JPH09152639A - Method and device for optical harmonic generation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide the method and device for optical harmonic generation which can lead high mean output out with high efficiency while preventing crystal destruction and crystal deterioration. SOLUTION: The laser light of wavelength λ emitted by a laser device 1 is branched by a half-mirror 5 into a 1st laser light and a 2nd laser light, which are made incident on nonlinear optical crystals 2a and 2b to generate higher harmonic lights of wavelength λ/2. The output lights of the nonlinear optical crystals 2a and 2b are spectrally diffracted by prisms 3a and 3b to extract higher harmonic lights of wavelength #;/2, and the polarization azimuth of one higher harmonic light is rotated by 90 deg. through a 1/2-wavelength plate 7; and the output lights of the prism 3a and 1/2-wavelength plate 7 are made incident on a polarization beam splitter 8 and the higher harmonic lights having mutually orthogonal polarization azimuths are put one over the other. Here, the polarization azimuths of the 1st and 2nd laser lights may be different from each other by using a polarization beam splitter instead of the half-mirror 5 without using the plate 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION An optical harmonic generator for wavelength-converting laser light output from a pulse laser to obtain a second harmonic, a fourth harmonic, a fifth harmonic, etc. is a light source device in semiconductor device manufacturing. Alternatively, as a light source for a stereolithography apparatus, a wide range of application is expected and its practical application is expected. The present invention relates to the above-described optical harmonic generation device and method, and more particularly, the present invention relates to an optical harmonic generation device and method capable of obtaining a high average output while preventing crystal breakage and crystal deterioration.

[0002]

2. Description of the Related Art A stereolithography apparatus is known as an apparatus for forming a three-dimensional object model. The stereolithography apparatus forms a three-dimensional model based on a figure data group obtained by slicing a numerical model (CAD data or three-dimensional measurement data) of the three-dimensional model on a horizontal plane at equal intervals in the height direction. The process of irradiating the photocurable resin with the light beam along the data and forming the thin plate solidified layer for each slice figure data is repeated, and the formed thin plate solidified layers are laminated to form an arbitrary three-dimensional model. It is a thing. A light source that generates a high average output at a high repetition frequency is required as a light source of the above-described stereolithography apparatus, and an ultraviolet laser device to which a harmonic generation technique using a nonlinear optical crystal is applied is being studied as the light source.

On the other hand, conventionally, an excimer laser device has been regarded as a promising light source for a lithographic apparatus in the manufacture of a semiconductor device. However, since the excimer laser device uses a highly reactive halogen gas as a source gas, maintainability, There was a problem in handling, and there was a problem in running cost because the raw material gas was frequently exchanged. For this reason, in recent years, an ultraviolet laser device that generates harmonics using the above-mentioned nonlinear optical crystal has been receiving attention.

FIG. 6 is a diagram showing a conventional example of a harmonic generator for generating a harmonic using the above-mentioned nonlinear optical crystal. In the figure, reference numeral 11 denotes a laser device that pulse-oscillates a laser beam having a wavelength λ, such as an Nd: YAG laser, and 12 denotes a non-linear optical crystal. ) Is incident, the nonlinear optical crystal 12 receives the fundamental wave λ and its second harmonic, the wavelength λ.
Generates / 2 light. Reference numeral 13 denotes a prism, and the prism 13 disperses the light having the wavelength λ and the light having the wavelength λ / 2, and emits the light having the wavelength λ and the light having the wavelength λ / 2 as shown in FIG. Of the light of wavelength λ and the light of wavelength λ / 2, by blocking the light of wavelength λ with the shield plate 14, only the light of wavelength λ / 2 can be extracted to the outside. That is, the wavelength λ at which the laser device 11 oscillates due to the nonlinear optical crystal 12
Can be converted into light of wavelength λ / 2.

[0005]

When the energy of one shot of a pulsed laser is E (J / pulse) and the number of shots during 1 second is N (pps: pulse per sec),
The average output (W) is expressed by the following equation. W = E × N Note that instead of the above N, the repetition frequency of one shot can be expressed by the following formula using f (Hz). W = E × f That is, in order to obtain a high average output from the laser device that oscillates in pulses, it is necessary to increase the energy of one shot and increase the repetition frequency. That is, in the optical harmonic generation device shown in FIG. 6, in order to obtain a high average output, it is necessary to secure high conversion efficiency at a high repetition frequency.

Further, in the above-mentioned optical modeling apparatus, etc.,
Since light is scanned to irradiate the photocurable resin, if the repetition frequency is low, the scanning speed becomes slow and the processing time increases. In the wavelength conversion of the pulse laser as described above, factors that impede the high conversion efficiency are phase shift (dephasing) caused by heating by the input wave (fundamental wave input) of the nonlinear optical crystal and the second harmonic generated inside the crystal. ), That is, an increase in the amount of phase mismatch is known.

FIG. 7 is a diagram showing a phase shift occurring in the above-mentioned nonlinear optical crystal. As shown in FIG. 7, the harmonics generated in Z1 and the harmonics generated in Z2 in the nonlinear optical crystal are shown in FIG. When the phases are different from each other, interference occurs, and when the amount of phase shift increases, they cancel each other out and the harmonic output decreases, so that the conversion efficiency decreases.

FIG. 8 is a diagram showing the temperature distribution in the crystal when light is incident on the nonlinear optical crystal. As shown in the figure, the temperature inside the crystal increases as it goes in the optical axis direction from the incident surface of the input wave. The temperature rises, and the nonuniformity of the temperature distribution causes the phase shift. Further, the temperature rise increases when the average output of the input wave is large, and further, even when the energy of one shot is the same, it increases as the repetition frequency thereof increases (that is, when the average output increases).

FIG. 9 is a diagram showing the energy and conversion efficiency of one shot extracted from the nonlinear optical crystal. The figure shows the energy of one shot of an input wave (fundamental wave input) having a wavelength of 532 nm and the extracted wavelength. The relationship between the output wave of 266 nm and the energy of one shot and the conversion efficiency thereof are shown. The solid line in the figure shows the case where the repetition frequency is 10 Hz, and the dotted line shows the case where the repetition frequency is 100 Hz. As is clear from the figure, even with the same crystal, when the repetition frequency is increased to increase the average output, the conversion efficiency is extremely reduced in the region where the energy of one shot of the fundamental wave input is large compared to the case where the repetition frequency is low. . This is an effect of phase dephasing due to heat generation due to absorption of the fundamental wave and the second harmonic in the nonlinear optical crystal as described above.

On the other hand, even if the fundamental wave is input to the nonlinear optical crystal, the average output of the obtained second harmonic wave has a certain limit. This is because the nonlinear crystal is damaged when the energy density (J / cm ² ) of the incident light on the nonlinear optical crystal exceeds a certain threshold value. Further, even if the energy density of the incident light is equal to or lower than the damage threshold value of the crystal described above, if the damage density is close to the damage threshold value, the crystal deterioration progresses rapidly and long-time operation becomes difficult. The present invention has been made in consideration of the above problems, the object of the present invention is to prevent the decrease in conversion efficiency due to the phase shift due to heat generation in the crystal (dephasing), and, crystal breakdown, An object of the present invention is to provide an optical harmonic generation method and device capable of extracting a highly averaged output with high efficiency by preventing crystal deterioration.

[0011]

In order to solve the above-mentioned problems, in the inventions of claims 1 to 4, the laser light emitted from the laser device is divided into two, and each of the laser light whose energy is halved is The harmonic light is generated by being separately input to the two harmonic generating means composed of a nonlinear optical crystal or the like.
Then, the two harmonic lights having different polarization directions are input to a polarization beam splitter or the like, and both are superposed and added.

In the invention of claims 1 to 4 of the present invention,
As described above, the laser light is split into the first laser light and the second laser light, the first and second harmonic light are generated from the first and second laser lights, and the polarization directions different from each other are generated. Since the first and second harmonic light beams are integrated, the conversion efficiency of the nonlinear optical crystal can be set near the maximum value, and the energy level of the laser light incident on one harmonic wave generation means can be set to the nonlinear optical wave. It can be set to a value sufficiently lower than the crystal damage threshold value, damage to the nonlinear optical crystal can be prevented, and stable operation for a long time becomes possible and reliability can be improved.

In the inventions of claims 1 and 3, after the laser light is branched, the polarization azimuth of one harmonic light is converted into an azimuth different from the polarization azimuth of the other harmonic light. It is suitable when using a laser device that emits light. Further, in the invention of claims 2 and 4, when the laser light is branched, the first and second laser lights having different polarization directions from each other are used. It is suitable for the case of using a laser device that emits randomly polarized laser light or orthogonally polarized laser light.

[0014]

1 is a diagram showing a first embodiment of the present invention. In the figure, 1 is a wavelength of 532 nm (hereinafter,
The laser device 1 is a laser device that pulse-oscillates light having a wavelength λ), and the laser device 1 includes, for example, as shown in FIG. 2, an Nd: YAG laser 1a that pulse-oscillates light having a wavelength of 1064 nm and a nonlinear optical crystal 1b. The second harmonic (wavelength 532 nm) can be obtained by making the light of wavelength 1064 nm emitted by the laser device 1 incident on the nonlinear optical crystal 1b.
Get. The light emitted from the nonlinear optical crystal 1b is dispersed by a prism (not shown) or the like, and only the second harmonic (wavelength 532 mm) is extracted from the laser device 1 (wavelength 10).
The 64 mm light is blocked by a shield plate or the like). The emitted light of the laser device 1 is linearly polarized light, and its polarization direction is, for example, in the vertical direction with respect to the paper surface as shown in the figure (indicated by a thick arrow in the figure).

Reference numerals 2a and 2b denote first and second non-linear optical crystals, and the non-linear optical crystals 2a and 2b have polarization directions when the light of wavelength λ having a polarization direction indicated by a thick arrow in the figure is incident. Direction light with wavelength λ and light with wavelength 266 nm (hereinafter referred to as wavelength λ / 2) whose polarization direction is orthogonal to the paper surface (indicated by a circle with a black dot in the figure). To do. 5 is a half mirror, 6a is a total reflection mirror,
The emitted light of the wavelength λ of the laser device 1 is branched by the half mirror 5, and the first laser light is converted into the first nonlinear optical crystal 2 described above.
a, the second laser light is reflected by the total reflection mirror 6a and then enters the second nonlinear optical crystal, and the respective nonlinear optical crystals 2a and 2b have wavelengths λ of polarization directions shown in FIG.
And the light of wavelength λ / 2 are emitted. It is an important precondition that the crystal length of each of the nonlinear optical crystals is optimized so that the conversion efficiency is maximized with respect to the energy of the laser light branched and halved.

Light emitted from the first and second nonlinear optical crystals 2a and 2b is incident on the first and second prisms 3a and 3b, and the prisms 3a and 3b emit light of wavelength λ and light of wavelength λ / 2. Spectrally. Of the light emitted from the prisms 3a and 3b, the light having the wavelength λ is blocked by the shielding plates 4a and 4b, and the light having the wavelength λ / 2 whose polarization direction is perpendicular to the paper surface is extracted. The light respectively enters the polarization beam splitter 8 and the total reflection mirror 6b.

The light incident on the total reflection mirror 6b is further 1
The light enters the 1/2 wavelength plate 7, and its polarization direction is rotated by 90 ° as shown in FIG. From the prism 3a and the half-wave plate 7, the wavelength λ
When the light of / 2 enters the polarization beam splitter 8, the polarization beam splitter 8 integrates the two lights without changing the polarization directions of the respective lights, and as a result, as shown in FIG. / 2 light is polarized beam splitter 8
Emitted from

FIG. 3 is a diagram for explaining the operation of this embodiment, which shows the case where this embodiment is applied to the nonlinear optical crystal having the characteristics shown in FIG. The axis shows the energy of one shot of incident light (wavelength 532 nm) to the nonlinear optical crystal, and the axis of ordinate shows the energy and conversion efficiency of one shot of the emitted light (wavelength 266 nm) of the nonlinear optical crystal. The energy of one shot of the light emitted from the laser device 1 is E3, the energy of one shot of the first laser light branched by the half mirror 5 is E1, and the energy of one shot of the second laser light is E3.
It is set to 2 (E3 = E1 + E2 unless the loss in the half mirror is taken into consideration). When the light of the above energies E1 and E2 is incident on the nonlinear optical crystals 2a and 2b, a second harmonic is generated from the nonlinear optical crystals 2a and 2b, and the energy of one shot of the second harmonic generated at that time is From 3 to a and b.

The light emitted from the nonlinear optical crystal 2a enters the polarization beam splitter 8 through the prism 3a, and the light emitted from the nonlinear optical crystal 2b passes through the prism 3b, the total reflection mirror 6b and the half wave plate 7. Since the polarization azimuths of the two lights that are incident on the polarization beam splitter 8 are orthogonal to each other, the energy of one shot of light emitted by the polarization splitter 8 is a + b. Here, when the incident light (fundamental wave) whose one-shot energy is E3 is not split as in the conventional case and is incident on the nonlinear optical crystal, the one-shot energy of the second harmonic becomes c according to FIG. .

As is clear from the figure, since a + b> c and E3 = E1 + E2 as described above, it can be seen that high conversion efficiency can be realized. On the other hand, when the influence of the temperature rise in the nonlinear optical crystal is small, the one-shot energy of the second harmonic with respect to the one-shot energy of the fundamental wave and the conversion efficiency are as shown in FIG. As described above, when one shot energy of the fundamental wave exceeds a certain threshold Eth, the nonlinear optical crystal is damaged and
Even if the shot energy is equal to or less than Eth, if the shot energy is close to Eth, crystal deterioration progresses rapidly and long-time operation becomes difficult. That is, the one-shot energy of the second harmonic cannot be set to a value close to f shown in FIG.

However, in the present invention, as is clear from the figure, f≈d + e, and E4 ≦ E.
Since th and E5 << Eth, one shot energy of the second harmonic having a value close to f can be obtained without damaging the crystal. Moreover, E4 ≦ Eth,
Since E5 << Eth, it is possible to operate for a long time without causing crystal deterioration.

FIG. 5 is a diagram showing a second embodiment of the present invention, and this embodiment shows an embodiment suitable for being applied to a laser device which emits randomly polarized light or orthogonally polarized light. In the figure, the same components as those shown in FIG. 1 are designated by the same reference numerals. In this embodiment, a polarization beam splitter 9 is used instead of the half mirror 5 of FIG. The light of 1 or 2 randomly polarized or orthogonally polarized is split into first and second lights whose polarization directions are orthogonal to each other, and the half-wave plate 7 in FIG. 1 is removed.

The first laser light split by the polarization beam splitter 9 is the same as in the first embodiment.
Light having a wavelength λ / 2 is generated by incidence on the nonlinear optical crystal 2a, and is incident on the polarization beam splitter 8 via the prism 3a. The second laser light split by the polarization beam splitter 9 is incident on the nonlinear optical crystal 2b to generate light of wavelength λ / 2, as in the first embodiment.
The light enters the polarization beam splitter 8 via the prism 3b and the total reflection mirror 7.

In this embodiment, since the light is split by using the polarization beam splitter 9, the polarization azimuth of the light of the wavelength λ / 2 generated by the first light and the second light is as shown in FIG. , They are orthogonal to each other. Therefore, it is possible to obtain lights having wavelengths λ / 2 orthogonal to each other without using the ½ wavelength plate 7 as in the first embodiment. Two wavelengths λ / 2 orthogonal to each other to the polarization beam splitter 8
When the light of (1) is incident, the polarization beam splitter 8 integrates the two lights as in the first embodiment, and the light of wavelength λ / 2 whose polarization directions are orthogonal to each other is emitted from the polarization beam splitter 8.

Also in this embodiment, the light emitted from the laser device 1 is branched and is incident on the first and second nonlinear optical crystals to obtain light of wavelength λ / 2, which is then polarized by the polarization beam splitter 8. As described above with reference to FIG. 3, it is possible to obtain high output with high conversion efficiency without damaging the crystal, and it is possible to operate for a long time without crystal deterioration. Become.

In the first and second embodiments, the case where the nonlinear optical crystal is provided in each of the optical paths where the laser light is branched into two to provide the second harmonic is shown. Is not limited to the above embodiment, and various modifications can be made, for example, by providing a plurality of nonlinear optical crystals in each of the optical paths to generate the fourth harmonic, the fifth harmonic, and the like. .

[0027]

As described above, in the present invention, the laser light is split into the first laser light and the second laser light, and the first and second laser lights are divided into the first and second harmonics. Since the wave light is generated and the first and second harmonic lights having different polarization directions are integrated, the energy level of the laser light incident on one harmonic generating means is almost equal to that of the conventional technique. Since it can be halved, the phase shift (depha
sing), and high conversion efficiency can be obtained. Further, the energy level can be set to a value sufficiently lower than the damage threshold of the nonlinear optical crystal, and high average output can be obtained while preventing damage and deterioration of the nonlinear optical crystal. Therefore, a high output and stable operation for a long time are possible, and reliability can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing an example of a laser device that emits light having a wavelength of 532 nm.

FIG. 3 is a diagram illustrating the operation of the first exemplary embodiment of the present invention.

FIG. 4 is a diagram illustrating the operation of the first embodiment of the present invention.

FIG. 5 is a diagram showing a second embodiment of the present invention.

FIG. 6 is a diagram showing a conventional example of a harmonic generator.

FIG. 7 is a diagram showing how a phase shift occurs in a nonlinear optical crystal.

FIG. 8 is a diagram showing a temperature distribution in a crystal when light is incident on the nonlinear optical crystal.

FIG. 9 is a diagram showing energy extracted from a nonlinear optical crystal and conversion efficiency.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser device 1a Nd: YAG laser 1b Nonlinear optical crystal 2a, 2b Nonlinear optical crystal 3a, 3b Prism 4a, 4b Shielding plate 5 Half mirror 6a, 6b Total reflection mirror 7 1/2 wavelength plate 8, 9 Polarizing beam splitter

Claims (4)

[Claims] 1. A laser beam is split into a first laser beam and a second laser beam, a first harmonic is generated from the first laser beam, and a second harmonic is generated from the second laser beam. After generating and converting the polarization azimuth of the second harmonic light to an azimuth different from the polarization azimuth of the first harmonic light, the first harmonic light and the second harmonic light whose polarization azimuth are converted are integrated. An optical harmonic generation method characterized by: 2. A laser beam is split into a first laser beam and a second laser beam having polarization directions different from each other, a first harmonic is generated from the first laser beam, and a second laser beam is generated. An optical harmonic generation method characterized in that a second harmonic is generated and the first harmonic light and the second harmonic light are integrated. 3. A laser device for emitting a laser beam, a branching unit for branching the laser beam emitted by the laser device into a first laser beam and a second laser beam, and a first laser beam to a first laser beam. First harmonic generating means for generating harmonic light, second harmonic generating means for generating second harmonic light from the second laser light, and polarization orientation of the second harmonic light as the first harmonic Polarization direction conversion means for converting the polarization direction of the higher harmonic light into a direction different from the polarization direction, and light integrating means for integrating the first higher harmonic light and the second higher harmonic light with the changed polarization direction. Optical harmonic generator. 4. A laser device for emitting a laser beam, and a branching unit for branching the laser beam emitted by the laser device into a first laser beam and a second laser beam having different polarization directions from each other. First harmonic generation means for generating a first harmonic light from the laser light, second harmonic generation means for generating a second harmonic light from the second laser light, and the first harmonic light And an optical integrating means for integrating the second harmonic light, the optical harmonic generating device.