Mode-Locked Lasers: Lower Average Powers in Shorter Pulses

Posted on 2008-03-26 as part of the Photonics Spotlight (available as e-mail newsletter!)

Permanent link: https://www.rp-photonics.com/spotlight_2008_03_26.html

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

Abstract: It is often experienced that those mode-locked lasers generating shorter pulses also offer lower average powers and pulse energies. The article explains some common reasons for that correlation.

Ref.: encyclopedia articles on mode-locked lasers, ultrafast lasers, ultrashort pulses

It is very common that those mode-locked lasers generating the shortest pulses tend to offer lower average output powers and pulse energies. This could be surprising, since there is no direct relation between the average power capability and the pulse duration. However, there are various reasons behind the mentioned correlation. Here are some examples:

Shorter pulses have a larger optical bandwidth, so that a laser gain media with a larger gain bandwidth are required. Unfortunately, such media tend to have a lower thermal conductivity. This is not surprising if they are glasses: the random order in a glass increases spectral broadening of electronic transitions, while also disturbing the flow of heat (phonons). Even for crystalline media, a broader gain bandwidth very often comes together with poor thermal properties.
An increased emission bandwidth of a gain medium also usually implies lower peak cross-sections. This means that the sigma–tau product is reduced, which tends to reduce the power efficiency. Furthermore, lower cross-sections lead to a higher tendency for Q-switching instabilities, and measures used to keep those under control can often further degrade the power efficiency.
Shorter pulses lead to higher peak powers (for a given pulse energy) and thus to stronger nonlinear effects. This can limit the pulse energy and peak power.

These are just some typical issues, which introduce trade-offs between different performance parameters of a laser design. Any laser design involves many different aspects with many (obvious or non-obvious) relations between those, and any dropped requirement (such as a particularly short pulse duration) may allow the designer to shift certain properties so that better performance in other aspects can be achieved. And of course the opposite holds as well: demanding more on one side can compromise other features – at least when the overall design is optimized, so that no room for general improvement is wasted anywhere.

Suppliers

The RP Photonics Buyer's Guide contains 45 suppliers for mode-locked lasers. Among them:

Stuttgart Instruments

The Stuttgart Instruments Primus is an ultrafast (fs) mode-locked oscillator, based on the solid-state technology. It provides a high average output power combined with a superior low noise level (shot noise limit above 300 kHz) and an excellent long-term stability.

The solid-state technology with 1040 nm central wavelength enables the excellent long-term stability by providing several watts of output power at 40 MHz pulse repetition rate and 450 fs pulse duration. Its superior low noise level reaches the shot noise limit above 300 kHz. In combination with the stability and output power, it enables ultrasensitive measurements and makes the Primus perfectly suited as pump source for frequency converters like the Stuttgart Instruments Alpha. The entire system is encapsulated in a solid CNC-cut and water-cooled housing, thus reaching excellent robustness against external perturbations.

EKSPLA

Due to their excellent stability and high output parameters, EKSPLA scientific picosecond lasers established their name as “Gold Standard” among scientific picosecond lasers. The innovative design of the new generation of picosecond mode-locked lasers features diode-pumping‑only technology, thus reducing maintenance costs and improving output parameters. Second, third, fourth and fifth (on some versions) harmonic options combined with various accessories, advanced electronics (for streak camera synchronization, phase-locked loop, synchronization of fs laser) and customization possibilities make these lasers well suited for many scientific applications, including optical parametric generator pumping, time-resolved spectroscopy, nonlinear spectroscopy, remote sensing, metrology and others.

Cycle

Cycle supplies tailored laser systems with unique features and affordable prices:

The SOPRANO-15 is Cycle’s state-of-the art femtosecond fiber lasers, designed to fulfill tasks such as OPO/OPA pumping, semiconductor testing, and materials analysis and processing. The SOPRANO-15 operates at a center wavelength of 1550 nm or 775 nm and pulse duration below 350 fs, establishing benefits in both industrial and scientific environments in 24/7 operation.

The SOPRANO-15 mini is designed to carry out tasks such as multiphoton microscopy, spectroscopy, semiconductor testing, and materials analysis. In addition to its dependable 24/7 operation, the SOPRANO-15 mini operates at a center wavelength of 1550 nm and typical pulse duration below 130 fs, establishing benefits in both industrial and scientific environments.

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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