Local oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the frequency of the local oscillator and frequency of the input signal of interest. These are the beat frequencies. Normally the beat frequency is associated with the lower sideband, the difference between the two.
Several local oscillators can be strung in series to form a local oscillator chain (LO chain).
Historically, most applications of local oscillators were widely used in radio physics over frequencies from kHz to GHz. After spreading of nonlinear optics, the same terms and principles apply also to the optical frequencies of order of 
Hz.