Disk laser

Disk laser

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A disk laser or active mirror (Fig.1.) is a type of solid-state laser
Solid-state laser
A solid-state laser is a laser that uses a gain medium that is a solid, rather than a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are also in the solid state, but are generally considered as a separate class from solid-state lasers .-Solid-state...

 characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. Despite their name, disk lasers do not have to be circular; other shapes have also been tried.

Disk lasers should not be confused with Laserdisc
Laserdisc
LaserDisc was a home video format and the first commercial optical disc storage medium. Initially licensed, sold, and marketed as MCA DiscoVision in North America in 1978, the technology was previously referred to interally as Optical Videodisc System, Reflective Optical Videodisc, Laser Optical...

s, which are a disk-shaped optical storage medium.

Disk lasers should not be confused with
Fiber laser disks,
which are a disk-shaped coils of a fiber laser
Fiber laser
A fiber laser or fibre laser 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, and thulium. They are related to doped fiber amplifiers, which provide light amplification without lasing...

,
pumped from the side.

Active mirrors and disk lasers



Initially, disk lasers were called active mirrors, because the gain medium of a disk laser is essentially an optical mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...

 with reflection coefficient
Reflection coefficient
The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave...

 greater than unity. An active mirror is a thin disk-shaped double-pass optical amplifier
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 from the cavity is suppressed...

.

The first active mirrors were developed in the Laboratory for Laser Energetics
Laboratory for Laser Energetics
The Laboratory for Laser Energetics is a scientific research facility which is part of the University of Rochester's south campus, located in Brighton, New York. The lab was established in 1970 and its operations since then have been funded jointly; mainly by the United States Department of...

  (USA).
Then, the concept was developed in various research groups,
in particular, the University of Stuttgart
University of Stuttgart
The University of Stuttgart is a university located in Stuttgart, Germany. It was founded in 1829 and is organized in 10 faculties....

 (Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

) for Yb:doped glasses.

In the disk laser, the heat sink does not have to be transparent, so, it can be extremely efficient even with large transverse size of the device (Fig.1.).
The increase in size allows the power scaling
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....

 to many kilowatts without significant modification of the design.

Limit of power scaling for disk lasers


The power of such lasers is limited not only by the power of pump available, but also by overheating, amplified spontaneous emission
Amplified spontaneous emission
Amplified spontaneous emission or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers....

  (ASE) and
the background round-trip loss.
To avoid overheating, the size should be increased with power scaling.
Then, to avoid strong losses due to the exponential growth
Exponential growth
Exponential growth occurs when the growth rate of a mathematical function is proportional to the function's current value...

 of the ASE
Amplified spontaneous emission
Amplified spontaneous emission or superluminescence is light, produced by spontaneous emission, that has been optically amplified by the process of stimulated emission in a gain medium. It is inherent in the field of random lasers....

, the transverse-trip gain
cannot be large.
This requires reduction of the gain ;
this gain is determined by the reflectivity of the output coupler and thickness .
The round-trip gain
Round-trip gain
Round-trip gain refers to the laser physics, and laser cavitys .It is gain, integrated along a ray, which makes a round-trip in the cavity....

  should remain larger than the
round-trip loss 
(the difference determines the optical energy,
which is output from the laser cavity at each round-trip).
The reduction of gain , in a given round-trip loss ,
requires increasing the thickness .
Then, at some critical size, the disk becomes too thick and cannot be
pumped above the threshold
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 threshold, the slope of power vs. excitation is orders...

 without overheating.

Some features of the power scaling can revealed from a simple model.
Let be the saturation intensity
Active laser medium
The active laser medium is the source of optical gain within a laser. The gain results from the stimulated emission of electronic or molecular transitions to a lower energy state from a higher energy state...

,

of the medium,
be the ratio of frequencies,
be the thermal loading parameter.
The key parameter

determines the maximal power of the disk laser.
The corresponding optimal thickness can be estimated with
.
The corresponding optimal size
.
Roughly, the round-trip loss should scale inversely proportionally to the cubic root of the power required.

An additional issue is the efficient delivery of pump energy.
In low round-trip gain, the single-pass absorption of the pump is also low. Therefore, recycling of pump energy is required for efficient operation. (See the additional
mirror M at the left-hand side of figure 2.) For power scaling
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....

,
the medium should be optically thin, with many passes of pump energy required; the lateral delivery of pump energy

also might be a possible solution.

Anti-ASE cap



In order to reduce the impact of ASE, an anti-ASE cap consisting of undoped material on the surface of a disk laser has been suggested.

Such a cap allows spontaneously emitted photons to escape from the active layer and prevents them from resonating in the cavity. Rays cannot bounce (Fig.3) as in uncovered disk. This could allow an order of magnitude increase in the maximum power achievable by a disk laser
. In both cases, the back reflection of the ASE from the edges of the disk should be suppressed. This can be done with absorbing layers, shown with green in Figure 4. At operation close to the maximal power, a significant part of the energy goes into ASE; therefore, the absorbing layers also should be supplied with heat sinks, which are not shown in the figure.


Key parameter for laser materials


The estimate of maximal power achievable at given loss , is very sensitive to . The estimate of the upper bound of , at which the desired output power is achievable is robust. This estimate is plotted versus normalized power
in figure 5. Here, is the output power of the laser, and
is dimensional scale of power; it is related with the key parameter
.
The thick dashed line represents the estimate for the uncovered disk. The thick solid line shows the same for the disk with undoped cap. The thin solid line represents the qualitative estimate without coefficients. Circles corresponds to the experimental data for the power achieved and corresponding estimates for the background loss . All future experiments and numerical simulations and estimates are expected to give values of , that are below the red dashed line in Fig.5 for the uncovered disks, and below the blue curve for the disks with anti-ASE cap. This can be interpreted as a scaling law for disk lasers
.

In the vicinity of the curves mentioned, the efficiency of the disk laser is low; most of the pumping power goes to ASE, and is absorbed at the edges of the device. In these cases, the distribution of the pump energy available among several disks may significantly improve the performance of the lasers. Indeed, some lasers reported using several elements combined in the same cavity.

Pulsed operation


Similar scaling laws take place for pulsed operation. In quasi continuous wave regime, the maximal mean power can be estimated by scaling the saturation intensity with the fill factor
Fill factor
Fill factor may refer to:*Fill factor , the ratio of maximum obtainable power to the product of the open-circuit voltage and short-circuit current*In vision science, the ratio of view areas to the object visible areas....

 of the pump, and the product of the duration of pump to the repetition rate. At short duration pulses,
more detailed analysis is required
.
At moderate values ot the repetition rate (say, higher than 1 Hz), the maximal energy of the output pulses is roughly inversely proportional to the cube of the background loss ; the undoped cap may provide an additional order of magnitude of mean output power, under the condition that this cap does not contribute to the background loss.
At low repetition rate (an in the regime of single pulses) and sufficient pump power, there is no general limit of energy, but the required size of the device grows quickly with increase of the required pulse energy
, setting the practical limit of energy; it is estimated that from a few joules to a few thousand joules can be extracted in an optical pulse from a single active element, dependently on the level of the background internal loss of the signal in the disk.
.

See also

  • VCSEL
    VCSEL
    The vertical-cavity surface-emitting laser, or VCSEL , is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers which emit from surfaces formed by cleaving the individual chip out of a...

  • VECSEL
    VECSEL
    A vertical-external-cavity surface-emitting-laser is a small semiconductor laser similar to a vertical-cavity surface-emitting laser...

  • Thermal shock
    Thermal shock
    Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...

  • round-trip gain
    Round-trip gain
    Round-trip gain refers to the laser physics, and laser cavitys .It is gain, integrated along a ray, which makes a round-trip in the cavity....

  • power scaling
    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....

  • gain medium