  Dual-comb spectroscopy is a promising method for precise optical spectrum
analysis with fast data acquisition speed. However, its implementation and
applications are often hindered by the complexity of optical comb systems.
Here, as a compact and robust system, femtosecond imbalanced time-stretch
spectroscopy (FITSS) with simple optical layout is proposed and demonstrated.
The time-stretch interferometry from one femtosecond laser builds a mapping
from the optical frequency domain to the radio frequency regime. In experiment,
the absorption line of a hydrogen cyanide cell is encoded in the probing arm of
a Mach-Zehnder interferometer (MZI). The down-converted radio frequency comb is
transformed from a periodically chirped waveform, which is the interferogram of
the MZI with different dispersion values on two arms. By turning the optical
filter, the spectrum over a wide range is analyzed.
