Why Fiber Amplifiers, not Fiber Lasers?

Posted on 2008-12-16. Permanent link: http://www.rp-photonics.com/spotlight_2008_12_16.html

Author: Dr. Rüdiger Paschotta, RP Photonics Consulting GmbH

Ref.: encyclopedia articles on fiber lasers, fiber amplifiers, high power fiber lasers and amplifiers, master oscillator power amplifier; spotlight 2008-06-06

Some time ago I wrote an article on "Fiber Lasers Which Are No Fiber Lasers". I explained that many "fiber lasers" are actually fiber amplifiers, or MOPAs = master oscillator power amplifiers, also called MOFAs = master oscillator fiber amplifiers. However, I forgot to discuss why this is the case. There are actually different reasons.

In fiber lasers, the optical intensities in the fiber core are very high, even for moderate power levels, because the effective mode area is rather small. For high power fiber lasers, the intensities are extremely high, even when large mode area fibers are used. In a fiber amplifier, the maximum occurring power in the fiber equals the output power (apart from some small coupling losses), whereas the intracavity power in a laser is higher. Particularly the average intensity in the fiber can be lower when an amplifier rather than a laser setup is used. This reduces the strength of nonlinear effects, which are often limiting the performance of fiber devices.

Another issue is that certain properties of the output would be difficult to control with a direct laser approach. This applies to the optical bandwidth, to wavelength tuning, and particularly to cases where short or ultrashort pulses are required. It is much easier to control such properties in a low-power seed laser, and to more or less preserve them in the fiber amplifier.

Unfortunately, there are also disadvantages of using an amplifier. Obviously, the complexity of the system gets larger, and usually one requires more than one pump source. A high-gain amplifier is particularly sensitive to optical feedback, so one often requires at least one optical isolator at the output. Also, an amplifier increases the laser noise. Finally, the output power may quite sensitively depend on the amplifier's pump power, unless strong gain saturation is achieved (as is often the case).