Lasers

Lasers The light from lasers differs from ordinary light in several important aspects. Ordinary light from a light bulb travels randomly in all directions (unless the bulb is equipped with an integral reflector that directs the light). The light is thus incoherent. Even when incoherent light is directed with a reflector, it still spreads rapidly. The light from a laser is temporary and spatially coherent.

This means that all of the wave-fronts of light are lined up in time and space (see Diagrams). The waves of light go up and down in sync, and travel in the same direction. Coherent light spreads less than other types of light. For example the beam of a tightly focused flashlight would spread between 2 degrees and 5 degrees over a 3 meter (10 ft) throw distance. The sides of a laser beam are almost parallel but the light still spreads slightly. This spread is called divergence and is measured in milliradians (mrad).

If a laser has a specified divergence of 5 mrad, then in the above example with a 3 meter throw (10 ft), a laser beam will spread only about 3/20 of a degree. This is a simplified explanation of the process of stimulated emission. If you are interested in more detailed information about this subject, you should consult a science or physics book. Let us take the HeNe laser as an example. If a glass tube were filled with a mixture of helium and neon gas; and an electrical current were applied to the electrodes, the gas would emit light energy.

This glowing gas is referred to as a plasma. You are already familiar with this glowing gas in the form of the neon signs you see at your favourite restaurants. We now have a neon tube but not a laser so let’s take a closer look at how the laser’s light is produced. Under normal conditions the electrons in a gas atom orbit at a fixed distance and pattern around the nucleus; this is the ground state or most stable configuration of the atom. When an electrical charge travels through the gas in the tube (energy is pumped into the gas), it excites or stimulates the atoms.

Some of the electrons absorb this energy by jumping up to the next stable orbit. This configuration is unstable. The electron wants to return to its regular orbit, the ground state. As the excited (stimulated) atoms in the gas relax back to the ground state, some of the energy that excited the electron(s) is emitted (released) in the form of random photons of light This is called spontaneous emission. This is how a neon sign (or other gas discharge light such as a mercury vapour lamp) produces light.

The photons travel rapidly in all directions. They are visible along the length of the neon tube or radiate outward from the light source. The spontaneous emission is not enough to cause lasing action. Lasers are very different from neon tubes in that they amplify the glowing effect via stimulated emission. Stimulated emission can only occur when there is a “population inversion” in the energy state of the lasing medium (in this case gas).

Laser tubes are designed in a long narrow configuration with a central bore. At either end of the bore there are mirrors. These mirrors must be held in precise alignment for the laser to work properly. In most HeNe lasers the mirrors are permanently attached or sealed onto the ends of the tube — sometimes referred to as hard seal technology. In higher power lasers the mirrors are usually not mounted on the ends of the tube itself, but on an external resonator that forms part of the laser frame.

This allows for changing the mirror optics or adding a littrow prism if a specific output wavelength (colour) is required. The mirrors must be perfectly aligned so that the emissions from the gas in the tube will be amplified. Some of the photons of light randomly emitted by the relaxing gas atoms will be travelling parallel to the bore (centre) of the laser tube. These photons will strike the mirror at the end of the tube and will be reflected back through the excited gas (plasma). When the photons travelling parallel to the bore are reflected from the mirrors, they oscillate back and forth between the mirrors.

An air-cooled laser tube with cooling fins – the connections for the cathode/filament are visible on the right. In their travels through the plasma, some photons strike other atoms that are in the excited state. The excited atoms are stimulated into relaxing to the ground state and releasing their duplicate photons. The groups of photons travel back and forth through the lasing medium (gas) reflecting from the mirrors at either end. They build up sufficient energy to overcome optical losses, then lasing begins. All of the above activities take place almost instantaneously (at the speed of light) when the tube is started. The mirrors form an optical “amplifier” allowing for the amplification and stimulation of the lasing medium (gas) in the cavity (tube) to produce light (photons).

If the mirrors were both totally reflective, the light would remain trapped inside the tube. In fact the high reflector is coated to 99.9% reflectivity (it should be 100% but nothing in life is perfect) so as to reflect the maximum amount of light. At the other end of the tube, the output reflector is coated between 90% and 97% reflectivity. Thus between 3% and 10% of the light in the tube is allowed to “leak” out as a laser beam which you see in light shows. This “leaking” light would drain all of the energy from the plasma if it were not for the electrical power that is continuously applied to the tube.

The electrical power keeps the plasma energised (ionised) and allows the laser to produce light continuously. Some types of lasers do have a cycle where energy is pumped into the lasing medium, then released in a short burst of laser energy. This type of laser is referred to as a pulsed laser and usually produces very high power levels. The most common type of laser used in professional light shows and displays is the Argon (Ar) laser. The argon laser gives a cyan coloured beam that can be broken into blue and green beams using a yellow dichotic or a prism.