JP2006066817A - Light source device and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To sufficiently suppress parasitic oscillation which occurs near a wavelength of a fundamental wave, and to improve output and stability in a light source device where a non-linear optical element having a periodic polarization inversion structure is installed in a solid-state laser resonator which oscillates at a vertical/horizontal multi-mode and in an image forming device. <P>SOLUTION: In the light source device 1, the non-linear optical element 5 having the periodic polarization inversion structure is disposed in the solid-state laser resonator 3 which oscillates at the vertical/horizontal multi-mode. When a higher harmonic by phase matching is generated, oscillation wavelength width of the fundamental wave by a laser medium 4 in the resonator 3 becomes phase matching width or below of the non-linear optical element 5. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technology for ensuring stability by suppressing parasitic oscillation in a light source device using a solid-state laser of vertical and horizontal multimode oscillation and an image generation device using the light source device.

  A solid-state laser is used when a high output is required, and a configuration using a semiconductor laser as a light source for excitation is known. For example, when irradiating a wavelength conversion element having a periodic polarization reversal structure with a beam, in order to accurately lock the oscillation wavelength of the semiconductor laser with respect to a wavelength that is phase-matched with the period of the domain inversion unit, An example is known in which a narrow-band bandpass filter (etalon or birefringence filter) is arranged between a mirror and a beam splitter after being branched by a beam splitter (see, for example, Patent Document 1). For example, Non-Patent Document 1 can be cited for second harmonic generation (SHG), quasi phase matching, etc. using a quasi phase matching element having a domain-inverted structure.

  In addition, in application to an image generation apparatus (such as a projector or a printer) using a laser light source and a light modulation element, a configuration including a laser light source, a beam expander, a spatial light modulator, and the like is known.

JP 2000-35599 A Martin M. Fejer, GA Magel, Dieter H. Jundt, Robert L. Byer, "Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances", IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL.28, NO.11, (USA) , November 1992, p.2631

  By the way, in a configuration in which a wavelength conversion element having a periodically poled structure is arranged in a vertical and horizontal multimode oscillation solid-state laser resonator, an oscillation line generated near the oscillation line having the strongest fundamental wave is a problem ( Parasitic oscillation).

  In a solid-state laser, when several oscillation lines are close to each other, the reflectivity of the mirror constituting the resonator is almost 100% in a certain wavelength range (the reflectivity is almost 100% at a specific wavelength). It does not mean that it shows high reflectivity with a certain wavelength width.), There is a possibility that they oscillate at the same time with respect to oscillation lines of close wavelengths.

  For example, when Nd: YAG is used as the laser medium, there is a possibility that parasitic oscillations such as 1052 nm, 1061 nm, and 1074 nm occur in the vicinity of the wavelength of 1064 nm.

  In the case of oscillation of only the fundamental wave, only the strongest oscillation line can be easily oscillated because the transmission wavelength width of the output mirror constituting the resonator can be made as wide as the reflectance of the mirror. However, for example, when a harmonic is extracted using a wavelength conversion element (for example, SHG element) having a periodically poled structure, the wavelength width (phase matching allowable width) is as narrow as several nanometers with a crystal length of 1 mm. When parasitic oscillation occurs together with the oscillation of harmonics (green light or the like), there is a risk that noise that causes a decrease in output and impairs stability.

  In view of this, the present invention is sufficient to prevent parasitic oscillation that may occur in the vicinity of the fundamental wavelength in a light source device and an image generation device in which a nonlinear optical element having a periodically poled structure is provided in a solid-state laser resonator that oscillates in a longitudinal and transverse multimode. It is an object to improve the output and stability by restraining to the above.

  In the light source device according to the present invention, in order to solve the above-described problem, a nonlinear optical element having a periodically poled structure is arranged in a solid-state laser resonator that oscillates in a vertical and horizontal multimode, and harmonics due to phase matching are generated. In addition, the oscillation wavelength width of the fundamental wave by the laser medium in the resonator is configured to be equal to or smaller than the phase matching width of the nonlinear optical element.

  In addition, an image generation apparatus according to the present invention includes an illumination light source configured to generate harmonics due to phase matching using a nonlinear optical element, and a configuration including an optical modulation unit that modulates light from the illumination light source. A non-linear optical element having a periodically poled structure is disposed in a solid-state laser resonator that oscillates in a multimode to generate harmonics due to phase matching. In this case, the oscillation wavelength width of the fundamental wave by the laser medium in the resonator is configured to be equal to or smaller than the phase matching width of the nonlinear optical element.

  Therefore, in the present invention, focusing on the relationship between the phase matching allowable width of the nonlinear optical element and the fundamental wave oscillation wavelength width, it is specified that the fundamental wave oscillation wavelength width is not larger than the phase matching allowable width. Therefore, it is possible to sufficiently suppress the oscillation line parasitic near the fundamental wave wavelength.

  According to the present invention, it is possible to suppress parasitic oscillation and guarantee stability, and to prevent a decrease in output and efficiency. For example, since the phase matching tolerance of a nonlinear optical element is almost inversely proportional to the crystal length, if the design is made so that the phase matching tolerance is larger than the oscillation gain width of the fundamental wave by selecting the element size, the device configuration becomes complicated. This is advantageous in terms of cost reduction without causing adverse effects such as downsizing. In addition, when applied to an image generation device that modulates light from an illumination light source using vertical and horizontal multimode lasers with a light modulation element, high image quality is achieved by improving the S / N ratio (signal-to-noise ratio). Is possible.

  In order to suppress parasitic oscillation, a configuration in which a wavelength selection element or a birefringence filter is disposed in a solid laser resonator is preferable, and oscillation lines around the fundamental wavelength can be sufficiently suppressed.

  In a nonlinear optical element having a periodic polarization inversion structure, it is possible to select a phase matching wavelength by changing the polarization inversion period. Therefore, it is possible to suppress parasitic oscillation by adopting a structure having a quasi-phase matching portion having a polarization inversion period corresponding to the parasitic oscillation wavelength and forming a phase matching portion for the fundamental wave and other oscillation lines.

  As for the nonlinear optical element disposed in the resonator, there is an element that is resistant to optical damage and excellent in long-term reliability, particularly when using a stoichiometric composition periodically poled lithium tantalate subjected to vapor phase equilibrium treatment. Since it is obtained and the wavelength conversion efficiency is high, high output light can be obtained stably.

  Further, in the form of outputting ultraviolet light, blue light, or green light by second harmonic generation using a nonlinear optical element or the like, by arranging a rare earth-added solid laser medium in an infrared resonator, for example, Visible light can be obtained based on high-power infrared light from the solid-state laser medium.

  The present invention relates to a light source device and an image generation device using vertical and horizontal multimode lasers. For example, an image display device (such as an image projection device) or a drawing device using a light modulation element, an image output device such as a printer, The present invention can be applied to various processing apparatuses, exposure apparatuses, and the like.

  1 and 3 show an example of a basic configuration according to the present invention.

  FIG. 1 shows a basic configuration example of a light source device.

  The light source device 1 includes an excitation light source 2, a laser medium 4 in a resonator 3 that oscillates in a vertical and horizontal multimode, and a nonlinear optical element 5 for wavelength conversion.

  A laser, a discharge lamp, or the like is used as the excitation light source 2, but it is preferable to use a semiconductor laser in view of downsizing, life, etc. (all-solid-state laser is possible).

For the laser medium 4, a rare-earth-added solid laser material is used, and examples thereof include Nd: YAG, Nd: YVO ₄ , Nd: GdVO ₄ , and Yb: YAG.

The nonlinear optical element 5 is used for wavelength conversion such as SHG and THG (third harmonic generation), or is used for sum frequency generation, optical parametric oscillation, and the like. The materials _{used, PP-KTiOPO 4, PP-} MgO: LiNbO 3, PP-MgO: S-LiNbO 3, PP-S-LiTaO 3 , etc., nonlinear optical materials and the like having the periodically poled structure. Here, “PP” means “Periodical Poling”, and “S” means “Stoichiometric”.

  In the configuration in which the nonlinear optical element 5 is arranged in the resonator 3 to generate harmonics by phase matching, the power density of the oscillation light confined inside the resonator 3 is high, and wavelength conversion with high efficiency is possible. Yes, continuous oscillation is possible.

  Excitation light output from the excitation light source 2 is applied to the laser medium 4, and the laser medium 4 is excited with, for example, an elliptical transverse mode pattern. A linear beam obtained in this manner is irradiated onto the nonlinear optical element 5, and the wavelength-converted linear light is output from the resonator 3.

  In the above configuration, for example, in the form in which a near-infrared laser is used as the excitation light source 2 and the laser medium 4 and the nonlinear optical element 5 (SHG element) are arranged in the infrared resonator, green light is generated by second harmonic generation. Alternatively, a linear beam of blue light or ultraviolet light can be output.

  In the present invention, the oscillation wavelength width of the fundamental wave by the laser medium 4 in the solid-state laser resonator is set to be equal to or smaller than the phase matching width of the nonlinear optical element 5.

  Hereinafter, the relationship between the phase matching tolerance and the oscillation gain will be described with reference to FIG.

  When the stoichiometric composition periodically poled lithium tantalate (PPSLT) is taken as an example of the material of the nonlinear optical element 5, the allowable phase matching width (FWHM) is 2.5 nm when the crystal is 1 mm long.

  Here, considering that the phase matching tolerance is almost inversely proportional to the crystal length (or the element length in the optical axis direction), for example, if the crystal length is 3 mm or less, the phase matching tolerance is 0. 83 nm or more.

  On the other hand, when Nd: YAG is assumed as the laser medium, for example, the oscillation line width of the fundamental wavelength of 1064 nm is 0.57 nm. Therefore, when the crystal length is 3 mm, the relationship of “phase matching allowable width (0.83)> oscillation line width (0.57)” is established.

FIG. 2 is a graph illustrating the phase matching allowable width indicated by the solid line G and the oscillation gain width indicated by the broken line g in comparison with each other. The vertical axis indicates the peak value at the horizontal axis “x = 0”. This is standardized to “1”. In this example, the graph line of the solid line G is “(sin (x) / x) ² ”, and its half-value width is 0.83 (nm). Also, the graph line of the broken line g is “exp (−x ² )”, and the half width is 0.57 (nm).

  In the relationship between the solid line G and the broken line g, as shown in the figure, when the allowable phase matching width is wider than the oscillation line width, there is no parasitic oscillation, but when the relationship between the two is reversed, the phase matching allowable range is outside the allowable range. Parasitic oscillation becomes a problem.

  When the crystal matching length is 1 mm when the crystal matching tolerance is expressed as “δλ” and the wavelength of the fundamental wave is expressed as “λ”, δλ is obtained from the following equation.

  In the above equation, “ne (λ)” represents the effective refractive index with respect to the fundamental wave, “ne (λ / 2)” represents the effective refractive index with respect to the second harmonic, and “d” is related to the wavelength. Represents differentiation.

  As the fundamental wavelength λ = 1064.15 (nm), δλ = 2.507 (nm) is calculated using the above equation.

  As described above, it is a necessary condition for eliminating parasitic oscillation that the allowable phase matching width defined by the crystal length of the nonlinear optical element is larger than the oscillation gain width of the fundamental wave. In an intracavity SHG laser that oscillates in a multimode, in order to make the SHG allowable wavelength width equal to or larger than the line width of the strongest oscillation line of the fundamental wave, the allowable wavelength width is almost inversely proportional to the crystal length. In this case, the crystal length may be 3 mm or less.

  In application to a one-dimensional illumination optical system, the output light of the light source device 1 passes through a subsequent optical system (not shown), for example, so that a predetermined intensity distribution (a constant range is flattened and the base is obtained). Illumination is performed on a portion to be irradiated (one-dimensional light modulation element or the like) with a so-called top hat shape distribution in which the intensity sharply decreases in the portion. The optical system includes an integrator optical system and an expander optical system. The former is configured using a one-dimensional lens array (a so-called fly-eye lens) in which a plurality of lens elements are arranged along a one-dimensional direction as is well known, and the latter is a linear shape extending in a predetermined direction. A configuration using an optical element such as a condenser lens or a field lens is provided to obtain illumination light.

  FIG. 3 schematically shows a configuration example of a main part of the image generating device 6 (excluding the one-dimensional illumination light source using the light source device 1).

  The one-dimensional light modulation elements 7R, 7G, and 7B are controlled in response to a signal from a drive circuit (not shown), and modulate light based on an image signal.

  In the application of the present invention, any one-dimensional light modulation element may be used. For example, a grating light valve element developed by US Silicon Light Machine (SLM) may be used. This element is configured using a reflective diffraction grating, and a large number of movable ribbons are arranged at a predetermined interval, and a fixed ribbon is arranged between adjacent movable ribbons. Then, by applying a drive voltage between the common electrode and the movable ribbon, the movable ribbon moves, and a diffraction grating for incident light is configured. That is, in this state (so-called pixel on), the first-order diffracted light is generated, and in the state where the movable ribbon is aligned with the fixed ribbon without moving (so-called pixel off), the first-order diffracted light is not generated (positive with respect to the incident light). Only reflection.) A configuration using specific diffracted light can be used for image display or the like with high diffraction efficiency (for example, a blazed element is preferable for high gradation image display).

  The one-dimensional light modulation elements 7R, 7G, and 7B are provided corresponding to the three primary colors R (red), G (green), and B (blue), respectively, and constitute light modulation means for modulating each color light. ing. The light modulated using these one-dimensional light modulation elements is mixed by the synthesizing unit 8 using an L prism or the like, and then passed through the optical system 9 and the optical scanning unit 10 in the subsequent stage to be irradiated 11 (for example, Projected onto a screen or a photoreceptor.

  The optical system 9 includes a spatial filter, a projection system, and the like. The light modulated using the one-dimensional light modulation element is passed through, for example, a Schlieren filter, and first-order diffracted light is selected.

  The light scanning means 10 scans the output light from the light modulation means in a predetermined direction (see the arrow “SS” in the figure), and uses a rotary reflecting mirror, and the reflected light is directed toward the irradiated portion 11. Irradiated. As for optical scanning, various forms using a galvanometer mirror, a polygon mirror, etc. can be mentioned.

  The one-dimensional image projected onto the irradiated portion 11 is developed into a two-dimensional image by optical scanning.

  The present invention is not limited to this example, and there are various types of changes in optical elements such as a configuration in which the positional relationship between the optical scanning system and the projection system is reversed and the projection system is arranged between the optical scanning means 10 and the irradiated portion 11. Implementation in the form is possible.

  Next, a specific configuration related to the light source device will be described.

  Examples of the excitation method include the following forms.

(A) End face excitation method (so-called end pump) in which excitation light is irradiated to an end portion in a direction along the laser output axis with respect to a laser medium.
(B) Side excitation method (so-called side pump) in which excitation light is irradiated from a side surface parallel to the laser output axis to the laser medium.

  First, the form (A) will be described.

  Examples of the excitation light source include a mode in which excitation is performed in an elliptical transverse mode pattern using a broad area LD (laser diode) having a wide emission region width, and in this case, a single or a plurality of LDs are used. . Moreover, the form which performs optical excitation using parallel light sources, such as a bar laser, and a micro lens array is mentioned.

  4 and 5, the laser medium is irradiated with excitation light using a semiconductor laser (bar laser), a collimator lens using a microlens, a condenser lens, or the like.

  4 shows a configuration viewed from the y-axis direction orthogonal to the x-axis and z-axis of the orthogonal coordinate system set on the paper surface, and FIG. 5 shows the x-axis and y-axis of the orthogonal coordinate system set on the paper surface. The configuration seen from the z-axis direction orthogonal to is shown, and the x-axis is set along the optical axis.

  A semiconductor laser is used as the parallel light source 13 constituting the excitation light source. The example shown in FIG. 6 (one-dimensional LD array) has an array structure in which a plurality of laser emitters are arranged one-dimensionally. For example, a laser diode having a quantum well structure of GaAlAs (gallium, aluminum, arsenic) can be used to obtain an output of a wavelength of 808 nm (nanometers) and about 40 W, and stripe-shaped emitters 13 a, 13 a having an opening width “wa”. Are formed in a direction along the pn junction surface with a predetermined interval (see “d” in the figure). Output light of a transverse mode pattern having a substantially elliptical shape is obtained from each emitter. The size “D” in the longitudinal direction of the light source unit is about 10 mm.

  A high output can be realized by paralleling a large number of emitters, and it is preferable to use a broad area structure with a light emitting region width of several tens to several hundreds μm.

  As shown in FIGS. 4 and 5, the output light beam of each emitter constituting the semiconductor laser array is collimated by two collimating lenses 14 and 15. For example, a lens unit manufactured by LIMO of Germany can be used. This lens unit is composed of a combination of cylindrical lenses that collimate the divergence of the “Fast Axis” and “Slow Axis” directions of the semiconductor laser, while the “Fast Axis Collimator” (FAC) is an aspheric cylindrical lens. On the other hand, “Slow Axis Collimator” (SAC) is a spherical cylindrical lens array that matches the emitter array pitch and divergence angle of the laser array. In other words, the collimating lens 14 closer to the semiconductor laser array is “FAC” and the other collimating lens 15 is “SAC”, and the collimating lens 14 collects the output light beam of each emitter in the xy plane of FIG. The collimated lens 15 collimates and collimates and collimates the output rays of each emitter in the xz plane of FIG. The divergence angle of the laser diode has a larger divergence angle in the xy plane than in the xz plane. However, since a separate cylindrical lens is used for each surface, the exit beam diameter is controlled independently. Thus, a desired beam shape can be obtained. Further, when the astigmatism is a problem when the light emitting region of the laser diode is large, it is preferable to use the above-described cylindrical lens for correction.

  The light transmitted through the collimating lenses 14 and 15 is converged by the condenser lens 16 at the subsequent stage and irradiated to one end of the laser medium 17 as a linear beam.

  A non-linear optical element 21 is provided for wavelength conversion by SHG or the like in the resonator formed by using the end face (reflecting face) of the laser medium 17 or a mirror attached thereto and reflecting means (18, 19, 20). It has been.

  Mirrors 18 (plane mirrors) and 19 (concave mirrors) disposed on the optical path between the laser medium 17 and the nonlinear optical element 21 are provided as reflecting means for turning back the optical path. In this example, the output light of the laser medium 17 reaches the mirror 18 along the x-axis direction, and the optical path is changed so that the light reflected by the mirror goes to the mirror 19. Then, the reflected light of the mirror 19 passes through the nonlinear optical element 21 and reaches the mirror 20 to be reflected. The apparatus size can be reduced by turning the optical path using the reflecting means (18, 19).

  In the case of the SHG element, the mirrors 18, 19, and 20 have a high reflectance with respect to the output light (fundamental wave) wavelength of the laser medium 17. Then, the light related to the effective output and the rear output (monitored by using a light detection means such as a photodiode, and the output control of the excitation light source is performed by a control circuit (not shown)) from the mirrors 19 and 20 respectively on the x axis. Along the opposite direction. The end face of the laser medium 17 or the attached mirror has a high reflectance with respect to the output light (fundamental wave) wavelength of the laser medium 17 and a high transmittance with respect to the incident light wavelength (excitation light wavelength). The

  The stray light can be reduced by minimizing the reflectance with respect to the second harmonic (SHG light) in the mirrors 18 and 19 for turning the optical path.

  As described above, high efficiency can be obtained by excitation light using a laser diode.

  When a parallel light source in which a plurality of laser emitters are arranged one-dimensionally is used, a lens array arranged between the parallel light source and the laser medium is used to set the optical axis of each lens element to each emitter. Can be excited with high efficiency (especially in the case of a one-dimensional transverse multimode, the shape of the laser diode beam pattern matches the laser oscillation light, resulting in higher efficiency). Effective.)

  Next, the said form (B) is demonstrated.

  Adoption of the side pump is effective for reducing the number of parts and reducing the cost by simplifying the configuration, and provides advantages such as fewer optical adjustment points. For example, the configuration modes shown below can be cited.

(B-1) Form using a parallel light source and two collimating lenses (see FIGS. 7 and 8)
(B-2) Form using parallel light source and one collimating lens (see FIGS. 9 and 10)
(B-3) A mode in which excitation light from a parallel light source is directly irradiated onto a laser medium without a collimating lens (see FIGS. 12 and 13).

  In the configuration example 22 shown in FIGS. 7 and 8, the excitation light is irradiated to the laser medium (Nd: YAG or the like) using a semiconductor laser (bar laser) and two collimating lenses.

  7 shows a configuration viewed from the y-axis direction orthogonal to the x-axis and the z-axis of the orthogonal coordinate system set on the paper surface, and FIG. 8 shows the x-axis and the orthogonal coordinate system set on the paper surface. A configuration viewed from the z-axis direction orthogonal to the y-axis is shown (only the laser medium is shown for the resonator), and the z-axis is set along the resonator optical axis.

  The parallel light source 13 that constitutes the excitation light source and the collimating lenses 14 and 15 are the same as the configuration examples of FIGS. 4 and 5, but are different in the following points.

The output axis of the laser medium 17 is set in the z-axis direction (long axis direction), and the excitation light from the parallel light source 13 is collimated by the collimating lenses 14 and 15 to the laser medium 17 from the x-axis direction. The mirrors 18 and 23 arranged on the z axis are positioned on opposite sides of the laser medium 17.

  In this example, in the laser medium 17 of slab-like crystal, the longitudinal direction (long axis direction) is the laser oscillation direction, and the light emitted from the parallel light source 13 is transmitted through the collimating lenses 14 and 15, so that the laser medium 17 Irradiated to the side surface (one side surface orthogonal to the x-axis) 17a. That is, the output light from each emitter of the LD constituting the parallel light source 13 is collected on the xy plane of FIG. 8 by the collimator lens 14, and within the xz plane of FIG. 7 by the microlens of the collimator lens 15. It is condensed in Then, the collimated parallel light beam is perpendicularly incident on the side surface 17 a of the laser medium 17.

Examples of the solid-state laser medium include Nd: YAG in which neodymium ions are doped with yttrium aluminum garnet (Y ₃ Al ₅ O ₁₂ ). As for the shape, a plate-like shape or an end surface in the major axis direction is inclined with respect to a plane perpendicular to the major axis so as to obtain an incident condition at a Brewster angle is used.

  A nonlinear optical element 21 is provided for wavelength conversion by SHG or the like in a resonator configured using the reflecting means (18, 19, 20, 23) shown in FIG. One of the laser oscillation light emitted from both end faces in the direction (z-axis direction) reaches the mirror 23 and is reflected in the opposite direction, and the other oscillation light is output toward the mirror 18.

  The light that has reached the mirror 18 along the z-axis direction is reflected by the mirror, and undergoes an optical path change toward the mirror 19. Then, the reflected light of the mirror 19 passes through the nonlinear optical element 21 and reaches the mirror 20 to be reflected.

  In the case of an SHG element, the mirrors 18, 19, and 20 have a high reflectance with respect to the output light (fundamental wave) wavelength of the laser medium 17 and a high transmittance with respect to the wavelength of the second harmonic. And the light which concerns on an effective output and back output (it is utilized for the output control of the light source for excitation as monitor light) is each radiate | emitted from the mirrors 19 and 20. The mirror 23 has a high reflectance with respect to the output light (fundamental wave) wavelength of the laser medium 17.

  In the configuration example 24 shown in FIG. 9, a laser medium is irradiated with excitation light using a semiconductor laser (bar laser) and one collimating lens (the collimating lens 15 is unnecessary). That is, the configuration differs from the above configuration example 22 in that the light emitted from the parallel light source 13 passes through the collimating lens 14 and is irradiated onto the laser medium 17. FIG. 9 shows a configuration viewed from the y-axis direction orthogonal to the x-axis and z-axis of the orthogonal coordinate system set on the paper surface.

  The laser light output from the parallel light source 13 passes through the collimator lens 14, is reflected by the reflecting mirror 25, and is irradiated from the side surface to the laser medium 17 (Nd: YAG).

  The resonator is configured by using a mirror (plane mirror) 23, a concave mirror 19A, and a mirror (plane mirror) 20A, and a laser medium 17 is disposed on the optical path between the mirror 23 and the concave mirror 19A arranged at an interval of 80 mm. Yes. A nonlinear optical element (PPSLT) 21 is disposed at a predetermined distance (26 mm) on the optical path between the concave mirror 19A and the mirror 20A.

  The mirror 23 and the concave mirror 19A have a high reflectance with respect to the fundamental wave, and the mirror 20A has a high reflectance with respect to the fundamental wave and harmonics.

  The fundamental wave output from the laser medium 17 enters a concave mirror 19A having a curvature radius of 50 mm at an incident angle of 10 °, and the reflected light passes through the nonlinear optical element 21 and is reflected by the mirror 20A.

  The harmonics generated by the nonlinear optical element 21 pass through the concave mirror 19A, and are collimated and output by a collimating lens (f30 mm) 26 positioned at a predetermined distance (5.1 mm) from the concave mirror.

  As shown in FIG. 10 (a diagram viewed from the z-axis direction), there is a configuration in which wavelength selection elements or birefringent filters for suppressing parasitic oscillation are positively arranged on the optical path in the solid-state laser resonator.

  In this example, by using Nd: YAG (length: 15 mm) as the laser medium 17 and PPSLT (length: 3 mm) as the nonlinear optical element 21 in the side surface excitation method, SHG light (532 nm) is generated from the fundamental wave (1064 nm). ) Is output.

  Of the end faces 17b and 17r of Nd: YAG, one of them 17b is an inclined surface processed to have a Brewster angle (61.2 °) with respect to the fundamental wave, and the other 17r is a reflecting surface by an HR coat. And has a high reflectance with respect to the fundamental wave (having the same effect as the mirror 23).

  The outgoing light from Nd: YAG passes through an optical element (wavelength selection element or birefringent filter) 27. In this example, a quartz plate having a thickness of about 1 to 3 mm is used, and is incident on the quartz plate with an incident condition at a Brewster angle, and the emitted light is directed to the concave mirror 19A (curvature radius 50 mm). As shown in FIG. 11 (viewed from the y-axis direction), the crystal orientation of the quartz plate is set to a rotation direction of about 45 ° with the vertical direction of the incident surface as the rotation axis, and satisfies the Brewster angle condition. It is arranged on the optical path with a tilted posture.

  The light that has reached the concave mirror 19A is incident on the nonlinear optical element 21 (PPSLT) after reflection (reflection angle 10 °). One end 21b of the end faces 21b and 21r of the PPSLT is an inclined surface processed to have a Brewster angle (64.9 °) with respect to the fundamental wave and the second harmonic, and the other 21r is an HR. It is a reflective surface by a coat, and has a high reflectivity with respect to the fundamental wave and harmonics (have the same effect as the mirror 20A). The harmonics generated by PPSLT are transmitted through the concave mirror 19A (output mirror) and output, and are collected by a collimator lens (not shown).

  As in this example, the allowable phase matching width (wavelength width) determined from the crystal length of PPSLT or the like is set to be equal to or larger than the line width of the strongest oscillation line of the fundamental wave, and other nearby oscillation lines are connected to the wavelength selection element or It is preferable to suppress using a birefringent filter.

  In the configuration example 28 shown in FIGS. 12 and 13, only a semiconductor laser (bar laser) is used, and a laser medium is brought close to the laser to irradiate excitation light without a collimating lens.

  12 shows a configuration viewed from the y-axis direction orthogonal to the x-axis and z-axis of the orthogonal coordinate system set on the paper surface, and FIG. 13 shows the x-axis and y-axis of the orthogonal coordinate system set on the paper surface. 2 shows a configuration viewed from the z-axis direction orthogonal to (resonator only shows a laser medium), and the z-axis is set along the optical axis of the resonator.

  In this example, since the laser medium 17 is directly excited using the parallel light source 13, no collimating lens is required, and the number of parts can be reduced. Moreover, since the optical alignment is not necessary, it is effective for reducing the number of manufacturing steps.

  However, when the irradiation light from the parallel light source 13 to the laser medium 17 is incident on the laser medium 17 with a relatively large divergence angle in the xy plane, this light is transmitted through the laser medium 17 to the outside. If it leaks, it will cause a decrease in efficiency.

  Therefore, it is preferable to provide an optical confinement means for the laser medium 17, and examples of the means include the following configurations.

-A form that uses total reflection-A form that forms a reflective film-A form that is provided with a reflecting member or the like

  For example, in an embodiment in which angle setting, polishing, or the like is performed so that light incident on the laser medium 17 is totally reflected inside, no additional means such as a reflecting member is required. Further, in a form in which a reflection film is formed on the outer surface of the laser medium 17 (excluding an end surface orthogonal to the z-axis) or a reflection member is provided, the light incident on the laser medium is converted into the reflection film or the reflection member. It is possible to confine efficiently by reflection on the surface.

  In the example shown in the upper right portion of the enlarged round frame in FIG. 13, light absorption with high efficiency is achieved by confining excitation light using a reflecting means (metal film, metal mirror, etc.) 29 with respect to the laser medium 17. Is realized, and the laser efficiency can be improved. The resonator configuration and the like are the same as those in the above configuration example 22.

  The collimating lenses are reduced one by one in the order of the above forms (B-1), (B-2), and (B-3), and the configuration is simplified accordingly. In (B-3), an optical confinement means is required. It is said.

  As described above, according to the mode (B), the configuration is simple and suitable for high output. And since excitation light can be disperse | distributed, exhaust heat (heat dissipation countermeasure of a laser medium) is easier than a form (A), and it is advantageous to a stability improvement, long lifetime, etc. In the case of one-dimensional multi-mode oscillation, it is possible to obtain the same efficiency as an end pump by devising the excitation method.

  As in (B-1) above, in the form of performing collimation by the lens array, it is possible to realize a highly efficient side pump by excitation with parallel light. If the oscillation mode size and the size of the excitation light are matched, high-efficiency oscillation similar to that of the end pump can be performed in one-dimensional transverse mode oscillation.

  In (B-2) above, a rod lens (collimator) is used to perform collimation in the xy plane so that the oscillation mode size and the size of the excitation light are matched in the vertical direction (z-axis direction). Can do.

  In the above (B-3), since no lens is used, it is effective for reducing the number of parts and cost.

  FIG. 14 is a perspective view schematically showing a configuration example 30 according to the form (B-3).

  In the three-dimensional orthogonal coordinate system set in the figure, the axis extending in the laser output axis direction of the rectangular laser medium 17 is the z axis, and the axis along the irradiation direction of the excitation light to the side surface of the laser medium 17 Is selected as the x axis, and the z axis and the axis orthogonal to the x axis are set as the y axis.

  Optical excitation of the laser medium 17 (Nd: YAG, YVO, etc.) in the resonator 32 is performed by side irradiation light from a light source 31 using a laser diode array, and laser oscillation light is reflected by reflecting means 33 (a plane mirror 33a and a concave mirror 33b). ) To irradiate the nonlinear optical element 34 with a linear beam.

  In the lateral excitation method, the lateral width of the laser medium becomes an aperture, and the beam lateral width is determined. The size of the longitudinal width can be determined by selecting the radius of curvature of the resonator mirror (concave mirror 33b).

  In each of the above-described configurations, the linear beam is transmitted through the non-linear optical element 34, so that the wavelength-converted linear output light is obtained. The nonlinear optical element 34 is preferably an optical device having a periodically poled structure. Compared with conventional nonlinear optical crystals, the nonlinear optical constant is large, high conversion efficiency is obtained, and mass production by wafer process technology is possible, which is advantageous for cost reduction. In application to nonlinear optical devices (such as SHG elements) having a periodically poled structure, when PPSLT subjected to vapor phase equilibration (VTE: Vapor Transport Equilibration) is used as a periodically poled material, it is highly resistant to optical damage. Since a device having excellent long-term reliability and high conversion efficiency can be obtained, high output light (SHG light or the like) of several watts or more can be stably obtained. Also, power scaling is possible in the horizontal direction by matching the horizontal multimode direction with the horizontal multimode direction of the slab laser.

In the following, an optical device manufactured by processing a substrate that is processed by a vapor phase equilibrium method using a ferroelectric material will be described using a stoichiometric composition periodically poled lithium tantalate (LiTaO ₃ ) as an example.

  As a manufacturing process, an optical device is generally manufactured through the following steps.

(1) Wafer input (2) VTE
(3) Single domain (4) Patterning (electrode formation, etc.)
(5) Periodic polarization reversal (polarization reversal by voltage application)
(6) Cutting / polishing / coating.

  First, a substrate (crystal substrate) is loaded into the apparatus to perform setting work.

  In the VTE process, for example, a platinum dish filled with raw material powder is placed in an alumina container, a substrate is placed on the raw material powder, and then the raw material powder and the periphery of the substrate are covered with a platinum crucible, and these are made of alumina. It is accommodated in a container and heated at a high temperature for a predetermined time.

  Then, after taking out the substrate that has been subjected to the high temperature treatment by VTE, it is made into a single domain by applying a predetermined voltage at a predetermined temperature, and an electrode (aluminum electrode, etc.) having a predetermined period (about several μm) in a lithography patterning process Form. In a periodic polarization reversal structure, for example, a comb-like electrode or a lattice-like electrode that matches the polarization periodic pattern is formed on a ferroelectric substrate, and a predetermined external DC voltage is formed between the paired electrodes (front-back electrode pair, etc.) Is applied. As a result, a domain structure having a required polarization inversion period is realized.

  Thereafter, the individual element portions formed on the substrate through a processing step such as polishing are separated, and an optical device (SHG element or the like) is completed.

  The thickness of the substrate using the periodically poled material is, for example, about 0.5 to 1 mm. A thick substrate has a problem that the processing time of VTE becomes long. There is no limit to the width (the width in the direction perpendicular to the light beam propagation direction and the thickness direction), and the lateral width can be made sufficiently large. This means that a nonlinear optical element having a sufficient lateral width can be arranged for the linear beam output from the laser medium. In other words, the width of the substrate can be increased with respect to the limit of crystal light density (light output per unit area), so that it is possible to easily cope with beam expansion in the width direction, and increase the output. (In the case where a certain limit is imposed on the substrate thickness, it is effective to widen the width direction of the substrate).

  When a linear beam of ultraviolet light, blue light, or green light is output by SHG using such a nonlinear optical element, a rare earth-added solid-state laser medium is disposed in the infrared resonator. High-power infrared output light from the solid-state laser medium is applied to the nonlinear optical element in the infrared resonator, and visible light is obtained by SHG. For example, oscillating light that is a source of light color (green or blue) can be obtained by the oscillation wavelength of rare earth.

  Next, a configuration (so-called multi-grating) having a plurality of quasi-phase matching (QPM) parts having a periodic polarization inversion structure with different polarization inversion periods for suppressing parasitic oscillation will be described. .

  In the case where the nonlinear optical element has a quasi phase matching portion having a polarization inversion period corresponding to the parasitic oscillation wavelength, FIG. 15 illustrates an optical element 35 in which three QPM portions are arranged (with respect to the substrate surface). Schematic plan view seen from the orthogonal direction).

  That is, in this example, the phase matching wavelengths are λ1, λ2, and λ3, respectively, and the QPM portions 36, 37, and 38 having polarization inversion periods corresponding to the respective wavelengths are arranged along the optical axis. For example, in the case of PPSLT, λ 1 = 1.0615 (μm), λ 2 = 1.06415 (μm), and λ 3 = 1.0737 (μm). In the Nd: YAG laser, the fundamental wave and the parasitic in the vicinity thereof are set. It corresponds to an oscillation line (1061 nm, 1074 nm, etc.).

  The QPM unit 36 positioned on the light incident side has a polarization inversion structure with a period Λ1, and the QPM unit 37 positioned with a predetermined interval “d1” from the QPM unit 36 has a polarization inversion structure with a period Λ2. The QPM unit 38 positioned on the emission side with a predetermined interval “d2” from the QPM unit 37 has a domain-inverted structure with a period Λ3. Each period (or reverse pitch) is calculated using the following equation.

  In FIG. 16, taking the case of PPSLT as an example, the horizontal axis represents the wavelength λ (unit: μm) and the vertical axis represents the polarization inversion period “Λ (λ)” (unit: μm). Shows the relationship.

  The values of the periods Λ1, Λ2, and Λ3 corresponding to the λ1, λ2, and λ3 are Λ1 = 7.94 (μm), Λ2 = 8 (μm), and Λ3 = 8.2 (μm), respectively.

  In the element using PPSLT, the phase matching wavelength can be selected by changing the polarization inversion period, and the phase matching is performed at each wavelength of 1061 nm for the period of 7.94 μm and 1074 nm for the period of 8.2 μm. Thus, it is possible to intentionally exert the effect of suppressing parasitic oscillation (depending on the loss of SHG light).

  In addition, in such a configuration including a plurality of QPM parts, the sum component of each harmonic output and other components, that is, interference between the parts, when each part exists independently with respect to the harmonic output. It is necessary to consider the component due to. Since the interference of harmonics generated in each QPM part is determined by their phase relationship, a non-QPM part (a part having no polarization inversion structure) is provided between the QPM parts, that is, in the range indicated by the intervals d1 and d2. Provided. That is, the non-QPM part is used for phase adjustment of harmonics generated in each QPM part, and the values of d1 and d2 are defined in consideration of the output fluctuation of the harmonics.

  In the above-described illumination to the one-dimensional light modulation element, by combining the one-dimensional direction (long axis direction) of the modulation element and the direction of the horizontal multimode, there is no need for profile conversion by a line generator or the like, and the number of parts can be reduced. Can be reduced.

  Further, for example, a common configuration for each color can be used for a beam expansion system used for one-dimensional illumination on a diffraction grating type one-dimensional spatial modulation element (grating, light, valve element, etc.). That is, it is preferable to use the beam expander having the same configuration for each color because the use of a beam expander having a different configuration for each color beam of R, G, and B causes a complication of the configuration. According to the configuration, the linear beam can be enlarged at a predetermined magnification and irradiated to the one-dimensional light modulation element with respect to the green and blue beams using the same optical configuration as that of the red beam. For example, there is a known optical system using four plano-convex lenses and one cylindrical lens (cylindrical lens), and a linear beam is formed in a plane including the major axis direction and the optical axis direction of the one-dimensional light modulation element. Is once condensed and then enlarged and shaped in the major axis direction.

  Assuming application to an image projection apparatus or the like having an illumination system for a diffraction grating type one-dimensional spatial modulation element using a vertical and horizontal multimode oscillation solid-state laser light source, one-dimensional direction is applied to illumination light to the element. In the (major axis direction of the modulation element), the requirement for the spatial coherence of the condensing characteristic is not strict, but in the direction orthogonal to the direction (minor axis direction of the modulation element), the beam is narrowed to a predetermined width (about several tens of μm). High coherence is required. By adopting the illumination optical system having the above-described configuration, a beam expanded in the one-dimensional direction is obtained by optical excitation in the transverse multimode in the major axis direction (longitudinal direction) of the one-dimensional modulation element in order to satisfy such a requirement. At the same time, the linear beam narrowed to almost the diffraction limit in the short axis direction of the one-dimensional modulation element can be irradiated to the element.

  According to the configuration described above, highly efficient illumination is possible, and a stable output without parasitic oscillation can be obtained.

It is explanatory drawing which shows the basic composition of the light source device which concerns on this invention. It is a figure for demonstrating the relationship between a phase matching allowable width | variety and an oscillation gain width | variety. It is explanatory drawing which shows the principal part about the structural example of the image generation apparatus which concerns on this invention. FIG. 5 is a diagram illustrating a configuration form according to the present invention together with FIG. 5, and is a diagram showing a configuration when viewed from a direction orthogonal to a plane including the arrangement direction of the emitters constituting the parallel light source. It is a figure which shows the principal part of a structure at the time of seeing from a different direction from FIG. It is the perspective view which illustrated the laser diode array as a parallel light source. FIG. 9 is a diagram showing another example of the configuration according to the present invention together with FIG. 8, and this diagram is a diagram showing a configuration when viewed from a direction orthogonal to the plane including the arrangement direction of the emitters constituting the parallel light source It is. It is a figure which shows the principal part of a structure at the time of seeing from a different direction from FIG. It is the figure which showed another example which concerns on this invention, and this figure is a figure which shows the structure at the time of seeing from the direction orthogonal to the surface containing the arrangement direction of the emitter which comprises a parallel light source. It is a figure which shows the structural example which provided the crystal plate in the solid-state laser resonator for parasitic oscillation suppression. It is a figure which shows the installation attitude | position of the crystal plate of FIG. 13 is a diagram showing still another example according to the present invention together with FIG. 13, which is a diagram showing a configuration when viewed from a direction orthogonal to a plane including the arrangement direction of the emitters constituting the parallel light source. . It is a figure which shows the principal part of a structure at the time of seeing from a different direction from FIG. It is the perspective view which showed the structural example of the light source device which concerns on this invention. It is explanatory drawing which shows an example of the nonlinear optical element which has several quasi phase matching parts. FIG. 6 is a graph illustrating the relationship between the wavelength λ of the fundamental wave and the polarization inversion period Λ (λ).

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Light source device, 3 ... Resonator, 4 ... Laser medium, 5 ... Nonlinear optical element, 6 ... Image generation apparatus, 7R, 7G, 7B ... Light modulation means, 17 ... Laser medium, 21 ... Nonlinear optical element, 32 ... Resonator, 34, 35 ... Nonlinear optical element, 36-38 ... Pseudo phase matching unit

Claims (12)

In a light source device configured to generate a harmonic by phase matching by arranging a nonlinear optical element having a periodically poled structure in a solid-state laser resonator that oscillates vertically and horizontally in a multimode,
A light source device characterized in that an oscillation wavelength width of a fundamental wave by a laser medium in a resonator is equal to or less than a phase matching width of the nonlinear optical element. The light source device according to claim 1,
A light source device characterized in that a phase matching tolerance defined by a crystal length of the nonlinear optical element is larger than an oscillation gain width of a fundamental wave. The light source device according to claim 1,
A light source device, wherein a wavelength selection element or a birefringent filter for suppressing parasitic oscillation is disposed in the solid-state laser resonator. The light source device according to claim 1,
The non-linear optical element includes a quasi-phase matching unit having a polarization inversion period corresponding to a parasitic oscillation wavelength. The light source device according to claim 1,
The substrate material of the nonlinear optical element is a stoichiometric composition periodically poled lithium tantalate subjected to a vapor phase equilibrium process. The light source device according to claim 1,
A light source device comprising: a solid-state laser medium doped with a rare earth element disposed in an infrared resonator; and outputting ultraviolet light, blue light, or green light by generating harmonics of the nonlinear optical element. An illumination light source configured to generate a harmonic by phase matching by arranging a nonlinear optical element having a periodically poled structure in a solid-state laser resonator that oscillates in a longitudinal and transverse multimode, and modulates light from the illumination light source In the image generation apparatus provided with the light modulation means,
An image generation apparatus, wherein an oscillation wavelength width of a fundamental wave by a laser medium in a resonator is equal to or less than a phase matching width of the nonlinear optical element. The image generation apparatus according to claim 7,
An image generating apparatus, wherein an allowable phase matching width defined by a crystal length of the nonlinear optical element is larger than an oscillation gain width of a fundamental wave. The image generation apparatus according to claim 7,
An image generating apparatus, wherein a wavelength selection element or a birefringent filter for suppressing parasitic oscillation is disposed in the solid-state laser resonator. The image generation apparatus according to claim 7,
The non-linear optical element includes a quasi-phase matching unit having a polarization inversion period corresponding to a parasitic oscillation wavelength. The image generation apparatus according to claim 7,
A substrate material of the nonlinear optical element is a stoichiometric composition periodically poled lithium tantalate subjected to a vapor phase equilibrium process. The image generation apparatus according to claim 7,
An image generating apparatus comprising: a rare-earth-added solid-state laser medium disposed in an infrared resonator; and outputting ultraviolet light, blue light, or green light by harmonic generation of the nonlinear optical element.

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