High spectral coherence in long-pulse and continuous free-electron laser: Measurements and theoretical limitations

Single mode operation was exhibited on the Israeli tandem free-electron laser (FEL). This enabled judicious measurements of narrow spectral linewidth, frequency chirp, and relaxation-oscillation effects. Exact 3D simulations of the FEL oscillator showed good agreement with the measurements, and permitted an estimation of the fundamental Schawlow-Towns linewidth limit of the FEL (including “α effect”) as well as technical noise linewidth limits. We estimate that with voltage-controlled stabilization high-power (10 kW) tunable (over 60% bandwidth) quasi-cw coherent [/(Δv/v_)_rms˜10⁻¹⁰] mm-FIR (far infrared) radiation is attainable in the tandem FEL.