JPS58195802A - Fiber type phase shifter - Google Patents

Abstract

PURPOSE:To obtain a fiber type phase shifter which has a small size and high performance and is simple in coupling system by using a single polarization fiber provided with a stress applying part having the coefft. of thermal expansion differing from that of a core as a phase shifter. CONSTITUTION:A stress applying part 2 is provided in addition to a core 1 and a clad 3 to a single polarization fiber. B2Os, etc. are added to the fiber and the fiber is cooled after drawing in order to make the coefft. of thermal expansion larger in the part 2' than in the clad 3; therefore, tensile stress is generated in the axial X direction in the core 1. The phase difference for the two polarization modes of the single polarization fiber can be made different by integer times of pi/2 by the double refraction occuring in the difference in stress in the core 1. Therefore, if such single polarization fiber is used, the fiber type phase shifter which has a small size and high performance and permits easy connection to an optical fiber is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a compact and high-performance fiber phase shifter that can be easily connected to an optical fiber.

(Background Art) A phase shifter that changes the polarization state of light is very important in various optical systems. Among them, a 1/2 wavelength plate that converts linearly polarized light into linearly polarized light with a desired polarization direction,
A 174 wavelength plate that converts linearly polarized light (circularly polarized light) into circularly polarized light (linearly polarized light) has a particularly wide range of applications.

Conventionally, a thin plate of calcite, mica, or quartz has been used as a phase plate such as a 1/2 wavelength plate or a 1/4 wavelength plate. In addition to crystals, phase plates using films of directional organic polymers are also used. However, since these conventional phase shifters are plate-shaped, a lens must be used when connecting to an optical fiber, resulting in a complicated coupling system. Furthermore, as mentioned above, phase plates using crystals have the disadvantage of being expensive.

(Problem to be solved by the invention) It is easier to connect with optical fibers, has high mass productivity,
Our objective is to provide a small, high-performance fiber phaser, and its features include a core, a cladding that surrounds it, and a portion of the cladding that is located at both ends of the core. It consists of an optical fiber that has a stress part that applies stress to the core part by inserting substances with different expansion coefficients, and the birefringence given by the stress in the core part creates two polarization modes of a single polarization fiber. The fiber-type phase shifter differs the phase constants for the two polarization modes so that the phase difference for the two polarization modes differs by an integral multiple of i, converts the incident linearly polarized light or circularly polarized light into a desired polarization state, and outputs the polarized light.

(Structure and operation of the invention) Hereinafter, the present invention will be explained in detail based on the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is the core of an optical fiber, 2 is a stress applying part, and 3 is a cladding.

The thermal expansion coefficient of the stress-applying part is larger than that of the cladding by adding 10 to 1mol1% of 8203. Therefore, in the process of cooling after drawing, the stress-applying part is inserted into the core in the X-axis direction. This causes a tensile stress. If the stress difference in the core is σ, (kg/1trd), then when the core is a perfect circle, the phase constants β, (x-direction polarization mode) and β2 (Y-direction polarization mode) for orthogonal polarization modes are wave mode) is given by . However, λ is the wavelength of light in vacuum, P is the photoelastic constant, and in the case of silica glass, P23.36 x 1
O-5 (md/kg) (given by 21. Core refractive index Δ = 0.75%, core radius a =
B20. of the stress applying part at 2.53 μm. The molar concentration of 18
When m01%, the mode birefringence defined by P・(σnyσy)−B is: B = 1.45 X 10−' (λ= 1.3,
E1771) (3). In a single polarized fiber having such birefringence, θ
When linearly polarized light tilted by (rad) is input, if the fiber length is l, the electric field components of the emitted light in the x and y axis directions are given by the following equation.

Ex2 A cos θ cos (ω one β11)
(41Ey=As1nθcos(ωt−β,l)
(5) Here, ω is the angular frequency of light and the amplitude of incident light. When time t is eliminated from equations (4) and (5), a 1 = A cosθ (7) a2
=A sin θ (8) δ=(β1−β, ll[9). Equation (6) generally represents elliptically polarized light,
The main direction of elliptically polarized light is ψ tan −' (tan 2θ・CO5δ)0
0) (Figure 2). From formula (6), δmi2mπ±π (m=o, 1+2+...)0
In the case of 1), ψ−0 03).

That is, it can be seen that a single polarization fiber whose phase difference δ satisfies the formulas (11) and (9);% formula % functions in the same way as a 172-wave plate. In addition, formula (6) smell δ′−2
mπ±-Cm=o, 1, 2. -・----) (1
412 θ−L (1 → In the case, 2 E: + E knee, αe. In other words, a single polarization fiber whose phase difference δ satisfies equation (14) is the same as a quarter-wave plate. I know it works.

Figure 4 shows that B = 1.45 x 10-', l
= 4.5 ill of single polarization fiber, λ-1,3
This is the result of measuring the polarization state of the emitted light when a μm linearly polarized laser light is input using the optical system shown in FIG. 3.

When the analyzer is rotated from 0° to 36σ, there are two angles at which the detector output is maximum, and the interval between them is 180°.
Met. Also, the ratio of the maximum ('r+mx) and minimum (■min) of the detector output is ■min/Imax-5Xl0-' (33dB)
It was αn. That is, it is shown that the single polarization fiber used in FIG. 3 functions as a 1/2 wavelength plate.

As an example of the present invention, a case was described in which a single-polarization single-mode optical fiber with a stress-applying structure shown in FIG. 1 was used.
Optical fibers that can be applied to the fiber phase shifter of the present invention are:
It goes without saying that the object of the present invention can also be achieved with optical fibers having other structures, such as an elliptical core or an elliptical jacket.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, a 1/2 wavelength plate or a 1/4 wave plate can be produced without using expensive materials such as crystals.
It is possible to realize a phase shifter such as a wave plate, and it can also be easily connected to other optical fibers using a V-groove splicer or fusion splicer, so it is a small and inexpensive phase shifter with very low splicing loss. It has the great advantage of being able to realize a container. Furthermore, since the fiber phase shifter of the present invention is highly flexible, it has the advantage that it can be used to measure polarization characteristics in narrow spaces or places that cannot be directly approached by humans.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the state of elliptically polarized light obtained when linearly polarized light is incident on the fiber phase shifter of the present invention, and FIG. 3 is a diagram showing an example of the phase shifter of the present invention. As an example, FIG. 4 is a diagram of an optical system for measuring the function as a 1/2 wavelength plate, and FIG. 4 is a diagram in which the polarization state of emitted light is measured by rotating an analyzer. 11111. , core 2...river/stress applying part 3
... Clad 4 ... Single polarizing fiber 5 ... Analyzer 6 ... Photodetector Patent applicant Nippon Telegraph and Telephone Corporation Patent application agent Patent attorney Megumi Yamamoto - Concave L Stem Figure 2 Figure 3 (2I4

Claims (1)

[Scope of Claims] (1) A core part, a clad part surrounding the core part, and a material having a coefficient of thermal expansion different from that of the clad part is inserted into a part of the clad part located at both ends of the core. It consists of an optical fiber having a stress applying part that applies stress to the core part, and the birefringence given by the stress in the core part makes the phase constants for the two polarization modes of the single polarization fiber different, so that the two polarization A fiber-type phase shifter is characterized in that the phase difference between the wave modes is changed by an integral multiple of the factor, and the incident linearly polarized light or circularly polarized light is converted into a desired polarization state and then emitted. (2. In the fiber phase shifter according to claim 1, δ-(β1-β2) l-2rnπ±πδ; phase difference β1.β2; phase constant l of two polarization modes; single polarization. A fiber-type phaser characterized in that the length m of the wave fiber satisfies an integer and converts incident linearly polarized light into linearly polarized light in a desired polarization direction and outputs the linearly polarized light. (3) Claim 1 In the fiber phase shifter, δ-(β1-β2)l=2myr±i δ; phase difference β1.β2; phase constant l of two polarization modes; length m of single polarization fiber; integer is satisfied. 1. A fiber phase shifter, characterized in that it converts between linearly polarized light and circularly polarized light.