Abstract
A photonics-based multi-band linearly frequency-modulated (LFM) waveform generator with reconfigurable center frequency, bandwidth and time duration is proposed and demonstrated. By introducing two coherent optical frequency combs (OFCs) with a frequency shift and different free spectral ranges (FSRs) as multi-frequency optical LOs, a set of LFM signals with different center frequencies will be generated if one of the combs is modulated by an intermediate-frequency (IF) LFM signal. The center frequencies of the generated RF-LFM signals can be flexibly tuned by adjusting the frequency shift between the two OFCs. In addition, by introducing a series of proper time delays to the LFM signals and combining them, a frequency-stepped LFM signal can be generated. Furthermore, when the bandwidth of the IF-LFM signal equals the difference of the comb FSRs, and the time duration of IF-LFM signal equals the time delay of the consecutive channels, a LFM signal with both bandwidth and time duration multiplied can be obtained. With N comb lines, the maximum achievable time-bandwidth product (TBWP) is N × N times of the applied IF LFM signal. A proof-of-concept experiment is carried out. A set of LFM signals with frequencies ranging from L to Ka bands are generated. By introducing proper time delays, a frequency-stepped LFM signal with frequency steps between 10 GHz and 20 GHz is also produced. In addition, LFM signals with the bandwidth and time duration multiplied by 2 and 5 are realized (4-GHz bandwidth, 2-μs time duration and 10-GHz bandwidth, 5-μs time duration), respectively. Correspondingly, the TBWPs are increased by 4 and 25 times.
Highlights
Modern radar systems are evolving to have multiple functions, demanding reconfigurable waveforms over multiple bands [1,2,3]
The center frequencies of the generated multi-band signals can be tuned by adjusting the frequency shift between the two optical frequency combs (OFCs)
The linearly frequencymodulated (LFM) signals centered at 5 GHz and 25 GHz, with the timebandwidth product (TBWP) multiplied by 25 (i.e., 5 × 104, 10-GHz bandwidth and 5-μs time duration), are realized by combining five time-delayed channels
Summary
Modern radar systems are evolving to have multiple functions, demanding reconfigurable waveforms over multiple bands [1,2,3]. One typical method to generate the LFM signal in the optical domain is to employ space-to-time mapping [6] or frequency-to-time mapping [7], which, can only produce RF signals with very small time duration (several nanoseconds). Another method is based on optical heterodyning of two linearly chirped optical pulses with different chirp rates [8,9,10,11,12]. We propose and experimentally demonstrate a photonic approach for multiband LFM waveform generation with reconfigurable bandwidth, center frequency and time duration. The LFM signals centered at 5 GHz and 25 GHz, with the TBWP multiplied by 25 (i.e., 5 × 104, 10-GHz bandwidth and 5-μs time duration), are realized by combining five time-delayed channels
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