Laser principles:lecture-7-
The first stage:
Mohammed Hamza:
TEMPORAL CHARACTERISTICS
OF
LASERS
One of the more
important characteristics of any laser is the temporal distribution of its
output. Continuous wave lasers produce a steady beam at
an essentially constant power output. Pulsed lasers emit their energy in short
bursts. Typical laser pulses may last several milliseconds or may be as short
as a few femtoseconds, depending upon the
methods used to shape the pulse and control its duration.
The coherence of a
laser beam is related to its temporal characteristics. For example, the
longitudinal coherence length is determined by the range of frequencies present
in the beam.
This module discusses the temporal
characteristics of lasers. In the laboratory, the student will measure the
duration and power of laser pulses.
PULSED LASERS:
Lasers may be
divided into two broad groups (1) continuous wave (CW) and (2) pulsed. A CW
laser is one whose power output undergoes little or no fluctuation with time.
It exhibits a steady flow of coherent energy. Helium neon and argon gas lasers are typical examples.
They are said to operate in the "CW mode." A larger group of lasers
has output beams that Undergo marked fluctuations; that is, the beams power
changes with time in a very noticeable fashion. They are said to operate in the
"pulsed mode." Nd:YAG solid crystal lasers and CO2 gas
lasers often, but not always, are operated in the pulsed mode.
Pulsed laser
operation may be further subdivided according to pulse length and methods for
producing such pulses. The following are the fourbasic operating modes for pulsed lasers: