JPS6040538A - Optical information reader - Google Patents

Abstract

PURPOSE:To stabilize follow-up control and to read the information of high quality by tilting the surface of a photodetector by a fixed angle and reducing the noise component which is produced by the reflected light going back to a semiconductor laser. CONSTITUTION:The diffracted light which is reflected on an information recording surface 7 is led to a photodetector 10 by a cylindrical lens 8 and a condenser lens 9 via an objective lens 6, a 1/4 wavelength plate 5 and a polarized beam splitter 4. In this case, the detector 10 is tilted by a prescribed angle phi which is decided by the numerical aperture of the lens 9. Thus the noise component which is produced by the reflected light going back to a semiconductor laser 1. This reader improves the S/N of both focusing and tracking signals, etc. and stabilizes the follow-up control. Thus it is possible to read the information of high quality.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an optical information reading device that optically reads information on an information record carrier, and particularly relates to an optical information reading device that optically reads information on an information record carrier, and in particular, a radar light beam containing information reflected from an information record carrier. The present invention relates to a photodetector that receives light and converts it into an electrical signal.

[Technical background of the invention]

Conventionally, an optical information reading device has a structure as shown in FIG. The circularly polarized laser beam passes through the /4 wavelength plate 5 and forms a minute spot on the information recording surface 7 by the objective lens 6. The laser beam reflected by the information recording surface 7 becomes circularly polarized light in the opposite direction, and the objective lens 6
The linearly polarized light is rotated by approximately 90 degrees from the linearly polarized wave plane of the light emitted from the 1-1 semiconductor laser when the 1/4 wavelength plate 5 is reversed. Then, the optical path is deflected at right angles by the polarizing beam splitter 4,
The light is focused by an astigmatic lens system formed by a cylindrical lens 8 and a condensing lens 9 onto a surface of a photodetector 10 disposed perpendicular to the optical axis. When the information recording surface 7 is located on the focal point plane of the objective lens 6, a substantially circular slit is formed on the photodetector 10 surface.

When the information recording surface 7 moves away from the focusing surface of the objective lens 6 and becomes far or near, the surface 4 on the photodetector 10? The plot changes from approximately circular to elliptical to linear. These optical signals are photoelectrically converted by a photodetector 1θ, and an electric signal 11
is obtained. Next, the operational amplifier 12 processes the reproduction signal (information signal) 13 and the control signal 14, and the control signal 14
The output from the servo amplifier 15 controls the objective lens 6 in the focusing direction (two axes) and the tracking direction (Y axis) using electromagnetic force or the like. In this way, accurate follow-up control is performed for the positional fluctuation of the information recording surface 7,
It is configured so that the reproduced signal 13 is not lost.

[Problems in the Background Art] In the conventional optical information reading device described above, when the light emitted from one end face of the semiconductor laser 1 is modulated and returns to this end face again, the semiconductor laser 1 exhibits a self-transmission effect or pumping effect. The laser output light fluctuates due to effects, changes in oscillation mode, etc. As a result, the noise component increases with respect to the reproduced signal or the focus, tracking, and control signals, which deteriorates the signal-to-noise ratio (Sc) and becomes a main factor that inhibits stable tracking control. Optical information processing devices that take advantage of this phenomenon have also been proposed (for example, Japanese Patent Publication No. 57-58735), but the stability of semiconductor lasers is still problematic and the device has not yet been put into practical use.

The following points can be considered as the causes of the light being returned to the semiconductor laser 1.

■Reflection from information recording surface 7.

■ Reflection from the plane of the optical element (polarizing beam splitter 4
．． (1/4 wavelength plate 5, one side of cylindrical lens 8, etc.) ■Reflection from the photodetector 10 side.

Among the above three items, the main element that moves the equivalent resonator length in synchronization with the disk movement on the surface forming the external resonator of the semiconductor laser 1 when the information recording surface 7, that is, the disk moves in two axial directions. The ones that hit are ■ and ■. This problem can be easily solved by tilting each end face within a few degrees with respect to the optical axis, or by applying multiple anti-reflection coatings.

Regarding (2), since the basic purpose is to obtain information from this reflective surface, it cannot be eliminated. In order to deal with this, it is necessary to develop a semiconductor laser 1 with characteristics that are resistant to reflected light, or to improve the characteristics of the optical isolator formed by the polarizing beam splitter 4 and the quarter-wave plate 5. . Regarding (2), it is currently difficult to obtain a laser that is strong against reflected light, and even if the characteristics of the optical isolator are improved, it is difficult to suppress the noise component due to this reflected light.

5- Experimental results have shown that it is usually necessary to keep the return ratio (the ratio of the return optical power to the semiconductor laser oscillation output power, expressed as a percentage) to about 0.001 inches or less. How the noise component due to the returned light exhibits characteristics with respect to changes in the position of the disk will be explained with reference to FIG.

According to the configuration of the optical system shown in FIG. 1, a 4-division 8l-PIN photodiode is used as a photodetector. If the difference between the sum of the outputs of the elements disposed on the diagonal of each of the four divided photodetectors and the difference between the respective sum outputs is used as a focus error signal, an S curve shown in A in FIG. 2 is obtained. The points of the positive and negative peak values are on the front and rear focal planes formed by the astigmatism lens system, and the stiff gap becomes the retraction range in which the objective lens can follow the disk.

Corresponding to this S curve, the characteristic of the noise component due to reflected light can be shown by B. This B characteristic curve shows that the noise increases at the starting point at the time of eye attraction. At @, the position of the disk is within the depth of focus of the objective lens 6, and therefore a substantially circular circle of least confusion is formed on the surface of the photodetector 10. This is mainly caused by reflection from the information recording surface 7, and reflection from the photodetector surface is added to the noise component, which becomes a factor that inhibits stable tracking control.

[Purpose of the invention]

The purpose of this invention is to stabilize focusing and tracking follow-up control, reduce loss of reproduced signals, and transmit high-quality information optically by reducing noise components generated when reflected light returns to the semiconductor laser. The object of the present invention is to provide a device with optical information h and y that can be read.

[Second Part 1 of the Invention] The present invention includes a semiconductor laser, an objective lens that converges a laser beam from the semiconductor laser to a minute spot on an information recording surface, and a reflected diffracted light of υ from the information recording surface of the semiconductor laser. an optical isolator that prevents it from returning to the laser;
An optical information reader comprising at least a condenser lens that condenses the reflected diffraction light, and a photodetector that photoelectrically converts the reflected diffraction light condensed by the condenser lens to obtain an electrical signal. In the device, the photodetector surface is set at a predetermined angle ψ with respect to the optical axis in order to reduce the output fluctuation of the semiconductor laser based on the reflected light υ from the photodetector surface, which has little effect even if an optical isolator is used. This is an optical information reading device that is fixed to a fixed part by tilting it by a certain angle and finely adjusting the position.

[Embodiments of the invention]

The optical information reading device of the present invention is constructed as shown in FIG. 3, and the same parts as in the conventional example (FIG. 1) are given the same reference numerals.

That is, as shown in the figure, a semiconductor laser 1, a collimator lens 3, and a deflection beam silicate which constitutes an optical isolator are used.
, a quarter wavelength plate 5, an objective lens 6, and an information recording surface 7 are arranged on the same axis at a predetermined interval, and an astigmatism lens system is arranged on the side of the deflection beam snoritter 4. A cylindrical lens 8 to be formed, a condensing lens 9, and a photodetector 10 are provided. The photodetector 10 is connected to an operational amplifier 12, and the operational amplifier 12 is connected to an amplifier circuit 15, and the amplifier circuit 15 is connected to the objective through an actuator 16, which is a driving means such as an electromagnetic force. It drives the lens 6.

In the above case, the surface of the photodetector 10 is inclined with respect to the optical axis by a predetermined angle ψ determined by the numerical aperture NA of the condenser lens 9, and this predetermined angle ψ is set within the following range. .

However, n is the refractive index of the medium surrounding the condenser lens 9 and the photodetector 10.

Note that the numerical aperture NA is expressed by the following equation.

NA=ns stone θ However, θ is the angle between the optical axis and the outermost luminous flux of the beam emitted from the condenser lens 9.

Now, during operation, the laser light beam 2-1 emitted from the semiconductor laser 1 becomes a parallel light beam at the collimator lens 3, and passes through the polarizing beam splitter 4 and quarter-wave plate 5, which form an optical isolator.
The objective lens 6 focuses the light onto the information recording surface 7 into a minute beam of light. - H The diffracted light reflected on the information recording surface 7 passes through an objective lens 6, a wavelength plate 5, a polarizing beam splitter 4, a cylindrical lens 8, a condenser lens 9, and is detected by a photodetector 10VC8. The photodetector 10 is inclined at a predetermined angle ψ (ζ11 to 12 degrees) with respect to the optical axis, and the electrical output signal 11 of the photodetector 10 is transmitted to the operational amplifier 1.
2, the signal is processed into a reproduction signal 13 and a control signal 14. This control signal 14 has means for obtaining two-axis control signals for focusing and tracking by an amplifier circuit 15, and driving the objective lens 6 in the two-axis and X-axis directions using electromagnetic force.

The astigmatic difference caused by the condenser lens 9 and the cylindrical lens 8 in this embodiment is approximately 14, and a Si MN structure 7-otodetector is disposed at the position of the circle of least confusion approximately in the middle of the astigmatism difference. There is. In this case, the numerical aperture NA of the condensing lens 9 is expressed as N. A = e n sin θ (where n is the refractive index of the condensing lens and the surrounding medium of the photodetector) and 1 to 1.
) ]ken = 1, NA = 0.16, and ψ = 9.
It is fixed to the mount part with the fixed optical system component within the range of 2 to 13.8 degrees. As the inclination angle ψ increases, the circle of least confusion becomes a pseudo-ellipse, making it difficult to adjust the position at the zero-crossing point of the S curve shown in Figure 2, and residual error components are likely to occur, making it difficult to adjust the position at the zero-crossing point of the S curve shown in Figure 2. It is easy to determine the waste points.

In the optical information collecting device configured as described above, the noise characteristics due to the V light returned to the semiconductor laser 1 by the reflected light are shown in Fig. 4, and the vertical axis represents the level of the reflected noise.
The horizontal axis shows the value (displacement μm) when the position of the disk is changed. If B (double-dashed line) is when the photodetector 10 is arranged approximately perpendicular to the optical axis as in the conventional case, then the inclination angle 11
It can be seen that the noise components of ■ and θ, which were large in the front and rear focal planes, have been reduced by about 30 dB or more at the front and rear focal planes, as shown by the solid line at .degree., and are also reduced at the just focus point@.

Also, the overall noise level is reduced. Since the signal component S is not reduced, both the control signal and the reproduced signal have a good S/N ratio, which contributes to stable follow-up control of the focus lens and tracking control, resulting in a high-quality reproduced signal. did it.

Incidentally, FIG. 5 shows a modification of this invention. In this modification, a collimator lens 3 is placed between a polarizing beam splitter 4 and an IA wavelength plate 5, and a light beam focused on an information recording surface 7 by an objective lens 6 is reflected on the information recording surface 2. It also serves to guide the light flux to the photodetector 10. Furthermore, as a detection method, when the distance between the objective lens 6 and the information recording surface 7 is slightly larger than the focal plane of the objective lens 6, the spot on the photodetector surface expands or converges. This is a control method commonly called the beam size method. Even in such an optical system, the light receiving surface of the photodetector 10 is determined by the numerical aperture NA of the collimator lens 3 [θ=sin-1('')]
It goes without saying that it is effective to use a predetermined inclination angle ψ.

〔Effect of the invention〕

According to the present invention, noise (displacement of the information recording surface) often occurs due to variations in the equivalent laser resonator length due to the information recording surface 7 and the photodetector 10 surface forming an external resonator with one end surface of the semiconductor laser. is the low frequency f0 noise that occurs every λ/2 of the laser oscillation wavelength, and the approximately 276'A noise due to the photodetector surface that occurs every λ/4 of the above displacement amount.
The variation (noise) of the semiconductor laser output light caused by the action of the zero surface is reduced by the focusing angle of the focusing lens 9 (θ-5in-'
-) It was possible to reduce this by tilting the surface of the photodetector 10 with respect to the optical axis. Along with this, the S/N of control signals such as focus and tracking error signals
Good and inexpensive follow-up control was obtained. Furthermore, the loss of keratinization signals is reduced, the error rate is reduced, and high-quality signals can be reproduced.

[Brief explanation of drawings]

Figure 1 is a schematic configuration diagram showing a conventional optical information reading device;
The figure is a characteristic curve diagram showing the relationship between the focus error signal and laser emission output fluctuation (reflection noise) and the amount of disk displacement in a conventional device. A schematic configuration diagram showing the device, FIG. 4 is a characteristic curve diagram showing the relationship between fluctuations in laser emission output (reflection noise) and disk displacement in the conventional device and the device of the present invention, and FIG. 5 is a characteristic curve diagram of the device of the present invention. It is a schematic block diagram which shows a modification. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2-1... Radiation beam from semiconductor laser, 2-2... Return light, 3... Collimator lens, 4... Polarizing beam splitter, 5... 1
74 wavelength plate, 6... objective lens, 7... information recording surface, 8... cylindrical lens, 9... condenser lens, 10... photodetector, 11... photoelectrically converted Electrical signal, 12... Operational amplifier, 13... Reproduction signal, 1
414-...Control signal, 15...Ser is amplifier circuit, 16...
- Actuator. Ill ff1fi agent Patent attorney Suzue Takehiko 15- Figure 1] Z Figure 2 Figure 3 Figure 4 Unevenness 1 Sedge displacement Figure 5

Claims (2)

[Claims] (1) A semiconductor laser, an objective lens that converges the Raded light beam from the semiconductor radar into a minute spot on the information recording surface, and a light that prevents reflected diffracted light from the information recording surface from returning to the semiconductor radar. An optical information reading device comprising at least an isolator, a condensing lens that condenses the reflected diffracted light, and a photodetector that photoelectrically converts the reflected diffracted light condensed by the condensing lens to obtain an electrical signal. An optical information reading device, wherein the photodetector surface is inclined with respect to the optical axis by a predetermined angle ψ, which is equal to the numerical aperture NA of the condenser lens. (2) The optical information reading device according to claim 1, wherein the predetermined angle ψ is set within the following range. However, n is the refractive index of the medium surrounding the condenser lens and photodetector.