Trace detection of hydrazines by optical homodyne interferometry

Waveguide based passively demodulated photothermal interferometer for light absorption measurements of trace substances

In this contribution, we present the development of a passively demodulated interferometer based on 3×3 waveguide couplers to measure light absorption of trace gases and aerosol particles via the photothermal effect. In contrast to a "classical" interferometer with two outputs, active quadrature control is not required to ensure a high sensitivity of the system. An algorithm for the evaluation of the photothermal interferometry signal from the outputs of asymmetric 3×3 couplers is detailed. The performance of the algorithm is demonstrated with NO₂ calibration experiments using couplers with different working principles (i.e., fused-fiber and planar-waveguide based). The results of a laboratory measurement campaign using aerosolized nigrosin are discussed, and the measured aerosol absorption is compared to a reference instrument. A noise analysis shows interferometer phase noise to be the primary noise component. Improvements to the setup are recommended, which should improve the current instrumental detection limit in terms of absorption coefficient to below the current value of 100Mm^-1 (1σ, 60 s). This corresponds to mass concentrations of about 10µg/m³ for submicrometer-size black carbon particles.

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

The interferometric cavity-assisted photothermal spectroscopy (ICAPS) method has been proven highly suitable for sensitive and compact gas detection by application of an optical cavity as transducer for photothermal spectroscopy. This work reports on the implementation of an overall fiber-coupled probe laser configuration detecting the reflectance of the individual interferometers in a balanced-detection ICAPS system. The layout greatly improves the overall sensor system robustness. Two identical 1 mm path length cavities were used for balanced detection, enabling sensor operation close to the fundamental limit of shot noise by efficiently cancelling excess noise. A quantum cascade laser served as a mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced refractive index variations. The metrological figures of merit for the sensor were investigated by SO₂ detection. For the targeted absorption band centered at 1380.93 cm^-1, a 3 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 4.5 × 10^-9 cm^-1 W Hz^-1/2.

Balanced-detection interferometric cavity-assisted photothermal spectroscopy

An optical cavity can be utilized as an excellent transducer for highly sensitive gas detection with the application of photothermal spectroscopy, featuring the beneficial property of an ultra-low absorption volume within a rugged sensing element. We report the novel implementation of balanced detection in Fabry-Perot photothermal interferometry via two identical 1 mm-spaced cavities. That way, excess noise limiting the sensitivity of previous cavity-based photothermal sensors was effectively rejected close to the fundamental limit of shot noise. A quantum cascade laser served as mid-infrared excitation source to induce refractive index changes in the sample, and a near-infrared fiber laser served as probe source to monitor the photo-induced variations. The metrological qualities of the sensor were investigated by SO₂ detection. For the targeted absorption band centered at 1380.93 cm^-1, a 5 ppbv minimum detection limit was achieved with a 1 s integration time, corresponding to a normalized noise equivalent absorption of 7.5 × 10^-9 cm^-1 W Hz^-1/2. Additionally, the sensor showed excellent long-term stability, enabling integration times of a few thousand seconds.

2f-wavelength modulation Fabry-Perot photothermal interferometry

Trace gas detection was performed by the principle of photothermal interferometry using a Fabry-Perot interferometer combined with wavelength modulation and second harmonic detection. The sensor employed a compact, low-volume gas cell in an overall robust set-up without the use of any moveable part. A quantum cascade laser was used as powerful mid-infrared excitation source to induce refractive index changes in the sample, whereas a near-infrared laser diode served as probe source to monitor the photo-induced variations. The functional principle of the selective sensor was investigated by detection of sulfur dioxide. For the targeted absorption band centered at 1379.78 cm^-1 a 1 σ minimum detection limit of about 1 parts per million by volume was achieved. The work demonstrates high potential for further sensor miniaturization down to a sample volume of only a few mm³. Limitations and possible improvements of the sensor regarding sensitivity are discussed.

Ultra-sensitive all-fibre photothermal spectroscopy with large dynamic range

Photothermal interferometry is an ultra-sensitive spectroscopic means for trace chemical detection in gas- and liquid-phase materials. Previous photothermal interferometry systems used free-space optics and have limitations in efficiency of light–matter interaction, size and optical alignment, and integration into photonic circuits. Here we exploit photothermal-induced phase change in a gas-filled hollow-core photonic bandgap fibre, and demonstrate an all-fibre acetylene gas sensor with a noise equivalent concentration of 2 p.p.b. (2.3 × 10⁻⁹ cm⁻¹ in absorption coefficient) and an unprecedented dynamic range of nearly six orders of magnitude. The realization of photothermal interferometry with low-cost near infrared semiconductor lasers and fibre-based technology allows a class of optical sensors with compact size, ultra sensitivity and selectivity, applicability to harsh environment, and capability for remote and multiplexed multi-point detection and distributed sensing.

Photothermal interferometry systems using free-space optics have limits in terms of light–matter interaction efficiency, size, optical alignment and integration. Here, Jin et al. use a gas-filled hollow-core photonic bandgap fibre to demonstrate an all-fibre gas sensor with ultrahigh sensitivity and dynamic range.

Folded Jamin interferometer: a stable instrument for refractive-index measurements

A novel two-beam interferometer for the measurement of a refractive index or its induced changes is described. This interferometer consists of only two optical elements and is largely insensitive to their movement. A laboratory prototype has been built. It uses a polarized version of the folded Jamin interferometer to allow for convenient phase adjustment.

Ultrasensitive spectral trace detection of individual molecular components in an atmospheric binary mixture

We present what is, to our knowledge, the first demonstration of the application of the laser homodyne interferometric technique to the quantitative identification of individual trace molecular constituents of a binary mixture in an ambient atmospheric background. Operation of the laser interferometric detection system to within a factor of 26 of the theoretical quantum noise limit, without extensive vibrationisolation, is observed. We realize the spectral identification of SF(6) and CF(2)Cl(2) mixed in various trace concentrations, without significant cross interference, using molecular spectral features overlapping the 10P CO(2) laser transitions.