How do I measure the power of a terahertz source using a calibrated detector?

Measuring the power of a terahertz source using a calibrated detector requires careful attention to the THz beam coupling, detector calibration, and atmospheric effects because THz power levels are typically very low (microwatts to milliwatts) and the measurement environment significantly affects the result. The measurement procedure is: select a calibrated detector (for absolute power measurement: a Thomas Keating (TK) power meter is the gold standard (uses a broadband absorber and thermopile; NIST-traceable calibration; response time approximately 1-3 seconds; sensitivity approximately 1 uW). Alternatively: a calibrated Golay cell or pyroelectric detector with a known responsivity curve. For relative measurements: a Schottky diode detector is faster but requires calibration against an absolute standard), set up the measurement (mount the THz source and detector on an optical bench with precise alignment; the THz beam must be fully captured by the detector aperture (if the beam is wider than the detector: use a focusing lens or mirror to concentrate the beam; if the beam is narrower: the detector captures all the power directly); the beam path should be enclosed in a dry nitrogen purge or vacuum to eliminate atmospheric absorption losses), account for atmospheric absorption (if measuring in ambient air: correct for the atmospheric absorption along the beam path; at 300 GHz in standard atmosphere (50% RH): absorption is approximately 5 dB/m. At 557 GHz (water absorption line): absorption is approximately 100+ dB/m. Purging the beam path with dry nitrogen (less than 1% RH) reduces the absorption to negligible levels), determine the detector's frequency-dependent responsivity (most THz detectors have a frequency-dependent sensitivity; use the calibration certificate to determine the responsivity at the source frequency; if the source is broadband: integrate the responsivity over the source spectrum), and calculate the power (P_source = V_detector / (R_v x eta_coupling), where V_detector is the measured voltage, R_v is the detector responsivity (V/W), and eta_coupling is the optical coupling efficiency between the source and detector).