Master Oscillator Fiber Amplifier

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), referring to a system encompassing a laser oscillator and a power amplifier, with the latter being a fiber amplifier. This fiber amplifier is typically a cladding-pumped high-power amplifier often based on ytterbium-doped fiber. The advantages of these fiber-based power amplifiers include:

• Achieving high output power with excellent power efficiency.
• A relatively straightforward cooling system.
• High beam quality, often nearing the diffraction limit.
• Substantial gain, easily reaching tens of decibels. For comparison, most bulk amplifiers with high average output power have significantly lower gain, often around 3 dB.

Figure 1:

Diagram of a single-stage core-pumped fiber MOPA.

To attain higher power levels, an additional amplifier stage with double-clad fiber can be incorporated. The seed laser diode can operate in a pulsed mode.

However, fiber-based systems have their disadvantages:

• Optical fiber nonlinearities can pose challenges in achieving very high peak powers and pulse energies in pulsed systems. For instance, in a nanosecond pulse system, a few millijoules of pulse energy is already considered high, whereas bulk lasers can provide much higher energies. For single-frequency systems, stimulated Brillouin scattering (SBS) can significantly restrict output power.
• Fiber amplifiers with high gain are sensitive to back-reflections from surfaces like workpieces. At elevated power levels, using a Faraday isolator to mitigate this issue is challenging, especially when high suppression of reflected light is needed.
• The polarization state is often indeterminate and unstable unless polarization-maintaining fibers are utilized.

Using a gain-switched laser diode (such as picosecond diode lasers) as a seed laser for a fiber MOPA is appealing. These devices compete with Q-switched lasers in applications like laser marking. Their advantages lie in their flexibility: it's simple to adjust not only pulse repetition rate but also pulse duration and shape, along with pulse energy.

An important feature of MOFAs is the relatively low saturation power of a large mode area double-clad fiber compared to typical output power. Thus, power extraction can be highly efficient, even with relatively low seed powers.

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