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 four basic operating modes for pulsed lasers:
• Normal pulsed mode.
• Q-switched mode.
• Mode locked.
• Cavity-jumped mode
NORMAL PULSED LASERS:
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 is composed of thousands of these short pulses.

Fig. 1 Normal pulsed showing longitudinal modes
giving rise to many spikes within the pulse width
the pulse with of 0.5 ms.
The total energy of the pulse and the total pulse duration remain essentially the same from shot to shot for such a laser. But the maximum output power reached during one pulse may be very different from that of the next. For this reason, such lasers often are classified according to energy per pulse and pulse duration. A rough approximation of maximum pulse power may be calculated from these values.
Q-SWITCHED LASERS:
Figure 2 shows a schematic diagram for a Q-switched laser. Several types of Q-switches are in common use, each type being suited to a particular type of laser and pulse domain. The Q-switch acts as a shutter within the laser cavity. When this shutter is closed, light passing through the active medium is blocked from reaching the HR mirror, or is reflected out of the cavity. Consequently, the high reflectivity (HR) mirror provides no feedback. The Q-switch introduces sufficient loss in the laser cavity to prevent lasing, which, in turn, allows the amplifier gain of the laser to increase far above the normal lasing threshold. When the Q-switch is opened that is, when feedback between the mirrors is restored lasing is initiated, and the energy stored in the active medium is subsequently released in one intense pulse.