JPS6348504A - Fiber coil type polarization eliminator - Google Patents

Abstract

PURPOSE:To eliminate a connection point by establishing a specific relation among the coherence distance between a 1st and a 2nd coils formed by winding single-mode optical fibers, the external diameters of the fibers, the diameters of the 1st and the 2nd coils, and the numbers of turns of those coils. CONSTITUTION:The 1st coil 1 formed by winding a single-mode optical fiber 3 N1 times to the diameter 2R1 and the 2nd coil 2 formed by winding the single- mode optical fiber N2 times to the diameter 2b are cascaded and arranged, the center axis of the 2nd coil is slanted at 45 deg. to the center axis of the 1st coil, and the specific relation is set among the coherence distance lc, the fiber external diameter 2b, the coil diameters 2R1 and 2R2 of the 1st and the 2nd coils, and the numbers N1 and N2 of turns of those coils. Therefore, the optical fibers have no connection point and become stable against external force such as vibration and acceleration. Further, when single-mode fibers with a large specific refractive index difference DELTA (e.g. DELTA>1.5%) are used, the bending loss does not increase even if the coil diameter are made small, so a small-sized polarization eliminator can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a device for eliminating coherence of laser light or light emitting diodes.

[Conventional technology]

The configuration of the conventional depolarizer using birefringent fiber is
As shown in the figure. The lengths of the birefringent fibers 4 and 5 are each L-
, and L,〉TJ, (3). Further, the birefringent fibers 4 and 5 are connected with their principal axes inclined at 45 degrees.

Now, if the propagation time difference (polarization mode dispersion) of each polarization mode of the birefringent fiber is τP (8eC/1,), then Δ/, -0τpLI (m) (4) is the coherence length of the light source 1e If the length is too long, the light will lose its coherence at the output end of the birefringent fiber 4. Here, C is the speed of light. However, if linearly polarized light is incident on the main axis of the birefringent fiber 7, the output end of the birefringent fiber 4 will be coherent? Therefore, in order to eliminate the polarization (or coherence) of light in any incident state, it is necessary to use a birefringent fiber 5 with a length of at least twice the length of LI as its main axis, as shown in Figure 2. must be connected at a 45 degree angle.

[Problem that the invention seeks to solve]

Conventional depolarizers require two birefringent fibers to be connected as described above, and have the disadvantage of being vulnerable to vibrations, acceleration, etc. because the strength of the connection is weak.

The object of the present invention is to eliminate such drawbacks of the conventional ones.
The object of the present invention is to provide a fiber-type depolarizer without connection points by utilizing birefringence produced by bending a single mode optical fiber.

[Means for solving problems]

To summarize the present invention, the present invention relates to a fiber coil type depolarizer, which eliminates the coherence of light with a coherence length le, in which a single mode with an outer diameter of 2b is used. A first coil in which the optical fiber is wound N1 times with a diameter of 2 R, and a second coil in which the optical fiber is wound N1 times with a diameter of 2 R, are arranged vertically, and the central axis of the second coil is aligned with the first coil. ), the coherence distance 1e, the fiber outer diameter 2b1, the first and second coil diameters 2R, 2R, and the number of turns N of each coil,
It is characterized in that N is set so as to satisfy the following relational expression: % expression % (1).

The most important feature of the present invention is that it utilizes the bending birefringence of an optical fiber. The difference from the conventional technology is that there is no fiber connection point.

FIG. 1 is a schematic diagram showing one embodiment of the device of the present invention),
1 is a first coil, 2 is a second coil], and 5 is a single mode fiber. The diameter of the first coil is 2R+,
The number of turns is N1, the diameter of the second coil is 2R1, and the number of turns is Nt.

Note that the outer diameter of the single mode fiber is 2b.

As shown in Figure 3, an optical fiber with an outer diameter of 2b has a bending radius of R.
When the fiber is bent at The propagation constant difference between the linearly polarized modes is ΔβBKIID ”” k
(nX ”' ny). However, k(=
2π/2: λ is the wavelength) is the wave number, n is the core ■ refractive index, R2
, Pu i is the Pockels coefficient, ν is Poisson's ratio and n
wm j, A 5 , Rs-CLl 21, PI3-
1270, y, -7s3o (ψenemy), νmQ, 186. Since the core of a single mode fiber is sufficiently small compared to its outer diameter, the stress felt by the light inside the core is lX1(b). By substituting each numerical value into equation (8), the birefringence caused by bending is expressed as: Polarization dispersion (delay time difference between X and Y polarization modes) is B!
X) is expressed using the ID. The shift of the wave packet in the double-sided mode due to the first coil is Δt, −clτpI−2πRI Nl . If the wave packet 9dl is larger than the coherence length 1e of the light source, the coherence can be eliminated. That is, o2 1.846 N+> le
(LIR.

The diameter and number of coils of the first coil may be set so as to satisfy the following conditions. Furthermore, as mentioned in the section on the prior art, in order to eliminate the coherence of incident light in all polarization states, the shift of the wave packet caused by the second coil must be 2 times the shift of the wave packet caused by the first coil. It must be more than double. That is, Δl, -1846-N, > 2Δl,
α◆ Must be a horse. Equation (6), the condition α◆ must be satisfied between the first and second coils.

〔Example〕

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples.

Example 1 FIG. 4 shows the refractive index difference Δ=IIQ, 55 polygons, core diameter 2a=
These are the results of measuring the bending birefringence BBnD and polarization mode dispersion τ of coils made with various diameters using single mode fibers of SO μm and outer diameter 21)-12 μm. That is, FIG. 4 shows polarization mode dispersion rp (ps
/km, vertical axis) and birefringence index 111311 Summer DI
(vertical axis) and R (horizontal axis) and (1/R)2(m-'
, horizontal axis).

The solid line is the theoretical value given by 2αQ and 20), and it can be seen that the measurement results are in good agreement with the theoretical value.

Using the above single mode fiber, R,=R,−2,!
A Sfi coil was produced, and the number of turns of the first and second coils was set to N, -21 (times) and N, ff142 (times).

R1xm 2.5 When B, Q to 2 and Figure 4 Yoshi l
Since it is %, the deviation of the wave packets due to the first coil and the second coil is Δl! -3α3 (μM%) αzuΔ12-6α
6 (μ duck) It is natural. FIG. 5A shows the polarization state when the main axes of the first coil and the second coil (YI axis and Y axis in FIG. 1) are tilted at 45 degrees with respect to each other. The measurement is performed using a coherence distance of 1 as a light source.
Using a semiconductor laser (wavelength: 1.3 .mu.m, spectrum width: .DELTA..lambda.-7 onm) with e=25 .mu.m, the laser light was incident on the depolarizer shown in FIG. 1, and the emitted light was measured through a polarizer. FIG. 5B is a measurement taken by passing laser light directly through a polarizer, and it can be seen that the intensity changes depending on the polarization direction. In contrast, in FIG. 5A, the light intensity is constant υ regardless of the direction of the polarizer, and it can be seen that the polarization e (in other words, coherence) is almost completely eliminated. Note that FIG. 5 is a graph showing the polarization state as a relationship between the polarizer angle (degrees, horizontal axis) and normalized output light intensity (vertical axis).

〔Effect of the invention〕

As explained above, the present invention has the advantage of being stable against external forces such as vibration and acceleration because there is no fiber connection point and a depolarization device can be created by simply combining two coils. .

In addition, as the optical fiber constituting the fiber coil, a single mode fiber with a large relative refractive index difference Δ (for example, Δ>
tS%) has the advantage that a compact depolarizer can be manufactured because the bending loss does not increase even if the coil diameter is made small.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing one embodiment of the device of the present invention, Fig. 2 is a configuration diagram of a conventional depolarization device, Fig. 3 is a schematic diagram for explaining stress due to bending, and Fig. 4 is a schematic diagram showing an embodiment of the device of the present invention. Graphs of actually measured birefringence and polarization mode dispersion (A) and theoretical values, and diagram (M) show the results when the depolarizer of the present invention is used (A) and when it is not used (A).
It is a graph which shows the difference in the polarization state of B). 1: first coil, 2: second coil, 5: single mode fiber, 4 and 5: birefringent fiber F? (mm) Fig. 4

Claims (1)

[Claims] 1. In a depolarizer that eliminates the coherence of light with a coherence length lc, a first coil in which a single mode optical fiber with an outer diameter of 2b is wound N_1 times with a diameter of 2R_1; and diameter 2
A second coil wound N_2 times with R_2 is arranged in cascade, the central axis of the second coil is inclined at 45 degrees with respect to the central axis of the first coil, and the coherence distance lc is outside the fiber. Diameter 2b, first and second coil diameters 2R_1, 2R
_2, and the number of turns N_1 and N_2 of each coil are expressed by the following relational expression: 0.846(b_2/R_1)N_1>lc(1)N_
A fiber coil type depolarizer characterized by being set to satisfy 2/R_2≧(2N_1)/R_1(2).