Train Of Light Pulses Research Articles

Optical frequency shifting of a mode-locked laser beam

Further progress in the development of an optical frequency shifter will be reported. In this device, light pulses emerging from a mode-locked He-Ne laser at a repetition rate of 100 MHz, are passed through an electro-optic phase modulator (LiNbO 3 ) driven by a 100 MHz sine wave phase-locked to the light pulse train. By properly adjusting the rf phase, the optical frequency of the light pulses can be up-shifted or down-shifted by an amount which is proportional to the maximum rate of change of the refractive index. Shifts as large as ±5 GHz have been observed with a scanning Fabry-Perot interferometer. Interferograms showing clearly resolved modes were recorded. The Gaussian-shaped mode spectrum of the mode-locked laser beam, about 1.5 GHz wide at half height, was shifted up or down along the frequency scale without any visible change in shape, in agreement with theory. For these measurements the beam made a single pass through a 3-cm long crystal of LiNbO 3 . The shifts are cumulative with the number of passes. Work on larger shifts produced after several passes through the crystal will be reported.

Flicker contours for intermittent photic stimuli of alternating duration.

Flicker studies generally employ a train of light pulses consisting of a single period of stimulation repeated serially. In a recent study, fusion points were measured using trains of light pulses in which alternate periods were of different duration. Fusion points obtained with this type of stimulation are best described by a fusion contour which identifies those combinations of two periods which, when alternated serially, result in a transition point between flicker and fusion. In the present study, data have been obtained on three flicker contours each of which identifies those combinations of two periods which, when alternated serially, have the same apparent rate of flicker. A matching technique was used to measure the subjective rate of trains of light pulses. Data points were obtained from observers who matched the apparent rate of a variable stimulus (composed of two periods of alternating duration) to the apparent rate of a standard stimulus (composed of a single period repeated serially). Standard rates of 33.3, 28.6, and 25 cps were used. The fusion contour for each of the two observers in the experiment is also shown. The fusion and flicker contours are not dichotomous: as the frequency of the standard stimulus is increased, the flicker contours tend to resemble more closely the shape of the fusion contour. The contours illustrate the limiting values within which various combinations of two alternating periods may have the same apparent rate: (1) The sum of the durations of the two alternating periods must be greater than the duration of the period of the standard stimulus. (2) The sum of the durations of the two periods must be less than twice the duration of the period of the standard stimulus.