MOPA (master oscillator power amplifier)
A laser architecture in which a low-power seed (the master oscillator) sets the pulse shape, wavelength and timing, and one or more amplifiers raise the power. A typical fiber MOPA marking laser gives 20–100 W average power with pulse widths adjustable from a few ns to several hundred ns at 1 kHz to a few MHz.
A MOPA (master oscillator power amplifier) separates the two jobs of a laser. The master oscillator is a small, well-controlled source, often a directly modulated laser diode, a DFB laser or a mode-locked oscillator, whose output defines the wavelength, linewidth, pulse shape and timing. A chain of optical amplifiers then raises the power by 30–50 dB while, ideally, preserving those properties. Seeding a 20 W ytterbium fiber amplifier with 10 mW of average seed power, for instance, is a gain of
usually split over two or three stages with isolators between them.
Fiber MOPA pulsed lasers
The most common MOPA in industry is the pulsed fiber laser used for marking, engraving and cleaning. A 1064 nm diode is driven with current pulses of arbitrary shape and length, then amplified in a preamplifier of ytterbium-doped fiber and a cladding-pumped power stage. Commercial units commonly offer pulse widths from about 2 ns to 500 ns, repetition rates from 1 kHz to several MHz, average powers of 20–100 W and pulse energies up to about 1–2 mJ.
The pulse energy is the average power divided by the repetition rate, . At 20 W:
- 100 kHz gives 0.2 mJ; spread over a 200 ns flat-top pulse, the peak power is 1 kW.
- 1 MHz gives 20 µJ; in a 20 ns pulse, the peak power is again 1 kW.
These figures assume the rated average power is available at every rate. At low repetition rates the long gaps between pulses let amplified spontaneous emission consume part of the stored energy, and the pulse energy is capped at a few times the amplifier's saturation energy, so the average power falls below its rated value. The pulsed laser calculator converts between these quantities.
Comparison with Q-switched sources
In a Q-switched fiber or solid-state laser, the pulse width depends on the gain and cavity length, and therefore on repetition rate: a Q-switched fiber laser that gives 100 ns pulses at 20 kHz gives longer, weaker pulses at higher rates. In a MOPA the seed electronics fix the pulse width and shape, so width, repetition rate and energy can be chosen almost independently, and pulses can be shaped (a short spike followed by a lower plateau, for example) to suit a given material. The first pulse of a burst can be held to the same energy as later ones, which matters for laser machining with galvanometer scanners. The costs are a more complex electronic seed, more ASE between pulses, and usually a lower maximum pulse energy than a Q-switched laser of similar average power.
Other MOPA systems
- Semiconductor MOPAs: a single-frequency diode seeding a tapered amplifier gives watt-level narrow-linewidth light for atom cooling and frequency doubling.
- Ultrafast MOPAs: a mode-locked oscillator seeds fiber or solid-state amplifiers, usually with chirped-pulse amplification to limit peak power in the amplifier.
- Narrow-linewidth CW MOPAs: a DFB fiber, diode or nonplanar ring oscillator (NPRO) seed amplified to hundreds of watts for spectral beam combining and gravitational-wave detectors.
- Gain-switched diode seeds give picosecond MOPAs with electronically set repetition rates.
Pitfalls
Back-reflections from the workpiece or from a fiber end can be amplified on the return trip and destroy the seed or the pump diodes, so isolators are placed at the output and between stages. Unseeded operation, if the seed fails, lets ASE build to the point of self-pulsing, which can damage the fiber; drivers interlock the pumps to the seed. In narrow-linewidth amplifiers, stimulated Brillouin scattering sets the power limit; in pulsed fiber amplifiers, self-phase modulation and stimulated Raman scattering broaden the spectrum as peak power rises. Large-mode-area fibers and short amplifier lengths push these thresholds up.
Common questions
What is the difference between a MOPA laser and a Q-switched fiber laser for marking?
A MOPA allows the pulse width to be set independently of repetition rate, typically from a few ns upward, which helps with color marking on stainless steel, marking anodized aluminum without damage and processing thin films. A Q-switched fiber laser has a pulse width set by its cavity and repetition rate, typically of order 100 ns, is simpler, and often delivers higher pulse energy.
Why not just build a larger oscillator?
A high-power oscillator must control mode quality, linewidth and pulse dynamics in the same cavity that handles the heat and power. Splitting the roles lets a low-power seed be optimized for spectral and temporal quality while the amplifier is optimized for efficiency and heat removal.
Does amplification degrade the seed?
Somewhat. Amplifiers add ASE noise and, at high peak power, nonlinear spectral broadening; saturation also reshapes a pulse because the leading edge sees more gain than the trailing edge. Seed pulse shapes are often pre-distorted to compensate.
References: Siegman, Lasers (University Science Books, 1986); Saleh & Teich, Fundamentals of Photonics 3rd ed. (Wiley, 2019); L. M. Frantz and J. S. Nodvik, J. Appl. Phys. 34, 2346 (1963).