| 1.
Monochromatic- Monochromatic means
single color - Lasers emit a light that consists of
a very narrow spectral range. |
| 2.
Directional- This refers to light divergence
over long distances. A laser is collimated and will
travel long distances without the beam spreading |
| 3.
Coherent- All of the light waves emitted by
a laser are in phase with each other. All the peaks
and valleys we would see on a scope would be perfectly
in line with each other. |
Non-Semiconductor
Lasers: A Lase media is required, either
a solid state material (crystal) or gas. The Lase media
is Stimulated by "pumping" energy into the Lase
media atoms causing them to release amplified coherent light
energy which we utilize as a laser beam.
PUMP TECHNOLOGIES - There are two basic "pumps"
or excitation mechanisms.
| Optical
- Is used in most solid state lasers. Wavelength specific
light is energized into a solid material, usually some
form of man made crystal. The “pump” light
is generated from special flash lamps or laser diodes
or diode arrays. |
| Electrical-
This type of pump uses either a DC or RF signal to excite
atoms. These are used mostly in gas lasers. Electrical
pumps usually consist of 2 electrodes in the center
of the optical cavity. |
| These excitation
mechanisms cause the electrons in an atom to absorb
energy and move to a higher energy level; upon obtaining
this elevated energy state they release it in order
to return to ground state. The energy is released in
the form of a photon, which is a short wave of light.
|
OPTICAL
CAVITY – These types of lasers must
contain some form of an optical cavity. There are several
types of optical cavities, but they all have the same principle
behind them. There are 2 basic components that form an optical
cavity.
| 1.
High Reflector- Consists of mirror
that will reflect over 99.9% of the light that comes
in contact with it. It is generally a concave mirror.
The mirror material is dependant upon what type of laser
media is used. |
| 2.
Output Coupler- The output coupler
is almost the same as the high reflector except it is
partially transparent to allow some laser light to emit
through it. This "leftover" light is what
we utilize as our laser beam. |
| These mirrors
are aligned inside the cavity in a way that allows the
released photons to "bounce" back and forth
between them. As the photons are moving through the
cavity the photons come into contact with other excited
atoms. This is when stimulated emission occurs. |
STIMULATED
EMISSION - Now we have used the excitation mechanism
to excite atoms and cause the electrons in these atoms to
release photons. These photons are gathering optical gain
by "bouncing" back and forth in the optical cavity
and gathering photons from other atoms. As these incident
photons come into contact with an excited atom they stimulate
those atoms to emit a photon which is identical to the incident
photon. When the optical gain reaches a certain point a
population inversion occurs, which means there are more
excited atoms than non-excited atoms. At this point stimulated
emission occurs and we get a usable laser beam emitting
from the output coupler.
NOTE- In order to speed up the emission process
laser manufacturers have implemented certain features that
keep the excited atoms just below stimulated emission or,
as in solid state lasers, use an electro-optical assembly
to attenuate the beam.
Trickle
Frequency- In gas lasers an electrical signal
is used as a type of excitation mechanism. In order
to cut the rise time of the laser output and to keep
relative power fluctuations low a tickle frequency
is used. A tickle frequency is used to excite the
atoms in the gas mixture enough to keep them just
below a population inversion and stimulated emission.
Therefore when the laser receives a signal for output
the rise time is in microseconds.
|
| Electro-optics-
In solid state lasers an optical excitation mechanism
is used. With most solid-state lasers some form of electro
or acousto-optic assembly is used to attenuate the beam
until the laser is ready to be fired. This means the
laser is always "on". The general concept
of both types of these attenuators is when a signal
is applied to them the beam passes through with little
or no refraction. When a signal is removed the crystal,
usually a liquid crystal sandwiched between optical
materials, the refraction index inside the crystal changes
and the laser beam cannot pass through the optical assembly.
This is a brief definition and there are many types
of optical attenuators used, but most operate on this
general principle. |
Semiconductor
Lasers (Direct
Diode Lasers, and Fiber Lasers):
A series of semiconductor diodes are optically coupled (diode
arrays, or optical fibers) which in combination produce
laser energy densities required for various industrial material
processes.
Laser
Diodes
 |
Laser
action (with the resultant monochromatic and coherent
light output) can be achieved in a p-n junction formed
by two doped gallium arsenide layers. The two ends of
the structure need to be optically flat and parallel
with one end mirrored and one partially reflective.
The length of the junction must be precisely related
to the wavelength of the light to be emitted. The junction
is forward biased and the recombination process produces
light as in the LED (incoherent). Above a certain current
threshold the photons moving parallel to the junction
can stimulate emission and initiate laser action. |
Diode
lasers are fabricated utilizing a specialized type of semiconductor
junction, and therefore share many of the advantages and
characteristics of other semiconductors and solid-state
devices. Although these lasers rely on electronic processes
that take place in a solid semiconductor medium, the basic
principles of laser action in diode lasers are no different
from those controlling the operation of other (non-semiconductor)
laser systems. In all lasers, it is necessary for energy
transitions to occur among electrons in the lasing medium,
and some of these must involve the emission of photons (categorized
as optical transitions). In order for these transitions
to result in emission of amplified light, the process of
stimulated emission must predominate over either spontaneous
emission or absorption. This situation is achieved under
the conditions of a population inversion in the active medium,
a process whereby the electron population of an upper energy
level is induced to grow larger than that of a lower level.
Most diode lasers are based on crystal wafers of group III-V
compounds from the periodic chart of elements. Those fabricated
from gallium arsenide and its derivatives typically lase
at wavelengths between 660 and 900 nanometers, and those
utilizing indium phosphide-based compounds produce wavelengths
between 1300 and 1550 nanometers.
As
the technology surrounding the diode laser has evolved,
dramatic improvements have been made in the efficiency,
spectral characteristics, and functional lifetime of the
devices. A primary objective in design of these lasers is
preventing internal loss of radiation due to excessive beam
spread from the small junction, where gain occurs. Through
various techniques for confining the beam, not only is the
efficiency and output power of the laser maximized, but
also certain other characteristics of the beam are affected
in a desirable manner.
Examples of High Power, High Brightness Direct Diode Laser
Arrays:
FIBER
LASER TECHNOLOGY – Diode Pumped
Single-mode, rare-earth-doped fiber lasers sources provide
high electrical-to-optical efficiency (up to 39% for Yb-doped
fiber amplifiers), small-signal gains as high as 105, and
low-threshold operation. The devices can achieve diffraction-limited
beam quality (M2 = 1) that is defined by the refractive-index
profile of the fiber and is thus insensitive to thermal
or mechanical fluctuations or optical power level. The glass
host broadens the optical transitions in the rare-earth
ion dopants, yielding continuous tunability; moreover, the
variety of possible rare-earth dopants such as Yb, Er, and
Tm yields broad wavelength coverage in the near-IR spectral
region. Fiber lasers can be diode pumped and further offer
low heat dissipation and facile heat removal (high surface-area-to-volume
ratio) and room-temperature operation. They also require
no consumables other than electrical power.
Recent advances have enabled dramatic power scaling of continuous-wave
(CW) and pulsed fiber sources, bringing the benefits of
this technology to a wide range of applications previously
dominated by other laser systems: materials processing,
lidar, and nonlinear frequency conversion, for example.
These developments have led to a surge of interest in fiber-based
laser systems for both industrial and military use.
High-Power
High-power fiber sources incorporate double-clad fiber (see
figure 1), in which the rare-earth-doped core is surrounded
by a much larger and higher-NA inner cladding. Light from
high-power multimode pump diode arrays can be launched efficiently
into the inner cladding, but the pump light is absorbed
only in the core, retaining the benefits of a single mode
gain region.
|
figure
1 Double-clad fiber consists of a rare-earth-doped
core surrounded by a much larger and higher-NA inner
cladding. |
|
