JPH06124475A - Optical information detection apparatus - Google Patents

Abstract

PURPOSE:To achieve that information contained in a beam is emphasized in its central part and to detect the information with high resolution by a method wherein a frequency conversion device utilizing a nonlinear optical crystal is incorporated in a detection optical system. CONSTITUTION:A frequency-raising conversion by a sum frequency mixture wherein reflected light from a magneto-optical disk 6 is used as signal light and a laser beam oscillated by solid-state laser crystals 19, 24 is used as pumping light is performed by a nonlinear optical effect with which nonlinear optical crystals 18, 23 are provided. At this time, since the conversion efficiency of a frequency depends on the intensity of the pumping light and is proportional to the optical density of each point of the Gaussian distribution of the pumping light which has been incident on the crystals 18, 23, the frequency of the signal light is raised according to the conversion efficiency in each point of the pumping light. As a result, the intensity distribution in the beam waist position of the signal light is emphasized in the central part, and the signal light can be detected with high resolution. In addition, when definite photons or higher are frequency-converted, a signal amplification function is caused and they can be detected with high resolution. Thereby, high-density recorded information can be detected with high quality.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for recording and reproducing information on an optical recording medium, and more particularly to an apparatus for detecting information recorded at high density with high quality.

[0002]

2. Description of the Related Art In order to detect optical information recorded on an optical recording medium with a high density, it is most common to use a laser beam having a short wavelength. A detection system combining apodization and a confocal optical system has also been devised.

[0003]

When short-wavelength laser light is used to detect optical information recorded on an optical recording medium with high density, the read signal strength is lowered particularly in a magneto-optical medium. There is a problem, and materials having a large Kerr rotation angle even at a short wavelength are being developed. In addition, in a detection system in which apodization and a confocal optical system are combined, alignment of pinholes or slits is indispensable, and positional displacement due to aging causes a problem.

[0004]

High resolution detection is achieved by incorporating a frequency conversion device using a nonlinear optical crystal in a detection optical system.

[0005]

Embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. A light beam emitted from a semiconductor laser 1 serving as a light source is converted into parallel light by a collimator lens 2, enters a beam splitter 3, and further passes through a polarization beam splitter 4 by an objective lens 5 to produce a magneto-optical disk 6
The recording surface is narrowed down to the diffraction limit.

A part of the light beam reflected by the magneto-optical disk 6 is reflected by the beam splitter 4 and enters the half-wave plate 11. A part of the light flux transmitted through the beam splitter 4 is reflected by the beam splitter 3, and the right angle prism 7, the converging lens 8 and the cylindrical lens 9 are included.
It is incident on the four-division photodetector 10 via, and a focus error signal by the astigmatism method and a track error signal by the push-pull method are generated.

With these signals, the beam spot, which is always narrowed to the diffraction limit, traces the track on the recording surface of the magneto-optical disk 6.

On the other hand, the half-wave plate 11 is rotated and adjusted so that the plane of polarization of the light beam incident on and transmitted through the half-wave plate 11 is 45 degrees with respect to the plane of incidence of the polarization beam splitter 12. ing.

Therefore, the reflected light from the magneto-optical disk 6 enters the converging lenses 16 and 21 with the same intensity to become convergent light, and its beam waist is located inside the nonlinear optical crystals 18 and 23. Has been done.

The semiconductor laser 13 is a high power laser which oscillates in a single longitudinal mode. The light flux emitted from the semiconductor laser 13 is converted into parallel light by the collimator lens 14, is incident on and transmitted through the half-wave plate 15, and is partially reflected by the polarization beam splitter 12 and partially transmitted. At this time, the half-wave plate 15 is rotationally adjusted so that the reflected light and the transmitted light have the same intensity. Here, the transmitted light becomes convergent light by the converging lens 16, and the reflecting mirror 17
And further excites the solid-state laser crystal 19 via the nonlinear optical crystal 18. That is, the reflecting mirror 17 and the reflecting mirror 20 form a resonator at the oscillation wavelength of the solid-state laser crystal 19.

That is, an antireflection coating for the wavelength of the semiconductor laser 13 is applied to the incident side of the reflecting mirror 17. The emitting side is provided with a coating having a non-reflection function for the wavelength of the semiconductor laser 13 and a high reflection function for the oscillation wavelength of the solid-state laser crystal 19 excited by the semiconductor laser 13. On the incident side of the reflecting mirror 20, a high reflection function for the oscillation wavelength of the solid-state laser crystal 19 and the magneto-optical disk 6 inside the nonlinear optical crystal 18 are provided.
An anti-reflection coating is applied to the wavelength of which the frequency is converted by the interaction with the signal light which is the reflected light from. On the emitting side, a non-reflection coating for the frequency-converted wavelength is applied.

The light beam from the semiconductor laser 13 reflected by the polarization beam splitter 12 becomes convergent light by the converging lens 21 as in the case of the light beam passing through the polarization beam splitter 12, enters the reflecting mirror 22, and further the nonlinear optical crystal. The solid-state laser crystal 24 is excited via 23. That is, the reflecting mirror 22 and the reflecting mirror 25 form a resonator at the oscillation wavelength of the solid-state laser crystal 24.

That is, an antireflection coating for the wavelength of the semiconductor laser 13 is provided on the incident side of the reflecting mirror 22. Further, a coating having a non-reflection function for the wavelength of the semiconductor laser 13 and a high reflection function for the oscillation wavelength of the solid-state laser crystal 24 excited by the semiconductor laser 13 is provided on the emission side. Further, on the incident side of the reflecting mirror 25, frequency conversion is performed by the high reflection function for the oscillation wavelength of the solid-state laser crystal 24 and the interaction with the signal light which is the reflected light from the magneto-optical disk 6 inside the nonlinear optical crystal 23. A non-reflective coating for different wavelengths is applied.

In this embodiment, the nonlinear optical crystals 18 and 23 are KTP, and the solid-state laser crystals 19 and 24 are Nd:
YAG was used. The wavelength of the semiconductor laser 1 as the light source and the high-power semiconductor laser 13 as the excitation light source is 809.
nm. Excited solid-state laser crystals 19 and 24
Oscillates at a wavelength of 1064 nm.

Frequency rising conversion by sum frequency mixing using reflected light from the magneto-optical disk 6 as signal light and laser light oscillating at 1064 nm of the solid-state laser crystals 19 and 24 as pump light is performed by the nonlinear optical crystals 18 and 23. It is made by the non-linear optical effect that it has. The converted wavelength is 4
It is 59 nm. Photodetectors 26 and 2 arranged on the transmission side and the reflection side of the polarization beam splitter 12, respectively.
7, the signal with the wavelength of 456 nm after conversion is detected.

In the frequency rising conversion by the sum frequency mixing as in the present embodiment, when the polarization beam splitter 12 converts the reflected light from the magneto-optical disk 6 which becomes the signal light into the intensity modulated signal, it becomes the signal intensity. Proportionally 459n
The signal of the wavelength converted into m is also modulated. Therefore,
The differential detection used in a normal magneto-optical disk is possible.

The most important point in the embodiment of the present invention is
Since the objective lens 5 and the converging lens 16 and the objective lens 5 and the converging lens 21 respectively form a confocal optical system, a pinhole is arranged at the beam waist position by the converging lenses 16 and 21, and the central portion of the signal light is arranged. It has the same effect as detecting only one and obtaining substantially higher resolution. However, in the method in which the pinholes are arranged, it is difficult to align them, and there is a high possibility that the pinholes will shift due to temperature changes. The intensity of the pump light oscillated from the solid-state laser crystals 19 and 24 has a Gaussian distribution, and inside the nonlinear optical crystals 18 and 23, KTP, the signal light that is the reflected light from the magneto-optical disk 6 and the frequency due to the sum frequency mixing Ascending conversion occurs. At this time, the frequency conversion efficiency depends on the intensity distribution of the pump light oscillated by the solid-state laser crystals 19 and 24. Therefore, since the pump light incident on the nonlinear optical crystals 18 and 23 is proportional to the light density at each point of the Gaussian distribution, the signal light that is the reflected light from the magneto-optical disk 6 has a conversion efficiency at each point of the pump light. The frequency rises. Therefore, the pump light fulfills a kind of masking effect, and the central portion is emphasized in the intensity distribution at the beam waist position of the signal light, which is the reflected light from the magneto-optical disk 6, and a substantially high resolution is achieved. The image is obtained. Further, the signal light itself, which is the reflected light from the magneto-optical disk 6, also has a masking effect (the appearance of the masking effect is shown in FIG. 2). When the beam system of the pump light at the beam waist position of the signal light located inside the nonlinear optical crystals 18 and 23 is sufficiently larger than the beam waist of the signal light, the masking effect of the signal light itself becomes dominant. , Mechanical adjustment is reduced, and the assembly is easy. Further, if the signal light is above a certain level, that is, the number of photons of the signal light is N1, the frequency of the signal light is W1, and the frequency of the pump light is W2, then W1 /
When the photons of (W1 + W2) N1 or more are frequency-converted to the sum frequency of the frequency of the pump light and the frequency of the signal light, the output is larger than that of the signal light. That is, it has a signal amplification function.

[0019]

[Effect] In frequency conversion using a nonlinear optical crystal,
The conversion efficiency is proportional to the incident intensity on the nonlinear optical crystal. Therefore, when the intensity distribution of the incident beam on the nonlinear optical crystal has a large center intensity like a Gaussian distribution beam, that is, the information of the frequency-converted beam in the frequency conversion device having the confocal optical system has the central part. Information is emphasized, high-resolution detection can be realized, and the margin regarding positional deviation is improved. In addition, when a certain number or more of photons are frequency-converted, a signal amplification effect appears, enabling highly sensitive detection.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the masking effect described in the embodiment.

[Explanation of symbols]

 1 Laser Light 2 Collimator Lens 3 Beam Splitter 4 Polarizing Beam Splitter 5 Objective Lens 6 Magneto-optical Disk 7 Right Angle Prism 8 Converging Lens 9 Cylindrical Lens 10 Four-division Photo Detector 11 1/2 Wave Plate 13 Semiconductor Laser 14 Collimator Lens 15 1/2 Wavelength Plate 16 Converging Lens 17 Reflecting Mirror 18 Nonlinear Optical Crystal 19 Solid State Laser Crystal 20 Reflecting Mirror 21 Converging Lens 22 Reflecting Mirror 23 Nonlinear Optical Crystal 24 Solid State Laser Crystal 25 Reflecting Mirror 26 Photodetector 27 Photodetector

Claims (4)

[Claims] 1. A light provided with a light source, a first means for converging a light beam emitted from the light source onto an optical recording medium, and a second means for detecting optical information reflected from the optical recording medium. In the information detection device, a confocal optical system is configured by the first and second means, a nonlinear optical crystal is arranged on the optical axis including the focal plane on the detection side, and the frequency of photons in the nonlinear optical crystal is changed. An optical information detecting device comprising a third means for converting. 2. The third means comprises a resonator including a solid-state laser crystal and a nonlinear optical crystal, and the solid-state laser crystal is excited by a semiconductor laser arranged outside the resonator, 2. The optical information detection device according to claim 1, wherein the frequency of the photon is converted in the method. 3. A polarization beam splitter is arranged between the semiconductor laser and the resonator, and a ½ wavelength plate is arranged between the semiconductor laser and the polarization beam splitter. 2. The optical information detection device described in 2. 4. The light according to claim 3, wherein a resonator is disposed on both the side where the light flux of the semiconductor laser passes through the polarization beam splitter and the side where the light flux is reflected by the polarization beam splitter. Information detection device.