Generation of linear frequency-modulated signals with improved time-bandwidth product based on an optical frequency comb.

A generation scheme for the linear frequency-modulated (LFM) signals with tunable carrier frequency and improved time-bandwidth product (TBWP) using an optical frequency comb (OFC) and a fiber Bragg grating (FBG) time delay line is proposed and demonstrated by simulation. In the scheme, intensity modulation is used to convert the continuous time-domain waveform to an optical pulse with a duty cycle of one-sixteenth, and two cascaded polarization modulators driven by two different radio frequency signals are used to generate 16-line OFC without an optical filter. Then the 16-line OFC is injected into the FBG time delay line, which consists of 16 discrete FBGs, and each spectral line is reflected by the specific FBG after different time delays. By properly designing the position of the FBGs, the 16 spectral lines of the 16-line OFC are separated in the time domain. The time-separated OFC is then modulated by a phase modulator driven by a periodic parabolic signal for introducing parabolic phase modulation. Subsequently, after the heterodyne beating between the phase-modulated optical signal and the local oscillator light wave emitted from a tunable laser source (TLS) in a photodiode, an electrical LFM signal with improved TBWP is generated. Simulation results show that the generated LFM signal has the bandwidth of 31.78 GHz and TBWP of 1365.33, and its center frequency can be tuned by varying the wavelength of the TLS.