Costas-coded linear frequency modulation waveform generation based on a VCO-controlled Fourier domain mode locking optoelectronic oscillator

A novel approach to generating tunable Costas-coded linear frequency modulation waveforms (LFMWs) from an optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The OEO works under Fourier domain mode locking, where the oscillation modes are selected by using a fast-scanning microwave bandpass filter based on phase-modulation-to-intensity-modulation conversion via stimulated Brillouin scattering. Costas coding is realized by using an open-loop voltage-controlled oscillator (VCO) with a fast tuning speed to generate an agile fast-scanning probe light via electro-optic frequency shift. The bandwidth of the generated Costas-coded LFMWs can be tuned by varying the voltage range of the low-frequency waveform applied to the VCO, and the center frequency can be finely tuned by varying the electro-optic frequency shift of the pump light. In the experiment, Costas-coded LFMWs with a 12-bit Costas code sequence of [9 5 7 12 6 4 1 8 11 10 2 3], a period of 20.39 µs and tunable frequency range are generated. The cross-correlation result with a time delay of 102.5 µs indicates that the generated Costas-coded LFMWs have excellent pulse-to-pulse coherence. In addition, the bad phase noise performance of the open-loop VCO has a negligible influence on the generated Costas-coded LFMWs. Benefited from employing an open-loop VCO with a fast tuning speed and a broad operation bandwidth, this approach has potential in generating agile broadband multi-format radar waveforms with low phase noise and excellent pulse-to-pulse coherence directly from an OEO.