*5.1. MZI PTS in ARHCF*

In an MZI-based PTS sensor configuration, the modulation of the RI induced by the gas molecules excitation is observed as the difference in the optical path length (hence the phase difference), that is experienced by the *probe* beam propagating in two arms of the interferometer [55]. In a typical MZI PTS sensor setup, the sensing arm of the interferometer consists of an absorption cell while the second is used as a reference and is free of gas molecules.

An ARHCF-aided configuration of such a sensor was demonstrated by Yao et al. in [50] and was aimed at detecting CO. The experimental setup of the sensor is presented in Figure 8. In this configuration, the *pump* laser operated at 2327 nm, which corresponds to the R(10) transition of CO in the 2v1 band, while the *probe* laser wavelength was set to 1533 nm. The *probe* beam was divided into two arms of the MZI and coupled together with the *pump* light into a 0.85 m long gas-filled hollow-core negative curvature fiber (HC-NCF) placed in the sensing arm. The dichroic mirror in the sensing arm was used to separate the remaining *pump* light from the *probe* beam that contained the information of the induced RI modulation. The MZI was set to operate in the quadrature point by implementing a piezoelectric transducer with a piece of a conventional single-mode fiber coiled on it. The *probe* beams leaving both arms of the MZI are combined using a fiber coupler, and subsequently, the beat note signal was detected by a photodetector. The interferometric signal contained information about the phase change of the *probe* light after passing through the heated gas sample in the fiber core. With the additional sinewave modulation applied to the *pump* laser injection current, the spectroscopic signal was retrieved using the well-known WMS method. The registered signal was free from the intermodal interference noise in the fiber, which allowed the sensor to obtain a normalized noise equivalent absorption coefficient (NNEA) at the level of 4.4 × <sup>10</sup>−<sup>8</sup> cm−<sup>1</sup> WHz−1/2 (90 ppmv), which gives an order of magnitude improvement in comparison to the similar sensor configuration utilizing a hollow-core capillary tube [55].

**Figure 8.** Schematic diagram of the MZI PTS sensor utilizing an HC-NCF as an absorption cell used to detect CO at 2327 nm. The fiber was filled with the gas analyte using gas filling cells placed at both ends. HC-NCF—hollow core negative curvature fiber (ARHCF), PID—proportional-integralderivative controller, PD—photodetector, LP—Flow-pass filter, FCfiber coupler, PCpolarization controller, PZT—piezo-electric transducer, DM—dichroic mirror. (**a**) Cross-section of the HC-NCF used in the experiment. (**b**) HC-NCF-delivered beam profile. (**c**) Profile of the beam emitted by the *pump* laser. Reprinted with permission from [50] © The Optical Society.

Further development of ARHCF-aided MZI PTS sensors was reported in [51]. The authors benefited from the unique property of the ARHCFs, which enables them to guide light with low loss both in the near- and mid-IR spectral band. The developed sensor configuration was similar to the one presented in Figure 8, however, the *pump* light was emitted from an ICL operating at 2778.48 cm<sup>−</sup>1, which corresponded to the strong transition of formaldehyde (CH2O), while the *probe* beam was delivered from a telecom DFB laser emitting light at 1.56 μm. The gas-laser interaction path in the sensing arm of the MZI was formed by a 1.2 m long ARHCF having an air core with a diameter of 65 μm, which was characterized by the attenuation of 0.6 dB/m and 0.43 dB/m at the *pump* and *probe* wavelengths, respectively. Hence, both wavelengths could be simultaneously coupled into the fiber and transmitted through it with low loss. The ICL was modulated with a sinusoidal signal with a frequency of 8 kHz, which enabled performing WMS-based spectroscopic signal retrieval at the harmonics of the modulation frequency. The authors proved that the demodulation of the photodetector signal at the 1st harmonic (1f detection) provided greater signal amplitude while maintaining a low noise level in comparison to the 2f signal when the sinewave modulation frequency was greater than 6 kHz. Furthermore, the background free behavior of the sensor was maintained utilizing the 1f detection scheme. The obtained SNR for the 1f signal reached 163, which was 2.4 times better compared to the value obtained for the 2f signal. The obtained SNR yielded an MDL of 0.18 ppmv, which corresponds to an NNEA of 4 × <sup>10</sup>−<sup>9</sup> cm−<sup>1</sup> WHz<sup>−</sup>1/2.
