Coherent light that is typically produced by lasers is light in which the electromagnetic waves maintain a fixed phase relationship over a period of time, and in which the phase relationship remains constant for various points in the plane that is perpendicular to the direction of propagation.
Coherence length of a light source characterizes the degree of temporal coherence of the emitted light, lC = ctC, where c is the light speed and tC is the coherence time, which is approximately equal to the pulse duration of the pulsed light source or inversely proportional to the wavelength bandwidth ДА of a CW light source, tC ~ A2/(cAA). A single frequency CW gas-discharge He-Ne laser with a narrow bandwidth ДА = 10-6 nm and wavelength A = 632.8 nm has a coherence length lC ~ 400 m; a multimode diode laser with ДА = 30 nm and A = 830 nm has lC ~ 23 pm. For a titanium sapphire laser with A = 820 nm, the bandwidth may be as big as 140 nm; therefore, coherence length is very short lC ~ 2 pm. The shortest lC ~ 0.9 pm is for a white light source (ДА = 400 nm). Coherence length is a fundamental parameter for optical coherence tomography (OCT); lower the lC value, the better is the image resolution that can be achieved. OCT systems based on a titanium sapphire laser or a white light source allows one to image skin with a subcellular resolution of 1-2 pm.
Monochromatic light is light of one color (wavelength) only, ideally produced by a CW single-frequency well-stabilized laser.
Quasi-monochromatic light is light that has a very narrow but nonzero wavelength (frequency) bandwidth; it can be presented as a group of monochromatic waves with a slightly different wavelength.
Nonmonochromatic light has a broad wavelength bandwidth and can be presented as many groups of monochromatic waves with different wavelengths.