Term PaperEE/OPE 454/553 – LASERSFall 2010TitleFiber LaserSubmitted byLin YangToDr. J. D. Williams1. IntroductionIt has been more than 40 years since the first GaAs semiconductor lasers was created in 1962. Nowadays, semiconductor lasers have been widely used in laser communications, CD storage, and laser inspection.With increasing continuous power output of semiconductor laser, the applications of it was extended farther. The main application range of the semiconductor laser is high power diode pumped solid state lasers (DPSSL). This technique integrated the advantage of semiconductor laser and the advantage of solid laser. DPSSL converts high-energy photons with short wavelength to low-energy photons with longer wavelength. Thus, a portion of energy converts into heat without radiation transition. Emission and removal of the energy is becoming a significant technique for diode pumped solid lasers. In literature and application, there are a lot of alternative methods.One method is to configure laser media to have a very slender shape like optical fiber. There are several reasons for this method. Firstly, the guiding of light allows extremely long gain regions providing good cooling conditions. Secondly, fibers have high surface area to volume ratio which allows efficient cooling. In addition, the fiber’s waveguide properties tend to reduce thermal distortion of the beam.The fiber laser in this context is the laser using an optical fiber as its active gain medium. In 1964, the first generation of glass laser in the world is fiber laser. It is toodifficult for common pump source such as gas discharge lamp to focus to the core of optical fiber since the core is so fine-grained. Therefore, there were no big achievements of fiber laser in the following 20 years. With the development of semiconductor laser pump technology and the needs of developing optical fiber communication, the flexibility of the erbium doped fiber amplifier (EDFA) was proved by British Southampton University and American Bell Laboratory in 1987. It amplifies optical signals by erbium-doped single-mode optical. EDFA is a significant component in optical fiber communication. The semiconductor laser must be single-mode for pumping it into the core of single-mode optical fiber (general diameter less than 10um), which makes single-mode EDFA difficult to achieve high power. The highest power output is no more than hundreds milliwatts.Putting the light pump into cladding was proposed in 1988 to enlarge the power output. The original design used a circle-shape inner cladding layer. However, the perfect symmetry of circle reduces the absorption efficiency of pump. Not until early 1990, the rectangle-shape inner cladding layer was invented. The laser conversion efficiency increased to 50 percent and power output reached 5 watts. The power of single-mode continuous laser output reached 110 watts in 1999 by using four 45 watts semiconductor lasers pumping in its both sides. In recent years, single optical fiber can produce thousands watts power output with the development of high power laser diode pump technology and double-clad fiber producing technique.2. Principles of Fiber LasersThe basic concept of fiber laser is using an optical fiber as the active gain medium of the laser, which is usually doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium. They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser. In addition, some lasers with a semiconductor gain medium (a semiconductor optical amplifier) and a fiber resonator have also been called fiber lasers (or semiconductor fiber lasers).2.1 Optical FiberOptical fiber is a glass fiber which is drawn into by SiO2 for matrix materials. It is widely applied in Optical Fiber Communication. The principle of its light transmitting is based on the principle of total internal reflection. A naked fiber generally consists of three parts. The core is silica glass with high refractive index (core diameter is generally 9-62.5μm); the middle is silica glass coating with low refractive index (core diameter is generally 125μm) and the most external is reinforced resin coating. Figure 1 shows this structure of a fiber.Figure 1: Structure of an optical fiberGenerally we have 2 types of optical fiber, single-mode optical fiber and multimode fiber.Single-mode optical fiber: the core is very fine (diameter is only 9±0.5μm) and only can transmit the light of a certain mode. Its dispersion between modes is very small and has the function of customize mode and mode-limit.Multimode fiber: the core is thicker (50±1μm) which can pass through a variety of modes of light, but its dispersion between modes is much larger and light transmitted is not pure.Types of fibers also can be classified by construction or functionalities.By construction, fiber can be classified as step index fiber, graded index fiber, PM fiber (polarization-maintaining fiber), photonic crystal fiber, and multi-core fiber. By functionalities fiber can be classified as passive fiber and active fiber. Figure 2 shows these different types of fibers.(a) (b)(c) (d)Figure 2: Cross-section of different types of fibers(a) Step index and graded index fiber; (b) multi-core fiber; (c) PMF; (d) PCFThe optical fiber used in high-power fiber laser is not common communication optical fiber, but a special optical fiber which is doped with various rare ions, structured more complexly and also resistant to high radiation - double-clad fiber. Figure 3 and 4 show the structures of Double-Clad Fiber with different shapes .Figure 3: Structure of Double-Clad Fiber (D shape)Figure 4: Different shapes of Double-Clad FiberComparing with common fiber, double-clad fiber has another coating outside its fiber core, and for pumping light it is multimode modulation. It can easily collect large amounts of photons in the fiber by absorbing high brightness and multi-mode pumped light, because it has larger diameter and light angle. Practice shows that the double-clad fiber with a D shape or rectangular shape cross-section has a coupling efficiency up to 95%. Thus this kind of double-clad fiber is widely used. As long aswe concerned about pulse optical fiber laser, a major topic is how to improve ability of a fiber of resistance of radiation. Currently around the world the monopoles capacity of fiber lasers can reach 20,000 W, but how can a hair size fiber bear such a high laser radiation? So some special ion must be considered to mix into fiber to prevent the fiber burning out. For instance even in nuclear fallouts circumstances, the cerium ions doped fiber will neither deform by high temperature nor lose its transmittance for dyeing. Table 1 shows most common laser-active rare-earth ions with the common hosts and important emission wavelengths:Table 1: Common used types of Rare-Earth-Doped FibersIon Common host glasses Important emissionwavelengths (μm) Neodymium(Nd3+) Silica, phosphate glasses 1.03-1.1, 0.9-0.95, 1.32-1.35 Ytterbium(Yb3+) Silica 1.0-1.1Erbium(Er3+) Silica, phosphate glasses,fluoride glasses1.5-1.6,2.7, 0.55Thulium(Tm3+) Silica, fluoride glasses 1.7-2.1, 1.45-1.53, 0.48, 0.8 Praseodymium(Pr3+) Silica, fluoride glasses 1.3, 0.635, 0.6, 0.52, 0.492.2 Principle of Fiber Laser2.2.1. Fiber Laser ConfigurationAs same as other common lasers, fiber lasers also consist of working materials, laser resonator and pumping source, as the figure shows below. Commonly fiber lasers are mostly developed on the basis of the fiber amplifier. It is made by using the REE doped fiber and a proper feedback system. The REE doped fiber works as thegain medium. There is a very slim fiber core in the fiber laser, which can form high-power density in the fiber and cause the population reversion under the action of outer light source and then output laser light. The working materials absorb different wave length pumplight and output a certain wave length laser by doped different ions (Er3+, Yb3+, Nd3+) into the fiber. The Yb3+ (or Er, Yb) doped fiber is wildly used in the recent high power laser systems because their obvious advantages such as, wider absorption spectrum, wider gain bandwidth and wider turning range.Figure 5: Simple fiber laser setup2.2.2. Fiber AmplifiersFiber amplifier is simply a doped strand of fiber (typically glass) with the required density of laser ions. As the figure shows below, the signals are amplified through interaction with the doping ions while pump laser are multiplexed into the doped fiber. Usually the pumping source is in kind of semiconductor laser diode.Figure 6: Simple Doped Fiber AmplifierHere only one type of laser amplifiers will be discussed according to the rare earth dopant type ---- Erbium (Er3+) Doped Fiber Amplifier (EDFA), which is the most common example. It can be efficiently pumped with a laser at a wavelength of 980 nm or 1,480 nm, and exhibits gain in the 1,550 nm regions. This laser amplifier is a three level-laser system (in order to have lasing action we need more than 2 energy levels to achieve the population inversion). Energy diagram for three-level system is shown in the figure below.Figure 7: A three-level laser energy diagramFigure 8: Energy diagram of an erbium-doped fiber laser An energy-level diagram for the erbium-doped fiber laser is shown in Figure 8. Optical pumping can be seen to occur from the ground state 4I15/2 to the 4I11/2 state at a wavelength of 0.98 um or directly to the 4I13/2 upper laser state at 1.48um. When the pumping occurs to the 4I11/2 level, rapid relaxation occurs to the upper laser levels. When pumping is direct to the upper laser state, rapid relaxation occurs to the lowest-lying levels of that state from which laser action is produced. The laser output then occurs in the region of 1.52-1.56 um. The lower level is similar to that of a dye laser in that it is part of the ground state within which the population is distributed according to the thermal. Consequently, the higher-lying levels of that state are not significantly populated and hence can serve as the lower level of a population inversion. The population then rapidly decays nonradiatively to the lower levels of that state.The transitions are taken place between the energy levels (4I13/2→4I15/2). It has a gain over a wavelength range centered near 1550 nm.Figure 9: Schematic setup of a simple erbium-doped fiber amplifier Two laser diodes (LDs) provide the pump power for the erbium-doped fiber. The pump light is injected via dichroic fiber couplers. The shape of the erbium gain spectrum depends both on the host glass and on the excitation level. Figure 2 shows data for a common type of glass, which is some variant of silica with additional dopants, for example, to avoid clustering of erbium ions.Figure 10: Gain and absorption (negative gain) of erbium (Er3+) ions in a phosphate glass for excitation levels from 0 to 100% in steps of 20%.3. Advantages and disadvantages of fiber laser3.1 The advantage of fiber lasers can be concluded as following six:∙Compact size : Because fibers can be bent and coiled and the light propagating in fibers is well shielded from the environment, for the same output powerfiber lasers are compact compared to rod or gas lasers with all-fiber setuplaser resonator, such as fiber Bragg gratings and fiber couplers ∙Light is already coupled into a flexible fiber: The fact that the light is already in a fiber allows it to be easily delivered to a movable focusing element. Thisis important for laser cutting, welding, and folding of metals and polymers.∙Fiber gain media allows wide wavelength tuning ranges and generates ultra pulses because fiber laser broadens laser transitions in glasses. In addition, it isnot necessary to stabilize the temperature of the pump diodes with broadspectral regions of fiber lasers due to fiber lasers’ good spectral regions withgood pump absorption∙High optical quality: The thermal distortion of the optical path is reduced or eliminated due to the fiber's waveguiding properties. Then the diffraction-limited, high-quality optical beam can be easily obtained whensingle-mode fibers are used, and sometimes also with slightly multimodefibers.∙High output power: Because fiber lasers can have very long active regions ,it can provide very high optical gain so that kilowatt levels of continuous outputpower can be supported because of the fiber's high surface area to volume ratio, which allows efficient cooling.∙Fiber lasers can be operated even on very “difficult” laser transitions such as upconversion lasers by guidance’s allowing high pump intensities to be applied over long lengths.3.2 Disadvantage:∙Alignment is important if the pump light has to be launched from free space into a single-mode core.∙Complicated temperature-dependent polarization evolution of most fibers not compatible with nonlinear polarization rotation mode locking.∙Nonlinear effects often limit the performance.∙Fiber is risky to be damaged at high powers.∙With limited gain and pump absorption per unit length, fiber is difficult to realize very short resonators.4. Some Variation and application of fiber laserThere are many different types of fiber laser; here we only discussed some of them.4.1 High-power Fiber LasersDue to a high surface-to-volume ratio and the guiding effect, which avoids thermo-optical problems even under conditions of significant heating, a single fiber can produce output powers of hundreds of watts, sometimes even several kilowatts from a single fiber.4.2 Upconversion Fiber lasersWhen operating on relatively “difficult” laser transitions, high pump intensities can be easily maintained over a long length. Thus, the gain efficiency achievable often makes it easy to operate even on low-gain transitions.In most cases, silica glass is not suitable for upconversion fiber lasers since the upconversion scheme requires relatively long lifetime of intermediate electronic levels. The lifetime is always very small in silica fibers due to the relatively large phonon energy of silica glass (→ multi-phonon transitions). Therefore, certain heavy-metal fluoride fibers such as ZBLAN (a fluorozirconate) are mostly used with low phonon energies.Most popular upconversion fiber lasers are based on thulium-doped fibers for blue light generation (Figure 11), praseodymium-doped lasers (possibly with ytterbium codoping) for red, orange, green or blue output, and green erbium-doped lasers.Figure 11: Level scheme of thulium(Tm3+) ions in ZBLAN fiber, showinghow excitation with an 1140-nm lasercan lead to blue fluorescence and laseremission.4.3 Narrow-linewidth Fiber LasersFiber lasers can be constructed to operate on a single longitudinal mode with a very narrow linewidth of a few kilohertz or even below 1 kHz. Laser resonator should be kept relative short to achieve long-term stable single-frequency operation without concerning temperature stability. Distributed-feedback laser (DFB laser) is a good example.4.4 Raman Fiber LasersA special type of fiber lasers is fiber Raman lasers. Its principal relies on Raman gain associated with the fiber nonlinearity. Such lasers use relatively long fibers which have increased nonlinearity, and typical pump powers of the order of 1 W. With several nested pairs of fiber Bragg gratings, the Raman conversion can be done in several steps by bridging hundreds of nanometers between the pump and output wavelength. For example, Raman fiber lasers can be pumped in the 1-μm region and generate 1.4-μm light as required for pumping 1.5-μm erbium-doped fiber amplifiers.4.5 Fiber Lasers with Semiconductor Optical AmplifiersThere are some lasers which have a semiconductor optical amplifier (SOA) as the gain medium in a resonator made of fibers. Even though the actual laser process does not occur in a fiber, they sometimes are still called fiber lasers. They typically emit relatively small optical powers of a few milliwatts or even less. Sometimes they exploit the very different properties of the semiconductor gain medium. Compared with a rare-earth-doped fiber, these lasers have much smaller saturation energy and upper-state lifetime. Rather than only generating coherent light, they can be used for information processing in optical fiber communications systems. For example, they can be used in the wavelength conversion of data channels based on cross-saturation effects.5. ConclusionThis paper reviewed the history of fiber laser, the basic principals of fiber laser, and some variance of fiber laser. This paper stated the different fiber material used as gain media, fiber-setting environment, and the advantage and disadvantage of fiber laser.6. References1. C. J. Koester and E. Snitzer, “Amplification in a fiber laser”, Appl. Opt. 3 (10),1182 (1964)2.Silfvast, W.T., 2004. Laser Fundamental, 2nd ed. Cambridge University Press,New York.3.Optics for Fiber Laser Applications by Emily Kubacki and Lynore M Abbott,CVI Laser, LLC. Technical Reference. 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