Fiber Lasers Which Are No Fiber Lasers

Posted on 2008-06-06. Permanent link: http://www.rp-photonics.com/spotlight_2008_06_06.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on fiber lasers, master oscillator fiber amplifier, fiber lasers versus bulk lasers

There are quite a few laser products which are marketed as fiber lasers, even though they are strictly no fiber lasers. In reality, they contain some kind of laser – which may or may not be a fiber laser – and a fiber amplifier for boosting the output power. In case of high power fiber laser products, you almost always actually obtain a so-called master oscillator fiber amplifier setup.

An imprecise statement on a product is not necessarily one for which the manufacturer should be criticized. In the discussed kind of case, the product often exhibits some typical properties of fiber lasers: a high beam quality combined with high output power, a compact setup, a high wall-plug efficiency, an undefined polarization state of the output (although there are also polarization-maintaining fiber devices), and a larger emission bandwidth (which may be irrelevant for the customer).

There is one more property, though, which the customer may not be aware of: the very much higher sensitivity to any optical feedback. A high-gain amplifier will also strongly amplify any light coming back e.g. from a workpiece, and send it toward the seed laser. The effect may not only be some malfunction, but easily a serious damage. This problem may be cured by including a Faraday isolator (or even two of those, to obtain a sufficient degree of isolation), but this is often not practical, as a bulky and expensive component would have to be added to the setup. The customer may then be unhappy to find only in the small print that any reflections going back into the device must be carefully avoided.

With the advent of fiber laser products, which are technically fairly variable, one rule has become significantly more important: don't buy a laser device before you sufficiently well understand its properties – either with in-house knowledge, or by getting help from an external consultant, who is independent of the manufacturers.