ABSTRACT
The ability to generate lower-noise microwaves has greatly advanced high-speed, high-precision scientific and engineering fields. Microcombs have high potential for generating such low-noise microwaves from chip-scale devices. To realize an ultralow-noise performance over a wider Fourier frequency range and longer time scale, which is required for many high-precision applications, free-running microcombs must be locked to more stable reference sources. However, ultrastable reference sources, particularly optical cavity-based methods, are generally bulky, alignment-sensitive and expensive, and therefore forfeit the benefits of using chip-scale microcombs. Here, we realize compact and low-phase-noise microwave and soliton pulse generation by combining a silica-microcomb (with few-mm diameter) with a fibre-photonic-based timing reference (with few-cm diameter). An ultrastable 22-GHz microwave is generated with -110 dBc/Hz (-88 dBc/Hz) phase noise at 1-kHz (100-Hz) Fourier frequency and 10-13-level frequency instability within 1-s. This work shows the potential of fully packaged, palm-sized or smaller systems for generating both ultrastable soliton pulse trains and microwaves, thereby facilitating a wide range of field applications involving ultrahigh-stability microcombs.
ABSTRACT
Simple multicolor electro-optic sampling-based femtosecond synchronization of multiple mode-locked lasers is demonstrated. Parallel timing error detection between each laser and a common microwave is achieved by wavelength division multiplexing and demultiplexing. The parallel timing error detection enables simultaneous femtosecond synchronization of more than two mode-locked lasers to the microwave oscillator, even when the lasers have different repetition rates. The residual root-mean-square (rms) timing jitter of laser-laser synchronization measured by an optical cross correlator is 2.6 fs (integration bandwidth, 100 Hz-1 MHz), which is limited by the actuator bandwidth in the laser oscillator. The long-term rms timing drift and frequency instability of laser-microwave synchronization are 7.1 fs (over 10,000 s) and 5.5×10-18 (over 2000 s averaging time), respectively. As a versatile and reconfigurable tool for laser-laser and laser-microwave synchronization, the demonstrated method can be used for various applications ranging from ultrafast x-ray and electron science facilities to dual- and triple-comb spectroscopy.
ABSTRACT
Frequency-stabilized optical frequency combs have created many high-precision applications. Accurate timing, ultralow phase noise, and narrow linewidth are prerequisites for achieving the ultimate performance of comb-based systems. Ultrastable cavity-based comb-noise stabilization methods have enabled sub-10-15-level frequency instability. However, these methods are complex and alignment sensitive, and their use has been mostly confined to advanced metrology laboratories. Here, we have established a simple, compact, alignment-free, and potentially low-cost all-fiber photonics-based stabilization method for generating multiple ultrastable combs. The achieved performance includes 1-femtosecond timing jitter, few times 10-15-level frequency instability, and <5-hertz linewidth, rivalling those of cavity-stabilized combs. This method features flexibility in configuration: As a representative example, two combs were stabilized with 180-hertz repetition rate difference and ~1-hertz relative linewidth and could be used as an ultrastable, octave-spanning dual-comb spectroscopy source. The demonstrated method constitutes a mechanically robust and reconfigurable tool for generating multiple ultrastable combs suitable for field applications.
