solid state laser
Post on 14-Jan-2016
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Solid-state lasers are lasers based on solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions, or semiconductor lasers. (Although semiconductor lasers are of course also solid-state devices, they are often not included in the term solid-state lasers.) Ion-doped solid-state lasers (also sometimes called doped insulator lasers) can be made in the form of bulk lasers, fiber lasers, or other types of waveguide lasers. Solid-state lasers may generate output powers between a few milliwatts and (in high-power versions) many kilowatts.
Optical Pumping and Energy Storage
Many solid-state lasers are optically pumped with flash lamps or arc lamps.
Such pump sources are relatively cheap and can provide very high powers.
However, they lead to a fairly lowpower efficiency, moderate lifetime, and
strong thermal effects such as thermal lensing in the gain medium. For such
reasons, laser diodes are very often used for pumping solid-state lasers.
Such diode-pumped solid-state lasers (DPSS lasers, also called all-solid-state
lasers) have many advantages, in particular a compact setup, long lifetime, and
often very good beam quality. Therefore, their share of the market is rapidly
rising.
The laser transitions of rare-earth or transition-metal-
doped crystals or glasses are normallyweakly allowed transitions, i.e.,
transitions with very low oscillator strength, which leads to long
radiative upper-state lifetimes and consequently to good energy storage,
with upper-state lifetimes of microseconds to milliseconds. For example, a laser
crystal pumped with 10 W of power and having an upper-state lifetime of 1 ms
can store an energy of the order of 10 mJ. Although energy storage is beneficial
for nanosecond pulse generation (see below), it can also lead to
unwanted spiking phenomena in continuous-wave lasers, e.g. when the pump
source is switched on.
Figure 1: Typical setups of solid-state bulk lasers, converting pump light (blue)
into laser light (red): end-pumped (top) and side-pumped (bottom) versions.
Pulse Generation
The long upper-state lifetimes makes solid-state lasers very suitable for Q
switching: the laser crystal can easily store an amount of energy which, when
released in the form of a nanosecond pulse, leads to a peak power which is
orders of magnitude above the achievable average power. Bulk lasers can thus
easily achieve millijoule pulse energies and megawatt peak powers.
In mode-locked operation, solid-state lasers can generate ultrashort pulses with
durations measured in picoseconds or femtoseconds (minimum: ≈ 5 fs, achieved
with Ti:sapphire lasers). With passive mode locking, they have a tendency for Q-
switching instabilities, if these are not suppressed with suitable measures.
Wavelength Tuning
In terms of their potential for wavelength tuning, different types of solid-state
lasers differ considerably. Most rare-earth-doped laser crystals, such
as Nd:YAG and Nd:YVO4, have a fairly small gain bandwidth of the order of
1 nm or less, so that tuning is possible only within a rather limited range. On
the other hand, tuning ranges of tens of nanometers and more are possible with
rare-earth-doped glasses, and particularly withtransition-metal-doped crystals
such as Ti:sapphire, Cr:LiSAF and Cr:ZnSe (→ vibronic lasers).
Types of Solid-state Lasers
Examples of different types of solid-state lasers are:
Small diode-pumped Nd:YAG (→ YAG lasers) or Nd:YVO4 lasers (→ vanadate
lasers) often operate with output powers between a few milliwatts (for
miniature setups) and a few watts. Q-switched versions generate pulses with
durations of a few nanoseconds, microjoule pulse energies and peak powers of
many kilowatts. Intracavity frequency doubling can be used for green output.
Single-frequency operation , typically achieved with unidirectional ring
lasers (e.g. NPROs = nonplanar ring oscillators) or microchip lasers, allows for
operation with very small linewidth in the lower kilohertz region.
Larger lasers in side-pumped or end-pumped configurations (see above), having
the geometry of rod lasers, slab lasers or thin-disk lasers, are suitable for output
powers up to several kilowatts. Particularly thin-disk lasers can still offer very
high beam quality, and also a high power efficiency.
Q-switched Nd:YAG lasers are still widely used in lamp-pumped versions. Pulsed
pumping allows for high pulse energies, whereas the average output powers are
often moderate (e.g. a few watts). The cost of such lamp-pumped lasers is lower
than for diode-pumped versions with similar output powers.
Fiber lasers are a special kind of solid-state lasers, with a high potential for high
average output power, high power efficiency, high beam quality, and
broad wavelength tunability. See also the articles on fiber lasers versus bulk
lasers and on high-power fiber lasers and amplifiers.
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