Maximum Permissible Exposure (MPE)

The level of laser radiation to which a person may be exposed without hazardous effects or adverse biological changes in the eye or skin.

Meniscus Lens

A lens which has one side convex, the other concave.

Metastable State

The state of an atom, just below a higher excited state, which an electron occupies momentarily before destabilizing and emitting light. The upper of the two lasing levels.


A unit of length in the International System of Units (SI) equal to one millionth of a meter.


An abbreviated expression for micro meter which is the unit of length equal to 1 millionth of a meter.


A term used to describe how the power of a laser beam is geometrically distributed across the cross section of the beam. Also used to describe the operating mode of a laser such as continuous or pulsed.

Mode Locked

A method of producing laser pulses in which short pulses (approximately 10-12 second) are produced and emitted in bursts or a continuous train.


The ability to superimpose an external signal on the output beam of the laser as a control.

Monochromatic Light

Theoretically, light consisting of just one wavelength. No light is absolutely single frequency since it will have some bandwidth. Lasers provide the narrowest of bandwidths that can be achieved.


Laser emission at several closely spaced frequencies.

M2 Factor (Quality Factor)

In most laser applications it is necessary to focus, modify, or shape the laser beam by using lenses and other optical elements. In general, laser-beam propagation can be approximated by assuming that the laser beam has an ideal Gaussian intensity profile, corresponding to the theoretical TEM00 mode. Coherent Gaussian beams have peculiar transformation properties that require special consideration. In order to select the best optics for a particular laser application, it is important to understand the basic properties of Gaussian beams. Unfortunately, the output from real-life lasers is not truly Gaussian (although helium neon lasers and argon-ion lasers are a very close approximation). To accommodate this variance, a quality factor, M2 (called the “M-square” factor), has been defined to describe the deviation of the laser beam from a theoretical Gaussian. For a theoretical Gaussian, M2=1; for a real laser beam, M2>1. Helium neon lasers typically have an M2 factor that is less than 1.1. For ion lasers, the M2 factor is typically between 1.1 and 1.3. Collimated TEM00 diode laser beams usually have an M2 ranging from 1.1 to 1.7. For high-energy multimode lasers, the M2 factor can be as high as 3 or 4. In all cases, the M2 factor, which varies significantly, affects the characteristics of a laser beam and cannot be neglected in optical designs.