Faraday compensator of reciprocal optical anisotropy utilizing a polarization ring interferometer

It is shown that a polarization ring interferometer containing a Faraday cell can be used to compensate the reciprocal anisotropy in round-trip optical circuits. It is established theoretically and experimentally that, unlike the case of conventional Faraday mirrors, the quality of the restoration of the polarization in a compensator based on a polarization ring interferometer is practically independent of the Faraday rotation angle. A deviation of the Faraday rotation angle from 45° leads only to an additional power loss. The novel compensators can be used in fiber-optic circuits with a wideband light source or with several sources having different wavelengths. Pis’ma Zh. Tekh. Fiz. 25, 57–63 (May 26, 1999)     

Measurement of linear velocity using a resonant ring interferometer with a low-coherence light source

It is demonstrated theoretically and experimentally that linear velocities of reflecting objects may be measured using a resonant ring interferometer with a low-frequency light source. The limiting sensitivity of this interferometer is estimated. It is shown that a resonant ring interferometer can be used to measure extremely low linear velocities corresponding to subhertz Doppler frequency shifts of light, which cannot be measured by conventional Doppler techniques based on direct measurements of the frequency difference between the initial and reflected waves. Pis’ma Zh. Tekh. Fiz. 25, 34–42 (December 12, 1999)     

Modulational method of eliminating the zero shift of a fiber ring interferometer induced by polarization nonreciprocity

A new method is proposed to eliminate the zero shift of a fiber ring interferometer induced by polarization nonreciprocity. This method is based on periodic modulation of the polarization state of nonmonochromatic radiation at the entrance to the interferometer ring system. Numerical estimates are given. Pis’ma Zh. Tekh. Fiz. 25, 78–82 (August 26, 1999)     

Measuring dispersion between two modes of an optical fibre using low-coherence spectral interferometry with a Michelson interferometer

Abstract

It is demonstrated experimentally that even if the spectral interference between two modes of an optical fibre excited by a low-coherence source is not resolved at its output by a spectrometer of a given resolving power, it is resolved in the Michelson interferometer configuration. In a tandem configuration of a dispersive Michelson interferometer and a two-mode optical fibre, the optical path difference (OPD) in the interferometer is adjusted close to the group OPD between modes to produce a low-frequency spectral modulation that can be processed. Thus, using the Fourier-transform method in processing the measured spectral modulations and subtracting the effect of the dispersive Michelson interferometer, the wavelength dependence of the group OPD between two modes of the optical fibre over a limited spectral region is obtained.     

Resonance ring interferometry with incoherent light

A new method of measuring the phase nonreciprocity in a passive ring resonator using a light source of low coherence is described. The method provides effective suppression of noise from the backscattering of light in the ring resonator and insensitivity of the interferometer to excursions of the resonance frequencies of the resonator due to reciprocal effects (e.g., thermal expansion) and also permits modulation and compensation of the phase nonreciprocity by using a device outside the resonator to shift the frequency of the light. One version of a low-coherent resonance ring interferometer is examined, viz., a two-transit asymmetric interferometer with a rotating mirror. Pis’ma Zh. Tekh. Fiz. 24, 24–29 (September 12, 1998)     

Excess noise of an Er/Yb fiber superfluorescent radiation source

A study is made of the noise in the output intensity of a high-power superfluorescent fiber radiation source generating 20 mW in the 1.55 μm region of the spectrum. The source is constructed in an all-fiber technology from a quartz fiber activated with ytterbium and erbium. It is found that the spectral density of the noise in the frequency range 100–500 kHz is uniform and corresponds to the excess noise of “thermal” sources. It is shown that the noise in the output intensity of the source can be compensated (decreased by an order of magnitude) in a two-channel scheme of photodetection, where it approaches a level close to the photon shot noise. Pis’ma Zh. Tekh. Fiz. 23, 1–6 (December 12, 1997)     

Dispersive spectral-domain two-beam interference analysed by a fibre-optic spectrometer

Abstract

The effect of a fibre-optic spectrometer on analysed spectral interference of two beams from a white-light source is studied theoretically and experimentally, including the effect of dispersion in an interferometer. First, the spectral interference law is expressed analytically under the condition of a Gaussian response function of a fibre-optic spectrometer, and then second, the theoretical analysis is accompanied by three experiments employing a fibre-optic spectrometer and a Michelson interferometer with different amounts of dispersion. Within one experiment the interference fringes are resolved over a wide spectral range and within two experiments the interference fringes are resolved only in a narrow spectral range around a wavelength at which the group optical path difference between interfering beams is zero. Knowing dispersion in the interferometer and the bandpass of the spectrometer, the positions of the interferometer mirror in the corresponding range are determined and good agreement between the recorded spectral interferograms and the theoretical ones is found.     

Optical amplification in a planar multichannel Mach-Zehnder interferometer based on metal-semiconductor-metal structures

This paper describes a new optoelectronic device—an optical transistor that provides power amplification of output visible radiation. The optical transistor is structurally based on a metallized semiconductor plate with a microchannel gap in the form of a planar Mach-Zehnder interferometer. A device of the light-light type is controlled by modulating the refractive index in one arm of the interferometer. Pis’ma Zh. Tekh. Fiz. 24, 17–21 (January 26, 1998)     

Recirculation fiber loop interferometer with a Faraday reflector

A new optical scheme of the recirculation fiber loop interferometer with a Faraday reflector and a broadband radiation source is suggested. It is experimentally established that this scheme provides for a more than tenfold decrease in the instrument zero response caused by a spurious polarization modulation in the phase modulator. Factors leading to additional shifts in the instrument zero response are revealed.     

Angular momentum of the fields of a few-mode waveguide: Conversion of the angular momentum

An analysis is made of the transformation of the angular momentum density in the field of an unstable IV vortex of a few-mode optical fiber. It is shown that the effect of mode dispersion of IV vortices is observed as the conversion of the polarization and orbital components of the electrodynamic angular momentum. The angular momentum defect may be recorded experimentally as a mechanical twist of the optical few-mode fiber. Formally the dispersion process resembles the conversion of the signs of the orbital and polarization components of the angular momentum density. A complex pseudopotential, whose real and imaginary parts characterize the field lines and lines of equal pseudopotential, is introduced to describe the energy flux density of the fiber vortex. The conversion of field states with equivalent partial ê ⁺ F ₁(R)exp{−iφ} and ê2⁻ F ₁(R)exp{+iφ} vortices was investigated experimentally. Pis#x2019;ma Zh. Tekh. Fiz. 23, 58–65 (November 26, 1997)     

Time-domain intermodal interference including higherorder dispersion effects

Abstract

The intermodal interference is analysed theoretically at the output of both a few-mode optical fibre alone and a tandem configuration of a Michelson interferometer with an optical fibre when higher-order intermodal dispersion effects are taken into account. It is revealed theoretically that in a special case of a two-mode optical fibre with a quadratic spectral dependence of the difference of propagation constants, which includes the equalization frequency with zero intermodal group delay, and fibre excitation by a broad-band light source, the time-domain visibility functions differ substantially from those corresponding to fibre excitation by a low-coherence light source. Moreover, measuring a two-mode optical fibre using time-domain and spectral-domain white-light interferometric techniques, some theoretical results are confirmed experimentally.     

Dynamics of field dislocations and disclinations in a few-mode fiber. IV. Formation of an optical vortex

The physical mechanisms responsible for the formation of an optical vortex in the field of a few-mode fiber have been investigated experimentally and theoretically. In an optical fiber with a parabolic refractive index profile an optical vortex is formed as a result of interaction between circularly polarized rotating pure edge dislocations of circularly polarized even and odd CP₁₁ modes. In a stepped-index fiber the formation of an optical vortex is also related to the simultaneous propagation of even and odd modes. The fields of these modes alter their structure over the fiber length and are not manifested by rotating edge dislocations. It has been found experimentally that a stable vortex does not alter its degree of polarization of the field at fiber lengths greater than 10 m. An unstable vortex, for which the product of the spin and the topological charge is always less than zero, periodically decays and recovers at a beat length of 0.65 m. It is noted that a stable optical vortex cannot be formed by orthogonally polarized LP₁₁ modes. This is because an optical vortex transfers additional angular momentum like the CP₁₁ modes whereas the LP₁₁ modes do not transfer additional angular momentum of the field. Pis’ma Zh. Tekh. Fiz. 23, 70–75 (March 12, 1997)     

Multiple-beam Interferometers: Analogy with Aberrated Optical Systems and Adjustment Tolerances Estimation

Abstract

In this paper we study multiple interference produced by an interferometer when illuminated by an extended quasimonochromatic source. We define a relative modulation factor, which arises from grouping the interference terms for the same frequency. This relative modulation factor measures the departure from ideal behaviour of the interferometer. We also show that for some cases it is possible to associate a single aberrated optical system to each frequency, just as can be done for the single interference case, as was shown in a previous work; this allows an extension of the formalism presented therein. As an example, we calculate, for the case of a double-grating interferometer, the tolerance for the spatial extension of the source, where the modulation factor is close to unity.     

Conversion of phase modulation of light into intensity modulation by means of an external fiber-optic interferometer

A new method of detecting phase modulation, based on using an “external” optical waveguide interferometer, is considered. This “detection” is weakly dependent on the length, position, and external conditions of the propagation channel for the phase-modulated light and, in many cases may prove more efficient than conventional methods of constructing phasemodulated fiber-optic systems. Pis’ma Zh. Tekh. Fiz. 23, 9–16 (May 26, 1997)     

Investigation of a glow discharge in a mixture of Ar and OH

A glow discharge in a mixture of argon and hydroxyl (OH) is investigated experimentally. It is shown that under certain discharge conditions a considerable portion of the electrical energy imparted to the discharge is used to generate resonant emission of OH molecules. The results obtained point to a realistic possibility of creating a new, ecologically friendly source of optical radiation, which can replace mercury lamps in the near future. “Applied Chemistry” State Scientific Center Pis’ma Zh. Tekh. Fiz. 25, 62–66 (April 26, 1999)     

In-fiber liquid-level probe based on Michelson interferometer via dual-mode elliptical multilayer-core fiber

Abstract

An in-fiber liquid-level probe fabricated from homemade dual-mode elliptical multilayer-core fiber (EMCF) was proposed and experimentally demonstrated. The proposed sensor simply consists of a segment of the EMCF with one end coated with silver film, and a Michelson interferometer is roughly established when light from single-mode fiber incident from the other end. The detected interference patterns, rather clean due to the few-mode property, shift as the liquid level due to strong interaction between high-order modes and measurands through evanescent waves. Both the propagation characteristics and operation principle of such a sensor were demonstrated in detail, and sensitivities of 33.48, 43.35, and 48.93 pm/mm corresponding to liquid indices of 1.333, 1.353, and 1.373 were successfully achieved with a 50-mm EMCF probe, respectively. Moreover, the proposed sensor had the potential to discriminate measurand index after proper calibration.     

Interferometry Based on the Partially Coherent Effect Lying between the Talbot and Lau Effects

Abstract

Based on the partially coherent effect that lies between the Talbot effect and the Lau effect, a novel type of interferometry is proposed in this paper for a general configuration. The resultant interferogram is characterized by two different grating-like carriers. Such an interferometer becomes a Talbot or a Lau interferometer by adjusting the source slit to one or other of the two extremes of coherence. The partially coherent effect and the performance of the interferometer are analysed in detail in terms of the ambiguity function. Experimental results are shown to be in good agreement with the predicted intensity distributions.     

External impact localization in a distributed intermodal fiber interferometer

An intermodal fiber interferometer in a bidirectional continuous excitation regime with an increasing number of modes propagating along the lightguide is analyzed. The potential to determine the point of fiber perturbation regardless of the conditions of impact is demonstrated experimentally.     

Topological birefringence and combined Rytov-Magnus effect

It is shown that the symmetry properties of the locally isotropic inhomogeneous medium of an optical fiber cause circular and linear topological birefringence. The circular birefringence δn _C in graded-index fibers is ∼(λ/ρ)² (where λ is the wavelength and ρ is the core radius), while the linear birefringence is δn _L ∼(λ/ρ)³. This topological birefringence is characterized not only by the polarization basis (as in crystals for example) but also by the magnitude and sign of the topological charge of the guided vortex. This topological birefringence forms the basis of the instability of the fiber IV vortex and is observed experimentally as the combined Rytov-Magnus effect. Pis’ma Zh. Tekh. Fiz. 25, 41–46 (March 12, 1999)     

Effect of particle mass on the behavior of stochastic ratchets

Nonlinear Brownian motion in stochastic rachets is investigated by electronic simulation methods. It is shown experimentally that it is possible to sort particles by mass using a discrete random process, binary noise, as the source of the nonequilibrium stochastic ratchets. Pis’ma Zh. Tekh. Fiz. 24, 47–53 (January 26, 1998)