Design and Signature Analysis of Remote Trace-Gas Identification Methodology Based on Infrared-Terahertz Double-Resonance Spectroscopy

The practicality of a newly proposed infrared-terahertz (IR-THz) double-resonance (DR) spectroscopic technique for remote trace-gas identification is explored. The strength of the DR signatures depends on known molecular parameters from which a combination of pump-probe transitions may be identified to recognize a specific analyte. Atmospheric pressure broadening of the IR and THz trace-gas spectra relaxes the stringent pump coincidence requirement, allowing many DR signatures to be excited, some of which occur in the favorable atmospheric transmission windows below 500 GHz. By designing the DR spectrometer and performing a detailed signal analysis, the pump-probe power requirements for detecting trace amounts of methyl fluoride, methyl chloride, or methyl bromide may be estimated for distances up to 1 km. The strength of the DR signature increases linearly with pump intensity but only as the square root of the probe power because the received signal is in the Townes noise limit. The concept of a specificity matrix is introduced and used to quantify the recognition specificity and calculate the probability of false positive detection of an interferent.

Figure 1

Schematic energy level diagram illustrating the DR remote sensing concept and the corresponding spectral and temporal evolution of the DR signal.

Figure 2

(a) Schematic diagram of the 100-m confocal and 1-km diffracting probe architecture for the DR chemical sensor. The red beam is the IR pump, and the blue and green beams are the transmitted and reflected THz probe from the respective antenna and retroreflector. (b) The dependence of noise power on probe power received at the detector for $b=B=5\mathrm{GHz}$ for the extremes in system noise temperature $T_S$. The inset presents a notional diagram of the THz transceiver.

Figure 3

(a) The atmospheric attenuation as a function of $T_{\mathrm{DP}}$ for the frequency region 50–500 GHz, and the fraction of power received at the detector for various $T_{\mathrm{DP}}$ at $R=100\mathrm{m}$ (b) and $R=1\mathrm{km}$ (c).

Figure 4

The convolution $C({\nu }_{\mathrm{pu}},{\nu }_o)$ of the spectral bandwidth of the laser pulse and the pressure-broadened linewidth of a rovibrational transition as a function of pump offset frequency $|{\nu }_o-{\nu }_{\mathrm{pu}}|$, compared with the Lorentzian line shape $L(\nu ,{\nu }_o)$ appropriate for a cw monochromatic pump.

Figure 5

IR-THz DR transition strength $\mathrm{\Delta }{\alpha }^{†}$ (a),(d),(g) for the standard case and the SNR for the standard (b),(e),(h) and long-range tropical (c),(f),(i) scenarios for ${\mathrm{CH}}_3\mathrm{F}$ (a)–(c), ${\mathrm{CH}}_3\mathrm{Cl}$ (d)–(f), and ${\mathrm{CH}}_3\mathrm{Br}$ (g)–(i). Each laser line labeled at the right is coincident with a transition that produces a $\mathrm{SNR}\ge 1$ for a 100–ppm m cloud.

Figure 6

(a) Detection-threshold pump energy per pulse as a function of probe power for various concentrations of ${\mathrm{CH}}_3\mathrm{F}$ with the standard scenario. (b) Scenario-dependent detection thresholds for a 100–ppm m cloud of ${\mathrm{CH}}_3\mathrm{F}$, ${\mathrm{CH}}_3\mathrm{Cl}$, and ${\mathrm{CH}}_3\mathrm{Br}$. Laser lines and probe frequencies used for analysis correspond to the strongest SNR given for each molecule in Tables 1, 2, 3. (c) Scenario-dependent minimum detection threshold for $\pi$-pulse excitation of ${\mathrm{CH}}_3\mathrm{F}$, ${\mathrm{CH}}_3\mathrm{Cl}$, and ${\mathrm{CH}}_3\mathrm{Br}$.

Figure 7

(Top row) Plots of $|\mathrm{\Delta }{\alpha }^{†}|$ as a function of pump wavelength and probe frequency for ${\mathrm{CH}}_3\mathrm{F}$, ${\mathrm{CH}}_3\mathrm{Cl}$, and ${\mathrm{CH}}_3\mathrm{Br}$. (Center row) Similar plots of SNR for the standard scenario. (Bottom row) Similar plots of SNR for the LRT scenario. Values for $\mathrm{SNR}<1$ are not plotted.

Figure 8

Plots of ${\mathrm{\Xi }}_{\min }(i)$ for $i={\mathrm{CH}}_3\mathrm{F}$, ${\mathrm{CH}}_3\mathrm{Cl}$, and ${\mathrm{CH}}_3\mathrm{Br}$ as a function of pump wavelength and probe frequency.