LIGHT IS SO fundamental to human existence that it was taken for granted in ancient time and not specifically listed in the classical elements of the universe.
We now understand of course, that light is electromagnetic radiation in a frequency range that is visible to the human eye. In 1917 physicist Albert Einstein predicted that light could also be produced by a process that he named ‘Stimulated Emission’.
Light produced in this way would be spatially coherent, collimated and monochromatic, meaning that it could be focused into a narrow beam capable of transmitting energy over distance and concentrating it in a small area. Enter the laser.
For 30 years laser was like a ‘solution waiting for a problem’ as it had found no real purpose, until the theory of holography (that could only be realised with laser light) was developed.
The acronym LASER, which stands for Light Amplification by Stimulated Emission of Radiation, is universally known today - but it may be of interest that the first stimulated emission device was dubbed MASER, for ‘Microwave Amplification by Stimulated Emission of Radiation’.
Most experts expected that gases would be the first substances to produce lasers, in optical and infrared wavelengths, yet it came as surprise when the first successful optical laser constructed by Maiman in 1960 employed a ruby crystal.
Today laser is used in communications, industry, medicine, and environmental care & research.
It has become one of the most powerful tools for scientists in physics, chemistry, biology and medicine throughout the world, while one of the latest fields of research is developing methods to cool and capture atoms by using laser.
At present this knowledge and technology has no specific relevance, but future applications will inevitably be based on today’s research.
Despite the fact that since their discovery literally thousands of types have been developed, lasers can still be broadly classified into five categories: Gas Discharge lasers, Semiconductor Diode lasers, Optically Pumped lasers, Diode Pumped Solid State (DPSS) lasers, and other types.
Helium neon lasers were the second type of laser to be discovered and the first to find high volume applications. Today, millions of these lasers function as barcode scanners in retail stores and numerous industrial applications.
Helium, as the major component in the gas mixture at 85%, provides the pumping medium to attain the necessary population inversion for laser action and neon is the actual lasing medium.
Noble-gas ion lasers (argon-ion and krypton-ion) are used in applications requiring high continuous-wave power in the visible, ultra violet and near infrared spectral regions.
High power, water-cooled units are found in many research laboratories and low power, air-cooled models are used in many OEM applications.
Carbon dioxide lasers exhibit laser action at several infrared frequencies, but none in the visible range. Capable of continuous output power above 10 kilowatts and extremely high power pulse operation, they are also the most efficient lasers, capable of operating at more than 30% efficiency.
Therefore CO2 lasers are chosen for a wide range of materials-processing applications, including cutting and welding of metals in fabrication. Unlike atomic lasers, CO2 lasers work with molecular transitions that can be populated thermally, so that increased gas temperature caused by the discharge will reduce the power output.
Excimer lasers take their name from the term ‘excited dimmer’ that refers to a molecular complex of two atoms, which is stable only in the electronically excited state.
Only available as pulsed lasers, these produce intense output in the ultraviolet and deep ultraviolet range and are extensively used in photolithography, micromachining and medical (refractive eye surgery) applications.
The very short wavelength of this laser enables an excimer beam to be focused to a spot diameter approximately 40 times smaller than a CO2 laser, with the same beam quality.
Semiconductor diode lasers
A laser diode, also known as an injection laser or diode laser, is a semiconductor device that produces coherent radiation when powered, by applying an electric potential to the material.
The low cost of mass-produced diode lasers makes them indispensible for mass-market applications, where other laser types would by unaffordable. Diode lasers are therefore, manufactured and sold in far greater numbers than any other type.
Applications of these lasers have been found in many different fields and can be categorised by the specific properties of light emission that they require.
The applications that primarily make use of the ‘directed energy’ property of an optical beam include the laser printers, bar-code readers, image scanning, illuminators, designators, optical data recording, laser surgery, and industrial machining.
Additionally, clinicians in medicine and dentistry have found the shrinking size of laser diode devices and their increasing user friendliness convenient for minor soft tissue procedures.
Optically pumped lasers
Optically pumped lasers use photons of light either from a lamp, or another laser as the energy source to excite electrons in the lasing medium to the upper energy levels.
The very first laser, based on a synthetic ruby crystal, was optically pumped and laser pumping is generally more efficient, due to the wavelength of the pump laser – which can be closely matched to specific absorption bands of the lasing medium, avoiding the build-up of unwanted heat.
The Neodymium-doped Yttrium Aluminium Garnet (or Nd:YAG) laser is the most common type of lamp-pumped laser and uses krypton-filled lamps, because the krypton emission is strong in the region between 750nm and 900nm.
Diode pumped solid state lasers
In order to address the optical difficulties experienced with diode lasers, a new concept was promoted in the 1980s by a group at Stanford University headed by Prof. Bob Byer which has been termed, ‘the diode-pumped solid state (DPSS) laser revolution’.
The discharge lamp for optically pumping the gain crystal in a traditional, high-efficiency, infrared laser was replaced by a diode laser source.
The infrared beam produced was efficiently converted by an intracavity non-linear crystal, into a visible beam with good mode.
Despite power losses the overall efficiency of this total electrical-to-optical conversion was several percent, compared with 0.1% efficiency of older gas lasers.
Industrial gases in laser applications
Ultra-high purity gases, doping gases and rare gases are required in the manufacture of many laser components and assemblies, using established semiconductor manufacturing techniques.
However, these high-tech operations have historically been concentrated in certain geographic areas such as ‘silicon valley’ in the US and various countries in Asia.
The application of lasers to cut and weld metals is of great interest to suppliers of industrial gases, because it has been rapidly implemented by manufacturing industries in most countries around the globe.
The vastly improved accuracy, shape precision, metallurgical quality, speed and cost efficiency of laser cut components, has enabled Laser Cutting to substantially replace oxy-fuel and other methods of cutting, in the manufacture of parts fabricated from mild steel, alloy steels, stainless steel, aluminium and titanium.
The advantages of laser welding include very narrow seam widths with considerably less distortion, compared with traditional welding methods.
Together with this change of technology, most fabricators now prefer to outsource the cutting process to specialist businesses, who not only deliver parts cut to specification, but offer design and optimisation services too.
Many of these laser cutting operators have enjoyed spectacular business growth during the past decade, and the high volumes of high purity nitrogen required to supply their consumption has been the incentive for considerable innovation by the gas supply industry.