Solid-state laser
A laser whose gain medium is a transparent crystal or glass doped with active ions such as Nd³⁺, Yb³⁺, Er³⁺, Ti³⁺ or Cr³⁺, excited by light from lamps or diode lasers. Upper-state lifetimes of microseconds to milliseconds let it store energy for Q-switched pulses; Nd:YAG at 1064 nm is the standard example.
A solid-state laser amplifies light in an insulating host, a crystal such as yttrium aluminium garnet (YAG), sapphire or yttrium vanadate, or a glass, into which from about 0.1 % to around 10 % of rare-earth or transition-metal ions are doped. The ions are the gain medium proper; the host holds them, sets their exact energy levels, and carries away heat. By convention the term excludes semiconductor lasers, although they are also solid, and doped-fiber lasers are usually treated as their own class. The first laser, Maiman's ruby laser of 1960, was a solid-state laser at 694.3 nm, and the family now spans the ultraviolet (through harmonics) to about 5 µm (Fe:ZnSe), with continuous powers from milliwatts to many kilowatts and pulse energies of tens of kilojoules per beam in fusion facilities.
Common media
| Medium | Wavelength | Upper-state lifetime |
|---|---|---|
| Ruby (Cr:Al₂O₃) | 694.3 nm | about 3 ms |
| Nd:YAG | 1064 nm | about 230 µs |
| Nd:YVO₄ | 1064 nm | about 100 µs |
| Yb:YAG | 1030 nm | about 0.95 ms |
| Er:glass | about 1535 nm | several ms |
| Ho:YAG | about 2090 nm | about 8 ms |
| Ti:sapphire | 650–1100 nm | 3.2 µs |
The rare-earth ions (Nd, Yb, Er, Tm, Ho) have 4f electrons shielded from the host by outer shells, so their lines are relatively narrow and change little from host to host. Transition-metal ions (Ti, Cr) couple strongly to the lattice and give broad, tunable bands; the Ti:sapphire laser is the main example.
Pumping and efficiency
Solid-state media are excited by optical pumping. Flashlamps and arc lamps were the only practical option until high-power diode lasers arrived in the 1980s; their broad spectra overlap the narrow absorption lines poorly, so most of the lamp energy becomes heat. Diode pumping at wavelengths matched to the absorption lines, such as 808 nm for Nd and 940 or 969 nm for Yb, raised efficiencies several-fold; the diode-pumped solid-state (DPSS) laser entry covers this. Lamps remain in use for large, high-energy pulsed systems because they are inexpensive per joule.
Even with ideal pumping, the quantum defect sets a floor on the heat deposited. The fraction is :
The smaller defect of ytterbium is the main reason Yb-doped media dominate high-average-power designs.
Energy storage
The long upper-state lifetime distinguishes these lasers from semiconductor lasers, whose carrier lifetimes are around a nanosecond. The energy stored in an inversion density is per unit volume; an inversion of ions per cm³ in Nd:YAG at 1064 nm stores
Holding the cavity at high loss while pumping, then switching the loss off, releases this energy as a single pulse; this is Q-switching, and it yields nanosecond pulses with megawatt peak powers from rods a few centimetres long. The same storage makes solid-state media good amplifiers for pulses from mode-locked oscillators.
Geometries and heat
Heat removal limits every design. In a rod cooled at its surface, the centre runs hotter than the edge, producing a thermal lens and stress birefringence that degrade beam quality as power rises. The main geometries trade these effects differently:
- Rod, side- or end-pumped: simple and robust, limited to tens to hundreds of watts in good beam quality.
- Slab: a rectangular crystal with the beam zigzagging between cooled faces, so each ray averages over the gradient.
- Thin disk: a disk about 0.1–0.3 mm thick cooled through one face, so the gradient runs along the beam.
- Fiber: doped glass fiber metres long, covered under fiber laser.
Pitfalls
Absorption lines of Nd-doped crystals are only a few nanometres wide, so a pump diode that drifts with temperature is absorbed less and the output falls. Many crystals are birefringent (Nd:YVO₄, sapphire) and must be oriented to the pump and laser polarization. Output figures quoted for a laser often refer to one operating point; beam quality and pointing can change with pump power because the thermal lens does.
Common questions
Is a semiconductor laser a solid-state laser?
Physically it is solid, but in laser terminology "solid-state laser" means an ion-doped insulating crystal or glass pumped optically. Semiconductor lasers are pumped electrically across a p-n junction and are classed separately as diode lasers.
What is the most common solid-state laser?
Nd:YAG and its close relative Nd:YVO₄, at 1064 nm and frequency-doubled to 532 nm, are the most widespread. Yb:YAG has grown in importance for high-power and ultrafast systems.
References: T. H. Maiman, Stimulated optical radiation in ruby, Nature 187, 493 (1960); W. Koechner, Solid-State Laser Engineering 6th ed. (Springer, 2006); A. E. Siegman, Lasers (University Science Books, 1986); O. Svelto, Principles of Lasers 5th ed. (Springer, 2010).