We show an actively Q-switched ytterbium-doped strictly all-fiber laser. Cavity loss modulation is achieved in a tapered optical fiber by core-to-cladding mode-coupling induced by travelling flexural acoustic waves. When the acoustical signal is switched-off, the optical power losses within the cavity are reduced, and then a laser pulse is emitted. Trains of Q-switched pulses were successfully obtained at repetition rates in the range 1-10 kHz, with pump powers between 59 and 88 mW, at the optical wavelength of 1064.1 nm. Best results were for laser pulses of 118 mW peak power, 1.8 μs of time width, with a pump power of 79 mW, at 7 kHz repetition rate.Fil: Villegas García, Irma Lorena. Universidad de Valencia; España. Centro de Investigaciones en Optica; MéxicoFil: Cuadrado Laborde, Christian Ariel. Universidad de Valencia; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Ópticas. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Centro de Investigaciones Ópticas. Universidad Nacional de La Plata. Centro de Investigaciones Ópticas; ArgentinaFil: Díez, Antonio. Universidad de Valencia; EspañaFil: Cruz, Jose Luis. Universidad de Valencia; EspañaFil: Martínez Gámez, M. A.. Centro de Investigaciones en Óptica; MéxicoFil: Andrés Bou, Miguel Vicente. Universidad de Valencia; Españ

Andrés Bou, Miguel Vicente

Cruz, Jose Luis

Cuadrado Laborde, Christian Ariel

Díez, Antonio

Martínez Gámez, M. A.

Villegas García, Irma Lorena

Laser Physics

English

LAReferencia - Red Federada de Repositorios Institucionales de Publicaciones Científicas Latinoamericanas

ISSN 1054660X, Laser Physics, 2011, Vol. 21, No. 9, pp. 1650–1655.© Pleiades Publishing, Ltd., 2011.Original Text © Astro, Ltd., 2011.16501 1. INTRODUCTIONQswitched fiber lasers have attractive applicationsin different fields, such as in medicine, remote sensingand material processing [1]. The mechanism of shortpulse emission is based on the modulation of the Qfactor of the cavity [2, 3], which can be done eitherpassively or actively. In the former the setups are simpler, but the repetition rate only varies with the pumppower of the medium gain [4, 5]. Further, they usuallyshow longterm instability, and frequently the amplitudes of the pulses are randomly modulated in time[6]. On the other hand, active Qswitching is independently and accurately controlled by an electrical signal, which triggers the modulator. The bulk approach,such as by electrooptic [7] or acoustooptic modulators [8], is not adapted to compact fiber laser systemsrequired nowadays; further they have large opticalcoupling losses and stringent alignment requirements[9]. It is also true that fiberpigtailed packaged electrooptic or acoustooptic modulators [10–12] could beused; with consequently lower power insertion losses,but still remaining relatively high (typically higherthan 3 dB). For these reasons, the allfiber approach isof permanent interest, being advantageous in terms ofcost, loss, packaging, robustness, and simplicity. Onesolution widely investigated has been enabled by theuse of fiber Bragg gratings (FBGs) as cavity mirrors[13]. In this way, the tuning of the wavelength of one ofthe FBGs has been used to achieve active Qswitching[14]. Among the typical tuning methods, we could1  The article is published in the original.mention the stretching of fiber Bragg gratings by magnetostrictive materials [15, 16], by piezoelectric actuators [17, 18], or by the interaction of longitudinalacoustic waves with the FBG [19, 20]. Other technique used for active Qswitching relied in the cavityloss modulation by the coretocladding modecoupling in optical fibers induced by travelling flexuralacoustic waves [21, 22].In this work we present an actively Qswitchedstrictly allfiber laser, where cavity loss modulation isachieved by intermodal modulation induced by travelling flexural acoustic waves in a tapered optical fiber.As opposed to [21, 22] which were focused at theerbium lasing wavelength, here we experimentallydemonstrate the possibility of using this modulatingtechnique at the technologically relevant ytterbiumlasing wavelength [10–12, 23–25]. The use of ytterbiumdoped fiber as a medium gain in fiber lasers is animportant subject of research also, among other reasons, because it has a high absorption at 976 nm, andfor this wavelength there is available a large range offiberpigtailed diode pump lasers with different characteristics. As the lasing band is very close to the pumpwavelength, then it also has among the highest quantum efficiencies (~95%). Very high doping concentrations are possible also, enabling very high single passgains and high slope efficiencies. However, very fewactively Qswitched ytterbiumdoped lasers with anallfiber configuration have been reported [16]. Themain difficulties arise from a relatively long timeresponse and a limited modulation depth of allfiberamplitude modulators. Here, we present the experiFIBEROPTICSYbDoped Strictly AllFiber Laser Actively QSwitchedby Intermodal AcoustoOptic Modulation1I. L. Villegasa, b, C. CuadradoLabordea, c, A. Díeza, J. L. Cruza, M. A. MartínezGámezb, and M. V. Andrésa, *a Departamento de Física Aplicada, ICMUV, Universidad de Valencia, Dr. Moliner 50, Burjassot E46100, Spainb Centro de Investigaciones en Óptica, Loma del Bosque 115, Col. Lomas del Campestre, León 37150, Guanajuato, Mexicoc CONICET, P.O. Box 3, Gonnet 1897, Buenos Aires, Argentina*email: Miguel.Andres@uv.esReceived March 16, 2011; in final form, March 21, 2011; published online August 3, 2011Abstract—We show an actively Qswitched ytterbiumdoped strictly allfiber laser. Cavity loss modulation isachieved in a tapered optical fiber by coretocladding modecoupling induced by travelling flexural acousticwaves. When the acoustical signal is switchedoff, the optical power losses within the cavity are reduced, andthen a laser pulse is emitted. Trains of Qswitched pulses were successfully obtained at repetition rates in therange 1–10 kHz, with pump powers between 59 and 88 mW, at the optical wavelength of 1064.1 nm. Bestresults were for laser pulses of 118 mW peak power, 1.8 µs of time width, with a pump power of 79 mW, at7 kHz repetition rate.DOI: 10.1134/S1054660X11170221LASER PHYSICS  Vol. 21  No. 9  2011YbDOPED STRICTLY ALLFIBER LASER 1651mental results that we have obtained in a research workfocused on the exploitation of infiber acoustooptics.In our proposed Qswitched fiber laser, when theacoustical signal is switchedoff, the optical powerlosses within the cavity are reduced, and then a laserpulse is emitted. With this configuration we obtainedtrains of Qswitched pulses in the range 1–10 kHz at1064.1 nm. Best results were for laser pulses of 120 mWpeak power, 1.8 μs of time width, for a pump power of83 mW, and 7 kHz of repetition rate.2. LASER SETUP AND QSWITCHING TECHNIQUEThe setup used for our Qswitched fiber laser isschematically illustrated in Fig. 1. The gain was provided by 0.65 m of a heavilydoped ytterbiumdopedsinglemode fiber—Nufern SMYSFHI, cutoffwavelength of 860 ± 70 nm, numerical aperture of0.11, and core absorption of 250 dB/m at 975 nm. Theactive fiber was pumped through a WDM coupler by apigtailed laser diode emitting at 980 nm, providing amaximum pump power of 110 mW. The infiberacoustooptic modulator (AOM) was spliced betweenFBG1—Bragg wavelength at 1064 nm, FWHM of0.23 nm, and 99.6% of maximum reflectivity—andFBG2—Bragg wavelength at 1064 nm, FWHM of70 pm, and 44% of maximum reflectivity, defining inthis way a Fabry–Perot cavity. The AOM in turn iscomposed of an RF source, a transversalmode piezoelectric disk, an aluminum horn, and a tapered singlemode optical fiber—Fibercore SM980 of low numerical aperture (0.13–0.15). The optical fiber wastapered down by the fusion and pulling techniqueusing a travelling flame, to obtain a taper waist with auniform diameter of 76 μm and 0.1 m length, for thisspecific case. The tip of the aluminum cone—with thepiezoelectric disc fixed to its base—was glued to anuncoated section of fiber near the taper, see Fig. 1.Finally, the optical fiber in the AOM was acousticallydumped in both extremes, in order to preventunwanted acoustical reflections.When an RF signal is applied to the piezoelectricdisc, a travelling flexural acoustic wave is launchedthrough the taper. If the acoustic wavelength matchesthe beatlength between the fundamental modeguided by the core and one of the optical modes supported by the cladding, then light coupled to the laterremains in the cladding downstream of the taper,being finally absorbed by the fiber coating [26]. Thus,the coupling of power from the fundamental mode toa cladding mode results in the appearance of an attenuation notch in the spectrum. When the acoustic frequency is varied, the periodicity of the perturbationalso does, and hence the phasematching condition isshifted to a different optical wavelength. Figure 2ashows the tunability of the notches caused by the coupling between the fundamental core mode and the firstthree cladding modes LP1m. The selected operatingpoint was at an optical wavelength of 1064.1 nm for anRF signal applied to the piezoelectric of 825 kHz.These measurements were made by illuminating thetaper with a broadband light source and detecting thelight transmitted through the taper with an opticalspectrum analyzer. As an example, the inset of Fig. 2ashows one of the measured transmission spectra for anapplied voltage to the piezoelectric of 26 V and a frequency of 885 kHz. The transfer of optical power fromthe fundamental core mode to one of the claddingmodes behaves periodically as a function of the acoustic power, which in turn is a function of the appliedvoltage to the piezoelectric. Figure 2b shows this effectfor the selected operation point marked in Fig. 2a. Asit can be observed, the transmittance decays by a maximum of 12 dB for an applied voltage to the piezoelectric of 50 V (peaktopeak measurement). Beyond thispoint, further increment in the applied voltage raisesthe transmittance again. Therefore, and bearing inmind its use as a Qswitching device, cavity loss modulation between 0 and 12 dB can be achieved by applying to the piezoelectric a sinusoidal signal at the frePump laser(980 nm)WDM FBG1YbdopedfiberRF sourceand amplifierADHornPiezoelectricADTaperAOMFBG2WDMOutputFig. 1. Qswitched fiberlaser setup. The acoustooptic modulator (AOM) is defined by the elements inside the dashed line; ADstands for acoustic dumper.1652LASER PHYSICS  Vol. 21  No. 9  2011VILLEGAS et al.quency of 825 kHz fullymodulated by a rectangularsignal, see the inset of Fig. 2b. This modulation produced onoff periods of the acoustic wave travel downthe fiber, which results in a modulation of the cavitylosses at the resonance wavelength. In passing, weshould mention that several tapers were fabricated inorder to better optimize this laser system, by usingFibercore SM980 of high numerical aperture (0.17–0.19), and Nufern SMYSFLO (a moderately ytterbiumdoped fiber), and in turn with different taperwaists. Despite this, the minimal decay in transmittance was always around the reported values, i.e.,between 10 and 16 dB.The switching time is one of the key parameters forany modulator intended to be used as a Qswitchingdevice in a fiber laser. Preferably, it should be as shortas possible. For this reason we measured the temporalresponse of this device, by detecting the transmittedlight, while simultaneously registering the modulatingsignal in an oscilloscope. It takes 25 μs toincrease/decrease the transmitted optical powerthrough the taper when the RF voltage applied to thepiezoelectric is switchedoff/on, respectively. Thistime corresponds reasonably well with the time it takesa flexural acoustical wave to travel down the taper, i.e.,0.1 m/3764 m/s ≅ 26 μs. This value is short enough toallow the use of this modulator as a Qswitching devicein a fiber laser, as it will be demonstrated in the following section. In principle, and for a given taper waist,shorter switching times could be achieved by decreasing the interaction length. Unfortunately, this simultaneously increases the acoustical power needed toreach the same optical coupling power, therefore thereis a tradeoff between both parameters.In summary, the basic parameters of the acoustooptic modulator are a time response of 25 μs and amodulation depth of about 12 dB. Although these values are modest for Qswitching applications, wedecided to set up the laser and investigate the characteristics of the optical pulses achievable with this typeof modulator when using an ytterbiumdoped fiber.3. THE ACTIVELY QSWITCHED YbDOPED ALLFIBER LASERIn this setup, both gratings are permanently tunedto the same wavelength by using two translationalstages, one for each FBG; see the setup in Fig. 1. Inthis way, for zero voltage applied to the piezoelectric,this laser emits in CW, since then the cavity losses areminimal, see the transmittance in Fig. 2b for 0 V. Figure 3 shows the optical power in CW emission as afunction of the pump power. There is a pump powerthreshold of 56 mW, about which the laser starts toemit, reaching a maximum output power of 3.4 mW,which in turn is determined by the maximum pumppower available in our setup (110 mW). The insetshows the spectrum of CW emission; its linewidth is of0.21 nm at a center optical wavelength of 1064.1 nm.Now, we discuss the Qswitched operation of thislaser. To this end, we modulated the cavity losses byapplying to the piezoelectric a fullymodulated sinusoidal signal at the frequency of 825 kHz and 50 V. Arectangular wave was used to modulate the RF voltagethat generates the acoustic wave. At a given frequencyof the modulating signal, we found always a maximalduty cycle—i.e., the fraction of time that the signal isin its high level—able to perform Qswitch correctly.120011501100105010009500.5 1.0 1.5 2.0 2.5 3.020−2−4−6−8−10−12−140 10 20 30 40 50 60 70Voltage, VAcoustic frequency, MHzResonance wavelength, nmTransmittance, dB0−5−10−151050 1060 1070Wavelength, nmTransmittance, dBTime, arb. unitsVoltage, arb. units(a) (b)Fig. 2. (a) Resonant optical wavelengths as a function of the acoustic frequency for the first three modecouplings. The insetshows a typical transmission notch caused by the first modecoupling by applying to the piezoelectric a sinusoidal signal at885 kHz and 26 V. (b) Transmittance at the selected operation point marked in (a)—i.e., 1064.1 nm and 825 kHz—as a functionof the applied voltage to the piezoelectric (solid scatter points), the curve represents a theoretical fitting according to a square sinefunction.Operating pointLASER PHYSICS  Vol. 21  No. 9  2011YbDOPED STRICTLY ALLFIBER LASER 1653If we decrease this duty cycle, then the cavity wouldstay in its high Q state longer, and more than one Qswitched pulse would be emitted in each time slot. TheQswitch repetition rate becomes determined by thefrequency of the modulating signal; with this configuration we reached continuous tuning of the Qswitchrepetition rate in the range 1–10 kHz. Figure 4ashows, as an example, a Qswitched optical pulsestrain at 1 kHz, together with the corresponding modulating signal, for a pump power of 59 mW. Figure 4bshows a detail of a single Qswitched optical pulse ofthe train shown in Fig. 4a with a time width (FWHM)of 3.72 μs. The pulses have a quasiGaussian profile;the fitting by this function is also shown in Fig. 4b. Theeffect of pump power on the Qswitched pulses, fordifferent repetition rates in the range 1–10 kHz, is3.53.02.52.01.51.00.5060 70 80 90 100 110Pump power, mWCW output power, mWLaser emission, arb. units1064 1065Optical wavelength, nmFig. 3. Output optical power in CW emission as a function of the pump power. The inset shows the emission spectrum.ModulatingsignalQswitchedoutputOptical power, arb. units1.0 1.5 2.0 2.5 3.0 3.5 4.0 5 10 15 20Time, msVoltage, arb. unitsOptical power, arb. units(a) (b)Fig. 4. (a) Modulating signal (above) at 1 kHz repetition rate together with the generated Qswitched laser output (below).(b) Detail of a single Qswitched pulse of (a) (scatter points) together with its corresponding fitting by a Gaussian function (solidcurve).Time, µs1654LASER PHYSICS  Vol. 21  No. 9  2011VILLEGAS et al.shown in Fig. 5. For each Qswitching frequency thereis a pump power threshold. Above threshold, the peakpower increases with pump power, and there is a corresponding reduction of pulse width. For a given frequency there is also a pump power level beyond whichthe emission is not stable and extra pulses appear; thecurves are truncated at that point. In order to overtakethis limitation, a modulator with improved modulation depth is required. In addition, our laser produceseasily multiple pulse emission, which is a wellknownproblem in actively Qswitched lasers with relativelylarge time responses. A large time response makes critical the adjustment of the duty cycle of the modulationvoltage when the pump power is increased, particularly at low repetition rates. Consequently, a fasterswitching response of the acoustooptic modulator isrequired in order to improve the operation of the laser.4. CONCLUSIONSWe have reported an actively Qswitched ytterbiumdoped strictly allfiber laser. Qswitching modulation is achieved by intermodal modulation inducedby flexural acoustic waves travelling in a tapered optical fiber. Qswitched pulses at 1064.1 nm were successfully obtained at repetition rates in the range 1–10 kHz, with pump powers between 59 and 88 mW.Best results were for laser pulses of 118 mW peakpower, 1.8 μs of time width, with a pump power of79 mW, at 7 kHz repetition rate.ACKNOWLEDGMENTSThis work has been financially supported by theMinisterio de Educación y Ciencia and the Generalitat Valenciana of Spain (projects TEC200805490 andPROMETEO/2009/077, respectively). C. CuadradoLaborde acknowledges the Secretaría de Estado deUniversidades e Investigación del Ministerio de Investigación y Ciencia (Spain).REFERENCES1. A. S. Kurkov, Ya. E. Sadovnikova, A. V. Marakulin, andE. M. Sholokhov, Laser Phys. Lett. 7, 795 (2010).2. E. M. Sholokhov, A. V. Marakulin, A. S. Kurkov, andV. B. Tsvetkov, Laser Phys. Lett. 8, 382 (2011).3. M. DelgadoPinar, A. Diez, J. L. 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Yb-doped strictly all-fiber laser actively Q-switched by intermodal acousto-optic modulation

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Yb-doped strictly all-fiber laser actively Q-switched by intermodal acousto-optic modulation

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