3628 OPTICS LETTERS / Vol. 34, No. 23 / December 1, 2009

Supercontinuum Q-switched Yb fiber laser usingan intracavity microstructured fiber

J. Cascante-Vindas, A. Díez,* J. L. Cruz, and M. V. AndrésDepartamento de Física Aplicada—ICMUV, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Spain

*Corresponding author: antonio.diez@uv.es

Received July 24, 2009; revised October 16, 2009; accepted October 26, 2009;posted November 2, 2009 (Doc. ID 114728); published November 19, 2009

We report on an intracavity configuration for supercontinuum generation in a Q-switched Yb fiber laser. Thesupercontinuum laser includes a section of microstructured fiber within the Q-switched laser cavity. With380 mW of pump power, the supercontinuum laser can emit broadband pulses of 6 �J energy and 10 nstemporal width, at repetition rates from few hertz up to 2 kHz. The supercontinuum spectrum spans over awavelength range in excess of 1.4 �m. © 2009 Optical Society of America

OCIS codes: 060.4005, 060.4370, 060.3510.

The most common scheme for supercontinuum (SC)generation is based on the use of a nonlinear opticalfiber pumped near its zero-dispersion wavelength(ZDW) by short light pulses. The advent of the micro-structured optical fibers (MOFs) [1] opened new pos-sibilities, since silica MOFs with small effective areaand ZDW well below 1.3 �m can be manufactured,which facilitates the extension of the spectrum to-ward visible wavelengths [2]. At present, low-cost su-percontinuum sources rely on an MOF pumped by asubnanosecond microchip laser [2,3], with the com-mon disadvantages of integrating a bulk laser and fi-ber components. In addition to alignment and me-chanical stability issues, an important limitationarises from the damage threshold of the input facet ofthe MOF that limits the peak power that can belaunched into the fiber, thus limiting the spectralbroadening and power density available at the outputof the fiber.

An interesting alternative is the use of fiber laserscombined with intracavity SC generation. The intra-cavity technique for broadband generation in the con-text of fiber lasers was first demonstrated in [4]where IR radiation due to cascaded Raman scatter-ing was reported within the multiwatt optical pump-ing scale. Recently, a similar configuration of a SC la-ser based on an Yb-doped nonlinear MOF wasdemonstrated [5], though the reported results weremodest and only light in the short IR �1000–1200 nm� and the visible �650–750 nm� domains wasgenerated. Here, we report our investigations on adiode-pumped Q-switched Yb-doped fiber laser emit-ting over a broadband spectrum using pump powersin the order of few hundreds of milliwatts. Intracav-ity SC generation was achieved by the insertion of anMOF section with proper chromatic dispersion prop-erties within the Q-switched laser cavity. In contrastto the previous reports, in our configuration the gainand the nonlinear media are different fibers.

Figure 1 shows the experimental arrangement ofthe SC Q-switched Yb fiber laser. A Fabry–Perot lasercavity was arranged using 2.5 m of single-modesingle-clad Yb-doped fiber (Nufern SM-YSF-HI) anda pair of fiber Bragg gratings (FBGs) whose Bragg

wavelength was 1064 nm. The bandwidth of the

0146-9592/09/233628-3/$15.00 ©

FBGs was 0.1 nm, and they were chosen with highreflectivity, �99%, to enhance the energy of theQ-switched pulses inside the laser cavity. The pair ofgratings provides feedback for the 1064 nm signal,which acts as a pump for the nonlinear processes,while other spectral components generated withinthe cavity will exit the laser with full transmission.The Yb-doped fiber was pumped through awavelength-division multiplexer (WDM) with a pig-tailed laser diode emitting at 976 nm, which provideda maximum pump power of 600 mW. Q-switchingwas achieved by using a pigtailed acousto-opticmodulator (3 dB insertion loss, 25 ns rise time).

The inset of Fig. 1 shows a scanning electron mi-croscope image of the MOF used in the experiments.The structural parameters of the MOF are �=5.7 �m, d /�=0.9, and 4.5 �m core diameter, where� is the pitch and d is the hole diameter. The MOFsupported several modes at 1064 nm. The ZDW of thefundamental mode was 1030 nm, so its dispersionwas anomalous at 1064 nm. The MOF section �3 m�was inserted between the acousto-optic modulator(AOM) and the output FBG. Because of the insertionloss of the AOM, placing the MOF in the opposite sideof the AOM, i.e., between the gain fiber and the AOM,would lead to stronger pulses in the MOF. However,the bandpass response of the AOM would filter mostof the spectral components of the generated SC. Thesetup shown in Fig. 1 has another advantage; therather narrow bandpass filter response of the AOM inconjunction with the wavelength response of theWDM prevents SC and amplified spontaneous emis-sion signals feeding back to the pump laser, which is

Fig. 1. SC Q-switched laser arrangement.

2009 Optical Society of America

December 1, 2009 / Vol. 34, No. 23 / OPTICS LETTERS 3629

essential to ensure good pulse-to-pulse and long-termstability of the emission.

The MOF was fusion spliced to the fiber pigtails ofthe output FBG and the AOM, both made of HI1060fiber, using a commercial arc fusion splicer (FujikuraFSM-20CSII). The discharge parameters, i.e., arcpower and arc duration, were set to minimize theholes’ collapse. The discharge current was 11.5 mA,which is approximately one third of the typical arcpower values used for splicing SMF-28 fibers, and thearc duration was 300 ms. Once the two fibers werefused together, several arcs were discharged on thesplice with the aim of reducing the holes’ diameter,and so reducing the mode field diameter mismatch.Especial attention was paid when doing those splicesin order to favor the excitation of the fundamentalmode in the MOF. The total insertion loss was about1 dB, including the MOF transmission loss and thesplice losses.

Figure 2 shows the spectral and temporal charac-teristics of the output radiation when the laser wasrunning at 1 kHz repetition rate. At the pump powerof 380 mW, the output pulse energy was 6.4 �J andthe temporal width was 10 ns, with an estimated in-tracavity peak power of about 2.5 kW. The pulse-to-pulse amplitude fluctuation remained below 5%. Thegenerated spectrum spans from 400 nm to 2000 nm,showing a very flat region from 1.2 to 1.6 �m, wherethe spectral flatness is ±0.5 dB. Output pulses withsimilar energy and spectral characteristics thanshown in Fig. 2 were achieved at repetition ratesfrom few hertz up to 2 kHz. Of course, a change inrepetition rate required adjusting the pump power in

Fig. 2. (Color online) (a) SC output spectrum for differentpump powers of 280, 320, and 380 mW. Black and gray (redonline) traces were measured with different optical spec-trum analyzers [ANDO AQ6314A (10 nm resolution) andAQ6375 (2 nm resolution), respectively]. (b) Output pulsemeasured with a 1 GHz bandwidth InGaAs photodetector.

Pump power, 380 mW.

order to optimize the performance of the system.Higher repetition rates required higher pump pow-ers. For example, at 2 kHz repetition rate, the pumppower required to obtain 6 �J energy pulses was420 mW.

The output of the SC radiation was emitted mostlyin the fundamental mode. Although the output radia-tion was emitted from a conventional fiber (HI-1060)whose cutoff wavelength is about 920 nm, the moded-ness of the SC radiation did not change significantlywith respect to the generated in the MOF, since thelength of the HI-1060 fiber was short (�50 cm) andbending was avoided. The observation of the visiblecomponents showed that only the blue component—corresponding to the peak centered at 478 nm, seeFig. 2—was generated in a higher-order mode, as aresult of multimode phase-matching in the MOF.

Figure 3(a) shows the short and long wavelengthedges of the SC spectrum measured at −20 dB, as afunction of the pump power of the Q-switched laser.The corresponding pulse energy measured with apiroelectric power meter is shown in Fig. 3(b). An SCspectrum spanning almost two octaves was gener-ated with 380 mW of pump power. Broader SC couldnot be generated, since higher pump powers led tounstable operation of the SC Q-switched laser. We be-lieve that at higher pump powers, the high cavitygain in conjunction with unwanted small back-reflections—for example, from splices between dis-similar fibers—led the laser to reach the cw emissionthreshold even when the AOM was off.

The spectral features observed in the SC spectrumat different stages of its development agree with thetypical nonlinear mechanisms reported for extracav-ity SC arrangements where an MOF is pumped withnanosecond duration pulses in anomalous dispersionregime and near the ZDW [3,6,7]. At low pump pow-ers, modulation instability (MI) bands were observed.At 280 mW of pump power [see Fig. 2(a)], the MIbands are separated about 22 nm from the 1064 nmsignal. At higher pump powers, it has been shownthat the development of MI leads to the formation ofsolitons [8] that propagate in the fiber experiencingself-frequency shift (SFS), owing to Raman scattering[7–9], and generating dispersive waves (DWs)[10,11]. SFS of solitons is responsible for the spread-

Fig. 3. (a) SC wavelength edges measured at −20 dB and

(b) pulse energy, as a function of pump power.

3630 OPTICS LETTERS / Vol. 34, No. 23 / December 1, 2009

ing of the SC into the IR, while the generation ofDWs extends the SC to the blue. Recently, it has beendemonstrated experimentally that the long- andshort-wavelength edges of the SC are related by thegroup index of the fiber [2,12]. A possible mechanismhas been pointed out in [13]: the SFS of a solitonpropagating in anomalous dispersion effectivelytraps blue radiation propagating with the samegroup index in a gravity-like potential and scattersthe blue radiation to shorter wavelengths in a cas-caded FWM-like process.

We found that the intracavity configuration pre-sents some advantages with respect to the conven-tional SC generation extracavity scheme usingMOFs. First, provided that the MOF insertion loss issmall, the pulse energy inside the laser cavity islarger than the laser’s output owing to the reflectiv-ity of the output grating. This fact has been furtherenhanced in our arrangement by using highly reflec-tive FBGs. As a result, broader SC spectrum can beobtained for a given 976 nm pump level. Second, theSC exits the system guided by a standard single-mode fiber, leading to a better beam shape quality.And third, the use of a highly reflective FBG as thelaser’s output coupler reduces dramatically the pres-ence of residual 1064 nm signal in the SC spectrum.This effect is shown in Fig. 4, where the output spec-trum around 1064 nm wavelength is compared withthe SC spectrum obtained with the conventional SCgeneration scheme. In the later case, the same pieceof MOF was pumped with a subnanosecond micro-chip laser (Teem Photonics SNP-20F-100); the pumppower was adjusted to obtain an SC spectrum with

Fig. 4. SC spectrum showing the residual 1064 nm signal,for (a) the intracavity SC Q-switched configuration and (b)the extracavity SC generation scheme. Both spectra wererecorded with 1 nm resolution.

similar span to that shown in Fig. 2. The average

pump power required was 40 mW, which correspondsto a pulse peak power of about 3.4 kW.

Finally, although being an intracavity arrange-ment, the limitation arising from the fiber end facetsdamage is not completely avoided because of the useof the bulk AOM for Q-switching.

In summary, we have reported on the intracavitysupercontinuum generation in a Q-switched Yb-doped fiber that incorporates a section of an MOFwithin the laser cavity. With pump powers in the or-der of few hundreds of miliwatts, the supercon-tinuum laser can emit broadband pulses of 6 �J en-ergy and 10 ns temporal width, at tunable repetitionrates from few hertz up to 2 kHz. The spectrum ofthe emitted pulses spans over a wavelength range inexcess of 1.4 �m. Additionally, the residual pump sig-nal is reduced significantly as a result of using FBGswith high reflectivity. Research is currently inprogress to increase the pulse energy and hence theextension of the supercontinuum, as well as to extentthe repetition rate to higher frequencies.

This work has been financially supported by theMinisterio de Ciencia e Innovación and the Generali-tat Valenciana of Spain (projects TEC 2008-05490and PROMETEO/2009/077, respectively).

References

1. P. St. J. Russell, Science 299, 358 (2003).2. J. M. Stone and J. C. Knight, Opt. Express 16, 2670

(2008).3. W. Wadsworth, N. Joly, J. Knight, T. Birks, F.

Biancalana, and P. Russell, Opt. Express 12, 299(2004).

4. S. V. Chernikov, Y. Zhu, and J. R. Taylor, V. P.Gapontsev, Opt. Lett. 22, 298 (1997).

5. A. Roy, M. Laroche, P. Roy, P. Leproux, and J. L.Auguste, Opt. Lett. 32, 3299 (2007).

6. J. M. Dudley, G. Genty, and S. Coen, Rev. Mod. Phys.78, 1135 (2006).

7. J. M. Dudley, L. Provino, N. Grossard, H. Maillotte, R.S. Windeler, B. J. Eggleton, and S. Coen, J. Opt. Soc.Am. B 19, 765 (2002).

8. S. M. Kobtsev and S. V. Smirnov, Opt. Express 13,6912 (2005).

9. G. Genty, M. Lehtonen, H. Ludvigsen, J. Broeng, andM. Kaivola, Opt. Express 10, 1083 (2002).

10. N. Akhemediev and M. Karlsson, Phys. Rev. A 51, 2602(1995).

11. G. Genty, M. Lehtonen, H. Ludvigsen, and M. Kaivola,Opt. Express 12, 3471 (2004).

12. L. Fu, B. K. Thomas, and L. Dong, Opt. Express 16,19629 (2008).

13. A. V. Gorbach and D. V. Skryabin, Nat. Photonics 1,

653 (2007).