JPH0688979A - Q switch/second harmonic generating combined element - Google Patents

Abstract

PURPOSE:To provide the Q switch/second harmonic generating combined element which is simple in constitution, is low in cost, is stable, is low in the loss of a resonator and does not degrade the oscillation characteristic of a laser. CONSTITUTION:This Q switch/second harmonic generating combined element has a laser diode light source 1 for excitation, a lens 2 for focusing the beam from this laser diode light source 1, an Nd:YVOA crystal 22 to be excited by irradiation with the focused beam, an optical crystal 23 which processes the beam past this Nd:YVOA crystal 22 to pass the ray back and forth therethrough and has an electrooptical effect and an output mirror 8 for this purpose. While the phase matching of a type II is maintained by utilizing the electrooptical effect of the optical crystal 23, the polarization of the fundamental wave of the Nd:YVOA crystal 22 is controlled. The polarization characteristic of the fundamental wave is changed by using the nonlinear optical crystal 23 which generates a higher harmonic wave by using the orthogonal electric field component and by utilizing the electrooptical effect of the optical crystal 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Q-switch / second harmonic generation composite element using a phase modulation element for laser light and a higher harmonic generation element. In particular, the present invention relates to a Q-switch / second-harmonic generating composite element used for a Q-switch laser of a type that generates a second harmonic inside a resonator (hereinafter, second-harmonic generating type inside a resonator).

[0002]

2. Description of the Related Art A solid-state laser pumped by a semiconductor laser (LD) is capable of high-power continuous wave oscillation, and is compact, highly efficient, and
It has characteristics of long life, excellent energy storage and frequency stability, and easy single longitudinal and transverse mode oscillation. Further, as compared with the discharge tube excitation type, the excitation efficiency is high due to the resonance excitation and the heat generation associated with the excitation is small, so that the oscillation mode can be highly stabilized, which is an excellent feature.

As the structure of such an LD pumped solid-state laser, the structure shown in FIG. 2 is usually used. That is, in addition to the pumping semiconductor laser 1, the condenser lens 2, the laser medium 4, the rear mirror 3, and the output mirror 8, the nonlinear optical crystal 7 for generating the second harmonic, the electro-optical crystal 6 for Q switch, and the polarizing plate 5 are used. It is configured. Since the electro-optic crystal 6 changes the phase of the fundamental wave in the resonator, as shown in FIG. 2, the polarization direction of the polarizing plate 5 is 45 ° with respect to the fast axis 21 and the slow axis 20 of the crystal.
Place it at an angle. When a high-voltage pulse is not applied to the electro-optical crystal 6 to which the high-voltage applying electrode 9 is attached on the upper and lower surfaces of the electro-optical crystal 6, the excitation light 10 excites the laser medium 4, so that the fundamental wave 11, 12 is generated. The slow axis 20 and the fast axis 21 of the electro-optic crystal 6 are arranged at an angle of 45 ° as shown. The polarization axis 19 of the polarizing plate 5 is in the vertical direction. The fundamental wave 12 is converted into a second harmonic wave 17 by the non-linear optical crystal 7 and is taken out from the output mirror 8 as outgoing light 18. The remaining fundamental wave 13 that has not been converted to the second harmonic is reflected by the output mirror 8,
It is returned as the reflected fundamental wave 14. In this configuration, the electro-optic crystal 6 functions as a quarter-wave plate (hereinafter, referred to as a λ / 4 plate) when the high voltage pulse is not applied by the electrode 9 and when the high voltage pulse is applied, It works as a zero-wave plate.

Such a conventional LD pumped solid state laser is
In the internal second harmonic oscillation type Q switch, in the second harmonic generating element, the polarization direction of the fundamental wave in the resonator is determined by the direction of the polarizing plate 5. That is, although the loss is small with respect to the electric field of the component according to the direction of the polarizing plate 5, the loss is large with respect to the component orthogonal to this, so that the electric field of the component allowed by the polarizing plate is set in the resonator. Only allowed. First, when a high voltage pulse is not applied, the electro-optic crystal 6 functions as a λ / 4 plate, so the fundamental wave 12 after passing through the electro-optic crystal becomes circularly polarized light, passes through the nonlinear optical crystal 7, and passes through the output mirror 8 Reflected again by the nonlinear optical crystal 7,
After passing through the electro-optic crystal 6, the fundamental wave electric field 16 becomes linearly polarized light that is orthogonal to the polarization direction 19 of the polarizing plate 5, and becomes a high loss state, that is, a resonator with a low Q value, which leads to laser oscillation. There will be no. Next, when a high voltage pulse is applied, the loss in the resonator decreases because it acts as a zero-wave plate, and the Q value increases, so the fundamental wave increases and laser oscillation occurs. However, the fundamental wave is confined in the resonator, and the second harmonic oscillating wave 17 is generated by the nonlinear optical crystal arranged in the resonator and taken out from the output mirror 8. At this time, since the strong fundamental wave in the resonator is used, highly efficient wavelength conversion can be performed.

In the above-described conventional resonator internal second harmonic generation type Q switch laser, the electro-optic crystal and the non-linear optical crystal are separately configured and used, but the resonator configuration is complicated. Therefore, there is a problem that not only the cost becomes high, but also unstable, the loss of the resonator increases, and the oscillation characteristic of the laser deteriorates.

[0006]

SUMMARY OF THE INVENTION Therefore, the present invention solves the above problems, has a simple structure, is inexpensive, is stable, has a low resonator loss, and has a low laser oscillation characteristic. It is an object of the present invention to provide a Q switch / second harmonic generation composite element without deterioration. The present invention also provides a Q switch / second harmonic generation composite element for internal second harmonic generation, which employs a system in which one KTP element can be used as both the phase modulation element and the second harmonic generation element. The purpose is to do.

[0007]

In order to solve the above technical problems, the present invention provides a pumping laser diode light source;
A lens for focusing a light beam from a laser diode light source; Nd: YVO which is irradiated and excited by the focused light beam
₄ crystal; an optical crystal having an electro-optical effect of reciprocally processing a light beam passing through the Nd: YVO ₄ crystal; and an output mirror therefor, and utilizing the electro-optical effect of the optical crystal,
The polarization of the fundamental wave of the Nd: YVO ₄ crystal is controlled while maintaining the type II phase matching, and a nonlinear optical crystal that generates high-order harmonics using orthogonal electric field components is used. Provided is a Q-switch / second harmonic generation composite element characterized by changing the polarization characteristic of a fundamental wave by utilizing an optical effect.

That is, the principle of changing the polarization characteristic of the fundamental wave by utilizing the electro-optic effect at the same time as the generation of the higher harmonics by using the nonlinear optical crystal which generates the higher-order high frequency by the orthogonal fundamental wave electric field component It was used. Therefore, the optical modulation / second harmonic generation element of the present invention can simultaneously perform the two functions of phase modulation by the electro-optical effect and generation of the second harmonic by the nonlinear optical effect. As a configuration for this, as shown in FIG. 1, a Nd: YVO ₄ crystal is used as the laser medium 22 and a mirror coating for a resonator is applied to its excitation surface, so that the rear mirror 3 in FIG. 2 is omitted. can do. Further, as the nonlinear optical crystal 23, KTP (KTiOPO ₄ ) is used.
2 is used for type II phase matching, the electrode 9 for applying a high voltage is provided in the c-axis direction 20, and the electro-optic crystal 6 of the Q switch in FIG. 2 can be omitted. At this time, the KTP crystal 23
The c-axis 24 of the Nd: YVO ₄ crystal is arranged at an angle of 45 ° with respect to the fast axis 21 and the slow axis 20 of the above, and when a high voltage is not applied, λ / 4 with respect to the fundamental wave. Adjust the angle of the crystal to act as a plate. K which is a biaxial crystal
Since the TP crystal has a wide second harmonic generation phase matching angle, it is possible to freely control the polarization characteristic of the fundamental wave from linearly polarized light to circularly polarized light while maintaining the type II phase matching. Further, not only the KTP crystal but also a non-linear optical crystal having similar properties can be used to configure the same device.

In the present specification, the Q switch of the present invention
As a second harmonic generation composite element, a semiconductor laser pumped N
The case of the intracavity second harmonic generation type Q switch laser in the d: YVO ₄ short cavity laser has been described, but the present invention can be applied even when the laser is another medium or the pumping method is different.

[0010]

The operation of the internal second harmonic generation type Q switch laser shown in FIG. 1 will be described to explain the operation of the Q switch / second harmonic generation composite element of the present invention. That is, the light beam from the pumping laser diode light source 1 is focused by the lens 2 and enters the Nd: YVO ₄ crystal 22. The Nd: YVO ₄ crystal is arranged on the c-axis 24 tilted at 45 ° and includes a partial reflection mirror 25. Therefore, the fundamental wave 11 polarized at 45 ° is incident on the KTP crystal 23 having the electro-optic effect having the electrodes 9 on the upper and lower surfaces, passes through, and then is reflected by the output mirror 8, and the fundamental wave 14 is obtained. , Again passes through the KTP crystal 23 and the Nd: YVO ₄ crystal 22, is reflected by the reflecting mirror 25, and again passes through the Nd: YVO ₄ crystal 22.
On the other hand, the second harmonic wave 17 generated in the KTP crystal 23 is emitted as output light 18 from the output mirror 8.

That is, FIG. 1 shows a Q switch according to the present invention.
It is a block diagram of a second harmonic generation composite element. The second inside
The operation of the harmonic generation type Q switch laser will be described. The Q-switched laser of FIG. 1 is a Nd: YVO ₄ crystal, which is a uniaxial crystal and has a high gain in the c-axis direction, that is, the roles of both the laser medium 4 and the polarizing plate 5 of FIG. It will have both. Therefore, when a high voltage is not applied, the angle of the crystal is adjusted as shown so that it acts as a λ / 4 plate with respect to the fundamental wave. Since the KTP crystal which is a biaxial crystal has a wide second harmonic oscillation phase matching angle, it is possible to freely control the polarization characteristic of the fundamental wave from linearly polarized light to circularly polarized light while maintaining the type II phase matching. it can. In addition, the biaxial KTP crystal not only has a large change in the refractive index even with a slight angle change,
Since the harmonic oscillation phase matching angle is also wide, the Nd: YVO ₄ fundamental wave (wavelength: 10
The polarization characteristics of 64 nm) can be freely controlled from linearly polarized light to circularly polarized light.

Further, the electro-optical effect is large when an electric field is applied in the c-axis direction, and phase modulation can be performed from 0 to λ / 4 by a relatively low applied electric field. Therefore, when the high voltage pulse is not applied, λ
By adjusting the angle of the crystal so as to act as a / 4 plate and applying a high voltage pulse, it is possible to act as a zero wavelength plate for the fundamental wave while maintaining the second harmonic oscillation phase matching. That is, the phase modulation of the Q switch and the second harmonic generation can be simultaneously performed by one KTP crystal.

That is, in the first embodiment, the laser medium 2
Nd: YVO ₄ crystal is used as 2, and a mirror coating 25 for a resonator is applied to its excitation surface.
Then, as the nonlinear optical crystal 23, KTP (KTi
OPO ₄ ) with Type II phase matching, c-axis direction 20
In other words, that is, the upper and lower surfaces of the Nd: YVO ₄ crystal are provided with electrodes 9 for applying a high voltage. At this time, the KTP crystal 23
The c-axis 24 of the Nd: YVO ₄ crystal is arranged at an angle of 45 ° with respect to the fast axis 21 and the slow axis 20 of. And
When no high voltage is applied, the angle of the crystal is adjusted so as to act as a λ / 4 plate with respect to the fundamental wave. That is, since the KTP crystal which is a biaxial crystal has a wide second harmonic generation phase matching angle, the polarization characteristic of the fundamental wave can be freely controlled from linearly polarized light to circularly polarized light while maintaining the type II phase matching. be able to. That is, by applying a high voltage pulse, it is possible to cause the fundamental wave to function as a zero wavelength plate while maintaining the second harmonic oscillation phase matching. The phase modulation element of the Q switch and the second harmonic generation element are combined into one KT.
It can be done simultaneously with P crystals.

In FIG. 1, when an Nd: YVO ₄ crystal is used instead of an Nd: YVO ₄ crystal, the gain is an isotropic crystal, so that polarization occurs in the resonator as in the conventional Q-switched laser. A plate (19 in FIG. 2) is required, which enables the Q switch / second harmonic generation operation even when an isotropic laser medium is used. Further, in FIG. 1, the angle of the KTP crystal is set so that the second harmonic generation phase matching is not achieved, and the reflectance of the output mirror 8 with respect to the fundamental wave is appropriately selected, whereby the Q switch of the fundamental wave with respect to the fundamental wave is obtained. It becomes possible to operate.

Next, the present invention will be specifically described with reference to examples, but the present invention is not limited thereto.

[0016]

EXAMPLE With the device configuration shown in FIG. 3, LD solid-state laser light is produced by utilizing the electro-optic effect of KTP crystal.
It was theoretically and experimentally studied that the polarization of the fundamental wave (wavelength = 1.06 μm) of the Nd: YVO ₄ crystal can be controlled while maintaining the II phase matching. As one of the applications, it was confirmed that the operation as the second harmonic generator and the Q switch element can be realized with one KTP placed inside the resonator, by making a composite optical element having the following configuration. . That is, it was confirmed that the operation of the internal second harmonic generation type Q switch laser and as the Q switch element as shown in FIG. 1 can be realized.

FIG. 3 shows an LD laser source for pumping (output is 76).
The light flux with a wavelength of 809 nm emitted from 1 (0 mW or less) is contained by the lens (focal length 3.3 mm) 2.
Is 1.1 atomic% and has a length of 500 μm and is converged on a Nd: YVO ₄ crystal (reflectance R> 99%, 1064,532 nm, transmittance T> 95%, 809 nm) 22, and then the electrodes 9, 9 KTP crystal 2 having a thickness of 1 mm, a length of 5 mm, and a width of 3 mm and having an electric field '
It is incident on 3 to control the polarization of the fundamental wave. Then, on the output side, an output mirror (R> 99%, 1064 nm, T>
An output light flux having a wavelength of 532 nm is emitted from an output concave lens having a concave surface of 95%, 532 nm).

This short-cavity laser has a maximum output of 308 mW (efficiency 40.5%) at the fundamental wave during continuous oscillation.
Thus, a characteristic of 69.5 mW (efficiency 9.1%) is obtained at the second harmonic. Since the Nd: YVO ₄ crystal is a uniaxial crystal, Q switch operation is possible without using a polarizing element.

FIG. 4 is a graph showing the applied voltage characteristic of the peak output of the second harmonic oscillation wave of the internal second harmonic generation type Q-switched laser of FIG. According to the analysis result, the λ / 4 voltage is about 740V, and the output is maximum around 750V in the experiment. Further, the harmonic output increased about 600 times as compared with the continuous output, the pulse width was about 20 nanoseconds, and the repetition was 100 Hz.

[0020]

As described above, the Q switch / second harmonic generation composite element of the present invention has the following remarkable technical effects. First, it is an object to provide a Q switch / second harmonic generation composite element having a simple structure, low cost, stable, low resonator loss, and no deterioration in laser oscillation characteristics. To do. Second, LD excitation N
The d: YVO ₄ crystal laser was improved in performance, and wavelength conversion and high output could be achieved. Thirdly, since one KTP element can be used as both the phase modulation element and the second harmonic generation element, the internal second harmonic generation type Q adopted in the method of the present invention is used.
Switched lasers have the advantage of simplifying the device. Fourthly, therefore, not only the stability of the laser resonance is enhanced, but also the resonator loss is reduced, so that highly efficient laser generation can be expected.

[Brief description of drawings]

FIG. 1 shows a configuration of a second internal harmonic generation type Q switch / second harmonic generation composite element having a function of Q-switch phase modulation and second harmonic generation according to the present invention.

FIG. 2 is a conventional general internal resonator second harmonic generation type Q.
The structure of a switch laser is shown.

FIG. 3 shows a structure of a second internal harmonic generating Q-switched laser of the present invention.

FIG. 4 is a graph showing the relationship between the peak output (comparison unit) of the KTP crystal and the applied voltage V _KTP (V) as a result of measurement with one example of the Q-switch / second harmonic generation composite element of the present invention. .

[Explanation of symbols]

1 Excitation semiconductor laser 2 Condensing lens 8 Output mirror 9 Electrode 18 Output light 20 Slow axis of KTP crystal 21 Fast axis of KTP crystal 22 Nd: YVO ₄ crystal 23 KTP crystal 24 Nd: YVO ₄ c-axis direction

 ─────────────────────────────────────────────────── ─── Continuation of the front page (71) Applicant 591059560 Takahiro Kobayashi 5-13-7 Bunkyo, Fukui-shi, Fukui Prefecture (72) Takinori Inoue 2-11-13 Miyuki, Fukui-shi, Fukui Prefecture 11 (72) Inventor Takao Kobayashi 5-13 Bunkyo, Fukui City, Fukui Prefecture Kamisato Dormitory 5-44

Claims (1)

[Claims] 1. A pumping laser diode light source; a lens for focusing a light beam from a laser diode light source; an Nd: YVO ₄ crystal which is irradiated with the focused light beam and excited; and a mirror for the Nd: YVO ₄ crystal. An optical crystal having an electro-optical effect, which is provided with a coating; and reciprocally processes a light beam that has passed through a Nd: YVO ₄ crystal; and an output mirror therefor, which utilizes the electro-optical effect of the optical crystal to perform type II phase matching Using the nonlinear optical crystal that controls the polarization of the fundamental wave of the Nd: YVO ₄ crystal and generates high-order harmonics by using the orthogonal electric field components, the electro-optical effect of the optical crystal is used. A Q-switch / second harmonic generation composite element characterized by changing the polarization characteristic of the fundamental wave.