JPH05235456A - Laser apparatus - Google Patents

Abstract

PURPOSE:To provide a laser apparatus capable of highly effectually generating a second harmonic wave with reduced variations without increasing the number of optical elements in an optical resonator. CONSTITUTION:A title laser apparatus is constructed with a thermistor 5, a Peltier element 6, and a second harmonic generation(SHG) unit which serve together to control the temperature of a nonlinear optical crystal 4 such that the phase shift of a fundamental wave when it passes through the nonlinear optical crystal 4 is an integral multiple of 2pi in order to generate a type 2 second harmonic wave.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to second harmonic generation of laser devices.

[0002]

2. Description of the Related Art Conventionally, a semiconductor laser pumped solid-state laser generates a small amount of heat during pumping, so that the beam quality is good, and a second harmonic can be easily obtained by inserting a nonlinear optical crystal into an optical resonator. You can The second harmonic obtained by this method has large output fluctuation due to the influence of sum frequency generation due to coupling between multiple longitudinal modes, and it can be used as it is for the light source of high-density optical memory or magneto-optical disk. I couldn't.

This phenomenon is called a green problem,
Research has continued. As one means for suppressing the output fluctuation, a method has been proposed in which a 1/4 wavelength plate is inserted in the optical resonator to oscillate the fundamental waves with polarizations orthogonal to each other. (Reference: T.Bear, J.Opt.Soc.Am, B 3-1175 (19
86) and M. Oka and S. Kubota, Opt. Lett. 13-805 (198
8)

In this method, it is necessary to mount an expensive quarter-wave plate on a holder having a rotating function and insert it into the optical resonator, which has a drawback that the resonator becomes long.
In addition, the 1/4 wave plate is AR (non-reflective) coated with respect to the fundamental wave and the second harmonic, but the loss of the optical resonator increases by the amount of the residual reflectance, and the high efficiency second harmonic is generated. No outbreak was obtained.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a laser device for generating a highly efficient second harmonic with a small output fluctuation without increasing the number of optical elements in an optical resonator. The purpose is.

[0006]

A laser device according to the present invention comprises a semiconductor laser, a condenser lens, a solid-state laser material, a temperature control element (Peltier element) having a thermistor for detecting the temperature of a nonlinear optical crystal, and a second device. It is composed of an SHG unit, which is an optical resonator for harmonic generation, and controls the temperature of the nonlinear optical crystal inserted in the optical resonator to generate the type 2 second harmonic, Is characterized by having a function of controlling the temperature of the nonlinear optical crystal so that the phase difference generated when the light passes through the nonlinear optical crystal becomes an integral multiple of 2π.

[0007]

Since the present invention is configured as described above, when the fundamental wave is transmitted through the nonlinear optical crystal, the nonlinear optical crystal has a phase difference caused by the birefringence of the nonlinear optical crystal that is an integral multiple of 2π. Since the temperature of the optical crystal is controlled, it is possible to generate a high-efficiency second harmonic that satisfies the phase matching condition and has a small reflection loss.

[0008]

First, an embodiment will be described with reference to FIG. Excitation light emitted from the semiconductor laser 1 is condensed by the SELFOX lens 2, and the solid laser material 3 (for example,
Nd: YVO ₄ crystal) and is absorbed. The end face of the solid-state laser material 3 on the semiconductor laser 1 side is provided with a coating 3a that is highly transmissive to the excitation light and highly reflective to the optical resonator and the second harmonic. The fundamental wave and the second wave are formed on the end face of the solid-state laser material 3 on the side of the nonlinear optical crystal 4.
It has a coating 3b that is highly transparent to harmonics. The nonlinear optical crystal 4 includes, for example, KTP (KTiOP
A single crystal such as O ₄ ) is used. The temperature of the non-linear optical crystal 4 is always detected by the thermistor 5 and is an optical resonator for generating a second harmonic wave having a Peltier element 6 S
The non-linear optical crystal 4 is constantly instructed by the HG unit 7,
It is configured to be maintained at a predetermined temperature. Also,
Both ends of the nonlinear optical crystal 4 are coated with a coating 4a that is highly transparent to the fundamental wave and the second harmonic.

The exit mirror 8 is provided with a special coating 8a which is highly reflective for the fundamental wave and highly transmissive for the second harmonic wave. An optical resonator is formed between the coating 3a surface of the end surface of the solid-state laser material 3 on the semiconductor laser 1 side and the coating 8a surface of the emission mirror 8, and laser oscillation of the fundamental wave is obtained. The fundamental wave linearly polarized with respect to the optical axis of the nonlinear optical crystal 4 placed in the resonator is incident at an angle of 45 °, but is decomposed in the two optical axes O and E directions and advances at each phase velocity. , The second harmonic linearly polarized in the E direction is generated.

The fundamental wave causes a phase difference by passing through the nonlinear optical crystal 4, and is generally elliptically polarized light. Since this fundamental wave is highly reflected by the coating 8a of the emission mirror 8, it is structured so as to be confined in the optical resonator, so that it is transmitted through the nonlinear optical crystal 4 many times and inside the optical resonator. , Different elliptically polarized mode fundamental waves exist, and the output becomes unstable. Therefore, the nonlinear optical crystal 4
If the Peltier element is used to control the temperature of so that the phase difference after transmission becomes an integral multiple of 2π, the second harmonic wave with less noise can be obtained without changing the polarization direction of the fundamental wave.

FIG. 2 is a graph showing the relationship between the temperature and the phase difference of the nonlinear optical crystal KTP. Here, KTP with a length of 5 mm was used. The horizontal axis shows the temperature of KTP, and the vertical axis shows the phase difference generated in the fundamental wave transmitted through KTP in radial units. The phase difference was measured by the Senarmont birefringence method. In the measured temperature range, the relationship between the temperature of KTP and the phase difference was linear. This is because the temperature coefficient of linear refractive index and the coefficient of linear expansion of KTP can be regarded as constant in this temperature range.

The second condition when the temperature of KTP is not controlled
Harmonic output was detected and measured with an oscilloscope using a pin photodiode, and fluctuations were observed when the cycle was 1 microsecond or less.

Also, the phase difference should be an integral multiple of 2π.
The output of the second harmonic when the temperature of KTP was controlled was measured with an oscilloscope. It was confirmed that the output fluctuation was significantly reduced and the high frequency noise was also significantly reduced compared to when the temperature of KTP was not controlled. did it.

[0014]

As the fundamental wave generated in the optical resonator does not change its polarization direction when passing through the temperature-controlled nonlinear optical crystal, the combination matching of fundamental waves having mutually perpendicular polarization planes ( It is possible to efficiently generate the second harmonic without any deviation from the so-called type 2) phase matching condition. Furthermore, since there is no elliptically polarized light component that changes with time in the optical resonator, the output fluctuation of the fundamental wave becomes small, and the second harmonic wave with less noise can be obtained.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing one embodiment of the structure of a laser device of the present invention.

FIG. 2 is a graph showing the relationship between the KTP temperature and the phase difference generated in the fundamental wave.

[Explanation of symbols]

 1 semiconductor laser 2 SELFOC lens 3 solid-state laser material 3a, 3b, 4a, 8a coating 4 nonlinear optical crystal 5 thermistor acid 6 Peltier element 7 SHG second harmonic generation unit 8 exit mirror

Claims (1)

[Claims] 1. In a semiconductor laser pumped solid-state laser device, the temperature of a nonlinear optical crystal inserted into an optical resonator for generating a type 2 second harmonic is controlled, and a fundamental wave is transmitted through the nonlinear optical crystal. A laser device having a function of holding a nonlinear optical crystal at a temperature at which a phase difference generated at that time becomes an integral multiple of 2π.