Power scaling of a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design. this means it can increase power without changing outside features! Usually, power scaling requires a more powerful

pump A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action, typically converted from electrical energy into hydraulic energy. Pumps can be classified into three major groups according to the method they u ...

source, stronger cooling, and an increase in size. It may also require reduction of the background loss in the laser resonator and, in particular, in the gain medium.

MOPA

The most popular way of achieving power scalability is the "MOPA" (Master Oscillator Power Amplifier) approach. The master oscillator produces a highly coherent beam, and an

optical amplifier An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback fr ...

is used to increase the power of the beam while preserving its main properties. The master oscillator has no need to be powerful, and has no need to operate at high efficiency because the efficiency is determined mainly by the power amplifier. The combination of several laser amplifiers seeded by a common master oscillator is essential concept of the High Power Laser Energy Research Facility.

Inherently scalable designs

Disk lasers

One type of solid-state laser designed for good power scaling is the disk laser (or "active mirror" ). Such lasers are believed to be scalable to a power of several kilowatts from a single active element in continuous-wave operation. Amplified spontaneous emission, overheating and round-trip loss seem to be the most important processes that limit the power of disk lasers. For future power scaling, the reduction of the round-trip loss and/or combining of several active elements is required.

Fiber lasers

Fiber laser A fiber laser (or fibre laser in British English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related t ...

s are another type of solid-state laser with good power scaling. The power scaling of fiber lasers is limited by

Raman scattering Raman scattering or the Raman effect () is the inelastic scattering of photons by matter, meaning that there is both an exchange of energy and a change in the light's direction. Typically this effect involves vibrational energy being gained by a ...

and

Brillouin scattering Brillouin scattering (also known as Brillouin light scattering or BLS), named after Léon Brillouin, refers to the interaction of light with the material waves in a medium (e.g. electrostriction and magnetostriction). It is mediated by the refractiv ...

, and by the fact that such lasers cannot be very long. The limited length of the

double-clad fiber Double-clad fiber (DCF) is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the ''core''. It is surrounded by the ''inner cladding'', which is surro ...

s limits the usable power of the multi-mode pump, because the pump is not absorbed efficiently in the fiber's active core. Optimization of the shape of the cladding can extend the limit of power scaling.

Fiber disk lasers

The limit of power scaling of fiber lasers can be extended with lateral delivery of the pump. This is realized in so-called fiber disk lasers . The pump in such a laser is delivered from side of a disk, made of coiled fiber with doped core. Several such disks (with a coolant between them) can be combined into a stack.

Problem of heat sink

The power scaling is limited by the ability to dissipate the heat. Usually, the thermal conductivity of materials designed for efficient laser action, is small compared to that of materials optimal for the heat transfer (

metals A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...

diamonds Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...

). For the efficient drain of heat from a compact device, the active medium should be a narrow slab; in order to give advantage to the amplification of light at wanted direction over the

ASE Ase may refer to: * Ase, Nigeria, a town in Delta State, Nigeria * -ase, a suffix used for the names of enzymes * Aṣẹ, a West African philosophical concept * American Sign Language (ISO 639-3 code: ase) See also * Åse (disambiguation) Å ...

, the energy and head would be withdrawn in orthogonal directions, as it is shown in figure. At low background loss (typically, at the level of 0.01 or 0.001) the heat and the light can be withdrawn in the opposite directions, allowing active elements of wide aperture. In this case, combining of several active elements is used for the power scaling.

Coherent addition and combining beams

Scalability can also be achieved by combining separate laser beams. Completely independent beams cannot usually be combined to produce a beam with higher radiance than each beam has alone. Beams can only be combined if they are

coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deri ...

with each other. Such beams can be combined actively or passively. In the passive combining (or

coherent addition Coherent addition (or coherent combining) of lasers is a method of power scaling. It allows increasing the output power and brightness of single-transversal mode laser. Usually, the term coherent addition applies to fiber lasers. As the ability ...

) of lasers, only the few

mode Mode ( la, modus meaning "manner, tune, measure, due measure, rhythm, melody") may refer to: Arts and entertainment * '' MO''D''E (magazine)'', a defunct U.S. women's fashion magazine * ''Mode'' magazine, a fictional fashion magazine which is ...

s common to all of the combined lasers can be above the

lasing threshold The lasing threshold is the lowest excitation level at which a laser's output is dominated by stimulated emission rather than by spontaneous emission. Below the threshold, the laser's output power rises slowly with increasing excitation. Above t ...

. Efficient passive combining of eight lasers has been reported. Further power scaling requires

exponential growth Exponential growth is a process that increases quantity over time. It occurs when the instantaneous rate of change (that is, the derivative) of a quantity with respect to time is proportional to the quantity itself. Described as a function, a q ...

of the gain bandwidth and/or length of the individual lasers. Active combining implies the real-time measurement of the

phase Phase or phases may refer to: Science *State of matter, or phase, one of the distinct forms in which matter can exist *Phase (matter), a region of space throughout which all physical properties are essentially uniform * Phase space, a mathematic ...

of individual lasers' output, and quick adjustment to keep them all in phase. Such adjustment can be done by adaptive optics, which is effective for suppression of phase noise at acoustic frequencies. Faster schemes based on all-optical switching are being researched.

References

Power scaling Power scaling of a laser is increasing its output power without changing the geometry, shape, or principle of operation. Power scalability is considered an important advantage in a laser design. this means it can increase power without changing outs ...