JP2677365B2 - Polarization variable device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] A communication system using an optical fiber, particularly a polarization variable device capable of controlling the polarization state. Polarization elliptical polarization circular polarization) for the purpose of providing a variable polarization device capable of rotatably supported with respect to a plane orthogonal to the optical axis between the input side and output side optical fibers coupled to the lens, A polarization variable device comprising a half-wave plate and a quarter-wave plate is constructed. [Field of Industrial Application] The present invention relates to a communication system using an optical fiber, and more particularly to a polarization variable device capable of controlling a polarization state. In recent years, in optical communication, a system is often constructed by combining a single mode optical fiber and a semiconductor laser (LD). The optical signal transmitted in such an optical communication system has high coherency and is so-called polarized light. Therefore, the optical device used in this optical communication system needs to clarify the polarization dependence of the characteristics. [Prior Art] Conventionally, a single mode optical fiber and a semiconductor laser (L
In the optical communication system in which D) is combined, as shown in FIG. 2, the polarization state of the characteristics is measured by simply twisting the optical fiber to change the polarization state. In FIG. 2, when the semiconductor laser (LD) and the device to be measured (coupler) are coupled by an optical fiber, the optical fiber is formed into a loop shape as shown and an arrow A
By adding a "twist" like that, the plane of polarization of the transmitted optical signal can be rotated. [Problems to be Solved by the Invention] However, in the prior art as shown in FIG. 2, the optical fiber is connectorized at both ends (B, C) (not shown) and fixed to the ferrule. Therefore, physical stress is applied particularly near both ends of the optical fiber, which may cause phase shift due to birefringence. Therefore, polarized light in a certain direction may be extremely elliptical. Such ellipticity can prevent the phase shift to some extent by the method of "twisting", but it has a problem that it takes time to adjust and it is difficult to fix it to an arbitrary polarization state. Therefore, it is an object of the present invention to provide a polarization variable device that can easily adjust the elliptical polarization of light while rotating the plane of polarization. [Means for Solving Problems] In order to solve such problems, according to the present invention, a first lens and a second lens are provided, and a space between the first lens and the second lens is provided. , That rotates orthogonally to the optical axis, a polarizer, a half-wave plate and a quarter-wave plate are arranged in this order or a polarizer, a quarter-wave plate and a half-wave plate in that order. A variable polarization device is provided. [Operation] According to the present invention, the light output from one optical fiber through the lens is transmitted through the polarizer, the half-wave plate, and the quarter-wave plate, and the polarization state is changed by each rotation. It is controlled to input through the other optical fiber lens. The polarizer obtains linearly polarized light having a high extinction ratio regardless of the polarization state of incident light. 1 /
The two-wave plate rotates the principal axis of incident polarized light by twice the rotation angle. Furthermore, the quarter-wave plate adjusts the ellipticity of polarized light according to the relationship between the angle between the principal axis of the crystal and the principal axis of incident polarized light and the ellipticity of polarized light. Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic view of an embodiment of the variable polarization device of the present invention. Input side optical fiber 11 and output side optical fiber
Reference numeral 21 is fixed to both ends of the housing 3 via the respective ferrules 12 and 22 so as to face each other, and is optically coupled via the lenses 13 and 23 so that their optical axes coincide with each other. Inside the housing 3, two lenses 13 facing each other,
Between 23, the polarizer 4 (for example, Glan-Thompson prism, etc.) 4, 1/2 wave plate 5, 1/4 from the input side optical fiber 11 side.
Each screw feed rotation mechanism is such that the wave plate 6 is rotatable about an optical axis and a plane orthogonal to the optical axis.
Supported by 71, 72, 73. Each screw feed rotation mechanism 71, 72,
Reference numeral 73 has rotary knobs 81, 82, and 83, respectively, so that the Glan-Thompson prism 4, the half-wave plate 5, and the quarter-wave plate 6 can be adjusted and fixed to desired rotation positions. The single mode (SM) input side optical fiber 11 is connected to a semiconductor laser (LD), and is also single mode (SM).
The output side optical fiber 21 is connected to an optical power meter (not shown) via a device under test (not shown). The Glan-Thompson prism 4 is constructed by laminating two calcite triangular prisms together.
By making the principal axis of polarization of D) and the axis of transmission polarization coincide with each other, linearly polarized light can be obtained with the maximum optical power, that is, with the maximum dynamic range of the measurement system. Therefore, the linearly polarized light is adjusted to that position and fixed there. By rotating the 1/2 wavelength plate 5, the principal axis of polarized light can be arbitrarily set, and the polarized light can be incident on the device under test (SW) with a necessary polarization axis. This 1/2
The wave plate 5 rotates the principal axis of the incident polarized light by twice the rotation angle of itself. 1/4 wave plate 6 is an optical fiber (optical fiber 21 on the output side)
In many cases, a local stress is locally applied to the birefringence, so that the birefringence is combined with a half-wave plate and rotated sequentially to give an ellipticity that cancels the elliptical polarization. adjust. By this, P or S
The polarized light can be incident on the device under measurement (coupler) as linearly polarized light. As described above, the required polarized light can be quantitatively given to the device under measurement (coupler), and accurate polarization dependency can be measured. In the above embodiment, the Glan-Thompson prism 4, the half-wave plate 5, and the quarter-wave plate 6 are arranged in this order from the side of the optical fiber 11 on the input side. The same effect can be obtained by arranging the wave plate 6 in the reverse order. As described above, according to the present invention, first, the polarizer (Glan-Thompson prism) 4 is rotationally adjusted, and then,
The "arbitrary polarization state" can be adjusted by rotationally adjusting the 1/2 wave plate and the 1/4 wave plate in this order or the 1/4 wave plate and the 1/2 wave plate in this order. The optical concept of the "arbitrary polarization state" in this case will be described with reference to the Poincare sphere shown in FIG. The Poincaré sphere is based on the fact that when the Stokes parameters S ₁ , S ₂ , and S ₃ are taken in the X, Y, and Z axis coordinates, all polarization states describe a sphere. That is, S ₀ ² = S ₁ ²
+ There is relationship S _{² 2} S _{3 2,} represents a sphere. It is mathematically clear from the Poincare sphere shown in FIG. 3 that it is necessary to arbitrarily set at least two independent polarizing elements in order to set all polarization states. . That is, the sphere is in phase with the plane and is described by the composition of two independent vectors. However, in the present invention, this mathematical basis is based on an incomplete light source in reality and a polarizer (Glan-Thompson prism), a half-wave plate, 1
In addition to specifying the three polarization elements of the / 4 wavelength plate, firstly, it corresponds to the connection of the light source with incomplete polarization, and the straight line is specified by the polarizer (Glan-Thompson prism), and secondly
By rotating the 1/2 wave plate from 0 ° to 45 ° about its optical axis, the polarization is changed along the equator of the Poincaré sphere. Rotate independently from 0 to 90 ° corresponding to the arbitrary setting of the plate to sweep the entire surface of the sphere. With this configuration, the coordinates of the spherical surface directly correspond to the rotation angle of the wave plate, the directions of polarization change are always orthogonal, and operations similar to longitude and latitude can be performed. Therefore, the adjustment can be performed by an operation similar to a general geographical sense such as a globe, and therefore the operation can be performed without necessarily having specialized knowledge about the Poincare sphere. Therefore, even in a factory or the like, even a non-specialist can easily give instructions for adjustment work, which is a great practical advantage. The second feature of the present invention is that the power fluctuation is suppressed. This is based on the condition that both ends are coupled to a single mode fiber, and it is assumed that an actual imperfect polarizer is attached to a rotation holder or the like and used. The polarizer (Glan-Thompson prism) is a compound prism of the same type of birefringent crystal and has both end surfaces parallel to each other.
Both the quarter-wave plate and the half-wave plate are also parallel on both end faces. When inserting a rotating optical element between optical fibers,
For example, surface wobbling becomes a problem. In the configuration of the present invention, instead of using the convergent light as shown in FIG. 4 (b), optical coupling is performed by parallel beams as shown in FIG. 4 (a). Thus, in the case of parallel beam lens coupling, the light beam is
There is almost no change due to misalignment in the parallel direction.
However, in the case of convergent light as shown in FIG. 4 (b), the change in optical power is unavoidable. FIG. 4C shows the positional deviation of the light rays when the angle of the polarizer is deviated. An optical element such as a polarizer (Glan-Thompson prism), a half-wave plate and a quarter-wave plate used in the present invention, which is capable of extremely high parallelism or perpendicularity to the optical axis. Therefore, when an apparatus is manufactured by applying the polarization tunable device of the present invention to realization, it is possible to provide a configuration in which a fluctuation in optical power is minimized by combining a parallel beam with an optical element having parallel end faces. It will be possible. [Effect of the Invention] According to the result of measurement using a certain polarization variable device configured according to the present invention, as compared with the case of adjusting the deflection surface by "twisting" the optical fiber as in the prior art. , The measurement time of polarization dependence was reduced to 1/2 to 1/10. Also, since it is possible to easily set any polarization state, not only the measurement of polarization but also the function as a polarization compensation device can be realized. For example, by making circular polarization, the polarization dependence of the device in the optical communication system can be realized. It has become possible to apparently ease the situation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of an embodiment of a polarization variable device of the present invention, and FIG. 2 is a conventional polarization variable device, which simply “twists” an optical fiber to change the polarization state. FIG. 3 shows an example of the above, FIG. 3 shows a Poincare sphere, and FIG.
(B) And (c) is a figure for explaining the influence by the position gap from the optical axis of a polarizer. 11, 21 ... Optical fiber, 12, 22 ... Ferrule, 13, 23 ... Lens, 3 ... Housing, 4 ... Glan-Thompson prism, 5 ... 1/2 Wave plate, 6 ... 1/4 Wave plate, 71, 72, 73 …… Torsion rotation mechanism, 81, 82, 83 …… Rotating knob.

   ────────────────────────────────────────────────── ─── Continuation of front page    (56) References JP-A-59-28116 (JP, A)                 Japanese Patent Sho 58-42445 (JP, B2)

Claims (1)

(57) [Claims] A polarizer, a half-wave plate and a quarter-wave plate, which are provided with a first lens and a second lens and rotate orthogonal to the optical axis, between the first lens and the second lens, Or a polarizer, a quarter-wave plate, and a half-wave plate in this order.