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.     

Influence of the polarization mode dispersion on the propagation of ultrashort optical pulses in single-mode fiber lightguides with very weak linear birefringence and random inhomogeneities

We consider the influence of the polarization mode dispersion, which is stipulated by the presence of random inhomogeneities in single-mode fiber lightguides, on the propagation of ultrashort optical pulses in the fiber communication lines with very weak linear birefringence. Evolution of the envelope of ultrashort optical pulses and their spectra as functions of the length of a single-mode fiber lightguide with very weak linear birefringence and random inhomogeneities are obtained by the method of mathematical simulation. An increase in the pulse duration is shown to be proportional to the square root of the length of a single-mode fiber lightguide. The numerical-simulation results are compared with the results of experimental measurements of the polarization mode dispersion.     

Nonholonomic relation between the electromagnetic field state and the torsion angle in a single-mode lightguide with linear birefringence

The relation between the electromagnetic field state and the torsion angle in a twisted single-mode fiber lightguide (SFL) with linear birefringence is considered for the case where the lightguide is stretched into a straight line. It is shown that this relation is nonholonomic, because of the noncommutativity of two infinitesimal but different changes in the length and torsion of the lightguide. This indicates that the relation between the radiation polarization and the torsion angle in a twisted lightguide is also nonholonomic, and so is the relation between the Pancharatnam geometric-optics phase increment due to the evolution of the light polarization state in an anisotropic optical medium and the SFL torsion angle. Institute of Applied Physics of the Russian Academy of Sciences, Research Institute for Applied Mathematics and Cybernetics, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 9, pp. 1125–1136, September, 1998.     

Influence of the irregularity of twisting of the birefringence axes of single-mode optical fibers on the polarization nonreciprocity of fiber ring interferometers

A particular example of the so-called minimum-configuration fiber ring interferometer whose loop comprises two different-length segments of a single-mode optical fiber with the same linear birefringence but different twisting is used to show that even in the case where the birefringence axes at the loop input coincide with the polarizer transmission direction, the phenomenon of polarization nonreciprocity of the fiber ring interferometer can emerge. It is shown that polarization nonreciprocity can be eliminated by the proper adjustment of the axes of a single-mode optical fiber, but the above adjustment should be changed if the waveguide temperature is changed. It is also shown that polarization nonreciprocity is a function of the light wavelength and the polarization nonreciprocity value in a fiber ring interferometer varies quasiperiodically in response to the temperature change in an optical fiber with random irregularities.     

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.     

Magneto-optic fiber Sagnac modulator based on magnetic fluids

A magneto-optic modulator with a magnetic fluid film inserted into an optical fiber Sagnac interferometer is proposed. The magnetic fluid exhibits variable birefringence and Faraday effect under external magnetic field that will lead to a phase difference and polarization state rotation in the Sagnac interferometer. As a result, the intensity of the output light is modulated under the external magnetic field. Moreover, the modulator has a high extinction ratio and can easily be integrated in a single-mode fiber system. The performance of the modulator is not affected by ambient temperature variation from room temperature to 40 °C.     

The possibility for a significant decrease in the zero drift of fiber ring interferometers with a circuit formed by a highly anisotropic optical waveguide and a broadband radiation source

It is shown that the zero drift of interference of counterpropagating waves at the output from a fiber ring interferometer can be completely eliminated in the first order of magnitude and significantly decreased in the second order of magnitude if a sufficiently broadband source of nonmonochromatic radiation is used in a fiber ring interferometer with a circuit formed by a highly anisotropic optical waveguide when the depolarization length of nonmonochromatic radiation in a single-mode fiber-optical waveguide becomes significantly shorter than the correlation length of random inhomogeneities. Numerical estimates are made.     

Increase in the correlation length of nonmonochromatic radiation during propagation in a single-mode optical fiber containing random inhomogeneities and the influence of this increase on the operation of a fiber ring interferometer

An analysis is made of the conditions under which the coupling of orthogonal polarization modes at random inhomogeneities in single-mode optical fibers leads to an increase in the correlation length of a source of nonmonochromatic radiation. It is shown that when long-base fiber ring interferometers with a single-mode fiber ring system possessing weak linear birefringence are used, the correlation length of the nonmonochromatic radiation at the interferometer exit is increased, which means that the interference pattern can have satisfactory visibility even when there is an appreciable difference between the interferometer arms as a result of the Sagnac effect due to the Earth’s rotation. The calculations were made by mathematical modeling of random inhomogeneities in the fiber. Zh. Tekh. Fiz. 69, 140–143 (July 1999)     

Simultaneous measurement of temperature and strain using a long-period fiber grating with a micro-taper

An all-fiber Mach–Zehnder interferometer (MZI) consisting of a long-period fiber grating with a micro-taper is proposed for simultaneous measurement of temperature and strain. The experimental results demonstrate that the temperature and strain sensitivities of the proposed MZI are 83 pm/°C and ?2.6 pm/με, respectively. The strain sensitivity is 20 times as high as that of a long-period fiber written by CO₂ laser pulses combined with a fiber bitaper. In addition, the interferometer requires only a common single-mode fiber, and it is easy to fabricate and is inexpensive for temperature and strain sensing applications.     

All-fiber optical gyroscopes on orthogonal polarizations

The expressions for the nonreciprocal phase at the output of a fiber ring interferometer (FRI) related to polarization phenomena are obtained in the general form. These expressions are used to analyze two systems of the FRIs on anisotropic elements without polarizer with one and two beam splitters with allowance for their phase anisotropy and losses. We formulate the requirements on the elements of circuits and their adjustment, which lead to minimization of the nonreciprocal polarization phase due to the single-loop realization of two independent orthogonal gyroscopic channels whose signals are added at the output. This allows us to use the above simple circuits for creating moderate-accuracy fiber gyroscopes. Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 11, pp. 1448–1460, November, 1998.     

Fiber Mach-Zehnder interferometer based on microcavities for high-temperature sensing with high sensitivity

A high-temperature sensor based on a Mach-Zehnder interferometer (MZI) in a conventional single-mode optical fiber is proposed and fabricated by concatenating two microcavities separated by a middle section. A femtosecond laser is used to fabricate a microhole on the center of a fiber end. Then a micro-air-cavity is formed by splicing the microholed fiber end with a normal fiber end. The interferometer is applied for high-temperature sensing, in the range of 500-1200?°C, with a sensitivity of 109 pm/°C that, to the best of our knowledge, is highest in silica fiber temperature sensors. Also, the interferometer is insensitive to external refractive index (RI), which is desirable for temperature sensors.     

Measurement of the chromatic dispersion of an optical fiber by use of a Sagnac interferometer employing asymmetric modulation

We describe a novel method, based on asymmetric modulation in a Sagnac interferometer, that measures the chromatic dispersion of single-mode fibers. The interferometer incorporates a phase modulator and a test fiber, so the dispersion can be determined from the interference fringe seen when a sweep rf signal is applied to the modulator. This technique provides picosecond temporal resolution without the need for any fast diagnostic equipment and is capable of accurately measuring the average dispersion of fibers several kilometers long.     

In-Fiber Fabry-Perot and Mach-Zehnder interferometers based on hollow optical fiber fabricated by arc fusion splicing with small lateral offsets

Two kinds of in-line all-fiber interferometers, including tip Fabry-Perot interferometer and compact Mach-Zehnder interferometer, are proposed and demonstrated by fusion splicing a short section of hollow optical fiber (HOF) to the end of single-mode fiber (SMF) and sandwiching a section of HOF in between two sections of SMF, respectively. In both interferometers, a small lateral offset is introduced to induce the optical path difference required to form the interferometer. Temperature responses of both types of interferometers are studied experimentally. It is anticipated that such easy making, compact and low-cost fiber-optic interferometers could find important applications in practice.     

Noise analysis of solid-core polarization-maintaining photonic interferometer fiber optic gyroscope

We report the prediction of the random walk coefficient of a solid-core single-mode polarization-maintaining photonic interferometer fiber optic gyroscope, a novel sensor that exhibits reduced thermal sensitivity owing to the stable temperature coefficient use of a photonic fiber. The random walk coefficient of the photonic interferometer fiber optic gyroscope is limited by detector thermal noise for detected powers under 1 μW and excess noise above 1 μW. Above this power level, the random walk coefficient is found to be independent of detected power and asymptotically to be 0.0029°/ \(\sqrt h \). This result is verified by the experiment.     

用三干涉臂马赫-曾德尔干涉仪制成的光纤密集型波分复用器

提出一种新型的光纤干涉仪 ,它是采用二个 3× 3单模光纤耦合器组成三光纤系统的马赫 -曾德尔 (Mach- Zehnder)干涉仪 ,当干涉仪的三条光纤干涉臂两两之间分别存在长度差Δ L和2Δ L时 ,干涉仪形成了一个波分复用间隔相等的三波分复用器 ,波分复用的波长间隔仅仅取决于ΔL的大小。实验样品的使用波段为 1.55μm,波长复用间隔为 1.6 nm,附加损耗为 0 .6 d B。     

Pulse compression in a longitudinally inhomogeneous fiber

A single-mode fiber with a linear or sinusoidal variation in the group-velocity dispersion is fabricated. The nonreciprocal effects and pulse compression due to the longitudinal oscillations of the fiber dispersion are experimentally demonstrated. The periodic modulation of the dispersion can be used to control the pulse dynamics.     

Application of fiber interferometer in coherent Doppler lidar

A novel coherent Doppler lidar (CDL) system based on single-mode fiber (SMF) components and instruments is presented to measure the speed of target. A fiber interferometer used in CDL system is reported.This fiber mixer is employed as a coherent receiver to resolve the shifts of signal-to-noise ratio (SNR) and mixing efficiency induced by backscattered field's wavefront error. For a certain wavelength, the maximum coupling efficiency between signal and SMF is determined by the ratio of pupil diameter to focal length of the coupling lens. The legible interference patterns and spectrum signals show that fiber interferometer is suitable to compensate for amplitude and phase vibrations. This robust coherent receiver can achieve improved CDL system performance with less transmitter power.     

An intrinsic fiber ring laser based sensor with saturable absorber

A novel configuration of fiber ring laser acoustic sensor, which adopts single-mode fiber as sensing element, is proposed and demonstrated. Linewidth less than 1 kHz is achieved by introducing a segment of erbium-doped fiber in the reflecting arm as saturable absorber. A polarization independent unbalanced Michelson interrogation interferometer and phase generated carrier demodulation technique are adopted to recover signal. Experiments demonstrate that the phase sensitivity is 0.018 rad/nm in good linearity and equivalent minimum detect able length change reaches 19.1 pm.     

Stimulated Brillouin scattering-induced phase noise in an interferometric fiber sensing system

Stimulated Brillouin scattering-induced phase noise is harmful to interferometric fiber sensing systems. The localized fluctuating model is used to study the intensity noise caused by the stimulated Brillouin scattering in a single-mode fiber. The phase noise structure is analyzed for an interferometric fiber sensing system, and an unbalanced Michelson interferometer with an optical path difference of 1 m, as well as the phase-generated carrier technique, is used to measure the phase noise. It is found that the phase noise is small when the input power is below the stimulated Brillouin scattering threshold, increases dramatically at first and then gradually becomes flat when the input power is above the threshold, which is similar to the variation in relative intensity noise. It can be inferred that the increase in phase noise is mainly due to the broadening of the laser linewidth caused by stimulated Brillouin scattering, which is verified through linewidth measurements in the absence and presence of the stimulated Brillouin scattering.     

High-temperature sensor using a Fabry-Perot interferometer based on solid-core photonic crystal fiber

A micro Fabry-Perot interferometer(M-FPI) is constructed by splicing a short section of polarizationmaintaining photonic crystal fiber(PM-PCF) to an end-cleaved single-mode fiber with controllable offset. Due to the high effective optical path difference induced by the solid core of the PCF,the M-FPI has an ultrasmall cavity of approximately 110μm.The temperature sensitivity within a range from 33℃to approximately 600℃is measured to be 13.8 pm/℃,which shows good agreement with the theoretical result.This proposed sensor has the advantages of ultracompact size and high stability.Therefore,it is suitable for various space-limited sensing applications in harsh environments.