Low Phase Noise Microwave Extraction From Mode-locked Er-fiber Laser

Ultra-low phase noise microwave is important for various scientific and engineering applications, for example, local oscillators, ultrafast electron sources, particle accelerators, high-resolution high-speed signal analysis instrumentation, high-performance radars and antennas, coherent communication and navigations. Although it has the advantage of low phase noise, the traditional microwave oscillator cannot meet the demand of high-frequency, broadband and high-performance because of its low-frequency and poor tenability. With the development of microwave photonics technology, the technology of generating high-performance microwave signal from optical frequency combs has been put forward because of its characteristics of ultra-low time jitter. However, it is necessary to suppress the optical amplitude noise to microwave phase noise (AM-PM) introduced by photo detector(PD). In most solutions,such nonlinearity can be overcome by state-of-the-art high-power PDs with optimized input optical power. Therefore, those solutions feature complex and high-cost.In this paper, we propose that low phase noise extraction can be achieved by a delay-matched frequency conversion pair combined with an intermediate frequency (IF) filter in between results in an equivalent narrowband microwave filter. This solution is based on the relationship between the AM-PM and the signal frequency. The AM-PM could be reduced while the high-frequency signal is converted to the intermediate frequency band. Besides, delay-matched frequency conversion pair is used to reduce the phase noise introduces by the local oscillator.In this paper, all-normal-dispersion erbium-doped mode-locked laser is set up and achieved the optical pulses with repetition frequency of 80MHz and pulse width of 407.2fs at first. Secondly, 9.978-GHz microwave source extracted from a mode-locked Er-fiber laser with-113dBc/Hz phase noise at 10 kHz offset frequency is experimentally demonstrated, when regardless the time jitter of mode-locked laser, the residual SSB noise is averagely around -155dBc/Hz under offset frequency larger than 1 kHz. Besides, large extraction tunability from 1 GHz to 10 GHz is also achieved and demonstrated. Equivalent narrowband filtering effectively could extract microwave signals with low phase noise and reduce the performance of photo detector.