JP3127861B2 - Optical parametric oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical parametric oscillator that performs wavelength conversion of laser light using optical parametric oscillation, and more particularly to a laser device used as a light source in a differential absorption laser radar device.

[0002]

2. Description of the Related Art Differential absorption laser radar (DIAL)
Then, the wavelength λ with a large absorption coefficient for the substance to be observed
Means for generating laser light of two wavelengths, _ON and a wavelength λ _{OFF having} a small absorption coefficient, are required. At DIAL,
Generally, a tunable laser device is used. In order to alternately generate two wavelengths of λ _ON and λ _OFF using one wavelength tunable laser device, it is necessary to mechanically change the angle of the nonlinear optical crystal or the wavelength dispersion element. Since the mechanical angle control has a slow control speed, the laser repetition frequency is limited when two wavelengths of λ _ON and λ _OFF are generated alternately. Therefore, one wavelength tunable laser device is connected to DI
It is not practical to use for AL.

Conventionally, two sets of laser devices each having a set of an excitation light source and a wavelength tunable laser are generally used as a DIAL laser light source, and one set has λ _ON and the other set has a λ _OFF laser. It was generating light. Since external control is performed so that laser pulses are alternately generated from the two laser devices, the laser can be generated by switching two wavelengths at high speed.

Japanese Patent Application Laid-Open No. 8-36202 discloses an example of a laser apparatus which alternately generates two wavelengths by alternately exciting two tunable lasers with one excitation light source.
There is an example disclosed in the issue . FIG. 5 shows the configuration of this laser device. A pulse light for exciting the wavelength tunable laser is generated from the excitation light source 12. The polarization rotation element 9 is externally driven so that the rotation amount of the polarization of the pulse light from the excitation light source 12 is alternately 0 ° and 90 ° for each pulse. The optical path of the pulse light rotated by 0 degree and 90 degrees is switched for each light pulse by the polarization selecting element 19, and alternately enters the two tunable lasers as pump light. The two tunable lasers alternately generate laser lights of different wavelengths by the incidence of pump light.

[0005]

[Problem to be Solved by the Invention] When the above-mentioned conventional laser device is used for DIAL, the output ports of the lasers for generating the laser beams having the wavelengths of λ _ON and λ _OFF are different.
Two sets of transmission optical systems are required. Since the transmission optical system generally uses an optical element such as a large-diameter lens, it is expensive. If laser beams of two wavelengths are output coaxially from the same aperture, DIAL can be configured with only one set of expensive transmission optical systems.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a laser device which generates two wavelengths of laser light alternately and coaxially from the same output port at high speed from pump light from one laser device. is there. This makes it possible to use a single set of transmission optics when used in DIAL, reducing the cost and size of DIAL equipment, and making it possible to reduce the size compared to the past by comparing only the laser generation part. Becomes

[0007]

In order to solve these problems, an optical parametric oscillator according to the present invention comprises a first resonator mirror and a second resonator mirror having different wavelengths due to optical parametric oscillation. It has a first nonlinear optical crystal and a second nonlinear optical crystal that generate signal light, and the first nonlinear optical crystal and the second nonlinear optical crystal can perform phase matching with pump light. Linearly polarized light incident on a resonator for optical parametric oscillation formed between the first resonator mirror and the second resonator mirror and generated in the form of a pulse is disposed so that the directions are orthogonal to each other. Means for alternately providing a rotation angle of 0 degree and 90 degrees in synchronization with the generation of the pump light pulse in the polarization direction of the pump light. Therefore, the optical signal light generated by the first nonlinear optical crystal and the signal light generated by the second nonlinear optical crystal are alternately output in the same direction.

Further, in this optical parametric oscillator, a means for alternately providing a rotation angle of 0 ° and 90 ° in the polarization direction of the signal light in synchronization with the generation of the signal light pulse is provided on the output side of the optical parametric oscillator. It is characterized by having.

[0009]

Next, embodiments of the present invention will be described in detail with reference to the drawings.

Referring to FIG. 1, first, the polarization rotator 9
There is. A Pockels cell, a Faraday rotator combining a solenoid coil and a magnetic material, or the like is used as the polarization rotation element 9. The polarization rotation element 9 controls the polarization rotation angle by a controller that drives the polarization rotation element 9. By applying a voltage having a waveform of (2) in FIG. 2 to the polarization rotator 9 (or flowing a current) to the pump light generated in a pulse shape as shown in (1) of FIG. Voltage (or current)
Can be set to 0 degree when is 0, and 90 degrees when the voltage is high (or high current). The space between the first resonator mirror 3 and the second resonator mirror 6 becomes a resonator for optical parametric oscillation. Between the first resonator mirror 3 and the second resonator mirror 6, a first nonlinear optical crystal 10 and a second nonlinear optical crystal 11 are arranged. By injecting pump light into a resonator including two nonlinear optical crystals, signal light is generated as output by optical parametric oscillation. When pump light having a specific wavelength is incident on the resonator, the first nonlinear optical crystal 10 generates a signal light having a wavelength λ _ON , and the second nonlinear optical crystal 11 generates a signal light having a wavelength λ _OFF. . The two non-linear optical crystals may have different cutting directions, or may be arranged at an angle at which signals of different wavelengths are generated when they are arranged with the same cutting direction. Further, the effects of the present invention can be obtained even if different types of crystals are used. First
Is arranged such that phase matching can be achieved in the polarization direction of the pump light before passing through the polarization rotator 9, and the second nonlinear optical crystal 11 has a polarization direction of the pump light of only 90 degrees. They are arranged so that phase matching can be achieved when they are rotated. By arranging the crystals in this manner, the voltage (or flowing current) applied to the polarization rotation element 9
Is zero, a signal light having a wavelength λ _ON is generated as shown in FIG. When a high voltage is applied to the polarization rotator 9 (or a high current flows), a signal light having a wavelength λ _OFF is generated as shown in FIG.

The laser beams having different wavelengths λ _ON and λ _OFF are generated by optical parametric oscillation using the same resonator between the first resonator mirror 3 and the second resonator mirror 6 as a resonator. Is output to

[0012]

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 3 is a schematic diagram of the configuration of the first embodiment of the present invention. Lithium niobate (LiN
A Pockels cell 1 in which electrodes are provided on a bO ³ ) crystal is used. The Pockels cell 1 is configured such that the pump alternates between two states: a state in which no voltage is applied, and a state in which a high-voltage is applied to act as a half-wave plate for light having a wavelength of 1.064 μm. It is driven by the Pockels cell driving device 2 in synchronization with the light pulse. Then, regarding the pump light having a wavelength of 1.064 μm , the polarization rotation angle is alternately changed to 0 degree and 90 degrees by the Pockels cell 1. Nonlinear optical crystals include KTP (KTiOP)
O ⁴ ) crystal, the first KTP crystal 4 and the second KTP crystal 4
In both KTP crystals 5, the cut-out direction of the crystal is θ = 70.
° and φ = 0 °. In this crystal cutting direction, pump light having a wavelength of 1.064 μm is applied to the first KTP crystal 4 and the
The polarization is parallel to the X-axis of the KTP crystal 5
, A signal light having a wavelength in the 1.6 μm band is generated. When the pump light passes through the Pockels cell and does not undergo rotation in the polarization direction, the first KTP crystal 4 is in a direction in which phase matching for optical parametric oscillation can be obtained and has a desired wavelength λ ^ON. They are arranged at an angle at which light is generated. When the pump light passes through the Pockels cell and the polarization direction is rotated by 90 degrees, the second KTP crystal 5 has a direction in which optical parametric oscillation can be phase-matched and has a desired wavelength λ ^OFF.
Are arranged at an angle at which the signal light is generated. this
As described above, the first KTP crystal 4 and the second KTP crystal 5
The direction of each X axis, that is, 0 degree and 9 degrees of the pump light
Phase matching is achieved for polarization directions that rotate alternately at 0 degrees
Orthogonal to each other so as to generate optical signals in different directions.
Has been placed. As described above, laser lights having wavelengths of λ ^ON and λ ^OFF are generated alternately. However, in the nonlinear optical crystal used in this embodiment, the polarization direction of the signal light is the same as the polarization direction of the pump light, so that the wavelength is λ ^ON and λ.
The polarization of the signal light that is ^{turned off} is orthogonal to each other.

FIG. 4 is a block diagram of a second embodiment of the present invention. In this embodiment, the first resonator mirror 3, the second resonator mirror 6, the first KTP crystal 4, and the second KTP crystal 5
Are the same as those used in the first embodiment, and the arrangement method is also the same. The oscillation wavelength is the same as in the first embodiment. The first Pockels cell drive 1a and the first Pockels cell drive 2a are also the same as the Pockels cell 1 and the Pockels cell drive 2 of the first embodiment. The signal light generated from the side of the second resonator mirror 6 enters the second Pockels cell 7. The second Pockels cell 7 is a pump that alternately switches between a state where no voltage is applied and a state where it acts as a half-wave plate for light of wavelength λ _OFF when a high voltage is applied. It is driven by the second Pockels cell driving device 8 in synchronization with the light pulse. As described above, the wavelengths are λ _ON and λ _OFF
Are generated alternately, and the polarizations of the laser light of the two wavelengths are generated so as to be in the same direction.

[0015]

The first effect can be generated while alternately switching the two wavelength laser lights at high speed. The reason is that two nonlinear optical crystals are placed in the resonator for optical parametric oscillation to generate two different wavelengths, and the polarization rotation angle of the pump light is changed by the polarization rotation element before the pump light enters the resonator. Is controlled in synchronization with the pulse of the pump light so that phase matching can be achieved with only one of the two nonlinear optical crystals.

The second effect is that laser beams of two wavelengths are
It can be generated coaxially from the same aperture. This is because two nonlinear optical crystals for generating laser beams of different wavelengths are provided in the same resonator.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing an embodiment of an optical parametric oscillator according to the present invention.

FIG. 2 is a time chart illustrating an operation of the optical parametric oscillator according to the embodiment of the present invention;

FIG. 3 is a conceptual diagram of a first embodiment of the optical parametric oscillator according to the present invention.

FIG. 4 is a conceptual diagram of a second embodiment of the optical parametric oscillator according to the present invention.

FIG. 5 is a conceptual diagram of a conventional laser device.

[Explanation of symbols]

 Reference Signs List 1 Pockels cell 1a First Pockels cell 2 Pockels cell drive 2a First Pockels cell drive 3 First resonator mirror 4 First KTP crystal 5 Second KTP crystal 6 Second resonator mirror 7 2 Pockels cell 8 Second Pockels cell driving device 9 Polarization rotating element 10 First nonlinear optical crystal 11 Second nonlinear optical crystal 12 Excitation light source 13 Polarization selection element 14 Reflecting mirror 15 1/2 wavelength plate 16a, b Resonance Mirrors 17a, b Laser media for tunable laser generation 18a, b Resonator mirror 19 Polarization selection element

────────────────────────────────────────────────── ─── Continued on the front page (58) Fields surveyed (Int. Cl. ⁷ , DB name) G02F 1/37-1/39 H01S 3/108-3/109 G01J 3/10 WPI (DIALOG) JICST file ( JOIS)

Claims (6)

(57) [Claims] 1. A first nonlinear optical crystal and a second nonlinear optical crystal that generate signal light having different wavelengths by optical parametric oscillation between a first resonator mirror and a second resonator mirror. Pump light polarization enabling phase matching
Arrange them so that their directions are orthogonal to each other, and alternate between 0 and 90 degrees.
An optical parametric oscillator that receives pump light . 2. A rotation angle of 90 ° and 0 ° is alternately applied to the plane of polarization of the linearly polarized light in synchronization with the generation of the pumping light pulse with respect to the input polarized light of the pulsed and linearly polarized pump light. 2. The optical parametric oscillator according to claim 1, further comprising a first polarization rotating means. 3. A second polarization rotation means for alternately providing rotation angles of 90 degrees and 0 degrees to a polarization plane of signal light generated as linearly polarized light by optical parametric oscillation in synchronization with the pump light pulse. The optical parametric oscillator according to claim 2. 4. The optical parametric oscillator according to claim 1, wherein at least one of the first and second nonlinear crystals is a potassium phosphate titanate (KT _i OPO ₄ ) crystal. 5. A method according to claim 1, wherein at least one of said first and second polarization rotating means is lithium niobate (LiNb).
O ₃₎ optical parametric oscillator according to any one of claims 2 to 4 is a Pockels cell which is provided an electrode on the crystal. 6. The optical parametric oscillator according to claim 2, wherein at least one of said first and second polarization rotating means is a Faraday rotator combining a solenoid coil and a magnetic material.