Attosecond timing jitter modelocked lasers and ultralow phase noise photonic microwave oscillators

Abstract

Photonic microwave oscillator based on optical frequency comb and ultrastable optical reference cavity represents the state-of-the-art solution to generate X-band microwaves of ultralow phase noise. Such high-quality microwave source enables a range of applications in which frequency stability and timing accuracy are essential to performance. Wide use of this technology, however, requires compact system architecture, low-term stability and low energy consumption, which drive the needs to develop high repetition-rate femtosecond lasers alternative to Ti:sapphire technology, and to explore a feasible means to achieve integrated photonic microwave oscillators. Ultrafast Cr:LiSAF lasers can be directly pumped with low-cost red laser diodes, and the electrical-to-optical conversion efficiency is as high as 10%. High repetition-rate femtosecond Cr:LiSAF lasers are developed with the help of semiconductor saturable absorber technology, efficient dispersion compensation mirror design algorithms, and heat management of the saturable absorber. The I-GHz Cr:LiSAF oscillator generates 55-fs pulses with 110 pJ pulse energy, which represents almost two orders of magnitude improvement in the output peak power over previous results. Timing jitter of 1 00-MHz Cr:LiSAF lasers is measured with a single-crystal balanced optical cross-correlator to be -30 as from 10 kHz to 50 MHz. Pump intensity noise coupled into phase noise through the self-steepening effect proves to be the major noise source. The most recent advance in silicon photonics and wafer-scale three-dimensional integration technology illuminates a pathway toward on-chip photonic microwave oscillators. Phase noise model of the proposed Erbium Silicon Photonics Integrated OscillatoR (ESPIOR) suggests that it is possible to achieve comparable noise performance with the Ti:sapphire-based system, without the need of carrier-envelope-offset frequency detection. A demonstration using fiber-optic components further indicates that it is practicable to realize optical frequency division and microwave readout in the proposed architecture. With the advancement of heterogeneous electronic-photonic integration, it would pave the way for an ultralow-noise microwave source fully integrated in a hybrid photonic-electronic chip on a silicon substrate.