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Abstract
Single-mode tunable quantum cascade lasers (QCLs) are promising for high-resolution and highly sensitive trace gases sensing across the mid-infrared (MIR) region. We report on the development of a tunable single-mode slot waveguide QCL array in the long wavelength part of the MIR regime (>12 µm). This laser array exhibits a tuning range of around 12 cm-1, from 735.3 to 747.3 cm-1. Using this developed single-mode tunable QCL, we demonstrate individual gas sensing, yielding the detection limit of 940 ppb and 470 ppb for acetylene and o-xylene, respectively. To verify the potential of the developed QCL array in multi-species gas detection, laser absorption measurements of two mixed gases of acetylene and o-xylene were conducted, showing the absorption features of the corresponding gases agree well with the theoretical predictions.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-infrared (MIR) laser absorption spectroscopy has been developed as an impactful and essential tool in the MIR molecular fingerprint region. Owing to a large amount of distinguished vibrational transition bands in this range, numerous technologies and applications related to MIR spectroscopy have been demonstrated, including trace gas sensing [1], breath analysis [2], fundamental physics [3], protein chemistry [4], atmospheric reactions [5], and other implementations like security [6] and imaging [7].
Thanks to the invention of quantum cascade laser (QCL) and its great progress in the recent years [8,9], trace gas sensing utilizing QCL sources has achieved high precision in the parts-per-trillion (ppt) range [10], high resolution up to 10−4 cm-1 [11], and high speed with sub-microsecond time resolution [4], in the wavelength range in the MIR below 12 µm. Despite this, due to the lack of high performance, widely tunable laser sources, trace gas spectroscopy in the long wavelength MIR range (>12 µm) has not been extensively explored.
Long wavelength MIR is attractive for gas sensing applications, because many molecules show strong absorptions of vibrational transitions. For example, acetylene (C2H2), which is an essential intermediate product from the hydrocarbon fuels [12], has primary absorption features from the near-infrared (NIR) to the long wavelength MIR range, and it shows the strongest absorption features near 730 cm-1 based on HIRTAN database [13]. More importantly, some volatile organic compounds pollutants from industrial processes, such as benzene, toluene, ethylbenzene, and o-xylenes, which are normally called BTEX, show strong absorption features in this region [14]. Compared with the short wavelength range, there are some inherent challenges in realizing high power long wavelength QCLs, such as the lower optical gain and higher waveguide loss. Therefore, many efforts have been done to find alternative laser sources. For instance, based on difference frequency generation (DFG), wide tuning range DFG lasers covering 12–15 µm have been reported [15,16]. However, the complicated optical setup and their low optical powers (below 110 µW) due to the limitation of nonlinear conversion efficiency, hinder the potential for compact and high sensitive trace gas sensing applications. With the distributed feedback (DFB) QCLs, the detection of acetylene has been studied previously [12,17]. Nevertheless, the wavelength tuning of the DFB QCL largely relies on adjusting the temperature of the laser chip, and a relatively small temperature tuning coefficient of 0.05 cm-1/°C is reported in Ref. [17]. Therefore, to meet the requirements of multi-species trace gas sensing, demonstration of a single-mode source with a wider tunable range in the long wavelength is appealing.
In this paper, we demonstrate a room temperature tunable single-mode QCL array from 735.3 to 747.3 cm-1, with a peak optical output power up to 200 mW. To overcome the QCL power degradation in the long wavelength range, the QCL epitaxial wafer is based on the high performance diagonal transition and three-phonon-resonance design [18]. The implementation of the tunable single-mode QCL operation is based on the two-section slot waveguide [19,20]. Compared with other types of single-mode tunable lasers, like distributed feedback QCLs [21] and external cavity QCLs [22], the slot waveguide QCLs show the advantages of the wide tuning range, free of external mechanical components, and uncomplicated fabrication process, which is potential for mass production. We applied our laser array for the measurements of C2H2 and o-xylene, and the mixture of these two gases. The detection limit of sub-ppm level is achieved for single gas measurement, and the absorption peaks of C2H2 and o-xylene are clearly identified in the reconstructed mixture gas absorption spectrum.
2. Experimental section
2.1 Tunable single-mode slot QCL array fabrication and characterization
The QCL epitaxial layers consist of lattice matched In0.53Ga0.47As quantum wells and In0.52Al0.48As barriers. The active region is based on our previous development of high performance long wavelength QCL design [18]. The slot waveguide design follows the same principle in [19,20] (details design parameters in Supplement 1). The slot pattern was defined using the standard lithography process. The slots were etched using the inductively coupled plasma reactive ion etching (ICP-RIE) in an Ar/Cl2 atmosphere to the depth of 3.5 µm. In Fig. 1, the laser array consists of five ridges, which were also defined using lithography and etched using ICP-RIE to the depth of 14 µm. 500 nm SiO2 isolation layer was deposited using plasma-enhanced chemical vapor deposition (PECVD). After window opening, the top Ti/Au contact was done by e-beam evaporation. The electrical separation of the different lasers in the array and two sections of individual lasers were realized by wet etching using the gold etchant. Afterwards, the sample was thinned down to around 200 µm and Ti/Au bottom contact was deposited. The sample was diced and epilayer-up bonded on the copper heatsink using indium.
Fig. 1. (a) Optical microscope image of a slot QCL array. (b) Electron micrograph of the laser facet from a fully processed device.
The temperature of the slot QCL array is stabilized to 20 °C using a thermoelectric cooler (TEC). Two pulse generators (AVR-4-B, AVTECH) separately pump the front and back section of the emitters in the array. The tunability of the slot waveguide QCLs arises from the Vernier effect, which is implemented by changing the pulse width of two pulse generators to achieve the refractive index variation between the front and back sections. It is noted that the method of tuning pulse width is different from the method of applying and tuning the DC current in the previous demonstration of slot waveguide QCL arrays [19,20]. Benefiting from the employed configuration, the total power consumption of the system will be reduced and the lasers will suffer from less thermal issue compared to applying the DC bias. In our experiment, two pulse generators are triggered with the frequency of 5 kHz. To ensure the synchronization of the falling edge of the pulse current, the delay is pre-characterized and introduced between two pulses with different pulse widths. The pulse widths are selected as short as possible based on two considerations. One is to suppress the thermal chirp induced linewidth broadening. Another reason is that the short pulse width results in high output peak power, which will increase the demodulated signal from lock-in amplifier. The wavelength scanning within the single laser is done by the LabVIEW software automatically. The driving conditions of the lasers achieving target wavelengths are calibrated. The information of the pulse widths, corresponding delay, and driving currents are recorded and controlled by the software during the wavelength scanning. The switching between the different emitters is done manually and the full automatization can be realized if employing the channel selection board [23].
During the light–current–voltage (LIV) characterization of the proposed laser array, each laser is characterized separately. The pulse widths of both the front and back sections are set to 200 ns, and the repetition rate is chosen to be 5 kHz. At 20 °C, five emitters in the array show similar output power, up to around 200 µW at roll over current, which is corresponding to the peak power of 200 mW as the duty cycle of the driving current is calculated to be 0.1% (details in Supplement 1). Considering the various pumping conditions during the wavelength tuning in the measurement, the average power consumption of single emitter is calculated to be 0.3 W.
In order to verify the successful design and fabrication of the slot waveguide QCLs, the supermodes of five emitters in the array are characterized by FTIR. In Fig. 2, experimentally measured supermodes shown in the upper side are compared with the simulated results shown in the bottom. Due to the limitation of the QCL active region gain bandwidth, not all the simulated supermodes are observed in the experiment. However, the fundamental supermodes and the corresponding switched modes agree well with the design value. It is noted that the mode spacings between some lasers have a small difference from the simulated one, which can be attributed to the estimation error of the effective refractive index used in the simulations and the fabrication error.
Fig. 2. Experimental characterized supermodes from the five lasers of the array compared with the simulated results. The solid lines are the fundamental supermodes and dashed lines are the supermodes after mode switching.
The continuous tuning spectrum of the laser array is also characterized using FTIR. The front and back sections of each laser are pumped by pulse current with the pulse width ranging from 100 ns to 1 µs (detailed driving condition in Supplement 1). By slightly changing the pumping current or the pulse width, the gap between the switched supermodes can be filled. In Fig. 3, a tuning range of around 12 cm-1 is achieved, from 735.3 to 747.3 cm-1. The lasing modes show the side mode suppression ratio (SMSR) larger than 17 dB. The narrow and steep gain profile may result in the serious mode competition, which reduces the SMSR for the lasing mode.
Fig. 3. Single-mode tuning spectra of the QCL array consists of five emitters, showing a tuning range around 12 cm-1, with side-mode-suppression-ratio (SMSR) larger than 17 dB.
2.2 Spectroscopy experimental setup
The experiment setup is shown in Fig. 4(a). The temperature of the QCL array on the TEC is fixed at 20 °C during all the measurements. The pulse generators are controlled by the computer to inject pre-calibrated currents with corresponding pulse widths into the QCL array to realize automatic wavelength scanning. The output beams of the QCLs are collimated and coupled into a long wavelength (Long wavelength coating for λ = 8–16 µm) hollow-core fiber (HCF, Guiding Photonics. Bore diameter of 1.5 mm and propagation loss of 0.2 dB/m), which serves as the miniaturized gas cell with a length of 5 m. The emitters have the same waveguide design and geometry parameters as the laser in Ref. [18], thus sharing the similar beam divergence as reported in Ref. [18]. Considering the beam divergence, a collimating lens with the effective focal length (EFL) of 6.35 mm is used to collimate the laser beam. Based on the optimal coupling condition of the HCF [24], a focusing lens with the effective focus length of 200 mm is chosen to focus and couple the beam into HCF with high coupling efficiency. The measured coupling efficiency is 0.98, which is consistent with the reported value with the similar setup [23]. The switching of different lasers is achieved by adjusting the laser position using a high precision stage. And the high coupling efficiency is ensured by monitoring and maximizing the N2 reference signal when doing the position adjustment.
Fig. 4. (a) Schematic diagram of the experimental setup. Lock-in, lock-in amplifier; PD, photodetector; HCF, hollow-core fiber; MFC, mass flow controller; FTIR: Fourier transform infrared spectrometers; M, folding gold mirror. The insert shows the beam profile near the fiber input (The position marked with a yellow dot in the setup). (b) Schematic of the customized fiber adapter. GM: gas mixture, HCF: hollow-core fiber, RC: rear cover, IMB: intermediate bracket, OR: O-ring, ZW: ZnSe windows, FC: front cover.
Figure 4(b) illustrates the customized inlet/outlet adaptors for securing the HCF and sealing the sensing system. The HCF was first secured to rear cover (RC) and the gap between HCF and RC are filled by silicone sealant. Successively, the RC was fixed to the intermediate bracket (IMB). To eliminate the gas leakage, the gaps between the RC, IMB and HCF were filled up with silicone sealant. After installing the O-ring, ZnSe window and front cover, the adapt was completely sealed and fixed by tightening the screws. After completing the installation of the HCF and adapters, the leakage test of the sensing system was also done. The window with the rubber O-ring is used to assure gas-tightness. Besides, the customized ZnSe window has an anti-reflection (AR) coating at the target wavelength range, for minimizing the transmission losses of the coupling light. The flow rates of the gases from the calibrated acetylene, o-xylene and N2 gas cylinders are regulated by the mass flow controllers (MFC) before the analytes are delivered to the HCF. To optimize the optical alignment, the laser beam was inspected by using an infrared beam profiler (WinCamD-IR-BB, DataRay Inc), which was placed close to the entrance of HCF. The obtained beam profile is exhibited in the insert figure. The folding gold mirror is used to calibrate the laser wavenumber by Fourier transform infrared (FTIR) spectrometer (VERTEX 70 v) before the spectroscopic experiment, and check the wavelength variation after the measurement. We noted that under the FTIR wavenumber accuracy, all the lasers in the array didn’t show the wavelength shifting before and after the experiment. The accuracy of FTIR is enough for the sensing of rather broad absorption features of o-Xylene and acetylene at ambient pressure. After passing through the HCF filled with target gas, the exiting beam is then collected by a thermoelectrically cooled MCT detector (PVMI-4TE-10.6, Vigo Systems, S.A.). The detector has a detectivity of 2 × 108 Jones at our target wavelength. For data acquisition with reduced noise, the detector signal is demodulated by a lock-in amplifier (HF2LI, Zurich Instruments) using the same reference with the repetition frequency of the laser pumping pulse. The time constant of the lock-in amplifier is chosen to be 35 ms for an optimized SNR.
3. Results and discussions
As described before, the HCF was secured to the inlet and outlet adapters by the silicone sealant as a compact gas cell. The assembled gas cell was idle for over 12 hours, as the acetic acid was released during the silicone curing process [25]. Given that the gaseous acetic acid dwelling in the HCF would mitigate the SNR, before implementing the trace-gas detection, the gas cell was continuously flushed with the gaseous N2 for one day. As the gas cell has been properly sealed, such step also ensures that interference of atmospheric absorbers on the absorption measurement of the target gas mixture was minimized. Throughout the discussions conducted here, all the absorption spectra were recorded at ambient temperature and pressure. The absorption cross-sections of two pure analytes within the tunning spectra coverage were simulated based on the HITRAN database, as illustrated in Fig. 5. It is revealed that the unique absorption peak of o-xylene is located at 741.03 cm-1. By contrast, the absorption signature of acetylene consists of multi peaks, significantly, the relatively high one is centered at 743.26 cm-1 in the tuning range. The absorption peak of o-xylene shows a linewidth of 0.5 cm-1, while the indicated absorption peak of acetylene displays a narrower linewidth of approximately 0.3 cm-1. Herein, pure acetylene and o-xylene as well as their binary mixtures in a carrier gas were prepared for trace gas demonstration. The combination of MFC and gas blending tube promised intake gases of HCF with a changeless flow rate. In addition, by customizing the flow rates of these two analytes independently, and balancing them in pure N2, we can obtain the gas mixture with the designed concentrations.
Fig. 5. The nebula blue and kelly green solid lines present, respectively, the absorption cross-section of acetylene and o-xylene simulated using HITRAN database. In the wavelength range of the tunable slot array QCLs, the strongest absorption peaks of the corresponding gases are labelled.
The determination of the two individual gases at trace levels was performed, at first. For the measurement of the absorbance of acetylene and o-xylene, the lasing wavenumber was respectively tuned to 743.27 cm-1 and 741.05 cm-1 to match the absorption peaks of the two gases. Remarkably, the current pulse width of the front and back section of laser 2 in the array was separately set to 200 ns and 100 ns for the wavenumber of 743.27 cm-1. In the same way, the driving current with the pulse width of 100 ns and 250 ns was correspondingly injected to the front and back section of the laser 4 in the array, resulting in the wavenumber to 741.05 cm-1. The nitrogen absorption spectra were collected 5 min prior to the analyte introduction into the gas cell to determine the standard deviation σSTD of the background signal. The limits of the detection (LOD) for each analyte were the concentrations evaluated by absorption signal equal to 3σSTD of the background noise. During either gas measurement series, the analyte was required to be diluted with N2 to concentrations of [10,20,30, 40, 50, 75] ppm at a total flow rate of 100 mL/min. Nitrogen gas was exploited to purge the residual analytes for sufficient time, as well as charge the gas cell as reference background gas between the injection of the analytes of different concentrations. The process is used to completely purge the residual analytes containing o-Xylene, which is found difficult to be fully removed by purging new analyte gas within short time in the experiment. After clearing the gas cell by purging N2, the time for charging the analyte was set to 1 min, to ensure the gas sample was homogeneously filled with the gas cell. Therefore, for each filling of new analyte, the absorption data and reference data of N2 were separately recorded. More specifically, to reduce background noise and low frequency fluctuation of the environment, the data acquisition time of 15 s was chosen for each data point. Figure 6 (a) shows the absorbance as a function of the acetylene and o-xylene concentration in N2, with linear fits. The absorbance intensities versus concentrations of the target gas presented a linear relationship that obeyed the Beer-Lambert law. The coefficients of determination (COD) R2 for acetylene and o-xylene were 0.995 and 0.999, respectively. In addition, the evaluated LODs were 940 ppb (acetylene) and 470 ppb (o-xylene) defined as 3σSTD of background noise.
Fig. 6. (a) The linear regression functions fitted to the peak value obtained for o-xylene (medium sea green solid ball) and acetylene (blue solid ball). (b)The experimental measured (magenta solid ball) and simulated absorption (red solid line) spectra of mixed acetylene and o-xylene with the same concentration of 50 ppm in N2, and the spectra were simulated by employing the HITRAN database.
In order to explore the multi-species trace gas sensing capability of our spectrometer, we have performed spectroscopy to measure the gas mixture of acetylene and o-xylene. The equal concentration of 50 ppm for both analytes was balanced in N2 with a total flow rate of 100 mL/min. The laser array was scanned through the wavenumber from 735.3 to 747.3 cm-1, by the LabVIEW program-controlled pulse generators using calibrated currents and pulse widths. In Fig. 6 (b), we show the measured and simulated absorption spectra of the binary gases with an identical concentration of 50 ppm. The experimental spectra were obtained by post-processing the data collected from the HCF filled with mixed gases and reference N2 gas, during the scanning of the laser wavenumbers. From the absorption spectra of pure acetylene and o-xylene in Fig. 5, despite some parts of the absorption spectra of the two analytes overlapping across the tunning range, the measured results well reconstructed the primary absorption peaks localized at 743.26 cm-1 and 741.03 cm-1 for acetylene and o-xylene. It could be seen that the average step size was around 0.37 cm-1 and the minimum step size is 0.13 cm-1. There is nonnegligible shift between experimental and simulated spectra near the absorption peak (around 747.5 cm-1). It was attributed to the broadening of the laser linewidth due to more severe thermal chirping. The thermal chirping is a common issue resulting in the linewidth broadening in QCLs [26]. The wavenumbers near 747.5 cm-1 are close to the edge of the laser gain, therefore wider pulse width and higher pumping power are required to tune the laser emission to the target wavenumbers. The well-reconstructed absorption features expect the wavenumbers around 747.5 cm-1 show that the linewidth broadening of demonstrated pulsed operated array is acceptable under the moderate pumping condition. And the system measurement precision is affected when the lasers are pumped in extreme conditions. It is noted that the as the active region of the QCL used the relative narrow gain design, a much wider tuning range is expected if a board gain design like bound-to-continuum [27] or continuum-to-continuum [28] design is adapted. Based on the tuning mechanism of the slot array waveguide QCLs, it is possible to achieve the continuous tuning [20]. The smaller tuning step is expected in the absorption measurement, if the wavelength accuracy of the spectrometer, and the stability and the minimum tuning step of the current supply can be further improved. The results revealed that the slot waveguide QCLs offer promising potential in applications of multi-gas spectroscopy.
4. Conclusion
In summary, we developed a mid-infrared laser spectrometer around 13.5 µm for individual and binary compounds trace gas sensing, by combining the compact tunable single-mode slot waveguide array QCLs and flexible hollow-core fiber gas cell. Sub-ppm LOD was achieved for single gas detection of acetylene and o-xylene, respectively. Additionally, taking the advantages of the single-mode tunable slot waveguide array QCLs, we well reconstructed the absorption spectra of the gas mixture of these two analytes. It is revealed that the homemade slot waveguide QCL array is a robust, compact, and powerful light source, especially for spectroscopic applications. Although the FTIR spectrometer is still a crucial part of the proposed system for the wavelength calibration and monitoring, the developed spectrometer has made significant contributions to the advancement of MIR spectroscopy. Our work has provided a possibility with great potential to monitor multiple trace gases simultaneously in the long-wavelength part of the MIR region, especially for volatile organic compounds.
Future work will be towards CW operation slot waveguide QCLs, which will greatly enhance the applicability and practicality of such laser source for various spectroscopy and sensing applications. It is potential to achieve CW operation by employing the low threshold current QCL designs, such as the recent demonstration of InAs-based QCLs [29]. Furthermore, additional efforts should be dedicated to customizing the compact current drivers and control electronics, which is an essential aspect to consider for portable and field-deployable spectroscopy systems.
Funding
National Research Foundation Singapore (NRF-CRP19-2017-01, NRF-CRP22-2019-0007); Agency for Science, Technology and Research (A18A7b0058, A2090b0144); National Medical Research Council (MOH-000927).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data that support the findings of this study are available from the corresponding author upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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