JP2006106104A - Depolarizing element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a depolarizing element which realizes miniaturization of the element and improvement in temperature stability. <P>SOLUTION: The depolarizing element converts polarized light into non-polarized light, and is provided with: a conversion medium 1 with optical anisotropy; an incident means (polarization maintaining optical fiber 3) for making the polarized light incident on the conversion medium 1 so that the incident light is included in a plane normal to the optical axis direction in the conversion medium 1 and is made incident on the medium from a direction oblique to the normal line direction of the incident plane 1a; and reflection layers 2a, 2b for reflecting the light propagated through the conversion medium 1 to return it almost in the shape of a letter V, and the polarized light made incident on the conversion medium 1 is separated into two optical components differing in polarization, and the two optical components are reflected two or more times different from each other by the reflection layers 2a, 3b and propagated through the conversion medium 1 while returning almost in the shape of a letter V, thereafter, the components are made to exit from the conversion medium at a position P where the optical paths of the two optical components are superimposed on each other. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a depolarizing element having a function of converting polarized light into non-polarized light.

For example, Raman amplifiers utilizing Raman scattering have recently become widespread as having excellent amplification characteristics.
On the other hand, the Raman amplifier has a problem that the polarization dependency of the gain is large, and a technique for canceling the polarization of the excitation light source with a depolarizer or the like is important.
In addition to the Raman amplifier, in the fields of optical communication and optical measurement (sensing), if the light source light is polarized, the system or the like may be adversely affected. In these fields, the depolarizing element is used.

As a conventional depolarizer, for example, a Lyot-Type one configured by fusing a polarization maintaining optical fiber by 45 ° is known.
A depolarizer using a birefringence crystal as shown in FIG. 6 has also been proposed (see Non-Patent Document 1 below). The crystal type depolarizer in this example is configured so that completely polarized light is incident perpendicularly from one surface (incident surface) of a birefringent crystal formed in a rectangular parallelepiped, and non-polarized light is emitted from a surface facing the incident surface. ing.
Kazami et al., “Development of Crystalline Depolarizer”, Furukawa Electric Times, January 2003, No. 111, p41-45.

The depolarizer formed by fusing the polarization-maintaining optical fiber by 45 ° has a problem that since the degree of depolarization per unit length is small, the fiber becomes long and it is difficult to reduce the size.
In the crystal type depolarizer, it is necessary to set the length of the birefringent crystal to such a length as to cause a phase difference (2m + 1) π between adjacent FP modes of incident light. The degree of depolarization per unit length is not very large. For this reason, in order to construct a practical depolarizer, a long crystal is required, which is difficult to put into practical use and cannot be reduced in size. Downsizing of the depolarizer is important because it contributes not only to cost reduction and space saving but also to improvement of temperature characteristics.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a depolarizing element capable of realizing downsizing of the element and improvement of temperature stability.

  In order to achieve the above object, the depolarizing element of the present invention is a depolarizing element that converts polarized light into non-polarized light, and includes a conversion medium having optical anisotropy, polarized light in the conversion medium, Incident means that is included in a plane whose normal direction is the optical axis direction of the conversion medium and that is incident obliquely with respect to the normal direction of the incident surface, and substantially reflects and reflects light propagating in the conversion medium. Reflection means for folding back in a V shape, and polarized light incident on the conversion medium is separated into two light components having different polarization states, and the two light components are different from each other by the reflection means. And the light is emitted from the conversion medium at a position where the optical paths of the two light components overlap each other.

  According to the present invention, it is possible to obtain a depolarizing element that can achieve downsizing and improved temperature stability.

1 to 3 show an embodiment of the depolarizing element of the present invention. FIG. 1 is a perspective view and FIG. 2 is a side view. In the figure, reference numeral 1 denotes a conversion medium, and 2a and 2b denote reflection layers (reflection means).
FIG. 3 is a cross-sectional view taken along a plane including an optical path formed in the conversion medium 1. The plane including the optical path is a plane along the line III-III in FIG. 1, and is a plane indicated by a symbol L2 in FIG.

The conversion medium 1 can be used without any particular limitation as long as it is a material having optical anisotropy, but is preferably a uniaxial crystal, for example, rutile (tetragonal), YVO ₄ , LN, calcite, or the like. . In this embodiment, rutile is used.
As for the shape of the conversion medium 1, it is preferable that at least the incident surface 1 a and the surface facing the incident surface 1 a (hereinafter referred to as the facing surface 1 b) are flat and parallel to each other. The conversion medium 1 in this embodiment is formed in a rectangular parallelepiped.
When the incident surface is an XY plane, the length direction of the incident surface 1a is the X direction, the direction included in the incident surface 1a and perpendicular to the X direction is the Y direction, and the normal direction of the incident surface is the Z direction. The optical axis (main optical axis) of the medium 1 is configured to be parallel to the Y direction. In a uniaxial crystal, the crystal axis c coincides with the optical axis. In this embodiment, the rutile crystal axis c (indicated by an arrow c in the figure) is parallel to the Y direction.

The reflective layers 2a and 2b are provided outside the incident surface 1a and the opposing surface 1b, respectively, and the reflective surfaces of the reflective layers 2a and 2b are parallel to the incident surface 1a and the opposing surface 1b, respectively. The reflectivity of the reflective layers 2a and 2b is preferably 95 to 98%, and more preferably close to 100% (total reflection).
The material of the reflective layers 2a and 2b is not particularly limited as long as a desired reflectance can be obtained. Preferable examples include a dielectric multilayer film in which layers made of SiO ₂ and layers made of Ta ₂ O ₅ are alternately laminated.

  In the present embodiment, a polarization maintaining optical fiber 3 is provided as an incident means. The polarization maintaining optical fiber 3 is configured to allow linearly polarized light (hereinafter referred to as polarization L1) emitted therefrom to enter the conversion medium 1 from a predetermined incident direction. The incident direction of the polarized light L1 is included in a plane (XZ plane) whose normal direction is the optical axis direction (Y direction) of the conversion medium 1, and is in the normal direction (Z direction) of the incident surface 1a. On the other hand, it is set in an oblique direction.

Also included in a plane of the traveling direction of the polarized light L1 and the normal direction, the direction of the optical axis of the conversion medium 1 (Y-direction) and a direction parallel to the y ₁ direction, the y ₁ direction contained within said surface the direction perpendicular to the x ₁ direction. The angle α (not shown) formed by the polarization azimuth of the polarized light L1 (the vibration direction of the electric field when the light beam is expressed as a wave) and the y ₁ direction in a plane whose normal direction is the traveling direction of the polarized light L1 is 0. It is greater than ° and less than 90 °. To obtain a practically preferable unpolarized is preferably an angle between the polarizing direction and y ₁ direction α is approximately 45 °, and more preferably 45 °. The allowable range is preferably about 45 ° ± 1 °. In the present embodiment, α = 45 ° is set.

Although the wavelength of the polarized light 1 is not specifically limited, For example, the wavelength of the range of 1520-1620 nm is used suitably.
The polarized light L1 incident on the conversion medium 1 is preferably parallel light, and a collimator is preferably provided as necessary.
Further, the reflection layer 2a is not provided at the incident position of the polarized light L1 on the incident surface 1a, and a non-reflection layer (not shown) is provided.

In the present embodiment, the polarizer 4 is provided on the optical path of the polarized light L1 between the polarization maintaining optical fiber 3 and the conversion medium 1. The polarizer 4 is configured to selectively transmit only the component of the polarized light L1 having an angle α of 45 ° with the y ₁ direction.

In the depolarizing element having such a configuration, the polarized light L1 incident from the oblique direction with respect to the incident surface 1 a is separated into two light components having different polarization states due to the birefringence of the conversion medium 1. That polarized light L1 and the polarization component oscillating direction of the electric field is the x ₁ direction, the vibration direction of the electric field is separated into a polarized light component that is the y ₁ direction. In the present embodiment, since the conversion medium 1 is made of a uniaxial crystal, the two polarization components are an ordinary ray and an extraordinary ray.
Since the two polarization components (ordinary ray and extraordinary ray) have different refractive indexes, different optical paths are formed in the conversion medium 1 as shown in FIG. Since the reflection layers 2a and 2b are provided outside the incident surface 1a and the opposing surface 1b, the two polarization components are propagated between the reflection layers 2a and 2b while being folded back in a substantially V shape. The optical paths of the two polarization components in the conversion medium 1 are formed in a plane (XZ plane) whose normal direction is the optical axis direction (Y direction). 2 indicates a surface including the optical paths of the two polarization components in the conversion medium 1.

The two polarization components are reflected by the reflective layers 2a and 2b a plurality of times different from each other, and the inside of the surface L2 (XZ plane) is turned in the X direction while turning back inside the conversion medium 1 in a substantially V shape. Then, the light is combined at the emission position P where the optical paths of the two polarization components overlap on the incident surface 1a and emitted from the conversion medium 1 (reference numeral L3 indicates the emitted light).
A delay occurs because there is a difference between the optical path lengths of the two polarization components in the conversion medium 1. In the present invention, the optical path length difference (delay amount) between the two is longer than the coherence distance (coherence time). Designed to be Therefore, when the two polarization components are combined at the emission position P to form the emission light L3, the two polarization components are in a state where coherence is eliminated.
The outgoing light L3 obtained in this way passes through a collimator (not shown) as necessary, and is then transmitted through the single mode optical fiber 5. In addition, the reflective layer 2a is not provided at the emission position P.

The optical path length difference (delay amount) between the two polarization components in the conversion medium 1 changes the angle θ1 between the polarization L1 and the normal direction of the incident surface 1a and / or the length Lz of the conversion medium 1 in the Z direction. Can be controlled.
The angle θ1 formed by the polarized light L1 incident on the conversion medium 1 and the normal direction of the incident surface 1a may be larger than 0, but if it is too small, the optical path before being reflected by the reflective layer 2a (or 2b) The two light beams are not sufficiently separated because the light paths after reflection substantially overlap. On the other hand, if θ1 is too large, the length (the distance Lx from the incident position to the exit position P in the X direction) of the conversion medium 1 necessary for obtaining a desired optical path length difference (delay amount) increases, and the element becomes large. It will become. Therefore, θ1 is preferably about 4 ° to 7 °.

In the present embodiment, the angle α formed by the polarization azimuth of the polarized light L1 incident on the conversion medium 1 and the y ₁ direction (parallel to the c-axis) is set to 45 °. Polarized light is obtained.
Here, “practically preferred non-polarized light” means that, for example, when a linear polarizer is provided on the optical path of the outgoing light L3 from the depolarizing element, the linear polarizer may be arbitrarily rotated around the optical axis. In the state where the amount of light emitted from the linear polarizer does not change, a quarter wavelength plate is provided on the optical path of the outgoing light L3 from the depolarizing element, and the linear polarizer is provided on the optical path of the quarter wavelength plate. In this case, the light quantity emitted from the linear polarizer does not change even when the quarter-wave plate and the linear polarizer are arbitrarily rotated around the optical axis.
That is, as shown in FIG. 4, since in the present embodiment has a alpha = 45 °, polarized light L1 incident from an oblique direction with respect to the incident surface 1a is, polarized light having a vibration direction of the electric field in the x ₁ direction When separated into a component (light intensity; Px) and a polarized light component (light intensity; Py) having an electric field oscillation direction in the y ₁ direction, the light intensity of both becomes equal (Px = Py).
Emitted light L3, the polarization component having the vibration direction of the electric field in the x ₁ direction (light intensity; Px) and the polarization component (light intensity; Py) with the vibration direction of the electric field in the y ₁ direction, but coherent Therefore, when Px and Py are equal (Px = Py), the above-mentioned “practically preferable non-polarized light” can be obtained.

On the other hand, when α is not 45 °, for example, when α = 30 °, as shown in FIG. 5, Px: Py = 1: √3, and Px and Py are not equal, but in the conversion medium 1 Is designed so that the optical path length difference (delay amount) of the two polarized light components is longer than the coherent distance (coherent time), so that the two polarized components (light intensity; Px, Py) are coherent. In a state where there is no property, the light is combined to become outgoing light L3.
Therefore, when α is not 45 °, the amount of emitted light varies when the quarter-wave plate and / or the linear polarizer is arbitrarily rotated around the optical axis, but α is larger than 0 °. In the range of less than 90 °, it is possible to obtain the emitted light L3 in a state where the coherence of the polarization component is eliminated. Such a state in which the coherence of the polarization component is eliminated is also included in the “non-polarized light” in the present invention.

The variation in the amount of emitted light when the quarter-wave plate and / or the linear polarizer is arbitrarily rotated around the optical axis increases as the value of α becomes larger or smaller than 45 °. The allowable range of the fluctuation range of the emitted light amount varies depending on the application of the depolarizer. Accordingly, the allowable range of α also varies depending on the accuracy required for the depolarizing element, but when “practically preferred non-polarized light” that hardly changes the amount of emitted light is required, a range of about 45 ° ± 1 ° is required. Is preferred.
In addition, in a case where the requirement for fluctuation of the amount of emitted light is not strict and non-polarized light is required in a state where the coherence of the polarization component is eliminated, α should be set in a range greater than 0 ° and less than 90 °. Can do.

In the above embodiment, as shown in FIG. 3, the gaps are provided between the reflective layers 2a and 2b and the incident surface 1a and the opposing surface 1b, respectively, but the reflective layers 2a and 2b are provided on the incident surface 1a. And may be in contact with the opposing surface 1b or may be integrated. When the gap is provided, if the gap width D is too large, the element becomes unnecessarily large. Therefore, the gap is preferably 1 mm or less, and more preferably 0.5 mm or less. Moreover, when providing the said gap | interval, it is preferable to provide a non-reflective coating layer on the incident surface 1a and the opposing surface 1b.
The incident means may be any means that can perform optical transmission while suppressing mode coupling between the polarization modes. Spatial light or the like can be used instead of the polarization maintaining optical fiber 3.
The emission position P may be any position where the optical paths of the two polarization components overlap, and the emission position P may be provided on the facing surface 1b.
Moreover, the reflective surfaces of the reflective layers 2a and 2b do not necessarily have to be parallel to each other, and an angle formed by the two can be within an allowable range.

In the depolarizing element of this embodiment, even if the conversion medium 1 is small, the optical path length of two polarization components separated in the conversion medium 1 can be increased. A difference (a delay amount) can be generated. Therefore, a small and high performance depolarizing element can be obtained. Further, by reducing the size of the element, the temperature stability of the element is improved.
Further, it is possible to obtain a plurality of types of delay amounts without changing the conversion medium 1 by changing the incident angle θ1 of the polarized light L1.

Example 1
The delay amount obtained when the polarization cancellation element shown in FIGS. 1 to 3 is configured under the following conditions was simulated. One of the refractive indexes of the two polarization components in the conversion medium 1 is 1.9447 and the other is 2.1486.
Distance Lx from the entrance position to the exit position P in the X direction: 13.7 mm
Length Lz of conversion medium 1 in the Z direction: 5.0 mm
Width D of the gap between the reflective layers 2a and 2b and the incident surface 1a and the opposing surface 1b: 0.3 mm
Incident angle θ1: 6.5 ° of polarized light L1
Reflectivity of the reflective layers 2a and 2b: 99%
Wavelength of polarized light L1: 1550 nm
At this time, of the two polarization components separated in the conversion medium 1, the component having the smaller number of reflections at the reflection layers 2a and 2b has a reflection number of 25 and the reflection surface of the reflection layer 2a (2b). The distance W between the reflection positions adjacent to each other is 0.53 mm. The optical path length difference between the two polarization components generated in the conversion medium 1 is 45.95 mm, and the delay amount is 153 psec (picoseconds).
Further, the value of the loss between coupled rays obtained by the formula for calculating the coupling efficiency of the Gaussian beam is 0.079 dB.

(Example 2)
The delay amount obtained when the polarization cancellation element shown in FIGS. 1 to 3 is configured under the following conditions was simulated. One of the refractive indexes of the two polarization components in the conversion medium 1 is 1.9447 and the other is 2.1486.
Distance Lx from the entrance position to the exit position P in the X direction: 13.7 mm
Length Lz of conversion medium 1 in the Z direction: 8.2 mm
Width D of the gap between the reflective layers 2a and 2b and the incident surface 1a and the opposing surface 1b: 0.3 mm
Incident angle θ1: 4.5 ° of polarized light L1
Reflectivity of the reflective layers 2a and 2b: 99%
Wavelength of polarized light L1: 1550 nm
At this time, of the two polarization components separated in the conversion medium 1, the component having the smaller number of reflections at the reflection layers 2a and 2b has a reflection number of 23 and the reflection surface of the reflection layer 2a (2b). The distance W between adjacent reflection positions is 0.599 mm. The optical path length difference between the two polarization components generated in the conversion medium 1 is 72.02 mm, and the delay amount is 240 psec (picoseconds).
Further, the value of the loss between coupled rays obtained from the Gaussian beam coupling efficiency calculation formula is 0.071 dB.

It is a perspective view which shows one Embodiment of the depolarizing element of this invention. FIG. 2 is a surface plan view of the depolarizing element in FIG. 1. Sectional drawing which follows the surface containing the optical path currently formed in the conversion medium of FIG. It is a figure for demonstrating the preferable angle of the polarization azimuth | direction of incident light. It is a figure for demonstrating the preferable angle of the polarization azimuth | direction of incident light. It is a schematic block diagram which shows an example of the conventional depolarizing element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Conversion medium 2a, 2b ... Reflection layer (reflection means)
3. Polarization-maintaining optical fiber (incident means)

Claims (2)

A depolarizing element that converts polarized light into non-polarized light,
A conversion medium having optical anisotropy;
Incident means for making polarized light incident on the conversion medium in a plane that is included in a plane whose normal direction is the optical axis direction of the conversion medium and that is incident obliquely with respect to the normal direction of the incident surface;
Reflecting means for reflecting the light propagating in the conversion medium and folding it back into a substantially V shape;
The polarized light incident on the conversion medium is separated into two light components having different polarization states, and the two light components are reflected by the reflection means a plurality of times different from each other, and the inside of the conversion medium is approximately V. A depolarizing element, which propagates while being folded back in a letter shape, and is emitted from the conversion medium at a position where the optical paths of the two light components overlap. An angle α between a direction parallel to the optical axis direction of the conversion medium and a polarization direction of the polarization incident on the conversion medium in a plane having a normal direction as a traveling direction of the polarization incident on the conversion medium. The depolarizing element according to claim 1, wherein is approximately 45 °.

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