Lasers are categorised on the basis of the “active medium” used to generate the laser beam. This medium may be a solid, liquid or a gas. Lasers with a solid medium are divided into crystal-type solids, termed solid state lasers, and semiconductor lasers. The following table lists some typical lasers and the wavelengths emitted by them.
Lasers produce electromagnetic radiation in the wavelength range 180nm in the ultra-violet to 1mm in the infra-red (a range which includes visible light). The light from a laser usually has low divergence i.e. it does not spread out much with distance. The light is referred to as collimated and forms a beam. This means that the light from a laser could still be hazardous at a large distance from the laser. If the beam from a laser is continuous (referred to as continuous wave or cw) the power output is measured in Watts (W) or milliwatts (mW). If the beam comes in short bursts the laser is pulsed and the output can be measured as the energy of each pulse in Joules (J). If these pulses are produced at regular time intervals (a constant train of pulses) the number of pulses per second (pps) is referred to as the pulse repetition rate. The output of a pulsed laser can also be measured as an average power by multiplying the energy per pulse by the pulse repetition rate. The energy and the duration of each pulse (the pulse length) determine the peak power (peak power = pulse energy/pulse length), which may be very high. For example a single pulse of 1mJ with a pulse length of 1 nanosecond will have a peak power of 1 megawatt. Hence pulsed lasers with low energy pulses and/or low repetition rates may have low average powers, but they could be extremely hazardous due to the high peak power in each individual pulse.
The light from a laser can normally be considered as being at a single wavelength - the light is referred to as being monochromatic. All the power in the laser is concentrated at the one wavelength. But not all lasers are monochromatic, some produce very short pulses of light with a very large range of wavelengths present and some lasers are tuneable - the wavelength can be changed.
The light from a laser is normally very coherent - the individual waves are in-step or in-phase along and across the beam. The combination of the low divergence, the mono-chromaticity and the coherence gives the laser light very high brightness and allows the beam to be focussed, leading to potentially very high irradiance (Wm- 2) or radiant energy (Jm-2).