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Mode-locked Lasers

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RP Photonics has a very deep expertise on mode-locked lasers, e.g. concerning the design of mode-locked bulk or fiber lasers, a comparison of different types of mode-locked lasers, of different mode-locking techniques, etc. Also, RP Photonics has powerful numerical software for designing and optimizing such lasers.

Definition: lasers which emit ultrashort pulses on the basis of the technique of mode locking

German: modengekoppelte Laser

Categories: lasers, light pulses

How to cite the article; suggest additional literature

A mode-locked laser is a laser to which the technique of active or passive mode locking is applied, so that a periodic train of ultrashort pulses is emitted. See the article on mode locking for more details on mode-locking techniques; the present article focuses more on the lasers themselves. The article on ultrafast lasers also gives some idea about current developments in ultrashort pulse generation.

As ultrashort pulses have a certain bandwidth, mode-locked lasers for short pulses (particularly in the sub-picosecond region) require a gain medium with a large gain bandwidth. Other desirable features are a not too high nonlinearity and chromatic dispersion, and (particularly for passive mode locking) high enough laser cross sections in order to avoid Q-switching instabilities.

Types of Mode-locked Lasers

The following types of lasers are attractive for mode locking:

cavity of mode-locked laser

Figure 1: Resonator setup of a typical femtosecond mode-locked solid-state bulk laser with low or medium output power. The gain medium can be made of a crystal or of glass. A prism pair is used for dispersion compensation, and passive mode locking is achieved with a SESAM.

Design Issues

The design of a mode-locked laser is generally a non-trivial task, and particularly so if extreme parameter regions for the pulse parameters are targeted. There is a complicated interplay of many effects, including dispersion and several nonlinear effects, and changing one design parameter often influences several others. (For example, in a soliton mode-locked laser a change in mode size in the laser crystal or of the cavity length changes the balance of nonlinearity and dispersion, and thus also the pulse duration.) As a result, it can be difficult to achieve simultaneously very short pulses, stable operation, and a high power efficiency. For given parameters of the gain medium, there can be certain restrictions on the achievable pulse parameters. A relatively trivial one is that a gain medium with a small gain bandwidth is not suitable for generating very short pulses. Certainly more surprising is e.g. the finding that mode-locked solid-state lasers have difficulties in combining a high pulse repetition rate with a high average output power, and that the additional requirement of generating sub-picosecond pulses makes this trade-off even much more demanding. Such constraints arise from a combination of effects and issues such as Q-switched mode locking and other kinds of instabilities, details of pulse shaping, and limitations of saturable absorbers, and are also influenced by seemingly totally unrelated issues such as the beam quality of the available pump source.

For such reasons, a very systematic process of laser development, based on a solid quantitative understanding of all the relevant physical details and on deep experience with typical limitations, is essential for efficient product development. A key point is to work out a detailed laser design and to check quantitatively a number of issues before engaging in experimental investigations. Without such preparations, there is a risk of getting into a combination of problems which can not simply be solved step by step.

Some Special Achievements

Some special achievements with passively mode-locked solid-state lasers are:

high repetition rate miniature laser

Figure 2: Miniature Er:Yb:glass laser setup for a pulse repetition rate of 50 GHz [15]. The cavity length is only 3 mm (from the output coupler to the SESAM). A modified setup allowed for even 100 GHz [20].

Higher Pulse Energies with Cavity Dumping

As explained in detail in the article on cavity dumping, a mode-locked laser can generate higher pulse energies of e.g. several microjoules at lower pulse repetition rates (e.g. 100 kHz or 1 MHz) by incorporation of a cavity dumper in the laser resonator. The basic principle is to form a high-energy pulse within the resonator while having low resonator losses, and then to couple out of the energy with the cavity dumper.

Typical Applications of Mode-locked Lasers

The following list gives some impression of the manifold applications of mode-locked lasers:

Bibliography

[1]F. Krausz et al., “Femtosecond solid-state lasers”, IEEE J. Quantum Electron. 28 (10), 2097 (1992)
[2]C. Spielmann et al., “Ultrabroadband femtosecond lasers”, IEEE J. Quantum Electron. 30 (4), 1100 (1994)
[3]U. Keller, “Ultrafast all-solid-state laser technology”, Appl. Phys. B 58, 347 (1994)
[4]S. Arahira et al., “Mode-locking at very high repetition rates more than terahertz in passively mode-locked distributed-Bragg-reflector laser diodes”, IEEE J. Quantum Electron. 32 (7), 1211 (1996)
[5]U. Morgner et al., “Sub-two cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser”, Opt. Lett. 24 (6), 411 (1999)
[6]D. H. Sutter et al., “Semiconductor saturable-absorber mirror-assisted Kerr lens modelocked Ti:sapphire laser producing pulses in the two-cycle regime”, Opt. Lett. 24 (9), 631 (1999)
[7]C. Hönninger et al., “Ultrafast ytterbium-doped bulk lasers and laser amplifiers”, Appl. Phys. B 69 (1), 3 (1999)
[8]R. Paschotta et al., “Progress on all-solid-state passively mode-locked ps and fs lasers”, Proc. SPIE 3616, 2 (1999)
[9]E. Sorokin et al., “Diode-pumped ultra-short-pulse solid-state lasers”, Appl. Phys. B 72, 3 (2001)
[10]L. Krainer et al., “Compact Nd:YVO4 lasers with pulse repetition rates up to 160 GHz”, IEEE J. Quantum Electron. 38 (10), 1331 (2002)
[11]U. Keller, “Recent developments in compact ultrafast lasers”, Nature 424, 831 (2003)
[12]E. Innerhofer et al., “60 W average power in 810-fs pulses from a thin-disk Yb:YAG laser”, Opt. Lett. 28 (5), 367 (2003)
[13]A. Fernandez et al., “Chirped-pulse oscillators: a route to high-power femtosecond pulses without external amplification”, Opt. Lett. 29 (12), 1366 (2004)
[14]F. Brunner et al., “Powerful RGB laser source pumped with a mode-locked thin-disk laser”, Opt. Lett. 29 (16), 1921 (2004)
[15]S. C. Zeller et al., “Passively mode-locked 50-GHz Er:Yb:glass laser”, Electron. Lett. 40 (14), 875 (2004)
[16]R. Paschotta et al., “Picosecond pulse sources with multi-GHz repetition rates and high output power”, New J. Phys. 6, 174 (2004)
[17]E. Sorokin and S. Naumov, “Ultrabroadband infrared solid-state lasers”, IEEE J. Sel. Top. Quantum Electron. 11 (3), 690 (2005)
[18]S. Gee et al., “Self-stabilization of an actively mode-locked semiconductor-based fiber-ring laser for ultralow jitter”, IEEE Photon. Technol. Lett. 19 (7), 498 (2007)
[19]S. V. Marchese et al., “Femtosecond thin disk laser oscillator with pulse energy beyond the 10-microjoule level”, Opt. Express 16 (9), 6397 (2008)
[20]A. E. Oehler et al., “100 GHz passively mode-locked Er:Yb:glass laser at 1.5 μm with 1.6-ps pulses”, Opt. Express 16 (26), 21930 (2008)
[21]P. Sévillano et al., “32-fs Kerr-lens mode-locked Yb:CaGdAlO4 oscillator optically pumped by a bright fiber laser”, Opt. Lett. 39 (20), 6001 (2014)
[22]R. Paschotta and U. Keller, “Passively mode-locked solid-state lasers”, in Solid-State Lasers and Applications (ed. A. Sennaroglu), CRC Press, Boca Raton, FL (2007), Chapter 7, pp. 259–318
[23]R. Paschotta, Field Guide to Laser Pulse Generation, SPIE Press, Bellingham, WA (2007)
[24]For German readers: R. Paschotta, “Ultrakurzpuls-Festkörperlaser – eine vielfältige Familie”, Photonik 01/2006, S. 70

(Suggest additional literature!)

See also: ultrafast lasers, mode locking, picosecond lasers, femtosecond lasers, titanium–sapphire lasers, mode-locked fiber lasers, mode-locked diode lasers, saturable absorbers, frequency combs, frequency metrology, pulses, ultrashort pulses, cavity dumping, timing jitter, laser design, laser development, Spotlight article 2008-03-26, Spotlight article 2010-03-22, Spotlight article 2010-07-27


Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!

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