Describes the advancements in active fiber laser systems. Outline of cladding pumping technique for enhanced power scaling; Improved energies and peak powers; Extended wavelength range; Frequency doubling with fiber sources

Hanna, D.C.

Minelly, J.D.

Richardson, D.J.

English

Southampton (e-Prints Soton)

1Southampton SO17 1BJ,  UKTelephone: +44 1703 593150e-mail:   light@orc.soton.ac.ukOptoelectronics Research CentreUniversity of SouthamptonFiber laser systems shine brightly David Richardson, John Minelly, and David Hanna More powerful semiconductor pump lasers, improved fiber fabrication, and better energy storage and power scalingtechniques are enabling lasers to produce tens of watts of high-quality output. Although it is now more than 30 years since lasing was first observed in a doped optical fiber,[1] only afterdevelopment of techniques for reliable fabrication of single-mode rare-earth-doped fibers during the mid-1980s didactive fiber devices really take off. The erbium (Er )-doped fiber amplifier (EDFA), for example - which provides3+gains of 30-40 dB for a few tens of milli-watts in the low-loss 1550nm third telecommunications window - hasvirtually eliminated loss as a factor limiting high-capacity optical communications and has opened the way toterabit-capacity communication systems. In fact, the commercial potential of the EDFA is such that the telecommunications industry has invested enormousresources in doped-fiber amplifier development over the past ten years. As a result, steady advances have occurredin fiber fabrication and design as well as in the understanding of the spectroscopic properties of rare-earth ions ina glass matrix. This research has also driven development of associated fiber and pump laser technology. Much of the development has involved fiber lasers themselves. Impressive performance characteristics have beenachieved for most regimes of fiber-laser operation, although large-scale commercialization of such devices forapplications other than communications has been limited--mostly by optical power requirements. Rapidly developingmarkets, such as medicine, aerospace, and material processing, require compact, all-solid-state sources of highaverage optical output power (5-10 W), or high pulse energies (millijoule) - performance characteristics notgenerally associated with fiber lasers. This situation is, however, changing rapidly. Figure 1. Double-clad fibers convert the output of a high-power semiconductor pump laser to a diffraction-limitedoutput beam in simple, end-pumped fiber laser cavities. Inset shows refractive index profile typical of acladding-pumped fiber. 2Significant advances in high-power diode pump lasers, refinement of power scaling and energy storage techniques,and fiber component fabrication, such as in-fiber Bragg gratings, are opening up the way to the development ofactive fiber systems that can deliver tens of watts of single-transverse and longitudinal-mode output power,millijoule pulse energies, and ultrashort pulses with peak powers in the 10-100 MW region. Furthermore, advancesin nonlinear optical materials are permitting these high-power fiber laser outputs to be efficiently converted to thevisible and near-infrared (IR) spectral regions. Cladding pumping enables power scaling The cladding pumping technique makes use of a double-clad fiber with a rare-earth-doped fiber core that is designedto be single-mode at the relevant lasing wavelength. The core is surrounded by a cladding layer heavily multimodeat the pump wavelength, which is itself surrounded by an outer cladding layer of still lower refractive index (see Fig.1) [2] The inner cladding layer is adapted in shape and dimensions to allow efficient end-coupling of the output fromhigh-power semiconductor diode stripes and arrays. Absorption of the pump laser output by the rare-earth ions and subsequent lasing within the core converts thehigh-power, low-brightness semiconductor-pump-laser input into intense diffraction-limited laser output. The highsurface-area-to-volume ratio of the fiber geometry effectively eliminates thermal problems, which are a criticallimiting factor in bulk high-power lasers. The design of a cladding-pumped fiber involves a compromise between launch efficiency--helped by a largercladding waveguide dimension - and pump rate, which requires smaller cladding waveguides. Furthermore, the pumpabsorption length depends on the shape and position of the core within the cladding structure and on thecore-cladding area ratio. Fibers with rectangular or circular cladding waveguides and with central or offset cores areused, and cladding dimensions as large as 100 x 300 µm have been successfully fabricated and used. Cladding pumping can be adapted to laser systems involving three atomic energy levels, such as erbium, andfour-level systems, such as neodymium (Nd ), although it is more suited to the latter. Before lasing can occur in3+three-level systems, sufficient inversion of the dopant population is required to overcome the loss due to unpumpedions - this results in higher laser thresholds. These can be on the order of several watts, although adding dopants likeytterbium (Yb ) with erbium to increase the pump rate can help. Multiple-watt output power levels have been3+achieved at 1.55 µm using such schemes. In four-level systems the fiber loss at the signal wavelength is determined only by the background fiber loss, whichallows thresholds of only a few tens of milliwatts. Currently, the highest output power for a cladding-pumped fibersystem was reported by workers at Polaroid Corp. (Cambridge, MA), who achieved 35-W output with a slopeefficiency of 65% from a simple end-pumped single-mode ytterbium-doped fiber laser operating as aquasi-four-level system at 1.1 µm [3]. In the near future expected improvements in diode beam-shaping andcombining techniques should yield a CW output of 100 W with this approach. Improved energies and peak powers Various pulsed fiber-laser designs offer a wide range of pulse durations and repetition rates and are already findingtheir way into the marketplace. Passive modelocking based on the inherent non-linearity and dispersion of the fibercan yield femtosecond pulses with nano-joule pulse energies. Active modelocking can provide picosecond, picojouleoperation at high (>40 GHz) repetition rates suitable for advanced communications. Q-switching can providenanosecond pulses at kilohertz repetition rates with typical energies of 1-10 µJ. For many applications, however, increasing the pulse energy and peak power is a key issue and, in this sense, thetightly confined mode of the fiber is a constraint. Energy storage is limited by the resulting high gain efficiency andconsequent self-gain saturation by amplified spontaneous emission (ASE). 3FIGURE 2. Diode-pumped, single-transverse-mode Q-switched fiber laser delivers 160-µJ, 50-ns pulses tunable from1530 to 1560 nm. Periodically poled lithium niobate (PPLN)-based frequency doublers and mixers enable efficientsecond- (66%) and third- (35%) harmonic generation from the fiber-laser output. Inset shows refractive index profiletypical of a large-mode-area erbium (Er )-doped fiber. 3+Peak power is limited to less than 10 kW by nonlinear effects such as Raman scattering and self-phase modulationcaused by the high internal fields and appreciable device lengths - typically more than 1 m. Recently though, fiber designs have been optimized for energy storage and extraction characteristics. Much is gainedsimply by increasing the spot size within the fiber. A multimode core allows for a significant increase in spot size,for example. Thus an erbium-doped system with a 20 µm core has produced 0.5 mJ, 10 ns pulses.[4] However, forapplications that require high beam quality, such as those concerned with short pulses, single-transverse modeperformance is necessary. By reducing the NA of the fiber, much-larger spot sizes can be obtained while maintainingsingle-mode operation. Issues arise because of the increased bend-loss sensitivity of the fiber, but by dropping toa NA of 0.06, close to the limit of fabrication tolerances, one can achieve a factor of ten increase in effective modearea with acceptable bend-loss conditions. This approach in high-concentration erbium-doped-fiber devices has enabled us to achieve mode areas larger than300 µm  and pulse energies approaching 0.2 mJ using both amplifier and simple diode-pumped Q-switched laser2systems (see Fig. 2) [5]. Peak powers exceeding 100 kW have been obtained this way without nonlinear spectraldistortion or fiber damage. More-elaborate fiber designs with optimized refractive index and dopant distributionspromise even better results. Placing the dopant in a ring around the fiber core so that it lies within the tails of themodal field distribution reduces the effects of ASE gain saturation and boosts energy storage capacity. The principle,recently demonstrated in a ytterbium-doped fiber-amplifier system, resulted in a factor of ten improvement overconventional amplifier designs.[6] This approach, combined perhaps with the cladding pumping concept, shouldallow millijoule pulse energies to be obtained. High-peak-power pulse generation has also been impacted by recent developments in fiber components.Photorefractive gratings can be written into the core of an optical fiber with fine depth and period control overmeter-long lengths, enabling fabrication of reflective grating filters with precise amplitude and phasecharacteristics.[7] Thus fiber Bragg gratings can be fabricated with enormous dispersion suitable for the stretchingand recompression of ultrashort pulses. Compact, fiber-based chirped-pulse amplification (CPA) schemes can thenbe readily implemented using these components (see Fig. 3). 4FIGURE 3. Fiber-based chirped-pulse amplification system illustrates the many advantages of fiber systems. Fibernonlinearity and dispersion combine to passively modelock the low-power oscillator. Chirped fiber gratings stretchand compress the pulse before and after amplification in a cladding-pumped large-mode-area erbium-doped fiber.Chirped-pulse amplification is based on temporally stretching (by factors of 10,000 or more) the pulse to beamplified prior to amplification. The pulse is then amplified before being compressed back to its initial duration.High peak powers, and hence nonlinear distortion, can be avoided within the amplifier, allowing for an enormousincrease in achievable output peak power. Large-mode-area fiber in the final-stage amplifier can further increasethe available output powers. This approach enabled our coworkers at IMRA (Ann Arbor, Ml) to achieve peak pulsepowers of more than 10 MW for 600 fs pulses using a master-oscillator-power amplifier (MOPA)-pumped,single-mode EDFA and more than 100 MW for 600 fs pulses by exciting a single mode of a cladding-pumpedmultimode EDFA. [8,9] Extended wavelength range Extending the wavelength coverage of fiber-laser output into the visible and further into the IR spectral regionsoffers increased application opportunities for fiber lasers. Reducing the phonon energy of the glass host can havea significant impact on the spectroscopic properties of rare-earth ions, opening up the opportunity for new lasertransitions and pump schemes. Impressive results have been achieved at visible wavelengths using upconversiontechniques with fluoride fibers in the red, green, and blue. An output of 200 MW at 480 nm was recently achieved,for example [10]. At longer wavelengths good results have been obtained around 2.7 µm with erbium-doped fluoridefiber,[11] but more-significant break-throughs are expected - particularly in the important 3-5 µm range based onnew glasses such as gallium lanthanum sulfide (GLS) [12]. The first lasing results have already been achieved forneodymium-doped multimode GLS fiber, and further results for single-mode operation are keenly anticipated. Alternative strategies exist for extending the wavelength of fiber-based lasers. The high powers and pulse energiesnow obtainable are suitable for wavelength translation by non-linear frequency conversion. Cascaded Ramanscattering in fiber is one route to obtaining efficient wavelength conversion. This was recently highlighted byresearchers at Lucent Technologies (Murray Hill, NJ) who obtained 5 W at 1.49 µm through cascaded Ramanscattering of the output of a 20-W cladding-pumped ytterbium-doped fiber laser operating at 1.1 µm [13]. 5Frequency doubling with fiber sources Perhaps the most exciting possibility, though, is the use of fiber sources to produce frequency conversion inquasi-phase-matched materials such as periodically poled lithium niobate (PPLN - see Laser Focus World, May1997, p. 105). This material allows phase matching from blue wavelengths to about 5 µm and has sufficientnonlinearity to allow very efficient conversion by harmonic generation, optical parametric oscillators (OPOs), andoptical parametric amplifiers (OPAs) at the power levels available from fiber-laser sources. Frequency-doubling efficiencies of several tens of percent have already been achieved by researchers at IMRA forfemtosecond pulses generated by erbium-based fiber-laser systems. These second-harmonic-generation (SHG)pulses have subsequently been used to pump OPAs operating out to 3 µm [14]. In the nanosecond region we haveachieved SHG conversion efficiencies of up to 83% with frequency-doubled pulse energies as high as 80 µJ usingsimple diode-pumped erbium-doped-fiber Q-switched systems. Such pulses are suitable for the pumping of PPLNOPOs. Efficient (>30%) sum-frequency generation to the green (512 nm) has also been demonstrated by mixingthe 1550 nm fundamental output and its second harmonic. High-efficiency multiple-watt frequency doubling fromCW cladding-pumped systems is a future prospect. In conclusion, recent key developments in fiber design and fabrication, fiber grating technology, high-powerdiode-pumped laser performance, and subsequent nonlinear frequency conversion are opening up many possibilitiesfor fiber-laser systems. High-power systems are appearing on the market through companies such as IRE Polus(Moscow, Russia), SDL (San Jose, CA), and Polaroid Corp. Fiber lasers are finally outgrowing their reputation forlow-power operation. References 1.  E. Snitzer, Proc. 3rd Intl. Conf. Solid-State Lasers, 999 (Paris, France, 1963). 2.  H. Po et al., Proc. OFC '89, Paper PD7, (Houston, TX, 1989). 3.  M. Muedel et al., Proc. CLEO '97, Paper CPD30, (Baltimore, MD, 1997). 4.  B. Desthieux et al., App. Phys. Lett 63, 586 (1993) 5.  D. Taverner et al., Opt Lett. Z2, 378 (1997). 6.  J. Nilsson et al., Opt Lett 22,1092 (1997). 7.  R. I. Laming et al., Proc OEC '97, Paper THA3, (Dallas, TX, 1997). 8.  D. Taverner et al., Proc. CI EO '96, Paper CFDS, (Anaheim, CA, 1996). 9.  J. D. Minelly et al., Proc. CLEO '97, Paper CFD4, (Baltimore, MD, 1997) 10. R. Paschotta et al., "230 MW of blue light from a thulium-doped upconversion fiber laser," to be publishedin J. Selected Topics in Quantum Electronics (1997). 11. S. Bedo et al., Optics Communication 116, 81 (1995). 12. T. Schweitzer et al., Proc. CLEO '97, Paper CWQ4, (Baltimore, MD, 1997). 13. D. Innis et al.. Proc. CLEO '97, Paper CPD31 (Baltimore, MD, 1997). 14. A. Galvanauskas et al., Opt. Lett. 22, 105 (1997). 

Fiber laser systems shine brightly

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