JP3203121B2 - Optical head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical head using an optical element for converting a beam into a beam which can reduce a beam diameter, in particular, a focused spot.

[0002]

2. Description of the Related Art In recent years, it has been desired that an optical information processing apparatus or the like can record, reproduce, or erase a large amount of information. For example, in a read-only optical disk device,
There is a demand for higher density information.

In an optical information processing apparatus such as an optical disk apparatus, a laser beam from a semiconductor laser as a light source is focused on an optical recording medium through a focusing lens to record, reproduce, or erase information. ing.

In such an apparatus, in order to increase the density of information, it is necessary to form a condensing spot as small as possible on an optical recording medium.

The light spot formed on the optical recording medium has a uniform numerical aperture of the converging lens, a wavelength of the beam of λ, and a uniform light intensity distribution in the direction perpendicular to the beam traveling direction (radial direction). Assuming a certain beam, the 1 / e ² width of the focused spot is given by 0.96λ / NA.

However, as shown in FIG. 9, the laser beam output from the semiconductor laser 101 has a light intensity distribution (radial direction) similar to a Gaussian distribution, and is condensed on the optical recording medium 103 by the condensing lens 102. There is a problem that the 1 / e ² width of the converged spot becomes larger than 0.96λ / NA.

However, the size of the converging spot on the optical recording medium 103 and the distance L (mm) between the semiconductor laser 101 and the converging lens 102 have a relationship as shown in FIG. Here, the aperture diameter of the condenser lens 102 is 3
mmφ, and the solid line shows the case of Gaussian light intensity distribution (radial direction).

As can be seen from FIG. 1, in order to solve the above-mentioned problem, a conventional semiconductor laser 101 and a focusing lens 1 are conventionally used.
02 is made sufficiently large, for example, in a compact disk device, this distance L is set to about 20 mm so that only the light at the center of the laser beam is transmitted through the condenser lens. This method, rather than using the entire laser beam, the beam transmitted through the condensing lens 102 the light intensity distribution consists of only a relatively homogeneous parts, the 1 / e ²
The width can be reduced to some extent.

[0009]

However, even with the method using only the light at the center of the laser beam as described above, a spot whose 1 / e ² width is substantially equal to 0.96λ / NA cannot be obtained, and In addition, there is a problem that the distance L between the light source and the condensing lens becomes large, so that the optical system becomes large. The same problem occurs when a light source other than a semiconductor laser is used as the light source, because the light output from the light source has a nonuniform light intensity distribution in the radial direction.

The present invention has been made in view of the above problems, and has as its object to provide an optical head using an optical element that can reduce the size of an optical system and can form a small converging spot. And

[0011]

[0012]

[0013]

[0014]

[0015]

[0016]

[0017]

[0018]

[0019]

[Means for Solving the Problems] The optical head of the present invention
A light source and a beam emitted from the light source at the center of the beam.
Eliminating nearby light increases beam intensity near the center.
An optical element for converting the beam into a constant main beam,
Irradiate the recording medium with the main beam converted by the optical element
An optical head, wherein the optical element
The light near the center is directed in a direction different from the main beam in the optical axis direction.
Diffracted as collimated light or convergent light as a split beam
Generate the main beam of zero order diffraction by removing
Consisting of a hologram element, diffracted by the hologram element
The intensity of the split beam, which is
It is characterized by monitoring.

[0020]

In particular, the hologram element is a transmission hologram element.

In particular, the hologram element is a reflection type hologram element.

[0023]

[0024]

In particular, a beam output from the light source is first incident on the optical element.

[0026]

[0027]

The optical head according to the present invention converts an incident beam into a beam having a constant beam intensity near the center. Therefore, even when the distance between the light source and the condenser lens is small, 1 / e ²
A small focused spot with a width approximately equal to 0.96 / NA is obtained
Therefore, it is possible to increase the density and reduce the size of information .

The optical element is a parallel beam as a split beam.
Or, because it consists of a hologram element that diffracts convergent light,
The output beam of the light source can be controlled by efficiently monitoring the beam intensity of the row light or the convergent light . When such a configuration is used in an optical head, a beam on the recording medium can be detected. Therefore, when a semiconductor laser is used as a light source, another end face different from the end face for outputting a beam for irradiating the recording medium is used. It is possible to control the beam irradiated on the recording medium with higher precision than by monitoring the beam output from the recording medium. Furthermore, when a semiconductor laser is used as a light source, a laser that outputs a beam only from one end face capable of high output can be used.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment according to the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic structural view showing an optical head of the present embodiment, and FIG. 2 is a schematic top view of a hologram surface of a hologram element used in the optical head.

In FIG. 1, reference numeral 1 denotes an optical recording medium such as a compact disk, and 2 denotes a semiconductor laser (light source) for outputting a laser beam.

Reference numeral 3 denotes a transmission type hologram element which splits light having a predetermined light intensity or more in a direction oblique to the optical axis of the incident beam and not to enter another optical element (diffraction in the first or -1 order). (Optical element), wherein the laser beam from the semiconductor laser 2 incident on the element 3 is converted into a beam having a constant beam intensity near the center by branching light having a certain light intensity or higher. Then, the light passes through the element 3 (0-order diffraction). FIG. 2 is a schematic top view of the hologram surface 4 of the hologram element 3, and the shape of the upper surface of the grating that forms the hologram surface has a smooth curve.

Reference numeral 5 denotes a condensing beam for converging a beam emitted from the hologram element 3 near the center and having a constant beam intensity on the optical recording medium 1 to form a condensed spot on the medium 1. Lens.

In the hologram element 3, the branched light is a parallel light, but may be a convergent light . The method of designing the hologram element 3 used in this embodiment will be described in detail below.

First, as shown in FIG.
The duty ratio r (X, Y), which means the ratio of the width of the unevenness of the lattice constituting the matrix, is obtained as follows. In the figure,
An alternate long and short dash line indicates the center of the lattice line (convex portion), and Λ indicates the width of the lattice line (convex portion).

As shown in FIG. 4, when the light intensity distribution u (X, Y) in a plane perpendicular to the optical axis of the beam (xy plane in the figure) gradually decreases from the center to the periphery (for example, , An output beam from a semiconductor laser). The vertical axis in FIG. 4 is standardized.

The diffraction efficiency of the zero-order diffracted light is generally given by the following equation (1).

Η₀(X, Y) = 1-ar (1-r),
a: constant (1) Therefore, a plane perpendicular to the optical axis at the center of the optical axis of the beam
The light intensity distribution within ₀(Constant), η₀= U₀
/ U, the duty ratio r is given by the following equation:
(2) required.

Ar ² −ar + 1−u ₀ / u = 0 (2) Note that the constant a is a duty ratio r at u (0, 0) = 1 that is an arbitrary value of 0 <r <1. What is necessary is just to obtain by setting. However, when r (0, 0) is set to a value close to 0.5, the difference between the duty ratios r (X, Y) of the central portion and the peripheral portion of the hologram surface is greatly different, and the production error of the hologram surface is reduced. It is desirable because it becomes smaller. For example, r (0,0)
= Time of 0.5, the constant a is 4 (1-u _0).

Next, as shown in FIG. 5, the hologram surface of the hologram element 3 is located at a distance Z ₀ from the light source 2, and light (branched light) having a certain intensity or more of the incident beam is applied to the hologram surface (in the figure). , X-axis direction) and the angle θ, the shape of the center of each grating line on the hologram surface can be obtained from the following equation (3).

[0040]

(Equation 1)

Where λ is the wavelength of the beam and m (m = 0, ±
..) Is the grid line number, f is the distance from the origin (0, 0, 0) to the convergence point or virtual divergent light source, θ is the parallel light or the convergent light is the hologram surface (FIG. Medium, x
S is convergence light and s = 1
You .

The width of the grating line on the hologram surface can be obtained from the following equation (4).

[0043]

(Equation 2)

When the split light becomes parallel light (f: infinity), the shape of each grid line on the hologram surface and the width of the grid line can be obtained from the following equations (5) and (6), respectively. These expressions are special cases of the general formulas (3) and (4).

[0045]

(Equation 3)

The height difference (depth) of the irregularities of the grating constituting the hologram surface is constant, for example, η ₀ (0,0) =
may be set in such a way that u _0.

FIG. 6 shows the light intensity distribution of the beam at each position in the optical head. Here, as an example, a case where the branched light is parallel light is shown.

FIG. 6A shows a light intensity distribution at a position A between the semiconductor laser 2 and the hologram element 3. From this figure, it can be seen that the light intensity distribution of the laser beam output from the semiconductor laser 2 is a Gaussian distribution approximation.

FIG. 6B shows the light intensity distribution of the light at the position B. From this figure, it can be seen that the light split by the hologram element 3 is light having a certain intensity or higher of the laser beam output from the semiconductor laser 2.

FIG. 6C shows a light intensity distribution at a position C between the hologram element 3 and the condenser lens 5. From this figure, it can be seen that the beam intensity near the center of the beam before entering the condenser lens 5 is constant (uniform).

As shown in FIG. 6, in this embodiment, the laser beam output from the semiconductor laser 2 and having a light intensity distribution approximate to a Gaussian distribution has a constant beam intensity near the center after passing through the hologram element 3. Therefore, the condensed spot formed on the optical recording medium 1 by being condensed by the condensing lens 5 is 1 independent of the distance.
/ E ² width is a small spot diameter close to 0.96λ / NA.

In the above optical head, λ = 635n
m, Z ₀ = 5 mm (semiconductor laser 2 and hologram element 3
Is 5 mm), the output laser beam of the semiconductor laser 2 has a Gaussian distribution (half-value angle: 34 ° × 12 °), and u ₀ = 0.
In the case of 5, the focused spot can be set to a value substantially equal to the analysis limit theory of 1.02 μm while the distance between the semiconductor laser 2 and the focusing lens 5 is set small.

A second embodiment according to the present invention will be described in detail with reference to the drawings. FIG. 7 is a schematic structural view showing the optical head of this embodiment.

In FIG. 7, 11 is an optical recording medium such as a compact disk, and 12 is a semiconductor laser (light source) for outputting a laser beam.

Reference numeral 13 denotes a reflection hologram element (optical element) for splitting light having a certain light intensity or higher. The laser beam from the semiconductor laser 2 incident on the element 13 splits light having a certain light intensity or higher. As a result, the beam is converted into a beam having a constant beam intensity near the center and reflected by the element 13.

Reference numeral 15 denotes a beam reflected by the hologram element 13 and having a constant beam intensity near the center near the optical recording medium 1.
1 is a condensing lens for forming a condensed spot on the medium 11 by condensing the light on the medium 1.

Also in the apparatus of this embodiment, the laser beam output from the semiconductor laser 12 and having a Gaussian distribution approximate light intensity distribution becomes a beam having a constant beam intensity near the center after reflection by the hologram element 13. The condensed spot formed on the optical recording medium 1 by being condensed by the condensing lens 15 has a 1 / e ² width of 0.
A small spot diameter substantially equal to 96λ / NA can be obtained.

In the above description, the light detecting section for reading information from the optical recording medium is omitted for the sake of simplicity. However, an optical head for performing tracking servo using a three-beam method is used in the third embodiment described below. Will be described in detail. FIG. 8 is a schematic configuration diagram of an optical head that performs tracking servo using the three-beam method of the present embodiment. In FIG. 8, electrodes, bonding wires, and the like are not shown.

In FIG. 8, reference numeral 21 denotes an optical recording medium composed of an optical disk such as a compact disk. 22 is an n ⁺ type S
A good heat conductive base made of a conductive semiconductor material such as i (silicon) or a conductive metal such as copper, the base having an upper plane portion 22a and an angle of 45 degrees with respect to the upper plane portion 22a. It has an inclined portion 22b.

Reference numeral 23 denotes n as a conductive heat sink for mounting a semiconductor laser (laser diode), which is die-bonded on the upper plane portion 22a and is electrically connected to the base 22.
^{It is a +} type Si semiconductor substrate.

Reference numeral 24 denotes a semiconductor laser provided with another electrode (not shown) on the upper surface which is die-bonded to an electrode (not shown) on one end surface of the semiconductor substrate 23 and is electrically connected to the substrate 23. The front end face is arranged so as to face the inclined portion 22b. The element 24 generates a laser beam in an active region (not shown) extending in parallel with the surface portion, and outputs a laser beam for detecting an optical recording medium signal from the front end face side of the element 24.

Reference numeral 25 denotes a signal detecting light detecting element (such as a PIN type photodiode) for detecting a return beam (reflected light) returned from the optical recording medium 21 and performing tracking servo, focusing servo and reproduction. Signal detecting light detecting portion), and the upper flat portion 2 of the base 22
The electrode side (not shown) is die-bonded on 2 a and is electrically connected to the substrate 22.

Reference numeral 26 denotes a transmission type hologram element (optical element) arranged in front of the front end face of the semiconductor laser 24 for branching light having a certain light intensity or higher. The laser beam is converted into a beam having a constant beam intensity near the center and transmitted through the element 26 by splitting light having a predetermined light intensity or higher.

Reference numeral 27 denotes a monitoring light detecting element for detecting the light branched by the transmission type hologram element 26 and monitoring the light intensity of the output beam from the semiconductor laser.

Reference numeral 28 denotes a reflection type hologram element (reflection type three-division diffraction grating) fixed to the inclined portion 22b. Next, the beam is split into ± 1st-order diffracted beams (hereinafter, the 0th-order beam is referred to as a main beam, the + 1st-order beam is referred to as a sub-beam X, and the −1st-order beam is referred to as a sub-beam Y) and reflected upward.

Reference numeral 29 denotes a transmission type (light transmission type) hologram element provided above the reflection type hologram element 28. The element 29 transmits (zero-order diffraction) the 0th-order and ± 1st-order beams (main beam and sub-beams X and Y) and reflects these 0th-order and ± 1st-order beams returned from the optical recording medium 21.
The next return beam (main beam and sub-beams X and Y) is diffracted in the first order (or −1 order) to reflect the reflection type hologram element 2.
The light is converged (condensed) on the light detecting portion of the light detecting element 25 located on the side of the light detecting element 8.

In the transmission type hologram element 29, the return beam diffracted by +1 order (or −1 order) by the element 29 converts its optical axis obliquely with respect to the optical axis of the incident beam. Light is condensed (astigmatism) so that the focal length is different between one direction orthogonal to the beam traveling direction and a direction orthogonal to the one direction. That is, the transmission hologram element 29 has the functions of a beam splitter, a converging lens, and a cylindrical lens.

Numeral 30 is provided above the transmission type hologram element 29 and transmits through the hologram element 29 (zero-order diffraction).
The 0-order and ± 1st-order beams (main beam and sub-beams X and Y) converge on the surface of the optical recording medium 21, and the main beam spot and the sub-beam spots X and Y on both sides of the main beam spot, respectively. This is an objective lens as a converging optical system for forming a beam spot Y. Here, the optical system of the optical head is arranged so that the main spot scans a track to be reproduced, and the sub-spots X and Y scan both sides of the main spot slightly over the track.

The photodetecting element 25 comprises a photodetector divided into four parts at the center for performing focusing servo using the astigmatism method, and a tracking servo using the three-beam method provided on both sides of the photodetector. The main beam diffracted by the hologram element 29 in the first (or −1) order is incident on the center of the four-divided light detecting unit. Has a well-known configuration in which the first-order (or -1st-order) diffracted sub-beams X and Y are similarly incident. The detection signals of these detection units are sent to an arithmetic circuit (not shown), and a focus error (FE) signal, a tracking error (TE) signal, and a reproduction signal are calculated.

The reproduction, tracking servo, focusing servo, and the like in such an optical head are performed as follows.

The laser beam output from the front end face side of the semiconductor laser 24 and having a certain intensity or higher is split by the transmission type hologram element 26, and this light is received by the monitor light detection element 27, An output beam from the semiconductor laser is controlled to be constant by an APC circuit (not shown) based on the corresponding signal. On the other hand, the beam output from the front end face side and transmitted through the transmission type hologram element 26 is split by the reflection type hologram element 28 into 0th and ± 1st order diffracted beams (main beam and sub beams X and Y) and at right angles. Reflected upward. The main beam and the sub-beams X and Y reflected upward are incident from one of the transmission hologram elements 29. After that, the main beam and the sub-beams X and Y diffracted (transmitted) by the element 29 in the 0th order are converged (condensed) on the optical recording medium 21 by the objective lens 30 as the main beam spot and the sub-beam spots X and Y described above. ) Is done. Return beams (main beam and sub-beams X and Y) containing information signals from these main spots and sub-spots X and Y pass through the objective lens 30 and then pass through the primary (or -1st-order) by the transmission hologram element 29. ), The main beam is incident on the four-divided photodetector of the photodetector 5, and the sub-beams X and Y are respectively incident on the tracking detector. Then, the signal obtained by the photodetector 25 is subjected to arithmetic processing by the arithmetic circuit (not shown) to obtain a reproduced signal, an FE signal, and a TE signal. Based on the FE signal and the TE signal, the objective lens 30 is driven by a driving mechanism (not shown) to perform tracking servo and focusing servo.

In such an optical head, as in the first and second embodiments, the laser beam output from the semiconductor laser 24 and having a light intensity distribution approximate to a Gaussian distribution is transmitted through the hologram element 26, and then transmitted near the center. Since the beam is converted into a beam having a constant intensity, the beam is focused by the focusing lens 30 and the focused spot formed on the optical recording medium 21 has a 1 / e ² width independent of the distance. This results in a small spot diameter substantially equal to 0.96λ / NA. In addition, the reflection type hologram element 28 separates the semiconductor laser 24 from the optical recording medium 21.
The optical path up to is bent. Therefore, the size and thickness can be further reduced.

In such an apparatus, the hologram element 26
Since the intensity of the beam from the semiconductor laser 24 is monitored using the light branched by the above, there is also an effect that the semiconductor laser 24 outputs only from one end face, that is, a semiconductor laser capable of high output can be used. However, when the split light is not monitored, a semiconductor laser output from both end faces may be used to monitor a beam output from the rear end face.

When the light diffracted by the hologram element 26 is not monitored, a light absorbing means for absorbing the light and a shielding means for shielding the light are provided so that the light does not become stray light. Is desirable.

As described above, the beam output from the semiconductor laser 24 is first transmitted through the hologram element 26, so that the beam intensity distribution can be more accurately uniformed and the focused spot can be minimized. .

Further, since the semiconductor laser 24 and the signal detecting light detecting element 25 are provided on the same upper plane portion 22a, wire bonding to these elements is easy.

In the above description, a hologram element for converting an incident beam into a beam near the center and having a constant beam intensity is described as one that diffracts in one direction. However, the hologram element is divided so as to diffract in a plurality of directions. A hologram surface may be used.

[0078]

The optical element for converting the beam near the center into a beam having a constant beam intensity may be provided between the light source and the condensing lens. As described above, the beam output from the light source is first applied to the optical element. Is preferable because the beam intensity distribution can be more accurately uniformed and the focused spot can be minimized.

[0080] The optical element of the present invention is not dependent on the course distance be arranged in various optical heads, 1 / e ² width 0.
A condensed spot having a small spot diameter substantially equal to 96λ / NA is obtained.

That is, a light source, a converging lens for converging (condensing) a beam output from the light source on an optical recording medium, and a beam of a beam emitted from the light source between the optical recording medium and the light source And an optical element for converting the beam into a beam whose diameter (focus spot) can be reduced. An optical element that converts the beam into a beam with a constant beam intensity may be used.

In the above description, only the portion where the beam intensity is constant enters the condenser lens and the 1 / e ² width is 0.96.
A small focused spot approximately equal to λ / NA was obtained. However, although the effect is reduced, some other non-uniform portion may enter the condenser lens.

[0083]

[0084]

According to the optical head of the present invention, the incident beam is
Since the beam is converted into a beam having a constant beam intensity near the center, even when the distance between the light source and the condensing lens is small, the beam intensity is 1
A small light-converged spot having an / e ² width substantially equal to 0.96λ / NA can be obtained, and high-density and small-sized information can be obtained.

Further , the optical element serves as a parallel beam as a branched beam.
Or, because it consists of a hologram element that diffracts convergent light,
The output beam of the light source can be controlled by efficiently monitoring the beam intensity of the row light or the convergent light . When such a configuration is used in an optical head, a beam on the recording medium can be detected. Therefore, when a semiconductor laser is used as a light source, another end face different from the end face for outputting a beam for irradiating the recording medium is used. It is possible to control the beam irradiated on the recording medium with higher precision than by monitoring the beam output from the recording medium. Furthermore, when a semiconductor laser is used as a light source, a laser that outputs a beam only from one end face capable of high output can be used.

[Brief description of the drawings]

FIG. 1 is a schematic structural view showing an optical head according to a first embodiment of the present invention.

FIG. 2 is a schematic top view of a hologram surface of a hologram element used in the embodiment.

FIG. 3 is a schematic sectional view of the hologram element.

FIG. 4 is a diagram showing a light intensity distribution in a radial direction of a beam output from a light source.

FIG. 5 is a schematic diagram showing an optical path of the hologram element.

FIG. 6 is a diagram showing a light intensity distribution in a radial direction of a beam in the optical head.

FIG. 7 is a schematic structural view showing an optical head of a second embodiment according to the present invention.

FIG. 8 is a schematic structural view showing an optical head of a third embodiment according to the present invention.

FIG. 9 is a schematic structural view showing a conventional optical head.

FIG. 10 is a diagram showing a relationship between a focused spot size and a distance L.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical recording medium 3 Hologram element (optical element) 5 Condensing lens 11 Optical recording medium 13 Hologram element (optical element) 15 Condensing lens 21 Optical recording medium 26 Hologram element (optical element) 27 Monitoring light detecting element 30 Condensing lens

Continuation of front page (72) Inventor Takao Yamaguchi 2-5-5 Keihanhondori, Moriguchi-shi, Osaka Sanyo Electric Co., Ltd. (56) References JP-A-3-3128 (JP, A) JP-A Sho 62- 18502 (JP, A) JP-A-63-223703 (JP, A) JP-A-62-147401 (JP, A) JP-A-62-157001 (JP, A) JP-A-63-222340 (JP, A) JP-A-62-236152 (JP, A) JP-A-7-141681 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G11B ^7/ 12-7/22 G02B 5/18 , 5 / 30,5 / 32

Claims (5)

(57) [Claims] 1. A light source and a beam emitted from the light source.
Near the center of the beam by removing light near the center
Optical element that converts the beam into a main beam with constant beam intensity
And the main beam converted by the optical element is recorded.
An optical head for irradiating a recording medium, wherein the optical element
Indicates that the light near the center of the incident beam is the optical axis of the main beam.
Diffracted as parallel or convergent light in different directions
The main order of the zero-order diffraction is eliminated by removing as a branched beam.
A hologram element for generating a beam;
The parallel light or the convergent light diffracted by the
Optical head characterized by monitoring the intensity of the branch beam
Good. 2. The hologram element according to claim 1, wherein the hologram element is a transmission hologram.
2. The optical head according to claim 1, wherein the optical head is an element.
De. 3. The hologram element is a reflection hologram.
2. The optical head according to claim 1, wherein the optical head is an element.
De. 4. Monitoring the light intensity of the split beam.
The optical head according to claim 1, 2 or 3, wherein
De. 5. The beam output from the light source is first
The light incident on the optical element,
The optical head according to 2, 3, or 4.