A practical guide to loss measurements using the Fourier transform of the transmission spectrum

TL;DR

This paper provides a practical guide for using Fourier transform techniques to analyze the loss characteristics and multimodedness of optical devices from transmission spectra, emphasizing data preparation and interpretation.

Contribution

It introduces detailed procedures and considerations for applying Fourier transform analysis to optical transmission data, including effects of sampling, windowing, and zero padding.

Findings

The waveguide is multimodal with two modes having similar group indices.

Different optical losses were identified for the two modes.

The method effectively reveals internal loss characteristics of optical components.

Abstract

Analyzing the internal loss characteristics and multimodedness of (integrated) optical devices can prove difficult. One technique to recover this information is to Fourier transform the transmission spectrum of optical components. This article gives instruction on how to perform the transmission measurement, prepare the data, and interpret the Fourier spectrum. Our guide offers insights into the influence of sampling, windowing, zero padding as well as Fourier spectrum peak heights and shapes which are previously neglected in the literature but have considerable impact on the results of the method. For illustration, we apply the method to a Bragg-reflection waveguide. We find that the waveguide is multimodal with two modes having very similar group refractive indices but different optical losses.