JPH069089B2 - Light head - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical head, and more particularly to an optical head for a video disc, a digital audio disc (compact disc), an optical disc, or the like.

(Prior Art) Video Disc, Digital Audio Disc,
As shown in FIG. 2, a conventional optical head for an optical disc or the like (hereinafter collectively referred to as an optical disc) has a semiconductor laser 16 as a light source, a collimating lens 18 for converting a semiconductor laser radiation 17 into a collimated light, Polarizing beam splitter
19, an isolator including a quarter-wave plate 20, a converging lens 21, a focus error detecting unit, and a tracking error detecting unit. There are various types of focus error detection means, and the critical angle method can be mentioned as the method most closely related to the method of the present invention. The critical angle method uses a critical angle prism 22 and its technical contents are described in the following documents.

Samurai, Ito, Shibata, pp. 634-6 of "Optics", Vol. 11, No. 6 (published in December 1982)
The article "Optical head for DAD" published on page 39 (Problems to be solved by the invention) In the above-mentioned conventional technology, a polarizing beam splitter 19 is used by laminating prisms coated with multilayer thin films. There are 5 surfaces, and the polarization characteristics are obtained with a multilayer thin film. Moreover, even the critical surface prism method shown in FIG. 2 requires polishing of 3 surfaces, which is problematic in that it is unproductive and expensive. It was Further, the quarter-wave plate 20 has a problem that it is expensive because it is made of quartz, which is expensive material, and is polished and laminated to a predetermined thickness, so that mass productivity is poor and the cost is high.

(Means for Solving Problems) The present invention relates to a semiconductor laser, a collimating lens, a means for polarization-splitting a laser beam, a linear polarization circular polarization conversion means, a converging lens, a focus error detection means, In the optical head including a tracking error detecting means, the means for polarization-splitting the laser beam is a transmission grating type polarization beam splitter for separating 0th-order diffracted light and 1st-order diffracted light according to the polarization direction of the incident light, and the linear polarization circle The polarization conversion means is a grating type 1/4 wavelength plate formed integrally with the beam splitter, and the focus error detection means and the tracking error detection means are arranged so that the light beam incident from the back surface of the diffraction grating is almost critical at the grating surface. After diffracting at an angle of refraction, it is almost totally reflected on the back surface of the grating and again at a critical diffraction angle on the grating surface, and then is incident on the diffraction grating. It is characterized in that the diffraction grating that emits light in the reflection direction of the beam and the photodetector in which the light receiving surface that receives the emission beam of the diffraction grating is divided into four parts are used.

(Operation) In the present invention, a critical angle prism, a polarizing prism, and a quarter-wave plate, which have problems in mass productivity and cost, are used, and a lattice type element having high mass productivity is used. Regarding the grating element corresponding to the critical angle prism, the inventor of the present invention has already applied for a focus error detecting device in Japanese Patent Application No. Sho 59-122262. Figure 3 shows
FIG. 6 is a cross-sectional view showing a relationship between incidence and diffraction of light with respect to a diffraction grating in order to explain the principle of the grating element corresponding to the critical angle prism.

In the figure, the lattice is enlarged from the actual size to show that 23 is a lattice plane.

In FIG. 3, incident light 24 is incident from the grating substrate surface 25 at the incident angle Θ ₁ shown. At the substrate surface, incident light is refracted,
When the refractive index of the substrate is n, the refraction angle Θ ₂ has the following relationship.

sin Θ ₁ = n sin Θ ₂ (1) The refracted light 26 is diffracted by the grating surface 23 according to the following equation, and is emitted into the air as diffracted light 27 shown by a dotted line.

_{nsinΘ 2 + sinΘ 2 = λ /} d (2) Here, lambda is the wavelength of light, d denotes the pitch of the grating.
Substituting equation (1) into equation (2) yields the following equation.

sin Θ ₂ + sin Θ ₃ = λ / d (3) Consider the case where λ / d is larger than 1 in the equation (3). When the incident angle Θ ₁ is decreased from 90 ° to 0 °, the diffraction angle Θ ₃ increases, and when the incident angle Θ ₁ is sin Θ ₁ = λ / d−1 (4), Θ ₂ Becomes 90 °, and the diffracted light does not come out into the air. At this time, the following equation is established in the substrate.

nsin Θ ₂ + nsin Θ ₄ = λ / d (5) Therefore, nsin Θ ₄ = 1 and reflected diffracted light 28 is generated. However, since sin Θ ₄ = 1 / n, the diffracted light 28 is totally reflected by the substrate surface 25, and the reflected light 29 is mirror-symmetrical with the diffracted light 28, so that the reflected diffracted light 30 is generated by the grating surface 23.

The reflected diffracted light 30 is refracted at the substrate surface 25 to become refracted light 31 and goes out into the air. As described above, when sin Θ ₁ > λ / d−1 with Θ ₁ in the equation (4) as a boundary, the diffracted light 31 does not occur,
Diffracted light 27 is generated from the lattice plane side. sin Θ ₁ <λ / d-1
In the case of, diffracted light 31 is generated, and diffracted light 27 on the grating surface side is not generated. Therefore, the change in the incident angle Θ ₁ can be known by detecting the intensity of the diffracted light 31.

That is, the same effect as the three-time total reflection in the critical angle prism can be obtained.

FIG. 4 shows the polarization dependence of the diffraction efficiency of a relatively deep transmission diffraction grating. Curve 32 shows the direction of the electric vector parallel to the groove of the grating, and curve 33 shows the direction of the electric vector of the grating. It represents the diffraction efficiency when it is perpendicular to the groove.

When the ratio of the wavelength to the grating pitch (wavelength / grating pitch) is 1.6 or more, particularly 1.8 or more, remarkable polarization characteristics are exhibited, and 0-order diffracted light and 1st-order diffracted light are used to act as a polarization beam splitter. You can

If the pitch of the dielectric grating is d and the wavelength used is λ, then λ /
The surface relief grating can be used as a wave plate by utilizing the fact that the d2 grating does not generate diffracted light and the refractive index is different in the direction orthogonal to the groove of the grating. Proposals and experiments of this type of wave plate are based on Flanders (DC, Flan
ders) by Applied Physics Laters (Appl
ied Physics Letter) Vol. 42, No. 6 (published March 15, 1983, pp. 492-494).

Examples of the optical head using the above-mentioned grating element will be described below.

(Embodiment) FIG. 1 is a diagram showing a basic configuration of an embodiment of the present invention.

The emitted light 2 of the semiconductor laser 1 is collimating lens 3
Is collimated into parallel light 4 and enters the polarization beam splitter grating 5.

Since the direction of polarization of the incident beam 4 is set so that the electric vector is parallel to the groove of the polarization beam splitter grating 5, it becomes the first-order diffracted light 6 and becomes the 1/4 wavelength plate. It is incident on the grating 7. 1/4 wavelength plate grating 7
The non-diffractive high-density grating described above produces a phase difference in orthogonal polarization directions, and as a result, converts incident linearly polarized light into a circularly polarized light beam 8. The light beam 8 is focused by the converging lens 9 on the disk surface 10.
Converge to point 11. The reflected light from the disk surface 10 passes through the converging lens 9 again, and becomes the light beam 6 converted into linearly polarized light by the 1/4 wavelength plate grating 7.

As a result, a light beam having an electron vector in the direction orthogonal to the grating groove is incident on the polarization beam splitter grating, the diffraction efficiency becomes almost 0, and 0-th order diffracted light 12 enters the critical angle diffraction grating 13. To do. In the critical angle diffraction grating 13, the incident beam undergoes critical angle diffraction and total reflection twice, and becomes a diffracted light 14 which is incident on the quadrant photodetector 15. When the disk surface 10 shifts in the optical axis direction, that is, when a focus error occurs, the incident beam to the critical angle diffraction grating 13 becomes parallel light and reaches the photodetector 15 asymmetrically. Focus error can be detected by taking it.

Further, the tracking error signal is obtained by the differential signal of the photodetector, which is also divided into two in the direction perpendicular to the paper surface. In this embodiment, the polarization beam splitter grating and the quarter-wave plate grating are integrally formed.

(Effects of the Invention) According to the present invention, a prism, which requires expensive polishing as in the conventional art, becomes unnecessary. The grating element of the present invention can be inexpensively duplicated as a replica, is lighter in weight than a prism, and has the effect of reducing the weight of an optical head.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an embodiment of the present invention, FIG. 2 is a diagram showing a configuration of a conventional optical head, FIG. 3 is a diagram for explaining the principle of a critical angle diffraction grating, and FIG. FIG. 4 is a diagram showing polarization dependence of first-order diffraction efficiency of a transmission type diffraction grating. In the figure, 1,16 ... Semiconductor laser, 3,18 ... Collimating lens, 5 ... Polarizing beam splitter grating, 7 ...
… 1/4 wave plate grating, 9, 21 …… Converging lens, 10 ……
Disk surface, 13 ... Critical angle diffraction grating, 15 ... Quadrant photodetector, 19 ... Polarizing beam splitter (prism), 20 ... 1/4 wavelength plate, 22 ... Critical angle prism,
23 …… Lattice surface, 24 …… Incident light, 27,31 …… Diffracted light, 32 …… Diffraction efficiency when the electric vector direction is parallel to the groove of the grating, 33 …… Electric vector direction is the groove of the grating The diffraction efficiencies when and are perpendicular are shown respectively.

Claims (1)

[Claims] 1. A semiconductor laser, a collimating lens, a means for polarization splitting a laser beam, a linearly polarized light circularly polarized light converting means, a converging lens, a focus error detecting means, and a tracking error detecting means. In the optical head, the means for polarization-splitting the laser beam is a transmission grating type polarization beam splitter for separating 0th-order diffracted light and 1st-order diffracted light according to the polarization direction of the incident light, and the linear polarization circular polarization conversion means is the beam splitter. A grating type quarter-wave plate formed integrally, the focus error detecting means and the tracking error detecting means, after diffracting the light beam incident from the back surface of the diffraction grating at the grating surface at a substantially critical diffraction angle, the grating Diffraction which is almost totally reflected on the back surface, again diffracted on the grating surface at a substantially critical diffraction angle, and then emitted in the reflection direction of the light beam incident on the diffraction grating. Son and optical head, characterized in that a light receiving surface divided into four photodetector for receiving the emitted beam of the diffraction.