Rare Earth Doped Optical Fiber Amplifier Biology Essay


Abstract-The erbium-doped fiber amplifier is emerging as a major enabler in the development of worldwide fiber-optic networks. The purpose of this paper is to present an introduction to the history of the rare earth doped optical fiber amplifier, as well as the context within which fiber amplifiers are having a very significant commercial impact. The emergence of the fiber amplifier foreshadows the invention and development of further guided wave devices that should play a major role in the continuing increase in transmission capacity and functionality of the fibers


RARE-EARTH-DOPED fiber lasers and fiber amplifiers have received great attention in recent years due to their efficiency, compactness, and, particularly, their potential to various applications including communication systems, biomedical equipment, materials processing, and fiber sensors. Although there are many different configurations, fiber lasers The field of rare-earth-doped fibre[1,2,3] lasers has expanded rapidly in recent years and there are now several groups working actively in the area. A variety of glass hosts, dopants and pump sources have been used, each with their own particular advantages and disadvantages. The goal is to achieve low-threshold, diode-laser-pumped operation of fibre lasers and amplifiers, particularly those operating in the second and third telecommunication windows. Fiber lasers and fiber amplifiers are nearly always based on glass fibers which are doped with laser-active rare earth ions (normally only in the fiber core). These ions absorb pump light, typically at a shorter wavelength than the laser or amplifier wavelength (except in upconversion lasers), which excites them into some metastable levels. This allows for light amplification via stimulated emission. Such fibers are often called active fibers. They are gain media with a particularly high gain efficiency, resulting mainly from the strong optical confinement in the fiber's waveguide structure. Rare earth doped glasses, as laser materials for amplifiers at 1500 nm, have received much attention[4], as they offer significant application possibilities in the area of optical communications. Silica fibers and germanium-doped silica fibers have been deployed in optical communications systems as Raman gain media where the transmission fiber itself becomes the amplifying medium. The Raman amplifier can operate over the entire telecommunications window from 1250 to 1650 nm whereas, the Erbium amplifiers (EDFAs) are limited to a 100 nm band centered around a wavelength of 1550 nm.

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Rare Earth Doped Fiber Amplifier Characteristics:

Rare earth doped fiber amplifiers are finding increasing importance in optical communications systems. Perhaps the most important version is erbium doped fiber amplifiers (EDFAs) due to their ability to amplify signals at the low loss 1.55 µm wavelength range.

Characteristics of EDFAs (advantages):

High power transfer efficiency from pump to signal power (> 50%).

Wide spectral band amplification with relative flat gain (>20 dB) - useful for WDM applications.

Saturation output > 1 mW (10 to 25 dBm).

Gain-time constant long (>100 msec) to overcome patterning effects and inter-modulation distortions ( low noise).

Large dynamic range.

Low noise figure.

Polarization independent.

Suitable for long-haul applications.

Disadvantages of EDFAs:

Relatively large devices (km lengths of fiber) - not easily integrated with other devices.

ASE - amplified spontaneous emission. There is always some output even with no signal input due to some excitation of ions in the fiber - spontaneous noise.

Cross-talk effects.

Gain saturation effects.

Wide spectral band amplification with relative flat gain (>20 dB) - useful for WDM applications.

Saturation output > 1 mW (10 to 25 dBm).

Gain-time constant long (>100 msec) to overcome patterning effects and inter-modulation distortions ( low noise).

Large dynamic range.

Low noise figure.

Polarization independent.

Suitable for long-haul applications.

Disadvantages of EDFAs:

Relatively large devices (km lengths of fiber) - not easily integrated with other devices.

ASE - amplified spontaneous emission. There is always some output even with no signal input due to some excitation of ions in the fiber - spontaneous noise.

Cross-talk effects.

Gain saturation effects.

Common Types of Rare-earth-doped Fibers

Table 1 shows the most common laser-active ions and host glasses and also typical emission wavelength ranges of rare earth-doped fibers.


Common host glasses

Important emission wavelengths

neodymium (Nd3+)

silicate and phosphate glasses

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1.03-1.1 μm, 0.9-0.95 μm, 1.32-1.35 μm

ytterbium (Yb3+)

silicate glass

1.0-1.1 μm

erbium (Er3+)

silicate and phosphate glasses, fluoride glasses

1.5-1.6 μm, 2.7 μm, 0.55 μm

thulium (Tm3+)

silicate and germanate glasses, fluoride glasses

1.7-2.1 μm, 1.45-1.53 μm, 0.48 μm, 0.8 μm

praseodymium (Pr3+)

silicate and fluoride glasses

1.3 μm, 0.635 μm, 0.6 μm, 0.52 μm, 0.49 μm

holmium (Ho3+)

silicate glasses, fluorozirconate glasses

2.1 μm, 2.9 μm

Table 1: Common laser-active ions and host glasses and important emission wavelength.


Recent years have witnessed an explosive and exponential growth in worldwide fiber networks. As of the end of 1997, the embedded fiber base was 69 million km in North America, 35 million km in Europe, 59 million km in Asia-Pacific, and 8 million km elsewhere, for a total of 171 million km, according to KMI Corporation, Newport, Rl. In 1997 alone, 38 million km of fiber were added worldwide. Additionally, by 1997, over 366,000 cable-km of fiber-optic undersea cable had been installed, up from 321,000 cable-km as of year-end 1996.[5] Currently fiber networks are used predominantly in long distance telephone networks, high-density metropolitan areas, and in cable television trunk lines. The next decade should witness a large increase in fiber networks for access applications, if the economics warrant it. Given the current high price of erbium-doped fiber amplifiers (US $10,000 and up at the time of this writing), they are used primarily in high-capacity backbone routes and are not yet slated for high-volume applications in the local loop. Optical amplifiers play an exceptionally important role in long haul networks. Prior to the advent of optical amplifiers, the standard way of coping with the attenuation of light signals along a fiber span was to periodically space electronic regenerators along the line. Such regenerators consist of a photo detector to detect the weak incoming light, electronic amplifiers, liming circuitry to maintain the timing of the signals, and a laser along with its driver to launch the signal along the next span. Such regenerators are limited by the speed of their electronic components. Thus, even though fiber systems have inherently large transmission capacity and bandwidth, due to their optical nature, they are limited by electronic regenerators in the event such regenerators are employed. Optical fiber amplifiers, on the other hand, are purely optical in nature and require no high-speed circuitry. The signal is not detected then regenerated; rather, it is very simply

optically amplified in strength by several orders of magnitude as it traverses the amplifier, without being limited by any electronic bandwidth. The shift from regenerators to amplifiers thus permits a dramatic increase in capacity of the transmission system. In addition, well-engineered amplified links can be upgraded in terms of bit rate from the terminal end alone, reusing the undersea cable and amplifiers. Since the introduction of optical amplifiers, rapid progress has been made in increasing the capacity of systems using such amplifiers.The first implementation of rare-earth-doped fiber amplifiers has been in long haul systems (Table 2), such as the TAT-12,13 fiber cable that AT&T and its European partners installed across the Atlantic in 1996. This cable, the first transoceanic cable to use fiber amplifiers, provides a near tenfold increase in voice and data transmission capacity over the previous transatlantic cable.






Bit Rate


of Basic



in Voice





1.1 MHz



Copper coax;


vacuum tubes



1.1 MHz






6 MHz



Ge transistors



30 MHz



Si transistors



30 MHz



Si transistors



280 Mb/s




optical fiber;



560 Mb/s




optical fiber;


TAT- 10

560 Mb/s





TAT- 11

TAT- 12,

560 Mb/s











Table 2. cable systems and capacity in simultaneous calls


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Concept of traveling wave optical amplifier was first introduced in 1962 by Geusic and Scovil.[6] After that shortly, optical fiber amplifiers were invented in 1964 by E. Snitzer,. He demonstrated a neodymium doped fiber amplifier at 1.06 fj,m. The fiber had a core of 10 /zm with a 0.75 to 1.5 mm cladding, a typical length of 1 m, and was wrapped around a flashlamp that excited the neodymium ions.[7] The fiber ends were polished at an angle to prevent laser oscillation, a technique that was used again by workers in the field more than twenty years later. This work lay dormant for many years thereafter. It emerged as an exceedingly relevant technological innovation after the advent of silica glass fibers for telecommunications. Snitzer also demonstrated the first erbium-doped glass laser. [8]. Interestingly, rare earth doped lasers in a small diameter crystal fiber had cores as small as 15 µm in diameter, with typical values in the 25 µm to 70 µm range. The cores were doped with neodymium, with a surrounding fused silica cladding. Lasing of this device was achieved for a laser wavelength of 1.06 µm. A laser was typically fabricated by polishing the end faces of the laser and coating them with dielectric coatings. The fiber was then aligned to a pump laser. In the case of a fiber with a core diameter of 35 µm, the laser pump threshold was as low as 0.6 mW of launched pump power at 890 nm. Lasing was even demonstrated with an LED pump.[9]


As the starting materials commercial oxides (>99% pure) were used. Three different glasses with one concentration of erbium oxide (1,75wt%) were obtained, designated as EF, KE and PE, respectively. They are presented in Table 3.


Glass composition

Weight of

mixed batches


Temperature of removing the OH- [oC]


temperature [oC]


temperature [oC]


SiO2 - PbO - B2O3 - Na2O -K2O- Er2O3


400 for 15 minutes




SiO2 - PbO - B2O3 - Na2O

- K2O - Al2O3 - Er2O3


400 for 15 minutes




P2O5 - Al2O3 - BaO - ZnO

- Na2O - MgO - Er2O3


400 for 15 minutes



Table 3. Compositions and characteristic temperatures of obtained glasses.

Additional aluminum oxide (Al2O3) in KE glass increases homogeneity of this glass matrix and prevents from clustering. Melted glasses were poured on a pre-heated brass mold. The samples were annealed for 12 hours in furnace, which temperature decreased gradually from the temperature of glass transition to room temperature.

Fabrication of rare earth doped fiber

Fabrication of suitable rare earth-doped fiber is one of the keys to creating an appropriate amplifier for a particular application. Fortunately, many of the methods used in fabricating low-loss silica transmission fiber can be used in this context. In most cases the concentration of rare earth is low enough that the fabrication methods do not entail a significant change in the fundamental structure of the underlying glass host. Rare earth doped fibers can be fabricated by a wide variety of methods, each suited for different amplifier design needs. The concentration of rare earth dopant ranges from very high (thousands of ppm) in multicomponent glasses, to less than 1 ppm in distributed erbium-doped fibers. The methods used to fabricate rare earth doped optical fiber are, in general, variations on the techniques used to produce low-loss communications grade fiber.[10] There is a strong incentive to maintain compatibility between standard low-attenuation silica-based fiber and rare earth doped fiber. The difficulty in delivering rare earth dopants to the reaction zones in conventional fiber preform fabrication methods is a fundamental result of the chemistry of the rare earth compounds. These halide compounds of rare earth ions are generally less volatile than the commonly used chlorides and fluorides of the index modifying ions (Ge, P, Al, and F). The rare earth halide materials therefore require volatilizing and delivery temperatures of a few hundred °C.[11] This requirement has stimulated the vapor and liquid phase handling methods to be discussed below.

Rare Earth Vapor Phase Delivery Methods

Methods to deliver rare earth vapor species to the reaction/deposition zone of a perform process have been devised for MCVD, VAD, and OVD techniques. The fabrication configurations employed for MCVD are shown in Figure 1. Rare earth dopants are delivered to an oxidation reaction region along with other index controlling dopants.

The low vapor pressure rare earth reactant is accommodated either by placing the vapor source close to the reaction zone and immediately diluting it with other reactants. The heated frit source (Figure 1, A) is made by soaking a region of porous soot, previously deposited on the upstream inner wall of an MCVD tube, with a rare earth chloride-ethanol solution. [12] Having been heated to 900°C and allowed to dry, the sponge becomes a vapor source. Two other source methods (Figures 1, B and C) use the heated chloride directly as a source after dehydration.[13] The dehydration is necessary in that most rare earth chlorides are in fact hydrated. The dehydration process may be accomplished by heating the material to near 900° C with a flow of Cl2, SOC12, or SF6- The attraction of the heated source injector method is that the rare earth reactant source is isolated from potentially unwanted reactions with the SiCU, GeCU, or POCla index-raising reactants. A variation of the heated chloride source method requires a two-step process referred to as transport-and-oxidation.[14] Using this material, the rare earth chloride is first transported to the downstream inner wall by evaporation and condensation, followed by a separate oxidation step at higher temperatures. The resulting single-mode fiber structure of a PaOs-SiC cladding and a YbaOa-SiOa core is one of the few reported uses of a rare earth dopant as an index-raising constituent. A 1 mole % Yb2Os~ SiC>2 core provided the 0.29 % increase in refractive index over the near silica index cladding. The aerosol delivery method (Figure 1, D) overcomes the need for heated source compounds by generating a vapor at the reaction site. [15] A feature of this method is the ability to create an aerosol at a remote location and pipe the resulting suspension of liquid droplets of rare earth dopant into the reaction region of the MC VD substrate tube with a carrier gas. The aerosols delivered this way were generated by a 1.5 MHz ultrasonic nebulizer commonly used in room humidifiers. Both aqueous and organic liquids have been delivered by this technique, allowing the incorporation of lead, sodium, and gallium as well as several rare earths. Given that most of the aerosol fluid materials contain hydrogen, dehydration after deposition is required for low OH content. Vapor transport of rare earth dopants may also be achieved by using organic compounds that have higher vapor pressures than the chlorides, bromides, or iodides. These materials can be delivered to the reaction in tubing heated to 200°C, rather than the several hundred °C requirements for chlorides. The application of this source to MC VD has been reported using three concentric input delivery lines (Figure 1, E). Multiple rare earth doping and high dopant levels are reported with this method, along with background losses of 10 dB/km and moderate OH levels of near 20 ppm. Rare earth vapor, aerosol, and solution transport may also be used to dope performs fabricated by the OVD or VAD hydrolysis processes. Such doping may be achieved either during the soot deposition or after the soot boule has been created.

Figure 1: Low vapor pressure dopant delivery methods for MCVD.


Erbium-doped fiber amplifiers are typically constructed by connecting fibers (erbiumdoped as well as transmission fiber) with other components necessary for the amplifier's operation. These other components are either passive (e.g., isolators) or active (e.g., pump lasers). Such bulk components usually have fiber pigtails to make it easier to integrate them in a fiber-based system by fusion splicing the fibers together. A typical two-stage amplifier is shown in Figure 2.

Fig 2 : Typical two-stage erbium-doped fiber amplifier. The various components needed are pump lasers, isolators, wavelength division multiplexers (WDM), filters, connectors, and various types of transmission fiber.

Many components of different type are clearly needed to obtain an amplifier with the desired performance characteristics. Fabrication of an intrinsically gain flattened Erbium-doped fiber amplifier (EDFA) based on a highly asymmetrical and concentric dual-core fiber, inner core of which was only partially doped. Phase-resonant optical coupling between the two cores was so tailored through optimization of its refractive index profile parameters that the longer wavelengths within the C-band experience relatively higher amplification compared to the shorter wavelengths thereby reducing the difference in the well-known tilt in the gains between the shorter and longer wavelength regions. The fabricated EDFA exhibited a median gain ≥ 28 dB (gain excursion below ± 2.2 dB within the C-band) when 16 simultaneous standard signal channels were launched by keeping the I/P level for each at -20 dBm/channel. Such EDFAs should be attractive for deployment in metro networks, where economics is a premium, because it would cut down the cost on gain flattening filter head. Erbium doped fiber amplifiers (EDFAs) exhibit large gain bandwidth and a single EDFA can amplify large amount of data without any gain narrowing effects. So, a single EDFA can be used to amplify several channels simultaneously in a dense wavelength division multiplexing (DWDM) system. However, the non-uniform gain spectrum in conjunction with the saturation effects of EDFAs cause increase in signal power levels and decrease in the optical signal-to-noise ratio (OSNR) to unacceptable values in systems consisting of cascaded chains of EDFAs . These features could limit the usable bandwidth of EDFAs and hence the amount of data transmission by the system. Accordingly various schemes of gain equalizing filters (GEFs) such as Mach-Zehnder filter [16], acousto-optic filter, long-period fiber-grating , fiber-loop mirror [17] , side-polished fiber based filter and so on have evolved in the literature. However, as is well known, one of the major drivers in a metro network design is low installation cost in addition to achieving low maintenance/ repair costs. Naturally one of the routes to achieve these objectives would be to use fewer components in the network. Use of an intrinsically gain flattened EDFA would cut down the cost on the GEF head. This motivated me to investigate design of a gain flattened EDFA by exploiting a wavelength filtering mechanism inherent in a co-axial dual-core fiber design scheme. There have been earlier reports in the literature of few schemes to achieve inherent gain flattening in an EDFA through a twin core EDF and also a co-axial dual core fiber design .However, these reported schemes are relatively complex; for example, the twin core fiber requires fabrication of two separate preforms followed by polishing and complex procedure to assemble them as a composite unit in a fiber draw tower, while the coaxial design requires i) an additional component in the form of a mode converter and ii) doping outer core with erbium, which is more demanding for the well known MCVD process of fiber fabrication.

Theoretical Analysis:

A schematic diagram of the RIP of the proposed fiber design is shown in Fig.3 . It consists of two highly asymmetric cores, an inner core with small index contrast and a much thinner outer core with a large index contrast while a matched index cladding connects the two cores. The parameter rd represents doping radius of the inner core, which is the only core doped with Erbium. The fiber parameters a, b, c, n1 and n2 were optimized such that the fundamental modes corresponding to the isolated cores were phase-matched at a wavelength near about 1533 nm, which we refer to as the phase matching wavelength (λP) for resonant coupling between the fundamental modes of the two co-axial cores. Thus as the wavelength changes from below to above λP, the mode field profile of the composite structure would undergo a significant change. For signals centered at wavelengths much shorter than λP, a large fraction of the signal power resides in the outer core. Fractional power in the inner core increases with increase in wavelength and finally for wavelengths longer than λP, the fractional power in the inner core becomes more than that in the outer core. Since only the inner core would be doped with Erbium ions, signals at those wavelengths longer than λP would significantly overlap with the erbium doped region, and hence experience relatively larger gain compared to wavelengths shorter than λP. As a result, the tilt in the gain spectrum between signals at the shorter and the longer wavelengths within the C-band would reduce leading to an effective flattening of the gain spectrum of the EDFA.

Fig 3. Schematic of the refractive index profile (RIP) of the proposed fiber.

Figure 4 shows the wavelength variation of mode effective index (neff) for the core (isolated inner core) and the ring modes (isolated outer core), as well as those of the LP01 and LP02 modes of the composite coaxial fiber. It can be seen from the figure that neff of the LP01 mode is close to that of the ring mode at wavelengths shorter than λP, while for wavelengths longer than λP it is closer to that of the core mode. Therefore, by optimizing the parameters for optimum λP and doping erbium in the inner core, we can achieve an increased overlap of the modal field with the doped region for the longer signal wavelengths, thus enabling higher gain in that wavelength region. Spectral dependence of the fractional powers in the two cores is shown in Fig. 5.

Fig 4 Variation of mode effective indices of core mode, ring mode, and the LP01 and LP02 modes of the fiber.

Fig 5 Wavelength dependence of the fractional power within the two individual cores

In order to obtain the most suitable index profile parameters commensurate to ease in our targeted fabrication of an inherently gain flattened EDFA by the MCVD method, the gain and other important characteristics of a co-axial dual core EDF were modeled through the standard three-level rate equation model [18]. We have assumed forward pumping at 980 nm wavelength via the LP02 mode and that the signal is launched into the LP01 mode only. The model also included the wavelength dependent forward and backward traveling amplified spontaneous emission (ASE). ASE has been determined at 100 wavelength points, spaced 1 nm apart, in the wavelength range 1500-1600 nm and the propagation effects were calculated for each of these sample wavelengths. The coupled nonlinear differential equations, which govern the propagation of ASE, signal and pump powers along the length of the fiber, are given by [18]

The emission and absorption factors, γe and γa are given by

where rd is the doping radius, I (ν, r) is the normalized intensity distribution at frequency ν and aσ and eσ are the wavelength-dependent absorption and emission cross-sections, respectively. Population densities in the ground (N1) and excited (N2) states respectively, are given by

with ρEr representing the concentration of Er3+ ions while Wa and We, respectively, are the total absorption and emission rates given by

where A21 is the spontaneous emission rate Is and Ip are the normalized intensities of the modes at the signal and pump wavelengths, respectively and Ps and Pp are the modal powers at the signal and pump wavelengths at a spatial location z along the fiber; SASE is the total of the forward and backward ASE power spectral densities. Modal intensities Is,p were obtained through the well-known Matrix method [19] and the variation of refractive index with wavelength was calculated using Sellemeir equations [20]. Equations (1 & 2) were solved over the bandwidth of 100 nm (1500-1600 nm) using fourth-order Runge-Kutta method with adaptive step size to obtain gain spectrum of the EDF. Equations (3 & 4) were solved by Simpson's method and considering the overlap integrals and the radial variation of the population density, transition rates and the modal field profiles at 41 points within the doping radius. Since both forward and backward ASEs were considered, the effect of the backward traveling ASE on the population inversion influences the propagation of the forward propagating light and vice versa, hence a large number of iterations were required to obtain a stable solution.


Here,some modeling results are compare with the experiments performed on different fiber structures and also different Er-Yb concentrations. To illustrate that modelling method not only works well for the normal double-cladding fiber but also accurately renders the situation in other complicated structures, consider here two different types of Er-Ybdoped phosphate glass fibers: 1) circular double-cladding fiber, 2.5%wt Er and 15%wt Yb and 2) three-core rectangular fiber amplifier with 3%wt Er and 15%wt Yb. These particular phosphate glass fibers have been developed to support very high erbium and ytterbium concentrations without the deleterious effects of ion clustering-to enable short-length ( cm), high-gain fiber amplifiers. We designed and produced core and cladding phosphate glasses with matching thermal properties such that their refractive indexes result in the designed numerical aperture of the fiber. Single-mode double-cladding phosphate glass fibers, as shown in Fig. 6, were fabricated using a rod-in-tube technique in which doped core (rod) and undoped cladding (tube) phosphate glasses were machined to fit together as a preform. The preform was then used in a fiber-drawing tower to produce hundreds of meters of amplifying fiber. The numerical aperture of the core and the cladding were 0.145

and 0.24 at 1550 nm, respectively. The core diameter was 6.6 m with Er ion concentration of 2.5%wt and co-doped with 15%wt Yb to provide a high absorption coefficient, and diameters of inner and outer clad are 60 and 108 m, respectively.

Fig. 6. Cross-sectional image of double-clad Er-Yb co-doped fiber.

A multicore array structure, as shown in Fig. 7, was produced in a method analogous to the usual optical fiber drawing for a single core. Three cylindrical holes arranged in a linear array were mechanically drilled in a cladding glass preform. Core glass was machined into rods and placed in cylindrical preforms made of the same cladding glass. The three core sections were placed in the three holes, and the entire structure was pulled much like single core optical fibers-resulting in monolithic amplifier arrays with active core elements containing very high erbium (3%wt) and ytterbium (15%wt) doping concentrations, and circular cross sections. The numerical aperture of the cores (0.15) was designed to match the numerical aperture of the single-mode fibers that carry the input signals. The pump cladding NA is 0.54 when the three-core fiber was embedded in an outer clad epoxy that was index matched to fused silica. The cores were arranged on a 30- m pitch with a rectangular shaped outer cladding that is 112 m 68 m. Fig. 5 shows a cross-sectional image of the cleaved surface.

Fig. 7. Cross-sectional image of three-core rectangular Er-Yb co-doped fiber.

Using the calculation process presented earlier, we can model cladding-pumped Er-Yb doped fiber amplifiers, not only in simple double-clad fibers but also in much more complicated structures. In Fig. 8(a), we show the simulation results of the pump propagation in the double-clad fiber, and the pump power in the core is in Fig. 8(b). As shown in Fig. 8, the multimode Using the calculation process presented earlier, we can model cladding-pumped Er-Yb doped fiber amplifiers, not only in simple double-clad fibers but also in much more complicated structures. In Fig. 8(a), we show the simulation results of the pump propagation in the double-clad fiber, and the pump power in the core is in Fig. 8(b). As shown in Fig. 8, the multimode

Fig. 8. (a) Pump propagation in double-clad fiber. (b) Pump power in fiber core versus propagation distance. The input pump power is 1 W with top-hat power distribution. Dashed curves are calculated by the EOF approximation, and filled circles are the experiment data.


The success of erbium-doped fiber amplifiers has been predicated on the commercial availability of reliable diode laser pumps with power sufficient to stimulate gain from the device. The first experiments with diode lasers were reported in 1989, and commercial lasers made a widespread appearance a few years later. There are several types of laser diode based pump lasers which have been demonstrated as pumps for erbium-doped fiber amplifiers:

• 1480 nm diode pump lasers

• 980 nrn diode pump lasers

• 800 nm diode pump lasers

• 670 nm diode pump lasers

• high power solid state lasers pumped by laser diode arrays

• high power fiber lasers pumped by laser diode arrays

• MOPA (Master Oscillator Power Amplifier) lasers

Of these, the most commonly used today are the 1480 nm and 980 nm diode lasers. 1480 nm lasers were the first diode lasers to demonstrate acceptable reliability for use in telecommunications systems. They have been used in the first optically amplified undersea cable systems such as TAT-12,13. 980 nm pump lasers, which provide a lower noise figure than 1480 nm, have taken a longer time to prove adequate reliability for integration into field deployable systems. The high power solid state and fiber lasers are used for high output power booster amplifiers. We briefly describe below some of the characteristics and typical operating parameters of these pump lasers. Diode lasers are often characterized by their L-I curves (light output power vs forward current across the diode). Some general comments can be made about these L-I curves. The typical shapes of L-I curves are shown in Figure 9.a. The light power increases with current until the heating of the semiconductor by the electrical power causes the output power to roll over. This phenomenon is typically reversible. COD (catastrophic optical damage) causes a permanent damage in the laser. It occurs at the facet of the laser where there can be localized heating due to laser light absorption. Excess carriers created at the surface recombine nonradiatively thus heating the volume affected. An avalanche effect can occur leading to increased absorption and finally meltdown at a point in the laser facet. The laser is damaged and the output power suddenly drops to a very low value. For systems with low carrier surface recombination velocity, such as is the case for 1480 nm lasers, this effect is negligible. 980 nm lasers containing Al based layers can, however, suffer significantly from this effect. Kinks, as shown in the right hand sketch of Figure 9.b, are caused by a lateral

mode change with a change in current. Such kinks are usually accompanied by a sudden change in the coupling efficiency to a single mode fiber pigtail. Kinks can be avoided by designing a laser to operate only in the fundamental mode. This can be accomplished by designing an active region in the shape of a stripe approximately 1 to 2 µm wide, or by creating a situation whereby the loss is very high for higher order modes. [21]

Fig 9.a

Fig 9. b