JPH04179934A - Second harmonics generator - Google Patents

Abstract

PURPOSE:To attain the miniaturization of a whole device and the reduction of cost by inclining the basic wave incident surface of non-linear optical material with reference to the optical axis of the basic wave and feeding back return- light to semiconductor laser on the same optical axis as the incident basic wave. CONSTITUTION:In a 2nd harmonics generator provided with the semiconductor laser 1 for generating the basic wave and the non-linear optical material 5 for converting the basic wave to the 2nd harmonics, the incident surface of non-linear otical material 5 is inclined with reference to the optical axis of the basic wave so that the basic wave may resonate in a ring-like state in the inner part. And return-light for stabilizing the oscillation frequency of the semiconductor laser 1 is fed-back to the semiconductor laser 1 from the non- linear optical material 5 on the same optical axis as the incident basic wave. Thus, the need of a light isolator, a current feed-back system and a feed-back system for outgoing light can be eliminated, and the miniaturization of the device, the simplification of manufacture process and the reduction of cost can be attained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a monolithic ring-type second harmonic generator that causes a fundamental wave to resonate in a ring shape inside a nonlinear optical material.

[Prior Art] FIG. 4 shows a conventional second harmonic generator using an external resonator.

21 is a semiconductor laser (LD) for fundamental wave generation, and 22
23 is a collimating lens, 23 is an optical isolator, 24 is a mode matching lens for matching the incident beam with the resonance mode in the external resonator composed of mirrors 25a and 25b, and 25a
, b is a mirror for fundamental wave resonance, 26 is a nonlinear optical material such as Nbos crystal, and 27 is a combination of the fundamental wave (hereinafter referred to as ω light) and the second harmonic (hereinafter referred to as 2ω light) in the nonlinear optical material. Temperature control Peltier element for phase matching, 2
8 is a gichroic mirror that transmits 2ω light and reflects ω light, 29 is a miscellaneous wavelength plate that rotates the polarization direction by 90° to return linearly polarized ω light to the LD, and 30 is an L
This is a mirror for locking the oscillation frequency by feedback of ω light to D21.

FIG. 5 shows another conventional example, in which the air isolator is omitted and the nonlinear optical material is placed inside a triangular ring-shaped external resonator.

31 is a semiconductor laser (LD) for fundamental wave generation;
8 is a collimating lens, 33 is a mirror for an external resonator that resonates the fundamental wave (hereinafter referred to as ω light), 38' is a mode matching lens that matches the resonant mode in the external resonator and the incident beam, 34 is a nonlinear optical material such as KNbOz crystal, 35 is a dichroic mirror that transmits the second harmonic (hereinafter referred to as 2ω light) and reflects ω light, and 32 is L.
This is a PZT element equipped with a mirror to lock the oscillation frequency by feedback of ω light to the DI. 36 is a photodiode (PD) that detects the ω light reflected by the dichroic mirror, and 37 is a controller that drives the PZT element 32 based on the detection signal of the PD 36 and adjusts the angle of the mirror provided therein.

[Problem to be solved by the invention] In the case of a conventional second harmonic generation device using an external resonator having the configuration shown in FIG.
An optical isolator 23 is used to remove the light returning to the optical system 1. This optical isolator 23 is very expensive, and for the 0.8 μm band optical isolator used here, only materials with a small Verdet constant have been put into practical use as materials with high transmittance. Become something. Therefore, when modularizing the device, the entire device becomes large and expensive. Furthermore, in order to stabilize the oscillation frequency of the semiconductor laser, it is difficult to adjust the optical axis to return the output light from the resonator to the semiconductor laser (optical feedback), and the adjustment process takes time. .

In the case of FIG. 5, there is a triangular ring-shaped external resonator for resonance of ω light, and a PZT element 32 for controlling ω light feedback from the external resonator to LDl by detecting the output ω light. , the dichroic mirror 5, the PD 36, and the controller 7, there are problems in that adjusting the optical axes thereof is complicated and the overall size of the device increases.

In addition, a monolithic ring type second harmonic generator that does not use an external resonator has been proposed, but in the conventional monolithic ring type, the current value input to the LD was controlled to stabilize its oscillation frequency. . Such types required complex controls to monitor the oscillation frequency and provide current feedback. Furthermore, if there is light returning from the nonlinear optical material to the LD, it will disturb the current feedback system and make precise control impossible, so it is necessary to provide an optical isolator on the optical axis between the nonlinear optical material and the LD. Ta.

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes a semiconductor laser for generating a fundamental wave, a nonlinear optical material that converts the fundamental wave into a second harmonic, and A second harmonic generation device having a nonlinear optical material, wherein the incident surface of the nonlinear optical material is inclined with respect to the optical axis of the fundamental wave so that the fundamental wave resonates in a ring shape, and the oscillation of the semiconductor laser The present invention provides a second harmonic generation device characterized in that return light for frequency stabilization is returned from the nonlinear optical material to a semiconductor laser along the same optical axis as the incident fundamental wave.

The oscillation frequency of the LD is stabilized by returning weak scattered light generated inside the nonlinear optical material to the LD.

As nonlinear optical materials, β-BaBaO4, KTiO
Nonlinear optical crystals such as PO, KH2PO, KNbO, crystals, and organic nonlinear optical materials can also be used. Although various solid state and gas lasers can be used as the light source, an LD is preferable in terms of compactness and weight reduction.

[Operation] By using the above-described configuration and frequency stabilization method, the present invention eliminates the need for an optical isolator, a current feedback system to the LD, and a feedback system using emitted ω light, thereby realizing miniaturization and cost reduction. Furthermore, there is no need to adjust the optical axis for ω light feedback, making it possible to improve productivity and reduce costs.

[Example] FIG. 1 shows an embodiment of the invention.

The light from a semiconductor laser (LD) 1 with a wavelength of 860 nm is made into parallel light by a collimator lens 2 (f=3.58),
Aberrations of the LDI are corrected using a ×4 anamorphic prism 3, and a KNbOa crystal 5, which is a nonlinear optical material, is used.
A mode for matching the resonant mode in the KNbOx crystal 5 with the incident beam is made to enter the KNbOx crystal 5 via a matching lens (f=135) 4. At this time, since the ω light incident surface of the KNbO crystal 5 is tilted with respect to the incident optical axis, the reflected light is reflected from the LD.
I won't go back to I. Inside the KNbOz crystal 5, resonance occurs in the clockwise direction in the figure, and the power of the ω light is multiplied. The ω light is converted into second harmonic 2ω light by the KNbOz crystal 5. At this time, the KNbO1 crystal 5 is maintained at 27° C. by a Peltier element so that phase matching can be achieved between the ω light and the 2ω light.

The ω light is incident with a polarization direction parallel to the b-axis of the crystal,
The 2ω light is emitted in a polarization direction parallel to the C axis perpendicular to the 2ω light.

As a frequency stabilization method, an optical feedback method is used here. In other words, K constituting the triangular ring resonator
NbO, a weak backward traveling wave (
Frequency stability is achieved by injecting approximately 30 to 50 dB of frequency (counterclockwise in the figure) into the LDI. The oscillation frequency of the LDI is automatically drawn to the resonant frequency of the resonator and becomes stable (locked).

FIG. 2 shows the spectrum of 2ω light when the oscillation frequency of the LDI is stabilized. The vertical axis is the relative intensity of light when the peak value is 1. Figure 3 shows the spectrum before stabilization, and the spectrum in Figure 2 has almost no output (unnecessary components) other than 428.52 nm, which is 1/2 the oscillation frequency of the LDI. ．．
Only the 52 nm spectrum has been sharpened.

The feedback method of the present invention has the advantage that the returned light due to scattering and the incident light are coaxial, so there is no need to adjust the optical axis.

Further, if such a second harmonic generation device is used as a data detection light source for a pickup of an optical recording medium such as an optical disk or a magneto-optical disk, it becomes possible to read high-density data.

[Effects of the Invention] The present invention provides an optical isolator, a current feedback system, and a current feedback system that have been conventionally inserted between a semiconductor laser and an external resonator.
Since a feedback system for the emitted ω light is not required,
Achieved miniaturization of the device, simplification of the manufacturing process, and cost reduction. Further, the ω light feedback for frequency stabilization of the semiconductor laser, which is the light source of the fundamental wave, eliminates the need for optical axis adjustment, and has the effect of improving productivity.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, FIG. 2 is a graph of the second harmonic spectrum when the semiconductor laser is locked, and FIG. 3 is a graph of the second harmonic spectrum when the semiconductor laser is not locked. This is a graph of the spectrum of the second harmonic, and the fourth harmonic.
5 and 5 are block diagrams of a conventional example. l...LD 5...KNbOs crystal 8 Gourd @ 9 Pe
Figure Q

Claims (2)

[Claims] (1) A second harmonic generation device having a semiconductor laser for generating a fundamental wave and a nonlinear optical material that converts the fundamental wave into a second harmonic, the nonlinear optical material having a fundamental wave inside the ring. The plane of incidence is tilted with respect to the optical axis of the fundamental wave so as to resonate in a shape, and the return light for stabilizing the oscillation frequency of the semiconductor laser is transmitted from the nonlinear optical material along the same optical axis as the incident fundamental wave. A second harmonic generation device characterized in that the second harmonic wave is fed back to a semiconductor laser. (2) A pickup for an optical recording medium using the second harmonic generator according to claim 1 as a light source for detecting recorded information.