Q-switched Lasers

Posted on 2006-09-16 as part of the Photonics Spotlight (available as e-mail newsletter!)

Permanent link: https://www.rp-photonics.com/spotlight_2006_09_16.html

Author: Dr. Rüdiger Paschotta, RP Photonics AG, RP Photonics AG

Abstract: This article explains the differences between the laser crystal materials Nd:YAG and Nd:YVO4 in the context of Q-switched lasers. Nd:YAG is most suitable for high pulse energies at low repetition rates, while Nd:YVO4 allows for the highest repetition rates and shortest pulses. The upper-state lifetime turns out to be of lower importance than is frequently believed.

Ref.: encyclopedia articles on Q switching, Q-switched lasers, saturation energy, upper-state lifetime, YAG lasers, vanadate lasers; G. J. Spühler et al., JOSA B 16 (3), 376 (1999)

Nd:YAG and Nd:YVO₄ (neodymium-doped yttrium vanadate) are belonging to the most popular laser gain media. While their properties for continuous-wave operation are overall quite similar, these media are rather different for use in Q-switched lasers (and by the way, also for mode locking, even though for entirely different reasons). As a rule of thumb, Nd:YAG is more suitable for high pulse energies, while Nd:YVO₄ is better for high pulse repetition rates.

But why exactly is this true? Many believe that the shorter upper-state lifetime of Nd:YVO₄ is the reason, but this is less than half of the truth. A shorter upper-state lifetime limits the energy storage and thus the achievable pulse energies. However, the high gain of Nd:YVO₄ can also be a limiting factor: for high pulse energies, it increases the demands on the Q-switch in terms of switching speed and hold-off loss.

Concerning high pulse repetition rates, the upper-state lifetime is actually not very important. It determines the rate of spontaneous emission, which is anyway weak in this regime. The factor of crucial importance is the high emission cross-section of Nd:YVO₄, which results in a low saturation fluence and (for a given mode area) in a low saturation energy. The latter enables to achieve a reasonably short pulse duration even for moderate pulse energies, as are possible at high repetition rates. The problem of Nd:YAG at high repetition rates turns out to be that the pulses get too long, or some pulses are left out due to too small gain. All this has nothing to do with the upper-state lifetime.

There are actually more differences between the materials. The very short pump absorption length of Nd:YVO₄ allows one to construct Q-switched microchip lasers with very short cavity length, so that pulse durations far below 1 ns can be achieved. The record in this respect is 37 ps, see the reference above (Spühler et al.). In general, Nd:YVO₄ will allow for shorter pulses than Nd:YAG for any given repetition rate.

Suppliers

The RP Photonics Buyer's Guide contains 97 suppliers for Q-switched lasers. Among them:

ALPHALAS

ALPHALAS offers actively and passively Q-switched DPSS lasers (IR, VIS & UV) with pulse durations < 1 ns, suitable for applications requiring high peak power and precise synchronization. Both types of lasers feature TEM₀₀ beam profile and compact design.

Passively Q-switched microchip lasers are mechanically extremely stable with very high pulse-to-pulse stability, pulse energies up to 1.5 mJ and > 2 MW peak power and pulse duration down to 500 ps.

The actively Q-switched lasers feature jitter < 500 ps with an option down to 200 ps. Repetition rates from single shot up to 100 kHz and average power up to 5 W are available.

Higher energies and powers can be reached using MOPA configurations.

Applications: micromachining, marking & cutting (e.g. diamonds), nonlinear optics, supercontinuum generation, time-resolved fluorescence measurements, DNA-analysis, pollution monitoring, LIBS, ignition of combustion engines.

Bright Solutions

Bright Solutions offers various Q-switched lasers:

Wedge – nanosecond Q-switched lasers for 266, 355, 532, 1064, 1570, 3100 nm (also multi-wavelength configurations), used e.g. for atmospheric LIDAR, monitoring, glass machining or lithography
Onda – compact monolithic nanosecond Q-switched lasers for 266, 355, 532 or 1064 nm, used e.g. for lens marking, plastic marking or intravolume glass marking
Sol – compact Q-switched lasers for 355, 532 or 1064 nm, up to 200 kHz, used e.g. for automotive fabrication, electronic machining, ID card writing and other industrial applications
Vento – sub-nanosecond MOPA lasers with pulse durations down to 500 ps, up to 200 kHz, up to 100 W average power at 1064 nm or 50 W at 532 nm, e.g. for LIDAR or PCB microprocessing
Aero – high energy lasers with up to 200 mJ at 1064 nm, 100 mJ at 532 nm, multi-wavelength configurations, custom beam shaping, application e.g. in atmospheric LIDAR, LIBS or nonlinear spectroscopy
ONE DPSS – miniaturized Q-switched lasers with up to 200 μJ and down to 3 ns, e.g. for atmospheric LIDAR and laser marking on plastics

EKSPLA

Short pulse duration, a wide range of customization options and high stability are distinctive feature of EKSPLA nanosecond Q-switched lasers. Employing latest achievements in laser technologies, our team of dedicated engineers designed wide range of products tailored for specific applications: from compact, simple and robust EKSPLA nanosecond Q-switched DPSS NL230 series lasers for OEM manufacturers to high energy customized flash-lamp multijoule systems for research laboratories.

Second (532 nm), third (355 nm), fourth (266 nm) and fifth (213 nm) (where available) harmonic options combined with various accessories and customization possibilities make these lasers well suited for many OEM and laboratory applications like OPO, OPCPA, Ti:sapphire and dye laser pumping, spectroscopy, remote sensing and plasma research.

This article is a posting of the Photonics Spotlight, authored by Dr. Rüdiger Paschotta. You may link to this page and cite it, because its location is permanent. See also the RP Photonics Encyclopedia.

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