Optimizing the modulation for evanescent-wave analysis with laser diodes (EWALD) for monitoring chlorinated hydrocarbons in water

Fiber evanescent-field analysis (FEFA) is a novel and promising sensor technique for on-line and in situ analysis of hydrocarbons in water. With a conventional IR light source and FTIR spectroscopy it allows multicomponent analysis, while the use of mid-infrared (MIR) tunable diode lasers (TDLs) results in sensitive and selective single-component analysis. A new modulation technique operates the diodes with short pulses in combination with base-current modulation. Conventionally these lasers are used for trace-gas analysis with spectral features of 10(-6) spectral resolution. For the new evanescent-wave analysis with laser diodes (EWALD) sensor application, broadband spectra must be scanned, which does not allow high-resolution techniques to be used. The laser emission bandwidth does not need to be small, 10(-3) to 10(-4) resolution is sufficient, but the wavelength scan range must be about 10(-2). In short-pulse mode, this can be achieved; furthermore, the laser can be used at higher operation temperature for more convenient application. The laser modulation characteristics are dependent on operation temperature, base-current, and pulse-current width and amplitude. The influence of these parameters and their optimization will be discussed.