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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 particularly attractive for mode locking:
- In the 1970s, dye lasers were routinely used, which were pumped with argon ion lasers. Laser dyes have a broad gain bandwidth, allowing for very short pulses. However, dye lasers have been largely replaced with solid-state lasers once these were able to deliver similar or better performance.
- Solid-state bulk lasers, based on ion-doped crystals or glasses, are today the dominant type of mode-locked lasers. They allow for very short pulses, very high pulse energies and/or average output powers, high or low pulse repetition rates, and high pulse quality. Some record achievements are listed below.
- Fiber lasers can also be mode-locked for generating very short pulses with potentially cheap setups. See the article on mode-locked fiber lasers for more details. High output powers are typically not achieved directly, but by using fiber amplifiers. The achieved pulse durations of ultrafast fiber lasers are often limited by nonlinearities or by higher-order dispersion, rather than by the gain bandwidth.
- Semiconductor lasers can be built as mode-locked diode lasers, mostly for applications in optical fiber communications. More recently, optically pumped passively mode-locked VECSELs have been demonstrated which can rival other solid-state lasers, particularly if a combination of relatively high output power, a multi-gigahertz pulse repetition rate, and possibly a short pulse duration (a few picoseconds or less) is required.
Due to the very different properties of those gain media, it is vital to select an appropriate medium for operation of a mode-locked laser in a particular parameter regime, e.g. concerning pulse duration, center wavelength and pulse repetition rate.
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:
- The very shortest pulses with durations below 10 fs (few-cycle pulses) are usually achieved with Kerr lens mode locking of a Ti:sapphire laser [6, 5].
- High average output powers of up to ≈ 80 W in sub-picosecond pulses  and pulse energies above 10 μJ have been obtained from passively mode-locked thin-disk lasers.
- Very high pulse repetition rates have been obtained with passively mode-locked miniature bulk lasers [10, 15, 20] and also with harmonically mode-locked fiber lasers. Even higher values of > 1 THz are possible with small laser diodes .
- Various kinds of lasers (normally with high pulse repetition rates) have reached quantum-limited timing jitter performance, thus outperforming many high-quality electronic oscillators.
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:
- The high pulse intensities are used for applications in material processing, such as micromachining, surface treatment, drilling holes, and three-dimensional laser prototyping.
- In the medical domain, mode-locked lasers may again be used for a kind of material processing, e.g. as a laser scalpel or in ophthalmology. However, there are also photochemical effects used e.g. for certain skin treatments, and important applications in imaging.
- Various methods of imaging, microscopy and spectroscopy, as applied in different domains, greatly profit from short pulses for various reasons.
- Short pulses allow for time-resolved measurements, e.g. electro-optic sampling measurements on integrated electronic circuits, or pump–probe measurements on semiconductor devices such as SESAMs.
- In the field of metrology, mode-locked lasers can be used for distance measurements, but also in frequency metrology (time keeping) and other fields.
- A number of processes for nonlinear frequency conversion are greatly facilitated by the high peak powers of mode-locked lasers, and are important e.g. for laser projection displays.
- Other fields with a large potential are microwave, millimeter-wave and terahertz optics, and picosecond optoelectronics.
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|||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)|
|||S. V. Marchese et al., “Femtosecond thin disk laser oscillator with pulse energy beyond the 10-microjoule level”, Opt. Express 16 (9), 6397 (2008)|
|||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)|
|||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)|
|||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|
|||R. Paschotta, Field Guide to Laser Pulse Generation, SPIE Press, Bellingham, WA (2007)|
|||For German readers: R. Paschotta, “Ultrakurzpuls-Festkörperlaser – eine vielfältige Familie”, Photonik 01/2006, S. 70|
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
and other articles in the categories lasers, light pulses
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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