JP3290474B2 - Optical isolator for semiconductor laser array - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a component of an optical transmission module used for optical fiber communication or the like.

[0002]

2. Description of the Related Art With the advance of optical communication using a semiconductor laser as a signal light source, high-speed and high-density signal transmission exceeding several hundred megahertz, which was impossible until now, has been put to practical use, and recent optical amplification has been realized. With the remarkable progress of technology,
A huge amount of information can be transmitted via an optical fiber without the need for photoelectric conversion, and in parallel with the technological advancement, there is an increasing demand for economical and low-cost construction systems. For high-speed optical communication for high-density transmission, up to 2.4 GBit / s has been implemented in an actual outdoor laying system, and 10 GBit / s has been confirmed at the laboratory stage, and the speeding up of digital signals has been promoted. In the future, higher transmission density is required, and parallel transmission including matching with a computer is expected.

As a semiconductor laser (LD) used for a high-speed signal, a distributed feedback laser (DFB-LD) or a distributed Bragg reflection laser (DBR-LD) is used as a single mode light source having an extremely narrow oscillation wavelength width. Can be Recently, an array LD in which a plurality of these narrow band LDs are constructed side by side has also been tried, and parallel communication has been confirmed at an experimental stage. This enables parallel optical communication, and at the same time, realizes a plurality of LD functions on one chip to form an array structure on the same substrate, so that the light source package can be incorporated into one module, thereby reducing the price of the optical communication system. It is expected to contribute to the development.

On the other hand, these narrow band LDs must be connected to an optical isolator for blocking reflected light in order to stabilize an oscillation signal. The optical isolator itself incorporates a Faraday rotator that rotates the plane of polarization of light by 45 degrees between a polarizer and an analyzer that are arranged to be rotated by 45 degrees with respect to each other. Faraday rotators are generally composed of rare-earth iron-based garnet crystals that exhibit a high Faraday effect, and are generated through complicated manufacturing processes such as crystal growth, rotation angle adjustment, and formation of an anti-reflection film, and other factors that hinder the cost reduction of optical isolators. It has become. Therefore, it is desired to use an optical isolator that allows a plurality of light beams to pass through each other, and an attempt to adopt an optical isolator in connection with an array LD is expected.

For example, Sekine et al., In the presentation of the C-275 lecture of the 1990 Spring Meeting of the Institute of Electronics, Information and Communication Engineers, used an ordinary commercially available optical isolator 1 to connect a 4-ch array LD2 to two confocal optical lenses 3, 4. The results of the coupling are reported (Fig. 2), and Sato et al. Presented the C-268 lecture at the 1992 IEICE Spring Conference, and the same number of array fibers as arrayed LDs with four LDs constructed in parallel. Are modularized with a pair of micro-flat lens arrays. However, there is no proposal for the optical isolator used, and when the number of parallel is further increased, the aperture diameter of the optical isolator must be increased, which inevitably increases the optical isolator diameter. As long as the isolator is used, the space occupied by the optical isolator portion is a factor that hinders parallelization.

[0006]

SUMMARY OF THE INVENTION The present invention provides a small-sized optical isolator which is applied between such an array LD and an array fiber and does not require an increase in the thickness of a module package. It has a rectangular cross-sectional structure flattened in the parallel direction. FIG. 1 shows a schematic structure of an example of the optical isolator of the present invention. A polarizer 5 having a rectangular light transmission cross-sectional shape and a rectangular analyzer 6 having the same shape as the polarizer and having a polarization direction rotated by 45 degrees with respect to the polarizer are arranged. This is an optical isolator for a semiconductor laser array comprising a rectangular Faraday rotator 8 having the same shape as a polarizer and an analyzer interpolated in the semiconductor laser array.

The technical problems of the optical isolator having this structure include adjustment of the polarization directions of the polarizer and the analyzer, the structure of a permanent magnet sufficient to magnetically saturate the Faraday rotator, and the polishing accuracy of the Faraday rotator. Adjustment of the polarization directions of the first polarizer and the analyzer cannot be performed in the process of assembling the optical isolator because the rectangular cross sections of the openings are rotationally displaced from each other. The base material is cut out so that the polarization direction is rotated. The manufacturing method is, for example, to adjust the polarizer and the analyzer to the orthogonal Nicol position, rotate one of them exactly 45 degrees, fix them to each other with an adhesive, etc. It can be formed by cutting into rectangular sections.

[0008] Of course, it is possible to form a pair of a polarizer and an analyzer rotated by 45 degrees with respect to each other with accuracy that can be adopted in the present invention by other methods. Only. That is, in the present invention, the polarizer,
It is meaningful to apply a polarizer-analyzer pair having such a shape that a 45-degree polarization direction difference can be automatically obtained by superimposing rectangular sections of the analyzer and aligning one side on a plane. Absorption-type polarizing elements are also effective in eliminating such adjustments and also in reducing the thickness of the optical path and contributing to the compact structure of the entire optical isolator. Of course, if the above-mentioned design points are overcome with other types of polarizers, the structure intended by the present invention can be established without inconvenience.

Second, it is necessary to study a permanent magnet for magnetically saturating the above-mentioned rectangular strip. FIG. 3 is a diagram showing the internal magnetic field distribution on the P plane which is the center of the inner surface of the permanent magnet 7 into which the rectangular section Faraday rotator is inserted, and has a width of 5.6 mm, a height direction of 3 mm, a length of 1.6 mm, and a magnet thickness. This is a result of analyzing a samarium-cobalt (Sm-Co) magnet of 0.8 mm, Br = 9KG, and an energy product of about 20MGOe as an example, and an almost uniform magnetic field is formed. In this case, the magnetic flux density is kept at 2KG or more over the entire cross section,
The Faraday rotator can be magnetized without any trouble.

The third problem is the polishing accuracy of the Faraday rotator. As already described in detail, in the present invention, since the polarizer analyzer pair itself is assembled with almost no adjustment, the accuracy of the polarization rotation angle is reduced. The extinction characteristic of is determined. A simple determination of the extinction characteristic is defined by the deviation angle △ θ with respect to 45 degrees, ignoring the polarization ellipse component generated when the light passes through the Faraday rotator. The extinction ratio ER is ER = −10 · log ₁₀ [sin ² (△ θ)]. If the ER value is 40 dB or more, the deviation angle △
θ must be limited to less than about 0.6 degrees. In the case of a Faraday rotator having a high Faraday rotation angle as used in the present invention, it is extremely effective for thinning the element itself, but there is a difficulty in controlling the rotation angle by polishing.

Actually, in the case of a Faraday rotator for a 1.31 μm band used for optical communication, the film thickness required for 45 ° rotation varies depending on the composition, but is approximately 200 to 300 μm.
Polishing accuracy is approximately ± 3μm to maintain below 0.6 degrees
Must be adjusted to the tolerance of Actual polishing tolerance is ±
A technique of controlling the thickness to 1 μm or less is possible, and if the polishing conditions are set, the reproducibility is sufficiently high. An optical isolator having a rectangular opening can be manufactured from the above technology integration. When defining the ratio of the width and height relative to the LD array to the degree of rectangle, if the ratio is 1.5 or more, it can be combined with a plurality of LD arrays, but the upper limit is not particularly defined. If the pitch of the fiber array is 250 μm, it can be connected to a 12-core fiber array by using an opening of 3 mm in the isolator having the rectangular through hole.

[0012]

EXAMPLE In order to form a rectangular rectangular polarizer analyzer pair,
Two pieces of polarizing glass are adhered to a disc-shaped glass substrate (diameter 2 inches), respectively, and placed on a separately prepared reference polarizing glass. One is adjusted to the orthogonal Nicol position, and the other is oriented to the substrate glass. After setting to the Nicol position, the head is rotated 45 degrees with reference to the rotary stage before orientation. Next, dicing is performed along the direction to a desired rectangular ratio. Since the dicing blade is adjusted to a position where there is no blur and the type of the blade is matched, the cut surface is finished with an accuracy comparable to optical polishing. Naturally, the conditions for cutting the polarizing glass and the Faraday rotator are different.

In this embodiment, the shape of the opening is 6 m wide.
m, the height was set to 1.4 mm, and a polarizer, analyzer and Faraday rotator were manufactured. The thickness accuracy of the Faraday rotator at this size was polished to within a tolerance of ± 1.2 μm on the same film surface, but was finished to ± 3 μm between other polished rectangular Faraday rotators at the same time. Was. Of course, it is obvious that such accuracy is a kind of technology that should be significantly improved by the polishing apparatus and its conditions, and that there is no factor that makes the gist of the present invention unrealizable. The Faraday rotator used in this embodiment is BiLu by the LPE method.
The film thickness required at 45 degrees in the 1.31 μm band of the DyGaFeO-based garnet film is 280 μm, and the accuracy of ± 3 μm corresponds to a deviation angle of ± 0.5 degrees or less.

As shown in FIG. 4, the size of the permanent magnet is
Two plate-shaped magnets 9 having a thickness of 8 mm, a thickness of 0.7 mm, and a length of 2 mm in the magnetic field direction were bonded and fixed to a metal jig 10 so that a gap between the magnets was 1.4 mm. Of course, there is no problem with a structure in which a rectangular magnet is formed by integral molding. FIG. 5 is a diagram showing a magnetic field distribution along the width direction of the center of the magnet. The flat part of about 1.9KG is about 6 in the center
mm is maintained, and it can be confirmed that a sufficient magnetic field strength can be achieved with the divided structure. In this embodiment, the residual magnetic flux density Br
= 11KG, SmCo sintered magnet with a maximum energy product of 28MGOe was applied.

The above components were inserted and fixed inside the rectangular box-shaped magnet. Polarized glass etc. is about 0.5 mm thick,
All parts were housed inside the magnet, and as a result, the optical isolator was finished in a flat structure with a height of 2.9 mm, a width of 9 mm, and a length of 2 mm in the light transmission direction. Even if they are mounted on a holder to protect them, they can be formed without much difference. Optical characteristics are distributed by coupling a collimator beam with a beam waste diameter of about 0.6 mm between two opposing fibers via a lens, inserting a rectangular optical isolator between them, and moving it by 1 mm in the width direction. Was measured. FIG. 6 shows the measurement results of the insertion loss (LF) and the reverse insertion loss (LB). Forward loss is less than 0.5dB,
The reverse loss is also maintained at 40 dB or more, confirming that the configuration of the present invention is useful as an optical isolator having an effective range of 6 mm or more in the width direction of the opening.

[0016]

The present invention provides an optical isolator which can be used for super-parallel transmission assuming the connection between an optical fiber and a computer while reducing the cost of an LD module. We can expect contribution.

[Brief description of the drawings]

FIG. 1 is a perspective view of an optical isolator decomposition example of the present invention.

FIG. 2 is a schematic diagram of a conventional array LD module.

FIG. 3 is a diagram illustrating a magnetic field strength analysis of a permanent magnet mounted on the present invention.

FIG. 4 is a perspective view of a structural example of a permanent magnet mounted on the present invention.

FIG. 5 shows a distribution diagram of a magnetic field intensity in the width direction inside the permanent magnet.

FIG. 6 shows a distribution diagram of the lateral optical characteristics of the optical isolator according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical isolator 2 4ch-array LD 3 Confocal system optical lens 4 Confocal system optical lens 5 Polarizer 6 Analyzer 7 Permanent magnet 8 Faraday rotator 9 Plate magnet 10 Metal jig

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G02B 27/28

Claims (2)

(57) [Claims] A polarizer 1. A light transmittance sectional shape is rectangular, the a polarizer equal shape, arranged analyzer rectangular having a direction of polarization direction is rotated 45 degrees with respect to the polarizer, during which A rectangular filter of the same shape as the polarizer and analyzer.
An optical element composed of an Faraday rotator is
Magnet with a rectangular through-hole into which an optical element can be inserted
Wherein the magnetization direction is parallel to the light transmission axis direction.
An optical isolator for a semiconductor laser array , comprising a hollow magnet body . 2. A polarizer having a rectangular light transmission cross-sectional shape.
And the same shape as the polarizer, and the polarization direction is
A rectangular analyzer with an azimuth rotated by 45 degrees
In the meantime, a rectangular filter having the same shape as the polarizer and analyzer
An optical element composed of an Faraday rotator is
A permanent magnet that interpolates the optical element from the
A pair of strip-shaped plate magnets at the lower part, and supporting the magnets
The optical element is constituted by a pair of metal fittings,
Magnet body, the magnetization direction is parallel to the light transmission axis direction
Characterized by being composed of a box-shaped hollow magnet body
For semiconductor laser arrays.