This study presents a reliability-oriented powercycling setup that integrates in-gel GaAs fiber-optic sensing with standard TSEP (Thermal Sensitive Electrical Parameter)-based junction temperature (Tj) estimation inside a Power Cycling Test Bench PCTB-E-1200-30 for industrial IGBT modules. The GaAs probe (OpSens OTG-PM) is equipped with a retractable ceramic guide to pierce silicone gel and, according to the vendor specifications, achieves ±0.8 ◦C system accuracy with full EMI immunity, thus enabling direct, local Tj sensing during cycling. We apply the method to an Infineon FF450R12ME4 (EconoDUAL 3, 1200V/450A) and crosscheck it against TSEP (measuring VCE at a constant Isense) natively supported by the PCTB. Using calibration data, we build an uncertainty budget for the optical channel and obtain an expanded (95%) uncertainty of ≈0.92 ◦C. Additionally, this study provides initial Zth transients and a two-pole Foster interpolation that establishes a quantitative baseline for future thermal-model validation under modified boundary conditions.