JPH04228124A - Optical pickup - Google Patents

Abstract

PURPOSE:To efficiently obtain a regenerative signal having a high S/N by providing plural optical detecting means and reading pit information from the difference of each output. CONSTITUTION:Light from a semiconductor laser is transmitted through the inside of an optical waveguide 6 on a semiconductor substrate 7 and converged on a spot at a condensing grating coupler 3a on an optical disk 4. The zero-th order reflected at the disk 4 becomes again a leading optical wave at the coupler 3a, is converged on a photodiode 5a for the zero-th order at a grating beam splitter 2, and becomes an output signal. Meantime, (+ or -)1st order diffracted light beams, which are generated by the diffraction of the pit 9 on the optical disk 4, are respectively returned to the opening area of (+ or -)1st order converging grating couplers 3b, 3c, again become the leading optical wave, and are converged to the photodiodes 5b, 5c for (+ or -)1st order light to become output signals. Namely, by means of the optical pickup, the zero-th order light and (+ or -)1st order light by the pit string 9 of the optical disk 4 are taken out separately and converged to the diodes 5a and 5b, 5c.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup for reading optically recorded information such as a CD (compact disc), and more particularly to an optical pickup suitable as an integrated optical pickup using an optical waveguide.

0002

2. Description of the Related Art FIGS. 10A and 10B are diagrams showing the principle of reading an optical disk using a conventional optical pickup (center aperture method). As shown in Figure (A), the emitted light from the semiconductor laser 11 passes through the half mirror 12,
(Objective) A spot is formed on the optical disk 14 by the lens 13. When the spot hits a pit, diffraction occurs due to the pit, and the 0 reflected by the optical disc 14
The secondary light returns to the aperture of the lens 13 as it is, but the ±1st-order diffracted light undergoes a phase shift proportional to the pit depth d and returns to regions 16 and 17 shifted from the aperture of the lens 13. Therefore, at the lens aperture, the 0th-order light and the 1st-order diffracted light interfere at the overlapping portion, and the intensity decreases due to the phase difference. The above is the case where the spot is on a pit, but if there is no pit, that is, the spot is on a mirror surface, almost no first-order diffracted light is generated, and the intensity decreases over the entire lens aperture. It disappears. In both cases, the reflected light entering the lens aperture is reflected by the half mirror 12 and focused onto the photodiode 15. As described above, since the intensity of reflected light within the lens aperture changes depending on the presence or absence of pits, an electrical signal corresponding to the change in intensity is obtained as the photodiode output.

Further, FIG. 10(B) is a diagram showing an example using an optical integrated pickup. This is shown in FIG. 10(A) mentioned above.
In contrast, the objective lens 13 is replaced with a condensing grating coupler 13A, the half mirror 12 is replaced with a grating beam splitter 12A, and an optical waveguide 20 is formed on a semiconductor substrate 19, and the reading principle is as shown in FIG. 10(A).
It is similar to

[0004] In signal detection using such an optical pickup, the portion of the ±1st-order diffracted light that does not enter the lens aperture is not used at all, and the efficiency of using reflected light is low. Furthermore, since the pit detection signal was detected as a sum signal of the outputs of a plurality of photodiodes, optical and electrical common-mode noises could not be removed, resulting in a decrease in the S/N ratio.

For this reason, an example of detecting pits by separately detecting the 0th order light and the diffracted light using separate sensors and taking the difference between the two is disclosed in Japanese Patent Publication No. 55-26529 (USP 3,913, 076) and others. As shown in FIG. 11, an optical filter 22 having a rectangular window 21 and an objective lens 23 allow the recording medium to be
The main detector 26
The 0th-order light and the higher-order diffracted light are read by the side detectors 27 and 28, and a signal is output.

[0006]

[Problems to be Solved by the Invention] In the method shown in FIG. 11, since the 0th order light and the diffracted light are divergent lights centered on the same point, the 0th order light and the diffracted light are separated by separate sensors. To detect separately by far field
A detector with a large area (
This increases the capacity of the detector, deteriorating the frequency characteristics and increasing the noise level. As described above, there are many restrictions on the shape of the sensor and its arrangement, and the problem remains that there is little freedom in design when actually manufacturing an optical pickup.

[0007]

[Means for Solving the Problems] In order to solve the above problems, the present invention provides an optical pickup for reading pit information from an optical information recording medium in which information is recorded by pits. formed and arranged so that the reflected light (or transmitted light) and the diffracted light are substantially separated, and
Diffraction light generated by the diffraction effect of the pits of the optical information recording medium and a first condensing area that guides the reflected light (or transmitted light) from the optical information recording medium to the first light detection means is converted into second light. a light collecting means having a second light collecting region that guides the light to the detecting means; a first light detecting means for detecting reflected light (or transmitted light) from the optical information recording medium; and a pit of the optical information recording medium. and second photodetection means for detecting diffracted light generated by the diffraction effect of the first and second photodetection means, and reads pit information from the difference between outputs of the first and second photodetection means. It is.

In the optical pickup configured as described above, the reflected light (or transmitted light) and the diffracted light from the optical information recording medium are condensed into a condenser having a first condensing area and a second condensing area. The pit information is separated through a means, detected by first and second photodetecting means, and pit information is read from the difference.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the optical pickup according to the present invention will be described in detail below with reference to the drawings. First, an example in which an optical pickup is configured with an optical integrated circuit using a grating lens will be described. FIG. 1 shows an example of an optical pickup constructed from an optical integrated circuit. The emitted light from the semiconductor laser 1 propagates within the optical waveguide 6 formed on the semiconductor substrate 7 and passes through the condensing grating coupler 3.
It is configured to connect spots on an optical information recording medium (optical disk) 4 by a.

The zero-order light reflected by the optical disk 4 passes through the condensing grating coupler (first condensing area) 3
The light becomes guided light again by the grating beam splitter 2, and is focused by the grating beam splitter 2 onto the zero-order light photodiode (first light detection means) 5a to become an output signal. on the other hand,
The ±1st-order diffracted light generated by the diffraction of the pits (columns) 9 on the optical disk 4 returns to the aperture areas of the condensing grating couplers (second condensing regions) 3b and 3c for ±1st-order light, respectively, and becomes the guided light again. The light is condensed onto photodiodes for ±1st order light (second light detection means) 5b and 5c, and is configured to become an output signal.

That is, the optical pickup includes a condensing grating coupler (first condensing area) 3a, a condensing grating coupler for ±1st order light (second condensing area) 3b, 3
c, the 0th-order light and the ±1st-order diffracted light from the pit row (9) of the optical disc 4 are separated and taken out, and the 0th-order light photodiodes (first light detection means) 5a and ±1st-order diffracted lights are respectively taken out.
Photodiode for secondary light (second light detection means) 5b, 5
It is configured so that the light is focused on c. The shape and arrangement of the condensing means will be described in detail later.

In the optical pickup constructed as described above, as shown in FIG.
output is strong, and the output of the zero-order photodiode 5a is relatively weak. On the other hand, if the spot is on a mirror surface without pits 9, there is almost no diffraction, so ±1
The outputs of the photodiodes 5b and 5c for the second order light almost disappear, and on the contrary, the output of the photodiode 5a for the zeroth order light becomes strong. In this way, the pit information appears in the outputs of the zero-order light photodiode 5a and the ±1st-order light photodiodes 5b, 5c in opposite phases. Therefore, the pit information (
An output signal can be obtained depending on the presence or absence of pits.

Regarding focusing error and tracking error detection, for example, Japanese Patent Application Laid-Open No. 61-29654
"Optical head device" described in No. 0 and JP-A-61-85
Like the "optical head device" described in No. 637, the outputs of four zero-order photodiodes (5a shown in Figure 1) at different detection positions are combined and each error signal is detected by addition and subtraction. Bye.

Next, the condensing grating coupler (first
The zero-order light and the ±1st-order diffracted light from the pit row (9) of the optical disk 4 are separated by the focusing grating couplers (second light focusing regions) 3b, 3c for ±1st-order light. The configuration (shape and arrangement of the light condensing means) for taking out the light will be explained with reference to FIGS. 3(A) and 3(B).

FIG. 3A shows how light is focused and diffracted in a cross section perpendicular to the pit row. Pit row spacing Λ
The light beams condensed on the surface of the optical disk are not only reflected along the same path as zero-order light, but also diffracted as ±1st-order diffracted light such that the centers of each light beam form an angle α with each other.

At this time, the angle α is expressed as α=sin-1(λ/Λ), where λ is the wavelength of light. In addition, the convergence/divergence angle θ of each luminous flux is shown in Figure 3 (
In B), the aperture of the light focusing means (light focusing area) in the direction perpendicular to the pit row (radial direction) is Lx, and the focal length is f.
When, θ=tan-1(Lx/2f). Therefore, the conditions under which the ±1st-order light does not overlap with the 0th-order light region are 2θ≦α and ∴2 tan-1 (Lx/2f)≦sin-1(λ
/Λ). In particular, when the equal sign holds, the regions are adjacent to each other without any interval.

For example, in an optical disc such as a CD (compact disc), Λ=1.6 μm, so
When λ=780nm and f=2mm, the aperture Lx is
Lx=2ftan{1/2 ・sin −1(λ/Λ)
}, it is sufficient to set Lx=1.04 mm. That is, the opening (radial direction) Lx of the condensing grating coupler (first condensing means) 3a is approximately 1.04 mm.
As shown in FIG. It is sufficient to arrange the lens so that the distance from the disk 4 is equal to the focal length f. With this configuration, the 0th-order light and the ±1st-order diffracted light are easily separated, and the 0th-order light region and the ±1st-order diffracted light region are adjacent to each other without any interval.

[0018] Incidentally, a grating lens (grati lens)
ng lens), see, for example, “D.h.
Eitmann, et al. , “Calculati
on and experimental verification
cation of two-dimensiona
l focusing grating couple
rs,”IEE J. Quantum Electro
n. , QE-17, p. 1257, 1981” “T.
Suhara, et al. , “Optical integrated circuitization of optical pickups”, O plus E, no. 76, Mar
．． Please refer to '86'.

The operation (function) of the optical pickup configured as above will be explained. Optical pickup is ±1
Almost the entire order diffracted light is detected by the condensing grating couplers for ±1st order light (second condensing means) 3b and 3c, and furthermore, only the intensity ratio of the 0th order light and the ±1st order diffracted light is detected without using interference. Since differential detection is performed using , the signal strength increases and the symmetry of the detected waveform improves. In addition, since reading is performed using differential detection, common mode noise can be suppressed.
The S/N ratio is improved. Therefore, pit information can be reliably reproduced from the optical disc.

That is, as shown in the simulations shown in FIGS. 4A and 4B, the present optical pickup has an extremely good read response waveform when compared with the conventional integrated optical pickup. (A) of the same figure shows the readout waveform of this optical pickup, and the amplitude L1 of the readout waveform when the pit length or pit interval is the shortest, and the amplitude L2 of the readout waveform when the pit length is the longest, and the amplitude values are large, and The amplitude changes are approximately symmetrical, and by setting the slice level to the center L0, it is possible to demodulate pit information extremely accurately. The same figure (B) shows the readout waveform of a conventional optical integrated pickup, and the amplitude R1 of the readout waveform when the pit length or pit interval is the shortest and the amplitude R2 of the readout waveform when the pit length is the longest are both the amplitude values. is small and the center values of the amplitude changes of both do not match, so the slice level cannot be set to the center of both amplitude values (center R of R1).
0), it is not easy to demodulate the pit information.

[0021] In addition, this optical pickup has an optimum aperture (Lx is set to approximately 1.04 mm) for a CD (compact disc) as described above.
As is clear from FIG. 5, when , the crosstalk from adjacent tracks is almost minimized. FIG. 5 is a diagram showing an analysis result of a read response in an optical integrated disk pickup (see pages 82 to 90 of "Optics" Vol. 18, No. 2). This shows the crosstalk change when the aperture (radial direction) is changed from 0 to 2 mm when Λ=1.6 μm and f=2 mm. Since crosstalk depends on the light collection characteristics in the radial direction,
The aperture Lx of the (0th order) condensing grating coupler (3a) of this optical pickup can be considered as the horizontal axis in the figure. C
As is clear from the figure, when the aperture is optimal for D (compact disc) (Lx is set to approximately 1.04 mm), crosstalk from adjacent tracks is almost minimized.

Furthermore, since the present optical pickup has a small opening (in the radial direction) of the output light focusing means, the allowable manufacturing error is relaxed, and the intensity distribution of the output light in the direction perpendicular to the pit row (in the radial direction) is optimized. Since the position of the laser light source is closer, the device can be made smaller.

In the above example, only the diffracted light in the radial direction is separated and detected, but this can also be extended to two dimensions. Since similar diffracted light is generated in the front and rear directions of the pit, an example in which a condensing means (condensing area) is provided for the diffracted light in the front and rear directions of the pit in the same way as for the diffracted light on both sides is shown in Figure 6. Shown below. The condensing grating coupler 3d is a newly provided fourth condensing means for the diffracted light in the front-rear direction. Concentrating grating coupler 3
The diffracted light collected at point d is detected by a photodiode 5d, added to the output of the diffracted light signal in the left and right direction, and the difference from the zero-order light is taken to obtain a pit detection signal.

Next, an example will be explained in which a bulk lens is used as the light condensing means. In FIG. 7, light emitted from a semiconductor laser 1 passes through an optical path changing element (for example, a half mirror) 2, and is focused onto a disk 4 by a central portion 3a of a lens 3. The zero-order light reflected by the disk 4 passes through the center portion (first condensing region) 3a of the lens 3 again, is bent by the optical path changing element 2, and is focused on the photodiode 5a, becoming a zero-order light output signal. If there is a pit 9 on the disk 4, its diffraction will generate ±1st-order diffracted light, which will be used for detection by both sides (second condensing area) 3b and 3c of the lens 3, respectively. The light is focused on the photodiodes 5b and 5c, and becomes a ±1st-order optical output signal. With the differential amplifier 8,
The difference between the sum of the detected ±1st-order optical signals and the 0th-order optical signal is taken, and a pit detection signal, that is, an information signal recorded on the disk 4 (presence or absence of pits 9) is reproduced.

Here, the lens (condensing means) 3 used
I will explain about it. FIGS. 8A and 8B show cross sections of the lens. In the conventional lens shown in the same figure (A), Y
-Y' is the lens center optical axis, and W is the reflecting surface (the surface of the disk 4). Furthermore, points P and Q are assumed to be conjugate points of the lens. Reflected light from point Q on the central optical axis converges on point P on the same axis, but reflected light from point Q' off the central optical axis of the same reflective surface converges on point O on the central optical axis of the lens. and Q', and is converged to a point P' at the same distance between the point P and the lens. Therefore, as a lens, as shown in the same figure (B), the lens parts 3a (first condensing area), 3b (second condensing area),
3c (second condensing region) is used. As already explained in FIG. 3, the lens portion 3a
Lens portions 3b and 3c (second light focusing region) are formed on both sides of the opening (radial direction) Lx of the first light focusing region, and the distance between the lens 3 and the disk 4 is the focal point. If arranged so that the distance is equal to f, the 0th order light and the ±1st order diffracted light can be easily separated, and the 0th order light region and the ±1st order diffracted light can be easily separated.
The first-order diffracted light region is adjacent to the first-order diffracted light region without any interval, and 0
It is possible to separate and detect the diffracted light of the order light and the ±1st order light.

Note that when a conventional lens 3 that utilizes refraction of a curved surface is used as a lens, a step between the center portion 3a and both side portions 3b and 3c is unavoidable, as shown in FIG. 8(B). However, there are problems such as light scattering in this part and manufacturing difficulties. Therefore, it is preferable that the lens be constituted by a grating lens shown in FIG. 7, which can realize the above-mentioned optical characteristics on the same plane.

Further, in the above example, only the diffracted light in the radial direction is separated and detected, but this can also be extended to two dimensions. Since similar diffracted light is generated in the front and rear directions of the pit, FIG. 9 shows an example in which the same condensing means is provided for the diffracted light in the front and rear directions of the pit as for the diffracted light on both sides. The lens 3 includes a lens portion 3a (first condensing region) and a lens portion 3b (second condensing region) formed by shifting the optical axis in the front-back direction of the pit (direction orthogonal to the radial direction). This is a condensing means that can also handle diffracted light in the front and rear directions. The diffracted light collected by the lens portion 3b (second condensing region) of the lens 3 is detected by the ±1st order diffracted light photodiode 5b, and the difference from the 0th order light is taken to obtain a pit detection signal.

As detailed above, FIGS. 1, 6, 7,
In the optical pickup configured as shown in FIG. 9, reflected light (or transmitted light) and diffracted light from the optical information recording medium 4 (pits 9) are transmitted to the first condensing area 3a and the second condensing area 3b. (
, 3c, 3d) are substantially separated through a condensing means (grating lens, bulk lens) 3, and detected by the first and second light detection means, respectively, and pit information can be read from the difference. It will be done. Therefore, there are fewer restrictions on the shape and arrangement of the light detection means (sensor),
This increases the degree of freedom in design when actually manufacturing an optical pickup.

The optical pickup according to the present invention can also produce similar effects in so-called transmission type optical discs and optical cards in which information is transmitted through the information recording surface.

[0030]

Effects of the Invention As detailed above, the optical pickup according to the present invention is an optical pickup that reads pit information from an optical information recording medium in which information is recorded by pits. Reflected light (or transmitted light)
a first condensing region formed and arranged so that the and diffracted light are substantially separated, and which guides the reflected light (or transmitted light) from the optical information recording medium to the first light detection means; The diffracted light generated by the diffraction effect of the pits of the optical information recording medium is
a second light focusing area that guides the light to the light detection means; and reflected light (or transmitted light) from the optical information recording medium.
a first light detection means for detecting the light, and a second light detection means for detecting the diffracted light generated by the diffraction effect of the pits of the optical information recording medium.
Since the pit information is read from the difference between the respective outputs of the first and second photodetecting means, the reflected light from the optical information recording medium (pit)
or the transmitted light) and the diffracted light are substantially separated via a light focusing means having a first light focusing region and a second light focusing region, and the first
and the second photo-detecting means, and pit information is read from the difference between the two. This makes it possible to efficiently obtain a reproduced signal with a high S/N ratio, and to avoid restrictions on the shape and arrangement of the photo-detecting means. This allows for a greater degree of freedom in design when actually manufacturing an optical pickup.

[Brief explanation of the drawing]

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

FIG. 2 is a diagram showing an output waveform of an optical pickup.

FIG. 3 is a diagram illustrating a condensing means (principle) of an optical pickup.

FIG. 4 is a diagram comparing output waveforms of an optical pickup according to the present invention and a conventional example.

FIG. 5 is a diagram illustrating crosstalk characteristics of an optical pickup.

FIG. 6 is a diagram showing a modification of the optical pickup according to the present invention. FIG. 7 is a diagram showing a modification of the optical pickup according to the present invention.

FIG. 8 is a diagram illustrating a condensing means (lens) of the optical pickup.

FIG. 9 is a diagram showing a modification of the optical pickup according to the present invention.

FIG. 10 is a diagram showing the principle of reading pit information with a conventional optical pickup.

FIG. 11 is a diagram showing the principle of reading pit information with a conventional optical pickup.

[Explanation of symbols]

1 Semiconductor laser, 2 Grating beam splitter, half mirror 3 (Grating) lens (focusing means), 3a 0th-order light focusing grating coupler, 0th-order light lens (first focusing area), 3b, 3
c, 3d ±1st order light condensing grating coupler,
± Lens for first-order light (second condensing area), 4 Optical disk (optical information recording medium), 5a Photodiode for zero-order light (first light detection means), 5b, 5c, 5d ±1
Photodiode for secondary light (second light detection means), 6
Optical waveguide, 7 (semiconductor) substrate, 8 Arithmetic unit, 9
pit.

Claims (1)

[Claims] 1. An optical pickup for reading pit information from an optical information recording medium in which information is recorded by pits, wherein reflected light (or transmitted light) and diffracted light from the optical information recording medium are substantially separated. formed and arranged so as to
Diffraction light generated by the diffraction effect of the pits of the optical information recording medium and a first condensing area that guides the reflected light (or transmitted light) from the optical information recording medium to the first light detection means is converted into second light. a light collecting means having a second light collecting region that guides the light to the detecting means; a first light detecting means for detecting reflected light (or transmitted light) from the optical information recording medium; and a pit of the optical information recording medium. a second light detection means for detecting diffracted light produced by the diffraction effect of the light, and pit information is read from the difference between the respective outputs of the first and second light detection means. pick up.