# A Review of Photothermal Detection Techniques for Gas Sensing Applications
## 4 Intra-Cavity PTS Gas Sensors
### Intracavity PT Detection in a Mode-Locked Laser Configuration  
An interesting approach to intracavity PT detection was proposed in Reference [16]. In this work, the Authors investigated the possibility of translating the PT-induced RI modulation directly into frequency deviation of a mode-locked laser. The schematic of the sensor is presented in Figure 12.  
The base of the sensor had standard configuration for a ring cavity Erbium-doped laser mode-locked by incorporating a semiconductor saturable absorber mirror (SESAM) [57]. The gain  
Figure 11: Experimental setup of the intracavity PT gas sensor. TC—temperature controller, CC—current controller, DAQ—data acquisition, EDF—erbium-doped fiber, WDM—wavelength division multiplexer, ISO—optical isolator, PLS—probe light source, HC-PBF IC—hollow core photonic bandgap fiber gas cell, BD—balanced detector, PC—polarization controller. Reprinted with permission from [56] C The Optical Society.  
Figure 12: Experimental setup of the ML intracavity PT gas sensor. PD—photodiode, FC—fiber coupler with 1% out-coupling ratio, Er\({}^{3+}\)—80-cm-long erbium doped fiber, WDM—wavelength division multiplexer, PUMP—980 nm singlemode pump laser, CIR—fiber circulator, SESAM—fiber pigtailed semiconductor saturable absorber mirror, COLL—fiber collimator, F—filter, DM—dichroic mirror. The gas absorption cell is 10 cm long. Reprinted with permission from [16] C The Optical Society.  
section was an 80-cm-long piece of erbium-doped fiber, which is optically pumped by a singlemode laser diode (\(\lambda\) = 980 nm) via a fiber wavelength division multiplexer (WDM). The fiber pigtailed SESAM was coupled to the cavity via a fiber circulator (CIR), which additionally ensured unidirectional operation of the laser. Parameters of the ML laser were monitored via a 1% fiber coupler with a photodiode. The gas sample was introduced into the ML laser via a 20-cm-long free-space open-path which was formed in the cavity with two fiber collimators. The gas under testing was carbon dioxide (CO\({}_{2}\)) and was pumped into a 10-cm-long absorption cell. The gas sample was excited via an auxiliary 100 mW laser (DFB diode with an additional thulium doped fiber amplifier) targeting an isolated absorption line located at \(\sim\)2003.5 nm. The _pump_ beam was co-aligned with the _probe_ beam of the ML laser using a dichroic mirror (DM). The ML laser had a pulse repetition frequency of \(f_{\textit{rep}}\) - 21.226 MHz. Exciting the CO\({}_{2}\) molecules in the path-length of the _probe_ beam of the ML laser resulted in modulation of the optical path-length, and hence a slight change in the repetition frequency. PT signal analysis was simplified by modulating the _pump_ laser and registering the resulting repetition frequency deviation via an FM demodulator and an LIA. In this simplified PT gas sensor, the Authors were able to achieve an NEC of 311 ppmv for CO\({}_{2}\) at ambient pressure. It is worth noting that compared to other fiber-based sensors, no interferometric read-out of the signal is required and due to its configuration it is possible to use mid-IR wavelength _pump_ sources to excite gas molecules.