JP2006163256A - Wavelength converting crystal and wavelength conversion optical system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength converting crystal emitting a wavelength-converted light beam. <P>SOLUTION: The wavelength converting crystal 1 is a CLBO (cesium lithium borate) crystal and puts a basic wave of laser light of 1,547 nm in wavelength and a 7th harmonic together to generate an 8th harmonic of 193 nm in wavelength. Such a wavelength converting element emits an 8th harmonic whose beam section is elliptic before because of a walk-off effect during the generation of the 8th harmonic. The incidence surface of the wavelength converting crystal 1 is perpendicular to incident light, but its projection surface 1a is cut obliquely to a plane perpendicular to the incident light. Consequently, the projection light is refracted by a projection-side end surface 1a, and a beam section of the projection light perpendicular to the optical axis is circular. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention converts wavelength at least part of incident light into light having a frequency twice that of the incident light, and converts at least part of incident light into light having a frequency of the sum and difference thereof. The present invention relates to a crystal and a wavelength conversion optical system using the wavelength conversion crystal.

  For example, Japanese Patent Laid-Open No. 2001-353176 (Patent Document 1) discloses a method in which long-wavelength light emitted from a solid-state laser is converted into short-wavelength light (for example, eighth harmonic) using a nonlinear optical crystal. Are listed.

JP 2001-353176 A

  Among such nonlinear optical crystals, wavelength conversion crystals such as LBO crystals and CLBO crystals have a problem of deformation of a light beam (hereinafter simply referred to as “beam”) due to walk-off. Of these, the incident light is uniformly affected even if the traveling direction is the abnormal direction of the wavelength conversion crystal, so the optical axis of the emitted light is incident on the output side end face of the wavelength conversion crystal. The beam itself is merely shifted in parallel with respect to the optical axis, and the beam itself is not deformed.

  However, for light generated by wavelength conversion, the converted light generated on the entrance side of the wavelength conversion crystal has a large amount of parallel shift accompanying the walk-off, and the converted light generated on the exit side of the wavelength conversion crystal is accompanied by the walk-off. The parallel shift amount is reduced. For this reason, for example, when incident light having a circular cross section is incident on the wavelength conversion crystal, the beam shape of the light generated by the wavelength conversion is elliptical.

  Therefore, conventionally, an optical system using a cylindrical lens is provided on the exit side of the wavelength conversion element, the beam shape is returned to a circular shape by this optical system, and the divergence angle and the beam diameter are made uniform. It was. An example is shown in FIG.

  In FIG. 6A, the elliptical beam emitted from the wavelength conversion crystal 31 is narrower (smaller) by the cylindrical lenses 32 and 33 (the superaxial direction of the elliptical beam on the emission end face of the crystal). ) Is substantially collimated, and the cylindrical lens 34 collimates the wide (large) side of the divergence angle (short axis direction of the elliptical beam on the exit end face of the crystal), and is adjusted so as to obtain a predetermined beam diameter. The

  However, a cylindrical lens is more expensive than a normal spherical lens, and there is a problem that an optical system becomes expensive when a large number of cylindrical lenses are used.

  In FIG. 6B, the divergence angle is made uniform by placing the cylindrical lens 32 for adjusting the divergence angle at a position where the beam diameters coincide with each other in the direction where the walk-off occurs and the direction where the walk-off does not occur. The shape is almost circular, and it is only necessary to adjust the spherical lenses 35 and 36 so as to obtain a predetermined beam diameter. In this case, only one cylindrical lens is required, but generally, the place where the beam diameter matches in the direction in which the walk-off occurs and the direction in which the walk-off does not occur is a place close to the wavelength conversion element. Become.

  The present invention has been made in view of such circumstances, and includes a wavelength conversion crystal capable of emitting a wavelength-converted light beam having a circular cross section, and a wavelength conversion optical system using the wavelength conversion crystal. The issue is to provide.

  The first means for achieving the above object is to convert at least a part of incident light into light having a frequency twice that of the incident light, or to add at least a part of a plurality of incident lights to their sum and difference. A wavelength conversion crystal that converts light having a frequency of a wavelength of which the wavelength-converted light is affected by a walk-off, and a surface from which the emitted light exits is a light beam caused by the influence of the walk-off. A wavelength conversion crystal (Claim 1) characterized by being cut obliquely with respect to a plane perpendicular to the optical axis of the incident light so as to compensate for a shape change.

  In this means, the surface from which the emitted light is emitted is cut obliquely with respect to the surface perpendicular to the optical axis of the incident light. Therefore, the outgoing direction of the outgoing light changes under the influence of refraction at the outgoing side end face. By changing the angle of the cut surface, the beam shape of the emitted light can be made circular when viewed from the optical axis direction of the emitted light. Thereby, the light generated by the wavelength conversion of the beam shape having a circular cross section can be extracted from the wavelength conversion crystal, and therefore, a cylindrical lens for beam shaping is not required.

  The second means for solving the problem is the first means, wherein the surface from which the emitted light is emitted is cut so as to be a Brewster cut surface with respect to the emitted light. (Claim 2).

  This means is effective when the light generated by the wavelength conversion is p-polarized light. That is, the surface from which the emitted light is emitted is cut so that it becomes a Brewster cut surface with respect to the emitted light (that is, the light generated by wavelength conversion incident from the inside of the crystal to the emission side end surface is incident at the Brewster angle. Therefore, this light is transmitted without being reflected by the emission side end face, and as a result, the light quantity loss at the crystal end face can be reduced.

  A third means for solving the problem includes at least one wavelength conversion crystal of either the first means or the second means (Claim 3). It is.

  In this means, since the cylindrical lens for beam shaping can be eliminated or reduced, the overall configuration can be simple and inexpensive.

  According to a fourth means for solving the above-mentioned problem, at least part of the incident light is converted into light having a frequency twice that of the incident light, or at least part of the plurality of incident light is summed and differenced between them. A wavelength conversion crystal that converts light having a frequency of a wavelength of which the wavelength-converted light is affected by a walk-off, and a light beam generated by the walk-off of the light emitted from the wavelength conversion crystal. A wavelength conversion optical system comprising a prism that compensates for a change in shape of the light source.

  A fifth means for solving the above-mentioned problem is the fourth means, characterized in that the exit-side end face of the prism is cut so as to be a Brewster cut face with respect to the emitted light. (Claim 5).

  A sixth means for solving the problem includes at least one wavelength conversion optical system of either the fourth means or the fifth means (Claim 6). ).

  In the first means to the third means, the emission side end face of the wavelength conversion crystal is cut obliquely with respect to the plane perpendicular to the optical axis of the incident light. Instead, by providing a prism on the light emission side of the wavelength conversion crystal, the same effect as that obtained by obliquely cutting the emission side end surface of the wavelength conversion crystal is obtained. Therefore, the same effects as the first to third means are obtained.

  According to the present invention, it is possible to provide a wavelength conversion crystal capable of emitting a wavelength-converted light beam having a circular cross section, and a wavelength conversion optical system using the wavelength conversion crystal.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a first example of a wavelength conversion crystal and an optical system using the same, which is an example of an embodiment of the present invention. The wavelength conversion crystal 1 is a CLBO crystal and is a wavelength conversion element that generates an eighth harmonic of a wavelength of 193 nm from a fundamental wave and a seventh harmonic of a laser beam having a wavelength of 1547 nm. In such a wavelength conversion element, conventionally, the emitted 8th harmonic has an elliptical beam cross section because of the walk-off when the 8th harmonic is generated.

  The incident-side end face of the wavelength conversion crystal 1 is perpendicular to the incident light, but the exit-side end face 1a is cut obliquely with respect to the plane perpendicular to the incident light. Therefore, the outgoing light is refracted at the outgoing side end face 1a, and the beam cross section of the outgoing light perpendicular to the optical axis is circular.

  FIG. 2 shows this relationship. In the following drawings, the same reference numerals are given to the components shown in the above-mentioned drawings, and the description thereof may be omitted. In FIG. 2, incident light is assumed to be incident perpendicularly to the incident side end face of the wavelength conversion crystal 1 as a circular beam having a diameter a. The emission-side end face 1a of the wavelength conversion crystal 1 is a plane inclined by α as shown in the figure with respect to a plane perpendicular to the optical axis of the incident light. That is, the emission side end face 1a is perpendicular to the paper surface. In the following description of FIG. 2, “upper and lower, left and right, and horizontal” simply indicate a positional relationship on the paper surface of FIG. 2.

In the wavelength conversion crystal 1, the traveling direction of the energy of the generated eighth harmonic wave is shifted by β in the downward direction of the drawing with respect to the incident direction of the incident light because of the walk-off. The size in the vertical direction of the paper increases as it goes to the exit side. When the vertical distance between the lowest point A and the highest point B of the emitted light emitted from the emission side end face 1a is b, and the horizontal distance from the incident side end face to the lowest point A is c,
b = a + c · tan β (1)
Therefore, the distance d between AB is d = b / cos α = (a + c · tan β) / cos α (2)
It becomes.

On the other hand, when the refractive index is n, the refraction angle γ with respect to the emission side end face 1a of the eighth harmonic is
sinγ = n · sinα (3)
Determined by the emission angle γ determined by. Using equations (1), (2), and (3), the beam diameter x of the emitted light is

It becomes.

  The above is the relationship with respect to the direction parallel to the paper surface. However, in the direction perpendicular to the paper surface, the beam does not spread or narrow due to refraction, and the beam size remains a.

  Therefore,

By determining the relationship between c and α so that the relationship is established, the cross-sectional shape of the emitted light can be made circular.

  In addition, by selecting α so as to have a Brewster angle, when the eighth harmonic wave is p-polarized with respect to the emission side end face 1a, the emission light is not reflected on the emission side end face 1a. Can be emitted from the emission side end face 1a. As described above, since there are two parameters for making the beam shape circular, c and α, even if α is determined to be the Brewster angle, the beam of the emitted light can be adjusted by adjusting c. The shape can be circular.

  In the wavelength conversion crystal 1 shown in FIG. 1, the length of the wavelength conversion crystal 1 and the inclination of the emission side end face 1a are determined so that this relationship is established. Accordingly, the shape of the eighth harmonic beam incident on the spherical lens 2 provided behind it is substantially circular and the divergence angle is also substantially uniform. Can be changed to The beam diameter can be changed depending on the position where the spherical lens 2 is placed.

  FIG. 3 is a diagram showing a second example of a wavelength conversion crystal which is an example of an embodiment of the present invention and an optical system using the same. In FIG. 3, the structure of the wavelength conversion crystal 1 is the same as that shown in FIG. 1, but the optical system is provided with two spherical lenses 3 and 4 thereafter. In this way, since the beam expander can be configured by the spherical lenses 3 and 4, a large beam diameter can be achieved with a short optical path length, and the restriction on the position where the spherical lens is placed can be relaxed.

  FIG. 4 is a diagram showing a third example of a wavelength conversion crystal and an optical system using the same, which is an example of an embodiment of the present invention. In this embodiment, the wavelength conversion crystal 1 has a rectangular parallelepiped shape as in the prior art, but a prism 5 is provided on the exit side to deflect the direction of the eighth harmonic wave. It is different from the one.

  It will be apparent to those skilled in the art that such a combination of the wavelength conversion crystal 1 and the prism 5 is equivalent to the wavelength conversion crystal 1 in which the emission-side end face 1a shown in FIGS. Will.

  In FIG. 4, the prism 5 is disposed away from the wavelength conversion crystal 1, but may be disposed so as to be in contact therewith. Also in FIG. 4, parallel light having a circular cross-sectional shape is obtained by placing the spherical lens 2 as in FIG.

  FIG. 5 is a diagram showing an eighth harmonic generation optical system according to an embodiment of the present invention. The fundamental wave amplified by a fiber amplifier or the like is incident on the wavelength conversion crystal 11, and a part of the fundamental wave is wavelength-converted to a double wave. Examples of the wavelength conversion crystal 11 include, but are not limited to, LBO, PPLN, PPKTP, and the like.

  The fundamental wave and the second harmonic wave emitted from the wavelength conversion crystal 11 are incident on the wavelength conversion crystal 12, and a part of the fundamental wave and the second harmonic wave is wavelength-converted to a third harmonic wave. Examples of the wavelength conversion crystal 12 include, but are not limited to, LBO, PPLN, PPKTP, and the like.

  The fundamental wave, the second harmonic wave, and the third harmonic wave emitted from the wavelength conversion crystal 12 are incident on the dichroic mirror 13, the fundamental wave and the second harmonic wave are transmitted, and the third harmonic wave is reflected. The second harmonic wave transmitted through the dichroic mirror 13 is reflected by the dichroic mirror 14, enters the wavelength conversion crystal 15, and a part of the second harmonic wave is wavelength-converted to the fourth harmonic wave. Examples of the wavelength conversion crystal 15 include, but are not limited to, LBO, PPLN, and PPKTP.

  The fourth harmonic wave emitted from the wavelength conversion crystal 15 is reflected by the dichroic mirror 16, becomes substantially coaxial with the third harmonic wave transmitted through the dichroic mirror 16 via the dichroic mirror 13 and the mirror 17, enters the wavelength conversion crystal 18, A part of the third and fourth harmonics is wavelength-converted to a seventh harmonic. Examples of the wavelength conversion crystal 18 include, but are not limited to, BBO.

  The 7th harmonic wave emitted from the wavelength conversion crystal 18 is reflected by the dichroic mirror 19 and becomes almost coaxial with the fundamental wave transmitted through the dichroic mirror 19 via the dichroic mirrors 13 and 14 and the mirrors 20, 21, and 22. The light is incident on the crystal 23, and a part of the fundamental wave and the seventh harmonic wave is wavelength-converted into 193 nm light which is an eighth harmonic wave. Examples of the wavelength conversion crystal 23 include, but are not limited to, LBO, CLBO, and BBO.

  The wavelength conversion crystal 23 has a structure as shown in FIG. 1 so that the deformation of the beam shape due to the walk-off when the eighth harmonic wave is generated is compensated. In the wavelength conversion crystal 15 as well, since the generated fourth harmonic wave becomes an elliptical shape due to the walk-off, conventionally this was compensated by two cylindrical lenses, but the emission side end face of the wavelength conversion crystal 15 is shown in FIG. As shown in FIG. 1, compensation may be performed by cutting diagonally.

It is a figure which shows the 1st example of the wavelength conversion crystal which is an example of embodiment of this invention, and an optical system using the same. It is a figure which shows the relationship of the light ray in the wavelength conversion crystal which is an example of embodiment of this invention. It is a figure which shows the 2nd example of the wavelength conversion crystal which is an example of embodiment of this invention, and an optical system using the same. It is a figure which shows the 3rd example of the wavelength conversion crystal which is an example of embodiment of this invention, and an optical system using the same. It is a figure which shows the 8th harmonic generation optical system which is embodiment of this invention. It is a figure which shows the example of the method of compensating the beam deformation by the conventional walk-off.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Wavelength conversion crystal, 2 ... Spherical lens, 3 ... Spherical lens, 4 ... Spherical lens, 5 ... Prism, 11 ... Wavelength conversion crystal, 12 ... Wavelength conversion crystal, 13 ... Dichroic mirror, 14 ... Dichroic mirror, 15 ... Wavelength Conversion crystal, 16 ... dichroic mirror, 17 ... mirror, 18 ... wavelength conversion crystal, 19 ... dichroic mirror, 20 ... mirror, 21 ... mirror, 22 ... mirror, 23 ... wavelength conversion crystal

Claims (6)

A wavelength conversion crystal that converts at least part of incident light into light having a frequency twice that of the incident light, or converts at least part of incident light into light having a frequency of the sum and difference thereof. The wavelength-converted crystal in which the wavelength-converted light is affected by the walk-off, and the surface of the incident light is compensated for so that the surface from which the emitted light is emitted compensates for the change in the shape of the light beam due to the influence of the walk-off. A wavelength conversion crystal characterized by being cut obliquely with respect to a plane perpendicular to the axis. 2. The wavelength conversion crystal according to claim 1, wherein a surface from which the emitted light is emitted is cut so as to be a Brewster cut surface with respect to the emitted light. A wavelength conversion optical system comprising at least one wavelength conversion crystal according to claim 1. A wavelength conversion crystal that converts at least part of incident light into light having a frequency twice that of the incident light, or converts at least part of incident light into light having a frequency of the sum and difference thereof. A wavelength conversion crystal whose wavelength-converted light is affected by the walk-off, and a prism that compensates for a change in the shape of the light beam caused by the walk-off of the light emitted from the wavelength conversion crystal. Conversion optical system. 5. The wavelength conversion optical system according to claim 4, wherein an outgoing side end face of the prism is cut so as to be a Brewster cut surface with respect to outgoing light. A wavelength conversion optical system comprising at least one wavelength conversion optical system according to claim 4 or 5.

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