JP2000241760A - Optical circulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive optical circulator in which the optical device part can be easily produced in a small size. SOLUTION: This circulator consists of a combination of at least a birefringence device and parallel plate optical devices including nonreciprocal polarization plane rotators 7, 8, and has a plurality of ports for entering and outgoing light. The circulator is produced by successively disposing along the propagation direction of light, a birefringent crystal 1 as a birefringence device, nonreciprocal polarization plane rotator 7, birefringent crystal 2, a pair of birefringent crystals 3, 4 as a birefringence device which is divided into two halves with respect to the propagation direction of light and then joined into a single element with the separation direction of polarized light opposite to each other, birefringent crystal 5, nonreciprocal polarization plane rotor 8, and birefringent crystal 6.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circulator, and more particularly, to an optical circulator as an optical passive component mainly used for arranging optical signal paths in the optical communication field.

[0002]

2. Description of the Related Art In an optical circulator, an optical element section (optical element) constituted by combining at least parallel plate optical elements such as a birefringent element, a non-reciprocal polarizing plane rotator, and a reciprocal polarizing plane rotator has conventionally been used. Passive components) are used. As this kind of conventionally known optical circulator, there is, for example, one disclosed in Japanese Patent No. 2539563.

This conventional optical circulator uses a half-wave plate as a reciprocal polarization plane rotator. The light is divided into two with respect to the light traveling direction, the relative optical axis angles are in a relationship of 45 °, and lights having polarization planes perpendicular to each other separated by a birefringent crystal are separated into １ ／ Incident on the wave plate, the polarization planes of which are aligned and separated by a birefringent crystal.
Two light beams are moved in the same direction, and the angles of the optical axes relative to each other are 45 °, and light beams having mutually perpendicular polarization planes separated by a birefringent crystal are separated into separate half-wave plates. , The polarization planes thereof are made perpendicular to each other, and recombined with a birefringent crystal that is a birefringent element.

[0004]

In the above-described conventional optical circulator, it is necessary to use at least two structural members that are divided into two or more in the light traveling direction. It should be noted that a １ ／ wavelength plate of quartz is mainly used for such a component. However, in such a configuration, when the divided boundary largely shifts, a beam may hit the boundary and cause a loss. In order to avoid this, it is necessary to increase the thickness of the birefringent crystal, and it has been difficult to reduce the size and cost.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and the technical problem thereof is that an optical element portion can be easily manufactured, the size can be reduced, and a low-priced material having excellent characteristics can be obtained. It is to provide a simple optical circulator.

[0006]

According to the present invention, there is provided an optical circulator having at least a plurality of light input / output ports by combining at least a parallel plate optical element including a birefringent element and a non-reciprocal polarization plane rotating element. A first birefringent crystal as the birefringent element and a first non-reciprocal polarization plane rotating element as a non-reciprocal polarization plane rotating element with respect to a light traveling direction which is one direction viewed from one of the light input / output ports. A reciprocal polarization plane rotator, a second birefringent crystal as the birefringent element, divided into two parts with respect to the light traveling direction, and the polarization separation directions are opposite to each other (that is, "opposite direction", the same applies hereinafter). The third as the birefringent element joined in a pair so that
And a fourth birefringent crystal, a fifth birefringent element as the birefringent element.
, A second non-reciprocal polarization plane rotator as the non-reciprocal polarization plane rotator, and a sixth birefringence crystal as the birefringence element are sequentially provided. An optical circulator is obtained.

[0007] The second birefringent crystal and the third birefringent crystal may have optical surfaces having substantially the same area and width.

The first birefringent crystal and the sixth birefringent crystal have substantially the same thickness, and the second birefringent crystal and the fifth birefringent crystal have substantially the same thickness. Wherein the thickness of the third birefringent crystal and the thickness of the fourth birefringent crystal are substantially the same, and the thickness of the first birefringent crystal: the thickness of the second birefringent crystal: The thickness of the third birefringent crystal is approximately √
(2) The ratio may be 1: 2.

[0009] The first to fourth birefringent crystals may be all made of the same material.

In the first non-reciprocal polarization plane rotator and the second non-reciprocal polarization plane rotator, the directions of rotation of the polarization planes are opposite to each other, and the first birefringent crystal and the sixth birefringent crystal are not rotated. The polarization separation directions of the birefringent crystals are directions different from each other by about 90 °, the polarization separation directions of the second birefringent crystal and the fifth birefringent crystal are opposite to each other, and the first birefringent crystal and The polarization splitting directions of the fourth birefringent crystal may be different from each other by approximately 45 °.

[0011] The direction of the magnetic field applied to the first non-reciprocal polarization plane rotator may be opposite to the direction of the magnetic field applied to the second non-reciprocal polarization plane rotator.

A permanent magnet having a hole is used as the magnetic field applying means, and the first non-reciprocal polarization plane rotator or the second
One of the non-reciprocal polarization plane rotators may be applied with a magnetic field by an internal magnetic field in the hole of the permanent magnet, and the other may be applied with a magnetic field by an external magnetic field of the permanent magnet.

[0013] At least one of the first and second non-reciprocal polarization plane rotators may be a Faraday rotator.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the optical circulator of the present invention will be described in detail with reference to the drawings.

First, an optical circulator according to an embodiment of the present invention will be described with reference to FIGS. That is, FIG. 1 is a perspective view showing the basic configuration and simple functions of this optical circulator. This optical circulator has an optical element portion configured by combining parallel plate optical elements including a birefringent element and a non-reciprocal polarization plane rotating element, and has a plurality of, more generally three or more, optical elements. Light input / output ports.

Specifically, the optical circulator includes a birefringent crystal 1, a non-reciprocal polarizing plane rotator 7 as a non-reciprocal polarizing plane rotating element, a birefringent crystal 2 as a birefringent element, a light input / output port. Birefringent crystals 3 and 4 as birefringent elements, which are divided into two parts with respect to a light traveling direction, which is one direction viewed from one, and are joined in pairs so that the polarization separation directions are opposite to each other. Birefringent crystal 5 as a refractive element,
A non-reciprocal polarizing plane rotator 8 as a non-reciprocal polarizing plane rotator and a birefringent crystal 6 as a birefringent element are arranged in this order. The birefringent crystal 1 has odd-numbered light input / output ports, and the birefringent crystal 6 has even-numbered light input / output ports.

Each of the birefringent crystals 1 to 6 is a parallel plate of rutile single crystal, and has a polarization separation direction of light traveling in a direction substantially parallel to the magnetic field H as indicated by an arrow. That is, this arrow indicates the direction of separation of extraordinary light that is desired to be viewed from the odd-numbered light input / output ports. The birefringent crystals 3 and 4 are formed by joining and fixing two rutile single crystals using, for example, an optical contact. The birefringent crystals 1 to 6 are also called walk-off crystals, and the polarization separation direction is also called a walk-off direction.

The birefringent crystals 1 to 6 are all made of the same birefringent crystal. Birefringent crystal 1, 2, 5, 6
Have optical surfaces of approximately the same area and have birefringent crystals 2 and 3
Have optical surfaces of approximately the same area. Birefringent crystals 1 and 6
The thickness of the birefringent crystals 2 and 5 is, for example, 643 μm, and the thickness of the birefringent crystals 3 and 4 is, for example, 910 μm.
Is, for example, 1286 μm. The ratio of the thickness of the birefringent crystals 1 and 6, the thickness of the birefringent crystals 2 and 5, and the thickness of the birefringent crystals 3 and 4 is √ (2): 1: 2
It has become.

The non-reciprocal polarization plane rotators 7 and 8 are each composed of a Faraday rotator, more specifically, a bismuth-substituted gadolinium iron garnet, and the arrow in the figure indicates the direction of Faraday rotation. More specifically, these non-reciprocal polarization plane rotators 7 and 8 rotate the polarization planes by 45 ° in opposite directions. The reciprocal polarization plane rotator 9 is composed of a quartz half-wave plate.

FIG. 2A collectively shows the direction of the separation of the extraordinary light or the extraordinary light component and the direction of the Faraday rotation as viewed from the odd-numbered light input / output ports in each optical element. FIGS. 2B to 2D show the polarization plane and the behavior of light of the optical circulator. That is, it shows the position in the optical plane and the polarization state of the polarization component before and after transmission in each optical element. More specifically, FIG. 2B shows a case where light travels from the first light input / output port 9 to the second light input / output port 10, and FIG. FIG. 2C shows a case where light travels from the third light input / output port 11 to the fourth light input / output port 12, and FIG. 2C shows a case where light travels from the third light input / output port 11 to the fourth light input / output port 12. Represents.

FIG. 2B clearly shows that all the polarized light components are emitted from one point, that is, the same place, from FIG. 2D. Therefore, if the thickness accuracy of each optical element is good, P
It can be understood that DL (Polarization Dependent Loss) does not occur and the number of light input / output ports can be set in any way as long as the size of the optical surface allows.

FIGS. 3A to 3C are diagrams showing the movement of the polarization component as viewed from the light input / output port side of the odd-numbered surface. Arrows shown by solid lines represent extraordinary light components in the birefringent crystal 1. The dotted arrows indicate the ordinary light components in the birefringent crystal 1. From the figure, when the light enters and exits from the odd-numbered light input / output ports to the even-numbered light input / output ports, the extraordinary light component in the birefringent crystal 1 passes through the birefringent crystal 3, and the ordinary light component passes through the birefringent crystal 4. It can be seen that the components transmit through the birefringent crystal, and therefore, the components have the same optical path length from the optical element in each path to the light input / output port. In other words, it can be seen that any polarization component is transmitted between the ports and has the same optical path length, and no polarization dispersion occurs in principle.

Further, the light traveling between the light input / output ports can be simply expressed as follows: port 1 → port 2 → port 3 →
Port 4 → ... The optical crystal used in the optical circulator in this embodiment has an extinction ratio of 55 dB for a birefringent crystal (rutile single crystal) and 45 dB for a nonreciprocal polarization plane rotator (bismuth-substituted gadolinium iron garnet).
It is.

Further, since an element having a low extinction ratio, such as a reciprocal polarizing plane rotator, ie, a quartz wave plate, is not used, like the two-stage optical isolator using two Faraday rotators, the center wavelength is not changed. An isolation characteristic of 55 dB or more can be obtained. Also, if the thickness of the bismuth-substituted gadolinium iron garnet, which is a non-reciprocal polarization plane rotator (Faraday rotator), is slightly shifted so that the Faraday rotation angle becomes 44.5 ° and 45.5 ° at a certain wavelength. , The isolation of the center wavelength is slightly degraded,
It operates as an isolator in a wider band.

FIG. 4 shows an optical circulator according to another embodiment. That is, the optical circulator forms the optical crystal stack 13 by fixing the optical element of the above embodiment with an organic adhesive, and more specifically, around the optical crystal stack 13, more specifically, the non-reciprocal polarization plane. A cylindrical permanent magnet 14 made of SmCo is arranged around the rotor 7.

The permanent magnet 14 has a hole in the center. Then, a magnetic field is applied to the non-reciprocal polarization plane rotator 7 by the internal magnetic field of the hole of the permanent magnet 14, and a magnetic field is applied to the non-reciprocal polarization plane rotator 8 by the external magnetic field of the permanent magnet 14. . As a result, a magnetic field having a direction opposite to that of the non-reciprocal polarization plane rotator 7 is applied to the non-reciprocal polarization plane rotator 8.

Here, the applied magnetic field required to saturate the nonreciprocal rotators (Faraday rotators) 7 and 8 is about 40
0 Oe. Therefore, by using the permanent magnet 14 made of the SmCo magnet, the magnetic field on the center axis of the cylinder outside the cylinder, that is, the external magnetic field is sufficiently saturated. Also, since this external magnetic field is opposite to the magnetic field inside the cylinder, that is, the above-mentioned internal magnetic field, the non-reciprocal rotors 7 and 8
The direction of the Faraday rotation at is opposite. Therefore, according to the configuration of FIG. 4 using the permanent magnet 14, the operation of the embodiment of FIG. 1 can be easily realized.

Around the optical crystal stack 15, made of SmCo or the like, the magnetization directions are opposite.
A permanent magnet with three holes is arranged, and one permanent magnet is arranged on the outer periphery of the non-reciprocal polarization plane rotator 7 and the other permanent magnet is arranged on the outer periphery of the non-reciprocal polarization plane rotator 8. You can also. Thereby, the non-reciprocal polarization plane rotator 7
And the direction of the magnetic field applied to the non-reciprocal polarization plane rotator 8 can be reversed.

The optical circulator of each embodiment shown in FIG. 5 is configured by adding a lens 15 and an optical fiber to the optical crystal stack 13 and the permanent magnet 14 shown in FIG. Here, the optical surface of the optical stack unit 13 is inclined about 4 degrees with respect to the light traveling direction. As the optical fiber, for example, a TEC fiber is used.

That is, the optical circulator of the embodiment shown in FIG. 5 has one lens 15 and one core T on one optical surface side.
An EC optical fiber 16 is provided, and a two-core TEC fiber 17 is provided on the other optical surface side. This embodiment operates as a three-port optical circulator. Here, in the embodiment of the present invention, there is only one structure in which the optical surface is divided, and since this structure is near the center of the optical crystal stack portion 13, light may hit the divided surface. Absent.

By adding a holder for accommodating a lens, an optical crystal stack, a magnet, and the like to the configuration shown in FIG. 5, an optical circulator as a completed product can be formed.

The optical circulator of the embodiment shown in FIG. 6 has lenses 18 and 19 and two-core TEC fibers 20 and 21 arranged on both optical surfaces, respectively. The optical circulator of this embodiment operates as a four-port optical circulator.

By adding a holder for accommodating a lens, an optical crystal stack, a magnet, and the like to the configuration shown in FIGS. 7 and 8, an optical circulator as a completed product can be formed.

[0034]

As described above, according to the optical circulator of the present invention, the first birefringent crystal, the first non-reciprocal polarizing plane rotator, the second birefringent crystal, A third and fourth birefringent crystal, a fifth birefringent crystal, a second non-reciprocal polarizing plane rotator, and a second non-reciprocal polarizing plane rotator that are divided into two and joined in pairs so that the polarization separation directions are opposite to each other. Since the birefringent crystals are arranged sequentially, the optical element in the optical circulator can be easily manufactured and downsized, and an optical circulator having excellent characteristics and low cost can be realized.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a basic configuration and simple functions of an optical circulator according to an embodiment of the present invention.

2A is an explanatory diagram showing the arrangement of optical crystals in the optical circulator of FIG. 1 and the polarization separation direction, Faraday rotation, and C-axis direction of an extraordinary light component incident from an odd-numbered port, and FIG. FIG. 6 shows the positions and polarization states of polarization components in the optical plane before and after transmission through each optical crystal viewed from the odd-numbered port side when light travels from the first light input / output port to the second light input / output port. (C) is an optical plane of polarization components before and after transmission through each optical crystal viewed from the odd-numbered port side when light travels from the second light input / output port to the third light input / output port. FIG. 4D is an explanatory view showing the position and the polarization state in (a), (d) is an optical plane of polarization components before and after transmission through each optical crystal when light travels from the third light input / output port to the fourth light input / output port. FIG. 4 is an explanatory diagram showing the position and the polarization state in FIG.

FIGS. 3A, 3B, and 3C are explanatory diagrams each showing the movement of a polarization component viewed from an odd-numbered light input / output port side.

FIGS. 4A and 4B are cross-sectional views each showing an optical circulator according to another embodiment of the present invention.

FIG. 5 is an explanatory diagram showing an optical circulator according to another embodiment of the present invention.

FIG. 6 is an explanatory diagram showing an optical circulator according to another embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 birefringent crystal 2 birefringent crystal 3 birefringent crystal 4 birefringent crystal 5 birefringent crystal 6 birefringent crystal 7 non-reciprocal polarization plane rotator 8 non-reciprocal polarization plane rotator 9 first light input / output port 10 2 light input / output port 11 third light input / output port 12 4th light input / output port 13 optical crystal stack 14 permanent magnet 15 lens 16 TEC fiber 17 TEC fiber 18 lens 19 lens 20 TEC fiber 21 TEC fiber

Claims (8)

[Claims] 1. An optical circulator comprising a combination of at least a birefringent element and a parallel plate optical element including a non-reciprocal polarization plane rotating element and having a plurality of light input / output ports, wherein: A first birefringent crystal as the birefringent element, a first non-reciprocal polarizing plane rotator as the non-reciprocal polarizing plane rotator, and A second birefringent crystal as an element, a third and a fourth birefringent element as the birefringent element divided into two with respect to the light traveling direction and joined in pairs so that the polarization separation directions are opposite to each other; Birefringent crystal,
A fifth birefringent crystal as the birefringent element, a second non-reciprocal polarizing plane rotator as the non-reciprocal polarizing plane rotator, and a sixth birefringent crystal as the birefringent element are sequentially provided. An optical circulator characterized in that: 2. The optical circulator according to claim 1, wherein said second birefringent crystal and said third birefringent crystal have optical surfaces having substantially the same area and width. 3. The thickness of the first birefringent crystal and the sixth birefringent crystal is substantially the same, and the thickness of the second birefringent crystal and the fifth birefringent crystal is The thicknesses of the third birefringent crystal and the fourth birefringent crystal are substantially the same, and the thickness of the first birefringent crystal: the thickness of the second birefringent crystal : The thickness of the third birefringent crystal is approximately √
(2) The ratio is 1: 2.
Or the optical circulator according to 2 above. 4. The optical circulator according to claim 1, wherein all of the first to fourth birefringent crystals are made of the same material. 5. The rotation direction of the polarization plane of the first non-reciprocal polarization plane rotator and the rotation direction of the polarization plane of the second non-reciprocal polarization plane rotator, and the first birefringent crystal and the second birefringent crystal. The birefringent crystal of No. 6 has a polarization separation direction different by about 90 °, and the polarization separation directions of the second birefringent crystal and the fifth birefringent crystal are opposite to each other. The optical circulator according to any one of claims 1 to 4, wherein the polarization separation directions of the crystal and the fourth birefringent crystal are different from each other by approximately 45 °. 6. The direction of a magnetic field applied to the first non-reciprocal polarization plane rotator is opposite to the direction of a magnetic field applied to the second non-reciprocal polarization plane rotator. The optical circulator according to any one of claims 1 to 5, wherein 7. A permanent magnet having a hole as a magnetic field applying means, and one of the first non-reciprocal polarization plane rotator or the second non-reciprocal polarization plane rotator is formed of a hole of the permanent magnet. The optical circulator according to any one of claims 1 to 6, wherein a magnetic field is applied by an internal magnetic field, and the other is applied by an external magnetic field of the permanent magnet. 8. The optical circulator according to claim 1, wherein at least one of said first and second non-reciprocal polarization plane rotators is a Faraday rotator.