SU1462219A1 - Method of measuring intermode dispersion in multimode light conduit - Google Patents

Abstract

The invention relates to methods for measuring dispersion in multimode fibers and allows for improved measurement efficiency and sensitivity. The bolonone fiber 2 excites from the source of coherent radiation 1, forms an amplitude mask 3 according to the speckle structure of the modes interfering at the output of the fiber, extends the fiber light guide with a tensile force, records the flux that passed the amplitude mask 3 as a function of the fiber length increment and in speed its variations judge intermode dispersion. 1 il. yu (L

Description

41

ABOUT)

N9

1 1 The invention relates to optical measurements, in particular, to a method for determining dispersion in multimode light guides, and can be used to control their optical properties,

The purpose of the invention is to increase the speed of measurement and its reliability.

The drawing shows a structural; scheme of the device that implements the proposed method;

A: The device contains a source 1: coherent optical radiation, a section of a multimode light guide 2, an amplitude mask 3, a photodetector 4, a device 5 for extending a light guide,: device 6 measuring the increment I of the length of the light guide,

i The method is carried out as follows

: A short section of a multimode fiber is excited by a continuous; Svetov1m signal from a coherent optical radiation source. How. It is known that a speckle pattern is formed at the output of a fiber segment, which is the result of the interference of all modes in a fiber. When changing the length of a fiber segment by, for example, stretching it, a change in the phase difference between the interfering modes occurs, which leads to a change in the speckle pattern; shift, resizing individual speckles. To register speckles that are chaotic in the direction of shifts and change their sizes, an amplitude mask is placed at the output end of the fiber section, made, for example, by a photoelectric method that weakens the interferopram parts with a high intensity and without attenuation passes areas with a low intensity. Amplitude mask recording (exposure) is performed at the initial length of the fiber. When the length of the fiber segment is changed, the phase difference between the interfering modes changes, as a result of which speckles shift in the interference pattern in the mask plane. Bright spots are displaced from the non-permeable portions of the mask and. the luminous flux, the transmitted optical fiber and the mask increases, but when the length is changed by a certain amount for a given optical fiber, it becomes constant and does not change with a further change in the length of the light segment

nineteen

water. The ratio of the initial light flux to the flux corresponding to the segment of the fiber of a modified length is 0.5,

By the rate of change of the radiation flux, the intermode dispersion is judged as the difference in the propagation time of the modes over a fiber of length Z, which can be characterized by the generalized time parameter ut (the generalized difference of the propagation times of modes) of a particular fiber determined by the formula

15

at K.z- (),

B Q

(one)

Where

(UZ

- average rate of relative change in radiation flux to a level of 0.5

five

p 0

five

0

five

on the amplitude mask; UZ is the increment of the length of the light guide on which the radiation flux changes to a level of 0.5; K - proportionality coefficient, established experimentally and. In general, it depends on the refractive index profile over the cross section of the light guide (fiber type) and the excitation condition of the light guide from the radiation source.

To clarify the formula (1), we consider the variance of two modes of a two-layer fiber; fundamental and superior axial. A change in the speckle pattern, when maxima occur at the intensity minima, and the radiation flux that passes the mask, reaches a level of 0.5 from the original, corresponds to the fundamental difference achieved by the modes and propagating at a critical angle of modes equal to L / 2 (where A is the radiation wavelength),

At the same time, the change in the length of the light-water segment can be expressed in the form

lg |.

1 U

Where

L 21121.

P,

g indicators

refraction of the core and the cladding of the fiber; On the other hand, dispersion as the difference in the time distribution

neither fundamental nor propagating at a critical angle of modes is determined by the formula

it

Zn --- b.

Thus, it is possible to determine the dispersion in terms of the rate of change of the radiation flux as follows,

.f - 7 () t - С

TO

bt KZ (-), b

WITH

In the real case, when there is interference of all modes and the refractive index profile over the cross section of the fiber is different from the stepped one, the calculation of the K coefficient is difficult, and the difference fct in the formula (1) g loses its specific content

and is a generalized (integral) characteristic of the intermode dispersion of the fiber.

Claims (1)

Invention Formula A method for measuring the intermode dispersion in a multimode fiber, i:. including excitation of the fiber by optical radiation, elongation of the fiber by tensile force, and detection of the transmitted radiation flux, characterized in that, in order to increase the measurement performance and its reliability, an amplitude mask corresponding to: -. - speckle structure interfering modes at the output of the fiber, register the transmitted fiber for the amplitude mask and for the speed of the radiation flux due to the increment of the fiber length, judge intermode r -. dispersions, while exciting the fiber with a coherent radiation flux.