Demodulation of Frequency Or Space Modulated Light
Department of Electrical Engineering, Stanford University., 1965 - Modulation (Electronics) - 192 pages
The generation and detection of a light beam which is deflected back and forth at a microwave rate in accordance with frequency modulation on the beam is considered. Deflection was achieved by sending the frequency modulated light through a dispersing element. The detection system consisted of a photocathode which emitted a transversely modulated electron beam into a transverse-field interaction circuit. General analyses were developed for the dispersion and electron beam-circuit interaction systems. The applicability of the instantaneous-frequency viewpoint was explored in detail, and a new condition for the validity of this viewpoint was derived. Phototube design procedures were formulated. Experiments demonstrated the dispersion and interaction mechanisms, including an unforeseen effect involving partial interception of the deflected light beam. The experiments supported the detailed theoretical analyses. An examination of the basic noise limitation revealed a significant advantage over certain other FM light detection systems. As optical modulation techniques improve, it is expected that this approach may represent a superior means of detection in optical communications systems. (Author).
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Basic FM light demodulation scheme
Illumination of photocathode by dispersed FM light
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amplitude modulation analysis aperture effect assume carrier cathode Chapter characteristic function chopped components consider coupled mode equations coupled mode formulation cross section cyclotron dc charge density dc current define DEMODULATOR derive desired signal detector discussed dispersing element electron beam envelope detector Eulerian finite FM light FM phototube FM sideband frequency modulation helix helix-on-rod homodyne illumination input interaction circuit kc/sec leakage lens light beam linearly polarized lineshape lock-in amplifier LOW-PASS FILTER microwave mixer modulating signal modulation index obtained parameters photocathode physical plane power output propagation constant QSS viewpoint result S/N ratio scheme shot noise shown in Fig sidebands signal-to-noise ratio significant small signal conditions space conversion criterion space conversion process spot movement spot-movement spot-size noise spurious signal SQUARE WAVE strict QSS condition strict small signal synchronism telescope term transverse modes tube unchopped noise uniform transverse field variables velocity weak QSS condition