Abstract
Gas detection with hollow-core photonic bandgap fibre (HC-PBF) and pulsed photothermal (PT) interferometry spectroscopy are studied theoretically and experimentally. A theoretical model is developed and used to compute the gas-absorption-induced temperature and phase modulation in a HC-PBF filled with low-concentration of C2H2 in nitrogen. The PT phase modulation dynamics for different pulse duration, peak power and energy of pump beam are numerically modelled, which are supported by the experimental results obtained around the P(9) absorption line of C2H2 at 1530.371 nm. Thermal conduction is identified as the main process responsible for the phase modulation dynamics. For a constant peak pump power level, the phase modulation is found to increase with pulse duration up to ~1.2 μs, while it increases with decreasing pulse duration for a constant pulse energy. It is theoretically possible to achieve ppb level detection of C2H2 with ~1 m length HC-PBF and a pump beam with ~10 ns pulse duration and ~100 nJ pulse energy.
Highlights
(b) light propagation direction hollow-core (c) silica-ring boundary y x z light propagation direction silica-ring boundary is proportional to pump light intensity instead of power, and for the same pump power level, a HC-PBF would offer a much higher light intensity due to its much smaller mode field diameter compared with the free-space approaches
The model includes three regions: an inner circular region with the diameter of the hollow-core which is filled with the gas sample to be measured, a silica ring region with a thickness equal to the wall-thickness of the hollow-core, and an outer gas region filled with the same gas
We focus on the case that the pump duration is considerably shorter than the loop delay time td; this ensures that the dynamics of PT phase modulation due to a single pump pulse could be observed clearly and directly from the output waveform
Summary
By fitting the leading and trailing edges of the phase modulation signal for the 2 μs duration of pump pulse (Fig. 4a,b), we obtained a time constant of 287 ns and 280 ns respectively which represents the characteristic time that the PT phase change will rise up to 1 − 1/e or decay down to 1/e of its maximum value. These values are about one quarter the value of tc[2] (i.e., tc2/4 ~ 275ns), showing that the thermal conduction is the main process that determines the rise and falling behavior of the phase modulation. Further work is needed to study the phase modulation with higher energy pulses
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