SU1387694A1 - Method of determining dispersion characteristics of low-mode optical fibre - Google Patents

Abstract

The invention relates to fiber optics and allows to simplify the measurement process. The method is carried out according to the single-arm interferometer scheme on a low-mode fiber using one of its mode groups as a reference beam. Laser 1 radiation is introduced into the optical fiber 3, in which the nonlinear stimulated Raman scattering occurs. At the output of the spectral device 5, the radiation is recorded on a photographic film. The modulation period is measured, which contains information about the difference in the arrival time of the output of individual mode group fibers. 2 Il. (R

Description

The invention relates to the field of fiber optics and can be used for the rapid analysis of the dispersion characteristics and the determination of the mode composition of the fibers.

The aim of the invention is to simplify the measurement process.

Figure 1 shows a diagram of the device for carrying out the proposed method, where 1 is a laser with a wide radiation spectrum, I is a device for inputting an optical fiber into an optical fiber, 3 is a sample of an optical fiber under study, 4 is a device for outputting radiation from a fiber, 5 is a spectral device , 6 - recording device. Figure 2 shows an example of a spectrogram obtained using the circuit in figure 1 for a two-mode optical fiber.

The method is based on the physical principle associated with the appearance of interference of light in the spectral region, which occurs when the path difference between the light beams exceeds the coherence length of the radiation source when using sources with a wide radiation spectrum and, accordingly, with a small coherence length. This condition can be fulfilled already with very small values of the delays between the signals.

The method uses a single-arm interferometer circuit with a low-mode optical fiber, in which the function of the reference beam is performed by one of the mode fiber groups, for example the mode group LP,. Using the example of a two-mode optical fiber, it can be shown that if the radiation source

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is characterized by the coherence time., and at the fiber output, the arrival times of the two honey groups differ by the values of dS and jC, then the radiation spectrum at the fiber output, unlike the source spectrum, will be modulated with a period over the wavelength U7 2 | Г (о / ufb- 6, where D is the difference of the constant propagation of the two mode groups; Ad is the central wavelength; 6 is the length of the optical fiber. Using an instrument with high resolution allows detection of modulation in the signal spectrum, the resulting spectrum has a period of frequency fi-1 d2, i.e. related with differences in the arrival times of the two mode groups and, consequently, the dispersion, the delay is related to the wavelength period as follows: L / s D.

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Rum is a nonlinear process of stimulated Raman scattering (SRS) of light. The nonlinear SSC process is necessary to expand the narrow pump spectrum. The spectrum thus obtained is wide enough to observe spectral modulation, which is a re-. the result of the interference of mode groups in the optical fiber. The spectrum expanded using SRS is observed and recorded on a photographic film at the output of the SCHR-23 monochromator, into which radiation is introduced from the output of the optical fiber under study. The monochromator has a high resolution of the order of 0.01 nm, so modulation in the frequency spectrum can be enabled on the film. The modulation period, D, is then measured, which contains information about the differences in time.

The period of spectral modulation can be estimated directly from the spectrogram or in the case of a fiber with more than two mode groups using the holographic reconstruction scheme.

An example of the method.

The method can be carried out on an installation that includes a pulsed laser source of YAG with a wavelength of L of 532 nm, with a pulse duration of 12 n and a radiation power in a separate pulse of Py, 10-20 kW. The optical fiber under investigation has a light-conductor diameter of 2–9 μm, a length of 1–3–10 m. Radiation was introduced into the optical fiber and was extracted using micro-lenses with magnification x20. When measuring, a WDR-23 spectral instrument was used.

The measurement process consists in introducing a high-power laser radiation from a source (pump radiation) into an optical fiber in which

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The arrival of fibers of individual mod groups in the output: th Lo / cD, where c is the speed of light. It is also possible to determine the difference of the constant propagation in the fiber of two separate mode groups AB 2 n, (, / y L. For an optical fiber of length 2–3.5 m, the spectral modulation period D D 0.56 nm was experimentally obtained for the center frequency 7 544 nm, the delay d s was 1.8 ps.

Claims (1)

Invention Formula The method of determining the dispersion characteristics of low-mode optical fibers, based on the irradiation of an optical fiber placed in one of the arms of the Machatzender interferometer, by radiation with a wide frequency band followed by spectral and Fourier analogue radiation, in order to simplify the measurement process The reference beam uses a mode of low-micting optical fiber. 1 Phi2. 2