In this work we present the design and characterization of a dual comb photoacoustic spectroscopy (DCPAS) set-up for ammonia detection in the near infrared. The system consists of a dual electro-optic (EO) comb generator that generates a multiheterodyne beating signal in the gas sample. The input to the dual EO comb generator is a laser diode tuned to a fixed wavelength within an absorption feature of ammonia (around 1531.6 nm, 6529 cm-1) and we show how the dual comb allows to perform PAS measurements and resolve the absorption features with high spectral resolution. We present results of the ammonia absorption line profile reconstruction with a bandwidth of 1 cm-1 and a resolution of 0.08 cm-1. Moreover, we show that dual comb technique based on electro-optic generation maximally simplifies the optimization of the multiheterodyne signal according the characteristic of the photoacoustic detection module. We present results using a resonant gas cell (pipe shape) and we show how easily the dual comb optical source is adjusted to generate multiherodyne beating tones within the band of resonance of the gas cell.
A sensitive photoacoustic detection approach employing a silicon cantilever is investigated for power measurement of electromagnetic radiation. The technique which is actuated by pressure waves generated through radiation-induced heat, depicts high sensitivity for a considerably large spectral range from 325 nm to 1523 nm. The implemented method shows linear response in the measurement of radiation power from 15 nW to 6 mW, demonstrating a dynamic range of almost six orders of magnitude. A numerical model has been developed to analyse and optimise the measurement sensitivity using different di…
We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Due to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of $^{14}\text{CH}_4$ is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon met…
We investigate theoretically the photoacoustic generation by a gold nanosphere in water in the thermoelastic regime. Specifically, we consider the long-pulse illumination regime, in which the time for electron-phonon thermalisation can be neglected and photoacoustic wave generation arises solely from the thermo-elastic stress caused by the temperature increase of the nanosphere or its liquid environment. Photoacoustic signals are predicted computed based on the successive resolution of a thermal diffusion problem and a thermoelastic problem, taking into account the finite size of the gold nano…
We demonstrate a 13-fold speed improvement in broadband cantilever-enhanced photoacoustic spectroscopy (CEPAS) by combining it with phase-controlled Fourier-transform spectroscopy (PC-FTS) instead of traditional Fourier-transform infrared spectroscopy (FTIR). PC-FTS is a modification of FTIR and capable of fundamentally faster interferogram acquisitions. The speed-improvement is beneficial for CEPAS, which is an especially sensitive version of the background-free photoacoustic spectroscopy technique. We used the PC-FTS-CEPAS technique to measure the absorption spectrum of methane in the mid-in…
We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. This is in contrast with conventional methods using absorbers or plasma generation using near-infrared laser light. The photoacoustic wave generation and plane wave propagation are observed u…
All-optical ultrasound detection bears unique advantages for photoacoustics, including wider detection bandwidth, higher signal-to-noise per unit area and lower susceptibility to electromagnetic noise. These benefits have established optical ultrasound detection as a key method for photoacoustic applications in biology and medicine. However, the use of free-space detectors renders this approach sensitive to optical aberrations, which can degrade the pressure sensitivity and result in deteriorated image quality. While spatial mode-filtering through fibers has been proposed to alleviate these pr…
MEMS Fiber-Tip Photoacoustic Spectrometer for In Situ Microscale Trace Gas Sensing
National Engineering Research Center of Next Generation Internet Access System, School of Optical and Electronic Information, Huazhong University of Science and Technology（华中科技大学光学与电子信息学院下一代互联网接入系统国家工程研究中心）；Optoelectronic Valley Laboratory（光谷实验室）
Histological images are critical in the diagnosis and treatment of cancers. Unfortunately, the current method for capturing these microscopy images require resource intensive tissue preparation that delays diagnosis for many days to a few weeks. To streamline this process, clinicians are limited to assessing small macroscopically representative subsets of tissues. Here, we present a combined photoacoustic remote sensing (PARS) microscope and swept source optical coherence tomography (SS-OCT) system designed to circumvent these diagnostic limitations. The proposed multimodal microscope provides…
Photoacoustic spectroscopy (PAS) based gas sensors with high sensitivity, wide dynamic range, low cost, and small footprint are desirable across a broad range of applications in energy, environment, safety, and public health. However, most works have focused on either acoustic resonator to enhance acoustic wave or optical resonator to enhance optical wave. Herein, we develop a gas sensor based on doubly resonant PAS in which the acoustic and optical waves are simultaneously enhanced using combined optical and acoustic resonators in a centimeter-long configuration. Not only the lower detection …
Dual-comb interferometry is a measurement technique that uses two laser frequency combs to retrieve complex spectra in a line-by-line basis. This technique can be implemented with electro-optic frequency combs, offering intrinsic mutual coherence, high acquisition speed and flexible repetition-rate operation. A challenge with the operation of this kind of frequency comb in dual-comb interferometry is its limited optical bandwidth. Here, we use coherent spectral broadening and demonstrate electro-optic dual-comb interferometry over the entire telecommunications C band (200 lines covering ~ 40 n…
We demonstrate dual-comb spectroscopy based on difference frequency generation of frequency-agile near-infrared frequency combs, produced with the help of electro-optic modulators. The combs have a remarkably flat intensity distribution and their positions and line spacings can be selected freely by simply dialing a knob. We record, in the 3-micron region, Doppler-limited absorption spectra with resolved comb lines within milliseconds. Precise molecular line parameters are retrieved. Our technique holds promise for fast and sensitive time-resolved studies e.g. of trace gases.
The repetition rate stabilization of an optical frequency comb based on diode-pumped solid-state laser technology is demonstrated using an intra-cavity electro-optic modulator. The large feedback bandwidth of such modulators allows disciplining the comb repetition rate on a cavity-stabilized continuous-wave laser with a locking bandwidth up to 700 kHz. This surpasses what can be achieved with any other type of actuator reported so far. An in-loop integrated phase noise of 133 mrad has been measured and the PM-to-AM coupling of the electro-optic modulator has been investigated as well.
