Balanced-detection interferometric cavity-assisted photothermal spectroscopy employing an all-fiber-coupled probe laser configuration

Simple approach for spectral beam combination of narrowband laser sources for spectroscopic applications

Spectral beam combination of multiple single mode laser sources employing narrowband spectral filters which are arranged on the perimeter of regular polygons is demonstrated. With this simple geometric design, co-alignment and co-propagation of the individual laser beams can be reasonably achieved. Spectroscopic applicability is displayed by spatial filtering, mode-matching, and subsequent coupling of the combined beams into a 76 m astigmatic mirror multipass cell.

Amplified Photothermal Phase Modulation for Carbon Dioxide Detection by Operating a Dual-Mode Interferometer at Destructive Interference

Photothermal interferometry is a highly sensitive spectroscopic technique for trace gas detection. However, the performance of the state-of-the-art laser spectroscopic sensors is still insufficient for some high-precision applications. Here, we demonstrate optical phase-modulation amplification by operating a dual-mode optical fiber interferometer at destructive interference for ultrasensitive carbon dioxide detection. With a 50 cm long dual-mode hollow-core fiber, amplification of photothermal phase modulation by a factor of nearly 20 is achieved, which enables carbon dioxide detection down to 1 parts-per-billion with a dynamic range of over 7 orders of magnitude. This technique could be readily used to improve the sensitivity of phase modulation-based sensors with a compact and simple configuration.

High-Q asymmetrically cladded silicon nitride 1D photonic crystals cavities and hybrid external cavity lasers for sensing in air and liquids

In this paper we show a novel design of high Q-factor silicon nitride (SiN) 1D photonic crystal (PhC) cavities side-coupled to curved waveguides, operating with both silica and air cladding. The engineering of the etched 1D PhC cavity sidewalls angle allows for high Q-factors over a wide range of upper cladding compositions, and the achievement of the highest calculated Q-factor for non-suspended asymmetric SiN PhC structures. We show the employment of these type of SiN PhC cavities in hybrid external cavity laser (HECL) configuration, with mode-hop free single mode laser operation over a broad range of injected currents (from 25 mA to 65 mA), milliwatts of power output (up to 9 mW) and side-mode suppression ratios in the range of 40 dB. We demonstrate the operation of these devices as compact and energy efficient optical sensors that respond to refractive index changes in the surrounding medium the measurement of sodium chloride (from 0% to 25%) and sucrose (from 0% to 25%) in aqueous solution. In HECL configuration, the RI sensor exhibits a 2 orders of magnitude improvement in detection limit compared to the passive microcavity. We also discuss the possibility for applying these devices as novel transducers for refractive index changes that are induced by analyte specific absorption of infrared radiation by the target analytes present in gas or liquid phase.

Sensitive mid-infrared photothermal gas detection enhanced by self-heterodyne harmonic amplification of a mode-locked fiber laser probe

In this work, a method of photothermal spectroscopic signal extraction is presented. The refractive index modulation readout is realized in a purely frequency detection-based approach, by demodulating the beatnotes of a mode-locked fiber laser operating at 1.56 µm. A unique and non-complex self-heterodyne harmonic amplification technique is employed, yielding an increase in the limit of detection by a factor of 22. The sensor's performance was evaluated by detecting nitric oxide at 5.26 µm, confirming the feasibility of separating the pump and probe sections of the device. The sensor reached a detection limit of 9.6 parts-per-billion by volume for an integration time of 136 s, with only a 20 cm-long laser-gas molecules interaction path length.

Shot-noise limited homodyne detection for MHz quantum light characterisation in the 2 µm band

Characterising quantum states of light in the 2 µm band requires high-performance shot-noise limited detectors. Here, we present the characterisation of a homodyne detector that we use to observe vacuum shot-noise via homodyne measurement with a 2.07 µm pulsed mode-locked laser. The device is designed primarily for pulsed illumination. It has a 3-dB bandwidth of 13.2 MHz, total conversion efficiency of 57% at 2.07 µm, and a common-mode rejection ratio of 48 dB at 39.5 MHz. The detector begins to saturate at 1.8 mW with 9 dB of shot-noise clearance at 5 MHz. This demonstration enables the characterisation of megahertz-quantum optical behaviour in the 2 µm band and provides a guide of how to design a 2 µm homodyne detector for quantum applications.

Compact photothermal self-mixing interferometer for highly sensitive trace detection

A self-mixing interferometer combined with the photothermal spectroscopy is utilized as a remarkable sensor for highly sensitive trace detection, featuring the beneficial property of a He-Ne laser with back-mounted photodiode, to the best of our knowledge, acting as an excitation laser, also as a probe laser, and even more, as a detector. Utilizing the novel implementation of the photothermal self-mixing (PTSM) interferometer with an external cavity modulation, the concentration of the sample is directly measured by the PTSM parameter extracted from the PTSM signal. The metrological qualities of the PTSM interferometer were investigated by methylene blue trace detection. For a low excitation power of 5 mW, a 7.7 nM of the limit of detection was achieved with a relative standard deviation of ∼3%. The compact and simple structure with high sensitivity has guiding significance to a robust analytical tool for the analysis of photosensitive compounds and in the detection of aquatic product hazards in aquaculture.

On the Limit of Detection in Infrared Spectroscopic Imaging

Infrared (IR) spectroscopic imaging instruments' performance can be characterized and optimized by an analysis of their limit of detection (LOD). Here we report a systematic analysis of the LOD for Fourier transform IR (FT-IR) and discrete frequency IR (DFIR) imaging spectrometers. In addition to traditional measurements of sample and blank data, we propose a decision theory perspective to pose the determination of LOD as a binary classification problem under different assumptions of noise uniformity and correlation. We also examine three spectral analysis approaches, namely, absorbance at a single frequency, average of absorbance over selected frequencies and total spectral distance - to suit instruments that acquire discrete or contiguous spectral bandwidths. The analysis is validated by refining the fabrication of a bovine serum albumin protein microarray to provide eight uniform spots from ∼2.8 nL of solution for each concentration over a wide range (0.05-10 mg/mL). Using scanning parameters that are typical for each instrument, we estimate a LOD of 0.16 mg/mL and 0.12 mg/mL for widefield and line scanning FT-IR imaging systems, respectively, using the spectral distance approach, and 0.22 mg/mL and 0.15 mg/mL using an optimal set of discrete frequencies. As expected, averaging and the use of post-processing techniques such as minimum noise fraction transformation results in LODs as low as ∼0.075 mg/mL that correspond to a spotted protein mass of ∼112 fg/pixel. We emphasize that these measurements were conducted at typical imaging parameters for each instrument and can be improved using the usual trading rules of IR spectroscopy. This systematic analysis and methodology for determining the LOD can allow for quantitative measures of confidence in imaging an analyte's concentration and a basis for further improving IR imaging technology.

Part-per-billion level photothermal nitric oxide detection at 5.26 µm using antiresonant hollow-core fiber-based heterodyne interferometry

In this work, I demonstrate a novel configuration of a photothermal gas sensor. Detection of nitric oxide at a wavelength of 5.26 µm was possible by constructing an absorption cell based on a self-fabricated antiresonant hollow core fiber characterized by low losses at both the pump and probe wavelengths. Proper design of the sensor allowed using the heterodyne interferometry-based signal readout of the refractive index modulation, which yielded a record noise equivalent absorption of 2.81×10^-8 cm^-1 for 100 s integration time for mid-infrared fiber-based gas sensors. The obtained results clearly demonstrate the full potential of using properly designed antiresonant hollow core fibers in combination with sensitive gas detection methods.