Performance analysis of a time-division-multiplexed fiber-optic gas-sensor array by wavelength modulation of a distributed-feedback laser

Abstract

The results of an investigation of the performance of a time-division-addressed fiber-optic gas-sensor array by means of wavelength modulation of a distributed-feedback ͑DFB͒ laser are reported. The system performance is found to be severely limited by the extinction ratio of the optical switch used for pulse amplitude modulation. Formulas that relate the cross-talk level to the extinction ratio of the switch, the modulation parameters of the DFB laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 methane gas sensors with a detection sensitivity of 2000 parts in 10 6 ͑ppm͒ ͑10-cm gas cell͒ for each sensor may be realized with a commercially available single Mach-Zehnder amplitude modulator ͑Ϫ35-dB extinction ratio͒. An array of 100 sensors with a 100-ppm detection sensitivity for each sensor may be realized if a double MachZehnder amplitude modulator is used.

Multiplexing of optical fiber gas sensors with a frequency-modulated continuous-wave technique

“…The system consists of N transmission-type gas cells connected in a forward-coupled ladder topology. 14 Light from the tunable laser source is modulated in intensity through the use of an external intensity modulator with a triangular chirped frequency carrier generated from a voltage-controlled oscillator and coupled into the gas sensor array. When light passes through the gas cells, gas concentration information is encoded onto the light intensity.…”

“…This would reduce the cost per sensing point and enhance the competitiveness compared with conventional electrical-chemical gas sensors. Spatial division multiplexing 13 ͑SDM͒ and time-division multiplexing 14 ͑TDM͒ techniques have been used for multiplexing fiber optic gas sensors. The SDM system exhibits a performance similar to that of a single-cell system, but a number of receivers and signal processing units need to be used.…”

Experimental demonstration of a fiber-optic gas sensor network addressed by FMCW

“…For economical consideration, sensor multiplexing must be implemented in order to reduce the cost-per-sensing-point where some expensive components can be shared by a number of sensors. Spatial-division multiplexing (SDM) [2] and time-division multiplexing (TDM) techniques [3] have been applied for multiplexing fiber-optic gas sensors. The SDM system is of similar performance to a single-sensor system but a number of receivers need to be used.…”

“…1) connected in a forward-coupled ladder topology [3]. Light from a New Focus external cavity tunable laser was modulated in intensity by use of a UTP single-polarization intensity modulator on which a triangular swept-frequency carrier generated from a voltage-controlled oscillator (VCO) was applied.…”

“…A polarization controller was used before the intensity modulator for optimizing the launched power. The splitting ratios of the couplers C , C were chosen to be 66 : 33 and 50 : 50, respectively, in order to balance the power level from each sensor [3]. When light passes through the gas cells, gas concentration information is encoded on to the light intensity.…”