JP2007073552A - Laser light generator and image formation apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser light generator which can stably oscillate a laser light and can be made small in size, and to provide a picture formation apparatus wherein the laser light generator is mounted as a light source. <P>SOLUTION: A plurality of laser lights having plural light axes are oscillated in an optical resonator 15 in the laser light generator 10. Therefore, even if external disturbance occurs in the laser light generator 10 or aging or the like occurs in the mirrors 5-7 of the optical resonator 15, the laser light generator 10 can stably generate a laser light. In addition, the optical resonator 15 is commonly provided with the mirrors 5, 6 and 7 for reflecting laser lights comprised of plural light axes, so that the laser light generator 10 can be made compact. Namely, the laser light generator 10 can be made small in size because of such a structure that laser lights comprised of plural light axes are generated in only one optical resonator 15. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laser beam generator for generating laser beam and an image generation device equipped with the laser beam generator as a light source.

  2. Description of the Related Art Conventionally, for example, an SHG (Second Harmonic Generation) type laser in a lateral single mode or lateral multimode optical resonator has been developed, and a laser beam generator has been commercialized. In such an intra-cavity SHG type laser beam generator, since a nonlinear optical element for wavelength conversion is arranged in the resonator, noise due to coupling between modes is generated. Inter-mode coupling is a phenomenon described in Journal of Optical Society of America B, Volume 3, Issue 9, pp. 1175-1180 (September 1986). When the oscillation wavelength of a laser medium is converted by a nonlinear optical element, laser light with a wavelength other than the oscillation wavelength of the laser medium is likely to oscillate due to loss of laser light of that wavelength if there is spatial hole burning or the like. . When the laser light of the other wavelength oscillates, the laser light of another wavelength or the original wavelength oscillates by the wavelength conversion element. By repeating this, a desired oscillation wavelength is generated or disappears and becomes unstable, resulting in noise.

In order to solve such a problem, there is a technique for reducing the noise caused by coupling between modes by increasing the length of the optical resonator to simultaneously oscillate laser beams in a longitudinal mode of about 100 to 500 (multi axial mode). It is disclosed (for example, see Patent Document 1).
US Pat. No. 5,444,749 (FIG. 1 etc.)

  However, in the technique of the above-mentioned Patent Document 1, the length of the resonator becomes long, so that the entire apparatus becomes large.

  On the other hand, in the transverse multi-mode intra-cavity SHG laser, the optical path length of each beam in the transverse mode is slightly different within the cavity, and therefore oscillates at different wavelengths. That is, since there are a large number of oscillation wavelengths in accordance with the number of longitudinal and transverse modes in the band selected by the wavelength selection element, noise is reduced accordingly. However, there is a possibility that the optical path distribution in the resonator may be shifted due to disturbances, fluctuations in the transverse mode, or changes over time of mirrors constituting the resonator, and there is a possibility that laser light cannot be generated stably.

  In view of the circumstances as described above, an object of the present invention is to stably oscillate laser light and to realize downsizing, and an image generation apparatus equipped with the laser light generator. Is to provide.

  In order to achieve the above object, a laser light generator according to the present invention generates an excitation light, and an excitation light generator capable of dividing the generated excitation light into a plurality of optical axes, and the divided excitation light Irradiating gain medium, and using the gain medium, for oscillating laser light having a plurality of optical axes for each of the divided excitation lights by optical amplification, at least for reflecting each laser light An optical resonator sharing one mirror, a wavelength controller provided in the optical resonator for controlling the wavelength of each oscillating laser beam, provided in the optical resonator, A nonlinear optical element for generating a second harmonic.

  In the present invention, laser beams having a plurality of optical axes oscillate independently in the optical resonator, and the second harmonic is generated by the nonlinear optical element. Thus, even if there are a change in transverse mode, a change with time of the mirror of the optical resonator, etc., the laser light can be stably generated by a plurality of independent laser beams. In the present invention, since at least two independent laser beams are generated by one optical resonator, it is possible to reduce the size.

  When there is one gain medium, the divided excitation light may be irradiated to a plurality of places in the one gain medium. Alternatively, there may be a plurality of gain media for each of the divided excitation lights. When there are a plurality of gain media, the number of optical axes of the divided pumping light may be set to be an integral multiple of the number of gain media. When there are a plurality of gain media, the gain media is often a fiber. However, the present invention is not necessarily limited to fibers, and there may be a plurality of crystalline or amorphous gain media.

  When the gain medium is a fiber and an FBG (Fiber Bragg Grating) is provided on the fiber, the optical resonator may have at least one mirror in addition to the BG. When the gain medium is a fiber, there are a plurality of fibers for each of a plurality of optical axes. If the gain medium is not a fiber, the optical resonator need only have at least two mirrors.

  In the “nonlinear optical element”, it is preferable that all laser beams divided by the excitation light generator are targets of the nonlinear optical effect, but this is not necessarily limited thereto. That is, it is only necessary to target laser beams of at least two axes (two) out of all the divided laser beams. The same can be said for the “wavelength controller”.

  It goes without saying that "reflection" is not limited to 100% reflection.

  “Wavelength region” means a wavelength band that is very small to some extent, and also includes a complete single wavelength.

  The mode of each laser beam to be oscillated may be a transverse multi-mode or a transverse single mode.

  In the present invention, the wavelength controller includes a wavelength selection element for aligning a center wavelength of each laser beam to a predetermined wavelength. Alternatively, the wavelength controller includes a wavelength selection element for selecting a laser beam having a different wavelength range for each of the oscillating laser beams. Thus, speckles can be reduced by selecting different wavelengths for each laser beam to be oscillated by the wavelength selection element.

  In the present invention, the nonlinear optical element has a polarization inversion structure formed with a different period for each laser beam. Thereby, for example, it is only necessary to prepare at least one nonlinear crystal, which contributes to miniaturization of the apparatus.

  In the present invention, the gain medium is a solid-state laser medium, and the solid-state laser medium has a first reflectance, and each of the laser beams passes through the first medium. And a reflection area region provided between the respective emission areas. Thereby, interference etc. of each laser beam can be suppressed and a plurality of laser beams oscillate stably. “Solid-state laser medium” means here a crystal, amorphous, or other solid-state laser medium, excluding a fiber laser.

  The present invention further includes a light shielding member for suppressing interference between the laser beams. Thereby, a plurality of laser beams oscillate stably.

  An image generation apparatus according to the present invention generates an excitation light, an excitation light generator capable of dividing the generated excitation light into a plurality of optical axes, a gain medium to which the divided excitation light is irradiated, Using a gain medium, an optical resonator for oscillating laser light having a plurality of optical axes for each of the divided excitation lights by optical amplification, and each laser provided in the optical resonator and oscillating A wavelength controller for controlling the wavelength of light; a non-linear optical element provided in the optical resonator for generating a second harmonic of each of the laser beams; and each of the generated optical elements for generating an image. And a modulation element for modulating the second harmonic laser beam.

  Examples of the modulation element of the image generation apparatus include GLV (Grating Light Valve), DMD (Digital Micro-mirror Device), liquid crystal, EL (Electro-Luminescence) element, and the like.

  As described above, according to the present invention, it is possible to stably oscillate laser light and realize downsizing.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 is a schematic diagram showing a laser beam generator according to an embodiment of the present invention.

  The laser beam generator 10 includes an excitation light source 2, a collimating lens system 4, a condenser lens 8, mirrors 5, 6, and 7 constituting an optical resonator 15, a laser medium 11 disposed in the optical resonator 15, a wavelength A selection element 13 and a wavelength conversion element (nonlinear optical element) 12 are provided. The excitation light source 2 generates excitation light for exciting the laser medium 11. The collimating lens system 4 uses the microlens array 3 to convert the light emitted from the excitation light source 2 into parallel light. The condensing lens 8 condenses the parallel laser light and makes it incident on a predetermined region of the laser medium 11.

  FIG. 2 is a perspective view showing the excitation light source 2. As the excitation light source, for example, a one-dimensional LD (Laser Diode) array (so-called bar laser) having a wide light emitting region width is used, and laser excitation light is generated in an elliptical transverse mode pattern. Such a one-dimensional LD array has an array structure in which a plurality of laser emitters are arranged one-dimensionally. For example, a laser diode 11 having a quantum well structure of GaAlAs (gallium / aluminum / arsenic) is used for the laser medium 11 containing Nd ions, and an output of about 797 to 810 nm and about 40 W is obtained. The material of the LD is not limited to this, and may be appropriately selected so as to match the absorption wavelength band of the laser medium 11. For example, a 910-980 nm semiconductor laser is used for the laser medium containing Yb ions.

  As shown in FIG. 2, striped emitters 2a, 2a,... Having an opening width w are formed in a direction along the pn junction with a predetermined pitch d. Output light of a substantially elliptical transverse mode pattern is obtained from each emitter 2a. For example, 25 emitters 2a having a width w of 100 μm are provided. Moreover, the pitch d of each emitter 2a is 400 micrometers, for example, and the size D of the longitudinal direction of a light source part is about 10 mm. The value and number of w, and the values of d and D are not limited to these values.

  As described above, the excitation light source 2 is configured by one or a plurality of bar lasers as shown in FIG. When there is one bar laser, the microlens array 3 is also configured in one dimension. When a plurality of bar lasers are arranged in a direction perpendicular to the arrangement direction of the emitters 2a, that is, when the emitters 2a are arranged two-dimensionally, the microlens array 3 is also configured two-dimensionally. Good. For example, a cylindrical lens is used as the condenser lens 8. Each of the condenser lenses 8 is arranged one-dimensionally or two-dimensionally, and the light emitted from the bar laser is divided by the number of the condenser lenses 8. In the example shown in FIG. 1, since there are two condenser lenses 8, excitation light having two optical axes is generated. Practically, it is preferable that there are more than two condenser lenses 8. The excitation light source 2, the collimating lens system 4, and the condenser lens 8 constitute an excitation light generator that can divide the excitation light into a plurality of optical axes.

  The laser medium 11 is excited and amplified by the reciprocating light between the mirrors 5 and 7 through the reflection of the mirror 6 constituting the optical resonator 15, and oscillates the laser light corresponding to each excitation light. The mode of each laser beam oscillated by the laser medium 11 may be a transverse multimode or a transverse single mode. The laser medium 11 has a wide gain wavelength range or several oscillation wavelengths depending on the area of the irradiation region of the excitation light source 2. Therefore, if wavelength control (wavelength selection) is not performed by the wavelength selection element 13 described later, some wavelengths may oscillate or stop at the same time and become unstable.

  As the laser medium 11, for example, Nd: YAG, Nd: YVO4, ceramic YAG, Yb: YAG, Yb glass or the like is used. When Nd: YAG is used, the fundamental wavelength of the oscillating laser light is 1064 nm. The laser medium 11 has a shape such as a cylindrical shape or a rectangular parallelepiped plate shape. The laser medium 11 is formed to be thin if the absorption coefficient is large and to be thick to some extent if the absorption coefficient is small in order to secure the amount of absorption and reduce the loss of the oscillated laser. In the present embodiment, since the excitation light composed of a plurality of optical axes is incident on the laser medium 11, an irradiation area is set for each excitation light. That is, a plurality of excitation light irradiation regions exist in one laser medium 11. Since the portion of the laser medium 11 irradiated with the excitation light generates heat and the temperature rises, a lens action occurs due to the temperature dependence of the refractive index (usually, the refractive index increases as the temperature rises). Therefore, each beam is separated by the confinement effect and oscillates in an array.

  The wavelength selection element 13 can make all the center wavelengths of the laser beams of the oscillating optical axes all close to each other, or conversely, have different wavelengths. In the above example, the center wavelength is, for example, around 1064 nm, but is not limited thereto. Examples of the wavelength selection element 13 include a diffraction grating, a volume hologram (such as a three-dimensional diffraction grating), a BRF (BiRefringent Filter) (birefringence filter), an etalon, or a combination of a plurality of these.

  Since the independent laser beam oscillates in an uncorrelated phase as in the present embodiment, the entire laser beam has a noise reduction effect when superimposed. That is, as the number of independent laser beams increases, the number of laser beams as a whole is less affected by disturbances, transverse mode fluctuations, or changes over time of the optical resonator 15, and the generation of laser light becomes more stable.

  For example, as described above, the wavelength selection element 13 may be configured to positively select slightly different wavelengths for each of the oscillating laser beams. In this case, as the wavelength selection element 13, for example, a plurality of volume holograms (three-dimensional diffraction gratings or the like) having different reflection or diffraction wavelength ranges for each oscillating laser beam are arranged. In this way, the wavelength selection element 13 is effective in reducing speckles by selecting different wavelengths for each laser beam to be oscillated. In this case, in particular, it is desirable that the distance between the central wavelengths in each reflection wavelength region is 1 nm or more.

  The nonlinear optical element 12 is an element that generates the second harmonic of the laser light oscillated by the laser medium 11. If the fundamental wave is 1064 nm as described above, the laser beam 19 of 534 nm is output. The mirror 7 reflects the laser beam having the fundamental wavelength and transmits the laser beam 19 having the second harmonic.

For example, the nonlinear optical element 12 is made of a crystal or amorphous having a nonlinear optical effect. Examples of the nonlinear optical element 12 include BBO ((β) Barium Borate), LBO (Lithium Triborate), CLBO (Cesium LBO), KTP (KTiOPO ₄ ), LiNbO 3 (Lithium Niobate), LiTiO 3 (Lithium Titanate), PPSLT, and the like. It is done.

  The nonlinear optical element 12 includes a type in which an artificial optical waveguide is formed on these nonlinear optical elements and a type having a polarization inversion periodic structure that enables quasi phase matching. This is a type in which the direction of spontaneous polarization of the crystal is periodically reversed. Examples of the type having the domain-inverted periodic structure include PPLN (Periodically Poled Lithium Niobate) and PPLT (Periodically Poled Lithium Tantalate). In addition, stoichiometric materials have been developed to reduce defects in wavelength conversion element materials and increase the optical damage threshold, and PPSLT (Periodically Poled Stoichiometric Lithium Tantalate) that combines these materials is now commercially available. I'm starting.

  Normally, when a single laser beam is wavelength-converted, a single type of polarization inversion period is created. FIG. 3 is a schematic diagram showing such a single-period type nonlinear optical element. The nonlinear optical element 16 has a polarization inversion periodic structure 16a. For the sake of explanation, the periodic structure 16a is drawn with diagonal lines. However, in an actual device, it is desirable that the periodic structure 16a is transparent at least at the wavelength used, and thus the domain-inverted periodic structure 16a is often transparent.

  On the other hand, the nonlinear optical element 12 according to the present embodiment has polarization inversion periodic structures 12a to 12e as shown in FIG. 4 in order to cope with laser light generated at a plurality of optical axes. In the case of this FIG. 4, it can respond to five laser beams. Of course, the number is not limited to five. As a result, for example, only one nonlinear crystal needs to be prepared, which contributes to downsizing of the apparatus.

  The conversion wavelength is slightly different depending on each cycle, but this is achieved by adjusting the laser oscillation wavelength by design, controlling the crystal angle and temperature, and finely adjusting the wavelength as necessary. Thereby, a state close to phase matching is maintained in each channel.

  As described above, in the present embodiment, laser beams having a plurality of optical axes oscillate independently within the optical resonator 15. Therefore, even if a disturbance occurs in the laser light generator 10 or there is a change with time of the mirrors 5 to 7 of the optical resonator 15, the laser light can be generated stably. Further, for example, since the optical resonator 15 shares the mirrors 5, 6, 7 and the like that reflect the laser light having a plurality of optical axes, the miniaturization can be realized. That is, since the laser beam generator 10 is configured to generate laser beams having a plurality of optical axes with one optical resonator 15, the laser beam generator 10 can be reduced in size.

The laser medium 11 can be further configured as follows. For example, since laser light having a plurality of optical axes is emitted from the laser medium 11, it is preferable that adjacent beams do not become one beam (mode). For this reason, the interval between the beams is set to be sufficiently wide with respect to the mode diameter of the oscillating laser beam (the diameter of the region where the beam intensity is 1 / e ² or more of the peak intensity).

  Alternatively, a partition may be provided between the beams. Specifically, as shown in FIG. 5, the incident surface (or / and the exit surface) 22 of each laser beam 26 of the laser medium 21 (the incident surface 22 is also the incident surface of the excitation light from the excitation light source). Are formed with a passage region 22a for the laser beam 26 and a reflection region 22b. In the example shown in FIG. 5, a reflection region 22 b is formed between two passage regions 22 a on one incident surface 22. Thus, by providing the partition part which consists of reflection area | region 22b, interference of each laser beam can be prevented.

  The passage region 22a and the reflection region 22b are formed by applying an AR (Anti Reflection) coating, for example, to the incident surface 22 in a state where a portion to be the reflection region 22b is masked.

  Conversely, the partition portion can also be formed by coating the reflective region 22b with a film having a higher reflectance than that of the passage region 22a.

  In the example shown in FIG. 5, there are two beams. However, when there are three, or four or more beams, it is only necessary to form a passing region 22 a corresponding to the number of beams. For example, the passage areas 22a may be arranged one-dimensionally, and a plurality of reflection areas 22b may be formed between the passage areas 22a. When the beam is irradiated two-dimensionally, for example, the passage regions 22a can be arranged in a lattice pattern.

  In FIG. 5, interference between the laser beams can also be prevented by arranging a light shielding member along the optical path at the front stage and / or the rear stage of the laser medium 11. The light shielding member may be any material, such as metal, resin, or other light shielding material. The light shielding member may have any shape such as a plate shape.

  FIG. 6 is a schematic view showing a laser beam generator according to another embodiment of the present invention. In the following description of the embodiments, the same members, functions, and the like included in the laser light generation apparatus 10 shown in FIG. 1 and the like will not be described or will be mainly described.

  The laser beam generator 30 according to the present embodiment includes the fiber 25 as a laser gain medium instead of the solid-state laser medium 11 as described above. The excitation light generated by the excitation light source 2 is divided by the condenser lens 8, and the divided laser beams are reflected to the mirror 6 via the fibers 25a and 25b, respectively. As the fiber 25, for example, a fiber doped with a rare earth element is used. In FIG. 6, the number of fibers 25 is two, but it goes without saying that the number of fibers 25 can be appropriately changed according to the number of optical axes of the divided excitation lights.

  When wavelength conversion is performed inside or outside the optical resonator 35, the fiber is preferably a polarization maintaining fiber. In order to improve the stability of laser oscillation and the wavelength conversion efficiency, it is desirable to use a single mode fiber. As in the case where the solid laser medium 11 is used, the wavelength of the excitation light needs to match the absorption wavelength band of the rare earth element doped in the fiber core. Since the core through which the oscillation laser beam propagates is small, especially in a single-mode fiber, it is several μm, so it is often difficult to narrow down the high-output excitation light in this. Therefore, in this case, a double clad fiber may be used. The double clad fiber is effective because the pump light is absorbed while propagating through the inner clad of the outer and inner clads.

  FIG. 7 is a schematic diagram showing a modification of the fiber type laser beam generator 30 shown in FIG. The laser beam generator 40 includes fibers 35a and 35b each having an FBG 43. The optical resonator 35 is configured by the FBG 43 and the mirrors 6 and 7. Thus, the presence of the FBG 43 eliminates the need for the mirror 5 shown in FIG. 6, and the laser light generating device 40 can be downsized. Further, since the FBG 43 has a wavelength selection function, it is not necessary to provide a wavelength selection element or the like, and further miniaturization is facilitated.

  For example, the center wavelength of the laser light in all the fibers 35a and 35b is set to be a predetermined wavelength. Or if the selection wavelength of FBG43 changes little for every fiber, a speckle will reduce.

  The present invention is not limited to the embodiment described above, and various modifications are possible.

  For example, in the embodiment shown in FIG. 1, the end pump method is used as the excitation method of the laser medium 11. The end pump method is a method of irradiating excitation light from a side surface substantially perpendicular to the laser output axis of the laser medium 11. However, the present invention is not limited to this, and a so-called side pump system in which excitation light is irradiated from a side surface substantially parallel to the laser output axis of the laser medium may be used.

  In the above embodiments, SHG has been described. However, the present invention is not limited to this, and the present invention can also be applied to an apparatus that generates THG (Third Harmonic Generation) or higher harmonics and sum frequencies.

  The laser beam generator 10 shown in FIG. 1 includes three optical resonator mirrors (mirrors 5, 6 and 7). However, for example, a configuration in which the mirror 6 is detached, the nonlinear optical element 12 is disposed at the subsequent stage of the wavelength selection element 13, and the mirror 7 is disposed at the subsequent stage of the nonlinear optical element 12 is also conceivable. In this case, the optical resonator only needs to have two mirrors (mirrors 5 and 7), and the resonator length can be shortened to achieve further miniaturization of the laser light generator. Similarly, in the fiber type laser light generator 40 shown in FIG. 7, the mirror 6 can be removed, and an optical resonator can be configured by the FBG 43 and the mirror 7 alone.

It is the schematic which shows the laser beam generator which concerns on one embodiment of this invention. It is a perspective view which shows an excitation light source. It is a schematic diagram which shows a single period type nonlinear optical element. It is a schematic diagram which shows a multiple period type nonlinear optical element. It is a perspective view which shows the laser medium in which the partition was provided between beams. It is the schematic which shows the laser beam generator which concerns on other embodiment of this invention. It is the schematic which shows the modification of the fiber type laser beam generator shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 2 ... Excitation light source 3 ... Micro lens array 4 ... Collimating lens system 5-7 ... Mirror 8 ... Condensing lens 10, 30, 40 ... Laser beam generator 11, 21 ... Laser medium 12 ... Wavelength conversion element (nonlinear optical element)
12a to 12e ... polarization inversion periodic structure 13 ... wavelength selection element 15, 35, 45 ... optical resonator 19 ... laser light (second harmonic)
22a ... Passing region 22b ... Reflection region 25, 25a, 25b, 35, 35a, 35b ... Fiber 43 ... FBG

Claims (8)

An excitation light generator capable of generating excitation light and dividing the generated excitation light into a plurality of optical axes;
A gain medium to which the divided excitation light is irradiated;
Using the gain medium, an optical resonator for oscillating laser light having a plurality of optical axes for each of the divided excitation lights by optical amplification;
A wavelength controller that is provided in the optical resonator and controls the wavelength of each of the oscillating laser beams;
A non-linear optical element provided in the optical resonator for generating a second harmonic of each of the laser beams. The laser light generator according to claim 1,
The wavelength controller includes a wavelength selection element for aligning a center wavelength of each laser beam to a predetermined wavelength. The laser light generator according to claim 1,
The wavelength controller includes a wavelength selection element for selecting a laser beam having a different wavelength range for each of the oscillating laser beams. The laser light generator according to claim 1,
The non-linear optical element has a polarization inversion structure formed with a different period for each laser beam. The laser light generator according to claim 1,
The gain medium is a solid laser medium,
The solid-state laser medium is
A passing region having a first reflectivity through which each of the laser beams passes;
A laser light generator, comprising: a second reflectance higher than the first reflectance, and a reflective region provided between the respective outgoing regions. The laser light generator according to claim 1,
A laser light generator, further comprising a light shielding member for suppressing interference between the laser lights. The laser light generator according to claim 1,
The gain medium is an optical fiber;
The optical fiber includes a Bragg grating provided in the optical resonator. An excitation light generator capable of generating excitation light and dividing the generated excitation light into a plurality of optical axes;
A gain medium to which the divided excitation light is irradiated;
Using the gain medium, an optical resonator for oscillating laser light having a plurality of optical axes for each of the divided excitation lights by optical amplification;
A wavelength controller that is provided in the optical resonator and controls the wavelength of each of the oscillating laser beams;
A non-linear optical element provided in the optical resonator for generating a second harmonic of each laser beam;
An image generating apparatus comprising: a modulation element that modulates the generated second harmonic laser beam to generate an image.