# A Review of Photothermal Detection Techniques for Gas Sensing Applications
## 3 Fiber-Based Photothermal Gas Sensors
### Pulsed PTS in Hollow-Core Fibers  
The HC-PBF-based PT gas layout was additionally investigated by [PERSON] et al. in a an excellent paper published in 2016 [45]. This work not only presents a novel approach to PT gas sensing but also clearly describes the thermal properties of gas particles excited in an HCF. All experimental data were confronted with numerical models. Moreover, Authors analyzed drawbacks pointed out in the work by [PERSON] et al. [41] and proposed an improved configuration of an HC-BPF-based photothermal gas sensor. The layout of the sensor is represented in Figure 8.  
Here, the Authors targeted the same gas as in the previous experiment using an ECDL exciting the P (9) absorption line of C\({}_{2}\)H\({}_{2}\) at 1530.37 nm. The emission of the diode was amplified by a  
Figure 8: Pulsed PT gas sensor employing an HC-PCF as a gas cell. HC-PBF—hollow core photonic bandgap fiber, ECDL—external-cavity diode laser with wavelength around 1530.371 nm, EDFA—erbium doped fiber amplifier, AOM—acoustic-optic modulator, BD—balanced detector, BS—broadband source, PC—polarization controller. Reprinted with permission from [45]. standard EDFA and additionally intensity modulated using an AOM. The pulse repetition rate was experimentally chosen and set to 500 Hz. Authors used the same 0.62-m-long HC-1550-02 fiber from NKT Photonics, which was filled with 7500 ppm C\({}_{2}\)H\({}_{2}\) in nitrogen. The PT-induced phase modulation was detected using an all-fiber Sagnac interferometer which was probed by a super-luminescent light emitting diode (SLED) [46] with a bandwidth of 41 nm centered around 1545 nm. The Sagnac interferometer was configured so that the SLED radiation is divided into a 3 \(\times\) 3 coupler and travels a common loop in opposite directions. The gas filled HC-PCF was inserted into the interferometer with an offset to its center. Authors used a 2-km-long single mode fiber as an optical delay line to ensure a constant, 10 \(\upmu\)s delay between the light traveling clockwise (CW) and counterclockwise (CCW) through the HC-PCF. The CW and CCW beams are combined at the 3 \(\times\) 3 coupler after traveling the interferometer and analyzed by a balanced photodetector. Compared to the experiment with the traditional MZ interferometer, both beams in the Sagnac interferometer travel through the same fiber-based loop. This limits the influence of environmental disturbances observed in previous experiments. The PT-induced phase shift was registered using a boxcar averager and analyzed for with different values of sample averaging. Ultimately, this configuration of a PT gas sensor reaches an NEC of 3.3 ppm of C\({}_{2}\)H\({}_{2}\), but was mainly limited by the number of times the registered signal was averaged (in this case 10,000). Both papers [41; 45] show that integrating HCF into PT gas sensors gives excellent results and should be further explored as an interesting alternative for other commonly used detection techniques. Unfortunately, currently commercially available HCF's have two main drawbacks which severely limit their implementation in PTS sensors and laser-based gas sensors in general. The first limitation is connected with the wavelength-dependent attenuation of such fibers. Drawn from standard silica glass, HCF's suffer from tremendous attenuation in the mid-IR wavelength region, especially above 3.5 \(\upmu\)m, mainly due to two photon absorption effects [47]. Transmission of a 1-cm-long fused silica glass has been plotted in Figure 9 in the function of a wavelength. This limitation is clearly pointed out by most of the researchers working on the implementation of HCF in laser-based gas spectrometers. Moreover, currently available HCF's suffer from several additional drawbacks, severely limiting their out-of-lab application not only in PTS gas sensors, but any mid-IR laser-based sensors. The issues are connected with small inner core diameters and not purely single-mode operation.