the device. Dark line defects are well known and are defects which can absorb both spontane- ous and stimulated radiation and thereby degrade the performance of the laser. As these dark line defects increase in number and are distributed through the crys- tal due to optical radiation, the device is damaged fur- ther and lasing may no longer occur. In addition, there is what is termed as filamenting. In filamenting, multiple portions of the gain region can lase independently due to such effects as nonuniform cur- rent injection into the gain region and poor uniformity of material. Instead of being uniformly radiating, the emission pattern occurs with many intense peaks. When this uncontrolled filamenting occurs, the local field intensity of laser light at the facet can exceed that which is necessary to cause catastrophic degradation. The use of an external cavity on a semiconductor laser can overcome the problems described above. Use of an external cavity will reduce stimulated emission in all directions except that defined by the external cavity and will also reduce spontaneous emission by clamping the gain at a value lower than that usually occurring in the monolithic device thereby reducing the production rate of such dark line defects. In addition, because there 10 15 20 25 30 35 45 50 55 65 2 is usually no spectral nor spatial hole burning in semi- conductor lasers, it is possible to extract nearly all of the multimode output power from a monolithic diode laser in a single frequency when the output is controlled in a single spatial mode using an external cavity. Such exter- nal cavity devices may then be easily tuned over a broad spectral bandwidth and frequency converted using techniques described below. SUMMARY OF THE INVENTION The external cavity laser according to one aspect of the invention includes a monolithic diode laser attached to one end of an external resonator. The external reso- nator has an optically aligned output coupling mirror attached at the end of the resonator opposite the laser and beam shaping optics disposed between the mirror and the laser. The optical elements are arranged so that only one spatial mode will oscillate. Another aspect of this invention includes a mono- lithic diode laser having an impurity level and which is attached to an external cavity such that radiative transi- tions involving the impurity level are selected. In one embodiment, the resonant cavity includes a spherical lens and a planar mirror. In another embodi- ment the cavity includes a cylindrical lens and a spheri- cal mirror. In still another embodiment, the cavity in- cludes a spherical lens and a cylindrical mirror. In yet another embodiment the cavity includes multiple spher- ical lenses and either a planar or cylindrical mirror. In an alternate embodiment, the beam shaping optics includes a waveguide. In this embodiment, a planar mirror is attached to the end of the waveguide opposite to the laser. In another alternate embodiment, the wave- guide contains a cylindrical lens between the laser and the planar mirror. In still another alternate embodiment, the waveguide contains a non-linear optical material between the mirror and the laser. The external cavity laser according to this invention employs a semiconductor laser attached to an external resonator which may have several forms. This con- struction creates a laser which operates with a single spatial mode. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1(a) is a schematic illustration of a side view of an optical cavity with a spherical lens. FIG. 1(b) is the top view of the optical cavity of FIG. 1(a). FIG. 1(e) is a side view of an optical cavity with a cylindrical lens. FIG. 1(d) is the top view of the optical cavity of FIG. 1(c). FIG. 1(e) is a side view of an optical cavity with a Spherical lens and cylindrical mirror. FIG. 1(f) is the top view of the optical cavity FIG. 1(e). FIG. 1(g) is a side View of an optical cavity of a two spherical lens system with a flat or cylindrical (shown in phantom) mirror. FIG. 1(h) is the top view of the optical cavity of FIG. 1(g); FIG. 2(a) is a schematic illustration of a side view of a single mode waveguide optical cavity resonator. FIG. 2(b) is the top view of the waveguide optical cavity of FIG. 2(a). FIG. 2(a) is a side view of a grazing incidence waveguide optical cavity. FIG. 2(d) is a top view of the waveguide optical cavity of FIG. 2(c). FIG. 2(a) is a side view of a grazing incidence waveguide optical cavity resonator containing a non-linear element. FIG 20‘) is a top view of the waveguide optical cavity of FIG. 2(a); ' FIG. 3(a) is a schematic illustration of a side view of a combination geometric and waveguide optical cavity. 5,327,447 3 FIG. 3(b) is top view of the combination geometric and waveguide optical cavity of FIG. 3(a); FIG. 4 is an exploded view of the components of the laser cavity; FIG. 5(a) is a schematic illustration of a side view of a single mode waveguide optical cavity resonator with the laser diode within the external cavity; FIG. 5(b) is a top view of the waveguide optical cavity of FIG. 5(a); FIG. 6(a) is an energy diagram showing a radiative transitiOn to an impurity band; FIG. 60;) is an energy diagram showing a radiative transition from an impurity band. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1(a), a semiconductor laser with a reflective back surface 11 and an antireflective front surface 13 is attached to an optical resonator consisting of a spherical lens 14 and a planar mirror 16. As the laser light diverges from the small dimension of the emitting facet 12, it is collected by the lens 14 located one focal length from the facet 12. The light so collected is colli- mated onto the planar mirror 16 through a mode aperao ture l7 and reflected back through the spherical lens 14 to be refocused onto the laser facet 12. The dimensions of the aperature are such that only the fundamental spatial mode will propagate in the laser resonator. Be- cause the mirror 16 is only partially reflecting, some of the laser light passes through for use. The laser light in FIG. 1(a) may be waveguided in the plane of the diode laser junction region but not necessarily in the same guiding region which occurs in the monolithic device. Guiding in a larger region may be accomplished by confinement in layers of different refractive index which occur above and/or below the usual region of the monolithic device. For devices with a large enough optical transparency region, there will be no guiding and the usual Guassian mode optics apply. Relating to FIG. 2(b), since the laser is constructed so that the width of the emitting region is much larger than the height, the fundamental mode is brought to a focus at mirror 16 through the mode aperature 17 in a manner consistent with the usual Gaussian optics for a laser resonator. In this case, the length of the diode is small compared to the length of the external cavity. When the length of the diode laser is not small compared to the length of the external cavity, the wave must be either 5 10 15 20 25 30 35 45 guided in both dimensions inside the diode or the diode - should be optically transparent such that a fundamental spatial mode of the external cavity may propagate in both dimensions inside the diode laser device. Dark line defects are thought to occur and propagate primarily because of the spontaneous emission within the diode laser itself. If the threshold for stimulated emission can be lowered by control of the diode laser cavity parameters such as an external cavity, the gain will be clamped at a lower level than usually occurs for the monolothic device thereby lowering the level of spontaneous emission. To suppress filamenting, an antireflection coating of sufficiently low reflectivity is applied to one or both sides of the diode laser depending on the external cavity geometry. The external cavity device can then be oper- ated in the fundamental spatial mode which will pro- vide a well defined field intensity at the diode laser facet without hot spots associated with filamenting, thereby allowing the laser to be driven near the limit of cata- strophic degradation. This, together with increasing the 50 55 60 65 4 size of the mode area in the diode laser over that for the monolithic device by the use of an external cavity will allow operation of the laser at higher power levels than that for the monolithic device. For the single mode laser to remain stable, the exter- nal cavity must be kept rigid. Transverse flexing of the external cavity structure should not allow the image of the external cavity mode to be displaced by more than approximately a few per cent of the mode waist width at the diode laser facet in order to insure stable single spatial mode Operation. The length of the cavity should not change according to the known laser art in order to insure stable, single frequency operation. Materials such as Zerodur TM and Super Invar TM which have a low or zero coefficient of thermal expansion at room tem- perature are used in the critical parts of the structure which define the cavity length. Referring to FIG. 4, an external cavity laser comprises a cavity spacer 410 which houses a lens assembly 412. The lens assembly 412 fits into a positioning barrel 420 which is moveabiy mounted in a positioning bracket 418. The barrel 420 is made so as to move along the axis of the cavity and thereby allow the focusing of the light from the laser diode 426. The laser diode 426 is mounted within a copper heat sink 422 and heat sink 422 and diode 426 are rigidly mounted to a mounting bracket 424. The mounting bracket 424 is movably mounted to an adjustment stop 428 which is affixed to the tabs 440 of the positioning bracket 418. A ceramic bracket having a low or zero coefficient of thermal expansion 430 is attached to the heat sink 422 to thermally isolate the diode 426 from the rest of the cavity. Two fiat seals 432 and 434 separate the positioning bracket 418 and the mirror bracket 442 respectively. The mirror bracket 442 holds a mirror 444 at one end of the cavity. The mirror 444 is constrained not to move relative to the cavity by use of a crushable gasket 416 and a mirror retraining bracket 438. The cavity so designed will be not only constant in length but will also not be susceptible to flexing which will change the optical alignment. It should be noted that any change in the length of the cavity does not result in misalignment since the image will still appear on axis with respect to the diode. It should also be mentioned that the laser cavity can include tuning etalons or gratings. For example, an etalon 436 with a free spectral range greater than twice the gain bandwidth of the diode is held in an etalon bracket 414 and secured in position within the mirror bracket 442. In this way tuning of the external cavity laser over its entire bandwidth can be accomplished by tilting the etalon. Referring to FIG. 1(c), the embodiment shown is similar to that of FIG. 1(a), except that cylindrical lens 18 is located one focal length from the laser facet 12. The light from the facet 12 is collimated through an aperture 17 onto a spherical mirror 20. The focal point of the spherical mirror is at the facet 12 of the laser 10. The cylindrical lens 18 is used because the facet 12 is . extended in one direction and need not be focused. The lens 18 is therefore oriented along the line of the facet 12. Referring to FIG. 1(d), the lack of focus with this orientation of the cylindrical lens 18 is readily apparent. Referring to FIG. 1(a), this is the embodiment shown in FIG. 1(a), but with a cylindrical mirror 22 located at a distance from the lens 14 equal to the focal length of the spherical lens 14 plus the focal length of the cylin- 5,327,447 5 drical mirror 22. In such an arrangement, the lens 14 in conjunction with the aperture 17 collirnates light in the direction of orientation of the mirror 22 so that the mirror acts as a planar reflector. Referring to FIG. 1(1), however, in the other direction, the light is defocused by the passage of the beam through the focal point of the lens 14, and is refocused by the mirror 22 upon reflection so as to be collirnated along the length of the facet 12. Referring to FIG. 1(g), a two-lens embodiment of the geometric optical cavity is shown. As previously de- scribed, the first spherical lens 14 is positioned one focal length away from the laser facet 12 and collimates the divergent beam. This collimated beam passes through an aperture 17 and through a second lens 24 located at a distance away from the first lens 14 equal to the focal length of the first lens 14 plus the focal length of the second lens 24. The second lens 24 refocuses the beam onto either a planar mirror 16 located at a distance of the focal length of the second lens 16 or on a cylindrical mirror 22 (shown in phantom), which is located at to a distance away from the second lens 24 equal to the focal length of the second lens 24 plus the focal length of the mirror 22. Referring to FIG. 10:), the top view of FIG. 1(g) shows the positioning of the lenses 14 and 24 so as to produce a collirnated beam on the facet 10 and mir- rors 16 or 22 (shown in phantom) while inverting the beam passing through the pair of lenses 14 and 24. Referring to FIG. 2(a), the diode laser 10 is attached to a waveguide optical cavity comprising a grazing incidence or a single mode waveguide 30 and a planar output coupling mirror 16. The waveguide 30 is at- tached at the laser so as to be symmetric about the emitting facet 12. The laser has a highly reflective coat- ing 11 on the surface away from the waveguide 30. The mirror 16 is located at a distance 1 from this highly reflective coating 11. Referring to FIG. 2(b), D is the width of the grazing incidence waveguide and is ap- proximately equal to the facet 12 width. For wide diode laser devices only a grazing incidence waveguide is a...
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