Page 263 - Tunable Lasers Handbook

6 Transition Metal Solid-state Lasers 223

is lost through a process referred to as amnplijied spontaneous emission (ASE). In
this process, a photon emitted spontaneously in the laser material will stimulate
several other active atoms in the upper laser level to emit their quantum of stcired
energy before it can escape from the laser material. Thus, a single spontaneously
emitted photon can cause several other atoms in the upper laser level to lose
their energy in a process that does not contribute to the laser output. As such,
lasers with high stimulated emission cross sections can be inefficient in the
pulsed mode. Conversely, lasers with low stimulated emission cross sections
also tend to be inefficient. In this case, the stimulated emission cross section is
so low that even the photons destined for laser output have difficulty stimulating
the active atoms in the upper laser level to emit. Although this can be overcome
bo a degree by having a high density of laser photons, this high density of laser
photons tends to aggravate laser induced damage problems.
Solid-state lasers can also have favorable size and reliability properties,
Solid-state lasers can be compact. A solid-state laser head. which is the optical
portion of the laser device. capable of producing an average output of several
watts. either pulsed or continuous wave, can be a hand-held item. The reliability
of solid-state lasers is primarily limited by the lifetime of the optical pump. Con-
tinuously operating arc lamps have lifetimes in the range of several hundred
hours. Pulsed flashlamps can have a lifetime from IO7 to 109 shots. With diode-
pumped lasers, these lifetimes can increase one or more orders of magnitude.
Because some of the improvements in solid-state lasers are predicated on
the use of laser diode pumping, it is reasonable to ask whether laser diodes
should be used directly. In many cases, the direct use of laser diodes is appropri-
ate. However. a primary advantage of the solid-state lasers is their utilily as an
optical integrator. Laser diodes are devices with a short upper laser level lifetime
and a limited amount of power. To obtain a high peak power or a large energy
per pulse requires many laser diodes to operate in concert. In addition, if good
beam quality or narrow spectral bandwidth is desired, all of the individual laser
diodes must be operated coherently, complicating the design of the laser diode
arrays.
Solid-state lasers on the other hand can integrate the output of many laser
diodes or laser diode mays. both spatially and temporally. in a single optical
device. Moreover, the solid-state laser material can store the power output of the
laser diodes efficiently. making the production of high-peak-power pulses possi-
ble. This spatial and temporal optical integration ability makes it substantially
easier to achieve a high peak power pulse or an output with particular beam qual-
ity or spectral bandwidth properties. Having the optical energy concentrated in a
single optical device. such as a laser rod, facilitates the production of a single-
transverse-mode, high-peak-power device.
In the following sections, transition metal and lanthanide series solid-state
lasers are compared and the physics germane to transition metal solid-state
lasers is outlined. Thereafter, a section is devoted to each of the more common