Abstract: Methods for measuring the diameters of transparent filaments with high precision, e.g., RMS standard deviations of less than 0.02 microns, are provided. The methods involve determining the average spatial frequency .omega. of the far-field interference pattern produced by illuminating the filament with a beam of laser light. The average spatial frequency is determined by performing a fast Fourier transform (FFT) on the interference data to obtain a coarse estimate for the average and then performing a set of discrete sequence Fourier transforms (DSFTs) in the region of the coarse estimate to obtain the desired high precision estimate of the average. Efficient on-line processing procedures are provided so that real time measurements can be performed on, for example, a moving optical waveguide fiber, at rates of 500 measurements/second and above.

Abstract: A tapped delay line (38) terminated with its characteristic impedance at both ends is connected to receive a differential input signal in intermediate taps (27, 29) of a delay line. Delay line taps (25, 27, 29, 31), as well as ends (21, 23) are connected symmetrically to two pair of summing networks (59, 61 and 83, 85) where voltage signal components having different delays from the input signal are linearly combined. One pair of summing networks (59, 61) produces a balanced differential output signal which is an amplitude equalized and differentiated version of the input signal, while the other pair of summing networks (83, 85) produces another balanced differential output signal which is a different amplitude equalized version of the input signal. Each version is needed in one of the channels of a dual channel signal detector for detecting the peaks of the variable amplitude input signal.