JPS6191621A - Optical lens device - Google Patents

Abstract

PURPOSE:To allow a lens system to respond with lightweight, small-sized, and simplified constitution by constituting a coupling lens for obtaining a double focus by using Fresnel zone plates which are grooved longitudinally and laterally. CONSTITUTION:Two cylindrical lenses used for an astigmatism type focus position detection system are realized by one Fresnel zone plate. Longitudinal grooves are formed in the Fresnel zone plate according to f1=(rn)<2>/nlambda, where lambdais the wavelength of light and (r) is the distance (r1=612, r2=613...) from the center to the (n)th recess and projection. Namely, a different focus position f1 is obtained by varying the number (n) of recesses and projections. Similarly, lateral grooves are formed according to the focus position f2=(r'n)<2>/nlambda. When the plate 602 of this constitution is irradiated with a parallel beam 601, the shape of the beam on the plate 602 varies as shown by 711a, 711b, and 711c, so the focus position is easily detected and the optical system is simplified and reduced in size.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to an optical lens device suitable for use in optical systems that require focus adjustment, such as optical disk devices, cameras, and laser beam printers.

[Background of the invention]

The above-mentioned device is an optical system composed of many lenses, and the lenses are mainly composed of glass.

In addition, many of the lenses have a lens drive mechanism because it is necessary to automatically adjust the focus.

An optical disk device will be described below as a typical example of such an optical system. The optical disk device was published in the magazine ``Hitachi Hyoron J''.
This is already well known in the August 1984 issue.

FIG. 2 shows the overall configuration of the optical system of a conventional optical disk device. The symbols of each part in the figure and their operations will be explained below. 1 is a laser diode serving as a light source. A collimation lens 2 converts the light beam from the laser diode 1 into parallel light. 3 is a polarizing beam splitter (hereinafter abbreviated as PBS) that: ・Transmits the output light of the collimation lens, and also transmits the λ/
4 Refracts the returning light from the plate. λ/4 plate 4 is PBS3
It is used to polarize the phase of light in order to easily distinguish between incident light and reflected light. 5 is an objective lens, which is used to condense incident light. 6 is a coupling lens, PBS
It receives the luminous flux from 3 and condenses it. The coupling lens 6 is composed of two semicylindrical lenses orthogonally crossed. 7 is a photodetector. The photodetector 7 indirectly detects the shape of the light spot of the output light L5 from the objective lens 5 by detecting the shape of the light spot of the incident light L6 from the coupling lens 6. 8 is an actuator, and the actuator 8 adjusts the focal position of the output light L5 of the objective lens 5 according to the output of the photodetector 7. Reference numeral 81 denotes a lens driving section, and the lens driving section 81 adjusts the position of the objective lens 5 based on the drive control output from the actuator 8. 9 is a disk on which information can be optically recorded, reproduced, erased, etc., and a portion thereof is shown. The disk 9 enables the above-mentioned recording, reproduction, erasing, etc. by irradiating a desired light spot on the disk surface with the output light L5 from the objective lens 5. 10 is a motor, and the disk 9 is driven by the motor 10.

FIG. 3 shows a conventional embodiment of the photodetector 7 shown in FIG. In the same figure, L6 indicates the output light L6 of the coupling lens 6 of FIG. 2. Pl.

P2. P3. P4 is a photodiode for converting the amount of light into an electric signal. When the light spot of the incident light L6 is a perfect circle, the photodiodes pi, P2 . The output voltages of pa and P4 are set to Vl, V2. V3. Assuming V4, these are set to be equal to each other. 71 and 72 are subtracters for tracking on the disk 9, respectively. By detecting the output signal of the difference (Vl - V2) between the output voltage v1 of the photodiode P1 and the output voltage v2 of the photodiode P2, the output light L5 from the objective lens 5 is indirectly irradiated onto a predetermined line. Determine whether or not the If the output light L5 of the objective lens 5 does not strike the information recording line of the disk 9 symmetrically, a difference will occur between the output voltages v1 and v2.

The deviation from the recording line on the disk 9 is detected based on the magnitude and sign of the difference, and a tracking control signal TAI for controlling the position of the objective lens 5 is output until the difference disappears.

Similarly to the subtracter 71, the subtracter 72 uses the output voltages 3 and v4 of the photodiodes P3 and P4 as input signals to obtain a tracking control signal TA2. 73 and 74 are adders, and 75 is a comparator, which output light L from the objective lens 5.
Reference numeral 5 denotes a portion for obtaining an output signal FA for autofocus control to adjust the depth of focus applied to the disk 9. This means that when the depth of focus matches the recording line of the disk 9, the incident light L6 is perfectly circular and the photodiodes PL, P2. P3. It is equally incident on P4 and has the largest amount of luminous flux, and the output voltage Vl.

V2. V3. V4 are equal and their magnitude is maximum. If the depth of focus deviates, the incident light L6 becomes elliptical, and the photodiodes P1, P2, . P3. P4
(7) Output voltage Vl, V2. Created with V3゜v4 (V
1+V3) and (V2+V4). This phenomenon is detected using adders 73, 74 and comparator 75, and the position of objective lens 5 is adjusted until the output signal FA becomes zero. FIG. 4 shows a conventional embodiment of a driving section for the objective lens 5 using tracking and autofocus control. Opening: 811°812.8
13.814 is a coil. 815 is a holder for the objective lens 5. 816, 817 are magnets, 818, 8
19 is a spring, and 820 is a frame. The objective lens 5 is fixed by a holder 815 and mounted to the frame 820 of the optical head by springs 818 and 819. magnet 8
16° 817 is used as one of the elements to determine the position of the objective lens 5. By making the holder 815 a magnetic material such as a piece of iron, the objective lens is moved by the attraction force of the magnets 816 and 817 and the tensile strength of the springs 818 and 819. The position of 5 has been determined. Based on the focus control output signal FA from FIG. 3, current is also applied to the coils 811 and 812 to move the position of the objective lens 5 up and down to obtain a desired depth of focus.

The tracking control signals TA and TB are each sent to the coil 81.
3, 814, the left and right positions of the objective lens 5 are adjusted, and desired tracking is performed.

As described above, according to the conventional method, it is possible to perform tracking and control the depth of focus to accurately irradiate a light spot onto a recording line of a disc.

As described above, the optical disk device is a very complicated optical system, and it is essential to precisely adjust the positions, angles, etc. of each part. Furthermore, regarding objects such as objective lenses that require automatic focus adjustment, as shown in FIG. 4, they have a large number of component parts and use glass lenses, resulting in large inertia and slow response.

[Purpose of the invention]

From the above, in the present invention, the lens is particularly lightweight and
It is an object of the present invention to provide an optical lens device that is simple, compact, and responsive.

[Summary of the invention]

In the present invention, a thin film-like micro Fresnel lens is used as the lens.

[Embodiments of the invention]

FIG. 6 is an overall structural diagram showing an example of the present invention.

The diverging light emitted from the laser diode 1 is turned into parallel light by a collimation lens 2, which is a micro Fresnel lens MFL, which will be described later, and transmitted through a polarizing beam splitter 3, where it becomes linearly polarized light by an objective lens 5, which is a MFL, which will be described later.
The lens is narrowed down to focus on the disk 9. Here, as is well known, the polarizing beam splitter is used in combination with a λ/4 plate to prevent the light reflected from the disk surface from returning to the laser diode 1. That is, the λ/4 plate 4 is a retardation plate that functions to convert linearly polarized light vibrating in a direction making 45 degrees to the optical axis included in the cut plane into circularly polarized light, and to convert circularly polarized light into linearly polarized light. When the light reflected from the disk passes through the λ/4 plate 4, the direction of the linearly polarized light is converted by 90 degrees from that at the time of incidence. Therefore, the reflected light from the disk does not return to the laser diode 1 side by the polarizing beam splitter 3, but the entire amount of light is refracted toward the photodetector 7 side. Since the coupling lens 6 is a micro Fresnel lens as described later, the shape of the spot formed on the photodetector 7 varies depending on the distance between the objective lens 5 and the disk 9, and this is captured as an increase or decrease in the signal of the detector. Therefore, using the signal from the photodetector 7, the distance between the objective lens 5 and the disk 9, that is, focusing, and the radial direction, that is, tracking can be controlled. This focusing and tracking is performed by a piezoelectric element 54 provided on a spring 53 supporting the objective lens 5.
．． This can be done by controlling the voltage applied to 56. The details will be described later.

As described above, according to the present invention, expensive collimation lenses and objective lenses that were conventionally required.

And not only can cylindrical lenses be made into inexpensive micro Fresnel lenses.

By directly joining them to the polarizing beam splitter 3, the time-consuming effort of optical axis adjustment can be saved.

Furthermore, by reducing the weight of the objective lens and supporting it with a soft spring 53, it becomes possible to control it with piezoelectric elements 54 and 56, simplifying the structure and improving frequency characteristics. Furthermore, looking at the overall configuration, the optical head from the laser diode 1 to the objective lens 5 including the actuator can be downsized.

FIGS. 1 and 5 show an astigmatism-type focal position detection optical system using a Fresnel zone plate and a configuration diagram of the Fresnel zone plate. This corresponds to coupling 6 in FIG. The astigmatic focal position detection system arranges two cylindrical lenses with different focal positions in orthogonal directions, and directs the transmitted beam from a long ellipse in a certain direction.
This is a method of detecting the focal point position by changing the focal point to a long ellipse in a direction perpendicular to that direction.

The feature of this configuration is that two orthogonal cylindrical lenses are realized by one Fresnel zone plate, making the optical system lightweight and compact.

The focal length f of the Fresnel zone plate is given by the following formula, where:
n is the number of concavities and convexities of the Fresnel zone plate, r is the distance from the center to the nth concavity and convexity, and λ is the wavelength of light. That is, by changing the value of n, that is, the number of concave and convex portions, different focal lengths can be provided.

A specific example is a composite Fresnel zone plate shown at 602 in FIG. The unevenness in the vertical and horizontal directions is formed at different pitches, giving different focal lengths in each direction.

When this composite Fresnel zone plate 602 is irradiated with a parallel beam of light 601, the shape of the output beam changes continuously as shown by 603 to 608. Assuming that when the objective lens of the optical disk device is at the focused position, a parallel beam is returned, and if the four-part sensor shown at 711 is installed at a perfect circle, that is, at the position 605, the focus will shift, When the parallel beam is distorted, the beam shape on the sensor changes as shown by 711a and 711c, and the defocus can be detected.

Figure 5 shows details of the composite prenel zone plate.

Vertical direction f-=(r,)''/nλ, which overlaps grooves with different pitches in the vertical and horizontal directions to serve as two cylindrical lenses with different focal lengths.
...It is given as darkness. rl is the maximum distance in the vertical direction from the center, and in the figure, rl is 612 and r is 613
,...r, are given by 615.

Similarly, the focal position f2 due to the horizontal groove is expressed by the following formula. r,' is the maximum horizontal distance from the center, r in the figure
, / is 619, r , / is 6201.・r, / is given by 622.

〔Effect of the invention〕

As described above in detail, according to the present invention, an optical lens device can be constructed easily and compactly.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram of an embodiment of a coupling lens which is the main part of the present invention, Fig. 2 is an overall configuration diagram of an optical system of a conventional optical disk device, and Fig. 3 is a configuration diagram of a conventional photodetector. , 4th
5 is an explanatory diagram of a presnel zone plate according to an embodiment of the present invention, and FIG. 6 is a diagram showing the entire configuration of an embodiment of the present invention. 1... Laser diode, 2... Collimation lens, 3... Polarizing beam splitter, 4... λ/4
Plate, 5...Objective lens, 6...Coupling lens, 7...Photodetector, 9...Disk.

Claims (1)

[Claims] 1. Light from a light source is irradiated onto a recording medium through a collimation lens, a polarizing beam splitter, and an objective lens, and the reflected light is transmitted through an objective lens, a polarizing beam splitter, and a coupling lens to obtain bifocal light. 1. An optical lens device for sensing with a photodetector, characterized in that the coupling lens is constituted by a Fresnel zone plate in which grooves are formed in the vertical and horizontal directions.