  We present a system for molecular spectroscopy using a broadband mid-infrared
laser with near infrared detection. Difference frequency generation of a
Yb:fiber femtosecond laser produced a mid-infrared (MIR) source tunable from
2100-3700 cm^-1 (2.7-4.7 microns) with average power up to 40 mW. The MIR
spectrum was upconverted to near-infrared wavelengths for broadband detection
using a two-dimensional dispersion imaging technique. Absorption measurements
were performed over bandwidths of 240 cm^-1 (7.2 THz) with 0.048 cm^-1 (1.4
GHz) resolution, and absolute frequency scale uncertainty was better than 0.005
cm^-1 (150 MHz). The minimum detectable absorption coefficient per spectral
element was determined to be 4.4 x 10^-7 cm^-1 from measurements in low
pressure CH_4, leading to a detection limit of 2 parts-per-billion. The
spectral range, resolution, and frequency accuracy of this system show promise
for determination of trace concentrations in gas mixtures containing both
narrow and broad overlapping spectral features, and we demonstrate this in
measurements of air and solvent samples.
