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Institut f¨ ur Photonik, TU Wien, Gusshausstr. 27/387, 1040 Vienna, Austria 

# Soliton absorption spectroscopy 

V. L. Kalashnikov ∗ and E. Sorokin † 

We analyze optical soliton propagation in the presence of weak absorption lines with much nar- rower linewidths as compared to the soliton spectrum width using the novel perturbation analysis technique based on an integral representation in the spectral domain. The stable soliton acquires spectral modulation that follows the associated index of refraction of the absorber. The model can be applied to ordinary soliton propagation and to an absorber inside a passively modelocked laser. In the latter case, a comparison with water vapor absorption in a femtosecond Cr:ZnSe laser yields a very good agreement with experiment. Compared to the conventional absorption measurement in a cell of the same length, the signal is increased by an order of magnitude. The obtained analytical expressions allow further improving of the sensitivity and spectroscopic accuracy making the soliton absorption spectroscopy a promising novel measurement technique. 

Light sources based on femtosecond pulse oscilla- tors have now become widely used tools for ultrashort studies, optical metrology, and spectroscopy. Such sources combine broad smooth spectra with diffraction- limited brightness, which is especially important for high- sensitivity spectroscopic applications. Advances in near- and mid-infrared femtosecond oscillators made possible operation in the wavelength ranges of strong molecu- lar absorption, allowing direct measurement of important molecular gases with high resolution and good signal-to- noise ratio [1]. At the same time, it was observed that such oscillators behave quite differently, when the absorb- ing gas fills the laser cavity or introduced after the output mirror [2, 3]. The issue has become especially important with introduction of the mid-IR femtosecond oscillators such as Cr:ZnSe [4], which operate in the 2–3 µ m wave- length region with strong atmospheric absorption. 

As an example, Figure 1 presents a typical spectrum of a Cr:ZnSe femtosecond oscillator, operating at normal atmospheric conditions. It is clearly seen, that the pulse spectrum acquires strong modulation features which re- semble the dispersion signatures of the atmospheric lines. Being undesirable for some applications, such spectral modulation might at the same time open up interest- ing opportunity of intracavity absorption spectroscopy. Compared with the traditional intracavity laser absorp- tion spectroscopy [5, 6] based on transient processes, this approach would have an advantage of being a well- quantified steady-state technique, that can be immedi- ately coupled to frequency combs and optical frequency standards for extreme accuracy and resolution. 

In this paper, we present a numerical and analytical treatment of the effect of a narrowband absorption on a femtosecond pulse, considered as a dissipative soliton. Such a treatment covers both, passively modelocked ul- 

## 1. INTRODUCTION 

<img src='content_image/421140.jpg'>

trashort pulse oscillators with intracavity absorbers, and soliton propagation in fibers with impurities. The the- oretical results are compared with the experiment for a femtosecond Cr:ZnSe oscillator operating at normal at- mospheric conditions. We prove that the spectral mod- ulation imposed by a narrowband absorption indeed ac- curately follows the associated index of refraction when the absorber linewidth is sufficiently narrow. 

