Spectra-Physics Tsunami - Appendix A: Modelocking Principles; General Principles of Laser Oscillation

A-1
Appendix A Modelocking
In this chapter, we provide a brief discussion of modelocking and the
regenerative modelocking 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 compensa-
tion techniques employed within the Tsunami are discussed since they ulti-
mately determine the output pulse width.
General
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 mir-
rors. The small group of frequencies that satisfy this condition are the lon-
gitudinal 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 continuous wave (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 destruc-
tively 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 80 to 82 MHz (refer to Figure A-1).