Ultra-Low Phase Noise Microwaves from Optical Signals.

摘要

Continuous-wave lasers locked to high-finesse optical reference cavities are oscillators that produce ∼500 THz optical signals with unprecedented stability. Indeed, sub-femtosecond fractional frequency instability at one second averaging can now be achieved. A self-referenced femtosecond laser frequency comb (FLFC) is used as a frequency divider to provide a phase-coherent link between optical and microwave domains, dividing the frequency down to the gigahertz range while also transferring the stability of the original signal. Photodetectors then convert the optical pulses into electronic signals. The resultant 10 GHz microwave signals have ultra-low phase noise below -100 dBc/Hz at 1 Hz offset, surpassing that of traditional microwave oscillators. This new approach offers significant improvement for many applications that rely on stable microwave signals, and may even create new measurement technologies otherwise unachievable with current signal sources.;In reality, fundamental and technical sources of noise in each stage of the optical-to-microwave generation process limit the ultimate achievable stability of the signal. Optical reference cavities are limited by environmental effects and thermal fluctuations, and FLFC dividers suffer from intrinsic timing jitter, amplitude noise, and limited stabilization servo bandwidth. However, it is the seemingly straightforward photodetection of optical pulses that proves to be the limiting factor in the ultimate noise floor of these signals.;In this thesis, I describe the noise limitations of each part of the optical-to-microwave scheme, particularly focusing on the noise limitations of photodetection. I will give a basic representation of these photodetection noise phenomena in terms of the physical behavior of optically-generated electrons in semiconductor photodiodes. The two main photodetection noise phenomena---shot noise and amplitude-to-phase conversion---will be thoroughly characterized in the context of generation of 10 GHz low phase noise signals. Finally, I will use this characterization of photodetector noise to choose optimal photodetectors and operating conditions to realize unprecedentedly low phase noise signals with a variety of optical-to-microwave generation schemes.