JPH09116219A - Laser light generating equipment and laser application equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize output by constituting a resonator in such a manner that the extinction ratio of a fundamental wave light which is emitted from a laser crystal having a wavelength sweeping range larger than or equal to a specific value and resonated in a resonator is equal to a specific value. SOLUTION: LiSAF crystal is used as a laser crystal 21. The left end surface 24 of the crystal is subjected to nonreflection coating whose reflectivity to excitation wavelength is at most 2%, and subjected to high reflection coating whose reflectivity to fundamental wavelength is at least 99%. In the right vicinity of the laser crystal 24, a double refraction filter 9 is arranged and so inclined that the incident surface contains the (c) axis of the laser crystal 24 and the optical axis and make the Brewster angle to the optical axis. In the right vicinity of the double refraction filter 9, nonlinear optical crystal 22 is arranged. By adjusting the optical axis of the laser crystal 21, the double refraction filter 9 and the nonlinear optical crystal 22, and setting the quenching ratio to be 3000 (at least 1000), SHG having a wavelength of 430nm is stably obtained with output error of ±6% or less, and stabilization of output is possible. SHG is second harmonic wave of a light.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of optoelectronics, and more particularly to a laser device used in a laser printer device, a particle counter device, a medical inspection device, a stereolithography device, an optical disk device and the like.

[0002]

2. Description of the Related Art With the progress of the advanced information age, demands for shorter wavelengths are increasing in order to satisfy the demands for recording density improvement and high speed printing in computer peripheral laser application devices such as optical disc devices and laser printer devices. There is. However, gas laser devices such as He-Cd (helium-cadmium) laser devices and Ar (argon) laser devices are the only light sources that satisfy the highly demanded blue region at the commercialization level. There was a problem that it could not be adapted to the mainstream office environment and residential environment.

Further, the laser device has a low conversion efficiency from input power to laser light, and most of the power consumption becomes heat, so that a cooling means is required and the size of the laser application device becomes large. Further, there is a problem that the deviation of the optical system due to the vibration of the cooling means deteriorates the reliability of the laser application device. Furthermore, due to deterioration of the gas, it is necessary to replace the gas or the tube containing it, which shortens the continuous use time, which shortens the continuous use time of the laser application device, and also requires replacement of the optical system when replacing the gas tube. There was a problem that adjustment was necessary.

On the other hand, a first laser beam (hereinafter referred to as a fundamental wave in some cases) which is an oscillating wave from the solid-state laser by inserting a nonlinear optical crystal inside the resonator of the solid-state laser.
By applying a wavelength conversion technique such as optical second harmonic generation (hereinafter simply referred to as SHG; Second Harmonic Generation) that converts the wavelength to 1/2 of Attempts have been made to overcome the problems of gas lasers. For example, Cr: LiSrAlF ₆ (chromium-added lithium strontium aluminum fluoride, which is a laser crystal capable of oscillating in the wavelength range of 800 to 900 nm band;
SH of solid-state laser using (hereinafter simply referred to as LiSAF)
The G method was proposed. (F. Ballenwall, P.J.,
F. S. Alan, “Blue light source with wavelength tuning by internal cavity type frequency doubling of Cr-doped LiSrAlF ₆ laser”
Applied Physics Communication Vol. 61, No. 20, pp. 2381 (1992), F. Balembois,
P. Georges, F. Salin, G. Roger, and A. Brun, "Tunable b
lue light source by intracavity frequency doubling
of a Cr-dope LiSrAlF ₆ laser ", Appl.Phys.Lett., Vol.
61, No. 20, p. 2381 (1992)). However, Balembois et al. Uses a Kr (krypton) laser as an excitation light source for a LiSAF crystal and does not solve the above-mentioned problems of He-Cd and Ar lasers.

On the other hand, the present inventors have proposed an SHG method of a LiSAF laser using a semiconductor laser as an excitation light source (Miyai, Makio, Furukawa, Sato, “Semiconductor laser excited Cr: LiS
10m using internal cavity type SHG method of rAlF ₆ laser
"W Blue Laser", 42nd Joint Lecture on Applied Physics,
28p-ZQ-10 (1995)).

[0006]

In a solid-state laser having a relatively wide wavelength sweep width of 10 nm or more as represented by LiSAF, for example, a wavelength sweep element causes a loss in a specific wavelength region to be smaller than a loss of another wavelength. It is configured to be relatively low to oscillate the laser light in the specific wavelength range. However, as a result of examination by the present inventors, when a non-linear optical crystal, that is, an element that converts a certain wavelength into a different wavelength, such as the specific SHG, is inserted in the resonator, that is, when a new loss occurs. To
It was found that the laser output and SHG output became unstable.

The present invention takes this point into consideration.
An object of the present invention is to provide a laser device whose output is stabilized in SHG using a solid-state laser having a relatively wide wavelength sweep width as represented by AF.

[0008]

That is, the present invention provides a laser light generator comprising a laser crystal having a wavelength sweep range of at least 10 nm or more and a resonator including the laser crystal, wherein the laser crystal emits light. In the laser light generator, the resonator is configured such that the extinction ratio of the fundamental wave light resonated in the resonator is 1000 or more.

Further, a laser crystal having a wavelength sweep range of at least 10 nm or more in the resonator, a wavelength sweep element for sweeping an oscillation wavelength of light emitted from the laser crystal, and a part of the light of the fundamental wave light. It is a laser light generator characterized by having a non-linear optical crystal for converting into a second laser light of a different wavelength. Further, as the laser crystal, Cr: LiSrAlF ₆ (abbreviated as chromium-added lithium strontium aluminum fluoride: LiSAF), Cr: LiSrGaF
₆ (lithium strontium gallium fluoride with chromium added), Ti: Al ₂ O ₃ (sapphire with titanium added), Cr: BeAl ₂
Use one of O ₄ (Alexandrite).
Further, as the non-linear optical crystal, LiB ₃ O ₅ (lithium triborate: abbreviated as LBO), CsLiB ₆ O ₁₀ (lithium cesium borate), β-BaB ₂ O ₄ (barium borate), KNbO ₃ (potassium niobate). , KLiNbO (lithium-niobate-potassium), or LiIO ₃ (lithium iodate).

The present inventors have used LiSAF as an example of a solid-state laser having a relatively wide wavelength sweep width of 10 nm or more.
The oscillation characteristics of the laser were examined. As a result, it was found that there is a relation between the polarization state of the solid-state laser output and the instability of the laser output. FIG. 1 is a diagram for explaining the correlation between the extinction ratio of the fundamental wave of the LiSAF laser and the SHG output stability. The extinction ratio can be changed, for example, by adjusting the angle between the C axis of the LiSAF crystal and the C axis of the LBO crystal that is a nonlinear optical crystal. From FIG. 1, the fluctuation of the SHG output is small in the region where the extinction ratio is considered to be linearly polarized light, and the fluctuation increases as the extinction ratio decreases. Quantitatively, when the extinction ratio decreases from 3200 to 900, the output fluctuation rate changes from 0.5% to 1.2%.
It turns out that it increases to%.

FIG. 2 is a diagram for explaining the correlation between the extinction ratio of the LiSAF laser and the number of longitudinal modes of the oscillation wavelength. It can be seen from FIG. 2 that when the extinction ratio decreases from 3200 to 900, the number of longitudinal modes increases from 3 to 9. Further, it was found that when the extinction ratio is reduced to 1000 or less, the longitudinal mode is separated into two groups of 5 and 4.

Generally, it is considered that noise due to mode competition easily occurs when the number of longitudinal modes of the oscillation wavelength increases. Therefore, a state in which the number of longitudinal modes is small and the extinction ratio is large is desirable for stabilizing the SHG output. Further, since it is considered that stability of ± 1% or less is required for a laser application device, it is understood from FIGS. 1 and 2 that the extinction ratio of solid-state laser output is preferably 1000 or more.

FIG. 3 is a diagram for explaining an example of the cause of the decrease in extinction ratio. As a laser crystal having a wavelength sweep width of at least 10 nm, other than a LiSAF crystal
There are laser crystals such as Cr: LiSrGaF ₆ (lithium strontium gallium fluoride with chromium added), Ti: Al ₂ O ₃ (sapphire with titanium added), and Cr: BeAl ₂ O ₄ (alexandrite), all of which are anisotropic. Therefore, it oscillates with linearly polarized light. In the configuration shown in FIG. 3, a birefringent filter is used as a typical wavelength sweep element. The birefringent filter is arranged at a Brewster angle at which loss is theoretically 0 in the polarization direction (in the plane of the paper) determined by the anisotropy of the laser crystal. Here, the birefringent filter is a crystal cut so that crystal axes having different refractive indices are in-plane and polished so that the parallelism between two planes and the surface accuracy are improved. If this plane is arranged at Brewster's angle with respect to the optical axis in the resonator, the polarization direction of the light emission of a specific wavelength will pass through the birefringent filter after being rotated an integral number of times, and This wavelength can be selected to cause laser oscillation by utilizing the effect of passing only a specific polarization direction without loss in the Brewster window formed by. Here, since the laser light of the above-mentioned specific wavelength reciprocates in the resonator many times, it becomes a linearly polarized light having a high extinction ratio in the Brewster window.

Now, let us consider a case where a loss is applied to a resonator at a specific wavelength. For example, consider a case where a non-linear optical crystal is arranged in a resonator and a part of the laser light of the specific wavelength (hereinafter referred to as a fundamental wave) is converted into SHG. In the first place, the fundamental wave is the light having the wavelength with the smallest loss amplified by the birefringent filter in the resonator. When a part of this fundamental wave is converted into SHG, a loss occurs in the fundamental wave.

FIG. 4 is a diagram showing a loss state near the fundamental wavelength and an oscillation wavelength. FIG. 4A shows that the fundamental wave oscillates at the wavelength with the smallest loss before being converted into SHG. FIG. 4B shows the state of the fundamental wave when the SHG is oscillating. In FIG. 4B, the loss of the fundamental wave wavelength converted to SHG (oscillation wavelength of FIG. 4A) increases, and the loss of the wavelengths of the adjacent wavelengths becomes relatively small. For this reason, the fundamental wavelength shifts to either of the adjacent second candidate wavelengths with small loss. Since the polarized light of the second candidate wavelength rotates and passes through the birefringent filter with a slight deviation from an integral multiple, it passes from orthogonally polarized light to elliptically polarized light in the anisotropic laser crystal and nonlinear optical crystal. The ellipticity of polarized light is called an extinction ratio. When light with a low extinction ratio enters a laser crystal having anisotropy, laser lights with different polarization states oscillate simultaneously. The resonator lengths are slightly different between different polarizations, and a difference occurs in the wavelength of the laser light depending on each polarization state. That is, as in the case of FIG. 2, the multimode of the longitudinal mode progresses, and the mode competition noise of the fundamental wave due to the multimode is generated. Such a decrease in the extinction ratio is a phenomenon that can occur even when the polarization plane determined by the anisotropy of the laser crystal and the Brewster window are not arranged at an accurate angle in FIG.

Further, when the shift from the fundamental wave wavelength to the second candidate wavelength explained in FIG. 4 is out of the wavelength allowable range of the SHG phase matching of the nonlinear optical crystal, the SHG output is not performed and the SHG output is obtained. It becomes 0. At this time, FIG.
The cause of the loss seen in FIG. 4 disappears and the state shown in FIG. Since the fundamental wavelength shift is repeated in this manner, the SHG output fluctuation is repeated. Also,
It is preferable that the laser light generator is mounted on a laser application device such as a laser printer device, a particle counter device, and an optical disk device.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) FIG. 5 is a view for explaining an embodiment of the present invention and shows the structure of a laser device. Semiconductor laser 11
The excitation beam 31 emitted from the laser beam is condensed inside the laser crystal 21 by the condensing optical system including the collimator 12 and the single lens 14, and excites the laser crystal 21. The semiconductor laser 11 is an AlGaInP-based semiconductor laser manufactured by SDL (Spectra Diode Lab.), And has an output of 500 mW and an oscillation wavelength of 680 n.
m. The excited laser crystal 21 oscillates a first laser beam 32 in the solid-state laser resonator 20 formed of an incident side resonator mirror 24 and an output mirror 25 formed on the left end surface thereof. At this time, the resonator structure 20 is a plano-concave resonator, the radius of curvature of the output mirror 25 is 150 mm, and the effective optical path length is slightly shorter than the radius of curvature.

The laser crystal 21 has a Cr addition amount of 1.5 mo.
A 1% LiSAF crystal (φ3 × 5 mm) was used. The left end face 24 of the crystal has a non-reflection (hereinafter simply referred to as AR; Anti-Reflection) coating having a reflectance of 2% or less with respect to the excitation wavelength,
A high reflection (hereinafter simply referred to as HR; High-Reflection) coating having a reflectance of 99% or more with respect to the fundamental wave wavelength was applied. A with a reflectance of 2% or less for the fundamental wavelength on the right end face of the crystal
R coating was applied. A birefringent filter 9 of a wavelength selection element is arranged immediately to the right of the laser crystal 24, and its incident surface includes the c-axis and the optical axis of the laser crystal 24 and is tilted so as to have a Brewster angle with respect to the optical axis. It was The wavelength of the fundamental wave was selected by rotating the birefringent filter 9 with respect to the normal to the incident boundary surface.

A nonlinear optical crystal 22 is arranged immediately to the right of the birefringent filter 9. LBO as a nonlinear optical crystal
Is cut out in the azimuth where the phase matching of TYPEI can be taken at a wavelength of 860 nm, and the reflectance at 860 nm is 1%.
The following AR coating was used. Also,
The non-linear optical crystal 22 is CLBO, BBO, KN, KL
Either N or LIO may be used. The laser crystal 21, the birefringent filter 9, and the non-linear optical crystal 22 are arranged so that the distance between them is as short as possible to improve the wavelength conversion efficiency. First, the optical axes of the laser crystal 21, the birefringent filter 9, and the nonlinear optical crystal 22 are adjusted to an extinction ratio of 3000 as shown in FIG.
G was stably obtained within an output error of ± 6%.

Next, for comparison, the optical axes of the laser crystal 21, the birefringent filter 9 and the non-linear optical crystal 22 are adjusted as shown in FIG.
The extinction ratio was adjusted to 800 as shown in FIG. Wavelength 4
Although SHG of 30 nm was obtained, the output was not stable and an output error of ± 23% was observed. The extinction ratio of the fundamental wave was higher and the SHG output was stable compared to the case where the laser crystal, the nonlinear optical crystal, and the birefringent filter were arranged in that order.

(Embodiment 2) FIG. 7 also explains an embodiment of the present invention, and shows the structure of a laser device as in FIG.
The excitation light source 11 and the focusing optical system are the same as in (Example 1). Inside the resonator, PBS (polarizing beam splitter) 2
It is the same except that 7 is inserted. PBS2
7 is an AR for the fundamental wave on two planes orthogonal to the optical axis.
A coating was applied, and the laser crystal 22 was arranged so that 99% or more of the fundamental wave of polarized light parallel to the c-axis direction was transmitted.
This PBS27 increases the extinction ratio of the fundamental wave,
G output error was reduced and stable output was obtained. Also in this system, the nonlinear optical crystal 22 is CLBO or BB.
Any of O, KN, KLN, or LIO may be used.

(Embodiment 3) FIG. 8 also explains one embodiment of the present invention, and shows the structure of a laser device as in FIG.
The excitation light source 11 and the condensing optical system are the same as in (Example 1). The laser crystal 21 and the non-linear optical crystal 22 used in the resonator are made of the same material as that of the first embodiment, but the end faces on the birefringent filter 9 side are cut to Brewster's angle. A total of 4 of these two surfaces and the birefringent filter surface.
Due to the Brewster window on the surface, the extinction ratio of the fundamental wave was increased, and the SHG output error was small and stable output was obtained.
Also in this system, the nonlinear optical crystal 22 is made of CLBO, B
Any of BO, KN, KLN, or LIO may be used.

(Embodiment 4) FIG. 9 is a diagram for explaining an application example in which one embodiment of the present invention is used in a laser printer. The SHG laser output 33 emitted from the SHG laser light source 100 described with reference to FIG.
After passing through an O; Acousto-Optical) modulator 51, a beam expander 52, a rotary polygon mirror 53, and an fθ lens 54, the light is condensed on a photosensitive drum 55. The AO modulator 51 modulates the SHG output 33 according to the image information, and the rotary polygon mirror 53
Scans horizontally (in the plane of the paper). With this combination, the two-dimensional information is recorded on the photosensitive drum 55 as a partial potential difference. The photosensitive drum 55 attaches toner according to the potential difference and rotates to reproduce information on a recording sheet. At this time, the photosensitive body applied to the photosensitive drum 55 was selenium (Se), and the output of the SHG laser light source 100 was set to 10 mW. Here, the photoreceptor may be other than Se.

(Embodiment 5) FIG. 10 is a view for explaining an application example in which an embodiment of the present invention is applied to an optical disk device. The optical disk device adopts a magneto-optical recording method. SH
SHG laser output 3 emitted from the G laser light source 100
3 is collimated after being expanded by the beam expander 52. The light partially bounced by the beam splitter 72 is captured by the front monitor 73. The beam that has passed through the beam splitter 72 is condensed on the medium 75 by the condensing optical system 74, and the reflected light is partially reflected by the beam splitter 72 and then separated into two beams, which are respectively taken into two detectors 76. . The SHG laser output 33 is displayed on the front monitor 73.
Is monitored to control the SHG laser output 33. Further, two detectors 76 after the beam splitter 72 perform autofocus, tracking, and signal detection. A constant magnetic field was applied to the medium 75, and recording was performed by modulating the SHG laser output 33 to raise the focus temperature to the Curie temperature of the medium 75 and reversing the magnetization. When the output is ON, the magnetic field of the medium is reversed, and when the output is OFF, the magnetic field is not reversed and signal recording is possible. The recording frequency was 10 MHz. Further, when reproducing the signal, the same SHG laser light source 100 as that used for recording was used to obtain a good reproduced signal.

[0025]

The present invention provides a second harmonic generation laser (S) using a laser crystal having a wavelength sweep range of 10 nm or more.
It is possible to improve the output stability of the (HG laser) and to apply it to an apparatus that requires a stable laser.

[Brief description of the drawings]

FIG. 1 is a diagram showing a correlation between an extinction ratio of a fundamental wave and stability of an SHG output.

FIG. 2 is a diagram showing an extinction ratio of a fundamental wave and a spectrum of a fundamental wave longitudinal mode at that time.

FIG. 3 is a diagram showing a method of using a birefringent filter as a wavelength selection element for a fundamental wave.

FIG. 4 is a diagram showing changes with respect to the wavelength in the resonator and the wavelength of the oscillation laser with and without the nonlinear optical crystal.

FIG. 5 is a diagram for explaining an embodiment of a laser device of the present invention.

FIG. 6 is a diagram showing an extinction ratio of a fundamental wave and an output stability of SHG.

FIG. 7 is a diagram for explaining one embodiment of the laser device of the present invention.

FIG. 8 is a diagram for explaining an embodiment of a laser device of the present invention.

FIG. 9 is a diagram for explaining an application example in which an embodiment of the present invention is used in a laser printer device.

FIG. 10 is a diagram for explaining an application example in which an embodiment of the present invention is applied to an optical disc device.

[Explanation of symbols]

9 birefringence filter, 11 semiconductor laser, 12
Collimator, 14 single lens, 20 resonator, 21
Laser crystal, 22 Non-linear optical crystal, 23 Wavelength tuning element, 24 Cavity mirror, 25 Cavity mirror, 26 Cavity mirror, 27 PBS (deflecting beam splitter), 31 Excitation beam, 32 fundamental wave, 33 SHG laser, 51 AOM , 52 beam expander, 53 rotating polygon mirror, 54 fθ
Lens, 55 Photosensitive drum, 72 Beam splitter, 73 Front monitor, 74 Condensing optical system, 75
Medium, 76 detector, 100 SHG light source.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication location // B41J 2/44 B41J 3/00 D (72) Inventor Yasunori Furukawa 5200 Mikageri, Kumagaya, Saitama Prefecture Hitachi Metals Co., Ltd. Magnetic Materials Research Center

Claims (8)

[Claims] 1. A laser light generator comprising a laser crystal having a wavelength sweep range of at least 10 nm or more and a resonator including the laser crystal, wherein a fundamental light emitted from the laser crystal and resonated in the resonator. The laser light generating device is characterized in that the resonator is configured so that the extinction ratio of is equal to or more than 1000. 2. A laser crystal having a wavelength sweep range of at least 10 nm or more in the resonator, a wavelength sweep element for sweeping an oscillation wavelength of light emitted from the laser crystal, and a part of the light of the fundamental wave light. The laser light generator according to claim 1, further comprising a nonlinear optical crystal that converts the light into a second laser light having a different wavelength. 3. The laser crystal is Cr: LiSrAlF ₆ (chromium-added lithium strontium aluminum fluoride),
Cr: LiSrGaF ₆ (chromium-added lithium strontium gallium fluoride), Ti: Al ₂ O ₃ (titanium-added sapphire), or Cr: BeAl ₂ O ₄ (Alexandrite) The laser light generator according to claim 1 or 2. 4. The non-linear optical crystal is LiB ₃ O ₅ (lithium triborate), CsLiB ₆ O ₁₀ (lithium cesium borate),
β-BaB ₂ O ₄ (barium borate), KNbO ₃ (potassium niobate), KLiNbO (lithium-potassium niobate), or
LiIO ₃ laser light generating apparatus according to claim 2 or claim 3, characterized in that any of a (lithium iodide acid). 5. A laser application apparatus using the laser light generator according to claim 1. Description: 6. The laser application apparatus according to claim 5, wherein the laser application apparatus is a laser printer apparatus. 7. The laser application apparatus according to claim 5, wherein the laser application apparatus is a particle counter apparatus. 8. The laser application apparatus according to claim 5, wherein the laser application apparatus is an optical disk apparatus.