Optical pulse generation by manipulations in FM spectra

Abstract

The method of optical pulse generation described here employs an optical switching of CW phase-modulated signal using Sagnac interferometer incorporating asymmetrically located phase modulator. The phase modulator provides a dual function: (i) optical spectrum broadening and (ii) phase shifting between counterpropagating fields to achieve the switching. Owing to nonreciprocity introduced by the phase modulator, the totally reflecting loop becomes totally transmitting, when an instantaneous differential phase shift ΔΦ=π between counterpropagating fields is attained. It was found that in the frequency domain this switching accomplishes spectral filtering required for pulse formation without the need for filters or etalons. FM-sidebands are fully coupled in sense of being tightly locked together with the phase characteristic corresponding to a constant amplitude in the time domain. By suppression of certain frequency sidebands, one can obtain the optical spectrum with frequency components having relative phases that correspond to optical pulses. In other words, spectral filtering allows the phase modulation to be converted into the intensity modulation. The square-pulse-phasc modulation used here instead of sinusoidal modulation is an efficient method for broadening the FM-spectrum. When pulse risetime τ is much shorter then modulation period T, the switching lime window becomes short and thus high value of duty cycle and shorter pulses can be obtained. Since edges of the electrical pulse could be well fitted by linear approximation, optical pulses generated at the edges are essentially chirp-free.