Optical Frequency Comb from Modelocked Lasers

Abstract

This chapter gives a basic introduction to optical frequency combs from modelocked lasers, for which the exact position of the carrier (i.e., the electric field below the pulse envelope) needs to be controlled. This carrier-envelope offset (CEO) was introduced in 1999 and is referred to as the carrier-envelope offset phase or the carrier-envelope phase. Normally, no locking mechanisms exist between envelope and carrier inside a modelocked laser oscillator, and the relative CEO phase experiences random fluctuations. This chapter gives a basic introduction to optical phase and frequency noise, starting from the basic mathematical principles, CEO phase noise measurement and stabilization techniques based on power spectral densities (PSDs), the connection with intensity noise and timing jitter, integrated noise, phase time and fractional noise, polynomial models with white and flicker noise, and Allan variance. The connection between the optical laser spectrum and phase noise is discussed in detail, using the useful concept of the \(\beta \)-separation line. The basic principle of active stabilization and the different laser technologies for optical frequency combs are reviewed.