JPH0158677B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention eliminates fluctuations in oscillation light intensity and noise caused by reflected light from optical fibers.
Regarding semiconductor laser modules.

With the advancement of optical fiber technology, the loss of lines has been reduced, making it possible to transmit large-capacity optical signals over long distances. In addition, improvements have been made to the optical coupling technology between the semiconductor laser, which is the signal source, and the optical fiber, making it possible to use light with high efficiency, and it is becoming possible to further increase the transmission distance. As the loss of optical fibers has been reduced and optical coupling efficiency has been improved, the following problems have come to light. Although the intensity of the reflected light generated at the input end face of the optical fiber that couples the output light of the semiconductor laser and between the other end of the optical fiber, that is, the connector connecting to the main transmission line, is small, it can be reflected by retracing the optical path. It is focused on the semiconductor laser end facet. The oscillation characteristics of the semiconductor laser change due to the incidence of this returned light. One is that the oscillation threshold current decreases, so the oscillation light intensity changes even if the semiconductor laser is driven with constant current. Furthermore, if there is a reflective surface at a short distance, such as from the input end of an optical fiber, a complex resonator formed by an external reflective surface is added to the optical resonator of the semiconductor laser. Changes in the output light intensity occur due to changes in the refractive index, temperature, etc. of the laser medium. Furthermore, when the reflected light from the output end face of the optical fiber enters the semiconductor laser again, the laser exhibits oscillation characteristics in which many longitudinal modes coexist, and slight changes in the ambient temperature or return light intensity can cause changes in the main oscillation mode. This may cause mode jumps, etc. All of these become noise in the oscillated light, and are a major cause of deterioration in the signal-to-noise ratio of communication systems.

In particular, the noise emitted by semiconductor lasers due to return light becomes an important problem in systems such as systems that receive and transmit optical analog signals rather than optical digital signal communication systems, broadband analog optical communication systems, optical video disks, and optical fiber grills. ing.

To eliminate this oscillation light noise of a semiconductor laser, it is necessary to take measures to prevent the return light that is the cause of the noise from entering the laser again, and many methods have been considered so far. One of them is to convert the emitted light from a semiconductor laser into a parallel light beam using a high NA/short focal length lens, and insert a polarizing prism and a quarter-wave plate into the optical path of this parallel light beam. The light beam is focused by a lens and coupled to an optical fiber.The optical axis of the polarizing prism transmits the oscillated light of the semiconductor laser, which has an oscillating electric field component in a direction parallel to the active layer, and refracts the component perpendicular to it. is selected so that it is removed out of the optical path. The light emitted from the laser and transmitted through the 1/4 wavelength plate becomes circularly polarized light.The reflected light reflected from the focusing lens and the end face of the fiber passes through the 1/4 wavelength plate in the opposite direction and is perpendicular to the direction of incidence. It becomes linearly polarized light and is removed by the aforementioned polarizing prism. In this method, reflected light whose polarization state does not change when reflected, such as from a lens surface or fiber input end face, that is, near-end reflection, can be removed by converting it into orthogonal polarized light using a quarter-wave plate. For reflected light that passes through the fiber, is reflected by the output end face, and then passes through the fiber again, that is, far-end reflected light, the polarization state of the light after passing through the fiber does not maintain circular polarization, so it must be removed. The problem is that it is not possible to

Instead of the above-mentioned 1/4 wavelength plate, a magneto-optic crystal or magneto-optic glass with the Faraday or Verdet effect is used, and the polarization of the light transmitted through this magneto-optic medium is a straight line having an angle of 45 degrees with the incident linearly polarized light. There is a method in which the light is polarized, a birefringent prism that transmits only this polarized light component is also provided on the output side, and the light is focused onto an optical fiber using a condenser lens. In this method, the polarized light component of the reflected light that passes through the output side prism is transmitted through the magneto-optic medium in the opposite direction and is rotated to a polarization perpendicular to that at the time of incidence. Since it is removed by the birefringent prism, re-injection into the semiconductor laser is prevented.
This configuration can provide high reverse loss, but both this configuration and the method described above first convert the emitted light from the semiconductor laser into a parallel beam using a lens, and then place a birefringent prism in the optical path of this parallel beam. Ya1/
It has a configuration in which four plates or a magneto-optical medium are inserted, and the light is again focused by a lens and condensed onto an optical fiber, and there are many optical components. Therefore, the arrangement is likely to shift due to changes in ambient temperature, making it unstable. For example, the width of the active region of a semiconductor laser is 2~
3 μm, and the core diameter of single mode fiber is about 10 μm. Therefore, the two lens diameters constitute a magnifying optical system with a magnification of 3 to 5 times. Therefore, lateral movement of the lens due to changes in ambient temperature is magnified and occurs on the end face of the optical fiber. Since the tolerance for optical axis deviation of a single mode fiber is narrow, about 1 to 2 μm, the coupling efficiency to the optical fiber easily decreases due to changes in ambient temperature. Further, the end face of the semiconductor laser is located at the focal point of a lens that converts the beam into a parallel beam. Therefore, if a slight reflection occurs at the entrance/exit surface of each component on the parallel beam optical path, the light will be efficiently focused on the light emitting section of the semiconductor laser. Therefore, it is necessary to apply high-performance anti-reflection treatment to the light input/output end faces of each component, which has the disadvantage of increasing the manufacturing cost of the components.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and provide an inexpensive semiconductor laser module with fewer components.

The semiconductor laser module of the present invention includes a semiconductor laser and a magneto-optic thin film isolator optically end face connected to the light emitting end face of the semiconductor laser, and the thin film isolator has a bonding surface between the substrate surface and the semiconductor laser. are arranged to form an angle of 45 degrees within the optically connected end surface, and the upper surface is divided into a portion covered with a metal film and a portion not covered in the light transmission direction. The optical waveguide has a single rib-shaped optical waveguide, and the composition of the crystal material constituting the optical waveguide is such that the phase constants of two propagation modes with orthogonal polarization planes propagating through the optical waveguide are degenerate. The configuration is such that the geometrical shape of the optical waveguide is set to a predetermined value. By adopting such a structure, it is possible to obtain a semiconductor laser module that is stable and has a simple structure, without causing unstable oscillation due to light returning to the semiconductor laser.

The present invention is based on the following facts. First, even if a TM wave, which is polarized light whose plane of polarization is rotated by 90 degrees, is input as return light to a semiconductor laser that is oscillating with TE polarized light, the laser output light does not change. (For example, a document showing this fact is shown in Proceedings of the 27th Applied Physics and Related Union Lectures, 3P-G-12, Spring 1981). By using a unique isolator and devising its optical arrangement, a semiconductor laser module can be constructed with fewer optical parts such as lenses.

The details of the present invention will be further explained with reference to the drawings. FIG. 1 is a diagram showing the configuration principle of one embodiment of the present invention.
is a semiconductor laser, 2 is a magneto-optic thin film isolator, 3 is an optical fiber, and 4 is the direction of an external magnetic field applied to the magneto-optic thin film isolator. The structure of this thin film isolator is such that a rib-shaped waveguide 5 is formed on a magneto-optic thin film 8 epitaxially grown on a non-magnetic substrate 7, and a metal film 6 is provided on a part of the waveguide. . The end face of the rib-shaped waveguide 5 of the magneto-optic thin film isolator 2 is placed in close proximity to the upper surface of the rib-shaped waveguide 5 of the semiconductor laser, and the end face of the rib-shaped waveguide 2 has a metal film on the upper surface. An optical fiber 3 is placed very close to the end face of the optical fiber 3. A line perpendicular to the light transmission direction of the magneto-optic thin film isolator and parallel to the non-magnetic substrate 7 and the bonding surface of the semiconductor laser 1 are arranged at an angle of 45°.

The behavior of light in this configuration will be explained. FIG. 2 is a diagram explaining the polarization state of light emitted from the semiconductor laser 1 and directed toward the optical fiber 3. The coordinate axes are the light transmission direction, the direction parallel to the bonding surface of the semiconductor laser, and the direction perpendicular to these. . Laser light 11 emitted from the semiconductor laser is a TE wave, which efficiently enters the rib-shaped waveguide 5 of the magneto-optic thin film isolator 2. The light wave traveling through the rib-shaped waveguide 5 becomes linearly polarized light 1 of 45 degrees at the boundary with the part whose upper part is covered with the metal film 6, as will be explained later.
2, the polarization is rotated by the Farade effect. Since the magneto-optic thin film isolator 2 forms an angle of 45° with the junction surface 9 of the semiconductor laser, this polarized light is a TE wave in the magneto-optic thin film isolator 2. Furthermore, the TE wave that travels through the rib-shaped waveguide and passes through the portion whose upper part is covered with the metal film 6 is hardly attenuated by the metal film. In addition, since the upper part is covered with a metal film, the phase velocity of the TE wave and the TM wave are different in the waveguide, so the linear polarization state 13 of 45° is maintained without polarization rotation, and the magneto-optic thin film isolator is emitted and enters an optical fiber 3 provided close to the emitting end face.

FIG. 3 is a diagram showing the behavior of polarized light with respect to the returned light.
There are 5 and 14 TM wave components orthogonal to it. Of the two polarized light components 14 and 15 that pass through the upper part of the rib-shaped waveguide of the magneto-optic thin film isolator 2 covered with the metal film 6, the TM wave 14 is rapidly attenuated by the effect of the metal film, and At the end of transmission through the part covered with the metal film, only the TE wave component becomes 16. Furthermore, the TE wave that travels toward the semiconductor laser 1 through the portion of the rib-shaped waveguide that is not covered with the metal film is polarized at the time of incidence due to the Faraday effect (Fig. 2, 11).
It undergoes rotation into polarized light 17 orthogonal to that of the polarized light 17. The return light that reaches the semiconductor laser end face is a TM wave orthogonal to the TE wave, which is the oscillation light. Therefore, even if this returned light enters the semiconductor laser again, the laser output light does not change as described above, and therefore, the oscillation light does not fluctuate or noise is generated.

The operation of the magneto-optical thin film isolator was disclosed in a patent application filed in 1983.
As described in 036990, it is as follows. Garnet magneto-optic single crystals, represented by Y ₃ Fe ₅ O ₁₂ , are optically transparent at light wavelengths of 1 to 2 μm.
Since it has a Faraday effect, it is useful as a material for optical isolators. In other words, the two light-transmitting surfaces of the bulk single crystal are polished to allow light to pass through,
If a magnetic field is applied in the light transmission direction and the transmission length is set appropriately (for example, about 2.4 mm for a Y ₃ Fe ₅ O ₁₂ crystal for a 1.3 μm light wave), the incident linearly polarized light will be at an angle of 45° from this when it exits. Linearly polarized light that is angularly polarized undergoes polarization rotation due to the Farade effect.
When this polarized light is transmitted through the crystal in the opposite direction, the polarized light at the reflective side becomes linearly polarized light orthogonal to the incident linearly polarized light. For this reason, it is well known that if a polarizing prism is provided on the input/output side and its crystal axis is appropriately determined, an optical isolator function, that is, an irreversible optical element with large reverse light transmission loss can be constructed.

In a thin film, its eigenpropagation mode TE
Since the phase velocity of the wave and the TM wave are different, polarization rotation does not occur in a normal configuration. Normally, the garnet crystal itself is optically isotropic, but when formed into a thin film, the phase constant of the TE wave changes.
larger than that of the TM wave. However, if the magneto-optical film to be epitaxially grown has a composition with a lattice constant larger than that of the substrate (for example, if Gd ₃ Ga ₅ O ₁₂ crystal is selected as _the substrate material, _Gd
When epitaxial growth of Fe ₅ O ₁₂ , 0.1<x<0.3 is selected, tensile stress is generated in the epitaxial film due to lattice constant mismatch, and this stress causes Birefringence occurs and the phase constant of the lowest order TM wave can be made larger than that of the TE wave.

In general, the phase constant of a waveguide also changes depending on the width of a channeled waveguide, such as a rib-shaped waveguide, for example. As described above, the composition and film thickness of the epitaxial film are set so that the phase constant of the TM wave is larger than that of the TE wave in the planar waveguide configuration, and the rib-shaped waveguide is formed by ion milling or the like. If the waveguide width is appropriately set, the phase constant of the TE wave and the phase constant of the TM wave can be degenerated. When a magnetic field is applied in the light transmission direction of this waveguide, irreversible characteristics can be obtained as in the case of the bulk single crystal described above.
As an example, Gd ₀ . ₂ Y ₂ . ₈ Fe ₅ O ₁₂ epitaxial film is used.
The waveguide width is 3.3 μm, and the waveguide length is the length in the light transmission direction of the portion 5 of the rib-shaped waveguide in Fig. 1 whose upper part is not covered with a metal film.
When the thickness is about 2.3 mm, the polarization characteristics shown in FIGS. 2 and 3 can be obtained.

In the configuration of this embodiment, the light emitted from the semiconductor laser is guided by the channel waveguide and guided to the optical fiber without passing through any optical components such as lenses.
Optical coupling can be achieved with an optical coupling loss of about 2 dB between the semiconductor laser and the channel waveguide of the above design example, and with an optical coupling loss of about 1.5 dB between the channel waveguide and the optical fiber.

When using a semiconductor laser, it is often desirable to use it as a parallel beam without transmitting it through an optical fiber. In this case, a parallel light beam can be obtained by using an optical lens in place of the optical fiber 3 of this embodiment and setting its focal point to be the light output end face of the magneto-optic thin film isolator.

In FIG. 1, which shows the configuration of the embodiment, the arrangement of the portion 6 covered with the metal film and the portion 5 not covered with the metal film of the rib-shaped waveguide of the magneto-optic thin film isolator 2 is reversed, that is, the semiconductor laser If the near side is covered with a metal film, only the TE component of the light polarization component that returns to the semiconductor laser 1 and re-enters remains, making it impossible to achieve the intended purpose.

As described above, according to the present invention, it is possible to obtain a highly reliable semiconductor laser module that is free from the influence of returning light to the semiconductor laser and has fewer optical components.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the principle configuration of an embodiment of the present invention, in which 1 is a semiconductor laser, 2 is a magneto-optic thin film isolator, 3 is an optical fiber, and 4 is an external magnetic field applied to the magneto-optic thin film isolator. FIGS. 2 and 3 are diagrams showing the behavior of polarization of light propagating in a magneto-optic thin film isolator.

Claims (1)

[Claims] 1 Consists of a semiconductor laser and a magneto-optic thin film isolator optically end-face connected to a light emitting end face of the semiconductor laser, and the thin film isolator has a substrate surface and a bonding surface of the semiconductor laser optically connected to each other. A single rib-shaped optical waveguide is arranged to form an angle of 45 degrees within the end face, and the upper surface is divided into two parts covered with a metal film and a part not covered with a metal film in the direction of light transmission. The composition of the crystal material constituting the optical waveguide and the geometric dimensions of the optical waveguide are such that the phase constants of two propagation modes with orthogonal polarization planes propagating through the optical waveguide are degenerate. A semiconductor laser module characterized in that: is set to a predetermined value.