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High-peak-power nanosecond pulse generation by stimulated Brillouin scattering pulse compression in a seeded Yb-doped fiber amplifier M. Laroche,* H. Gilles, and S. Girard Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), ENSICAEN, CNRS, CEA/IRAMIS, Université de Caen, 14050 CAEN cedex, France *Corresponding author: [email protected] Received October 8, 2010; revised November 22, 2010; accepted November 26, 2010; posted December 6, 2010 (Doc. ID 136339); published January 12, 2011 We demonstrate the generation of nanosecond and multikilowatt peak-power pulses in a double-clad Yb-doped fiber amplifier seeded by a spectrally narrowed gain-switched laser diode. Injected pulses with 100 ns duration were simultaneously compressed and amplified by the combination of high amplifier gain and stimulated Brillouin scat- tering. A maximum peak power of 20 kW has been obtained, corresponding to a single-pass gain of þ57 dB in terms of peak power. Part of this output signal was also converted into IR continuum light by splicing a length of single- mode fiber at the end of the fiber amplifier. © 2011 Optical Society of America OCIS codes: 140.3510, 190.4370, 140.3538. Over the past few years, there has been an increasing in- terest in fiber laser sources capable of delivering high- peak-power pulses (>10 kW) in a diffraction-limited beam. High efficiency and high spatial beam quality make these laser sources suitable for applications such as micromachining, range finding, or remote sensing. A fi- ber laser can be easily Q switched using an acousto-optic or electro-optic modulator but generally at the loss of the all-fiber scheme in the case of high-peak-power genera- tion [1,2]. In addition, it is difficult to achieve a narrow linewidth or short pulse duration (<50 ns) with such fi- ber sources. Master-oscillatorpower-amplifier (MOPA) systems based on a double-clad fiber amplifier seeded by a modulated laser diode appear to be very attractive as they offer much better flexibility in terms of pulse shape, repetition rate, and spectral width [3,4]. However, owing to the relatively low peak power delivered by a laser diode seed, a gain of more than 40 dB is required to reach a multikilowatt peak-power level, which can be achieved only by using a complicated multistage fiber amplifier [5]. In these systems, the final-stage amplifier often requires the use of a large-mode-area fiber to both reduce the onset of the amplified stimulated emission (ASE) and the nonlinear effects. As a consequence, the output is not purely single ode. On the other hand, high peak powers have also been obtained in a Q-switched cladding-pumped fiber laser by using the fast dynamics of stimulated Brillouin scattering (SBS). Whereas laser-pulse compression by SBS in a li- quid or gas cell has been well known for now more than 25 years (see, for example, [6]), the interest in SBS dy- namics in a fiber laser is more recent [7,8]. It is now ad- mitted that SBS can provide a strong feedback in the laser cavity, which tends to generate a nanosecond pulse able to extract most of the energy from the fiber ampli- fier. This process was demonstrated in different Q- switched systems, such as self-Q-switched [7], actively Q-switched [8], and passively Q-switched [9]. The main drawbacks are the rather poor pulse stability, the strong temporal jitter in the pulse repetition rate, and frequent damages owing to unpredictable giant pulses with a peak power exceeding the silica damage threshold. In this contribution, we propose a scheme for the gen- eration of high peak power in a cladding-pumped Yb- doped fiber amplifier with single-mode output. The key idea is to use SBS pulse compression in a fiber amplifier seeded by a long pulse from a spectrally narrowed laser diode. It is experimentally demonstrated that the combi- nation of the amplifier gain and the cascaded Stokes conversion was responsible for the generation of nano- second pulses with peak powers of 20 kW. The major drawbacks of conventional fiber MOPA sys- tems are the low energy saturation and the inherent strong ASE, which limits the single-pass gain value around þ35 dB. Therefore, multikilowatt output peak power would require an initial seed peak power of a few watts and even more for a gain-saturated amplifier, which can- not be achieved using commercial single-mode laser diodes. To overcome this limitation, we show here that SBS, which was originally identified as a detrimental ef- fect in a fiber amplifier, can be exploited to extract much more energy compared to conventional MOPA systems. It is important to mention that, using this scheme, the power of the amplified pulse has to exceed the SBS threshold, which depends (among other factors related to the fiber) on the initial pulse duration and on the spectral width of the pulse. To lower the SBS threshold and trigger a giant pulse, it was thus necessary to use a spectrally narrowed laser diode because of the narrow Brillouin gain band- width (20 MHz). Moreover, the duration of the seed pulse has to exceed the phonon lifetime (10 ns [10]) to allow sufficient phonon accumulation. The experimental setup is shown in Fig. 1 and de- scribed as follows. A single-element, FabryPerot (FP) laser diode operating at a wavelength near 1045 nm is gain switched by a square pulse using an arbitrary pulse generator. The repetition rate was adjusted between 100 Hz and 5 kHz during the experiments. Single- frequency operation was achieved using an external cav- ity comprising a 7030 dichroic beam splitter and a diffraction grating (1200 lines=mm, blazed at 1 μm) in January 15, 2011 / Vol. 36, No. 2 / OPTICS LETTERS 241 0146-9592/11/020241-03$15.00/0 © 2011 Optical Society of America

High-peak-power nanosecond pulse generation by stimulated Brillouin scattering pulse compression in a seeded Yb-doped fiber amplifier

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High-peak-power nanosecond pulse generation bystimulated Brillouin scattering pulse

compression in a seeded Yb-doped fiber amplifierM. Laroche,* H. Gilles, and S. Girard

Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), ENSICAEN,CNRS, CEA/IRAMIS, Université de Caen, 14050 CAEN cedex, France

*Corresponding author: [email protected]

Received October 8, 2010; revised November 22, 2010; accepted November 26, 2010;posted December 6, 2010 (Doc. ID 136339); published January 12, 2011

We demonstrate the generation of nanosecond and multikilowatt peak-power pulses in a double-clad Yb-dopedfiber amplifier seeded by a spectrally narrowed gain-switched laser diode. Injected pulses with 100 ns duration weresimultaneously compressed and amplified by the combination of high amplifier gain and stimulated Brillouin scat-tering. A maximum peak power of 20 kW has been obtained, corresponding to a single-pass gain of þ57 dB in termsof peak power. Part of this output signal was also converted into IR continuum light by splicing a length of single-mode fiber at the end of the fiber amplifier. © 2011 Optical Society of AmericaOCIS codes: 140.3510, 190.4370, 140.3538.

Over the past few years, there has been an increasing in-terest in fiber laser sources capable of delivering high-peak-power pulses (>10 kW) in a diffraction-limitedbeam. High efficiency and high spatial beam quality makethese laser sources suitable for applications such asmicromachining, range finding, or remote sensing. A fi-ber laser can be easily Q switched using an acousto-opticor electro-optic modulator but generally at the loss of theall-fiber scheme in the case of high-peak-power genera-tion [1,2]. In addition, it is difficult to achieve a narrowlinewidth or short pulse duration (<50 ns) with such fi-ber sources. Master-oscillator–power-amplifier (MOPA)systems based on a double-clad fiber amplifier seededby a modulated laser diode appear to be very attractiveas they offer much better flexibility in terms of pulseshape, repetition rate, and spectral width [3,4]. However,owing to the relatively low peak power delivered by alaser diode seed, a gain of more than 40 dB is requiredto reach a multikilowatt peak-power level, which canbe achieved only by using a complicated multistage fiberamplifier [5]. In these systems, the final-stage amplifieroften requires the use of a large-mode-area fiber to bothreduce the onset of the amplified stimulated emission(ASE) and the nonlinear effects. As a consequence,the output is not purely single ode.On the other hand, high peak powers have also been

obtained in a Q-switched cladding-pumped fiber laser byusing the fast dynamics of stimulated Brillouin scattering(SBS). Whereas laser-pulse compression by SBS in a li-quid or gas cell has been well known for now more than25 years (see, for example, [6]), the interest in SBS dy-namics in a fiber laser is more recent [7,8]. It is now ad-mitted that SBS can provide a strong feedback in thelaser cavity, which tends to generate a nanosecond pulseable to extract most of the energy from the fiber ampli-fier. This process was demonstrated in different Q-switched systems, such as self-Q-switched [7], activelyQ-switched [8], and passively Q-switched [9]. The maindrawbacks are the rather poor pulse stability, the strongtemporal jitter in the pulse repetition rate, and frequent

damages owing to unpredictable giant pulses with a peakpower exceeding the silica damage threshold.

In this contribution, we propose a scheme for the gen-eration of high peak power in a cladding-pumped Yb-doped fiber amplifier with single-mode output. The keyidea is to use SBS pulse compression in a fiber amplifierseeded by a long pulse from a spectrally narrowed laserdiode. It is experimentally demonstrated that the combi-nation of the amplifier gain and the cascaded Stokesconversion was responsible for the generation of nano-second pulses with peak powers of 20 kW.

The major drawbacks of conventional fiber MOPA sys-tems are the lowenergy saturation and the inherent strongASE, which limits the single-pass gain value aroundþ35 dB. Therefore, multikilowatt output peak powerwould require an initial seed peak power of a few wattsand even more for a gain-saturated amplifier, which can-not be achieved using commercial single-mode laserdiodes. To overcome this limitation, we show here thatSBS, which was originally identified as a detrimental ef-fect in a fiber amplifier, can be exploited to extract muchmore energy compared to conventional MOPA systems. Itis important tomention that, using this scheme, the powerof the amplified pulse has to exceed the SBS threshold,which depends (among other factors related to the fiber)on the initial pulse duration and on the spectral width ofthe pulse. To lower the SBS threshold and trigger a giantpulse, it was thus necessary to use a spectrally narrowedlaser diode because of the narrow Brillouin gain band-width (−20 MHz). Moreover, the duration of the seedpulse has to exceed the phonon lifetime (∼10 ns [10])to allow sufficient phonon accumulation.

The experimental setup is shown in Fig. 1 and de-scribed as follows. A single-element, Fabry–Perot (FP)laser diode operating at a wavelength near 1045 nm isgain switched by a square pulse using an arbitrary pulsegenerator. The repetition rate was adjusted between100 Hz and 5 kHz during the experiments. Single-frequency operation was achieved using an external cav-ity comprising a 70∶30 dichroic beam splitter and adiffraction grating (1200 lines=mm, blazed at 1 μm) in

January 15, 2011 / Vol. 36, No. 2 / OPTICS LETTERS 241

0146-9592/11/020241-03$15.00/0 © 2011 Optical Society of America

a Littrow configuration. Note that this part of the setupmight be fully fiberized by using a pigtailed FP laserdiode, by replacing the diffraction grating by a Bragggrating and the beam splitter by a fiber coupler. For pulseduration of between 10 and 200 ns, the spectral linewidthof the external-cavity diode laser (ECDL) was measuredto be less than 0:05 nm, limited by the resolution of theoptical spectrum analyzer (OSA). After transmissionthrough a free-space isolator, the signal from the ECDLwas coupled into 6-m-long, 6 μm core diameter(NA ¼ 0:15), Yb-doped double-clad fiber. The pumpabsorption coefficient of the fiber at 975 nm is1:5 dB=m. The fiber amplifier is pumped at 975 nm bya pigtailed multimode laser diode through a pump/signalfiber combiner. Both fiber ends were cleaved with anangle of ∼15° in order to reduce the onset of parasiticlasing. With this configuration, a maximum small-signalgain of þ33 dB was measured at λ ¼ 1045 nm.Stable giant pulses with a duration of ∼3 ns and a peak

power exceeding 20 kW were generated for a minimumseed pulse duration of 50 ns at the maximum injectedpeak power of 36 mW. The traces of the output pulseswere measured through neutral density filters and usinga high-speed silicon photodiode (rise time of 100 ps) di-rectly connected to a fast digital oscilloscope (1 GHzbandwidth). Filters and the photodiode were previouslycalibrated in order to precisely measure instantaneouspowers. An example of a typical trace and a zoom inof the pulse formation region (inset) are given in Fig.2. Because of the stochastic nature of SBS, pulse-to-pulsepower fluctuations with a mean deviation of 6% wereobserved and additional small satellite pulses may alsoappear. One can note that a satellite pulse may containup to 25% of the total energy but does not significantlyreduce the main pulse peak power or change the pulseduration. These instabilities might also be partly attribu-ted to self- and cross-phase modulations induced by the

optical Kerr effect [10]. On the other hand, the pulse jitterwas less than 5 ns at a repetition rate of 3 kHz, hencemuch better than other passively or self-Q-switched fiberlaser using SBS dynamics (see, for example, [9]). At theamplifier output, the critical instantaneous power for astrong SBS effect was observed experimentally to be30 W, which is indeed lower than previous observationsin a Q-switched fiber laser based on a similar Yb-dopedfiber [11]. Once this power threshold is reached, SBS willinduce strong signal depletion, corresponding to a powerconversion into a backscattered Stokes wave. As alreadyexplained in [12], the next step is a cascading effect inwhich several consecutive Stokes lines are generated.

To better understand the mechanism of pulse forma-tion, the optical spectra of the laser emission in the for-ward and backward directions were recorded andcompared to the initial spectrum of the ECDL (Fig. 3).These spectra reveal a structure with sharp lines sepa-rated by Δλ ¼ 0:12 nm, which corresponds to twice theBrillouin frequency shift in silica glass (ΔλSBS ¼ 0:06 nmnear 1050 nm). It can also be observed that the Stokeslines alternate between the forward and backward direc-tions, which confirms the cascaded Stokes generationinto the seeded amplifier. Output spectra also containa few anti-Stokes lines. Finally, the initial spectral lineat the ECDL wavelength appears to be the most ampli-fied, but it is important to mention that the experimentalspectrum is integrated other several pulses and may notreflect the spectrum of a single isolated pulse. We alsobelieve that two consecutive Stokes waves may ex-change energy through a four-wave mixing process,which could provide additional energy to the pulse. How-ever, numerical modeling of the SBS dynamics wouldcertainly be required to go further in the interpretationof the complete pulse compression process.

For 1:3 W of launched pump power, the maximumpeak power was obtained at a pulse repetition rate of4 kHz and the average output power was 235 mW. A highrepetition rate would be stopped from generating a giantpulse by a lack of amplifier gain, whereas a lower repeti-tion rate would strongly increase ASE intensity. In addi-tion, we estimated that 120 mW of output power wasemitted in the backward direction, hence representing39% of the total output power.

At the peak power near 20 kW, the forward outputspectrum also shows the presence of stimulated Raman

Fig. 1. (Color online) Experimental setup of the Yb-dopedfiber MOPA.

Fig. 2. Typical pulse shape at the output of the fiber amplifier(injected pulse peak power, 36 mW). Inset, zoom in the initialpulse formation.

Fig. 3. Spectra of the ECDL emission and the generatedBrillouin–Stokes lines in the forward and backward directions.

242 OPTICS LETTERS / Vol. 36, No. 2 / January 15, 2011

scattering Stokes lines at 1092 and 1146 nm and the onsetof continuum generation between 1 and 1:3 μm (Fig. 4).Nonetheless, 90% of the output power is contained intothe Brillouin–Stokes, hence in a spectral bandwidth ofless than 0:5 nm. To further extend the width of the con-tinuum, a 2 m length of high-NA, single-mode fiber at1050 nm (Nufern UHNA1) was spliced at the output ofthe Yb-doped fiber. In this configuration, the spectrumstrongly broadens into a flat supercontinuum between1130 and 1750 nm (limited by the OSA spectral range)and also presents a better defined Raman line at 1092 andan additional one at 1146 nm. The mechanism of super-continuum formation is now well known and can befound in [13], for example. Output power was reducedto 135 mW in which the supercontinuum represents76% of the power. This power reduction increased withthe length of single-mode fiber and is due to leaky guidedmodes at the longest wavelengths. No light was observedin the visible domain, but we believe that the use of amicrostructured fiber in place of the single-mode fiberwould lead to efficient white-light generation.

In conclusion, we have demonstrated a method togenerate high-peak-power nanosecond pulses in a dif-fraction-limited beam. Based on the combination of again-switched ECDL and a Yb-doped fiber amplifier, acompressed pulse of 3 ns with 20 kW of peak power isobtained using a low-cost, potentially all-fiberized setup.It was also shown that a flat supercontinuum extendingfrom 1130 to 1750 nm can be generated by simply addinga short length of single-mode fiber, hence without the useof microstructured or tapered fibers.

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Fig. 4. Supercontinuum spectra (a) at the output of the fiberamplifier and (b) after 2 m of high-NA, single-mode fiber at1050 nm.

January 15, 2011 / Vol. 36, No. 2 / OPTICS LETTERS 243