JP2014222263A - Wavelength conversion element and wavelength conversion laser apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem that, when a fundamental wave is oscillated in a plurality of laser oscillation modes on a waveguide, a conventional optical wavelength conversion element cannot satisfy phase matching conditions for the plurality of laser oscillation modes of the fundamental wave, and cannot efficiently convert the wavelength of the fundamental wave.SOLUTION: The wavelength of a fundamental wave in a plurality of laser oscillation modes is efficiently converted with a simple configuration by disposing between nonlinear optical materials an optical material having a lower refractive index than that of the nonlinear optical materials composing a wavelength conversion element to reduce an effective refractive index difference between laser beams of a zero-order mode and of a primary order mode.

Description

  The present invention relates to a wavelength conversion element for laser light and a wavelength conversion laser device.

  In a device that displays a color image, such as a printer or a projection television, light sources of three colors R (red), G (green), and B (blue) are required as light sources. In recent years, as these light sources, laser light in the 1.3 μm band, 1 μm band, and 900 nm band is used as the fundamental wave laser light, and the fundamental laser light is a second light having a half wavelength (double frequency) using a nonlinear material. A wavelength conversion laser device (laser oscillator) that converts to a harmonic (SHG (Second Harmonic Generation)) has been developed.

  In SHG, in order to efficiently extract laser light having a desired wavelength, it is desirable to realize high conversion efficiency from fundamental laser light to second harmonic laser light. As an element that performs wavelength conversion while satisfying the phase matching condition for correcting the phase shift between the fundamental laser beam and the second harmonic laser beam, for example, a quasi phase matching (QPM (Quasi Phase Matching)) element using a periodic structure is used. Are known. In this QPM wavelength conversion element, an optical waveguide is formed on a periodically poled lithium niobate (PPLN) which is a nonlinear optical crystal, and the polarization is periodically inverted along the waveguide direction.

In addition, as a method of widening the phase matching bandwidth, which is the tolerance of the phase matching condition with respect to temperature, a structure in which the pitch of the polarization inversion period is gradually changed (a structure in which the periodic structure of the polarization inversion is changed into a chirp shape) A QPM wavelength conversion element has been proposed.

The optical wavelength conversion element described in Patent Document 1 has a periodic domain-inverted structure formed in a nonlinear optical crystal. The domain-inverted structure has a single periodic part (single-period part) and the period gradually changes. And a chirp period portion.

JP 2000-321610 A

  However, in the above conventional technique, when the fundamental wave oscillates in the higher-order laser oscillation mode (higher-order mode) in the waveguide, the effective refractive index is different, so that the phase matching condition for the fundamental wave of the higher-order mode is different. The fundamental wave of the higher order mode cannot be wavelength-converted efficiently. Therefore, there is a problem that the wavelength conversion efficiency for a plurality of laser oscillation modes is lowered.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a wavelength conversion element and a wavelength conversion laser device that efficiently convert wavelengths of a plurality of fundamental waves of a laser oscillation mode with a simple configuration.

  In order to achieve the above object, a wavelength conversion element according to the present invention is a planar waveguide type wavelength conversion element that includes a nonlinear optical material and performs wavelength conversion of laser light propagating in the element. The optical material has a plurality of polarization inversion layers in the traveling direction of the laser beam, and the wavelength conversion element includes a plurality of optical materials having different refractive indexes in a direction substantially perpendicular to the traveling direction of the laser beam. And

  The wavelength conversion element according to the present invention can efficiently convert the wavelengths of the fundamental waves of a plurality of laser oscillation modes with a simple configuration.

The side view which shows the structure of the wavelength conversion laser apparatus which concerns on Embodiment 1 of this invention. 1 is a top view showing a configuration of a wavelength conversion laser device according to Embodiment 1 of the present invention. The perspective view which shows the structure of the wavelength conversion element which concerns on Embodiment 1 of this invention. The side view which shows the 0th-order mode laser beam, the 1st-order mode laser beam, and refractive index distribution in the wavelength conversion element which concerns on Embodiment 1 of this invention. The side view which shows the 0th-order mode laser beam, the 1st-order mode laser beam, and refractive index distribution in the wavelength conversion element which concerns on Embodiment 2 of this invention. The side view which shows the 0th-order mode laser beam, the 1st-order mode laser beam, and refractive index distribution in the wavelength conversion element which concerns on Embodiment 3 of this invention. The perspective view which shows the structure of the wavelength conversion element which concerns on Embodiment 4 of this invention.

Embodiment 1 FIG.
FIG. 1 shows a first embodiment of the present invention. FIG. 2 is a side view showing the configuration of the wavelength conversion laser device according to the first embodiment. It is a top view which shows the structure of the wavelength conversion laser apparatus concerning. 1 and 2, reference numeral 7 denotes an optical axis representing the laser oscillation direction. In each figure, the same numerals indicate the same or corresponding parts.

  In FIG. 1, reference numeral 100 denotes a planar waveguide type wavelength conversion laser device, which is a semiconductor laser 30, a solid-state laser element 20, and a wavelength conversion element (waveguide type wavelength conversion element) that is the main feature of the first embodiment of the present invention. ) 10. The wavelength conversion laser device 100 changes the refractive index distribution in the direction perpendicular to the principal surface of the nonlinear optical material so that the fundamental wave oscillated in a plurality of laser oscillation modes, for example, the 0th-order mode and the first-order mode can be wavelength-converted. It is a laser oscillator. The wavelength conversion laser device 100 is used as a light source of a laser display device or an optical memory device in the field of optical information processing, for example.

In the following, for convenience of explanation, the optical axis 7 is the z-axis direction, the direction perpendicular to the top surface of the wavelength conversion laser device 100 is the y-axis direction, and the wavelength conversion element 10 is the direction perpendicular to both the y-axis and the z-axis. Such a width direction will be described as the x-axis direction.

  The semiconductor laser 30, the laser medium 21, the optical material 1, and the nonlinear optical materials 2 and 3 each have a substantially rectangular flat plate shape, and one plane so that the upper surface of each flat plate is parallel to the xz plane. It is juxtaposed inside. The laser medium 21 is close to the semiconductor laser 30 at the end face 25a perpendicular to the z-axis, and close to the optical material 1 and the nonlinear optical materials 2 and 3 at the end face 25b perpendicular to the z-axis facing the end face 25a. It is disposed between the optical material 1 and the nonlinear optical materials 2 and 3. The optical material 1 and the nonlinear optical materials 2 and 3 have an end surface 11 a and an end surface 11 b perpendicular to the optical axis 7, and the end surface 11 a is disposed close to the end surface 25 b of the laser medium 21. On the other hand, the end surfaces 11b of the optical material 1 and the nonlinear optical materials 2 and 3 are end surfaces on the side from which the second harmonic laser beam L is emitted.

  The adjacent surface where the semiconductor laser 30 and the laser medium 21 are close to each other has substantially the same surface shape (generally rectangular shape) in the semiconductor laser 30 and the laser medium 21, and the laser medium 21 and the optical material 1, the nonlinear optical material 2, 3 has a substantially rectangular shape having substantially the same surface shape in the laser medium 21, the optical material 1, and the nonlinear optical materials 2 and 3.

  In other words, in the wavelength conversion laser device 100, the semiconductor laser 30 is arranged such that the emission surface of the semiconductor laser 30, the end surfaces 25a and 25b of the laser medium 21, the end surfaces 11a and 11b of the optical material 1 and the nonlinear optical materials 2 and 3 are parallel to each other. A solid-state laser element 20 and a wavelength conversion element 10 are disposed. A cooling heat sink (not shown) may be bonded to the semiconductor laser 30.

  The semiconductor laser 30 outputs one to a plurality of LD (Laser Diode) lights from one to a plurality of active layers. When the semiconductor laser 30 outputs a plurality of LD lights, it emits the LD lights in an array by a multi-emitter semiconductor laser or the like, and causes the solid-state laser element 20 to perform multi-emitter oscillation. The semiconductor laser 30 has a width in the x-axis direction substantially equal to that of the laser medium 21 and outputs the excitation light substantially uniformly in the x-axis direction. When the semiconductor laser 30 is a multi-emitter semiconductor laser, the active layers are arranged in the x-axis direction of the laser light emission surface. In this case, the semiconductor laser 30 outputs a plurality of LD lights from a plurality of active layers, and the solid-state laser element 20 can obtain the LD lights from a plurality of active layers arranged in the x-axis direction. The LD light output from the semiconductor laser 30 is incident on the laser medium 21 in the direction of the optical axis 7 of the laser medium 21 from the end face 25a.

The solid-state laser element 20 is an element that oscillates fundamental laser light, and includes a laser medium 21 and clads 22 and 23 that are low refractive index portions. The end face 25a of the laser medium 21 is a total reflection film that reflects the fundamental laser light, and the end face 25b of the laser medium 21 is an antireflection film that transmits the fundamental laser light. End surfaces 11a of the optical material 1 and the nonlinear optical materials 2 and 3 are partially reflecting films that are optical films that transmit the fundamental laser beam and reflect the second harmonic laser beam L. The optical material 1 and the nonlinear optical material 2 The third end face 11b is a partial reflection film that is an optical film that reflects the fundamental laser beam and transmits the second harmonic laser beam L. These total reflection film, antireflection film, and optical film are produced, for example, by laminating dielectric thin films. When the excitation light output from the semiconductor laser 30 is incident from the end face 25a of the laser medium 21, the total reflection film on the end face 25a is an optical film that transmits the excitation light and reflects the fundamental laser light.

  As the laser medium 21, a general solid-state laser material can be used. The laser medium 21 is, for example, Nd: YAG, Nd: YLF, Nd: Glass, Nd: YVO4, Nd: GdVO4, Yb: YAG, Yb: YLF, Yb: KGW, Yb: KYW, Er: Glass, Er: YAG Tm: YAG, Tm: YLF, Ho: YAG, Ho: YLF, Tm, Ho: YAG, Tm, Ho: YLF, Ti: Sapphire, Cr: LiSAF, Pr: YAG, Pr: YLF.

The clads 22 and 23 have a refractive index smaller than that of the laser medium 21 and are bonded to the upper surface and the lower surface of the laser medium 21 on the surfaces parallel to the xz plane, that is, the upper surface of the clad 23 and the lower surface of the clad 22, respectively. ing. The clads 22 and 23 are produced by, for example, a method of depositing a film made of an optical material as a raw material on the laser medium 21 or a method of optically bonding the optical material to the laser medium 21 by optical contact or diffusion bonding. A cooling heat sink (not shown) may be bonded to the lower surface side of the clad 23.

The wavelength conversion element 10 is an element that converts the oscillated fundamental laser light into second harmonic laser light and emits the converted second harmonic laser light. The wavelength conversion element 10 has a planar waveguide type waveguide structure, and has an optical material 1, a nonlinear optical material 2, 3 and a cladding 4, 5 which are low refractive index portions. As the wavelength conversion element 10, a QPM wavelength conversion element, that is, a quasi phase matching wavelength conversion element is used. The nonlinear optical materials 2 and 3 convert the wavelength of the fundamental laser beam incident from the laser medium 21 side and output the second harmonic laser beam L. That is, by utilizing the nonlinearity of the optical material, the wavelength of the fundamental laser beam is converted to the wavelength of the second harmonic laser beam. The nonlinear optical materials 2 and 3 are, for example, MgO-added LiNbO3, MgO-added LiTabO3, stoichiometric LiNbO3, stoichiometric LiTaO3, KTP, etc. having a periodic inversion polarization structure.

Embodiment 1 Then, the wavelength conversion element 10 includes a plurality of optical materials having different refractive indexes in a direction substantially perpendicular to the traveling direction of the laser light. Specifically, the upper surface and the lower surface of the optical material 1 having a refractive index smaller than those of the nonlinear optical materials 2 and 3 are respectively defined as the lower surface of the nonlinear optical material 2 and the upper surface of the nonlinear optical material 3 (each of the surfaces parallel to the xz plane). To join). The clads 4 and 5 have a refractive index smaller than that of the nonlinear optical materials 2 and 3, and are bonded to the upper surface of the nonlinear optical material 2 and the lower surface of the nonlinear optical material 3 (each of which is a plane parallel to the xz plane). To do.

For joining the optical material 1 and the nonlinear optical materials 2 and 3 and joining the claddings 4 and 5 and the nonlinear optical materials 2 and 3, for example, a film using the optical material as a raw material is deposited on the nonlinear optical materials 2 and 3. It is manufactured by a method, a method of optically bonding an optical material to the nonlinear optical materials 2 and 3 by optical contact or diffusion bonding, or a combination thereof.

  FIG. 3 is a perspective view showing the configuration of the wavelength conversion element 10. As shown in FIG. 3, the wavelength conversion element 10 includes nonlinear optical materials 2 and 3 and is a planar waveguide type element that performs wavelength conversion of laser light propagating in the wavelength conversion element. The nonlinear optical materials 2 and 3 of the wavelength conversion element 10 have a plurality of polarization inversion layers 6 in the traveling direction of the laser light. The polarization inversion layer 6 is obtained by inverting the polarization direction of a single crystal dielectric material polarized in a certain direction. In the nonlinear optical materials 2 and 3, non-polarization inversion regions and polarization inversion layers 6 that are polarization inversion regions are alternately formed. Thereby, the domain-inverted layers 6 are periodically formed in the nonlinear optical materials 2 and 3. Each polarization inversion layer 6 has a substantially flat plate shape and is sandwiched between clads 4 and 5 so that the surface of the flat plate is parallel to the x-axis direction and the y-axis direction.

A fundamental laser beam as a fundamental wave from the laser medium 21 is incident on the nonlinear optical materials 2 and 3 from the end face 11a side, and propagates in turn in the non-polarization inversion regions and the polarization inversion regions arranged alternately. . The incident fundamental wave laser beam is converted into the second harmonic laser beam by the nonlinear effect of the nonlinear optical materials 2 and 3. The crystal axis angle, temperature, period of inversion polarization, etc. of the nonlinear optical materials 2 and 3 are optimized in advance so that the fundamental laser beam is converted into the second harmonic laser beam. A part of the fundamental laser light incident on the nonlinear optical materials 2 and 3 is converted into second harmonic laser light and output to the outside from the end face 11b.

  The fundamental laser beam that remains in the nonlinear optical materials 2 and 3 without being converted into the second harmonic laser beam is totally reflected by the end face 11b and passes through the nonlinear optical materials 2 and 3 again. It is converted into a harmonic laser beam. The second harmonic laser beam generated by converting a part of the remaining fundamental laser beam is totally reflected by the end face 11a and output as laser light to the outside from the end face 11b.

  FIG. 4 is a distribution diagram of the intensity distribution and the refractive index n of the laser light L0 and the laser light L1 when the fundamental wave laser light L0 of the 0th order mode and the laser light L1 of the first order mode are incident in the wavelength conversion element 10. is there. The zero-order mode has a maximum intensity at the center of the element in the y-axis direction, so the influence of the refractive index of the optical material 1 is large. On the other hand, the primary mode has a small intensity at the center of the element in the y-axis direction, so the influence of the refractive index of the optical material 1 is small.

Embodiment 1 of the present invention Then, the optical material 1 having a lower refractive index than the nonlinear optical materials 2 and 3 is disposed at the center of the wavelength conversion element 10 in the y-axis direction. The effective refractive index of the laser beam L0 is small. On the other hand, the change in the effective refractive index of the laser beam L1 in the first-order mode is small compared to the conventional technique without the optical material 1. As a result, by disposing the optical material 1 between the nonlinear optical materials 2 and 3, the difference between the effective refractive index of the zero-order mode laser light L0 and the effective refractive index of the first-order mode laser light L1 is reduced. Can do.

  Thus, the first embodiment of the present invention. Then, the wavelength conversion element 10 includes a plurality of optical materials having different refractive indexes in a direction substantially perpendicular to the traveling direction of the laser beams L0 and L1, so that the effective refractive index of the zero-order mode and the first-order mode in the wavelength conversion element 10 are obtained. It is possible to reduce the difference in effective refractive index.

The optimum period of inversion polarization is determined from the wavelength and the effective refractive index. Therefore, in the wavelength conversion element 10 that forms the period of inversion polarization optimized for the 0th-order mode, the difference between the effective refractive index of the 1st-order mode and the effective refractive index of the 0th-order mode becomes small. It is possible to increase the wavelength conversion efficiency.

  The first embodiment. The configuration of the wavelength conversion laser device 100 illustrated in FIGS. 1 and 2 has been described, but configurations other than those illustrated in FIGS. 1 and 2 may be used. For example, the wavelength conversion element 10 may be configured to include only one of the clads 4 and 5. Further, the solid-state laser element 20 may be configured to include only one of the clads 22 and 23, or may be configured to dispose a substrate on the outer side of the clads 4 and 5 or on the clads 22 and 23. The wavelength conversion laser device 100 is not limited to an internal wavelength conversion method, that is, a configuration in which a wavelength conversion element is installed inside the resonator, but is an external type wavelength conversion method, that is, a configuration in which the wavelength conversion element is installed outside the resonator. It may be. When the wavelength conversion laser device 100 is an internal wavelength conversion method, the fundamental laser beam oscillates between the end face 25 a of the solid-state laser element 20 and the end face 11 b of the wavelength conversion element 10. On the other hand, when the wavelength conversion laser device 100 is an external type wavelength conversion method, the fundamental wave laser light oscillates between the end face 25 a of the solid-state laser element 20 and the end face 25 b of the solid-state laser element 20.

The optical material 1 and the nonlinear optical materials 2 and 3 may be designed so as to satisfy a phase matching condition for correcting the phase of the laser light in the 0th order mode and the 1st order mode in the wavelength conversion element. In this case, the wavelength conversion of the 0th-order mode and the first-order mode laser light can be performed more efficiently. Further, the laser beam may be a laser beam of a higher-order mode than the zero-order mode and the first-order mode.

Embodiment 2. FIG.
Embodiment 1 FIG. In the present embodiment, the optical material 1 having a refractive index smaller than that of the nonlinear optical materials 2 and 3 is disposed in the wavelength conversion element 10, whereas in the present embodiment, an optical material having a refractive index larger than that of the nonlinear optical material is disposed. The configuration is disclosed.

In the second embodiment of the present invention, as shown in FIG. 5, the optical materials 8 and 9 having a refractive index larger than that of the nonlinear optical materials 12, 13, and 14 are placed at positions where the intensity of the laser light L1 in the first-order mode is high. Deploy. With such an arrangement, the optical materials 8 and 9 have a greater influence on the laser beam L1 in the first-order mode than the laser beam L0 in the 0th-order mode whose intensity is small at that position. Thereby, compared with the case where there is no optical material 8 and 9, the increase amount of the effective refractive index of 1st-order mode becomes larger than the increase amount of the effective refractive index of 0th-order mode. As a result, it is possible to reduce the difference between the effective refractive index of the 0th-order mode, the effective refractive index, and the primary mode, and it is possible to increase the wavelength conversion efficiency of the primary mode.

Embodiment 3 FIG.
Embodiment 1 FIG. And 2. In the present embodiment, an optical material having a refractive index larger or smaller than that of the nonlinear optical material is disposed in the wavelength conversion element 10, whereas in this embodiment, an optical material having a refractive index larger than that of the nonlinear optical material and an optical material having a smaller refractive index than that of the nonlinear optical material are used. A configuration in which both are arranged is disclosed.

In the third embodiment of the present invention, as shown in FIG. 6, both the optical material 15 having a small refractive index and the optical material 16 having a large refractive index with respect to the nonlinear optical material 17 are provided. The arrangement of the optical material 15 is the same as in the first embodiment of the present invention, and the arrangement of the optical material 16 is the same as in the second embodiment of the present invention. As described above, by arranging the optical material 15 having a small refractive index and the optical material 16 having a large refractive index between the nonlinear optical materials 17, the effects of both Embodiments 1 and 2 of the present invention are utilized. Thus, the effect of reducing the difference in effective refractive index between the 0th-order mode and the 1st-order mode can be increased, and the wavelength conversion efficiency of the 1st-order mode can be increased.

Embodiment 4 FIG.
Embodiment 1 FIG. 2. And 3. In the present embodiment, a configuration in which the period is gradually changed is disclosed in contrast to the case where the polarization inversion period is constant.

  Embodiment 4 of the present invention. Then, as shown in FIG. 7, the nonlinear optical materials 18 and 19 of the wavelength conversion element 40 have a polarization inversion layer 24 having a structure in which the polarization inversion period is gradually changed. In the structure having an optimal constant period determined from the effective refractive index for a specific mode of the fundamental wave, the conversion efficiency to the second harmonic of other modes having different effective refractive indexes is low. By gradually changing the polarization inversion period, it is possible to increase the conversion efficiency to the second harmonic for other modes. That is, compared with the case where the polarization inversion period shown in FIG. 3 is uniform, the effect on the waveguide structure can be relaxed.

  Embodiment 4 FIG. The wavelength conversion element according to the first embodiment has a configuration in which the polarization inversion period is gradually changed, and the first embodiment. 2. By making the effective refractive index close to each other between the fundamental wave modes such as 3 and 3, the conversion efficiency of a plurality of modes can be further increased.

  1, 8, 9, 15, 16: Optical material 2, 3, 12, 13, 14, 17, 18, 19: Nonlinear optical material 4, 5, 22, 23: Clad, 6, 24: Polarization inversion layer , 7: optical axis, 10, 40: wavelength conversion element, 11a, 11b, 25a, 25b: end face, 20: solid state laser element, 21: laser medium, 30: semiconductor laser, 100: wavelength conversion laser device, L: first Second harmonic laser light, L0: 0th order mode laser light, L1: 1st order mode laser light

Claims (8)

The fundamental wave propagating through the planar waveguide in the first mode is wavelength-converted to a harmonic by a nonlinear optical material having a periodically inverted polarization structure corresponding to the fundamental wave of the second mode lower than the first mode. A wavelength conversion element,
A wavelength conversion comprising: a refractive index adjusting means for adjusting a refractive index distribution of the planar waveguide so as to reduce a difference in effective refractive index between the fundamental wave of the first mode and the second mode. element. The wavelength conversion element according to claim 1, wherein the refractive index adjusting unit includes a first optical material having a refractive index smaller than that of the nonlinear optical material. 3. The wavelength conversion element according to claim 1, wherein the refractive index adjusting unit includes a second optical material having a refractive index larger than that of the nonlinear optical material.   4. The wavelength conversion element according to claim 1, wherein the nonlinear optical material has a periodic inversion polarization structure in which a polarization inversion period is gradually changed. 5. The first mode and the second mode are a first-order mode and a zero-order mode, respectively, and the first optical material is arranged to reduce the effective refractive index of the zero-order mode, and the second optical The wavelength conversion element according to claim 2 or 3, wherein the material is disposed so as to increase an effective refractive index of the first-order mode. 6. A wavelength conversion laser device comprising: the wavelength conversion element according to claim 1; and a light source having a resonator that oscillates the fundamental wave. The wavelength conversion laser device according to claim 6, wherein the wavelength conversion element is disposed inside the resonator. The wavelength conversion laser device according to claim 6, wherein the wavelength conversion element is disposed outside the resonator.

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