Ultra-narrow-linewidth mode-locked lasers with wide wavelength tunability can be versatile light sources for a variety of newly emergent applications. However, it is very challenging to achieve the stable mode locking of substantially long, anomalously dispersive fiber laser cavities employing a narrowband spectral filter at the telecom band. Here, we show that a nearly dispersion-insensitive dissipative mode-locking regime can be accessed through a subtle counterbalance among significantly narrowband spectral filtering, sufficiently deep saturable absorption, and moderately strong in-fiber Kerr nonlinearity. This achieves ultra-narrow-linewidth (a few gigahertz) nearly transform-limited self-starting stable dissipative soliton generation at low repetition rates (a few megahertz) without cavity dispersion management over a broad tuning range of wavelengths covering the entire telecom C-band. This unique laser may have immediate application as an idealized pump source for high-efficiency nonlinear frequency conversion and nonclassical light generation in dispersion-engineered tightly light-confining microphotonic/nanophotonic systems.

We propose and demonstrate an agile X-band signal synthesizer with ultralow phase noise based on all-fiber-photonic techniques for radar applications. It shows phase noise of ?145dBc/Hz (?152dBc/Hz) at 10 kHz (100 kHz) offset frequency for 10 GHz carrier frequency with integrated RMS timing jitter between 7.6 and 9.1 fs (integration bandwidth: 10 Hz–10 MHz) for frequencies from 9 to 11 GHz. Its frequency switching time is evaluated to be 135 ns with a 135 pHz frequency tuning resolution. In addition, the X-band linear-frequency-modulated signal generated by the proposed synthesizer shows a good pulse compression ratio approximating the theoretical value. In addition to the ultrastable X-band signals, the proposed synthesizer can also provide 0–1 GHz ultralow-jitter clocks for analog-to-digital converters (ADC) and digital-to-analog converters (DAC) in radar systems and ultralow-jitter optical pulse trains for photonic ADC in photonic radar systems. The proposed X-band synthesizer shows great performance in phase stability, switching speed, and modulation capability with robustness and potential low cost, which is enabled by an all-fiber-photonics platform and can be a compelling technology suitable for future X-band radars.