A fiber laser (or fibre laser in British English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. 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.
Advantages and applications
An advantage of fiber lasers over other types of lasers is that the laser light is both generated and delivered by an inherently flexible medium, which allows easier delivery to the focusing location and target. This can be important for laser cutting, welding, and folding of metals and polymers. Another advantage is high output power compared to other types of laser. Fiber lasers can have active regions several kilometers long, and so can provide very high optical gain. They can support kilowatt levels of continuous output power because of the fiber's high surface area to volume ratio, which allows efficient cooling. The fiber's waveguide properties reduce or eliminate thermal distortion of the optical path, typically producing a diffraction-limited, high-quality optical beam. Fiber lasers are compact compared to solid-state or gas lasers of comparable power, because the fiber can be bent and coiled, except in the case of thicker rod-type designs, to save space. They have lower cost of ownership. Fiber lasers are reliable and exhibit high temperature and vibrational stability and extended lifetime. High peak power and nanosecond pulses improve marking and engraving. The additional power and better beam quality provide cleaner cut edges and faster cutting speeds.
Design and manufacture
Unlike most other types of lasers, the laser cavity in fiber lasers is constructed monolithically by fusion splicing different types of fiber; fiber Bragg gratings replace conventional dielectric mirrors to provide optical feedback. They may also be designed for single longitudinal mode operation of ultra narrow distributed feedback lasers (DFB) where a phase-shifted Bragg grating overlaps the gain medium. Fiber lasers are pumped by semiconductor laser diodes or by other fiber lasers.
Many high-power fiber lasers are based on double-clad fiber. The gain medium forms the core of the fiber, which is surrounded by two layers of cladding. The lasing mode propagates in the core, while a multimode pump beam propagates in the inner cladding layer. The outer cladding keeps this pump light confined. This arrangement allows the core to be pumped with a much higher-power beam than could otherwise be made to propagate in it, and allows the conversion of pump light with relatively low brightness into a much higher-brightness signal. There is an important question about the shape of the double-clad fiber; a fiber with circular symmetry seems to be the worst possible design. The design should allow the core to be small enough to support only a few (or even one) modes. It should provide sufficient cladding to confine the core and optical pump section over a relatively short piece of the fiber.
Recent developments in fiber laser technology have led to a rapid and large rise in achieved diffraction-limited beam powers from diode-pumped solid-state lasers. Due to the introduction of large mode area (LMA) fibers as well as continuing advances in high power and high brightness diodes, continuous-wave single-transverse-mode powers from Yb-doped fiber lasers have increased from 100 W in 2001 to over 20 kW. In 2014 a combined beam fiber laser demonstrated power of 30 kW.
High average power fiber lasers generally consist of a relatively low-power master oscillator, or seed laser, and power amplifier (MOPA) scheme. In amplifiers for ultrashort optical pulses, the optical peak intensities can become very high, so that detrimental nonlinear pulse distortion or even destruction of the gain medium or other optical elements may occur. This is generally avoided by employing chirped-pulse amplification (CPA). State of the art high-power fiber laser technologies using rod-type amplifiers have reached 1 kW with 260 fs pulses  and made outstanding progress and delivered practical solutions for the most of these problems.
However, despite of the attractive characteristics of fiber lasers, several problems arise when power scaling. The most significant are thermal lensing and material resistance, nonlinear effects such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), mode instabilities and poor output beam quality.
The main approach to solving the problems related to increasing the output power of pulses has been to increase the core diameter of the fiber. Special active fibers with large mode were developed to increase surface-to-active-volume ratio of active fibers and, hence, improve heat dissipation enabling power scaling.
Moreover, specially developed double cladding structures have been used to reduce the brightness requirements of the high-power pump diodes by controlling pump propagation and absorption between the inner cladding and the core.
Several types of active fibers with a large effective mode area (LMA) have been developed for high power scaling including LMA fibers with a low-aperture core, micro-structured rod-type fiber  helical core  or chirally-coupled fibers, and tapered double-clad fibers (T-DCF). The mode field diameter (MFD) achieved with these low aperture technologies  usually does not exceed 20–30 μm. The micro-structured rod-type fiber has much larger MFD (up to 65 μm ) and good performance. An impressive 2.2 mJ pulse energy was demonstrated by a femtosecond MOPA  containing large-pitch fibers (LPF). However, the shortcoming of amplification systems with LPF is their relatively long (up to 1.2 m) unbendable rod-type fibers meaning a rather bulky and cumbersome optical scheme. LPF fabrication is highly complex requiring significant processing such as precision drilling of the fiber pre-forms. The LPF fibers are highly sensitive to bending meaning robustness and portability is compromised.
In addition to the types of mode locking used with other lasers, fiber lasers can be passively mode locked by using the birefringence of the fiber itself. The non-linear optical Kerr effect causes a change in polarization that varies with the light's intensity. This allows a polarizer in the laser cavity to act as a saturable absorber, blocking low-intensity light but allowing high intensity light to pass with little attenuation. This allows the laser to form mode-locked pulses, and then the non-linearity of the fiber further shapes each pulse into an ultra-short optical soliton pulse.
Semiconductor saturable-absorber mirrors (SESAMs) can also be used to mode lock fiber lasers. A major advantage SESAMs have over other saturable absorber techniques is that absorber parameters can be easily tailored to meet the needs of a particular laser design. For example, saturation fluence can be controlled by varying the reflectivity of the top reflector while modulation depth and recovery time can be tailored by changing the low temperature growing conditions for the absorber layers. This freedom of design has further extended the application of SESAMs into modelocking of fiber lasers where a relatively high modulation depth is needed to ensure self-starting and operation stability. Fiber lasers working at 1 µm and 1.5 µm were successfully demonstrated.
Graphene saturable absorbers have also been used for mode locking fiber lasers. Graphene's saturable absorption is not very sensitive to wavelength, making it useful for mode locking tunable lasers.
Dark soliton fiber lasers
In the non-mode locking regime, a dark soliton fiber laser was successfully created using an all-normal dispersion erbium-doped fiber laser with a polarizer in-cavity. Experimental findings indicate that apart from the bright pulse emission, under appropriate conditions the fiber laser could also emit single or multiple dark pulses. Based on numerical simulations the dark pulse formation in the laser may be a result of dark soliton shaping.
Multi-wavelength fiber lasers
Multi-wavelength emission in a fiber laser demonstrated simultaneous blue and green coherent light using ZBLAN optical fiber. The end-pumped laser was based on an upconversion optical gain media using a longer wavelength semiconductor laser to pump a Pr3+/Yb3+ doped fluoride fiber that used coated dielectric mirrors on each end of the fiber to form the cavity.
Fiber disk lasers
Another type of fiber laser is the fiber disk laser. In such lasers, the pump is not confined within the cladding of the fiber, but instead pump light is delivered across the core multiple times because it is coiled in on itself. This configuration is suitable for power scaling in which many pump sources are used around the periphery of the coil.
- "Growing adoption of laser cutting machine market in the US through 2021, due to the need for superior-quality products: Technavio". Business Wire. Feb 2, 2017. Retrieved 2020-02-08.
- Shiner, Bill (Feb 1, 2016). "Fiber lasers continue to gain market share in material processing applications". SME.org. Retrieved 2020-02-08.
- Shiner, Bill (Feb 1, 2006). "High-power fiber lasers gain market share". Industrial Laser Solutions for Manufacturing. Retrieved 2020-02-08.
- Zervas, Michalis N.; Codemard, Christophe A. (September 2014). "High Power Fiber Lasers: A Review". IEEE Journal of Selected Topics in Quantum Electronics. 20 (5): 219–241. Bibcode:2014IJSTQ..20..219Z. doi:10.1109/JSTQE.2014.2321279. ISSN 1077-260X.
- Phillips, Katherine C.; Gandhi, Hemi H.; Mazur, Eric; Sundaram, S. K. (Dec 31, 2015). "Ultrafast laser processing of materials: a review". Advances in Optics and Photonics. 7 (4): 684–712. Bibcode:2015AdOP....7..684P. doi:10.1364/AOP.7.000684. ISSN 1943-8206.
- Popov, S. (2009). "7: Fiber laser overview and medical applications". In Duarte, F. J. (ed.). Tunable Laser Applications (2nd ed.). New York: CRC.
- S. Bedö; W. Lüthy; H. P. Weber (1993). "The effective absorption coefficient in double-clad fibers". Optics Communications. 99 (5–6): 331–335. Bibcode:1993OptCo..99..331B. doi:10.1016/0030-4018(93)90338-6.
- A. Liu; K. Ueda (1996). "The absorption characteristics of circular, offset, and rectangular double-clad fibers". Optics Communications. 132 (5–6): 511–518. Bibcode:1996OptCo.132..511A. doi:10.1016/0030-4018(96)00368-9.
- Kouznetsov, D.; Moloney, J.V. (2003). "Efficiency of pump absorption in double-clad fiber amplifiers. 2: Broken circular symmetry". JOSA B. 39 (6): 1259–1263. Bibcode:2002JOSAB..19.1259K. doi:10.1364/JOSAB.19.001259.
- Kouznetsov, D.; Moloney, J.V. (2003). "Efficiency of pump absorption in double-clad fiber amplifiers.3:Calculation of modes". JOSA B. 19 (6): 1304–1309. Bibcode:2002JOSAB..19.1304K. doi:10.1364/JOSAB.19.001304.
- Leproux, P.; S. Fevrier; V. Doya; P. Roy; D. Pagnoux (2003). "Modeling and optimization of double-clad fiber amplifiers using chaotic propagation of pump". Optical Fiber Technology. 7 (4): 324–339. Bibcode:2001OptFT...7..324L. doi:10.1006/ofte.2001.0361.
- D.Kouznetsov; J.Moloney (2004). "Boundary behaviour of modes of a Dirichlet Laplacian". Journal of Modern Optics. 51 (13): 1362–3044. Bibcode:2004JMOp...51.1955K. doi:10.1080/09500340408232504.
- Filippov, V.; Chamorovskii, Yu; Kerttula, J.; Golant, K.; Pessa, M.; Okhotnikov, O. G. (2008-02-04). "Double clad tapered fiber for high power applications". Optics Express. 16 (3): 1929–1944. Bibcode:2008OExpr..16.1929F. doi:10.1364/OE.16.001929. ISSN 1094-4087. PMID 18542272.
- Filippov, Valery; Kerttula, Juho; Chamorovskii, Yuri; Golant, Konstantin; Okhotnikov, Oleg G. (2010-06-07). "Highly efficient 750 W tapered double-clad ytterbium fiber laser". Optics Express. 18 (12): 12499–12512. Bibcode:2010OExpr..1812499F. doi:10.1364/OE.18.012499. ISSN 1094-4087. PMID 20588376.
- "Many lasers become one in Lockheed Martin's 30kW fiber laser". Gizmag.com. Retrieved 2014-02-04.
- Müller, Michael; Kienel, Marco; Klenke, Arno; Gottschall, Thomas; Shestaev, Evgeny; Plötner, Marco; Limpert, Jens; Tünnermann, Andreas (2016-08-01). "1 kW 1 mJ eight-channel ultrafast fiber laser". Optics Letters. 41 (15): 3439–3442. Bibcode:2016OptL...41.3439M. doi:10.1364/OL.41.003439. ISSN 1539-4794. PMID 27472588.
- Koplow, Jeffrey P.; Kliner, Dahv A. V.; Goldberg, Lew (2000-04-01). "Single-mode operation of a coiled multimode fiber amplifier". Optics Letters. 25 (7): 442–444. Bibcode:2000OptL...25..442K. doi:10.1364/OL.25.000442. ISSN 1539-4794. PMID 18064073.
- Limpert, J.; Deguil-Robin, N.; Manek-Hönninger, I.; Salin, F.; Röser, F.; Liem, A.; Schreiber, T.; Nolte, S.; Zellmer, H.; Tünnermann, A.; Broeng, J. (2005-02-21). "High-power rod-type photonic crystal fiber laser". Optics Express. 13 (4): 1055–1058. Bibcode:2005OExpr..13.1055L. doi:10.1364/OPEX.13.001055. ISSN 1094-4087. PMID 19494970.
- Wang, P.; Cooper, L. J.; Sahu, J. K.; Clarkson, W. A. (2006-01-15). "Efficient single-mode operation of a cladding-pumped ytterbium-doped helical-core fiber laser". Optics Letters. 31 (2): 226–228. Bibcode:2006OptL...31..226W. doi:10.1364/OL.31.000226. ISSN 1539-4794. PMID 16441038.
- Lefrancois, Simon; Sosnowski, Thomas S.; Liu, Chi-Hung; Galvanauskas, Almantas; Wise, Frank W. (2011-02-14). "Energy scaling of mode-locked fiber lasers with chirally-coupled core fiber". Optics Express. 19 (4): 3464–3470. Bibcode:2011OExpr..19.3464L. doi:10.1364/OE.19.003464. ISSN 1094-4087. PMC 3135632. PMID 21369169.
- "AEROGAIN-ROD HIGH POWER YTTERBIUM ROD FIBER GAIN MODULES". Retrieved 14 January 2020.
- Eidam, Tino; Rothhardt, Jan; Stutzki, Fabian; Jansen, Florian; Hädrich, Steffen; Carstens, Henning; Jauregui, Cesar; Limpert, Jens; Tünnermann, Andreas (2011-01-03). "Fiber chirped-pulse amplification system emitting 3.8 GW peak power". Optics Express. 19 (1): 255–260. Bibcode:2011OExpr..19..255E. doi:10.1364/OE.19.000255. ISSN 1094-4087. PMID 21263564.
- Li N.; Xue J.; Ouyang C.; Wu K.; Wong J. H.; Aditya S.; Shum P. P. (2012). "Cavity-length optimization for high energy pulse generation in a long cavity passively mode-locked all-fiber ring laser". Applied Optics. 51 (17): 3726–3730. Bibcode:2012ApOpt..51.3726L. doi:10.1364/AO.51.003726. PMID 22695649.
- H. Zhang et al., "Induced solitons formed by cross polarization coupling in a birefringent cavity fiber laser" Archived 2011-07-07 at the Wayback Machine, Opt. Lett., 33, 2317–2319. (2008).
- D.Y. Tang et al., "Observation of high-order polarization-locked vector solitons in a fiber laser" Archived 2010-01-20 at the Wayback Machine, Physical Review Letters, 101, 153904 (2008).
- H. Zhang et al., "Coherent energy exchange between components of a vector soliton in fiber lasers", Optics Express, 16,12618–12623 (2008).
- Zhang H.; et al. (2009). "Multi-wavelength dissipative soliton operation of an erbium-doped fiber laser". Optics Express. 17 (2): 12692–12697. arXiv:0907.1782. Bibcode:2009OExpr..1712692Z. doi:10.1364/oe.17.012692. PMID 19654674.
- Zhang, H; Tang, DY; Zhao, LM; Bao, QL; Loh, KP (28 September 2009). "Large energy mode locking of an erbium-doped fiber laser with atomic layer graphene". Optics Express. 17 (20): 17630–5. arXiv:0909.5536. Bibcode:2009OExpr..1717630Z. doi:10.1364/OE.17.017630. PMID 19907547.
- Han Zhang; Qiaoliang Bao; Dingyuan Tang; Luming Zhao; Kianping Loh (2009). "Large energy soliton erbium-doped fiber laser with a graphene-polymer composite mode locker" (PDF). Applied Physics Letters. 95 (14): P141103. arXiv:0909.5540. Bibcode:2009ApPhL..95n1103Z. doi:10.1063/1.3244206. Archived from the original (PDF) on 2011-07-17.
-  Archived February 19, 2012, at the Wayback Machine
- Zhang, H.; Tang, D. Y.; Zhao, L. M.; Wu, X. (27 October 2009). "Dark pulse emission of a fiber laser" (PDF). Physical Review A. 80 (4): 045803. arXiv:0910.5799. Bibcode:2009PhRvA..80d5803Z. doi:10.1103/PhysRevA.80.045803. Archived from the original (PDF) on 2011-07-17.
- Baney, D. M., Rankin, G., Change, K. W. "Simultaneous blue and green upconversion lasing in a diode-pumped Pr3+/Yb3+ doped fluoride fiber laser,"Appl. Phys. Lett, vol. 69 No 12, pp. 1622-1624, Sept 1996.
- Ueda, Ken-ichi (1998). "Optical cavity and future style of high-power fiber lasers". Proceedings. 3267 (Laser Resonators): 14. Bibcode:1998SPIE.3267...14U. doi:10.1117/12.308104.
- K. Ueda (1999). "Scaling physics of disk-type fiber lasers for kW output". Lasers and Electro-Optics Society. 2: 788–789. doi:10.1109/leos.1999.811970. ISBN 978-0-7803-5634-4.
- Ueda; Sekiguchi H.; Matsuoka Y.; Miyajima H.; H.Kan (1999). "Conceptual design of kW-class fiber-embedded disk and tube lasers". Lasers and Electro-Optics Society 1999 12th Annual Meeting. LEOS '99. IEEE. 2: 217–218. doi:10.1109/CLEOPR.1999.811381. ISBN 978-0-7803-5661-0.
- Hamamatsu Photonics K.K. Laser group (2006). "The Fiber Disk Laser explained". Nature Photonics. sample: 14–15. doi:10.1038/nphoton.2006.6.