Marek T. Wlodarczyk

Evanescent Field Spectroscopy With Optical Fibers For Chemical Sensing

A simple fiber optic chemical sensor is reported which may be used to detect in situ and in real time concentrations of gases and liquids. The sensor, comprised of a single LED source and a cladding-free multimode fiber, operates on the principle of evanescent field and differential spectroscopy. Exhibiting a linear response to relative humidity, this sensor selectively detects water vapor while remaining insensitive to source power fluctuations.

Wavelength referencing in single-mode microbend sensors

The strong wavelength dependence of microbending losses in single-mode fibers is utilized in wavelength referencing for fluctuation-free microbend sensors. Through fiber size and microbend periodicity, optimization of the wavelength dependence can be increased dramatically. The reference and signal wavelengths can be provided through the spectrum of a light-emitting diode.

Fiber-optic sensor employing thin-film-coating optical spectrum modulation

We describe a novel sensing approach and experimental results from the use of three optical wavelengths to probe temperature-induced optical spectrum modulation of thin-film optical coatings. This approach provides a sensor design with integrated pressure- and temperature-sensing capabilities, active pressure sensitivity correction, and environmental error compensation, all embodied in one simple sensor construction. The miniature sensor design is intended primarily for dynamic pressure measurement in normal- to high-temperature environments.

Fiber optic pressure sensor for combustion monitoring and control

This paper describes a diaphragm-based, multi-wavelength fiber optic pressure sensor that is suitable for combustion monitoring and control applications. Through the simultaneous optical detection of sensor temperature and diaphragm deflection, temperature induced inaccuracies are significantly reduced. Design criteria for high temperature sensors are outlined and initial experimental results are presented.

Bragg Effect Refractometer

A novel optical waveguide refractometer is reported suitable for on-line and in situ monitoring of index of refraction. The device exhibits theoretical sensitivity dramatically higher than the sensitivity of the previously reported fiber optic refractometers. Based on the Bragg effect in the periodically corrugated dielectric waveguide, the refractometer can detect variations in the index of refraction of the medium in contact with the waveguide. It is found that more than two orders of magnitude higher theoretical sensitivity can be accomplished with the Bragg-type refractometer compared to the one based on the reflection principle.

Microbending Losses Of Metal Coated Single Mode, Multimode, And Cladding-Free Fibers

Under optimized conditions the microbending sensitivity is comparable in metal coated multimode and single mode fibers. Microbending losses in step index fibers may show significant wavelength dependence and this dependence is quite similar for single and multimode fibers. The spectral characteristics of the losses depend not only on the fiber parameters but also on the periodicity of microbending. An inexpensive cladding-free fiber exhibits two-region displacement sensitivity. The maximum sensitivity of the single region fiber can exceed the maximum sensitivities of conventional fibers.

Quasi-optics of scattering and absorption in planar waveguides

A quasi-optical approach is developed to investigate the effects of absorption and surface and volume scattering on the attenuation of guided waves in planar waveguides. Through the use of quasi-optics it is possible to calculate the guide extinction coefficient by following a zigzagging ray in the guide. It is found that at shorter wavelengths the film attenuation dominates the cladding attenuation despite the much smaller relative value of the film absorption coefficient. The absorption spectra of cladding material are more distorted for fundamental modes than for higher-order guided modes. Scattering losses, particularly those that are due to surface roughness, depend more strongly on optical wavelength than do absorption losses. In the cases of both surface scattering and volume scattering, the losses are increased at shorter wavelengths and are larger for the TM modes.

Quasi-Optics Of Random Scattering In Planar Waveguides

A quasi-optical technique is developed to investigate random surface and volume scattering of guided waves in planar waveguides. Through the use of quasi-optics, the guide extinction coefficient is calculated in a simple manner by following a zigzagging ray in the guide. The analysis demonstrates that the scattering losses resulting from random surface roughness depend more strongly on optical wavelength than the index inhomogeneity-induced losses. For a typical correlation length both surface and volume scattering losses increase at shorter wavelengths. The surface roughness-induced losses are larger for low order TM modes than the TE modes of the same order but the differences between the polarizations decrease with increased wavelength and mode order.

Analysis Of Microbending Losses On A Single-Mode, Step-Index Fiber At Higher-Order Harmonics Of Deformation Spectrum

A new analytical technique for analyzing microbending losses of step-index, single-mode fibers is reported. The technique is applicable to fibers with arbitrary N.A., large deformation periods and radiation escaping at arbitrary angles to fiber axis. Two dimensional fiber deformations can be studied and information on radiation loss, propagation directions and polarization content of scattered radiation can be obtained. The effect of fiber radius on radiation loss and the distribution of radiation power between different space harmonics is investigated. It is found that radiation loss depends strongly on fiber radius and that the maximum loss for different space harmonics occurs at the same radius. At the fiber radius resulting in maximum loss, the radiated power is sharply peaked around optimum period lengths for both fundamental and higher-order space harmonics. It is shown that the fraction of radiated power in a higher-order space harmonic can be larger than that in the fundamental space harmonic at large deformation periods. The results indicate that microbending sensors do not necessarily have to operate at the fundamental harmonic of deformation spectrum.