JP2014032300A - Nonlinear wavelength conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase the efficiency and service life of a light source by suppressing the optical damage of the emission end surface of a nonlinear wavelength conversion element.SOLUTION: An end cap 102 which comprises a crystal which is the same kind of nonlinear optical crystal and is slightly different in cut angle, and does not satisfy a phase matching condition, a crystal which does not have a secondary optical nonlinear shape, a crystal which has the same zenith angel and a different azimuth angle, etc., but does not contributes to wavelength conversion is joined to the emission end surface 104 of a nonlinear optical crystal 101, to avoid that the damage of an end surface caused by a harmonic wave causes an optical loss in a fundamental wave.

Description

The present invention relates to a laser light source using a nonlinear wavelength conversion technique, and relates to extending the life of the light source device.

Laser light sources using nonlinear wavelength conversion in nonlinear optical crystals have been actively applied in recent years, and their reliability has been an issue. The present invention provides a means for joining an end cap material that does not contribute to wavelength conversion to the exit end face of a nonlinear optical crystal and avoiding damage to the end face due to harmonics resulting in optical loss with respect to the fundamental wave. .

One of the limitations of the reliability of laser light sources using nonlinear wavelength conversion in nonlinear optical crystals is damage to the end face of the nonlinear optical crystal that is irradiated with the laser light focused to a small spot. Even in the case of a wavelength conversion laser device using a resonator, even if it is not damaged, the efficiency of the device is greatly reduced.

In particular, in applications where ultraviolet light is generated using fundamental waves in the infrared to visible range, damage due to ultraviolet light having a high photon energy becomes more prominent, and most of the end face damage is caused by ultraviolet light. The optical loss with respect to the fundamental wave (input light) in is the main cause of performance degradation of the apparatus.

In addition, there were cases in which damage occurred more significantly when ultraviolet light and visible light were simultaneously incident on the end face. An object of the present invention is to solve the above problems and to greatly extend the device life of a nonlinear wavelength conversion laser light source. Hereinafter, a conventional laser light source will be described.

The nonlinear optical constant of a nonlinear optical crystal that can be used in the ultraviolet region is at most about several pm / V, and the normalized conversion efficiency is on the order of 10 ⁻⁴ / W. That is, in order to obtain a harmonic output of 1 W, it is necessary to input a fundamental wave of 100 W. In the case of a pulse light source having a high spire value, it is still possible to obtain practical conversion efficiency, but in the case of a continuous wave laser light source, it is essential to enhance the fundamental wave by an optical resonator structure. For this enhancement, it is essential that the optical loss of the optical resonator is small, the optical loss increases, the enhancement factor decreases, the conversion efficiency decreases, and the harmonic output decreases. Limit the life of the device.

US5027361: “Efficient laser harmonic generation reducing a low-loss external opticalresonator”

W. J. Kozlovsky, C. D. Nabors, and R. L. Byer, "Efficientsecond harmonic generation of a diode-laser-pumped CW Nd: YAGlaser using monolithic MgO: LiNbO3 external resonant cavities," IEEE Journal of Quantum Electronics QE-24, 913-919 (1988)

In a wavelength conversion laser apparatus, particularly an apparatus that efficiently generates harmonics using an optical resonator in a continuous wave laser apparatus as shown in FIG. The goal is to keep losses low.
The fundamental wave 1 having a single frequency is incident on the input coupling mirror 105, and a part of the fundamental wave 1 is reflected and enters the light receiving element 8. By generating an error signal from the light receiving element 8 and returning it to the precision positioning element 9, the resonator is kept in resonance. In the resonator constituted by the input coupling mirror 105 and the high reflection mirror 106, the fundamental wave power 6 enhanced by several tens to several hundred times circulates. The harmonic wave 3 generated by the nonlinear wavelength conversion element 7 that is arranged inside the resonator and receives the fundamental wave enhanced by the resonance effect is transmitted through the resonator mirror 106 or separated by some wavelength discriminating element and separated from the resonator. Is taken out. The degree of this enhancement is almost inversely related to the optical loss in the resonator, and for higher degree of enhancement, i.e. for high conversion efficiency and high harmonic output, the optical loss of the resonator should be kept low. is important. However, the end face of the wavelength conversion element is gradually damaged by the long-time use due to the strong light intensity in the resonator and the harmonics with high photon energy accordingly, increasing the optical loss and converting the harmonics. The efficiency is reduced and the life of the laser light source device is limited.

FIG. 3 shows the relationship between the intensity distribution of the fundamental wave and the harmonic wave due to the birefringence effect generally called walk-off. A light beam having polarization called extraordinary light in a crystal has a wave vector and a pointing vector which are not parallel, and the wave front normal and the direction in which energy propagates are different. This is shown in FIG. In the case of the light intensity distribution as shown in FIG. 4 thus obtained, the output end face or the antireflection coating applied on the output end face is damaged by the presence of harmonics or harmonics and fundamental waves at the same time. May increase optical loss. Even if the increased loss is only a few percent, the optical loss of the optical resonator is very large, and there is room for improvement for the wavelength conversion laser device. Therefore, a means that does not increase the optical loss at least with respect to the fundamental wave and does not reduce the wavelength conversion efficiency is strongly desired. In the present invention, the fundamental wave and the harmonic are spatially separated at the output end face of the wavelength conversion element. Thus, the above problem is solved.

A medium (end cap) that does not contribute to nonlinear wavelength conversion is joined to the end face of the nonlinear crystal. Since the joint surface needs to be optically transparent, the end cap needs to have a refractive index as close as possible to the nonlinear crystal. Further, it is desirable that the mechanical properties such as the coefficient of thermal expansion are close. In one embodiment, the end cap does not contribute to nonlinear wavelength conversion but has birefringence, and has the effect of spatially separating harmonics and fundamental waves having different polarizations. Therefore, the fundamental wave and harmonics are spatially separated on the exit surface of the end cap, and the fundamental wave and harmonics do not overlap on the end surface on which the antireflection coating is applied. An end cap is bonded to the end face of the nonlinear crystal, and the anti-reflection coating is applied to the exit end face of the end cap to prevent damage to the anti-reflection coating, or damage to the anti-reflection coating due to harmonics causes optical loss with respect to the fundamental wave. Do not become. For the same purpose, the exit end face of the end cap may be processed so as to have a Brewster angle with respect to the fundamental wave.

According to the present invention, the optical loss of the fundamental wave does not increase due to the damage of the end face due to the harmonics, and the end face damage caused by both the harmonics and the fundamental wave does not occur, thereby greatly extending the life of the laser light source device. It becomes possible.

Configuration diagram of wavelength conversion device with end cap Overview of resonator type wavelength converter Harmonics generated in a conventional birefringent crystal spread in one direction by walk-off and emerge from the exit end face as a flat beam Fundamental and harmonic beam patterns on the exit surface of the wavelength conversion device of the prior art Wave pattern of fundamental wave and harmonic wave at the end face of wavelength conversion device with end cap When the end-cap walk-off direction of the fundamental and harmonic beam patterns at the output end face of the wavelength conversion device with end cap is 90 degrees different from the walk-off direction of the nonlinear wavelength conversion crystal

DESCRIPTION OF SYMBOLS 1 Fundamental wave 2 Residual wave (remaining without conversion) 3 Harmonic wave 4 Fundamental wave intensity distribution 5 Harmonic wave intensity distribution 6 Resonator input coupling mirror 7 Nonlinear wavelength conversion element 8 Light receiving element 9 Precision positioning element 101 Nonlinear Optical crystal 102 End cap 103 Incoming end face 104 Outgoing end face 105 Resonator input coupling mirror 106 High reflection resonator mirror 201 Polarization of fundamental wave (perpendicular to the paper surface)
202 Harmonic polarization (parallel to the paper)
301 Fundamental beam pattern and position 302 Fundamental beam pattern and position 303 Harmonic beam pattern and position without end cap 304 Fundamental beam pattern and position without end cap

FIG. 1 shows a wavelength conversion element in which a nonlinear optical crystal and an end cap material are joined, and a walk-off state. The end cap material does not contribute to wavelength conversion and has a walk-off effect.

For example, when β-barium borate is used as a nonlinear optical crystal, the end cap material that does not contribute to wavelength conversion is a cut angle of the same kind of nonlinear optical crystal, such as β-barium borate (BBO), which is often used for ultraviolet generation. May be slightly different and may not satisfy the phase matching condition, or α-barium borate (α-BBO) having no second-order optical nonlinearity. What is important is that the refractive indexes of the nonlinear optical crystal and the end cap material do not differ greatly. This is intended to minimize the residual reflection at the joint surface.

In that sense, even if the cut angle of the BBO crystal is changed, the refractive index of ordinary light does not change, so it makes sense to use BBO with a different cut angle as an end cap material, and the secondary index with the same refractive index. Α-BBO crystals that do not have mechanical nonlinearity are also suitable end cap materials.

Further, even in the case of β-BBO of uniaxial crystal having the same zenith angle θ, if the azimuth angle φ is different by 30 degrees, the effective nonlinear optical constant becomes zero, so that it can be said to be suitable as an end cap material. In this case, not only the refractive index is exactly the same for both polarized light, but also the mechanical properties such as the thermal expansion coefficient are exactly the same, which is very suitable as an end cap material.

If the end cap is α-BBO having the same cut angle or β-BBO having different azimuth angles, for example, if a harmonic wave having a wavelength of 266 nm is generated from a fundamental wave having a wavelength of 532 nm, the walk-off angle Is 85 mrad similarly for the nonlinear optical crystal. Typically, the fundamental is focused into a beam with a Gaussian radius of about 30 to 50 microns for a nonlinear crystal. This size is determined by the optical configuration of the resonator in the resonant wavelength converter, and sufficient reproducibility is ensured industrially.

However, by appropriately selecting the thickness of the end cap material, it is possible to create a state in which the harmonic and fundamental wave beams do not overlap on the end cap emission end face or the overlap is extremely small. The relative movement amount of the beam is proportional to the thickness. For example, if the walk-off angle is ρ and the thickness is t, the parallel movement amount of the abnormal light is ρt.

As described above, for example, since it is necessary to move about 50 to 100 microns so as not to affect the optical path of the fundamental wave (input light) having a radius of 50 microns, since ρ = 85 mrad, t = 2 mm or so. It will be good. Further, in order to further increase the parallel movement amount of the beam and avoid the overlapping of the beams in the previous optical element, it is possible to cope with this by appropriately selecting the thickness of the end cap material. The exit end face of the end cap may be provided with an antireflection coating in order to keep the optical loss low. Alternatively, the end face may be processed so that the Brewster is incident on the fundamental wave. The anti-reflection coating applied is the same as that applied directly to the nonlinear optical crystal in the past, or the harmonics and fundamental wave are emitted from the end cap, regardless of whether it is an uncoated Brewster cut or similar surface treatment. Since there is no overlap on the end faces, damage to the end face or coating caused by harmonics or both harmonics and fundamental waves can be prevented or at least does not increase the optical loss of the fundamental.

It is important that the interface between the nonlinear crystal and the end cap is optically transparent and strong. For this purpose, it is important that the refractive index of the material is not significantly different and that the mechanical properties are not significantly different. However, the bonding method itself must be carefully selected to satisfy these properties. . Possible methods include optical contact, surface activated bonding, capillary bonding, diffusion bonding, fusion, and adhesion.

With the configuration described above, spatial separation of the fundamental wave and the harmonic wave is realized on the emission end face of the wavelength conversion element. In such a configuration, damage to the end face or coating caused by simultaneous irradiation of harmonics and fundamental waves can be prevented, and damage to the end face or coating caused only by harmonics is caused by damage marks of the fundamental wave. It does not overlap with the path and does not increase the optical loss of the fundamental in any damage. Thereby, it is possible to suppress a decrease in conversion efficiency due to an increase in optical damage, and it is possible to extend the life of the laser light source device.

Examples of the present invention are shown below.

In the present invention, the case where the second harmonic (ultraviolet light) with a wavelength of 266 nm is generated from the fundamental wave (input light, green light) with a wavelength of 532 nm using β-barium borate (BBO) will be described as an example. The range is of course not limited to this, and is widely applicable in general. In order to generate the second harmonic efficiently, it is necessary to satisfy the phase matching condition, that is, the phase velocity of the fundamental wave and the second harmonic are equal, and this is achieved by utilizing natural birefringence in BBO. . Specifically, in the propagation direction of θ = 47.5 degrees, the refractive indexes of ordinary light having a wavelength of 532 nm and extraordinary light having a wavelength of 266 nm are equal, and a second harmonic having a wavelength of 266 nm is effectively generated. Due to the birefringence of the BBO crystal, extraordinary light with a wavelength of 266 nm has a walk-off of about 85 mrad, and moves away from ordinary light with a wavelength of 532 nm that propagates as ordinary light in a plane including the C axis of the BBO crystal.
The concept of this process is shown in FIG. The extraordinary light having a wavelength of 266 nm generated over the entire length of the crystal has a flat pattern as a result, and a pattern as shown in FIG. 4 is formed on the emission end face of the wavelength conversion element having no end cap. On the other hand, in the present invention, the nonlinear wavelength conversion element includes a nonlinear optical crystal and an end cap joined thereto. The end cap does not contribute to wavelength conversion, but has the effect of spatially separating ultraviolet light and green light by walk-off.

FIG. 5 shows the beam patterns and positions of the fundamental wave and the harmonic wave on the wavelength conversion element emission end face in one embodiment of the present invention. The polarization direction of the extraordinary light of the end cap is the same as the polarization direction of the extraordinary light of the nonlinear wavelength conversion crystal, and the walk-off direction of the end cap is the same as the walk-off direction of the nonlinear crystal. Due to the end cap, the harmonics move relative to the major axis direction of the beam pattern from the position of the fundamental wave by an amount indicated by D = ρt in the figure. Here, ρ is the walk-off angle of abnormal light in the end cap, and t is the thickness of the end cap. When D is approximately equal to or larger than the beam radius of the fundamental wave, the fundamental wave and the harmonics hardly overlap each other at the wavelength conversion element emission end face, and the two-wavelength coexistence of the antireflection coating applied to the end face, particularly the end face. Damage can be prevented. Further, the optical loss due to the damage of the end face or the antireflection coating due to the harmonic does not affect the fundamental wave, and the optical loss with respect to the fundamental wave can be kept low.

FIG. 6 shows beam patterns and positions of the fundamental wave and the harmonic wave at the wavelength conversion element emission end face of another embodiment of the present invention. The polarization direction of the extraordinary light of the end cap is the same as the polarization direction of the ordinary light of the nonlinear wavelength conversion crystal, and the walk-off direction of the end cap is orthogonal to the walk-off direction of the nonlinear crystal. The fundamental wave is moved by the end cap relative to the minor axis direction of the harmonic beam pattern from the harmonic position by an amount indicated by D = ρt in the figure. Here, ρ is the walk-off angle of abnormal light in the end cap, and t is the thickness of the end cap. When D is equal to or larger than the beam radius of the fundamental wave, the fundamental wave and the harmonic do not overlap at the output end face of the wavelength conversion element, and damage due to the coexistence of two wavelengths of the antireflection coating applied to the end face, particularly the end face. Can be prevented. Further, the optical loss due to the damage of the end face or the antireflection coating due to the harmonic does not affect the fundamental wave, and the optical loss with respect to the fundamental wave can be kept low.

Claims (18)

Satisfying phase matching, input wave of wavelength λ1 or input wave of wavelengths λ1 and λ2 is incident and generates λ3 which is the second harmonic of wavelength λ1, λ4 which is the sum frequency of wavelengths λ1 and λ2, or λ5 which is the difference frequency Non-linear wavelength conversion element comprising: a non-linear optical crystal that is bonded to an output end face of the non-linear optical crystal, and an end cap material that does not contribute to wavelength conversion and prevents damage to the output end face of the non-linear optical crystal The end cap of claim 1 is intended to spatially separate the fundamental wave (input wave) and the harmonic wave (output wave) and prevent laser damage caused by the coexistence of both wavelengths. The end cap according to claim 1 spatially separates the fundamental wave (input wave) and the harmonic wave (output wave), and end face damage caused by the harmonic wave (output wave) occurs in the optical path of the fundamental wave (input wave). The purpose is to prevent the optical loss of the fundamental wave (input wave) from increasing without overlapping The nonlinear optical crystal according to claim 1 and the end cap material are bonded by any one of optical contact, surface activation bonding, capillary bonding, diffusion bonding fusion, and a highly transparent adhesive. Conversion element 2. The wavelength conversion element according to claim 1, wherein the end cap material of claim 1 has birefringence and spatially separates the input wave λ1 / 2 and the output wave λ3 / 4/5 having different polarizations. 6. The wavelength conversion element according to claim 4, wherein the thickness of the end cap material according to claim 5 is such that the spatial separation of the input wave and the output wave due to the birefringence effect is at least twice the beam radius of the fundamental wave. The thickness of the end cap material according to claim 5 is more than twice the beam radius of the input wave on the wavelength discriminating element on which the spatial separation of the input wave and the output wave due to the birefringence effect separates the output wave from the input wave. The wavelength conversion element according to claim 4, 6. The wavelength conversion element according to claim 5, wherein the polarization axis of the end cap material is bonded to the nonlinear optical crystal within one degree. A non-linear wavelength conversion element characterized in that an antireflection coating is applied to the output end face of the non-linear wavelength conversion element of claim 1 The nonlinear wavelength conversion element according to claim 1, wherein the output end face of the nonlinear wavelength conversion element has a Brewster angle with respect to the fundamental wave and is processed so as to reduce reflection loss. The reflectance of the interface represented by R = ((n1-n2) / (n1 + n2)) ^ 2 is small in the difference in refractive index with respect to the wavelength and polarization of the input wave between the nonlinear optical crystal and the end cap material of claim 1 Nonlinear wavelength conversion element characterized by being selected to be 0.1% or less 2. The nonlinear wavelength conversion element according to claim 1, wherein the nonlinear optical crystal is β barium borate. 13. The nonlinear wavelength conversion element according to claim 12, wherein the end cap material is β barium borate, and the phase matching condition is not satisfied by changing the cut angle. 13. The nonlinear wavelength conversion element according to claim 12, wherein the end cap material is α barium borate. 13. The nonlinear wavelength conversion element according to claim 12, wherein the end cap material is β barium borate cut at substantially the same zenith angle θ (± 5 degrees), but the azimuth angle φ is 30 degrees (± 5) with that of the nonlinear optical crystal. Degrees) are different (the absolute value of φ can be defined in multiple ways depending on the coordinates, and the absolute value is not discussed here), the effective nonlinear constant is zero, the thermal expansion coefficient in the surface, etc. Non-linear wavelength conversion element characterized in that it does not contribute to wavelength conversion while matching the mechanical characteristics of 2. The nonlinear wavelength conversion element according to claim 1, wherein the nonlinear optical crystal is cesium lithium borate (CLBO). 17. The nonlinear wavelength conversion element according to claim 16, wherein the end cap material is CLBO, and the phase matching condition is not satisfied by changing the cut angle. The nonlinear wavelength conversion element according to claim 16, wherein the end cap material is CLBO cut at substantially the same zenith angle θ (± 5 degrees), but the azimuth angle φ is 45 degrees (± 5 degrees) with that of the nonlinear optical crystal. A non-linear wavelength conversion element that is different, has an effective non-linear constant of zero, does not contribute to wavelength conversion while matching in-plane thermal expansion coefficient and other mechanical characteristics.