Question

Why are lasers monochromatic?

Answer 1

Today the term 'laser' refers to coherent light sources, so the answer is almost tautological; lasers are by definition coherent! But how do lasers achieve this? Coherence means that the light waves matched everywhere; think of a many parallel sine waves, lined up perfectly. How this coordination occurs depends on the type of laser. In a HeNe laser the existing photons "stimulate" Neon atoms to emit additional photons in lock-step. In a free-electron laser, the mechanism is different, but the picture is the same; electrons group into microbunches, and the electrons in the microbunches all emit cooperatively. In both cases, the cooperative production of radiation leads to coherence.

The monochromatic aspect of lasers is essential for coherence. Again, think of coherence in terms of parallel waves, lined up perfectly. If the periods differ, then if the waves align at one point, they fall out of step elsewhere. (Think of trying to align two sine waves with different periods over infinite time. It's not possible!) So coherence requires some degree of monochromaticity. Again the mechanism depends on the laser type; in the HeNe laser, the energy difference of two electronic states determines the energy of the emitted photons, which determines the wavelength of the emitted light. In a free-electron laser, the emitted photon energy is determined by the resonant condition of the undulator. In both cases, the smaller the range of wavelengths produced, the higher the degree of coherence that can be achieved.

Answer 2

The light from a laser comes from one atomic transition with a single precise wavelength. So the laser light has a single spectral color and is almost the purest monochromatic light. however, the laser light is not exactly monochromatic.