Master Oscillator Fiber Amplifier

The term master oscillator fiber amplifier (MOFA, MOPFA, or fiber MOPA) is a variation of the term master oscillator power amplifier (MOPA), meaning a system where the power amplifier is a fiber amplifier. The latter is usually a cladding-pumped high-power amplifier, often based on an ytterbium-doped fiber. The main attractions of such fiber-based power amplifiers are:

• A high output power can be achieved with a high power efficiency.
• The cooling system can be relatively simple.
• The beam quality can be high; it is often close to diffraction-limited.
• The gain can easily be as high as tens of decibels. For comparison, most bulk amplifiers, particularly those with high average output power, have a much lower gain.

Figure 1: Setup of a single-stage core-pumped fiber MOPA. For higher power levels, a second amplifier stage with double-clad fiber may be added. The seed laser diode may be operated in a pulsed regime.

However, the use of fibers also has disadvantages:

• Various kinds of fiber nonlinearities can make it difficult to reach very high peak powers and pulse energies in pulsed systems. For example, a few millijoules of pulse energy in a nanosecond pulse system are already considered high for a fiber device, whereas bulk lasers can provide much higher energies. In single-frequency systems, stimulated Brillouin scattering (SBS) can severely limit the output power.
• Due to the high gain, fiber amplifiers are relatively sensitive to back-reflections e.g. from a workpiece. At high power levels, it is not easy to use a Faraday isolator for solving this problem.
• The polarization state is often undefined and unstable, unless polarization-maintaining fibers are used.

It can be attractive to use a gain-switched laser diode as seed laser for a fiber MOPA. Such devices compete with Q-switched lasers, e.g. for application in laser marking. Their advantages partly lie in their flexibility concerning output formats: it is easy to modify not only the pulse repetition rate but also the pulse duration and shape, and of the course the pulse energy.

A special aspect of MOFAs is that the saturation power even of a large mode area double-clad fiber is low compared with the typical output power. Therefore, the power extraction can be as efficient as in a fiber laser, even for relatively low seed powers.

See also: master oscillator power amplifier, amplifiers, fiber amplifiers, high-power fiber lasers and amplifiers, Spotlight article 2008-09-24, Spotlight article 2008-12-16
and other articles in the categories optical amplifiers, fiber optics and waveguides, lasers

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