Compact Optoelectronic THz Frequency Domain Spectroscopy System for Refractive Index Determination based on Fabry-Perot Effect

In this work, we present a compact optoelectronic THz frequency domain spectroscopy (FDS) system for determination the real part of the refractive index of a semiconductor wafer with a given thickness. The concept is based on the detection of transmission maxima, which appear due to Fabry-Perot interferences inside the wafer and which depend on the refractive index of the semiconductor material. This all-fiber based THz FDS setup consists of two external cavity laser diodes and an uni-traveling-carrier photodiode (UTC-PD) module on the emitter side, while a Schottky barrier diode (SBD) is used as THz receiver. Since we don't need any additional lenses and because of the small device dimensions, this setup is compact in size, compared with traditional bulky TDS systems. We prove our THz FDS concept by characterizing of a semi-insulating iron-doped indium phosphide (InP:Fe) wafers with different thicknesses within a frequency range from 220 GHz up to 450 GHz. Based on the determination of the free spectral range (FSR) between the Fabry-Perot transmission maxima, a refractive index of 3.475 for this frequency region is obtained. Additional THz time domain spectroscopy experiments match the THz FDS results very well and confirm our results. Furthermore, analytic calculations are in excellent agreement with the measurements. A planned transfer of this THz FDS approach to a completely hybrid or monolithic integration of all photonic devices in a compact module could be offer a very small and full mobile THz spectroscopy setup.