Absorbtion of Energy:The electrons in an atom are circling the
nucleus in many different orbits.These orbits have different energy
levels of the atom.If we apply some heat to an atom, we might
expect that some of the electrons in the lower-energy orbitals
would transition to higher-energy orbitals farther away from the
nucleus.
Absorption of energy: An atom absorbs energy in the form of
heat, light, or electricity. Electrons may move from a lower-energy
orbit to a higher-energy orbit.
Stimulated emission:Definition: a quantum effect, where photon
emission is triggered by other photons The stimulating agent is a
photon whose energy (E3-E2) is exactly equal to the energy
difference between the present energy state of the atom, E3 and
some lower energy state, E2. This photon stimulates the atom to
make a downward
transition and emit, in phase, a photon identical to the
stimulating photon. The emitted photon has the same energy, same
wavelength, and same direction of travel as the stimulating photon;
and the two are exactly in phase. Thus, stimulated emission
produces light that is monochromatic, directional, and coherent.
This light appears as the output beam of the laser.
Spontaneous emission:Definition: quantum effect, causing the
spontaneous decay of excited states of atoms or ions An atom in an
excited state is unstable and will release spontaneously its excess
energy and return to the ground state. This energy release may
occur in a single transition or in a series of transitions that
involve intermediate energy levels. For example, an atom in state
E3 of Figure 8 could reach the ground state by means of a single
transition from E3 to El, or by two transitions, first from E3 to
E2 and then from E2 to E1. In any downward atomic transition, an
amount of energy equal to the difference in energy content of the
two levels must be released by the atom. In ordinary light sources,
individual atoms release photons at random. Neither the direction
nor the phase of the resulting photons is controlled in any way,
and many wavelengths usually are present. This process is referred
to as "spontaneous emission" because the atoms emit light
spontaneously, quite independent of any external influence. The
light produced is neither monochromatic, directional, nor
coherent.
LASER:A laser is a device that controls the way that energized
atoms release photons. "Laser" is an acronym for light
amplification by stimulated emission of radiation, which describes
very succinctly how a laser works.
Einstine Equation:hf=E2-E1 h is Planck's constant, f is the
freq. of the incident photon, E2-E1 is difference of energy between
orbits
Three-Level Laser:Here's what happens in a real-life,
three-level laser.
TYPES OF LASERS:(45)
Lasers may be classified according to the type of active medium,
excitation mechanism, or duration of laser output. We dicuss there
only HeNe gas LASER.
GAS LASERS: Helium- neon (HeNe) LASER:Laser
(most frequently use) with its familiar red beam (Fig.6). The
laser medium is a mixture of helium and neon gases. An electrical
discharge, in the form of direct current or radio
frequency current, is used to excite the medium to a higher
energy level. The pumping action takes place in a complex and
indirect manner. First the helium atoms are excited by the
discharge to two of the excited energy levels (Fig.7). These two
levels happen to be very close to the 3s and 2s levels of the neon
atoms. When the excited helium atoms collide with the neon atoms,
energy is exchanged, pumping the neon atoms to the respective
levels. The atoms at the neon 3s level eventually drops down to the
2p level, as a result of stimulated emission, and light of
wavelength 632.8 nm is emitted. The atoms at the 2s level, on the
other hand, drops to the 2p level by emitting light at 1.15 nm.
However , the atoms at the 3s level may instead drop down to the 3p
level, by emitting light at 3.39 mm. 632.8nm is in the visible
range.
Figure 6 : He-Ne Gas Laser
Semiconductor LASER: Laser Diodes:Light emitters are a key
element in any fiber optic system. This componentconverts the
electrical signal into a corresponding light signal that can be
injected into the fiber. The light emitter is an important element
because it is often the most costly element in the system, and its
characteristics often strongly influence the final performance
limits of a given link.Figure 1 - Laser Diodes Convert an
Electrical Signal to Light
Laser Diodes are complex semiconductors that convert an
electrical current into light. The conversion process is fairly
efficient in that it generates little heat compared to incandescent
lights. Five inherent properties make lasers attractive for use in
fiber optics. 1. They are small. 2. They possess high radiance
(i.e., They emit lots of light in a small area). 3. The emitting
area is small, comparable to the dimensions of optical fibers. 4.
They have a very long life, offering high reliability. 5. They can
be modulated (turned off and on) at high speeds.
Table 1 offers a quick comparison of some of the characteristics
for lasersand LEDs. These characteristics are discussed in greater
detail throughout this article and in the article on light-emitting
diodesTable 1 - Comparison of LEDs and Lasers Characteristic LEDs
Lasers Linearly proportional to drive Proportional to current above
Output Power current the threshold Drive Current: 50 to 100 mA
Threshold Current: 5 to 40 Current Peak mA Coupled Power Moderate
High Speed Slower Faster Output Pattern Higher Lower Bandwidth
Moderate High Wavelengths Available 0.66 to 1.65 m 0.78 to 1.65 m
Narrower (0.00001 nm to 10 Spectral Width Wider (40-190 nm FWHM) nm
FWHM) Fiber Type Multimode Only SM, MM Ease of Use Easier Harder
Lifetime Longer Long Cost Low ($5-$300) High ($100-$10,000)
Laser diodes are typically constructed of GaAlAs (gallium
aluminumarsenide) for short-wavelength devices. Long-wavelength
devices generally incorporate InGaAsP (indium gallium arsenide
phosphide).
Structure And Operation:
Laser Diode Performance CharacteristicsSeveral key
characteristics lasers determine their usefulness in a given
application. These are:
Peak Wavelength: This is the wavelength at which the source
emits the mostpower. It should be matched to the wavelengths that
are transmitted with the least attenuation through optical fiber.
The most common peak wavelengths are 1310, 1550, and 1625 nm.
Spectral Width: Ideally, all the light emitted from a laser
would be at the peakwavelength, but in practice the light is
emitted in a range of wavelengths centered at the peak wavelength.
This range is called the spectral width of the source.
Emission Pattern: The pattern of emitted light affects the
amount of light thatcan be coupled into the optical fiber. The size
of the emitting region should be similar to the diameter of the
fiber core. Figure 2 illustrates the emission pattern of a
laser.
Power: The best results are usually achieved by coupling as much
of a source'spower into the fiber as possible. The key requirement
is that the output power of the source be strong enough to provide
sufficient power to the detector at the receiving end, considering
fiber attenuation, coupling losses and other system constraints. In
general, lasers are more powerful than LEDs.
Speed: A source should turn on and off fast enough to meet the
bandwidthlimits of the system. The speed is given according to a
source's rise or fall time, the time required to go from 10% to 90%
of peak power. Lasers have faster rise and fall times than
LEDs.Figure 2 - Laser Emission Pattern
Linearity is another important characteristic to light sources
for some applications.Linearity represents the degree to which the
optical output is directly proportional to the electrical current
input. Most light sources give little or no attention to linearity,
making them usable only for digital applications. Analog
applications require close attention to linearity. Nonlinearity in
lasers causes harmonic distortion in the analog signal that is
transmitted over an analog fiber optic link.
Lasers are temperature sensitive; the lasing threshold will
change with thetemperature. Figure 3 shows the typical behavior of
a laser diode. As operating temperature changes, several effects
can occur. First, the threshold current changes. The threshold
current is always lower at lower temperatures and vice versa. The
second change that can be important is the slope efficiency. The
slope efficiency is the number of milliwatts or microwatts of light
output per milliampere of increased drive current above threshold.
Most lasers show a drop in slope efficiency as temperature
increases. Thus, lasers require a method of stabilizing the
threshold to achieve maximum performance. Often, a photodiode is
used to monitor the light output on the rear facet of the laser.
The current from the photodiode changes with variations in light
output and provides feedback to adjust the laser drive current.
Figure 4a shows the behavior of an LED, and Figure 4b shows the
behavior of a laser diode. The plots showthe relative amount of
light output versus electrical drive current. The LED outputs light
that is approximately linear with the drive current. Nearly all
LED's exhibit a "droop" in the curve as shown in Figure 4b. This
nonlinearity in the LED limits its usefulness in analog
applications. The droop can be caused by a number of factors in the
LED semiconductor physics but is often largely due to self-heating
of the LED chip.
OUTPUT COUPLER:The output coupler allows a portion of the laser
light contained between the two mirrors to leave the laser in the
form of a beam. One of the mirrors of the feedback mechanism allows
some light to be transmitted through it at the laser wavelength.
The fraction of the coherent light allowed to escape varies greatly
from one laser to another--from less than one percent for some
helium-neon lasers to more than 80 percent for many solid-state
lasers.
Laser-to-Fiber Coupling:The Laser-to-Fiber Coupling System
consists of the FiberBench Base, a LaserPort and a FiberPort, with
one output FiberCable with cleaved distal end . The Laser-to-Fiber
Coupling System is "empty" (containing no Optical Component
Modules), and is used for directly coupling a
diode laser output into a fiber. LaserPorts are available for
the following laser types: 5.6 mm, 9.0 mm or TO3.
Fiber amplifiersDefinition: Optical Amplifiers with doped fibers
as gain media Fiber amplifiers are optical amplifiers based on
optical fibers as gain media. In most cases, the gain medium is a
fiber doped with rare-earth ions such as erbium ( EDFA =
erbium-doped fiber amplifier), neodymium, ytterbium ( YDFA),
praseodymium, or thulium. This active dopant is pumped (fed with
energy) with light from a laser, such as e.g. a fiber-coupled laser
diode; in almost all cases, the pump light propagates through the
fiber core together with the signal to be amplified. A special
breed of fiber amplifiers are Raman amplifiers .
Gain and Output PowerDue to the possible small mode area and
long length of an optical fiber, a high gain of tens of decibels
can be achieved with a moderate pump power, i.e., the gain
efficiency can be very high. The high surface-to-volume ratio and
the robust single-mode guidance also allow for very high output
powers with diffraction-limited beam quality, particularly when
double-clad fibers are used. However, high power fiber amplifiers
usually have a moderate gain in the final stage, partly due to
power efficiency issues; one then uses amplifier chains where the
preamplifier provides most of the gain and a final stage the high
output power.
Fig.: Schematic setup of a simple erbium-doped fiber amplifier.
Two laser diodes (LDs) provide the pump power for the erbium-doped
fiber. Two pig-tailed optical isolators strongly reduce the
sensitivity of the device to back reflections.
Raman Amplification:
Raman amplification is based on stimulated Raman scattering
(SRS), a nonlinear effect in fiber-optical transmission that
results in signal amplification if optical pump waves with the
correct wavelength and power are launched into the fiber.
Erbium Fiber Amplifiers:Fiber amplifiers based on erbium-doped
single-mode fibers (acronym: EDFAs) are widely used in long-range
optical fiber communications systems for compensating the loss of
long fiber spans. The best gain efficiency (order of 10 dB/mW) and
lowest noise figure is achieved for pumping at 980 nm, while
pumping at 1450 nm can lead to a higher power efficiency. The
maximum gain typically occurs in the wavelength region around
1530-1560 nm, but this depends on parameters like fiber length,
erbium concentration, and on pump and signal intensity; such
parameters are used to optimize EDFAs for a particular wavelength
region, such as e.g. the telecom C or L band. A good flatness of
the gain in a wide wavelength region ( gain equalization) can be
obtained by using optimized glass hosts (e.g. tellurides, or some
combination of amplifier sections with different glasses) or by
combination with appropriate optical filters. A high gain in a
shorter length can be achieved with ytterbium-sensitized fibers. In
addition to the erbium dopant, these contain some amount of
ytterbium (typically much more ytterbium than erbium). Ytterbium
ions may then be excited e.g. with 1064-nm or 980-nm pump light and
transfer their energy to erbium ions. For a proper choice of the
material composition of the fiber core, this energy transfer can be
rather efficient. One can also use double-clad fibers of this type
for very high output powers.
Hybrid (EDF and Raman) amplification:Hybrid (EDF and Raman)
amplification has been used successfully in recent designs to
obtain the necessary optical signal-to-noise ratio (OSNR) for
highcapacity dense wavelength division multiplexing systems (DWDM)
or to achieve very large amplifier spacing in, for example, festoon
applications. Figure 5 shows a possible design of a hybrid
EDF/Raman amplifier. The doped fiber is pumped remotely via the
transmission fiber where Raman amplification occurs.
Figure 5. Hybrid EDF/Raman Amplifier
The transversal power distribution of the signal over an
amplified fiber span is strongly dependent on the applied
amplification scheme and can be controlled by the Raman pump power
and pump direction. Figure 6 shows the transversal span power
profile employing different hybrid EDF/Raman amplification
schemes.Figure 6. Span Power Profile for EDFABased Systems (1),
System Using Hybrid Schemes with Backward Raman Amplification Only
(2), and Bidirectional Raman Amplification (3)
By properly selecting pump laser wavelengths, transmission fiber
lengths, and types, many optimization targets can be
reachedflattening of the EDFA gain through an optimized design of
the frequency-dependent Raman gain, for example. Optimization can
be achieved using numerical simulation.
Erbium-Doped Fiber versus Raman Amplification:Table 1.
Comparison of Raman and Doped-Fiber Amplifier Characteristics