Education of the republic of uzbekistan tashkent state technical university named after islam karimov

№3/2021 year 
Technical science and innovation
204 
This result seems quite understandable, since in the case of the "cut-off" wavelength λc > 
0.6 microns, the fiber light guide operates in a low-mode mode (more than two modes), 
moreover, it is clear that the channeling conditions of higher (in order) optical modes in 
comparison with the fundamental mode do not strongly depend on the nature of external 
influences (in particular, force) on the fiber, which leads to a sharper dependence of the phase 
shift on the deformation of the optical fiber. 
Note that a similar situation is also observed with uniaxial transverse deformation of the 
fiber Fig. 1, where the dependence of δ(F) on the amplitude of uniaxial transverse deformation 
in the case of a single-mode fiber with λc = 0.6 microns has a much smaller slope. 
It should be said that in the light of the proposed hypothesis about the significantly greater 
efficiency of phase modulation of light radiation in low-mode fibers compared with single-
mode fiber fibers, the results of measurements of phase shifts with uniaxial transverse 
deformations performed for different sections of the fiber become obvious. 
But especially sensitive to the mode of operation of the fiber (single-mode and low-mode) 
is the case of a phase shift between orthogonal modes during twisting of the optical fiber. It is 
clear that at small twisting angles of the fiber (≤ 1 deg/cm), the deformations of the fiber 
associated with its elongation or transverse deformations are very small and their contribution 
to the mechanism of ellipticity modulation can be neglected. In this case, orthogonal circularly 
polarized modes NOT║ (+) and NOT║ (-) channeled in the core of the fiber acquire a phase 
difference proportional to the twist angle β, and linearly polarized radiation experiences a 
rotation of the polarization plane [6]. 
But due to the unequal nature of the attenuation of circular modes NOT║(+) and NOT║(-) 
when channeling in a fiber, the transmitted light radiation is also characterized by elliptical 
polarization, moreover, in the case of low-mode operation under external influences that 
significantly change the propagation conditions of modes of higher orders (TE0, TM0, etc. – 
modes), the efficiency of ellipticity modulation can significantly increase compared to induced 
optical activity in single-mode operation. 
Indeed, the results obtained by us of measuring the ellipticity angle ψ with a twisted fiber 
(Fig. 3) showed that this is the case. Fig. 3 shows the results of measuring the phase shift δ 
(ellipticity angle ψ) in a low-mode VS (λc > 0.6 microns) with the preservation of polarization 
when it is twisted into small angles (≤1 deg /cm), and it can be seen that this dependence is 
linear in the specified range of twisting angles (measurements were performed both with an 
increase in the degree of twisting of the fiber and with its decrease) [5]. 
Fig
. 3. Dependence of the ellipticity angle Ψ (phase shift δ) in the fiber with the 
preservation of polarization when it is twisted at small angles on the load value F 
The ratio of the ellipticity angle to the twisting angle is ψ /β ≈ 1.3, which is more than an 
order of magnitude higher than the similar coefficient obtained in [7,6], when measuring the 
angle of rotation of the plane of polarization of light in a twisted single-mode fiber.