What is a master oscillator power amplifier?

Master Oscillator Power Amplifier

The term master oscillator power amplifier (MOPA) refers to a configuration consisting of a master laser (or seed laser) and an optical amplifier to boost the output power. A special case is the master oscillator fiber amplifier (MOFA), where the power amplifier is a fiber device. In other cases, a MOPA may consist of a solid-state bulk laser and a bulk amplifier, or a tunable external-cavity diode laser and semiconductor optical amplifier.

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Advantages of MOPA

• With a MOPA instead of a simple laser oscillator, it can be easier to reach required performance in terms of linewidth, wavelength tuning range, beam quality, or pulse duration if high power is necessary. This is because various performance aspects are decoupled from the generation of high powers. For instance, a gain-switched laser diode can be used as a seed laser. The absence of additional optical components such as wavelength tuning elements in a high-power laser resonator can be advantageous, as they can be placed in the oscillator where they do not have to withstand high optical intensities.
• For other kinds of modulation, such as intensity or phase modulation, it may be advantageous to modulate the low-power seed laser, or to use an optical modulator between the seed laser and the power amplifier, rather than modulating a high-power device directly. Slower power modulation can be achieved by adjusting the amplifier's pump power without significantly affecting the pulse duration or wavelength.
• The combination of an existing laser with an existing amplifier may be simpler than developing a new laser with higher output power.
• Optical intensities are lower in an amplifier compared to the intracavity intensities in a laser.

Disadvantages of MOPA

• The setup complexity is higher.
• The wall-plug efficiency is often lower but may be higher if the approach allows for the removal of lossy optical elements from the high-power stage.
• The resulting laser noise tends to be higher since an amplified source cannot reach the shot noise level. However, the effects of drifts in the seed power may be suppressed if the amplifier operates in a strongly saturated regime.
• A MOPA is highly sensitive to back-reflections, which are amplified again before entering the master laser. This feedback sensitivity is often mitigated by placing a Faraday isolator behind the amplifier. For high-power pulsed devices, this can introduce serious limitations.

MOPA architectures are also used for pulsed laser sources. In this case, the amplifier may act as a reservoir of energy. If a pulse from the seed laser extracts a significant fraction of stored energy, gain saturation occurs, leading to a drop in amplifier gain during the pulse. This can deform the temporal pulse shape. The pulse shape from the seed source may be tailored to obtain the desired pulse shape after amplification.

Master Oscillator Power Amplifier (MOPA)

Master Oscillator (MO)

The master oscillator is the heart of the MOPA system. It generates a low-power seed signal that is tailored to achieve the performance required by the application. Depending on the application, the characteristics of the seed signal are optimized for specific wavelengths, linewidths, or pulse durations.

Power Amplifier (PA)

The power amplifier boosts the seed signal to high power and energy levels to meet the demands of applications such as throughput or material removal rates. There are several design options for the power amplifier stage to meet the widely varying demands of industrial, scientific, or medical applications. For instance, bulk amplifiers are well-suited to generate high-energy pulses. While fiber-based amplifiers are limited in energy, they are well-suited to generate high power levels. It is common to combine multiple amplifier stages to reach the required performance.

MOPA Advantages

High Power and Energy Levels: Laser powers and energy levels can be scaled over several orders of magnitude by selecting and combining the appropriate power amplifier stages.

Superb Beam Quality: The oscillator's pristine beam quality is maintained in the amplifier stage, enabling excellent surface quality and clean cuts, precise welds, accurate measurements, and pristine images.

Spectral Purity: The amplified laser maintains the MO's narrow linewidth and stable wavelength, crucial for applications like spectroscopy and optical communications.

Versatility: Different MO and PA combinations can be tailored to meet specific needs, from ultra-short pulses to continuous wave (CW) operations, wavelength tuning, power or energy level modulation, or pulse picking.

MOPA Applications

Material Processing: Enables precise cutting, drilling, and welding.

LIDAR: Facilitates accurate 3D environmental mapping.

Pumping Other Lasers: Enhances other laser systems' capabilities.

Medical Imaging: Generates high-power, ultra-fast pulses for advanced imaging, diagnostics, and therapies.

Empowering Innovation with MOPA

MOPA systems, at the forefront of laser technology, offer unparalleled power, quality, and versatility. Integral in driving advancements across various sectors, they provide precision where it matters most.

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