JP2963220B2 - Second harmonic generator and optical recording medium pickup - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a second harmonic generator using a nonlinear optical material having an external resonator.

[0002]

2. Description of the Related Art A second harmonic generator using a conventional external oscillator is shown in FIG.

Reference numeral 21 denotes a semiconductor laser (hereinafter referred to as an LD) for generating a fundamental wave, reference numeral 22 denotes a collimating lens, reference numeral 23 denotes an optical isolator, and reference numeral 24 denotes a resonance mode in an external resonator constituted by mirrors 25a and 25b. A mode matching lens for matching a beam, 25a and 25b are mirrors for fundamental wave resonance, 2
Reference numeral 6 denotes a nonlinear optical material such as a KNbO ₃ crystal, etc., and reference numeral 27 denotes a temperature control for performing phase matching of a fundamental wave (hereinafter referred to as ω light) and a second harmonic (hereinafter referred to as 2ω light) by temperature in the nonlinear optical material. A Peltier device, 28 is a dichroic mirror that transmits 2ω light and reflects ω light, 29 is a λ / 2 plate for rotating the polarization direction by 90 ° to return linearly polarized ω light to the LD 21, and 30 is an LD 21 This is a mirror for locking the oscillation frequency by the ω light feedback to the mirror.

[0004]

The conventional second harmonic generator using an external resonator has a configuration as shown in FIG. 2, and removes the return light from the incident mirror 25a to the LD 21. An optical isolator 23 is used for the operation.
This optical isolator 23 is very expensive, and as the optical isolator in the 860 nm wavelength band used here, only a material having a small transmittance and a small Verdet constant has been practically used. It will be big.

[0005] Therefore, when modularized, there is a problem that the whole apparatus becomes large and expensive. Also, output light from the resonator is returned to the semiconductor laser to stabilize the oscillation frequency of the semiconductor laser (optical feedback).
Optical axis adjustment is difficult, and it takes time to perform the adjustment work.

Further, since the 2ω light generated by the nonlinear optical material exits from both the mirror 25a on the incident side of the ω light and the mirror 25b on the output side of the 2ω light, the conversion efficiency to 2ω light is reduced. There was a problem.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has a light source for generating a fundamental wave of linearly polarized light, an external resonator and a second harmonic. In the second harmonic generation device including a nonlinear optical material that converts a wave into a wave, two λ / 4 plates are arranged in the external resonator with the nonlinear optical material interposed therebetween, and the two λ / The crystal axes of the four plates are inclined by 45 ° with respect to the polarization direction of the fundamental wave ,
A fundamental wave is provided on the optical axis on the emission side of the second harmonic outside the external resonator.
Dichroic mirror for reflection, light source and external resonator
A polarizing beam splitter on the optical axis between
The fundamental wave reflected by the
Not on the optical axis to enter the litter and return to the light source
It that provides a second harmonic generating device according to claim which comprises a mirror. Further, the polarizing beam splitter and the outside
The crystal axis is the polarization direction of the fundamental wave on the optical axis between
Another λ / 4 plate inclined at 45 ° to
A second harmonic generator as described above is provided. Ma
In addition, the second harmonic generator described above is
Provision of optical recording medium pickup used as detection light source
I do.

An embodiment of the present invention will be described with reference to FIG. Arrows and the like provided along the optical axis in FIG. 1 indicate the polarization state of the ω light in the forward path and the return path. A KNbO ₃ crystal 10 or the like of a nonlinear optical material is arranged on the optical axis of ω light inside a standing wave type external resonator. At this time, the KNbO ₃ crystal 1
The two λ / 4 plates are arranged so that their crystal axes are tilted by 45 ° with respect to the polarization direction of the ω light with 0 interposed therebetween.

On the optical axis between the light source LD1 and the external resonator, another polarizing beam splitter 5 and another λ whose crystal axis is inclined by 45 ° with respect to the polarization direction of the ω light from the LD1 side.
A / 4 plate 6 is arranged, and functions as an optical isolator so that the ω light reflected by the resonance mirror 8A does not return to the LD1. This polarization beam splitter 5 and λ / 4
It is preferable that the plate 6 be closely attached and integrated, since the size can be further reduced.

Accordingly, a large and expensive optical isolator such as a conventional Faraday rotator is not required. Also,
The stabilization of the frequency of LD1 is based on the second harmonic outside the external resonator.
The dichroic mirror 12 on the optical axis on the exit side of
The mirror 13 reflects only light (fundamental wave) and is not on the optical axis , and light between the LD 1 (light source) and the external resonator.
The ω light is converted to LD1 by the polarization beam splitter 5 on the axis.
It is done by returning to .

As a nonlinear optical material, β-BaB ₂ O
_{4, KTiOPO 4, KH 2 PO} 4, LiNbO 3, M
Non-linear optical crystals such as gO: LiNbO ₃ and organic non-linear optical materials can also be used. As a light source, various solid and gas lasers can be used, but an LD is preferable in terms of compactness and light weight. Nonlinear optical material and two λ / 4 plates 9
If A and 9B are integrated using an adhesive having substantially the same refractive index, the size can be reduced and the temperature characteristics of the λ / 4 plate can be controlled.

[0012]

The linearly polarized ω light emitted from the LD 1 is converted into a λ / 4 plate 6.
As a result, the light becomes clockwise circularly polarized light and enters the external resonator. The ω light is linearly polarized by the λ / 4 plate 9A on the ω light incidence side in the external resonator, and passes through the KNbO ₃ crystal 10 in a polarization direction parallel to the b axis of the crystal axis to generate 2ω light. KNbO ₃
The ω light that has passed through the crystal 10 becomes clockwise circularly polarized light at the λ / 4 plate 9B, and is reflected as counterclockwise circularly polarized light at the resonance mirror 8B.

On the return path, ω becomes counterclockwise circularly polarized light.
The light has a polarization direction perpendicular to the b-axis on the λ / 4 plate 9B,
Since phase matching cannot be achieved, KNbO ₃ is generated without generating 2ω light.
The light passes through the crystal 10 and becomes counterclockwise circularly polarized light at the λ / 4 plate 9A.

By repeating the resonance of the ω light as described above, it is possible to output the 2ω light in one direction.

A part of the ω light entering the external resonator on the outward path is reflected by the mirror 8A and returns to the LD1 side. However, since such reflected ω light is left-handed circularly polarized light, it passes through the λ / 4 plate 6 and becomes a polarization direction orthogonal to the polarization direction of the original ω light of the LD 1. Therefore, the reflected ω light cannot pass through the polarization beam splitter 5, and functions as an optical isolator.

[0016]

FIG. 1 shows an embodiment of the present invention. The ω light having a wavelength of 860 nm of the LD 1 whose temperature is controlled by the Peltier element 2 is converted into parallel light by the collimator lens 3, and the aberration of the LD 1 is corrected by using the anamorphic prism pair 4. The polarization beam splitter 5 and the λ / 4 plate 6 function as an optical isolator, and the ω light is a mode matching lens for matching an incident beam with a resonance mode in an external resonator composed of resonance mirrors 8A and 8B. Through 7, the light enters a standing wave type external resonator.

At this time, the crystal axes of the two λ / 4 plates 9A and 9B arranged in the external resonator with the KNbO ₃ crystal 10 interposed therebetween are tilted by 45 ° with respect to the polarization direction of the ω light, so that the external The polarization state of the ω light in the resonator is orthogonal to the forward path and the return path.

Here, on the light entrance / exit surface of the KNbO ₃ crystal 10, an antireflection film (AR coating) for ω light and 2ω light is provided.
The KNbO ₃ crystal 10 itself has a wavelength of 860.
As the Roh link Ritika Le phase matching in nm, is thermostated at 27 ° C. by the Peltier element 11.

The phase matching of the KNbO ₃ crystal 10 is realized only when the polarization direction of the ω light is parallel to the b axis of the crystal axis (coincident with the polarization direction in the forward path in the figure), and at that time, 2ω light is converted into one direction ( Outgoing direction). On the contrary, no 2ω light is generated in the backward direction, so that the internal loss of the 2ω light in the external resonator can be reduced to 比較 of that emitted in the two directions. Conversion efficiency can be improved.

In the second harmonic generator using such an external resonator, the resonance frequency of the external resonator is matched with the oscillation frequency of the LD, that is, the oscillation frequency of the LD which is inherently unstable in frequency. It is very important to stabilize (lock).

As a method of stabilizing the frequency, an optical feedback method is used here. That is, the dichroic mirror 1
2 separates the ω light and the 2ω light, and only the ω light is
And return to LD1 by the polarizing beam splitter 5, and
1 and the resonance frequency of the external resonator can be locked.

The dimensions of each component are as shown in Table 1.

[0023]

[Table 1]

If such a second harmonic generator is used as a data detection light source for pickup of an optical recording medium such as an optical disk or a magneto-optical disk, high-density data can be read.

[0025]

According to the present invention, since the 2ω light can be generated and emitted from the nonlinear optical material in only one direction, the conversion efficiency of the ω light to the 2ω light can be improved. Since there is no need for a large and expensive optical isolator such as a Faraday rotator inserted between them, there is an excellent effect that downsizing of the device, simplification of the manufacturing process, and cost reduction are achieved.

[Brief description of the drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a block diagram of a conventional example.

[Explanation of symbols]

Reference Signs List 1 LD 2 Peltier element 3 Collimator lens 4 Anamorphic prism pair 5 Polarized beam splitter 6 λ / 4 plate 7 Mode matching lens 8A Mirror for resonance 8B Mirror for resonance 9A λ / 4 plate 9B λ / 4 plate 10 KNbO ₃ Crystal 11 Peltier device 12 Dichroic mirror 13 Mirror

Claims (3)

(57) [Claims] 1. A second harmonic generator comprising: a light source for generating a linearly polarized fundamental wave; and a nonlinear optical material having an external resonator and converting the fundamental wave to a second harmonic. the inside vessel two lambda / 4 plate across the non-linear optical material is arranged, the crystal axes of the two lambda / 4 plate are tilted 45 ° to the polarization direction of the fundamental wave, the Second outside the external resonator
A dichroic beam for reflecting the fundamental wave is placed on the optical axis on the output side of the harmonic.
Polarization mirror on the optical axis between the light source and the external resonator.
Reflected by the beam splitter and the dichroic mirror
Incident on the polarized beam splitter,
A second harmonic generator , comprising: a mirror that is not on the optical axis for returning to a light source . 2. A 4 on the optical axis between said polarization beam splitter and the external resonator, forming Akirajiku is to the polarization direction of the fundamental wave
2. The second harmonic generator according to claim 1, further comprising another λ / 4 plate inclined by 5 °. 3. An optical recording medium pickup using the second harmonic generator according to claim 1 as a light source for detecting recorded data.