Broadband intermodal fiber interferometer for sensor application: fundamentals and simulator.

An intermodal fiber interferometer using the light from an incoherent broadband source has been considered analytically and implemented as a laboratory device. It was shown that this optical scheme could be used to measure external perturbations that cause a change in the optical length of a multimode fiber. The use of an optical spectrum analyzer and correlation functions approach in extracting the utility signal made it possible to achieve a linear response to the measured external perturbation and effective fading mitigation. A pair of integral coefficients was introduced: the contrast coefficient for characterization of the coherency of the operation regime, and the fading coefficient for estimating the signal stability against non-signal parasitic influences. Analytical expressions for the utility signal parameters were derived in dependence on the parameters of the light source, multimode fiber, and optical spectrum analyzer. The relationships among fiber length, width of the light source spectrum, and frequency resolution of the optical spectrum analyzer were stated for the optimum regime of interferometer operation. The simulation of the external perturbations performed at the elaborated device proved the applicability of the proposed scheme as a sensor of various physical quantities.

Intermodal fiber interferometer with frequency scanning laser for sensor application.

An intermodal fiber interferometer with an optical frequency scanning light source is considered to attain two main goals: signal fading elimination and achieving a linear response to external fiber perturbations. It is demonstrated that the interferometric signal traces repeatedly generated by the laser frequency scans can be used to average the target signal by calculating their autocorrelation function. A correlation approach is studied for signal processing, and various correlation function modifications are proposed and theoretically analyzed to achieve complete signal reconstruction. In particular, the relation between the autocorrelation function and the averaged amplitude characteristic is demonstrated. The efficiency of the proposed interferometer's scheme and the correlative signal processing are confirmed experimentally for the case of a sine-shaped fiber length modulation. We believe that the proposed method will be useful in real-time sensing applications.

Rayleigh backscattered radiation produced by an arbitrary incident mode in multimode optical fibers.

The recently developed diffraction technique of analytical investigation of the Rayleigh backscattering produced by an incident fundamental mode in a multimode optical fiber with an arbitrary refractive index profile is generalized to admit an arbitrary incident mode, either radial or azimuthal. The relative powers of all backscattered modes are determined with explicit formulas via the properly normalized transverse distributions of the incident and backscattered mode fields within the fiber cross section. The regularities conditioned by azimuthal indices are expressed via a universal set of coefficients, and dependences on the radial mode indices are estimated numerically. Excitation coefficients are shown to be symmetrical for any pair of incident and backscattered modes.

Influence of a variable Rayleigh scattering-loss coefficient on the light backscattering in multimode optical fibers.

The recently developed diffraction technique of analytical investigation of the Rayleigh backscattering produced by an incident fundamental mode in a multimode optical fiber with an arbitrary refractive index profile is supplemented by taking into account the Rayleigh scattering-loss coefficient, which could be variable within the fiber cross section. The relative changes in various radial and azimuthal modes' excitation levels, due to some typical radial dependences of this coefficient, are computed for the quadratic- and step-index fibers. It is stated that the excitation efficiency could either rise or decay for different modes. The effect of the variable Rayleigh scattering-loss coefficient is shown to be more noticeable in the fibers with a quadratic refractive index profile, whereas it is negligible in actual multimode step-index fibers.

Rayleigh backscattering from the fundamental mode in step-index multimode optical fibers.

The Rayleigh backscattering produced by an incident fundamental mode in a multimode step-index optical fiber was analyzed using a recently developed diffraction technique. The complete set of backward propagating modes, both radial and azimuthal, was determined and regarded. For this type of fiber, normalized mode functions were constructed in an explicit form, thus providing a unified power scale to characterize the relationships between various excited modes. The dependencies of the mode excitation efficiencies on the technical parameters of the fiber and the frequency of the launched radiation were studied. A comparison of the mode excitation efficiencies was performed with those in the fiber with quadratic refractive index profiles.

Rayleigh backscattering from the fundamental mode in multimode optical fibers.

Rayleigh backscattering produced by an incident fundamental mode in a multimode optical fiber is analyzed using a diffraction technique, with a full set of backward-propagating modes being taken into consideration. Explicit formulas are derived for mode excitation efficiency via radial distributions of the mode fields, and it is proved that only half-azimuthal modes are backscattered by the incident wave of a fixed polarization. Advanced analytical expressions are developed for fibers with a quadratic refractive-index profile, and mode groups of even numbers, composed of modes with equal propagation constants, are stated to be excited with equal efficiencies.