JP3694812B2 - Optical circulator - Google Patents

Description

[0001]
[Industrial application fields]
The present invention is an optical circulator that changes the path of transmitted light in the forward direction and the reverse direction in optical communication and optical information processing, and in particular, operates on light in all polarization states for transmitted light in at least one direction. It relates to an optical circulator.
[0002]
[Prior art]
An optical circulator is an element that separates both forward and reverse transmitted light when performing bidirectional communication using a single optical fiber in an optical communication system, and exchanges transmitted light among a plurality of optical fibers. Used. In general, a three-terminal optical circulator is used for installing at the laser emission end to separate transmitted light in the reverse direction, and a four-terminal optical circulator is used for exchanging transmitted light between optical fibers.
[0003]
The configuration of a conventional optical circulator will be described based on the conventional examples shown in FIGS. FIG. 6A is a diagram showing a transmission path (behavior) of transmitted light when forward transmitted light enters the optical circulator from the input / output end. FIG. 6 shows an example of a three-terminal type optical circulator, and FIG. 7 shows an example of a four-terminal type optical circulator. In FIG. 6A, the optical circulator includes an input / output end 13, birefringent crystal flat plates 21, 22, 23, a Faraday rotator 24, an input end 14, and an output end 15. In FIG. 6A, when the direction of transmitted light from the left side to the right side is a forward direction, the incident light IL incident from the incident / exit end 13 ₁ Are separated into two types of polarized light by the birefringent crystal flat plate 21. That is, incident light IL ₁ Is separated into forward transmitted light 25a having the first polarization component and forward transmitted light 25b having the second polarization component, and sequentially transmitted through the Faraday rotator 24, the birefringent crystal flat plate 22, and the birefringent crystal flat plate 23. To do. The birefringent crystal flat plate 23 synthesizes the forward direction transmitted lights 25a and 25b to produce the combined transmitted light LL. ₁ Is sent to the output end 15. The light reaches the emission end 15 and is emitted from the emission end 15.
[0004]
On the other hand, FIG. 6B is a diagram showing a transmission path (behavior) of transmitted light when transmitted light in the reverse direction from the incident end enters the optical circulator. Incident incident light IL ₂ Are classified into two types of polarized light by the birefringent crystal flat plate 23. That is, incident light IL ₂ Is separated into a reverse transmission light 26a having the first polarization component and a reverse transmission light 26b having the second polarization component, and sequentially transmitted through the birefringent crystal flat plate 22, the Faraday rotator 24, and the birefringent crystal flat plate 21. To do. The two types of separated polarized light, that is, the reverse direction transmitted light 26a and the reverse direction, and the transmitted light 26b are not combined, and only the reverse direction transmitted light 26b is finally emitted from the incident / exit end 13. On the other hand, the reverse direction transmitted light 26a reaches the x position in the figure and cannot be emitted. When transmitted return light is incident in the opposite direction from the exit end 15, no separated light component reaches the entrance / exit end 13 as shown in FIG. 6B, so that the incident light passes through the optical circulator at all. I can't.
[0005]
In the case of the four-terminal optical circulator shown in FIG. 7A, similarly to the above-described three-terminal optical circulator, incident light IL incident from the incident / exit end 16 and the incident / exit end 17 respectively. _Three , IL _Four Is separated by the birefringent crystal flat plate 23 without attenuation of the separated light, that is, forward transmitted light 27a and 27b, 27c and 27d, and LL _Three , LL _Four To the incident / exit end 18 and the incident / exit end 19. Further, as shown in FIG. 7B, transmitted light IL in the reverse direction from the incident / exit end 18 and the incident / exit end 19 is transmitted. _Five , IL ₆ Is incident, only the reverse direction transmitted light 28a and the reverse direction transmitted light 28d, which are the separated lights, reach the incident / exit end 17 and the incident / exit end 16, respectively.
The remaining transmitted light separated by the birefringent crystal flat plate 23, that is, the reverse direction transmitted light 28b and the reverse direction transmitted light 28c, cannot reach the input and output ends 16 and 17 on the forward direction side. In the middle, that is, the beam cannot reach the x position in the figure and emit light.
[0006]
[Problems to be solved by the invention]
The conventional optical circulator has the following two problems. The first problem relates to the optical path difference of transmitted light. The transmitted light from the left side to the right side (forward direction) in the figure is separated into two types of polarized light components whose polarization planes are orthogonal to each other at a halfway birefringent crystal flat plate 21, and then again at the birefringent crystal flat plate 23. Synthesized. However, the optical path lengths of the separated two polarized light beams that pass through the total of three birefringent crystal flat plates are not the same. In general, the refractive index when polarized light passes through a birefringent crystal plate is determined by the relationship between the plane of polarization and the crystal axis of the crystal plate. In this case, the polarization component in a specific direction of the transmitted light is separated and transmitted in an oblique direction with respect to the crystal surface (abnormal light), and the other transmitted light component is transmitted as it is perpendicular to the crystal surface ( Joko). At this time, the total thickness of the birefringent crystal flat plates through which both are transmitted is naturally the same, but since the refractive indexes of the two in each crystal flat plate are different, the total of the transmitted optical path lengths of the respective polarizations is generally equal. There is no.
[0007]
When the relative thickness of the birefringent crystal flat plate 21 is √2t, the relative thicknesses of the birefringent crystal flat plate 22 and the birefringent crystal flat plate 23 are each t. The total of the optical path lengths of the forward transmitted light 25a and the forward transmitted light 25b excluding the portion of the Faraday rotator 24 in FIG. 6A is nt + (√2 + 1) n′t and (√2 + 1) nt + n′t, respectively. The difference between the two is √2t | n−n ′ |.
[0008]
Here, n and n ′ are the ordinary light and extraordinary light refractive indexes of the birefringent crystal flat plate, respectively.
[0009]
When n ≠ n ′, this value does not become zero, and at that time, in the optical circulator presented as a conventional example, it can be seen that the optical path lengths of the polarization components separated from the transmitted light in the forward direction do not coincide with each other. If this transmitted light is parallel light, there is no problem, but in a general application, a condensing lens (not shown) is installed between the incident / exit end 13 and the birefringent crystal flat plate 21, and the end face of the exit end 15. The transmitted light is condensed so that the focal point is arranged on the surface. In such a case, due to the difference in optical path length, the transmitted light is condensed at two points shifted in the optical axis direction and cannot be condensed at one point. In actual use, the output end 15 is often an element having a very narrow condensing area, such as the end face of the optical fiber. The mismatch of the condensing points due to this optical path difference is caused by attenuation (insertion loss) of transmitted light. Appears.
Normally, in addition to the above-mentioned lens, another condensing lens is often used between the birefringent crystal flat plate 23 and the exit end 15 and the entrance end 13. There is no solution to the first problem. This problem occurs not only in the three-terminal optical circulator shown in FIG. 6, but also in the four-terminal optical circulator shown in FIG.
[0010]
The second problem relates to the ability to separate transmitted light. Originally, in FIG. 6A, the transmitted light in the forward direction is emitted only from the emission end 15 and is not emitted from the adjacent incident end 14 at all. However, in practice, the two separated two polarized lights are each elliptically polarized while passing through the Faraday rotator 24. As an example, when the transmitted light is a laser beam of 1.3 to 1.5 μm and the Faraday rotator is composed of a magnetic garnet film and a permanent magnet for magnetically saturating it, its ellipticity (considered as the extinction ratio) May be about 40 to 50 dB. Therefore, in this case, light having an intensity of −40 to −50 dB as compared with the light emitted from the emission end 15 is emitted from the incident end 14 as leakage light of the transmitted light.
[0011]
The second problem in the conventional optical circulator described above is similarly caused in the four-terminal optical circulator shown in FIG. In this case, the leakage light of the forward transmitted light from the incident / exit end 16 that originally exits from the incident / exit end 18 leaks the forward transmitted light from the incident / exit end 17 that exits from the incident / exit end 19. Light exits from the entrance / exit end 18.
[0012]
From the above, there has been a demand for the development of an optical circulator having a structure in which there is no optical path difference of forward transmitted light and the leakage light of forward emitted light can be reduced as compared with a conventional optical circulator. The present invention proposes a new optical circulator structure that can solve this problem.
[0013]
[Means for Solving the Problems]
According to the present invention, an optical element unit comprising two first birefringent crystal flat plates, two second birefringent crystal flat plates, and first and second 45 ° Faraday rotators, It is composed of an incident / exit end disposed on one end side, an exit end and an incident end disposed on the other end side, A birefringent plate is disposed before and after each 45 ° Faraday rotator, and the first and second birefringent crystal plates are disposed between the first and second 45 ° Faraday rotators, The first birefringent crystal flat plate and the second birefringent crystal flat plate; thickness Ratio is 1: √2-1, A reverse magnetic field is applied to the first and second 45 ° Faraday rotators, and Faraday rotation is performed. An optical circulator characterized in that the directions are opposite to each other is obtained.
[0014]
[Action]
In the optical circulator of the present invention, the two Faraday rotators act nonreciprocally with respect to transmitted light in the forward direction and in the reverse direction. The direction of rotation of each element is opposite to each other, and the rotation angle is 45 °. On the other hand, the two types of birefringent crystal flat plates, each having two sheets, have a ratio of separation distance between extraordinary rays and ordinary rays set to 1: √2-1 (= tan (22.5 °)), respectively. The separation direction is an integral multiple of 45 °, and acts reciprocally on transmitted light in the forward direction and the reverse direction. By combining these non-reciprocal working elements and reciprocal working elements, for forward light, it is selected so that it emits to the same point regardless of polarization and has one type of optical path length, On the other hand, the light in the reverse direction is configured to take a different optical path from the transmitted light in the forward direction. As a result, for the forward light, all the transmitted light from the input / output end reaches the input / output end (or output end) on the exit side, and the light traveling in the reverse direction is incident on the forward transmitted light. A part of the light is emitted from an incoming / outgoing end different from the incoming / outgoing end which is a mouth.
[0015]
【Example】
An embodiment of an optical circulator according to the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing the configuration of an optical element constituting the optical circulator in the first embodiment of the present invention. The optical circulator is composed of a total of six optical elements: a birefringent crystal flat plate 1, a Faraday rotator 5, a birefringent crystal flat plate 3, a birefringent crystal flat plate 2, a Faraday rotator 6, and a birefringent crystal flat plate 4. Two examples are shown below, but the configuration of the optical elements is common. The two Faraday rotators 5 and 6 include magnetic field applying means such as permanent magnets. Here, the white arrows illustrated below the birefringent crystal flat plates 1, 2, 3, and 4 indicate the direction of separation of extraordinary light having an extraordinary light component of forward incident light. Further, the curved arrows shown below the Faraday rotators 5 and 6 indicate the direction of the Faraday rotation of the transmitted light. The ratio of the thickness (separation distance) of each birefringent crystal flat plate 1, 2, 3, 4 is 1: √2-1: 1: √2-1 from the forward direction, and the direction of separation of the extraordinary light components (Open white arrow) is vertically upward, obliquely downward 45 °, obliquely upward 45 °, and horizontally leftward in order from the forward direction to the transmission direction. The forward direction is a direction from left to right in FIG. Further, the rotation directions (solid arrows) of the two Faraday rotators 5 and 6 are 45 ° counterclockwise and 45 ° clockwise in the same direction in the transmission direction. A dotted arrow passing through each optical element is the direction of transmitted light in the forward direction.
[0016]
Next, the transmitted light behavior when transmitted light is incident on the optical element will be described with reference to FIG. FIG. 2A is an explanatory diagram regarding the spot position of the separated light when forward transmitted light is incident. Each point in the figure represents the position (coordinates) of the separated light in the plane of each birefringent crystal flat plate.
[0017]
At the incident point O of the transmitted light to the birefringent crystal flat plate 1, the extraordinary light component of the two polarized lights separated here moves to the point P1. Next, after passing through the Faraday rotator 5, the separated light at the point P 1 moves to the point P 2 on the birefringent crystal flat plate 3. On the other hand, the remaining ordinary light component does not move from the point O, but this light becomes extraordinary light in the next birefringent crystal flat plate 2, so that it moves from the point O to the point P 3 and passes through the Faraday rotator 6. The refractive crystal plate 4 moves to the point P2. During this time, since the separated light component that has already been moved to the point P2 behaves as ordinary light consistently, the position from the point P2 does not move. Therefore, the incident light is synthesized again at the point P2 in the birefringent crystal flat plate 4 and passes through the optical circulator without attenuating the separated light.
[0018]
On the other hand, FIG. 2B is an explanatory diagram when transmitted light in the reverse direction enters the optical circulator. Here, the case where the transmitted light in the reverse direction is incident on the point P2 at the same position as the emission point in the forward direction will be described as an example. First, incident light is separated into two kinds of polarized light by the birefringent crystal flat plate 4, and the extraordinary light component of the birefringent crystal flat plate 2 is ordinary light, the birefringent crystal flat plate 3 is extraordinary light, and finally The birefringent crystal flat plate 1 is transmitted as ordinary light, and the position reaches the point P4 after passing through the birefringent crystal flat plate 1 via the point P3. On the other hand, the ordinary light component of the two polarized lights separated by the birefringent crystal flat plate 4 has the next birefringent crystal flat plate 2 as extraordinary light, the birefringent crystal flat plate 3 as normal light, and the last birefringent crystal flat plate. 1 passes through the birefringent crystal flat plate 1 via a point P5 in the figure, and finally reaches a point P6 in the figure. Since the points P4 and P6 are far away from the point O which is the incident point of the transmitted light in the forward direction, it can be seen that the transmitted light from the reverse direction cannot reach the forward incident position at all. This is a transmission path that is emitted from the exit end 15 of the three-terminal optical circulator shown in FIG. 4B described below and proceeds to the entrance / exit end 13 side of the two polarized light separated by the birefringent crystal flat plate 4. 2 shows the position of each polarized light (separated light) when the polarized light passes through the birefringent crystal flat plates 4, 2, 3 and 1, and the plane of each flat plate is the xy plane. Therefore, the path of the ordinary light component of the transmitted light from the emission end 15 is P2 → P5 → P6, and the path of the abnormal light component is P2 → P3 → P4.
[0019]
By the way, the total of the movement amounts of the two separated polarization components in the forward direction (point O → point P1 → point P2 and point O → point P3 → point P2) is exactly the same, and the transmitted optical element ( Since the sum of the thicknesses of the birefringent crystal flat plate and the Faraday element naturally matches, the sum of the optical path lengths of the two types of polarized light is also equal. Therefore, it can be said that the configuration of the optical element of this patent is consistent with the above-mentioned problem to be solved. Further, when the transmitted light travels in the opposite direction, the transmission loss of the reflected return light increases, and high isolation can be obtained.
[0020]
Next, specific movement (behavior) of transmitted light will be described with reference to FIGS. FIG. 4 shows an example related to a three-terminal optical circulator, and FIG. 5 shows an example related to a four-terminal optical circulator.
[0021]
FIG. 4A is a diagram showing a transmission path (behavior) of transmitted light when transmitted light is incident on the constituent optical elements of the three-terminal optical circulator in the reverse direction. As shown in FIG. 4A, the entrance end 14 and the exit end 15 are arranged on the same side, and the exit end 15 emits all the polarization components of the transmitted light passing therethrough regardless of the direction. . The transmitted light incident from the incident / exit end 13 is separated into two types of polarized light by the birefringent crystal flat plate 1, that is, forward transmitted light 9 b and forward transmitted light 9 a, and then the Faraday rotator 5 and the birefringent crystal flat plate 3. Then, the light passes through the birefringent crystal flat plate 2 and the Faraday rotator 6, and is then synthesized again in the birefringent crystal flat plate 4 and emitted from the emission end 15.
[0022]
FIG. 4B is a diagram showing a transmission path (behavior) of transmitted light when transmitted light is incident on the constituent optical elements of the three-terminal optical circulator in the reverse direction. In FIG. 4B, the transmitted light incident from the incident end 14 at a position different from the output end 15 is two types of polarized light, that is, the reverse direction transmitted light 10a and the birefringent crystal flat plate 4 as in the forward direction. The reverse direction transmitted light 10b is separated. Thereafter, the light passes through the Faraday rotator 6, the birefringent crystal flat plate 2, the birefringent crystal flat plate 3, the Faraday rotator 5, and the birefringent crystal flat plate 1 in this order, and one separated light from the incident / exit end 13 (reverse direction transmission in the figure). Only the light 10b) is emitted, and the other separated light (reverse direction transmitted light 10a in the figure) finally reaches the x position in the figure and does not exit from the inside of the optical circulator. Since the position of the incident end 14 is originally determined at a position where a part of the transmitted separated light is emitted from the incident / exit end 13, this is natural. As for the behavior of the transmitted return light from the output end 15, as in the case of the optical circulator described above as the conventional example, the two polarized lights separated in the middle, that is, both the reverse polarized transmitted light 10c and the reverse transmitted light 10j are It reaches the point marked with x in the figure without reaching the entrance / exit end 13, and cannot exit.
[0023]
FIG. 5A is a diagram showing a transmission path (behavior) of transmitted light transmitted from the forward direction to the four-terminal optical circulator. As shown in FIG. 5A, the incident / exit ends 18 and 19 are arranged on the same side, and emit all the polarized components of transmitted light to be transmitted regardless of their directions. As in the case of the three-terminal optical circulator, the transmitted light incident from the input / output end 16 is separated into two types of polarized light, that is, forward transmitted light 11a and forward transmitted light 11b by the birefringent crystal flat plate 1. Thereafter, the respective separated lights, that is, the forward transmitted lights 11 a and 11 b are sequentially transmitted through the Faraday rotator 5, the birefringent crystal flat plate 3, the birefringent crystal flat plate 2, and the Faraday rotator 6, and again in the birefringent crystal flat plate 4. The combined light is emitted from the incident / exit end 18. Further, when transmitted light is incident from the incident / exit end 17, the separated light, that is, forward transmitted light 11 c and 11 d is emitted from the incident / exit end 19 through the same process as described above.
[0024]
FIG. 5B is a diagram showing a path (behavior) of transmitted light incident on the four-terminal optical circulator from the reverse direction. The transmitted light incident from the incident / exit end 18 is separated by the birefringent crystal flat plate 4 into two types of polarized light, that is, the reverse direction transmitted light 12a and the reverse direction transmitted light 12b as in the forward direction. Thereafter, the respective separated lights, that is, the reverse direction transmitted lights 12a and 12b are sequentially transmitted through the Faraday rotator 6, the birefringent crystal flat plate 2, the birefringent crystal flat plate 3, the Faraday rotator 5, and the birefringent crystal flat plate 1 in order. Only one separated light, that is, the reverse direction transmitted light 12b is emitted from the end 16, and the other separated light, that is, the reverse direction transmitted light 12a finally reaches the position x in the drawing and does not exit from the inside of the optical circulator. . Even when transmitted light in the reverse direction is incident from the incident / exit end 19, only a part thereof, that is, only the reverse direction transmitted light 12c is emitted from the incident / exit end 17, similarly to the above, and the other separated light, that is, the reverse direction transmitted light 12d. Does not exit from inside the optical circulator. 4 and FIG. 5, the direction of magnetic field application (arrow H) in the Faraday rotators 5 and 6 is opposite to each other. This is because the directions of the Faraday rotations in the Faraday rotator 5 and the Faraday rotator 6 are mutually opposite. It corresponds to the reverse direction. Note that the dotted arrows from the birefringent crystal flat plate 2 to the birefringent crystal flat plate 4 in FIG. 4A are actually overlapped when viewed from the side, but are illustrated as viewed from obliquely above for the sake of explanation. . In FIG. 4B, the solid line arrows extending from the birefringent crystal flat plate 4 to the birefringent crystal flat plate 3 are actually overlapped when viewed from the side, but are illustrated as viewed obliquely from above. . The same applies to FIGS. 5A and 5B.
[0025]
Next, a configuration example of an actual optical circulator according to the optical element configuration of the three-terminal and four-terminal optical circulator in the present invention described above will be shown.
[0026]
In the three-terminal optical circulator shown in FIGS. 4A and 4B, the optical element has a rutile crystal as a birefringent crystal plate, a bismuth-substituted magnetic garnet film as a Faraday rotator, and Sm-Co permanent as a magnetic field applying means. Using a magnet, a non-reflective coating is applied to the light transmission surface of each optical element. In addition, a total of three light incident / exit ends are optical fiber end faces, and coupling lenses are disposed between the optical fiber end faces and the optical elements facing the optical fiber end faces for coupling of transmitted light. The wavelength of transmitted light is 1.55 μm in both forward and reverse directions, the rotation angle by the Faraday rotator is 45 °, and the thickness of the birefringent crystal plate is 1.500 mm, 0.621 mm, 1.500 mm in order from the forward direction. 0.621 mm. The ratio of the thick crystal to the thin crystal is 1: √2-1, and the optical axis direction of each crystal is inclined by 48 ° with respect to the normal of the light transmission surface, and the separation distance of the extraordinary light is maximum. It has become.
[0027]
The optical characteristics of each element used are as follows.
[0028]
The characteristics of a Faraday rotator using a magnetic garnet film are a Faraday rotation angle of 44.8 °, an insertion loss of 0.08 dB, and an extinction ratio of 45 dB.
[0029]
The characteristics of the birefringent crystal flat plate using rutile crystal are that the ellipticity of separated ordinary light is 50 dB and the insertion loss is 0.05 to 0.1 dB.
[0030]
In the three-terminal optical circulator shown in FIGS. 4A and 4B made using these optical elements, forward insertion loss is 0.9 dB or less, and light is incident from the light emitting end 15 in the reverse direction. Attenuation of 50 dB or more of return light transmitted to the end 13 was obtained. This value is larger than the extinction ratio of the magnetic garnet film used, and cannot be realized at all in a conventional optical circulator using only one conventional Faraday rotator. The insertion loss of the transmitted light (in principle, half of which attenuates) from the light incident end 14 to the light incident end 13 is not particularly measured.
[0031]
Similarly, in the four-terminal optical circulator shown in FIGS. 5A and 5B, the insertion loss of the forward direction is 0.9 dB or less, and the transmitted return light from the light emitting end 18 to the light incident end 16 in the reverse direction is reduced. An attenuation of 50 dB or more is obtained. This figure is also something that could not be realized at all in the past. In addition, the insertion loss of the transmitted light (in principle, half of which attenuates) from the light incident end 18 to the light incident end 17 is not measured.
[0032]
FIG. 3 shows an optical element configuration of an optical circulator in a second embodiment different from the case of FIG. The optical elements used are six from the forward direction (left side of the figure): a birefringent crystal flat plate 1, a Faraday rotator 5, a birefringent crystal flat plate 3, a birefringent crystal flat plate 2, a Faraday rotator 6, and a birefringent crystal flat plate 4. It is. The white arrow written below each birefringent crystal flat plate represents the direction of separation of forward incident light having an extraordinary light component. Moreover, the arrow described below the Faraday rotator represents the direction of the Faraday rotator of the transmitted light in each element. A dotted arrow passing through each optical element indicates the direction of transmitted light in the forward direction. Even if the above-described element configuration is used instead of the optical element configuration shown in FIG. 1, an optical circulator having the same effect as described above can be configured.
However, there is a slight difference in the relative position of the incident / exit end within the optical element surface.
[0033]
As described above, in the optical circulators according to the first and second embodiments of the present invention, the optical path lengths of the separated two polarized light beams in the forward direction are equal and the optical attenuation amount of the transmitted light in the reverse direction is 50 dB or more. These values are obtained, and none of these can be realized by a conventional optical circulator. Accordingly, the optical circulator according to the present invention is expected to make a great contribution in the future use of the optical circulator for optical communication and the like. In this example, the ratio of the separation distance of each birefringent crystal flat plate was two: 1-√2-1: 1: √2-1 and √2-1: 1: √2-1: 1. However, even in the case of 1: 1: √2-1: √2-1 and √2-1: √2-1: 1: 1, the same effect as described above can be obtained.
[0034]
【The invention's effect】
As described above, based on the content of the claims, by making the optical path length of the incident light separated and combined at the time of transmission equal, the transmission loss at the outgoing end of the forward transmission light that has conventionally occurred can be reduced. By removing the cause and using two Faraday rotators in the interior, it is possible to improve the transmission attenuation of the reflected return light (measured value of 50 dB or more in the embodiment).
[0035]
In addition, the use of this optical circulator can greatly improve the optical characteristics compared to conventional products, so there is no high-performance product that has conventionally required an optical circulator, such as optical communication. It is expected that the use of optical circulators will become more active in the field where the use has been abandoned, and the characteristics and reliability of the entire system will be greatly improved.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of an optical element constituting an optical circulator in a first embodiment of the present invention.
FIG. 2 (a) is a diagram showing the position of the center of transmitted light within the optical element surface when forward light is incident from the left side, and FIG. 2 (b) is the right side. It is the figure shown about the case where the transmitted light of the reverse direction enters from.
FIG. 3 is a diagram showing a configuration of an optical element constituting the optical circulator in the second embodiment.
4A is a diagram showing a transmission path of transmitted light when transmitted light is incident on the constituent optical elements of the three-terminal optical circulator of FIG. 1 in the forward direction; FIG. FIG. 4B is a diagram illustrating a transmission path of transmitted light when transmitted light in the reverse direction is incident from the incident end or the output end of FIG.
FIG. 5A is a diagram showing a transmission path (behavior) of transmitted light when transmitted light transmitted from the forward direction enters the four-terminal optical circulator, and FIG. FIG. 6 is a view showing a transmission path of transmitted light when transmitted light in the reverse direction is incident from the incident / exit end or the incident / exit end of FIG.
6A is a diagram showing a transmission path of transmitted light when forward transmitted light is incident on a conventional three-terminal optical circulator; FIG. 6B is a diagram illustrating FIG. It is the figure shown about the permeation | transmission path | route of this transmitted light when the transmitted light of the reverse direction injects from the entrance / exit end of a).
7A is a diagram showing a transmission path of transmitted light when forward transmitted light is incident on a conventional four-terminal optical circulator, and FIG. 7B is a diagram illustrating FIG. It is the figure shown about the permeation | transmission path | route of this transmitted light when the transmitted light of the reverse direction injects from the entrance / exit end of a).
[Explanation of symbols]
1,2,3,4,21,22,23 Birefringent crystal flat plate
5, 6, 24 Faraday rotator
9a, 9b, 11a, 11b, 11c, 11d, 25a, 25b, 27a, 27b, 27c, 27d Forward transmitted light
10a, 10b, 10c, 10d, 12a, 12b, 12c, 12d, 26a, 26b, 26c, 26d, 28a, 28b, 28c, 28d Reverse transmitted light
13, 16, 17, 18, 19 Input / output end
14 Incident end
15 Output end

Claims (6)

An optical element portion comprising two first birefringent crystal flat plates, two second birefringent crystal flat plates, and first and second 45 ° Faraday rotators, and one end side of the optical element portion It is composed of an entrance and exit end, an exit end and an entrance end arranged on the other end side,
Birefringent plates are arranged before and after each 45 ° Faraday rotator,
The first and second birefringent crystal plates are disposed between the first and second 45 ° Faraday rotators;
The thickness ratio of the first birefringent crystal flat plate and the second birefringent crystal flat plate is 1: √2-1, and the first and second 45 ° Faraday rotators are opposite to each other. An optical circulator, wherein a magnetic field is applied and the directions of Faraday rotation are opposite to each other. An optical element portion comprising two first birefringent crystal flat plates, two second birefringent crystal flat plates, and first and second 45 ° Faraday rotators, and one end side of the optical element portion The first and second input / output ends, and the third and fourth input / output ends arranged on the other end side,
Birefringent plates are arranged before and after each 45 ° Faraday rotator,
The first and second birefringent crystal plates are disposed between the first and second 45 ° Faraday rotators;
The thickness ratio of the first birefringent crystal flat plate and the second birefringent crystal flat plate is 1: √2-1, and the first and second 45 ° Faraday rotators are opposite to each other. An optical circulator, wherein a magnetic field is applied and the directions of Faraday rotation are opposite to each other.   3. The optical circulator according to claim 1, wherein the optical element section includes the first birefringent crystal flat plate, the first 45 ° Faraday rotator, the second birefringent crystal flat plate from the input and output ends, An optical circulator, wherein the first birefringent crystal flat plate, the second 45 ° Faraday rotator, and the second birefringent crystal flat plate are arranged in this order.   3. The optical circulator according to claim 1, wherein the optical element section includes the second birefringent crystal flat plate, the first 45 ° Faraday rotator, the first birefringent crystal flat plate from the input and output ends, An optical circulator, wherein the second birefringent crystal flat plate, the second 45 ° Faraday rotator, and the first birefringent crystal flat plate are arranged in this order.   3. The optical circulator according to claim 1, wherein the optical element section includes the first birefringent crystal flat plate, the first 45 ° Faraday rotator, the first birefringent crystal flat plate from the input and output ends, An optical circulator, wherein the second birefringent crystal flat plate, the second 45 ° Faraday rotator, and the second birefringent crystal flat plate are arranged in this order.   3. The optical circulator according to claim 1, wherein the optical element section includes the second birefringent crystal flat plate, the first 45 ° Faraday rotator, the second birefringent crystal flat plate from the input and output ends, An optical circulator, wherein the first birefringent crystal flat plate, the second 45 ° Faraday rotator, and the first birefringent crystal flat plate are arranged in this order.

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