JP3428437B2 - Wavelength converter - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the generation of a harmonic (a second harmonic, a third harmonic, a fourth harmonic, etc.) by injecting light of a predetermined wavelength such as laser light into a non-linear optical crystal to generate a wavelength. A wavelength conversion device that performs conversion, for example, second harmonic generation (SH
G) A device, a parametric oscillator for synthesizing a sum frequency and a difference frequency of two or more electromagnetic waves, and the like.

[0002]

2. Description of the Related Art In recent years, a light source of ultraviolet laser light has been used in a technology utilizing short-wavelength light (especially coherent light), for example, in a lithography apparatus for semiconductor manufacturing or a stereolithography apparatus for forming a three-dimensional object model. Has been done. As a light source of this ultraviolet laser light, for example, an excimer laser device or the like is used, but since the excimer laser device uses a halogen gas having high reactivity, it is troublesome to handle such as maintenance and the cost is high. was there.

As a countermeasure against this, a nonlinear optical crystal is SHG.
(Second Harmonic Generation) has been proposed as a technique for obtaining short wavelength light. For example, there is an optical harmonic generation technique in which laser light (1064 nm) of an Nd: YAG laser is used as a fundamental wave and is incident on a plurality of nonlinear optical crystals to generate a second harmonic (532 nm) to obtain light of 1/2 wavelength. Are known.

The wavelength conversion efficiency to a harmonic in a non-linear optical crystal is mainly determined by the physical constants of the material itself, and in order to improve the wavelength conversion efficiency, a method devised on the light source side, for example, , A method such as increasing the power density of incident light or deforming the beam shape of light (elliptical), a method devised on the material side,
That is, there is a method of performing phase matching by applying temperature or pressure to the crystal as a property of the material itself.

However, there is a limit to the improvement of the wavelength conversion efficiency due to these reasons for the following reasons. In the above nonlinear optical crystal, it is necessary to set so as to satisfy a phase matching angle such that dispersion (refractive index with respect to wavelength) becomes equivalent in the arrangement of the incident angle and the crystal. It is known that there is a certain limit on the effective length of the crystal due to the walk-off separation between the normal ray and the extraordinary ray.

That is, the fundamental wave (laser light) that passes through the nonlinear optical crystal propagates as an ordinary ray, but the generated harmonic wave propagates as an extraordinary ray, and therefore propagates in a direction different from the fundamental wave due to birefringence. However, there is a problem that a walk-off phenomenon occurs in which the overlapping portion of the fundamental wave and the harmonic becomes gradually smaller. Due to this walk-off phenomenon, the area for contributing to the area where harmonics are generated is reduced, and the efficiency of wavelength conversion is reduced, making it difficult to obtain high-intensity harmonics.

Therefore, conventionally, for example, as shown in FIG. 7, a pair of non-linear optical crystals, that is, a first non-linear optical crystal C1 and a second non-linear optical crystal C2 are used, and the c-axis ( There has been proposed a wavelength conversion technique in which the optical axis) is arranged so as to be inverted with respect to the center of the traveling direction of the incident light L1, thereby compensating the influence of the walk-off of the generated harmonics H1 and H2.

[0008]

However, the above-mentioned wavelength conversion technique has the following problems. That is, since the light incident on the nonlinear optical crystal has a constant beam diameter, as shown in (a) and (b) of FIG. C1
The harmonic H1 generated in 1 and the harmonic H2 generated in the second nonlinear optical crystal C2 on which the light L1 is incident next are shifted from each other at the exit end. Therefore, even if such a pair of nonlinear optical crystals is used, the effect of walk-off cannot be completely compensated in practice, so that high-intensity harmonics cannot be obtained.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a wavelength conversion device capable of reducing the influence of walk-off and obtaining harmonics with high conversion efficiency.

[0010]

The present invention has the following features to attain the object mentioned above. That is, in the wavelength conversion device according to claim 1, a crystal unit, which is arranged in series and includes three or more nonlinear optical crystals that generate harmonic light by allowing light having a predetermined wavelength to enter from one end side and transmit the light,
Each nonlinear optical crystal of the crystal unit is arranged such that the direction of the c-axis in each crystal is alternately inverted with respect to the traveling direction of the light, and the first crystal to which light is first incident and The first crystal includes a second crystal to which light is incident next, and a non-linear optical crystal to which light is sequentially incident after the second crystal. For the nonlinear optical crystal, a technique is adopted in which the crystal length is set to be the same as that of either the first crystal or the second crystal having the same c-axis direction.

In this wavelength converter, each of the nonlinear optical crystals in the same optical path crystal group is set to the same crystal length as either one of the first crystal and the second crystal having the same c-axis direction. Therefore, the harmonic light generated in each nonlinear optical crystal of the same crystal group in the optical path has the same crystal length as the first harmonic light.
Will pass through exactly the same optical path as that of the harmonic light generated in the second crystal or the second crystal. For example, when four nonlinear optical crystals are arranged in series, the harmonic light generated in the third nonlinear optical crystal is in the same optical path as the harmonic light generated in the first crystal, and the fourth nonlinear optical crystal The harmonic light generated in the crystal has the same optical path as the harmonic light generated in the second crystal. That is, in the crystal group having the same optical path, the walk-off compensation can be performed, the wavelength conversion efficiency can be improved, and high-intensity harmonic light can be obtained.

According to a second aspect of the present invention, in the wavelength converter of the first aspect, the first crystal and the second crystal are the harmonic light generated in the first crystal and the second crystal.
And the harmonic light generated in the crystal of the second crystal is set to a crystal length in which at least a part of the second crystal overlaps with the other crystal,
A technique is adopted in which the distance between the nonlinear optical crystals of the crystal unit is set to a distance at which the phases of the harmonic light generated in the respective nonlinear optical crystals match.

In this wavelength conversion device, the intervals between the nonlinear optical crystals of the crystal unit are set to the distances at which the phases of the harmonic lights generated in the respective nonlinear optical crystals are matched, so that the harmonic lights overlap each other. The parts strengthen each other and further improve the conversion efficiency.

According to a third aspect of the present invention, there is provided the wavelength conversion device according to the first aspect, wherein the first crystal and the second crystal are the harmonic light generated in the first crystal and the second crystal.
And the harmonic light generated in the crystal of the second crystal is set to a crystal length in which at least a part of the second crystal overlaps with the other crystal,
A technique is used in which a vacuum state is maintained between the crystals of each nonlinear optical crystal of the crystal unit.

In this wavelength conversion device, since the crystals of the nonlinear optical crystals of the crystal unit are in a vacuum state,
The waveguiding medium between crystals is non-dispersive, and the influence of the distance between crystals on the phase is eliminated, so that the harmonic light beams strengthen each other without adjusting the phase matching separately, further improving the conversion efficiency. To do.

A wavelength converter according to a fourth aspect is the wavelength converter according to the first aspect, wherein the first crystal and the second crystal are the harmonic light generated in the first crystal and the second light.
And the harmonic light generated in the crystal of the second crystal is set to a crystal length in which at least a part of the second crystal overlaps with the other crystal,
A technique in which a birefringent medium for matching the phase of the harmonic light generated in each nonlinear optical crystal is arranged between the crystals of each nonlinear optical crystal of the crystal unit is adopted.

In this wavelength converter, a birefringent medium for matching the phase of the harmonic light generated in each of the nonlinear optical crystals is arranged between the crystals of the nonlinear optical crystals of the crystal unit, so that the dispersion between the crystals is dispersed. Is intentionally adjusted by a predetermined birefringent medium (for example, calcite) to achieve phase matching, strengthen each other at the portions where the respective harmonic lights overlap with each other, and further improve the wavelength conversion efficiency.

A wavelength converter according to a fifth aspect is the wavelength converter according to any one of the first to fourth aspects, in which a plurality of the crystal units are arranged in series so that the light can be sequentially incident, and each of the crystals. A technique is adopted in which only the harmonic light generated in the unit is provided for each crystal unit, and a harmonic light separating means is provided for separating the light from the light by a spectroscope.

Since this wavelength conversion device is provided with a harmonic light separating means for extracting only the harmonic light generated in each crystal unit from the light for each crystal unit by a spectroscope, the generated harmonic light is It can be taken out while maintaining the output before being incident on the next crystal unit and being internally attenuated.

According to a sixth aspect of the present invention, there is provided the wavelength conversion device according to the fifth aspect of the present invention, in which the harmonic light integrating means integrates the harmonic light beams extracted by the crystal unit by the harmonic light separating means into one. Is adopted.

In this wavelength conversion device, since the harmonic light unifying means for unifying the respective harmonic light extracted for each crystal unit by the harmonic light separating means into one is provided, a high output as a combined wave of the respective harmonic light is provided. It is possible to obtain higher harmonic light.

A wavelength converter according to a seventh aspect is the wavelength converter according to the sixth aspect, wherein a harmonic wave for converting each of the harmonic lights extracted for each crystal unit by the harmonic light separating means into polarization directions different from each other. The technology in which the polarization conversion means is provided is adopted.

Since this wavelength conversion device is provided with the harmonic polarization conversion means for converting the respective harmonic lights extracted for each crystal unit by the harmonic light separation means into polarization directions different from each other, the harmonic polarization conversion means Since the respective harmonic lights are converted into polarization directions different from each other and then integrated into one, it is difficult for the harmonic lights to cancel each other at the time of integration, and it is possible to obtain a higher-harmonic composite wave. That is, when the polarization directions of the respective harmonic lights are the same, since the respective harmonic lights generate a phase difference due to the difference in the optical path lengths, they weaken each other at the time of integration and the output of the composite wave decreases. In the present invention, since the polarization directions of the respective harmonic lights are different from each other at the time of integration, the influence of the phase difference is significantly reduced.

The wavelength converter according to claim 8 is the wavelength converter according to claim 6, wherein the light incident on the crystal units and transmitted therethrough is converted into polarization directions different from each other. The technology in which the conversion means is provided is adopted.

Since this wavelength conversion device is provided with a light polarization conversion means for converting the above-mentioned light sequentially incident on and transmitted through each crystal unit into different polarization directions for each crystal unit, it is generated in each crystal unit. The harmonic lights have different polarization directions, and when integrated, the influence of the phase difference is significantly reduced, and high-power harmonic lights can be obtained.

According to a ninth aspect of the wavelength conversion apparatus, in the wavelength conversion apparatus according to any of the first to eighth aspects, the nonlinear optical crystal is Li ₂ B ₄ O ₇ .

In this wavelength converter, since the nonlinear optical crystal is Li ₂ B ₄ O ₇ having a relatively high damage resistance threshold,
High-power laser light that is hard to be thermally damaged can be made incident. In addition, since the walk-off phenomenon is relatively small and the so-called walk-off angle is small in Li ₂ B ₄ O ₇ , the region that effectively contributes to the generation of harmonics is widened and the harmonic generation efficiency is improved. .

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION A first embodiment of a wavelength conversion device according to the present invention will be described below with reference to FIGS. 1 to 5.

As shown in FIG. 1, the wavelength conversion device 1 of the first embodiment includes a laser device 3 for emitting a YAG second harmonic wave 2 having a wavelength of 532 nm and a YAG second harmonic wave (laser light) 2 arranged in series. The first YAG is made incident from the side and transmitted.
First crystal unit 5 for generating fourth harmonic wave (harmonic light) 4A and second YAG fourth harmonic wave (harmonic light) 4B
A and the second crystal unit 5B, and the first YAG4 harmonic 4A and the second YAG4 generated in each crystal unit.
For each crystal unit, only the harmonic wave 4B is applied to the YAG second harmonic wave 2 to the first YAG fourth harmonic wave separator (spectrometer) 6A and the second harmonic wave 4B.
Harmonic light separating means 7 for separating and extracting by the YAG fourth harmonic wave separator (spectrometer) 6B, and harmonic integrating means 8 for integrating each YAG fourth harmonic wave extracted for each crystal unit by the harmonic light separating means 7 into one. And a harmonic polarization conversion means 9 for converting each YAG fourth harmonic extracted by each crystal unit by the harmonic light separation means 7 into polarization directions different from each other.

The laser device 3 causes a laser beam having a wavelength of 1064 nm generated by an Nd: YAG laser (not shown) to be incident on an LBO (Li ₂ B ₄ O ₇ ) crystal (not shown) to generate a second harmonic. A YAG second harmonic wave 2 having a wavelength of 532 nm, which is a wave, is generated. This YAG second harmonic 2 is a fundamental wave in the wavelength conversion device 1.

The first crystal unit 5A and the second crystal unit 5B are arranged in series as shown in FIG.
YAG4
Four nonlinear optical crystals that generate harmonics, that is, FIG.
As shown in FIG. 5, the c-axis direction c in each crystal is alternately inverted with respect to the traveling direction X of the YAG second harmonic wave 2 at a predetermined angle θ, and the
First crystal 5a on which the second harmonic wave 2 is incident and first crystal 5a
The same optical path as that of the second crystal 5b to which the YAG second harmonic wave 2 is incident after a and the third crystal 5c and the fourth crystal 5d to which the YAG second harmonic wave 2 is sequentially incident after the second crystal 5b. And a crystal group 10. further,
The third crystal 5c and the fourth crystal 5d of the same optical path crystal group 10 are set to have the same crystal length l as the first crystal 5a and the second crystal 5b.

First crystal unit 5A and second crystal
The first, second, third and fourth connections in the unit 5B
Crystals 5a, 5b, 5c and 5d are LBO (Li₂
B_FourO ₇) It is a crystal and is arranged in series on the optical path of the YAG second harmonic wave 2.
It is placed. The first YAG fourth harmonic separator 6A
Is a first crystal unit 5A and a second crystal unit 5B.
Is arranged on the optical path of the YAG second harmonic wave 2 between
The G4 harmonic separator 6B is the same as that of the second crystal unit 5B.
It is arranged on the optical path of the YAG second harmonic 2 at the other end.
It

The conversion efficiency (4th harmonic output / 2nd harmonic input) with respect to the crystal length l of each nonlinear optical crystal is shown in FIG.
As shown in FIG. 3, the crystal length l of the first, second, third and fourth crystals 5a, 5b, 5c and 5d becomes constant when the crystal length (lx) is exceeded. Is within a region proportional to the square and the square of the crystal length l
It is set within the range up to x. That is, this is to improve the conversion efficiency by using the crystal length of the region where the conversion efficiency is high.

The first crystal 5a and the second crystal 5b are the second harmonic 4a of the YAG generated in the first crystal 5a and the second crystal 4a.
And the YAG fourth harmonic wave 4b generated in the crystal 5b is set to a crystal length l at which at least a part of the second crystal 5b overlaps with the other side of the second crystal 5b. Further, the distances d1, d2, d3 between the nonlinear optical crystals in the first crystal unit 5A and the second crystal unit 5B are set to distances in which the phases of the harmonics generated in the respective nonlinear optical crystals are matched. .

The first YAG fourth harmonic wave separator 6A and the second YAG fourth harmonic wave separator 6B are composed of the first crystal unit 5A and the second crystal unit 5B.
YAG fourth harmonic 4A and second YAG fourth harmonic 4B (Y
The second harmonic of the AG second harmonic: the wavelength of 266 nm is separated from the YAG second harmonic 2 and the optical path thereof is changed to be extracted to the outside.

The harmonic polarization conversion means 9 includes a second YA
Second YA separated by G4 harmonic separator 6B
Λ / 2 crystal wave plate 11 arranged on the optical path of the G4 harmonic 4B
Is equipped with. The λ / 2 crystal wave plate 11 is a second YA
When transmitting the G4 harmonic wave 4B, the polarization direction is rotated by 90 °.

The harmonics integrating means 8 is the first YAG 4
First YAG 4 separated by the harmonic separator 6A
A third YAG fourth harmonic wave separator 6C for changing the optical path of the fourth harmonic wave 4A, a first YAG fourth harmonic wave 4A from the third YAG fourth harmonic wave separator 6C, and a second YAG fourth harmonic wave separator 6B from the second YAG fourth harmonic wave separator 6B. The polarization beam splitter 12 is provided at a position where the optical path of the YAG fourth harmonic 4B intersects. The polarization beam splitter 12 integrates a first YAG fourth harmonic 4A and a second YAG fourth harmonic 4B whose polarization directions are orthogonal to each other to generate a composite wave 13.

Next, the wavelength conversion method of light in the wavelength conversion device 1 of the first embodiment will be described.

First, the laser device 3 emits a YAG second harmonic wave 2 having a wavelength of 532 nm, which is a polarization of only a longitudinal wave, and the first
The light is incident on the crystal unit 5A from one end side thereof. At this time, when the YAG second harmonic wave 2 first enters the first crystal 5a, as shown in FIGS. 2 and 5 (a), the YAG fourth harmonic wave 4a is generated with a constant walk-off angle, and the other end side Is emitted from. Next, YAG second harmonic 2 and YAG
When the fourth harmonic wave 4a is incident on the second crystal 5b, as shown in FIGS. 2 and 5 (b), the YAG has a walk-off angle whose sign is opposite to that of the case of the first crystal 5a.
The fourth harmonic wave 4b is generated and emitted from the other end side together with the YAG second harmonic wave 2.

At this point, the YAG fourth harmonic 4a and the YAG4
The second harmonic wave 4b is shifted from the second crystal 5b in a partially overlapped state, and is incident on the next third crystal 5c together with the transmitted YAG second harmonic wave 2. And the third crystal 5c
As shown in FIGS. 2 and 5C, a part of the YAG second harmonic wave 2 incident on the same optical path as that of the YAG fourth harmonic wave 4a has the same walk-off angle as that of the first crystal 5a. Converted to YAG fourth harmonic 4c. That is, in the third crystal 5c, the YAG fourth harmonic 4a and the YAG fourth harmonic 4
c is superposed and guided, and is superposed and emitted from the other end side. In the third crystal 5c, the YAG fourth harmonic 4b
Is also guided with a walk-off angle similar to that of the YAG fourth harmonic 4c.

Further, Y emitted from the third crystal 5c
The AG fourth harmonic 4a, 4b, 4c and the YAG second harmonic 2 are
After being incident on the fourth crystal 5d, as shown in FIGS. 2 and 5 (d), a part of the YAG second harmonic wave 2 has a walk-off angle similar to that of the second crystal 5b, and thus the YAG fourth harmonic wave is generated. Four
It is converted into a YAG fourth harmonic 4d in the same optical path as b. In the fourth crystal 5d, the YAG fourth harmonics 4a and 4c are also Y
The wave is guided with a walk-off angle similar to that of the AG fourth harmonic 4d.

Therefore, from the other end side of the fourth crystal 5d, the YA consisting of the YAG fourth harmonic waves 4a, 4b, 4c and 4d is formed.
The G4 harmonic wave 4A and the transmitted YAG second harmonic wave 2 are emitted. At this time, the YAG fourth harmonic 4a and the YAG fourth harmonic 4
c, and the YAG fourth harmonic 4b and the YAG fourth harmonic 4d are
The YAG fourth harmonic waves 4a and 4c and the YAG fourth harmonic waves 4b and 4d are emitted in a state where they are partially overlapped and emitted.

In addition, YAG4
The inter-crystal distances d1 and d are set so that the phase difference of harmonics does not occur.
Since 2 and d3 are adjusted, it is possible to obtain a high-intensity beam from the exit end by the interference of the four YAG fourth harmonics and the mutual interference of the two and d3. In this way, a part of the YAG second harmonic wave 2 is converted into the first YAG fourth harmonic wave 4A having the wavelength of 266 nm in the first crystal unit 5A, and the first YAG fourth harmonic wave 4A is converted into the YAG second harmonic wave 2A.
And is emitted from the other end side.

The first YAG fourth harmonic 4A is the YAG
It is emitted as a polarized wave of only the transverse wave in a state rotated by 90 ° with respect to the polarization direction of the second harmonic wave 2. And the first YAG4
The second harmonic wave 4A and the second harmonic wave YAG 2 are incident on the first YAG fourth harmonic wave separator 6A, the first YAG fourth harmonic wave 4A is separated from the YAG second harmonic wave 2, and the optical path is changed.

Next, the first YAG fourth harmonic separator 6A
YAG second harmonic wave 2 transmitted through the second crystal unit 5B
Is incident on one end side of the second crystal unit 5B in the same manner as the first crystal unit 5A.
Due to the second, third and fourth crystals 5a, 5b, 5c and 5d, a part of the YAG second harmonic 2 has a second wavelength of 266 nm.
Is converted into the YAG fourth harmonic 4B, and the second YAG fourth harmonic 4B is emitted from the other end side together with the YAG second harmonic 2.
It should be noted that the second YAG fourth harmonic 4B is emitted as a polarization of only the transverse wave in a state of being rotated by 90 ° with respect to the polarization direction of the YAG second harmonic 2.

Then, the second YAG fourth harmonic 4B and Y
The AG 2nd harmonic wave 2 is incident on the second YAG 4th harmonic wave separator 6B, the second YAG 4th harmonic wave 4B is separated from the YAG 2nd harmonic wave 2, and the optical path is changed. Furthermore, this second YA
The G4 harmonic 4B is transmitted through the λ / 2 crystal wave plate 11, and the polarization azimuth is rotated by 90 ° at this time to be converted into a longitudinal polarized wave.

The first YAG fourth harmonic 4A has a third Y
The optical path is changed by the AG quadruple wave separator 6C and is incident on the polarization beam splitter 12. On the other hand, the second YAG fourth harmonic 4B whose polarization direction is rotated is also incident on the polarization beam splitter 12. Therefore, in the polarization beam splitter 12, the transverse first YAG fourth harmonic 4A and the longitudinal second YAG fourth harmonic 4B whose polarization directions are orthogonal to each other are integrated, and the composite wave 13 of the wavelength 266 nm is integrated into the polarization beam splitter. It is emitted from 12.

In the wavelength conversion device 1, the third crystal 5c and the fourth crystal 5d of the same optical path crystal group 10 are the same as the first crystal 5a and the second crystal 5b having the same c-axis direction. Since the same crystal length l is set, the third crystal 5c and the fourth crystal 5d of the same optical path crystal group 10 are formed.
YAG fourth harmonics 4c and 4d generated in Y are generated in the first crystal 5a and the second crystal 5b having the same crystal length.
The light passes through exactly the same optical path as the AG fourth harmonics 4a and 4b. That is, in the crystal group 10 having the same optical path, walk-off compensation can be performed, wavelength conversion efficiency can be improved, and a high-strength YAG fourth harmonic wave can be obtained.

The inter-crystal distances (distances) d1, d2 and d3 of the respective nonlinear optical crystals of the first crystal unit 5A and the second crystal unit 5B are the phases of the YAG fourth harmonic generated in the respective nonlinear optical crystals. Are set to match each other, so that the YAG fourth harmonics are strengthened at a portion where they overlap each other, and the conversion efficiency is further improved.

In addition, from the first crystal unit 5A and the second crystal unit 5B arranged in series, one end side of YA
Since the G2 overtone 2 is made incident and sequentially transmitted through each crystal unit, and each crystal unit sequentially generates the first YAG fourth overtone 4A and the second YAG fourth overtone 4B,
The YAG second harmonic 2 which is the fundamental wave is not divided and is converted into the first YAG fourth harmonic wave 4A by the first crystal unit 5A and is incident on the second crystal unit 5B, which is one fundamental wave. The second harmonic is generated with high conversion efficiency in the order of incidence.

Furthermore, YAG generated in each crystal unit
YAG from the second harmonic to YAG for each crystal unit
Since it is separated by the fourth harmonic wave separator and taken out, the generated first YAG fourth harmonic wave 4A is transferred to the next second crystal unit 5B.
It can be taken out while maintaining the output before it is incident on and attenuated internally.

Further, each Y extracted for each crystal unit
Since the AG 4th harmonic is integrated into one, a high-output second harmonic can be obtained as the composite wave 13 of each YAG 4th harmonic. Then, each YAG4 taken out for each crystal unit
Since the harmonics are converted into polarization directions different from each other and then integrated into one, the influence of the phase difference is significantly reduced, it is difficult for the harmonics to cancel each other during integration, and a higher output YAG fourth harmonic composite wave 13 can be obtained. it can.

Next, a second embodiment of the wavelength conversion device according to the present invention will be described with reference to FIG.

The wavelength conversion device 21 of the second embodiment is shown in FIG.
As shown in FIG. 3, a laser device 3 that emits a YAG second harmonic wave 2 having a wavelength of 532 nm and a YAG second harmonic wave 2 that is arranged in series are made incident from one end side to be transmitted therethrough and the first YAG fourth harmonic wave 4A
And a first crystal unit 5A and a second crystal unit 5B that generate a second YAG fourth harmonic wave 4B, and only the YAG fourth harmonic wave generated in each crystal unit from the YAG second harmonic wave 2 to the first YAG4. Harmonic light separating means 7 separated and taken out by the harmonic wave separator (spectrometer) 6A and the second YAG 4th harmonic wave separator (spectrometer) 6B, and each YAG 4th harmonic wave taken out for each crystal unit by the harmonic light separating means 7 Harmonic unifying means 8 for unifying
An optical polarization conversion means 22 is provided for converting the YAG second harmonic wave 2 which is sequentially incident on and transmitted through each crystal unit into different polarization directions for each crystal unit.

The laser device 3, the first crystal unit 5A, the second crystal unit 5B, the harmonic light separation means 7 (the first YAG fourth harmonic wave separator 6A and the second Y unit).
AG 4th harmonic separator 6B) and harmonic integration means 8
The (third YAG fourth harmonic separator 6C and the polarization beam splitter 12) is the same as that of the wavelength conversion device 1 in the first embodiment.

The light polarization conversion means 22 is the laser device 3
And the first crystal unit 5A between the YAG second harmonic 2 and the first λ / 2 crystal wave plate 22A for rotating the polarization direction of the YAG second harmonic 2 by 90 °, and the third YAG.
A second λ / 2 crystal arranged on the optical path of the first YAG fourth harmonic 4A between the fourth harmonic separator 6C and the polarization beam splitter 12 and rotating the polarization direction of the first YAG fourth harmonic 4A by 90 °. And a wave plate 22B.

Next, a wavelength conversion method of light in the wavelength conversion device 21 of the second embodiment will be described.

First, the laser device 3 emits a YAG second harmonic wave 2 having a wavelength of 532 nm, which is a polarization of only a longitudinal wave, and the first
It is incident on the λ / 2 crystal wave plate 22A. At this time, Y
The polarization azimuth of the second harmonic wave 2 of AG is rotated by 90 ° and converted into the polarized wave of only the transverse wave. Thereafter, the polarization-converted YAG second harmonic wave 2 is incident on the first crystal unit 5A from one end side thereof, and the first, second, third, and fourth crystals 5
By a, 5b, 5c, 5d, a part of the YAG second harmonic wave 2 is converted into the first YAG fourth harmonic wave 4A having a wavelength of 266 nm, and the first YAG fourth harmonic wave 4A is emitted from the other end side together with the YAG second harmonic wave 2. To be done.

It should be noted that the first YAG fourth harmonic 4A is a YAG
It is emitted as a polarized wave of only the longitudinal wave in a state of being rotated by 90 ° with respect to the polarization direction of the second harmonic wave 2. And the first YAG4
The second harmonic wave 4A and the second harmonic wave YAG 2 are incident on the first YAG fourth harmonic wave separator 6A, the first YAG fourth harmonic wave 4A is separated from the YAG second harmonic wave 2, and the optical path is changed.

Next, the first YAG fourth harmonic separator 6A
YAG second harmonic wave 2 transmitted through the second crystal unit 5B
To the YAG in the second crystal unit 5B in the same manner as in the first crystal unit 5A.
A part of the second harmonic wave 2 is converted into a second YAG fourth harmonic wave 4B having a wavelength of 266 nm, and the second YAG fourth harmonic wave 4B is converted to YAG2.
It is emitted from the other end side together with the harmonic wave 2. The second Y
The AG fourth harmonic 4B is emitted as a polarized wave of only a longitudinal wave in a state of being rotated by 90 ° with respect to the polarization direction of the YAG second harmonic 2.

Then, the second YAG fourth harmonic 4B and Y
The AG 2nd harmonic wave 2 is incident on the second YAG 4th harmonic wave separator 6B, the second YAG 4th harmonic wave 4B is separated from the YAG 2nd harmonic wave 2, the optical path is changed, and the polarization beam splitter 12 is inserted.
Is incident on.

On the other hand, the first YAG fourth harmonic wave 4A has its optical path changed by the third YAG fourth harmonic wave separator 6C and is incident on the second λ / 2 crystal wave plate 22B. At this time, the polarization displacement of the first YAG fourth harmonic wave 4A is rotated by 90 ° and is converted again into the transverse wave polarization. In addition, the second YAG fourth harmonic 4B has a second λ
While passing through the / 2 crystal wave plate 22B, the polarization azimuth is rotated by 90 ° at this time, and is converted into a longitudinal polarized wave.

After that, the polarization-converted second YAG fourth harmonic 4B is incident on the polarization beam splitter 12. Therefore, in the polarization beam splitter 12, the first shear wave
The YAG fourth harmonic 4A and the longitudinal second YAG fourth harmonic 4B are integrated, and the combined wave 13 having a wavelength of 266 nm is emitted from the polarization beam splitter 12.

In the wavelength converter 21, the YAG second harmonic wave 2 is applied from one end side of the first crystal unit 5A and the second crystal unit 5B arranged in series, as in the wavelength converter 1 of the first embodiment. Since the first YAG fourth harmonic 4A and the second YAG fourth harmonic 4B are sequentially generated in each crystal unit after being incident and sequentially transmitted through each crystal unit, the fundamental YAG second harmonic 2 is not divided. First
Will be converted into the first YAG fourth harmonic wave 4A by the crystal unit 5A and will be incident on the next second crystal unit 5B, and harmonics will be generated with high conversion efficiency in the order of incidence based on one fundamental wave. .

In addition, YAG4 generated in each crystal unit
YAG2 to YAG4 for each crystal unit
Since it is separated and taken out by the harmonic separator, the first
The YAG fourth harmonic wave 4A can be taken out in a state where the output is maintained before the YAG fourth harmonic wave 4A is incident on the next second crystal unit 5B and is internally attenuated. Then, since each YAG fourth harmonic extracted for each crystal unit is integrated into one, each YA
High-output harmonics can be obtained as the G4 harmonic composite wave 13.

Further, since the YAG second harmonic wave 2 which is sequentially incident on and transmitted through each crystal unit is converted into a different polarization direction for each crystal unit and then is incident, the second harmonic light generated in each crystal unit is different. It becomes the polarization direction. Therefore, at the time of integration, the influence of the phase difference is significantly reduced, and the high-output YAG fourth harmonic composite wave 13 can be obtained.

The present invention also includes the following embodiments. (1) In each of the above embodiments, the YAG fourth harmonic wave 4a generated in the first, second, third and fourth crystals 5a, 5b, 5c and 5d in the first and second crystal units 5A and 5B,
In order to perform the phase matching of 4b, 4c, and 4d, the distance between the crystals is adjusted, but other phase matching means may be used in a vacuum state between the crystals. That is, since the crystals are in a vacuum state, the waveguide medium between the crystals is non-dispersive, and the influence of the inter-crystal distance on the phase is eliminated, so that each YAG can be adjusted without separately adjusting the phase matching.
It is possible to reinforce each other in a portion where the fourth harmonic waves overlap each other, and further improve the conversion efficiency.

(2) As another phase matching means, the first, second, third and fourth crystals 5a, 5b, 5 are provided between the respective crystals.
YAG 4th harmonics 4a, 4b, 4c, 4
A birefringent medium that matches the phase of d may be provided. That is, it is possible to intentionally adjust the dispersion between crystals by a predetermined birefringent medium such as calcite to achieve phase matching, to strengthen each other at the portions where the respective fourth harmonics of YAG overlap each other, and further improve the wavelength conversion efficiency. .
In this case, the adjustment of the phase matching is performed depending on the type, thickness and arrangement of the birefringent medium to be arranged.

(3) The first, second, third and fourth crystals 5a, 5b, 5c and 5d are set to have the same crystal length l, but the first crystal 5a and the third crystal 5c And the second crystal 5b and the fourth crystal have the same crystal length.
If the crystal length of the pair of crystals is the same as that of the crystal 5d, the crystal lengths of the two pairs may not be the same. That is, the first crystal 5a and the third crystal 5c are set to the crystal length l1,
The second crystal 5b and the fourth crystal 5d may be set to a crystal length l2 different from the crystal length l1. Therefore, among the nonlinear optical crystals arranged in series, every other nonlinear optical crystal may be set to have the same crystal length.

(4) As a nonlinear optical crystal, Li₂B_FourO
₇However, other nonlinear optical crystals may be used. example
For example, as a conventionally used nonlinear optical crystal, L
iB ₃O_Five, KTP, KNbO₃, KDP or the like. Na
As mentioned above, Li₂B _FourO₇Are thermal characteristics and
Excellent in terms of fork / off angle.

(5) YA having a wavelength of 532 nm as a fundamental wave
Although the wavelength conversion for generating the YAG fourth harmonic having the wavelength of 266 nm is performed using the G2 harmonic, the fundamental wave of another wavelength may be used. For example, wavelength conversion may be performed to generate a YAG second harmonic wave having a wavelength of 532 nm using a laser light having a wavelength of 1064 nm as a fundamental wave.

(6) First crystal unit 5A and second crystal unit
The crystal unit 5B of No. 3 has the same optical path crystal group 10
Two non-linear optical crystals of a first crystal, a second crystal, a third crystal, and a fourth crystal 5d.
Although it is composed of four nonlinear optical crystals 5b, 5c, and 5d, a crystal unit composed of three or more nonlinear optical crystals may be used as long as the same optical path crystal group is composed of one or more nonlinear optical crystals.

(7) Although the first crystal unit 5A and the second crystal unit 5B are arranged in series, three or more crystal units may be arranged in series. In the serial arrangement, even if the optical path of the fundamental wave is bent by a mirror, a lens, or the like, the crystal units may be sequentially arranged in series on the optical path of the fundamental wave.

(8) A temperature adjusting mechanism such as a Peltier element may be additionally provided in order to suppress heating due to higher harmonics generated inside the nonlinear optical crystal and control the crystal to a constant temperature. (9) The wavelength conversion devices of the above embodiments may be arranged in parallel, and the second harmonics generated from each may be combined by the polarization beam splitter as in the above. That is,
You may synthesize | combine a 2nd harmonic by arranging three or more said wavelength converters.

[0075]

The present invention has the following effects. (1) In the wavelength conversion device according to claim 1, the nonlinear optical crystals in the same optical path crystal group have the same c-axis direction.
Since the crystal length is set to be the same as that of either one of the first crystal and the second crystal, the harmonic light generated in each nonlinear optical crystal of the same crystal group of the optical path has the same crystal length as the first crystal. The harmonic light generated in the crystal or the second crystal passes through exactly the same optical path, and walk-off compensation can be realized in the same optical path crystal group. Therefore, the aperture length (walk of the incident light beam diameter
It is possible to artificially increase the apparent coherence length (ratio to the off-angle), improve the wavelength conversion efficiency to the point just before the diffraction limit, and obtain high-intensity harmonic light.

(2) In the wavelength converter according to claim 2,
Since the distance between the nonlinear optical crystals of the crystal unit is set to a distance at which the phases of the harmonic lights generated in the respective nonlinear optical crystals match, they strengthen each other at the portions where the harmonic lights overlap with each other, and the conversion efficiency is further improved. improves.

(3) In the wavelength converter according to claim 3,
Since the non-linear optical crystals of the crystal unit are kept in a vacuum state between the crystals, the waveguide medium between the crystals is non-dispersive, and the influence of the inter-crystal distance on the phase is eliminated, and the phase matching is not adjusted separately. However, it is possible to reinforce each other in the portions where the respective harmonic lights overlap with each other, and further improve the conversion efficiency.

(4) In the wavelength converter according to claim 4,
Since a birefringent medium for matching the phase of the harmonic light generated in each nonlinear optical crystal is arranged between the crystals of each nonlinear optical crystal of the crystal unit, the dispersion between the crystals is intentionally controlled by a predetermined birefringent medium. It is possible to achieve phase matching by adjusting to, strengthen each other at the portions where the respective harmonic lights overlap with each other, and further improve the wavelength conversion efficiency.

(5) In the wavelength converter according to claim 5,
Since harmonic light separating means for separating only the harmonic light generated in each crystal unit from the light by the spectroscope for each crystal unit is provided, the laser light itself, which is the fundamental wave, is not divided, and the crystal units are sequentially separated. It is possible to convert into high harmonics with high efficiency, and it is possible to efficiently take out only the generated harmonic light before it is incident on the next crystal unit and attenuated.

(6) In the wavelength converter according to claim 6,
Since the harmonic light unifying means for unifying the respective harmonic light beams taken out for each crystal unit by the harmonic light separating means is provided, a high output harmonic wave can be obtained as a composite wave of the respective harmonic light beams.

(7) In the wavelength converter according to claim 7,
Harmonic polarization conversion means for converting each harmonic light extracted for each crystal unit by the harmonic light separation means into different polarization directions is provided, so it is difficult to cancel each other at the time of integration, and synthesis of higher output harmonics You can get the waves.

(8) In the wavelength converter according to claim 8,
Since light polarization conversion means for converting the light sequentially incident on and transmitted through each crystal unit into different polarization directions for each crystal unit, the harmonic light generated in each crystal unit has different polarization directions, It is difficult to cancel each other at the time of integration, and it is possible to obtain a higher-power harmonic composite wave.

(9) In the wavelength conversion device according to claim 9,
Since the nonlinear optical crystal is Li ₂ B ₄ O ₇ which has a relatively high damage resistance threshold and a small walk-off angle, it is hard to be thermally damaged and a high-power laser beam can be made incident. The region that effectively contributes to the generation of waves is widened and the harmonic generation efficiency is improved.

[Brief description of drawings]

FIG. 1 is a schematic layout diagram showing a first embodiment of a wavelength conversion device according to the present invention.

FIG. 2 is a schematic layout diagram showing a crystal unit in the first embodiment of the wavelength conversion device according to the present invention.

FIG. 3 is a schematic layout diagram showing the c-axis direction of each nonlinear optical crystal in the crystal unit in the first embodiment of the wavelength conversion device according to the present invention.

FIG. 4 is a graph showing the relationship between the conversion efficiency and the crystal length of the nonlinear optical crystal in the first embodiment of the wavelength conversion device according to the present invention.

FIG. 5 is an arrangement diagram showing beam positions on the other end side of each nonlinear optical crystal in the crystal unit in the wavelength conversion device according to the first embodiment of the present invention.

FIG. 6 is a schematic layout diagram showing a second embodiment of the wavelength conversion device according to the present invention.

FIG. 7 is a schematic layout diagram showing a conventional example of a wavelength conversion device according to the present invention.

FIG. 8 is an arrangement diagram showing beam positions at output ends of respective nonlinear optical crystals in a conventional example of the wavelength conversion device according to the present invention.

[Explanation of symbols]

1, 21 Wavelength converter 2 YAG second harmonic (light) 3 Laser devices 4a, 4b, 4c, 4d YAG fourth harmonic (harmonic light) 4A First YAG fourth harmonic (harmonic light) 4B Second YAG fourth harmonic (harmonic) Wave light 5a First crystal (nonlinear optical crystal) 5b Second crystal (nonlinear optical crystal) 5c Third crystal (nonlinear optical crystal) 5d Fourth crystal (nonlinear optical crystal) 5A First crystal unit 5B 2 crystal unit 6A 1st YAG 4th harmonic separator (spectrometer) 6B 2nd YAG 4th harmonic separator (spectrometer) 7 Harmonic light separating means 8 Harmonic integrating means 22 Optical polarization converting means d1, d2, d3 Crystal unit Interval of each nonlinear optical crystal of l Crystal length

Continuation of front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/37

Claims (9)

(57) [Claims] 1. A crystal unit comprising three or more nonlinear optical crystals that are arranged in series and generate harmonic light by allowing light of a predetermined wavelength to enter from one end side and transmit the light, and each nonlinear optical crystal of the crystal unit comprises: The c-axis direction of each crystal is alternately inverted with respect to the traveling direction of the light, and the first crystal to which light is first incident and the light to be incident next to the first crystal are incident. A second crystal and a group of non-linear optical crystals having the same optical path on which light is sequentially incident after the second crystal. Each non-linear optical crystal of the same group of optical paths has a c-axis direction. A wavelength conversion device, wherein the crystal length is set to be the same as that of either one of the first crystal and the second crystal that are the same. 2. The wavelength conversion device according to claim 1, wherein the first crystal and the second crystal are composed of a harmonic light generated in the first crystal and a harmonic light generated in the second crystal. The length of the second crystal is set so that at least a part thereof overlaps with the other end of the second crystal, and the distance between the nonlinear optical crystals of the crystal unit is set to a distance at which the phases of the harmonic light generated in the respective nonlinear optical crystals match. A wavelength conversion device characterized by being set. 3. The wavelength conversion device according to claim 1, wherein the first crystal and the second crystal are composed of harmonic light generated in the first crystal and harmonic light generated in the second crystal. A wavelength conversion device characterized in that a crystal length is set such that at least a part of the second crystal overlaps at the other end side thereof, and a vacuum state is maintained between the crystals of the respective nonlinear optical crystals of the crystal unit. 4. The wavelength conversion device according to claim 1, wherein the first crystal and the second crystal are composed of a harmonic light generated in the first crystal and a harmonic light generated in the second crystal. A crystal length is set such that at least a part of the second crystal is overlapped on the other end side, and a phase of harmonic light generated in each nonlinear optical crystal is matched between the crystals of the nonlinear optical crystals of the crystal unit. A wavelength conversion device comprising a refraction medium. 5. The wavelength conversion device according to claim 1, wherein the plurality of crystal units are arranged in series so that the light can be sequentially incident, and only the harmonic light generated in each of the crystal units is generated. A wavelength conversion device, characterized in that harmonic light separating means for separating and extracting the light from the light by a spectroscope is provided for each crystal unit. 6. The wavelength conversion device according to claim 5, further comprising a harmonic integration unit that integrates each of the harmonic lights extracted for each crystal unit by the harmonic light separation unit. Wavelength converter. 7. The wavelength conversion device according to claim 6, further comprising: a harmonic polarization conversion means for converting each of the harmonic lights extracted by the crystal light unit by the harmonic light separation means into polarization directions different from each other. A wavelength conversion device characterized by the above. 8. The wavelength conversion device according to claim 6, further comprising a light polarization conversion means for converting the light, which is sequentially incident on and transmitted through each of the crystal units, into polarization directions different from each other for each crystal unit. A wavelength conversion device. 9. The wavelength conversion device according to claim 1, wherein the nonlinear optical crystal is Li ₂ B ₄ O ₇ .