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.

Fig. 2 Q-switched laser schematic.
There are generally four types of Q-switches in use:
mechanical, acousto-optic, electro-optic, and dye.
Figure 3 compares the operation of a pulsed laser in the Q-switch mode to the operation of the same laser in the normal pulsed mode. Without the Q-switch, the amplifier gain reaches the lasing threshold at t1, and lasing begins. The lasing process removes energy from the active medium in the form of the spiked output of a normal pulse. The amplifier gain and output power of the normal pulsed mode laser are indicated by dotted lines.

Fig. 3 Operation of a Q-switch
The values for the Q-switched mode are indicated by solid lines. The Q-switch prevents internal feedback of the beam and maintains a loop gain value of zero until the energy stored in the active medium has reached a maximum value. At time t2 in Figure 3, the amplifier gain is many times the maximum gain value in a normal pulsed laser, due of course, to the large population inversion created. When the Q-switch is opened, loop gain rises rapidly to a large value – in some cases, the value may be several hundred. This large increase in loop gain can produce intense standing waves in many cavity modes, and all the stored energy is released in the resulting pulse. Q-switched pulses range in duration from a few seconds to several hundred nanoseconds. And the peak power of a Q-switched pulse may be several thousand times greater than that of the same laser without a Q-switch. While Q-switching reduces the total energy of the pulse, the pulse width is shortened even more.
The "Q" in Q-switching stands for "quality factor" and is a carryover from electronics. The quality of a cavity is defined as the ratio of the amount of energy stored in that cavity in the form of a standing wave to the amount of energy lost for all reasons during a round trip of the cavity. When the Q-switch of a laser is closed, there is no feedback and thus no standing wave. The loss is very high and thus the quality factor is zero. When the Q-switch is opened, a strong standing wave is formed, causing the loss to be reduced. The Q-switch receives its name from the fact that it allows the "Q" of the cavity to be "switched" from (feedback blocked) a low value to a high value. (feedback restored) However, actual calculations of the quality factor are seldom made for laser cavities.
No mechanical shutter can open fast enough for effective Q-switching. The simplest mechanical method of achieving Q-switching at the necessary rate is the rotation of the HR mirror. The HR mirror is mounted on the shaft of a motor that has a rotational rate of 30,000 rpm or greater. Once during each revolution, the mirror is aligned for nanoseconds and the laser pulse is produced. As indicated in the caption of Figure 2, one can use acousto-optic, electro-optic and dye switches, in addition to mechanical switches, to effectively Q-switch the cavity.
MODELOCKED LASERS:
Figure 4 illustrates the output of a modelocked laser. This output consists of a train of laser pulses, each pulse having a duration of from picoseconds to a fraction of a nanosecond, depending upon the laser. The separation of the pulses is equal to the time required for light to make one round trip around the laser cavity, from mirror to mirror and back again. If the distance between mirrors is l, then the roundtrip time is where c is the speed of light in the cavity.