JPH10239723A - Second harmonic generating device and laser application device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a low-noise 2nd harmonic generating device, by controlling the oscillation frequency of basic wave to the minimum value of phase matching wavelength characteristics of nonlinear optical crystal(SHG crystal) and generating a 2nd harmonic. SOLUTION: The exciting beam 31 emitted by a semiconductor laser 11 is converged by a convergence optical system 12 to excite solid-state laser crystal 4. The solid-state laser crystal 4 generates a 1st oscillation wave as the basic wave and a laser resonator consisting of an incidence-side laser mirror 3 as a curvature mirror and a laser mirror 7 which is formed on the projection end surface of the SHG crystal 6 and reflects the 1st oscillation wave generates the basic wave as the 1st oscillation wave. A birefringent filter formed of one crystal plate is used for a wavelength control element 5, and arranged at the Brewster angle to the optical axis and rotated on the optical axis to perform wavelength control, thereby matching the wavelength of the basic wave with the minimum value of phase matching wavelength characteristics of the SHG crystal 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a second harmonic generation device and a laser application device represented by a laser printer device, a fine particle detection device, an optical shaping device, an optical recording device, etc. using the second harmonic generation device. About.

[0002]

2. Description of the Related Art Along with the progress of the advanced information age, in the field of optical recording such as an optical disk device and a laser printer device, there is an increasing demand for a shorter wavelength in order to satisfy a demand for a higher recording density and a higher speed printing. I have. However, as a light source that satisfies the blue region wavelength (wavelength 400 to 480 nm), which is often required at the product level, there are only gas laser devices such as a He-Cd (helium-cadmium) laser device and an Ar (argon) laser device. It was unsuitable for mounting on a large size because of its large power consumption. Also,
The gas laser device is actually mounted as a light source in some laser printer devices, but for the reasons described above, there is a possibility that it will be an obstacle in promoting miniaturization and low power consumption in the future.

To solve this problem, optical second harmonic generation (Seco
nd Harmonic Generation: Hereafter referred to as SHG. ) Has been proposed to shorten the wavelength. This S
The study of the practical application technology of the HG light source has progressed with the increase in the output of the semiconductor laser as the excitation light source. The background is this SH
The G light source does not require a discharge like a conventional gas laser, it is compact and low power consumption, and the high reliability (output stability, long life) of the SHG light source that depends on the output stability and long life of the pumping semiconductor laser ) Can be realized.

As an SHG light source having an output wavelength equivalent to that of the gas laser, for example, as shown in FIG. 8, a nonlinear optical crystal 6 (SHG crystal) is disposed inside a resonator of a solid laser using an oscillation wave of the solid laser as a fundamental wave. Internal resonator type SHG method. In this internal resonator type SHG method, the resonator forming the solid-state laser has a configuration in which mirrors 3 and 7 having high reflection at the oscillation wavelength of the solid-state laser crystal 4 are arranged at both ends, so that reflected return light from outside the resonator is formed. The characteristic is that the fluctuation of the oscillation wavelength due to disturbances such as the above is extremely small.

In recent years, a laser device using a LiSAF (Cr: LiSrAlF ₆ ; chromium-added lithium strontium aluminum fluoride) crystal as a laser crystal oscillating at a wavelength of 750 to 1000 nm in a semiconductor laser-excited wavelength variable solid-state laser device is disclosed in US Pat. , 349.

[0006]

SUMMARY OF THE INVENTION The present inventors use the laser light from a LiSAF crystal pumped by a semiconductor laser as a first oscillation wave (fundamental wave) and use a nonlinear optical crystal as a second oscillation wave to produce SHG light in the blue region. After examining the method of generating the following, the following problems were found.

FIG. 1 is a view showing an example of a second harmonic generator using a LiSAF crystal and a nonlinear optical crystal. Excitation light 31 from the semiconductor laser 11 of FIG. 1 passes through the first laser mirror 3 on the incident side and is incident on the LiSAF crystal 4. The first laser mirror 3 is composed of a mirror film made of a dielectric multilayer film that reflects 99% or more of the fundamental wave 32 oscillated from the LiSAF crystal 4, between the mirror film 7 formed on the emission side of the SHG crystal 6. Constitute a laser resonator.
The SHG light 33 is emitted by controlling the fundamental wave in the resonator to the phase matching wavelength of the SHG crystal 6 by the birefringent filter 5 which is a wavelength control element. The second laser mirror 7 is provided with a coating that reflects 99% or more of the fundamental wave and transmits SHG light.

The problem in FIG. 1 is that the output SHG light contains high frequency component noise. The cause of this noise is considered as a common problem of the internal cavity type SHG laser, and multiple longitudinal oscillations of the fundamental wave oscillating in the resonator cause multiple oscillations in the oscillation intensity distribution of the solid-state laser. It is considered that the fundamental modes become longitudinal modes and their intensities change with each other, so that each fundamental mode causes an intensity competition, and the SHG output fluctuates accordingly. (T.Baer, "Large-amplitude fluctuations
due to longitudal mode copling in diode-pu
mped intracavity-doubled Nd: YAG lasers "J. Opt.
Soc.Am.B3,1175 1986)

[0009]

Means for Solving the Problems The present inventors can solve the above-mentioned conventional problems by converting the longitudinal mode of the fundamental wave oscillation in the resonator into a single mode. However, even if the mode is not necessarily set to the single mode, the SHG noise can be reduced. It has been found that it can be reduced. That is, the present inventors have found out that the SHG noise can be sufficiently reduced by strictly grasping the phase matching wavelength characteristic of the nonlinear optical crystal that generates SHG light and adjusting the fundamental wave wavelength in the resonator to the characteristic. That is, the present invention provides a wavelength control element for controlling the wavelength of a first oscillation wave and a non-linear optical element for wavelength-converting the first oscillation wave as a fundamental wave into a second harmonic wave as a second oscillation wave. A second harmonic generation device including a resonator having a crystal, wherein the second harmonic is generated by controlling the oscillation wavelength of the fundamental wave to a minimum value of the phase matching wavelength characteristic of the nonlinear optical crystal. It is a second harmonic generator.

In addition, the oscillation wavelength of the fundamental wave is controlled to a minimum value within a wavelength range from the maximum value of the generation efficiency of the second harmonic to 20% of the maximum value in the phase matching wavelength characteristic of the nonlinear optical crystal to obtain the second harmonic. A second harmonic generation device characterized by generating a harmonic.

Also, LBO (LiB ₃ O ₅ ), BBO (β-BaB ₂ O ₄ ) and BBO (β-BaB ₂ O ₄ ),
_{CLBO (CsLiB 6 O 10),} CBO (CsB 3 O 5)
A second harmonic generator using at least one of the following.

[0012] One or three birefringent crystals inclined at Brewster's angle are disposed between the LiSAF crystal and the nonlinear optical crystal as a wavelength control element for controlling the wavelength of the first oscillation wave. Thus, the wavelength can be easily controlled. It is known that when a birefringent crystal is inserted into a resonator as a wavelength control element for controlling the oscillation wavelength of a solid-state laser crystal, the oscillation threshold of the resonator increases due to loss. This is because the oscillation characteristics of the laser resonator depend on the loss in the resonator, and the insertion of a birefringent crystal increases the loss and increases the threshold before oscillation. Furthermore, if the transmission bandwidth of the wavelength control element is narrow, the amount of light that is not transmitted but is reflected by the wavelength control element and becomes a loss increases, so that the loss increases and the oscillation threshold value increases. As a countermeasure against this, there is an effect that the transmission wavelength width can be widened by reducing the thickness of the birefringent crystal, and the oscillation threshold can be lowered by reducing the loss.

Further, when a single crystal plate is used as the birefringent crystal, the wavelength interval of laser oscillation with respect to the thickness of the birefringent crystal is:
As the thickness increases, the interval between laser oscillation wavelengths decreases. This is because if the thickness is large, the interval between adjacent laser oscillation wavelengths becomes narrow, and a laser such as a LiSAF laser that can oscillate in a wide band may oscillate at two or more oscillation wavelengths at the same time. The reflection bandwidth of a general laser mirror is about 50 nm.
Since the oscillation wavelength interval of about nm or more is required, the thickness of the birefringent crystal must be 3 mm or less. If the thickness of the crystal is too thin, wavelength control becomes difficult. Desirably, it is 0.4 mm or more. Here, the laser oscillation wavelength interval is an interval between wavelengths at which laser oscillation can be performed at the same time, and depends on a gain characteristic with respect to the wavelength of the laser medium. If the gain characteristic is wide, the wavelength range in which laser oscillation can be performed is wide, so that laser oscillation may occur simultaneously depending on the transmission wavelength interval of the wavelength control element. In the present invention, by using a plurality of birefringent crystals in combination, it is possible to control the oscillation wavelength width and interval with higher accuracy, and it is possible to increase the accuracy of adjusting the phase matching wavelength of the nonlinear optical crystal, which is preferable.

The solid laser crystal is made of LiSAF (Cr: LiSrAlF
₆ ; when a chromium-added lithium strontium aluminum fluoride crystal is used, the first oscillation wave has a wavelength of 8
It can be generated in the range of 00 to 900 nm, and can generate the second oscillation wave in the blue range (wavelength of 400 to 450 nm). In addition, LiSGAF (Cr: L
iSrGaF ₆ ; when chromium-added lithium strontium gallium fluoride crystal is used, the first oscillation wave is 80
It can be generated in the range of 0 to 1000 nm and has a wavelength of 4
A second oscillation wave of 00 to 500 nm can be generated. By employing these means, it is possible to realize a small-sized, low-power-consumption, low-noise second harmonic generator which is a feature of the SHG method.

In the present invention, a useful laser application device such as a laser printer device, a fine particle detection device, an optical shaping device, an optical recording device, etc. can be constituted by using the above-mentioned second harmonic generation device having the above features.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) FIG. 1 is a diagram for explaining an embodiment of the present invention. The excitation beam 31 emitted from the semiconductor laser 11 is condensed by the condensing optical system 12 and excites the solid-state laser crystal 4. The semiconductor laser 11 is AlGa manufactured by US SDL.
The output power is 500 mW and the oscillation wavelength is 670 nm using an InP semiconductor laser. Further, the condensing optical system 12 used two cylindrical lenses and a single lens (f = 30 mm). The solid-state laser crystal 4 to be excited generates a first oscillation wave as a fundamental wave, and a first oscillation wave formed on the exit side face of the SHG crystal 6 with the first laser mirror 3 on the incident side as a curvature mirror. A fundamental wave, which is a first oscillation wave, is generated by a laser resonator composed of a reflecting second laser mirror 7. A laser crystal 4, a wavelength control element 5, and an SHG crystal 6 are arranged in the laser resonator. The first laser mirror 3 transmits 85% or more of the excitation light wavelength from the semiconductor laser and has a total reflectance of 99% or more with respect to the fundamental wavelength (hereinafter simply referred to as HR; High-Reflection). Coated. This resonator structure was a concave flat resonator, and the radius of curvature of the first laser mirror 3 was 20 mm, and the resonator length (L) was 18 mm. As the laser crystal 4, a rectangular parallelepiped LiSAF crystal (3 mm long × 3 mm wide × 5 mm thick) with a Cr addition amount of 1.5 mol% is used, and the crystal end face has a reflectance of 2 with respect to the excitation light wavelength and the fundamental wave wavelength. % Of anti-reflection coating. The SHG crystal 6 is a rectangular parallelepiped LBO crystal having a length of 3 mm × a width of 3 mm × a thickness of 5 mm.

The emission side, ie, the rear end face of the LBO crystal 6 is coated with an HR coating having a reflectance of 99% or more for the fundamental wavelength and an AR coating having a reflectance of 1% or less for the SHG wavelength, and a second laser mirror is provided. 7 was set. Also, LB
An AR coating having a reflectance of 0.2% or less with respect to the fundamental wavelength was applied to the incident side of the O crystal 6, that is, the front end face. As the wavelength control element 5, a birefringent filter made of a single quartz plate having a thickness of 1 mm is used. The wavelength is controlled by rotating the optical axis around the optical axis by arranging it at a Brewster angle with respect to the optical axis.
The wavelength of the fundamental wave is adjusted to the phase matching wavelength of the LBO crystal 6, which is an HG crystal.

A nonlinear optical crystal L such that the SHG conversion is performed by type I phase matching at a fundamental wavelength of 860 nm.
FIG. 2 shows a result of measuring a phase matching wavelength characteristic of the LBO crystal in which a BO (LiB ₃ O ₅ ) crystal was cut to 26.2 ° and the cut length was 5 mm. 2, the half width of the phase matching wavelength is as wide as about 4 to 5 nm. FIG. 3 shows the result of measuring the phase matching wavelength characteristic in more detail. FIG.
It can be seen from the above that the phase matching wavelength characteristic repeatedly repeats a maximum and a minimum at intervals of 0.02 to 0.06 nm.
The cut out LBO crystal was placed in the second harmonic generator of FIG. 1 described above, and SHG light was generated. The result of FIG. 4 was obtained. FIG. 4 shows that the SHG output is 20
The oscillation wavelength characteristic of the fundamental wave when tuned to a wavelength such that it becomes mW (maximum) is a Michelson interferometer type high-resolution optical spectrum analyzer (Advantest Co., Ltd .: Model Q
8347), and four longitudinal modes oscillate at intervals of about 0.05 nm. FIG. 5 shows high-frequency noise characteristics when the SHG output at this time was received by a high-speed Si photodiode and the output was measured by an RF spectrum analyzer. In FIG. 5 and FIG. 7 described later, (a) is a zero level when light is not received, and (b) is a frequency characteristic when SHG light is received. Noise (shaded area) in a frequency region of 2 MHz or less is shown. Has occurred.

The wavelength control element 5 sets the fundamental wave wavelength of FIG.
As shown in the figure, when the position was slightly shifted from about 0.025 nm, the SHG output dropped to about 12 mW (about 60%). FIG. 7 shows the high-frequency noise characteristics at this time. FIG.
It can be seen from FIG. 5 that the noise at 2 MHz or less has been significantly reduced. That is, only 0.02
By shifting the oscillation wavelength of the 5 nm fundamental wave, the SHG output is reduced by the shift, but it can be seen that high frequency noise is greatly reduced. This is presumably because the longitudinal modes of the four fundamental waves are phase-matched to the minimum values in the phase matching wavelength characteristics in FIG. FIG. 5 shows a case where the oscillation wavelength of the fundamental wave matches the maximum value of the phase matching wavelength of the SHG crystal, and mode competition occurs intensely, so that a lot of noise is generated. As shown in the figure, SH in the wavelength range where the SHG efficiency is slightly reduced
It has been found that the degree of mode competition can be reduced and the noise can be reduced by adjusting the wavelength of the fundamental wave to the minimum value of the phase matching wavelength characteristic of the G crystal. S effective for this noise reduction
The preferred range of the HG efficiency is the hatched area A in FIG. That is, it is preferable to adjust the oscillation wavelength of the fundamental wave to a minimum value within a wavelength range from the maximum value of the generation efficiency of the second harmonic to 20% of the maximum value in the phase matching wavelength characteristic of the nonlinear optical crystal. For example, SHG output is 1mW
In this case, a low noise SHG output can be easily obtained by selecting the minimum value in the region A, which is extremely practical. This noise reduction effect is achieved by BBO (β-BaB ₂ O ₄ ) and CLBO (Cs
LiB ₆ O ₁₀ ) and CBO (CsB ₃ O ₅ ) can also be obtained in the same manner as LBO.

In the present invention, for example, the first laser mirror 3, the laser crystal 4, and the wavelength control element 5 of FIG. 1 are installed on the same structural member 8, the SHG crystal 6 is installed on the structural member 9,
It is preferable to fix them on a Peltier element which is a temperature control element so as to control the temperature of the entire resonator. Even when the temperature is slightly shifted to ± 0.5 ° C. or less when the SHG output is maximum, the wavelength is slightly shifted and a low-noise SHG output can be obtained because the resonator condition changes. Further, the excitation light 31 is converted into X, Y, and Z by a focusing optical system.
In either of the two directions or slightly more than two directions, the whole of the resonator structural members 8 and 9 may be moved in X, Y and Z directions.
Even if it is slightly moved in any of the directions or in two or more directions by about several μm, the resonator condition changes, so that the oscillation wavelength shifts and a low-noise SHG output can be obtained.

(Embodiment 2) FIG. 9 is a view for explaining an example in which the second harmonic generator according to the present invention is applied to a laser printer. The SHG output 33 emitted from the second harmonic generator described in the first embodiment is an acousto-optic (hereinafter AO: Ac).
out-of-optical) modulator 401, folding mirror 402, beam expander 403, rotating polygon mirror 4
04, and is condensed on the photosensitive drum 406 through the fθ lens 405. The AO modulator 401 controls the SH according to the image information.
The G output 33 is modulated, and the rotating polygon mirror 404 scans the beam in the horizontal (in the plane of the paper) direction. With this combination, two-dimensional information is recorded on the photosensitive drum 406 as a partial potential difference. The photosensitive drum 406 rotates by attaching toner according to the potential difference, and can reproduce information on a recording sheet.

(Embodiment 3) FIG. 10 shows an example in which the second harmonic generator of the present invention is applied to a particle inspection apparatus,
FIG. 3 is a diagram illustrating an apparatus for detecting fine particles on a wafer. The SHG output 33 emitted from the second harmonic generator described in the first embodiment enters the optical head 500, is condensed to the diffraction limit by the turning mirror 502 and the condensing lens 503, and is irradiated on the Si wafer 501. Light 50 scattered from light collected to a wavelength order of 0.4 μm
5 is received at each position by the photodetector 504, the intensity is recorded, and the distribution of fine particles on the wafer surface is measured by moving the optical head 500 from the center to the end of the rotating wafer 501. It is useful because it can detect fine particles up to about 1/10 or less of the SHG wavelength.

(Embodiment 4) FIG. 11 is a view for explaining an example in which the second harmonic generator according to the present invention is applied to an optical shaping apparatus. The second harmonic generator described in Example 1 was used as a light source. Example 1 A container was filled with a blue cured resin 601 and
The SHG output 33 emitted from the second harmonic generation device described in (2) is two-dimensionally scanned as a laser beam 603 on the liquid surface by a mirror 602. At this time, the blue cured resin 60
No. 1 cures only at the liquid surface portion where the light is absorbed. When the formation of one fault is completed, the elevator 604 descends, and continuously forms the next fault. By this operation, a three-dimensional model 605 having a desired shape was produced.

(Embodiment 5) FIG. 12 is a view for explaining an example in which the second harmonic generation device of the present invention is applied to an optical recording device. The optical disk device employs a magneto-optical recording method. SH emitted from the second harmonic generator described in the first embodiment
The G laser light 33 becomes parallel light after being expanded by the beam expander 701. The light partially rejected by the beam splitter 702 is captured by a front monitor photodetector 708 to monitor the SHG laser beam 33 and control the output. The beam transmitted through the beam splitter 702 is condensed on a medium 705 by a condensing optical system 704, and the reflected light is partially reflected by a beam splitter 702 and then separated into two beams by a beam splitter 706 to be separated into two light detectors. 70
7 to perform auto focus and signal detection. A constant magnetic field is applied to the medium 705 and SH
Recording is performed by modulating the G laser beam 33, raising the focal point temperature to the Curie temperature of the medium 705, and reversing the magnetization. When the laser beam is ON, recording is performed by reversing the magnetization of the medium. The recording frequency was 10 MHz. Also, at the time of the reproduction signal, a good reproduction signal was obtained using the same SHG laser beam.

[0025]

As described above, the present invention employs a characteristic configuration in which the oscillation wavelength of the fundamental wave is controlled to the minimum value of the phase matching wavelength characteristic at which the second harmonic of the nonlinear optical crystal is generated. A high-efficiency, low-noise second harmonic generation device and various laser application devices can be realized, and are industrially extremely useful.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an embodiment of the present invention.

FIG. 2 is a diagram illustrating phase matching wavelength characteristics of an LBO crystal.

FIG. 3 is a diagram illustrating details of a phase matching wavelength characteristic of an LBO crystal.

FIG. 4 is a diagram illustrating an oscillation wavelength of a conventional fundamental wave.

FIG. 5 is a diagram illustrating a high-frequency noise characteristic of a conventional SHG output.

FIG. 6 is a diagram illustrating an oscillation wavelength of a fundamental wave according to the present invention.

FIG. 7 is a diagram illustrating the high-frequency noise characteristics of the SHG output according to the present invention.

FIG. 8 is a diagram illustrating an internal cavity type SHG laser device.

FIG. 9 is a diagram illustrating an example of a laser printer according to the present invention.

FIG. 10 is a diagram illustrating an example of a particle inspection apparatus according to the present invention.

FIG. 11 is a diagram illustrating an example of an optical shaping apparatus according to the present invention.

FIG. 12 is a diagram illustrating an example of the optical recording device of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 11 Semiconductor laser, 12 Condensing optical system, 3rd laser mirror, 4 laser crystal, 5 wavelength control element, 6
SHG crystal, 7 second laser mirror, 8 structural members,
9 structural members, 31 excitation beam, 32 fundamental beam, 33 SHG output, 401 AO modulator, 402,
502 folding mirror, 403 and 701 beam expanders, 404 rotating polygon mirror, 405 fθ lens, 4
06 photosensitive drum, 500 optical head, 501 wafer, 503 condenser lens, 504, 707, 708
Photodetector, 505 scattered light, 601 blue cured resin, 6
02,703 mirror, 603 laser beam, 604 solid model, 702,706 beam splitter, 704
Condensing optical system, 705 Optical disk medium.

Claims (10)

[Claims] 1. A wavelength control element for controlling a wavelength of a first oscillation wave, and a non-linear optic for wavelength-converting the first oscillation wave as a fundamental wave into a second harmonic wave as a second oscillation wave. A second harmonic generation device comprising a resonator having a crystal, wherein the second harmonic is generated by controlling the oscillation wavelength of the fundamental wave to a minimum value of the phase matching wavelength characteristic of the nonlinear optical crystal. Second harmonic generator. 2. The oscillation wavelength of the fundamental wave is controlled to a minimum value within a wavelength range from the maximum value of the generation efficiency of the second harmonic to 20% of the maximum value in the phase matching wavelength characteristic of the nonlinear optical crystal. The second harmonic generator according to claim 1, wherein the second harmonic is generated. 3. The method according to claim 1, wherein the nonlinear optical crystal is LBO (LiB ₃
O ₅ ), BBO (β-BaB ₂ O ₄ ), CLBO (CsL
_3. The second harmonic generator according to claim 1, wherein at least one of iB ₆ O ₁₀ ) and CBO (CsB ₃ O ₅ ) is used. 4. 4. A wavelength control element for controlling the wavelength of the first oscillation wave, wherein the wavelength control element has a Brewster angle of one to three.
The second harmonic generator according to any one of claims 1 to 3, wherein one of the birefringent crystals is used. 5. A solid-state laser crystal disposed in the resonator, LiSAF (Cr: LiSrAlF ₆ ; chromium-doped lithium strontium aluminum fluoride) or LiSAF.
GaF:; second harmonic generating device according to any one of claims 1 to 4, wherein the use of (Cr LiSrGaF ₆ lithium fluoride, strontium gallium chromium added). 6. A laser application device using the second harmonic generator according to any one of claims 1 to 5. 7. The laser application device according to claim 6, wherein the laser application device is a laser printer device. 8. The laser application device according to claim 6, wherein the laser application device is a particle detection device. 9. The laser application apparatus according to claim 6, wherein the laser application apparatus is an optical shaping apparatus. 10. The laser application device according to claim 6, wherein the laser application device is an optical recording device.