Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VII. Zero drift in a fiber ring interferometer with a circuit formed by a twisted optical fiber with weak natural linear birefringence

Zero drift and fading of the interference signal determined by polarization nonreciprocity at the exit from a fiber ring interferometer (FRI) of great length designed for recording a number of general relativity (GR) effects and made on the basis of a uniformly twisted single-mode optical fiber (SMF) with weak natural birefringence are considered. The calculations are carried out by numerical simulation using the model of random coupling between orthogonal polarization modes in an SMF, which was proposed in the first part of this work. The so-called minimum arrangement of an FRI with a nonmonochromatic radiation source, including a linear polarizer located between two beam splitters, is considered. Numerical estimates are made. It is shown that the FRI with a circuit on the basis of a twisted SMF has a significantly smaller zero drift and significantly smaller fading of the interference signal than the FRI with a circuit formed by an untwisted SMF. It is shown that the FRI produced on the basis of a twisted SMF provides the polarization sensitivity necessary for detecting the GR effects under study.     

Mathematical simulation of random coupling of polarization modes in single-mode optical fibers: IX. Zero drift in a fiber ring interferometer with a loop made of an optical fiber with strong linear birefringence

Numerical simulation of random inhomogeneities in a strongly anisotropic single-mode optical fiber (SMOF) forming a loop in a fiber ring interferometer (FRI) designed according to the minimal scheme is used to obtain the dependences of an interference signal at the FRI output and the zero shift and drift of an interference pattern of counterpropagating waves on the angles of orientation of the axes of linear birefringence in an SMOF at both inputs of the FRI loop. It is shown that the use of a superluminescent diode as a radiation source makes it possible to increase the sensitivity of fiber-optic gyroscopes fabricated on the basis of FRIs by 1–2 orders of magnitude and to obtain the values of real sensitivity required for navigation purposes.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: X. Verification of the ergodic hypothesis for fiber ring interferometers

The problem of the validity of the ergodic hypothesis as applied to a fiber ring interferometer (FRI) is considered on the basis of a comparison between magnitudes of the zero drift of an FRI calculated upon changing temperature of the fiber and upon random realizations of inhomogeneities in a single-mode optical fiber (SMF). The physical nature and statistical characteristics of random inhomogeneities in an SMF, types of polarization nonreciprocity in an FRI, and thermo-optical parameters of an SMF are analyzed. An algorithm for calculation of the zero drift of an FRI on changing temperature is proposed. The conditions under which the ergodic hypothesis is satisfied in an FRI are formulated. In particular, it is shown that many random inhomogeneities have to be placed on the depolarization length of polychromatic radiation in the SMF loop of an FRI; otherwise, the zero drift of the FRI calculated by the method of averaging over an ensemble of independent realizations may significantly exceed its actual value. Numerical estimations are made. It is shown that thermostabilization of an FRI with a polychromatic radiation source may significantly reduce its zero drift.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XV. Dependence of integral statistical parameters of polarization modes in single-mode optical fibers with random inhomogeneities on the wavelength of light

Spectral dependences of integral statistical elements of the Jones matrix of an extended segment of a single-mode optical fiber (SMOF) with random inhomogeneities, whose length considerably exceeds the correlation length of random inhomogeneities, are found by the method of mathematical modeling. The boundaries of the ranges of optical wavelengths are determined, within the limits of which the parameters under consideration remain quasi-constant. This allows the derivation of analytical expressions for the degree of polarization of nonmonochromatic radiation in an SMOF and for the zero drift in fiber ring interferometers with the help of the approximate method of small perturbations. It is shown that the small-perturbation method leads to adequate results for SMOFs with strong and intermediate linear birefringence.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: Integrated statistical properties of polarization modes of single-mode optical fibers in the presence of random inhomogeneities

Using mathematical modeling, an explicit form is found for the Jones matrix of a segment of a single-mode optical fiber with random inhomogeneities, whose length is considerably greater than the correlation length of random inhomogeneities. It is shown that parameters of this matrix are of a statistical nature. A rational representation of the matrix is proposed. It is shown that, under certain conditions, one of the matrix parameters may be treated as constant, whereas the other parameter is assumed to be continuously distributed on the interval [0, 2π]. As the latter parameter is changed, which corresponds to a change from one random realization of inhomogeneities in a single-mode fiber to another, the ellipticity and the azimuth of the major axis of the polarization ellipse of natural polarization modes of a single-mode fiber simultaneously change.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XII. Dependence of the zero drift in fiber ring interferometers in orthogonal polarization modes on the range of temperature variation in an optical fiber

Temperature dependences of the zero drift in a fiber ring interferometer (FRI) without a polarizer in the presence of random inhomogeneities in a single-mode optical fiber in the FRI loop are obtained by numerical simulation. The results are compared with known experimental data. It is shown that FRIs using a superluminescent diode as a source of nonmonochromatic radiation and a loop made of a single-mode optical fiber with weak linear birefringence can find application as low-accuracy fiber-optic gyroscopes and FRIs with a loop made of a single-mode optical fiber with strong linear birefringence can be used as medium-accuracy fiber-optic gyroscopes.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XVI. Depolarization and repolarization of polychromatic radiation in single-mode optical fibers with random inhomogeneities

The dependences of the degree of polarization of polychromatic radiation on the length of a single-mode optical fiber (SMF) with random inhomogeneities have been obtained by mathematical modeling. The case is considered where radiation having both polarization modes excited with equal weights of linear polarization is first introduced into a depolarizer of polychromatic radiation (a SMF segment with high linear birefringence) and arrives at an SMF with low linear birefringence. It is shown that the degree of polarization of radiation after transmission through the first segment becomes significantly suppressed and remains almost constant upon propagation through the second segment, after which it begins to sharply increase at some length; i.e., repolarization of radiation occurs. It is shown that repolarization of radiation depends weakly on the angle made by the axes of unperturbed linear birefringence of the first and second segments. The conditions for the length of the first segment (depolarizer) under which the degree of polarization remains minimum throughout the second segment are determined.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XIII. Conditions of applicability of the ergodic hypothesis for fiber ring interferometers

Some additional conditions of applicability of the ergodic hypothesis to fiber ring interferometers (FRIs) with a loop consisting of a single-mode optical fiber (SMOF) with random inhomogeneities are considered. It is shown by mathematical modeling that the change in the phase difference of counterpropagating waves at the FRI output with the SMOF temperature is not a stationary random process. However, in a fairly narrow temperature range, this dependence can be assumed to be locally stationary. The conditions determining this temperature range are formulated. It is shown for a fairly large ensemble of independent realizations of random inhomogeneities in an SMOF that, even when all conditions of ergodicity are satisfied with a large margin, there will always be at least one realization violating strict ergodicity. Thus, only conditional (approximate) ergodicity occurs in this case. Nevertheless, in calculation of the FRI zero drift in this situation, averaging over an ensemble of independent realizations of random inhomogeneities in the SMOF of an FRI loop can be performed with sufficient accuracy. As a result, calculations are simplified significantly. In the general case, when at least one of the conditions of ergodicity is not satisfied, averaging over temperature for each realization with subsequent averaging over the entire ensemble should be performed. It is shown also that, within this problem, we can speak only about quasi-ergodicity or emulation of ergodicity, since a change in the temperature of the SMOF of an FRI loop and successive enumeration of independent realizations of random inhomogeneities in the SMOF loop are radically different random processes. The parameters characterizing quasiperiodic temperature changes in the phase difference of counterpropagating waves at the FRI output are refined.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XI. Dependence of the zero drift of fiber ring interferometers on the interval of temperature variation of the fiber

The dependence of the value of the zero drift of a fiber ring interferometer (FRI), which is caused by a change in polarization nonreciprocity as a consequence of a change in the random coupling of polarization modes at inhomogeneities of the FRI loop, on the value of the interval of possible temperature variation of the single-mode optical fiber (SMF) of the FRI loop is considered. It is shown that the value of the zero drift of an FRI with a nonmonochromatic radiation source—a superluminescent diode—in a rather broad interval of temperature variations (about 100 K) is proportional to the value of the interval itself and inversely proportional to the linewidth of the radiation source. Numerical estimations are made, and they show that, for practical values of the interval of temperature variation in a room, the value of the zero drift may be much smaller than the values calculated according to the method of averaging over independent realizations of random inhomogeneities in the SMF of an FRI loop. When temperature stabilization is used, the value of the zero drift of an FRI can be additionally reduced by one to two orders of magnitude.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: XIV. Spectral characteristics of nonreciprocal phase difference of counterpropagating waves at the output of a fiber ring interferometer at a change in the fiber temperature

Distributions of the spectral density of the dependence of the phase difference of counterpropagating waves in a fiber ring interferometer (FRI) on the temperature of the single-mode optical fiber (SMOF) in the FRI loop (temperature spectra of the FRI zero drift) due to polarization nonreciprocity have been obtained by Fourier analysis for different spectral linewidths of the FRI radiation source and different linear birefringences of the SMOF in the FRI loop. It is shown that a change in the temperature of the SMOF in the FRI loop changes mainly the SMOF linear birefringence. This effect leads to a change in the phases of the radiation that is transferred from one polarization mode to another at those points in the SMOF where the random twisting of the axes of unperturbed linear birefringence of the SMOF changes. The effect of the magnitude of the jump under consideration, its location with respect to the nearest FRI loop end, and the magnitude of the cross-correlation (visibility) function of the radiation transmitted along the slow and fast SMOF axes from the loop end to the point where a jump in twisting of the SMOF axes occurs on the character of the temperature spectra of the FRI zero drift is determined. It is shown that in the case where the depolarization length of nonmonochromatic radiation in an SMOF is smaller than the average length of the SMOF segment on which random twisting is constant, the qualitative and quantitative characteristics of the temperature spectra of the FRI zero drift depend strongly on the polarization state of the radiation at the input of the FRI loop. The reason for this difference is explained.     

Mathematical modeling of random coupling between polarization modes in single-mode optical fibers: VI. Evolution in the degree of polarization of nonmonochromatic radiation traveling in a twisted optical waveguide

An analysis is made of the evolution of polarization of nonmonochromatic radiation travelling through single-mode optical fibers in the presence of random coupling between orthogonally polarized modes, which is caused by the random twist of axes of natural linear birefringence of an SMF, and in the presence of a regular axis twist. It is shown that the depolarization length of nonmonochromatic radiation in an SMF increases with increasing regular SMF twist, regardless of the presence of a random axis twist. Using the Monte Carlo method, the dependences of the mathematical expectation and the mean-square deviation of the degree of polarization of nonmonochromatic radiation on the fiber length for SMFs with different linear birefringences and regular axis twists are calculated. It is shown that the fiber length on which the degree of polarization of nonmonochromatic radiation reaches its limiting value increases and the limiting value itself decreases with an increasing regular twist. It is also shown that an induced SMF twist is able to improve parameters of a fiber interferometer, in particular, to decrease random phase changes and intensity fadings of the interference signal, which are caused by random coupling between polarization modes.     

Effect of orthogonal-mode random coupling on polarization characteristics of single-mode fiber lightguides (SMFL) and SMFL-based ring interferometers Part II. Zero drift in fiber ring interferometers with nonmonochromatic radiation sources

A method is proposed for calculation of the zero drift and Fedding interference signal at the output of a fiber ring interferometer (FRI) based on a single-mode fiber lightguide (SMFL) with a nonmonochromatic radiation source. The fiber length is divided into sections that are equal to the depolarization length in the SMFL. It is shown that when the FRI is made of an anisotropic SMFL with weak orthogonal-mode coupling, the calculation results agree with those obtained earlier by other calculation methods. Results for an FRI made of a weakly anisotropic SMFL with strong orthogonal-mode coupling are obtained for the first time.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 11, pp. 1473–1480, November, 1994.     

Effect of orthogonal-mode random coupling on polarization characteristics of single-mode fiber lightguides (SMFL) and SMFL-based ring interferometers. Part III. Zero drift in ring interferometers with nonmonochromatic-radiation depolarizers

A method is proposed for calculation of the zero drift and Fedding interference signal at the output of a fiber ring interferometer with a circuit made of a weakly anisotropic single-mode fiber lightguide (SMFL) and a depolarizer made of an anisotropic SMFL with a nonmonochromatic radiation source. The fiber is divided into sections equal to the depolarization length in the SMFL. Four FRI circuits all of whose parameters are the same with the exception of the location of the depolarizer are compared theoretically for the first time. Numerical estimates are made. It is shown that an FRI with a Lyot depolarizer located between the polarizer and the circuit is best from a practical point of view. The required precision of assembly of the elements of the Lyot depolarizer is considerably lower than that in the other FRI circuits.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 12, pp. 1567–1575, December, 1994.     

Effect of random coupling of orthogonal modes on polarization characteristics of single-mode fiber optical waveguides (SMF) and SMF-based ring interferometers. I. Change in the degree of polarization of nonmonochromatic radiation propagating through a single-mode fiber optical waveguide

We propose a method for calculating the degree of polarization of nonmonochromatic radiation as it propagates through a single-mode fiber optical waveguide (SMF), based on dividing the length of the fiber into segments equal to the depolarization length of the radiation in the SMF. We have obtained the dependences of the degree of polarization of the radiation on the length of the fiber for an SMF with strong and weak anisotropy. In the case of anisotropic SMF's characterized by weak coupling of orthogonal modes, the results match those obtained previously by other calculation methods. The results for weakly anisotropic SMF's characterized by strong coupling of orthogonal modes are obtained here for the first time.Institute of Applied Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, No. 10, pp. 1311–1320, October, 1994.     

Zero shift in fiber ring interferometer with wideband radiation source

It is shown that the polarization zero shift of a fiber ring interferometer (FRI) is chiefly determined by the Stokes components of the radiation, which are circularly and linearly polarized at an angle of 45° to the axis of the polarizer. A birefringent element between the polarizer and coupler in conjunction with a wideband source results in disappearance of the FRI zero shift, regardless of the mutual orientations of the polarizer, coupler, and birefringent element.Moscow Physics and Technology Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 34, No. 9, pp. 1001–1010, September, 1991.     

Methods for calculating the temperature zero drift initiated in fiber ring interferometers by random inhomogeneities in single-mode optical fibers

The specific features of the existing methods used for calculating the temperature zero drift initiated in fiber ring interferometers (FRIs) by linear coupling between polarization eigenmodes at random inhomogeneities in single-mode optical fibers are analyzed. The ranges of applicability of each method are determined. It is demonstrated that numerical simulation with a variation in the temperature of the single-mode optical fiber of the FRI loop is the most complex and, at the same time, the most universal method.     

Effect of random orthogonal-mode coupling on polarization characteristics of monomode fibers (MF) and MF-based ring interferometers part IV. Zero drift in simplified fiber ring interferometers with nonmonochromaticradiation depolarizers

We considered four fiber-ring-interferometer (FRI) schemes without polarizers made of weakly anisotropic monomode fibers which differ from one another in the location of depolarizer of nonmonochromatic Lyot radiation from a highly anisotropic MF. The results of calculations conducted by numerical simulation with allowance for the h-parameter of the circuit and depolarizer MF show that the FRI zero drift is practically the same for all the four FRI circuits and strongly depends on the accuracy of setting of the angle of mutual orientation of the Lyot depolarizer elements. If the Lyot depolarizer is located at the FRI input (before the first beam splitter or between the first and second beam splitters), the interference signal fadings at the FRI output are practically not eliminated. If, however, the Lyot depolarizer is inside the FRI circuit (at one of its ends or in the middle), the interference signal fadings are, in practice, entirely eliminated, which makes these FRI circuits preferable.Institute of Applied Physics and Institute of Microstructure Physics Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 12, pp. 1293–1304, December, 1995.     

Low-loss coupling between two single-mode optical fibers with different mode-field diameters using a graded-index multimode optical fiber

We present a method for ultra-low-loss coupling between two single-mode optical fibers with different mode-field diameters using multimode interference in a graded-index multimode optical fiber. We perform a detailed analysis of the interference effects and show that the graded-index fiber can also be used as a beam expander or condenser. The results are important for devices in which optical fibers with different mode-field diameters are coupled in series, such as in ultra-short-pulse fiber ring lasers, or in optical fiber communication links.     

Modulation of the phase difference between polarization modes in single-mode fibers

Mechanisms of polarization modulation in a single-mode fiber that modulate the phase difference between polarization modes without affecting their amplitudes are considered. A coefficient that characterizes the efficiency of cylindrical piezoceramic modulators and is independent of their resonant properties is introduced. Analytical expressions for this coefficient for different modulation mechanisms are derived. The lateral pressure on the fiber is shown to provide the highest efficiency. For isotropic fibers, a modulator with a squeezing covering, which increases significantly its efficiency, is studied. For anisotropic fibers, the most appropriate way of phase difference modulation is longitudinal extension, in which case the birefringence axes do not have to be matched. In most cases, the measured and predicted efficiencies are in good correspondence.