JP4070053B2 - Optical circulator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical circulator used in an optical communication system, an optical measurement field, and the like.
[0002]
[Prior art]
An optical circulator is one of non-reciprocal optical circuit elements that are important in fields such as optical communication and optical measurement. The optical circulator has at least three ports, whereas the optical circulator is a two-port type on the incident side and the emission side, for example, when it has three ports represented by numbers 1, 2, and 3, Light traveling in the direction 1 → 2, 2 → 3, 3 → 1 in the forward direction has low loss, and light traveling in the direction 1 → 3, 3 → 2, 2 → 1 in the reverse direction is transmitted as a high loss output. To do.
[0003]
As such an optical circulator, for example, an optical circulator disclosed in International Publication No. WO97 / 22034 is known.
[0004]
[Problems to be solved by the invention]
By the way, the optical circulator has a structure in which light is transmitted from one optical waveguide array opposed to the other along the optical axis to the other optical waveguide array. was there. In addition, if the number of ports is increased, the number of components increases accordingly, which increases the size and makes it difficult to increase the number of ports.
[0005]
The present invention has been made in view of the above points, and it is an object of the present invention to provide an optical circulator that is smaller in number of components than a conventional product even if the number of ports is increased.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, in the optical circulator according to the present invention, a birefringent member, a first phase member, a polarization rotator, a composite, and the like are arranged between an array in which at least three or more optical fibers are arrayed and a lens. A birefringent member is disposed, and the second phase member and the reflector are disposed on the opposite side of the lens with respect to the lens.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the optical circulator according to the present invention will be described in detail with reference to FIGS.
As shown in FIG. 1, the optical circulator 1 includes a birefringent member 3, a first phase member 4, a polarization rotator 5, and a composite between an array 2 in which four optical fibers are arranged and a lens 7. A birefringent member 6 is disposed, and a second phase member 8 and a reflector 9 are disposed on the opposite side of the lenses 7 of the constituent members 3 to 6. As the birefringent member 3 to the second phase member 8, it is desirable to use a light incident / exit surface having an antireflection coating such as SiO2 / TiO2.
[0008]
Here, in FIGS. 3A and 3B, the arrangement from the birefringent member 3 to the reflector 9, the light traveling direction is the Z axis, and the horizontal direction in the plane perpendicular to the Z axis is the X axis, The polarization state of light when the vertical direction is the Y axis is shown. These X-axis to Z-axis are the same in other drawings attached to this specification.
In the array 2, optical fibers 2 b are arrayed and fixed in each of four V grooves (not shown) formed on the surface of the substrate 2 a at equal intervals and in parallel (within 3 degrees of parallelism). Here, for convenience of distinction in the following description, the four optical fibers 2b are referred to as ports P1 to P4, respectively, as shown in FIG. At this time, the array 2 has the first optical fiber, where a is the distance from the central axis AcX of the lens 7 to the nearest optical fiber 2b (port P1), and d is the width separated by the composite birefringent member 6. The distance wn to the nth optical fiber 2b with reference to 2b is defined as wn = Lf * n = 2 {a + 2 (n-1) d} + d (n = 1, 2, 3,...) Set to. However, Lf is an interval between the adjacent optical fibers 2b, and is determined in consideration of the back focus of the lens 7 and the crystal length of the member constituting the optical circulator 1 and the relationship between the crystal length and the separation width. In this embodiment, it is 126 μm. Set.
[0009]
In addition, the plurality of optical fibers 2b has an MFD whose divergence angle is reduced by heating the birefringent member 3 side, treating the core with a TEC (thermal expanded core), and expanding a mode field diameter (MFD). It has an enlarged portion 2c. The MFD enlargement unit 2c is not limited to the optical fiber subjected to the TEC process, and a distributed refractive index type fiber or a minute lens may be used. On the other hand, the array 2 is not limited to four or more optical fibers as long as three or more optical fibers can be arranged at equal intervals and in parallel. For example, it is possible to use various types such as ceramics for multi-fiber connectors and ferrules made of SUS, a plurality of optical fibers fixed to a V-groove array, or a plurality of optical fibers molded with a synthetic resin. .
[0010]
The birefringent member 3 combines the light emitted from the optical fiber 2b into the ordinary ray and the extraordinary ray, and the polarized light reflected by the reflector 9, and the interval between the optical fibers 2b is changed from the ordinary ray to the extraordinary ray. It is made to correspond to the separation width to be separated from the light beam. As the birefringent member 3, a birefringent crystal plate or a uniaxial optical crystal (polarizer) is used. There are crystals such as. The birefringent member 3 is preferably rutile in consideration of the availability and the reliability of the crystal itself by processing the optical axis to around 42 to 50 degrees with respect to the incident light so that the minimum crystal length is obtained. In the birefringent member 3, the crystal length t is set to a relationship of Lf = about 0.1t with respect to the distance Lf of the optical fiber 2b. In this embodiment, the optical axis is 2 mm square and about 1.3 mm thick. The polarization separation element cut at 45 was used.
[0011]
The first phase member 4 rotates the polarization plane of the polarized light transmitted through the birefringent member 3 by 45 degrees, and a composite reciprocal polarization rotator, a composite half-wave plate or the like is used. For example, TBIG, GBIG There are garnet and crystal. The composite reciprocal polarization rotator is desirably as thin as possible, such as a zero-order single plate or a primary single plate. When a higher-order wave plate is used, wavelength characteristics and temperature characteristics are deteriorated. When the first phase member 4 uses a composite half-wave plate, as shown in FIG. 3A, the half-wave plate 4a in which the optical axis AOP1 is tilted by -2.5 degrees with respect to the Y-axis. And a half-wave plate 4b in which the optical axis AOP2 is inclined +22.5 degrees with respect to the Y-axis are overlapped in the Y-axis direction on the bonding surface 4c and bonded with an optical adhesive such as an ultraviolet curing type. The first phase member 4 is careful so that the adhesive used does not protrude from the joint surface 4c to the light incident / exit surface.
[0012]
In the present embodiment, as the first phase member 4, a 0th-order single plate quartz wavelength plate having an optical axis AOP1 having an inclination of −22.5 degrees and an optical axis AOP2 having an inclination of +22.5 degrees is joined with a dicing saw. The chip was cut so that the chipping accuracy was 0.02 mm or less.
The polarization rotator 5 is a non-reciprocal polarization rotator that rotates the polarization plane of polarized light that has passed through the first phase member 4 and is as thin as possible with an optical rotation angle of about 45 degrees in the used wavelength band. As the polarization rotator 5, for example, garnet, TBIG (terbium bismuth iron garnet), GBIG (gadolinium bismuth iron garnet) or the like can be used. In the present embodiment, a clockwise garnet is used with respect to the light traveling direction. However, when a counterclockwise garnet is used, the first phase member 4 has an optical axis AOP1 of +22.5 degrees and an optical axis AOP2 of A slope of -22.5 degrees may be used.
[0013]
The polarization rotator 5 may be replaced with the position of the first phase member 4 in FIG. When arranged in this way, the polarization rotator 5 can be used where the beam diameter of light is small because the divergence angle of transmitted light is constant. For this reason, in the optical circulator 1, when the polarization rotator 5 is arranged in this way, the possibility of beam kicking due to assembly errors or component processing errors is reduced.
[0014]
The composite birefringent member 6 switches the ordinary ray to the extraordinary ray and the extraordinary ray to the ordinary ray, respectively, and makes half the interval of the optical fiber 2b coincide with the separation width for separating the ordinary ray and the extraordinary ray. As the composite birefringent member 6, a composite birefringent crystal plate or a composite uniaxial optical crystal (polarizer) is used. There are crystals such as (αBaB2O4). For example, as shown in FIG. 4A, the composite birefringent member 6 includes a crystal plate 6a in which the optical axis AOP1 is inclined by θ1 = about −42 to −50 degrees with respect to the Z axis and the optical axis AOP2 in the Z axis. On the other hand, the crystal plate 6b tilted by θ2 = about +42 to +50 degrees is overlapped in the X direction on the bonding surface 6c and used by being bonded with an optical adhesive. The composite birefringent member 6 is disposed so that the crystal axis is orthogonal to the crystal axis of the birefringent member 3.
[0015]
At this time, the composite birefringent member 6 is careful not to allow the adhesive to protrude from the joint surface 6c to the light incident / exit surface. The composite birefringent member 6 uses a crystal plate 6a, 6b cut with a dicing saw so that the chipping accuracy is 0.1 mm or less, and the adhesive is made as thin as possible to reduce the pyramidal error. (Pyramidal error) is suppressed. In this embodiment, two sabal plates having a thickness of about 0.63 mm obtained by cutting rutile optical axes AOP1 and AOP2 at ± 45 degrees with respect to the Z axis are inverted and bonded together at the joint surface 6c as shown in FIG. Used.
[0016]
Here, as shown in FIG. 4B, the composite birefringent member 6 has the optical axis AOP1 inclined by θ1 = about +42 to +50 degrees with respect to the Z axis, contrary to the case of FIG. 4A. The crystal plate 6a and the crystal plate 6b in which the optical axis AOP2 is inclined by θ2 = about −42 to −50 degrees with respect to the Z axis may be overlapped in the X direction on the bonding surface 6c and bonded with an optical adhesive. In this case, in the four optical fibers 2b, the optical fiber 2b close to the central axis Ac of the lens 7 is the ports P3 and P4, and the far optical fiber 2b is the ports P1 and P2, contrary to the case shown in FIG. .
[0017]
The lens 7 cancels and collimates the polarization mode dispersion of incident light, and an aspheric lens, a ball lens, a plano-convex lens, a distributed refraction lens, or the like can be used. However, as the lens 7, a lens having a back focus in which the constituent members 3 to 6 can be arranged between the array body 2 and the lens 7 is used. In this embodiment, an aspheric lens having a focal length f = 1.8 mm is used.
[0018]
The second phase member 8 is a retardation plate such as a quarter-wave plate, and a zero-order single plate or a two-order zero-crystal plate is suitable. If a higher-order wave plate is used, wavelength characteristics and temperature characteristics are obtained. Becomes worse. The second phase member 8 may be a non-reciprocal polarization rotator with 45 degree rotation, for example, a garnet such as GBIG. In the present embodiment, a quarter wavelength plate having the optical axis AOP set at 45 degrees with respect to the polarization direction is used as the second phase member 8.
[0019]
The reflector 9 is a reflector that reflects light that has passed through the second phase member 8. In the present embodiment, for example, a reflector that is coated with SiO2 / TiO2 on the surface of the base 9a is used.
As described above, the optical circulator 1 according to the present invention includes the birefringent member 3, the first phase member 4, and the polarization rotator 5 between the array body 2 in which the four optical fibers 2 b are arrayed and the lens 7. And the composite birefringent member 6 is arrange | positioned and the 2nd phase member 8 and the reflector 9 are arrange | positioned on the opposite side with respect to the lens 7 of the structural members 3-6. That is, the optical circulator 1 has a structure in which light incident from any one of the optical fibers 2 b of the array 2 is reflected by the reflector 9 to be folded.
[0020]
Therefore, in the optical circulator 1, for example, the light incident on the port 1 of the four optical fibers 2b is formed on the plane including the X and Y axes, as shown in FIG. It propagates to the member 8, is reflected by the reflector 9, and is emitted to the ports 2 of the four optical fibers 2b with low loss as shown in FIG. 2 (b). At this time, in FIGS. 2 (a) and 2 (b), the circles indicate polarized light oscillating in the Z-axis direction perpendicular to the paper surface with respect to the light traveling direction, and the arrows indicate X parallel to the paper surface with respect to the light traveling direction. The polarized light vibrates in the axial direction.
[0021]
However, since the light incident on the port 4 of the four optical fibers 2b has a large loss, it does not travel in the reverse direction and is emitted to the port 1. Similarly, light incident on the optical fiber 2b is emitted from port 2 → port 3, port 3 → port 4, but does not travel in the reverse direction. Thus, the optical circulator 1 has non-reciprocal characteristics with respect to the light propagation direction.
[0022]
At this time, as defined in FIGS. 3A and 3B, the light traveling direction is the Z axis, the horizontal direction in the plane orthogonal to the Z axis is the X axis, and the vertical direction is the Y axis. When viewed from the 9 side, the horizontal direction is the X axis and the vertical direction is the Y axis. For convenience of explanation, the horizontal direction is divided into 13 and 1 to 13, and the vertical direction is divided into 5 and expressed as a to e. Then, the position of the port 1 is “6d” as shown in FIG. This relationship is the same in the following FIGS. 5B to 6G. Reference sign AcY is the central axis of the lens 7 in the Y-axis direction, and reference sign AcX is the central axis of the lens 7 in the X-axis direction.
[0023]
Therefore, for example, the light incident on the port 1 propagates through the optical fiber 2b and is emitted to the birefringent member 3, and as shown in FIG. 5 (b), the birefringent member 3 is always perpendicular to the crystal axis. Separated into light rays and parallel extraordinary rays.
Next, the first phase member 4 has the optical axis AOP1 tilted by −22.5 degrees with respect to the Y axis and the optical axis AOP2 tilted by +22.5 degrees with respect to the Y axis. When the extraordinary rays pass through the first phase member 4, the polarization directions are rotated by 45 degrees as shown in FIG. 5C, and the polarization directions are the same. Here, in the figure, reference numeral 4c denotes a joint surface between two zeroth-order single-plate quartz wave plates.
[0024]
Next, these lights pass through the polarization rotator 5 and are rotated counterclockwise as shown in FIG. 5 (d) to be aligned with polarized light parallel to the crystal axis of the birefringent member 3. Thereafter, these lights are incident on the composite birefringent member 6. At this time, since the compound birefringent member 6 has a crystal axis orthogonal to the crystal axis of the birefringent member 3, as shown in FIG. Will pass as an ordinary ray.
[0025]
Next, the light that has passed through the composite birefringent member 6 is refracted by a predetermined angle by the lens 7, but the polarization state does not change as shown in FIG. 5 (f). The refraction angle at this time is determined by the center position of the light from the central axes AcX and AcY of the lens 7 and the focal length f of the lens 7.
Next, the light that has passed through the lens 7 enters the second phase member 8. Since the second phase member 8 has the optical axis AOP set at 45 degrees with respect to the polarization direction, the light passing therethrough is right circularly polarized light as shown in FIG.
[0026]
Then, the light that has become the right circularly polarized light is reflected by the reflector 9 to the opposite side to the incident angle, so that the phase is changed to become the left circularly polarized light shown in FIG. Thus, in the optical circulator 1, light is reflected by the reflector 9, and the entire optical path length is shortened. Next, the reflected left circularly polarized light passes through the second phase member 8 again, and is orthogonal to the light (see FIG. 5 (f)) that has passed through the lens 7, as shown in FIG. 6 (b). Horizontally polarized light.
[0027]
Next, the horizontally polarized light passes through the lens 7 again, and is emitted at a position symmetrical to the case of FIG. 5F with respect to the central axis AcY of the lens 7 as shown in FIG. 6C. . At this time, the polarization state does not change before and after passing through the lens 7.
Thereafter, the polarized light that has passed through the lens 7 passes through the composite birefringent member 6 and the beam position is shifted to the right in the horizontal direction as an extraordinary ray as shown in FIG. 6 (d).
[0028]
Next, after passing through the polarization rotator 5, these polarized lights are rotated 45 degrees to the left as shown in FIG. 6 (e). This state is the same as the polarization state after passing through the first phase member 4 from the birefringence member 3 side, as shown in FIG. 5C, although the beam position in the Y-axis direction is different.
Then, when these lights pass through the first phase member 4, as shown in FIG. 6F, the polarization directions are respectively rotated by 45 degrees, and the polarization directions become orthogonal directions.
[0029]
Then, after passing through the birefringent member 3, these lights are respectively combined as an ordinary ray and an extraordinary ray, and as shown in FIG. 6 (g), in the port 2 corresponding to “9d”. The light enters the optical fiber 2b. Here, in the figure, the circles “1d”, “4d”, “6d”, and “11d” indicate positions corresponding to other ports, respectively.
[0030]
Hereinafter, also in the other ports 2, 3, and 4, light is propagated between the birefringent member 3 and the reflector 9 in the same manner as described above.
As described above, in the optical circulator 1, the birefringent member 3, the first phase member 4, the polarization rotator 5, and the composite birefringent member 6 are disposed between the array body 2 and the lens 7, and the component member 3. The second phase member 8 and the reflector 9 are arranged on the opposite side to the lenses 7 to 6. For this reason, the optical circulator 1 can be reduced in size by reducing the number of components by half as compared with a structure that does not reflect light, even if the number of optical fibers, and thus the number of ports, is large. Moreover, since the light reflected by the surface of the structural members 3-6 diverges by 2 times the reflection angle, it becomes difficult to inject into the optical fiber 2b. For this reason, the optical circulator 1 does not need to take a return loss countermeasure.
[0031]
【The invention's effect】
According to the first aspect of the present invention, even if the number of ports is increased, the number of components does not increase, and a small-sized optical circulator can be provided compared to the conventional product.
[Brief description of the drawings]
FIG. 1 is a plan view showing a configuration of an optical circulator according to the present invention.
2 is a cross-sectional view (a) showing a polarization state of incident light in a cross section along a port of a plurality of optical fibers in the optical circulator of FIG. 1, and a cross section showing a polarization state of reflected light returning from a reflector. FIG.
3 shows the arrangement from the birefringent member to the reflector in the optical circulator of FIG. 1, the light traveling direction is the Z axis, the horizontal direction in the plane perpendicular to the Z axis is the X axis, and the vertical direction is the Y axis. FIG. 6 is a perspective view showing the polarization state of light when divided into two parts, the first half side (a) and the second half side (b).
4 is a perspective view showing a first arrangement of optical axes of two crystal plates constituting the composite birefringent member in the optical circulator of FIG. 1, and a perspective view showing a second arrangement; It is.
5 is an explanatory diagram showing a first half side of a light state after passing through each constituent member in the optical circulator of FIG. 1; FIG.
6 is an explanatory diagram showing the second half of the state of light after passing through each component in the optical circulator of FIG. 1. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Optical circulator 2 Array 2b Optical fiber 2c MFD expansion part 3 Birefringence member 4 First phase member 5 Polarization rotator 6 Compound birefringence member 7 Lens 8 Second phase member 9 Reflector AcX, AcY )
AOP optical axis AOP1, AOP2 optical axis P1-P4 port

Claims (5)

In an optical circulator comprising three or more ports and outputting light input from an optical fiber (2b) according to any port to an optical fiber according to a different port,
Between at least three or more of said optical fiber (2b) is arranged sequence body (2) and lens (7), a birefringent member (3), the first phase member (4), the polarization rotator (5) The composite birefringent member (6) is disposed, and the second phase member (8) and the reflector are disposed on the opposite side of the members (3, 4, 5, 6) with respect to the lens (7) . (9) and are arranged ,
The light input from the optical fiber (2b) related to any one of the ports in the array (2) is transmitted on the birefringence member (3), the first phase member (4), and the polarization in the forward path. It passes through the wave rotator (5), the composite birefringent member (6), the lens (7), and the second phase member (8) in this order, and is reflected by the reflector (9) and reflected. In a later return path, the second phase member (8), the lens (7), the composite birefringent member (6), the polarization rotator (5), the first phase member (4), An optical circulator configured to pass through the birefringent member (3) in this order and output to the optical fiber related to the different port in the array (2). In an optical circulator comprising three or more ports and outputting light input from an optical fiber (2b) according to any port to an optical fiber according to a different port,
Between at least three or more of said optical fiber (2b) is arranged sequence body (2) and lens (7), a birefringent member (3), the first phase member (4), the polarization rotator (5) The composite birefringent member (6) is disposed, and the second phase member (8) and the reflector are disposed on the opposite side of the members (3, 4, 5, 6) with respect to the lens (7) . (9) and are arranged ,
The light input from the optical fiber (2b) related to any one of the ports in the array (2) is, in the forward path, the birefringent member (3), the polarization rotator (5), the first The phase member (4), the composite birefringent member (6), the lens (7), and the second phase member (8) in this order and reflected by the reflector (9), In the return path after reflection, the second phase member (8), the lens (7), the composite birefringent member (6), the first phase member (4), the polarization rotator (5), An optical circulator configured to pass through the birefringent member (3) in this order and to be output to the optical fiber (2b) relating to the different port in the array (2). In the optical path from the optical fiber (2b) related to any one of the ports in the array (2) to the optical fiber (2b) related to the different port in the array (2), The optical circulator according to claim 1 or 2, wherein a polarization state of light passing through the refraction member (3) and a polarization state of light passing through the birefringence member (3) in the return path are the same. In the optical path from the optical fiber (2b) related to any one of the ports in the array (2) to the optical fiber (2b) related to the different port in the array (2), the composite in the forward path The optical circulator according to claim 3, wherein the polarization state when passing through the birefringent member (6) and the polarization state when passing through the composite birefringent member (6) in the return path are orthogonal to each other. A ratio of 2 to 1 is provided between the distance (Lf) between the adjacent optical fibers in the array (2) and the separation distance (d) between the forward path and the return path in the composite birefringent member (6). The optical circulator according to claim 1 or 2.

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