Generation of pulses and multiplying their repetition rate using the temporal fractional Talbot effect

Abstract

The generation of pulses from a periodic phase-modulated continuous wave (CW) laser field, which is transmitted through a group-delay-dispersion (GDD) circuit, is considered. A time lens (TL), consisting of a quadratic phase modulator and a GDD circuit is proposed in combination with temporal array illuminators (TAI) using another GDD circuit. The time lens producing field compression into pulses is realized for a particular value of the normalized fractional Talbot length (NFTL) L/LT=P1/Q1, where L is the physical length of the GDD circuit, L_T is the Talbot length, P1=1, and Q₁ is an integer. The length of the GDD circuit is selected to convert a given parabolic phase-modulated CW laser field into short pulses repeated with a phase modulation period T in accordance with the chirp radar concept. If NFTL is increased by 1/Q2, where Q2=4, 6, or 8, the pulse sequence period is shortened as T/2, T/3, and T/4, respectively. This is because the additional GDD circuit with NFTL 1/Q2, performs repetition rate multiplication of the initially prepared pulse sequence as TAI does. The maximum multiplication number considered in this paper is 12, which makes it possible to reduce the time interval between pulses by a factor of 12 and obtain a repetition rate 120 GHZ of picosecond pulses generated by phase modulation with frequency f=1/T=10 GHz. The proposed method of pulse sequence generation with a discretely tunable period provides a new tool for optical signal processing in optical communication.