Fiber Laser Sudip Nepal Abstract: A fiber laser is a laser in which the gain medium is an optical fiber doped with rare earth elements. On Doping rare earth elements the loss can be reduced and special properties of the laser can be achieved. The main purpose of doping of rare earth element is to amplify light without lasing. Fiber laser have wide range of spectral range and tunability. High power can be obtained from fiber laser. The wave can be guided as required using the fiber crystal. Low cost, easy build up with high applications of fiber laser attracted many researcher. There are variety of fiber laser depending upon the cavity direction, fiber used and doped elements. Introduction: In fiber laser the laser is passed through the fiber already and easy to pass and operate it. They are high power laser and can be built on low cost. Fiber lasers are used in medicine, communication, spectroscopy, chemical compound detection. The loss in fiber laser is minimized by using small cavity length. The fiber laser consists of glass fiber, highly reflective dichroitic mirror, output coupling mirror and semiconductor laser for pumping mechanism. According to the type of doped rare earth elements and doping percentage per weight the wavelength of the fiber laser can vary from 0.7µm to 3 µm. Many fibers can be used on fiber laser. Fiber reduces the pump power necessary for the laser. Poly crystal fibers are advantageous fiber than others. The mode of the guided wave depends upon the structure of poly crystal fiber. Using correct PCF structure the desired mode of the wave can be guided. Furthermore, mode and dispersion of fiber laser can be controlled. In fiber laser pumping is done using low power laser and the output is laser with high power. It is often termed as laser amplification. Pumping Mechanism:For the pumping of the fiber laser other laser sources are used. The pumping should be done according to energy level of doped material. In erbium doped fiber laser pumping is done using the laser of wavelength 800, 980 and 1480nm. For the pumping at 800 nm semiconductor diode laser can be used. Ti- Sapphire laser is one option for the pumping at 980 nm. Other laser that can be used is styryl-9 dye laser. For the pumping at 1480 nm semiconductor lasers can be used.
Gain Medium:For the fiber laser many gain media are used . Solid state gain medium is one gain medium in which rare earth doped crystals, ceramic media and glasses are used. Most of the time doping is done using trivalent elements and rarely divalent elements. Some commonly doped elements with host media and emission of wavelength is stated in a table below.
Ion
Common host media
Important emission wavelengths
neodymium (Nd3+)
YAG, YVO4, YLF,silica
1.03–1.1 μm, 0.9–0.95 μm, 1.32– 1.35 μm
Ytterbium (Yb3+)
YAG,tungustate , silica
1.0–1.1 μm
erbium (Er3+)
YAG, silica
1.5–1.6 μm, 2.7 μm, 0.55 μm
thulium (Tm3+)
YAG, silica, fluoride glasses
1.7–2.1 μm, 1.45–1.53 μm, 0.48 μm, 0.8 μm
holmium (Ho3+)
YAG, YLF, silica
2.1 μm, 2.8–2.9 μm
praseodymium (Pr3+)
silica, fluoride glasses
1.3 μm, 0.635 μm, 0.6 μm, 0.52 μm, 0.49 μm
cerium (Ce3+)
YLF, LiCAF, LiLuF, LiSAF, and similar 0.28–0.33 μm fluorides
Table source: - http://www.rp-photonics.com/rare_earth_doped_gain_media.html.
Atomic level involved:Atomic level involved in fiber laser depends upon the element doped on it. For Erbium doped laser there are three atomic levels involved whereas in Neodymium doped laser there are four levels. The diagram below indicates the atomic transition in three level gain system of laser. In this system the transition from level 3 to level 2 is rapid decay and the decay from
level 2 to level 1 is slow laser decay. The laser transition gives the laser of 1.54 µm for erbium doped three level fiber laser. The unsaturated inversion in three level laser is calculated as,
Fig: - Three level laser. Source: - wikipedia.org The diagram below indicates the four atomic level of a laser system. In this system the transition from level 3 to level 2 is slow laser transition. In this transition of Nd doped fiber laser the output laser gives the wavelength of 1.06 µm. For four level laser the population inversion is given by
Fig: - Four level laser. Source: - Wikipedia.org. For the fiber laser the cavity lifetime is in the order of 10ns and the population inversion lifetime is the order of 100 ns. The transition strengths of Erbium doped laser for laser transition is tabulated below Glass AI/P Silica AI Silica Ge/Al Silica Silicate L22
Lifetime(ms) 10.8 10.2 10.2 14.5
Gain Tellurium based erbium doped fiber laser has two peaks at 1568 and 1602 nm. The signal gains corresponding to it are 36 and 38 dB respectively. The tellurium based laser has more gain in comparison to the other based erbium doped fiber laser (6).
Figure source ref (6). Figure shows the Signal-gain spectra of a tellurite-based EDFA with a 0.9-m-long fiber for various input-signal power levels.
Type of broadening:-
In fiber laser both homogeneous and non-homogeneous line broadening takes place. In erbium doped fiber laser it is operated at room temperature and the broadening is homogeneous. In inhomogeneous broadening the establishment of laser may be difficult as saturation can occur only at specific wavelengths. The cause of homogeneous broadening is lifetime broadening. The broadening depends upon the fiber and the doped materials.
Pump and laser wavelengths (Tunability/spectral range):In fiber laser the pump can be tuned at the center around 800nm, 980 nm and 1480 nm. The pump wavelength depends upon the atomic level and the transition. The atomic level and transition depends upon the doped element. The spectral range of fiber laser depends upon the doped elements. For example the spectral range of Mode-locked ytterbium fiber laser is 980-1070-nm.
Efficiency:Slope efficiency of the laser is the slope obtained by plotting output power versus input power. The laser efficiency is not 100% due to unwanted loses like heat, absorption, scattering transmission etc. The efficiency of erbium doped fiber laser for different wavelength is stated as Pump Wavelength 810 980 1460
Efficiency (%) 92 79 53
Fiber laser can be operated in single as well as multi-mode. When it is operated in single mode then diffraction limited beam quality can be obtained. Pump claddings of double-clad fibers lasers work on multimode. Fiber lasers are continuous mode high power lasers. They have high ratio of surface area to volume such that cooling is allowed and they produce continuous wave. For the fiber ring laser maximum output power obtained is 38 mw at the pump power of 480 mw. The average power is 38 mw for single pulse operation. For fiber laser
maximum peak is obtained as 10kw and the average power for 200 Hz is 134mw. The power of fiber laser can vary from some mw to hundreds of kw. The length of the cavity can be varied on fiber laser. The cavity length of fiber laser depends upon the wavelength. Distributed Bragg reflector cavity are operated in small cavity length. Spectral purity:The narrowness of wavelength separation and line width with less degree of phase noise defines high degree of spectral purity of the laser. The spectral purity of laser depends upon laser ion pump power cavity loss etc. Generally fiber laser are fabricated on which they oscillate around the optical line width of 1 to 10nm . In single mode DBR fiber laser the wavelength separation nm be from 0.25nm to 10nm. Divergence:Laser beam has Gaussian profile. They have small width. The point where width is minimum called the waist of the beam. As the distance travelled by laser beam increases the width of laser beam also increases. The divergence of the laser beam is the measure of how fast is the beam growing with respect to the distance travelled. The divergence angle is given by Θ=λ/πω0 Where λ is wavelength and w0 is the beam waist. The divergence of the laser beam can be some mrad.
Coherence length:- The length on which travelling a beam maintains coherent angle is coherent length. Fiber lasers have high coherence length in comparison with He-Ne laser with around 25cm and semiconductor laser with around 100m. They can even exceed the coherence length of 100km. Applications:Fiber laser are high power continuous wave. As they are passed through the laser they can be directed on movable object. They can be used for different purposes due to some salient feature. Fiber laser can be used for marking , cutting , welding and folding of metals and polymers. They are also used on imaging and microscopy, spectroscopy and molecule detection. Due to high power nature they can be used as weapons. Fiber lasers can be used for the telecommunication systems.
Conclusion:Fiber laser are high power laser on which semiconductor or diode laser is used as pumping material. The gain medium of the laser is fiber with doping of rare earth elements. They have high range of tuanbility. Pumping can be done with different wavelength according to the energy levels of doped elements. They have different atomic level according to the doped elements. In fiber laser both homogeneous and inhomogeneous broadening takes place. Fiber lasers have high range of applications. The coherent length of fiber laser is large in comparison with other laser types.
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