Optical frequency combs: Properties and applications.

摘要

The powerful ideas originally behind optical frequency combs (OFC) are now materializing in experiments which are revolutionizing optical frequency metrology, optical-atomic clocks and coherent control. Optical frequency combs are emitted by pulsed lasers, such as Titanium:Sapphire (TiS) modelocked lasers, whose pulse durations are on the order of femtoseconds. Within these lasers, approximately 106 discrete optical frequencies or modes are made coherent with one another through nonlinear processes in the cavity's gain medium. The power of the OFC is that each one of its frequencies, which are in the vicinity of 250 THz may be known very precisely by extracting two radio frequency parameters. The first parameter is the spacing between the frequency modes, given by the repetition rate frep of the laser pulses, and the second is the starting point of the modes (from a dispersionless cavity), given by the offset frequency, f 0. When these two parameters are held fixed using electronic phase-locked loops, the optical frequency comb modes are held fixed and the OFC may be used as an optical frequency 'ruler'.;I study fundamental properties of the OFC, both in the time and frequency domain. In the time domain, the optical pulses undergo periodic recompression due to cavity dispersion. The balance between the competing mechanisms of linear dispersion and the nonlinear Kerr effect ensures that the pulse is sustained. I demonstrate that the pulse dynamics inside the femtosecond laser cavity are well modeled by an asymptotic theory for dispersion-managed solitons where only one fitting parameter is used. In the frequency domain, it is possible to extract very low phase noise signals in the optical and microwave domains from OFCs that are phase stabilized to narrow linewidth optical references. I demonstrate that for time scales of less than 1 second, the OFC exhibits phase stability in the range of mHz/Rad1/2 over its entire spectral bandwidth. I show that we can predict the scaling of the phase stability of the optical frequency comb modes with the frequency difference from the optical lock point using the radio frequency (RF) phase locked parameters of the OFC. In the microwave domain, I demonstrate the coupling between amplitude noise on the TiS's pump source to amplitude and phase noise on microwave signals extracted from OFCs. I mitigate amplitude noise effects by implementing a high-bandwidth phase-locked loop to suppress the noise by leveraging the sensitivity of the noise offset frequency to pump power. I obtain approximately 30 dB of noise suppression (approaching Shot noise levels) of the TiS optical power and this directly results in reduced noise in the microwave signal. Finally, as a demonstration of the use of the powerful ideas behind OFCs, I have applied the OFC as an optical frequency 'ruler' to measure very precisely frequency differences, by referencing two CW diode lasers to the OFC to generate phase-stable and broad-bandwidth radiation in the terahertz regime.