Cavity ringdown spectroscopy (CRDS), first invented for use with pulsed lasers
Opto-Electronic Advances, Volume. 7, Issue 11, 240077-1(2024)
Agile cavity ringdown spectroscopy enabled by moderate optical feedback to a quantum cascade laser
Cavity ringdown spectroscopy (CRDS), relying on measuring the decay time of photons inside a high-finesse optical cavity, offers an important analytical tool for chemistry, physics, environmental science, and biology. Through the reflection of a slight amount of phase-coherent light back to the laser source, the resonant optical feedback approach effectively couples the laser beam into the optical cavity and achieves a high signal-to-noise ratio. However, the need for active phase-locking mechanisms complicates the spectroscopic system, limiting its primarily laboratory-based use. Here, we report how passive optical feedback can be implemented in a quantum cascade laser (QCL) based CRDS system to address this issue. Without using any phase-locking loops, we reflect a moderate amount of light (–18.2 dB) to a continuous-wave QCL simply using a fixed flat mirror, narrowing the QCL linewidth from 1.2 MHz to 170 kHz and significantly increasing the laser-cavity coupling efficiency. To validate the method’s feasibility and effectiveness, we measured the absorption line (P(18e), 2207.62 cm?1) of N2O in a Fabry–Perot cavity with a high finesse of ~52000 and an inter-mirror distance of 33 cm. This agile approach paves the way for revolutionizing existing analytical tools by offering compact and high-fidelity mid-infrared CRDS systems.
Introduction
Cavity ringdown spectroscopy (CRDS), first invented for use with pulsed lasers
where α is the absorption coefficient of the gas inside the cavity, c is the speed of light, and τ0 and τ are the ringdown time of the cavity in vacuum and the presence of gas, respectively. Hence, CRDS is less sensitive to the intensity noise of the laser source compared to direct absorption spectroscopy. Over the past decades, CRDS has been well adapted to different types of lasers such as distributed feedback (DFB) diode lasers, external cavity diode lasers (ECDLs), quantum cascade lasers (QCLs) and interband cascade lasers (ICLs), for many spectroscopic and sensing applications
High-sensitivity CRDS typically employs an optical cavity with a higher finesse or a larger inter-mirror distance. This evitably reduces the transmission width of the cavity mode down to kHz level, significantly narrower than the linewidth of single-mode tunable lasers (i.e., DFB lasers), normally in the MHz range. Hence, only a fraction of the incident light can be effectively coupled into the optical cavity, leading to a noisy or distorted output signal
Utilizing optical feedback, which reflects emitted light coherently back into the electromagnetic field inside the laser cavity, proves to be an effective approach for linewidth reduction or laser stabilization
Moderate (φ = −30 dB to −10 dB) or strong (φ > −10 dB) optical feedback is mostly avoided in optical systems due to the risk of coherence collapse, which easily destabilizes the laser and significantly broadens the laser linewidth
In this work, we demonstrate an agile method for performing low-noise CRDS by employing moderate optical feedback into a QCL. Using a continuous-wave DFB-QCL with MHz-level linewidth and sub-mW emission power, we seed it with moderate optical feedback (−18.2 dB), leading to a significant linewidth narrowing without the need for any phase control. As a proof-of-concept experiment, we tune the QCL wavelength across a mid-infrared absorption line (4.53 μm) of nitrous oxide (N2O), which is diluted to ppb levels and filled in a high-finesse (~52000) optical cavity. Our method paves the way for the advancement of next-generation CRDS using QCLs or other semiconductor lasers, offering a straightforward configuration suitable for a wide range of spectroscopic applications.
Principle and experimental setup
Figure 1.
Results and discussion
Characterization of QCL linewidth and ringdown signal
We first quantitatively investigated the effect of linewidth reduction using the passive optical feedback approach. To assess the QCL linewidth
Figure 2.
We then directed the first-order QCL beam into the optical cavity, as shown in
Figure 3.
To showcase the effectiveness of the proposed method, we reduced the QCL power to a very low level of 0.4 mW by using a lower injection current to investigate the capability of observing the ringdown events under low-power conditions. Upon the photodetector signal reaching the threshold voltage (i.e., 80 mV), the incident laser was promptly interrupted by the AOM to initiate an intracavity ringdown.
Figure 4.
Spectrometer performance
The QCL-based CRDS system is characterized by measuring the absorption line of N2O. Over the spectral range covered by the DFB-QCL (2207-2212 cm−1), we aim to exploit the strong N2O absorption line, P(18e), centered at 2207.62 cm−1. This particular absorption line exhibits minimal spectral interference from other atmospheric molecules, which is illustrated in Supplementary information Section 3 through spectral simulation. To demonstrate the high sensitivity and reliability of the developed CRDS system, we conducted measurements of ppb-level N2O mixtures by diluting a N2O/N2 cylinder with a certified volume fraction of 1.2 ppm N2O. However, we found that the ultrahigh-purity (> 99.999%) N2 gas cylinder used for dilution contains about 74 ppb N2O and 85 ppb CO as impurities (see Supplementary information Section 4). A similar discovery of impurities in ultrahigh-purity N2 gas cylinders has also been reported previously
Figure 5.
Conclusion
In conclusion, our research demonstrates that moderate optical feedback to a QCL enables agile and low-noise CRDS. By employing an AOM to split the QCL beam into two paths, we used the zero-order beam for optical feedback and directed the first-order diffraction to the optical cavity for ringdown measurements. Without using any active phase control of the reflected light, we demonstrated a considerable reduction in linewidth using moderate optical feedback (–18.2 dB). We verified the effectiveness of the spectrometer by resolving a mid-infrared absorption line of ppb-level N2O with a high SNR. Considering the similar behavior of ICLs
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Qinxue Nie, Yibo Peng, Qiheng Chen, Ningwu Liu, Zhen Wang, Cheng Wang, Wei Ren. Agile cavity ringdown spectroscopy enabled by moderate optical feedback to a quantum cascade laser[J]. Opto-Electronic Advances, 2024, 7(11): 240077-1
Category: Research Articles
Received: Apr. 5, 2024
Accepted: Aug. 6, 2024
Published Online: Feb. 21, 2025
The Author Email: Wang Cheng (WRen), Ren Wei (CWang)