Non-linear frequency-sweep correction of tunable electromagnetic sources

TL;DR

This paper presents a universal pre-distortion method to significantly reduce non-linearity in tunable electromagnetic sources during frequency-sweep modulation, enhancing accuracy in applications like spectroscopy.

Contribution

The authors introduce a simple, universal pre-distortion technique that minimizes non-linear frequency responses in tunable electromagnetic sources, demonstrated on QCLs and ozone spectroscopy.

Findings

Non-linearity reduced by a factor of ten using a single distortion parameter.

Frequency scale error decreased by two orders of magnitude in ozone spectrum measurement.

Method applicable to various electromagnetic sources, broadening potential applications.

Abstract

Tunable electromagnetic sources, such as voltage controlled oscillators (VCO), micro electromechanical systems (MEMS) or diode lasers are often required to be linear during frequency-sweep modulation. In many cases, it might also be sufficient that the degree of the non-linearity can be well controlled. Without further efforts, these conditions are rarely achieved using free running sources. Based on a pre-distortion voltage ramp, we develop in this letter a simple and universal method that minimizes the non-linear frequency response of tunable electromagnetic sources. Using a current-driven Quantum Cascade Laser (QCL) as an example, we demonstrate that the non-linearity can easily be reduced by a factor of ten when using a single distortion parameter $\gamma$. In the investigation of the IR absorption spectrum of ozone at 10\,$\mu$m, an even better reduction of the frequency scale error by two orders of magnitude is obtained by using the pre-distortion method to generate an essentially purely quadratic sweep frequency dependency which can be inverted easily to retrieve precise molecular line positions. After having tested our method on a variety of electromagnetic sources, we anticipate a wide range of applications in a variety of fields.