Absorption and wavelength modulation spectroscopy of NO2 using a tunable, external cavity continuous wave quantum cascade laser Andreas Karpf* and Gottipaty N. Rao Department of Physics, Adelphi University, Garden City, New York 11530, USA *Corresponding author: [email protected] Received 16 September 2008; revised 5 December 2008; accepted 9 December 2008; posted 10 December 2008 (Doc. ID 101685); published 9 January 2009 The absorption spectra and wavelength modulation spectroscopy (WMS) of NO2 using a tunable, external cavity CW quantum cascade laser operating at room temperature in the region of 1625 to 1645 cm−1 are reported. The external cavity quantum cascade laser enabled us to record continuous absorption spectra −1 of low concentrations of NO2 over a broad range (∼16 cm ), demonstrating the potential for simulta- neously recording the complex spectra of multiple species. This capability allows the identification of a particular species of interest with high sensitivity and selectivity. The measured spectra are in excellent agreement with the spectra from the high-resolution transmission molecular absorption database [J. Quant. Spectrosc. Radiat. Transfer 96, 139–204 (2005)]. We also conduct WMS for the first time using an external cavity quantum cascade laser, a technique that enhances the sensitivity of detection. By employing WMS, we could detect low-intensity absorption lines, which are not visible in the simple ab- sorption spectra, and demonstrate a minimum detection limit at the 100 ppb level with a short-path ab- sorption cell. Details of the tunable, external cavity quantum cascade laser system and its performance are discussed. © 2009 Optical Society of America OCIS codes: 000.2170, 010.1120, 120.6200, 280.3420, 300.1030, 300.6340. 1. Introduction damaging of these pollutants and have multiple im- The accurate and precise monitoring of trace gases pacts on the environment and public health. They has applications in a wide range of fields, including play important roles in the formation of photochemi- the detection of environmental pollutants, tracking cal smog, the formation of tropospheric ozone, and of contaminants in closed environmental systems, the formation of acid rain. medical diagnostics, defense, and homeland security. Laser-based techniques for monitoring environ- Real-time trace gas detection is particularly impor- mental pollutants in the air have several advantages tant in environmental science (e.g., in atmospheric over chemical and other techniques, especially their physics/chemistry concerning air quality control, as ability to provide real-time monitoring capabilities well as for complying with Environmental Protection and greater sensitivity. Until recently, the ap- Agency [1] air quality regulations and safety moni- proaches have included (i) observing the visible and near-IR overtones of molecular transitions (which toring). In these applications, the concentrations of are much weaker than the fundamental mid-IR tran- pollutants typically range from several parts per mil- sitions) using room-temperature tunable diode lasers lion to the parts-per-trillion level and thus require [2–5]; (ii) observing the strong mid-IR transitions techniques that are both highly sensitive and selec- using tunable lead–salt diode lasers (which require tive. Nitrogen oxides (NOx) are some of the more cryogenic cooling and are typically multimode) [6] or strong tunable molecular laser systems such as 0003-6935/09/020408-06$15.00/0 CO lasers (which require cryogenic cooling and pro- © 2009 Optical Society of America vide only limited tuning around the molecular lines 408 APPLIED OPTICS / Vol. 48, No. 2 / 10 January 2009 and overtones) [7,8]. The advent of the quantum cas- The output power at 1632:5 cm−1 (where we con- cade laser in the mid-IR region covering the 4–24 μm ducted our WMS) was 8 mW. Broad-range tuning range has provided an attractive source for investi- is achieved entirely by rotating the external cavity gating the spectroscopy of trace gases in the atmo- grating via a stepper motor; no current or tempera- sphere and constructing portable gas sensors [9]. ture tuning was necessary. The system may be set to Using quantum cascade lasers (which can operate one of six preset tuning rates, the slowest of which in a room-temperature environment using thermo- was 3:125 cm−1=s (the tuning rate used to record electric cooling), one can access the strong mid-IR our NO2 absorption spectra). A piezoelectric transdu- fundamental rotational–vibrational transitions of cer (PZT) positioned between the stepper motor and molecular species [10], thus offering improved the grating itself allows one to tune smoothly at any sensitivity over techniques based on detecting the desired rate over a smaller range (1 cm−1) as well as overtones of these lines. By employing an external modulate the laser frequency with a sinusoidal sig- cavity arrangement, a quantum cascade laser offers nal (allowing us to perform WMS). Additionally, its Δν ∼ 0:001 −1 a narrow linewidth ( cm ), highly stable room-temperature operability and compact size and reproducible tunable CW output, and a wide make it a suitable candidate for a portable device continuous tuning range, all of which are essential for in situ measurements. for the study of complex spectra, as is the case in We carried out absorption spectroscopy of NO2 investigating trace gas components in the atmo- using a short-path absorption cell (12:5 cm) filled sphere [11]. with different concentrations of NO2. The schematic The current work focuses on using a quantum cas- of the experimental arrangement is shown in Fig. 1. cade laser operating at room temperature to detect Two rectangular, flat, protected gold mirrors were NO2 with high sensitivity and selectivity. The pro- ’ aligned to reflect the beam back and forth nine times ject s aim is to ultimately develop a portable, self- through the cell, creating a homemade multipass cell contained device that may be used for high-precision, that increased the path length to 1:125 m. The signal in situ atmospheric measurements. To this end, a was detected using a two-stage, TE-cooled, IR photo- short-path absorption cell was used with a multipass voltaic detector (PVI-2TE-8 manufactured by Vigo), configuration, and wavelength modulation techni- which can be operated in a room-temperature ques were employed to enhance the sensitivity. In wavelength modulation spectroscopy (WMS) [12–15], environment. The detectors were optically immersed the laser output is modulated at a frequency (f ), and in a high refractive index, hyperhemispherical lens. Phase-sensitive detection was done using a chopper detection is done at higher harmonics of the fre- 3:9 quency (e.g., 2f ) so that the excess laser noise, which running at kHz and a lock-in amplifier (Stanford varies as 1=f , is reduced, thereby enhancing the Research Systems SR830 DSP) with the time con- 3 signal-to-noise ratio. Specifically, the modulation fre- stant set to ms. quencies are lower than the absorption linewidth. The NO2 mixtures used were prepared by loading the experimental cell with a precalibrated 1% mix of NO2 and 99% N2 (mixed and certified by National 2. Experimental Details Specialty Gases to Æ2% of the specified concentra- The quantum cascade laser source was purchased tion) to a desired pressure. Additional pure N2 was from Daylight Solutions (Model TLS-CW-MHF) and then added to increase the pressure to the desired −1 has a tuning range of 1625 to 1645 cm . The external final value. For example, for the 50 ppm concentra- cavity is of Littrow configuration, which allows a tion used for the absorption spectroscopy, the wide range of mode-hop-free tuning, provides a nar- row linewidth (∼0:001 cm−1) and is thus well suited for spectroscopic measurements. Because of manu- facturing issues with the antireflective coating on the laser chip, the mode-hop-free tuning range did not span the entire 20 cm−1 tuning range of the laser system, but rather it was limited to the region be- tween 1629 and 1645 cm−1. We were able to record −1 a continuous NO2 spectrum over this 16 cm range without mode hops. Between 1625 and 1629 cm−1, the laser tuned over short ranges (of the order of 1 cm−1) before hopping to another mode. The continu- ity and linearity of the laser tuning were monitored using an etalon and the laser controller’s display screen, as well as by comparing recorded spectra with line positions identified in the high-resolution transmission molecular absorption (HITRAN) data- base [16]. The laser power varies as a function of tun- ing, with a minimum output power of 5 mW at Fig. 1. Schematic of the experimental setup employed for absorp- −1 −1 1625 cm and a maximum of 18 mW at 1645 cm . tion and WMS. 10 January 2009 / Vol. 48, No. 2 / APPLIED OPTICS 409 −1 experimental cell was first loaded with 1:3 Æ 1 mbar and 1632:511 cm (note that the ðν1 ν3 ν3Þ– of the precalibrated 1% mix of NO2 before additional ðNKa KcÞ level notation is used). The separation of −1 pure N2 was added to reach a final pressure of the lines within this doublet is 0:003 cm , which 260 Æ 20 mbar. Because of limitations in the accu- is much less than the pressure broadened widths racy of our vacuum/mixing apparatus, the mixed con- of the lines at 120 mbar, meaning that it should show centrations are expected to be accurate to Æ20%. The as a single absorption “line.” We measured the mixing apparatus was tested by generating several HWHM of the resulting unresolved line to be −1 concentrations of NO2 and comparing the recorded ∼0:018 cm and used this value for purposes of de- absorption line intensities with the calculated inten- termining the modulation depth of the sine wave.