CW Nd:YAG LASER SYSTEMS
Components of a CW Nd:YAG Laser
CW Nd:YAG lasers are available in output from a few milliwatts to as high
as a kilowatt in power. Although they vary considerably in size and
complexity of design, all have the same basic elements shown in Figure 1.
The active medium is an Nd:YAG laser rod. It is optically pumped by a
continuous-pumping lamp and is placed between two external mirrors that
form the optical cavity for the laser beam. This section discusses each of the
basic subsystems of such a laser.
Fig. 1
Basic design of CW Nd:YAG lasers.
Laser Rod
The rods used for CW operation are usually from one to four millimeters in
diameter and have lengths from one inch to about six inches. Smallerdiameter rods are preferred because they present fewer cooling problems
than larger rods. The rod ends are usually antireflection coated from the
Nd:YAG wave-length of 1.06 microns. The rod is mounted inside a quartz
or glass water jacket. Cooling is provided by water flow directly across the
rod surface.
Optical Cavity
The optical cavity of the Nd:YAG laser usually consists of two mirrors
mounted separately from the laser rod. Several cavity configurations may be
used, but all employ at least one spherical mirror. Both long radius and long
radius hemispherical cavities are commonly employed. In some systems,
shaping of the beam within the cavity is desirable, and two mirrors with
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different radii of curvature are used. The HR mirror has a reflectivity of
about 99.9% and the output coupler transmission varies from less than one
percent on small lasers to about eight percent on larger ones. The optical
cavities of Nd:YAG lasers are often equipped with an adjustable or
interchangeable aperture for selection of multimode or TEM00 mode
operation.
Optical Pumping System
ARC-LAMP
Arc lamps are gas-discharge devices similar to flashlamps in design but
intended to be operated continuously. There are two main types of arc
lamps: linear lamps and short-arc lamps. The linear lamps are very like
flashlamps, except for electrode design, having pointed cathodes instead of
the rounded cathodes used in flashlamps. They are cylindrical in shape and
have bore diameters that typically lie in the range between 4 and 7 mm and
typically have arc lengths between 5 and 15 cm. They are filled with gas at a
pressure usually between one and three atmospheres. They are well-suited to
act as the pump sources for solid-state lasers, because the shape of the
spectral emission is well-matched to the absorption of laser rods. A typical
arrangement for pumping a solid-state laser with a linear arc lamp is
illustrated in Figure 1.
Fig. 1
Typical configuration for continuous arc-lamp
pumping of a solid-state laser
In short-arc lamps, the distance between electrodes is much shorter,
typically less than one millimeter, so that these devices act essentially as
point sources of light. They are suited for applications like illumination and
exposure of photoresist. The structure of linear lamps and short-arc lamps is
compared in Figure 2.
2
Fig. 2
Typical forms for linear arc lamp (top) and short-arc lamp (bottom)
Arc lamps may be filled with a number of different gases, including
krypton, xenon, mercury, and mercury-xenon. The linear krypton-filled arc
lamp emits a spectrum that is well-matched to the absorption of Nd:YAG.
Linear krypton arc lamps are used almost universally for pumping
continuous Nd:YAG lasers and continuously pumped, repetitively pulsed
Nd:YAG lasers. Since the Nd:YAG laser is the most common solid-state
laser, for our purposes we will emphasize the linear krypton arc lamp.
Krypton Arc Lamps
Krypton arc lamps emit a spectrum of light that is marked by a number of
strong spectral lines in the 750-900-nm region. A typical example of the
spectrum of a continuous krypton arc lamp is shown in Figure 3. This
spectrum is similar to the spectrum of the krypton flashlamp with relatively
low input energy
Typical spectral distribution of light emission
from a continuous krypton arc lamp
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Because the peak absorption of Nd:YAG is in the 800-nm region, krypton
arc lamps make very effective pumps for Nd:YAG lasers.
The useful light output of krypton arc lamps for excitation of Nd:YAG
lasers increases rapidly with lamp input power, bore diameter, and gas fill
pressure. Figure 4 shows how the light output of a krypton arc lamp
increases with input power for a fixed bore diameter of 4 millimeters at
several different fill pressures. Output increases rapidly with fill pressure up
to about 4 atmospheres, but increases only slowly at higher pressures.
Light output useful for pumping an Nd:YAG laser for continuous
krypton arc lamps for various fill pressures.
Figure 5 shows the useful light output as a function of lamp input power for
krypton arc lamps filled to 6 atmospheres cold pressure and with several
bore diameters. The output is a strong function of bore diameter up to a
diameter around 4 millimeters, but increases more slowly at larger
diameters.
4
Light output versus lamp input power for krypton arc lamps
with the indicated bore diameters
Krypton arc lamps are available as both linear lamps and short-arc lamps,
but the linear type is used for excitation of solid-state lasers. Table 1 shows
characteristics of some representative high-power rod-seal continuous
krypton arc lamps, suitable for pumping of solid-state lasers, to give an idea
of what is commercially available. Rod-seal construction will be discussed
in the section on mechanical characteristics.
Table 1. Characteristics of Commercially Available
High-Power Continuous Krypton Arc Lamps.*
Bore
Diamete
r
(mm)
Arc
Lengt
h
(mm)
Maximu
m
Average
Input
Power
(W)
Maximu
m SteadyState
Voltage
(V DC)
Maximu
m SteadyState
Current
(A DC)
Minimu
m
Starting
Anode
Voltage
(V DC)
Minimu
m
Trigger
Voltage
(kV)
4
48
2200
104
20
1800
25
4
51
2100
95
20
2000
25
4
51
2200
110
20
2000
25
4
76
3200
160
20
2500
25
5
4
76
3200
155
20
2500
30
6
76
3000
120
40
2500
30
6
76
5000
122
40
2500
30
6
102
5500
140
38
3000
30
*Based on information from EG&G Electro-Optics
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