A. V. Medvedev

EFPI signal processing method providing picometer-level resolution in cavity length measurement

In the current article an advanced method of EFPI baseline measurement by use of spectral function approximation is proposed. The method provides an increase in EFPI baseline measurement precision and computational acceleration. The method bases on two foundations provided by analysis of low-finesse Fabry-Perot interferometer model: reduction of search domain and taking into account the most informative spectral intervals, providing a greater impact on the residual of measured and theoretical EFPI spectral functions. Proposed signal processing method resulted in the EFPI baseline measurement resolution less than 50 pm for the cavity length values between 20 and 700 μm.

Fiber-optic polarization interferometer with an additional phase modulation for electric field measurements

A fiber-optic polarization interferometer with two electrooptic elements (sensor and additional modulator) designed for measuring electric field intensity is suggested and analyzed. The paper focuses on the theoretical and experimental demonstration of the principles of operation of fiber-optic circuits that generate and process the optical signals and provide measurement. The advantage offered by our interferometer is that it allows, in contrast to conventional configurations, additional modulation of the phase difference between interfering waves by a modulator in the instrumental part of the device and the use of available methods of phase detection developed for fiber-optic interferometric sensors. A laboratory prototype has been successfully tested.

Digital demodulation methods for fiber interferometers

A principle of interference signal demodulation using an additional harmonic modulation of phase difference between interfering beams based on digital signal processing is suggested. This demodulation principle allows implementation of processing algorithms using different ratios between modulation and discretization frequencies N. The case N = 3 is considered in detail. The expressions allowing calculation of the phase difference that can be useful for development of interferometric sensors are derived.

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.

Multichannel Signal Detection in a Multimode Optical-Fiber Interferometer: Ways to Reduce the Effect of Amplitude Fading

For a multimode optical-fiber interferometer with a multichannel detector system, two alternative methods of signal processing in the presence of amplitude fading are studied theoretically and experimentally: (a) the summation of the signal magnitudes over various channels; (b) the selection of the channel whose signal modulus has the largest amplitude. It is shown that both methods can significantly reduce the signal amplitude fading at the interferometer output. A statistical model of the multimode interferometer is suggested assuming independent signal variations in different channels. For a ten-channel system with photodiode detectors, the computation and the measurements demonstrate that both variants of the data processing yield more than three-fold decrease in the ratio of the rms deviation under the fading conditions to the mean output signal.

Dependence of the signal of a multimode fiber-optic interferometer on the mode power distribution

Multimode fiber-optic interferometer (MFI) signals formed under conditions of external actions and a change in the optical power distribution between modes along the fiber have been studied. It is experimentally established that parameters of the MFI signal significantly depend on the number of propagating modes in the region of external action upon the fiber.

Polarization modulation of light in an optical waveguide under lateral compression

It is shown that a lateral pressure applied to a circular single-mode optical waveguide changes the phase difference between two orthogonally polarized modes and thereby modulates the polarization state of light (elastooptic effect). The efficiency of polarization modulation is characterized by a strain-to-phase difference conversion factor as applied to polarization modes of the fiber. An expression relating the phase difference between the modes and the displacement of squeezing plates is obtained and checked in real experiments and numerically. The efficiency of conversion of lateral pressure to phase difference between the polarization modes is shown to be fairly high, on the order of 104 rad/(μm m).

Two Mechanisms of Phase Modulation in Multimode Fiber-Optic Interferometers

The mechanisms of phase modulation of optical radiation in multimode light guides are treated, namely, the first mechanism with equal relative phase increments for all modes and the second mechanism including the mode interaction and characterized by different relative phase increments for each mode. It is demonstrated theoretically that, in the case of several modulating effects on the light guide, the behavior of signals of modal interference from each one of those effects upon variation of the parameters of the medium surrounding the fiber depends on the type of effect. The variations of signals from the effects of the first type are in full agreement. Signals from effects of the second type or of different types may vary differently. In experiments with an intermode fiber-optic interferometer 500 m long, modulators associated with the mechanism of the first type exhibited a high correlation (97%) of the behavior of signals upon drift of the light guide temperature. The variations of signals from modulators of the second type and of different types were almost uncorrelated (6 and 1%).

Remote interferometer with polarizing beam splitting

A scheme of an optoelectronic system built around a remote Mach-Zehnder interferometer was developed and experimentally studied. The output signal of the interferometer is detected under pseudoheterodyne conditions using the polarizing beam splitting and a sawtooth phase modulation. It is demonstrated that the system is insensitive to external noise in the forward and backward optical channels.

Transformation of modal power distribution in multimode fiber with abrupt inhomogeneities

A transformation operator is constructed for the calculation of the modal power distribution changes caused by abrupt inhomogeneity in multimode fibers. Simplification and practical applicability of the analysis are achieved by using the mode-continuum approximation. Thus, the model enables taking into account the fiber inhomogeneities of the following types: fiber lateral offset, core diameter mismatch, numerical aperture difference, and joints of fibers with different index profiles. The admission of an arbitrary refractive index profile, input modal power distribution, and parameters of fiber mismatch is an advantageous feature of the model. The operator is applied to calculations of power loss at joints between different multimode fibers, and the results of the calculations are shown to be in a good agreement with experimental data.