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