JPH04256932A - Secondary higher harmonic generator and pickup for optical recording medium - Google Patents

Abstract

PURPOSE:To improve the efficiency of conversion from omega light to 2omega light by generating and emitting 2omega light from a nonlinear optical material only in one direction, to miniaturize a device and to reduce the cost of the device by eliminating the need for a large-sized, expensive optical isolator such as a Faraday rotator interposed conventionally between a light source for a fundamental wave and an external resonator. CONSTITUTION:The crystal axes of two lambda/4 plates 9A and 9B arranged in the external resonator by holding KNbO3 crystal 10 therebetween are slanted at 45 deg. to the polarizing direction of the omega light and then the polarization states of the omega light in the external resonator on its going and return ways intersects orthogonally with each other. The phase matching of the KNbO3 crystal 10 is realized only when the polarizing direction of the omega light is parallel to the crystal axis (b) and the 2omega light is emitted in a forwarding direction at this time. The 2omega light is not generated in the opposite returning way direction and the efficiency of conversion from the omega light to the 2omega light is improved.

Description

[Detailed description of the invention]

0001

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

0002

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

21 is a semiconductor laser (hereinafter referred to as LD) for fundamental wave generation, 22 is a collimating lens, 23 is an optical isolator, and 24 is a resonance mode and an incident light in an external resonator composed of mirrors 25a and 25b. 25a, b are mirrors for fundamental wave resonance; 26 is a nonlinear optical material such as KNbO3 crystal; 2
7 is the fundamental wave (hereinafter referred to as ω light) in nonlinear optical materials.
and a second harmonic (hereinafter referred to as 2ω light) by temperature. 28 is a temperature control Peltier element that transmits 2ω light and reflects ω light; 29
In order to return the linearly polarized ω light to the LD21, the polarization direction is changed to 9.
It is a λ/2 plate for 0° rotation, and 30 is LD21
This is a mirror for locking the oscillation frequency by feedback of ω light to the oscillator.

0004

[Problems to be Solved by the Invention] A conventional second harmonic generation device using an external resonator has a configuration as shown in FIG. 2, and eliminates light returning from the incident mirror 25a to the LD 21. An optical isolator 23 is used for this purpose. This optical isolator 23 is very expensive, and for the optical isolator in the 860 nm wavelength band used here, only materials with a small Verdet constant have been put into practical use as materials with high transmittance. It becomes something big.

[0005] Therefore, when modularizing the device, there is a problem that the entire device becomes large and expensive. In addition, in order to stabilize the oscillation frequency of the semiconductor laser, the output light from the resonator is returned to the semiconductor laser (optical feedback).
Therefore, it is difficult to adjust the optical axis, and the adjustment process takes time.

Furthermore, since the 2ω light generated in the nonlinear optical material exits from both the mirror 25a on the input side of the ω light and the mirror 25b on the exit side of the 2ω light, the conversion efficiency to the 2ω light decreases. There was a problem.

[0007]

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems, and includes a light source for generating a fundamental wave of linearly polarized light and an external resonator, and converts the fundamental wave into a second harmonic. In a second harmonic generation device equipped with a nonlinear optical material that converts into waves, two λ/4 plates are arranged in the external resonator with the nonlinear optical material sandwiched therebetween, and the two λ/4 plates are arranged with the nonlinear optical material in between. The present invention provides a second harmonic generation device characterized in that the crystal axes of the four plates are tilted at 45 degrees with respect to the polarization direction of the fundamental wave.

An embodiment of the present invention will be explained based on FIG. 1. Arrows and the like drawn along the optical axis in FIG. 1 indicate the polarization state of the ω light on the outbound and return passes. A nonlinear optical material such as a KNbO3 crystal 10 is placed on the optical axis of the ω light inside the standing wave type external resonator. At this time, two λ/4 plates sandwich the KNbO3 crystal 10 in such a way that their crystal axes are tilted at 45 degrees with respect to the polarization direction of the ω light.

On the optical axis between the LD1 of the light source and the external resonator, there is a polarizing beam splitter 5 from the LD1 side and another λ beam whose crystal axis is tilted by 45 degrees with respect to the polarization direction of the ω light.
A /4 plate 6 is arranged and functions as an optical isolator to prevent the ω light reflected by the resonance mirror 8A from returning to the LD1. This polarizing beam splitter 5 and λ/4
It is preferable to integrate the plates 6 in close contact with each other because this will further reduce the size.

Therefore, there is no need for a large and expensive optical isolator such as a conventional Faraday rotator. Also,
The frequency of LD1 is stabilized by dichroic mirror 12.
This is accomplished by reflecting only the ω light at , and then returning it to the LD 1 using the mirror 13 and the polarizing beam splitter 5 . To stabilize the frequency of this LD1, the dichroic mirror 12 is eliminated, and the reflected ω light from the external resonator is detected with a polarizing beam splitter, etc., and the reflected ω light is minimized.
This may be done by changing the current injected into D1.

[0011] As a nonlinear optical material, β-BaB2
O4, KTiOPO4, KH2PO4, LiN
Nonlinear optical crystals and organic nonlinear optical materials such as bO3, MgO:LiNbO3, etc. can also be used. As a light source,
Various solid state and gas lasers can also be used, but LD is preferable in terms of compactness and weight reduction. If the nonlinear optical material and the two λ/4 plates 9A and 9B are integrated using an adhesive with approximately the same refractive index, they can be made smaller and the temperature characteristics of the λ/4 plates can be controlled. may be converted into

0012

[Operation] The linearly polarized ω light emitted from the LD1 is transmitted to the λ/4 plate 6
The light becomes clockwise circularly polarized light and enters the external resonator. The ω light becomes linearly polarized light by the λ/4 plate 9A on the ω light incident side in the external resonator, and passes through the KNbO3 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 by the λ/4 plate 9B, and is reflected by the resonance mirror 8B as counterclockwise circularly polarized light.

On the return trip, ω became counterclockwise circularly polarized light.
The light is polarized in a direction perpendicular to the b-axis by the λ/4 plate 9B,
KNbO3 does not generate 2ω light because phase matching cannot be achieved.
The light passes through the crystal 10 and becomes counterclockwise circularly polarized light by 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 incident on 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 counterclockwise circularly polarized light, when it passes through the λ/4 plate 6, the polarization direction becomes perpendicular to the polarization direction of the original ω light of the LD 1. Therefore, the reflected ω light cannot pass through the polarizing beam splitter 5, which functions as an optical isolator.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention. The 860 nm wavelength ω light 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 aberrations of the LD 1 are corrected by using the anamorphic prism pair 4. The polarizing beam splitter 5 and the λ/4 plate 6 function as an optical isolator, and the ω light is transmitted through a mode matching lens for matching the resonant mode in the external resonator composed of resonant mirrors 8A and 8B with the incident beam. 7 and enters the standing wave type external resonator.

At this time, by tilting the crystal axes of the two λ/4 plates 9A and 9B placed in the external resonator with the KNbO3 crystal 10 sandwiched therebetween by 45° with respect to the polarization direction of the ω light, the external resonance can be achieved. The polarization state of the ω light inside the device becomes orthogonal on the outbound and return trips.

Here, an antireflection film (AR coating) for ω light and 2ω light is applied to the light input/output surface of the KNbO3 crystal 10, and the KNbO3 crystal 10 itself has a wavelength of 8
The temperature is controlled to 27° C. by the Peltier element 11 so that non-critical phase matching is achieved at 60 nm.

Phase matching of the KNbO3 crystal 10 is achieved only when the polarization direction of the ω light is parallel to the b-axis of the crystal axis (corresponding to the polarization direction in the outgoing path in the figure), and in that case, the 2ω light is directed in one direction (in the outgoing path). direction). Since 2ω light is not generated in the opposite return direction, the internal loss of the 2ω light inside the external resonator can be reduced to 1/2 compared to that of the 2ω light emitted in two directions. The conversion efficiency can be improved.

In a second harmonic generation device using such an external resonator, it is possible to match the resonant frequency of the external resonator and the oscillation frequency of the LD, that is, to adjust the oscillation frequency of the LD, which is inherently unstable in terms of frequency. It is very important to perform stabilization (lock).

[0021] As a frequency stabilization method, an optical feedback method is used here. That is, dichroic mirror 1
2 separates the ω light and 2ω light, and sends only the ω light to the mirror 13.
and returns to LD1 by polarizing beam splitter 5, and LD
The oscillation frequency of 1 and the resonance frequency of the external resonator can be locked.

Note that the dimensions of each part are as shown in Table 1.

[0023]

[Table 1]

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

[0025]

Effects of the Invention The present invention can generate and emit 2ω light in only one direction from a nonlinear optical material, which improves the efficiency of converting ω light into 2ω light. Since a large and expensive optical isolator such as a Faraday rotator inserted between the devices is no longer necessary, the device has excellent effects such as miniaturization of the device, simplification of the manufacturing process, and reduction in cost.

[Brief explanation of the drawing]

FIG. 1: Block diagram of an embodiment of the present invention.

[Figure 2] Block diagram of conventional example

[Explanation of symbols]

1 LD 2 Peltier element 3 Collimator lens 4 Anamorphic prism pair 5 Polarizing beam splitter 6　λ/4 plate 7 Mode matching lens 8A Resonance mirror 8B Mirror for resonance 9A λ/4 plate 9B λ/4 plate 10 KNbO3 crystal 11 Peltier element 12 Dichroic mirror 13 Mirror

Claims (3)

[Claims] 1. A second harmonic generation device comprising: a light source for generating a fundamental wave of linearly polarized light; and a nonlinear optical material having an external resonator and converting the fundamental wave into a second harmonic; Two λ/4 plates are placed inside the device with the nonlinear optical material sandwiched between them, and the crystal axes of the two λ/4 plates are tilted at 45 degrees with respect to the polarization direction of the fundamental wave. Characteristics of the second harmonic generator. 2. On the optical axis between the light source and the external resonator,
A second harmonic generator in which a polarizing beam splitter and another λ/4 plate whose crystal axis is tilted at 45 degrees with respect to the polarization direction of the fundamental wave are arranged. 3. A pickup for an optical recording medium using the second harmonic generator according to claim 1 as a detection light source for recorded data.