Experimental and theoretical characterization of nonuniform temperature effects in heater-tuned DBR laser diodes: ASE-FFT reflectance measurement and temperature-dependent coupled mode theory

Heater-tuned distributed Bragg reflector laser diodes (DBR-LDs) with strongly thermally confined passive waveguides achieve high wavelength tuning efficiency but are theoretically expected to suffer from pronounced nonuniform temperature distributions along the cavity that distort the DBR reflectance spectrum. However, in such structures, the nonuniform temperature distributions and the resulting DBR reflectance spectra have not been experimentally verified. Furthermore, the in situ measurement of DBR reflectance lacks a rigorous mathematical basis, and conventional coupled mode analyses, which assume a uniform coupling coefficient, remain insufficient. To address these issues, we present an integrated experimental and theoretical characterization of a fabricated heater-tuned DBR-LD featuring a reverse-mesa beneath an etched-mesa buried heterostructure waveguide. The nonuniform temperature distribution along the cavity is experimentally verified by thermoreflectance measurements and quantitatively benchmarked against three-dimensional electro-thermal finite-element simulations. Building on the amplified spontaneous emission fast Fourier transform (ASE-FFT) technique, which was established for gain extraction in Fabry–Pérot laser diodes, we propose an ASE-FFT-based reflectance measurement method for heater-tuned DBR-LDs, for which an explicit mathematical formulation identifying the extracted quantity as the gain-reflectivity product is derived. To interpret the measured spectra, a temperature-dependent coupled mode theory (T-CMT) is developed, incorporating a spatially varying coupling coefficient and effective refractive index profiles determined from the computed temperature distribution. The two are complementary: the measurement method yields the DBR reflectance spectra directly from the measured ASE, independently of the T-CMT, while the T-CMT reproduces these spectra and thereby accounts physically for the measured redshift, side-lobe growth, and stop-band width broadening.