Enhancement and shape control of weak molecular absorption signal with chirped-pulse mid-IR lasers
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Detection of weak narrowband absorption by molecular lines is usually a topic of linear-optical technology. In this paper we demonstrate nonlinear-optical enhancement and shape control of atmospheric absortion lines using a broadband ultrashort-pulse oscillator and nonlinear fiber propagation. The absorption signal is generated inside the passively mode-locked mid-IR Cr:ZnS laser. The nonlinear interaction with the broadband ps-long pulse causes the narrow-band ns-long absorption signal to acquire a 90° phase shift with respect to the pulse . The signature of such an absorber on the output spectrum is a dispersion-like modulation, which can be much stronger than absorption modulation itself. This effect has been experimentally observed using the atmospheric water vapour absorption lines around 2.5 μm in Cr:ZnSe laser [1, 2]. Additional modulation increase has been predicted to occur near the spectrum edge if the laser is operating in a chirped-pulse (dissipative soliton) regime.