JP2528094B2 - Fiber type polarizer - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical fiber sensor and a fiber other polarizer used for optical communication.

(Prior Art) Those that pass only polarized light in a certain direction can generate and detect polarized light, and are referred to as a polarizer and an analyzer depending on the application. The former is used to make plane polarized light from natural light, and the latter is used to detect the plane of polarization.

FIG. 8 is an explanatory view of a conventionally used polarizer, and 1 is a Glan-Thompson prism in which calcite as shown in the drawing is adhered in opposite directions with Canadian balsam.

Of the natural light incident on the polarizer, only the plane polarized light of the P component is transmitted.

However, in order to use such a conventional bulk polarizer in a fiber sensor, a lens must be used for coupling light with the optical fiber. When a lens or a bulk polarizer is used, the optical axis easily shifts due to vibration, heat, etc., and the amount of light fluctuates, so there is a drawback in that the reliability of the system is poor.

(Problems to be Solved by the Invention) In order to eliminate the above-mentioned drawbacks, it is an object of the present invention to provide a fiber-type polarizer using polarization-dependent bending loss in fiber.

(Means for Solving Problems) A PANDA fiber satisfying the following conditions is wound around a drum to form a fiber type polarizer.

That is, the wavelength of incident light is λ, and relative refractive index difference ΔΔ = (n ₁ ² −n ₂ ² ) / 2n ₁ ² between the core and the cladding of the PANDA fiber (where n ₁ and n ₂ are respectively The refractive index of the core and the clad), the longest cutoff wavelength λ of the cutoff wavelengths of the higher-order mode group having an electric field distribution other than the Gaussian waveform of the PANDA fiber
Design so that c satisfies the condition of 4.3 × 10 ² Δ + 0.18 ≦ λ / λc ≦ 4.3 × 10 ² Δ + 0.8.

The present invention is composed entirely of optical fibers, and differs from conventional polarizers in that it can be optically coupled to other optical fibers without using a lens or the like.

FIG. 1 is a diagram showing an embodiment of the present invention. The fiber type polarizer shown in FIG. 1 is obtained by winding a stress-applying PANDA fiber 2 on a drum having a diameter D.

FIG. 2 is a cross-sectional view of the PANDA fiber, in which stress applying portions 5 having a coefficient of thermal expansion different from that of the cladding 4 are arranged on both sides of the core 3. Specifically, a glass in which 15 mol% of B ₂ O ₃ is doped in quartz is used for the stress applying portion.

 FIG. 3 is an explanatory view of a method for manufacturing a PANDA fiber.

In FIG. 3, reference numeral 6 is a core, and the stress-applying base material 8 is inserted into a brif ohm 7 which is drilled with an ultrasonic drill.
Heat to 2100 ° C with heater 9 and draw.

Fig. 4 shows the refractive index distribution considering the photoelastic effect due to stress in PANDA fiber. n ₁₀ and n ₂₀ are the refractive indices of the core and the cladding when stress is not considered, respectively.

FIG. 4 shows the refractive index distribution n _x , n on the y axis in FIG.
_y is shown. The Figure 4, although the core and the core near a _{n x> n y (1)} , it can be seen that the in Kuratsudo a _{n x ≒ n y (2)} . In the figure, β _x / k or β _y / k is the normalized propagation constant of the x and y polarized waves. When such a PANDA fiber is bent with a radius R (= D / 2) as shown in Fig. 5, the phase velocity of the plane wave in the fiber is Given in. Where v _∞ is when there is no bending (R =
∞) is the phase velocity. Note that E (r) shown in FIG. 5 is the electric field distribution of light.

The phase velocity of the plane wave in the cladding increases with the increase of r, and at the position of r = r _o , the phase velocity in the medium v _cl = c / n _cl
It is radiated into the clad over. Here, n _cl is the refractive index of the cladding, and c is the speed of light in vacuum. The position r _{O at which} radiation begins is v _R (r _O ) ＝ c / n _cl Given in. As shown in Fig. 4, β _x / k−β _y / k = B (5) (B: mode birefringence), so the position r _o ^(x) of radiated x polarization and y polarization It can be seen that there is the following difference between the radiated positions r _o ^(y) of.

In the case of this embodiment, B = 4.5 × 10 ⁻⁴ , D = 2R = 4.5 cm
Therefore, r _o ^(x) −r _o ^(y) ≈ 7 μm (7). Therefore, it can be seen that the y-polarized wave is emitted from a position closer to the core than the x-polarized wave.

Since the electric field in the cladding decreases exponentially, the y-polarization suffers a larger bending loss, whereas the x-polarization receives almost no bending loss and the bending loss has polarization dependence. I understand.

FIG. 6 shows the fiber polarizer shown in FIG. 1 (Δ = 0.24%).
2 shows the wavelength dependence of the extinction ratio and the loss of x polarization. When 2R = 4.5 cm, number of turns N = 4, and length of PANDA fiber L = 1.5 m, the extinction ratio is 20 dB or more, and the loss of x polarization is
The wavelength range below 1 dB is 1.48 μm λ1.61 μm (8) (hatched area in Fig. 6).

The cutoff wavelength of the PANDA fiber used is λ _c = 1.1 μm
Because Becomes

FIG. 7 shows the results of experiments conducted by variously changing the relative refractive index difference Δ, the cutoff wavelength λ _c , the mode birefringence B, the bending radius R and the number of turns N. In the area above the straight line a in the figure, the extinction ratio is
The region is 20 dB or more, and the region below the straight line b is a region that satisfies the condition that the loss of x polarization is 1 dB or less.

That is, the portion surrounded by the diagonal lines in FIG. 7 is a region that satisfies the characteristics as a polarizer.

 The straight lines a and b can be approximated by the following formula as an empirical formula.

Straight line a: λ / λ _c = 4.3 × 10 ² Δ + 0.18 (10) Straight line b: λ / λ _c = 4.3 × 10 ² Δ + 0.8 (11) Therefore, the range enclosed by the diagonal lines is Is represented.

After all, when a fiber whose relative refractive index difference is Δ is used when the wavelength of the light source is λ, the cutoff wavelength λ _c is calculated by the formula (12).
It can be seen that the design should be made so that

(Effects of the Invention) As is clear from the above description, the fiber-type polarizer of the present invention is small and lightweight, and optical coupling with other fibers can be easily achieved by a connector or fusion splicing. There are advantages.

[Brief description of drawings]

FIG. 1 is an embodiment of the present invention, FIG. 2 is a cross-sectional view of the PANDA fiber, FIG. Fig. 5 is a diagram for explaining the radiation loss due to the curved fiber, and Fig. 6 shows the wavelength dependence of the extinction ratio of the fiber polarizer and the loss of x polarization in the case of 2R = 4.5cm and N = 4. FIG. 7 is a diagram showing a region satisfying an extinction ratio of 20 dB or more and a loss of 1 dB or less, and FIG. 8 is an explanatory diagram of a conventional bulk-type Glan-Thompson polarizer. 1 …… Gran-Thompson prism 2 …… PANDA fiber 3 …… Core 4 …… Clad 5 …… Stress applying part 6 …… Core 7 …… Perforated preform 8 …… Stress applying base material, 9 …… Heater 10 ... core

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-58-189848 (JP, A) ELECTRONICS LETTE RS Vol. 19, No. 17, PP. 679-680

Claims (1)

(57) [Claims] 1. A PANDA fiber is a fiber-type polarizer wound around a drum, wherein the PANDA fiber has the wavelength of incident light of λ and the P
Relative refractive index difference between the core and the cladding of ANDA fiber against ^{_{Δ Δ = (n 1 2 -n}} 2 2) / 2n 1 2 ( although, n ₁ and n ₂ are respectively the refractive index of the core and the cladding), The longest cutoff wavelength λc of the cutoff wavelengths of the higher-order mode group having an electric field distribution other than the Gaussian waveform of the PANDA fiber
Is a fiber-type polarizer characterized by satisfying the condition of 4.3 × 10 ² Δ + 0.18 ≦ λ / λc ≦ 4.3 × 10 ² Δ + 0.8.