ليزر Nd-YAG

2.3 Nd :YAG
The Nd :YAG laser is by far the most commonly used type of solid-state laser.
Neodymium-doped yttrium aluminum garnet (Nd :YAG) possesses a combination
of properties uniquely favorable for laser operation. The YAG host is hard, of
good optical quality, and has a high thermal conductivity. Furthermore, the cubic
structure of YAG favors a narrow fluorescent linewidth, which results in high
gain and low threshold for laser operation. In Nd :YAG, trivalent neodymium
substitutes for trivalent yttrium, so charge compensation is not required.

Physical Properties
Pure Y3Al5O12 is a colorless, optically isotropic crystal that possesses a cubic
structure characteristic of garnets. In Nd :YAG about 1% of Y3+ is substituted by
Nd3+. The radii of the two rare earth ions differ by about 3%. Therefore, with
the addition of large amounts of neodymium, strained crystals are obtained—
indicating that either the solubility limit of neodymium is exceeded or that the

TABLE 2.2. Physical and optical properties of Nd :YAG.

lattice of YAG is seriously distorted by the inclusion of neodymium. Some of the
important physical properties of YAG are listed in Table 2.2 together with optical
and laser parameters.
Laser Properties
The Nd :YAG laser is a four-level system as depicted by a simplified energy level
diagram in Fig. 2.2. The laser transition, having a wavelength of 1064.1 nm, originates
from the R2 component of the 4F3/2 level and terminates at the Y3 component
of the 4I11/2 level. At room temperature only 40% of the 4F3/2 population
is at level R2; the remaining 60% are at the lower sublevel R1 according
to Boltzmann’s law. Lasing takes place only by R2 ions whereby the R2 level
population is replenished from R1 by thermal transitions. The ground level of
Nd :YAG is the 4I9/2 level. There are a number of relatively broad energy levels,
which together may be viewed as comprising pump level 3. Of the main pump
bands shown, the 0.81 and 0.75?m bands are the strongest. The terminal laser
level is 2111 cm?1 above the ground state, and thus the population is a factor of
exp(_E/kT) ? exp(?10) of the ground-state density. Since the terminal level is
not populated thermally, the threshold condition is easy to obtain.
The upper laser level, 4F3/2, has a fluorescence efficiency greater than 99.5%
and a fluorescence lifetime of 230?s. The branching ratio of emission from 4F3/2
is as follows: 4F3/2 ? 4I9/2 = 0.25, 4F3/2 ? 4I11/2 = 0.60, 4F3/2 ? 4I13/2 =
0.14, and 4F3/2 ? 4I15/2 < 0.01. This means that almost all the ions transferred
from the ground level to the pump bands end up at the upper laser level, and 60%
of the ions at the upper laser level cause fluorescence output at the 4I11/2 manifold.



FIGURE 2.2. Energy level diagram of Nd :YAG.


At room temperature the main 1.06?m line in Nd :YAG is homogeneously
broadened by thermally activated lattice vibrations. The spectroscopic cross section
for the individual transition between Stark sublevels has been measured to
be ?(R2 ? Y3) = 6.5 × 10?19 cm2. At a temperature of 295 K, the Maxwell–
Boltzmann fraction in the upper Stark sublevel is 0.427, implying an effective
cross section for Nd :YAG of ?
?
(4F3/2 ? 4I11/2) = 2.8 × 10?19 cm2. The effective
stimulated-emission cross section is the spectroscopic cross section times the
occupancy of the upper laser level relative to the entire 4F3/2 manifold population.
Figure 2.3 shows the fluorescence spectrum of Nd3+ in YAG near the region of
the laser output with the corresponding energy levels for the various transitions.
The absorption of Nd :YAG in the range 0.3 to 0.9?m is given in Fig. 2.4(a). In
Fig. 2.4(b) the absorption spectrum is expanded around the wavelength of 808 nm,
which is important for laser diode pumping. Thermal properties of Nd :YAG
are summarized in Table 2.3. Under normal operating conditions the Nd :YAG
laser oscillates at room temperature on the strongest 4F3/2 ? 4I11/2 transition
at 1.0641?m. It is possible, however, to obtain oscillation at other wavelengths
by inserting etalons or dispersive prisms in the resonator, by utilizing a specially



FIGURE 2.3. Fluorescence spectrum of Nd3+ in YAG at 300K in the region of 1.06 ?m
[10], [11].
designed resonant reflector as an output mirror, or by employing highly selective
dielectrically coated mirrors. These elements suppress laser oscillation at
the 1.06?m wavelength and provide optimum conditions at the wavelength desired.
With this technique laser systems have been built which utilize the 946 and
1330 nm transitions.




FIGURE 2.4. (a) Absorption spectrum of Nd :YAG from 0.3 to 0.9?m and (b) expanded
scale around 808 nm.

TABLE 2.3. Thermal properties of Nd :YAG.


FIGURE 2.8. Output from a Nd:YVO4 and Nd :YAG laser as a function of diode pump
temperature and wavelength [13].


3.6.2 Diode Side-Pumped Nd : YAG Laser
The system illustrated here produces an energy per pulse of about 0.5 J at a repetition
rate of 40 Hz. Critical design issues for this laser include heat removal
from the diode arrays and laser rod and the overlapping of the pump and resonator
mode volumes. In side-pumped configurations, laser-diode arrays are not
required to be coherent, and pump power can be easily scaled with multiple arrays
around the circumference of the rod or along its axis. Instead of one diode array

pumping the laser crystal, this particular laser employs 16 diode arrays located
symmetrically around the rod. As shown in Fig. 3.19 the diode pumps are arranged
in four rings, each consisting of four arrays. Since each array is 1 cm long,
the total pumped length of the 6.6 cm × 0.63 cm Nd :YAG crystal is 4 cm. This
arrangement permits the incorporation of large water-cooled heat sinks required
for heat dissipation, and it also provides for a very symmetrical pump profile. An
eight-fold symmetry is produced by rotating adjacent rings of diodes by 45?. A
photograph of the extremely compact design is also shown in Fig. 3.19. The symmetrical
arrangement of the pump sources around the rod produces a very uniform
pump distribution, as illustrated in Fig. 3.20. The intensity profile displays
the fluorescence output of the rod taken with a CCD camera. In Fig. 3.21, the
output versus optical pump input is plotted for long pulse multimode and TEM00


FIGURE 3.19. Cross section (left) and photograph (right) of diode-pumped Nd :YAG laser
head.


FIGURE 3.20. Pump distribution of a 16-diode array side-pumped Nd :YAG crystal (each
line represents a 10% change in intensity).

mode operation. Also shown is the output for Q-switch TEM00 mode operation.
The resonator configuration for the long pulse, multimode operation is depicted
in Fig. 3.22. The TEM00 mode performance was achieved with a variable reflectivity
mirror and a concave–convex resonator structure which will be described in
Chapter 5.
The multimode laser output can be expressed by