JP2000357833A - Wavelength conversion laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a wavelength conversion laser device capable of controlling the intensity of a higher harmonic output by maintaining a pumping input to be constant. SOLUTION: In a wavelength conversion laser device using a wavelength conversion crystal 4 or both a wavelength conversion crystal 4 and a wavelength selection element, electrodes 6 are installed on at least one out of the wavelength conversion crystal 4 and the wavelength selection element, and a voltage is applied to the electrodes 6 to generate an electric field. By the change of refractive index of the crystal 4 or the element which is caused by the electrooptic effect, the phase matching condition of a fundamental wave light and a higher harmonic wave light is changed and the intensity of the higher harmonic wave light is controlled, while the intensity of the fundamental wave light is maintained to be constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a light source for short-wavelength light,
The present invention relates to a wavelength conversion laser device used for an optical disk, a printing device, and the like, and more particularly to a wavelength conversion laser device that controls the intensity of a harmonic output using an electro-optic effect of a wavelength conversion crystal or a wavelength selection element.

[0002]

2. Description of the Related Art Conventionally, the following method is generally used as a method for controlling the intensity of the harmonic output of a wavelength conversion laser device. As shown in FIG. 5, an optical resonator (hereinafter, referred to as a resonator) including a mirror M1, a laser medium 23, and a wavelength conversion crystal 24.
A method of controlling the intensity of the harmonic output by adjusting the pump laser beam 10 incident on the resonator), a method in which a laser beam of another wavelength variable in output is input from outside the resonator, and a wavelength conversion crystal in the resonator is used. A method of controlling the intensity of the harmonic output by controlling the intensity of the laser light from outside the resonator as a converted output of the wavelength light of the sum frequency or the difference frequency with the fundamental light generated in the resonator, This is a method in which a modulator utilizing the electro-optic effect or acousto-optic effect is arranged, and the intensity of the laser beam output from the resonator is kept constant and the intensity of the harmonic output extracted from the modulator is controlled. .

[0003]

The conventional wavelength conversion laser apparatus uses the above-described method for controlling the output intensity of the harmonic wave, but has the following disadvantages. That is, in the method of adjusting the excitation laser light, a thermal effect by the excitation light is given to the laser medium, the wavelength conversion crystal, and the like by changing the excitation input. Such a thermal effect disturbs the intensity of the harmonic output and the stability of the wavelength distribution, and adversely affects the response in high-speed control. Further, there is a problem that the control becomes unstable because the intensity of the harmonic output changes nonlinearly with respect to the excitation input due to the fluctuation of the thermal effect on the wavelength conversion crystal. In particular, in a longitudinal mode single type wavelength conversion laser device using a wavelength selection element, there is a problem that a phenomenon called mode hop occurs in which an oscillation mode instantaneously shifts to another mode due to a change in thermal effect. Furthermore, when the harmonic output is controlled near zero, the control is performed around the oscillation limit value of the fundamental light, and when the harmonic output rises from zero, the response is delayed, so-called relaxation oscillation occurs, and it tends to be unstable. There's a problem.

In the method of irradiating a laser beam of another wavelength whose output is variable from outside the resonator, it is necessary to perform wavelength conversion efficiently by performing phase matching. Therefore, a distributed feedback (DFB) semiconductor laser is usually used as an external laser. It is necessary to use an expensive single-mode semiconductor laser such as the one described above, and it is difficult to manufacture because the external laser and the resonator need to be mode-matched. There is also a problem. Further, since an extra laser is disposed outside, there is a problem that the laser device is enlarged and the cost is increased. And, in a method using a modulator utilizing an electro-optic effect or an acousto-optic effect,
Since the modulator is arranged outside the resonator, there is a problem that the device itself becomes large.

The present invention has been made in view of such circumstances, and it is possible to control the intensity of harmonic output without changing the excitation input, and to increase or decrease the mode hop or the output near zero. It is an object of the present invention to provide a wavelength conversion laser device in which an instability factor for output control such as relaxation oscillation generated by the above is removed.

[0006]

In order to achieve the above object, a wavelength conversion laser device according to the first aspect of the present invention comprises a laser medium for generating a fundamental wave light for generating a harmonic light, and a laser medium for generating the fundamental wave light. In a wavelength conversion laser device including an excitation source for excitation and an optical resonator for oscillating fundamental light, and a wavelength conversion crystal for converting the fundamental light into harmonic light in the optical resonator, an electro-optical effect is obtained. Electrodes are provided on a pair of opposing surfaces of the wavelength conversion crystal having a voltage, a voltage is applied between the electrodes to generate an electric field, and the phase matching condition is changed by a change in the refractive index of the wavelength conversion crystal due to the electro-optic effect, thereby producing a harmonic output. Is performed.

According to a second aspect of the present invention, there is provided a wavelength conversion laser device according to the first aspect, wherein a wavelength selection element is disposed in an optical resonator to limit an oscillation wavelength width and to provide a phase matching condition. Is controlled to control the harmonic output.

A wavelength conversion laser device according to a third aspect of the present invention is arranged such that a wavelength conversion crystal and a wavelength selection element are arranged in an optical resonator, so that one of the longitudinal modes of an oscillation mode determined by a resonator mode or the like is obtained. In the wavelength conversion laser device of which only one is selected, only the wavelength selection element, or each of the wavelength selection element and the wavelength conversion crystal is provided with electrodes on a pair of opposing surfaces, and a voltage is applied between the electrodes to generate an electric field,
Control the phase matching condition determined by the change in the refractive index due to the electro-optic effect and the oscillation spectrum wavelength determined by the change in the cavity length, or control the wavelength selection region, or use at least one of the functions to control the harmonics. It is characterized in that the output is controlled.

The wavelength conversion laser device of the present invention is configured as described above, and the harmonic output can be controlled while the excitation input is kept substantially constant, so that a stable output wavelength conversion laser device can be obtained. .

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a wavelength conversion laser device according to the present invention will be described below with reference to the drawings. FIG. 1 is a schematic structural diagram of a first embodiment of the present invention. As shown in FIG. 1, a semiconductor laser (LD) 1 irradiates the laser medium 3 with its output light as excitation light via a converging optical system lens 2. On one end surface of the laser medium 3, a high-reflectance coat for transmitting the output wavelength of the semiconductor laser 1 and reflecting the stimulated emission light (fundamental wave) from the laser medium 3 with a high reflectivity is formed. A resonator is constituted by the high reflectance coat film and the coupling mirror 5, and a wavelength conversion crystal 4 such as a KNbO ₃ crystal having an electrode 6 on a pair of opposing surfaces is provided in the resonator. . On the other hand, the fundamental wave beam sampler 11b extracts a part of the fundamental wave light from the oscillation laser light 8, and the photodetector 12 detects the fundamental wave light and feeds it back to the semiconductor laser 1. The harmonic beam sampler 11a transmits the harmonic light from the oscillating laser light 8 and extracts a part of the harmonic light, and the photodetector 13 detects the harmonic light and feeds it back to the voltage controller 7.

In the above configuration, when the excitation laser light 10 from the excitation semiconductor laser 1 is incident on the laser medium 3 via the converging optical system lens 2, a fundamental wave light is generated from the laser medium 3 and the resonance occurs. Laser oscillation occurs in the vessel, and the laser oscillation is amplified to a large power oscillation laser beam 8. A part of the oscillation laser beam 8 is used as a fundamental wave beam sampler 11.
The fundamental wave power fed back to the semiconductor laser 1 for excitation via the optical detector b and the photodetector 12 and stored inside the resonator is controlled to be constant. The fundamental light is converted by the wavelength conversion crystal 4 into higher harmonic light having a certain phase difference with respect to the fundamental light. In this case, the intensity of the fundamental light is I _w , the wavelength is λ _w , the intensity of the harmonic light is I _{2 w} , the thickness of the wavelength conversion crystal 4 is L, the refractive indices for the fundamental light and the harmonic light are n _w , n _2w , and a constant. Let K be the wavelength conversion efficiency η
(= I _2w / I _w ) has a relationship represented by the following equation (1), and the conversion efficiency η becomes maximum when the phase matching condition of n _2w = n _w is satisfied. ^{_{η = L 2 KI w × SIN}} 2 (4πL (n 2w -n w) / λ w) / (4πL (n 2 w -n w) / λ w) 2 ...... (1) formula

This phase matching condition is determined by the wavelength conversion crystal 4
It can be obtained by changing the alignment of the wavelength conversion crystal 4 with an appropriate constant voltage applied to the electrode 6 and adjusting the output so that the maximum harmonic output is obtained. If the voltage of the electrode 6 is changed from this state, the phase matching condition is broken, the wavelength conversion efficiency is reduced, and the harmonic output changes. A part of the harmonic output 9 is fed back to the voltage controller 7 via the photodetector 13, and a voltage control signal is applied from the voltage controller 7 to the electrode 6, whereby the intensity of the harmonic output 9 is increased. Can be controlled stably.

In this case, the temperature of the wavelength conversion crystal 4 is controlled by using a thermo module (not shown), and the phase matching condition is optimized while a certain maximum voltage is applied to the wavelength conversion crystal 4. By performing processing and temperature control of the wavelength conversion crystal 4 so that the voltage difference between them is 0 volt and the harmonic output becomes zero, output calibration can be performed simply.

As described above, when the output is controlled by utilizing the reactivity of the wavelength conversion crystal 4 having a good electro-optic effect, the conversion efficiency to the harmonic light is controlled while the fundamental light is kept almost constant. Harmonic output can be increased or decreased, so that unstable phenomena such as relaxation oscillation that occur when the laser medium 3 is suddenly excited from a state in which the laser medium 3 is not excited do not occur. High-speed control is possible within the range.

FIG. 2 is a schematic configuration diagram of a second embodiment of the present invention. As shown in FIG. 2, this device is obtained by disposing a wavelength selection element 14 such as an etalon in the resonator of the wavelength conversion laser device of FIG. 1 showing the first embodiment. In this configuration, when the voltage applied to the pair of electrodes 6 formed on the wavelength conversion crystal 4 is changed,
The wavelength of the oscillation spectrum corresponding to the refractive index of the wavelength conversion crystal 4 and the transmission characteristic of the etalon are relatively shifted, the resonator loss is increased or decreased, and the intensity of the harmonic output is controlled. According to this method, when the same wavelength conversion crystal as the wavelength conversion crystal 4 in the first embodiment is used, the range of the voltage applied to the electrode 6 is made smaller, and the intensity of the harmonic output is controlled. In addition, when the voltage conversion crystal is used within the same voltage use range, the intensity of the harmonic output can be controlled similarly to the first embodiment even if a wavelength conversion crystal having a smaller electro-optic effect is used. .

Further, the intensity of the harmonic output can be controlled by causing the harmonic light to oscillate in a single mode using the configuration shown in FIG. This single mode oscillation can be generated by matching the gain curve (a) of the laser device, the resonance frequency (b) of the resonator, and the resonance frequency of the wavelength selection element shown in FIG. For example, an etalon is used as the wavelength selection element 14, and its angle with respect to the optical axis is finely adjusted.
As shown in (C), by adjusting the resonator spacing, the resonance frequency changes, and a single mode oscillation (d) can be obtained. When the voltage applied to the pair of electrodes 6 of the wavelength conversion crystal 4 is changed in the single mode oscillation state, the transmission characteristics and the refractive index of the wavelength conversion crystal 4 change corresponding to the refractive index of the wavelength selection element 14. When the wavelength characteristics of the corresponding oscillation spectrum are relatively shifted, the resonator loss increases and decreases, and the intensity of the harmonic output light can be controlled.

FIG. 4 is a schematic configuration diagram of a third embodiment of the present invention. This device comprises a wavelength selection element 15 having electrodes 16 provided on a pair of opposing surfaces of a wavelength selection element 14 used in the second embodiment shown in FIG. 2, and a voltage control for supplying a voltage to the electrodes 16. And a container 17. In this configuration, when a constant voltage is applied to the electrode 6 of the wavelength conversion crystal 4 and the voltage applied to the electrode 16 of the wavelength selection element 15 is changed, the phase condition and the resonator corresponding to the refractive index of the wavelength conversion crystal 4 are changed. The intensity of the harmonic output changes due to the relative shift between the oscillation spectrum characteristic determined by the length and the transmission characteristic that changes with the refractive index of the wavelength selection element 15. By feeding back a part of the harmonic output to the voltage controller 17 via the harmonic beam sampler 11a and the photodetector 13, the harmonic output can be controlled stably. By changing the voltage applied to the wavelength conversion crystal 4, the variable range of the harmonic output can be expanded.

When a constant voltage is applied to the electrode 16 of the wavelength selection element 15 and the voltage applied to the electrode 6 of the wavelength conversion crystal 4 is changed, the phase that changes corresponding to the refractive index of the wavelength conversion crystal 4 changes. The intensity of the harmonic output changes due to the relative shift between the oscillation spectrum characteristic determined by the matching condition and the resonator length and the transmission characteristic corresponding to the refractive index of the wavelength selection element 15. By feeding back a part of the harmonic output to the voltage controller 7 via the harmonic beam sampler 11a and the photodetector 13 as shown by a dotted line, the harmonic output can be controlled stably. The wavelength selection element 15
By changing the voltage applied to the electrode 16, the variable range of the harmonic output can be expanded.

[0019]

The wavelength conversion laser device according to the present invention is configured as described above, and controls the intensity of the harmonic output by utilizing the electro-optic effect of the elements necessary for the configuration of the laser.
It is possible to suppress an increase in size and cost. In addition, since operation can be performed without changing the excitation input energy, thermal effects other than the external temperature are kept constant in output control, resulting in instability in output control, hopping in vertical mode, and near zero output. The occurrence of relaxation oscillation or the like can be prevented.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a first embodiment of a wavelength conversion laser device of the present invention.

FIG. 2 is a configuration diagram showing a second embodiment of the wavelength conversion laser device of the present invention.

FIG. 3 is a characteristic diagram of a laser device for describing a second embodiment of the wavelength conversion laser device according to the present invention.

FIG. 4 is a schematic configuration diagram showing a third embodiment of the wavelength conversion laser device of the present invention.

FIG. 5 is a configuration diagram showing a conventional wavelength conversion laser device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Excitation semiconductor laser 2 ... Condensing optical system 3, 23 ... Laser medium 4, 24 ... Wavelength conversion crystal 5 ... Coupling mirror 6, 16 ... Electrode 7, 17 ... Voltage controller 8 ... Oscillation laser beam 9 ... Harmonic output 10 ... Excitation laser beam 11a ... Harmonic beam sampler 11b ... Basic wave beam sampler 12, 13, ...・ Photodetector 14, 15 ・ ・ ・ Wavelength conversion element M1 ・ ・ ・ Lens

Claims (3)

[Claims] 1. A laser medium for generating fundamental light for generating harmonic light, an excitation source for exciting the laser medium, and an optical resonator for oscillating the fundamental light. In a wavelength conversion laser device in which a wavelength conversion crystal for converting fundamental wave light to harmonic light is disposed, electrodes are provided on a pair of opposing surfaces of the wavelength conversion crystal having an electro-optic effect, and a voltage is applied between the electrodes to apply an electric field. A wavelength conversion laser device, wherein a phase matching condition is changed by a change in a refractive index of a wavelength conversion crystal generated by the electro-optic effect to control a harmonic output. 2. The device according to claim 1, wherein a wavelength selection element is arranged in the optical resonator to limit an oscillation wavelength width.
The wavelength conversion laser device as described in the above. 3. A wavelength conversion laser device in which only one of longitudinal modes of an oscillation mode determined by a resonator mode or the like is selected by disposing a wavelength conversion crystal and a wavelength selection element in an optical resonator. An electrode is provided on a pair of opposing surfaces for only the wavelength selection element or each of the wavelength selection element and the wavelength conversion crystal, and a voltage is applied between the electrodes to generate an electric field, which is determined by a refractive index change due to the electro-optic effect. Controlling the oscillation spectrum wavelength determined by the phase matching condition and the change in the resonator length, or controlling the wavelength selection region, or controlling the harmonic output using at least one of the functions. Wavelength conversion laser device.