  We use two femtosecond Erbium-doped fiber lasers with slightly different
repetition rates to perform a modern type of Fourier transform spectroscopy
without moving parts. The measurements are done in real time, and it takes less
than 50 microseconds. We work with somewhat different spectral outputs from two
Erbium-doped fiber lasers and employ spectral filtering based on a 2f-2f
grating setup to select the common spectral region of interest, thereby
increasing the signal-to-noise ratio. The interferogram is taken with a 20 cm
long gas cell, containing a mixture of acetylene and air at atmospheric
pressure, and is fast-Fourier-transformed to obtain the broadband spectral
fingerprint of the gas.

Optics