JPH0678965B2 - Inspection method for single polarization maintaining optical fiber - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for inspecting the position of a stress applying portion of a single polarization maintaining optical fiber used in the fields of optical communication and optical fiber sensors in a non-contact manner.

(Prior Art) Along with the progress of optical fiber manufacturing technology, a single-mode optical fiber has been developed that preserves linearly polarized light stably over a long distance along the principal axis, and is called a single polarization-maintaining optical fiber. , Is expected to create new advances in the field of optical communication fiber sensors.

Optical fibers having various structures have been proposed and developed as single polarization-maintaining optical fibers. Among these, PANDA
Fiber (T. Hosaka, et al. Electron. Lett., Vol. 17, No. 15, p
p.530-531 (1981)] has a structure in which stress applying parts are provided on both sides of the core of a single-mode optical fiber, and non-axisymmetric residual stress is applied to the core part by this stress, resulting in low loss. Promising as a single polarization maintaining optical fiber with low crosstalk.

When a single polarization maintaining optical fiber is used as a component of optical communication or an optical fiber sensor, its linear polarization maintaining property is required. Furthermore, in the fabrication of a single polarization maintaining optical fiber coupler with extremely small crosstalk and the fabrication of a fiber coil for a single polarization maintaining optical fiber gyro, the position of the stress applying part is accurately detected and the alignment is performed. There is a need.

FIG. 3 shows a procedure for manufacturing a general single polarization maintaining optical fiber coupler. FIG. 3A is a cross-sectional view of a single polarization maintaining optical fiber, and the single polarization maintaining optical fiber has a core 1
And a clad 2 surrounding the core 1, and stress applying portions 3 arranged on opposite sides of the core 1 and having a coefficient of thermal expansion different from that of the clad 2. As shown in FIG. 3 (b), the procedure for producing the single polarization maintaining optical fiber coupler is to remove the jackets of the two single polarization maintaining optical fibers and arrange them in parallel as bare fibers 10. Then, the spindle 4 in the direction connecting the center of the stress applying portion 3 and the center of the core 1
With the microscope 11, the stress applying part 3 is moved from the side surface of the optical fiber so that the main axes (hereinafter referred to as the main axes of the stress applying parts) are aligned parallel to each other.
To observe. At this time, the optical fiber 10 is immersed in the refractive index matching liquid 12, and further, if necessary, auxiliary illumination is performed by the polarized light 13 or the ultraviolet light 14 to detect the main axis of the stress-applying portion, and the optical fiber 10 is moved to the central axis thereof. Rotate along 5 to position.

Then, as shown in FIG. 3 (c), a part 15 of the optical fiber 10 is heated and fused at a high temperature, and further stretched to form a coupler section 16 as shown in FIG. 3 (d). A polarization maintaining optical fiber coupler.

As an important characteristic of the single polarization maintaining optical fiber coupler,
There are characteristics of insertion loss and crosstalk, and strength characteristics of the stretched portion. The main axis arrangement of the stress applying part does not significantly affect the insertion loss, but has a significant effect on crosstalk. In addition, the index matching liquid used for the main axis alignment
The use of 12 may increase the loss of the coupler due to dust and fine particles contained in the refractive index matching liquid.

As mentioned above, for the purpose of reducing crosstalk,
The use of the index matching liquid 12 for the main axis alignment may cause a disadvantage that the loss of the coupler increases.
In order to improve the yield in the production of couplers, there has been a demand for a method of inspecting the stress applying portion in the principal axis direction that does not require the index matching liquid 12.

On the other hand, in the production of the fiber coil for a single polarization maintaining optical fiber gyro, in order to reduce the occurrence of drift equivalent to the rotation of the earth due to the Faraday effect due to the geomagnetism Although a coil is used, the main axis twist of the stress-applying part during coil production causes the drift (reference: Tabe, Hotate, IEICE Technical Report OQE 85-91, 1985).

There is no principal axis direction detection method for arranging the main axis of the stress applying part at a desired position when manufacturing a single polarization maintaining optical fiber coil. A principal axis inspection method was strongly desired.

(Problems to be Solved by the Invention) The present invention provides a method for inspecting the stress applying portion of a single polarization maintaining optical fiber in the principal axis direction in a non-contact manner.

(Means for Solving the Problems) The present invention is directed to irradiating white light or laser light from the side surface of a single polarization-maintaining optical fiber and rearward of light transmitted through an optical fiber composed of a core and a stress imparting portion. Detects the interference pattern of scattered light, or the interference pattern of light scattered forward by the core of the optical fiber and the core or stress applying part of the internal structure,
Rotate the optical fiber along the central axis of the optical fiber,
Inspect the principal axis direction of the stress applying part.

The conventional technology does not require a refractive index matching liquid, which was indispensable for detecting the main axis direction of the stress applying part of the single polarization maintaining optical fiber, and the main axis direction of the stress applying part can be inspected without contact. It is very effective in the production of crosstalk single polarization maintaining optical fiber couplers and optical fiber gyro coil.

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

FIG. 2 is a diagram for 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. 2A, white light or laser light 20 is made incident on the single polarization maintaining optical fiber 10. The incident light forms a focal point 21 behind the exit point of the optical fiber due to the lens effect of the optical fiber, and then is magnified and projected on the screen 22. On the other hand, 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 shaft 4 of the stress applying portion of the optical fiber is rotated. By quantifying the relationship between the change in the intensity of one or more points of the interference pattern 23 or the change in the intensity distribution of the distribution shape and the rotation angle of the spindle 4, the direction of the spindle of the stress imparting portion can be detected.

The present invention will be described in detail below with reference to specific examples in the case of FIG. 2 (a), that is, the case of using forward scattered light.

FIG. 1 is a diagram for explaining a specific embodiment of the present invention, in which (a) is an optical system, and (b) and (c) are measurement of light intensity distribution shapes on screens 22b and 22a. For example, (d) is a diagram showing light intensity changes at two points of the current-voltage converters 35a and 35b. 5 mW HeNe laser light is used as the laser light 20, the laser light is divided into two directions by the semi-transparent mirror 30, and the direction changing mirror 31 is used.
At a and 31b, the direction of the laser light is changed so as to be incident on the single polarization maintaining optical fiber 10 at an angle difference of 90 °. The direction-changed laser light forms the focal points 21a and 21b by the 20 × objective lenses 32a and 32b. The objective lenses 32a and 32b are the single polarization maintaining optical fiber 10
They are installed at positions where light can be emitted toward the single polarization-maintaining optical fiber 10 up to a range slightly larger than the outer diameter. Then, the forward scattered light distribution is observed on the screens 22a and 22b placed 20 cm in front of the single polarization maintaining optical fiber 10, and the light intensity is measured through the PIN diodes (UDT Company PIN 5D) 34a, 34b
The signal was processed by the current-voltage converters 35a and 35b. A DC polarization motor (SAWAMURA Electric MM13B-JI-1500) was used to rotate the single polarization-maintaining optical fiber 10 with an accuracy of 0.1 °. It was supported by a V-groove type vacuum chuck.

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 imparting 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, a core diameter of 6.5 μm, a relative refractive index difference Δ = 0.4% of the core part, a cutoff wavelength λ _C = 1.1 μm, a stress applying part diameter of 40 μm, and stress applying A PANDA type with a relative refractive index difference Δ = −0.4% was used.

FIGS. 1 (b) and (c) show screens 22b and 22a, respectively.
The above shows the forward scattered light distribution, where θ is 90 °.

Comparing the case of FIG. 1 (b) and the case of FIG. 1 (c), the whole intensity distribution is higher in FIG. 1 (b), and the brightness and darkness thereof are also higher than those in FIG. 1 (c). It is clear.

Furthermore, the intensities that appear due to higher-order diffraction in the peripheral portion also clearly show a significant difference.

FIG. 1 (d) shows the outputs of the current-voltage converters 35a and 35b when the optical fiber is rotated. From this result, 35
It can be seen from both signals a and 35b that the output intensity changes ± 10 to 20% with an angle change of ± 1 °. As a result of eliminating the noise of the light source by differentially amplifying the outputs from the current-voltage converters 35a and 35b, the angle θ = 90 ° between the principal axis of the light beam and the principal axis of the stress applying portion is ± The angle could be detected with an accuracy of 0.2 °.

Although the above-mentioned embodiments describe the case where the forward scattered light is used, it goes without saying that the present invention is also effective in the case of FIG. 2B, that is, the case where the back scattered light is used.

Next, using the present invention, the principal axes of the stress applying portions of the 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 way has an average of −40 dB, and when the alignment error of the principal axis of the stress applying portion is estimated from this value, it is about 0.6 ° or less.

No increase in loss due to dust or the like was observed with the conventional method.

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 the PANDA fiber. Normal outer diameter 50 mm
A φ PANDA fiber base material was heated and melted at a high temperature of 2000 ° C. in a resistance heating furnace to form a single polarization maintaining optical fiber with an outer diameter of 200 μm at a speed of 20 m / min, which was drawn out from the resistance heating furnace. The optical system shown in FIG. 1 (a) of the present invention was attached 1 m below the resistance heating furnace, and the principal axis direction of the stress applying part was detected. Then, the winding drum was rotated to wind the optical fiber so that the direction of the main axis was constant. The optical fiber on this drum was coated and cured with an ultraviolet curable resin, and then the optical fiber was cut and the arrangement of the cross section was inspected. As a result, it was confirmed that the principal axis direction of the stress applying portion was controlled with an angle array error of 1 ° or less.

Needless to say, the single polarization-maintaining optical fiber having a structure in which the stress-applying part has a non-circular shape can also detect the direction of the principal axis determined by the stress-applying part by the above inspection method.

(Effects of the Invention) As described above, according to the method for inspecting a single polarization maintaining optical fiber of the present invention, since the principal axis direction of the stress applying portion can be detected in a non-contact manner, the single polarization maintaining optical fiber coupler. When manufacturing the above, the manufacturing yield and strength characteristics can be improved, and when manufacturing the optical fiber gyro coil, the twisting of the spindle of the stress applying portion can be prevented, and so on.

[Brief description of drawings]

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 screens 22b and 22a in the embodiment of FIG. 1 (a). FIG. 1 (d) shows a state, and FIG. 1 (d) is a current-voltage converter in the embodiment of FIG. 1 (a).
35a and 35b are diagrams showing changes in light intensity at two points, FIG. 2 is a diagram for explaining the inspection principle of the present invention, and FIGS. 3 (a), (b), (c), and (d) are It is a figure which shows the manufacture procedure of a general polarization-maintaining optical fiber coupler. 1 ... Core, 2 ... Clad 3 ... Stress applying part, 4 ... Sensor applying main axis 5 ... Optical fiber central axis 10 ... Single polarization maintaining optical fiber 11 ... Microscope, 12 ... Refractive index matching liquid 13 …… Polarized light, 14 …… UV light 15 …… Part of the optical fiber 16 …… Coupler 20 …… White light or laser light 21,21a, 21b …… Focus 22,22a, 22b …… Screen 23 …… Interference pattern, 30 …… Semi-transparent mirror 31a, 31b …… Direction changing mirror, 32a, 32b …… Objective lens 33a, 33b …… Hole 34a, 34b …… PIN diode 35a, 35b …… Current-voltage converter

Claims (2)

[Claims] 1. A single polarization-maintaining optical fiber having a core, a clad surrounding the core, and stress-applying portions having a coefficient of thermal expansion different from that of the clad on opposite sides of the core. Pattern of the forward scattered light that has radiated white light or laser light from the side of the optical fiber and that has been transmitted through the optical fiber, or the interference pattern of the back scattered light that is scattered on the incident side of the light by the surface and internal structures of the optical fiber. Is detected, the optical fiber is rotated along the central axis of the optical fiber, and the principal axis direction of the stress applying portion is inspected. 2. A bidirectional interference pattern generated by forward scattering or backscattering by irradiating white light or laser light from side surfaces in two directions different from each other by 90 ° with respect to the central axis of a single polarization maintaining optical fiber. The method for inspecting a single polarization-maintaining optical fiber according to claim 1, wherein the inspection is performed.