Generation of Linear Frequency-Modulated Waveforms by a Frequency-Sweeping Optoelectronic Oscillator

In this paper, a simple scheme for linear frequency-modulated (LFM) waveform generation based on a frequency-sweeping optoelectronic oscillator is proposed and demonstrated. The OEO is built up with an optically injected semiconductor laser and the oscillation frequency can be tuned by adjusting the optical injection strength. By applying an injection strength controller in the OEO for rapid frequency sweeping, an LFM microwave waveform can be generated. When the sweep period of the output frequency matches with the round-trip time of the OEO cavity, signal quality of the generated LFM waveform can be significantly enhanced by the high Q optoelectronic oscillation. In the experiment, an LFM signal with a bandwidth as large as 7 GHz, a chirp rate reaching 0.18 GHz/ns, and a time-bandwidth product (TBWP) up to 2804.2 is generated. The corresponding electrical spectrum is a frequency comb with a contrast as high as 47 dB. Based on this system, an improved scheme for extending the frequency and bandwidth of the generated LFM signal is proposed by employing a polarization modulator to implement microwave photonic frequency multiplication. With this method, an LFM waveform with a TBWP as large as 13839.1 (bandwidth 15.6 GHz; temporal period 887.12 ns) is obtained.