# A Review of Photothermal Detection Techniques for Gas Sensing Applications
## 3 Fiber-Based Photothermal Gas Sensors
### PTS in Hollow-Core Fibers  
Hollow core fibers (HCF) have attracted much attention from researchers working on laser-based gas sensors [34]. Numerous experiments were published demonstrating the usefulness of such structures in gas sensors [34; 35; 36; 37; 38; 39; 40]. The problem of achieving reasonable PTS sensor performance was investigated by several Groups worldwide. Some researchers addressed the issue of PTS sensitivity by incorporating hollow-core fibers into the gas sensors. The appropriate design of an HCF-based PTS sensor addresses several crucial issues: optimized _pump_ and _probe_ beam overlap and beam size throughout the entire gas-laser path-length, limited gas volume required for the analysis and overall stability of the system. One of the most interesting experiments was presented by [PERSON] et al. [41]. The schematic of the setup is presented in Figure 7.  
Figure 7: (a) HC-PCF-based PTS gas sensor. FC—fiber coupler, PZT—piezoceramic transducer, OC—optical circulator, EDFA—erbium doped fiber amplifier, LPF—low-pass filter, HPF—high-pass filter DAQ—digital acquisition card, DFB—distributed feedback laser diode. (b) Amplitude of the second harmonic signal in function of gas concentration. Reprinted with permission from [41].  
The base of the presented sensor was a fiber-based MZ interferometer, similar to the ones explained in the previous experiments. A singlemode external cavity diode laser with a wavelength of 1556.59 nm was used as the _probe_ laser. Here, the Authors decided to use phase detection for the PT-induced RI modulation. This method is significantly more sensitive compared to pure frequency demodulation, but requires using a piezo ceramic transducer (PZT) [42] in order to enable quadrature detection of the phase variations. The _probe_ radiation was split into two arms--reference and measurement. The reference arm fiber was coiled around a PZT and stretched to maintain a 90 degree phase difference between the interfering beams. The measurement beam was coupled into a 0.62-m-long commercially available hollow-core photonic bandgap fiber (HC-PBF) [43; 44], manufactured by NKT Photonics (HC-1550-02). The fiber had a core diameter of only 11 \(\upmu\)m. Authors used but-coupling between the SMF28 standard fiber and the HC-PBF. A distributed feedback (DFB) semiconductor laser diode was used as the _pump_ source which targeted the P (9) line of acetylene (C\({}_{2}\)H\({}_{2}\)) located at 1530.37 nm. The _pump_ laser was modulated at \(f_{m}\) = 50 kHz and the resulting PT-induced phase-shift signal detected by the MZ interferometer was analyzed by an LIA at the second harmonic of the \(f_{m}\). The Authors achieved an NEC of 2 ppb. Moreover, the sensor had a very wide dynamic range, nearly six orders of magnitude (\(5.3\times 10^{5}\)), which was superior to any previously reported fiber-based gas sensor at that time (inset in Figure 7). Nevertheless, the Authors admitted that the achieved results would be unobtainable in out-of-lab applications. The main identified issue was the feed-back loop controlling the quadrature operation of the MZ interferometer. In-lab experiments were conducted in an controlled environment, with limited mechanical and acoustic noise. Real-world applications would suffer from all sorts of disturbances influencing the MZ interferometer, and this would affect the stability of the quadrature phase feed-back loop and thus significantly increase the overall noise floor of the sensor.