engleski

Frequency comb spectroscopy and applications

Optical frequency combs are an excellent tool for frequency metrology and high precision spectroscopy. They are produced by mode-locked lasers, whose optical spectrum consists of a series of narrow equidistant modes spanning hundreds of terahertz of bandwidth. The frequency of the nth comb mode is given by, where is the repetition rate of the laser and is the carrier envelope offset frequency. Thus, the frequency of each comb line can be controlled by measuring and adjusting the two radio frequencies. This straightforward connection between optical and radio frequency domains makes the frequency combs an excellent tool for precision metrology. In addition to serving as frequency references, optical frequency combs can be used to perform spectroscopy directly [1]. In particular, direct frequency comb spectroscopy (DFCS) combines the vast spectral coverage of a mode-locked laser with the extremely high spectral resolution provided by each individual comb component. The technique is virtually equivalent to a simultaneous measurement with tens of thousands of narrow laser lines, which allows high resolution multiplexed detection of many gaseous species. Moreover, due to its regular spectrum, the frequency comb can be efficiently coupled into an external high finesse cavity, providing enormous enhancement of the interaction length with the sample. Cavity-enhanced (CE-) DFCS can reach absorption sensitivities down to 10*(^-10) cm-1 Hz-1/2 per spectral element [2] and has thus the potential to become a real-time, highly sensitive, broad-bandwidth, high resolution technique for applications such as breath analysis, atmospheric research, studies of cold molecules and process control in semiconductor manufacturing. In this talk, the principles of the technique as well as possible detection schemes and comb-to-cavity coupling strategies of CE-DFCS will be presented. Initial demonstrations of DFCS were limited to the near-infrared wavelength range, which was dictated by the working range of the available mode-locked lasers. The recent development of frequency comb sources in the mid-infrared wavelength range provides access to fundamental transitions of a wide range of molecules and higher sensitivity in terms of gas concentrations. We will present a broadband high resolution mid-infrared frequency comb-based Fourier transform spectrometer operating in the important molecular fingerprint spectral region of 2100 - 3600 cm-1 (2.8 - 4.8 µm) [3]. Due to the high spectral brightness of the frequency comb and the access to strong molecular transitions, gas concentrations at low part-per-billion (ppb) levels have been measured in a multi-pass cell with acquisition times less than 1 minute for various species which are of importance for breath analysis and atmospheric studies. The current absorption sensitivity, 2 × 10(^-8) cm-1 Hz-1/2 per spectral element, is expected to improve by two orders of magnitude by an implementation of an external cavity, bringing the detection limits down to the part-per-trillion (ppt) range. [1] A. Marian, M. C. Stowe, J. R. Lawall, D. Felinto and J. Ye: ''United time-frequency spectroscopy for dynamics and global structure'', Science 306, 2063-2068 (2004). [2] M. J. Thorpe and J. Ye: ''Cavity-enhanced direct frequency comb spectroscopy'', Appl. Phys. B 91, 397-414 (2008). [3] F. Adler, P. Maslowski, A. Foltynowicz, K. C. Cossel, T. C. Briles, I. Hartl and J. Ye: ''Mid-infrared Fourier transform spectroscopy with a broadband frequency comb'', Opt. Express 18, 21861-21872 (2010).

frequency comb; cavity enhanced spectroscopy; broadband; high sensitivity

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