Apparatus and method for producing blue-green light radiation

Abstract: Apparatus and method for producing coherent blue-green-­ light radiation having a wavelength of essentially 490-500 nm. A diode laser (10), such as a strained-layer InGaAs/GaAs diode laser, provides a 980-1,000 nm beam (12), and a nonlinear crystal (19) of KTP (KTiOPO₄) produces coherent radiation (22) by noncritically phase-matched second-harmonic generation (SHG) of said beam. The beam preferably has a wavelength of essentially 994 nm for generating radiation having a wave­ length of essentially 497 nm. The crystal is disposed within an optical resonator (18) and the frequency of the laser is locked to that of the resonator. Alternatively, two diode lasers (10,11) are oriented to provide orthogonally polarized beams each with a wavelength of 980-1,000 nm but within essentially 1 nm of each other, and the KTP crystal is orient­ ed with its a and c axes parallel to the orthogonally polar­ ized beams. The KTP crystal may have an associated optical waveguide along which the beam is propagated to enhance SHG efficiency.

Description: Background of the Invention Field of the Invention This invention relates to a laser source and method for producing coherent blue-green-light radiation, and more particularly relates to a miniature solid-state laser source and method for producing said radiation by second-harmonic generation of the output of a semiconductor diode laser in a nonlinear crystal that permits noncritical phase matching over a wide range of temperatures, angles and input wavelength, rendering it especially suitable for optical storage applications. Blue-green lasers are desirable because they permit a significant enhancement of the storage density of an optical recording system. However, fundamental material fabrication problems are encountered in developing diode lasers operating at wavelengths <600 nm (i.e., in the blue-green range). Insofar as we are aware, there are only two reported examples of direct frequency doubling of diode laser radiation by second-harmonic generation (SHG). T. Taniuchi et al. reported in CLEO 87, Paper WP6, discloses generation of essentially 420 nm radiation by SHG of a GaAlAs diode laser using a LiNbO₃ waveguide. However, this material does not permit phase-matched SHG at 420 nm, and the blue-green light has a curved wavefront that cannot be focussed to a diffraction-limited spot and hence is unsuitable for optical storage and many other uses. Also, LiNbO₃ has a relatively low optical damage threshold due to photorefractive processes. J.C. Baumert et al, in APPLIED OPTICS, Volume 24, page 1299 (1985) reported that frequency doubling of 860 nm GaAlAs diode lasers can be achieved with KNbO₃ at room temperature; but this material has many disadvantages that severely limit its suitability for practical devices. For example, KNbO₃ is difficult to grow and undergoes two phase transitions between the growth temperature and room temperature. Also the temperature and wavelength tolerances for phase matching are extremely narrow, and therefore would require a laser of very specific wavelength and also precise temperature stabilization of the SHG crystal. T.Y. Fan et al, in APPLIED OPTICS, Volume 24, page 2390 (1987) reported that frequency doubling of 1064 nm Nd:YAG diode lasers can be achieved with a nonlinear crystal of essentially KTiOPO₄ (KTP). This is achieved by non-critically phase matched second-harmonic generation of the beam. Frequency doubling at other frequencies is not disclosed. Other related approaches use diode-laser pumped Nd:YAG lasers to generate 530 nm radiation by phase-matched SHG since suitable nonlinear materials are available for this process. Similarly, frequency mixing of an 808 nm diode laser and a 1.06 µm Nd:YAG laser is possible in the commercially available nonlinear material KTiOPO₄ (KTP), as disclosed in U.S. Patent 4,791,631 issued December 13, 1988, assigned to the assignee of the present invention. There is a need for a practical nonlinear material which can be cut for noncritically phase-matched SHG of the shortest wavelength possible and combined with a semiconductor diode laser that operates at the phase-matching wavelength. Insofar as applicants are aware, coherent radiation has never heretofore been provided by SHG of a diode laser in a nonlinear crystal consisting essentially of KTP. Summary of the Invention The present invention is defined in the attached claims. Towards this end and according to the invention, a miniature blue-green laser source has been developed that has two key components: (a) a semiconductor diode laser recently developed by others and having a wavelength of essentially 980-1,000 nm; and (b) a crystal of KTP that applicants unexpectedly found to permit noncritical type II phase-matched SHG of essentially 980-1,000 nm radiation over a wide range of temperatures, input wavelengths and angles of the input beam to the crystal. The apparatus and method embodying the invention produce coherent blue-green-light radiation having a wavelength of essentially 490-500 nm. A diode laser, such as a strained-layer InGaAs/GaAs, semiconductor laser, provides an essentially 980-1000 nm beam, and a nonlinear crystal of essentially KTP produces the blue-green radiation by noncritically phase-matched SHG of said beam. The beam preferably has a wavelength of 994 nm for generating 497 nm radiation. Also the frequency of the laser is preferably matched and locked to that of an optical resonator within which the crystal is disposed. Brief Description of the Drawing Fig. 1 is a schematic diagram of a resonator-enhanced apparatus for generating blue-green light, according to one embodiment of the invention; Fig. 2 is a schematic diagram of such an apparatus according to another embodiment of the invention; Fig. 3 is a schematic diagram of a resonator-enhanced apparatus for generating blue-green light, according to a modified embodiment; and Fig. 4 is a schematic diagram of an apparatus for generating blue-green light using an optical waveguide according to a variation of the invention. Description of Preferred Embodiments As illustrated in Fig. 1, the apparatus embodying the invention comprises two strained-layer InGaAs/GaAs diode lasers 10, 11 with collimation optics and modulated output beams 12, 13, respectively, that each provide essentially 994 nm fundamental radiation. Beams 12, 13 are orthogonally polarized and directed to a conventionally coated polarization beamsplitter 14. Beamsplitter 14 combines the 994 nm beams 12, 13 into a beam 15 that is directed to a standard beamsplitter 16. Beamsplitter 16 passes the 994 nm fundamental radiation via a focussing lens 17 to a passive Fabry-Perot resonator 18. Resonator 18 is resonant at a wavelength of 994 nm. It may be formed by placing a crystal 19 consisting of essentially KTP between two mirrors 20, 21 which are highly reflective at 994 nm; or, if preferred, resonator 18 may be formed by polishing spherical and/or flat surfaces at opposite ends of the crystal and depositing highly reflective coatings thereon. Mirrors 20, 21 have facing surfaces that reflect the 994 nm; and mirror 21 transmits the 497 nm radiation. The birefringence of KTP crystal 19 causes the resonator 18 to have two sets of resonant modes. Laser diode 10 is linearly polarized along the a-axis of the KTP crystal and is frequency locked, in the manner hereafter described, to the a-axis polarized resonance of the resonator. Laser diode 11 is polarized along the c-axis of the crystal and is similarly frequency locked to the c-axis polarized resonator resonance. The beam 15 of essentially 994 nm radiation is focussed by lens 17 into, and is propagated along the b-axis of, KTP crystal 19, thereby producing a beam 22 of coherent blue-green-light radiation at essentially 497 nm by noncritically phase-matched SHG of beam 15. The frequencies of these two resonances may not be identical; however, as long as they are within the 1.4 nm wide phase-matching bandwidth for SHG, beam 22 will be efficiently produced. The diode lasers 10, 11 are frequency locked to the resonances of the resonator 18 by respective feedback loops that comprise a polarization beamsplitter 25 and two detection and feedback circuits 26, 27. Beamsplitter 16 directs a beam 28 of 994 nm radiation to beamsplitter 25 which then splits said beam into an a-axis polarized beam 29 and a c-axis polarized beam 30. Beam 29 is directed to detection/feedback circuit 26 which may, for sake of illustration, be of the type disclosed in the April 1987 issue of "IEEE Journal of Lightwave Technology" at pp. 485 et seq. This circuit 26 comprises briefly means (not shown) for generating a signal indicative of deviations in the frequency of the beam 12 from the frequency of the resonator 18 and for adjusting the injection current to diode laser 10 and/or its temperature to maintain the laser frequency at the resonator frequency. Beam 30 is directed to detection/feedback circuit 27 which comprises similar means (not shown) to adjust the injection current to diode laser 11 and/or its temperature. As illustrated in Fig. 2, the apparatus embodying the invention comprises a single strained-layer InGaAs/GaAs diode laser 60 with collimation optics and output beam 61 to provide essentially 994 nm fundamental radiation of linear polarization. Crystal 64 of essentially KTP is part of a passive resonator 65. Mirrors 66, 67 at opposite ends of crystal 64 are highly reflective at 994 nm. Mirror 67 is highly transmissive at 497 nm. The nonlinear crystal 64 is oriented such that its a- and c- axes are at an angle of 45 degrees to the polarization of the essentially 994 nm beam 61 for Type II noncritically phasematched second harmonic generation. The phaseplate 69 is oriented such that it corrects the polarization of the 994 nm radiation emerging from the crystal 64 so that the 994 nm radiation has a polarization direction of 45 degrees with respect to the a- and c- axes of the KTP crystal 64 when it re-enters the crystal after a round-trip pass through the resonator. The diode laser 60 is frequency locked to the resonance of the resonator 65 by a feedback loop which comprises a beam splitter 62 and the detection and feedback circuitry 71. The feedback circuitry 71, as described in connection with the embodiment of Fig. 1, adjusts the injection current to diode laser 60 and/or its temperature. As illustrated in Fig. 3, the apparatus embodying the invention comprises a single strained-layer InGaAs/GaAs diode laser 40 with collimation optics and modulated output beam 41 to provide essentially 994 nm fundamental radiation. Beam 41 is directed through a dichroic beamsplitter 42 and a focussing lens 43 into a crystal 44 of essentially KTP that forms part of a passive resonator 45. Mirrors 46, 47 (or highly reflective coatings) at opposite ends of crystal 44 are highly reflective at 497 nm; but mirror 47 has a residual small transmission at 497 nm adjusted for maximum blue-green output power for the characteristics of the particular KTP crystal used. Beam 41 is polarized at 45° to the a- and c- axes of the crystal 44. Resonator 45 is designed to be resonant at the second-harmonic wavelength of essentially 497 nm. Since the second-harmonic radiation is linearly polarized along the a-axis of crystal 44, the second-harmonic radiation generated within the crystal can excite a similar a-axis polarized resonant mode of resonator 45. Thus, a beam 48 of coherent blue-green-light radiation at essentially 497 nm is produced by noncritically phased-matched SHG of beam 41 as focussed by lens 43. The wavelength of the generated second-harmonic radiation can be controlled by tuning the fundamental diode laser wavelength so that the essentially 497 nm radiation generated is frequency locked to match the resonant wavelength of resonator 45 by directing beam 49 to detection/feedback circuitry 50 which, as in the earlier described embodiment, adjusts the injection current to diode laser 40 and/or its temperature. It should be noted that the efficiency obtained with the configuration of Figs. 1 and 2 are essentially equivalent to the square of the finesse of their passive resonators 18, 65, respectively. The efficiency enhancement for the Fig. 3 configuration is essentially equal to the finesse of its resonator 45. The efficiency enhancement is thus less for the Fig. 3 configuration, but that configuration is simpler than that of Fig. 1 wherein only one diode laser needs to be frequency locked to a resonant mode of the resonator. The resonator 18 or 45 may, if desired, be replaced by a ring resonator comprising three or more reflecting surfaces. These surfaces may be provided by external mirrors or by polished crystal surfaces, such as described in the June 1988 issue of the "IEEE Journal of Quantum Electronics" at pp. 913 et seq. If desired, and as illustrated in Fig. 4, an optical waveguide 50 may be applied to an edge of a KTP crystal 51. A diode laser 52 with collimation optics and modulated output beam 53 provides essentially 994 nm fundamental radiation. Beam 53 is directed through a focussing lens 54 into the waveguide to enhance the SHG efficiency in producing a beam 55 of coherent blue-green-light radiation at essentially 497 nm. Note that with this embodiment, no locking of laser frequency to resonator frequency is needed. It should also be noted that, according to a feature of the invention, the phase matching of the type II laser herein described is achieved by tuning the frequency of the diode laser source to that of a passive resonator by fine adjustment of the laser injection current. This is in contrast to the prior art method wherein the resonator frequency is tuned to that of the laser source by applying fields to the resonator. While the invention has been shown and described with respect to preferred embodiments thereof especially suitable for use in optical storage applications, the invention also is highly desirable for use in printers and color displays.

Claims: Apparatus for producing coherent blue-green light radiation comprising: at least one diode laser (10, 11, 40, 52, 60) for providing a beam (12, 13, 41, 53, 61); and a nonlinear crystal (19, 44, 51, 64) of essentially KTiOPO₄ (KTP) which produces the coherent radiation by non-critically phase-matched second harmonic conversion of said beam; characterised in that: the beam provided by the at least one diode laser has a wavelength of essentially 980-1,000 nm so that the blue-green light radiation has a wavelenght of essentially 490-500nm; and the apparatus further comprises a resonator (18, 45, 65) resonant at a wavelength of essentially 980-1,000 nm and in which said crystal is disposed. An apparatus as claimed in claim 1, wherein the diode laser (10, 11, 40, 52, 60) is a strained layer InGaAs/GaAs diode laser. An apparatus as claimed in claim 1 or claim 2, wherein the beam (12, 13, 41, 53, 61) has a wavelength of essentially 994 nm for generating radiation having a wavelength of essentially 497 nm. An apparatus as claimed in any preceding claim, wherein the resonator (18, 45, 65) consists of a pair of reflectors (20, 21, 46, 47, 66, 67) disposed adjacent opposite sides of the crystal. An apparatus as claimed in any preceding claim, further comprising means (20, 21, 46, 47, 66, 67) for matching the light frequency of the essentially 980-1,000 nm beam to the resonant frequency of the resonator (18, 45, 65). An apparatus as claimed in any one of claims 1 to 5, wherein the coherent radiation is propagated along the b axis of the crystal. An apparatus as claimed in any one of claims 1 to 5, wherein: the at least one diode laser is two diode lasers oriented to provide orthogonally polarised beams of essentially 980-1,000 nm wavelength; and the non-linear crystal is oriented with its a- and c- axes parallel to the polarisation direction of said orthogonally polarised beams. An apparatus as claimed in any one of claims 1 to 5, including locking means comprising: means (26, 27) for each laser (10, 11) for generating a signal indicative of deviations in the frequency of that laser beam from a respective preselected frequency; and means (26, 27), including feedback means, for each laser for controlling the temperature and/or injection current of that laser beam according to variations in said signal to maintain the laser beam at its respective frequency. An apparatus as claimed in any one of claims 1 to 5, wherein said beam is polarised at essentially 45° to the a- and c- axes of said crystal. An apparatus as claimed in claim 9, wherein: the resonator further comprises a phaseplate (69) which is oriented such that the polarisation of the essentially 980-1,000 nm radiation is at an angle of essentially 45 degrees with respect to the a- and c- axes of the crystal when it re-enters said crystal after a round trip pass through the resonator; and the apparatus further comprises: means (62,71) for locking the frequency of the diode laser to a resonant frequency of the resonator. An apparatus as claimed in claim 1, wherein said non-linear crystal comprises an optical waveguide oriented such that the beam is propagated along said waveguide for generating the coherent radiation by noncritically phase-matched second harmonic generation of said beam. A method for producing coherent blue-green light radiation comprising the steps of: using at least one diode laser (10, 11, 40, 52, 60) for providing a beam (12, 13, 41, 53, 61); and producing said radiation by noncritically phase-matched second harmonic generation of said beam in a nonlinear crystal (19, 44, 51, 64) of essentially KTiOPO₄ (KTP); characterised in that: the blue-green light radiation has a wavelength of essentially 490-500 nm; the beam provided by the at least one diode laser has a wavelength of essentially 980-1,000 nm; and the crystal is disposed in a resonator (18, 45, 65) resonant at a wavelength of essentially 980-1,000 nm.

Inventors: HARDER CHRISTOPH STEPHAN, LENTH WILFRIED, MEIER HEINZ PETER, RISK WILLIAM PAUL, HARDER, CHRISTOPH STEPHAN, LENTH, WILFRIED, MEIER, HEINZ PETER, RISK, WILLIAM PAUL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: G02F1
IPC codes: H01S5, G02F1, H01S3