JPH11202260A - Optical isolator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a cancel type polarization independent optical isolator to have high optical characteristics. SOLUTION: This device is constituted by overlapping a first 172 wavelength board 3 at a part and one side of a Faraday rotator 4 and overlapping a second 1/2 wavelength board 5 at the part, which the first 1/2 wavelength board 3 does not overlap, on the other side of the Faraday rotator 1. In this case, the total sum of areas of chipping existent on an optical plane near an end face partitioning the parallel beams of the respective first and second 1/2 wavelength boards 3 and 5 by half can be less than 100 μm<2> within the range irradiated with beams.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization independent optical isolator.
The present invention relates to a new type of polarization independent optical isolator that does not cause polarization dispersion or signal light separation.

[0002]

2. Description of the Related Art As is well known, an optical isolator is a non-reciprocal optical device having a function of transmitting signal light in a forward direction and blocking reflected return light in a reverse direction. Of these, polarization-independent light of a type that does not depend on the polarization state of light and is used to prevent an optical resonator structure from being formed in an optical amplifier made of an optical fiber or the like doped with a rare earth element. Various types of isolators have been proposed so far. For example, a so-called wedge-shaped optical isolator as disclosed in JP-B-61-58809 is one of them, and a pair of birefringent plates and one is used.
The optical isolator function part can be constituted by two 45 ° Faraday rotators, and the number of components of the optical isolator can be reduced, thereby promoting the cost reduction.

However, in the case of the wedge-shaped optical isolator,
Since slight separation between ordinary light and extraordinary light, polarization dispersion, and the like are observed, JP-A-5-297321 and JP-A-6-11 are disclosed.
As disclosed in Japanese Unexamined Patent Publication No. 664 and No. Hei 8-505961, a birefringent plate called a compensating plate is combined to eliminate the separation of ordinary light and extraordinary light and the polarization dispersion. However, since adding a compensator increases the cost, any optical isolator still has room for improvement,
It was a step now.

[0004] Recently, a polarization independent optical isolator having a new structure which does not require an expensive birefringent crystal plate has been proposed (1997 IEICE General Conference C-).
3-98). FIG. 1 shows the configuration of this optical isolator.

The optical isolator includes a first optical fiber 1 through which light in a forward direction propagates, a first lens 2 that converts light emitted from the first optical fiber 1 into parallel light,
A first half-wave plate 3 that transmits only half of the parallel light
A 45 ° Faraday rotator 4 that transmits all the parallel light, and a second half-wave plate 5 that transmits the other half of the parallel light that has not passed through the first half-wave plate 3. ,
A second lens 6 and a second optical fiber 7 are arranged in this order, and the first and second half-wave plates 3 and 5 are provided with at least one straight side 3 for dividing parallel light into half.
a and 5a, and the slow axis angles of the two half-wave plates 3 and 5 are shifted by 45 ° or 135 °.

As described above, in the optical isolator, the light emitted from the single mode optical fiber is collimated by a lens and divided into halves, and each light is sequentially transmitted to a 45 ° Faraday rotator and a half-wave plate at the upper and lower sides. Is changed to allow transmission. Here, the direction in which light travels right in FIG.
1 in a plane perpendicular to the z-axis and the x-axis,
Assuming that the axis and the horizontal direction are the y axis, the slow axis angle of the half-wave plate from the x axis is 22.5 ° on the upper side and −22.5 ° on the lower side.
°. At this time, the electric field distribution and the polarization state of the forward wave and the backward wave are as shown in FIGS. 2A and 2B when the x-polarized light incidence is shown by a solid line and the y-polarized light incidence is shown by a wavy line. The light having an even symmetric electric field distribution emitted from the optical fiber becomes the original even symmetric even if condensed by the right lens, and is coupled to the right optical fiber. Also, the electric field distribution of the light emitted from the right optical fiber traveling in the opposite direction is initially even symmetric, but by passing through the optical function part,
Since the x-polarized light and the y-polarized light have opposite polarities in the upper half and the lower half, the electric field distribution condensed by the lens has an odd symmetry, and is not coupled to the left single-mode optical fiber. Therefore, a polarization independent optical isolator is obtained. In addition, in FIG.
90 indicates the direction of the plane of polarization.

However, an optical isolator of the above type is
Since the boundary end face of the half-wave plate is located at the center of the optical surface, it may affect the optical characteristics, particularly the extinction performance, and it has been desired to solve this point.

[0008]

Means for Solving the Problems and Embodiments of the Invention The present inventor has conducted intensive studies in order to meet the above-mentioned demands. In particular, the extinction performance is greatly affected, and the total area of chipping existing on the optical surface in the vicinity of the end face is set to 100 μm ² or less in a range where the beam shines, and the center line average roughness of the end face is set to 1. It has been found that a polarization independent optical isolator excellent in extinction performance can be obtained by setting the thickness of the end face to 0 μm or less and the maximum amount of surface sag of the end face to 1.0 μm or less.

Therefore, according to the present invention, as a first configuration, a first half-wave plate is superposed on a part of one surface of the Faraday rotator, and another surface of the Faraday rotator is provided as a first surface. In a polarization independent optical isolator in which a second half-wave plate is superimposed on a portion where a half-wave plate is not superimposed, the first and second half-wave plates are separated from each other.
This provides an optical isolator in which the total area of chipping existing on the optical surface near the end face that divides the parallel light of the two-wavelength plate in half is 100 μm ² or less in the range where the beam is applied. Having.

As a second configuration, a first half-wave plate is superposed on a part of one surface of the Faraday rotator, and another half of the Faraday rotator is provided with a first half-wave plate.
In a polarization independent optical isolator in which a second half-wave plate is superimposed on a portion where no wave plate is superimposed, the parallel light of the first and second half-wave plates is divided in half. Provided is an optical isolator having a center line average roughness of the end face of 1.0 μm or less, which also has high optical characteristics.

Further, as a third configuration, a first half-wave plate is superimposed on a part of one surface of the Faraday rotator, and another half of the Faraday rotator is provided with a first half-wave plate.
In a polarization independent optical isolator in which a second half-wave plate is superimposed on a portion where no wave plate is superimposed, the parallel light of the first and second half-wave plates is divided in half. The present invention also provides an optical isolator having a surface sagging amount of 1.0 μm or less at the end face, which also has high optical characteristics.

Further, by combining the above-described configurations, it is possible to further improve the optical characteristics.

Hereinafter, the present invention will be described in more detail.
The optical isolator of the present invention has a configuration as shown in FIG. 1, for example, and converts a first optical fiber through which light in the forward direction propagates and a light emitted from the first optical fiber into parallel light. A first lens, a first half-wave plate that transmits only half of the parallel light, a 45 ° Faraday rotator that transmits all parallel light, and a transmission through the first half-wave plate A second half-wave plate that transmits the remaining half of the parallel light that has not been transmitted, a second lens, and a second optical fiber are arranged in this order, and the first and second 1/2 wavelength plates are arranged in this order.
The two-wavelength plate is an optical isolator having at least one straight side for dividing parallel light into halves, and in which the slow axis angles of the two half-wave plates are shifted by 45 ° or 135 °. . The basic principle of this optical isolator is that, as described above, the light emitted from the single mode optical fiber is collimated by a lens, the light is split into upper and lower halves, and each light is placed on a 45 ° Faraday rotator and a half-wave plate. By changing the order of transmission on the lower side and the lower side, the electric field distribution of each of the forward wave and the backward wave can be made the same even symmetry as originally, and oddly different symmetry as originally. Therefore, the light condensed by the lens becomes a cancellation type polarization independent optical isolator that couples to the single mode optical fiber in the case of the forward wave, but does not couple to the single mode optical fiber in the case of the backward wave. It is.

Here, in the present invention, as shown in FIG. 3, the optical surfaces near the end surfaces (end surfaces) 3b and 5b that divide the parallel light of the first and second half-wave plates 3 and 5 in half. Face 3
c, and the total sum of the areas of the tipping present in 5c, 100 [mu] m ² or less in the range where the beam is striking, preferably 80 [mu] m ² or less, more preferably to 30 [mu] m ² or less. In FIG. 3, reference numeral 8 denotes a spacer. Further, chipping is a general term for a minute missing portion that occurs near an edge when a base material of a half-wave plate is cut into a shape having at least one straight side.

Also, in the present invention, apart from the above method, the JIS of the end faces 3b, 5b of the half-wave plates 3, 5 is used.
The center line average roughness according to B0601 is 1.0 μm or less,
Preferably, it is 0.3 μm or less. In this case, the center line average roughness is preferably 1.0 μm or less, and the total chipping is preferably 100 μm ² or less as described above.

Further, separately from the above method, the end face 3
The amount of surface sag of b and 5b is set to 1.0 μm or less at maximum, preferably 0.8 μm or less. In this case, it is preferable to define the surface sagging amount as described above and to define one of the chipping total area and the center line surface roughness, particularly preferably both, as described above.

In this case, the half-wave plate can be formed of a known material such as quartz, and the Faraday rotator and other components can also be formed of a known material.

[0018]

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

Example 1 A 1.6 mm square 45 ° Faraday rotator provided with an antireflection film against air was cut out from a Bi-substituted rare earth iron garnet film manufactured by Shin-Etsu Chemical Co., Ltd. ) A set of 1.6 × 0.8 mm quartz half-wave plates with a anti-reflection coating against air, processed to a slow axis angle of + 22.5 °, and 1.6 ×
A set of SUS spacers having a size of 1.6 × 0.05 mm and an optical surface of 1.5 mmφ is fixed with a thermosetting silicone resin 8 manufactured by Shin-Etsu Chemical Co., Ltd. as shown in FIG. Co., Ltd., was inserted into a cylindrical body having an inner diameter of 2.3 mm, an outer diameter of 4 mm, and a thickness of 2 mm, and was fixed with the silicone resin to form an optical isolator function unit. A set of optical isolator function units was prepared. Then, 300 μm of the optical surface of each optical isolator function part near the end face is used.
The approximate sum of the chipping areas in the range where the beam of mφ is applied is estimated while observing with an optical microscope.
A single mode optical fiber connected to each collimator is connected to an optical switch. One of the terminal pairs on the opposite side of the optical switch has a wavelength of 1.55 μm. For the light source,
The other was connected to a power meter to produce a system for measuring optical characteristics, and the insertion loss of these optical isolator function units in both forward and reverse directions was measured.

The results are shown in Table 1. From the results in Table 1,
The reverse insertion loss tends to decrease and the forward insertion loss tends to increase as the sum total of the chipping areas increases. In addition, it is understood that the total sum of the chipping areas needs to be 100 μm ² or less, particularly when the reverse insertion loss is to be 33 dB or more.

[0021]

[Table 1]

Example 2 A 1.6 mm square 45 ° Faraday rotator provided with an antireflection film for air was cut out from a Bi-substituted rare earth iron garnet film manufactured by Shin-Etsu Chemical Co., Ltd. ) A set of 1.6 × 0.8 mm quartz half-wave plates with a anti-reflection coating against air, processed to a slow axis angle of + 22.5 °, and 1.6 ×
A set of SUS spacers having a size of 1.6 × 0.05 mm and an optical surface of 1.5 mmφ is fixed with a thermosetting silicone resin 8 manufactured by Shin-Etsu Chemical Co., Ltd. as shown in FIG. Co., Ltd., was inserted into a cylindrical body having an inner diameter of 2.3 mmφ, an outer diameter of 4 mmφ, and a thickness of 2 mm, and was fixed with the silicone resin to form an optical isolator function unit. A set of optical isolator function units was prepared. At this time, several types of different cutting blades are used to cut a quartz half-wave plate of 5 × 3.5 mm size manufactured by Cymalec Co., Ltd.
It was cut into a size of 0.8 mm square, and an optical isolator function part was produced by changing the surface roughness of the cut end surface. Then, a quartz plate having a large cross-sectional area (width 0.5 mm) separately cut as a dummy with the different types of blades described above is fixed to glass to stabilize, and the center line surface roughness is measured by a stylus method. The center line surface roughness of the cut end face of the isolator function part was set. For five sets, each optical isolator function part has an optical surface near the end face of 300 μm.
The sum of the chipping areas in the area where the φ beam is applied,
It was estimated while observing with an optical microscope. Then 300μ
A single mode optical fiber connected to each collimator is connected to an optical switch. One of the terminal pairs on the opposite side of the optical switch has a wavelength of 1.55 μm. For the light source,
The other was connected to a power meter to produce a system for measuring optical characteristics, and the insertion loss of these optical isolator function units in both forward and reverse directions was measured.

The results are shown in Tables 2 and 3. From the results shown in Table 2, the reverse insertion loss tends to decrease and the forward insertion loss tends to increase as the center line surface roughness increases. And
In particular, if the reverse insertion loss is to be increased to 33 dB or more, it is understood that the center line surface roughness needs to be 1.0 μm or less. Further, from the results in Table 3, it is understood that the characteristics are further improved when the center line surface roughness is within the above range and the total area of the chipping is within the above range.

[0024]

[Table 2]

[0025]

[Table 3]

Example 3 A 1.6 mm square 45 ° Faraday rotator provided with an antireflection film for air was cut out from a Bi-substituted rare earth iron garnet film manufactured by Shin-Etsu Chemical Co., Ltd. ) A set of 1.6 × 0.8 mm quartz half-wave plates with a anti-reflection coating against air, processed to a slow axis angle of + 22.5 °, and 1.6 ×
A set of SUS spacers having a size of 1.6 × 0.05 mm and an optical surface of 1.5 mmφ is fixed with a thermosetting silicone resin 8 manufactured by Shin-Etsu Chemical Co., Ltd. as shown in FIG. Co., Ltd., was inserted into a cylindrical body having an inner diameter of 2.3 mmφ, an outer diameter of 4 mmφ, and a thickness of 2 mm, and was fixed with the silicone resin to form an optical isolator function unit. A set of optical isolator function units was prepared. At this time, both end surfaces in the longitudinal direction of a quartz half-wave plate manufactured by Cymalec Co., Ltd. having a size of 5 × 3.5 mm were rough-polished at the end surfaces to generate surface sagging. Then, the amount of surface sag on the end surfaces on the sides separating the optical surfaces of the two half-wave plates of each optical isolator function unit was observed and measured using an optical microscope. For the five sets, the total of the chipping areas of the respective optical isolator function parts in the range where the optical surface near the end face is irradiated with a beam of 300 μmφ is estimated while observing with an optical microscope. Changes the size of the abrasive grains for edge polishing so that the center line surface roughness to be finished can be controlled, and a large-area (0.5 mm wide) dummy quartz plate polished separately with each abrasive grain is fixed to glass. Then, the center line surface roughness of each polished end face was determined from the value obtained by measuring the center line surface roughness by a stylus method. Then, it is set at the center of a pair of collimators that create a parallel light of 300 μmφ, and a single mode optical fiber connected to each collimator is connected to an optical switch.
One of the terminal pairs on the opposite side of this optical switch is 1.55 μm
A light source having a wavelength was connected to a power meter, and the other was connected to a power meter to prepare a system for measuring optical characteristics, and the insertion loss of these optical isolator function units in both forward and reverse directions was measured.

Tables 4 and 5 show the results. From the results shown in Table 4, as the surface sag increases, the forward insertion loss particularly tends to increase. Then, this forward insertion loss is reduced to 0.4
If it is attempted to reduce the surface sag to 4 dB or less,
It can be seen that it is necessary to make it smaller than μm. Further, from the results in Table 5, it is understood that the characteristics are further improved when the total area of the chipping areas and the center line surface roughness are defined in the above ranges.

[0028]

[Table 4]

[0029]

[Table 5]

[0030]

The cancellation type polarization independent optical isolator of the present invention has high optical characteristics.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a configuration of an optical isolator of the present invention.

FIG. 2 is a diagram illustrating an electric field distribution of the optical isolator, and FIG.
Is a forward wave, and (b) is a backward wave.

FIG. 3 is a partially enlarged perspective view of the optical isolator.

[Explanation of symbols]

 Reference Signs List 1 optical fiber 2 lens 3 first half-wave plate 3a straight side 3b end face 4 Faraday rotator 5 second half-wave plate 5a straight side 5b end face 6 lens 7 optical fiber

Claims (5)

[Claims] 1. A first Faraday rotator having a first surface
Of the Faraday rotator, and the second half-wave plate is superimposed on a portion of the other surface of the Faraday rotator where the first half-wave plate is not superimposed. In the independent optical isolator, the sum of the chipping areas existing on the optical surface near the end face that divides the parallel light of the first and second half-wave plates into halves is 100 μm in a range where the beam hits. An optical isolator characterized by being ² or less. 2. A first part of one surface of the Faraday rotator.
Of the Faraday rotator, and the second half-wave plate is superimposed on a portion of the other surface of the Faraday rotator where the first half-wave plate is not superimposed. 2. An optical isolator according to claim 1, wherein a center line average roughness of an end face of said first and second half-wave plates which divides parallel light into half is 1.0 μm or less. 3. A first part of one surface of the Faraday rotator.
Of the Faraday rotator, and the second half-wave plate is superimposed on a portion of the other surface of the Faraday rotator where the first half-wave plate is not superimposed. 2. An optical isolator according to claim 1, wherein an amount of surface sag of an end face of said first and second half-wave plates which divides parallel light into half is at most 1.0 [mu] m or less. 4. The optical isolator according to claim 1, wherein the center line average roughness of an end face of the first and second half-wave plates which divides parallel light into half is 1.0 μm or less. An optical isolator characterized by the above-mentioned. 5. The optical isolator according to claim 2, wherein an amount of sag of an end surface of the first and second half-wave plates which divides parallel light into half is 1.0 μm at the maximum.
m or less.