Real-time dual frequency comb spectroscopy in the near infrared

Abstract

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 µs. 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.Ever since the femtosecond frequency comb was invented in the late 1990s [1], there have been ongoing revolutions in the field of spectroscopy. Many methods have been and are being developed to utilize the regular comb structure of millions of laser modes in spectroscopy ranging from the XUV to the mid IR [2][3][4][5]. Among these, dual frequency comb spectroscopy (DFCS) emerges as a promising, highly sensitive, superior fast spectroscopy with high resolution to complement the traditional Fourier Transform Spectroscopy (FTS) [2,[6][7][8][9][10][11][12]. Especially in the near and mid IR, a plethora of greenhouse and other gases have molecular fingerprint spectra that can be studied with DFCS based mainly on Er-or Yb-doped fiber lasers and their wavelength ranges extended by optical parametric oscillation process [13], supercontinuum, or difference frequency generation [14].DFCS uses two femtosecond frequency combs with slightly different repetition rates. In the time domain, pulse pairs arrive at a photo detector (PD) with a linear increasing time delay. Each time a pulse pair overlaps in time, like the zero optical path difference in FTS, the central burst of an interferogram is formed. Subsequent pairs of pulses impinge on the PD with varying delay, analogous to the delay introduced by FTS, except no moving mirror parts are needed. As a result, the PD records an interferogram formed by many pulse pairs of various delay. Because pulse pairs repeatedly move through each other, a new interferogram starts to form as soon as the previous is completed. In the frequency domain, comb lines of one source beat with the same order comb lines of the other source, and the optical frequency information is down converted to the radio frequency range, which is the Fourier transform of the interferogram. After frequency up conversion and calibration, the optical spectrum is recovered.Compared to traditional FTS, DFCS is extremely fast, and an interferogram can be recorded in less than 25 µs [9]. A fast oscilloscope can display an interferogram and the broadband spectrum by Fast Fourier Tranform (FFT) in real time. If the repetition rates and carrier envelope offset (CEO) frequencies of both combs are well stabilized, several seconds of the interference signal can be transformed to a spectrum with high resolution at high signal-to-noise ratio (SNR) ...

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