JP4721654B2 - Wavelength conversion laser device - Google Patents

Description

  The present invention relates to a wavelength conversion laser device that makes two laser beams having different frequencies incident on a nonlinear optical crystal and outputs a laser beam having a difference frequency between incident laser beams, and in particular, a wavelength conversion laser capable of efficiently obtaining an infrared laser. Relates to the device.

  Conventionally, by mixing two pump lights with different wavelengths in a nonlinear optical crystal, infrared light can be generated using the principle of difference frequency generation, in which long wavelength coherent light corresponding to the difference in pump light frequency is generated. Infrared light generators that generate light are known. In this infrared light generator, the first pumping light (pump light) having a short wavelength is wavelength-converted in the nonlinear optical crystal and generates a difference frequency light with the second pumping light (signal light) having a variable wavelength. The output of infrared light is determined by the laser light on the short wavelength side.

  For example, a device that generates a difference frequency light (infrared light) using a wavelength variable laser obtained from a Ti: Sapphire laser generator as pump light and a laser having a wavelength of 1.064 μm obtained from an Nd: YAG laser generator as signal light. It has been known. In this apparatus, the output of the infrared light depends on the output of the Ti: Sapphire laser, but the Ti: Sapphire laser needs to be excited by the second harmonic of the Nd: YAG laser having a wavelength of 0.532 μm. In order to increase the output, it is inevitable to increase the size of the entire apparatus.

On the other hand, Patent Document 1 discloses a high-power, compact, and wavelength tuning that is configured by using an Nd: YAG laser as the pump light on the short wavelength side and a Cr: forsterite laser as the signal light on the long wavelength side. A possible infrared light generator is disclosed.
FIG. 12 is a block diagram of the infrared light generator disclosed in Patent Document 1. In FIG.

In the disclosed infrared generator, two Nd: YAG laser devices are synchronously driven by pulses generated by the pulse generator. One Nd: YAG laser is incident on the nonlinear optical crystal using a pulsed laser with a wavelength of 1.064 μm as pump light, and the other Nd: YAG laser is a pulsed laser as an excitation light source for a Cr: forsterite laser. To supply.
The pulse laser incident on the Cr: forsterite laser is adjusted to have a predetermined beam diameter by a telescope constituted by a lens, and then divided by a beam splitter and incident on both sides of the Cr: forsterite laser crystal. Side excitation.
The laser crystal is disposed between the output mirror and the reflecting mirror, and a dispersion prism is provided between the laser crystal and the reflecting mirror. The reflecting mirror is a rotating mirror that adjusts the direction of reflection so that only the selected light from the wavelength-dispersed light by the dispersion prism returns to the output mirror, and the light of the selected wavelength resonates in the resonator. So that laser oscillation occurs.

The Cr: forsterite laser is a tunable solid-state laser that can select a wavelength in the range of 1.15 to 1.35 μm, and supplies the generated tunable laser light as signal light to the nonlinear optical crystal.
Thus, the Nd: YAG laser is incident on the nonlinear optical crystal using a pulse laser with a wavelength of 1.064 μm as pump light, and the laser selected by the Cr: forsterite laser within the wavelength range of 1.15 to 1.35 μm is used as signal light. When incident on the nonlinear optical crystal, it selectively generates infrared light having a wavelength range of 5 to 14 μm related to the difference frequency between the pump light and the signal light.
JP 2002-287190 A

As the nonlinear optical crystal for generating the difference frequency, a chalcopyrite crystal such as AgGaS ₂ crystal, AgGa ₂ S ₄ crystal, AgGaSe crystal, AgSe crystal, or the like can be used. The difference frequency light is generated by modulating the pump light to be incident with the signal light and outputting it. The intensity of the output light depends on the light energy incident on the optical crystal. Therefore, in order to obtain strong output light, it is preferable to increase the intensity of the laser light incident on the incident surface of the crystal as much as possible.

However, these nonlinear optical crystals for difference frequency may be damaged when laser light exceeding a certain intensity is incident. Therefore, an appropriate threshold value is set so that laser light having energy higher than the threshold value is not incident. Must be.
Since the spatial energy distribution of laser light used as pump light or signal light is a normal distribution type, the total amount of energy incident on the crystal is greatly reduced if the peak value does not exceed the set threshold value. The smaller the half width of the laser beam to be used, the larger the amount of energy decrease around the peak, and the smaller the amount of energy that can be injected into the crystal.

  In addition, since the intensity of the output light depends on the light energy incident on the optical crystal, it is preferable that the actual incident area on the crystal plane be as large as possible. Since the laser light having a circular distribution in which the optical energy in the plane perpendicular to the axis has an energy peak in the optical axis is incident, the contribution of the peripheral part of the crystal incidence plane and crystal volume is small, and the difference frequency light The output could not be increased sufficiently.

Therefore, the problem to be solved by the present invention is to provide a small wavelength conversion laser device that increases the difference frequency light output by effectively using the entire difference frequency nonlinear optical crystal. It is another object of the present invention to provide a wavelength conversion laser device capable of achieving both optical axis stability and spectral line narrowing of a wavelength tunable laser used for generating difference frequency light.

In order to solve the above problems, the wavelength conversion laser device of the present invention includes a nonlinear optical crystal that outputs laser light having a difference frequency between incident laser light when laser light having two different wavelengths is incident as pump light and signal light. A wavelength conversion laser device comprising an aperture that transmits the vicinity of the optical axis of laser light incident on the nonlinear optical crystal, and an image relay that transfers an image of the laser light in the aperture to the incident surface of the nonlinear optical crystal. Features.
The image relay is preferably provided at least in the incident path of the pump light, but more preferably provided in the incident path of the signal light.

In the image relay, a first convex lens having a focal length f1 and a second convex lens having a focal length f2 are arranged so as to share a focal point on the optical axis, and a reference located at a distance X1 in front of the first convex lens. Between the surface and the image transfer surface at the distance X2 behind the second convex lens,
M = f2 / f1
X2 = M (f1 + f2) −M ² X1
When the above relationship is established, the two-dimensional image existing on the reference plane is enlarged and reduced as it is and transferred to the position of the image transfer plane. Note that M is a magnification.

Since the laser light emitted from the laser device has a normal distribution with the energy centered on the optical axis, it is expanded in the direction perpendicular to the axis using a collimator, etc., and the energy distribution near the peak is moderated. When projected onto the aperture above, the energy distribution in the direction perpendicular to the optical axis of the laser light passing through the aperture has a substantially trapezoidal shape. If the image relay is appropriately arranged according to the above equation, the energy distribution state at the aperture position (reference plane) can be transferred to the incident surface (image transfer plane) of the nonlinear optical crystal, so that the input energy threshold of the nonlinear optical crystal can be set. On the other hand, if the peak value of the energy is adjusted so as to have an appropriate margin, since the trapezoidal energy distribution is obtained, significantly larger energy can be injected into the crystal as a whole as compared with the case of the normal distribution state. Therefore, according to the wavelength conversion laser device of the present invention, it is possible to obtain output light having a high energy level.
Further, according to the image relay, even if a slight shift occurs in the optical axis direction, the positions of the reference plane and the image transfer plane are not changed, and a two-dimensional image on the reference plane is transferred onto the image transfer plane. Therefore, even when the optical axis of the laser device fluctuates, the fluctuation of the laser beam on the image transfer surface is small, and a stable laser output can be obtained.

The shape of the aperture laser beam passage portion can be made to correspond to the shape of the incident surface of the nonlinear optical crystal.
Since the image at the aperture can be transferred to the entrance surface of the optical crystal using an image relay, when the entrance surface of the optical crystal is a rectangle, the aperture shape of the aperture is adjusted to a similar rectangle, and the magnification is adjusted. Energy efficiency can be improved by making laser light incident on the entire surface of the optical crystal incident surface. Further, the conversion efficiency can be improved by fully utilizing the corners of the optical crystal.

It is preferable to insert an attenuator in the resonator of the wavelength tunable laser generator for signal light so that the spectrum narrowing and energy adjustment of the signal light can be performed.
There is an optimal combination of pump light and signal light intensity corresponding to the output difference frequency light. For example, if it is preferable that the pump light is 15 mJ with respect to a certain difference frequency light and the signal light is 5 mJ, the signal input to the nonlinear optical crystal even if the capacity of the signal light output device is 20 mJ. Light needs to be attenuated to 5 mJ.
On the other hand, the narrower the spectral width of the signal light, the more accurate difference frequency light can be obtained, and the conversion efficiency is improved and the difference frequency output is increased.

In the conventional method, the output of the signal light laser generator is reduced by a filter. However, in this method, the output is reduced while maintaining the chromatic dispersion state of the large output device, so that the spectrum width is not narrowed.
On the other hand, when the optical energy is attenuated in the resonator of the laser device, the mode near the center wavelength with a high gain is selectively oscillated, and the total oscillation output is reduced. The output can be improved.

  As the attenuator used in the resonator, a diffraction grating, an aperture, or the like can be used, but a glass plate that can adjust the inclination of the incident surface with respect to the optical axis can also be used. Since the transmittance of incident light to the glass plate depends on the angle of incidence on the glass surface, the laser output of the laser generator is adjusted by adjusting the energy of the laser light that passes through the glass plate by changing the glass surface angle with respect to the optical axis. Can be easily controlled.

Note that a pair of glass plates may be provided in the laser beam path, and the pair may be arranged so as to be symmetrical with respect to a plane perpendicular to the laser beam path.
When the light passes through the glass plate, the optical axis moves in parallel. However, when the light passes through a pair of symmetrically arranged glass plates, the optical axis returns to the original direction, so that the structure of the optical system becomes simple. Note that the angle of the pair of glass plates can be adjusted symmetrically by using a simple gear mechanism or the like.

  In addition, the resonant mirror part in the resonator of the wavelength tunable laser generator has a concave lens characteristic in the wavelength dispersion direction and does not have power in the direction perpendicular thereto, and a cylindrical concave lens that has no power in the wavelength dispersion direction and does not have power in the wavelength dispersion direction. It is preferable to be constituted by a combination of cylindrical concave mirrors having concave mirror characteristics. In such a configuration, the dispersion performance is improved by the action of the concave lens in the wavelength dispersion direction. On the other hand, the direction perpendicular to this is the direction related to the stability of the output, but the stability is improved by the action of the concave mirror. As described above, the resonator parameters can be independently selected for the two directions, so that both the optical axis stability and the spectrum narrowing can be achieved.

It is optically equivalent to the above combination, a cylindrical convex lens having a convex lens characteristic in a direction perpendicular to the chromatic dispersion direction and a convex mirror characteristic in the chromatic dispersion direction and having a power in a direction perpendicular thereto. It is clear that the same effect can be obtained even if it is constituted by a combination of cylindrical convex mirrors that do not have.
Therefore, in the wavelength conversion laser device of the present invention, the optical resonator that outputs the wavelength tunable laser light disperses the light output from the laser medium according to the wavelength, and the wavelength of the light incident from the wavelength dispersive element. A reflecting mirror that has a concave surface in the direction perpendicular to the dispersion direction and reflects incident light, and is disposed between the reflecting mirror and the output mirror and has a concave lens characteristic in the chromatic dispersion direction and is dispersed by the wavelength dispersion element An optical element that further disperses the emitted light, and the reflector is configured to reflect the light toward the output mirror while suppressing fluctuations in the optical axis of the light in a direction perpendicular to the wavelength dispersion direction. Features.
In addition, the reflecting mirror reflects the incident light with a convex surface with respect to the wavelength dispersion direction of the incident light, and the optical element disposed between the reflecting mirror and the output mirror is perpendicular to the wavelength dispersion direction It has convex lens characteristics and suppresses fluctuations in the optical axis of light in the direction perpendicular to the wavelength dispersion direction, so that the reflector further disperses the light dispersed by the wavelength dispersion element and reflects the light to the output mirror side. It may be configured as follows.

AgGaS ₂ crystals, nonlinear optical crystal such as AgGaSe ₂ crystals, laser beam and the nonlinear optical phenomena for outputting laser light (DFG) of when the input laser light difference frequency ω3 frequency ω2 (= ω1-ω2) frequency .omega.1 For example, in Patent Document 1, a laser beam having a wavelength of 1.064 μm and a tunable laser beam having a wavelength of 1.15 to 1.35 μm are mixed with a nonlinear optical crystal made of chalcopyrite crystal such as AgGaS ₂ crystal. Infrared light generators are disclosed that obtain wavelength-converted wavelength-adjusted infrared light in the range of 5 to 14 μm.
The present invention improves the output by incorporating an image relay into such a wavelength conversion laser device.
Hereinafter, the present invention will be described in detail using examples.

FIG. 1 is a conceptual block diagram of a wavelength conversion laser device according to one embodiment of the present invention.
The wavelength conversion laser device of the present embodiment is based on the infrared light generation device disclosed in Patent Document 1 shown in FIG. 12 and has a main feature in which an aperture and an image relay are further added. In order to improve efficiency, the internal structure of the signal light laser generator is improved. Therefore, since the basic structure of the laser generator for the laser incident on the nonlinear optical crystal is described in detail in Patent Document 1, the features of the invention will be mainly described in detail in this specification.

The wavelength conversion laser device of this embodiment includes a first laser device 1 that generates laser light having a short frequency ω1, a second laser device 2 that generates laser light having a longer frequency ω2, and a nonlinear optical crystal 3. Furthermore, an aperture and an image relay are provided between these two laser devices and the nonlinear optical crystal, respectively.
The nonlinear optical crystal 3 is a light mixing type nonlinear optical crystal such as an AgGaS ₂ crystal. When laser beams having frequencies ω1 and ω2 are input, a difference frequency light (DFG) having a frequency ω3 (= ω1−ω2) of a difference between the frequencies. ) Is output.
The difference frequency light emitted from the optical crystal 3 is basically generated by the wavelength conversion action of the pump light, and the energy of the difference frequency light depends on the energy of the pump light. Further, since the frequency of the difference frequency light is the frequency difference between the pump light and the signal light, it can be adjusted by changing the frequency of the signal light.

  For example, an Nd: YAG laser is used as the first laser device 1 and a pulse laser having a wavelength of 1.064 μm is input to the optical crystal 3 as pump light having a relatively large energy, and 1.. When a Cr: forsterite laser that generates laser light in the range of 15 to 1.35 μm is used as the second laser device, for example, an output laser adjusted to a wavelength of 1.284 μm is input to the optical crystal 3 as signal light contributing to wavelength conversion, The optical crystal 3 outputs difference frequency light having a wavelength of 6.21 μm in addition to the input laser light. If these output lights are passed through the Ge filter 4 for wavelength separation and only the long-wavelength difference frequency light (infrared light) is extracted to the outside, the wavelength conversion laser device becomes an infrared light generator.

In the wavelength conversion laser device of the present embodiment, the output of the first laser device 1 is widened in a direction perpendicular to the optical axis by a widening device such as a collimator 5 to moderate the energy distribution near the peak, and then the output laser is output by the aperture 6. Cut out a flat part near the optical axis.
The energy of the laser beam output from the first laser device 1 has a Gaussian distribution, takes a maximum value in the vicinity of the axis, and rapidly decreases in intensity as it goes to the periphery. Therefore, when the output laser beam is directly applied to the optical crystal 3, the peak value cannot be made very high because the energy at the peak needs to be below a sufficiently safe level with respect to the damage threshold of the crystal. Since the energy is Gaussian distributed, the energy level in the region away from the optical axis is lowered, so that the total amount of energy input to the optical crystal 3 cannot be increased.

In this embodiment, a portion where the energy distribution is gentle near the optical axis of the output laser is further flattened by widening, and then projected onto the aperture 6. Since the aperture 6 cuts out the flat optical axis portion, the energy distribution of the laser light after passing through the aperture 6 has a substantially trapezoidal shape, and the energy of the laser light is averaged in the transverse direction. FIG. 1 shows a state in which the energy distribution changes while the output laser of the first laser device 1 passes through the collimator 5.
The laser beam transmitted through the aperture 6 is image-transferred onto the incident surface of the optical crystal 3 by the image relay 7.

FIG. 2 is a diagram illustrating the configuration of the image relay.
In the image relay, a first convex lens Ls1 having a focal length f1 and a second convex lens Ls2 having a focal length f2 are arranged so as to share the focal point on the optical axis with the same optical axis, and in front of the first convex lens Ls1. Between the reference surface (A surface) at the position of the distance X1 and the image transfer surface (B surface) at the position of the distance X2 behind the second convex lens Ls2.
M = f2 / f1
X2 = M (f1 + f2) −M ² X1
The relationship is established.

When the image relay is used, the two-dimensional image on the reference surface is directly enlarged and reduced by M times and transferred to the image transfer surface.
In the image transfer by the image relay, as shown by the dotted line in the figure, the position of the reference plane and the image transfer plane does not change even if a slight deviation occurs in the optical axis direction, and the two-dimensional image on the reference plane is Transferred onto the image transfer surface. Therefore, when configuring an optical system, the image relay can be used as a robust optical structure.

When the position of the aperture 6 is the reference plane, the incident surface of the nonlinear optical crystal 3 is the image transfer surface, and the image relay 7 is appropriately arranged according to the above formula, the energy distribution state in the aperture 6 is transferred to the incident surface of the nonlinear optical crystal 3. can do.
Therefore, if the energy value on the trapezoidal top surface of the trapezoidal energy distribution of the laser light incident on the optical crystal 3 through the aperture 6 and the image relay 7 is adjusted to be an appropriate value with respect to the damage threshold of the optical crystal, The energy level can be approached as much as possible over the entire range of incident energy, and the total amount of input energy to the crystal is significantly increased.

In addition, since image transfer by an image relay has a small change on the image transfer surface even if the optical axis direction is slightly deviated, the fluctuation of the laser beam on the image transfer surface is small even when the optical axis of the laser device changes.
By utilizing the aperture and the image relay in this way, the intensity of the difference frequency light output from the crystal can be greatly enhanced by effectively utilizing the volume of the nonlinear optical crystal 3.

Further, in the image transfer by the image relay 7, the shape of the aperture of the aperture 6 can be changed to the incident surface of the optical crystal 3 since the laser output shape at the aperture of the aperture 6 can be transferred to the incident surface of the optical crystal 3. By adjusting the image transfer magnification to a similar shape, laser energy can be injected into the entire incident surface. For example, if the incident surface is rectangular, if the aperture opening is a similar rectangle, the laser light is incident on the entire surface of the incident surface, and the entire volume of the optical crystal 3 acts to efficiently generate the difference frequency light. Can do.
Compared to the conventional case where a laser beam having a circular cross section is incident on a rectangular incident surface, the efficiency is improved by a factor of about 4 by flattening the energy distribution and making the aperture aperture suitable.

The second laser device 2 is a device that supplies a pulse laser having a frequency lower than that of the first laser device 1 to the nonlinear optical crystal 3 as signal light.
In this embodiment, a Cr: forsterite laser device using Cr: forsterite as a laser medium is used for the second laser device 2. The second laser device 2 includes a resonator in which a dispersion prism 23 is accommodated between an output mirror 22 and a reflecting mirror 24 provided with a Cr: forsterite laser medium 21 interposed therebetween.

Although not shown in FIG. 1, as in FIG. 12, another Nd: YAG laser that is synchronously driven by the same pulse generator is used as an excitation light source for a Cr: forsterite laser medium. . Further, the wavelength of the output laser can be selected in the range of 1.15 to 1.35 μm by adjusting the posture of the rotating mirror and the laser medium.
The Cr: forsterite laser apparatus 2 of this embodiment further incorporates an attenuator 25 in the resonator in order to narrow the output laser band and stabilize the optical axis.

  The intensity of the signal light does not have to be strong, but there is a value that balances with the intensity of the pump light according to the difference frequency light output. For example, 5 mJ should be adjusted to an appropriate value for 15 mJ pump light. I must. Normally, a filter is inserted in the path of the output light to reduce it to an appropriate value. However, in this method, the wavelength spectrum of the oscillation light is proportionally reduced after being obtained in a Gaussian distribution state. In FIG. 5, the dispersed wavelength fluctuation width is preserved as it is, and the spectral line width is not narrowed.

Here, the power of the difference frequency light increases as the spectrum width of the frequency ω2 of the signal light is narrower.
FIG. 3 is a graph for explaining the relationship between the spectral line width of the signal light and the output of the difference frequency light when the difference frequency light is generated by the nonlinear optical crystal. The figure shows the spectral line width of the frequency ω2 of the signal light on the horizontal axis and the vertical axis on the case where the difference frequency light of wavelength 6.21 μm is obtained from the pump light of wavelength 1.064 μm and the signal light of wavelength 1.284 μm. The difference frequency light output is plotted against this.

From the figure, it can be seen that an output of about 350 μJ is expected when there is almost no spectral line width, whereas the output is halved to 200 μJ when the spectral line width becomes 1 nm, and decreases to 120 μJ when the spectral line width becomes 2 nm.
Therefore, when the input energy is constant, a higher difference frequency light output is obtained when the spectrum width of the signal light frequency ω2 is narrower. Furthermore, when using a nonlinear optical crystal with a small damage threshold such as AgGaS ₂ , the energy of the laser that can be input to it is limited. Therefore, narrowing the spectral width is an effective means for improving the differential frequency light output. Become.

FIG. 4 is a diagram for explaining a resonator used in the second laser apparatus 2 in the present embodiment, in which the output is adjusted by introducing an attenuator therein.
In this embodiment, the Nd: YAG laser excites the laser medium 21 with a constant intensity as large as possible to suppress the time fluctuation of the pulse, that is, the occurrence of so-called jitter. Further, the attenuator 25 is inserted in the resonator to resonate. Reduce output directly by reducing light. As described above, in the laser apparatus 2 of the present embodiment, the light resonating inside the resonator is directly attenuated to reduce the output of the laser apparatus from 20 mJ to 5 mJ.
When the P-polarized light that resonates in the resonator is reduced, the light impinging on the bottom of the frequency distribution of the excitation light cannot resonate. Accordingly, the spectrum of the output light gathers at the center, and the spectral line width becomes narrow, so that the output light can be narrowed.

Although a diffraction grating and an aperture can be used for the attenuator 25, a glass plate formed of optical glass such as BK7 is used in this embodiment.
The reflectance of transparent flat glass changes depending on the incident angle. FIG. 5 is a graph showing the relationship between the incident angle of P-polarized light on the surface of a glass plate made of BK7 and the reflectance at the surface. Since the refractive index of BK7 is 1.504 near the wavelength of 1.3 μm, the Brewster angle of BK7 is 56.4 °.
At a Brewster angle of 56.4 °, the surface reflectance becomes zero and all incident light is transmitted. On the other hand, the surface reflectance at an incident angle of 0 ° is 0.04, and the transmitted light is hardly attenuated by one pass. However, since it reciprocates many times in the resonator, it passes through the glass plate many times. The transmitted light is reduced exponentially, and the intensity of the output light is sufficiently reduced.

Therefore, as shown in FIG. 4, a glass plate is rotatably provided in the optical axis in the resonator, and the incident angle θ is changed by adjusting the inclination of the glass plate with respect to the optical axis, thereby changing the light angle. The intensity of output light can be controlled by adjusting the transmittance.
In addition, as shown in FIG. 6, when a light ray permeate | transmits the one glass plate 25, the phenomenon in which an optical axis shifts | deviates parallel occurs. Moreover, the amount of deviation varies depending on the inclination of the glass plate.

Therefore, as shown in FIG. 7, when the same glass plate 25 ′ is arranged in plane symmetry along the optical axis so that light rays pass through the two glass plates, the glass plate is inclined regardless of the inclination of the glass plate. The optical axis shift is generated in the opposite direction by the same amount and canceled, and the optical axis of the light transmitted through the two glass plates comes to be on the extension of the initial optical axis. Therefore, a resonator optical system can be easily constructed by using a configuration in which a pair of glass plates are arranged symmetrically in this way.
When this method is adopted, a mechanism for tilting the pair of glass plates 25 and 25 'at an equal angle is required, but this can be easily realized by, for example, a rack and pinion as shown in FIG.

As described above, when the attenuator 25 is incorporated in the resonator and the intensity of light reciprocating in the resonator is directly adjusted, the intensity of the output light can be adjusted and the output is reduced. In addition, the output light can be narrowed, and the quality of the output light can be improved.
Furthermore, when using an attenuator in which a pair of BK7 glass plates are arranged symmetrically, the reflection loss on the surface of one glass changes in the range of 0 to 0.04, so it passes through a pair of glass plates once. By simply doing this, the reflection loss can be changed from 0 to 0.16, and the output light intensity can be greatly adjusted.

Further, the resonator of the second laser device used in the present embodiment further stabilizes the optical axis and narrows the spectrum by providing an optical system to the reflecting mirror portion.
FIG. 9 is a conceptual diagram illustrating the configuration of the resonator portion.
FIG. 9A is a drawing depicting the normal of the incident surface of the laser medium 21 and the traveling locus of the light on the surface including the incident optical axis, and FIG. 9B is the normal of the incident surface of the laser medium and the incident light. It is drawing which showed the run locus | trajectory of light from the direction which belongs to the surface containing an axis | shaft. If FIG. 9A is a plan view, FIG. 9B is an elevation view.
The locus of light passing through the dispersion prism 23 is represented by a straight line for simplicity.

A lens 26 is disposed in front of the rotating reflecting mirror 24.
The rotating reflecting mirror 24 is a plane mirror in the wavelength dispersion direction of FIG. 9A, and a cylindrical mirror that is a concave mirror in the direction perpendicular to the wavelength dispersion direction in FIG. 9B is a concave lens with respect to the optical axis direction of FIG. 9A, and a cylindrical lens that is a plate glass with respect to the optical axis direction of FIG. 9B.
Conventionally, a flat reflecting mirror is generally used as the resonator mirror, but the optical axis of the plane mirror is likely to fluctuate, and it is necessary to realign each time the wavelength is changed. Further, when trying to stabilize the optical axis using a concave spherical mirror, there is a problem that the spectral line width is widened.

  In FIG. 9A, the resonance light generated from the laser medium 21 draws a locus dispersed according to the wavelength by the dispersion prism 23, enters the concave lens 26, and further enters the reflecting mirror 24 with the dispersion direction strengthened. Of the incident light, only the light ray that has entered the original trajectory and can be returned to the laser medium 21 is incident on the reflecting surface of the reflecting mirror 24 substantially perpendicularly. Therefore, the wavelength of the laser light emitted by the inclination of the reflecting mirror Can be determined. At this time, as a result of strengthening the dispersion tendency by the concave lens 26, only the light rays that can resonate in the resonator are very close to the target wavelength, so that the spectral lines can be narrowed.

In FIG. 9B, when the laser optical axis fluctuates in a direction parallel to the drawing, if the reflecting mirror 24 has a concave surface in this fluctuating direction, the fluctuation of the optical axis becomes small. The axis stabilizes. In addition, since the lens 26 functions equivalent to a plane mirror in this direction, it is not related to optical axis stabilization.
When the reflecting mirror 24 and the lens 26 have such a structure, the resonator parameters can be set independently in two directions, so that both stabilization of the optical axis and narrowing of the spectral line can be achieved.

9A and 9B is replaced with an optically equivalent convex cylindrical lens, and the concave cylindrical lens 26 is replaced with an optically equivalent convex mirror. Needless to say, the same effect can be obtained even if it is replaced. In this case, a convex mirror is disposed behind to form a rotating reflecting mirror.
Further, a telescope configured by combining a cylindrical concave lens and a cylindrical convex lens, an anamorphic prism combined with a wedge-shaped prism, or the like is inserted between the laser medium 21 and the dispersion prism 23, and a beam incident on the dispersion prism 23 is inserted. By enlarging in the wavelength dispersion direction, it is possible to further enhance wavelength dispersion and further promote spectrum narrowing.

As shown in FIG. 1, the signal light emitted from the output mirror 22 with the wavelength selected by the second laser device 2 is cut out only in the very vicinity of the optical axis by the aperture 8 and is nonlinear by the image relay 9. The image is transferred to the incident surface of the optical crystal 3. A reflecting mirror 10 and a beam splitter 11 are interposed between the image relay 9 and the incident surface of the nonlinear optical crystal 3, and can be incident at the same position as the pump light.
As in the case of pump light, if the shape of the aperture 8 is similar to the incident surface of the nonlinear optical crystal 3, signal light can be injected more effectively.

The signal light emitted from the second laser device 2 is dispersed according to the wavelength, but by cutting out a part near the optical axis of the output light by the aperture 8, the spectral line can be narrowed. .
Further, the energy of the signal light emitted from the second laser device 2 has a Gaussian distribution, but the energy distribution of the signal light in the aperture 8 is substantially trapezoidal, so that the energy incident on the nonlinear optical crystal 3 Is sufficiently large and can effectively perform the wavelength conversion action of the pump light.

FIG. 10 is a cross-sectional view showing an example of an aperture used in the wavelength conversion laser device of this embodiment.
If the laser that does not transmit through the aperture is reflected by the laser incident on the aperture, the reflected laser hits the surrounding article and damages it. Therefore, it is necessary to take some measures to give up the influence.

The aperture shown in FIG. 10 includes a hole 31, an absorber 32, and a cover 33. The hole 31 is provided with a taking-in opening 34 that can cut out the vicinity of the optical axis of the incident beam. The outer peripheral edge of the taking-in opening 31 is a mirror-like conical inclined surface 35, and is formed at the center of the cover 33. A large incident port 36 that transmits almost the entire amount of the incident beam is provided, the absorber 32 is provided around the hole 31, and is configured to form a recess having a conical inclined surface on the back of the cover 33. The
The absorber 33 and the cover 34 are narrow at the entrance of the center and gradually become wider, forming a black body structure that absorbs all incident light without returning it to the incident direction.

Of the light incident on the entrance 36, all light components that do not pass through the take-in opening 35 are reflected by the inclined surface 36 and are incident on the black body structure and absorbed, so that other components are not damaged. .
In addition, when making the intake opening 34 into a shape other than a circle such as a rectangle, it goes without saying that the shape of the inclined surface 35 of the outer peripheral edge or the inclined surface forming the black body structure is changed in accordance with the shape of the opening 34. Absent.

FIG. 11 is a cross-sectional view of another example aperture used in the wavelength conversion laser device of the present embodiment.
The aperture shown in FIG. 11 is used when the load is lower than that of FIG. 10, has a simpler structure, and is composed of two parts, a hole 37 and a cover 38. The hole body 37 is provided with an intake opening, a conical reflecting wall is formed around the intake opening, and the cover 38 is provided with an entrance opening for receiving an incident beam as in FIG. .
The aperture of FIG. 11 is fixed by screwing with a male screw provided on the outer periphery of the hole and a female screw provided on the inner periphery of the cover, and the space between the hole and the cover is reflected as a black body structure 39. The incident light is absorbed so that other components are not damaged.

  The wavelength conversion laser device of this embodiment improves the wavelength dispersion of the aperture transmitted light by equalizing the energy distribution by transmitting the vicinity of the optical axis of the laser light incident on the nonlinear optical crystal that has the aperture and performs optical mixing. Further, an image relay is provided to transfer an image of the laser beam in the aperture to the incident surface of the nonlinear optical crystal, so that larger energy can be injected into the optical crystal. Therefore, the intensity and quality of the obtained difference frequency light are improved. Even when an optical crystal having a low damage threshold is used, sufficiently strong difference frequency light can be generated.

Furthermore, if the aperture shape is similar to the incident surface of the optical crystal for image transfer, light energy can be incident more efficiently.
Also, the spectral line width of the output light of the laser device is narrowed by providing an attenuator in the resonator of the laser device for signal light, improving the configuration of the reflecting mirror, or incorporating a beam expansion mechanism, etc. Therefore, the quality can be improved, the intensity of the output light of the wavelength conversion laser device can be increased, and the spectral width can be narrowed.

It is a block diagram explaining the wavelength conversion laser apparatus which concerns on one Example of this invention. It is drawing explaining the structure of the image relay used for a present Example. It is a graph explaining the relationship between the spectral line width of the signal light and the output of the difference frequency light when the difference frequency light is generated by the nonlinear optical crystal in this example. It is explanatory drawing of the resonator which introduce | transduced the attenuator used for the signal light laser apparatus in a present Example. It is a graph which shows the relationship between the incident angle of P polarized light with respect to the glass plate surface, and surface reflectance. It is an effect | action figure regarding one example of the attenuator provided in a resonator in a present Example. It is an effect | action figure regarding another example of the attenuator provided in a resonator in a present Example. It is a mechanism figure which shows the example of the drive mechanism of an attenuator in a present Example. It is a conceptual diagram explaining the structure of the resonator used for a present Example. It is sectional drawing explaining an example of the aperture used for a present Example. It is sectional drawing explaining another example of the aperture used for a present Example. It is a block diagram of the wavelength conversion laser apparatus in a prior art.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st laser apparatus 2 2nd laser apparatus 3 Nonlinear optical crystal 4 Ge filter 5 Collimator 6 Aperture 7 Image relay 8 Aperture 9 Image relay 10 Reflective mirror 11 Beam splitter 21 Laser medium 22 Output mirror 23 Dispersion prism 24 Reflective mirrors 25 and 25 '' Attenuator (or glass plate)
26 Lens 31 Hole 32 Absorber 33 Cover 34 Take-in Open 35 Conical Inclined Surface 36 Entrance 37 Hole 38 Cover 39 Black Body Structure

Claims (5)

A wavelength conversion laser device that outputs laser light having two different wavelengths incident on a nonlinear optical crystal and outputs difference frequency light, wherein at least one of the laser beams is a wavelength tunable laser beam and outputs the wavelength tunable laser beam. With an optical resonator,
The optical resonator, in order from the output side,
An output mirror having a constant transmittance;
And Les over The media,
A wavelength dispersion element that disperses the light output from the laser medium according to the wavelength;
A reflecting mirror having a concave surface with respect to a direction perpendicular to the direction of wavelength dispersion of incident light and reflecting incident light ;
The resonator further includes an optical element that is disposed between the reflecting mirror and the output mirror, has a concave lens characteristic with respect to a wavelength dispersion direction, and further disperses light dispersed by the wavelength dispersion element,
The reflecting mirror suppresses fluctuations in the optical axis of light in a direction perpendicular to the wavelength dispersion direction, and reflects the light to the output mirror side .
Wavelength conversion laser device. A wavelength conversion laser device that outputs laser light having two different wavelengths incident on a nonlinear optical crystal and outputs difference frequency light, wherein at least one of the laser beams is a wavelength tunable laser beam and outputs the wavelength tunable laser beam. With an optical resonator,
The optical resonator, in order from the output side,
An output mirror having a constant transmittance;
And Les over The media,
A wavelength dispersion element that disperses the light output from the laser medium according to the wavelength;
A reflecting mirror that has a convex surface with respect to the wavelength dispersion direction of the incident light and reflects the incident light;
The resonator is further disposed between the reflecting mirror and the output mirror, has a convex lens characteristic with respect to a direction perpendicular to the wavelength dispersion direction, and an optical axis of light in the direction perpendicular to the wavelength dispersion direction. Including optical elements that suppress fluctuations ,
The reflecting mirror further disperses the light dispersed by the wavelength dispersion element and reflects the light to the output mirror side .
Wavelength conversion laser device. An aperture that is disposed on the optical path of the laser beam output from the optical resonator and transmits a portion near the optical axis of the laser beam;
The wavelength conversion laser device according to claim 1 or 2 . The reflecting mirror is a rotating reflecting mirror that selects the wavelength of light emitted by the inclination of the reflecting surface.
The wavelength conversion laser device according to claim 1 or 2 . The optical resonator comprises an attenuator for adjusting the intensity of light reciprocating in the optical resonator;
The wavelength conversion laser device according to claim 1 or 2 .

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