SEMICONDUCTOR LASER APPARATUS WITH ISOLATOR

Abstract: A laser diode (2) is mounted on a block (101) forming part of a base structure (1). The diode is hermetically sealed within a cap (3) provided with a window (4) for the emission of light. The p-n junction plane of the laser diode (2) is parallel to the vertical surface on which it is mounted and light is emitted through an end surface of the diode in a direction normal to the window (4). In order to prevent return light from disturbing the diode and creating noise, the window (4) is formed of magneto-optical material which is magnetically saturated by an external annular magnet (5). The thickness of the window (4) is such that the plane of polarization of the emitted light is rotated 45° by Faraday rotation. A further 45° rotation is imposed on any returning light so that there is a 90° rotation relative to the plane in which the laser diode (2) is active. In other embodiment the magneto-optical material is deposited on the inside of a glass window or on the end surface of the diode.

Description: The present invention relates to a semiconductor laser apparatus widely employed in optical devices such as an optical communication apparatus, optical data processing apparatus, or optical instrument, and, more particularly to a semiconductor laser apparatus of a structure which does not induce noise even if reflected light re-enters the same semi-conductor laser. The semiconductor laser has recently been used in an increasing number of apparatus, typical examples of which are a pickup for optical discs and an optical data transmission terminal. In a terminal for optical data transmission, for example, the driving current of a semiconductor laser is modulated by data signals to be transmitted, the output light from the laser is focussed by a lens and is coupled to an optical fiber which is a transmission line of the optical signals. In a pickup of an optical disc such as an optical video disc, semiconductor laser beam is irradiated on the surface of an optical disc through a plural number of lenses in a fine spot to follow up signal track. The light reflected from the recording pit of the optical disc surface is deflected in the direction of the optical path by a half mirror or a polarizing prism inserted between a collimating and a focusing lenses or between a laser diode and a collimating lens, and then detected by light detectors. With these optical devices, optical noise problems have occurred when light reflected from optical components in the transmission path returns to the semiconductor laser to cause instability of oscillation modes, thereby generating noise. In an optical data transmission terminal, for instance, the light emitted from a semiconductor laser is reflected from the both sides of a LD-fiber coupling lens, or the optical fiber ends and returns to the semiconductor laser again. In a pickup of an optical video disc, the light reflected from a disc substrate as well as from optical members provided in the optical path returns and enters the semiconductor laser and causes noise as described in the above. Noise deteriorates the quality of optical data transmission, and of images reproduced by the disc. When the S/N of the image reproduced by a video disc is assumed to be 45 dB, the permissible noise level required for a semiconductor laser ranges from -135 dB/Hz to -140 dB/Hz. When there is no return light, the noise level is maintained at the range from -140 to -150 dB/Hz with a strong single axial mode, but if there is return light in 0.1% - 1%, the level deteriorates to -110 to -120 dB/Hz accompanied with decline in the pictorial image S/N to 42 dB, thus remarkably deteriorating the pictorial image quality. A simple optical isolator comprising the combination of a polarization beam splitter and a quarter-wave length plate is often used in a pickup of conventional optical video discs in order to restrict the return of the reflected light. The principle of the above isolator lies in that the light which has gone through and come out from a quarter-wave length plate perpendicularly intersects the polarized light before it enters said plate and the return of the light is deflected to be eliminated by a polarization beam splitter. However, isolation obtained by such an isolator is limited to the range of 20 - 25 dB at most even when the reflection in an optical disc plane is not accompanied by changes in polarizing conditions. 0.3 - 1% of light is always returned to the semiconductor even though the transmittance of a collimating lens which collimates the beam from the semiconductor laser is maintained at 30%. As is well known, the surface of an optical disc is protected with a resin film. As such protective films are not always uniform in refractive index birefringence, the polarized plane of the reflected light becomes rotated. A polarization beam splitter can therefore not completely remove the return of the light, and the light returning to the semiconductor laser increases to thereby enhance noise. In order to reduce noise generation due to the axial mode instability of the semiconductor laser, there has been proposed a method of superimposing a high-frequency current of 500 MHz - 1 GHz on a DC driving current of the semiconductor laser to reduce the coherence of the laser light even if there is ca. 0.1 - 1% return light. (See, for example, Nikkei Electronics, 1983, 10, 10, pp. 173 - 194 (in Japanese)). This method, however, is detrimental as it requires an additional circuit for superposing high frequency current as well as a sophisticated technique for packaging the circuit with a semiconductor laser because the high-frequency circuit should be matched with the semiconductor laser in impedance; the cost becomes inevitably high. There is known another method for reducing noise which uses a semiconductor laser having an oscillation characteristics to cause oscillation of 1 GHz on the emitted light simply by applying a DC current instead of superimposing the high-frequency signals on the driving current. (S. Matsui et al, "Suppression of feedback-induced noise in short cavity V-channeled substrate inner stripe lasers with self-oscillation", Appl. Phys. Lett. 43 (3), 1 August 1983, pp. 219 - 221). The mechanism of such self-oscillation has not been elucidated yet. Effective oscillation characteristics cannot be obtained unless the composition and thickness of the active layer of a semiconductor laser as well as those of the cladding layer adjacent to and between said layer and the substrate are controlled to remain within a closely restricted range. The currently available technique of growing a crystal in the semiconductor laser manufacturing process does not have sufficient control over these factors nor reasonable yield, resulting in a high price. The return of light to a semiconductor may be prevented by inserting in the optical path an optical isolator as an element which has a non-reversible transmission characteristics. Light wavelengths of a semiconductor laser employed in an optical video disc, etc. range within 0.78 - 0.85 micrometre. Isolators to be employed in the above wavelength range in such a method have the structure of repeatedly transmitting a light beam through a high density lead glass inserted in an intensive magnetic field (See, for example, M. Saki et al, "Optical Isolators for Fiber-Optic Communications", Paper of Technical Group, TG OQE 78-133, pp. 25 - 30, IECE of Japan (in Japanese).). However, such an isolator has a size as large as a match box, is of a considerable weight and costs much. Moreover, such an isolator requires a powerful permanent-magnet to provide a high magnetic field, which inevitably causes considerable magnetic leakage to outside. The isolator of this type cannot therefore be suitably used in a pickup for an optical disc or in optical communication devices. As described in the above, the prior art methods have drawbacks in one way or another. It is known from DE-A- 2 900 356 to provide a semiconductor laser diode mounted on a stem, with a cap having a window through which oscillation light from the semiconductor laser diode passes, for hermetically sealing the semiconductor laser diode, the window having antireflection coatings on both surfaces thereof. An antireflection coating is also known for a laser diode from DE-A- 3 231 443. Such antireflection layers have the purpose of stopping reflection taking place at the window of the laser diode structure; US-A- 4 375 910 describes an optical isolator which uses a Faraday rotator sandwiched between two elongate rod lenses and respective birefringent plates. It relies on the co-operation of the Faraday rotator with the birefringent plates to shift the reflected beam sideways so that it does not return and impinge on the laser itself. The present invention is defined in the appended claims to which reference should now be made. As is well known, a ferro magnetic magneto-optical material having spontaneous magnetization saturated by an external magnetic field acts as a non-reversible element when a magnetic field is applied so as to orient the spontaneous magnetization in the direction of the light transmission. Since the element is located in a position immediately subsequent to the light emitting spot of a semiconductor laser, the reflected light from the incident and emanating planes of an optical member such as a lens, through which the laser light transmits in the process, is provided with a non-reversible characteristic. Therefore, the noise by the return of light is not induced, and thus an extremely low noise characteristic can be obtained. Brief Description of the Drawings: Fig. 1 is a perspective and partially exploded view of the first embodiment of the laser package according to this invention. Fig. 2 is a graph indicating the wavelength dependency of Faraday rotation and light absorption coefficient of a magneto-optical crystal employed in the first embodiment of the present invention. Fig. 3 is a block diagram showing a laser package of the first embodiment used as a pickup for a video disc. Fig. 4 is a block diagram showing the laser package of the first embodiment used in an optical communication device. Fig. 5 is a block diagram showing the laser package of the first embodiment for a video disc provided with an isolator which comprises a quarter-wave length plate and a polarization prism. Figs. 6 and 7 are perspective views of the second and third embodiment of the present invention respectively. Fig. 8 is a perspective view of a laser diode employed in Fig. 7. Fig. 1 is a perspective view which is partially exploded in cross section of a semiconductor laser apparatus with isolator packaged according to the first embodiment of the present invention. The reference numeral 1 refers to a stem for mounting, 2 a semiconductor laser diode (LD), 3 a cap to seal LD, 4 a ferro-magnetic magneto-optical crystal member and 5 a ring magnet generating a sufficient magnetic field to saturate spontaneous magnetization of the magneto-optical crystal member. The magneto-optical crystal member forms the window in a window opening 20 of the cap and has a thickness sufficient to rotate polarized light by 45° in saturated magnetization against the laser light vertically entering the crystal member from the cap. This magneto-optical crystal member is a non-reversible element which gives the rotation by 90° to the light which first passes out through the member and then passes back therethrough because of reflection. The stem 1 includes a base 13 attached to a flange, lead rods 15, 16 fixed on the base through insulating members 14, a lead rod 17 directly fixed on the base and a cooling block 19. Except for the members 14, all other materials of the stem are of copper. The laser diode is fused to the cooling block through a silicon heat sink 21. The soft soldering, and the gold-and-tin alloying methods are usually employed. The lead rod 15 attached to the stem and the laser diode 2 are connected by a thin gold wire 18 by the wire bonding method. The p-n junction plane of the LD 2 is positioned parallel to the heat sink plane and the direction of the laser beam is oriented perpendicular to the plane of the magneto-optical member. A light detector 22 which monitors the light emanating from the end face of the LD opposite to the magneto-optical member is provided on the base member 13 and one electrode thereof is connected to the lead rod 16 and the other is grounded. The magneto-optical member 4 is attached in advance to the window 20 of the cap 3 by soft soldering, low melting point glass soldering, etc. The cap is fused to the stem 1 by metal fusing, and then a hollow cylindrical magnet 5 is placed over the cap 3 from outside. Fig. 2 shows the magneto-optical characteristics of the magneto-optical crystal plate or a garnet crystal with the composition of Gd 3-x Bi x Fe₅O₁₂, which is used in the embodiment shown in Fig. 1. This crystal has a Faraday rotation power of 10,000 deg/cm and light absorption coefficient of 100 cm⁻¹, i.e. 434 dB/cm, around the wavelength of 0.78 or 0.83 micrometre of the emitted light from the semiconductor laser employed in an optical disc. The thickness of the crystal which rotates the transmission light by 45° is 45 micrometres at the wavelength of 0.78 micrometre. Light absorption at this point is 2 dB. Faraday rotation angle of 45° can be obtained for a wavelength of 1.3 micrometres which is used generally as the light source for optical communications if the thickness of the crystal is set at ca. 150 micrometres. The increase in optical loss in this case is approxmately 1 dB. A magneto-optical crystal plate employed in the present invention may be prepared by the liquid phase epitaxy (LPE) method on a substrate such as a neodymium-gallium-garnet crystal or a gadolinium-gallium-garnet crystal a part of which being substituted with calcium, zirconium or magnesium of thickness of ca. 0.1 - 0.3 mm. Fig. 3 shows the LD device 200 of Fig. 1 employed in an optical pickup. The LD device 200 emits 0.78 micron light through the magneto-optical crystal plate 4. A collimating lens 6 is arranged to oppose the crystal plate to focus output light in parallel beams. The reference numeral 7 denotes a half mirror, and 8 an objective lens which focuses collimated beams emanating from the half mirror on the surface of an optical disc 9. A semiconductor laser diode 2 emits linear polarized light having an electric field component parallel to the p-n junction plane of the laser diode. The light transmitting through the magneto-optical crystal plate 4 to which a magnetic field is applied in the direction of light transmission is subjected to Faraday rotation. The thickness of the plate 4 is set at 45 micrometres so as to provide a linear polarized light with an angle at 45° to the incident polarization. One half of the light focussed in parallel by the lens 6 transmits straight through the half mirror 7, converged by the lens 8, reflected by the disc surface and re-enters the half mirror again. One half of the light which re-entered the half mirror advances straight to the semiconductor laser through the lens 6. The other half is deflected by 90° in its path by the half mirror and led to an optical receiver system 25 which receives focus or tracking error signals, or disc recording signals. The light returning to the LD package is further subjected to Faraday rotation of 45° when it transmits through the crystal plate 4 toward the semiconductor laser 2. In other words, the polarization direction of the light re-entering the semiconductor laser 2 is perpendicular to that of the light emanating from the said laser, having no adverse effect on the laser oscillation. Although a pickup accompanied by the laser apparatus of the present invention adds an absorption loss of 2 dB to a conventional one, it provides an isolation of 49 dB, which is 34 dB greater than that of the conventional pickup. Fig. 4 shows the first embodiment of the present invention employed as the light source of an optical communication device. As the LD shown in Fig. 1, a laser which oscillates the wavelength of 1.3 micrometres is used in this embodiment. A magneto-optical crystal member is set at 150 micrometres to provide 45° of Faraday rotation to the light of the wavelength of 1.3 micrometres. The reference numeral 10 denotes a rod lens and 11 an optical fiber. The light emanating from the semiconductor laser 2 transmits through the magnetized magneto-optical crystal plate 4 and is coupled to the optical fiber through a coupling lens 10. The light reflected from the incidence and emanation ends of the lens and optical fiber transmits through the magnetic optical crystal plate again as a polarized light perpendicular to the emanating light so that, as described in the foregoing, noise is not induced even if this light re-enters the semiconductor laser. Fig. 5 shows an embodiment of the LD device 200 provided further with a polarized light isolator comprising a polarizing prism 23 and a quarter-wave length plate 24 in combination. Since the isolation of a simple isolator combining a polarizing prism and a quarter-wave length plate is usually at least -15 dB and the isolation of a magneto-optical element is -35 dB; the total of at least -50 dB isolation, in other words, less than 0.001% return is obtained. Such low amount of returning light will not cause noise. Fig. 6 is a perspective view of the second embodiment of the present invention, partially exploded and cross sectioned. The difference from the first embodiment shown in Fig. 1 lies in that, instead of using the magneto-optical single crystal as the window of the cap 3, a garnet film 4b is vapor-deposited on an optical glass window 4a by sputtering method. The thickness of the garnet film is determined in such a way as to subject the polarized light transmitting through the film under the applied magnetic field of magnet 5 to 45° of Faraday rotation. The garnet film employed in the embodiment may be prepared, for example, as in the following. Using an ordinary RF sputtering apparatus or planar magnetron sputtering apparatus, an amorphous film is formed on a substrate of optical glass, by sputtering a sintered compact which is prepared by sintering powder of bismuth oxide, yttrium oxide, or iron oxide under argon or oxygen atmosphere. The film is crystalized in a thermal process in the atmosphere or in oxygen. Besides the above sputtering method, the film may be formed by other methods such as MBE method (molecular-beam epitaxial method) and PVD method which is widely known as the ionized beam sputtering method. Compared to the film prepared by the ordinary LPE method, the amount of bismuth substitution can be greatly increased in a garnet film Bi x Y 3-x Fe₅O₁₂ in which yttrium is substituted with bismuth. Faraday rotatability increases in proportion to the increase in the substituted amount of bismuth. For example, in a film prepared by LPE method, substitution of bismuth is x ≃ 1 at most, whereas the amount can be increased to x ≃ 3 by sputtering method, which increased Faraday rotatability almost 3 times as much. With the composition of x ≃ 1, the thickness of the material required to achieve 45° rotation of the polarized light of the wavelength of 0.8 micrometre is 45 micrometres whereas with x ≃ 3 material, only 15 micrometres of thickness can achieve the same result. This level of thickness can be easily realized by the present film forming techniques. Furthermore, it is reported that addition of cobalt to the film increases Faraday rotatability. If oxygen in the composition of R₃Fe₅O₁₂ (R is a rare-earth atom) is substituted with a sulphur atom, to obtain for example, the resultant composition of R₃Fe₅S 7.5 O 4.5 , Faraday rotatability is further increased. By selecting a suitable composition, film thickness required for 45° rotation of polarized light can be further reduced. Fig. 7 shows the third embodiment of the present invention. This embodiment is different from the first embodiment in that an ordinary optical glass 4a instead of the magneto-optical crystal member 4 is used as the window of the cap 3 and a different LD 12 is used instead of LD2. Details of LD 12 are shown in Fig. 8. Reference numeral 101 in the figure denotes a semiconductor laser chip and 102 a laser active region contributing to laser oscillation of the semiconductor laser. One end face of the chip is cleavaged and the other end face is provided with a dielectric film 103 of non-magneto-optical substance. A garnet film 104 having magneto-optical characteristic is formed on said dielectric film 103. The film 103 is an optical buffer layer which through addition of a garnet film thereon prevents reflectivity of the end face of the semiconductor laser from decreasing to a degree as low as to inhibit laser oscillation. The film is formed, for example, from silicon oxide (SiO₂) by sputtering method to obtain a half wavelength. This LD 12 is placed in the cap in such a way as to oppose the garnet film surface 104 to the optical glass plate 4a (Fig. 7). In this LD device 200, polarization of the light oscillating under the magnetic field provided by the magnet 5 is subjected to 45° of Faraday rotation by transmitting through the garnet film 104. The light reflected from the reflective surfaces existing and returning to the semiconductor laser chip 101 is rotated by 45° further when it transmits through said garnet film 104 again in the opposite direction. Since the polarization direction of the light reflected is perpendicular to that of the oscillating light, noise which otherwise might be caused by the returning light is not induced even if it re-enters the semiconductor laser. While in Fig. 8 the dielectric film 103 is shown between the garnet film 104 and the end face of the semiconductor laser, the magneto-optical material may alternatively be deposited directly on the end face of the semiconductor laser diode without any optical matching layer.

Claims: A semiconductor laser apparatus with an optical isolation function, comprising: a semiconductor laser diode (2) mounted on a stem (1), and a cap (3) provided with a window (4 or 4a) through which oscillation light from the semiconductor laser diode passes, for hermetically sealing the semiconductor laser diode; a ferro-magnetic magneto-optical film member (4, 4b, 104) located immediately subsequent to the light emitting portion of the semiconductor laser diode and having spontaneous magnetization orientated in the direction of the oscillation light transmission such that between the laser diode and the film member, the polarization of the light that is reflected back to the laser diode through the film member is changed by 90° with respect to the oscillation light emitted directly by the light emitting portion of the laser diode; and means (5) for applying a magnetic field in the direction of the oscillation light transmission in a manner to saturate the spontaneous magnetization of the film member. Apparatus according to claim 1, in which the ferro-magnetic magneto-optical film member constitutes the window (4). Apparatus according to claim 1, in which the ferro-magnetic magneto-optical film member (4b) is located on the inside of the window (4a). Apparatus according to claim 1, in which the ferro-magnetic magneto-optical film member (104) is located on the end face of the diode (12). Apparatus according to claim 4, comprising an optical matching layer (103) between the end face of the diode (12) and the ferro-magnetic magneto-optical film member (104). Apparatus according to claim 3, 4 or 5, in which the film member is a deposited film.

Inventors: HIBIYA, TAKETOSHI, OHTA, YOSHINORI, SAKAGUCHI, MITSUHITO, SAKUMA, ISAMU
Applicants: NEC CORPORATION
Application year: 1985
Main classification: H01S3
IPC codes: H01S5, G02F1, G02B6