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We describe a non-intrusive, open-path, fast-response compact sensor for simultaneous measurements of nitrous-oxide (N 2 O) and carbon-monoxide (CO) primarily designed for UAV applications. N 2 O is the third most important anthropogenic greenhouse gas, but the spatial and temporal distributions of N 2 O emissions are poorly quantified. On the other hand, CO is an important tracer to distinguish between fossil fuel and biogenic sources. We use a 4.5 micron thermoelectrically-cooled, distributed feedback, continuous wave quantum cascade laser as a mid-infrared radiation source to scan CO and N 2 O transitions centered at 4538.9 nm and 4539.8 nm respectively. Detection was achieved by a thermo-electrically (TE) cooled 5 micron Indium-Phosphide (InSb) infrared detector. For the first time in this application, a compact cylindrical cell with a pattern configuration to minimize the sensor size with a pathlength of 10 meters (2.54 cm radius mirrors, 25 cm basepath). Wavelength modulation spectroscopy was employed to achieve high sensitivity detection. The detection limit of 10 -5 fractional absorbance was achieved at a 10 sec. averaging time. This is equivalent to less than 1 ppbv of N 2 O and 2 ppbv of CO out of 320 ppbv and 200 ppbv ambient levels respectively. In summary we report a cryogen-free, consumable-free sensor that can operate with 10s W of electrical power and packaged in a small shoe-box size which is ideal for UAV or airborne applications.

We report a compact, portable, low power tunable diode laser based sensor for a fast, non-intrusive measurement of temperature on airborne-based vehicles. The proposed sensor design avoids common problems in existing sensors such as adiabatic compression of the ambient airstream, thermal inertia of the sensing element, and impinging cloud particles. These effects are quite common in the conventional temperature sensors used in most aerial vehicles for ambient temperature measurements. The molecular oxygen transitions are measured using a 765 nm wavelength range vertical cavity surface emitting laser in the spectral region of two closely spaced oxygen transitions, centered at 13069.95 cm -1 and 13068.07 cm -1 respectively, according to HITRAN database. Another advantage of the proposed sensor design is that it can simultaneously detect additional trace gas species along with in-situ temperature measurements. For example, in this design we detect carbon dioxide concentration using a 2000 nm wavelength laser. The two laser beams are co-aligned and coupled into a single 20 cm multipass cell. The absorption signal (from both carbon-dioxide and oxygen) was detected simultaneously on a 2 micron photodetector. Second harmonic (Nf, N=2) detection, using wavelength modulation spectroscopy was employed to enhance the sensitivity of measurements. The sensor can readily be miniaturized and consumes less than 2 W of power, ideal for use of unmanned aerial systems and other airborne platforms.

A basic wireless laser spectroscopic sensor network for monitoring of trace-gases will be presented. The prototype lowpower sensor nodes targeting carbon dioxide are based on tunable diode laser absorption spectroscopy and operate using a 2 μm VCSEL and a 3.5 m Herriott multi-pass cell. The sensor system, which employs real-time wireless communications, is controlled by custom electronics and can be operated autonomously. The sensor core electronics performs molecular concentration measurements using wavelength modulation spectroscopy with an active laser frequency locking to the target transition. The operating sensor node consumes approximately 300 mW of electrical power and can work autonomously for up to 100 hours when powered by a 10.5 Ah Lithium-ion polymer battery. Environmentally controlled long term (12 hours) stability tests show sensor node detection limit of ~0.286 ppm with 1 second integration time and the ultimate minimum detectable fractional absorption of 1.5x10-6 is obtained after 3500 seconds averaging time. The sensor node performance results and preliminary tests in a basic network configuration are discussed.

Measurement of the isotopic composition of atmospheric methane is a valuable tool for understanding the sources and sinks of the global carbon budget. One promising carbon isotope ratio measurement technology is optical spectroscopy using inter-band cascade (IC) lasers. Ongoing development of these light sources has the goal of providing, from a package operating near room temperature, a single mode laser source in the wavelength range of 3 mm. The spectral features of methane are sufficiently strong at this wavelength that a path length of about 100 m should suffice for measuring 12- and 13-C isotopes in air without pre-concentrating the sample. Experimental IC lasers are described and their use for isotope sensing by wavelength modulation spectroscopy is evaluated.

The recent development of a dense pattern, multiple pass optical cell based on cylindrical mirrors makes possible a differential spectroscopic method that removes (nearly) all common mode features including laser noise, laser distortion, and unwanted optical interference fringes (etalons). The cylindrical mirror cell is similar to other astigmatic cells in that the beam enters through a hole drilled in the center of one mirror. Key differences, however, include the property that for most re-entrant beam trajectories have N passes (where N/2 is odd) through the cell, the N/2 spot is always located at the center of the far mirror. In the differential cell approach, a pellicle beamsplitter located just behind a hole in the far mirror transmits a portion of the beam and reflects the remainder to continue the second set of N/2 passes before exiting through the entrance hole. The two beams - one exiting at the far mirror after N/2 passes, the other exiting at the entrance mirror following N passes - are the reference and sample beams, respectively, applied to a noise canceler circuit. Proof-of-principle experiments reported here using near-infrared measurements of methane absorbance show the differential method does work. The optical system used, however, introduced excessive astigmatism in the beam reflected from the pellicle beam splitter because of the displace of the pellicle from the cell mirror surface. That astigmatism made it difficult to align the return beam for the complete set of N/2 passes and to focus the exit beam onto the photodiode detector. Design improvements are discussed.