JPH088480A - Laser device - Google Patents

Abstract

PURPOSE:To enable a laser device to be lessened in SHG noise and kept stable in output by a method wherein an etalon is provided inside a resonator, and an injection current is controlled by feedback of the spectrums of fundamental waves obtained through double refraction of a nonlinear optical crystal. CONSTITUTION:An inner resonator type SHG laser resonator is composed of an Nd:YV 0412 fixed to a holder 13 of a Peltier device 19, a KTP 15 fixed to a holder 14, an etalon 16, and an output mirror 17 fixed to a holder 18. The exciting light of a semiconductor laser 10 is made to impinge on the end face of the holder 13 to excite the Nd:YV 0412 to excite the fundamental wave, and SHG is oscillated by the KTP 15. Then, the fundamental wave is split by a beam splitter 20 and further by a polarized beam splitter 24 and is made to impinge on photodetector element 25 and 26 respectively. The second harmonic is split by a beam splitter 21 and is made to impinge on a photodetector element 27. Intensity data of the lights impinging on the photodetector elements 25 to 27 are inputted to a control circuit 37 so as to make the fundamental wave linearly polarized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an internal cavity type wavelength conversion laser using a nonlinear optical crystal, and more particularly to stabilization of laser output or reduction of noise.

[0002]

2. Description of the Related Art There is an increasing demand for compact short wavelength lasers for high density optical discs and high definition laser printers. SH using a nonlinear optical crystal as a short wavelength laser
There is a G (second harmonic generation) laser. In particular, the method of pumping the internal cavity SHG laser having the nonlinear optical crystal 4 inside the laser cavity as shown in FIG. 2 with the semiconductor laser 1 from the end face of the solid-state laser crystal 3 has various applications due to the small size and high efficiency oscillation. Is expected. By the way, the laser of FIG. 2 is a standing wave type solid-state laser and generally oscillates in a longitudinal multimode unless special means is provided. In the internal cavity SHG laser, when the fundamental wave oscillates in the longitudinal multimode, mode competition in the cavity causes SHG noise during wavelength conversion in the nonlinear optical crystal, and the SHG output becomes unstable.

In order to reduce SHG noise and stabilize the output, it is effective to oscillate the fundamental wave in the single longitudinal mode. Therefore, insert an etalon (an element that applies reflective coating on both sides of a transparent parallel plate and uses that multiple reflection to select a wavelength) inside the resonator, and then use another wavelength (longitudinal mode).
The method of suppressing the laser oscillation of was considered. This is because the effective optical path length in the etalon is changed by tilting the etalon, and the selected wavelength of the etalon is matched with one of the many resonator modes (the discrete wavelengths that satisfy the resonance condition) to create a single mode. This is a method of oscillating. By this method, the fundamental wave is oscillated in the longitudinal single mode, and the stable output SHG
Is obtained. An example of this is the IEEE Journal of Quantum Electronics (IEEE Journ
al of Quantum Electronics), vol. QE-10, No.2, p2
53 (1974)), Spring 1993 40th Joint Lecture on Applied Physics Lecture Proceedings No 3, p996, 31p-Z-4 and the like (Fig. 3).

[0004]

A nonlinear optical crystal such as KTP (KTiOPO4) that performs phase matching of TYPE2 when SHG occurs has birefringence, so that the polarization of light passing through the crystal changes due to temperature change. . On the other hand, since the laser crystal generally has a gain depending on the polarization direction, the laser output changes when the polarization of the fundamental wave inside the resonator changes. Since this variation cannot be controlled by the wavelength selectivity of the etalon, the conventional SHG laser device using this has an SH
G noise occurs and the output becomes unstable.

Another problem is that the decrease of the output and the change of the wavelength due to the change with time of the excitation semiconductor laser result in the decrease of the excitation energy absorbed by the laser crystal, resulting in the decrease of the SHG output.

The present invention takes this point into consideration. In a wavelength conversion laser device represented by an internal cavity type SHG laser, an etalon is used inside the cavity and the polarization of the fundamental wave selected by the etalon is kept constant. To stabilize the fundamental wave single mode oscillation, S
HG noise is reduced and output is stabilized. In addition, the output and wavelength of the pumping light are controlled to handle changes over time, and SH
It is to provide a laser device in which the G output does not decrease.

In addition to the above, the present invention considers that a difference in performance due to variations in laser device parts can be adjusted and made constant after assembly, and when the laser device is incorporated into another device and applied, it is compact. An object is to provide a laser device that can take a convenient form.

[0008]

An optical path length of a resonator is changed to move a resonator mode within a wavelength range selected by an etalon. Further, the nonlinear optical crystal is made into a full-wave plate or a half-wave plate by controlling birefringence. This makes it possible to obtain stable longitudinal single mode oscillation in which the polarization direction of the fundamental wave in the laser crystal is constant. Further, the injection current and the temperature of the semiconductor laser are controlled with respect to the decrease in the output and the change in the wavelength due to the change with time of the excitation semiconductor laser. These series of controls determine the control direction by monitoring the output SHG and fundamental wave. At this time, if control is performed by a programmable LSI, it is possible to perform programming that optimizes the difference in control parameters due to component variations for each laser device after device assembly. Further, the laser device can be downsized by using the LSI.

[0009]

In order to suppress the mode competition of the fundamental wave, it is necessary to increase the reflectance of the etalon surface and narrow the wavelength selection width. The movement of the resonator mode due to the change in the resonator length is large with respect to this wavelength selection width, and even a slight change in the resonator length causes the resonator mode to deviate from the etalon wavelength selection width. Therefore, the optical path length of the resonator is controlled by controlling the resonator temperature and the resonator mode is kept within the etalon wavelength selection width. Of course, the allowable temperature range in this case is small.
On the other hand, the temperature tolerance for controlling a birefringent nonlinear optical crystal such as KTP into a full-wave plate or a half-wave plate is relatively large.

Therefore, in order to satisfy these two conditions, first, the temperature of the nonlinear optical crystal is set to a full-wave plate or a half-wave plate, and the temperature is finely adjusted in the vicinity of the temperature to make it within the etalon wavelength selection range. Move the resonator mode. This series of operations is performed while referring to the P and S polarized outputs of the fundamental wave and the SHG output. That is, in order to set the temperature at which the nonlinear optical crystal becomes a full-wave plate or a half-wave plate, the P and S polarization outputs of the fundamental wave are monitored, and one is maximum and the other is minimum (linear polarization). To become). Further, when the resonator mode is moved within the wavelength selection width of the etalon, the temperature of the resonator is changed so that the SHG output becomes maximum.

By the above operation, stable SH without noise
G output is obtained. However, when the specified SHG output cannot be obtained by this, the pumping power is lowered or the pumping laser wavelength is deviated, so the injection current and temperature of the pumping semiconductor laser are gradually increased while monitoring the SHG output. Adjust alternately. By the above operation, stable SHG output without output reduction can be obtained.

[0012]

【Example】

(Example 1) As shown in FIG. 4, Nd: YVO4 12 fixed to a holder 13 on a Peltier element 19, KTP (KTiOPO4) 15 fixed to a holder 14, etalon 16, holder 18
The internal mirror type SHG laser resonator is constituted by the output mirror 17 fixed to. At this time, the c-axis of 12 and the c-axis of 15 are set to be 45 °. The left surface of 12 and the left surface of 17 are highly reflective coatings for the wavelength of 1064 nm, and the left surface of the mirror 17 is further anti-reflection coating for 532 nm. The excitation light of the semiconductor laser 13 having a wavelength of about 810 is made to enter from the end surface of the holder 13 by the condenser lens 11.
Excite Nd: YVO4 12. As a result, 1
The fundamental wave of 064 nm oscillates, and the KTP 15 oscillates SHG of 532 nm. Two different wavelength lasers, a fundamental wave and SHG, are emitted from the mirror 17.
The fundamental wave of the emitted laser is beam splitter 20.
And the polarized beam splitter 24 separates the polarized light into P and S polarized lights, which are incident on the light receiving elements 25 and 26, respectively.

The SH transmitted through the beam splitter 20
A part of G is separated by the beam splitter 21, and the light receiving element 27
Incident on. The information of the light intensity entering 25, 26 and 27 is converted into an electric signal and input to the control circuit 37. When the light intensity changes, 37 issues an instruction to the temperature controller 38 to control the resonator temperature to increase or decrease so that the SHG light intensity increases and the fundamental wave becomes linearly polarized. If the SHG output cannot be restored only by controlling the resonator temperature, an instruction is issued to the semiconductor laser temperature controller 29 to change the semiconductor laser temperature so that the SHG output becomes the maximum temperature. If the SHG output still does not return to the original value, the laser driver 28 is instructed to increase the semiconductor laser injection current and return the SHG output to the original value.

(Embodiment 2) In a device as shown in FIG. 1, the resonator is constructed on a Peltier element as in Embodiment 1.
The output laser from this resonator is made incident on the light receiving element 43 from the left using the beam splitters 40, 41 and 42 for kicking out the fundamental wave S-polarized light, the fundamental wave P-polarized light and a part of the SHG.
A resonator mounted on these Peltier elements, a beam splitter, a light receiving element, a semiconductor laser for excitation, and a condenser lens are housed in a housing 45 and sealed for the SHG output 50. 45 and 42 are bonded and there is no movement of air. The electric signal from 43 is input to an LSI (Large Scale Integrated Circuit) 46.
In 46, control parameters such as an initial cavity temperature, a semiconductor laser temperature, an injection current, etc., which are matched with the cavity, and a control algorithm are programmed in advance. According to the instruction of 46, the resonator temperature, the semiconductor laser temperature, and the semiconductor laser injection current are controlled to obtain a stable SHG output.

[0015]

According to the present invention, a small internal resonator type SHG laser using an etalon can be operated with stable output without noise. Because the output is stable and the size is small, optical disk system, laser beam printer,
It can be effectively used in other fields.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing a first embodiment of the present invention.

FIG. 2 is an explanatory view of a structure of a conventional semiconductor laser pumped internal cavity type SHG laser.

FIG. 3 is an explanatory view of a structure of a conventional semiconductor laser pumped internal cavity type SHG laser using an etalon.

FIG. 4 is an explanatory diagram showing a second embodiment of the present invention.

[Explanation of symbols]

10 ... Semiconductor laser, 11 ... Focusing lens, 12 ... Nd: YV
O4, 13 ... Nd: YVO4 holder, 14 ... KTP (KTiOPO4)
Holder, 15 ... KTP (KTiOPO4), 16 ... Etalon,
17 ... Output mirror, 18 ... Output mirror holder, 27 ... Light receiving element, 28 ... Semiconductor laser driver, 29 ... Temperature controller, 31 ... Wiring to thermistor, 32 ... Wiring for semiconductor laser injection current, 33 ... Wiring for semiconductor laser monitor , 35 ... Peltier current wiring, 36 ... Peltier current wiring, 40 ... Beam splitter, 41 ... Beam splitter, 42 ... Beam splitter, 43 ... Light receiving element, 45 ...
Enclosure, 46 ... LSI, 50 ... SHG output window.

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display area H01S 3/131 3/16

Claims (13)

[Claims] 1. A wavelength conversion laser using a non-linear optical crystal inside a laser resonator using an etalon, which is excited by a semiconductor laser having means for controlling birefringence of the non-linear optical crystal and a resonator optical path length. And laser equipment. 2. A laser device according to claim 1, further comprising means for controlling the birefringence of the nonlinear optical crystal and the optical path length of the resonator by referring to the powers of the fundamental wave laser and the wavelength-converted laser to be output. 3. A laser device as set forth in claim 2, wherein a semiconductor laser is used to control one or both of temperature and injection current with reference to the powers of the fundamental wave laser and the wavelength-converted laser to be output. 4. The laser device according to claim 2, wherein a large-scale integrated circuit is used for control. 5. The laser device according to claim 3, wherein the large scale integrated circuit is used for control. 6. The laser device according to claim 2, wherein the temperature control element controls the birefringence of the nonlinear optical crystal and the optical path length of the resonator. 7. The laser device according to claim 6, wherein one temperature control element controls the birefringence of the nonlinear optical crystal and the cavity optical path length. 8. A laser device according to claim 3, wherein the temperature control element controls the birefringence of the nonlinear optical crystal and the cavity optical path length. 9. The laser device according to claim 8, wherein the temperature control element controls the birefringence of the nonlinear optical crystal and the cavity optical path length. 10. The laser device according to claim 4, wherein the birefringence of the nonlinear optical crystal and the optical path length of the resonator are controlled by a temperature control element. 11. The laser device according to claim 10, wherein one temperature control element controls the birefringence of the nonlinear optical crystal and the cavity optical path length. 12. The laser device according to claim 5, wherein the temperature control element controls the birefringence of the nonlinear optical crystal and the cavity optical path length. 13. The laser device according to claim 12, wherein the birefringence of the nonlinear optical crystal and the optical path length of the resonator are controlled by a temperature control element.