Abstract
We theoretically and experimentally investigate a phase-modulated frequency-shifting loop (PM-FSL) that includes an electro-optic phase modulator (EOPM) and a gain element, and is seeded by a single-frequency laser. By slightly detuning the modulation frequency of the EOPM off an integer multiple of the fundamental loop frequency, we generate an output waveform that exhibits a series of pulse doublets modulated at radio frequency (RF). We prove that the series consists of sinusoidal frequency-modulated (SFM) pulse doublets whose repetition rate and bandwidth are easily reconfigurable. We report the generation of the SFM waveforms with bandwidth above 7 GHz (limited by the detection bandwidth) by simply tuning the input RF tone over a span of a few kHz in the vicinity of 14.58 MHz (the round-trip frequency). The system is modeled using a time-delayed interference model that accounts for the modulation function of the EOPM, the loop delay time, and the detuning parameter. The model explains the formation of SFM pulse doublets and effectively reproduces all the experimental waveforms. This well-defined waveform may find applications in RF-optical signal processing and radar systems.
Highlights
W AVEFORMS whose instantaneous frequency or phase varies nonlinearly with time are of primary importance for applications such as pulse compression radar [1], radiofrequency (RF) signal processing [2]–[3] and communications [4]
In line with the theoretical results obtained with fm = p fc + Δf, the time response of the phase-modulated frequency-shifting loops (FSLs) in the small detuning case consists of sinusoidal frequency-modulated (SFM) pulse doublets with a repetition rate equal to the RF frequency applied to the electro-optic phase modulator (EOPM)
We have proposed and experimentally demonstrated a phasemodulated FSL for the generation of SFM waveforms
Summary
W AVEFORMS whose instantaneous frequency or phase varies nonlinearly with time are of primary importance for applications such as pulse compression radar [1], radiofrequency (RF) signal processing [2]–[3] and communications [4]. In contrast to the case of linear frequency modulation, pulse compression radar with nonlinear frequency modulation does not require amplitude weighting for the suppression of the resulting sidelobes, and filter adjustment with much steeper edges but low time-sidelobes becomes possible [5]–[6]. In this way, the losses in the signal-to-noise ratio. We further investigate a phase-modulated frequency-shifting loop (PM-FSL) in which a common electro-optic phase modulator (EOPM) replaces the AOFS.
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