JPS6232532B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical recording/reproducing device that utilizes output light from a semiconductor laser. Semiconductor lasers have the following characteristics: their output light can be directly modulated by electrical signals without using an optical modulator, and they are small compared to other lasers. It is being used as a light source for various devices, such as a light source for optical communication, and its practical use is also being considered as a light source for playback of optical video discs.

Generally, when a semiconductor laser is used as a light source for an optical recording/reproducing device, light with a certain high output power is required. In other words, when using a silver salt film as an optical recording material, which requires development and fixing processing after light irradiation, a very high output power is not required, but real-time recording materials that do not require development and fixing processing, such as metal When using a thin film or a thin pigment film, it is possible to
A recording energy of 1000 mj/cm ² is required, and the output power of the light source is also required to be several tens of mW or more. In order to achieve a sufficient effect with the above-mentioned recording energy on the recording material, it is necessary to make the transmission efficiency of the light as high as possible in the optical system that narrows the output light of the semiconductor laser to a small diameter. Since the output light of a semiconductor laser essentially has a spread, special consideration is required in this respect. Further, the beam diameter of the light irradiated onto the optical recording material needs to be narrowed down to the smallest possible beam diameter in order to satisfy the above-mentioned recording energy condition and to record and reproduce signals with as high a density as possible.

In general, the cross-sectional shape of the light beam at the light emitting surface of a high-power semiconductor laser is 1 to 2 μm in the thickness direction of the bonded surface (direction perpendicular to the bonded surface) and 1 to 2 μm in the direction parallel to the bonded surface. It has a flat shape of about 10 μm. Also, the light beam emitted from it has a fairly large divergence angle,
For example, it has an expanse of ±15° to 25° in the direction perpendicular to the bonding surface and ±5° to 10° in the direction parallel to the bonding surface, so it needs to be treated differently from a general He--Ne gas laser. One example of a conventional optical system that uses a semiconductor laser having such properties as a light source for reproducing an optical video disk is shown in FIG.

In FIG. 1, reference numeral 1 indicates a semiconductor laser, and the angle indicates the spread angle at a portion where the energy of the semiconductor laser is half its maximum value. 2 is a condensing lens with a short focal length used as an objective lens of a microscope or the like. In this conventional example, an objective lens with a small numerical aperture (NA ₂ ) is used as the condensing lens 2 to collect only the central portion of the light from the semiconductor laser. That is, by reducing the NA ₂ of the condenser lens 2, the condenser lens 2 is also made to function as a slit. In the figure, θ is the spread angle of light that can be collected by the condenser lens 2, and has a relationship of sin θ/2=NA ₂ . In the above example, a focusing lens with NA ₂ = 0.07 is used, so θ/2 = Sin ^-1 (0.07) ≒ 4°, which means that light within a spread of approximately ±4° is focused. .
The semiconductor laser 1 is placed slightly outside the focus of the condenser lens 2, and forms an image of a portion of the light source at a point Q on the optical axis Z. 3 is an aperture lens, similar to condenser lens 2,
It consists of a microscope objective lens, etc. A large numerical aperture (NA ₃ ) of this aperture lens 3 is used in order to improve the performance of the aperture. In this example
An aperture lens with NA=0.65 is used. In this way, the image at point Q is formed at point R as an image with a minute diameter. The information recording surface of the video disc is placed at this point R.

The optical system shown in FIG. 1 attempts to collect and narrow down only the light at the center of the light emitted by the semiconductor laser 1 having the flat light emitting surface using the condenser lens 2. The optical system shown in FIG. The rate at which optical power is transmitted to point R) is extremely low, at a few percent. In addition, in order to increase the transmission efficiency, the condenser lens 2 has a numerical aperture (NA ₂ ).
If an objective lens with a large diameter is used, the aperture diameter of the light at point R cannot be made small, which is a contradictory factor. In addition, the optical system shown in FIG. 1 requires a sufficiently long optical path length in order to fully demonstrate the lens performance of each of the objective lenses 2 and 3, and the aperture diameter of the light becomes small, but the optical path length is large. This has the disadvantage that the entire optical system becomes large.

In an optical recording/reproducing device, it is necessary to maintain a constant distance between the objective lens 3 and the disk surface (recording surface) in order to irradiate the information recording surface of the video disk with a minute light beam focused to a certain size. be.
For this purpose, it is well known that so-called focus control is performed in which the objective lens 3 is made movable in the optical axis direction and moved in accordance with the surface wobbling of the disk. This focus control is performed by receiving the reflected light from the disk surface with a photoelectric conversion element and using the output signal thereof.

In FIG. 1, reflected light from the disk surface passes through an objective lens 3, is separated from incident light by a half mirror 4, and is guided to a photoelectric conversion element 6 via a cylindrical lens 5.

When the distance between the objective lens 3 and the disk surface changes, the shape of the reflected beam on the photoelectric conversion element 6 changes. Information corresponding to this change is included in the output signal of the photoelectric conversion element 6, and after being processed by a known method, this output signal becomes a focus control signal for moving the objective lens 3 in the optical axis direction. It is also known that the output signal of the photoelectric conversion element 6 is used as an information signal or a tracking control signal.

In the optical device as described above, the present invention focuses the light from the flat light source into a nearly circular light beam with a minute diameter, downsizes the device, and improves the efficiency of light transmission from the light source to the recording medium. expansion of the optical element retention mechanism due to temperature fluctuations,
It is intended to be a structure that is optically stable against shrinkage.

An embodiment of the present invention will be described with reference to FIGS. 2 and 3. The optical system shown in Figure 2 is a semiconductor laser 1.
A cross section with respect to light parallel to the joint surface of No. 1 is shown. The semiconductor laser 11 has a flat light emitting surface as described above. 12 is a semiconductor laser drive circuit that modulates the output light of the semiconductor laser, and
Controls the optical output of recording light and reproduction light. A condenser lens 13 converts the emitted light of the semiconductor laser 11 having a spread into a parallel light beam. 14 is a concave cylindrical lens, and 16 is a convex cylindrical lens, which are arranged so as to have maximum curvature in the cross section with respect to light exiting in a direction parallel to the cemented surface. 15 is a polarizing beam splitter provided in the optical path between the cylindrical lenses 14 and 16, and 17 is a λ/4 plate. A tracking mirror 18 changes the optical path and performs tracking control using a known technique. 1
Reference numeral 9 irradiates a light beam onto a recording medium (disk) coated with an optical recording material using an aperture lens. disk 2
0 rotates around a rotation shaft 22 connected to a motor 21 and moves in its radial direction. Reference numeral 23 denotes a known voice coil type structure, which drives the aperture lens 19 in a direction perpendicular to the surface of the disk 20 in response to the surface wobbling of the disk.
9 is maintained constant. A single convex lens 24 converts the reflected light from the disk 20 into semiconductor laser beams l ₁ , l ₂ ,
It performs a lens action on the light l ₅ separated from the optical path of l ₃ and l ₄ . 25 is a PIN with a detection surface divided into four parts.
A photodiode (shown in a perspective view) is used to detect the signal recorded on the disk, the focus error signal, and the tracking control error signal.

Next, the aperture optical system shown in FIG. 2 will be explained with reference to FIG. Figure 3a shows a cross section along the optical axis in the direction (⊥ direction) perpendicular to the junction surface of the semiconductor laser.
Figure b shows a cross section along the optical axis in a direction (direction) parallel to the bonding surface. First, the operation of the semiconductor laser 11 in the direction perpendicular to the junction surface in FIG. 3a will be explained.
The size of the light emitting surface in the direction perpendicular to the bonding surface is 2 μm,
Let the spread angle in that direction be ₁ = 15° at the point where the energy is half the value. When the focal length of the condenser lens 13 is set to f ₁₃ and the light emitting surface of the semiconductor laser 11 is placed at the focal point of the condenser lens 13, the condenser lens 1
As the output light of 3, a parallel light beam having a light beam width φ ₁ shown in FIG. 3a can be obtained. For example, if the numerical aperture (NA ₁₃ ) of the condensing lens 13 is 0.5, this lens condenses all the light having a spread of ₁ = 30 degrees and generates a parallel light beam, so the condensing lens is Almost all of the light in the direction perpendicular to the bonding surface can be transmitted without loss due to vignetting. And the luminous flux width φ ₁ of the parallel luminous flux is approximately 2×
It is given by f ₁₃ ×sin ₁ . This parallel light beam is condensed by an aperture lens 19 placed at a point t on the optical axis Z, and an image with a minute diameter is formed at a point u on the optical axis. Therefore, if the entrance aperture of the aperture lens 19 is made larger than _φ1 , almost all of the light will be transmitted to the point u without any loss due to eclipse caused by the optical element. The focal length of the aperture lens 19 is
If f ₁₉ , the size of the image in the direction perpendicular to the junction surface of the semiconductor laser formed at point u, IL, is given by the following equation (1) from a geometrical perspective.

I⊥=f ₁₉ /f ₁₃ ×2 (μm) (1) Next, the optical system in the direction parallel to the cemented surface shown in FIG. 3b will be explained. The size of the light emitting surface in this direction is 8μ
m, the spread angle of light is ₂ at the point of half energy
=5°. When the light emitting surface of the semiconductor laser 11 is placed at the focal point of the condensing lens 13, parallel light with a beam width _φ2 is obtained. Then, φ ₂ ≒ 2 × f ₁₃ × sin 5 °, and as mentioned above, if a condensing lens with numerical aperture (NA ₂ ) = 0.5 is used, light in this direction will also be eclipsed by the condensing lens without loss. Most of the light can be focused. The diameter φ ₂ is considerably smaller than φ ₁ in FIG. 3a. If this φ ₂ is stopped directly with the diaphragm lens 19 in the same way as shown in Fig. 3a, then u on the optical axis will be
An image of I=f ₁₅ /f ₁₂ ×8 (μm) is formed at the point, and as a result, a strip-shaped light similar to 2 μm × 8 μm is formed at u. Therefore, the concave and convex cylindrical lenses 14 and 16 change the magnification of the light in the direction parallel to the cemented surface, adjust the beam width φ ₃ of the parallel beam entering the aperture lens 19, and form a substantially circular beam at point u. Form a close image. Concave cylindrical lens 1 in Figure 3b
4 (focal length f ₁₄ ) receives a parallel beam of beam width φ ₂ and creates a virtual image thereof at point l. The distance between this point l and the position r of the concave cylindrical lens 14 is = _f14 . Therefore, the concave cylindrical lens 14 generates light that spreads from point l, and the convex cylindrical lens 16 (focal length
f ₁₆ ). The convex cylindrical lens 16 is located at a position where the position on the optical axis Z of this convex cylindrical lens 16 is S, and the distance between l and S is = _f16 .
As shown in FIG. 3b, a parallel beam row with a beam width φ ₃ is obtained. The relationship between φ ₃ and φ ₂ is expressed by the following equation (2).

φ ₃ /φ ₂ =lS/lr=f ₁₆ /f ₁₄ (2) This parallel light beam φ ₃ is focused by the objective lens 19 to generate an image at point u on the optical axis. The size I ₁₁ of the geometric optical image in this direction is expressed by the following equation (3).

I= _f19・_f14 / _f13・_f16 ×8μm...
(3) Therefore, from equations (1) and (3), f ₁₄ /f ₁₆ = 1/4
...(4) If you choose the focal length of each concave and convex cylindrical lens, φ ₁ ≒ φ ₃ , and u in Figure 3
It is possible to obtain a substantially circular or substantially positive image at a point where I⊥≒=I.

In FIG. 3b, by arranging each cylindrical lens so that point l is the focal plane of the concave cylindrical lens 14 and the focal plane of the convex cylindrical lens 16, the luminous flux width φ ₂ This converts parallel light with a beam width of φ3 to parallel light with a beam width of _φ3 . Therefore, if only the mutual distance between the concave and convex cylindrical lenses is kept constant, these lenses can be
Since it has the feature that it can be placed at any position between 9 and 9, the degree of freedom in design is increased in the arrangement of optical components. In addition, in FIG. 3, the distance between the condenser lens 13 and the concave cylindrical lens 14 and the distance between the convex cylindrical lens 16 and the diaphragm lens 19 can be set to arbitrary lengths since the light beam between them is parallel light. Therefore, by shortening the above, the optical system can be made significantly smaller.

In FIG. 3a, the total lateral magnification m _L of the optical system in the direction perpendicular to the junction surface of the semiconductor laser is m _L = f ₁₉ /f
It is given by ₁₃ . On the other hand, the overall vertical magnification m _T is m _T =
It is given by (f ₁₉ /f ₁₃ ) ² . Therefore, the focal length of the condenser lens 13, f ₁₃ and the aperture lens 19
When the focal length f ₁₉ of is made equal, the vertical magnification m _T becomes 1. This means that if the distance between the semiconductor laser 11 and the condensing lens 13 changes by 1 μm due to expansion or contraction due to changes in ambient temperature, the distance between the aperture lens 19 and the aperture beam will change by 1 μm.
This means that in the optical recording/reproducing apparatus shown in FIG. 2, the distance between the aperture lens 19 and the aperture point R varies depending on the temperature.
This means that the diameter of the light beam irradiated onto the disk 20 varies depending on the temperature, and the optical recording characteristics on the disk are significantly deteriorated in terms of recording energy and maximum recording/reproducing frequency. Also, when reproducing signals that have already been recorded on the disk 20, if the diameter of the light beam irradiated onto the disk becomes larger than a predetermined size, the reproduction frequency characteristics may deteriorate or signal crosstalk between tracks may increase. The playback characteristics will deteriorate significantly. Further, variations in the distance between the objective lens 19 and the aperture point R cannot be compensated for by known focus control techniques.

The present invention makes the focal length f _{13 of the condenser lens 13} longer than the focal length f ₁₉ of the aperture lens 19 (f ₁₃ >
_f10 ) to solve the problem of characteristic deterioration due to temperature changes. Now, f ₁₃ = 9mm,
If f ₁₄ = 4.5 mm, the total vertical magnification is m _T = (9/4.5
) ² =4, and the 4 μm variation in between is reduced to a 1 μm variation in between. Therefore, in the apparatus of FIG. 2, the focal length f ₁₃ of the condenser lens 13
Making the focal length f19 longer than the focal length _f19 of the aperture lens 19 is an important requirement for making the optical recording/reproducing device thermally stable.

As described above, the device of the present invention includes a first optical means (condensing lens 13) that condenses the output light of the semiconductor laser 11 having a spread and converts it into parallel light, and a It consists of a concave cylindrical lens 14 and a convex cylindrical lens 16, each having a curved surface in a direction, and arranged so that the distance between the two cylindrical lenses is determined by the difference in their focal lengths, and the width of one of the parallel lights is expanded. and a third optical means (aperture) that focuses the parallel light obtained by the second optical means into a small diameter light beam and irradiates it onto the recording medium (disk 20). In an optical recording/reproducing device comprising a lens 19), the focal length of the first optical means (condensing lens 13) is made longer than the focal length of the third optical means (diaphragm lens 19). This has the excellent effect of providing an optical device with stable characteristics against temperature fluctuations.

[Brief explanation of the drawing]

Fig. 1: A diagram showing a conventional optical device using a semiconductor laser, Fig. 2: A diagram showing an embodiment of the device of the present invention, Fig. 3: A diagram illustrating the operation of the optical system of the present invention.
a is a cross-sectional view taken along the optical axis in a direction perpendicular to the bonding surface of the semiconductor laser, b is a cross-sectional view taken along the optical axis in a parallel direction, Symbols, 11... Semiconductor laser, 12... Drive circuit, 13 ... Condensing lens, 14 ... Concave cylindrical lens, 15 ... Polarizing beam splitter, 1
6...Convex cylindrical lens, 17...λ/4
Plate, 18...Tracking mirror, 19...Aperture lens, 20...Disk (recording medium), 21...
... Disk motor, 22 ... Rotating shaft, 23 ... Voice coil type structure, 24 ... Single convex lens, 25
...Photodiode.

Claims (1)

[Claims] 1. A first optical means that condenses the output light of a semiconductor laser having a spread and converts it into parallel light; a second optical means that converts the output light of the parallel light into parallel light with an expanded width; In an optical system comprising a third optical means that focuses parallel light obtained by the optical means into a micro-diameter light beam and irradiates it onto a recording medium, the focal length of the condensing lens constituting the first optical means An optical recording/reproducing device characterized in that the focal length of the aperture lens constituting the third optical means is longer than the focal length of the aperture lens constituting the third optical means.