A Chaotic Optical Cavity Combined With a Quantum Cascade Laser for Chemical Vapor Sensing Abhishek Agrawal, Allen Hsu, * Pat Whitworth, # Evgenii Narimanov, and Claire Gmachl Department of Electrical Engineering and MIRTHE, Princeton University, Princeton, NJ 08544, USA * Current address: Massachusetts Institute of Technology, Cambridge, MA 02139 # Current address: Tulane University, New Orleans, LA 70118 Email address: abhishek@princeton.edu Abstract: A novel multi-pass optical cavity with partially-chaotic ray dynamics has been combined with a Quantum Cascade laser for sensing of ethanol. The 4-cm diameter cavity shows an optical path length in the mid-infrared of ~4.5-m. ©2007 Optical Society of America OCIS Codes: (140.4780) Optical resonators, (140.1540) Chaos 1. Introduction Sensitive mid-infrared laser absorption spectroscopy often involves overlapping the gas sample under test with the probing laser light in a multiple-pass optical cavity. Such an arrangement increases the effective path-length of the light in the cavity and, hence, in Beer’s law maximizes the overall absorption. Commercially available and often- used optical multi-pass cells are the so-called White or Herriott cells.[1] While conveniently available, they are still rather bulky and prone to misalignment, as they typically consist of two separate, opposed mirrors. We recently proposed a novel, quasi-chaotic optical cavity for multi-pass measurements.[2] A single hollow body is formed with a quadrupolar shape and coated for high reflectivity. The particular shape of the inner surface allows for a ray dynamics that repeatedly refocuses the incident beam for a range of incident angles and hence allows long path- lengths over a small volume. The advantages of this type of multi-pass resonator are its small size and volume, a few cm size cavity results in a few meters of path-length in the experiment, its single surface shape, resulting in less sensitivity towards optical misalignment, and a potential for low-cost manufacturing. Here, we present experimental results for such a multi-pass optical cavity used in combination with a λ ~ 8 μm Quantum Cascade (QC) laser; a first demonstration of sensing of ethanol vapor is also presented. 2. Experimental Setup Our multi-pass cavity has the shape of a three-dimensional quadrupole, defined in terms of its average radius R 0 and the deformation parameter ε as )) 2 cos( 1 ( ) ( 0 θ ε θ + = R R in the standard spherical coordinates (R, θ, ϕ), and with R 0 = 2.54 cm, and ε = 0.16. It is rotational symmetric around a major axis. The cavity has been fabricated from two halves of an acrylic plastic shell by diamond-turning, and gold is uniformly deposited inside the cavity. A circular aperture 2 mm in diameter is drilled along the line ϕ = 54.4 0 at one point in the cavity. Light from a λ ~ 8 μm QC laser is coupled into the cavity using a telescope of Ge lenses. Light coupled out of the cavity is focused through a Fig. 1. Schematic of the experimental setup; QCL stands for Quantum Cascade laser. Fig. 2. Photograph of the optical cavity (back) and the collimating lenses for the QC laser (front). The cavity is made from a hollow acrylic body coated inside with gold. Refraction on the acrylic makes the cavity appear more spherical than it is in reality. Fig. 3. Schematic of the ray dynamics inside the quasi-chaotic cavity. A multi- pass beam (blue line) is bouncing repeatedly inside the cavity; the bounce points are indicated by the red dots. a2525_1.pdf JThD87.pdf