protocol for the modulation of the laser drive signal and the demodulation of the detector response. The analyser was configured for hydrogen measurements in nitrogen, with an environmental temperature range of -20°C to +50°C, and a sample cell pressure range of 10- 25 psia. Instrument design The TDLAS instrument evaluated in this work was an AMETEK 5100HD analyser (see Figure 1 ) which was modified to operate with a multi- pass Herriott cell, having an optical path length of 20 metres and a volume of 1 litre. A schematic representation of the instrument is shown in Figure 2 . The measurement of hydrogen was performed with a distributed feedback (DFB) laser. This laser produced an optical power of approximately 3 MW, and optical attenuators were used to reduce the output power to usable levels. The output of the laser was coupled into single-mode optical fibres, which in turn were connected to a fibre- optic beam splitter. The splitter was used to divide the optical power in a 50/50 ratio for use in the sample and reference measurements, respectively. Gradient refractive index (GRIN) lenses, with a beam divergence of 1.8 mrad (milliradians), were used to collimate the output of the single-mode fibres and direct
the resulting beams through the sample and reference cells. The sample and reference cells each contained 0.5 mm 2 InGaAs-photodiode detectors, which were connected to separate input channels of the electronics unit. With this configuration, it was possible to make simultaneous measurements of unknown samples and known references, which were used to lock the output wavelengths for both lasers. The sample cell temperature was controlled with an accuracy of +/- 0.1°C and could be set in the range of 60°C by setting the temperature of the oven in the sample cell compartment. The reference cell is not located in the sample compartment, but the temperature was maintained above 40°C. The laser and photodiodes were also located in the main electronics compartment, isolated from the heated sample oven. The wavelength modulation spectroscopy (WMS) experiment was implemented by using a digitally sampled sine function, summed with a staircase, and the resulting signal was used to drive the tunable DFB laser diode. Signals produced by the detectors were digitised prior to applying signal processing (such as phase-sensitive detection and smoothing). In contrast with the common practice of using second harmonic
Mirror
Mirror
Multi pass sample cell
GRIN lens
Beam splitter
Photodiode
Laser diode
Reference cell
GRIN lens
Photodiode
Temperature control
Electronics unit
Current control
Figure 2 Block diagram for TDLAS measurement
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