MOJAVE, CA 93501

HWG-CRDS with Laser Scanning

Develop a trace gas sensor with ~1 second sampling resolution and  > 10 times improvement in minimum detection limit over more traditional absorption spectroscopy techniques. Our proposed technique is to incorporate pulsed cavity ring down laser absorption spectroscopy in a hollow waveguide (HWG-CRDS). In conventional CRDS, a gas cavity is charged with a laser (typically mid-IR), and the time rate of decay of laser energy is observed as light “bounces” between two highly reflective end mirrors (99.9% to 99.999% typical). The rate of decay is dependent on the rate of energy loss due to gas absorption in the cell at a particular wavelength. In the HWG-CRDS approach, a ~103-104 signal enhancement is made my injecting light energy through an aperture into a cavity constrained by the hollow waveguide. Pulsed light is used to remove the requirement for precision tuning of the cavity normally used to prevent constructive/destructive wave interference. A portion of this work includes the development of a very high-speed data acquisition system to effectively monitor the laser light pulses bouncing between the two end mirrors.

HWP-CRDS 1.66 μm CH4 experiment.

Photo of an exemplary HWG-CRDS waveguide recently designed
and fabricated utilizing the Bragg waveguide model described herein.

Exemplary 3 mm i.d. Bragg waveguide using 30 layers of alternating
dielectric layers of SiO2 and TiO2.

Exemplary transverse modal electric fields (for 1° propagation angle)
associated with a small subset of the eigenvalue solutions.

Complex eigenvalues of Bragg HWG shown in Fig. 3 plotted as the imaginary component of
mode ‘‘eigenvalue’’ (in dB/m) vs. the real component (in propagation angle). (A) TE/TM bounding attenuation
modes (l = 0) over entire propagationangle range of HWG. (B) Modal ‘‘eigenvalues’’ for 1≤l≤10 and
shallow propagation angles. Superimposed is the equivalent cavity linear attenuation for a 20 cm CRDS
cavity with 99.95% reflective mirrors.

Principal Investigator:

Christopher Dryer, Colorado School of Mines

Institutional Principal Investigator:

Greg Mungas


-One Patent Pending


  • G. Mungas, C. Dreyer, Cavity Ringdown Spectroscopy in a Hollow Bragg Waveguide – Electromagnetic Theory and Modeling, Applied Spectroscopy, Vol. 63, (11). (2009)

  • C. B. Dreyer, G. S. Mungas, Development Progress of Pulsed Cavity Ringdown Laser Absorption Spectroscopy in a Hollow Waveguide for Trace Gas Detection. Lunar and Planetary Science Conference XXXVIII  #2369 (2007).

  • G. Mungas, C. Dreyer. Pulsed Cavity Ringdown Laser Absorption Spectroscopy in a Hollow Waveguide. IEEE Aerospace Conference 2006, IEEEAC#1476