JP2000241761A - Optical circulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small-sized inexpensive optical circulator in which the number of structural parts divided with respect to the propagation direction of light is decreased as much as possible and thin birefringent crystals can be used. SOLUTION: This optical circulator consists of a combination of parallel plate optical devices. Along the propagation direction of light as one direction seen from the odd number of plural ports for entering and outgoing light, the circulator has the optical device part consisting of a first birefringent crystal 1, first 45 deg. Faraday rotator 5, second birefringent crystal 2 and third birefringent crystal 3, second 45 deg. Faraday rotator 6, and fourth birefringent crystal 4. The second and third birefringent crystals 2, 3 are joined to form a pair so that the polarized light components separated from each other by the first birefringent crystal 1 separately enter and outgo from the respective crystals with the separation direction of polarized light opposite to each other. The optical device part has a port of an odd number of entering and outgoing light on the side of the first birefringent crystal 1 and has a port of an even number for entering and outgoing light on the side of the fourth birefringent crystal 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art Conventionally, in this type of optical circulator,
At least a structure combining a birefringent element, a non-reciprocal polarization plane rotator, and a parallel plate optical element including a reciprocal polarization plane rotator is used.

For example, in the case of an optical circulator described in Japanese Patent No. 2539563, a half-wave plate is used as a reciprocal polarization plane rotator, and its operation principle is divided into two in the light traveling direction. The light having an angle of 45 degrees relative to each other has a relationship of 45 degrees, and lights having polarization planes perpendicular to each other separated by a birefringent element (birefringent crystal) are separated into １ ／
Wavelength light is incident, the polarization planes are aligned, and the two lights separated by the birefringent crystal move in the same direction. Further, the relative optical axis angles have a relationship of 45 degrees, and the birefringence Light having polarization planes perpendicular to each other separated by a crystal is
The light is incident on the wave plate, the polarization planes thereof are made perpendicular to each other, and the light is combined again with a birefringent crystal.

[0004]

In the case of the above-described optical circulator, at least two components which are divided into two or more in the light traveling direction are required. To avoid this, the thickness of the birefringent crystal must be increased in order to avoid the loss. However, such a configuration has a problem that it becomes a major obstacle to realizing the miniaturization and cost reduction of the entire optical circulator. .

SUMMARY OF THE INVENTION The present invention has been made to solve such problems, and its technical problem is to apply a thin birefringent crystal by minimizing the number of components divided in the light traveling direction. It is another object of the present invention to provide an optical circulator capable of realizing a reduction in size and cost.

[0006]

According to the present invention, there is provided an optical circulator having a plurality of light input / output ports, a birefringent crystal as a plurality of birefringent elements, and a plurality of Faraday rotators. A first birefringent crystal, a first 45-degree Faraday rotator, and a first birefringent crystal are separated from each other in a light traveling direction that is one direction viewed from one of the plurality of light input / output ports. And a second birefringent crystal and a third birefringent crystal joined in a pair so that the polarized light components enter and exit separately, and the polarization separation directions are opposite to each other, and a second 45-degree Faraday rotation Thus, an optical circulator having an optical element portion in which the element and the fourth birefringent crystal are arranged in this order is obtained.

Further, according to the present invention, there is provided an optical circulator in which a polarization dispersion compensating plate for compensating polarization dispersion is provided in the optical path of the optical element section.

Further, according to the present invention, in any of the above-described optical circulators, an optical circulator having an optical surface having substantially the same area and width as the second birefringent crystal and the second birefringent crystal can be obtained. In this optical circulator, the thicknesses of the first birefringent crystal and the fourth birefringent crystal are substantially the same, and the thicknesses of the first birefringent crystal and the fourth birefringent crystal are equal to the second birefringent crystal. A ratio of the thickness of the refracting crystal or the third birefringent crystal to approximately 2 ^1/2 : 2, and the first to fourth birefringent crystals, the first 45-degree Faraday rotator, and the second 45-degree Faraday It is preferable that both the rotor and the polarization dispersion compensating plate are formed of parallel flat plates.

On the other hand, according to the present invention, in any one of the above-described optical circulators, the Faraday rotation directions of the first 45-degree Faraday rotator and the second 45-degree Faradata rotator are opposite to each other, The first birefringent crystal and the fourth birefringent crystal have a polarization separation direction of substantially 90 degrees with each other, and the first birefringent crystal, the third birefringent crystal, the second birefringent crystal, and the fourth An optical circulator having a polarization separation direction of approximately 45 degrees with each other is obtained.

On the other hand, according to the present invention, in any one of the optical circulators described above, a permanent magnet having an air gap is provided as a magnetic field applying means, and the first 45-degree Faraday rotator and the second 45-degree Faraday rotator are provided. In either case, a magnetic field in the air gap is applied, and in the other, an optical circulator to which a magnetic field outside the permanent magnet is applied is obtained.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The optical circulator according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing a basic configuration and simple functions of an optical circulator according to a first embodiment of the present invention. However, each optical element in FIG. 1 shows the polarization separation direction and the Faraday rotation direction regarding the extraordinary light component of the transmitted light incident from the odd-numbered light input / output ports among the plurality of light input / output ports. I have.

This optical circulator has a plurality of light input / output ports and is constituted by a plurality of birefringent crystals as birefringent elements and a plurality of Faraday rotators. The first birefringent crystal 1 corresponds to the light traveling direction which is one direction as viewed from the odd-numbered one.
And the first 45-degree Faraday rotator 5 and the polarized components separated by the first birefringent crystal 1 are separately input and output, and are joined in a pair so that the polarization separation directions are opposite to each other. It has an optical element portion in which a second birefringent crystal 2, a third birefringent crystal 3, a second 45-degree Faraday rotator 6, and a fourth birefringent crystal 4 are arranged in this order. This optical element portion has odd-numbered light input / output ports on the first birefringent crystal 1 side, and has even-numbered light input / output ports on the fourth birefringent crystal 4 side.

Among these, each of the birefringent crystals 1 to 4 is made of a rutile single crystal, and a polarization separation direction of light traveling in a direction substantially parallel to a magnetic field H as indicated by an arrow (odd-numbered light input / output light). (The polarization separation direction of the extraordinary light component viewed from the port), and the second birefringent crystal 2 and the third birefringent crystal 3 have optical surfaces of almost the same area and width by using an optical contact. The two rutile single crystals are fixed, and the polarized light components separated by the first birefringent crystal 1 or the fourth birefringent crystal 4 are separately incident on and emitted from a pair of rutile single crystals. Each of the first birefringent crystal 1 and the fourth birefringent crystal 4 has the same thickness of 910 μm, and the thickness of the second birefringent crystal 2 and the third birefringent crystal 3 is 1286 μm.
m. Therefore, the ratio of the thickness of the first birefringent crystal 1 and the fourth birefringent crystal 4 to the thickness of the second birefringent crystal 2 or the third birefringent crystal 3 is approximately 2 ^1/2 : 2. ing.
Note that the first birefringent crystal 1 and the fourth birefringent crystal 4 can be interchanged. The first 45-degree Faraday rotator 5 and the second 45-degree Faraday rotator 6 are made of bismuth-substituted gadolinium iron garnet, and each rotate the polarization plane by 45 degrees according to the Faraday rotation direction indicated by the arrow.

That is, here, the Faraday rotation directions of the first 45 degree Faraday rotator 5 and the second 45 degree Faraday rotator 6 are opposite to each other, and the first birefringent crystal 1 and the fourth The polarization splitting direction of the birefringent crystal 4 is maintained at approximately 90 degrees with respect to the birefringent crystal 4, and the first birefringent crystal 1, the third birefringent crystal 3, the second birefringent crystal 2, and the fourth birefringent crystal 4 are different from each other. Are configured so that the polarization separation directions are approximately 45 degrees with respect to each other. However, the first birefringent crystal 1 and the fourth birefringent crystal 4 are the same when turned over, and the second birefringent crystal 2 and the Since the birefringent crystal 3 is the same as the rutile single crystal, the optical element portion (optical crystal stack) of the optical circulator is made of three types of parallel flat plates having different thicknesses including the 45-degree Faraday rotators 5 and 6. Can be configured.

FIG. 2 shows the polarization plane of this optical circulator and the behavior of light. FIG. 2A shows the polarization separation direction of the extraordinary light component in each optical element viewed from the odd-numbered light input / output ports. FIG. 4B shows the optical surface of the polarization component before and after transmission in each optical element when light travels from the first light input / output port P1 to the second light input / output port P2. (C) of FIG. 4 shows the position of the polarization component in the optical element before and after transmission through each optical element when light travels from the second light input / output port P2 to the third light input / output port P3. FIG. 3D shows the position in the optical plane and the polarization state, and FIG. 4D shows the positions from the third light input / output port P3 to the fourth light input / output port P4.
This relates to the position on the optical surface of the polarization component before and after transmission through each optical element and the polarization state when light travels.

Here, referring to FIGS. 2 (b) to 2 (d), all the polarization components are converged on one point, and if the thickness accuracy of each optical element is good, PDL (Polarization)
Dependent Loss) does not occur,
In addition, it can be seen that the number of light input / output ports can be arbitrarily set as long as the size of the optical surface permits. The light traveling between the light input / output ports can be simply represented by a first light input / output port P1.
→ second light input / output port P2 → third light input / output port P
3 → the fourth light input / output port P4 →.

In the case of this optical circulator, the extinction ratio of the rutile single crystal used for each of the birefringent crystals 1 to 4 is 55 d
B. The extinction ratio of the bismuth-substituted gadolinium iron garnet used for the first 45 degree Faraday rotator 5 and the second 45 degree Faraday rotator 6 is 45 dB, and a material having a low extinction ratio such as a quartz wave plate is used. Therefore, an isolation characteristic of 55 dB or more can be obtained at the center wavelength, similarly to a two-stage optical isolator using two ordinary Faraday rotators. Further, the thickness of the bismuth-substituted gadolinium iron garnet used for the 45-degree Faraday rotators 5 and 6 is slightly shifted so that the Faraday rotation angles are set to 44.5 degrees and 45.5 degrees at a specific certain wavelength. For example, although the isolation at the center wavelength is slightly deteriorated, it operates as an isolator over a wider band.

FIG. 3 is a perspective view showing a basic configuration and simple functions of an optical circulator according to a second embodiment of the present invention. However, also in this case, each optical element in FIG. 3 shows the polarization separation direction and the direction of Faraday rotation regarding the extraordinary light component of the transmitted light incident from the odd-numbered port among the plurality of light input / output ports. .

In the case of this optical circulator, the optical circulator has a configuration similar to that of the optical circulator of the first embodiment shown in FIG. In contrast, the first birefringent crystal 11 and the first 4
A second birefringence in which the 5-degree Faraday rotator 15 and the polarized light components separated by the first birefringent crystal 11 are separately input and output, and are joined in pairs so that the polarization separation directions are opposite to each other. Crystal 12 and third birefringent crystal 1
3, the second 45 ° Faraday rotator 16 and the fourth birefringent crystal 14 in this order, except that the second birefringent crystal 12 and the third birefringent crystal 13 and the second 45 A polarization dispersion compensating plate 17 made of a rutile single crystal for compensating the polarization dispersion of the optical path of the optical element portion is provided between the polarization dispersion compensator 17 and the Faraday rotator 16. However, since the polarization dispersion compensating plate 17 does not rotate the polarization plane, the position of the polarization component before and after transmission through each optical element, the polarization state, and the position of the light input / output port are the same as those in the first embodiment. The relationship is similar to that described above.

FIG. 4 shows the behavior of some light of this optical circulator as viewed from the first birefringent crystal 11 side (the odd-numbered light input / output port side). FIG. FIG. 3B relates to one port, FIG. 4B relates to another port, and FIG. 5C relates to another port.

Here, referring to FIG. 4 (a), it is the case from the odd-numbered port to the even-numbered port. The incident light from the port P1 has an extraordinary light component in the first birefringent crystal 11. It can be seen that the light exits from the port P2 via an optical path length that is longer than the ordinary light component by an optical path length corresponding to the polarization separation distance by the second birefringent crystal 12. Referring to FIG. 4B, the case is from the even-numbered port to the odd-numbered port. The incident light from the port P2 is such that the extraordinary light component in the fourth birefringent crystal 14 is smaller than the ordinary light component It can be seen that the light exits from the port P3 via an optical path length increased by the optical path length corresponding to the polarization separation distance by the third birefringent crystal 13. Further, referring to FIG. 4C, as the case from the odd-numbered port to the even-numbered port, the incident light from the port P3 behaves in the same manner as when traveling from the port P1 to the port P2. It can be seen that the light is emitted from. Second birefringent crystal 12 and third
Since the birefringent crystals 13 have the same thickness and the absolute value of the optical path length difference is equal, the polarization dispersion can be canceled by inserting the polarization dispersion compensator 17. Here, the case where the polarization dispersion compensating plate 17 is provided between the second birefringent crystal 12 and the third birefringent crystal 13 and the second 45-degree Faraday rotator 16 has been described. In addition, the same effect can be obtained by disposing between the first 45-degree Faraday rotator 15 and the second birefringent crystal 12 and the third birefringent crystal 13.

FIGS. 5A and 5B show the arrangement of each optical element and each component when this optical circulator is assembled for a product. FIG. 5A is related to a front view, and FIG. Is related to a side view with a partial cross section.

Here, the optical element portion according to the arrangement of the second embodiment is fixed with an organic adhesive to form the optical crystal stack 23, and the Sm around the first Faraday rotator 15 is formed.
A Co magnet 22 is provided. That is, the SmCo magnet 22 as the magnetic field applying means here arranges the first 45-degree Faraday rotator 15 in the gap and applies a magnetic field to the second 45-degree Faraday rotator 16 in the outside. It is configured to apply a magnetic field. Note that the configuration here is reversed, and the second 45-degree Faraday rotator 16 is arranged in the gap of the SmCo magnet 22, and the first
The external magnetic field may be applied to the 45-degree Faraday rotator 15.

FIG. 6 shows an optical crystal stack 2 about the light traveling direction in the product body of the above-described optical circulator.
3 shows the relationship between the position in the optical crystal stack 23 at the central portion of No. 3 and the magnetic field strength.

FIG. 6 shows the comparison between the magnetic field intensity region E1 at the position of the first 45-degree Faraday rotator 15 and the magnetic field intensity region E2 at the position of the second 45-degree Faraday rotator 16. Even if the direction of the magnetic field is opposite to that of the first 45-degree Faraday rotator 15, a sufficient intensity magnetic field for saturation is applied to the second 45-degree Faraday rotator 16, so that one SmCo It is understood that the operation of the optical circulator can be sufficiently realized by the magnetic field generated by the magnet 22.

FIG. 7 shows a basic configuration of an example of an optical system device using the above-described optical circulator product body. In this optical system device, a single-core TE
C fiber 25, lens 24, SmCo magnet 2
2 stacking optical crystal stack 23, and 2-core TEC
The fibers 26 are arranged in this order, and the optical surface of the optical crystal stack 23 is inclined by about 4 degrees with respect to the light traveling direction.

In the case of such an optical system device, only one of the second birefringent crystal 12 and the third birefringent crystal 13 of the optical crystal stack 23 has a structure in which the optical surface is divided. Since it is near the center of the crystal stack 23, there is little possibility that light will collide with the divided plane.

FIG. 8 shows a basic configuration of another example of an optical system device using the product body of the optical circulator described above. In this optical system device, a two-core TEC fiber 26 is used instead of the one-core TEC fiber 25 as compared with the one shown in FIG. The difference is that it works.

In any case, in such an optical system apparatus, the optical crystal stack 23 provided with the lens 24 and the SmCo magnet 22 is housed in a holder, so that an actual optical circulator can be used as a finished product. The form will be completed.

[0031]

As described above, according to the optical circulator of the present invention, a birefringent crystal as a parallel plate birefringent element and a 45-degree Faraday rotator as a non-reciprocal polarizing plane rotator, or Among the constituent members constituted by the combination of the optical elements including the refractive crystal polarization dispersion compensating plate, the constituent parts divided in the light traveling direction are the second birefringent crystal as the birefringent element and the third birefringent crystal. A birefringent crystal is formed as a pair of two birefringent crystals in such a manner that the polarization separation directions are opposite to each other. Therefore, a thin birefringent crystal can be applied. This makes it easy to manufacture each optical element and can be provided at a low price as a small configuration having excellent optical characteristics.

[Brief description of the drawings]

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

FIGS. 2A and 2B show the polarization plane and the behavior of light of the optical circulator shown in FIG. 1. FIG. 2A shows the polarization separation direction or Faraday of an extraordinary light component in each optical element as viewed from odd-numbered light input / output ports. (B) shows the position and polarization in the optical plane of the polarization components before and after transmission in each optical element when light travels from the first light input / output port to the second light input / output port. About the state,
(C) relates to the position and polarization state of the polarization component in the optical plane before and after transmission in each optical element when light travels from the second light input / output port to the third light input / output port, and (d). ) Relates to the position on the optical surface and the polarization state of the polarization component before and after transmission in each optical element when light travels from the third light input / output port to the fourth light input / output port.

FIG. 3 is a perspective view showing a basic configuration and simple functions of an optical circulator according to a second embodiment of the present invention.

FIG. 4 shows the behavior of some light as viewed from the first birefringent crystal side (the odd-numbered light input / output port side) of the optical circulator shown in FIG. 3, where (a) shows one port. (B) relates to another port, and (c) relates to another port.

5A and 5B show the arrangement of each optical element and each component when the optical circulator shown in FIG. 3 is assembled for a product, wherein FIG. It concerns a side view in cross section.

6 shows the relationship between the position in the optical crystal stack portion at the center of the optical crystal stack portion with respect to the light traveling direction in the product body of the optical circulator shown in FIG. 3 and the magnetic field strength.

FIG. 7 shows a basic configuration according to an example of an optical system configured using the product body of the optical circulator shown in FIG.

8 shows a basic configuration according to another example of the optical system configured using the product body of the optical circulator shown in FIG.

[Explanation of symbols]

 1-4,11-14 birefringent crystal 5,6,15,16 45 degree Faraday rotator P1-P4 light input / output port 17 polarization dispersion compensator 22 magnet made of SmCo 23 optical crystal stack 24 lens 25 1-core TEC Fiber 26 2-core type TEC fiber

Claims (7)

[Claims] 1. An optical circulator having a plurality of light input / output ports and comprising a plurality of birefringent crystals as a plurality of birefringent elements and a plurality of Faraday rotators. The first birefringent crystal, the first 45-degree Faraday rotator, and the polarized light components separated by the first birefringent crystal are separately separated from each other with respect to the light traveling direction as one direction viewed from one. A second birefringent crystal and a third birefringent crystal, which are input and output and joined in pairs so that the polarization separation directions are opposite to each other, a second 45-degree Faraday rotator, and a fourth birefringence An optical circulator having an optical element portion in which crystals are arranged in this order. 2. An optical circulator according to claim 1, wherein a polarization dispersion compensating plate for compensating polarization dispersion is provided in an optical path of said optical element section. 3. The optical circulator according to claim 1, wherein said second birefringent crystal and said second birefringent crystal have optical surfaces of substantially the same area and width. . 4. The optical circulator according to claim 3, wherein said first birefringent crystal and said fourth birefringent crystal have substantially the same thickness, and said first birefringent crystal and said fourth birefringent crystal have different thicknesses. The ratio of the thickness of the birefringent crystal to the thickness of the second birefringent crystal or the thickness of the third birefringent crystal is approximately 2 ^1/2 : 2, and the first to fourth birefringent crystals, The optical circulator, wherein each of the first 45-degree Faraday rotator, the second 45-degree Faraday rotator, and the polarization dispersion compensating plate are formed of parallel flat plates. 5. The optical circulator according to claim 4, wherein the first to fourth birefringent crystals and the polarization dispersion compensator are made of a birefringent crystal of the same material. 6. An optical circulator according to claim 1, wherein the first 45-degree Faraday rotator and the second 45-degree Faraday rotator have Faraday rotation directions opposite to each other. Orientation, the first birefringent crystal and the fourth birefringent crystal have a polarization separation direction of about 90 degrees with each other, and the first birefringent crystal and the third birefringent crystal are A second birefringent crystal and the fourth birefringent crystal;
And the birefringent crystal of which each have a polarization separation direction of about 4
An optical circulator, wherein the angle is 5 degrees. 7. The optical circulator according to claim 1, further comprising a permanent magnet having an air gap as a magnetic field applying means, wherein the first 45-degree Faraday rotator and the second 45-degree Faraday rotator are provided. An optical circulator wherein one of the Faraday rotators is applied with a magnetic field in the gap, and the other is applied with a magnetic field outside the permanent magnet.