JPH10199008A - Multibeam optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make light beam emitting intervals very small and to enable a high speed recording and reproducing in a high density without enlarging the device constitution by arranging emitting beams of laser diodes at minute intervals. SOLUTION: This multibeam optical head for simultaneously recording and reproducing plural bits of information on a recording medium is equipped with plural laser diodes 1 for emitting their source becomes and optical fibers 8 installed in the same number as these plural laser diodes to be used as a light guide path for beams emitted from the laser diodes respectively. Emitting ends of the optical fibers 8 are arranged at equal intervals on a straight line by a processed substrate 9.

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 a laser beam, and more particularly, to a multi-beam optical head for recording and reproducing by forming a plurality of light spots on a recording medium using a plurality of laser beams. About.

SUMMARY OF THE INVENTION According to the present invention, in a multi-beam optical head for simultaneously recording and reproducing a plurality of pieces of information on a recording medium, the emission beams of the individually oscillating laser diodes are shorter than the required beam emission interval. By forming a light source with a small beam emission interval in which optical fibers are guided to optical fibers having a small diameter and the emission ends of the optical fibers are arranged, the interval between the light spots is reduced, and thus information can be placed on the recording medium without gaps. In addition to recording at a density, this can be reproduced.

[0003]

2. Description of the Related Art In recent years, the information processing capability of computers has been dramatically improved, and high-speed processing of a large amount of information such as highly accurate image information has become possible. Accordingly, high-speed processing of large-capacity data is also required in a device that records and reproduces the information on a recording medium.

[0004] Optical recording is characterized by high-density recording, but lags behind other types of recording methods in terms of high-speed data processing. In optical recording, one of the methods for improving the transfer rate and enabling high-speed processing is to perform recording and reproduction with a plurality of beams (multi-beams) at the same time. As an apparatus for realizing high-speed processing by recording and reproducing with a multi-beam, for example, as described in “IEICE Technical Report, MR90-31, 37-44 (1990)”, a plurality of laser diodes are used. An optical disk apparatus using a laser diode array in which are arranged at equal intervals is known.

In this apparatus, a four-beam laser diode array is used, and a beam emission interval is set to 100 μm. If the beam emission interval is set smaller than 100 μm, the output cannot be controlled stably due to heat generated during laser oscillation. Accordingly, the interval between the light spots condensed by the optical head has a size obtained by multiplying the beam emission interval by the magnification of the optical system (about ３ to ５), and is about 20 to 33 μm. In the case of the above-mentioned conventional optical disk device, the light spot interval is 22 μm. When recording on an optical disc at this light spot interval is considered, the track pitch is 22 μm in a light spot array arranged linearly from the center of the optical disc toward the circumferential direction. Cannot be recorded. In other words, to perform high-density recording with multiple beams using a laser diode array,
Since the light spot interval cannot be reduced, the track pitch finally recorded on the recording medium must be reduced by some method.

In order to realize high-density recording, the conventional optical disk apparatus employs a method as shown in FIG. In FIG. 4, 101 is an optical disk, 10
2 denotes a light spot array, and 103 denotes a track guide groove formed on the optical disk 101. As shown in the drawing, the light spot array 102 is composed of four light spots A, B, C, and D formed by a laser diode array. The optical disc 101 is arranged on the optical disc 101 while maintaining a relative positional relationship such that it crosses obliquely. As described above, the track pitch is effectively narrowed by arranging the light spot array 102 obliquely with respect to the track guide groove 103. For example, even if the beam emission interval of the laser diode array is 100 μm, finally 2 μm
A track pitch of the order of magnitude is realized, and high-density recording on the optical disc 101 is enabled.

However, this conventional method is effective only when the number of beams of the laser diode array is relatively small, and is difficult to apply when the number of beams is further increased.

On the other hand, in recent years, for example, “Jpn. J. Appl., Phy
s., vol. 32, 5428-5432 (1993) "
There has been proposed an optical tape recording apparatus which realizes high-density optical recording with a tape-type memory and aims at an ultra-large-capacity memory.

In this apparatus, as shown in FIG. 5, an arc-shaped track 112 is formed on an optical tape 111 by using a rotary optical head, and four optical spots A, B,
Recording and reproduction are performed by a light spot array 113 composed of C and D.

However, in this optical tape recording apparatus, the direction of the light spot train 113 is the same as the light spot train 113 while rotating.
13 cannot be arranged diagonally. For this reason, the light spot interval becomes the track pitch as it is, and in order to perform high-density recording, it is absolutely necessary to make the light spot interval very small to about the track pitch.

In order to realize the minute light spot interval, there is a method of reducing the magnification of the optical system of the optical head. However, in this method, since the numerical aperture on the object point side is reduced, the utilization efficiency of the laser beam is low. , A large laser output power is required. For example, in order to realize a light spot interval of 2 μm using a laser diode array having a beam emission interval of 100 μm, a magnification of 1/50 is required. Assuming that the numerical aperture is 0.5, the numerical aperture on the object point side is 0.01, and the angle of taking in the laser beam is only 0.57 °. Usually, the emission angle of the output beam of the laser diode is about 20 ° to 30 ° at full width at half maximum, so that the utilization efficiency of the laser beam is 1% or less, and an optical head using such an optical system is not practical. .

In addition, the use efficiency of a laser beam can be increased by using a microlens array in the above-mentioned optical system (for example, see Jpn. J. Appl., Phys., Vol. 35, 375-379 (1)
996)).

However, this method has a limitation in increasing the number of beams for improving the recording speed. For example, if the number of beams of the laser diode array is set to 31, the distance between the laser beams at both ends is determined by the beam emission interval × (number of beams−1), which is 30 times the beam emission interval. The beam emission interval of the laser diode array currently being developed is formed at 50 to 100 μm, and when the number of beams is 31, the beam emission interval at both ends is about 1.5 to 3 mm. in this way,
In order to capture laser beams from widely distributed light emitting points, the optical head must use an optical system such as a stepper used for LSI fabrication, and the increase in the number of beams increases the size of the optical recording device. Let me do it.

[0014]

According to the conventional example described above, in the optical disk device, in order to realize high-density recording, the light spot array is arranged obliquely with respect to the track guide groove 103. Since a narrow track pitch is ensured, there is a problem that the number of beams is limited to a relatively small number.

In an optical tape recording apparatus using a rotary optical head, a small light spot interval can be realized by reducing the magnification of the optical system of the optical head. However, there is a problem that the utilization efficiency of a laser beam is poor, a large laser emission power is required, and the laser is not suitable for practical use.

Further, in the method of improving the utilization efficiency of a laser beam by using a microlens in the optical system, there is a problem that the optical recording apparatus is increased in size, and the increase in the number of beams is limited.

As described above, in order to avoid introducing an optical system having an extremely small magnification and increasing the size of an optical recording apparatus, it is sufficient that the beam emission interval of the laser diode array can be reduced to about 10 μm. In the high-power laser diode described above, a technique for making the beam emission interval smaller than the above-mentioned size has not been realized so far.

The present invention has been made in view of the above circumstances, and an object of the present invention is to dispose the emission beams of a laser diode at minute intervals, thereby minimizing the beam emission interval without increasing the size of the device configuration. To provide a multi-beam optical head capable of high-density and high-speed recording and reproduction.

[0019]

First, the concept of the present invention will be described. In the multi-beam optical head of the present invention, an outgoing beam from a plurality of laser diodes is formed by an optical fiber having a diameter smaller than a required beam emission interval. By using the exit end of those optical fibers as a light source,
An object of the present invention is to realize a multi-beam optical head having a light source having a minute beam emission interval, which is difficult to realize with a laser diode array. For this purpose, in the multi-beam optical head according to the present invention, the emission ends of the optical fibers are arranged at equal intervals on a straight line.

Therefore, in order to achieve the above object, the present invention provides a multi-beam optical head for simultaneously recording and reproducing a plurality of pieces of information on a recording medium, comprising: a plurality of laser diodes for emitting light from a light source; An optical fiber, which is provided in the same number as the plurality of laser diodes and is used as an optical waveguide of a beam emitted from each laser diode, is provided.

According to a second aspect of the present invention, in the multi-beam optical head according to the first aspect, the emission ends of the optical fibers are arranged at equal intervals on a straight line.

[0022]

FIG. 1 is a block diagram showing an embodiment of a multi-beam optical head according to the present invention.

As shown in FIG. 1, the multi-beam optical head includes a plurality of laser diodes 1 for emitting light from a light source.
And a coupling optical system 7 including a collimator lens 2, a beam splitter 3, two condenser lenses 4, 5, and a photodiode 6, and a multi-beam light source including an optical fiber 8, a processing substrate 9, and an optical fiber array 10. And an optical head optical system 18 including a collimator lens 12, a beam splitter 13, a quarter-wave plate 14, an objective lens 15, a condenser lens 16, and a photodiode 17. The coupling optical system 7 is provided individually for each optical fiber 8, and receives the laser light emitted from the laser diode 1 and couples the laser light to the incident end of the optical fiber 8. The multi-beam light source 11 guides the light emitted from the coupling optical system 7 to a plurality of optical fibers 8, and arranges the emission ends in a line at equal intervals. The optical head optical system 18 forms a light spot 20 on a recording medium 19 with light emitted from the multi-beam light source 11 to record information or reproduce recorded information.

As described above, the multi-beam optical head of the present invention uses the optical fiber 8 to which the output beams of the laser diode 1 are combined, and uses a laser diode array by a method described below with reference to a specific example. A light source having a minute beam emission interval that cannot be realized is configured so that recording and reproduction can be performed at a minute light spot interval.

In FIG. 1, the output beams of the laser diode 1 individually oscillated by the recording signal are transmitted to a coupling optical system 7 composed of a collimator lens 2, a beam splitter 3, a condenser lens 4, a condenser lens 5, and a photodiode 6. Is coupled to the input end of the optical fiber 8. In the coupling optical system 7, the beam splitter 3, the condenser lens 5, and the photodiode 6 are used for light output control (APC).

As described above, the output end of the optical fiber 8 is
The optical fiber array 10 is arranged at equal intervals on a straight line and fixed by the processing substrate 9.

FIG. 2 shows an emission end of the optical fiber array 10 used as the multi-beam light source 11. As shown in FIG. 2, for example, a silicon substrate on which a V-groove has been processed by a photolithography method or the like is used as a processing substrate 9 which has been processed in advance, and optical fibers 8 are aligned at regular intervals on a straight line using this as a guide plate. It is arranged and used as the multi-beam light source 11. The optical fiber 8 includes a core 8a and a clad 8b. Between each optical fiber 8,
Barrier 8 for preventing crosstalk between optical fibers 8
c is provided. The barrier 8c is unnecessary when it is not necessary to consider the occurrence of crosstalk between the optical fibers 8.

FIG. 3 shows the embodiment of the present invention in more detail by using an example in which four optical fibers 8 are used. The optical fiber array 10 has four optical fibers 8 arranged at equal intervals on a straight line. The optical fiber array 10 is placed at the front focal length of the collimating lens 12 having a focal length f1 (here, 48 mm), and the focal length f
2 (here, 4 mm) objective lens 15 is a recording medium 1
It is installed so as to collect light at 9. In this configuration, the light spot interval on the recording medium 19 is
F2 / f1 times the optical fiber pitch (here, 1/1
2 times). By using such an optical system, it is possible to adjust the light spot interval to a target track pitch by selecting the focal lengths f1 and f2 of the two lenses 12 and 15.

Here, the following four requirements are listed as main items required for a multi-beam optical head using an optical fiber. The first requirement is that the wavefront aberration of the beam emitted from the optical fiber is small and can be focused to the diffraction limit. The second requirement is that the light spot interval can be set to 2 μm or less, and the beam use efficiency can be kept high. A third requirement is to be able to introduce a control system for keeping the beam intensity constant. The fourth requirement is that a countermeasure against the influence of return light can be taken.

Next, it will be examined whether the multi-beam optical head in this embodiment satisfies the above four requirements.

<< Regarding the First Requirement >> If the wavefront of the beam incident on the optical fiber is not preserved, it is not possible to form an image of a minute light spot. Mode optical fiber must be used. In a single mode optical fiber, the wavefront of the incident beam is preserved. That is, the requirement is satisfied by using a single mode optical fiber as the type of optical fiber to be used. << Regarding the second requirement >> Although there is a problem of the beam emission interval and the emission angle at the optical fiber emission end, the beam emission interval is the arrangement pitch (optical fiber array pitch) when the optical fibers are arranged. The optical fiber array pitch depends on the size of the optical fiber. As mentioned above, the optical fiber is composed of the core and the cladding surrounding it,
The diameter of the optical fiber is determined by the size of both.
First, consider the core diameter of the optical fiber.

The distribution of the electromagnetic field allowed in the optical fiber is:
This is called the mode of the optical fiber. When the number of modes is 1, the optical fiber is a single mode optical fiber. The condition that the number of modes is 1 is that the V number is 2.4
It is when it is smaller than 05. The V number is represented by the following equation.

V = K ₀ × a × NA where K ₀ is the wave number in vacuum and is represented by 2π / λ, λ is the wavelength of the laser diode used, a is the core radius, and N is
A is the numerical aperture of the optical fiber.

Wavelength λ = 780 nm, numerical aperture NA = 0.1
Then, when the core radius a is about 3 μm or less, a single mode is established. Therefore, if the core diameter is smaller than about 6 μm, a single mode is obtained.

Next, the size of the clad required for the optical fiber will be considered. The beam propagating in the optical fiber is
It propagates while leaking not only to the core but also to the cladding.

Therefore, the size of the clad may be determined by considering how far the propagating beam leaks from the core to the clad.

In consideration of this, when calculating the minimum required diameter W of the optical fiber, the beam pattern in the single mode optical fiber is approximated by a Gaussian distribution, and the diameter W
Is approximated by the following equation.

[0038]

## EQU1 ## W = a (0.65 + 1.619V ^-1.5 +2.8)
79V ^-6 ) Therefore, when V = 2.405, the diameter W is about 10% larger than the core diameter. That is, the diameter of the optical fiber needs to be at least 10% larger than the core diameter.

From the above consideration, the diameter of the optical fiber is set to 10
Even if it is about μm, it is 67% larger than the core diameter of about 6 μm, and it is understood that there is no problem. Therefore, it is possible to physically process the diameter of the optical fiber to a diameter of 10 μm by, for example, an etching method, and to arrange these end faces in a straight line at a pitch of 10 μm.

On the other hand, the outgoing angle at the outgoing end of the optical fiber will be considered. As shown in FIG. 3, it is now assumed that the track pitch Tp of the recording medium 19 is 2 μm. Optical fiber 8
Is 10 μm, if the beam diameter after passing through the collimating lens 12 with the focal length f1 is 4 mm, the focal length f2 of the objective lens 15 is 4 mm, and the wavelength λ of the laser diode 1 used is 780 nm, If the magnification M (= f2 / f1) of the optical head optical system 18 is １ ／, Tp = 2 μm. Therefore, the focal length f1
Is 20 mm. The emission angle rad of the beam emitted from the optical fiber 8 having the core diameter of 2a is rad = λ / (π × 2
a) Therefore, at the aperture of the collimating lens 12, the beam diameter becomes f1 × rad, and in order to obtain a beam diameter of 4 mm, the core diameter is about 2.5 μm. If the core diameter is large, the emission angle becomes small and the focal length f1 can be made large, so that the magnification of the optical system of the optical head becomes small,
A narrow track pitch of 2 μm or less can be realized.

In the embodiment shown in FIG. 3, the core diameter is 6
The case of μm is shown. The collimating lens 12 and the objective lens 15 use lenses having an effective aperture of 4 mm. If the focal length f1 of the collimating lens 12 is 48 mm, the beam diameter after passing through the collimating lens 12 is about 4 mm. At this time, the magnification M of the optical head optical system 18
Is 1/12, and if the fiber pitch Fp is 24 μm, the light spot interval is 2 μm. If the fiber pitch Fp is 10 μm, the light spot interval is 0.8 μm.
It can be 3 μm.

As described above, when the wavelength of the laser diode 1 is 780 nm, the core diameter can be selected from a range of 2.5 μm to 6 μm. It is possible to meet the requirements. Here, the wavelength of the laser diode 1 is 78
Although the case of 0 nm was calculated, a similar design is possible at other wavelengths.

<< Third Requirement >> The light output control (APC) of the third requirement is, as shown in the configuration example of FIG. 1, provided in the coupling optical system 7 in the beam splitter 3, the condenser lens 5,
This is possible by introducing a control optical system including the photodiode 6. Alternatively, an aspheric lens is used instead of the collimating lens 2 and the condensing lens 4,
A method is also possible in which a beam splitter is inserted between the optical fiber 8 and control light. Alternatively, after transmission through the optical fiber array 10, an optical system for control may be arranged and the laser diode 1 may be driven and controlled in a time-division manner.

<< Fourth Requirement >> The fourth requirement, that is, countermeasures against noise caused by return light, can be addressed by driving the laser diode 1 which is used in the optical disk device and has been subjected to high frequency superposition. Alternatively, a self-excited oscillation laser, an isolator, a polarizing beam splitter (PBS) + ／ wavelength plate, or the like may be used.

As described above, this embodiment using an optical fiber is
It has been clarified that the above four conditions required by a multi-beam optical head using an optical fiber are satisfied.

The beam emitted from the optical fiber 8 forms a circular beam pattern. Since the beam intensity distribution pattern also has a Gaussian distribution, the optical head using the optical fiber 8 does not need to use an optical element such as an anamorphic prism for making the beam pattern circular.

As described above, according to the present invention, the light emitted from the laser diode can be arranged at the beam emission intervals which are difficult to realize with the laser diode array, and the number of beams can be greatly increased. Further, since the light spot interval can be used as a track pitch as it is, high-density and high-speed recording can be performed in an optical disk device or an optical tape recording device.

[0048]

As described above, according to the first and second aspects of the present invention, by arranging the emission beams of the laser diode at minute intervals, the beam emission interval can be reduced without increasing the size of the device configuration. This enables high-density and high-speed recording and reproduction.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a multi-beam optical head according to the present invention.

FIG. 2 is a configuration diagram showing a multi-beam light source of the multi-beam optical head according to the present invention.

FIG. 3 is an explanatory diagram showing the operation of the multi-beam optical head according to the present invention.

FIG. 4 is an explanatory diagram showing an arrangement of light spots in a conventional optical disk device.

FIG. 5 is an explanatory diagram showing an arrangement of light spots in a conventional optical tape recording device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Laser diode 2,12 Collimating lens 3,13 Beam splitter 4,5,16 Condensing lens 6,17 Photodiode 7 Coupling optical system 8 Optical fiber 9 Processing board 10 Optical fiber array 11 Multi-beam light source 14 1/4 wavelength plate 15 Objective lens 18 Optical head optical system 19 Recording medium 20 Light spot

Claims (2)

[Claims] In a multi-beam optical head for simultaneously recording and reproducing a plurality of information on a recording medium, a plurality of laser diodes for emitting light from a light source, and the same number of the plurality of laser diodes are provided. An optical fiber used as an optical waveguide of an output beam. A multi-beam optical head, comprising: 2. The multi-beam optical head according to claim 1, wherein the output ends of the optical fibers are arranged at equal intervals on a straight line.