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Master Oscillator Power Amplifier

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Acronym: MOPA

Definition: a laser system consisting of a seed laser and a laser amplifier for boosting the output power

More specific term: master oscillator fiber amplifier

Categories: article belongs to category laser devices and laser physics laser devices and laser physics, article belongs to category optical amplifiers optical amplifiers

DOI: 10.61835/11k Cite the article: BibTex plain text HTML Link to this page! LinkedIn

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 of a tunable external-cavity diode laser and semiconductor optical amplifier.

Although a MOPA configuration is in principle more complex than a laser which directly produces the required output power, the MOPA concept can have certain advantages:

With a MOPA instead of a simple laser oscillator, it can be easier to reach the required performance e.g. in terms of linewidth, wavelength tuning range, beam quality or pulse duration if the required power is very high. This is because various performance aspects are decoupled from the generation of high powers. This gives extra flexibility, e.g. when a gain-switched laser diode (→ picosecond diode lasers) is used as a seed laser. Note also that it can be advantageous to avoid the presence of additional optical components such as wavelength tuning elements in a high-power laser resonator; with a MOPA architecture, one can place these in the oscillator, where they do not have to withstand high optical intensities, do not spoil the power efficiency, etc.
The same aspects apply to other kinds of modulation, e.g. intensity or phase modulation: it may be advantageous to modulate the low-power seed laser, or to use an optical modulator between seed laser and power amplifier, rather than to modulate a high-power device directly. Slower power modulation may be done by adjusting the amplifier's pump power, without significantly affecting e.g. the obtained pulse duration or wavelength.
The combination of an existing laser with an existing amplifier (or an amplifier chain) may be simpler than developing a new laser with higher output power.
The optical intensities are lower in an amplifier, compared with the intracavity intensities in a laser.

However, the MOPA approach can also have disadvantages:

The complexity of the setup is higher.
The wall-plug efficiency is often lower. However, it may also be higher, e.g. if that approach allows to remove lossy optical elements from the high-power stage.
The resulting laser noise tends to be higher, since an amplified source can not reach the shot noise level (→ amplifier noise). Effects of drifts of the seed power may be suppressed, however, if the amplifier is operated in a strongly saturated regime.
A MOPA can be highly sensitive to back-reflections, which are amplified again before entering the master laser. This feedback sensitivity can often be cured only by placing a Faraday isolator behind the amplifier. Particularly for high-power pulsed devices, this can introduce serious limitations.

MOPA architectures are also used for pulsed laser sources. In that case, the amplifier may be used as a reservoir of energy. If a pulse from the seed laser extracts a significant fraction of the stored energy, the effect of gain saturation is relevant: the amplifier gain drops during the pulse. This can lead to a deformation of the temporal pulse shape. In some cases, the pulse shape from the seed source is tailored so as to obtain the desired pulse shape after amplification.

More to Learn

Optical amplifiers

Master oscillator fiber amplifier

Amplifier noise

Decoupling Pulse Duration and Pulse Energy

Why Fiber Amplifiers, not Fiber Lasers?

All-in-one Concepts versus Modular Concepts

Suppliers

The RP Photonics Buyer's Guide contains 25 suppliers for seed lasers. Among them:

Menlo Systems

Menlo Systems' femtosecond fiber lasers based on Menlo figure 9^® patented laser technology are unique in regard to user-friendliness and robustness. We offer solutions for scientific research as well as laser models engineered for OEM integration.

HÜBNER Photonics

The VALO Series of ultrafast fiber lasers are unique in their design offering among the shortest femtosecond pulses and highest peak powers which can be obtained from a compact turn-key solution. They can be used as seed lasers for various amplifier systems. Pulse durations of <50 fs are achieved using novel fiber laser based technology. The ultrashort pulse durations combined with computer controlled group velocity dispersion pre-compensation, allow users of the VALO lasers to achieve the highest peak power exactly where its needed, which makes the lasers ideal for use in multiphoton imaging, advanced spectroscopy and many other applications.

<50 fs pulse duration
up to 2 W output power
very low noise
integrated pre-compensation dispersion module

AeroDIODE

SHIPS TODAY: Fiber-coupled seed laser diodes (emitting at 1030 nm, 1064 nm, 1550 nm ) are offered as stock items or associated with a CW laser diode driver or pulsed laser diode driver. They are compatible with our high speed nanosecond pulsed drivers or low noise laser diode driver for ultra-narrow linewidth single frequency emission of DFB laser diode modules. The single-mode laser diode can reach high powers up to 500 mW in the nanosecond pulse regime. Most turn-key diode & driver solutions are optimized for single-shot to CW performances with pulse width lengths down to 1 ns. The laser diode precision pulses are generated internally by an on-board pulse generator, or on demand from an external TTL signal.

TOPTICA Photonics

TOPTICA’s ultrafast fiber lasers family “FemtoFiber smart” is available as picosecond or femtosecond version. The lasers are based either on ytterbium-doped or erbium-doped fiber laser architecture. These systems are dedicated to applications ranging from seed laser purposes, biophotonics to terahertz generation and two-photon polymerization.

MPB Communications

Single frequency fiber lasers exhibit reduced frequency noise, excellent stability and long coherence length due to their short cavity design.

MPBC has a line of short cavity single frequency lasers available at wavelengths ranging from 910 nm to 1300 nm. With an all-fiber optical cavity architecture and a mechanical package that is thermally and acoustically isolated, these lasers are known for their quality and robustness. Active stabilisation implementing Pound–Drever–Hall (PDH) locking is used to stabilise frequency. Wavelength tunability is achieved by temperature and piezo.

These seed lasers can be used in conjunction with MPBC's single frequency amplifiers to increase the output power while maintaining the critical seed parameters, such as narrow linewidth, reduced frequency noise and long coherence lengths.

Thorlabs

Thorlabs manufactures an extensive selection of ultrafast lasers and related products for control and characterization. Applications from nonlinear excitation and amplifier seeding to THz and supercontinuum generation are served by a family of products covering a spectral range from 700 – 4500 nm. Our femtosecond laser offerings include fiber lasers, and our picosecond lasers include gain-switched and microchip lasers. Complimenting these laser systems is a suite of ultrafast optics, including nonlinear crystals, chirped mirrors, low-GDD optics, and related products for pulse measurement, pre-compensation, and dispersion measurement.

ALPHALAS

Single-frequency microchip, NPRO and DFB diode CW lasers are available for seeding bulk amplifier chains or fiber amplifiers for generating high power laser radiation at 1030, 1047, 1053, 1064 and 1342 nm wavelengths. Another application is seeding of high-power single-frequency pulsed Q-switched lasers for holographic and interferometric applications.

Picosecond pulse diode lasers from ALPHALAS can be applied for seeding of fiber and regenerative amplifiers.

RPMC Lasers

Serving North America, RPMC Lasers offers precision seed lasers with narrow linewidth and single longitudinal mode (SLM) for high spectral purity, customizable wavelengths, and powers to optimize amplification with reduced gain thresholds.

Flexible fiber-coupling includes single- and multimode options, stabilized and ruggedized for lab or field use, with adjustable pulse widths and rep rates for versatile seeding in stable, reliable setups.

Turnkey solutions deliver high-peak power and low jitter in compact, low-SWaP designs, featuring user-friendly controls and diagnostics for synchronized, high-performance applications.

Let RPMC help you find the right seed laser today!

Questions and Comments from Users

2022-01-21

What happens if the oscillator stops working in a high power MOPA structure?

The author's answer:

That can be problematic in some cases. For example, if there is a fiber amplifier which then (lacking gain saturation) develops a very high gain, even amplifier damage can result.

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