Thursday, December 23, 2010

Rare Earth Metals: Ideal Optically Pumped Lasing Material?

Given that rare earth metals based optically pumped lasers are widely used since the 1960s and until now proof that rare earth metals are an ideal optically pumped lasing material?

By: Ringo Bones

In the 1960s till now, optically pumped lasers produce higher outputs – in terms of both energy and peak power – than those in the discharge excited gas lasers or electron injection lasers or semiconductor lasers. Even as far back as 1964, optically pumped lasers were already capable of producing pulse energies of 2,000-joules and peak powers of 5-gigawatts or 5-billion watts. Optically pumped lasers are still the widely used type both in practical development and in laboratory research. In optically pumped lasers, the atoms in the laser material are indirectly excited by light from an intense light source – such as a xenon flash lamp – that is often coiled, snake-like, around the material. Like the other types of lasers, optically pumped lasers may either operate in a pulsed or continuous wave. A pulsed laser produces a burst of radiation that lasts about a thousandth – or in some cases, a few billionths – of a second. In a continuous-wave laser, the intensity achieved is much less, but the laser beam comes out in a continuous stream.

Although there are perhaps 20 or so optically pumped solid-state laser materials, like those calcium chloride crystals doped with samarium that can produce laser beams intense enough to burn metal or bounce off the Moon, 4 are the most dominant: ruby, neodymium doped glass, neodymium-doped calcium tungstate, and dysprosium-doped calcium fluoride. The ruby and glass lasers normally operate at room temperatures, and in the pulse mode. The neodymium-doped calcium tungstate and dysprosium-doped calcium fluoride can readily be operated to emit continuous waves; however, the dysprosium-doped calcium fluoride must be cooled to cryogenic temperatures about that of liquid air.

There is even a class of organic liquids, such as nitrobenzene, that exhibits simulated Raman emission when optically pumped by another laser. A plastic laser, developed in 1963, uses europium chelates dissolved in a clear plastic fiber as the lasing material to produce crimson pulses when excited by ultraviolet light. Lasers of this sort are also used as a light source in Raman spectroscopy – a spectroscopic technique of great value in organic chemistry based on an effect discovered by the Indian physicist Sir Chandrasekhara Venkata Raman – and has greatly facilitated studies in that field.

Ever since as far back as the 1960s, medical-grade optically pumped lasers were already used in eye surgery where it is used to treat retinal detachment especially if the retina – the light-sensitive tissue at the back of the eye tears loose from the eyeball – blindness may result. When a laser beam is focused through the lens of the eye, the intense burning ray may cause scar tissues to form at the point of separation, reattaching the retina by fusing it to the underlying tissue. While today’s medical-grade optically pumped lasers used in ophthalmological applications are in the more popular cost-effective Lasik eye surgery. Optically pumped lasers are proof yet again that almost all of our high-tech tools are very dependent on rare earth metals in their construction and operation.


  1. Actually, Sir Chandrasekhara Venkata Raman won the Nobel Prize in Physics in 1930 - way before lasers were invented. When it comes to lasers, aren't semiconductor lasers - despite of their relatively low power output, at least the one's being sold to us anyway - the most efficient type? I mean they have an efficiency as high as 50% whereas optically pumped lasers only have a maximum efficiency of 4% - though I haver yet to see a semiconductor laser using rare-earth elements as the primary lasing material.

  2. Semiconductor lasers were already around in the mid-1960s where a special type of photodiode - made of intermetallic compounds - is used as a semiconductor laser. This device consists of a tiny p-n junction of gallium arsenide (the same type used in the night-vision camera in that "notorious" Paris Hilton video) with the highly polished front and back faces cut parallel to each other perpendicular to the junction plane. When high-current pulses are sent through the device, intense coherent light in the infrared region is emitted perpendicular to the polished surfaces along the p-n junction. Efficiencies approaching 100 per cent are possible for converting electrical energy to light.