Differential Phase Modulation Research Articles

Coherence-tuned interrogation of a remote elliptical-core, dual-mode fiber strain sensor

Two dual-mode fibers in tandem act as unbalanced sensor and recovery interferometers. We investigate how coherence-tuned interrogation of the remote sensor fiber is affected by the coherence properties of the multimode laser diode source and the modal group index difference of the dual-mode fibers. The parameters for optimized sensor performance are ascertained. We focus particularly on the recovery of small optical phase modulations induced by periodic strain. Using homodyne phase tracking, stability of the detected ac signal is achieved to within /spl plusmn/10/sup -3/ in the presence of additional, quasistatic phase drifts of large amplitude. The minimum detectable differential modal phase modulation is 5 /spl mu/rad rms//spl radic/Hz at 70 Hz. As an example, remote interrogation of a piezoelectric quartz high-voltage transducer is demonstrated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Automatic alignment of optical interferometers

We present a description of a system for automatic alignment of optical interferometers. The technique relies on using differential phase modulation to permit the detection of the phase difference between two fundamental-mode Gaussian beams at the output of an interferometer. Measurements of the spatially varying phase difference between the two beams by use of one or more multielement photodiodes permits information to be derived about the mismatch in overlap between the phase fronts at the output of the interferometer.

Phase-sensitive fibre-optic monoptodes for chemical sensing

A highly sensitive measurement method and miniaturized all-fibre sensing device for the detection of small changes in refractive index caused by chemical or biochemical reactions are proposed. The optical principle is based on the differential phase modulation between the two orthogonal polarization modes of a single-mode fibre via evanescentfield interaction with sensitive thin films deposited on the side-polished fibre, a so-called ‘monoptode’. Using the matrix method for an approximated planar five-layer waveguide, the influence of the main parameters of the system, such as film thickness, refractive indices and wavelength of operation, on the polarimetric interferometer efficiency has been theoretically studied. As an example, the method is applied to optimize theoretically the waveguide parameters of a humidity-sensing optode using porous TiO 2-SiO 2 thin film.

Photoinduced refractive-index changes in two-mode, elliptical-core fibers: sensing applications

Photoinduced refractive-index changes in two-mode, elliptical-core optical fibers are shown to affect the differential phase modulation between the LP(01) and the LP(11)(even) modes. This change in beat length is dependent on the amount of strain induced in the fiber while the grating is being formed. We present experimental results that agree with conventional coupled-mode theory and propose the use of such sensors for weighted and distributed applications.

Fluorescence depolarization of cis- and trans-parinaric acids in artificial and red cell membranes resolved by a double hindered rotational model.

Although steady-state anisotropy measurements of phase-sensitive probes provide a qualitative description of the phase behavior of biomembranes, there is little information about the physical state of lipid domains. We have developed a ground-state double hindered rotator model (DHR) for fluorescence anisotropy decay, in which probes possess separate rotational correlation times and r infinity in each phase. To validate the model, multifrequency differential phase angles (delta) and modulation amplitudes (lambda) were measured in a two-compartment cuvette with combinations of POPOP, TMA-DPH, and DPH in isotropic solvents and in DPPC liposomes. Rotational parameters obtained by fitting the DHR model were similar to those of a single hindered rotator model fitted to data obtained separately for each probe. As predicted by the model, negative delta and decreasing lambda with increasing modulation frequency were obtained when fluorophores in isotropic solvents were paired with fluorophores in DPPC liposomes. The rotational parameters of the phase-sensitive fluorophores cis-parinaric (cPnA) and trans-parinaric (tPnA) acid in DPPC/DMPC (1:0, 0:1, and 1:1) liposomes were determined at 15-40 degrees C. Two lifetimes (1 and 3 ns) were obtained above the phase transition temperature (Tc); greater than 95% of the fluorescence intensity was described by two lifetimes (3-9 and 12-32 ns) below Tc. Negative delta values were obtained when solid-phase lipid was present. r infinity varied from 0.26-0.32 below to 0.11-0.14 above Tc; at intermediate T, where two phases coexists, r infinity values were approximately 0.23 and approximately 0.31. These data indicate very hindered PnA rotation in solid-phase lipid.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of a single-mode optical fibre piezoceramic phase modulator

The behaviour of a cylindrical single-mode optical fibre piezoceramic phase modulator was investigated in the frequency range below resonance. The integral and differential phase modulation efficiencies are reported for different geometrical dimensions of the modulator and for various piezoelectric ceramic materials. p ]The results obtained show that a phase-modulation depth in excess of ≪ can be easily achieved using commercially available piezoelectric cylinders, with driving voltage amplitude as low as 5 V.

Differential phase-shift keying applied to chirp data signals

Chirp communication systems have been constructed that use signals having ascending and descending linear frequency sweeps to define mark and space data states, respectively. Such signal states are approximately orthogonal, and are generally detected noncoherently. The paper describes preliminary investigations into a method of differential phase modulation applied to chirp data signals. This improves the chirp-system performance in white noise and single carrier interference, and essentially retains the inherent protection that chirp signals have in multipath conditions. Also, improved bandwidth utilisation may be achieved with multiphase operation.

Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform

The Fourier transform data communication system is a realization of frequency-division multiplexing (FDM) in which discrete Fourier transforms are computed as part of the modulation and demodulation processes. In addition to eliminating the bunks of subcarrier oscillators and coherent demodulators usually required in FDM systems, a completely digital implementation can be built around a special-purpose computer performing the fast Fourier transform. In this paper, the system is described and the effects of linear channel distortion are investigated. Signal design criteria and equalization algorithms are derived and explained. A differential phase modulation scheme is presented that obviates any equalization.

Phase-Locked Sampling Instruments

This paper describes a new approach to the design of RF instruments which are capable of measuring phase difference, level, distortion, and amplitude modulation of high-frequency signals from about 1 Mc/s to 1000 Mc/s or higher. The basic block consists of a modified phase-lock loop with a search and reset circuit. The input high-frequency signal is sampled, the frequency of the sampled output is locked to a reference frequency of a few kc/s, and the output waveform is a sampled replica of the input waveform. Using such a loop, measurements of RF signals can be made easily by a constant low-frequency readout circuit. Synchronization is obtained automatically for any input frequency without the need of a separate trigger circuit. Limitations as well as applications of the sampling phase-lock loop are briefly discussed.