Fiber laser 1

Fiber laserA fiber laser or fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earthelements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, and thulium. They are related todoped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulatedRaman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.

Advantages and applications

A laser cutting machine with a 2kW continuouswave fiber laser

The advantages of fiber lasers over other types include: 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 movablefocusing element. This is important for laser cutting, welding, andfolding of metals and polymers.

High output power: Fiber lasers can have active regions severalkilometers long, and so can provide very high optical gain. Theycan support kilowatt levels of continuous output power because ofthe fiber's high surface area to volume ratio, which allows efficientcooling.

High optical quality: The fiber's waveguiding properties reduce or eliminate thermal distortion of the optical path,typically producing a diffraction-limited, high-quality optical beam.

Compact size: Fiber lasers are compact compared to rod or gas lasers of comparable power, because the fiber canbe bent and coiled to save space.

Reliability: Fiber lasers exhibit high vibrational stability, extended lifetime, and maintenance-free turnkeyoperation.

High peak power and nanosecond pulses enable effective marking and engraving. The additional power and better beam quality provide cleaner cut edges and faster cutting speeds. Lower cost of ownership. Fiber lasers are now being used to make high-performance surface-acoustic wave (SAW) devices. These lasers

raise throughput and lower cost of ownership in comparison to older solid-state laser technology.Fiber laser can also refer to the machine tool that includes the fiber resonator.Applications of fiber lasers include material processing (marking, engraving, cutting), telecommunications,spectroscopy, medicine, and directed energy weapons.

Design and manufactureUnlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicingdifferent types of fiber; fiber Bragg gratings replace conventional dielectric mirrors to provide optical feedback.Another type is the single longitudinal mode operation of ultra narrow distributed feedback lasers (DFB) where aphase-shifted Bragg grating overlaps the gain medium. Fiber lasers are pumped by semiconductor laser diodes or byother fiber lasers. Q-switched pulsed fiber lasers offer a compact, electrically efficient alternative to Nd:YAGtechnology.

Fiber laser 2

Double-clad fibers

Double-clad fiber

Many high-power fiber lasers are based on double-clad fiber. The gain medium formsthe core of the fiber, which is surrounded by two layers of cladding. The lasing modepropagates in the core, while a multimode pump beam propagates in the inner claddinglayer. The outer cladding keeps this pump light confined. This arrangement allows thecore to be pumped with a much higher-power beam than could otherwise be made topropagate in it, and allows the conversion of pump light with relatively low brightnessinto a much higher-brightness signal. As a result, fiber lasers and amplifiers areoccasionally referred to as "brightness converters." There is an important question aboutthe shape of the double-clad fiber; a fiber with circular symmetry seems to be the worstpossible design. The design should allow the core to be small enough to support only a few (or even one) modes. Itshould provide sufficient cladding to confine the core and optical pump section over a relatively short piece of thefiber.

Power scaling

10,000W SM Laser

Recent developments in fiber laser technology have led to a rapid andlarge rise in achieved diffraction-limited beam powers fromdiode-pumped solid-state lasers. Due to the introduction of large modearea (LMA) fibers as well as continuing advances in high power andhigh brightness diodes, continuous-wave single-transverse-modepowers from Yb-doped fiber lasers have increased from 100W in 2001to >20kW. Commercial single-mode lasers have reached 10kW inCW power. In 2014 a combined beam fiber laser demonstrated powerof 30kw.

Mode locking

Passive mode locking

Nonlinear polarization rotation

When linearly polarized light is incident to a piece of weakly birefringent fiber, the polarization of the light willgenerally become elliptically polarized in the fiber. The orientation and ellipticity of the final light polarization isfully determined by the fiber length and its birefringence. However, if the intensity of the light is strong, thenon-linear optical Kerr effect in the fiber must be considered, which introduces extra changes to the lightpolarization. As the polarization change introduced by the optical Kerr effect depends on the light intensity, if apolarizer is put behind the fiber, the light intensity transmission through the polarizer will become light intensitydependent. Through appropriately selecting the orientation of the polarizer or the length of the fiber, an artificialsaturable absorber effect with ultra-fast response could then be achieved in such a system, where light of higherintensity experiences less absorption loss on the polarizer. The NPR technique makes use of this artificial saturableabsorption to achieve the passive mode locking in a fiber laser. Once a mode-locked pulse is formed, thenon-linearity of the fiber further shapes the pulse into an optical soliton and consequently the ultrashort solitonoperation is obtained in the laser. Soliton operation is almost a generic feature of the fiber lasers mode-locked by thistechnique and has been intensively investigated.

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Semiconductor saturable absorber mirrors (SESAMs)

Semiconductor saturable absorbers were used for laser mode-locking as early as 1974 when p-type germanium isused to mode lock a CO2 laser which generated pulses ~500 ps . Modern SESAMs are III-V semiconductor singlequantum well (SQW) or multiple quantum wells grown on semiconductor distributed Bragg reflectors (DBRs). Theywere initially used in a Resonant Pulse Modelocking (RPM) scheme as starting mechanisms for Ti:Sapphire laserswhich employed KLM as a fast saturable absorber . RPM is another coupled-cavity mode-locking technique.Different from APM lasers which employ non-resonant Kerr-type phase nonlinearity for pulse shortening, RPMemploys the amplitude nonlinearity provided by the resonant band filling effects of semiconductors . SESAMs weresoon developedinto intracavity saturable absorber devices because of more inherent simplicity with this structure.Since then, the use of SESAMs has enabled the pulse durations, average powers, pulse energies and repetition ratesof ultrafast solid-state lasers to be improved by several orders of magnitude. Average power of 60 W and repetitionrate up to 160GHz were obtained. By using SESAM-assisted KLM, sub-6 fs pulses directly from a Ti: Sapphireoscillator was achieved. A major advantage SESAMs have over other saturable absorber techniques is that absorberparameters can be easily controlled over a wide range of values. For example, saturation fluence can be controlled byvarying the reflectivity of the top reflector while modulation depth and recovery time can be tailored by changing thelow temperature growing conditions for the absorber layers . This freedom of design has further extended theapplication of SESAMs into modelocking of fiber lasers where a relatively high modulation depth is needed toensure self-starting and operation stability. Fiber lasers working at ~ 1m and 1.5m were successfullydemonstrated.[1][2][3][4][5]

Graphene saturable absorbers

Graphene is a one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycombcrystal lattice. Optical absorption from graphene can become saturated when the input optical intensity is above athreshold value. This nonlinear optical behavior is termed saturable absorption and the threshold value is called thesaturation fluency.[citation needed] Graphene can be saturated readily under strong excitation over the visible tonear-infrared region, due to the universal optical absorption and zero band gap.[6] This has relevance for the modelocking of fiber lasers, where wideband tunability may be obtained using graphene as the saturable absorber.[7] Dueto this special property, graphene has wide application in ultrafast photonics.[8][9] Furthermore, comparing with theSWCNTs, as graphene has a 2D structure it should have much smaller non-saturable loss and much higher damagethreshold. Self-started mode locking and stable soliton pulse emission with high energy have been achieved with agraphene saturable absorber in an erbium-doped fiber laser. Atomic layer graphene possesses wavelength-insensitiveultrafast saturable absorption, which can be exploited as a full-band mode locker. With an erbium-dopeddissipative soliton fiber laser mode locked with few layer graphene, it has been experimentally shown thatdissipative solitons with continuous wavelength tuning as large as 30nm (15701600nm) can be obtained.

Active mode locking

Active mode-locking is normally achieved by modulating the loss (or gain) of the laser cavity at a repetition rateequivalent to the cavity frequency, or a harmonic thereof. In practice, the modulator can be acousto-optic orelectro-optic modulator, Mach-Zehnder integrated-optic modulators, or a semiconductor electro-absorptionmodulator (EAM). The principle of active mode-locking with a sinusoidal modulation. In this situation, opticalpulses will form in such a way as to minimize the loss from the modulator. The peak of the pulse wouldautomatically adjust in phase to be at the point of minimum loss from the modulator. Because of the slow variationof sinusoidal modulation, it is not very straightforward for generating ultrashort optical pulses (< 1ps) using thismethod.For stable operation, the cavity length must precisely match the period of the modulation signal or some integer multiple of it. The most powerful technique to solve this is regenerative mode locking i.e. a part of the output signal of the mode-locked laser is detected; the beatnote at the round-trip frequency is filtered out from the detector, and

Fiber laser 4

sent to an amplifier, which drives the loss modulator in the laser cavity. This procedure enforces synchronism if thecavity length undergoes fluctuations due to acoustic vibrations or thermal expansion. By using this method, highlystable mode-locked lasers have been achieved. The major advantage of active mode-locking is that it allowssynchronized operation of the mode-locked laser to an external radio frequency (RF) source. This is very useful foroptical fiber communication where synchronization is normally required between optical signal and electroniccontrol signal. Also active mode-locked fiber can provide much higher repetition rate than passive mode-locking.Currently, fiber lasers and semiconductor diode lasers are the two most important types of lasers where activemode-locking are applied.

Dark soliton fiber lasersIn the non-mode locking regime,the first dark soliton fiber laser has been successfully achieved in an all-normaldispersion erbium-doped ber laser with a polarizer in cavity. Experimentally finding that apart from the bright pulseemission, under appropriate conditions the ber laser could also emit single or multiple dark pulses. Based onnumerical simulations we interpret the dark pulse formation in the laser as a result of dark soliton shaping.[10]

Multiwavelength fiber lasersRecently,multiwavelength dissipative soliton in an all normal dispersion fiber laser passively mode-locked with aSESAM has been generated. It is found that depending on the cavity birefringence, stable single-, dual- andtriple-wavelength dissipative soliton can be formed in the laser. Its generation mechanism can be traced back to thenature of dissipative soliton.

Fiber disk lasers

3 fiber disk lasers

Another type of fiber laser is the fiber disk laser. In such, the pumpis not confined within the cladding of the fiber (as in thedouble-clad fiber), but pump light is delivered across the coremultiple times because the core is coiled on itself like a rope. Thisconfiguration is suitable for power scaling in which many pumpsources are used around the periphery of the coil.

References[1] H. Zhang et al, Induced solitons formed by cross polarization coupling in a

birefringent cavity fiber laser (http:/ / www. sciencenet. cn/ upload/ blog/ file/2009/ 1/ 2009130111724898121. pdf), Opt. Lett., 33, 23172319.(2008).

[2] D.Y. Tang et al, Observation of high-order polarization-locked vector solitons in a fiber laser (http:/ / www3. ntu. edu. sg/ home2006/zhan0174/ Observation of High-Order Polarization-Locked Vector Solitons in a Fiber Laser. pdf), Physical Review Letters, 101, 153904(2008).

[3] H. Zhang et al, Coherent energy exchange between components of a vector soliton in fiber lasers, Optics Express, 16,1261812623 (2008).[4] H. Zhang et al, Multi-wavelength dissipative soliton operation of an erbium-doped fiber laser (http:/ / www. opticsinfobase. org/ abstract.

cfm?URI=oe-17-15-12692), Optics Express, Vol. 17, Issue 2, pp.12692-12697[5] L.M. Zhao et al, Polarization rotation locking of vector solitons in a fiber ring laser (http:/ / www. sciencenet. cn/ upload/ blog/ file/ 2009/ 2/

200921017037656137. pdf), Optics Express, 16,1005310058 (2008).[6] Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko and A. C. Ferrari, ACS Nano," Graphene

Mode-Locked Ultrafast Laser (http:/ / dx. doi. org/ 10. 1021/ nn901703e)"[7] Z. Sun, D. Popa, T. Hasan, F. Torrisi, F. Wang, E. Kelleher, J. Travers, V. Nicolosi and A. Ferrari, Nano Research," A stable, wideband

tunable, near transform-limited, graphene-mode-locked, ultrafast laser (http:/ / dx. doi. org/ 10. 1007/ s12274-010-0026-4)"[8] Qiaoliang Bao, Han Zhang, Yu Wang, Zhenhua Ni, Yongli Yan, Ze Xiang Shen, Kian Ping Loh,and Ding Yuan Tang, Advanced Functional

Materials," Atomic layer graphene as saturable absorber for ultrafast pulsed lasers (http:/ / www3. ntu. edu. sg/ home2006/ zhan0174/ AFM.pdf)"

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[9] F. Bonaccorso, Z. Sun, T. Hasan and A. C. Ferrari, Nature Photonics," Graphene photonics and optoelectronics (http:/ / dx. doi. org/ 10. 1038/nphoton. 2010. 186)"

[10] Han Zhang, Dingyuan Tang, Luming Zhao and Wu Xuan, Dark pulse emission of a fiber laser (http:/ / www3. ntu. edu. sg/ home2006/ZHAN0174/ pra. pdf)" PHYSICAL REVIEW A 80, 045803 2009

Article Sources and Contributors 6

Article Sources and ContributorsFiber laser Source: http://en.wikipedia.org/w/index.php?oldid=601781036 Contributors: Alex-engraver, Brad101, Conscious, Delusion23, Dicklyon, DoctorKubla, Domitori, Dougher, Elenao,Eranredick, FiberLaser7, Flex3000, Furries, HarDNox, Hqb, INSAR, Igfmnbo, Ippopotamus, Kdaly100, Lfstevens, Lhasapso, Mchsvosm, Mnmngb, MrDrBob, Mrba70, Northryde, NufernWiki,PassPort, Rchandra, Rick.neff, Rjwilmsi, SMAir2009, SMasters, Sergiusz Patela, Sfan00 IMG, Shaddack, Some standardized rigour, Srleffler, Stickee, Thaddeusw, VarunPius, Vectorsoliton,Wtshymanski, Yanje03, 44 anonymous edits

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Fiber laserAdvantages and applicationsDesign and manufactureDouble-clad fibersPower scalingMode lockingPassive mode lockingActive mode locking

Dark soliton fiber lasersMultiwavelength fiber lasers

Fiber disk lasersReferences

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