MIR-Pump NIR-Probe Fiber-Optic Photothermal Spectroscopy With Background-Free First Harmonic Detection

Abstract

We demonstrated MIR-pump NIR-probe photothermal spectroscopy with the first harmonic (PTS-1f) detection of formaldehyde, one of the most common volatile organic compounds (VOCs), in a silica hollow-core negative curvature fiber (HC-NCF). The photothermal gas sensor adopts a mid-infrared interband cascade pump laser at 3.6 µm and a near-infrared fiber probe laser at 1.56 µm. At the optimal modulation frequency (8 kHz) and modulation index (1.8) of the pump laser, we obtained a normalized noise equivalent absorption (NNEA) coefficient of 4×10 -9 cm -1 WHz -1/2 . The use of HC-NCF with an inner diameter of 65 µm enables the sensitive photothermal detection even for a very low pump power of microwatts. The background-free PTS-1f detection was observed to enhance the sensitivity by a factor of 2.4 compared to the second harmonic (2f) detection. A theoretical model was established in this work to interpret the experimental results.

Oxygen Gas Sensing with Photothermal Spectroscopy in a Hollow-Core Negative Curvature Fiber

“…However, for the PTI system studied here, only the pump power absorbed by gas molecules will contribute to the PT phase modulation, hence the influence of residual laser intensity modulation on its non-absorption background is negligible. Thus, we demodulate the 1f component from PD2 by using a lock-in amplifier, which has the largest amplitude among all the harmonic components of wavelength modulation without strong background signal [21].…”

A Review of Antiresonant Hollow-Core Fiber-Assisted Spectroscopy of Gases

“…Furthermore, the ability to guide light in dissimilar wavelength regions allows ARHCFs to transmit simultaneously both the pump and probe light in the gas-filled core, providing the perfect overlap between them. So far, several configurations of the PTS sensors utilizing ARHCFs as absorption cells have been developed based on the Mach-Zehnder (MZI) and Fabry-Perot (FPI) interferometer setups and shown to provide an exceptional detection capability [22,26,[50][51][52][53][54].…”

“…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 × 10 −8 cm −1 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]. 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.…”

“…Another approach to measuring the photothermal effect refers to the application of an FPI, which enables efficient detection of the phase change of the propagating probe beam after passing through the heated gas sample, hence experiencing the locally modulated RI. The intensity of the beam exiting the Fabry-Perot cavity can be determined based on the equation [44,56]: 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.…”

Photothermal Gas Detection Using a Mode-Locked Laser Signal Readout

“…In this paper, the improvement in the ML laser cavity configuration and the signal processing methods were investigated, yielding a substantial enhancement of its performance. The proposed approach to PT-induced signal readout is unique and significantly dissimilar compared to numerous previously published configurations, where the spectroscopic signal is encoded in amplitude variations due to the continuous wave (CW) interferometric nature of the readout [11,17,[20][21][22].In our proposed PTS sensor, the gas particles under test are enclosed in a gas cell, which is a part of a standard linear cavity fiber ML laser. The excitation of the gas via the auxiliary pump laser and the resulting RI modulation affects the optical pathlength of the ML laser.…”