JPS638529A - Inspection of optical fiber which holds single polarized wave - Google Patents

Abstract

PURPOSE:To inspect the direction of the principal axis of a portion to which a stress is applied in noncontact manner by irradiating light from the side of an optical fiber to be inspected and detecting that the interfere pattern of light scattered by the optical fiber is changed by the rotation of the optical fiber. CONSTITUTION:White light or a laser beam 20 is led to objective lenses 32a and 32b via direction changing mirrors 31a and 31b, respectively, and the light or the beam throttled thereby is irradiated on a single polarized wave holding optical fiber 10. The distribution of forward scattering light transmitted through the optical fiber 10 is observed on screens 22a and 22 and the light intensities of the central portions of the screens are detected and processed by PIN diodes 34a and 34b and current-to-voltage converters 35a and 35b via holes 33a and 33b, respectively. Since, when the optical fiber 10 is rotated about a principal axis, the outputs of the converters 35a and 35b are changed, the direction of the principal axis of a portion which is provided on both sides of the optical fiber 10 and to which a stress is applied can be inspected in noncontact manner.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for non-contactly inspecting the position of a stress-applying portion of a single polarization-maintaining optical fiber used in the fields of optical communications and optical fiber sensors.

(Conventional technology) With advances in optical fiber manufacturing technology, single-mode optical fibers that stably preserve linearly polarized waves along their principal axis over long distances have been developed, and are called single-polarization-maintaining optical fibers. It is expected that this will lead to new advances in the fields of optical communications and optical fiber sensors.

Optical fibers having various structures have been proposed and developed as single polarization maintaining optical fibers. With this power), PA
NDA fiber (T, Ho5aka, etc. Electro
n.

Lett, Vol. 17. tl&115. pp.5
30-53H1981)) has a structure in which stress applying parts are provided on both sides of the core of a single mode optical fiber, and this stress applies non-axisymmetric residual stress to the core part, resulting in low loss and low crosstalk. It is expected to survive as a single polarization-maintaining optical fiber.

When a single polarization-maintaining optical fiber is used as a component of optical communication or an optical fiber sensor, linear polarization-maintaining property is required. Furthermore, in the production of single-polarization-maintaining optical fiber couplers with extremely low crosstalk and the production of single-polarization-maintaining optical fiber gyro fiber coils, the position of the stress-applying part must be accurately detected and aligned. There is a need.

FIG. 3 shows the procedure for manufacturing a -C type single polarization maintaining optical fiber coupler. FIG. 3(a) is a cross-sectional view of a single polarization-maintaining optical fiber, and the single polarization-maintaining optical fiber includes a core 1, a cladding 2 surrounding the core 1, and a cladding 2 arranged on opposite sides of the core I. The stress applying section 3 has a thermal expansion coefficient different from that of the cladding 2. The procedure for manufacturing a single polarization maintaining optical fiber coupler is as shown in FIG. 3(b), by removing the jackets of two single polarization maintaining optical fibers and arranging them in parallel as bare fibers 10. Next, the main axis 4 (
Hereinafter, this will be referred to as the main axis of the stress applying section. ) are aligned parallel to each other, using the microscope 11 to remove the stress applying part 3 from the side surface of the optical fiber.
Observe. At this time, the optical fiber 10 is connected to the refractive index matching liquid 1.
If necessary, auxiliary illumination is performed using polarized light 13 or ultraviolet light 14, the main axis of the stress-applying part is detected, and the optical fiber IO is rotated along its central axis 5 for positioning.

Next, as shown in FIG. 3(c), a portion 15 of the optical fiber 10 is heat-fused at high temperature and further stretched to form a coupler portion 16 as shown in FIG. 3(d). It is a polarization-maintaining optical fiber coupler.

The important characteristics of a single polarization maintaining optical fiber coupler are:
There are insertion loss, cocoon stalk characteristics, and strength characteristics of the stretched portion. Although the main axis arrangement of the stress applying section does not have a large effect on insertion loss, it has an important effect on crosstalk. Further, the use of the refractive index matching liquid 12 used for main axis alignment may result in increased loss of the coupler due to particulates such as dust and clumps contained in the refractive index matching liquid.

As mentioned above, for the purpose of reducing crosstalk,
If the refractive index matching liquid 12 is used for main axis alignment, there may be a problem that the loss of the coupler increases.
There has been a desire for a method for inspecting the stress-applying portion in the principal axis direction, which does not require the following.

jT, When manufacturing a fiber coil for a single polarization maintaining optical fiber gyro, a single polarization maintaining optical fiber is used to reduce the drift of the optical fiber gyro equivalent to the rotation of the earth due to the Faraday effect caused by the geomagnetism. Although a coil is used, twisting of the main axis of the stress-applying part during coil production causes drift (Reference: Nitabe, Maintenance, IEICE Technical Report OQE 85-91, 1985).

There is no method of detecting the direction of the principal axis of the stress-applying part to align it at a desired position when manufacturing a single polarization-maintaining optical fiber coil, and the stress-applying part There was a strong desire for an inspection method in the principal axis direction.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for non-contactly inspecting the principal axis direction of a stress-applying portion of a single polarization-maintaining optical fiber.

(Means for Solving the Problems) The present invention irradiates white light or laser light from the side of a single polarization maintaining optical fiber, and the rear of the light transmitted through the optical fiber consisting of a core and a stress-applying part. Detects the interference pattern of scattered light or the interference pattern of light scattered forward by the core or stress applying part of the surface and internal structure of the optical fiber,
By rotating the optical fiber along the central axis of the optical fiber,
Inspect the main axis direction of the stress applying part.

Conventional technology does not require a refractive index matching liquid, which was essential for detecting the principal axis direction of the stress applying section of a single polarization maintaining optical fiber, and the principal axis direction of the stress applying section can be inspected without contact, resulting in low It is very effective for producing crosstalk single polarization maintaining optical fiber couplers and for producing coils for optical fiber gyros.

Hereinafter, the present invention will be explained in detail with reference to specific examples.

FIG. 2 is a diagram explaining the inspection principle of the present invention, (a
) shows the case where forward scattered light is used, and (b) shows the case where back scattered light is used.

In FIG. 2(a), white light or laser light 20 is introduced into a single polarization-maintaining optical fiber 10. In FIG. The incident light is focused 21 behind the output point of the optical fiber due to the lens effect of the optical fiber, and is then enlarged and projected onto the screen 22. On the other hand, the light that has not entered the optical fiber is also projected onto the screen, and these lights interfere with each other to form an interference pattern 23 on the screen. This interference pattern 23 changes when the main axis 4 of the stress applying portion of the optical fiber is rotated. By quantifying the relationship between the intensity change at one or more points of the interference pattern 23 or the intensity distribution change in the distribution shape and the rotation angle of the main shaft 4, the direction of the main axis of the stress applying portion can be detected.

Hereinafter, the present invention will be described in detail based on a specific example for the case shown in FIG. 2(a), that is, the case where forward scattered light is used.

FIG. 1 is a diagram for explaining a specific embodiment of the present invention, in which (a) shows the optical system, and (b) and (c) show the measurement of the shape of the light intensity distribution on the screens 22b and 22a. example,
(d) is a diagram showing changes in light intensity at two points of the current-voltage converters 35a and 35b. 51 I as laser beam 20
Using f e Ne laser light, the laser light is divided into two directions with a semi-transparent mirror 30, and the directions are changed respectively v131a, 31
At b, the direction of the laser beam is changed so that it is incident on the single polarization maintaining optical fiber 10 with an angle difference of 90°. The laser beam whose direction has been changed is focused at focal point 2 by 20x objective lenses 32a and 32b.
Connect 1a and 21b. The objective lenses 32a and 32b are each installed at a position where they can irradiate light toward the single polarization-maintaining optical fiber 10 to a range slightly larger than the outer diameter of the single-polarization-maintaining optical fiber 10. Next, the forward scattered light distribution was measured 20 cm from the single polarization maintaining optical fiber 10.
Observation was made using screens 22a and 22b placed in front, and a hole 33a with an outer diameter of 3n+++φ in the center of the screen.
The light intensity is transmitted through the PIN diode (UDT PIN 5D) 34a through the PIN diode 33b.

34b, and signal processing was performed by current-voltage converters 35a and 35b. In addition, a DC gear motor (MURA electric MM13B-J) is used to rotate the single polarization-maintaining optical fiber 10.
1-1500) with an accuracy of 0.16, and furthermore, the optical fiber was supported by a groove type vacuum chuck to prevent optical axis fluctuation of the optical fiber during rotation.

The angle between the principal axis of the optical axis (the optical axis of the objective lens 32b shown in FIG. 3(a)) and the principal axis 4 of the stress applying portion of the single polarization maintaining optical fiber was defined as θ.

The single polarization maintaining optical fiber 10 has an outer diameter of 200 μm.
, core diameter 6.5 μm, relative refractive index difference of core part 7 = 0.4%
, cutoff wavelength λ. = 1.1 μm 1 A PANDA type was used in which the diameter of the stress applying portion was 40 μm and the relative refractive index difference A of the stress applying portion was −〇, 4%.

FIGS. 1(b) and 1(c) show the screen 22, respectively.
b.

The forward scattered light distribution on 22a is shown in the case where Q is 90°.

Comparing the cases in Figure 1(b) and Figure 1(c), the overall intensity distribution is higher in Figure 1(b), and its brightness is also higher than in Figure 1(c). It is clear.

Furthermore, the intensity that appears due to higher-order diffraction in the peripheral area is also bright.
There is a significant difference in education.

FIG. 1(d) shows the outputs of the current-voltage converters 35a and 35b when the optical fiber is rotated. From this result, it can be seen that the output intensity changes by approximately 10 to 20% with an angular change of ±1° from either signal 35a or 35b. As a result of differentially amplifying the outputs from the current-voltage converters 35a and 35b and removing noise from the light source, the angle θ-9 between the principal axis of the light beam and the principal axis of the stress applying section is
The angle could be detected with an accuracy of ±0.26 with respect to the setting of 0''.

Although the above embodiments have been described using forward scattered light, it goes without saying that the present invention is also effective in the case shown in FIG. 2(b), that is, when back scattered light is used.

Next, using the present invention, the main axes of the stress applying parts of two single polarization maintaining optical fibers were aligned in parallel, and a single polarization maintaining optical fiber coupler was manufactured.

The crosstalk of the coupler manufactured in this manner is 140 dB on average, and when the alignment error of the principal axis of the stress applying portion is estimated from this value, it is approximately 0.6° or less.

Increased loss due to dust, etc., which was observed with conventional methods, was not observed.

As another example of the present invention, a fiber coil for an optical fiber gyro was manufactured. The inspection method of the present invention was introduced during the drawing process of PANDA fiber. A normal PANDA fiber base material with an outer diameter of 50 mmφ was heated and melted at a high temperature of 2000°C using a resistance heating furnace, and a single polarization-maintaining optical fiber with an outer diameter of 200 μm was produced at a speed of 20 m/min, which was pulled out from the resistance heating furnace. . The optical system shown in FIG. 1(a) of the present invention was installed 1 m below the resistance heating furnace to detect the principal axis direction of the stress applying part.

Next, the winding drum was rotated to wind up the optical fiber so that the direction of the main axis was in a fixed direction. The optical fibers on this drum were coated and cured with an ultraviolet curable resin, and then the optical fibers were cut and the cross-sectional arrangement was inspected.

As a result, it was confirmed that the main axis direction of the stress applying portion was controlled with an angular alignment error of 1° or less.

It goes without saying that even for single-polarization maintaining optical fibers in which the shape of the stress-applying portion is not circular, the direction of the principal axis determined by the stress-applying portion can be detected by the above-described testing method.

(Effects of the Invention) As explained above, according to the method for testing a single-polarization maintaining optical fiber of the present invention, the direction of the principal axis of the stress-applying portion can be detected without contact. When manufacturing sopras, it is possible to improve the manufacturing yield and strength characteristics, and when manufacturing optical fiber gyro coils, it has great effects such as preventing twisting of the main axis of the stress applying part.

[Brief explanation of the drawing]

FIG. 1(a) is a diagram showing an optical system of an embodiment of the present invention, and FIGS. 1(b) and 1(c) are light intensity distributions on the screens 22b and 22a in the embodiment of FIG. 1(a). A diagram showing the state, FIG. 1(d) is a diagram showing light intensity changes at two points of the current-voltage converters 35a and 35b in the embodiment of FIG. 1(a), and FIG. 2 is a diagram showing the inspection principle of the present invention. Figures 3(a), 3(b), 3(c), and 3(d) are diagrams for explaining the manufacturing procedure of a general polarization-maintaining optical fiber coupler. 1... Core 2... Clad 3...
Stress applying part 4...Main axis 5 of stress applying part...
Central axis of optical fiber 10...Single-polarization maintaining optical fiber 11...Microscope 12...Refractive index matching liquid 13...Polarized light 14...Ultraviolet light 15...
...Part 16 of the optical fiber 10...Coupler section 20...White light or laser light 21, 21a, 21b...Focus 22, 22a, 22b...Screen 23...Interference pattern 30... Semi-transparent mirrors 31a, 31b・
...Direction changing mirror 32a, 32b...Objective lens 3
3a, 33b--holes 34a, 34b--PTN diode 35a, 3
5b...Current-voltage converter Figure 1 I, 1 on screen 22b, 1 on screen 22a, 1 on screen 22a. 2
Figure (a) Figure 3゛(

Claims (1)

[Claims] 1. A single polarized wave having a core, a cladding surrounding the core, and stress applying portions having a thermal expansion coefficient different from that of the cladding on opposite sides of the core. White light or laser light is irradiated from the side of the optical fiber being held, and the interference pattern of the forward scattered light transmitted through the optical fiber or the backscattered light of the light scattered toward the light incident side by the surface and internal structures of the optical fiber 1. A method for inspecting a single polarization-maintaining optical fiber, comprising detecting an interference pattern of the fiber, rotating the optical fiber along the central axis of the optical fiber, and inspecting the main axis direction of the stress applying part. 2.9 to each other with respect to the central axis of a single polarization-maintaining optical fiber
The single polarization device according to claim 1, characterized in that white light or laser light is irradiated from the sides in two directions different by 0°, and interference patterns in two directions generated by forward scattering or backward scattering are detected. Inspection method for wave-maintaining optical fiber.