JP2006292942A - Second higher harmonic generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a second higher harmonic generator capable of generating a second harmonic with stable output by regulating a resonating polarization of a fundamental wave to one direction. <P>SOLUTION: The second higher harmonic generator 1 has an excitation light source 2 which generates excitation light L0, a fundamental wave generation crystal 3 which receives the excitation light L0 and generates the fundamental wave, a nonlinear optical crystal 4 which generates the second higher harmonic L3 of the fundamental wave, and an optical resonator 7 having a pair of reflecting mirrors 5, 6 which are arranged opposite to each other by interposing a fundamental wave generation crystal 3 and a nonlinear optical crystal 4 between them. The reflecting mirrors 5, 6 have grating sections 20 arranged with grating layers at specified intervals and reflect only the fundamental wave having the polarization direction parallel to the orientation direction of the grating layers. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a second harmonic generator that generates a second harmonic of a fundamental wave by wavelength conversion using, for example, a nonlinear optical crystal.

  Green laser light sources are used for medical, dental, pointer, and measurement applications. Currently, the green laser is generally a solid-state laser using a nonlinear optical crystal that generates second harmonics (SH waves). When using the second harmonic generation by a solid-state laser with a low optical output of about 100 mW or less, there are many configurations (intracavity) in which a nonlinear optical crystal is disposed in an optical resonator of light (fundamental wave) having a wavelength of 1064 nm. Used (see, for example, Patent Document 1). The optical resonator is composed of a pair of reflecting mirrors. For example, the reflecting mirror is formed of a dielectric multilayer film.

In order to realize the second harmonic generation, it is necessary to satisfy the phase matching condition. One of the methods is angle tuning, which is classified into type I and type II (see, for example, Patent Document 2). In Type II, the principal axis of the fundamental wave generating crystal that generates the fundamental wave and the principal axis of the nonlinear optical crystal are arranged to be inclined by 45 °. In the fundamental wave generating crystal, a fundamental wave having a polarization direction along the principal axis is usually generated.
JP 2002-344048 A JP-A-7-131101

  However, when the output from the excitation light source is increased, not only the polarization direction along the principal axis but also the fundamental wave in the polarization direction orthogonal to the principal axis is generated from the fundamental wave generating crystal. When two or more types of fundamental waves having different polarization directions are generated by the fundamental wave generating crystal, each fundamental wave having a different polarization direction resonates in the resonator, and as a result, laser light having three or more types of polarization directions from the nonlinear optical crystal. There is a problem that (second harmonic) is generated and the output becomes unstable.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to generate a second harmonic with a stable output by defining the polarization direction in which the fundamental wave resonates in one direction. The object is to provide a second harmonic generator.

  In order to achieve the above object, a second harmonic generator of the present invention includes an excitation light source that generates excitation light, a fundamental wave generation crystal that generates the fundamental wave upon incidence of the excitation light, A non-linear optical crystal that generates a second harmonic, and an optical resonator having a pair of reflecting mirrors facing each other with the fundamental wave generating crystal and the non-linear optical crystal interposed therebetween, the reflecting mirror having a constant interval And a grating portion in which a grating layer is disposed, and reflects only the fundamental wave having a polarization direction parallel to the extending direction of the grating layer.

In the second harmonic generator of the present invention described above, when the excitation light from the excitation light source enters the fundamental wave generating crystal, the fundamental wave generating crystal resonates in an optical resonator composed of a pair of reflecting mirrors. The fundamental wave oscillates. Since the optical resonator reflects only the fundamental wave having the polarization direction parallel to the extending direction of the grating layer, only the fundamental wave having the polarization direction oscillates.
When this fundamental wave passes through the nonlinear optical crystal, a second harmonic is generated by the nonlinear optical crystal. The second harmonic passes through the reflecting mirror disposed on the nonlinear optical crystal side and is output to the outside.

  According to the present invention, it is possible to realize a second harmonic generator capable of generating the second harmonic with a stable output by defining the polarization direction in which the fundamental wave resonates in one direction.

  Embodiments of a semiconductor device according to the present invention will be described below with reference to the drawings.

(First embodiment)
FIG. 1 is a perspective view showing the configuration of the second harmonic generator 1 according to the present embodiment. FIG. 2 is a side view of the second harmonic generator 1 shown in FIG. 1 as viewed from the x direction.

  The second harmonic generator 1 according to this embodiment includes an excitation light source 2, a fundamental wave generation crystal (solid laser medium) 3 pumped by the excitation light source 2, a nonlinear optical crystal 4, a fundamental wave generation crystal 3, and And an optical resonator 7 including a pair of reflecting mirrors 5 and 6 facing each other with a nonlinear optical crystal 4 interposed therebetween.

  The excitation light source 2 is composed of, for example, a semiconductor laser element. As the excitation light source 2, for example, a semiconductor laser element that emits excitation light (laser light) L0 having a wavelength of 808 nm is used. For example, the excitation light source 2 emits excitation light L0 in the z direction in the figure. In FIG. 1, the emission direction of the excitation light L0 is defined as the z direction, and the two directions orthogonal to the z direction are defined as the x direction and the y direction.

The fundamental wave generating crystal 3 is pumped by the excitation light L0 from the excitation light source 2, and oscillates the fundamental wave (laser light) L1. The wavelength of the fundamental wave L1 is 1064 nm, for example. The fundamental wave generating crystal 3 is, for example, Nd: YAG (Y ₃ Al ₅ O ₁₂ ), Nd: YVO _4, or Nd: GdVO ₄ . The fundamental wave generating crystal 3 has, for example, a dimension in the x direction and the y direction of 1 mm and a dimension in the z direction of 0.7 mm.

The nonlinear optical crystal 4 converts the wavelength of the fundamental wave L1 oscillated by the fundamental wave generation crystal 3 to generate a second harmonic (laser light having a double frequency) L2 of the fundamental wave. Nonlinear optical crystal 4 is, for example, KTP (KTiOPO ₄ ), BBO (β-BaB ₂ O ₄ ), LN (LiNbO ₃ ), LT (LiTaO ₃ ), KN (KNbO ₃ ), LBO (LiB ₃ O ₅ ), It is formed by KDP (KH ₂ PO ₄ ). The nonlinear optical crystal 4 has, for example, a dimension in the x direction and the y direction of 1 mm and a dimension in the z direction of 2.2 mm. The nonlinear optical crystal 4 is bonded to the end face of the fundamental wave generating crystal 3 with an ultraviolet curable adhesive.

  FIG. 3 is a diagram showing the arrangement of the optical axis of the fundamental wave generating crystal 3 and the optical axis of the nonlinear optical crystal 4.

  In order to realize the second harmonic generation, it is necessary to satisfy the phase matching condition. One of the methods is angle tuning, which is classified into type I and type II. In the present embodiment, the type II phase matching condition is satisfied. For this reason, the principal axis 3a of the fundamental wave generating crystal 3 that generates the fundamental wave and the principal axis 4a of the nonlinear optical crystal 4 are disposed so as to be inclined by 45 °. The main axis is a high refractive index axis. For example, when the principal axis 3a of the fundamental wave generating crystal 3 is set in the x-axis direction, the principal axis 4a of the nonlinear optical crystal 4 is set to be inclined by 45 ° from the x-axis in the xy plane. In the fundamental wave generating crystal, a fundamental wave L1 having a polarization direction along the principal axis is usually generated.

  One reflecting mirror 5 constituting the optical resonator 7 is formed on the end face of the fundamental wave generating crystal 3 on the excitation light source 2 side. The reflecting mirror 5 transmits the excitation light L0 emitted from the excitation light source 2 and reflects the fundamental wave L1 obtained from the fundamental wave generating crystal 3. The reflecting mirror 5 reflects only the fundamental wave L1 in the polarization direction along the principal axis of the fundamental wave generating crystal 3.

  The other reflecting mirror 6 constituting the optical resonator 7 is formed on the end face of the nonlinear optical crystal 4 on the emission side of the second harmonic L2. The other reflecting mirror 6 transmits the second harmonic L2 wavelength-converted by the nonlinear optical crystal 4 and reflects the fundamental wave L1. The reflecting mirror 6 reflects only the fundamental wave L1 in the polarization direction along the principal axis of the fundamental wave generating crystal 3.

  FIG. 4 is a cross-sectional view for explaining the configuration of the reflecting mirror 5 (6). 4 corresponds to a cross-sectional view of the end face of the fundamental wave generating crystal 3 in the case of the reflecting mirror 5, and corresponds to a cross-sectional view of the end face of the nonlinear optical crystal 4 in the case of the reflecting mirror 6. FIG.

  Each of the reflecting mirrors 5 and 6 includes a first reflecting portion made of the dielectric multilayer mirror 10 and a second reflecting portion made of the grating portion 20 formed in the uppermost layer of the dielectric multilayer mirror 10.

The dielectric multilayer film mirror 10 has two low-refractive index films 11 and high-refractive index films 12 alternately having different refractive indexes (n) and thicknesses of λ / 4n (λ: wavelength of light to be reflected). (About 20 to 40 layers) A mirror having a reflectance of nearly 100% by being laminated. The low refractive index film 11 is formed by, for example, a SiO ₂ film, and the high refractive index film 12 is formed by, for example, a TiO ₂ film or a Ta ₂ O ₅ film. The dielectric multilayer mirror 10 is formed by sputtering, for example.

  Both the dielectric multilayer mirror 10 of the reflecting mirror 5 and the reflecting mirror 6 need to reflect the fundamental wave L1. Therefore, for example, when the refractive index of the low refractive index film 11 is n1 and the wavelength of the fundamental wave L1 is λ, the film thickness of each low refractive index film 11 is set to λ / 4n1. When the refractive index of the high refractive index film 12 is n2 and the wavelength of the fundamental wave L1 is λ, the film thickness of each high refractive index film 12 is set to λ / 4n2.

  FIG. 5A is a cross-sectional view showing the configuration of the grating portion 20, and FIG. 5B is a plan view of the grating portion 20.

  The grating portion 20 is formed on the upper surface of the dielectric multilayer mirror 10. The grating portion 20 has a grating layer 21 formed in a stripe shape. The stripe direction of the grating layer 21 is formed parallel to the optical axis of the fundamental wave generating crystal 3.

  By making the pitch P of the grating layer 21 as large as the wavelength of the fundamental wave L1, it is reflected by the fundamental wave in the polarization direction parallel to the stripe of the grating layer 21 and the fundamental wave in the polarization direction perpendicular to the stripe. A difference appears in the rate. The magnitude of the reflectance depends on the refractive index (n), the width W, and the film thickness d of the grating layer 21.

  For example, when the wavelength of the fundamental wave L1 is 1064 nm, the pitch P of the grating layer 21 is set to about 1064 nm, the width W of the grating layer 21 is set to several tens nm to several hundreds nm, The film thickness d is set to 40-50 nm. As a result, the grating section 20 has a high reflectance for the fundamental wave in the polarization direction parallel to the stripe of the grating layer 21 and a low reflectance for the fundamental wave in the polarization direction perpendicular to the stripe of the grating layer 21. It becomes.

  As a result, only the fundamental wave L1 in the polarization direction parallel to the stripe of the grating layer 21 is resonated by the optical resonator 7 including the reflecting mirrors 5 and 6 including the grating unit 20, and single polarization mode oscillation of the fundamental wave is realized. Is done.

  The grating layer 21 is formed, for example, by patterning the low refractive index film 11 or the high refractive index film 12 constituting the dielectric multilayer mirror 10. The patterning is performed by electron beam drawing and etching (dry etching or wet etching).

  The grating portion 20 only needs to be formed with portions having different refractive indexes at regular intervals due to the formation of the grating layer 21. Therefore, in addition to the case where the grating layer 21 has a physical stripe shape as described above, the low refractive index film 11 or the high refractive index film 12 is ion-implanted with hydrogen, helium, etc. It can also be realized by forming a grating layer 21 having a shape.

  Next, the operation of the second harmonic generator 1 according to the present embodiment will be described with reference to FIG.

  When excitation light L0, which is pumping light emitted from the excitation light source 2, is incident on the fundamental wave generating crystal 3, it resonates in an optical resonator 7 composed of the reflecting mirror 5 and the reflecting mirror 6 to generate fundamental waves. Only the fundamental wave L1 having a polarization direction parallel to the optical axis 3a of the crystal 3 oscillates. At this time, the fundamental wave L1 having a polarization direction different from that of the optical axis 3a of the fundamental wave generating crystal 3 is not oscillated.

  When the fundamental wave L1 passes through the nonlinear optical crystal 4, the fundamental wave L1 is wavelength-converted by the nonlinear optical crystal 4, and the second harmonic L2 of the fundamental wave L1 passes through the reflecting mirror 6 to generate the second harmonic. Output from the device 1. When the fundamental wave L1 with a wavelength of 1064 nm is oscillated, a second harmonic L2 with a wavelength of 532 nm is obtained, and so-called green laser light can be output.

  As described above, in the present embodiment, the reflecting mirrors 5 and 6 constituting the optical resonator 7 include the grating unit 20 that reflects only the fundamental wave L1 in one polarization direction. One polarization mode oscillation can be realized. Therefore, the second harmonic L2 having a single polarization direction can be output.

  Therefore, even if the injection current to the excitation light source 2 is increased to increase the output of the excitation light L0, the second harmonic L2 can be generated with a stable output. In addition, the second harmonic L2 can be generated with a stable output without being affected by the FFP (far field pattern) characteristics and the oscillation wavelength characteristics of the excitation light source 2 and the absorption and emission characteristics of the fundamental wave generating crystal 3. Furthermore, since the linearity of the polarization direction of the second harmonic L2 is improved, the polarization of the second harmonic L2 can be used in the application.

  In the present embodiment, one reflecting mirror 5 constituting the optical resonator 7 is formed on the end surface of the fundamental wave generating crystal 3 on the excitation light source 2 side, and the other reflecting mirror 6 constituting the optical resonator 7 is The nonlinear optical crystal 4 is formed on the end face of the second harmonic L2 emission side. Since there is no need to provide separate parts as the reflecting mirror 5 and the reflecting mirror 6, there is an advantage that the number of parts can be reduced and the second harmonic generator 1 can be downsized.

(Second Embodiment)
FIG. 6 is a perspective view showing the configuration of the second harmonic generator 1 according to the second embodiment.

  In the second embodiment, the pair of reflecting mirrors 5 and 6 constituting the optical resonator 7 are arranged with a predetermined distance from the fundamental wave generating crystal 3 and the nonlinear optical crystal 4. In the present embodiment, unlike the first embodiment, separate components are provided as the reflecting mirror 5 and the reflecting mirror 6.

  Similarly to the first embodiment, the reflecting mirrors 5 and 6 include a first reflecting portion formed of the dielectric multilayer mirror 10 and a second reflecting portion 20 formed of the grating portion 20 formed on the uppermost layer of the dielectric multilayer mirror 10. Part. In addition, these 1st reflection parts and 2nd reflection parts are formed on a transparent substrate.

  The reflecting mirror 5 may be formed with the grating part 20 and the dielectric multilayer mirror 10 in order from the excitation light source 2 side, or may be formed in the order of the dielectric multilayer film mirror 10 and the grating part 20.

  Similarly, the reflecting mirror 6 may be formed with the grating part 20 and the dielectric multilayer mirror 10 in order from the light output side, or may be formed in the order of the dielectric multilayer film mirror 10 and the grating part 20.

  The second harmonic generator 1 according to the present embodiment can achieve the same effects as those of the first embodiment.

(Third embodiment)
FIG. 7 is a perspective view showing the configuration of the second harmonic generator 1 according to the third embodiment.

  The third embodiment is an example in which the fundamental wave generating crystal 3 and the nonlinear optical crystal 4 are not joined and are arranged at a predetermined interval. The distance between the fundamental wave generating crystal 3 and the nonlinear optical crystal 4 is, for example, several μm to several tens of μm.

  One reflecting mirror 5 constituting the optical resonator 7 is formed on the end face of the fundamental wave generating crystal 3 on the excitation light source 2 side, and the other reflecting mirror 6 constituting the optical resonator 7 is the second reflecting mirror 6 of the nonlinear optical crystal 4. It is formed on the end face of the second harmonic L2 emission side.

  The second harmonic generator 1 according to the present embodiment can achieve the same effects as those of the first embodiment.

(Fourth embodiment)
FIG. 8 is a perspective view showing the configuration of the second harmonic generator 1 according to the fourth embodiment.

  The fourth embodiment is an example in which the fundamental wave generating crystal 3 and the nonlinear optical crystal 4 are not joined and are arranged at a predetermined interval. The distance between the fundamental wave generating crystal 3 and the nonlinear optical crystal 4 is, for example, several μm to several tens of μm.

  A pair of reflecting mirrors 5 and 6 constituting the optical resonator 7 are arranged at a predetermined interval with respect to the fundamental wave generating crystal 3 and the nonlinear optical crystal 4. In the present embodiment, unlike the first embodiment, separate components are provided as the reflecting mirror 5 and the reflecting mirror 6.

  Similarly to the first embodiment, the reflecting mirrors 5 and 6 include a first reflecting portion formed of the dielectric multilayer mirror 10 and a second reflecting portion 20 formed of the grating portion 20 formed on the uppermost layer of the dielectric multilayer mirror 10. Part. In addition, these 1st reflection parts and 2nd reflection parts are formed on a transparent substrate.

  The reflecting mirror 5 may be formed with the grating part 20 and the dielectric multilayer mirror 10 in order from the excitation light source 2 side, or may be formed in the order of the dielectric multilayer film mirror 10 and the grating part 20.

  Similarly, the reflecting mirror 6 may be formed with the grating part 20 and the dielectric multilayer mirror 10 in order from the light output side, or may be formed in the order of the dielectric multilayer film mirror 10 and the grating part 20.

  The second harmonic generator 1 according to the present embodiment can achieve the same effects as those of the first embodiment.

The present invention is not limited to the description of the above embodiment.
In the present embodiment, the reflecting mirrors 5 and 6 have been described as having a configuration including the first reflecting portion made of the dielectric multilayer mirror 10 and the second reflecting portion made of the grating portion 20, but a metal mirror as the first reflecting portion. May be adopted. In addition to the dielectric film, a metal film may be used as the grating layer 21 constituting the grating portion 20.

In the present embodiment, an example in which green laser light is obtained as the second harmonic has been described. In addition, second harmonics of other wavelengths can be obtained.
In addition, various modifications can be made without departing from the scope of the present invention.

It is a perspective view which shows the structure of the 2nd harmonic generator which concerns on 1st Embodiment. It is the side view which looked at the 2nd harmonic generator shown in FIG. 1 from the x direction in the figure. It is a figure which shows arrangement | positioning of the optical axis of a fundamental wave generating crystal, and the optical axis of a nonlinear optical crystal. It is sectional drawing which shows the structure of a reflective mirror. (A) is sectional drawing which shows the structure of a grating part, (b) is a top view of a grating part. It is a perspective view which shows the structure of the 2nd harmonic generator which concerns on 2nd Embodiment. It is a perspective view which shows the structure of the 2nd harmonic generator which concerns on 3rd Embodiment. It is a perspective view which shows the structure of the 2nd harmonic generator which concerns on 4th Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... 2nd harmonic generator, 2 ... Excitation light source, 3 ... Fundamental wave crystal, 4 ... Nonlinear optical crystal, 5 ... Reflective mirror, 6 ... Reflective mirror, 7 ... Optical resonator, 10 ... Dielectric multilayer mirror , 11 ... Low refractive index film, 12 ... High refractive index film, 20 ... Grating part, 21 ... Grating layer

Claims (7)

An excitation light source that generates excitation light;
A fundamental wave generating crystal that generates the fundamental wave upon incidence of the excitation light; and
A nonlinear optical crystal that generates a second harmonic of the fundamental wave;
An optical resonator having a pair of reflecting mirrors facing each other through the fundamental wave generating crystal and the nonlinear optical crystal;
The reflection mirror has a grating part in which grating layers are arranged at regular intervals, and reflects only the fundamental wave having a polarization direction parallel to the extending direction of the grating layer. The pair of reflectors is
A dielectric multilayer mirror in which films having different refractive indexes are repeatedly laminated;
The second harmonic generator according to claim 1, further comprising: a grating portion formed on the dielectric multilayer mirror. The second harmonic generator according to claim 1, wherein the pair of reflecting mirrors are formed on both end faces of the fundamental wave generating crystal and the nonlinear optical crystal. The second harmonic generator according to claim 1, wherein the pair of reflecting mirrors are disposed at a distance from the fundamental wave generating crystal and the nonlinear optical crystal. The second harmonic generator according to claim 1, wherein an end face of the fundamental wave generating crystal and an end face of the nonlinear optical crystal are joined. The second harmonic generator according to claim 1, wherein the fundamental wave generating crystal and the nonlinear optical crystal are arranged with a space therebetween. The second harmonic generator according to claim 1, wherein an optical axis of the fundamental wave generating crystal and an optical axis of the nonlinear optical crystal are inclined.

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