JPH0143873Y2 - - Google Patents

Description

[Detailed description of the invention] (Field of industrial application) The present invention is a semiconductor laser device used in many optical application devices such as optical communication equipment, optical information processing equipment, and optical measurement equipment. The present invention relates to a semiconductor laser device having a structure that does not induce the generation of noise even if it is re-injected into the semiconductor laser.

(Prior Art) Semiconductor lasers have been used in many devices in recent years, and representative devices include terminal equipment for optical information transmission and optical pickups in optical disk devices. Examples of its usage include the following. For example, in a terminal equipment for optical information transmission, the driving current of a semiconductor laser is modulated according to the information signal to be transmitted, and the output light of the semiconductor laser is focused by a lens or the like, and then sent to an optical fiber, which is an optical signal transmission path. Combine and output. Furthermore, in the pickup of an optical disk device, such as an optical video disk, semiconductor laser light is irradiated by a plurality of lenses so that a minute bright spot follows a signal track on the surface of the optical disk. The optical path of the reflected light from the recording pit on the optical disk surface is deflected by a semi-transparent mirror or a deflection prism inserted between the lens groups or closer to the laser, and the light is incident on a photodetector.
Performing signal detection.

(Problem that the invention aims to solve) In these optical application devices, the light reflected from various parts of the light transmission path re-enters the semiconductor laser that is the light source, which causes competition in the oscillation modes of the semiconductor laser. There is a problem that noise is generated. For example, in optical information transmission terminal equipment, the oscillated light of a semiconductor laser is focused on an optical fiber, so it is reflected from the light input/output surface of a lens, the light input end face of the optical fiber, and the end face of a connector installed in the middle of the optical fiber. Then, these lights enter the semiconductor laser again. Furthermore, in the pickup of an optical video disk, in addition to the reflected light from the optical parts provided in the path of the light beam, the reflected light from the disk substrate returns to the semiconductor laser and induces the generation of the above-mentioned noise.
The generation of noise causes deterioration of optical information transmission quality, deterioration of disc reproduction signal, deterioration of disc reproduction image quality, etc. In conventional optical video disk pickups, a simple optical isolator consisting of a polarizing beam splitter and a quarter-wave plate is used to suppress the light returning to the semiconductor laser.
The principle of this simple optical isolator is that since the light that has traveled back and forth through the quarter-wave plate is orthogonal to the polarization before entering the quarter-wave plate, the returning light is polarized and removed by a polarizing beam splitter. Even when reflection on the optical disk surface does not involve a change in polarization state, the isolation obtained with the above isolator is approximately 20 to 25 dB at most, and the transmittance of the collimating lens that collimates the semiconductor laser light is reduced by 30%.
Even in terms of degree, 0.3 to 0.1% of return light is always generated to the semiconductor laser. As is well known, a protective film made of resin is formed on the surface of an optical disk. This non-uniformity of the protective film causes a rotation in the plane of polarization of the reflected light. For this reason, the polarization removal of the returned light by the polarizing beam splitter becomes insufficient, and the amount of returned light to the semiconductor laser further increases, resulting in an increase in the amount of noise generated.

Even if the return light to the semiconductor laser is about 0.1 to 1%, a high frequency of 500 MHz to 1 GHz is added to the semiconductor laser drive current in addition to the DC component as a way to suppress noise caused by competition between the axial modes of the semiconductor laser. A known method is to reduce the coherency of the laser beam by superimposing the laser beam, but it requires an additional circuit for high frequency superimposition, and because the frequency is high, it requires advanced mounting technology with the semiconductor laser to match the electrical impedance. However, it has the disadvantage that the price becomes high as a result.

There is also a known method of reducing noise by using a semiconductor laser that has laser oscillation characteristics that cause self-excited vibration of about 1 GHz in the oscillated light simply by applying a direct current without superimposing a high-frequency signal on the drive current. ing. The mechanism of this self-oscillation has not yet been fully elucidated. The composition and thickness of the active layer of the semiconductor laser and the composition and thickness of the cladding layer adjacent to the active layer and between the substrate are controlled in extremely limited areas.
Effective oscillation characteristics cannot be obtained unless conditions are set with extremely good controllability. Current crystal growth techniques used in the process of manufacturing semiconductor lasers have drawbacks such as insufficient controllability, low yields, and, as a result, high prices.

As a measure to prevent light from returning to the semiconductor laser, it is conceivable to insert an element optical isolator having irreversible light transmission characteristics into the optical path. The oscillation wavelength of semiconductor lasers used in optical video disks and the like is in the range of 0.78 to 0.85 μm.
An isolator proposed for use in this wavelength range has a structure in which a light beam is multiple-diffracted and transmitted through high-concentration lead glass inserted into a strong magnetic field. However, this type of isolator has a large volume, about the size of a pine box, and is heavy.
It's expensive. In addition, since it requires a strong magnetic field and uses a strong permanent magnet, the leakage magnetic field to the outside is also strong, making it unsuitable for use in optical pickups for optical disk devices or in optical communication equipment. .

As described above, all of the conventional techniques have drawbacks.

This invention eliminates the above-mentioned conventional difficulties,
It is an object of the present invention to provide a semiconductor laser device having high S/N characteristics at low cost.

(Means for solving the problem) The present invention includes a built-in semiconductor laser, and a garnet film formed by a film forming method is provided on the emission end face of the oscillation light of the semiconductor laser. The semiconductor laser with an isolator is characterized in that it is provided with means for applying a magnetic field in a direction in which the oscillation light of the semiconductor laser is transmitted.

(Operation) The present invention solves the problems of the prior art by adopting the above-mentioned configuration. Since a garnet film that operates as a non-reciprocal element is installed on the emission end face of the semiconductor laser, all of the light reflected from the interface of each optical component installed in the optical path as the emitted laser light travels is reflected back. undergoes irreversible transmission characteristics. Therefore, noise is not induced by light returning to the semiconductor laser, and since the garnet film is formed by PVD (Physical Vapor Deposition), including sputtering and co-evaporation, conventional liquid The film can contain a much higher concentration of bismuth than garnet films formed by chemical crystal growth techniques such as phase crystal growth. Therefore, the polarization rotation of 45° in one-way transmission, which is necessary to provide irreversible transmission characteristics, can be generated with a thin film thickness. By adopting the above configuration, extremely excellent low noise characteristics of a semiconductor laser can be obtained with a simple structure.

(Example) Examples of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram of a semiconductor laser showing an embodiment of the present invention. 1 is a semiconductor laser chip, and 2 is a laser active region that contributes to laser oscillation of the semiconductor laser. A dielectric film 3 made of a non-magneto-optical material is provided on one end face where laser resonance occurs, and a garnet film 4 having magneto-optic properties is formed on the dielectric film 3. A magnetic field 5 is applied to the garnet film 4 in a direction through which the oscillation light of the semiconductor laser chip 1 is transmitted. Since the dielectric film 3 has the garnet film 4 added thereon, the reflectance of the end face of the semiconductor laser is as follows.
This is an optical buffer layer provided to prevent laser oscillation from being degraded to the extent that it is inhibited, and is formed of silicon oxide (SiO ₂ ), for example, by sputtering to a thickness of about 1/2 wavelength. The horizontally linearly polarized light oscillated by the semiconductor laser chip 1 passes through the garnet film 4.
When transmitted through , it undergoes a Faraday rotation of 45°. The light reflected from the reflective surface existing in the optical path of this laser oscillation light and returned to the semiconductor laser chip 1 is further transmitted through the garnet film 4 in the opposite direction.
Subject to 45° rotation. Since the polarization of the reflected light is direct from the oscillation light, noise induced by the returned light does not occur even if the reflected light enters the semiconductor laser again.

FIG. 2 is a diagram in which the isolator-equipped semiconductor laser 12 of FIG. 1 is applied to an optical pickup. 11
13 is a stem in which the semiconductor laser 12 is mounted such that the emission surface provided with a garnet film faces the same so that the oscillation light can be extracted through a window 14; 13 encapsulates the semiconductor laser 12; It is a cap for An annular magnet 15 is fitted into the cap 13 for applying a magnetic field to the garnet film in the direction of light transmission. 16 is a collimating lens that converts the emitted light from the semiconductor laser 12 into a parallel beam. 17 is a half mirror. Reference numeral 18 denotes an objective lens that focuses the collimated beam that has passed through the half mirror and irradiates and focuses the beam onto the optical disk.

The semiconductor laser 12 oscillates with linearly polarized light having an electric field component parallel to the junction surface. Magnetic field 1 in the direction of light transmission
The light transmitted through the garnet film on the end face of the semiconductor laser to which 5 is applied is collimated by the lens 16, and 1/2 of its intensity passes straight through the half mirror 17, is focused by the lens 18, and is focused on the disk. The light is reflected by the surface 19 and enters the half mirror 17 again. 1/2 of the light that has entered the half mirror 17 in the opposite direction travels straight through the lens 16 toward the semiconductor laser 12 . When this light passes through the garnet film provided on the semiconductor laser 12 again, it is further subjected to Faraday rotation by 45 degrees. That is, the polarization of the light that enters the semiconductor laser 12 in the opposite direction is orthogonal to the light that exits the semiconductor laser 12. It is well known that even if polarized light perpendicular to the oscillation polarized light is incident on the semiconductor laser 12, no return optical noise is induced.

FIG. 3 is a diagram showing the configuration of still another embodiment of the present invention, which is an example applied to a light source section of an optical communication device.
20 is a rod lens, and 21 is an optical fiber.
The light emitted from the semiconductor laser 12 passes through the garnet film and is coupled to an optical fiber 21 via a lens 20. Input/output surface of the lens 20, optical fiber 21
The reflected light generated from the incident surface of the optical fiber 20 and the other end of the optical fiber 20 will pass through the garnet film again, resulting in polarized light perpendicular to the emitted light. The generation of noise is not induced.

As an example, the garnet film used in the present invention can be manufactured as follows. Using an ordinary RF sputtering device or planar magnetron sputtering device, sputtering is performed in an argon or oxidizing atmosphere using a sintered body made by sintering powders such as bismuth oxide, yttrium oxide, or iron oxide as a target. Then, an amorphous film is formed on the semiconductor laser end face from above the dielectric film. This is crystallized by heat treatment in air or oxygen atmosphere. As a film forming method, in addition to the sputtering method, many film forming methods generally called PVD methods such as MBE method (molecular beam epitaxial method) and ionization beam evaporation method can be used.

In the garnet film Bi _x Y _3-x Fe ₅ O ₁₂ produced by this film-forming method, in which yttrium is replaced with bismuth, the amount of bismuth substitution can be made much larger than when produced by the normal liquid phase growth method. I can do it. As the amount of normal bismuth substitution increases, the Faraday rotation ability increases proportionally. For example, when fabricating by liquid phase growth method, the amount of bismuth substitution is at most about x~1, but with sputtering method, about x~3 can be achieved. Therefore, Faraday rotation ability is also 3
It will increase nearly twice as much. A material with a composition of x~1 and a wavelength of ~
The thickness of the material required to rotate the polarization of a 0.8 μm light wave by 45 degrees is 45 μm, but for a material with x ~ 3,
It only needs to be about 15 μm, which is a thickness that can be fully realized using film-forming technology. Furthermore, it has been found that the Faraday rotation ability increases when cobalt is introduced as a film composition atom. Furthermore, in the composition of R ₃ Fe ₅ O ₁₂ (R is a rare earth atom) in which oxygen is replaced with a sulfur atom, _for example, the composition R ₃ Fe ₅ S _7.5 O _4.5 , the Faraday rotation ability is further increased _. is becoming clear. By choosing an appropriate composition among these,
The film thickness required for 45° polarization rotation can be further reduced.

(Effects of the invention) As described above, according to the invention, the noise induced by the return light to the semiconductor laser can be suppressed with a simple structure.
This has the effect of making it possible to obtain a semiconductor laser with an isolator that provides extremely low noise characteristics.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the configuration of an embodiment of the semiconductor laser device of the present invention, and FIGS. 2 and 3 are configuration diagrams showing applied examples. DESCRIPTION OF SYMBOLS 1... Semiconductor laser chip, 2... Laser active region, 3... Dielectric film, 4... Garnet film, 5
...Bias magnetism, 12...Semiconductor laser, 15
……magnet.

Claims (1)

[Scope of utility model registration request] It has a built-in semiconductor laser, and has a garnet film formed by a film formation method on the emission end face of the oscillation light of the semiconductor laser, and a magnetic field is applied to the garnet film in the direction in which the oscillation light of the semiconductor laser is transmitted. 1. A semiconductor laser device with an optical isolator, characterized in that it is provided with means for applying .