By Tony Hoult
Nobody would disagree that real change is occurring in the laser industry. In the broadest context, what is happening is a shift from the conventional technique of assembling lasers using free space optical components to a technique based on splicing together fiber based components. It is now very widely accepted that this brings a range of benefits, not least of which are dramatic improvements in reliability, stability and ease of use. This shift can be seen across the whole spectrum of the laser industry from multi-kilowatt lasers to low power ultra-short pulse length lasers where more and more lasers use the key word ‘fiber’ in their descriptions.
But let us take a step back; the phrase fiber laser (or the English spelling ‘fibre’ laser) arrived in the laser lexicon in 1962 with the invention of the earliest devices by Elias Snitzer, he was the first to show that a laser beam can actually be generated within the fiber itself, hence the term ‘fiber laser’. Despite a slow start low power fiber lasers developed rapidly to play a key role in the telecom boom (and bust) in the late 90’s. Since then very rapid progress has been made in scaling up laser power, recently an announcement was made of the development of a 100 kW fiber laser! This scaling up of power, performance and reliability has brought rapid penetration of the industrial laser market at the expense of the incumbent old technology lasers and real commercial success has come to those leading this technology – fiber lasers have become a hot topic. As a result, the phrase ‘fiber laser’ and even the word ‘fiber’ itself is being applied far too loosely in the multi-kilowatt laser market to associate some laser products with this success that are clearly not truly fiber lasers. The aim of this article is to clarify what is a ‘fiber laser’ and what is not.
The problem is that the word ‘fiber’ is sometimes used inappropriately in a product title or description to identify a laser that is not a ‘fiber laser’, it is simply a ‘fiber delivered’ laser. A ‘fiber delivered’ laser beam is one that is generated using conventional free space optics technology via other solid state media such as rods , discs or slabs and is then focused down into a fiber and delivered to the workpiece via this flexible fiber optic cable. This ‘fiber delivery’ technique has been in use with flashlamp-pumped solid state lasers for almost 30 years and does indeed reduce many problems associated with building lasers systems in that the laser can be remote from the actual workstation. More recently this technique is now employed by direct diode lasers where complex optical techniques are used to combine very many individual diode laser beams into a fiber for delivery to the workpiece. Although these lasers are still ‘free space’ devices in one sense, the diode lasers can be squeezed into a much smaller ‘free space’ and the benefits of fiber delivery are maintained.
Other solid state free space laser resonators based on slabs or discs of doped optical crystals have also been developed to avoid some of the issues associated with large diameter rod lasers but NONE of these other high power laser types are fiber lasers – sometimes the names of fiber delivered lasers can be truly misleading.
A real fiber laser is a totally different disruptive technology that has many major attributes that none of these other laser types can match. The very essence of a real fiber laser, and this does bear repeating is that the beam is actually generated in the fiber itself. This is achieved by writing optical gratings into the fiber and these act as the partially and totally reflecting mirrors required to make up the laser resonator. This active fiber or gain medium within which the laser beam is generated is actually drawn down contiguously with layers of cladding. Pump light is introduced into the cladding layer by splicing larger core diameter fibers from laser diodes into the cladding and the outer cladding confines this pump light. This elegant solution to the problem of generating laser beams is scaleable simply by increasing the length of the active fiber, by increasing the number of pump diodes or for high power applications where focusability is not so important, using optical combiners to combine many beams together. In this way the average power of fiber lasers is being increased to average powers far in excess of anything available from other technologies.