Frequency-derived distributed optical-fiber sensing technique: theory and characterization.

Frequency-derived distributed optical-fiber sensing is a method for remote measurement of the spatial distribution of linear birefringence in an optical fiber, allowing a corresponding measurement of those external measurands that influence this birefringence. The method employs a pump-probe scheme, which, by use of the optical Kerr effect, generates an optical modulation of the probe beam, with a modulation frequency whose temporal variation maps the spatial distribution of birefringence. We provide a complete theoretical analysis of this method by using Jones calculus and graphic representation on the Poincaré sphere. The relevant characterization of the technique and some experimental results are also presented; these show good agreement with the theory.

Measurement of axial stress at high spatial resolution in ultraviolet-exposed fibers.

High-spatial-resolution measurements of axial-stress profiles of UV-irradiated fibers are reported, providing diagnostic information and a technique for the direct observation of UV-written grating structures. Measurements have been made with a spatial resolution of approximately 0.3 microm, which is capable of resolving detail within the pitch of the gratings.

Power measurement of noise-initiated Brillouin scattering in optical fibers for sensing applications.

We discuss the measurement of noise-initiated Brillouin scattered power in optical fibers and its application to distributed sensing systems. In particular, we consider the use of Brillouin scattering in the nonlinear regime, demonstrating a novel processing technique that compensates for the nonlinear growth of the scattered signals. The signal-to-noise ratio performance of this technique is evaluated, highlighting the importance of the noise contributed by the random statistics of the scattered field and yielding the conditions for optimum system operation.

Temperature and strain dependence of the power level and frequency of spontaneous Brillouin scattering in optical fibers.

The strain dependence of the optical power of Brillouin scattering in optical fibers has been measured for the first time to our knowledge. Together with measurements of the dependence of Brillouin power on temperature and the variation of Brillouin frequency with temperature and strain, we demonstrate, for what we believe to be the first time, the feasibility of a simultaneous temperature and strain sensor based on spontaneous Brillouin scattering.

Single-shot distributed optical-fiber temperature sensing by the frequency-derived technique.

We present, for the first time to our knowledge, a distributed optical-fiber temperature sensor, based on a pulsed laser, that provides distributed temperature measurement by use of a single pulse propagating in an optical fiber. The system uses the frequency-derived technique based on the optical Kerr effect. The performance of the system is investigated for the temperature range 33-150 degrees C. A linear relationship between the temperature and the derived frequency is obtained. The best temperature resolution was determined to be +/-1.2 degrees C. The best measured spatial resolution was 0.56 m.

Simultaneous strain and temperature sensing with photogenerated in-fiber gratings.

A new technique for simultaneous strain and temperature sensing is demonstrated. The approach employs two different types of photogenerated fiber grating, namely, a fiber Bragg grating and a fiber polarization-rocking filter. The method relies on the different dependencies of the fiber refractive index and birefringence on strain and temperature. Both of these measurands can be determined from the effect that they have on the resonant wavelength of each grating. The information is provided in the frequency domain, avoiding the problem of limited unambiguous signal range associated with the use of competing optical fiber interferometers.

Distributed optical-fiber sensing by four-photon mixing pulse walk-off.

We propose a distributed optical-fiber sensing scheme using four-photon mixing pulse walk-off. We use a copropagating system in which only a pump wave is launched into the fiber. We analyze the performance by including the effects of fiber birefringence, group-velocity dispersion, and self-phase modulation. A short system with high sensitivity and good resolution can be built. The useful operating length is limited by self-phase-modulation-induced pulse broadening.

Characteristics and applications of birefringent-fiber Kerr-couplers.

We consider the effects of the pump power and the interaction length on the efficiency of polarization coupling induced by the optical Kerr effect in high-birefringence fibers. An efficiency of up to 100% is predicted for suitable combinations of the length and the strength of the birefringence grating in the fiber. The device is rapidly tunable, highly selective in wavelength, and independent of any thermal drift in the fiber. Potential applications include real-time tuning in wavelength-division demultiplexers, wavelength selection in optical spectrum analyzers, and band rejection in tunable notch filters.

Distributed opticalfiber sensor for spatial location of mode coupling by using the optical Kerr effect.

We describe a method for determining the locations of discrete mode-coupling points along a polarization-maintaining fiber using a pump-probe architecture based on the optical Kerr effect.

Compact Mach-Zehnder fiber interferometer incorporating photoinduced gratings in elliptical-core fibers.

A new type of Mach-Zehnder interferometer has been fabricated in an elliptical-core high-birefringence fiber. The directional couplers of a conventional fiber interferometer are replaced by two photoinduced polarization rocking filters written on the same fiber. The resonant wavelength of the interferometer was 787 nm, with an operational bandwidth of 18 nm. The device can be used in two different modes for temperature sensing.

Phase-matched polarization coupling in high-birefringence fibers through the optical Kerr effect.

We describe the use of the optical Kerr effect in a simple and novel way to write a transient polarization coupler in high-birefringence fiber. A conversion efficiency of as much as 0.4% is demonstrated, with a coupling bandwidth of approximately 0.4 nm. The method is fast and independent of any thermal drift. Potential applications include real-time tuning in wavelength-division demultiplexers and wavelength selection in optical spectrum analyzers.

Holographically written reflective polarization filter in single-mode optical fibers.

A new type of internally written grating filter is proposed and demonstrated in single-mode optical fibers. A photoinduced rocking birefringence is created by polarization interference between two counterpropagating and orthogonal linear-polarization modes at 514.5 nm. This birefringent grating has the correct submicrometer pitch for efficient phase-matched reflection of a linearly polarized mode into the orthogonally polarized counterpropagating mode. The phenomenon represents a new form of holography in which the image (the orthogonal counterpropagating mode) is reconstructed by scattering off a hologram consisting of a periodic birefringence.

Frequency-derived distributed optical-fiber sensing: Rayleigh backscatter analysis.

Frequency-derived distributed optical-fiber sensing is a powerful and convenient method for measuring the spatial distribution of birefringence in a high-birefringence fiber. The method relies on the special statistical characteristics of Rayleigh backscatter for its action, and these are analyzed in the context of the sensing arrangement, with an emphasis on the physical mechanisms. Implications for system design are also discussed.

Indium-coated D-shaped-fiber polarizer.

A fiber polarizer has been made using an etched D-shaped fiber coated with indium. For a 39-dB extinction ratio, 0.2-dB loss has been obtained. An analysis shows that the effect of coupling between the polarized modes within the polarizer is to produce a saturation in the observed extinction ratio beyond a critical length.