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Solid-state Lasers

Author: the photonics expert

Acronym: SSL

Definition: lasers based on solid-state gain media (usually ion-doped crystals or glasses)

More general term: lasers

More specific terms: doped insulator lasers, all-solid-state lasers, bulk lasers, fiber lasers, semiconductor lasers

Category: article belongs to category laser devices and laser physics laser devices and laser physics

DOI: 10.61835/qcw   Cite the article: BibTex plain textHTML   Link to this page   LinkedIn

Solid-state lasers are lasers based on solid-state gain media such as crystals or glasses doped with rare earth or transition metal ions. Semiconductor lasers are also solid-state lasers, but they are not always meant with that term.

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.

The first solid-state laser – and in fact the first of all lasers – was a pulsed ruby laser, demonstrated by Maiman in 1960 [1]. Later on, however, other solid-state gain media were preferred because of their superior performance. A major problem with ruby is its pronounced three-level nature.

Optical Pumping

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 low power efficiency, moderate lifetime, and strong thermal effects such as thermal lensing in the gain medium.

Laser diodes are now most 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.

Energy Storage

The laser transitions of rare-earth or transition-metal-doped crystals or glasses are normally weakly allowed transitions, i.e., transitions with very low oscillator strength, which leads to long 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.

end-pumped laser
side-pumped laser
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.

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.

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 light 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). Some passively mode-locked solid-state lasers have a tendency for Q-switching instabilities, but these can usually be 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 with transition-metal-doped crystals such as Ti:sapphire, Cr:LiSAF and Cr:ZnSe (→ vibronic lasers).

Typical Solid-state Lasers

Examples of different types of solid-state lasers are:

More to Learn

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Suppliers

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Bibliography

[1]T. H. Maiman, “Stimulated optical radiation in ruby”, Nature 187, 493 (1960) (first experimental demonstration of a laser); https://doi.org/10.1038/187493a0
[2]R. L. Byer, “Diode laser-pumped solid-state lasers”, Science 239, 742 (1988); https://doi.org/10.1126/science.239.4841.742
[3]G. Huber, C. Kränkel, and K. Petermann, “Solid-state lasers: status and future”, J. Opt. Soc. Am. B 27 (11), B93 (2010); https://doi.org/10.1364/JOSAB.27.000B93
[4]D. C. Hanna and W. A. Clarkson, “A review of diode-pumped lasers”, in Advances in Lasers and Applications (eds. D. M. Finlayson and B. Sinclair), pp. 1–18, Taylor & Francis, New York(1999)
[5]W. Koechner, Solid-State Laser Engineering, 6th edn., Springer, Berlin (2006)
[6]A. Sennaroglu (ed.), Solid-State Lasers and Applications, CRC Press, Boca Raton, FL (2007)
[7]R. Paschotta, Field Guide to Lasers, SPIE Press, Bellingham, WA (2007)
[8]R. Paschotta, “Operation regimes of solid-state lasers”, chapter in Handbook of solid-state lasers: Materials, systems and applications, editors: B. Denker, and E. Shklovsky, Woodhead Publishing (2013), ISBN 0 85709 272 3

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Questions and Comments from Users

2022-06-24

Could you please explain why most solid state laser can generate pulses shorter than the average lifetime of the excited state.

The author's answer:

The upper-state lifetime is not relevant because it is the lifetime in the absence of light. With a short intense pulse, you can extract the stored energy within a much shorter time.

2022-06-26

In some real solid-state continuous-wave lasers it is possible to observe mode hopping. What is the reason of that?

The author's answer:

See the article on mode hopping.

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