JPH11174378A - Optical isolator and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make manufacturable an optical isolator which has excellent optical characters without making any special adjustment by bringing the end surfaces of joint units into contact with each other and then making extremely small the gap or overlap of a 1/2-wavelength plate and to make adaptable the optical isolator to mass-production by cutting a large-size 1/2-wavelength plate and a Faraday rotator to specific size after sticking them together. SOLUTION: The 1/2-wavelength plates of the 1st joint unit A and 2nd joint unit B having the 1/2-wavelength plates and Faraday rotators adhered and fixed on optical surfaces are arranged oppositely to each other and the end surfaces P and Q perpendicular to the optical surfaces of the units are brought into contact; and a half of incident light is transmitted through the 1/2-wavelength plate of the 1st joint unit and the remaining half of the light is transmitted through the 1/2-wavelength plate of the 2nd joint unit. In this case, the angle that the phase delay axis D of the 1/2-wavelength plate of the 1st joint unit and the phase delay axis D of the 1/2-wavelength plate of the 2nd joint unit contain is 45 deg. and the angle of rotation of the Faraday rotator is 45 deg..

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a first half-wave plate, a Faraday rotator and a second half-wave plate which are sequentially arranged, and one-half of the incident light is the first one-wave plate. A half-wave plate is transmitted, and the remaining half of the light is transmitted through a second half-wave plate.
The present invention relates to a polarization-independent optical isolator in which an angle between a slow axis of a half-wave plate and a rotation angle of a Faraday rotator is 45 ° and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years,
With the rapid development of optical amplification technology in optical communication systems, there is an increasing demand for miniaturization and cost reduction of polarization-independent optical isolators that are components thereof. As a polarization independent optical isolator satisfying such requirements, NTT
An isolator that does not use a birefringent crystal plate has been proposed (1997 IEICE General Conference C-3-9)
8).

This isolator has a structure in which optical elements are arranged in the order of a half-wave plate / a Faraday rotator / a half-wave plate. The two half-wave plates are arranged facing each other. , So that only half of the incident light passes therethrough.

In general, an inexpensive quartz plate can be used as a half-wave plate. The half-wave plate made of quartz has a thickness of about 90 μm, and can be considerably thinner than a birefringent element used in a conventional polarization-independent optical isolator (using rutile). The thickness of the obtained birefringent element is about 800 μm). Because of these features,
Practical application as a small and low-cost optical isolator is desired.

One of the important requirements in putting this optical isolator into practical use is that as the gap (or overlap) formed when two half-wave plates are projected in the optical axis direction becomes closer to 0, the light is extinguished. This means that an optical isolator having an excellent ratio and insertion loss can be obtained (if the gap or overlap between the two wave plates exceeds 2 μm, sufficient optical characteristics cannot be obtained). However, if the processing accuracy of parts is raised to the submicron order, an increase in cost is unavoidable. Usually, when a commonly used dicer is used, the dimensional accuracy of the cutting process is about ± 5 μm. Therefore, it has been a problem to develop an optical isolator having sufficient optical characteristics even if components having such a dimensional accuracy are used.

[0006]

Means for Solving the Problems and Embodiments of the Invention In order to solve the above problems, the present invention provides (1) a joining unit 2 in which a half-wave plate and a Faraday rotator are adhesively fixed on an optical surface. Are arranged such that the respective half-wave plates of the first bonding unit and the second bonding unit are located on the opposite sides, and the end faces perpendicular to the optical surface of each unit are in close contact with each other. In this structure, half of the incident light passes through the half-wave plate of the first junction unit, and the remaining half of the light passes through the half-wave plate of the second junction unit. The angle between the slow axis of the half-wave plate of the first joining unit and the slow axis of the half-wave plate of the second joining unit is 45.
, A polarization independent optical isolator characterized in that the rotation angle of the Faraday rotator is 45 °, and (2) 1 /
A cutting unit in which the two-wavelength plate and the Faraday rotator are bonded on the optical surface is manufactured, and the cutting unit is cut into a predetermined size, and the obtained two cutting units are used as first and second joining units. Used, each half-wave plate of the first joining unit and the second joining unit were arranged so that they were located on opposite sides, and the end faces perpendicular to the optical surface of each unit were brought into close contact with each other. A method for manufacturing the polarization independent optical isolator according to the above (1) is provided.

According to the present invention, the gap (or overlap) between two half-wave plates can be adjusted to a practically minimum even if a polarization-independent optical isolator is constituted by components having poor dimensional accuracy. This makes it possible to obtain an isolator having good optical characteristics. As a result, the material cost can be reduced and the assembly yield can be improved, and the cost and size of the isolator can be reduced.

Hereinafter, the present invention will be described in more detail. FIG. 1 shows the structure of a polarization independent optical isolator to which the present invention is applied. In the figure, reference numeral 1 denotes a Faraday rotator, on each side of which a first half-wave plate 2 and a second half-wave plate 3 are provided. Lenses 4 and 5 and optical fibers 6 and 7 are provided in front of and behind 2, 3, respectively. In the drawing, Lc is an optical axis (center line), and L is an optical path.

[0009] This optical isolator is 1/1 of the incident light.
2 is a structure that transmits the first half-wave plate 2 and the remaining half of the light is transmitted through the second half-wave plate 3. The angle between the phase axis and the slow axis of the second half-wave plate 3 is 45 °, and the rotation angle of the Faraday rotator 1 is 4 °.
5 °.

As shown in FIG. 2, this optical isolator has a gap s (see FIG. 2A) formed when two half-wave plates 2 and 3 are projected in the optical axis direction. Overlap p
(See FIG. 2B) is desired to be as small as possible.
However, it is not easy to make the gap s (or the overlap p) zero. In other words, a process is required in which the tolerance of the members to be used is close to zero, or the gap s (or the overlap p) is adjusted to zero at the microscope level.

Therefore, as a result of examining the structure of an optical isolator that can obtain good optical characteristics with simple adjustment even when a component having a dimensional accuracy of about ± 5 μm is used,
As shown in FIGS. 2A and 2B, by adjusting the angle between the optical surface of the isolator and the optical axis, one half of the two
The gap s (or overlap p) between the wave plates can be apparently minimized. However, this method has the following problems. In other words, as the angle between the optical surface and the optical axis deviates from 90 °, the optical axis side end faces 2 of the half-wave plates 2 and 3
Light passing through a and 3a increases. These end faces 2a, 3a
Does not pass through the original half-wave plate, which causes deterioration of the extinction ratio and insertion loss as shown in FIG.

On the other hand, as shown in FIG.
The two joining units A and B in which the wave plate 20 and the Faraday rotator 10 are adhered and fixed on the optical surface are divided into the half wave plates 20 of the first joining unit A and the second joining unit B.
Are disposed on opposite sides of each other, and the end faces P and Q perpendicular to the optical surfaces of the units A and B are brought into close contact with each other to form an optical isolator as shown in FIG. 1 (specifically, FIG. 6). Thus, the above problem can be solved.

Here, in FIG. 4, the Faraday rotator 10 and the half-wave plate 20 are bonded together in advance with an adhesive 30, and the two bonding units A and B are brought into close contact with each other at end faces P and Q. Have been. Faraday rotator 1
The 0 and 波長 wavelength plates 20 are each 1 so that they are adjacent to each other.
The half-wave plate 20 is arranged on the opposite side.
Further, the slow axis D of the half-wave plate 20 and the close end face P,
The angle between the boundaries of Q is 22.5 °,
The angle formed by the slow axis D of the two-wavelength plate 20 is 45 °. The rotation capability of the Faraday rotator is 45 °. In this case, the contact end surfaces P and Q are parallel to the optical axis Lc direction.

Therefore, in the optical isolator of the present invention, the gap between the two half-wave plates can be reduced to the level of the surface accuracy (surface roughness and flatness) of the end surfaces only by bringing the end surfaces into close contact during assembly. It is possible. In general, the surface accuracy can be made one or more digits smaller than the dimensional accuracy, and two
It is possible to make the gap between the half-wave plates sufficiently small.

For the close contact of the end surfaces, an adhesive may be used, or it may be merely pressed mechanically. It is desirable that the end face is previously flattened by polishing the end face, but good optical characteristics can be obtained even when the end face is used as it is.

In the case of manufacturing the optical isolator as described above, as shown in FIG. 5, a large cutout unit C in which the half-wave plate 20 and the Faraday rotator 10 are bonded on the optical surface is manufactured. The cutting unit C is cut into a predetermined size, and the obtained two cutting units C are used as the first and second joining units A and B, and the first joining unit A and the second joining unit B are used. It is preferable that the half-wave plates are disposed on opposite sides of each other so that the end surfaces P and Q perpendicular to the optical surfaces of the units A and B are in close contact with each other. The process is labor-saving.

[0017]

EXAMPLES The present invention will be described below in detail with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

FIG. 6 shows the structure of the isolator according to the present embodiment. A quartz plate (90 μm thick) is used as a half-wave plate.
m, a side (slow axis in a direction of 22.5 ° with respect to end faces P and Q), and a Faraday rotator (thickness: 350 μm) Bi
Substituted Fe garnet was used. Each dimension is 4
mm × 12 mm. Each optical element was provided with an anti-reflection film for air and an anti-reflection film for adhesion.

A half-wave plate and a Faraday rotator are bonded with a silicone adhesive to form a cutout unit, and then cut into a size of 1.9 mm × 3.8 mm.
A total of six joining units were created. One pair of the obtained joining units was bonded and fixed to the pedestal 40. At this time, the adhesive was cured while applying pressure from both sides so that the end faces P and Q of the joining unit were sufficiently adhered to each other. One pair of the joining units fixed to the pedestal was inserted and fixed in the magnet 50 to form an isolator function unit 60. The functional unit 60 was combined with the collimator lenses 4 and 5 and the single mode optical fibers 6 and 7 to form the optical system shown in FIG. The position of the isolator function was finely adjusted so that the extinction ratio was optimized, and the extinction ratio was measured.

The wavelength used for this optical system is as follows.
31 μm. FIG. 8 shows the extinction ratio of ten isolators similarly prepared. For comparison, the extinction ratio of the isolator having the structure shown in FIG. 7 was plotted. As is clear from FIG. 8, it was confirmed that the isolator of the present invention had a higher extinction ratio and less variation than the comparative example.

The procedure for producing the isolator of the comparative example is as follows. The half-wave plate and the Faraday rotator used were of the same material as in the example. An antireflection film against air was applied to both surfaces of each optical element. The half-wave plate is 1.9 mm x 3.8 mm, and the Faraday rotator is 3.8
It was cut out to a size of mm × 3.8 mm. These are 1 /
The two-wavelength plate / spacer / Faraday rotator / spacer / 1/2 wavelength plate were adhered and fixed in this order (adhesion was performed without interposing an adhesive in the optical path). The assembly configured as described above was installed in a cylindrical magnet to function as an isolator. This isolator function part was arranged in an optical system composed of a collimator lens and an optical fiber. The position of the isolator function was adjusted so that the extinction ratio became optimal.

Here, in FIG. 7, reference numeral 80 denotes a spacer which is formed in a ring shape having a circular hole 81 as shown in FIG. 7B. The magnet 50 is shown in FIG.
It has the cylindrical shape shown in (C).

In this comparative example, a gap s formed when two half-wave plates 2 and 3 are projected in the optical axis direction.
Depends on the cutting accuracy of the half-wave plate and is expected to be around 5 μm. The tilt of the isolator with respect to the optical axis is
Not particularly

[0024]

According to the present invention, it is possible to minimize the gap or overlap of the half-wave plate by bringing the end faces of the joining unit into close contact with each other, and as a result, special adjustment can be made. Without this, an optical isolator having good optical characteristics can be manufactured. Further, by laminating a large-sized half-wave plate and a Faraday rotator and then cutting it to a predetermined size, it is possible to cope with mass production.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a structure of an optical isolator to which the present invention is applied.

2A and 2B are explanatory diagrams of a conventional method for adjusting an optical isolator. FIG. 2A shows a state in which there is a gap between optical axis side end faces of a half-wave plate, and FIG. (B) shows a state in which there is an overlap between the end faces on the optical axis side of the half-wave plate, and (B ') shows a state in which it is inclined with respect to the optical axis.

FIG. 3A is an explanatory view of a state where light passes through an end face on the optical axis side of a half-wave plate, and FIG. 3B is a partial explanatory view thereof.

4A and 4B are views for explaining a bonding mode of the bonding unit of the present invention, wherein FIG. 4A is a sectional view and FIG. 4B is an explanatory view of an optical surface.

FIG. 5 illustrates an example of the production method of the present invention.
(A) is a cross-sectional view of a cutting unit, (B) is a cross-sectional view of a cutting unit obtained by cutting the cutting unit, and (C) is a cross-sectional view of a state in which two cutting units are closely attached and assembled.

FIG. 6 shows an optical isolator according to one embodiment of the present invention, wherein (A)
FIG. 2 is a side sectional view, and FIG. 2B is a front view of an isolator function unit.

7A and 7B show a conventional optical isolator, wherein FIG. 7A is a side sectional view, FIG. 7B is a sectional view of a spacer, and FIG. 7C is a sectional view of a magnet.

FIG. 8 is a graph showing the extinction ratio of each sample.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Faraday rotator 2 1st 1/2 wavelength plate 3 2nd 1/2 wavelength plate 4, 5 Lens 6, 7 Optical fiber 10 Faraday rotator 20 1/2 wavelength plate 30 Adhesive A 1st joining unit B Second joining unit P, Q Joining end face D Slow axis L Optical path Lc Optical axis Lp Line perpendicular to optical surface s Separation distance between optical axis side end faces of 1/2 wavelength plate p Optical axis of 1/2 wavelength plate Overlap distance between side end faces

Claims (2)

[Claims] 1. A bonding unit comprising a half-wave plate and a Faraday rotator bonded and fixed on an optical surface, and a half-wave plate of a first bonding unit and a half-wave plate of a second bonding unit. It is arranged so as to be located on the opposite side, and the end faces perpendicular to the optical surface of each unit are in close contact with each other, and half of the incident light is transmitted through the half-wave plate of the first junction unit. , The remaining half of the light is １ ／ of the second joint unit
It is a structure that transmits the wave plate, and is one of the first joining units.
The angle between the slow axis of the half-wave plate and the slow axis of the half-wave plate of the second joining unit is 45 °, and the rotation angle of the Faraday rotator is 45 °. Dependent optical isolator. 2. A cutout unit in which a half-wave plate and a Faraday rotator are bonded on an optical surface is manufactured, and the cutout unit is cut into a predetermined size. And the second joint unit, each 1 / of the first joint unit and the second joint unit
2. The polarization-independent optical isolator according to claim 1, wherein the two wavelength plates are arranged on opposite sides of each other, and the end faces perpendicular to the optical surfaces of the respective units are brought into close contact with each other. Method.