Photoacoustic Substitution Method for Calibrating Subterahertz Attenuation in Free Space

A terahertz (THz) attenuation calibration system is developed using a photoacoustic (PA) detector for free-space beams. Based on the principle of the PA detector, the system measures the THz attenuation by substituting radio frequency (RF) attenuation with audio frequency (AF) attenuation. The AF attenuation at 1 kHz is directly calibrated using an inductive voltage divider (IVD) as the reference standard. The calibration is demonstrated for metalized film attenuators up to 20 dB at 0.11 THz. A Gunn oscillator with a corrugated horn antenna is used as the source, and its radiated beam is collimated using an off-axis parabolic mirror. Sources of uncertainties are also discussed in detail, and the expanded uncertainty is found to be 0.40-0.46 dB (k = 2).