A general theory for fiber-optic, evanescent-wave spectroscopy and sensors is presented for straight, uncladded, step-index, multimode fibers. A three-dimensional model is formulated within the framework of geometric optics. The model includes various launching conditions, input and output end-face Fresnel transmission losses, multiple Fresnel reflections, bulk absorption, and evanescent-wave absorption. An evanescent-wave sensor response is analyzed as a function of externally controlled parameters such as coupling angle, f number, fiber length, and diameter. Conclusions are drawn for several experimental apparatuses.

Theory of fiber-optic, evanescent-wave spectroscopy and sensors.

A fibre-optic evanescent field absorption sensor based on a U-shaped sensing probe is described. The influences of fibre core diameter, bending radius of the probe and the refractive index of the fluid on the sensitivity of the sensor are evaluated experimentally. The results are compared with the theoretical results obtained using geometrical optics and based on two-dimensional treatment. A good qualitative agreement is found between them.

Fibre-optic evanescent field absorption sensor based on a U-shaped probe

An optical fiber-based evanescent gaseous ammonia sensor is designed and developed. The sensing dye, bromocresol purple (BCP), is immobilized in the substitutional cladding using sol–gel process. The sensing properties of the optical fiber sensor to gaseous ammonia at room temperature are presented. This newly developed ammonia sensor exhibits good reversibility and repeatability. The effect of different carrier gases, argon, nitrogen, and air on sensing properties of the ammonia sensor is investigated. The sensor with air as carrier gas has the best response time and sensitivity. In order to improve the response time of the optical fiber evanescent ammonia sensor, an elevated ambient temperature is applied and thoroughly investigated. A fast response time of 10 s was obtained at 55.5 °C with the carrier gas of air or argon. These experimental results have demonstrated that a fast response optical fiber evanescent gaseous ammonia sensor can be constructed by applying slightly elevated ambient temperature.

Optical fiber-based evanescent ammonia sensor

New developments and applications of tunable IR lead salt lasers

Diode laser based absorption spectroscopy (DLAS) is widely used for gas detection in variety of applications across the energy, petrochemical and mining industries. Recent developments in near and mid infrared diode lasers have improved the sensitivity of gas measurement based on high resolution spectra of target species. The availability of near-infrared diodes that can operate at room temperature has expanded the application of spectroscopy technique in hand-held gas detection devices. DLAS in conjunction with optical fibres has also been applied for the distributed sensing and monitoring of various gases in real-time basis. This paper, after introducing methane characteristics and semiconductor diode lasers, comprehensively reviews development in spectroscopic methane sensing techniques in accordance with methane absorption lines in the near infrared spectrum.

A review of developments in near infrared methane detection based on tunable diode laser

A novel fiber optic evanescent wave spectroscopy (FEWS) system is based on a tunable diode laser (TDL) source, on a polymer‐coated AgClBr infrared transmitting fiber, and on a mercury cadmium telluride detector. This system was used for sensing low levels of chlorinated hydrocarbons in water. The detection limit was 100 μg/l (100 ppb), which is an improvement by a factor of 50 in comparison to a similar system that is based on a Fourier transform infrared spectrometer, and the measurement time was reduced by a factor of 3. The TDL‐FEWS system shows significant potential for in situ monitoring of ground water.

Fiber optic sensor for chlorinated hydrocarbons in water based on infrared fibers and tunable diode lasers

A novel fiber-optic evanescent wave sensor is presented. The sensor, composed of a lead salt tunable infrared laser and a multimode uncladded silver halide infrared optic fiber, was used for rapid measurement of sulfur hexafluoride pressures at a few-percent detection limit. High-precision detection and identification of a large variety of gases are possible.

Fiber-optic evanescent wave sensor for gas detection.

A new fiber‐optic sensor system, comprised of an IR tunable diode lasersource and an unclad multimode IR fiber, is described. Design considerations, theoretical predictions, and experimental verification of the sensor performance are presented. This sensor has a relatively high sensitivity, and it will be very useful for monitoring and controlling i n s i t u and in real time, concentrations and chemical transformations of liquids and pastes.

Fiber‐optic‐based evanescent field chemical sensor using tunable diode lasers for the midinfrared spectral region

A simple fiber-optic chemical sensor which may be used to detect in-situ and in real time concentrations and chemical transformation of liquids and pastes is reported. The sensor, comprised of a tunable IR diode laser source and a cladding-free multimode IR fiber, operates on the principle of evanescent field absorption spectroscopy. The sensor's response is theoretically analyzed and experimentally verified.

Fiber-Optic-Based Evanescent Field Infrared Spectroscopy Using Tunable Diode Lasers

Evanescent wave spectroscopy measurements were performed using unclad silver halide fibers and tunable laser sources such as lead salts diode lasers and a CO2 laser. Spectra of liquids and gases were recorded. The system S/N ratio and resolution were better than those of FTIR spectrometers. This demonstrates the potential use of tunable lasers in fiberoptic evanescent wave spectroscopy.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

U. Schiessl

Papers

Fiber optic sensor for chlorinated hydrocarbons in water based on infrared fibers and tunable diode lasers

Fiber-optic evanescent wave sensor for gas detection.

Fiber‐optic‐based evanescent field chemical sensor using tunable diode lasers for the midinfrared spectral region

Fiber-Optic-Based Evanescent Field Infrared Spectroscopy Using Tunable Diode Lasers

Liquid and gas fiber optic evanescent-wave spectroscopy by tunable lasers