Broadband interferometric subtraction of optical fields.

We present a Mach-Zehnder-like interferometer capable of simultaneous super-octave (950 - 2100 nm) destructive interference with an intensity extinction of 4 × 10. Achromatic nulling is achieved by unbalancing the number of Fresnel reflections off optically denser media in the two interferometer arms. With a methane gas sample in one interferometer arm, we isolate the coherent molecular vibrational emission from the broadband, impulsive excitation and quantitatively examine the potential improvement in detectable concentration, compared to direct transmission geometry. The novel concept will benefit sensing applications requiring high detection sensitivity and dynamic range, including time-domain and frequency-domain spectroscopy.