What is the role of photonic techniques in generating millimeter wave and terahertz signals?

Photonic techniques are the primary method for generating coherent signals in the millimeter-wave (30-300 GHz) and terahertz (0.3-10 THz) frequency ranges, where electronic oscillators struggle to produce sufficient power or spectral purity: (1) Optical heterodyning: two phase-coherent laser beams (frequencies f₁, f₂) illuminate a high-speed photodetector. The photocurrent contains a beat tone at f_beat = |f₁ - f₂|. By setting the frequency difference to the desired mmWave or THz frequency: any frequency from DC to the PD bandwidth is generated. For mmWave (30-300 GHz): standard InGaAs PIN PDs or UTC PDs (bandwidth: 40-100 GHz). Output power: -10 to +10 dBm. For THz (0.3-3 THz): photomixers (specialized THz PDs or photoconductors). Output power: -30 to -10 dBm (much lower). (2) Optical frequency comb: a mode-locked laser or electro-optic comb generator produces multiple evenly spaced optical frequencies (f₀, f₀+f_rep, f₀+2f_rep, ...). The repetition rate f_rep is typically 10-100 GHz. Selecting two comb lines and beating them on a PD generates a mmWave signal at N × f_rep (where N is the index difference). All comb lines are phase-coherent (derived from the same laser), so the beat signal has very low phase noise. (3) Photonic multiplication: modulate the RF signal onto the optical carrier. The modulator generates harmonics (at 2f, 3f, 4f... of the RF input). Filter to select the desired harmonic. Detect on a PD. Effective multiplication: ×2, ×4, ×6 of the input RF frequency. Example: 10 GHz input → ×6 → 60 GHz output. (4) Key applications: 5G/6G millimeter-wave systems (28, 39, 60, 100+ GHz): photonically generated mmWave carriers distributed via fiber to remote radio heads. THz imaging and spectroscopy: THz signals generated by photomixing for non-destructive testing, security screening, and material characterization. Radio astronomy: phase-coherent LO distribution to telescope arrays (e.g., ALMA uses photonic LO generation at 90-900 GHz). Radar: 77 GHz and 140 GHz radar signals generated with low phase noise using photonic techniques.