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Simultaneous measurement of NO and NO(2) by dual-wavelength quantum cascade laser spectroscopy.

The concept of a multi-wavelength quantum cascade laser emitting at two or more spectrally well-separated wavelengths is highly appealing for applied spectroscopy, as it allows detecting several species with compact and cost-efficient optical setups. Here we present a practical realization of such a dual-wavelength setup, which is based on a room-temperature quantum cascade laser emitting single-mode at 1600 cm-1 and 1900 cm-1 and is thus well-suited for simultaneous NO and NO2 detection. Operated in a time-division multiplexed mode, our spectrometer reaches detection limits of 0.5 and 1.5 ppb for NO2 and NO, respectively. The performance of the system is validated against the well-established chemiluminescence detection while measuring the NOx emissions on an automotive test-bench, as well as upon monitoring the pollution at a suburban site.


Introduction
The excellent sensitivity, specificity and speed of state-of-the-art mid-infrared spectrometers based on single-mode quantum cascade (QC) [1][2][3] or interband cascade (IC) [4,5] lasers has secured their leading position for trace gas detection in environmental sciences [6][7][8], medical diagnosis [9-11], and industrial process control [12]. The strength of this technique rests upon the availability of distributed feedback (DFB) lasers with extremely narrow spectral emission that can be tailored for any frequency in the entire fingerprint region from 3 to 25 µm. However, one major limitation of using DFB-QC lasers is their rather small spectral coverage of typically 10 cm −1 , which generally leads to one-laser-one-compound measurement strategy. Much broader single-mode tuning ranges in the order of 100 cm −1 have been recently achieved using external cavity (EC) configurations [13] or MIR optical combs [14,15]. In the case of the EC lasers, the large tunable bandwidth is achieved at the expense of a drastic reduction of the measurement rate, which is typically between 1-100 Hz [16] compared to more than 10 kHz with DFB lasers. On the other hand, MIR dual comb spectroscopy realized using novel heterostructure QC lasers [17] give a promise of both, bandwidth and speed, but immaturity of this method still hinders its use in practical application.
Commercial implementations of multi-compound spectrometers therefore rely on multiple DFB laser modules, each selected for a given target gas, whose outputs are combined into a single multi-wavelength beam using macroscopic optical elements, such as beam splitters or dichroics [18], dispersive elements [19] or metallic reflectors [12]. In this way, for instance, up to four QC lasers were combined through a four-face pyramidal mirror to measure five different components of cigarette smoke [18], and highly time-resolved molecular plasma diagnostics was accomplished using three lasers coupled by a series of four off axis parabolic mirrors [12]. All these approaches, however, imply a significant amount of optical elements that are highly sensitive to their alignment. Furthermore, a temperature controller and driving electronics is required for each individual laser, which increases the overall footprint and the cost of the instrument. In this context, it is highly appealing to have a MIR laser source with all advantages of a DFB laser, which emits at two or more wavelengths associated with molecular transitions of different gas species.
In this paper, we demonstrate a dual-wavelength spectrometer, based on a DFB laser emitting sequentially at 1600 cm −1 and 1900 cm −1 . These two wavelengths were chosen as they coincide with the strongest absorption bands of NO 2 and NO that are both relevant pollutants and their simultaneous detection is of a great practical interest. Since both frequencies are generated in the same optical waveguide, their emission directions are strictly identical, providing for multi-species detection without the need for multiple laser modules or beam combining optics.

Dual-wavelength quantum cascade laser
Although hardly achievable in conventional semiconductor diode lasers, multi-wavelength emission, i.e. emission at two or more spectrally well-separated wavelengths from the same active ridge, is feasible with QC lasers. The first working dual-wavelength lasers were demonstrated more than a decade ago [20-22]. However, only due to newly developed predictive physical models [23] and improvements in fabrication technology, designs maintaining good quantum efficiency and gain performance for both lasing wavelengths have been realized.
The dual-wavelength QC laser employed in the present work is a single-mode, roomtemperature device based on a common active waveguide (see Fig. 1), which allows for sequential operation at two, spectrally well-separated wavelengths. The option of sequential operation is a necessary prerequisite for use in laser spectrometers, since it prevents any overlaps and cross-talks between absorption signals from the two spectral ranges. Detailed description of the active region design, the QC layer structure as well as the laser processing is reported elsewhere [24].
The laser operates in pulsed mode in a wide temperature range with an upper limit of 30 • C. Figure 2(a) shows the peak output power versus the injection current at the heatsink tempera-ing the heatsink temperature from 20 • C to -20 • C. Figure 3 shows the tunable wavelength range superimposed with the absorption spectra of NO and NO 2 calculated using the molecular parameters from the HITRAN database [27]. While the spectral emission of the front laser is in perfect agreement with the design, a slight mismatch for the 1900 cm −1 section prevents us from reaching the ideal 1900.08 cm −1 NO line. Nevertheless, the heatsink temperature of 0 • C gives simultaneous access to a strong absorption doublet of NO at 1890.71 cm −1 and to a NO 2 absorption line at 1599.14 cm −1 . Being weaker only by a factor of 1.3 and 2.5, respectively, these lines are of a comparable intensity as the originally targeted lines and exhibit a relatively small overlap with other interfering gases, in particular water. Fast electric tuning over these lines is achieved by applying subthreshold current ramp of 0 to 100 mA.

Laser driving
Taking advantage of the fact that both laser sections can be operated at the same driving conditions, the laser is driven using a single pulser unit combined with a radio-frequency compatible switch to time-multiplex the laser sections. Compared to the option of driving the respective sections with two laser drivers, our approach has the advantage of reduced hardware cost, but involves a custom solution for switching short and high-current driving pulses.
The pulser generates both the pulses (5 ns width and 1 MHz repetition rate) and the subthreshold current ramp (0.025 to 0.065 A, 10 kHz) for fine frequency tuning. The latter is optimized to sweep only a narrow spectral region around the targeted absorption lines in order to densely sample the absorption features and thus increase the precision of the measurement. The pulser output is fed into a switch based on reed-relays, which is characterized by a low capacitive coupling between the output channels even for 5-ns short laser pulses. The drawback is a rather long down-time of the switch of 300 µs that clearly hinders switching between subsequent spectral scans. An effective solution to overcome this issue while maintaining both high data rates and a thermal equilibrium of the system is to record several spectra while the switch is in a given position. However, the switching speed must still be faster than the thermal relaxation of the system, i.e. the laser heat sink must remain at an apparently constant temperature despite the fact that the heat dissipation in the respective laser sections may differ. Based on these considerations, an optimal switching rate of 100 Hz was identified. The first three spectra after each switching event are discarded due to thermal stabilization of the active region, which yields 44 spectra that are recorded within 5 ms for each laser section ( Fig. 4(a)).

Spectroscopic setup
The experimental setup is schematically shown in Fig. 4(b). The highly divergent output beam from the QC laser is collimated by a high NA = 0.56 antireflection coated aspheric lens and passed through a 1 mm-thick sapphire window. The latter acts as an optical short-pass filter with a soft cutoff at 6 µm, reducing the power from the stronger front laser section (1600 cm −1 ) by a factor of 5. The power-equalized laser beam is then directed into an astigmatic mirror Herriott multipass gas cell (AMAC-36, Aerodyne Research, Inc., USA), where it undergoes 182 reflections corresponding to an optical path of 36 m. The nearly parallel beam leaving the cell is then refocused onto a fast thermoelectrically cooled MCT detector with a 10 MHz bandwidth and a noise-equivalent power of 1 pW Hz −1/2 (PVI-4TE-6, Vigo SA, Poland).
Pulsed operation of QCL typically exhibit pulse-to-pulse amplitude variations of few percents [25] that mainly reflect the current fluctuations of the driving electronics. In order to compensate for such amplitude variations, we employ the technique of pulse normalization using a reference optical path with temporal gating as proposed by [25]. The 120 ns time delay due to the 36 m path difference makes it possible to temporally resolve and normalize the reference and the signal pulses using a single detector, thus reducing the signal noise by as much NO 2 and NO, respectively, has proven to be sufficient to closely reproduce the concentration variations of both pollutants. Dübendorf is a suburban site in a densely populated area, dominated by industrial and commercial buildings and a dense road network. The highest NO and NO 2 levels were measured in early morning due to rush-hour traffic and accumulation in the low mixing layer, typically found during the winter months. After about 8:30, the concentration of NO gradually decreased to values often not exceeding a few ppb, with isolated peaks that can be attributed to transient pollution events. These events are much more pronounced for NO than for NO 2 because NO is directly emitted during combustion processes, while NO 2 is mainly a secondary pollutant formed during atmospheric transport through the oxidation of NO. Therefore, NO 2 is more representative of the urban background and shows much less variations than NO. During the observed measurement period, NO 2 concentrations were always above 15 ppb with a second rise after 16:00, indicating the evening traffic increase. The measured concentrations of both nitrogen oxides were rather low for an urban site in winter season, with NO 2 well below the Swiss legal daily limit value of 80 µg/m 3 (42 ppb).

Conclusion
The present work is a successful demonstration of sensitive and specific multi-species detection performed with a recently developed dual-wavelength QC laser. Emitting at two, spectrally largely distinct (300 cm −1 ) frequencies, this laser provides for simultaneous detection of NO and NO 2 down to ppb levels. Moreover, operated using a single laser driver and requiring no beam combining optics, it considerably reduces the footprint, complexity and power consumption of the instrument, compared to systems based on multiple laser modules. As such, it is particularly suitable e.g. for portable NO and NO 2 automotive emission measurements systems (PEMS).
While the current laser prototype still operates in a pulsed mode and is not fully matched to the best-suited absorption features for NO and NO 2 detection, the second generation dual-wavelength lasers promises continuous emission at exactly the targeted wavelengths of 1900 cm −1 and 1600 cm −1 . Further advances in the laser design and fabrication shall include the extension of the active region design to new spectral regions, which is likely to trigger a new trend in multi-species laser spectrometer development.