LASER-9

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
Figure 1 shows graphically the output pulse of a solid state laser operating in the normal pulsed mode. Such a pulse has a nominal duration of from a tenth of a millisecond to several milliseconds. The pulse is composed of many small pulses, each lasting about 50 ns. Module 1-6, "Lasing Action," discusses the variations in amplifier gain that lead to this spiking in the laser output. But there is another factor that must be considered to account for the large number of spikes present and their overlapping. Solid state lasers typically have a laser line width of 30 GHz or greater and therefore, operate on a hundred or more longitudinal modes. [Recall Examples E and H in Module 1-7. There it was shown that a typical Md:YAG laser has a mode spacing of of 258 MHz (Example E) and, if the fluorescent linewidth of the Nd:YAG laser is 30GHz, then the number of longitudinal modes is calculated to (Example H).]Each of these longitudinal modes exhibits a spiking behavior independent of the behavior of the other modes. The total output pulse