ASE-FFT-based DBR reflection spectrum reconstruction and internal reflection analysis in monolithically integrated...

This study proposes a non-destructive method for analyzing internal reflection characteristics in a monolithically integrated tunable electro-modulated laser (TEML). An amplified spontaneous emission (ASE)-based fast Fourier transform (ASE-FFT) is applied to below-threshold spectra to obtain a distance-domain reflection map. Selective inverse FFT of the distributed Bragg reflector (DBR) section then reconstructs its intrinsic Bragg spectrum on-chip, without external light sources or interferometers. Experiments show 21% DBR reflectivity at 1542.2 nm and a ∼3.2 nm redshift of the Bragg wavelength under a 50 mA DBR current. An antireflection coating at the output facet reduces reflection by ∼11 dB, and residual active-passive interface reflections are quantified. Harmonic amplitude ratio analysis estimates cavity loss, and electro-absorption modulator bias produces absorption-induced spectral narrowing with a baseline rise in the distance-domain response. Beyond TEML, the ASE-FFT method offers a low-complexity, real-time tool for spatially resolved reflection mapping and spectral characterization of photonic integrated circuits.