Solid state laser

from Wikipedia, the free encyclopedia

As a solid-state laser optically excited are laser referred whose reinforcing (active) medium consists of a crystalline or glassy ( amorphous ) solid body consists. This so-called host material or host crystal contains the laser-active ions in a certain concentration ( doping ) .

Solid-state lasers are pumped with light or infrared radiation .

function

The host crystal or a glass is doped with ions of a foreign substance. These foreign ions are the actual active medium of the solid-state laser.

The electron levels of these ions used for lasering lie within the d orbital ( titanium , chromium , cobalt ) or f orbital ( neodymium , erbium , ytterbium ). These orbitals are not involved in chemical bonds. The carrier material (host crystal, glass) therefore has only a minor influence on the laser properties of the ions.

In order to achieve energy absorption in the active medium, electrons must be raised to a higher energy level. This process is called pumping . Solid-state lasers are always optical; H. by radiation, pumped . The effective pump light wavelength results from the energy difference between the lower and upper energy levels, which is characteristic of the doping ions.

The energy level achieved by pumping does not match the upper and lower laser level (one speaks of 3 or 4 level lasers ). B. the lower laser level very quickly through lattice relaxation into the lower energy level, a population inversion required for lasing can be achieved much more easily , since the lower laser level is hardly filled.

It is also helpful if the electrons remain in the upper laser level for a long time - their energy can then be released suddenly as a light pulse with a Q-switch .

The operating mode can be continuous (" continuous wave ", CW) or pulsed. The pulsing can be done by pumping (flash lamps) or an intra-cavity optical switch ( Q - switch ). If you combine both (high pump peak power through flash lamp, then release of the energy stored in the upper laser level by opening the Q switch), peak powers of a few megawatts can be achieved within a few nanoseconds.

By amplification and impulse expansion and contraction, powers in the petawatt range can be achieved in a few femtoseconds. Solid-state lasers can generate the highest peak powers and the shortest pulse durations of all types of laser.

Common host materials / host crystals

  • Al 2 O 3 ( corundum , sapphire ) (e.g. ruby (chromium doping), titanium: sapphire laser )
    • Advantage: high thermal conductivity, high strength
    • Disadvantage: Doping usually leads to pump wavelengths at which direct pumping with laser diodes is not possible
  • alexandrite
  • Fluorides like YLF, BYF or KYF
  • Glass (rod shape or fiber laser )
    • Advantage: easy production even in large dimensions
    • Disadvantages (not with fiber lasers!): Low thermal conductivity, low strength
  • Sesquioxides such as Sc 2 O 3 and Lu 2 O 3
  • YAG ( yttrium - aluminum - garnet laser) doping Nd ( Nd: YAG laser ), Er, Yb
    • Advantage: high thermal conductivity, high strength, low absorption
    • Disadvantage: expensive
  • Yttrium vanadate (YVO 4 ), doping Nd
    • One of the most efficient laser crystals currently available.

Frequently used doping materials

  • Chromium was the doping material of the first laser of the ruby laser (694.3 nm (red)). Due to its low efficiency, it is rarely used today
  • Erbium wavelength 1.5 or 3 µm, pumps at 980 nm, so-called "eye-safe" laser, used for laser rangefinders and in medicine
  • Neodymium , 1064 nm, the most important commercial solid-state laser: Nd: YAG laser , at 1064 nm (infrared), or doubled in frequency at 532 nm (green). Also possible are: Nd: glass and Nd: YLF lasers.
  • Praseodymium A relatively new laser ion. Very interesting because of several transitions in the visible spectral range (444 nm (blue), 480 nm (blue), 523 nm (green), 605 nm (orange) and 640 nm (red)). The transition at 523 nm should be emphasized here, with which green laser radiation can be generated without the detour via a frequency doubling of the Nd: YAG laser
  • Titanium An important mode-locked solid-state laser: Titanium: sapphire laser , 670–1100 nm (red-infrared), suitable for pulses in the fs range due to broadband amplification
  • Ytterbium , 1030 nm, allows a high efficiency of over 50% in laser operation. However, this requires narrow-band pumping with laser diodes (940 nm). The most important material with this doping is the Yb: YAG laser , e.g. B. highly doped as a disk laser with a wavelength of 1030 nm.

Forms of the active medium

Pumping arrangements

Pump lamp of a solid-state laser, krypton filling, water-cooled, approx. 2 kW

Pumping takes place z. B. by illuminating the laser medium (laser rod) with intense light sources such as gas discharge lamps (arc lamps or flash lamps ).

The pulsed laser emission - with the solid-state lasers pumped with flash lamps - often shows a highly irregular structure. The statistical peaks of the emitted laser intensity are called spikes and are due to the falling (during emission) and rising of the electron density in the higher energy level.

The gas discharge lamps used must have as high a spectral component as possible at the pump wavelength (generally in the near infrared NIR ). They are krypton or xenon arc lamps with tungsten electrodes, which are arranged individually or in pairs parallel to the rod.

The laser rod and lamps are usually water-cooled (deionized water washes around the lamp and rod).

The laser rod must be illuminated as evenly as possible. This can be achieved with internal reflectors made of a gold layer or half-shells made of a diffusely reflective white ceramic.

The laser rod must be protected from the harsh ultraviolet radiation of the lamps - a protective glass tube is used for this.

Pumps with diode lasers

Since semiconductor lasers of sufficient power have become available, solid-state lasers are often optically pumped with laser diodes of suitable wavelengths. This has made it possible to implement completely new types of solid-state lasers ( fiber lasers and disk lasers ), but there are also advantages for conventional solid-state lasers through pumping with laser diodes.

advantages

  • Laser diodes have a very high degree of efficiency and they can work exactly on the pump wavelength - this increases the overall efficiency of solid-state lasers from 1–3% (lamp-pumped) to 10–25% (diode-pumped).
  • Due to the higher pump efficiency, the heating of the laser rod is reduced, there are less mechanical stresses, therefore the damage threshold is higher and higher power can be generated with the same rod size.
  • the lower heating of the rod reduces the lens effect caused by the inhomogeneous temperature distribution - the beam quality and stability increase significantly.
  • Laser diodes have a longer service life (> 10,000 h) than arc lamps (several 100 h), so the maintenance cycles are longer.
  • Some solid-state lasers can also be operated in continuous wave mode (cw) with laser diodes. Only pulsed operation is possible with flash lamps.

disadvantage

  • Laser diodes are much more expensive than arc lamps, so the investment is higher.
  • Laser diodes show degradation associated with a decrease in their efficiency to 80% of the initial value after about 10 ... 20,000 hours of operation and must then be replaced.
  • In contrast to flash lamps, laser diodes break relatively suddenly, so that often additional replacement diodes are installed, which can be used until the device is next serviced.
  • Laser diodes are much more sensitive than flash lamps and more difficult to cool.

Pumping fiber and disk lasers

With fiber and disk lasers, the problem of thermal influences on the optical properties does not apply - they can therefore be used to generate high powers with good beam quality. However, the pump radiation must be concentrated on small areas, which is why pumping is only possible with diode lasers.

With disk lasers, the pump radiation passes through the disk several times by being directed back onto the disk several times with a prism reflector in order to be absorbed as completely as possible.

In the case of fiber lasers , the focused pump radiation passes through the end face of the fiber into the latter (end-pumped) or a cladding of the fiber guides the pump radiation along the active (doped) fiber core. The reverse arrangement (pump radiation in the core) is also possible.

Resonator

A resonator is required (except for fiber lasers) and, like other lasers, consists of a 100% mirror (end mirror) and a partially transparent mirror (output mirror). Dielectric interference mirrors are suitable for the laser wavelength, since metal mirrors do not withstand the beam intensity or have excessive losses.

Inside the mirror is the anti-reflective crystal rod and possibly other optical components such. B. Crystals for frequency doubling / multiplication or for Q-switching .

Applications

In addition to carbon dioxide lasers, solid-state lasers are the lasers most frequently used in industry for material processing.
Typical applications are:

  • Cutting (especially thinner materials and precision machining, continuous or pulsed operation)
  • Drilling (pulsed laser)
  • Engraving (pulsed, Q-switched, deflection with scanners)
  • Welding (spot welding with flash lamp-pumped lasers, seam welding in CW mode)
  • Soldering (cw soldering, pulse soldering)
  • Clean
  • Hardening

There are other diverse applications in the scientific field. The lasers with the shortest pulse lengths and the highest peak powers are solid-state lasers.

history

The first laser ever built, developed by Maiman in 1960, was a solid-state laser - a lamp-pumped ruby laser .

Lamp- pumped Nd: YAG lasers for continuous and pulsed operation as well as Nd: glass lasers for very high pulse energies were the most important representatives of solid-state lasers in industry and research for many years.

Since around 1995, the possibility of pumping with laser diodes has enabled a multitude of new types of solid-state lasers and active materials to conquer numerous new applications in research and industry.

Outstanding results are short pulse lasers down to the sub-picosecond range, miniaturized frequency-doubled solid-state lasers (e.g. green laser pointers) and the extremely good focusability of disk and fiber lasers, the long working distances (e.g. metal welding) or enable good cutting performance.

Due to their increased efficiency, beam quality and power, solid-state lasers are often replacing the medium-power industrial CO 2 lasers . B. the beam transmission with optical fibers is possible and the absorption on metals is better.

literature

  • Walter Koechner : Solid State Laser Engineering . 6th edition, Springer, 2006 (first 1976).
  • Walter Koechner, Michael Bass : Solid State Lasers: a graduate text. Springer, 2003.

Web links