JPH0715077A - Harmonic generator - Google Patents

Abstract

(57) [Abstract] [Purpose] When the frequency control of the semiconductor laser (LD) is performed by the optical feedback method, the frequency fluctuation permissible range (rocking range) of the LD is expanded to further stabilize the harmonic output. [Structure] Conventional locking range of about 2 GHz (LD
1. Allowable range of injection current fluctuation and allowable temperature fluctuation range are about ± 1.4.
mA, ± 0.06 ° C) and end face reflectance of 2%, about 3.4 GHz (about ± 2.4 mA, about ± 0.0)
1 ° C).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a harmonic generator for generating a harmonic by making a fundamental wave of a semiconductor laser incident on a nonlinear optical material such as KNbO ₃ crystal.

[0002]

2. Description of the Related Art A conventional second harmonic generator is shown in FIG. The conventional second harmonic generator is a semiconductor laser (resonator side end face reflectance R _f = 5%, rear face reflectance R _r = 90%,
Hereinafter, referred to as LD) 1, a monolithic resonator 4 composed of KNbO ₃ crystal itself, LD 1 and temperature control devices (Peltier elements) 2a and 2b of the resonator 4 respectively, and a coupling optical system 3. .

The frequency control (frequency stabilization) for matching the oscillation frequency of the LD 1 with the resonance frequency of the resonator 4 causes the counterclockwise resonance light generated by the scattered light inside the resonator 4 to return to the return light 5.
The optical feedback method (rocking) for returning to LD1 is performed. In FIG. 4, 6a and 6b are antioxidant films.

[0004]

In the conventional device, when the frequency control of the LD is performed by the optical feedback method, the LD frequency fluctuation permissible width (locking range) is narrow, so that the locking is prevented by disturbance such as environmental temperature change. There is a problem that the harmonic output is likely to be destabilized and become unstable.

[0005]

The present invention has been made to solve the above-mentioned problems, and includes a semiconductor laser for generating a fundamental wave and a nonlinear optical material for converting the fundamental wave into a harmonic wave. A harmonic generation device comprising a resonator for resonating a fundamental wave with a plurality of reflection surfaces, wherein the end face reflectance of the semiconductor laser on the fundamental wave emission side is reduced. It is provided.

In the present invention, the resonator may be a monolithic resonator in which a plurality of surfaces of a non-linear optical material are provided with reflecting surfaces of the fundamental wave by a dielectric multilayer film, and the resonators are arranged on the optical axis between the plurality of resonance mirrors. It may be a discrete resonator in which a nonlinear optical material is arranged. The present invention can be applied not only to the second harmonic generator but also to higher harmonic generators.

[0007]

The harmonic generator of the present invention can increase the amount of return light returned to the LD by lowering the end face reflectance of the LD, and as a result, the LD frequency fluctuation allowable width (rocking range). Can be expanded.

[0008]

FIG. 1 shows the basic structure of the device of the present invention. In FIG. 1, the same parts as those in FIG. 4 of the conventional example are designated by the same reference numerals, and the description thereof will be omitted. The oscillation wavelength is 860 nm,
The light of the LD (resonator side facet reflectivity R _f = 2%, rear face reflectivity R _r = 99%) 1 adjusted by using the Peltier element 2a is converted by the coupling optical system 3 into a KN having an optical path length of 12 mm.
The incident light was made to match the resonance mode of the bO ₃ resonator (monolithic resonator) 4. A low reflection film 7a is provided on the emission surface of the fundamental wave of the LD1, and a high reflection film 7b is provided on the surface (rear surface) opposite to the emission surface of the LD. As the low-reflection film 7a, a single film of amorphous silicon, alumina or the like, or a film formed by combining them is preferably used.

The rear surface reflectance of the LD is set to 99% in order to suppress the increase of the threshold current as much as possible. Further, the light directly reflected on the incident surface of the resonator 4 does not return to the LD because the optical axis is different. Fundamental wave (lambda ₁ = 860 nm) is resonant multiplication clockwise in FIG. 1 in the resonator 4, the second harmonic (lambda ₂
= 430 nm). At this time, the KNbO ₃ resonator 4 satisfies the phase matching condition and the resonance condition,
The temperature was controlled by the Peltier device 2b so that the temperature was maintained at 27 ° C. The fundamental wave is incident in the polarization direction parallel to the b-axis of the crystal, and the second harmonic is emitted in the polarization direction orthogonal to it.

In the second harmonic generator using such a resonator, it is necessary to make the resonance frequency of the resonator and the oscillation frequency of the LD match. Here, counterclockwise (reverse to the resonance light in FIG. 1) resonance light generated from the resonator is returned to the LD, and optical feedback control is performed to lock the oscillation frequency of the LD to the resonance frequency of the resonator.

2 (a) and 2 (b) show schematic diagrams of the principle. In these figures, the case where the returning light does not exist and the case where the returning light exists are shown by a broken line and a solid line, respectively. Figure 2
The horizontal axis of (a) is the oscillation frequency (ω _N ) of the LD alone, which is determined by the current injected into the LD and the temperature, and the vertical axis is the composite resonator system (LD + resonator when there is return light from the resonator). ) Is the frequency (ω).

FIG. 2B shows the resonator transmission spectrum at this time. By returning the light having the information of the resonance frequency from the resonator to the LD, the oscillation frequency of the LD is drawn to the resonance frequency of the resonator and is locked at that frequency. At this time, the resonator transmission spectrum has a band called a rocking range. Within this range, even if the LD injection current or temperature changes and the frequency of the LD itself shifts, the LD frequency and the resonance frequency of the resonator are equal. I can't release the lock.

This rocking range can be expanded by reducing the end face reflectance (R _f ) of the LD on the resonator side. FIG. 3 shows the relationship between the end surface reflectance (R _f ) of the LD and the rocking range. The values of the rocking range in the figure are both standardized by the value when the experimental value and the calculated value are 5%. From this figure, the end face reflectance is 2%
The rocking range could be expanded by about 1.7 times by reducing to 10.

That is, in the conventional case, a rocking range of about 2 GHz (corresponding to the LD injection current fluctuation allowance and temperature fluctuation allowance is about ± 1.4 mA and ± 0.06).
By setting the end face reflectance to 2% as in this example, the rocking range is about 3.4 GHz (LD injection current fluctuation allowable range, temperature fluctuation allowable range is about ± 2.4 mA, about ± 0). It was effective in improving the stability of the entire system, controlling the current, and relaxing the temperature control accuracy.

Further, as shown in FIG. 3, the rocking range can be expanded by further lowering the end face reflectance.

In this embodiment, KNbO ₃ crystal was used as the monolithic resonator, but LiNbO ₃ and β-BB were used.
The structure of the present invention is also effective in a monolithic resonator formed of a non-linear optical crystal such as O, LBO, or KTP. Furthermore, regarding LD, Fabry-Perot type, D
It is effective for both FB type and DBR type.

[0017]

According to the present invention, compared with the conventional second harmonic generator, the rocking range is expanded and the optical feedback is strengthened with the reduction of the end facet reflectivity of the LD, and the allowable range of the frequency fluctuation of the LD is about. It expanded about 1.7 times and improved the stability of the entire system.

[Brief description of drawings]

FIG. 1 is a side view showing a basic configuration of a second harmonic generation device of the present invention.

FIG. 2 illustrates an optical feedback method, and FIG. 2A illustrates an LD in the case where the return light from the resonator is present and in the case where the return light is not present.
A graph showing the relationship between a single frequency and a composite frequency,
(B) is a graph showing the relationship between the frequency of the LD alone and the transmitted light intensity of the resonator.

FIG. 3 is a graph showing a relationship between an LD end face reflectance and a rocking range.

FIG. 4 is a side view of the basic configuration of a conventional second harmonic generation device.

[Explanation of symbols]

 1: Semiconductor laser (LD) 2a: Temperature controller (Peltier element) 2b: Temperature controller (Peltier element) 3: Coupling optical system 4: Monolithic resonator 5: Return light from resonator 6a: Antioxidant film 6b : Antioxidant film 7a: Low reflective film 7b: High reflective film

Claims (1)

[Claims] 1. A harmonic generator comprising a semiconductor laser for generating a fundamental wave, and a resonator which includes a nonlinear optical material for converting the fundamental wave into a harmonic wave and resonates the fundamental wave with a plurality of reflecting surfaces. 2. A harmonic generation device according to claim 1, wherein the end face reflectance of the semiconductor laser on the fundamental wave emission side is reduced.