JPH0990281A - Polarization independent type optical isolator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical isolator which is hardly degraded in isolation characteristics in spite of the presence of a change in temp., light wavelength, etc., and a variation in the characteristics of Faraday elements, is further small in polarization mode dispersion and has excellent characteristics. SOLUTION: This optical isolator consists of a first double refraction element B1 , first Faraday element F1 , second double refraction element B2 . second Faraday element F2 , third double refraction element B3 , third Faraday element F3 and fourth double refraction element B4 . The optical isolator is constituted in the following manner: The polarized light separating directions of the first and second double refraction elements B1 , B2 incline about 45 deg. with each other in the plane perpendicular to transmitted light and the ratio of the polarized light sepns. of the first and second double refraction elements B1 , B2 is 1/2<1/2> . The polarized light separating directions of the third and fourth double refraction elements B3 , B4 incline about 45 with each other in the plane perpendicular to transmitted light and the ratio of the polarized light sepn. distances of the third and fourth double refraction elements B3 , B4 is 1/2<1/2> . In addition, the rotating angle of the polarization directions of the transmitted light by the first, second and third Faraday elements F1 to F3 is about 45 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention prevents light waves emitted from a laser serving as a light source from returning to the light source due to various causes in optical communication, broadcast wave transmission using light, measurement by light, and the like. The present invention relates to an optical isolator used for powering.

[0002]

2. Description of the Related Art Conventionally, an optical isolator used for such a purpose has one or two Faraday elements.
In many cases, one sheet was used, and further, a large polarization mode dispersion was present.

[0003]

In the conventional optical isolator, the characteristics of the optical isolator depend on the characteristics of the Faraday element due to temperature changes, variations in the oscillation wavelength of the laser used as the light source, and variations in the characteristics of the Faraday element. There is a problem that the isolation decreases. Moreover, since the optical path lengths of two orthogonal polarization components of the lights separated in the optical isolator are different, there is a problem that the polarization mode dispersion is large.

An object of the present invention is to change the temperature and the wavelength of light,
Another object of the present invention is to provide an excellent optical isolator in which the isolation of the optical isolator hardly deteriorates due to the variation in the characteristics of the Faraday element and the polarization mode dispersion is small.

[0005]

According to the present invention, a first birefringent element, a first Faraday element, a second birefringent element, and a second Faraday element are arranged in this order in the light transmission direction. , A third birefringent element, a third Faraday element, and a fourth birefringent element, wherein the polarization separation directions of the first and second birefringent elements are about 45 each other in a plane perpendicular to the transmitted light. The polarization separation direction of the first and second birefringent elements is substantially 1/2 ^1/2 , and the polarization separation direction of the third and fourth birefringent elements is transmitted light. About 45 degrees relative to each other in a plane perpendicular to, and the ratio of the polarization separation distances of the third and fourth birefringent elements is substantially 1/2 ^1/2 , and An optical isolator having a rotation angle of about 45 degrees in the polarization direction of transmitted light by the second and third Faraday elements is obtained. That.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION As a means for solving the conventional problems, the present invention combines three Faraday elements and four birefringent elements as in the embodiments described below and separates them in an optical isolator. It is designed so that the optical path lengths of two polarized light components orthogonal to each other are the same. In this way,
Despite changes in temperature and laser oscillation wavelength, no change in the isolation characteristics of the optical isolator was observed,
Further, the present invention provides an excellent optical isolator having a small polarization mode dispersion for light transmitted in the forward direction.

Specific embodiments of the present invention will be described below with reference to examples.

(Embodiment) FIG. 1 is a perspective view showing an embodiment of the present invention. In FIG. 1, the direction indicated by arrow A ₁ is the light transmission direction of the optical isolator (hereinafter referred to as the forward direction), and the direction indicated by arrow A ₂ is the light transmission blocking direction of the optical isolator (hereinafter referred to as the reverse direction). ). Code B
₁ , B ₂ , B ₃ , and B ₄ are birefringent elements made of rutile single crystal. Reference numerals F ₁ , F ₂ and F ₃ are Faraday elements made of terbium bismuth iron garnet single crystal.

In the birefringent elements B ₁ , B ₂ , B ₃ and B ₄ , the crystal axes and the surface of the element form an angle of approximately 48 degrees, and the plate of the birefringent elements B ₁ and B ₃ is The thickness is 1 mm, and the thickness of the birefringent elements B ₂ and B ₄ is 1.41 mm.

The polarization separation directions and distances of the birefringent elements B ₁ , B ₂ , B ₃ and B ₄ are in the directions of arrows shown in FIG. FIG.
(A) is a case of transmitted light in the forward direction, in the birefringent element B ₁ , the direction is at 3 o'clock of the hour hand of the watch, the distance is 100 μm, and in the birefringent element B ₂ , at 10 o'clock of the hour hand of the watch. In the direction of 30 minutes, the distance is 141 μm, in the birefringent element B ₃ , the distance is 100 μm in the direction of the hour hand of the clock at 9 o'clock, and in the birefringent element B ₄ , it is 10:30 of the hour hand of the clock. In the direction of, the distance is 141 μm.

FIG. 2B shows the case of transmitted light in the opposite direction.
The polarization separation distance is the same as the forward direction, but the separation direction is opposite.

The Faraday elements F ₁ , F ₂ and F ₃ have wavelengths of 1.
Saturation magnetization is performed by permanent magnets arranged around the Faraday elements F ₁ , F ₂ , and F ₃ so that the polarization direction of the light of 55 μm rotates by 45 degrees. When viewed from the forward direction, the rotation direction is the direction indicated by the arrow in FIG. 2, and the Faraday elements F ₁ and F ₂ are in the clockwise direction,
The Faraday element F ₃ has a counterclockwise rotating direction.

FIG. 3 and FIG. 4 show the polarization separation states of light transmitted in the forward direction and the backward direction.

As shown in FIG. 3, regarding the birefringent element B ₁ , the position on the forward incidence side is 1, the position between the birefringent element B ₁ and the Faraday element F ₁ is 2, and the Faraday element F ₁ is The position between the birefringent element B ₂ is 3, the position between the birefringent element B ₂ and the Faraday element F ₂ is 4, and the position between the Faraday element F ₂ and the birefringent element B ₃ is 5. The position between the refracting element B ₃ and the Faraday element F ₃ is 6, the position between the Faraday element F ₃ and the birefringent element B ₄ is 7, and the position on the forward exit side with respect to the birefringent element B ₄ 8
As shown.

Next, the respective positions 1 to 8 of the light transmitted in the forward direction
FIG. 4A shows the polarization state viewed from the forward direction in FIG.

First, at the position 1, light incident from one point in the forward direction is unpolarized and is represented as a polarization component orthogonal to the two directions. At position 2, the birefringent element B ₁ causes a horizontal polarization component (extraordinary light component of rutile crystal).
Moves 100 μm in the horizontal direction, and the polarization component in the vertical direction (the ordinary light component of the rutile crystal) remains at the original position. At the position 3, the polarization components are all rotated 45 degrees clockwise by the Faraday element F ₁ . At the position of 4, the birefringent element B ₂ causes only the polarized component tilted by 45 degrees clockwise from the horizontal direction at 10 o'clock 3 o'clock of the clock hand.
It moves 141 μm in the direction of 0 minutes. At the position of 5, the polarization components are all rotated 45 degrees clockwise by the Faraday element F ₂ . In the position of 6, the birefringent element B
_{By 3} , only the horizontal component moves 100 μm in the direction of the hour hand 9 o'clock of the clock. At the position of 7, the Faraday element F ₃ causes any of the polarization components to rotate counterclockwise by 45 degrees.
It is rotating around. At position 8, due to the birefringence element B ₄ , only the polarization component inclined 45 degrees counterclockwise from the horizontal direction moves 141 μm in the direction of the hour hand 1:30 of the clock, and the polarization component of the other orthogonal polarization component is moved. Match the light. As a result, the light transmitted in the forward direction passes through the element of the optical isolator without being separated into the polarized components.

Next, the respective positions 1 to 8 of the light transmitted in the opposite direction
FIG. 4B shows the polarization state viewed from the forward direction in FIG.

First, at the position 8, the light incident from one point in the opposite direction is unpolarized and is represented as a polarized component orthogonal to the two directions. At the position 7, the birefringent element B ₄ causes only the polarized component inclined by 45 degrees counterclockwise from the horizontal direction to move 141 μm in the direction of 7:30 of the short hand of the clock. At the position of 6, all polarization components are rotated by 45 degrees counterclockwise by the Faraday element F ₃ . At the position 5, the birefringent element B ₃ moves only the horizontal polarization component by 100 μm in the direction of 3 o'clock of the hour hand of the clock. At the position 4, all polarization components are rotated 45 degrees clockwise by the Faraday element F ₂ . At the position 3, the birefringent element B ₂ causes only the polarized component tilted 45 degrees clockwise from the horizontal direction to move 141 μm in the direction of 4:30 of the short hand of the clock. At the position 2, the polarization components are all rotated 45 degrees clockwise by the Faraday element F ₁ . At the position 1, the birefringent element B ₁ moves only the horizontal component by 100 μm in the 9 o'clock direction of the hour hand of the clock. As a result of the above,
When the light transmitted in the opposite direction passes through the element of the optical isolator, it is split into polarized light components and does not return to the position 1 in FIG. 4A.

FIG. 5 shows the structure of the isolator of the present invention. A single mode fiber 11 with a PC connector and a GRIN lens 12 are provided on both sides of the optical isolator 14.
The laser light having a wavelength of 1.55 μm emitted from the single mode fiber 11 is narrowed down to a diameter of about 60 μm by the GRIN lens, and the narrowed position 13 and the center of the optical isolator 14 substantially coincide with each other. Arrange to do. The end surface of the single mode fiber 11 on the lens side, the GRIN lens 12, each birefringent element forming the optical isolator 14, and each Faraday element are provided with a non-reflection coating film for light having a wavelength of 1.55 μm.

Light of a semiconductor laser having an oscillation wavelength of 1.55 μm was transmitted through the single mode fiber 11 in the forward direction of the element of the optical isolator to the optical isolator configured as described above. Optical isolator temperature 0 ℃
When the temperature is changed to -60 ° C, when one Faraday element is used, the isolation deteriorates from 50 dB to 20 dB, and when two Faraday elements are used, the isolation deteriorates from 50 dB to about 40 dB, compared to three. When the Faraday element was used, no change in the isolation value of 50 dB was observed. In the case of an optical isolator using three Faraday elements, the oscillation laser wavelength is 1.51 ~
No change was observed in the value of isolation even when changed in the range of 1.59 μm. Furthermore, the polarization mode dispersion (PMD) of the light transmitted in the forward direction was measured with a PMD measuring instrument manufactured by Applied Photoelectric Co., Ltd., and it was 0.05 picoseconds or less in the case of the present invention. This is because the horizontal and vertical components of the orthogonal polarization components of the light passing through the optical isolator are designed so that the optical path lengths in the optical isolator are the same. On the other hand, the polarization mode dispersion was 3 picoseconds in the optical isolators having different optical path lengths in the commercially available optical isolators for the two orthogonal polarization components.

[0021]

According to the optical isolator of the present invention, the isolation characteristic is hardly deteriorated due to changes in temperature, light wavelength, etc. and variations in the characteristics of the Faraday element.
Furthermore, since the optical path lengths of two orthogonal polarization components of the lights separated in the optical isolator are the same, it is possible to provide an optical isolator characterized by a small polarization mode dispersion and excellent characteristics.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an array of birefringent elements and Faraday elements in an optical isolator of the present invention.

FIG. 2 is an explanatory diagram of polarization separation and polarization rotation of a birefringent element and a Faraday element in the optical isolator of the present invention.

FIG. 3 is an explanatory diagram of the numbering of positions for explaining the functions of the birefringent element and the Faraday element that form the optical isolator of the present invention.

FIG. 4 is an explanatory view of polarization separation and polarization rotation of forward and backward transmitted light at each numbered position shown in FIG.

FIG. 5 is an explanatory diagram of a configuration of an optical isolator according to the present invention.

[Explanation of symbols]

11 single mode fiber 12 GRIN lens 14 optical isolator A ₁ arrow indicating the light transmission direction (forward direction) A ₂ arrow indicating the light transmission blocking direction (reverse direction) B ₁ , B ₂ , B ₃ , B ₄ birefringent element F ₁ , F ₂ , F ₃ Faraday element

Claims (1)

[Claims] 1. A first birefringent element, a first Faraday element, a second birefringent element, a second Faraday element, a third birefringent element, and a third birefringent element, which are arranged in this order in the light transmission direction. Faraday element and a fourth birefringent element, wherein the polarization separation directions of the first and second birefringent elements are inclined by about 45 degrees in a plane perpendicular to the transmitted light. The polarization separation ratio of the second birefringent element is substantially 1/2 ^1/2 , and the polarization separation directions of the third and fourth birefringent elements are about 45 degrees in a plane perpendicular to the transmitted light. Inclined and said third
And the ratio of the polarization separation distances of the fourth birefringent element is substantially 1
/ 2 ^1/2 , and the rotation angle of the polarization direction of the transmitted light by the first, second and third Faraday elements is about 45 degrees.