JPH05282694A - Optical head - Google Patents

Abstract

PURPOSE:To set the oscillation wavelength range narrow by providing a narrow band pass filter, which is a combination of a calcopearlite crystal and an analyzer, as a wavelength cut filter of the light beam from a light source. CONSTITUTION:A narrow band pass filter 31 is placed between a collimator lens 13 and a shaping prism 15. The laser beams outputted from a laser diode 11 become parallel beams by the collimator lens 13 and are made incident to the filter 31. The transmitted beams of the filter 31 are corrected by a prism 15, become a true circle and are spot irradiated on an optical disk 61 by an objective lens 19 through a beam splitter 17. The reflected light beams from the disk 61 are made incident to the lens 19 again, lead to a detection system 25 through the beam splitter 17, a half wavelength plate 21 and a condenser lens 23, focus error and tracking error signals are detected and information reading and writing are performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head of an optical recording device (including a magneto-optical recording device).

[0002]

2. Description of the Related Art In an optical head of an optical recording apparatus, it is ideal that light of a single wavelength from a laser light source is used to focus on a recording medium with an objective lens to write information on the recording medium or read the recorded information. Target. However, in reality, since the laser has temperature dependence, when the laser oscillates in a single mode, mode hopping occurs, the wavelength is not stable, and the light is returned from the recording disk. Therefore, a high-frequency current is superimposed on the direct current for laser oscillation so that the laser oscillates in multiple modes, and multimode oscillation is performed.

On the other hand, however, the multimode oscillation causes a problem that the wavelength of the emitted light is not stable and the spot size focused on the recording disk becomes unstable, and the S / N ratio deteriorates. Will end up.

Conventional techniques for disposing an optical filter in an optical head are described in Japanese Patent Laid-Open Nos. 3-209638 and 2-68737.

In Japanese Patent Laid-Open No. 3-209638, a filter having a transmission peak wavelength equal to the oscillation peak wavelength of the semiconductor laser is inserted in the optical system between the semiconductor laser and the objective lens to prevent chromatic aberration of the lens. It is described that the influence of the returning light of the semiconductor laser caused by the above is prevented. However, this filter is a multilayer film filter in which low-refractive index dielectrics and high-refractive index dielectrics are alternately laminated, and if the wavelength selection width of the filter is wide, it is easily affected by the return light. Further, this filter is basically of a transmittance changing type, and it is inevitable that a minute amount of light returns as return light. Further, since the sum of the transmittance and the reflectance at a certain wavelength is almost 100% in this multilayer filter, the reflectance is extremely high in the vicinity of the wavelength selection region, and this reflected light adversely affects the laser as return light. .. Therefore, since it is necessary to attach the filter obliquely so that the light beam obliquely enters, the dependency of the transmission center wavelength of the filter on the incident angle becomes a problem, and it becomes difficult to attach or adjust the filter to the optical head. ..

On the other hand, Japanese Patent Laid-Open No. 2-68737 discloses that
By placing an optical filter with high transmittance in the central part and low in the peripheral part in the reflected light path of the optical pickup, it is stated that crosstalk of reproduced signals can be prevented without significantly changing the structure of the optical pickup. Has been done. However, this method has a drawback that the amount of light in the entire wavelength range is reduced because the spot size is reduced by the light intensity.

[0007]

SUMMARY OF THE INVENTION The present invention provides an optical head having an emission system having a stable oscillation wavelength and reducing the influence of returning light.

[0008]

The optical head of the present invention comprises:
A narrow band transmission filter in which a chalcopyrite type crystal and an analyzer are combined is provided as a wavelength cut filter of a light beam from a light source.

[0009]

FIG. 1 is an explanatory view showing an embodiment of an optical head of the present invention, except that a narrow band transmission filter 31 is inserted between a collimator lens 13 and a shaping prism 15 and a conventional light beam is used. It has the same structure as the optical head in the recording apparatus.

Laser light output from the laser diode 11 (semiconductor laser) is collimator lens 1
The light beam is collimated into a parallel light beam at 3 and enters the narrow band transmission filter 31. The light passing through the narrow band transmission filter 31 is corrected by the shaping prism 15 into a perfect circle, passes through the beam splitter 17, and is spot-irradiated onto the optical disc 61 by the objective lens 19. The reflected light from the optical disc 61 enters the objective lens 19 again, and the beam splitter 17 and the half-wave plate 2
1. The light is guided to the detection system 25 via the condenser lens 23, the focus error signal and the tracking error signal are detected, and the information is read and written.

FIG. 2 is an explanatory view showing an example of the configuration of the narrow band transmission filter 31, which is composed of a polarizer 33 and an analyzer 35 whose transmission axes 37 are orthogonal to each other, and a chalcopyrite type crystal 41 inserted between them. It is configured.

A chalcopyrite type crystal is a ternary compound semiconductor having a chalcopyrite structure and is composed of AgGa.
Group I-III-VI ₂ semiconductors such as S ₂ and AgGaSE ₂ ,
Most of II-IV-V ₂ group semiconductors have a chalcopyrite type crystal structure (Horinaka et al., Applied Physics, Volume 60,
No. 2, 112-118 (1991)). The chalcopyrite type crystal exhibits birefringence, but exhibits the property that the birefringence becomes zero only for a certain specific wavelength. And, as shown in FIG. 3, it has a point where the birefringence becomes zero near room temperature. Moreover, in most of the transmission wavelength region, the chalcopyrite type crystal does not change the polarization state of incident light because the effect of birefringence is sufficiently larger than the effect of optical activity, but the polarization of the wavelength at which the birefringence becomes zero. The optical rotation state changes in the vicinity. Therefore, by appropriately setting the crystal orientation and the thickness, the narrow band pass filter 31 as shown in FIG. 2 can be constructed by utilizing the property that the birefringence becomes zero at a specific wavelength and the optical rotatory power. .. As for the incident light, only the polarized light component which coincides with the direction of the transmission axis 37 of the polarizer 33 passes through the polarizer 33, and only the light of a certain specific wavelength of the transmitted light is not birefringent and the chalcopyrite crystal Through 41,
At that time, the polarization plane is rotated by 90 degrees and passes through the analyzer 35 to form a narrow band transmission filter. Since the laser light from the laser diode 11 can be regarded as linearly polarized light, the polarizer 33 can be omitted.

When a high frequency superposition is applied to the laser diode 11 to cause multimode oscillation, the laser diode 11
The oscillating wavelength range of the light emitted from the device is expanded as shown in FIG. 4, but the narrow band filter 31 using the chalcopyrite type crystal shifts it from the optical axis of the optical system by birefringence except for a part of the wavelength range. It is possible to narrow the wavelength of the emitted light by cutting. Since the wavelength range in which the birefringence becomes zero in the chalcopyrite type crystal is very narrow, it becomes an oscillation state close to a single mode as shown in FIG. 4, and coherent light with a stable wavelength can be emitted to the optical system. ..

Further, the return light of the optical disc 61 having a wavelength different from that of the emission system is deviated from the optical axis by the birefringence of the chalcopyrite type crystal 41, so that it does not return to the laser diode 11 as the return light, The effect of returning light can be reduced. Further, the chalcopyrite type crystal, unlike an interference filter composed of a multilayer film, is basically not a variable transmittance type, has a small influence of reflected light, and has an advantage that the center wavelength does not depend on the incident angle. ..

The narrow band transmission filter 31 of the present invention
In FIG. 1, it is arranged at the rear stage of the collimator lens 13, but it can also be arranged at another part of the optical system of the optical head.

[0016]

According to the optical head of the present invention, even at a wavelength slightly deviated from the original oscillation of the laser light source, it is possible to use birefringence to shift the laser beam from the optical axis of the optical head and cut it. It is possible to configure an emission system having the above-mentioned oscillation wavelength, and particularly when the laser light source is oscillated in multimode, the oscillation wavelength range can be set narrow.

Further, the return light from the optical recording medium, which has a wavelength different from that of the emission system, can be deviated from the optical axis by birefringence, thereby preventing adverse effects such as wavelength instability of the laser light source due to the return light. it can.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of an optical system of an optical head of the present invention.

FIG. 2 is an explanatory diagram showing an example of a narrow band transmission filter using chalcopyrite tetragonal crystals.

FIG. 3 is a graph showing an example of a relationship between a central wavelength and a temperature at which birefringence becomes zero in a chalcopyrite type crystal.

FIG. 4 is a graph showing wavelength cut characteristics in the present invention in multimode oscillation.

[Explanation of symbols]

 11 laser light source 13 collimator lens 15 shaping prism 17 beam splitter 19 objective lens 21 half-wave plate 23 condensing lens 25 signal detection system 31 narrow-band transmission filter 33 polarizer 35 analyzer 37 transmission axis 41 chalcopyrite type crystal 61 optical disk

Claims (1)

[Claims] 1. An optical head comprising a narrow band transmission filter in which a chalcopyrite type crystal and an analyzer are combined, as a wavelength cut filter for a light beam from a light source.