150 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 2, JANUARY 15, 2014

High Power Supercontinuum Generation in FluorideFibers Pumped by 2 μm Pulses

Jacek Swiderski, Maria Michalska, Christelle Kieleck, Marc Eichhorn, and Gwenael Mazé

Abstract— High power supercontinuum generation in fluoridestep-index optical fibers pumped with 2-µm pulses deliveredby a gain-switched thulium-doped fiber laser and amplifiersystem is reported. An output average power of 1.25 W in the∼1.8–4.15 µm spectral band and 1.82 W in the ∼1.8–3.65 µmband was achieved for two fluorozirconate fibers of differentparameters. The first demonstration of Watt-level supercontin-uum generation in a fluoroindate fiber is also demonstrated.The achieved output SC powers are the highest ones ever reportedfor fluoride fibers pumped with 2-µm gain-switched opticalpulses.

Index Terms— Supercontinuum generation, fiber lasers, dopedfiber amplifiers, fiber nonlinear optics.

I. INTRODUCTION

SUPERCONTINUUM (SC) generation, referring to sig-nificant broadening of a spectrum of an optical signal

when passing through a nonlinear medium, has been thesubject of intense research recently. In particular, mid-infrared(mid-IR) SC sources are considered to be attractive for manyapplications, such as infrared microscopy [1], medicine [2],countermeasure, and spectral fingerprinting [3]. To ensurecontinuum generation in the 2–5 μm spectral band, tellurite(e.g. [4], [5]), chalcogenide (e.g. [6], [7]), and fluoride [8]–[10]fibers have been successfully adopted as nonlinear media.They have high nonlinearity and relatively low attenuationin the mid-IR region, where silica based fibers are nottransparent [11]. Broadband SC generation in the soft-glassfibers, using a variety of pumping sources delivering fem-tosecond [4], [8], picosecond [6], [12] and nanosecond[9], [13] pulses, has also been demonstrated. However, mostof the reports concern experiments with high peak power fem-tosecond lasers or optical parametric amplifiers (e.g. [8], [14]),where output average pump power and thus output SC power islimited to mW-level, which may be an obstacle for some appli-cations requiring sufficient brightness of broadband sources.To achieve high average SC power, longer pump pulses on the

Manuscript received September 27, 2013; revised October 30, 2013;accepted November 7, 2013. Date of publication November 8, 2013; date ofcurrent version December 26, 2013. This work was supported by the PolishNational Science Centre under Grant 724/N-MIFL/2010/0.

J. Swiderski and M. Michalska are with the Institute of Opto-electronics, Military University of Technology, Warsaw 00-908, Poland(e-mail: jswiderski@wat.edu.pl; mmichalska@wat.edu.pl).

C. Kieleck and M. Eichhorn are with the Division III - Laserand Electromagnetic Technologies, French-German Research Institute ofSaint-Louis, Saint-Louis F-68301, France (e-mail: christelle.kieleck@isl.eu;marc.eichhorn@isl.eu).

G. Mazé is with Le Verre Fluoré, Campus KerLann, Bruz F-35170, France(e-mail: gmaze@leverrefluore.com).

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2290376

Fig. 1. Experimental setup for SC generation.

picosecond or nanosecond scale are needed. Sparse literaturereports on high-power (>1 W) SC generation [12], [15–19]show that this technology is still in its infancy.

In this letter, we report an elegant method for broadbandmid-IR SC generation in fluorozirconate (ZBLAN) and flu-oroindate (InF3) fibers with the use of high-power, pulsed,all-fiber laser source, in which the original seed pulsesare generated by a resonantly pumped, fast gain-switchedTm3+-doped fiber laser (TDFL) operating at λ = 1994.5 nmand then amplified in a Tm3+-doped fiber amplifier (TDFA).

II. EXPERIMENTAL RESULTS

The scheme of the experimental setup is shown in Fig. 1.The TDFL was core-pumped by a three stage all-fiber

MOPA system, seeded by a 1.55-μm pulsed laser diode,delivering ∼100 ns pulses at the repetition rate of 26 kHz.The optical cavity of the TDFL was formed by a 20-cmlong, single-mode, double-clad Tm3+-doped fiber (TDF) witha core/clad diameter of 10/130 μm (0.15/0.46NA) and twofiber Bragg gratings with reflectivit of >99% and 90% atλ = 1994.5 nm for the high reflector and output coupler,respectively. The TDFL delivered a train of ∼25 ns gain-switched pulses with up to 0.4 W of average output powerat λ = 1994.5 nm (FWHM linewidth, ∼1 nm). By suitableadjustment of pump power it was possible to achieve theoutput pulse train resembling simultaneous gain-switchingand mode-locking operation with 100% modulation depth,as reported in [13]. The peak power of the highest sub-pulses recorded within a gain-switched pulse envelope was∼1 kW. In the next step, the output from the TDFL, afteroptical isolation, was directly fusion spliced to the TDFA,built with the use of a ∼4.5-m long large-mode-area (LMA)TDF, characterized by a core/clad diameter of 25/250 μm andcorresponding numerical apertures of 0.1/0.46. It was claddingpumped in co-propagation configuration by a 793-nm, 30-Wlaser diode, which radiation was launched into the gain fibervia a (2+1)×1 pump combiner with a signal feedthrough

1041-1135 © 2013 IEEE

SWIDERSKI et al.: HIGH POWER SUPERCONTINUUM GENERATION 151

Fig. 2. Typical output pulse train of the 2-μm pump laser system operatingat 26 kHz repetition rate and delivering maximum output power of 4.8 W. Theinset shows three highest sub-pulses from the train of the recorded amplifiedgain-switched pulse.

(signal input port: 10/125 μm, 0.15/0.46 NA; pump inputports: 105/125 μm, 0.22 NA; output port: passive double-clad25/250 μm fiber, 0.11/0.46NA). The output of the LMA TDFwas first equipped with an in-home made mode field adaptor(fiber taper) with a 25/250 μm (0.11/0.46 NA) fiber at the inputand 11/125 μm (0.11 NA) fiber at the output, providing themode conversion to a single mode fiber (0.5-m long SM2000).All the pump system components were fusion spliced, thusmaking it all-fiber. Finally, the 2-μm amplified pulses werelaunched into a nonlinear fiber.

The maximum average output power provided by the 2-μmlaser system, measured at the SM2000 fiber output, was 4.8 W.Fig. 2 presents a typical output pulse at 26 kHz repetitionrate. The amplified gain-switched pulse envelope containeda train of 20-25 sub-pulses exactly spaced at the TDFLcavity round-trip time (frequency, 190 MHz). The durationof the fully modulated, most intense mode-locked-resemblingpulses varied from ∼500 to 900 ps and their origin can beidentified as the beating of laser longitudinal modes [20], [21].The maximum energy of the gain-switched pulse (calculatedby dividing the average output power by the repetition rate)was 185 μJ, whereas the energy of the highest peak in thetrain was 22.3 μJ. The peak power of the three highest sub-pulses, estimated by assuming a Gaussian shape of each sub-pulse, was determined to be 30.5 kW, 19.5 kW and 18 kW(inset in Fig. 2). It goes to show that applying a suitablydesigned gain-switched TDFL followed by a TDFA it is possi-ble to obtain high peak power pulses which, in the context ofSC generation, may reduce the dependence of zero dispersionwavelength (ZDW) on SC spectrum broadening [22].

In the first experiment two ZBLAN fibers of differentparameters were used. The first one marked as ZBLAN-1had a length of 20 m, a core/clad diameter of 6.8/125 μm,a NA of 0.23, a ZDW at 1.9 μm and a cut-off wavelengthλcut−off at 2.04 μm. The other one, annotated as ZBLAN-2,had a length of ∼7 m and was characterized by a core/claddiameter of 8/125 μm, a NA of 0.3, a ZDW at 1.54 μm anda λcut−off at 3.15 μm. The silica and fluoride fibers were

Fig. 3. SC emission spectrum generated in the ZBLAN-1 fiber. Top insetshows the attenuation curve of the fiber and bottom inset shows SC powerevolution in different spectral bands as a function of lunched pump power.

Fig. 4. Spectral emission recorded at the output of the ZBLAN-2 fiber. Topinset shows the attenuation curve of the fiber and bottom inset shows SCpower evolution in different spectral bands versus lunched pump power.

coupled together using lenses with an efficiency of ∼60%.Furthermore, the coupling ends of SM2000 and fluoride fiberswere angle cleaved to eliminate back reflections.

The SC radiation, generated out of the fluoride fibers, wasdirected to detection systems covering the wavelength rangeunder investigation. For 1200-2400 nm spectral region weused an optical spectrum analyzer (Yokogawa, AQ6375), towhich optical signal was delivered by a multimode fluoridefiber. For longer wavelengths the light was focused at theentry slit of a grating monochromator (Princeton Instruments,SP-2300) with a thermo-electrically cooled HgCdTe detector(PVI-3TE, VIGO System). In order to suppress high orderdiffraction peaks, appropriate band pass filters placed beforethe measurement system were used.

The output spectra, generated from the ZBLAN fibers, formaximum recorded output SC power, are presented in Fig. 3and Fig. 4. The bottom insets present SC average output

152 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 26, NO. 2, JANUARY 15, 2014

power recorded in different bands versus launched pumppower whereas upper insets show the attenuation curves ofthe fluoride fibers.

In case of ZBLAN-1 fiber the spectrum covers more thanone octave in frequency, that is from ∼1.8 to 4.15 μm with aresidual peak at the pump wavelength (Fig. 3). The maximumaverage output power was measured to be 1.25 W, for ∼2.2 Wof launched pump power. It can be noticed that 48.5% ofthe power (0.6 W) corresponded to wavelengths longer than2.4 μm and 24.6% (0.31 W) of the power was detected forwavelengths longer than 3 μm. The dip in the spectrum around2.8 μm corresponds to OH ion absorption in the nonlinearmedium and detection system. The 10 dB spectral flatness wasmaintained in the range of ∼2 to 3.8 μm. The pump peak atλ =1994.5 nm was not included into the calculation of thebandwidth. Another characteristic feature of the spectrum isthat it is effectively broadened to ∼3.8 μm and then the signalintensity falls rapidly, which can be attributed to the increasingfiber attenuation >100 dB/km at ∼4 μm and >1000 dB/kmat 4.5 μm. This can be also confirmed by decreasingthe power slope efficiency for higher pump power levels(bottom inset in Fig. 3). Besides, due to the fact that thefluoride fiber was coiled on a 20-cm diameter spool a fiberbend-induced loss contributed to the overall attenuation [9],which has been shown to be significant for the spectrum red-shifting even for a fiber coil radius of 40 cm [17].

By replacing the ZBLAN-1 fiber with a ∼7-m longZBLAN-2 fiber, SC extending from ∼1.8 to 3.6 μm with1.82 W of average power, for ∼2.2 W of pump power launchedinto the fiber is generated (Fig. 4). The output power forλ > 2.4 μm is 0.71 W with a power ratio of 39% withrespect to the total SC power. The long-wavelength edge ofthe spectrum is shorter than that recorded for the ZBLAN-1fiber, but nevertheless the two spectra are similar. The lessefficient spectral extension can be explained by much highermaterial losses of the ZBLAN-2 (top inset in Fig. 4). Forinstance, the attenuation of the ZBLAN-2 fiber at 2.5 μm and3 μm was ∼90 dB/km and ∼200 dB/km, respectively, whereasthe attenuation for the ZBLAN-1 for these wavelengths was∼1.5 dB/km and ∼80 dB/km.

In the second experiment an 8-m long indium fluoride fiberwith a core/clad diameter of 16.7/125 μm, a NA of 0.17 anda ZDW at 1.83 μm was used as a nonlinear medium. It wasmechanically spliced with the SM2000 fiber providing > 90%of coupling efficiency. Furthermore, to remove the residualcladding modes the exposed InF3 fiber clad was covered withhigh refractive index optical glue.

The spectrum measured at the fiber output is plotted inFig. 5. The top inset shows the power scaling of the SCsource in different spectral regions and the bottom inset depictsfiber attenuation. The output spectrum extended from ∼1.7to 3 μm, measured at 45 dB below the peak. Contrary tothe spectra measured for the ZBLAN fibers, the continuumis not so broad and its long wavelength tail steeps muchmore slowly, which probably results from high overall materiallosses comparing with the ZBLAN fibers. Furthermore, sincethe nonlinear coefficient γ is inversely proportional to theeffective mode area Aeff of a nonlinear waveguide, the spectral

Fig. 5. SC emission spectrum generated in the fluoroindate fiber . Top insetshows SC power evolution in different spectral bands versus lunched pumppower and bottom inset presents the attenuation curve of the fiber.

extension was also affected by large core diameter of the fiber.In our case the γ parameter for the InF3 fiber was approxi-mately 4 times lower comparing to the fluorozirconate basedfibers.

For pump power of 2.85 W, the output SC power scaledup linearly (top inset in Fig. 5) reaching the level of 1.02 W(166 mW for λ > 2.4 μm). When we increased the pumppower to 3.26 W we achieved 1.17 W of output power butunexpectedly at this power level, fiber damage occurred at∼65 cm distance from the fiber input. The cause of this failureis not clear, because power handling limit for fluoride fiberswas reported to be over 10 W [17]. It is worth adding thatthe fiber end facet was unaffected. Therefore, we believe thatthe damage was caused by fiber coating degradation or thepresence of some absorbing inclusions inside the fiber corearea [23].

The material ZDW of fluorozirconate fibers is reported to be∼1.63 μm [24] whereas the ZDW of the fluoroindate fibersis ∼1.8 μm [25], however the total fiber dispersion profilealso depends on fiber core diameter and NA aperture [26],thus allowing the pump wavelength to be placed in theanomalous dispersion region close to the ZDW. In our casethe ZBLAN-1, ZBLAN-2 and InF3 fibers had the ZDW at1.9 μm, 1.54 μm and 1.83 μm, respectively, which meansthat they were pumped in anomalous part of dispersion D,where dispersion slope is dD/dλ < 0. Therefore, mechanismsleading to the continuum generation in this case rely pri-marily on modulation instability leading to the pump pulsebreakup and the generation of a distributed spectrum of manysolitons that are further red-shifted when interacting with anonlinear medium [27]. It can be also noticed that in case ofInF3 fiber a small shifting of the entire spectrum to shorterwavelengths (1.5–1.8 μm) was recorded, whereas this featurewas not observed in case of ZBLAN fibers used in theexperiment. This spectrum range represents contribution fromphase-matched dispersive waves, generated at wavelengthsshorter than the ZDW of the nonlinear fiber [28]. Since we did

SWIDERSKI et al.: HIGH POWER SUPERCONTINUUM GENERATION 153

not measure dispersion characteristics for the used fluoridefibers we cannot give an adequate information about theirinfluence on SC generation process. However, as reported in[26], [29] this issue can be critical for SC spectrum red-shiftingin ZBLAN fibers.

III. CONCLUSION

In conclusion, high power mid-IR SC generation in fluo-ride (both ZBLAN and InF3) fibers pumped by 2-μm gain-switched pulses, delivered by a Tm-doped fiber laser andamplifier system, is reported. An overall SC power of upto 1.25 W in the band of ∼1.8 to 4.15 μm from the 20-mlong ZBLAN fiber with a 6.8 μm core diameter was obtained.Pumping the 7-m long ZBLAN fiber with a core diameter of8 μm it was possible to achieve 1.82 W of average SC powerin the ∼1.8–3.65 μm spectral band. Furthermore, we presentwhat is, to the best of our knowledge, the highest mid-IRsupercontinuum power, generated out of InF3 fiber pumpedat ∼2 μm. The output power and spectrum bandwidth of thedeveloped SC systems should be scalable by applying higherpump power with simultaneous shortening of the pump pulsewidth.

REFERENCES

[1] S. Dupont, C. Petersen, J. Thogersen, C. Agger, O. Bang, andS. R. Keiding, “IR microscopy utilizing intense supercontinuum lightsource,” Opt. Express, vol. 20, no. 5, pp. 4887–4892, Feb. 2012.

[2] K. Ke, C. Xia, M. N. Islam, M. J. Welsh, and M. J. Freeman,“Mid-infrared absorption spectroscopy and differential damage invitro between lipids and proteins by an all-fiber-integrated super-continuum laser,” Opt. Express, vol. 17, no. 15, pp. 12627–12640,Jul. 2009.

[3] C. F. Kaminski, R. S. Watt, A. D. Elder, J. H. Frank, andJ. Hult, “Supercontinuum radiation for applications in chemical sens-ing and microscopy,” Appl. Phys. B, vol. 92, no. 3, pp. 367–378,Sep. 2008.

[4] P. Domachuk, et al., “Over 4000 nm bandwidth of mid-IR supercontin-uum generation in sub-centimeter segments of highly nonlinear telluritePCFs,” Opt. Express, vol. 16, no. 10, pp. 7161–7168, May 2008.

[5] D. Buccoliero, H. Steffensen, O. Bang, H. Ebendorff-Heidepriem,and T. M. Monro, “Thulium pumped high power supercontinuumin loss-determined optimum lengths of tellurite photonic crystalfiber,” Appl. Phys. Lett., vol. 97, no. 6, pp. 061106-1–061106-3,Aug. 2010.

[6] R. R. Gattass, L. B. Shaw, V. Q. Nguyen, P. C. Pureza, I. D. Aggarwal,and J. S. Sanghera, “All-fiber chalcogenide-based mid-infrared super-continuum source,” Opt. Fiber Technol., vol. 18, no. 5, pp. 345–348,Sep. 2012.

[7] C. W. Rudy, A. Marandi, K. L. Vodopyanov, and R. L. Byer, “Octave-spanning supercontinuum generation in in situ tapered As2S3 fiberpumped by a thulium-doped fiber laser,” Opt. Lett., vol. 38, no. 15,pp. 2865–2868, Aug. 2013.

[8] G. Qin, et al., “Ultrabroadband supercontinuum generation from ultra-violet to 6.28 μm in a fluoride fiber,” Appl. Phys. Lett., vol. 95, no. 16,pp. 161103-1–161103-, Oct. 2009.

[9] C. Xia, et al., “Mid-infrared supercontinuum generation to 4.5 μmin ZBLAN fluoride fibers by nanosecond diode pumping,” Opt. Lett.,vol. 31, no. 17, pp. 2553–2555, Sep. 2006.

[10] J. Swiderski and M. Michalska, “Over three-octave spanning supercon-tinuum generated in a fluoride fiber pumped by Er & Er:Yb-doped andTm-doped fiber amplifiers,” Opt. Laser Technol., vol. 52, pp. 75–80,Nov. 2013.

[11] T. Izawa, N. Shibata, and A. Takeda, “Optical attenuation in pure anddoped fused silica in the IR wavelength region,” Appl. Phys. Lett.,vol. 31, no. 33, pp. 33–35, Jul. 1977.

[12] M. Eckerle, C. Kieleck, J. Swiderski, S. D. Jackson, G. Maze, andM. Eichhorn, “Actively Q-switched and mode-locked Tm3+-dopedsilicate 2 μm fiber laser for supercontinuum generation in fluoride fiber,”Opt. Lett., vol. 37, no. 4, pp. 512–514, Feb. 2012.

[13] J. Swiderski, M. Michalska, and G. Maze, “Mid-IR supercontinuumgeneration in a ZBLAN fiber pumped by a gain-switched mode-lockedTm-doped fiber laser and amplifier system,” Opt. Express, vol. 21, no. 7,pp. 7851–7857, Apr. 2013.

[14] C. Agger, et al., “Supercontinuum generation in ZBLAN fibers—Detailed comparison between measurement and simulation,” J. Opt. Soc.Amer. B, vol. 29, no. 4, pp. 635–645, Apr. 2012.

[15] W. Yang, B. Zhang, K. Yin, X. Zhou, and J. Hou, “High power allfiber mid-IR supercontinuum generation in a ZBLAN fiber pumped bya 2 μm MOPA system,” Opt. Express, vol. 21, no. 17, pp. 19732–19742,Aug. 2013.

[16] O. P. Kulkarni, et al., “Supercontinuum generation from ∼1.9 to 4.5 μmin ZBLAN fiber with high average power generation beyond 3.8 μmusing a thulium-doped fiber amplifier,” J. Opt. Soc. Am. B, vol. 28,no. 10, pp. 2486–2498, Oct. 2011.

[17] C. Xia, et al., “10.5 W time-averaged power mid-IR supercontin-uum generation extending beyond 4 μm with direct pulse patternmodulation,” IEEE J. Sel. Topics Quantum Electron., vol. 15, no. 2,pp. 422–434, Mar. 2009.

[18] M. Kumar, et al., “Stand-off detection of solid targets with diffusereflection spectroscopy using a high-power mid-infrared supercontinuumsource,” Appl. Opt., vol. 51, no. 15, pp. 2794–2807, May 2012.

[19] A. M. Heidt, et al., “Mid-infrared ZBLAN fiber supercontinuum sourceusing picosecond diode-pumping at 2 μm,” Opt. Express, vol. 21, no. 20,pp. 24281–24287, Oct. 2013.

[20] P. Myslinski, J. Chrostowski, J. A. K. Koningstein, and J. R. Simpson,“Self-mode locking in a Q-switched erbium-doped fiber laser,” Appl.Opt., vol. 32, no. 3, pp. 286–290, Jan. 1993.

[21] J. Swiderski and M. Michalska, “Generation of self-mode-locked resem-bling pulses in a fast gain-switched thulium-doped fiber laser,” Opt. Lett.,vol. 38, no. 10, pp. 1624–1626, May 2013.

[22] C. Larsen, S. T. Sorensen, D. Noordegraaf, K. P. Hansen, K. E. Mattsson,and O. Bang, “Zero-dispersion wavelength independent quasi-CWpumped supercontinuum generation,” Opt. Commun., vol. 290,pp. 170–174, Mar. 2013.

[23] M. Eichhorn, “Numerical modeling of Tm-doped double-clad fluo-ride fiber amplifiers,” IEEE J. Quantum Electron., vol. 41, no. 12,pp. 1574–1581, Dec. 2005.

[24] P. W. France, S. F. Carter, M. W. Moore, and C. R. Day, “Progress influoride fibres for optical communications,” Brit. Telecommun. Technol.J., vol. 5, no. 2, pp. 28–44, Apr. 1987.

[25] A. Soufiane, F. Can, H. L’Helgouach, and M. Poulain, “Material disper-sion in optimized fluoroindate glasses,” J. Non-Cryst. Solids, vol. 184,pp. 36–39, May 1995.

[26] I. Kubat, C. S. Agger, P. M. Moselund, and O. Bang, “Mid-infraredsupercontinuum generation to 4.5 μm in uniform and tapered ZBLANstep-index fibers by direct pumping at 1064 or 1550 nm,” J. Opt. Soc.Am. B, vol. 30, no. 10, pp. 2743–2757, Oct. 2013.

[27] M. N. Islam, G. Sucha, I. Bar-Joseph, M. Wegener, J. P. Gordon, andD. S. Chemla, “Femtosecond distributed soliton spectrum in fibers,”J. Opt. Soc. Amer. B, vol. 6, no. 6, pp. 1149–1158, Jun. 1989.

[28] D. V. Skryabin and A. V. Yulin, “Theory of generation of new frequen-cies by mixing of solitons and dispersive waves in optical fibers,” Phys.Rev. E, vol. 72, no. 1, pp. 016619-1–016619-10, Jul. 2005.

[29] S. Dupont, P. M. Moselund, L. Leick, J. Ramsay, and S. R. Keiding,“Up-conversion of a megahertz mid-IR supercontinuum,” J. Opt. Soc.Amer. B, vol. 30, no. 10, pp. 2570–2575, Oct. 2013.