Laser Beam Properties, Coherence, and Efficiency

Calculating Laser Beam Size

It is simple to calculate the size of a laser beam. For example, if the divergence is 2 mils, after the beam has traveled 10 meters, its radius is 20 mm and its diameter is 40 mm.

Laser Coherence and Wave Properties

Electromagnetic radiation is an oscillatory phenomenon. The electric and magnetic fields oscillate in a sine wave over time. However, these oscillations do not remain constant for long and are not always the same at all points in the space occupied by the light. The degree of relationship between these oscillations at different times and in different positions defines the coherence of radiation.

The coherence of the radiation determines the visibility of interference phenomena (the constructive superposition of waves) that are used in certain laser measurement methods. Lasers have a very high degree of coherence in comparison with other light sources because of their generation mechanism. While many applications of lasers do not require coherent light, consistency is essential for:

• Holography
• Precision measurement

In a comparison between a regular light source and a laser, ordinary light disperses, whereas laser light travels in a specific direction.

Efficiency and Power Requirements

Besides wavelength, power output, and pulse rate features, there are two major factors to consider: efficiency and power requirements. Lasers differ greatly in how efficiently they convert supplied energy (usually in the form of electricity) into light.

Like other light sources, lasers are often not very efficient in generating light; the best conversion of input energy to light is about 20%. There are several types of lasers that convert only 0.01% to 0.001% of the input energy into light.

Efficiency is a very important consideration for higher power output. There is not much problem if a 1 mW laser produces 1 watt of lost heat because it is easy to dissipate, but it is very difficult to remove the 1 million watts of waste heat produced when a 1 kW (1000 W) laser operates at the same low efficiency.

Internal Operation and Optical Components

The internal operation of lasers depends on physics. Laser emission depends on how atoms and molecules emit light, which is determined by their internal structure. Similarly, lasers are optical devices, and the use of optics is required, including:

• Laser mirrors
• Lenses
• Prisms
• Mirrors to direct the beam