Fiber Optics: Research and Development

Abstract

With twisted stacks of N polarizers P or retarders R, the polarization of a light beam can be cycled around the Poincaré sphere on N similar arcs of great P or small R circles. We calculate the phase changes around these cycles (geometric for P; geometric + dynamical for R). In the continuum limit N → ∞ of a smoothly twisting medium, a P stack forces the light to follow its changing polarization, and the phase is the solid angle of the associated loop on the sphere; for an R stack, on the other hand, it is only in the adiabatic limit of slow twist (where the dynamical phase is large) that the geometric phase corresponds to that of the loop specified by the changing eigenpolarization of the medium. The predicted phase shifts are observed as fringe shifts in an interferometer for N=2, 3 and 4.     

A 120-MHz Bandwidth Linear Signal Transmission System Using Fiber Optics

A complete linear fiber-optic-signal transmission system with an upper frequency limit greater than 150 MHz is described. The transmitter accepts signals in the range of 1 to 15 mV and amplitude modulates an LED. The emitted light signal is carried via fiber optics to a receiver employing a silicon avalanche detector and an amplifier which increases the signal level to the hundreds of millivolts range. Sine-wave bandwidth is 30 kHz-120 MHz (voltage response down to 70 percent), and phase distortion is low, thus preserving pulseshapes. For a transmission path consisting of 15 m of low-cost fiber cable, the dynamic range is 30 dB with harmonic content 30 dB below signal.