Phase noise and frequency are sometimes interchangeably used to describe the spectral noise of a laser, although in different but related units of measure. They describe the signal source’s frequency and phase instability and are directly related as frequency is just the time rate of change of phase. Phase noise is most often measured in units of dBc/Hz or rad2/Hz where frequency noise in units of Hz2/Hz or Hz/√Hz.  For applications where the laser is to be locked to an external cavity or an atomic transition, power spectral density of the phase noise is useful to determine the widest locking bandwidth possible.