Three-page invited OFC paper, Anaheim, March 2001. The PDF title is Frequency response metrology for high-speed optical receivers; its exact byline is Paul D. Hale, Tracy S. Clement and Dylan F. Williams, National Institute of Standards and Technology. Page 1, Heterodyne measurements: two single-frequency lasers near 1319 nm, variable attenuators, beam splitter, polarizer and single-mode fibre produce mode-matched beating at a tunable difference frequency. Pages 1–2, equations 1–4 and Figure 1, separate the optical modulation model from electrical loss and impedance-mismatch corrections. No current instrument specifications are inferred from this historical apparatus.