Active modulation of intracavity laser intensity with the Pound-Drever-Hall locking for photoacoustic spectroscopy

Modulation-free laser stabilization technique using integrated cavity-coupled Mach-Zehnder interferometer

Stable lasers play a significant role in precision optical systems where an electro-optic laser frequency stabilization system, such as the Pound-Drever-Hall technique, measures laser frequency and actively stabilizes it by comparing it to a frequency reference. Despite their excellent performance, there has been a trade-off between complexity, scalability, and noise measurement sensitivity. Here, we propose and experimentally demonstrate a modulation-free laser stabilization method using an integrated cavity-coupled Mach-Zehnder interferometer as a frequency noise discriminator. The proposed architecture maintains the sensitivity of the Pound-Drever-Hall architecture without the need for any modulation. This significantly simplifies the architecture and makes miniaturization into an integrated photonic platform easier. The implemented chip suppresses the frequency noise of a semiconductor laser by 4 orders-of-magnitude using an on-chip silicon microresonator with a quality factor of 2.5 × 106. The implemented passive photonic chip occupies an area of 0.456 mm2 and is integrated on AIM Photonics 100 nm silicon-on-insulator process.

Clamp-type quartz tuning fork enhanced photoacoustic spectroscopy

In this Letter, clamp-type quartz tuning fork enhanced photoacoustic spectroscopy (Clamp-type QEPAS) is proposed and realized through the design, realization, and testing of clamp-type quartz tuning forks (QTFs) for photoacoustic gas sensing. The clamp-type QTF provides a wavefront-shaped aperture with a diameter up to 1 mm, while keeping Q factors > 10⁴. This novel, to the best of our knowledge, design results in a more than ten times increase in the area available for laser beam focusing for the QEPAS technique with respect to a standard QTF. The wavefront-shaped clamp-type prongs effectively improve the acoustic wave coupling efficiency. The possibility to implement a micro-resonator system for clamp-type QTF is also investigated. A signal-to-noise enhancement of ∼30 times has been obtained with a single-tube acoustic micro resonator length of 8 mm, ∼20% shorter than the dual-tube micro-resonator employed in a conventional QEPAS system.

High-sensitivity miniature dual-resonance photoacoustic sensor based on silicon cantilever beam for trace gas sensing

We report a miniature dual-resonance photoacoustic (PA) sensor, mainly consisting of a small resonant T-type PA cell and an integrated sensor probe based on a silicon cantilever beam. The resonance frequency of the miniature T-type PA cell is matched with the first-order natural frequency of the cantilever beam to achieve double resonance of the acoustic signal. The volume of the designed T-type PA cell is only about 2.26 cubic centimeters. A PA spectroscopy (PAS) system, employing the dual-resonance photoacoustic (PA) sensor as the prober and a high-speed spectrometer as the demodulator, has been implemented for high-sensitivity methane sensing. The sensitivity and the minimum detection limit can reach up to 2.0 pm/ppm and 35.6 parts-per-billion, respectively, with an averaging time of 100 s. The promising performance demonstrated a great potential of employing the reported sensor for high-sensitivity gas sensing in sub cubic centimeter-level spaces.

Sensitive and single-shot OH and temperature measurements by femtosecond cavity-enhanced absorption spectroscopy

In many low-temperature plasmas (LTPs), the OH radical and temperature represent key properties of plasma reactivity. However, OH and temperature measurements in weakly ionized LTPs are challenging, due to the low concentration and short lifetime of OH and the abrupt temperature rise caused by fast gas heating. To address such issues, this Letter combined cavity-enhanced absorption spectroscopy (CEAS) with femtosecond (fs) pulses to enable sensitive single-shot broadband measurements of OH and temperature with a time resolution of ∼180 ns in LTPs. Such a combination leveraged several benefits. With the appropriately designed cavity, an absorption gain of ∼66 was achieved, enhancing the actual OH detection limit by ∼55× to the 10¹¹ cm^-3 level (sub-ppm in this work) compared with single-pass absorption. Single-shot measurements were enabled while maintaining a time resolution of ∼180 ns, sufficiently short for detecting OH with a lifetime of ∼100 μs. With the broadband fs laser, ∼34,000 cavity modes were matched with ∼95 modes matched on each CCD pixel bandwidth, such that fs-CEAS became immune to the laser-cavity coupling noise and highly robust across the entire spectral range. Also, the broadband fs laser allowed simultaneous sensing of many absorption features to enable simultaneous multi-parameter measurements with enhanced accuracies.

Mid-infrared intracavity quartz-enhanced photoacoustic spectroscopy with pptv - Level sensitivity using a T-shaped custom tuning fork

Resonant optical power buildup inside a high finesse cavity is exploited to boost the sensitivity in quartz-enhanced photoacoustic spectroscopy (QEPAS) for CO, N₂O and H₂O detection, operating at a wavelength of 4.59 µm. A quartz tuning fork with T-shaped prongs optimized for QEPAS has been employed. Exploiting the high signal-to-noise ratio attainable with this tuning fork together with an optical power amplification of ~100 enabled by efficient optical feedback locking, limits of detection (3σ, 10 s integration) of 260 ppt and 750 ppt for CO and N₂O have been reached.

All-optical high-sensitivity resonant photoacoustic sensor for remote CH4 gas detection

This paper presents an all-optical high-sensitivity resonant photoacoustic (PA) sensor to realize remote, long-distance and space-limited trace gas detection. The sensor is an integration of a T-type resonant PA cell and a particular cantilever-based fiber-optic acoustic sensor. The finite element simulations about the cantilever vibration mode and the PA field distributions are carried out based on COMSOL. The all-optical high-sensitivity resonant PA sensor, together with a high-speed spectrometer and a DFB laser source, makes up of a photoacoustic spectroscopy (PAS) system which is employed for CH₄ detection. The measured sensitivity is 0.6 pm/ppm in the case of 1000 s average time, and the minimum detection limit (MDL) reaches 15.9 parts per billion (ppb). The detective light source and the excitation light source are all transmitted by optical fibers, therefore remote and long-distance measurement of trace gas can be realized. Furthermore, the excitation light source and the acoustic sensor are designed at the same side of the PA cell, the sensor may be used for space-limited trace gas detection.