JPS61132925A - Semiconductor laser module with optical isolator - Google Patents

Abstract

PURPOSE:To mount easily an optical isolator without degrading the efficiency of coupling to an optical fiber by inserting the optical isolator consisting of a polarizer, a Farady rotator consisting f a magnetic body and a magnet, and an analyzer between the first and the second converging rod lenses. CONSTITUTION:With respect to an optical coupler which couples a semiconductor laser 21 and an optical fiber 22 optically, an optical isolator 26 is inserted between the first converging rod lens 23 whose end face on the side of the semiconductor laser 21 is a convex surface and the second converging rod lens 24 whose both end faces are plane surfaces, and the optical isolator 26 consists of a polarizer 27, a magnetic material thin film 28 and a magnet 30 which constituted the Farady rotator, and an analyzer 29, and the plane of polarization of an output light 25 is rotated at 45 deg. by the Farady rotator. By this constitution, the reflected light from the external is prevented from being charged again to the semiconductor laser 21 with the capacity of 0.7dB insertion loss and 25dB isolation.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a module, and particularly to a semiconductor laser module with an optical inulator that prevents reflected light from the outside from being recombined into the semiconductor laser.

[Prior art]

Semiconductor lasers (LDs) have many features such as low current drive and high-speed direct modulation, and are seen as promising light sources for various types of original fiber communication systems, including long-distance, high-capacity optical fiber communication systems. It is known that when the optical output is reinjected due to reflection or the like, unstable operating characteristics are exhibited. Therefore, in order to realize a high quality and highly reliable optical fiber communication system, it is necessary to prevent the reinjection of the LD output power.

Re-injected light is mainly caused by reflection due to refractive index mismatch in the transmission path, so for the above purpose, for the optical fiber incidence direction, for example, ■ formation of an anti-reflection film, ■ incidence It is effective to tilt the surface or make it more convex, or (2) insert an optical isolator consisting of a 174 wavelength plate and a polarizer between the LD and the optical fiber.

However, all of these measures are antireflection measures only for the input surface of the optical fiber, and cannot eliminate reflections from other surfaces (the connecting portion and the output surface of the optical fiber). To remove reflected light at the optical fiber connection, there are three methods: ■ Forming an antireflection film on the two optical fibers to be connected, ■ Tilting the input and output surfaces, and ■ Using a refractive index matching body (liquid, film, etc.). Scissors,■
Further measures such as using close-fitting connectors or fusion splicing are required. These measures are considered undesirable because they are not only troublesome but also impose various restrictions on the configuration of the optical fiber transmission line.

Therefore, in order to realize stable LD operation in optical fiber communication, it is considered desirable to use an optical isolator that utilizes the Faraday effect, which can prevent reflected light from returning.

A representative example of this wavelength 1 μm band is Kobayashi-Koshiro's IEE Journal Op ΦQuantum Electronics (IEEE J+Quant).
um Electronics), 1980 QE-
There is an optical isolator described in Vol. 16, page 11. FIG. 2 shows a cross-sectional view of the structure of an optical isolator using a Rochon prism. This is a YIG crystal (YsFesOt), which is a bulk magnetic material 11 with a Faraday rotation angle of 10.
A Rochon prism with a polarizer 12° and an analyzer 13 is placed on both sides of the x). Generally, when light passes through the magnetic body 11 surrounded by the magnets 14, the plane of polarization rotates, and the rotation angle is expressed by the following equation.

θ=FIIl@M/M8 j is the passing distance sM8 is the saturation magnetization, and is a component of the magnetization in the passing direction of the MRI element. Now, apply a magnetic field in the direction of light passage,
When saturated, θ=Fl, and the Faraday rotation angle becomes the largest. In YIG crystal, at wavelength L3μm, F is 22
Because it is 0°/calculation.

In order to obtain a rotation angle of the polarization plane of 45°, the length of the magnetic body is approximately 2IIIm. Therefore, this magnetic material 11
Polarizer 12. Analyzer 13. Including the magnet 14, the length of the optical isolator is 4 to 5 m, so the distance between the semiconductor laser and the optical fiber must not be large. An effective way to increase this distance is to
There is a method of shortening the lens pitch and using a lens with a long focal length.

However, shortening the gap, gap, and gap will shift the coupling system with the optimal lens pitch, thereby degrading the coupling efficiency to the optical fiber, resulting in the problem that the effect of inserting an optical isolator cannot be fully utilized. there were.

[Purpose of the invention]

An object of the present invention is to provide a semiconductor laser module with an optical isolator that eliminates the above-mentioned drawbacks and can be easily mounted without deteriorating the coupling efficiency to the source fiber.

[Structure of the invention]

A semiconductor laser module with an optical inulator according to the present invention includes a semiconductor laser and an end face facing the semiconductor laser, each having a convex curved surface whose refractive index decreases with a substantially square distribution with respect to the distance from the central axis. In a non-confocal fiber coupler, the first and second focusing rod lenses each have a first focusing rod lens attached to the lens, and a second focusing rod lens having both flat end surfaces and one end surface close to the optical fiber. Between the focusing rod lenses, there is a polarizer, a Faraty rotator consisting of a magnetic material and a magnet,
It is characterized by the structure in which an optical isolator is inserted from the analyzer, and the magnetic material can be formed from a magnetic thin film formed by epitaxial growth.

[Principle and operation]

In the present invention, two focusing rod lenses are used as means for optically coupling the semiconductor laser and the optical fiber. The first focusing rod lens, which has a convex curved surface on the side closer to the semiconductor laser, magnifies the output light from the semiconductor laser, and the second focusing rod lens, which has a flat end face, reduces the output light to a spot size of 7 eye diameters. A non-confocal coupling method is used in which the optical axis direction is adjusted to match. The second focusing rod lens and the optical fiber are integrated in contact with each other. Since the distance between the first and second focusing rod lenses is usually about 5 cm, it is possible to easily insert an optical isolator with a length of about 4 m between them. A particularly important optical isolator is the Faraday rotator. When the wavelength of the output light is 1.3 μm, in the case of a bulk crystal which is a magnetic material, a thickness of about 2 m and 1 degree is required.

Therefore, when the magnetization is saturated, a large demagnetizing field is generated, and an external magnetic field of about 2000 Qe (Oersteds) is required, so a large magnet is required. However, since a bulk crystal is used, there is an advantage that the wavelength range when achieving high isolation can be expanded. On the other hand, if a magnetic thin film formed by epitaxial growth is used instead of a bulk crystal, there is almost no need to consider the effects of demagnetizing fields, so a small magnet can be used, and the optical isolator can therefore be made small and inexpensive. realizable. In this case, the wavelength range of high isolation is relatively narrow because it depends on the size of the isolation or the size of the crystal. Therefore, an optical isolator that only needs to be inserted between the first and second focusing rod lenses can be realized in any configuration depending on performance, size, and price. Furthermore, since the first focusing rod lens functions to magnify the image, even if an optical isolator is inserted, the coupling efficiency to the optical fiber will not be deteriorated.

〔Example〕

Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows a side sectional view of a semiconductor laser module with an optical isolator which is an embodiment of the present invention.

The optical coupler for optically coupling the semiconductor laser 21 and the optical fiber 22 has a first end whose end face on the semiconductor laser 21 side is a convex curved surface.
and a second focusing rod lens 24 having flat end surfaces. The refractive index distribution of the focusing rod lens used in this example is n = nQ (1-7
−a r2).

Here, n6 represents the refractive index on the central axis 50, a represents the focusing parameter, and r represents the distance from the central axis 50. For focusing rod lenses, nQ is 15 to 1.63. a is 0.1
~0.2 m, and r is in the range H of 0.5 to 1 ttll.

The first focusing rod lens 23 magnifies the output light 25 from the semiconductor laser 21 by about 10 times, and the second focusing rod lens 24 adjusts the direction of the optical axis 51 to match the spot size of the original fiber 22. do. Further, the original fiber 22 used had a core diameter of 10 μm and a to-off wavelength of 1 μm, and the semiconductor laser 21 had an oscillation wavelength of 1.3 μm. In this case, since the distance between the lenses is approximately 5+m, the optical isolator 26 can be inserted between the lenses.

The optical isolator 26 includes a polarizer 27177, a magnetic thin film 28 and a magnet 30, which constitute a Rade rotator. In this embodiment, the analyzer 29 is fixed close to the flat end surface side of the first focusing rod lens 23.

Most of the output light 25 from the semiconductor laser 21 is linearly polarized TE mode, and the ratio of normal TE mode to TM mode is about 25 dB. Therefore, the polarizer 27 is arranged with its polarization planes aligned so that the output light 25 can be sufficiently transmitted. The magnetic thin film 28 is a 330 μm thick 0YIG thin film manufactured by epitaxial growth. YIG
The sides of the thin film are surrounded by hollow magnets 30 so that a magnetic field is applied in a direction perpendicular to the film surface. The plane of polarization of the output light 25 is rotated by 45° by this 7A2D rotator.

The analyzer 29 is arranged so that the polarization directions thereof are inclined at 45 degrees with respect to the polarizer 27 so that the output light 25 that has passed through the magnetic thin film 28 can sufficiently pass therethrough.

Inserting the optical isolator 26 having the above-mentioned configuration between the lenses makes it possible to prevent reflected light from the outside from being re-injected into the semiconductor laser 21 with an insertion loss of 0.7 dB and an isolation performance of 25 dB. did it.

In this example, since the magnetic thin film 30 manufactured by the epitaxial growth method is used, the strength of the saturation magnetic field required to rotate the plane of polarization by 45'' is only about 3ooOe, and therefore the optical isolator is thin and small. It can be mounted in a .26 gold optical coupler.In addition, the deterioration of coupling efficiency due to the presence or absence of an optical isolator is reduced by the insertion loss of the isolator.
．． Since it was possible to suppress the noise to only 7 dB, it was found that the coupling efficiency was not deteriorated more than necessary.

In addition to the typical embodiments described above, several modifications of the embodiments of the present invention are possible. For example, in this embodiment, the polarizer 27
, a Faraday rotator, and an analyzer 29 are used, but the polarizer 27 may be omitted in consideration of the fact that the output light 25 of the semiconductor laser 21 is linearly polarized light.

In this case, the linearly polarized light corresponding to the reflected light dTM mode will be re-injected into the semiconductor laser 21, but since the resonator of the semiconductor laser 21 is occupied by TE mode light, it will not be affected in any way. do not have.

Further, in the above embodiment, the optical isolator 26 is arranged close to the flat end surface side of the first focusing rod lens, but if it is located between the first focusing rod lens 23 and the second focusing rod lens 240, For example, any arrangement may be used. For example, no difference in characteristics can be seen even if the components of the optical isolator 26 are separated into two or three parts and placed in the space between the lenses, or if they are placed separately and close to each other on the flat end surfaces of the two lenses. do not have. Furthermore, in this embodiment, the magnetic thin film 2
Although YIG was used as 8, it goes without saying that it is not particularly limited to YIG. Furthermore, although the first and second converging rod lenses 23 and 24 were used as optical couplers, a combination of a ball lens and a ball lens or a combination of a ball lens and a convergence rod lens may be used instead. There is no particular limitation except that a confocal coupling circuit is used.

Further, in the above embodiment, the magnetic thin film 30 was limited, but 3
Needless to say, a bulk crystal may be used to obtain high isolation of 0 dB or more.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to obtain a semi-dedicated thermosilicator with an original isolator in which an optical isolator can be easily mounted without deteriorating the coupling efficiency with the original fiber.

[Brief explanation of drawings]

FIG. 1 is a side sectional view of an embodiment of the present invention, and FIG. 2 is a sectional view showing an example of the structure of a conventional original isolator. lO°°°°゛°Faraty rotor, 11...
Bulk magnetic material, 12...Polarizer, 13 Old...Analyzer, 14...Magnet, 21...
Semiconductor laser, 22... Optical fiber, 23...
...First focusing rod lens, 24゛°°°second
Focusing rod lens, 25...output light. 26... Optical isolator, 27... Polarizer, 28... Magnetic thin film, 29...
Analyzer, 3o... Magnet, 5o... Central axis, 51... Optical axis.

Claims (2)

[Claims] (1) A semiconductor laser and a first convergence feature with a convex curved surface, in which the end face facing the semiconductor laser is formed in a convex curved shape, and the refractive index decreases with approximately square distribution with respect to the distance from the central axis. A non-confocal optical coupler comprising a rod lens and a second focusing rod lens having flat end surfaces and one end surface close to an optical fiber, wherein the first focusing rod lens and the second focusing rod lens A semiconductor laser module with an optical isolator, characterized in that an optical isolator consisting of a polarizer, a Faraday rotator made of a magnetic material and a magnet, and an analyzer is inserted between the focusing rod lenses. (2) The semiconductor laser module with an optical isolator as set forth in claim 1, wherein a magnetic thin film formed by an epitaxial growth method is used as the magnetic material of the Faraday rotator.