JP2594856B2 - Non-reciprocal light element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element used for optical communication and optical information processing, for specifying the traveling direction of light.

[0002]

2. Description of the Related Art Generally, a semiconductor laser diode is used in a bidirectional optical communication system or an optical fiber amplifier circuit. In such a system, an element (optical element) for specifying the traveling direction of the light is necessary to prevent the light returning to the semiconductor laser diode and efficiently guide the light to the light receiving device. As such an optical element, a four-terminal polarization-independent optical circulator has been conventionally known.

[0003] Here, with reference to FIG. 3, a polarization independent optical circulator (hereinafter simply referred to as an optical circulator) will be outlined.

The optical circulator is a 45-degree Faraday rotator 39 and a 45-degree optical rotation element (a half-wave plate may be used).
40 is provided. On the left side of the 45-degree Faraday rotator 39, a first dielectric multilayer polarizing beam splitter 35 is disposed, and the beam splitter 35 is connected to a pair of ports 31.
And 33 are provided. Similarly, a second dielectric multilayer polarizing beam splitter 36 is disposed on the right side of the 45-degree optical rotation element 40, and the beam splitter 36 has a pair of ports 32 and 34. Further, a first reflection mirror 37 is disposed facing the 45-degree Faraday rotator 39 and the beam splitter 35, and the 45-degree optical rotation element 40
And the second reflecting mirror 3 facing the beam splitter 36.
8 are arranged.

[0005]

Incidentally, since the above-mentioned optical circulator uses a polarization beam splitter using a dielectric multilayer film, that is, the dielectric multilayer film polarization beam splitter has a low degree of linear polarization of the reflection component. There is a problem that it is difficult to obtain sufficient isolation.

An object of the present invention is to provide a non-reciprocal optical element which can obtain a sufficiently high isolation and can be easily assembled.

[0007]

According to the present invention, there are provided first to fifth birefringent crystals, first and second Faraday rotators, and the first to fifth birefringent rotators are sequentially arranged along a predetermined direction. The first to fifth positions are positioned, the first and fifth birefringent crystals are disposed at the first position, and the first Faraday rotator is disposed at the second position. , The second and third birefringent crystals are disposed at the third position,
The second Faraday rotator is provided at the fourth position, the fourth birefringent crystal is provided at the fifth position, and the first to fifth birefringent crystals are provided. The crystal orientation of the first to fifth birefringent crystals is selected such that the amount of shift with respect to normal incidence extraordinary light in the body is proportional to the thickness of the first to fifth birefringent crystals in the predetermined direction. The thickness of the fourth birefringent crystal is defined as t ₁ ,
And when the thickness of the third birefringent crystal is t ₂ , the thickness (t ₃ ) of the fifth birefringent crystal is t ₃ = t ₁ ± 2
A non-reciprocal optical element characterized by being set to ^1/2 × t ₂ is obtained.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

Referring to FIG. 1 and FIG.
Is vertically incident on the first birefringent crystal flat plate 4 via the first lens 11. This birefringent crystal plate 4
In, the optical axis direction is determined so that the extraordinary light is shifted downward in the figure in parallel with the incident light and emitted. The ordinary light is transmitted without being refracted by the first birefringent crystal flat plate 4 and is incident on the first Faraday rotator 9. The ordinary light is emitted by the first Faraday rotator 9 after being rotated 45 degrees counterclockwise as viewed from the optical fiber 1 side. The second birefringent crystal flat plate 5 is arranged so as to receive the light emitted from the first Faraday rotator 9 as ordinary light. As a result, the light emitted from the first Faraday rotator 9 is not refracted and the second 2 is transmitted through the birefringent crystal flat plate 5. Then, the transmitted light from the second birefringent crystal flat plate 5 is rotated by the second Faraday rotator 10 counterclockwise by 45 degrees when viewed from the first optical fiber 1 side. As a result, the polarization direction becomes the vertical direction in the figure, and is incident on the fourth birefringent crystal flat plate 7 as extraordinary light. It has shifted vertically downward.

On the other hand, light incident on the first birefringent crystal plate 4 as extraordinary light shifts vertically downward with respect to the incident light as described above. Then, the polarization plane is rotated counterclockwise by 45 degrees by the first Faraday rotator 9 and is incident on the third birefringent crystal flat plate 6 as ordinary light. As a result, the third birefringent crystal flat plate 6 is transmitted without being refracted. Then, the light emitted from the third birefringent crystal flat plate 6 is rotated by 45 degrees in the polarization plane counterclockwise by the second Faraday rotator 7. As a result, the polarization direction becomes horizontal, and the fourth birefringent crystal flat plate 7 is transmitted as ordinary light without being refracted.

Incidentally, since the amount of shift of extraordinary light by the first and fourth birefringent crystal flat plates 4 and 7 is set to be the same, the incident light from the first optical fiber 1 is once separated. After passing through the fourth birefringent crystal flat plate 7, they are overlapped and coupled to the second optical fiber 2 via the second lens 12.

Next, the light applied from the second optical fiber 2 to the fourth birefringent crystal flat plate 7 will be described.

Referring to FIGS. 1 and 2, horizontal polarized light is ordinary light with respect to the fourth birefringent crystal flat plate 7, and is transmitted without being refracted by the fourth birefringent crystal flat plate 7. On the other hand, the vertically polarized light is extraordinary light with respect to the fourth birefringent crystal flat plate 7, and is shifted vertically upward by the fourth birefringent crystal flat plate 7. The outgoing light from the fourth birefringent crystal flat plate 7 has its polarization plane rotated by 45 degrees by the second Faraday rotator 10 and is inclined by 45 degrees with respect to the horizontal plane. When light is incident from the second optical fiber 2 side, the second birefringent crystal flat plate 5 has a polarization plane rotated 45 degrees counterclockwise from vertically above when viewed from the first optical fiber 1 side. The light is shifted parallel to the upper left, which is the same direction as the polarization direction, as the extraordinary light.

The lower light transmitted through the Faraday rotator 10 is incident on the birefringent crystal plate 6 as extraordinary light, and when viewed from the first optical fiber 1 side by the birefringent crystal plate 6, is counterclockwise from the horizontal direction. It is shifted parallel to the lower left, which is the direction rotated by 45 degrees. At this time, the birefringent crystal flat plate 5
The shift amounts of the extraordinary light due to (6) and (6) are set to be equal. Thereafter, the upper and lower lights, which have been rotated counterclockwise by 45 degrees when viewed from the first optical fiber 1 side by the Faraday rotator 9, become horizontal polarized light and vertical polarized light, respectively.

Light incident on the birefringent crystal plate 8 with horizontal polarization passes through the birefringent crystal plate 8 without being refracted as ordinary light. On the other hand, light incident on the birefringent crystal flat plate 8 as vertically polarized light is shifted vertically upward and parallel as extraordinary light. here,
The amount of extraordinary light shift due to the birefringent crystal flat plates 4 and 7 is d ₁ ,
When the extraordinary light shift due to the birefringent crystal flat plates 5 and 6 is d ₂ , the shift d ₃ of extraordinary light from the birefringent crystal flat plate 8
Is defined as d ₃ = d ₁ +2 ^1/2 × d ₂ (usually, the shift amount of these crystal axes is proportional to the thickness of each birefringent crystal flat plate, Therefore, the thickness of the first and fourth birefringent crystal flat plates is set to t.
₁ , when the thickness of the second and third birefringent crystal flat plates is t ₂ , the thickness t ₃ of the fifth birefringent crystal flat plate is t ₃ = t ₁ +2
^1/2 × t ₂ ). In this way, the light emitted from the birefringent crystal flat plate 8 overlaps and becomes one,
Is coupled to the third optical fiber 3 via the lens 13. Note that the light from the third optical fiber 3 is not coupled to the second optical fiber 2, and similarly, the light from the first optical fiber 1 is not coupled to the third optical fiber 3. (This is apparent from FIGS. 1 and 2).

As described above, the light incident from the first optical fiber 1 is emitted to the second optical fiber 2, and the light incident from the second optical fiber 2 is emitted to the third optical fiber 3. It is not coupled to another optical fiber (such a non-reciprocal optical element is called a three-terminal polarization independent non-reciprocal optical element).

In the above three-terminal polarization independent non-reciprocal optical element, the Faraday rotator has a thickness of 1.31 μm and a thickness of
Using a bismuth-substituted iron garnet thick film of 0.34 mm as a birefringent crystal flat plate, a rutile crystal as a uniaxial birefringent crystal, the C axis, which is one of the crystal axes of the birefringent crystal flat plate, is 48 It was processed so that it could only be tilted. When the thickness of the birefringent crystal flat plates 4 and 7 is set to 5.00 mm, the thickness of the birefringent crystal flat plates 5 and 6 is set to 7.07 mm, and the thickness of the birefringent crystal flat plate 8 is set to 15.00 mm, 45 dB or more Isolation was obtained.

Note that both the second and third birefringent crystal flat plates 5 and 6 are rotated 90 degrees in the same direction with respect to the axis perpendicular to the optical surface.
Is rotated by degrees, the magnetic field direction of the first and second Faraday rotator 9 and 10 both are inverted, the extraordinary light shift amount d3 of the fifth birefringent crystalline _{plate-8 d 3 = d 1 -2 1} / ^Two
It may be × d _2. In other words, since the shift amount with respect to the vertical incident extraordinary light in these birefringent crystal flat plate crystal orientation to be proportional to the thickness of the birefringent crystal plate-are selected, t ₃ is t ₃ = t ₁ -2 ^1/2 × t ₂ .

[0019]

As described above, in the present invention, five birefringent crystals and two Faraday rotators are used to determine the thickness relationship of the birefringent crystals, so that a sufficient Not only can you ensure isolation,
A three-terminal polarization-independent non-reciprocal optical element can be configured, and as a result, there is an advantage that assembly is easy. Further, by bonding the birefringent crystal and the Faraday rotator with an optical adhesive, a small three-terminal polarization independent non-reciprocal optical element can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing one embodiment of a non-reciprocal optical element according to the present invention.

FIG. 2 is a diagram for explaining the operation of the non-reciprocal optical element shown in FIG.

FIG. 3 is a diagram showing an example of a conventional four-terminal polarization independent optical circulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 1st optical fiber 2 2nd optical fiber 3 3rd optical fiber 4 1st birefringent crystal flat plate 5 2nd birefringent crystal flat plate 6 3rd birefringent crystal flat plate 7 4th birefringent crystal flat plate Reference Signs List 8 fifth birefringent crystal flat plate 9 first 45 degree Faraday rotator 10 second 45 degree Faraday rotator 11 first lens 12 second lens 13 third lens 31 first port 32 second Port 33 Third port 34 Fourth port 35 First dielectric multilayer polarizing beam splitter 36 Second dielectric multilayer polarizing beam splitter 37 First reflecting mirror 38 Second reflecting mirror 39 45 degree Faraday rotation Element 40 45-degree optical rotation element (or half-wave plate)

Claims (1)

(57) [Claims] A first birefringent crystal and first and second Faraday rotators, the first to fifth positions being sequentially positioned along a predetermined direction; The first and fifth birefringent crystals are disposed at the first position, the first Faraday rotator is disposed at the second position, and the second Firade rotator is disposed at the third position. And the third birefringent crystal is disposed, the second Faraday rotator is disposed at the fourth position, and the fourth birefringent crystal is disposed at the fifth position. And a shift amount of the first to fifth birefringent crystals with respect to the abnormally incident normal light is proportional to a thickness of the first to fifth birefringent crystals in the predetermined direction. the crystal orientation is selected such, the thickness of the first and the fourth birefringent crystal and t _1, the second When the thickness of the fine the third birefringent crystal was t _2, the thickness of the fifth birefringent crystal (t ₃₎ is set to ^{_{t 3 = t 1 ± 2 1/2}} × t 2 A non-reciprocal light element characterized by being performed.