A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2024.240077 , discusses agile cavity ringdown spectroscopy enabled by moderate optical feedback to a quantum cascade laser.

Optical Feedback is an important laser control technology that can produce a rich variety of nonlinear dynamic behaviors through the coupling of the optical field reflected by a mirror with the laser's optical field. Appropriate optical feedback can narrow the laser linewidth and stabilize the laser frequency, playing a crucial role in precision spectroscopy. Cavity ring-down spectroscopy (CRDS) with optical feedback allows for sensitive and precise trace gas detection. However, commonly used semiconductor lasers are very sensitive to feedback light, and only by precisely controlling the intensity and phase of the feedback light can the effect of linewidth reduction be achieved. Weak optical feedback (feedback rate much less than 1%) is normally required, and more importantly, it is necessary to ensure coherent superposition of the feedback light with the laser’s optical field. When the feedback intensity is too high (feedback rate larger than 0.1%-1%), the laser can enter a chaotic state, leading to a dramatic increase in laser linewidth. Therefore, semiconductor laser spectroscopy typically uses optical isolators to avoid excessive optical feedback.

Quantum Cascade Laser (QCL) is a semiconductor laser source based on intersubband transitions in multiple quantum wells, with the emission wavelength determined by the energy difference between subbands. The wavelength can be controlled by designing the thickness of the quantum well layers. Mid-infrared QCLs can cover the fingerprint spectral region of many gas molecules, and their compact size, low power consumption, and high precision in wavelength tuning make them highly suitable for gas analysis, environmental monitoring, and medical diagnostics. Unlike traditional semiconductor lasers, QCLs have a linewidth broadening factor close to zero, allowing them to remain stable under moderate and strong optical feedback conditions. This feature makes QCLs promising for addressing the current bottleneck in optical feedback laser spectroscopy systems that require complex phase-locking loops.

On the occasion of the 30^th anniversary of the invention of the QCL, Professor Wei Ren's research group from The Chinese University of Hong Kong, in collaboration with Professor Cheng Wang's research group from ShanghaiTech University, proposed a method for narrowing the QCL linewidth via optical feedback without the need for any phase-locking loops. This approach not only reduces the complexity of traditional CRDS systems, but also significantly enhances the measurement signal-to-noise ratio (SNR), providing a new insight for mid-infrared trace gas detection.

Figure 1 depicts the schematic diagram of the proposed QCL-based CRDS with moderate optical feedback. The QCL beam undergoes polarization control using a half-wave plate (HWP), and is then split by an acousto-optic modulator (AOM): the zero-order (undiffracted) portion is used for optical feedback, while the first-order beam is sent to a Fabry–Perot cavity for CRDS measurements. A plane mirror (PM) reflects the zero-order beam to the QCL, wherein the optical feedback rate can be finely adjusted via a polarizing beamsplitter (PBS) and monitored by a power meter (P) to measure the idle light. The first-order diffraction used for CRDS detection is injected into the Fabry–Perot cavity, which comprises two plano-concave dielectric mirrors. To ensure the ringdown events occur at each laser frequency, the longitudinal mode of the cavity is swept using the PZT that is attached to one of the cavity mirrors. When the laser-cavity resonance is attained, the AOM rapidly cuts off the incident light, and the ringdown signal is captured by a photodetector (PD).

The researchers quantitatively investigated the effect of linewidth reduction using the passive optical feedback approach. To assess the QCL linewidth, the laser frequency noise is converted into intensity noise by tuning the laser frequency to an absorption line of CO and the frequency-intensity conversion factor is shown in Fig. 2a. Figure 2b illustrates that the free-running QCL exhibits a linewidth of about 1.2 MHz, decreasing to a minimum of 170 kHz at a feedback rate of -16.8 dB. Figure 2c plots the cavity-transmitted signal at different feedback rates. The cavity-transmitted signal is enhanced significantly at the feedback rate of -18.2 dB and -14.8 dB, which proves that the moderate optical feedback without any phase-locking loops can reduce the QCL linewidth, increase the coupling efficiency, and thus significantly improve the SNR of the ringdown event.

To verify the spectral measurement performance, the researchers measured the absorption line of nitrous oxide (N₂O), a greenhouse gas, at 2207.62 cm⁻¹ (shown in Fig. 3a). The measured spectrum can be well fitted by the Lorentzian function with a fitting residual within 3%, showing the high-precision of spectral measurements. In addition, the comparison of the measured concentrations of N₂O with the nominal concentrations in the range of 1-10 ppb validates the measurement accuracy of the proposed method (shown in Fig. 3b).

This research addresses the bottleneck issues of complexity and poor reliability in field applications associated with traditional optical feedback CRDS systems. It provides new insights for ultra-sensitive gas detection based on QCLs. This method holds the potential to be extended to other types of mid-infrared semiconductor lasers, such as interband cascade lasers, making it highly valuable for practical applications.

Keywords: cavity ringdown spectroscopy / optical feedback / quantum cascade laser / gas sensing
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The Advanced Laser Diagnostics Laboratory at The Chinese University of Hong Kong (CUHK) was established in 2014, focusing on laser spectroscopy, optical fiber sensing, and combustion diagnostics. The team is led by Dr. Wei Ren, who is currently a Professor in the Department of Mechanical and Automation Engineering, Director of Energy and Environmental Engineering Program, and Assistant Dean of the Faculty of Engineering at CUHK. Dr. Ren has led over 30 research projects funded by National Natural Science Foundation of China, Hong Kong Research Grants Council, and the Innovation and Technology Commission of Hong Kong. He has published more than 130 papers in academic journals such as Nature Communications, Light: Science & Applications, and Opto-Electronic Advances. Dr. Ren serves as the associate chair of the Laser Spectroscopy Committee of the Optical Society of China, the co-editor of Applied Physics B, the senior member of Optica, the section co-chair of the International Conference on Information Optics and Photonics (CIOP), and committee member of Optica Sensing Congress and FiOLS. Homepage: https://www.rengroup.mae.cuhk.edu.hk/


Professor Cheng Wang, Associate Professor, Doctoral Supervisor at ShanghaiTech University. He received his Ph.D. degree in Optoelectronics from Institut National des Sciences Appliquées de Rennes, France in 2015 and then joined City University of Hong Kong as a Senior Research Assistant. He joined the School of Information and Science and Technology of ShanghaiTech University in 2016. He was a visiting researcher at Télécom ParisTech, France, Technische Universität Berlin, Germany, and Politecnico di Torino, Italy, respectively. His research interests include reservoir computing, chaotic lasers and low-noise lasers. he has published more than 50 journal papers, including Light: Science & Applications and Optica, and more than 80 international conference papers. He is a member of the Editorial Advisory Board of APL Machine Learning (2024-now) and a member of the program committee of SPIE Photonics West.Homepage: https://shanghaitech-sist-chengwang.myfreesites.net/

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Nie QX, Peng YB, Chen QH et al. Agile cavity ringdown spectroscopy enabled by moderate optical feedback to a quantum cascade laser. Opto-Electron Adv 7, 240077 (2024). doi: 10.29026/oea.2024.240077