JP2000241759A - 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 and its size can be suppressed. SOLUTION: This circulator consists of a combination of parallel plate optical devices including a birefringence device, nonreciprocal polarization plane rotator 6 and reciprocal polarization plane rotator 7, and has plural ports for entering and outgoing of light. The circulator is produced by successively disposing a birefringence crystal 1 as a birefringence device, nonreciprocal polarization plane rotator 5, birefringence crystals 2, 3 as a birefringence device which is divided into two parts with respect to the propagation direction of light and joined to form a pair with the opposite separation direction of polarized light to each other, a nonreciprocal polarization plane rotator 6, a reciprocal polarization plane rotator 7, a birefringence crystal 4 or the like.

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, a plurality of light beams are formed by combining at least a birefringent element, a non-reciprocal polarization plane rotation element, and a parallel plate optical element including a reciprocal polarization plane rotation element. In an optical circulator having an input / output port, a first birefringent element as the birefringent element with respect to a light traveling direction defined as one direction viewed from one of the light input / output ports
Birefringent crystal, a first non-reciprocal polarization plane rotator as the non-reciprocal polarization plane rotator, and 2 with respect to the light traveling direction.
The second and third birefringent crystals as the birefringent elements, which are divided and joined in pairs so that the polarization separation directions are opposite to each other (that is, “opposite direction”, the same applies hereinafter); A second non-reciprocal polarizing plane rotator as the rotative polarizing plane rotator, a reciprocal polarizing plane rotator as the reciprocal polarizing plane rotator, and a fourth birefringent crystal as the birefringent element. An optical circulator characterized by the above is obtained.

According to the present invention, at least a birefringent element,
A non-reciprocal polarization plane rotator and a parallel plate optical element including a reciprocal polarization plane rotator are combined to form an optical circulator having a plurality of light input / output ports, as viewed from one of the light input / output ports. A first birefringent crystal as the birefringent element with respect to the light traveling direction as one direction,
A first non-reciprocal polarization plane rotator as the non-reciprocal polarization plane rotator, the first non-reciprocal polarization plane rotator being 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; Second and third birefringent crystals as birefringent elements, reciprocal polarization plane rotator as the reciprocal polarization plane rotation element, and second non-reciprocal polarization plane rotation as the non-reciprocal polarization plane rotation element And a fourth birefringent crystal as the birefringent element are sequentially arranged, thereby obtaining an optical circulator.

[0008] In addition to the optical element, a polarization dispersion compensating plate may be provided before any of the optical elements constituting the optical element section.

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

The thickness of the first 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 is The ratio may be approximately Δ (2): 2.

The first non-reciprocal polarization plane rotator and the second non-reciprocal polarization plane rotator have the same direction of rotation of the polarization plane, and the first birefringent crystal and the fourth birefringent crystal have the same rotation direction. The polarization separation direction of the birefringent crystal is a direction different by about 90 °,
The polarization split directions of the first birefringent crystal and the third birefringent crystal may be different by approximately 45 °.

As a means for applying a magnetic field to the first and second non-reciprocal polarization plane rotators, a permanent magnet having a hole may be used.

[0013] At least one of the first and second non-reciprocal polarization plane rotators may be a 45 ° Faraday rotator.

The reciprocal polarization plane rotator may be a half-wave plate.

[0015] The second and third birefringent crystals may have substantially the same polarization separation distance.

[0016]

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.

(Embodiment 1) First, an optical circulator of Embodiment 1 will be described with reference to FIGS. 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 a birefringent element, a non-reciprocal polarization plane rotator, and a parallel plate optical element including a reciprocal polarization plane rotator, and a plurality of more general optical elements. Typically, it has three or more light input / output ports.

Specifically, a birefringent crystal 1 as a birefringent element whose polarization separation direction is substantially perpendicular to a light traveling direction which is one direction viewed from one of the light input / output ports, And a birefringent crystal 2 as a birefringent element which is divided into two parts with respect to the light traveling direction and joined in pairs so that the polarization separation directions are opposite to each other. 3. Non-reciprocal polarization plane rotator 6 that does not depend on the light traveling direction, reciprocal polarization plane rotator 7 that depends on the light traveling direction, as a birefringent element whose polarization separation direction is substantially perpendicular to the light traveling direction 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 4 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. More specifically, the polarization separation direction as viewed from the odd-numbered light input / output ports, that is, the direction of separation of extraordinary light or extraordinary wavelength light components, the birefringent crystal is also called a walk-off crystal, The polarization separation direction is also called a walk-off direction.

The birefringent crystals 2 and 3 are formed by joining and fixing two rutile single crystals using, for example, an optical contact. These birefringent crystals 2 and 3 have optical surfaces of substantially the same area and width, and have substantially the same thickness. Specifically, the thickness of the birefringent crystals 2 and 3 is 128
6 μm. On the other hand, the thickness of birefringent crystals 1 and 4 is 910
μm. Therefore, the ratio of the thickness of the birefringent crystals 1 and 4 to the thickness of the birefringent crystals 2 and 3 is √ (2): 2.

The non-reciprocal polarization plane rotators 5 and 6 are each composed of a Faraday rotator, more specifically, a bismuth-substituted gadolinium iron garnet, and have a direction of Faraday rotation indicated by an arrow in the figure. 45 °
The polarization planes are rotated in the same direction. The reciprocal polarization plane rotator 7 has a direction of the C axis indicated by an arrow in the figure, and is specifically formed of a quartz half-wave plate.

Here, the birefringent crystals 1 and 4 are the same when turned over, and the birefringent crystals 2 and 3 are also the same.
The optical crystal stack shown in FIG. 1 includes a non-reciprocal polarizing plane rotator (bismuth-substituted gadolinium iron garnet), a birefringent crystal (rutile crystal) having a thickness of 910 μm, and a thickness of 1286.
It can be realized by four types of members: a birefringent crystal (rutile crystal) of μm, and a reciprocal polarization plane rotator (quartz half-wave plate).

FIGS. 2A to 2D show the polarization plane and the behavior of light of the optical circulator. That is, the arrows in each optical element shown in FIG. 2A indicate the direction of the separation of the extraordinary light component and the direction of the Faraday rotation viewed from the odd-numbered light input / output ports. FIGS. 2B to 2D show the positions and polarization states in the optical plane of the polarization components 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 8 to the second light input / output port 9, and FIG. FIG. 2D shows a case where light travels from the third light input / output port 10 to the third light input / output port 10.
The case where light travels to 1 is shown.

FIG. 2B clearly shows that all the polarized light components are focused on one point from FIG. 2D. Therefore,
If the thickness accuracy of each optical element is good, PDL (Polarizati
On Dependent Loss) does not occur, and it can be understood that the number of light input / output ports can be set in any way as long as the size of the optical surface allows. The optical circulator according to this embodiment can obtain an isolation characteristic of 50 dB or more at the center wavelength, like the two-stage optical isolator using two non-reciprocal polarization plane rotators (Faraday rotators). it can.

(Embodiment 2) An optical circulator according to Embodiment 2 will be described with reference to FIGS. FIG. 3 is a diagram showing an arrangement of each optical element constituting the optical circulator. Here, the birefringent crystal 12 becomes the birefringent crystal 1,
The non-reciprocal polarization rotator 16 is a non-reciprocal polarization rotator 5.
The birefringent crystals 13 and 14 are birefringent crystals 2 and 3
The non-reciprocal polarization plane rotator 17 is used for the non-reciprocal polarization plane rotator 6, and the reciprocal polarization plane rotator 18 is used for the reciprocal polarization plane rotator 7.
In addition, the birefringent crystal 15 corresponds to the birefringent crystal 4 respectively.

The difference from the first embodiment is that
Polarization dispersion compensator 19 made of parallel plate of rutile single crystal
Is added after the birefringent crystals 13 and 14.
Since the polarization dispersion compensating plate does not rotate the polarization plane, the positions of the polarization components in the optical plane, the polarization state, and the positions of the light input / output ports before and after transmission through each optical element are the same as those in the first embodiment.

FIGS. 4A to 4C 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. As can be seen from the figure, in the case of transition from the odd-numbered light input / output port to the even-numbered light input / output port, the extraordinary light component in the birefringent crystal 12 is larger than the ordinary light component by the polarization separation in the birefringent crystal 13. The light exits through the optical path length. In addition, in the case of transition from the even-numbered light input / output port to the odd-numbered light input / output port, the extraordinary light component in the birefringent crystal 15 has more optical paths than the ordinary light component by the polarization separation in the birefringent crystal 14. Emitted through a long distance. Since the birefringent crystals 12 and 15 have the same thickness, the absolute values of the optical path length differences are equal. Therefore, the polarization dispersion can be canceled by inserting the polarization dispersion compensator 19. The polarization dispersion compensating plate 19 can be disposed before or after each optical element if attention is paid to the direction of the optical axis.

Third Embodiment FIG. 5 shows an optical circulator according to a third embodiment. In this optical circulator, the optical element of the second embodiment is fixed with an organic adhesive to form an optical crystal stack section 24.
A permanent magnet 25 made of samarium cobalt (SmCo) having a hollow cylindrical shape, that is, a hole is provided around the periphery of the magnet 4.

Here, it is generally known that the extinction ratio of a non-reciprocal polarization plane rotator (Faraday rotator) is deteriorated when the light traveling direction and the direction of the magnetic field do not match. For example, in the case of bismuth-substituted gadolinium iron garnet, this problem can be avoided to some extent by performing a heat treatment at about 1200 ° C. after the completion of crystal growth, but such treatment leads to an increase in cost. Therefore, as in the present embodiment, the optical element portion is provided in a hollow cylindrical magnet, and the length of the magnet is set to about twice that between the non-reciprocal polarization plane rotators, so that the light traveling direction is reduced. And suppressing the deviation of the direction of the magnetic field can be effectively performed. Although not shown, the optical crystal stack portion 24 and the permanent magnet 25 are bonded to each other via a spacer such as ceramic between them,
It is possible to effectively suppress deterioration of characteristics due to stress caused by a difference in thermal expansion coefficient.

(Embodiment 4) The optical circulator shown in FIG. 6 and FIG. 7 is configured by adding a lens and an optical fiber to the optical crystal stack 24 and the permanent magnet 25 shown in Embodiment 3. Here, the optical surface of the optical crystal stack unit 24 is inclined about 4 degrees with respect to the light traveling direction. As the optical fiber, for example, a TEC fiber is used.

Specifically, in the optical circulator of FIG. 6, one lens 18 and one core TEC are provided on one optical surface side.
An optical fiber 27 is provided, and a two-core T
An EC fiber 28 is provided. This optical circulator operates as a three-port optical circulator.
The optical circulator of FIG. 7 includes lenses 29 and 30 and a two-core TEC fiber 3 on both optical surfaces, respectively.
1 and 32 are arranged. The optical circulator of this embodiment operates as a four-port optical circulator.

In these optical circulators, there is only one structure in which the optical surface is divided, and since this structure is located at the center of the optical crystal stack portion, there is little possibility that light will fall on the divided surface. By adding a holder for accommodating a lens, an optical crystal stack, a magnet, and the like to the configuration shown in FIGS. 6 and 7, an optical circulator as a completed product can be formed.

(Embodiment 5) FIG. 8 shows an optical crystal stack 33 and a permanent magnet 34 similar to those of the embodiment 3, plus prisms 35 to 38 and lenses 42 to 45 before each light input / output port. This is an optical circulator configured by adding fibers 46 to 49.

The prisms 35 to 38 are made of total reflection mirrors 39 and 40 and glass 41 whose optical surface is AR-coated, and have the effect of increasing the distance between the light input / output ports. Thereby, each lens and optical fiber can be adjusted independently, and an optical circulator having excellent characteristics can be realized.

In the optical element stack section 24 of the optical circulator of the embodiment shown in FIG. 6, the thickness of each birefringent crystal and the polarization dispersion compensator is determined by the distance between the cores of the two-core optical fiber. On the other hand, the embodiment of FIG. 8 has an effect that the thickness of each birefringent crystal and the polarization dispersion compensator can be determined relatively freely. However, as for this thickness, the ratio of the thickness of the birefringent crystal 1 to the thickness of the birefringent crystal 2 needs to be in the range of √ (2): 2. And, by this effect, the beam diameter can be made relatively large, and a design with a margin against optical damage can be made.

The positions of the non-reciprocal polarization plane rotator 6 and the reciprocal polarization plane rotator 7 can be interchanged in the optical crystal stack of the embodiment described above, for example, in the configuration shown in FIG. Effects can be obtained.

[0038]

As described above, according to the optical circulator of the present invention, for example, the first birefringent crystal, the first non-reciprocal polarizing plane rotator, and the optical circulator are divided into two parts with respect to the light traveling direction. Second and third birefringent crystals, a second non-reciprocal polarization plane rotator, a reciprocal polarization plane rotator, and a fourth birefringent crystal joined in pairs so that the polarization separation directions are opposite to each other. Are sequentially arranged, so that an optical element in the optical circulator can be easily manufactured, and a low-cost optical circulator having excellent characteristics 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 a first 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. 3C is a diagram illustrating the position and polarization state of the polarization component on the optical surface before and after transmission through each optical crystal when light travels from the first light input / output port to the second light input / output port. FIG. 3D is an explanatory view showing the position and polarization state of the polarization component in the optical plane before and after transmission through each optical crystal when light travels from the second light input / output port to the third light input / output port. It is explanatory drawing which showed the position in the optical surface of the polarization component before and after each optical crystal transmission, and the polarization state when the light progresses from a 3rd light input / output port to a 4th light input / output port.

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

4 (a), (b), and (c) are explanatory diagrams showing the movement of a polarization component as viewed from a light input / output port side of an odd-numbered surface in the optical circulator of FIG.

FIGS. 5A and 5B are cross-sectional views illustrating an optical circulator according to a third embodiment of the present invention.

FIG. 6 is an explanatory diagram illustrating an optical circulator according to a fourth embodiment of the present invention.

FIG. 7 is an explanatory diagram showing another optical circulator according to the fourth embodiment of the present invention.

FIG. 8 is an explanatory diagram showing an optical circulator according to a fifth embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 1 birefringent crystal 2 birefringent crystal 3 birefringent crystal 4 birefringent crystal 5 non-reciprocal polarization plane rotator 6 non-reciprocal polarization plane rotator 7 reciprocal polarization plane rotator 8 first light input / output port 9 second Light input / output port 10 third light input / output port 11 fourth light input / output port 12 birefringent crystal 13 birefringent crystal 14 birefringent crystal 15 birefringent crystal 16 non-reciprocal polarization plane rotator 17 non-reciprocal polarization Plane rotator 18 reciprocal polarization plane rotator 19 polarization dispersion compensator 20 first light input / output port 21 second light input / output port 22 third light input / output port 23 fourth light input / output port 24 optics Crystal stack 25 Permanent magnet 26 Lens 27 TEC fiber 28 TEC fiber

Claims (10)

[Claims] An optical circulator comprising a combination of at least a birefringent element, a non-reciprocal polarization plane rotation element, and a parallel plate optical element including a reciprocal polarization plane rotation element, and having a plurality of light input / output ports, A first birefringent crystal as the birefringent element and a first non-reciprocal as the 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 polarization plane rotator, a second and a third birefringent crystal as the birefringent element, which are divided into two with respect to the light traveling direction and joined in a pair so that the polarization separation directions are opposite to each other; A second non-reciprocal polarization plane rotator as the non-reciprocal polarization plane rotator, a reciprocal polarization plane rotator as the reciprocal polarization plane rotator, and a fourth as the birefringent element
An optical circulator characterized by sequentially disposing birefringent crystals of the above. 2. An optical circulator having at least a birefringent element, a non-reciprocal polarization plane rotation element, and a parallel plate optical element including a reciprocal polarization plane rotation element and having a plurality of light input / output ports, A first birefringent crystal as the birefringent element and a first non-reciprocal as the 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 polarization plane rotator, a second and a third birefringent crystal as the birefringent element, which are divided into two with respect to the light traveling direction and joined in a pair so that the polarization separation directions are opposite to each other; A reciprocal polarization plane rotator as a reciprocal polarization plane rotator, and a second reciprocal polarization plane rotator as the non-reciprocal polarization plane rotator.
Non-reciprocal polarization plane rotator, the fourth as the birefringent element
An optical circulator characterized by sequentially disposing birefringent crystals of the above. 3. The optical circulator according to claim 1, wherein a polarization dispersion compensating plate is provided before any of the optical elements constituting the optical element section in addition to the optical element. 4. The method according to claim 1, wherein the second birefringent crystal and the third birefringent crystal have optical surfaces having substantially the same area and width. Optical circulator. 5. The thickness of the first 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. Saga approximately (2): 2
The optical circulator according to any one of claims 1 to 4, wherein: 6. The first non-reciprocal polarization plane rotator and the second non-reciprocal polarization plane rotator have the same direction of rotation of the polarization plane, and the first birefringent crystal and the second birefringent crystal have a same rotation direction. 4. The polarized light separating directions of the birefringent crystal of No. 4 are different by about 90 °, and the polarized light separating directions of the first birefringent crystal and the third birefringent crystal are different by about 45 °. The optical circulator according to claim 1. 7. A permanent magnet having a hole as a means for applying a magnetic field to said first and second non-reciprocal polarization plane rotators. Optical circulator. 8. The optical circulator according to claim 1, wherein at least one of the first and second non-reciprocal polarization plane rotators is a 45 ° Faraday rotator. 9. The optical circulator according to claim 1, wherein the reciprocal polarization plane rotator is a half-wave plate. 10. The optical circulator according to claim 1, wherein said second and third birefringent crystals have substantially the same polarization separation distance.