Tsunami - Beckman Institute Laser Resource Center

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Appendix A
Mode Locking
General
In this chapter, we provide a brief discussion of mode locking and the
regenerative mode locking technique employed in the Tsunami laser. A
description of the group velocity dispersion (GVD) found within the cavity
is also presented, and the role of nonlinear effects (due to intense pulses
passing through the gain medium) is considered. Finally, GVD compensation
techniques employed within the Tsunami are discussed since they
ultimately determine the output pulse width.
In any laser system, the allowed oscillating wavelengths (or frequencies)
are determined by two factors: the longitudinal modes determined by the
laser cavity (subject to threshold conditions) and the gain-bandwidth of
the laser medium. In a laser cavity, the electric field of the oscillating
optical frequencies must repeat itself after one round-trip; i.e., the oscillating
wavelengths must satisfy a standing wave condition in the laser cavity
or an integral number of half-wavelengths must exactly fit between the end
mirrors. The small group of frequencies that satisfy this condition are the
longitudinal modes of the laser. The gain-bandwidth of the laser medium
is determined by its atomic or molecular energy levels. Atomic gas lasers
tend to have relatively narrow bandwidth, while molecular dye and solid
state systems exhibit broader bandwidth.
In a cw or free-running laser, the longitudinal modes operate independently.
Cavity perturbations cause some modes to stop oscillating and
when they re-start they have a different phase. Thus, the laser output
comprises various randomly phased mode frequencies. In a mode-locked
laser, the longitudinal modes must be "locked" in phase, such that they
constructively interfere at some point in the cavity and destructively interfere
elsewhere in order to create a single circulating pulse. Each time this
intracavity pulse reaches the partially reflective output coupler, an output
pulse is produced. The time between the output pulses is the time it takes
for the cavity pulse to make one complete round trip. For a Tsunami
system, this corresponds to about 12.2 ns. The output pulse frequency, or
repetition rate (rep rate), is 82 MHz (refer to Figure A-1).