JPS627609B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical device of an optical reproducing apparatus that optically reproduces information recorded on a recording medium.

As already known as an optical reproduction device,
Video disk, document file, PCM
A wide variety of applications are being considered, such as audio disks, but one of the things that has been desired in order to reduce the size and weight of conventional playback devices is to reduce the size and weight of the signal and pick-up systems that consist of optical systems. It was hot. In recording/playback devices or playback-only devices that use gas lasers and ion lasers that are currently in common use, the size of the laser often ranges from several tens of centimeters to over a meter, resulting in heavy weight. However, it weighs several hundred grams or several kilograms, so there is no hope of reducing the weight.
However, recent advances in semiconductor technology have made it possible to miniaturize optical systems using semiconductor lasers. One of the typical conventional optical systems is shown below.
Let's explain an example.

FIG. 1 shows one of the well-known optical systems using a semiconductor laser. A light beam emitted from a semiconductor laser 1 passes through a polarizing beam splitter 2, enters a coupling lens 3, and becomes a parallel beam. The parallel beam passes through a quarter-wave plate 4, is converged by an objective lens 5 almost to the diffraction limit of light, and irradiates a recording medium (optical disk) 6 containing an information signal. The reflected light beam modulated by the information signal passes through the objective lens 5, the quarter wavelength 4, and the coupling lens 3 again, and is now reflected by the polarizing beam splitter 2 in a perpendicular direction. The exit surface of the polarizing beam splitter 2 in this direction has a biprism shape, and the light beam passing through it is split and sent to the photodetector 7.
incident on . The photodetector has a quadrant shape and is located within the image formation plane of the information signal, and the light beam split by the biprism is divided into (7 of 1) and (7 of 2) of the quadrant photodetector. It is incident on the boundary line and on the boundary line between (7-3) and (7-4). This method detects the optical disk position, which is collectively called the Foucault method or knife-edge method, and the output of the photodetector is calculated using a circuit as [(7-1) + (7-4)] - [(7- 2) +(7 of 3)] By setting the error in the distance between the optical disk 6 and the objective lens 5, a detection signal as shown in FIG. 2 can be obtained. Therefore, so-called focusing servo, which controls the position of the optical disk, can be performed using this detection signal. In addition, since the light beam is divided by a far field, the information can be tracked by using the circuit as [(7 of 1) + (7 of 2)] - [(7 of 3) + (7 of 4)]. It is possible to obtain a tracking error signal indicating the relationship between the distance and the focal point.
Therefore, it is possible to perform so-called tracking servo for controlling the information track position of the optical disk using this detection signal. The mechanism for performing such focusing servo and tracking servo is as follows:
For example, the method disclosed in JP-A-53-120403 can be used, so the explanation of the mechanism will be omitted.

A drawback of the above-mentioned method is that, as is clear from Figure 2, the dynamic range in which the error signal can be obtained depends on the distance between the optical disk and the objective lens, and becomes extremely large as the distance between the optical disk and the objective lens increases. It is a narrow thing. For example, if the numerical aperture (NA) of the objective lens is 0.5 and the numerical aperture of the coupling lens is 0.15, the magnification of the optical system will be approximately 3.3 times. Since the distance between the polarizing beam splitter 2 and the photodetector 7 is on the order of several millimeters or less in a compact optical system, the dynamic range of focusing is (1/3.3 ² ), that is, about 10 It is on the order of several hundred microns or less. As a practical matter, in an optical system that is becoming smaller, the shorter the distance between the polarizing beam splitter and the photodetector, the easier it will be to reduce the size and weight. If the optical system is designed with this in mind, the actual focusing dynamic range will be less than 200μ, which will cause considerable practical problems.

From the above points, it can be inferred that desirable characteristics can be obtained by increasing the distance between the polarizing beam splitter and the photodetector.

In the present invention, in order to increase the distance between the photodetector and the polarizing beam splitter, the light beam emitted from the polarizing beam splitter is transmitted through the polarizing beam splitter again, and is directed to the vicinity of the other end face of the polarizing beam splitter. This is an attempt to make the light incident on a photodetector. As a system similar to this type of optical system, the present inventor's patent application No. 54-160979
There is also a patent application No. 146166/1983.

The basic differences between the present invention and the prior invention are as follows:
In the prior invention, a polarizing beam splitter is inserted into the parallel light beam, and the method according to the present invention cannot be made as efficiently as the method according to the present invention in terms of miniaturization of the optical system. That is, in the method of the prior invention, a concave mirror-shaped reflecting mirror, for example, was required to convert the parallel light beam into a convergent light beam. Therefore, it is impossible to form a total reflection mirror by performing vapor deposition on a quarter-wave plate as shown in the embodiments of the present invention.

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 3 is a diagram showing an outline of the present invention. The light beam emitted from the semiconductor laser 1 passes through the polarizing beam splitter 2, the coupling lens 3, and the quarter-wave plate 4, and is then converged by the objective lens 5 to the diffraction limit of light, and is focused on the optical disk 6. Emits information signals. The light beam is modulated by the information signal, reflected, passes through the objective lens 5, quarter-wave plate 4, coupling lens 3, and is reflected by the polarizing beam splitter 2. The reflected light beam is incident on the second quarter wave plate 10, and some of the light beams are incident on the second quarter wave plate 10.
The polarizing beam splitter 2 is reflected by a reflecting mirror provided by vapor deposition on approximately half of the back surface of the beam splitter or by a total reflection mirror 11 provided separately.
and enters the two-split photodetector 12. The remaining light beam that is not reflected by the total reflection mirror 11 enters the photodetector 9. This photodetector 9 is
The photodetector can be divided into two parts in the direction perpendicular to the drawing. Then, since the photodetector 9 is set at the far-field position of the information signal, a far-field tracking error signal can be obtained by a known method. Further, the light beam incident on the photodetector 12 converges a part of the light beam in the far field and is incident on the boundary line of the photodetector 12 which is a two-split photodetector. Since a part of the light beam is incident on the boundary line of the two-split photodetector, it is possible to obtain an error signal from this photodetector 12 for the focusing servo called the aforementioned Foucault method or knife-edge method. .
The error signal for this focusing servo is
As shown in FIG. 4, the dynamic range is increased in the direction in which the optical disk moves away from the optical disk. According to the previous calculation example, the numerical aperture of the objective lens is
0.5, and the numerical aperture of the coupling lens is 0.15.
If the size of the polarizing beam splitter 2 is 5 mm square and the thickness of the quarter-wave plate 10 is about 0.5 mm, it is possible to obtain a focusing dynamic range of about 400 μm or more. This value is approximately twice the value of 200 μm in the example shown in FIG. 1, and the lateral size of the optical system is reduced by more than 1.5 mm. This lateral size is especially important when the optical system is integrated, focusing servo,
In terms of mechanical design for tracking servo and the like, it is desirable to keep the size as small as possible, and the features of the method according to the present invention work effectively.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing an example of an optical system of a conventional playback device, FIG. 2 is an explanatory diagram of the same operation, FIG. 3 is a configuration diagram of an optical system showing one embodiment of the present invention, and FIG. It is an operation explanatory diagram. 1... Semiconductor laser, 2... Beam splitter, 3... Coupling lens, 4, 10... Quarter wavelength plate, 5... Objective lens, 6... Disk, 9, 12... Light detection Vessel, 11...Reflector.

Claims (1)

[Claims] 1. Light from a light source is irradiated onto a recording medium through a beam splitter, a first quarter-wave plate, and an objective lens, and the reflected light from the recording medium is transmitted through the objective lens and a quarter-wavelength plate. a mirror that irradiates the beam splitter through the beam splitter and reflects the reflected light in a right angle direction, and a mirror that reflects a part of the luminous flux of the reflected light through a second quarter-wave plate; and a first photodetector. The light reflected by the mirror is irradiated onto a second photodetector via the second quarter-wave plate and the beam splitter 2, and the tracking servo is transmitted from the first photodetector. An optical device for a reproducing device, characterized in that a focusing servo signal is obtained from a second photodetector.