JPH08110496A - Optical circulator - Google Patents

Abstract

PURPOSE: To obtain an optical circulator of a structure in which the optical path difference of forward transmitted light does not exist and which is capable of decreasing the leaking light of forward exist light as compared with the optical circulator of the conventional type. CONSTITUTION: This optical circulator is composed of an optical element part consisting of first double refractive crystal flat plates 1, 2, second double refractive crystal flat plates 3, 4 and first and second 45 deg. Faraday rotators 5, 6, an incident and exit end 13 arranged on one end side of the optical element part and an exit end 15 and incident end 14 arranged on the other end side. The ratio of the separating distances of the first double refractive crystal flat plates 1, 2 and the second double refractive crystal flat plates 3, 4 is 1:√2-1 and the directions of rotation by the first and second 45 deg. Faraday rotators 5, 6 are reverse from each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circulator for changing the path of transmitted light in the forward direction and the reverse direction in optical communication and optical information processing, and more particularly to all polarized light for transmitted light in at least one direction. The present invention relates to an optical circulator that operates with respect to light in a state.

[0002]

2. Description of the Related Art In an optical communication system, an optical circulator separates both forward and backward transmitted light when performing bidirectional communication using a single optical fiber, and transmits the transmitted light between a plurality of optical fibers. Used as an element for replacement.
Generally, a three-terminal optical circulator is used to install the laser at the laser emission end to separate transmitted light in the opposite direction, and a four-terminal optical circulator is used to exchange transmitted light between optical fibers.

Based on the conventional example shown in FIGS. 6 and 7,
The configuration of a conventional optical circulator will be described. Figure 6
(A) is a diagram showing a transmission path (behavior) of the transmitted light when the transmitted light in the forward direction from the entrance / exit end enters the optical circulator. FIG. 6 shows an example of a 3-terminal type optical circulator, and FIG. 7 shows an example of a 4-terminal type optical circulator. In FIG. 6A, the optical circulator comprises an input / output end 13, a birefringent crystal flat plate 21,
22 and 23, a Faraday rotator 24, an entrance end 14, and an exit end 15. When the direction of the transmitted light from the left side to the right side in FIG. 6A is the forward direction, the incident light IL ₁ incident from the entrance / exit end 13 is separated by the birefringent crystal flat plate 21 into two types of polarized light. That is, the incident light IL ₁ is the forward transmitted light 25 having the first polarization component.
a and a forward transmitted light 25b having a second polarization component, and is separated into a Faraday rotator 24, a birefringent crystal flat plate 22,
The light passes through the birefringent crystal flat plate 23 in order. Birefringent crystal flat plate 2
3 combines the forward direction transmitted lights 25a and 25b and sends the combined transmitted light LL ₁ to the emission end 15. It reaches the emission end 15 and is emitted from the emission end 15.

On the other hand, FIG. 6B is a diagram showing a transmission path (behavior) of the transmitted light when the transmitted light in the opposite direction from the incident end enters the optical circulator. The incident incident light IL ₂ is classified into two types of polarized light by the birefringent crystal flat plate 23. That is, the incident light IL ₂ is separated into the backward transmitted light 26a having the first polarization component and the backward transmitted light 26b having the second polarization component, and the birefringent crystal flat plate 2
2, the Faraday rotator 24, and the birefringent crystal flat plate 21 are sequentially transmitted. The two types of polarized light separated, that is, the backward transmitted light 26a and the backward transmitted light 26b are not combined,
Only the backward transmitted light 26b is finally emitted from the incident / emission end 13. On the other hand, the backward transmitted light 26a cannot reach the x position in the figure and be emitted. In addition, when the transmitted return light enters in the opposite direction from the exit end 15, FIG.
As shown in (b), since there is no separated light component reaching the entrance / exit end 13, incident light cannot pass through the optical circulator at all.

In the case of the four-terminal type optical circulator shown in FIG. 7 (a), similarly to the above-mentioned three-terminal type optical circulator, incident lights IL ₃ and IL ₄ respectively incident from the entrance / exit end 16 and the entrance / exit end 17 are incident. Is its separated light, that is,
The forward transmitted lights 27a and 27b, 27c and 27d are respectively combined in the birefringent crystal flat plate 23 without being attenuated and reach the entrance / exit end 18 and the entrance / exit end 19 as LL ₃ and LL ₄ . Further, as shown in FIG. 7B, transmitted light IL ₅ , I ₅ in the opposite direction from the entrance / exit end 18 and the entrance / exit end 19 is transmitted.
When L ₆ is incident, only the backward transmitted light 28a and the backward transmitted light 28d, which are the respective separated lights, reach the entrance / exit end 17 and the entrance / exit end 16, respectively. The remaining transmitted light separated by the birefringent crystal flat plate 23, that is, the backward transmitted light 28b and the backward transmitted light 28c cannot reach the input / output ends 16 and 17 on the forward direction side, and thus the optical circulator. In the middle, that is, the X position in the figure cannot be reached and the light cannot be emitted.

[0006]

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 kinds of polarization components whose polarization planes are orthogonal to each other by the birefringent crystal flat plate 21 on the way, and then again by the birefringent crystal flat plate 23. Is synthesized. However, the optical path lengths of the separated two polarized lights when passing through a total of three birefringent crystal flat plates are not the same. In general, the refractive index when polarized light passes through a birefringent crystal flat plate is determined by the relationship between the plane of polarization and the crystal axis of the crystal flat plate. In this case, the polarized component of the transmitted light in a specific direction is separated and transmitted obliquely to the crystal surface (extraordinary light), and the other transmitted light component is transmitted as it is perpendicular to the crystal surface ( Tsunemitsu). At this time, the total thickness of the birefringent crystal flat plates that both transmit is naturally the same, but since the refractive indexes of the two in each crystal flat plate are different, the total transmission optical path length of each polarized light is generally equal. There is no.

The relative thickness of the birefringent crystal flat plate 21 is √2
Letting t be the relative thicknesses of the birefringent crystal flat plate 22 and the birefringent crystal flat plate 23, respectively. Forward transmitted light 25 excluding the Faraday rotator 24 in FIG.
The total optical path lengths of a and the forward transmitted light 25b are nt + (√2 + 1) n′t and (√2 + 1) nt + n ′, respectively.
t, and the difference between them is √2t | n−n ′ |.

It should be noted that n and n'are the refractive indexes of the ordinary and extraordinary rays of the birefringent crystal flat plate, respectively.

When n ≠ n ', this value does not become zero, and at that time, in the optical circulator presented as a conventional example, it is found that the optical path lengths of the polarized components separated in the forward transmitted light do not coincide with each other. . If this transmitted light is parallel light, there is no problem, but in a general application, a condenser lens (not shown) is provided between the input / output end 13 and the birefringent crystal flat plate 21.
Is installed, and the transmitted light is condensed so that the focal point is arranged on the end face of the emission end 15. In such a case, due to the difference in the optical path lengths, the transmitted light cannot be condensed at two points deviated in the optical axis direction and condensed at one point. In actual use, an element having a very narrow condensing area such as an end face of an optical fiber is often used at the emitting end 15, and the discrepancy of condensing points due to this optical path difference causes attenuation (insertion loss) of transmitted light. Appears. Usually, in addition to the above lens, another condensing lens is often installed between the birefringent crystal flat plate 23 and the exit end 15 and the entrance end 13 for use. There is no solution to the first problem. This problem occurs not only in the 3-terminal optical circulator shown in FIG. 6 but also in the 4-terminal optical circulator shown in FIG.

The second problem relates to the ability of separating transmitted light. Originally, in FIG. 6A, the transmitted light in the forward direction is emitted only from the emitting end 15, and the incident end 14 adjacent to the emitting end 15.
Is not emitted at all. However, in reality, each of the two kinds of separated two polarized lights while passing through the Faraday rotator 24 becomes an elliptically polarized light. As an example, the transmitted light is 1.3
When the Faraday rotator is composed of a magnetic garnet film and a permanent magnet for magnetically saturating it with a laser beam of ~ 1.5 μm, its ellipticity (which may be considered as an extinction ratio) is about 40 to 50 dB. Will be. Therefore, in this case, compared with the light emitted from the light emitting end 15, −40 to −5.
The light having the intensity of 0 dB is emitted from the incident end 14 as the leaked light of the transmitted light.

The second problem with the conventional optical circulator described above also occurs in the four-terminal type optical circulator shown in FIG. In this case, the leak light of the forward transmitted light from the entrance / exit end 16 that originally exits from the entrance / exit end 18 leaks the forward transmitted light from the entrance / exit end 19 that exits from the entrance / exit end 19. Light is emitted from each of the entrance and exit ends 18.

From the above, it is desired to develop an optical circulator having a structure in which there is no optical path difference of forward transmitted light and leakage light of forward outgoing light can be reduced as compared with a conventional optical circulator. It was The present invention proposes a new optical circulator structure that can solve this problem.

[0013]

According to the present invention, an optics comprising two first birefringent crystal flat plates, two second birefringent crystal flat plates, and first and second 45 ° Faraday rotators. The first birefringent crystal flat plate and the second birefringent crystal flat plate, which are composed of an element part, an entrance / exit end arranged on one end side of the optical element part, and an exit end and an entrance end arranged on the other end side. An optical circulator characterized in that a ratio of separation distances of birefringent crystal flat plates is 1: √2-1 and rotation directions of the first and second 45 ° Faraday rotators are opposite to each other. can get.

[0014]

In the optical circulator of the present invention, the two Faraday rotators act nonreciprocally on the forward and backward transmitted light. The rotation directions of the respective elements are opposite to each other, and the rotation angles thereof are 45 °. On the other hand, the ratio of the separation distances for the extraordinary ray and the ordinary ray for the two types of birefringent crystal flat plates of two pieces each is 1: √2-1 (= tan
(22.5 °)) and the separation direction is 4
It is an integral multiple of 5 ° and acts reciprocally on the transmitted light in the forward and reverse directions. By combining these non-reciprocal action elements and reciprocal action elements, for forward light, the light is emitted to the same point regardless of the polarization and the optical path length is selected to be one type, On the other hand, for the light in the reverse direction, an optical path different from that of the transmitted light in the forward direction is taken.
As a result, for light in the forward direction, all transmitted light from the entrance / emission end reaches the entrance / emission end (or exit end) on the exit side, and light traveling in the opposite direction is incident on the forward transmitted light. A part of the light exits from the entrance / exit end that is different from the entrance / exit end that is the mouth.

[0015]

Embodiments of the optical circulator according to the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing a configuration of an optical element that constitutes an optical circulator in the first exemplary embodiment of the present invention. The optical circulator is composed of a total of 6 optical elements including 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 types of examples are shown below, but the configurations of the optical elements are common. Further, the two Faraday rotators 5 and 6 include magnetic field applying means such as a permanent magnet. Here, the outline arrows shown under each of the birefringent crystal flat plates 1, 2, 3 and 4 represent the direction of separation of the extraordinary light having the extraordinary light component of the forward incident light. The curved arrows below the Faraday rotators 5 and 6 represent the directions of Faraday rotation of transmitted light. Each birefringent crystal flat plate 1,
The ratio of the thickness (separation distance) of 2, 3, 4 is 1: √ from the forward direction.
2-1: 1: √2-1, and the direction of separation of these extraordinary light components (white arrows) is vertically upward, diagonally right downward 45 °, and diagonally right upward 45 from the forward direction toward the transmission direction. °, horizontal to the left. The forward direction is the direction from left to right in FIG. Further, the rotation directions (solid arrows) of the two Faraday rotators 5, 6 are 45 ° counterclockwise and 45 ° clockwise, respectively, in the same manner as the transmission direction. The dotted arrows passing through each optical element indicate the forward direction of transmitted light.

Next, the behavior of the transmitted light when the transmitted light is incident on this optical element will be described with reference to FIG. FIG. 2A is an explanatory diagram regarding the spot position of the separated light when the forward transmitted light is incident. Each point in the figure represents the position (coordinates) of the separated light within the plane of each birefringent crystal flat plate.

Incident point O of transmitted light on the birefringent crystal flat plate 1
Then, the extraordinary light component of the two polarized lights separated here is the point P1.
Move to. Next, after passing through the Faraday rotator 5, the separated light at the point P1 on the birefringent crystal flat plate 3 moves to the point P2. 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 the point O
To the point P3, and after passing through the Faraday rotator 6, the birefringent crystal flat plate 4 moves to the point P2. During this time, the separated light component that has already moved to the point P2 consistently behaves as ordinary light, and therefore the position does not move from the point P2. Therefore, the incident light is recombined at the point P2 in the birefringent crystal flat plate 4, and passes through the optical circulator without attenuating the separated light.

On the other hand, FIG. 2 (b) is an explanatory view when the transmitted light in the opposite direction is incident on the optical circulator. Here, the point P where the transmitted light in the reverse direction is at the same position as the emission point in the forward direction
The case where the light beam is incident on 2 will be described as an example. First, the incident light is separated into two types of polarized light by the birefringent crystal flat plate 4, and the extraordinary light component among them is the next birefringent crystal flat plate 2 as ordinary light, further the birefringent crystal flat plate 3 as extraordinary light, and finally. Since it passes through the birefringent crystal flat plate 1 of FIG.
After passing through the birefringent crystal flat plate 1 via 3, the point P4 is reached. On the other hand, the ordinary components of the two polarized lights separated by the birefringent crystal flat plate 4 are the next birefringent crystal flat plate 2 as extraordinary light, the birefringent crystal flat plate 3 as ordinary light, and the last birefringent crystal flat plate. Since 1 transmits as extraordinary light, its position finally reaches point P6 in the figure after passing through the birefringent crystal flat plate 1 via point P5 in the figure. Since the points P4 and P6 are far apart from the point O, which is the incident point of the transmitted light in the forward direction, it is understood that the transmitted light from the reverse direction cannot reach the incident position in the forward direction at all. This is a transmission path that is emitted from the emission end 15 of the three-terminal optical circulator shown in FIG. 4B described below and proceeds to the side of the input / output end 13 of the two polarized lights separated by the birefringent crystal flat plate 4. In Fig. 3, when each polarized light is transmitted through the birefringent crystal flat plates 4, 2, 3 and 1, the position of each polarized light (separated light) is shown with the plane of each flat plate as the xy plane. Therefore, the path of the ordinary component of the transmitted light from the emission end 15 is P2 →
P5 → P6, and the path of extraordinary light components is P2 → P3 → P
It becomes 4.

By the way, the total sum of the movement amounts of the positions of the two types of 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 is transmitted. Since the sum of the thicknesses of the optical elements (the birefringent crystal flat plate and the Faraday element) is naturally the same, the sum of the optical path lengths of the two types of polarized light is also the same. Therefore, it can be said that the configuration of the optical element of this patent is in accordance with the spirit of the problems to be solved. If the transmitted light travels in the opposite direction,
The transmission loss of the reflected return light becomes large, and high isolation can be obtained.

Next, a specific movement (behavior) of transmitted light will be described with reference to FIGS. 4 and 5. 4 is 3
An example relating to a terminal optical circulator, and FIG. 5 is an example relating to a 4-terminal optical circulator.

FIG. 4A is a diagram showing the transmission path (behavior) of the transmitted light when the transmitted light is incident on the constituent optical elements of the three-terminal optical circulator in the opposite 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 their orientations. . The transmitted light incident from the entrance / exit end 13 is separated into two types of polarized light by the birefringent crystal flat plate 1, that is, the forward transmitted light 9b and the forward transmitted light 9a, and then the Faraday rotator 5 and the birefringent crystal flat plate 3 are separated. After passing through the birefringent crystal flat plate 2 and the Faraday rotator 6, they are combined again in the birefringent crystal flat plate 4 and emitted from the emission end 15.

FIG. 4B is a diagram showing the transmission path (behavior) of the transmitted light when the transmitted light is incident on the constituent optical elements of the three-terminal optical circulator in the opposite direction. Figure 4 (b)
In the same manner as in the case of the birefringent crystal flat plate 4 in the forward direction, the transmitted light incident from the incident end 14 at a position different from the emitting end 15 has two types of polarized light, that is, the backward transmitted light 10a and the backward transmitted light 10b. Is separated into After that, 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 are sequentially transmitted, and only one of the separated lights (reverse direction transmitted light 10b in the figure) is emitted from the entrance / exit end 13 and the other separated light (reverse direction transmitted light in the figure). 10a) finally reaches the X position in the figure and does not exit from the inside of the optical circulator. It should be noted that the position of the incident end 14 is determined at a position where a part of the transmitted / separated light is emitted from the incident / emission end 13 in the first place. Regarding the behavior of the transmitted return light from the exit end 15, as in the case of the optical circulator described above as a conventional example, the two polarized lights separated in the middle, that is, the reverse polarized light transmitted light 10c and the reverse direction transmitted light 10j are both , The point marked by X in the figure is reached without reaching the input / output end 13, and the light cannot be emitted.

FIG. 5A is a diagram showing a transmission path (behavior) of transmitted light which has been transmitted from the forward direction to the 4-terminal optical circulator. As shown in FIG. 5A, the entrance and exit ends 18, 19
Are arranged on the same side, and all the polarized components of the transmitted light that is transmitted are emitted regardless of their orientations. As in the case of the three-terminal optical circulator, the transmitted light that has entered from the input / output end 16 has two types of polarized light in the birefringent crystal flat plate 1, that is, the forward transmitted light 11a and the forward transmitted light 11b.
Is separated into After that, the respective separated lights, that is, the forward transmitted lights 11a and 11b are separated into Faraday rotator 5, birefringent crystal flat plate 3, birefringent crystal flat plate 2, and Faraday rotator 6.
Through the birefringent crystal flat plate 4 and are combined again to be emitted from the entrance / exit end 18. Also, when the transmitted light is incident from the incident / emission end 17, the separated light, that is, the forward transmitted light 11c, 11d is emitted from the incident / emission end 19 through the same process as above.

FIG. 5B is a diagram showing the path (behavior) of the transmitted light which is incident on the 4-terminal optical circulator from the opposite direction. The transmitted light incident from the entrance / exit end 18 is separated into two types of polarized light, that is, the backward transmitted light 12a and the backward transmitted light 12b, by the birefringent crystal flat plate 4 as in the case of the forward direction. After that, the respective separated lights, that is, the backward transmitted lights 12a and 12b are 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 to enter and exit. Only one of the separated lights, that is, the backward transmitted light 12b is emitted from the end 16,
The other separated light, that is, the backward transmitted light 12a finally reaches the X position in the figure and is not emitted from the inside of the optical circulator. Even when the transmitted light in the reverse direction is incident from the incident / emission end 19, a part thereof, that is, the transmitted light in the reverse direction 12
Only c is emitted from the entrance / emission end 17, and the other separated light, that is, the backward transmitted light 12d is not emitted from the inside of the optical circulator. The directions of applying magnetic fields (arrows H) in the Faraday rotators 5 and 6 in FIGS. 4 and 5 are opposite to each other. This means that the directions of Faraday rotation in the Faraday rotator 5 and the Faraday rotator 6 are opposite to each other. It corresponds to the opposite direction. It should be noted that the dotted arrows extending from the birefringent crystal flat plate 2 to the birefringent crystal flat plate 4 in FIG. 4A actually overlap when viewed from the side, but for the sake of explanation,
It is shown as seen from diagonally above. The solid line arrows from the birefringent crystal flat plate 4 to the birefringent crystal flat plate 3 in FIG. 4B actually overlap each other when viewed from the side, but for illustration purposes, they are shown as viewed from diagonally above. . Figure 5
The same applies to (a) and FIG. 5 (b).

Next, a configuration example of an actual optical circulator according to the optical element configuration of the 3-terminal and 4-terminal optical circulator in the present invention described above will be shown.

The structure of the optical element is shown in FIGS.
In the three-terminal optical circulator of (b), a birefringent crystal flat plate is a rutile crystal, a Faraday rotator is a bismuth-substituted magnetic garnet film, and S is a magnetic field applying means.
An m-Co permanent magnet is used to apply a non-reflective coating to the light transmitting surface of each optical element. The total of three light input / output ends are optical fiber end faces, and coupling lenses are arranged between the optical fiber end face and the optical element facing the optical fiber end face for coupling of transmitted light. The wavelength of the transmitted light is 1.55 μm in both the forward and reverse directions, the rotation angle by the Faraday rotator is 45 °, and the thickness of the birefringent crystal flat plate is 1.500 mm, 0.
It is 621 mm, 1.500 mm, and 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 48 with respect to the normal to the light transmitting surface.
It is tilted, and the separation distance of extraordinary light is maximum.

The optical characteristics of each element used are as follows.

The characteristics of the Faraday rotator using the magnetic garnet film are that the Faraday rotation angle is 44.8 °, the insertion loss is 0.08 dB, and the extinction ratio is 45 dB.

The characteristics of the birefringent crystal flat plate using the rutile crystal are that the ellipticity of the separated ordinary ray is 50 dB and the insertion loss is 0.05.
Is about 0.1 dB.

FIG. 4 produced using these optical elements.
In the three-terminal optical circulator shown in FIGS. 4A and 4B, the insertion loss in the forward direction is 0.9 dB or less, and the attenuation amount of the transmitted return light from the light emitting end 15 in the reverse direction to the light incident end 13 is 50 dB or more. Got This value is larger than the extinction ratio of the magnetic garnet film used, and is 1 for the conventional Faraday rotator.
This cannot be realized at all in the conventional optical circulator using only one sheet. The transmitted light from the light incident end 14 to the light incident end 13 (half of which is attenuated in principle)
The insertion loss was not measured.

Similarly, in the four-terminal optical circulator shown in FIGS. 5A and 5B, the insertion loss in the forward direction is 0.9 dB or less, and the light is transmitted from the light emitting end 18 to the light incident end 16 in the reverse direction. A return light attenuation of 50 dB or more is obtained. This numerical value is also something that could not be realized in the past. Note that the insertion loss of the transmitted light from the light incident end 18 to the light incident end 17 (half of which is attenuated in principle) was not measured.

Further, FIG. 3 shows a second example different from the case of FIG.
The optical element configuration of the optical circulator in the example of FIG. The optical element to be used is 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 from the forward direction (left side of the figure). Is. The white arrow below each birefringent crystal flat plate indicates the direction of separation of the forward incident light having an extraordinary light component. The arrow below the Faraday rotator indicates the direction of the Faraday rotator of the transmitted light in each element. The dotted arrow penetrating each optical element is the direction of transmitted light in the forward direction. An optical circulator having exactly the same effect as described above can be configured by using the above-described element configuration instead of the configuration of the optical element shown in FIG. However, there are some differences in the relative positions of the entrance and exit ends within the optical element plane.

As described above, in the optical circulators according to the first and second embodiments of the present invention, the optical path lengths of the split two polarized lights in the forward direction are the same, and the optical attenuation of the transmitted return light in the reverse direction is the same. As a result, a numerical value of 50 dB or more was obtained, and none of these could be realized by the conventional optical circulator. Therefore, the optical circulator of the present invention is expected to make a great contribution in the future where the optical circulator is used in optical communication and the like. In this example, the separation distance ratio of each birefringent crystal flat plate was 1-√.
2-1: 1: √2-1 and √2-1: 1: √2-1: 1
2 are shown, but 1: 1: √2-1: √2-1 and √
Even in the case of 2-1: √2-1: 1: 1, the same effect as above can be obtained.

[0034]

As described above, based on the contents of the claims, by equalizing the optical path lengths of the incident lights separated / combined at the time of transmission, the emission of the forward transmitted light that has conventionally occurred By eliminating the cause of the transmission loss at the end and by using two Faraday rotators inside it, the transmission attenuation of the reflected return light is improved (measured value 50d in the embodiment
B or more) can be achieved.

Further, by using this optical circulator, the optical characteristics can be greatly improved as compared with the conventional product, so that there is a high performance product such as optical communication, which has conventionally required the optical circulator. Therefore, it is expected that the use of optical circulators will become active in fields where the use of optical circulators has been abandoned, and the characteristics and reliability of the entire system will be greatly improved.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an optical element that constitutes an optical circulator in a first exemplary embodiment of the present invention.

FIG. 2 (a) is a diagram showing a position of a center of transmitted light in an optical element plane when forward incident light is incident from the left side, and FIG. 2 (b) is a 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 that constitutes an optical circulator in a second example.

4 (a) is a diagram showing a transmission path of the transmitted light when the transmitted light is incident on the constituent optical element of the three-terminal optical circulator of FIG. 1 in the forward direction, and FIG. )
FIG. 4 is a diagram showing a transmission path of transmitted light when the transmitted light in the opposite direction is incident from the incident end or the emission end of FIG.

FIG. 5A is a diagram showing a transmission path (behavior) of the transmitted light when the transmitted light transmitted from the forward direction is incident on the 4-terminal optical circulator, and FIG. FIG. 6 is a diagram showing a transmission path of transmitted light in the case where transmitted light in the opposite direction is incident from the incident / emission end or the incident / emission end of FIG.

6 (a) is a diagram showing a transmission path of transmitted light when forward transmitted light is incident on a conventional 3-terminal optical circulator, and FIG. 6 (b) is a diagram showing FIG. It is the figure which showed the permeation | transmission path | route of this transmitted light when the transmitted light of a reverse direction injects from the entrance-emission end of a).

FIG. 7 (a) is a diagram showing a transmission path of transmitted light when forward transmitted light is incident on a conventional 4-terminal optical circulator, and FIG. It is the figure which showed the permeation | transmission path | route of this transmitted light when the transmitted light of a reverse direction injects from the entrance-emission 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, 25
a, 25b, 27a, 27b, 27c, 27d Forward transmitted light 10a, 10b, 10c, 10d, 12a, 12b, 1
2c, 12d, 26a, 26b, 26c, 26d, 28
a, 28b, 28c, 28d Reverse direction transmitted light 13, 16, 17, 18, 19 Input / output end 14 Input end 15 Output end

Claims (6)

[Claims] 1. An optical element section comprising two first birefringent crystal flat plates, two second birefringent crystal flat plates, and first and second 45 ° Faraday rotators, and one end of the optical element section. Side and the exit end and the entrance end arranged on the other end side, and the ratio of the separation distances of the first birefringent crystal flat plate and the second birefringent crystal flat plate is 1: √2-1
And the rotation directions of the first and second 45 ° Faraday rotators are opposite to each other. 2. An optical element part comprising two first birefringent crystal flat plates, two second birefringent crystal flat plates, and first and second 45 ° Faraday rotators, and one end of the optical element part. The first and second entrance / exit ends arranged on the other side, and the third and fourth entrance / exit ends arranged on the other end side.
The ratio of the separation distance between the birefringent crystal flat plate and the second birefringent crystal flat plate is 1: √2-1, and the directions of rotation by the first and second 45 ° Faraday rotators are opposite to each other. An optical circulator characterized by: 3. The optical circulator according to claim 1 or 2, wherein the optical element section includes the first birefringent crystal flat plate, the first 45 ° Faraday rotator, and the second optical element section from the input / output end. An optical circulator in which a birefringent crystal flat plate, the first birefringent crystal flat plate, the second 45 ° Faraday rotator, and the second birefringent crystal flat plate are arranged in this order. 4. The optical circulator according to claim 1 or 2, wherein the optical element portion includes the second birefringent crystal flat plate, the first 45 ° Faraday rotator, and the first An optical circulator in which a birefringent crystal flat plate, the second birefringent crystal flat plate, the second 45 ° Faraday rotator, and the first birefringent crystal flat plate are arranged in this order. 5. The optical circulator according to claim 1 or 2, wherein the optical element section includes the first birefringent crystal flat plate, the first 45 ° Faraday rotator, and the first An optical circulator in which a birefringent crystal flat plate, the second birefringent crystal flat plate, the second 45 ° Faraday rotator, and the second birefringent crystal flat plate are arranged in this order. 6. The optical circulator according to claim 1 or 2, wherein the optical element section includes the second birefringent crystal flat plate, the first 45 ° Faraday rotator, and the second An optical circulator in which a birefringent crystal flat plate, the first birefringent crystal flat plate, the second 45 ° Faraday rotator, and the first birefringent crystal flat plate are arranged in this order.