JPH06291402A - Harmonic generator - Google Patents

Abstract

(57) [Summary] [Purpose] When the light is returned to the LD to lock it,
The optical phase is optimized so that a highly stable second harmonic output can be obtained. [Structure] Of the output light from a monolithic resonator 15, only a fundamental wave is reflected by a dichroic mirror 16.
The fundamental wave is adjusted to an optimum light quantity through the lens 17 and the ND filter 18, and the phase is adjusted by the folding mirror 20 to form an LD.
The light is incident on the emission facing surface 12 of the light source.

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 converting a fundamental wave emitted from a light source such as a semiconductor laser into a harmonic in a resonator.

[0002]

2. Description of the Related Art In recent years, various devices have been proposed to obtain a wavelength-converted second harmonic or third harmonic by passing a fundamental wave emitted from a semiconductor laser or the like through a non-linear optical material such as KNbO ₃ crystal. There is. In these devices, a nonlinear optical material is arranged in a resonator composed of a plurality of reflecting surfaces, and a fundamental wave is confined in the resonator to be amplified so that harmonics are efficiently generated.

As a resonator, a reflecting film is provided on the end face of a non-linear optical material, and a monolithic resonator for resonating inside the reflecting film and a plurality of mirrors are arranged to form the resonator. An external resonator in which a non-linear optical material is arranged is known. In recent years, in order to downsize the device and improve the conversion efficiency to higher harmonics, the mainstream is shifting from the external resonator type to the monolithic type that resonates the fundamental wave inside the nonlinear optical material. I am doing it.

FIG. 2 shows, as an example of a conventional harmonic generator, a second harmonic generator using a monolithic resonator.

The second harmonic generator 1 is composed of a semiconductor laser (hereinafter referred to as LD) 2, a collimating lens 3, a mode matching lens 4, and a nonlinear optical material 5 made of KNbO ₃ crystal or the like. The LD 2 emits the fundamental wave 6 having a wavelength of 860 nm, for example. The two surfaces on the left and right in the figure of the nonlinear optical material 5 are polished into spherical shapes.

Of these, the surface on the left side of the drawing is the incident surface of the fundamental wave 6, and a spherical mirror 8 is formed on this surface, which partially transmits the fundamental wave 6 and reflects the second harmonic wave 7. There is. The surface on the right side of the drawing is an exit surface for the second harmonic wave 7, and a spherical mirror 9 that reflects the fundamental wave 6 and transmits the second harmonic wave 7 is formed on this surface. Further, the lower surface of the non-linear optical material 5 in the figure forms a plane mirror 10 that totally reflects both the fundamental wave 6 and the second harmonic wave 7.

In the above structure, the fundamental wave 6 having a wavelength of 860 nm emitted from the LD 2 is collimated by the collimator lens 3, passes through the mode matching lens 4, and passes from the point A of the spherical mirror 8 of the nonlinear optical material 5. Incident. At this time, the fundamental wave 6 is incident on the crystal axis a at a specific angle θ so that the fundamental wave 6 incident on the point A travels in parallel with the crystal axis a of the nonlinear material 5.

This fundamental wave 6 is generated by two spherical mirrors 8 and 9.
, And a plane mirror (total reflection mirror) 10 are reflected by a ring type at points A, B, and C in the ring resonator to be amplified. When the fundamental wave 6 passes through the nonlinear optical material 5 in the direction of the crystal axis a, a part of the fundamental wave 6 has a wavelength of 430n.
It is converted into the second harmonic wave 7 of m and emitted from the point B of the spherical mirror 9. The nonlinear optical material 5 is temperature-controlled by a Peltier element or the like in order to meet the phase matching condition and stabilize the conversion efficiency to the harmonic. By using such a harmonic generator, the fundamental wave can be efficiently converted into a harmonic.

[0009]

In the above-mentioned conventional second harmonic generator, the LD oscillation frequency is locked to the resonator frequency by returning a slight amount of the counter-rotating fundamental wave light generated in the resonator to the LD. A method called optical feedback control for stabilizing the output is used. In this case, the return light is very weak and the amount of light cannot be adjusted,
The problem is that it is difficult to obtain sufficient stability.

Therefore, an object of the present invention is to use the fundamental wave light of the resonator mode included in the output light as the return light from the resonator to the LD to set the optimum light quantity and adjust the phase with a piezoelectric element or the like. Then, it is to provide a harmonic wave generating device capable of stabilizing the output and adjusting the output by directly entering the LD emission facing surface.

[0011]

In order to achieve the above object, a harmonic generator of the present invention includes a semiconductor laser for generating a fundamental wave and a resonator including a nonlinear optical crystal for converting the fundamental wave into a harmonic. The harmonic generator provided is characterized in that only the fundamental wave light contained in the light emitted from the resonator is incident on the semiconductor laser emission facing surface to obtain a highly stable harmonic output.

Further, in the present invention, when the fundamental wave from the resonator is incident on the emission facing surface of the semiconductor laser, an ND filter is inserted in the optical path, and by adjusting this, the light quantity of the fundamental wave is optimized and stabilized. This is what you get.

Further, when the fundamental wave from the resonator is made incident on the emission facing surface of the semiconductor laser, a mirror to which a piezoelectric element is bonded is inserted in the optical path, and by adjusting this, the phase of the fundamental wave light is made variable. , To adjust the output.

[0014]

The harmonic generating device of the present invention is arranged so that only the fundamental wave of the output light from the resonator is incident on the LD emission facing surface by using the mirror, the lens, the ND filter and the piezoelectric element.

Here, when the fundamental wave light included in the output light from the resonator is not directly incident on the LD, only the weak return light generated in the resonator is LD on the route opposite to the path from the LD to the optical system. Will return to. The returned light from this resonator is very weak and depends on the crystal and processing quality, and is a physical quantity that cannot be adjusted. Therefore, when the LD frequency is locked to the resonator frequency, it is difficult to secure the output stability because there is a large variation between the resonators and the locking is weak.

On the other hand, the second harmonic output light can be stabilized by condensing the fundamental wave contained in the output light with the lens, turning it back with the mirror, and directly entering the LD emission facing surface. Further, in this case, by inserting an ND filter between the lens and the mirror, the amount of incident light can be optimized and high stability can be realized. Further, by adjusting the phase of the folding mirror with a piezoelectric element or the like, in addition to stabilization, output adjustment is possible.

[0017]

FIG. 1 shows an embodiment in which the present invention is applied to a second harmonic generation device. The present invention is not limited to the second harmonic generation device and can be applied to the third harmonic generation device.

The second harmonic generator 11 comprises an LD 12 as a laser light source, a collimator lens 13, a mode matching lens 14, and a monolithic resonator 15 which are sequentially arranged. Further, the output light from the monolithic resonator 15 transmits only the second harmonic by the dichroic mirror 16 and reflects the fundamental wave. This fundamental wave is condensed by the lens 17, adjusted to an optimum amount of return light by the ND filter 18, and then phase-adjusted by the folding mirror 20 to which the piezoelectric element 19 is adhered to perform the LD1.
The light is incident on the emission-opposing surface of No. 2.

The LD 12 has a wavelength of 860n in this embodiment.
m, single longitudinal, single transverse mode, which emits a fundamental wave with little astigmatism is used. Further, 5% and 90% of antireflection films are applied to the emission end face and the emission opposite face by a vacuum deposition method, respectively. The collimator lens 13 collimates the fundamental wave 21 emitted from the LD 12 into a parallel beam, and the mode matching lens 14 narrows the beam to match the resonance mode in the monolithic resonator 15 with the incident beam. Monolithic resonator 15
In this embodiment, KNbO ₃ is used as the nonlinear optical crystal.

One end face of the monolithic resonator 15 located on the incident side of the fundamental wave 21 is formed in a spherical shape, and a reflection film for reflecting the fundamental wave by 92% is vapor-deposited near the point A on this face. It is regarded as a spherical mirror. On the other hand, the end face of the resonator 15 located on the emission side of the second harmonic is also formed into a spherical shape, and the fundamental wave is near the point B on this face.
A reflective film that reflects 9% or more and transmits 90% or more of the second harmonic is vapor-deposited to form a spherical mirror. The plane including the point C of the resonator 15 is formed so as to satisfy the condition of total reflection.

Using this second harmonic generation device, blue output light of 10 mW or more was obtained from the mirror 16 which transmits 99% or more of the second harmonic. On the other hand, the fundamental wave light reflected by the mirror 16 is 1 to 2 mW, and after the output light is condensed by the lens 17, it is 0.5 to 0.5% by using the filter 18 which transmits 50%.
The mirror 2 adjusted to about 1 mW and bonded with the piezoelectric element 19
The light was incident on the emission facing surface of the LD 12 by 0. As a result,
A stable blue output exceeding 10 mW was obtained for 10 hours or more. Further, the second harmonic output can be adjusted by controlling the piezoelectric element 19, and the output ON-OF can be adjusted.
F control was also possible.

[0022]

As described above, according to the present invention,
The fundamental wave included in the output from the resonator is input to the LD emission facing surface by using a mirror and a lens, so that a stable second
Harmonic output is obtained. Further, by using the ND filter and the piezoelectric element together, it is possible to optimize the return light amount and the optical phase, realize higher stabilization of the output, and configure the second harmonic generation device capable of adjusting the output. A practical information reading device for an optical recording medium can also be realized by using the second harmonic generation device as a light source for information detection.

[Brief description of drawings]

FIG. 1 is a side view showing an embodiment of a harmonic generator of the present invention.

FIG. 2 is a side view showing an example of a conventional second harmonic generation device.

[Explanation of symbols]

 11: Second harmonic generation device 12: Semiconductor laser (LD) 13: Collimating lens 14: Mode matching lens 15: Monolithic resonator 16: Dichroic mirror 17: Condensing lens 18: ND filter 19: Piezoelectric element 20: Folding mirror 21: Basic wave

Claims (3)

[Claims] 1. A harmonic generator comprising a semiconductor laser for generating a fundamental wave and a resonator including a non-linear optical crystal for converting the fundamental wave into a harmonic, and a fundamental contained in light emitted from the resonator. A harmonic generation device characterized in that a high-stable harmonic output is obtained by causing only waves to enter the semiconductor laser emission facing surface. 2. When a fundamental wave from a resonator is made incident on the emission facing surface of a semiconductor laser, an ND filter is inserted in the optical path and adjusted to optimize the light quantity of the fundamental wave and to provide stable output. The harmonic generator according to claim 1, which is obtained. 3. When the fundamental wave from the resonator is incident on the emission facing surface of the semiconductor laser, a mirror to which a piezoelectric element is bonded is inserted in the optical path, and the phase of the fundamental wave is made variable by adjusting the mirror. The harmonic generator according to claim 1, wherein the output is adjusted.