Asynchronous upconversion sampling of frequency modulated combs

We demonstrate an asynchronous optical sampling technique for the temporal characterization of frequency modulated combs. On the basis of a mid-infrared quantum cascade laser frequency comb, we measure both its instantaneous intensity and optical frequency.

Optical frequency combs (OFCs) [1] are coherent sources whose spectrum consists of a set of discrete, equally spaced spectral modes.Following Fourier's theorem such spectra are produced by any periodic amplitude modulation in time domain, with the most prominent example being a regular train of ultrashort pulses.On the other extreme, also periodic frequency modulated light fields associated with a constant intensity give rise to OFCs.In recent years, novel semiconductor laser sources were identified to have such a predominantly frequency modulated output.This included quantum cascade [2], interband cascade [3], quantum dot [4] and quantum dash [5] lasers, but also conventional diode lasers [6].With the emergence of these sources also temporal characterization techniques experienced a rebirth [7,8].
Optical sampling techniques permit to measure light fields directly in time domain.This approach has proven particularly successful in the terahertz frequency range [9], but can also be applied to shorter wavelength ranges provided that the sampling pulses are considerably shorter than the time scales of the field to be measured.In this work, we use such an optical sampling technique to measure both the instantaneous intensity and frequency of a frequency modulated OFC.Near-infrared ultrashort pulses are employed to asynchronously sample the intensity of a mid-infrared quantum cascade laser (QCL) frequency comb [2].With the help of a tunable optical bandpass filter, we acquire a spectrogram of one repetition period of the comb.The experimental configuration is shown in Figure 1.
The obtained results are shown in Figures 2 a and b.For the full QCL spectrum transmitted through the optical filter, we observe a close to constant intensity waveform, while for the bandpass filtered spectrum, we observe a picosecond pulse.By linearly tuning the spectral bandpass filter from the low to high frequency side of the QCL spectrum, we observe a close to linear increase in the pulses group delay.The obtained results are in excellent agreement with phase-resolved spectral measurements [7] as shown in Figures 2 c and d.
We interpret the results in Figure 2 in the framework of the generalized nonlinear Schrödinger equation [10].In this picture, the frequency modulated nature of the emitted field arises from a predominantly phase-dependent potential term, physically originating from cross-steepening non-linearities.This frequency modulation follows a close to linear chirp, with the sign of the chirp determined by the sign of the effective dispersion.By independently measuring the intracavity dispersion, we expect the chirping direction observed in Figure 2 b.Asynchronous upconversion sampling is not limited to the characterization of frequency modulated combs.In fact it also allows to measure short and ultrashort [11] mid-infrared pulses given the high temporal resolution (∼ 100 fs).This makes it a versatile tool for the study of novel frequency comb sources.Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

Fig. 1 .Fig. 2 .
Fig. 1.Asynchronous upconversion sampling.(a) A modelocked (MLL) is used to asynchronously sample the output of a QCL frequency comb.(b) Optical sampling is accomplished using sumfrequency generation in AgGaS 2 .TBP: tunable optical bandpass filter, ISO: optical isolator, PM: pick-off mirror, BC: beam combiner, RF: Radio frequency synthesizer, FPD: Fast (∼ 15 GHz) photodiode, M: frequency mixer, PS: phase shifter, PBS: polarizing beam splitter, LP: low pass filter, BP: optical bandpass filters, APD: avalanche photodiode.(c) In order to assign each intensity sample to its respective waveform position, the frequency difference Δ f is additionally acquired in the experiment.It corresponds to the difference between the QCL repetition frequency f QCL rep and the high beattone k × f MLL rep of the MLL lying closest in frequency.