Harmonic Mode Locking
Pulse trains with high pulse repetition rate are sometimes obtained with the technique of harmonic mode locking, where multiple ultrashort pulses are circulating in the laser resonator with a constant temporal spacing (see Figure 1). This technique is often applied in high (multi-gigahertz) pulse repetition rate fiber lasers, since their resonators can not be made short enough to achieve a high repetition rate with a single pulse (→ fundamental mode locking).
Harmonic mode locking is associated with some technical challenges:
- Additional means may be required for achieving a constant pulse energy. Without special measures, there may be fluctuating pulse energies, or even pulse drop-out (missing pulses).
- The circulating pulses are not always mutually phase-coherent, which can matter under certain circumstances.
- In the case of passively mode-locked lasers, it can also be difficult to obtain a stable pulse spacing, i.e. a low timing jitter.
Various kinds of instabilities are related to so-called supermode noise. If N identical pulses are circulating in the resonator with equal phase, only every Nth resonator mode is excited. Supermode noise means that stable oscillation on such a subset of resonator modes is not achieved; the laser may hop to different sets of modes, or exhibit simultaneous oscillation on different mode sets over longer times. The beat notes involved are associated with increased high-frequency laser noise, e.g. in the form of increased timing jitter.
There are a variety of methods for suppressing supermode noise. These involve measures such as inserting various types of intracavity spectral filters and/or using electronic feedback systems, or exploit nonlinear and dispersive effects. In many cases, the setup of a harmonically mode-locked laser becomes more sophisticated due to such requirements. On the other hand, once supermode noise is effectively suppressed, harmonically mode-locked lasers have the potential for substantially lower laser noise (e.g. timing jitter and phase noise), compared with fundamentally mode-locked lasers. This is essentially because they tentatively require less laser gain per unit time, and gain is what mostly introduces timing jitter.
A variation of harmonic mode locking is called rational harmonic mode locking. Here, the modulation frequency is the round-trip frequency times the ratio of two integers. This also enforces a higher pulse repetition rate. In some cases, very high repetition rates have been achieved, but often with a non-constant pulse energy.
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