Tsunami - Beckman Institute Laser Resource Center

Tsunami
L-
0
6
a
C
3
+
n
5
I-
- 2L
c
+
I\
Time -
) \ I \ ) \
n,
Figure A-1: Typical output of a mode-locked laser, where L is the
cavity length and c is the velocity of light.
Mode Locking Techniques
A variety of approaches have been used to obtain a train of mode-locked
pulses from different laser systems including active mode locking, passive
mode locking, additive pulse mode locking, and self mode locking.
Active mode locking is by far the most common approach used to obtain
short optical pulses (picosecond duration) from solid state or gas lasers. A
loss modulation is applied to the laser cavity at the same frequency as the
pulse repetition rate. This is equivalent to introducing an optical shutter
into the laser cavity--only light that arrives at the shutter at precisely the
correct time passes through and is amplified in the gain media. Since the
shutter is closed at all other times, a second pulse cannot be formed.
The most common active mode-locking element is an acousto-optic modulator
(AOM), which is placed inside the optical cavity close to one of the
end mirrors. The modulator comprises a high quality optical material
(such as quartz) with two highly polished surfaces that are parallel to the
direction of light propagation. Attached to one of these surfaces is a piezoelectric
transducer that is driven at an rf frequency to generate an acoustic
wave within the modulator (Figure A-2). Using the reflection off the
opposite surface, a standing acoustic wave is generated within the modulator.
This induces a time-dependent refractive index grating along an axis
perpendicular to the light propagation. As the light interacts with this
grating, a portion of it is both diffracted and shifted in frequency by an
amount equal to the acoustic frequency. After passing through the modulator,
the diffracted and undiffracted rays are reflected back through the
modulator where a portion of each beam is diffracted once again.
If the rf drive is at frequency U ~L, the acoustic grating generated by the -"
standing wave will turn on and off at a rate of 2wmL. The value for 2amL
- -