JP2000228559A - Optical device - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To effectively suppress noise caused by return light to a self-excited resonance type laser element and improve the quality of a reproduced signal. SOLUTION: A self-excited oscillation frequency fsp of a self-excited resonance type laser device determined under the influence of return light satisfies the following expression (1). (M−0.25) · (c / 2Lex) ≦ fsp ≦ m ·
(C / 2Lex) (1) (where, Lex is the external resonator length, m is 1 or an integer of 1 or more, and c is the speed of light.)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device incorporating a self-excited oscillation laser element as a light source.

[0002]

2. Description of the Related Art In an information medium such as an optical disk, a semiconductor laser element is incorporated in an optical system such as an optical pickup as a light source for reproducing and writing recorded information.

In such an optical information device, a problem called return optical noise has been pointed out. That is, this return light noise is a phenomenon that occurs when light emitted from a semiconductor laser element is reflected by an optical disk and then a part of the reflected light returns to the original semiconductor laser element itself. is there.

[0004] When this return light noise occurs, laser oscillation becomes unstable. To reduce this, some measures have been taken mainly from the viewpoint of suppressing return light.

On the other hand, according to the technique disclosed in Japanese Patent No. 2670046, the self-excited oscillation frequency fo derived from the material and structure of the semiconductor laser element is made to coincide with the reciprocal fext of the reciprocating cycle of the external resonator. For the first time, the return optical noise can be suppressed below the allowable value. That is, the modulation of the self-excited oscillation and the modulation by the external resonator produce a synergistic effect, and the degree of modulation of the self-excited oscillation becomes large. As a result,
It is said that noise can be reduced.

[0006]

However, the effect of the prior art is still insufficient. This is because the self-excited oscillation frequency fsp varies from the self-excited oscillation frequency fo without an external resonator due to the properties of the external resonator, particularly, the reflectance Rex of the external reflector and the external resonator length Lex. Even if the fo of the excited oscillation laser alone is made to coincide with the ext, the effect described in the above publication (ie,
Due to the synergistic effect of the modulation of the self-excited oscillation and the modulation by the external resonator, the modulation degree of the self-excited oscillation becomes large and the effect of reducing the noise cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made to improve the above circumstances, and its purpose is different from a conventional concept that attempts to suppress return light to a laser element as much as possible. An object of the present invention is to provide an optical device that can reliably suppress noise while actively using it.

[0008]

That is, the optical device of the present invention incorporates a laser element that performs self-sustained pulsation, and the light emitted from this laser element is reflected by the optical system, and a part of the reflected light. Is returned to the laser element via the optical path length Lex, and the self-excited oscillation frequency fsp of the laser element determined under the influence of the returned reflected light satisfies the following equation (1). This is not only an optical system such as an optical pickup, but also a light source itself.) (M−0.25) · (c / 2Lex) ≦ fsp ≦ m · (c / 2Lex) (1) (where m is 1 or an integer of 1 or more, and c is the speed of light.)

The technical idea of the present invention is that the above equation (1) does not originate in the material or structure of a laser device such as a semiconductor laser device, but forms a pulse-like signal at a timing convenient for a laser device which self-oscillates by itself. It is based on the principle that the return light is incident, which triggers the next or m-th subsequent pulse. (The rate equation for analyzing the operation of a semiconductor laser with return light is: A differential difference equation including a delay (2Lex / c) is obtained. If this equation has a periodic solution (a solution corresponding to self-excited vibration), the period T is slightly larger than (2Lex / c) (m = 1).
Is also shown from the result of the present inventors' numerical analysis. ].

In the present invention, since fsp is set so as to satisfy the above equation (1), the modulation of the self-excited oscillation and the modulation by the external resonator produce a very excellent synergistic effect, and (1) RIN (Relative Noise Intensity) is smaller than in the case where the expression is not satisfied, and the quality of the reproduced signal is improved.

Also, there is an effect that pulses of self-oscillation are aligned neatly, and when a pulse train is used for communication or optical information processing, jitter (time shift of pulse interval) is reduced. Errors can be reduced and these qualities can be improved.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, it is preferable that the optical system has an external resonator having a light reflecting surface. The light reflecting surface may be a signal recording surface of the information recording medium.

As a typical structure of the optical device of the present invention, a light source composed of a semiconductor laser element, an optical disk having a signal recording surface, a beam splitter provided in an optical path therebetween, and light from the beam splitter are provided. It is preferable that an optical pickup is provided for the optical pickup, comprising: an objective lens for condensing light on the optical disc; and a photodetector for receiving a part of the reflected light from the optical disc via the beam splitter.

When a semiconductor laser element is incorporated as a light source into the optical device of the present invention, the semiconductor laser element has a gain region in which an optical gain is generated by current injection and light generated by this gain oozes out. It is preferable to provide a saturable absorption region having no optical gain.

Preferably, the gain region is an active layer, and the saturable absorption region is disposed near the active region.

Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the drawings.

FIG. 6 shows an optical disk reproducing apparatus having an optical pickup according to the first embodiment of the present invention.
A semiconductor laser element (LD) 1 as a light source is connected to a power supply 2, and a coupling lens 4, a beam splitter 5, and an objective lens 6 are arranged in this order on an optical path between the optical disk 3.

Further, the beam splitter 5 is optically connected to a photodetector (PD) 8 via a sensor lens 7, and an amplifier 9 is electrically connected to the photodetector 8.

The structure of the semiconductor laser device 1 is not particularly limited as long as it can self-oscillate, but for example, the structure shown in FIG. 7 is preferable. In the semiconductor laser device 1 shown in FIG. 1A, the electrodes (anodes) 1a and 1b
Between the (cathode), a substrate 1c, a lower cladding layer 1d, an active layer 1e, an upper cladding layer 1g on which a ridge 1f is formed, and a current confinement layer 1h are laminated in this order. The active layer 1e includes a gain region 15 and a saturable absorption region 16 facing the ridge 1f.

The semiconductor laser device 1 shown in FIG. 2B has a point that a layer having a saturable absorption region 16 is provided in a ridge 1f and is separated from an active layer 1e having a gain region 15. , (A).

In the optical disk reproducing apparatus according to the first embodiment, the optical disk 3 forms an external resonator with respect to the semiconductor laser device 1, and Lex shown in the figure is the external resonator length, that is, the optical disk 3 as an external reflecting mirror. 2 shows the optical path length between the reflective surface and the semiconductor laser device 1.

The light emitted from the semiconductor laser device 1 enters a beam splitter 5 via a coupling lens 4, and from there, further passes through an objective lens 6.
Incident on the recording surface (reflection surface) of.

The reflected light from which information has been picked up on the recording surface of the optical disk 3 returns to the beam splitter 5 via the objective lens 6 again, and a part of the reflected light enters the photodetector 8 via the sensor lens 7 and is further amplified. After passing through No. 9, it is extracted as a reproduction signal.

On the other hand, a part of the reflected light from the beam splitter 5 passes through the coupling lens 4 and enters the semiconductor laser device 1 as return light.

The self-excited oscillation frequency fsp of the semiconductor laser element 1 depends on whether the semiconductor laser element 1 is a single body or when the semiconductor laser element 1 is incorporated in an optical system as described above, that is, from the optical disk 3 which is an external reflecting mirror. And it varies depending on the external resonator length Lex and the reflectance Rex of the external reflecting mirror.

Then, the noise of the return light is converted into the relative noise intensity RI.
Viewed at N, it changes according to Lex and Rex.

The smaller the value of RIN, the better. Therefore, when conditions for that purpose were examined, it was found that fsp determined by Lex and Rex should satisfy the above equation (1).

That is, the present inventors have determined that the average light output and R
When the interdependency between RIN and Lex was examined by changing ex variously, the results shown in the graphs of FIGS. 1 to 5 were obtained.

Further, when the fsp near the RIN minimum point in each graph was examined, the results shown in Table 1 were obtained.

[0030]

According to Table 1, it is understood that the above equation (1) is satisfied by setting m to an appropriate value in the vicinity of any RIN minimum point.

It should be noted that as described above, when the semiconductor laser device is coupled to an external resonator, its fsp
Changes from fo in the case of a single semiconductor laser element, and fsp> fo.

If fsp is set so as to satisfy the above equation (1), a very excellent synergistic effect is produced by the modulation of the self-excited oscillation and the modulation by the external resonator.
As compared with the case where the expression is not satisfied, RIN is reduced, and the quality of the reproduced signal is improved. In addition, since the pulses of self-oscillation are neatly aligned, when the pulse train is used for communication or optical information processing, jitter is reduced, errors in communication or optical information processing are reduced, and these qualities can be improved. is there.

Next, FIGS. 8 to 12 show embodiments of an optical device which can apply a modulated light output obtained by applying the device of the present invention to optical communication and information processing.

First, in the optical device of the second embodiment shown in FIG. 8, a semiconductor laser device (LD) connected to the power source 2
1 is a reflection mirror (reflectance Rex) 20 via a lens 21
(External resonator), Lex shown in the figure indicates the optical path length, that is, the external resonator length (the same applies to the following embodiments). A lens 2 is provided after the semiconductor laser device 1.
2, an optical isolator 24 and an optical modulator 25 via a lens 23 are provided. The optical isolator 24 is connected to the semiconductor laser device 1 from the optical modulator 25.
It is provided so that the reflected light does not enter.

The light emitted from the semiconductor laser device 1 is
The light is self-oscillated under the action of the return light from the external resonator, enters the optical modulator 25 via the optical isolator 24 and the like, where it is modulated by a modulation signal and output. The pulse train of light before and after the optical modulator 25 is, for example, as shown in the figure.
The light pulse train having a small jitter becomes a pulse train modulated as shown in the figure after passing through the optical modulator 25 (the broken line in the figure indicates a pulse lost by the modulation).

The light thus modulated can be used for an optical information system or for optical communication as shown in FIG. In FIG. 2A, a light pulse train from the light modulator 25 is incident on the optical fiber 27 by the coupling lens 26, and the light pulse train transmitted through the optical fiber 27 is received by the coupling lens 29 from the emission end of the optical fiber 27. The light enters the element 30. (B) shows an example in which the free space 28 is replaced without using an optical fiber.
9. After passing through the light receiving element 30 and the amplifier 31, it is taken out.

Next, FIG. 10 shows an optical device according to a third embodiment. Here, a beam splitter 5 is provided at a stage subsequent to the semiconductor laser device 1, and a reflecting mirror (reflectance Rex) is sandwiched therebetween. 20 forms an external resonator with respect to the semiconductor laser device 1. The rest of the device configuration is the same as in FIG.

FIG. 11 shows an optical device according to a fourth embodiment. The difference from the third embodiment is that the beam splitter 5 and the reflecting mirror 20 are omitted, and a half mirror (reflectance Rex) 34 is disposed in the optical path instead.
In this case, the external resonator is formed by the half mirror 34.

Further, the optical device of the fifth embodiment is shown in FIG.
Shown in In this optical device, the incident side of the optical modulator 25 is configured as a reflecting mirror (reflectance Rex), and an external resonator is formed with a simpler configuration with respect to the semiconductor laser element 1 with the lens 35 as the center. Is a major feature.

As described above, in any of the embodiments (2 to 8), the relationship between the external resonator length of the optical device, that is, the optical path length Lex, and the self-excited oscillation frequency fsp of the semiconductor laser device has been described (1). As long as the expression is satisfied, the effects of the present invention described above can be obtained.

The present invention is not limited to the above-described embodiment, and can be variously modified based on the technical idea.

For example, in the first embodiment, an example of an optical disk is described as an information recording medium, but other information recording media, for example, an optical tape may be used. In addition to the optical disk reproducing device, the technical idea of the present invention can be applied to an information recording device or an erasing device.

Further, the semiconductor laser element used as the light source may be a known one. AlGaAs is a typical example
System, InGaAsP system, AlGaAsSb system, AlGa
InP, GaInNAs, AlGaNAs, Al
GaInN-based, ZnCdSe / ZnMgSSe-based, and the like can be used. Semiconductor laser elements other than LD and laser elements other than semiconductor laser elements can be used as long as they can self-oscillate. Examples thereof include a solid-state laser in which a saturable dye is placed in the optical path in the resonator of the laser element, or a dye laser, for example, a ruby laser in which an alcohol solution of a blue dye cryptocyanine is placed. The semiconductor laser device is not limited to the above-described current injection gain type, but may be a refractive index waveguide type.

On the other hand, in the optical disk reproducing apparatus shown in FIG. 6, the arrangement of equipment from the beam splitter 5 to the amplifier 9 is omitted, and a half mirror is provided in place of the optical disk 3; If a wavelength converter is arranged on the side opposite to the objective lens 6), it is even possible to obtain a harmonic having a desired wavelength.

[0046]

As is clear from the above, according to the optical device of the present invention, the self-excited oscillation frequency of the laser element determined by the influence of the return light is determined by the specific condition represented by the above equation (1). Since the setting is made to satisfy, the modulation of the self-excited oscillation and the modulation by the external resonator produce a very excellent synergistic effect, the relative noise intensity is reduced, and the reproduced signal can be effectively improved. .

Furthermore, the pulses of self-oscillation are neatly aligned, and jitter is reduced when the pulse train is used for communication or optical information processing, so that the quality of communication and optical information processing can be improved.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between an external resonator length Lex and a relative noise intensity RIN in an optical disc reproducing apparatus according to an embodiment of the present invention.

FIG. 2 shows an average optical output mW and a reflectance Re in the same device.
It is a graph which shows the relationship between Lex and RIN when x is changed.

FIG. 3 shows an average power mW and a reflectance Rex in the apparatus.
7 is a graph showing the relationship between Lex and RIN when changing.

FIG. 4 shows an average output mW and a reflectance Rex in the apparatus.
7 is a graph showing the relationship between Lex and RIN when the value is changed.

FIG. 5 shows an average output mW and a reflectance Rex in the apparatus.
7 is a graph showing the relationship between Lex and RIN when changing.

FIG. 6 is a schematic diagram of an optical disc reproducing apparatus according to the first embodiment of the present invention.

FIG. 7 shows a semiconductor laser element used as a light source in the optical disc reproducing apparatus, where (A) is a cross-sectional view thereof;
(B) is a sectional view of another example.

FIG. 8 is a schematic diagram of an optical device showing a second embodiment of the present invention.

FIG. 9 is a schematic configuration diagram showing an optical communication system of the optical device.

FIG. 10 is a schematic configuration diagram of an optical device according to a third embodiment of the present invention.

FIG. 11 is a schematic configuration diagram of an optical device according to a fourth embodiment of the present invention.

FIG. 12 is a schematic configuration diagram of an optical device according to a fifth embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser element (LD), 1a ... electrode (anode), 1b ... electrode (cathode), 1c ... substrate, 1d ... lower cladding layer, 1e ... active layer, 1f ... ridge, 1g ... upper cladding layer, 1h ... Current constriction layer, 2 power supply, 3 optical disk, 4 coupling lens, 5 beam splitter, 6 objective lens, 7 sensor lens, 8 photodetector, 9 amplifier, 15 gain area, 16 Saturable absorption region, 20 ... Reflector, 21, 22, 23, 26, 29, 3
2, 33, 35 ... lens, 24 ... optical isolator, 25
... optical modulator, 27 ... optical fiber, 28 ... free space, 30
... Light receiving element, 31 ... Amplifier, 34 ... Half mirror

Claims (6)

[Claims] 1. A laser device that performs self-pulsation is incorporated, light emitted from the laser device is reflected by an optical system, and a part of the reflected light is returned to the laser device via an optical path length Lex. An optical device in which a self-excited oscillation frequency fsp of the laser element determined under the influence of the returned reflected light satisfies the following expression (1). (M−0.25) · (c / 2Lex) ≦ fsp ≦ m · (c / 2Lex) (1) (where m is 1 or an integer of 1 or more, and c is the speed of light.) 2. The optical device according to claim 1, wherein the optical system includes an external resonator having a light reflecting surface. 3. The optical device according to claim 2, wherein the light reflection surface is a signal recording surface of an information recording medium. 4. A light source comprising a semiconductor laser element, an optical disk having the signal recording surface as the light reflecting surface, a beam splitter provided in an optical path therebetween, and light from the beam splitter being transmitted to the optical disk. An objective lens for focusing, and a photodetector that receives a part of the reflected light from the optical disc via the beam splitter, and configured as an optical pickup,
The optical device according to claim 3. 5. An optical device incorporating, as a light source, a current injection type semiconductor laser element that performs self-sustained pulsation, wherein the semiconductor laser element has a gain region in which an optical gain is generated by current injection, and a gain region in the gain region. The optical device according to claim 1, further comprising: a saturable absorption region in which light that oozes but does not have optical gain. 6. The optical device according to claim 5, wherein the semiconductor laser element has an active layer serving as the gain region and the saturable absorption region near the active layer.