JPH08212582A - Optical pickup device - Google Patents

Abstract

PURPOSE: To obtain an optical pickup device with an excellent CMRR by reducing or eliminating the optical path difference of light beam. CONSTITUTION: This device has a split optical system 6 splitting a light beam from a medium 5 in two and two photodetectors 11, 12 receiving the light beams split into two beams by the optical system 6 respectively and applying photoelectric transduction to the beams and reproduces a signal recorded on the medium 5 by taking an output difference between the photo detectors 11, 12. The split optical system 6 acts like a polarization beam splitter that splits an incident light beam into a light reflected in a split face and a light transmitted through the split face, and correction optical systems 9, 10 are provided to one or both the light beams split in two in order to reduce or eliminate a difference between the optical path length of each of both the split beams reaching the photo detectors 11 and 12.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention irradiates a signal recorded on a medium with a light beam, divides the transmitted or reflected light into two light beams, photoelectrically converts each of them, and then takes a differential between the two signals. The present invention relates to an optical pickup device that reproduces a signal.

[0002]

2. Description of the Related Art As a method of reproducing a signal recorded on a medium, a light beam is irradiated, the transmitted or reflected light is divided into two light beams, each of which is photoelectrically converted, and the two signals are differentiated. The system for reproducing a signal can remove common-mode noise with a simple structure, and is therefore used, for example, in a pickup device for a magneto-optical disk. According to this method, for example, disk noise such as scratches on the surface of the magneto-optical disk and laser noise such as laser noise caused by the laser light source, which are generated before the light beam is divided into two and are present in equal amounts in both beams, have a simple structure. Can be removed.

FIGS. 4 and 5 are sectional views showing the structure of the main part of a conventional magneto-optical disk pickup device. 1 is a laser diode, 2 is a collimator lens, 3 is a beam splitter, 4 is an objective lens, 5 is a recording medium, 6 and 16 are polarized beam splitters, 7 is one light beam, 8 is the other light beam, 11 is a first photodetector, 12 is a second photodetector, 13, 17, Reference numeral 18 is a printed circuit board.

[0004]

Conventionally, in a pickup device for a magneto-optical disk, a split surface is usually set at 45 ° with respect to the incident light as a member for dividing the reproducing light into two as shown in 16 of FIG. Using a polarized beam splitter. Therefore, since the bottom surfaces of the outer enclosures of the first and second photodetectors 11 and 12 are not on the same plane, both photodetectors cannot be mounted on the same substrate, and the positioning of each photodetector cannot be performed. However, there are disadvantages that the structure is complicated, the structure is complicated, positioning work is required, the number of steps is increased, and wiring is required for electrical connection between the substrates. Further, in order to overcome this drawback, the shape of the polarization beam splitter as a member for splitting the reproduction light into two can be set as shown in 6 of FIG. 5, but in this method, the recording medium and both photodetectors are used. Since the optical path lengths of the two signals do not match, CMRR [Common Mode Rejection Rat]
[io]] gets worse.

CMRR means the common mode voltage rejection ratio. Generally, when a signal is transmitted by two paths, 2
If the noise on the two paths is exactly the same, subtraction can eliminate the noise. However, in general, the noise cannot be reduced to 0 because it is not an ideal system. The degree of deviation from this ideal state is represented by CMRR. A voltage A is applied to the + and-inputs of the subtractor, and the output at that time (ideally 0)
It is usually expressed as -20log (B / A) [dB] with respect to B.

Therefore, in an ideal system, CMRR is ∞. Here, the first factor that deteriorates CMRR in the optical pickup device is the mismatch of the gains of the two paths (two paths each including both the optical system and the electric signal system). Reference numeral 2 is a mismatch of the phase characteristics of the two paths (two paths including both the optical system and the electric signal system). These discrepancies are caused by a difference in optical path length in the optical system and a difference in frequency characteristics of the detector and the amplifier in the electric signal system. Of these, the first factor is generally AGC.
The [Auto Gain Control] circuit can automatically make the coincidence, but the second factor cannot be automatically compensated.

Therefore, how much the reduction in CMRR due to the second factor is allowed will be examined below. First, consider the electric signal system. In general, the CMRR of a differential amplifier is large enough that it need not be taken into consideration. Although the mismatch of the phase characteristics of the preamplifier is quite large, if the characteristics of the amplifier are extended to a sufficiently high frequency band, the CMRR can be about 40 dB. That is, since the CMRR of the entire electric signal system is limited to about 40 dB by the preamplifier, it is meaningless to make the other parts larger than that.

Next, the optical system will be considered. Since the optical system has stray light, the CMRR of the well designed optical system is 30d.
It is said to be about B. That is, since the CMRR of the entire optical system is limited to about 30 dB by stray light, it is meaningless to make other parts larger than that. Therefore, in the case where two photodetectors are provided on one plane as shown in FIG. 5, the CMRR when the electric signal system and the optical system are combined is limited to about 30 dB by the optical system. Decrease in CMRR due to a factor, that is, inconsistency in phase characteristics of two paths (two paths each including both an optical system and an electric signal system) (difference in optical path length in an optical system, detector and amplifier frequency in an electric signal system) The decrease in CMRR due to the difference in characteristics) is sufficient as long as it does not decrease the overall CMRR below 30 dB. Here, in order to reduce the difference in the optical path length of the optical system by the method shown in FIG. 5, it is possible to reduce the shape of the polarization beam splitter 6, but reducing the shape reduces the manufacturing cost of the polarization beam splitter 6. It will dramatically increase, and practically it will be difficult to commercialize.

FIG. 6 shows the actual magneto-optical pickup device having the structure shown in FIG.
It is a figure showing the result of having calculated the relation with R using the optical path length difference as a parameter. However, since the purpose is to investigate only the effect due to the difference in optical path length, the CMRR determined by the differential amplifier, the optical system, etc. is calculated as an ideal one (infinity).

The decrease in CMRR due to the difference in optical path length is due to the phase difference between the two signals, and the phase difference is relative to the difference in optical path length.

[0011]

(Equation 3)

From the above relationship, the optical path length difference that gives a constant phase difference, that is, a constant CMRR, is

[0013]

[Equation 4]

As shown in FIG. 6, when the value of 40 dB is used as a guideline for the CMRR determined by the optical path difference, the optical path difference is 100 m.
In case of m, 50mm, 25mm and 12.5mm, each is about 5M
It can be seen that the values are 40 dB at Hz, 10 MHz, 20 MHz, and 40 MHz, respectively. Next, the allowable optical path length difference of the actual magneto-optical pickup device is obtained, and the constant k in the above equation is obtained.

It has been described above that even in an apparatus having an optical path length difference of 0, the CMRR is limited to about 40 dB by the preamplifier and further to about 30 dB due to stray light of the optical system. Therefore, the constant k is determined on the basis of the following. (1) CMRR = 30d as a magneto-optical pickup device
Achieve B. (2) To achieve the above (1), C determined only by the difference in optical path length
Keep MRR at 40 dB.

As described above, the factors that influence the CMRR are the stray light, which is the largest factor, the differential amplifier, the preamplifier, the optical path length difference, etc., and therefore the CMR is determined only by the optical path length difference.
R is 10 dB from the CMRR of the magneto-optical pickup device
It is reasonable to set it low. Therefore, the value of k is obtained from FIG. 6 as follows. Optical path length difference d = 50m
Since m, f = 10 MHz and CMRR = 40 dB,

[0017]

(Equation 5)

Therefore, if the maximum frequency of this device is fmax,

[0019]

(Equation 6)

This inequality is a general relational expression. For example, in the case of a magneto-optical pickup device with fmax = 18 MHz

[0021]

(Equation 7)

It can be understood that the following is good. FIG. 7 shows an actual magneto-optical pickup device having the structure shown in FIG.
It is a figure showing the result of having calculated the relation between the frequency of a reproduced signal and CMRR using the optical path length difference as a parameter. However, in FIG. 7, calculation is performed in consideration of CMRR determined by a differential amplifier, an optical system, etc. in order to investigate not only the effect of the difference in optical path length but also the effect of the current device.

An allowable optical path length difference is obtained for the magneto-optical pickup device having the performance shown in FIG. According to FIG. 7, when the optical path length difference is 0 mm and the reproduced signal is 18 MHz, CMRR
Is about 30 dB, but with an optical path length difference of 33 mm, the CMRR is less than 30 dB when the recording / playback signal is 18 MHz, so when obtaining the 18 MHz playback signal, the optical path length difference is 33 mm.
It should be sufficiently smaller, at least 1/2 the size of 16.5 mm.

The magneto-optical pickup device having the performance shown in FIG.

[0025]

(Equation 8)

Therefore, in the case of this device, in the case of the magneto-optical pickup device of the maximum frequency fmax = 18 MHz of the reproduction signal,

[0027]

[Equation 9]

It can be understood that the following is good. This inequality is obtained from various conditions when manufacturing the device (for example, the amount of stray light in the optical system, the adjustment limit of the electric signal system, etc.), and is a value that is generally appropriate from the current technical level. The present invention solves the above-mentioned drawbacks and allows two photodetectors to be mounted on one printed board, and the optical path difference between the two light beams can be reduced or eliminated to obtain a good magneto-optical characteristic. The purpose is to obtain a pickup device.

[0029]

SUMMARY OF THE INVENTION The main point of the present invention is to provide an optical path length correction optical system for reducing or eliminating the difference in the optical path lengths of two divided light beams to a photodetector.

[0030]

1 is a first embodiment of the present invention in which a light beam emitted from a semiconductor laser 1 is focused on a recording medium 5 via a collimator lens 2, a beam splitter 3 and an objective lens 4. Tie The light beam reflected by the recording medium 5 undergoes rotation of the plane of polarization in accordance with the information written on the recording medium, and again enters the polarized beam splitter 6 via the objective lens 4 and the beam splitter 3. Then one substrate 1
Total reflection prisms 9 and 10 are used to align the optical path lengths between the photodetector 11 and the photodetector 12 mounted on the recording medium 3 and the recording medium 5. Due to this structure, the positions of the total reflection prisms 9 and 10 are adjusted so that the optical path lengths of the two photodetectors and the recording medium are equal, and the bottom surfaces of the two photodetectors are on the same plane. Is possible.

FIG. 2 shows a second embodiment of the present invention, in which the light beam emitted from the semiconductor laser 1 reaches the recording medium 5 and reaches the beam splitter 3 again in the same way as in the first embodiment. Is the same. The light emitted from the beam splitter 3 enters the photodetectors 11 and 12 via the modified polarized beam splitter 14. Due to this structure, the recording media 5 to 2
The two photodetectors 11 and 12 are not required to be increased in order to match the two optical path lengths to the two photodetectors 11 and 12, and by appropriately changing the shape of the modified polarization beam splitter 14. To the photodetectors 11 and 1 on the printed circuit board 13 while maintaining the same optical path length difference between
There is an advantage that the interval of 2 can be changed and the pattern design of the printed circuit board 13 becomes easy.

FIG. 3 shows a third embodiment of the present invention in which the light beam emitted from the semiconductor laser 1 reaches the recording medium 5 and reaches the beam splitter 3 again in the first and second directions.
Is the same as the embodiment described above. The light beam 7 that has passed through the polarized beam splitter 6 enters the photodetector 12 through the high refractive index member 15 in order to match the optical path lengths of the photodetectors 11 and 12 and the recording medium 5, and the light beam 8 remains as it is. It is incident on the detector 11. Due to such a structure, in the present embodiment, it is possible to achieve the object of the present invention simply by adding the high refractive index member 15 having a simple shape (such as a prism or a cylinder) to the polarized beam splitter 6 which is easy to manufacture and inexpensive. There is.

[0033]

As described above, according to the present invention, since the optical path length correction optical system for reducing or eliminating the difference in the optical path lengths of the two divided light beams to the photodetector is provided, the two photodetectors are provided. Can be mounted on a single printed board, which has an advantage in mounting, and an optical pickup device having a good CMRR can be obtained by reducing the optical path length difference between the two light beams or eliminating the optical path difference.

In the first and third embodiments shown in FIGS. 1 and 3, since the polarization beam splitter 6 is relatively simple and easy to manufacture, the total number of prisms 9 and 10 or the simple total reflection prisms 9 and 10 is used. Since it is only necessary to add the high refractive index member 15 having a shape (square column, column, etc.), there is an advantage that the entire pickup is inexpensive. Further, in the second embodiment shown in FIG. 2, the shape of the modified polarized beam splitter 14 is appropriately changed so that both optical path lengths from the recording medium 5 to the photodetectors 11 and 12 are kept the same on the printed circuit board 13. There is an advantage that the distance between the photodetectors 11 and 12 can be changed appropriately, the degree of freedom in pattern design of the printed circuit board 13 is increased, and the design is very easy.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical pickup device according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of an optical pickup device according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a configuration of an optical pickup device according to a third embodiment of the present invention.

FIG. 4 is a sectional view showing a configuration of a main part of a conventional magneto-optical disk pickup device.

FIG. 5 is a cross-sectional view showing a configuration of a main part of a conventional magneto-optical disk pickup device.

FIG. 6 is a diagram showing a result of calculating the relationship between the frequency of a reproduction signal and CMRR of the conventional magneto-optical disk pickup device shown in FIG. 5 by using an optical path difference as a parameter.

FIG. 7 is a diagram showing a result of calculating a relationship between a frequency of a reproduction signal and CMRR of the conventional magneto-optical disk pickup device shown in FIG. 5 by using an optical path difference as a parameter.

[Explanation of symbols]

 1 ---- laser diode, 2 ---- collimator lens, 3 ---- beam splitter, 4 ---- objective lens, 5 ---- recording medium, 6, 16 ---- polarized light Beam Splitter, 7, 8 ---- Light Beam, 9, 10 ---- Total Reflection Prism, 11, 12 ---- Photo Detector, 13, 17, 18 ---- Printed Circuit Board, 14- --- Modified polarized beam splitter, 15 ---- High refractive index material.

Claims (3)

[Claims] 1. A photodetector comprising: a splitting optical system for splitting a light beam from a medium into two parts; and two photodetectors for respectively receiving and photoelectrically converting the split light beam by the optical system. In the optical pickup device that reproduces the signal recorded on the medium by taking the output difference of, the splitting optical system splits the incident light beam into light reflected by the split surface and light transmitted therethrough. In order to reduce or eliminate the difference in the optical path length of the bisected light beam to the photodetector, a correction optical system is provided in one or both of the optic paths of the bisected light beam. An optical pickup device characterized by the above. 2. The difference d in the optical path length after correction by the correction optical system is The optical pickup device according to claim 1, wherein the optical pickup device satisfies the above condition. 3. The difference d in the optical path length after correction by the correction optical system is The optical pickup device according to claim 1, wherein the optical pickup device satisfies the above condition.