Mid-infrared supercontinuum-based upconversion detection for trace gas sensing

: Recent advancements of mid-infrared (MIR) supercontinuum light sources have opened up new possibilities in laser-based trace gas sensing. While the supercontinuum sources inherently support wide spectral coverage, the detection of broadband absorption signals with high speed and low cost is traditionally limited by the MIR detector arrays. In this work, we demonstrate that this limitation can be circumvented by upconverting the MIR signal into the near-infrared (NIR) region, where cost-effective silicon-based detector arrays can be utilized to measure broadband absorption. We also show that, by combining a MIR supercontinuum source with a MIR-to-NIR upconverter and an astigmatic multipass cell, fast detection (~20 ms) of ethane with sub-ppmv sensitivity can be achieved at room temperature. For multi-species detection, a least-square global fitting method is presented, showing a promising potential for applications such as environmental monitoring and biomedical research.


Introduction
The detection of gas-phase molecular species at low concentration levels supports a wide variety of industrial, environmental, and biological applications.Among various types of detection methods such as mass spectrometry and chemical sensing, MIR laser absorption spectroscopy provides an essential opportunity for selective, sensitive, and non-destructive detection [1][2][3][4][5].On one hand, the characteristic rotational-vibrational transitions generally show very strong absorption patterns in the so-called "MIR fingerprint region".On the other hand, the fast development of stable, bright and compact laser sources allows the detection of these patterns at high speed and relatively low cost.Therefore, trace gas sensing based on laser absorption techniques is a fast progressing field.
Nowadays, there are various types of MIR light sources that are commercially available for trace gas sensing, such as quantum cascade lasers [6,7], interband cascade lasers [8], and nonlinear sources such as difference frequency generation sources [9], as well as optical parametric oscillators [10,11].The narrow linewidth and high brightness of these light sources make them ideal for measuring the individual absorption lines of some molecular species.However, for the detection of larger molecules, which can have overlapping rotational absorption lines within a vibrational transition band causing broad absorption, either scanning detection or multiple-laser integration is generally adopted, inevitably reducing the detection speed or increasing the instrumental cost.Furthermore, it is even more challenging to detect simultaneously multiple molecular species having overlapping absorption patterns.Consequently, it is advantageous to apply a broadband light source to measure broadband absorption patterns in order to increase the detection speed and to reduce the cost.
Traditional MIR broadband sources such as globars may be considered, but the low spectral power density and poor directionality limit their practical applications.More recent advancement in MIR frequency combs shows more promise [12], covering the molecular fingerprint region with high brightness comparable to synchrotron sources [13].New detection methods such as dual comb spectroscopy [14] and Vernier spectroscopy [15] have been developed in the MIR region, and both broad spectral coverage and high detection sensitivity have been demonstrated.However, the system complexity and cost are still significantly higher, as compared to conventional methods.
Another type of broadband MIR light source emerging as a new candidate for absorption spectroscopy is the supercontinuum (SC) source [16,17].The high brightness of this source makes it superior as compared to synchrotron sources [18], although it generally does not show the shot-to-shot coherency of the frequency combs.In fact, the state-of-the-art SC source can also provide improved coherency [19].Nevertheless, since high coherency is not a prerequisite for direct absorption spectroscopy, the phase noise does not require sophisticated opto-electronic control while generating the SC beam, thus making the SC sources promising for broadband MIR spectroscopy in terms of system complexity and cost.A SC source providing an ultra-broad 1.4 -13.3 μm spectral coverage has been reported by Petersen and associates in 2014, largely covering the MIR "fingerprint region" [20].Furthermore, bright SC sources providing >200 μW/nm spectral power density have recently become commercially available, supporting the development of SC-based trace gas sensing.
While the development of MIR SC sources is remarkably fast in recent years, sensitive detection of the broadband absorption signal at high speed and low cost is still challenging.Standard measurement methods involve the use of a Fourier transform spectrometer [21] or a scanning-grating-based spectrometer [22].However, these methods are typically limited in acquisition speed due to mechanical movement.The utilization of a photodetector array, rather than a single-point detector, can significantly improve the measurement speed, as all the spectral elements can be captured simultaneously across the spectrum [23,24].However, unlike in the visible and NIR regions, fast and sensitive MIR detector arrays still require liquid nitrogen or Stirling cooling, and such detectors are consequently less energy efficient and less cost effective.
In spite of the large effort to develop the MIR detector arrays for direct measurement, the above limitation can be actually circumvented by upconverting the MIR signals into the NIR region, leveraging the mature silicon technology.Historically, this approach was viewed as inefficient due to the low conversion quantum efficiency.However, following a recent intracavity upconversion breakthrough achieving ~20% quantum efficiency at room temperature [25], an upconversion-enabled array spectrometer has been reported [26].Methane detection was also demonstrated [27], although covering only a narrow spectral window.For broadband detection, an aperiodically poled ZnO:LiNbO 3 waveguide was utilized to upconvert light in the range of 2.5 -4.5 μm [28].More recently, the intra-cavity upconversion technique was also applied for broadband applications [29], leading to a successful commercialization of upconversion-based MIR spectrometers, which also opens up the possibility for practical broadband trace gas sensing applications.
In the present work, we combine the unique advantages of the MIR SC source and the MIR-to-NIR upconversion technique to develop a broadband trace gas sensor.Fast detection (~20 ms) of ethane with ~15 ppb•s −1 sensitivity was achieved.For multi-species measurement, an ultra-broad (>1300 nm) spectral range was obtained.A comprehensive analysis algorithm was developed, showing the promising potential for broadband trace gas analysis.

Experimental setup
In order to measure broadband absorption features in the MIR region, a broadband light source is required, preferably with high power and high directionality.In general, such an output can be optical fibers generation) [

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Funding
The FLAIR project of the HORIZON 2020 program (732968), the QCAP project of the Interreg North-West Europe program (NWE 363), and the division of Applied and Engineering Sciences associated with the Netherlands Organization for Scientific Research (14709).