JPH06164045A - Laser apparatus and second-harmonic generation method - Google Patents

Abstract

PURPOSE:To provide a method, for SHG, wherein its output fluctuation is small and its efficiency is high without additionally inserting an optical element increasing a reflection loss into an optical resonator and to provide a laser device. CONSTITUTION:A semiconductor-laser excitation solid-state laser device is provided with a means 5 which controls the temperature of a solid-state laser crystal 3 in such a way that a phase difference caused when fundamental waves are transmitted through the solid-state laser crystal 3 becomes (2mpi+3pi/ 2) and with a means 9 which controls the temperature of a nonlinear optical crystal in such a way that a phase difference caused when the fundamental waves are transmitted through the nonlinear optical crystal 7 becomes (2m'pi) [where (m) and (m') are integers] or a method for SHG by the phase matching of type II uses the means 5 and the means 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to second harmonic generation (SHG) of a laser device.

[0002]

2. Description of the Related Art A semiconductor laser pumped solid-state laser has a good beam quality because it generates a small amount of heat during pumping, and a second harmonic can be easily obtained by inserting a nonlinear optical crystal into an optical resonator. Are known. When the excitation input is increased to obtain a high output, the fundamental wave also starts oscillating in multiple longitudinal modes. The output of the second harmonic wave has a large output fluctuation due to the influence of the sum frequency generation due to the coupling between the longitudinal modes of multiple fundamental waves, and it is required for high-density optical memory and magneto-optical disk that require high stability as it is. Could not be used as a light source

As a means for suppressing the output fluctuation of the second harmonic, a quarter-wave plate is inserted in the optical resonator so that the fundamental waves oscillate with polarized lights orthogonal to each other and the coupling between the fundamental waves is suppressed. Method [Reference: T. Bear, J.
Opt. Soc. Am., B3-1175 (1986) and M. Oka & S. Kubot
a, Opt. Lett., 13-805 (1988)], a method of inserting a Brewster plate into an optical resonator to oscillate the fundamental wave in a single longitudinal mode [Reference: Nagai et al., Report of the Laser Society of Japan ,
RTM-90-38 (1990)] and the like have been proposed.

[0004]

In the above method, in the former method, it is necessary to mount an expensive quarter-wave plate on a holder having a rotating function and insert it into an optical resonator. Insertion of a quarter wave plate increases the reflection loss. In the latter method, a reflection loss due to the Brewster plate occurs in the generated second harmonic. As described above, both methods increase the reflection loss and increase the efficiency of the second harmonic generation (S) by additionally inserting an optical element in the optical resonator.
There was a problem that HG) could not be obtained. Furthermore, since both methods were developed for laser crystals that do not have polarization characteristics,
The laser crystal has anisotropy like Nd: YVO ₄ crystal,
There is a problem that it is not effective when the fundamental wave is polarized and oscillates.

By the way, the inventor of the present application has previously proposed a laser device for controlling the temperature of a nonlinear optical crystal (Japanese Patent Application No. 4-36133). Compared to the case of not controlling the temperature of the nonlinear optical crystal, the proposed laser device was able to generate a remarkably stable second harmonic, but in terms of the generation of a precisely controlled second harmonic. I still have a problem.

In view of the above-mentioned circumstances, the present invention provides a highly efficient second harmonic generation with a small output fluctuation without additionally inserting an optical element for increasing the reflection loss in the optical resonator. It is an object of the present invention to provide a laser device and a method for generating a second harmonic wave.

[0007]

A laser device of the present invention is a solid-state laser device excited by a semiconductor laser,
It is characterized by having a function of controlling the temperature of the solid-state laser crystal and a function of controlling the temperature of the nonlinear optical crystal.

Further, the second harmonic generation method of the present invention uses a solid-state laser device excited by a semiconductor laser to type II.
In the second harmonic generation method based on the phase matching of, the phase difference generated when the fundamental wave passes through the solid-state laser crystal is (2
mπ + 3π / 2), and the temperature of the nonlinear optical crystal is controlled so that the phase difference generated when the fundamental wave passes through the nonlinear optical crystal becomes (2m′π). Is controlled (m and m ′ are integers).

Furthermore, the second harmonic generation method of the present invention uses a solid-state laser device excited by a semiconductor laser to perform type II
In the second harmonic generation method based on the phase matching of, the phase difference generated when the fundamental wave passes through the solid-state laser crystal is (2
The temperature of the solid-state laser crystal is controlled so that mπ + π / 2), and the temperature of the nonlinear optical crystal is controlled so that the phase difference generated when the fundamental wave passes through the nonlinear optical crystal becomes (2m′π). Is controlled (m and m ′ are integers).

[0010]

1 is a diagram showing the construction of the laser device of the present invention. In the figure, 1 is a semiconductor laser, 2 is a cylindrical gradient index lens (trade name SELFOC lens), 3
Is a solid-state laser crystal (eg Nd: YVO ₄ ). 4 is a thermistor, 5 is a Peltier element or heater for controlling the temperature of the solid-state laser crystal 3, 6 is a heat sink for absorbing heat, 7 is a nonlinear optical crystal [eg KTP (KTP (KTP (KTP
TiOPO ₄ )], 8 is a thermistor, 9 is a Peltier element or heater for controlling the temperature of the nonlinear optical crystal 7, 10 is a heat sink for absorbing heat, 11 is a coating which will be described later, and the second harmonic wave is transmitted therethrough. It is an emission mirror that emits light from the plane side. Among the above-mentioned components, the semiconductor laser 1, the selfoc lens for condensing, the solid-state laser crystal 3, the nonlinear optical crystal 7, and the emission mirror 9 are arranged on the same optical axis.

In the laser device shown in FIG. 1, the excitation light emitted from the semiconductor laser 1 is condensed by the SELFOC lens 2 and enters the solid-state laser crystal 3 to be absorbed. The end face of the solid-state laser crystal 3 on the semiconductor laser 1 side is coated so as to be highly transmissive to the excitation light and highly reflective to the fundamental wave and the second harmonic. On the other hand, the end face of the solid-state laser crystal 3 on the side of the nonlinear optical crystal 7 is coated so as to have high transmission for the fundamental wave and the second harmonic. Further, the solid-state laser crystal 3 is provided with a Peltier element or heater 5 for controlling the temperature of the solid-state laser crystal 3, a thermistor 4 and a heat sink 6 for radiating heat. When the heater is used instead of the Peltier element, the heat sink 6 is unnecessary.

Both end faces (on the side perpendicular to the optical axis) of the nonlinear optical crystal 7 are coated so as to have high transmission for the fundamental wave and the second harmonic. Further, like the solid-state laser crystal 3, the nonlinear optical crystal 7 is also provided with a Peltier element or heater 9, a thermistor 8 and a heat sink 10. Further, the concave surface of the emission mirror 11 is coated so as to be highly reflective for the fundamental wave and highly transmissive for the second harmonic wave. As a result of such coating, an optical resonator is constituted by the end surface of the solid-state laser crystal 3 on the semiconductor laser 1 side and the concave surface of the emission mirror 11, the fundamental wave oscillates, and the nonlinear optics provided in the optical resonator. The crystal 7 converts the fundamental wave into the second harmonic, and the second harmonic passes through the emission mirror 11 and is emitted from the plane side of the emission mirror 11.

In the second harmonic generation by the type II phase matching, the fundamental wave oscillated from the solid-state laser crystal 3 as linearly polarized light has an angle π with respect to the optical axis of the nonlinear optical crystal 7.
It is incident at / 4 rad. The fundamental wave is decomposed into the two optical axes O and E of the nonlinear optical crystal 7, advances at respective phase velocities, and generates a second harmonic wave linearly polarized in the E direction.
When the fundamental wave is transmitted through the nonlinear optical crystal 7, a phase difference is generated due to the birefringence of the crystal, and is generally elliptically polarized light.

In the above case, the length of the nonlinear optical crystal 7 is L, the refractive indices in the optical axes O and E are n _o , n _e , and respectively.
When the wavelength of the fundamental wave in vacuum is λ, the phase difference δ is simply given by the following equation (1). δ = (2π / λ) (n _o −n _e ) L (1) The refractive indices n _o and n _e and the length of the nonlinear optical crystal 7 are functions of temperature. Therefore, the nonlinear optical crystal 7 has the formula (1)
By performing an appropriate temperature control set by, the phase difference δ occurring in the fundamental wave can be made an integral multiple of (2π).
At this time, the polarization state of the fundamental wave does not change even after passing through the nonlinear optical crystal 7. When the fundamental wave oscillates in a plurality of longitudinal modes, the wavelengths thereof are different from each other, so that only one longitudinal mode light has a phase difference of an integral multiple of (2π). Light in other longitudinal modes becomes elliptically polarized light.

The polarization state of the fundamental wave in the device and method of the present invention will be described (in the following description, m and m ').
Is an integer). When the linearly polarized fundamental wave emitted from the solid-state laser crystal 3 passes through the nonlinear optical crystal 7, is reflected by the concave surface of the emission mirror 11, passes through the nonlinear optical crystal 7 again, and returns to the solid-state laser crystal 3, Light having a phase difference of an integral multiple of 2π (hereinafter referred to as A wave) remains linearly polarized light with an azimuth angle of (π / 4) and does not change. Light having a phase difference other than that (hereinafter referred to as B wave) is converted into elliptically polarized light having a long-axis azimuth angle of (π / 4) and an ellipticity angle of (δ / 2). Since the phase difference of the solid-state laser crystal 3 is also a function of temperature, appropriate temperature control is performed on the solid-state laser crystal 3 and the phase difference generated in the B wave is (2mπ + 3π / 2).
, The light that has traveled back and forth through the solid-state laser crystal 3 is converted into linearly polarized light with an azimuth angle of (π / 4 + δ / 2). Since the polarization direction of the A wave coincides with the optical axis of the crystal, it has no relation to the temperature of the solid-state laser crystal 3 and its polarization state does not change.

When the B wave goes back and forth through the nonlinear optical crystal 7, it becomes elliptically polarized light. The azimuth angle of the major axis of this ellipse is (π / 4 + δ / when the phase difference δ is as small as (π / 8).
2). Further, when the B wave travels back and forth through the solid-state laser crystal 3, it is elliptically polarized light, and the azimuth angle of its long axis is (π / 4
−δ / 2). B that has reciprocated through the nonlinear optical crystal 7
The wave becomes elliptically polarized with a long axis azimuth of (π / 4). This B wave becomes elliptically polarized light whose azimuth angle of the major axis is (π / 4), is emitted from the solid-state laser crystal 3 first, and becomes equal when it reciprocates through the nonlinear optical crystal 7.

As described above, in the optical resonator, there are A wave which is stable linearly polarized light and B wave which is elliptically polarized light whose polarization state changes periodically. The B wave, which is elliptically polarized,
The azimuth angle of the major axis of the ellipse is from (π / 4-δ / 2) to (π / 4
Although it vibrates between + δ / 2), the output is stable because the phase difference δ is small. Even when the phase difference δ produced by the solid-state laser crystal 3 is (2mπ + π / 2), the azimuth angle of the major axis of the B wave that is elliptically polarized light is from (π / 4−δ / 2) to (π /
It vibrates between 4 + δ / 2). At this time as well, the output of the B wave is stable.

When the temperature that causes the phase difference δ of (2mπ + 3π / 2) and (2m′π) in the solid laser crystal 3 and the nonlinear optical crystal 7 is higher than the ambient temperature, the temperature control of the crystal is A heater can be used instead of the expensive Peltier element. The output characteristics of the second harmonic were the same regardless of whether the Peltier element or the heater was used.

When the temperature of the solid-state laser crystal 3 and the nonlinear optical crystal 7 is controlled, the set temperature is defined by the equation (1) as described above, but the crystal length L is precisely measured to the order of microns. Is difficult. However, if the approximate value of the set temperature is obtained from the formula (1) and the temperature control of the crystal is adjusted while referring to the situation of the output fluctuation of the second harmonic,
It turns out that sufficient results are obtained.

As the solid-state laser crystal 3 used in the laser device and the second harmonic generation method of the present invention, the above-mentioned Nd: Y is used.
Besides VO ₄ crystals, there are Nd: LiYF ₄ crystals, Nd: YAlO ₃ crystals, and the like. As the nonlinear optical crystal 7, there are RTP crystal, KTA crystal, KDP crystal, etc. in addition to the above-mentioned KTP crystal.

As is clear from the above description, when the temperature of the solid laser crystal 3 is not controlled, the output of the A wave is stable, but the B wave is generated at the time of reciprocating in the solid laser crystal 3. Since the phase difference is not appropriate, the azimuth angle of the major axis of the ellipse changes greatly each time the solid-state laser crystal 3 and the nonlinear optical crystal 7 reciprocate. Therefore, the output of the B wave becomes unstable and the fluctuation becomes large. If the intensity of the output of the fundamental wave fluctuates, the intensity of the second harmonic wave will also be unstable and the fluctuation will increase, and a stable second harmonic wave output cannot be obtained.

[0022]

Example 1 A solid-state laser crystal 3 having a thickness of 1 mm in the optical axis direction, a width of 3 mm, and a Nd: YVO ₄ crystal having a length of 3 mm, and a nonlinear optical crystal 7 having a length of 5 mm in the optical axis direction and a width of 2 mm, A 2 mm long KTP crystal was used. A Peltier device was used for temperature control, and a test was performed by a laser device having the configuration shown in FIG. The temperature of the Nd: YVO ₄ crystal was controlled at 37.0 ° C. and the temperature of the KTP crystal was controlled at 35.0 ° C. to generate a second harmonic green laser with a wavelength of 0.53 μm.
A silicon photodiode was used to continuously detect the output of the second harmonic. FIG. 2 shows the result, which shows the change with time of the second harmonic output.

(Comparative Example 1) A solid laser crystal 3 having a thickness of 1 mm in the optical axis direction, a width of 3 mm, and a length of 3 mm of Nd: YVO ₄ crystal, and a nonlinear optical crystal 7 having a length of 5 mm in the optical axis direction, a width of 2 mm, and a vertical length A 2 mm KTP crystal was used. A Peltier device was used for temperature control, and a test was performed by a laser device having the configuration shown in FIG. The temperature of Nd: YVO ₄ crystal is 30.0 ℃ to 4
The temperature of the KTP crystal was controlled to 35.0 ° C by changing the temperature to 0.0 ° C, and the output of the generated laser was detected. FIG. 3 shows the results and shows the dependence of the second harmonic output on the Nd: YVO ₄ crystal temperature.

(Comparative Example 2) As the solid-state laser crystal 3, an Nd: YVO ₄ crystal having a thickness of 1 mm, a width of 3 mm and a length of 3 mm in the optical axis direction, and a nonlinear optical crystal 7 having a length of 5 mm in the optical axis direction, a width of 2 mm and a length. A 2 mm KTP crystal was used. A Peltier device was used for temperature control, and a test was performed by a laser device having the configuration shown in FIG. The temperature of KTP crystal is controlled to 35.0 ℃, and N
The temperature of the d: YVO ₄ crystal was not controlled (from approximately 38.0 ° C to 4
The output of the generated laser was continuously detected. FIG. 4 shows the result, which shows the change with time of the second harmonic output.

Comparing FIG. 2 with FIGS. 3 and 4, the temperature of the KTP crystal, which is a nonlinear optical crystal, is properly controlled, and
By controlling the temperature of the Nd: YVO ₄ crystal, which is a solid-state laser crystal, it is possible to generate a laser with a small output fluctuation.

[0026]

As described above, the laser device and the second harmonic generation method according to the present invention have a temperature suitable for each of the solid laser crystal and the nonlinear optical crystal when the fundamental wave oscillates in a plurality of longitudinal modes. By controlling, a stable standing wave of the fundamental wave is generated in the optical resonator, and a stable second harmonic wave with less noise is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a laser device of the present invention.

FIG. 2 is a diagram showing a temporal change of a second harmonic output when appropriate temperature control is performed on both the solid-state laser crystal and the nonlinear optical crystal (Example 1).

FIG. 3 is a diagram showing a temporal change of a second harmonic output when the temperature of the solid-state laser crystal is changed by appropriately controlling the temperature of the nonlinear optical crystal (Comparative Example 1).

FIG. 4 is a diagram showing a temporal change of a second harmonic output when suitable temperature control is performed only on a nonlinear optical crystal (Comparative Example 2).

[Explanation of symbols]

 1 Semiconductor Laser 2 SELFOC Lens 3 Solid Laser Crystal 4 Thermistor 5 Peltier Element or Heater 6 Heat Sink 7 Nonlinear Optical Crystal 8 Thermistor 9 Peltier Element or Heater 10 Heat Sink 11 Ejection Mirror

Claims (3)

[Claims] 1. A semiconductor laser pumped solid-state laser device having a function of controlling the temperature of a solid-state laser crystal and a function of controlling the temperature of a nonlinear optical crystal. 2. In a method for generating a second harmonic by phase matching of type II using a solid-state laser device excited by a semiconductor laser, a phase difference generated when a fundamental wave passes through a solid-state laser crystal becomes (2mπ + 3π / 2). To control the temperature of the solid-state laser crystal so that the phase difference generated when the fundamental wave passes through the nonlinear optical crystal becomes (2 m′π). Characteristic second harmonic generation method (m and m'are integers). 3. A method of generating a second harmonic by phase matching of type II using a solid-state laser device excited by a semiconductor laser, wherein a phase difference generated when a fundamental wave passes through a solid-state laser crystal is (2 mπ + π / 2). To control the temperature of the solid-state laser crystal so that the phase difference generated when the fundamental wave passes through the nonlinear optical crystal becomes (2 m′π). Characteristic second harmonic generation method (m and m'are integers).