End pumped solid-state laser

Abstract: An end pumped solid-state laser is comprised of a resonator (RS) formed of a pair of mirrors (5, 6) and a laser rod (7) disposed between the pair of mirrors (5, 6), an exciting light source (1) for generating a pumping light which excites the laser rod (7) and a converging optical element (4) for converging the pumping light from the exciting light source (1) so that the converged pumping light is irradiated on an end face of the laser rod (7). The wavelength of the light generated from the exciting light source (1) is widened in band and/or a converging optical element (4) having a large wavelength dispersion is employed, whereby heat generation in the optical axis direction of the laser rod (7) is uniformed so as to generate a laser beam of large power.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention generally relates to laser generating apparatus and, more particularly, is directed to an end pumped solid-state laser. Description of the Prior Art In Optics Letters, Vol.13 no.4, April 1988, p.306-308 a conventional end pumped solid-state laser is described comprising a resonator formed of a pair of mirrors and a laser rod (i.e., Nd:YAG laser rod), a laser diode for emitting a pumping light which excites the laser rod and a condenser lens for converging the pumping light from a plurality of pumping light sources so that the converged light is irradiated on the end face of the laser rod. The laser beams from the plurality of laser diodes are introduced into optical fibers forming an optical fiber bundle and the laser beam converged by this optical fiber bundle is irradiated through the condenser lens to the end face of the laser rod. However, if the pumping light of large power is radiated on the end face of the laser rod, then temperature gradients of the radial and optical axis directions of the laser rod are increased to deteriorate the efficiency of the laser rod. Therefore, there is a limit on increasing the output power of the end pumped solid-state laser. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved end pumped solid-state laser in which the aforementioned shortcomings and disadvantages of the prior art can be eliminated. More specifically, it is an object of the present invention to provide an end pumped solid-state laser in which a laser rod uniformly generates heat in the optical axis direction thereof so as to generate a laser beam of large power. As an aspect of the present invention, an end pumped solid-state laser is comprised of a resonator formed of a pair of mirrors and a laser rod disposed between the pair of mirrors, an exciting light source for generating a pumping light which excites the laser rod and a converging optical element for converging the pumping light from the exciting light source so that the converged pumping light is irradiated on an end face of the laser rod. The wavelength of the light generated from the exciting light source is widened in band and a converging optical element having a large wavelength dispersion is employed, whereby heat generation in the optical axis direction of the laser rod is uniformed so as to generate a laser beam of large power. The present invention is defined in claim 1. An end-pumped solid-state laser according to the preamble of claim 1 is known from Patent Abstracts of Japan, vol.14, no.348 (E-957) [4291], 27th July 1990 ; & JP-A-2 122 581. Embodiments of the invention are set out in the dependent claims. The above and other objects, features, and advantages of the present invention will become apparent in the following detailed description of illustrative embodiments thereof to be read in conjunction with the accompanying drawings, in which like reference numerals are used to identify the same or similar parts in the several views. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing an arrangement of an end pumped solid-state laser according to an embodiment of the present invention; FIG. 2 is a cross-sectional view illustrating an example of an optical guide bundle used as an optical guide in the present invention; FIG. 3 is a graph graphing a wavelength spectrum of a laser diode and a heat absorbing characteristic of the laser rod; FIG. 4 is a schematic diagram showing an example of a temperature control circuit for laser diode used in the present invention; FIGS. 5A, 5B and 5C are graphs graphing temperature distribution of the laser rod and a schematic diagram; and FIG. 6 is a schematic diagram showing the end pumped solid-state laser according to a second embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described with reference to the accompanying drawings. FIG. 1 generally shows an end pumped solid-state laser according to a first embodiment of the present invention. It will be seen in FIG. 1 that a laser resonator RS is comprised of a pair of opposing concave mirror (input mirror) 5 and plane mirror (output mirror) 6 and a columnar laser rod (Nd:YAG laser rod) 7 disposed between the concave mirror 5 and the plane mirror 6 so as to make its central axis become coincident with the optical axis. Seven semiconductor lasers (i.e., laser diodes) 1 are provided to excite the laser rod 7 and laser beams from these laser diodes 1 are supplied to an optical fiber bundle 2 which is shown more fully in FIG. 2. As shown in FIG. 2, this optical fiber bundle 2 is composed of bundles of seven optical fibers 3 in response to the number of laser diodes, that is, seven in this embodiment. Output laser beams from the optical fiber bundle 2, i.e., a bundle of lights for laser pumping L₁ are converged by a condenser lens 4 and irradiated through the concave mirror 5 onto the laser rod 7 at a small area of the end face thereof. In this embodiment, the condenser lens 4 might be a kinoform lens or a holographic element whose wavelength dispersion is large and therefore the pumping laser beam coincident with the optical axis of the laser rod 7 is focused at the position corresponding to the wavelength thereof within the laser rod 7. Thus, the laser rod 7 generates a laser beam having its own wavelength by the excited beam. This laser beam is repeatedly propagated between the mirrors 5 and 6 in a reciprocal fashion, thereby a laser beam L₂ of the lateral mode conforming to the profile of the pumping light L₁ being generated. A part of the oscillated laser beam L₂ is passed through the mirror 6 and generated to the outside as an output laser beam L₃. FIG. 3 shows a wavelength spectrum of the laser beam generated from the laser diode 1 and a light absorption curve of Nd⁺³ ion in the laser rod 7 of the Nd:YAG laser. As is clear from FIG. 3, the laser rod 7 has excellent light absorption relative to the laser beam having a wavelength of about 808 nm from the laser diode 1 and also has excellent light absorption relative to the laser beam having a wavelength of about 804 nm. FIG. 4 shows an example of a temperature control circuit 8 which controls the temperature such that the wavelength of the oscillated light output from the laser diode 1 may fall within a predetermined range. In FIG. 4, like parts corresponding to those of FIG. 1 are marked with the same references and therefore need not be described in detail. In this embodiment, the temperature control circuit 8 is constructed such that a Peltier effect element 9 and a temperature sensor 10 are unitarily formed with the laser diode 1. When the rise of temperature in the laser diode 1 is detected by the temperature sensor 10, then the laser diode 1 is cooled by the Peltier effect element 9. In FIG. 4, reference numeral 11 designates a heat sink which is used to cool the laser diode 1 and reference numeral 12 designates a laser element. The temperature control circuits 8 having respective semiconductor lasers constructing the laser diodes 1 have setting temperatures which are slightly different from each other with the result that central wavelengths of the laser beams generated from a plurality of laser diodes 1 are deviated by these temperature control circuits 8. Accordingly, the band of the pumping laser beam generated from the optical fiber bundle 2 is widened so as to cover the absorbing band (800 nm to 810 nm) of Nd³⁺ ion. Alternatively, in order to obtain the pumping laser beam whose band is widened, it is possible to employ a plurality of laser diodes which generate laser beams whose central wavelengths are different from each other. If the band of the wavelength of the pumping laser beam is widened, then the converged points of laser beams of respective wavelengths on the laser rod optical axis are displaced so that, as compared with the case such that the band of the wavelength of the pumping laser beam is narrow, an average heat generation coefficient of Nd³⁺ ion in the laser rod 7 is lowered. This is because it is avoided that the heat generation within the laser rod 7 is concentrated in the optical axis direction in response to the dispersion of the converged points. FIGs. 5A and 5B show simulation results of temperature distribution relative to the light absorption coefficient of the laser rod. More specifically, FIGS. 5A, 5B show simulation results in which temperature distributions in the position r of the radial direction in z = 0, 1, 2, 3, 4, 5 mm are calculated by a simulation based on the finite-element method when the average light absorption coefficient α is 6 cm⁻¹ and 3.3 cm⁻¹ where r is the position of the radial direction of the laser rod 7 and z is the position of the laser rod 7 in the optical axis direction (central axis direction). In FIGS. 5A, 5B, ΔT on the ordinate represents the amount (°C) of temperature raised per Watt by the pumping laser beam of the laser rod 7. In the equation shown in FIG. 5C, N represents the refractive index of the laser rod 7 and T represents the temperature (°C), respectively. From the temperature distribution curves in FIGS. 5A, 5B, it will be appreciated that the temperature gradient in the radial direction of the laser rod 7 becomes gentle in accordance with the increase of Z. In the above simulation, the deviation of the converged point of each wavelength of the pumping laser beam irradiated on the laser rod 7 is not taken into consideration. However, if a lens having a large wavelength dispersion (having strong wavelength dispersion) is utilized as the condenser lens 4, then it is possible to locate the converged point of each wavelength of the pumping laser beam on the optical axis. In addition, if the central wavelengths of the laser beams generated from the plurality of laser diodes 1 are deviated, then the band of the pumping laser beam generated from the optical fiber bundle 2 can be widened. Therefore, by utilizing the lens whose wavelength dispersion is large as the condenser lens 4 which converges the pumping laser beam, the heat generation points of the laser rod 7 can be made more uniform. FIG. 6 shows a second embodiment of the end pumped solid-state laser according to the present invention. In FIG. 6, like parts corresponding to those of FIG. 1 are marked with the same references and therefore need not be described in detail. FIG. 6 shows a part of the end pumped solid-state laser according to the second embodiment of the present invention and, in this embodiment, the kinoform lens is utilized as the condenser lens 4 having a large wavelength dispersion. The kinoform lens is a phase hologram lens which has coded informations as the change of the surface relief. A stereo-type kinoform lens has a configuration of Fresnel lens plate formed of rings whose optical thickness is changed in a quadratic function fashion and also whose optical thickness is changed by one wavelength each time it crosses each zone. The kinoform lens looks like well the Fresnel lens but they are not the same. The concavity and convexity of the kinoform lens has a height about twice the wavelength. Further, the kinoform lens makes effective use of light interference in different zones, while the Fresnel lens is adapted to bend a light. The surface configuration of the kinoform lens has the same function as a blaze angle on the diffraction grating. From a theory standpoint, the kinoform lens is different from the amplitude zone plate in which the efficiency is about 5 % and has a quadratic function phase angle in which the efficiency is 100 %. The above-mentioned explanation is based on Sinclair Optics Design Notes, 1990, Winter, Vol. 1, No. 1. Assuming that Fr is the focal length when Λr represents the reference wavelength of the kinoform lens, then the focal length F in the wavelength Λ is given by the following equation (1): 1/F = (Λ/Λr) (1/Fr) The equation (1) yields the following equation (2) with respect to a focal length difference ΔF (= F - Fr) and a wavelength difference ΔΛ (= Λ - Λr). ΔF/F = -ΔΛ/Λr Let it now be assumed that Λr = 819 nm and that ΔΛ = - 10 nm, then F = 20 mm (Λ = Λr) may be satisfied in order to realize ΔF = 0.25 mm. Further, assuming that a magnification of the converging system is given as m = 1.75, then the change of the image surface distance corresponding to ΔF = 0.25 mm is given as ΔD = 2 mm by the tracing of laser beam, which is about a half of the length 5 mm of the typical Nd:YAG laser rod 7. If the wavelength in the range of about 800 to 810 nm band of the laser beam from the laser diode 1 is shorter, then the light absorption coefficient of Nd in the Nd:YAG laser rod 7 is lowered more but the focal length is increased so that the converged point in which the pumping laser beam is converged on the optical axis of the laser rod 7 becomes distant. This is more convenient in order to uniform the heat generation point on the laser rod 7 from a spatial standpoint. Furthermore, the converging optical element having large wavelength dispersion is not limited to the kinoform lens and might be diffraction grating such as the holographic element and so on.

Claims: An end pumped solid-state laser comprising: a resonator (RS) having a pair of mirrors (5, 6) and a laser rod (7) disposed between said pair of mirrors (5, 6); an exciting light source formed of a plurality of laser light sources (1) having different central wave-lengths and which generates a pumping light to excite said laser rod (7); an optical guide (2, 3) for guiding a laser beam from said exciting light source (1) into said laser rod (7); and a converging optical element (4) for converging said laser beam guided by said optical guide (2, 3) and for irradiating said converged laser beam on one end face of said laser rod (7) so as to excite said laser rod (7) characterised by said converging optical element having a large wavelength dispersion characteristic whereby heat generation in the optical axis direction of the laser rod is uniformed so as to generate a laser beam of large power. An end pumped solid-state laser of claim 1, wherein said laser light source (1) is a laser diode. An end pumped solid-state laser of claim 1 or 2, wherein said laser light source includes a temperture sensor and a cooling circuit, in which said cooling circuit (9) is controlled (8) in response to an ouput of said temperature sensor and said laser light source (1) is maintained at a predeterminded temperature. An end pumped solid-state laser of claim 3, wherein a control temperature set to one laser light source (1) is different from those set to other laser light sources (1), whereby the wavelength of output laser light from said excited light source (1) is widened. An end pumped solid-state laser of anyone of claims 1 to 4, wherein said optical guide is composed of a plurality of optical fibers (3) in which laser beams from said light sources (1) become incident on the one ends of said optical fibers (3) and the output laser light of said excited light source is generated from the bundled (2) other ends of said optical fibers (3). An end pumped solid-state laser of claims 6, wherein said converging optical element (4) is a kinoform lens. An end pumped solid-state laser of claim 6, wherein said converging optical element (4) is a diffraction grating. An end pumped solid-state laser of claim 8, wherein said diffraction grating is a holographic element.

Inventors: KANEDA YUSHI C O SONY CORPORAT, KUBOTA SHIGEO C O SONY CORPORA, KANEDA, YUSHI, C/O SONY CORPORATION, KUBOTA, SHIGEO, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1991
Main classification: H01S3
IPC codes: H01S5, H01S3