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Additive-pulse Mode Locking

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Acronym: APM

Definition: a technique for mode locking a laser, using a nonlinear interaction in an external resonator

Alternative term: coupled-cavity mode locking

More general term: mode locking

Category: article belongs to category light pulses light pulses

DOI: 10.61835/7z8 Cite the article: BibTex plain text HTML Link to this page! LinkedIn

Additive-pulse mode locking [1, 2] (sometimes also called coupled-cavity mode locking) is a technique for passive mode locking of lasers (typically bulk lasers), used for generating short optical pulses with durations of picoseconds or femtoseconds.

The general principle of additive-pulse mode locking is to obtain an artificial saturable absorber by exploiting nonlinear phase shifts in a single-mode fiber. The fiber is contained in a resonator, which has the same round-trip time as the laser resonator and is coupled to it with a semi-transparent dielectric mirror. The pulses returning from the fiber resonator into the main laser resonator interfere with those pulses which already are in the main resonator. For proper adjustment of the resonator lengths, there is constructive interference near the peak of the pulses, but not in the wings because the latter have acquired different nonlinear phase shifts in the fiber. As a result, the peak of the circulating pulse is enhanced, whereas the wings are attenuated.

Figure 1: Schematic setup of an additive-pulse mode-locked laser.

The strength of the APM technique is that it makes it possible to obtain fairly short pulses without using very special optical components. It can also work in different wavelength regions. For these reasons, additive-pulse mode locking was widely used in the early days of passively mode-locked lasers; it was applied to various bulk lasers, color center lasers, and fiber lasers. However, the resonator length adjustment is usually rather critical, questioning the practicability of the technique for commercial products. In some mode-locked high-power lasers, additive-pulse mode locking was reported to be self-adjusting, but the exact mechanism does not seem to have been reported. In some fiber lasers, APM has been reported to occur with self-matched resonator lengths due to an automatic adjustment of the oscillation wavelength [8].

A Modified Scheme for Fiber Lasers

Additive mode locking can also be applied to mode-locked fiber lasers in different forms. In particular, one can use a nonlinear fiber loop mirror, often also containing a piece of active fiber, which provides a power-dependent transmission to an output port or power-dependent reflection. The fiber loop mirror contains a fiber coupler, which is first use for splitting a pulse into two, propagating through the ring in opposite directions, and then for interferometrically combining them again. In effect, this is quite similar to what happens in the above described type of bulk laser, although in the latter case one has a nonlinear resonator which is not a ring resonator.

Another frequently used technique is based on nonlinear polarization rotation in a fiber. That method is also somewhat related to additive-pulse mode locking, although an external resonator is not required. Two polarization modes, which are coupled with each other by nonlinear effects, are interferometrically combined at a polarizing element, where a similar interference effect occurs as in additive-pulse mode locking.

Optimizing Mode-locked Fiber Lasers

Design optimization of a mode-locked laser, whatever mode-locking mechanism it is built on, is a complex task. Having a suitable simulator is essential for getting detailed insight as the basis for any optimized design. For example, find out how exactly the pulses propagate inside the laser, and how sensitive your design is to variations of various parameters. The RP Fiber Power software is an ideal tool for such work.

More to Learn

Bibliography

[1]	J. Mark et al., “Femtosecond pulse generation in a laser with a nonlinear external resonator”, Opt. Lett. 14 (1), 48 (1989); https://doi.org/10.1364/OL.14.000048
[2]	E. P. Ippen et al., “Additive pulse mode locking”, J. Opt. Soc. Am. B 6 (9), 1736 (1989); https://doi.org/10.1364/JOSAB.6.001736
[3]	L. Y. Liu et al., “Self-starting additive-pulse mode locking of a Nd:YAG laser”, Opt. Lett. 15 (10), 553 (1990); https://doi.org/10.1364/OL.15.000553
[4]	F. Krausz et al., “Self-starting additive-pulse mode locking of a Nd:glass laser”, Opt. Lett. 15 (19), 1082 (1990); https://doi.org/10.1364/OL.15.001082
[5]	K. Tamura et al., “Self-starting additive pulse modelocked erbium fibre ring laser”, Electron. Lett. 28, 2226 (1992); https://doi.org/10.1049/el:19921430
[6]	H. A. Haus et al., “Additive-pulse modelocking in fiber lasers”, IEEE J. Quantum Electron. 30 (1), 200 (1994); https://doi.org/10.1109/3.272081
[7]	L. E. Nelson et al., “Ultrashort-pulse fiber ring lasers”, Appl. Phys. B 65, 277 (1997); https://doi.org/10.1007/s003400050273
[8]	D. W. Huang et al., “Fiber-grating-based self-matched additive-pulse mode-locked fiber lasers”, IEEE J. Quantum Electron. 35 (2), 138 (1999); https://doi.org/10.1109/3.740734

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