Sensitivity of SWIFT spectroscopy

Abstract

SWIFT spectroscopy (Shifted Wave Interference Fourier Transform Spectroscopy) is a coherent beatnote technique that can be used to measure the temporal profiles of periodic optical signals. While it has been essential in understanding the physics of various mid-infrared and terahertz frequency combs, its ultimate limits have not been discussed. We show that the envelope of a SWIFTS interferogram is physically meaningful and is directly related to autocorrelation. We derive analytical expressions for the SWIFTS signals of two prototypical cases—chirped pulses from a mode-locked laser and a frequency-modulated comb—and derive scaling laws for the noise of these measurements, showing how it can be mitigated. Finally, we confirm this analysis by performing the first SWIFTS measurements of near-infrared pulses from femtosecond lasers, establishing the validity of the technique for highly-dispersed sub-picojoule pulses.

Toward new frontiers for terahertz quantum cascade laser frequency combs

“…Phase measurements on THz QCLs to assess a genuine FC operation regime, performed with both SWIFT spectroscopy and FACE, proved that FWM establishes a tight phase relation among the modes emitted by the analyzed devices, resulting in stable Fourier modal phases [39][40][41][42]. The retrieved phase relations among the modes are never trivial (i.e., not linear, like in pulsed operation combs), leading to a frequency and amplitude modulated laser emission, different from that obtained by passive modelocking pulsed operation.…”

“…One option to assess the phases is the shifted wave interference Fourier-transform (SWIFT) spectroscopy [40,41] that gives access to the phase domain by measuring the phase difference between adjacent FC modes. Such a procedure allows retrieving the phase relation of continuous portions of the FC spectrum by a cumulative sum on the phases.…”

Physics and technology of Terahertz quantum cascade lasers

“…The presence of a single and sharp (linewidths in the kHz range) IBN represents a good precursor for a genuine comb operation. However, such a technique lacks information about the stability of the Fourier phases, that can be extracted via alternative optical techniques as the shifted-wave interference Fourier-transform spectroscopy (SWIFTS) [131,132], which gives access to the phase domain by measuring the phase difference between adjacent FC modes or via the Fourier analysis of the comb emission (FACE) [123,133]. Relying on a multi-heterodyne detection scheme, this technique is capable of real-time tracing the phases of the modes emitted by the FC, thus working even if the comb presents spectral gaps.…”

Mid-infrared, long-wave infrared, and terahertz photonics: introduction

“…In addition to coherent emission, the feature issue also presents advances in the understanding of radiative relaxation in silicon-based heterostructures [5], structures for selective absorption and thermal emission [6], and techniques for understanding the physics of MIR and terahertz frequency combs [7]. Ciano et al study the radiative relaxation of n-type Ge/SiGe quantum wells that are optically excited using a free-electron laser [5].…”