JPH0695050A - Faraday rotor for nonreciprocal optical device and its production - Google Patents

Abstract

PURPOSE:To miniaturize the sectional area of a ray direction and to microminiaturize the Faraday rotor by previously forming plural grooves of a desired shape and size on a substrate surface, growing and packing R1RIG films therein and cutting out the films orthogonally with the longitudinal direction thereof. CONSTITUTION:The plural rectangular groove parts having >=20mum width and depth are previously formed by etching or grinding, etc., on the surface of the wafer-shaped substrate 1 consisting of garnet single crystal. The films of liquid epitaxial crystals are formed in these groove parts up to the positions where the thicknesses of the films formed therein correspond to the original substrate surface and the liquid epitaxial crystal films are removed down to the substrate surface. The liquid epitaxial Bi substd. rare earth garnet crystals grown in the remaining groove parts are cut in the direction approximately orthogonal with the longitudinal direction of the remaining groove parts in order to obtain the length at which the Faraday rotating angle necessary for the nonreciprocal optical device is obtainable and the cut faces are optically polished in order to form incident and exit faces for rays. Antireflection films 3 are formed on these surfaces. Then, the full-surface antireflection film is formed on the R1RIG parts and the reliability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is mounted on a transmission module for optical transmission used for optical fiber communication or the like, a non-reciprocal optical device mounted between optical transmission fibers, or a sensor utilizing various magneto-optical effects. Faraday rotator.

[0002]

2. Description of the Related Art With the progress of optical communication using a semiconductor laser as a signal light source, high-speed and high-density signal transmission exceeding several hundreds of megahertz, which has been impossible up to now, has been put into practical use, and a recent optical amplification technology has been developed. Due to the remarkable progress, a huge amount of information can be transmitted through the optical fiber without the need for photoelectric conversion, and in the future, along with the technical sophistication, the demand for economical cost reduction of the constituent system is increasing. In these optical communication systems, a reflected return light blocking optical isolator used for signal stabilization has been recognized as an indispensable component, and various commercially available optical isolators have been applied.
However, although various components such as a semiconductor laser are being promoted to be low in price along with the expansion of demand, the price of the optical isolator is still high, which prevents cost reduction of the entire system.

The components of an optical isolator are basically
It is composed of two polarizing elements (polarizer and analyzer), Faraday rotator, permanent magnet and exterior holder. In these components, polarizing elements and Faraday rotators dominate the cost. In particular, the Faraday rotator is expensive. Faraday rotators currently used in optical isolators are yttrium iron garnet: YIG (Y ₃ Fe ₅ O ₁₂ ) produced by a floating zone crystal growth method and Bi-substituted rare earth iron garnet: BiRIG (BiRIG) in which a part of the rare earth element is replaced with Bi. However, part of iron is Ga, Al, Ca, Mg, Ti,
It may be replaced with other substances such as Zr, but it is simplified and BiRI
There is G).

At the beginning of the development of optical isolators, bulk YIG crystals were used in most optical isolators.
Compared with the Faraday rotator made of glass, which has been used before that, it has been remarkably miniaturized and has excellent optical characteristics, which has contributed to the expansion of demand for optical isolators. On the other hand, various production methods have been attempted since it was confirmed that the substitution of Bi has the effect of increasing the Faraday rotation ability, but the liquid phase epitaxial growth method (LPE method) is now generalized. Initially, the production was expanded as a Faraday rotator for optical isolators up to a film thickness of several μm on the substrate, and it is a material that is expected from the viewpoint of mass productivity and economy.

[0005]

However, the problem with BiRIG by the LPE method is that the crystallinity deteriorates as the growth film thickness increases, and pits, segregation, cracks, etc. are induced, making it impossible to maintain the optical characteristics. The occurrence of such defects depends on the film forming conditions, but empirically the film thickness is 300
It has been observed that there is a gradual generation from around μm and a gradual expansion to the entire film surface. The Faraday rotation of BiRIG by the LPE method currently on the market is approximately 2000 degrees / c.
When a magneto-optical rotator with a Faraday rotation angle of 45 degrees is applied to the optical nonreciprocal part, the required film thickness is 1310 nm.
The wavelength band is about 200 μm, and the long wavelength band of 1550 nm is about 400 μm.

In practice, since the above-mentioned crystal defect portion is excluded and the homogeneous crystal portion is selected and used, the yield of the Faraday rotator is low in the Faraday rotator for the 1550 nm band, which requires a thick film. As a result, it has been an obstacle to the cost reduction of the Faraday rotator. Moreover, the currently adopted L
For the BiRIG film by the PE method, a method of cutting the substrate surface into a checkerboard pattern into a required size as shown in FIG. 2 is adopted. When an LPE film is formed on a 2-inch substrate and a 2 mm square Faraday rotator is manufactured, the blade thickness of the cutting device is also taken into consideration to be about 290, and even if multiple growth furnaces are adopted in parallel, the price is low. It is not the production capacity and production method that are reflected in the realization.

[0007]

SUMMARY OF THE INVENTION In order to reduce the price of a Faraday rotator, the present invention is a BiRIG method using the LPE method.
In the step of forming the film, a plurality of trench grooves having a desired shape and size are formed in advance on the substrate surface, the BiRIG film is grown and filled in the grooves, and the grooves are cut in a direction substantially orthogonal to the groove longitudinal direction. By expanding the number of Faraday rotators taken out per sheet, and by reducing the cross-sectional area in the direction of the light beam to a size equivalent to that of the optical fiber so that it can be used in close proximity to the fiber, the actual size of the Faraday rotator required for nonreciprocal optical devices can be It consists of minimization.

That is, the first point of the present invention is that T.Ao
Reported by yama et al. (NEC Research & Development NO.6
6 July 1982 / P44-49), the light is transmitted not in the film growth direction but in the direction perpendicular to the film growth direction within the film thickness that does not cause crystal deterioration due to thickening during LPE growth. It is not necessary to grow to a film thickness that induces crystal deterioration because the utilization method is used. As a result, it is premised that the film-forming yield of the conventional method can be significantly improved, but by forming a trench groove with a width of 20 μm or more and a depth of 500 μm or less on the substrate surface and growing BiRIG by the LPE method in the groove. is there. Here, the structure of T. Aoyama is that the LPE grown film is simply cut in a state in which it is attached to the substrate, and the light beam is propagated in the in-plane direction of the substrate as it is.
The G side causes surface sagging or chipping, and it is difficult to form a Faraday rotator with a minute cross section as desired in the present invention.

On the other hand, in the case of the trench groove structure proposed by the present invention, the cut pieces are protected on three sides by the substrate garnet crystal, and accurate transmission cross-section polishing can be realized. However, since the transmission cross section in the light ray direction is limited by the film thickness, the maximum effective light ray diameter is necessarily limited to 500 μm or less. Of course, if the film thickness of 500 μm or more can be grown without crystal defects, no problem will occur in the reproducibility of the present invention.
The object of the present invention is satisfied to the extent not exceeding. On the other hand, the minimum ray effective diameter is regulated from the core diameter of the optical fiber to be connected to the non-reciprocal optical device, and is determined from the size that can shield 10 μm of the single mode fiber diameter, but considering the ray propagation diameter. 20 μm or more is desirable.

[0010]

[Example] As a substrate material, a part of constituent elements having a wafer thickness of 1.2 mm was replaced with Ca, Mg, Zr or the like to obtain a lattice constant of 12.49
The surface of the substitutional type Gd ₃ Ga ₅ O ₁₂ (SGGG) crystal wafer, which has reached a certain level, was subjected to trench groove processing using a diamond blade, and an etching treatment was carried out to smooth the surface in a phosphoric acid solution. FIG. 3 shows a local cross-sectional structure of the substrate surface after the groove processing. The trench groove had a substrate portion of 400 μm left on both sides, the groove width was 400 μm, and a cutting margin of about 300 μm was considered so that the film thickness in the light transmitting direction could be 45 ° at 1550 nm. 30 trench grooves could be formed on the 2-inch substrate.

Next, BiRIG is grown on the groove side by LPE, and then the surface of the LPE growth side is ground and polished to produce the first SG.
It was removed until the surface of the GG substrate appeared. The Faraday rotation capability of BiRIG formed in the trench groove was about 1100 degrees / cm in the wavelength band of 1550 nm. Therefore, it takes about 400 μm to achieve a Faraday rotation angle of 45 degrees, and one Faraday rotator can be formed with 700 μm including the cutting margin. It was found that about 1400 elements can be formed on the entire substrate, which is five times as many as the conventional method. FIG. 1 shows the structure of the Faraday rotator 2 after being cut out from the substrate 1 and optically polished.

The surface accuracy of the BiRIG portion is processed to λ / 10 or less according to the measurement by the interferometric surface measuring device, and almost no chipping of the edge portion is recognized. In FIG. 1, the semicircular hatched portion is the antireflection film 3, and since the antireflection film can be formed while holding the substrate 1 portions surrounding the three sides,
The entire surface of the BiRIG portion can be an antireflection film, and a highly reliable film can be obtained. When removing the substrate portion, it is removed at this point.

When applied to an optical isolator for a semiconductor laser (LD) module, the laser beam must be sufficiently narrowed because the Faraday rotator has a small cross section. In the conventional configuration, the LD emission light beam is transmitted through one lens arranged at the confocal position, propagates through a polarizer, a Faraday rotator, and an analyzer, and passes through a lens arranged on the fiber side to obtain a single mode fiber (SMF). ) To. Therefore, in order to suppress the spatial propagation loss because the propagation distance becomes large, it is necessary to expand the LD light beam once and converge it on the SMF core part, and it is also necessary to enlarge the optical isolator component parts. It is desirable to consider an optical coupling system in order to apply a child.

On the other hand, FIG. 4 shows a single lens combination in which one lens 5 is arranged on the LD 4 side, and in this case SMF.
Just before 6, the light rays are sufficiently converged, and the optical isolator 7 can be used without any problem even if the transmission part has a small aperture diameter. FIG. 5 shows the structure of an optical isolator equipped with the Faraday rotator of the present invention. The Faraday rotator 2 has a rectangular section of 0.4mm and 0.3mm with the substrate part surrounding three sides removed, and an outer diameter of 1.4m.
Ring type SmCo magnet 8 with m, inner diameter 0.5mm and thickness 2mm in the magnetic field direction
The analyzer 9 was arranged on the light exit side of the magnet, and the polarizer 10 was fixed on the entrance side at the optimum optical position after rotation adjustment. The length from the SMF end to the lens side exit end is about 1.6m
When the beam is coupled to the SMF by an aspherical lens or the like, the beam diameter is already 200 μm or less, and the beam can be coupled to the SMF without loss in the isolator section.

[0015]

INDUSTRIAL APPLICABILITY The present invention makes a great contribution to providing a low-cost structure of an LD module, and at the same time, is applied to a long wavelength of 1550 nm band where uniform crystal growth was difficult in the conventional manufacturing process. The Faraday rotator can be easily manufactured by the manufacturing method, and it is possible to provide a Faraday element having excellent reliability, which is expected to make a great contribution to optical transmission in the future.

[Brief description of drawings]

FIG. 1 shows a perspective view of the structure of a Faraday rotator element of the present invention.

FIG. 2 is an explanatory view of an example of cutting out a conventional LPE method Faraday rotator.

FIG. 3 shows a perspective view of a cross-sectional structure after processing a substrate trench groove according to the present invention.

FIG. 4 shows a schematic view of an LD module using a single lens convergent beam.

FIG. 5 is an exploded view of an optical isolator for an LD module using the Faraday rotator of the present invention. 1 substrate 2 Faraday rotator 3 antireflection film 4 LD (semiconductor laser) 5 lens 6 single mode fiber 7 optical isolator 8 ring magnet 9 analyzer 10 polarizer 11 fixed ring

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[Procedure amendment]

[Submission date] December 2, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 shows a perspective view of the structure of a Faraday rotator element of the present invention.

FIG. 2 is an explanatory view of an example of cutting out a conventional LPE method Faraday rotator.

FIG. 3 shows a perspective view of a cross-sectional structure after processing a substrate trench groove according to the present invention.

FIG. 4 shows a schematic view of an LD module using a single lens convergent beam.

FIG. 5 is an exploded view of an optical isolator for an LD module using the Faraday rotator of the present invention.

[Explanation of symbols] 1 substrate 2 Faraday rotator 3 antireflection film 4 LD (semiconductor laser) 5 lens 6 single mode fiber 7 optical isolator 8 ring magnet 9 analyzer 10 polarizer 11 fixed ring

Claims (3)

[Claims] 1. A Faraday rotator for a nonreciprocal optical device comprising a liquid phase epitaxial Bi-substituted rare earth iron-based garnet crystal film formed on the surface of a wafer-shaped garnet single crystal substrate, wherein the surface of the wafer-shaped garnet single crystal substrate is 20.
In a plurality of rectangular grooves having a width and depth of μm or more,
Liquid phase epitaxial Bi-substituted rare earth iron garnet crystal formed to a position where the film thickness corresponds to the surface of the substrate, and the Faraday rotation angle required for a nonreciprocal optical device was obtained in a direction approximately orthogonal to the longitudinal direction of the groove. A Faraday rotator that has been cut into pieces and has been subjected to optical polishing to make the cut surfaces into the entrance and exit surfaces of light rays and an antireflection film is applied. 2. A surface of a wafer-shaped garnet single crystal substrate is previously etched in a Faraday rotator for a nonreciprocal optical device, which is formed on a surface of a wafer-shaped garnet single crystal substrate and comprises a liquid phase epitaxial Bi-substituted rare earth iron-based garnet crystal film. Or after forming a plurality of rectangular groove portion having a width and depth of 20μm or more by grinding or the like, liquid phase epitaxial crystal film formation is performed up to a position where the film thickness formed in the groove portion corresponds to the original substrate surface, The liquid phase epitaxial crystal film is removed up to the substrate surface, and the liquid phase epitaxial Bi-substituted rare earth iron-based garnet crystal grown in the residual groove portion has a Faraday rotation angle necessary for the nonreciprocal optical device, so that the length can be obtained. The remaining groove is cut in a direction substantially orthogonal to the longitudinal direction, and optical polishing is performed to make the cut surface the light entrance and exit surface. A method of manufacturing a Faraday rotator, which comprises applying an anti-reflection film. 3. The Faraday rotator according to claim 1, wherein a substrate crystal forming a groove wall is removed and a cross-sectional area in a light transmitting direction is equivalent to a cross section of an optical fiber.