High power all-fibered femtosecond master oscillator power amplifier ...

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oscillator power amplifier at 1.56 μm. D. A. Gaponov,1,2,* L. V. Kotov,2 M. E. Likhachev,2 M. M. Bubnov,2 A. Cabasse,3 J.-L. Oudar,5. S. Fevrier,3,4 D. S. Lipatov ...
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High power all-fibered femtosecond master oscillator power amplifier at 1.56 μm D. A. Gaponov,1,2,* L. V. Kotov,2 M. E. Likhachev,2 M. M. Bubnov,2 A. Cabasse,3 J.-L. Oudar,5 S. Fevrier,3,4 D. S. Lipatov,6 N. N. Vechkanov,6 A. N. Guryanov,6 and G. Martel1 1

CORIA-CNRS UMR 6614, Avenue de l’Université, BP12, 76801 Rouen, France

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Fiber Optics Research Center of the Russian Academy of Sciences, 38 Vavilov Street, Moscow, 119333, Russia 3 Université of Bordeaux, CEA, CNRS, CELIA (Centre Lasers Intenses et Applications), UMR5107, F-33400 Talence, France 4

Xlim CNRS, UMR7252, F-87060 Limoges, France

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LPN-CNRS, Route de Nozay, 91460 Marcoussis, France 6 Institute of Chemistry of High Purity Substances of the RAS, 49 Tropinin Street, Nizhny Novgorod 603950, Russia *Corresponding author: [email protected] Received February 17, 2012; revised June 4, 2012; accepted June 22, 2012; posted June 25, 2012 (Doc. ID 163215); published July 25, 2012 Direct amplification of output from chirped pulse oscillator (CPO) to 3.3 W of average power (pulse energy of 118 nJ in 20 ps pulse duration before compression) was achieved in a properly designed cladding pumped large mode area Er-doped fiber. Various configurations of CPO cavity with different FWHM of output spectrum and pulse duration were investigated. Fourier limit compression with 480 fs pulse duration and 32 kW peak power has been obtained for pulses with 14.8 nm FWHM spectrum. Subsequent nonlinear compression in a standard SMF-28 fiber yielded pulses as short as 145 fs. © 2012 Optical Society of America OCIS codes: 060.2320, 140.3510, 140.4050.

During the last decade the energy from ultrafast femtosecond (fs) fiber oscillators was gradually increased to reach the microjoule level (within 700 fs pulse) in Ybdoped fibers (i.e., around 1 μm) [1]. Gigawatt peak power was also achieved in ultrafast fs amplifiers (within 480 fs pulse [2]). These outstanding results were made possible thanks to large mode area (LMA) fibers. In particular, rod-type photonic crystal fibers with core up to 100 μm in diameter where single transverse mode operation could still be provided [2] are the favorite platform at the wavelength of 1 μm. On the other hand, if we are going to shift towards longer wavelengths in the IR range, the power scaling is not so evident and faces a number of difficulties, such as dispersion management, manufacturing of efficient doped LMA fibers, powerful pump sources, etc. Thus, in case of Er-doped fibers, the deliverable output energy characteristics are two orders of magnitude lower. Up to date, the best value of pulse energy from Er-doped fiber ultrafast oscillators at 1.56 μm is approaching 10 nJ [3]. In turn, the output pulse peak power from ultrafast Erdoped fiber chirped pulse amplifiers (CPAs) reaches the MW level (within 600 fs pulse) [4] at 1.6 μm using bulky elements and with cladding pumping, or hundreds of kW (within 530 fs pulse) [5] using resonant corepumping. Best values of average output power are 2 W within 6 ps pulses [6] and 1.5 W within 530 fs pulses [5]. In this Letter, we demonstrate a simple and robust allfiber format master oscillator power amplifier (MOPA) based on a newly developed cladding pumped LMA step-index Er-doped fiber [7]. Compared to the stateof-the-art CPA schemes, we simplified the set-up by keeping only the main amplifier stage exploiting the benefit of the already chirped output pulse from CPO cavity [3,8]. Different regimes of amplification (linear and nonlinear) were investigated by varying the cavity configuration yielding record values in both linear and nonlinear amplification regimes. 0146-9592/12/153186-03$15.00/0

The experimental set-up of a usual co-propagating MOPA scheme is depicted in Fig. 1. The seed signal is generated by a CPO similar to the one described in our previous work [3]. We should stress here that a key feature of a CPO is that the net-normal dispersion of the cavity yields already chirped pulses, thus eliminating the prerequisite for an external stretcher prior to the amplifier. This, together with the relatively high average output power (∼0.22 W) from the CPO avoids the need for the pre-amplifier stage usual in high power CPA schemes [8]. The injected seed pulse average power was estimated to be 100 mW taking into account splice (taper/active fiber) and isolator losses in total of 3.5 dB. As a pump source for the amplifier stage we used a multimode pigtailed diode (fiber core diameter of 105 μm, NA  0.2) delivering up to 30 W at 976 nm. The seed pulse and pump were combined thanks to a 2  1 × 1 combiner. Two isolators (marked as ISO in Fig. 1) were used to prevent parasitic feedback which could disturb our CPO. For the amplification stage we used singlemode purely Er-doped double clad fiber developed in [7]. Fabrication of the large core (D  22 μm, NA ∼ 0.07) doped by 1000 ppm Er2 O3 becomes possible owing to utilization of the new P2 O5 -Al2 O3 -SiO2 glass matrix as a host for Er3 ions [9]. According to the measured refractive index profile of the fiber, the cut-off wavelength was calculated to be 1600 nm. At the operating wavelength of 1560 nm

Fig. 1. (Color online) Fully fibered experimental set-up. Dots represent fusion splices. CPO, master oscillator; ISO, isolators. © 2012 Optical Society of America

August 1, 2012 / Vol. 37, No. 15 / OPTICS LETTERS

the LMA fiber was made singlemode by proper coiling (with 8 cm curvature radius). An extremely high numerical aperture teflon coating (NA ∼ 0.6) allowed us to decrease the first cladding diameter down to 95 μm. The resulting cladding pump absorption at 976 nm was as high as 1.3 dB ∕ m. The pump cladding cross-section is square. In continuous wave regime the highest slope efficiency of 31% was achieved at 1585 nm using a 12 m fiber length. Amplification of shorter wavelengths (1560 nm) resulted in appearance of significant part of amplified spontaneous emission (ASE) in the output spectrum. To suppress the ASE and minimize nonlinear effects the fiber length was reduced to 5 m. In turn, this results in a pump absorption efficiency drop down to 50%–60% (depending on the diode current which was temperature not stabilized) as a penalty [7]. Finally, a 6 ∕ 125 μm to 4.5 ∕ 95 μm double-clad fiber taper was used to couple the pump and signal from the pump combiner into the LMA Er-doped fiber, thus conferring our MOPA a true fully-fibered architecture. First, we investigated the linear amplification regime with highly stretched pulses. The length of dispersion compensation fiber (DCF) inside the CPO cavity (see [3]) was increased to 1 m resulting in largely stretched 20 ps pulses with 4.7 nm spectral bandwidth at 28 MHz repetition rate (compared to [3], here the WDM and coupler lengths inside the CPO cavity have been reduced, resulting in a repetition rate of 28 MHz even with longer pieces of DCF fiber). With the maximum available pump power at the MOPA output we obtained 3.3 W of signal average power (118 nJ pulse energy) with 15% slope efficiency with respect to the launched pump power [see Fig. 2(a)]. As shown in Fig. 2(a) the input and output spectra have similar FWHM presuming the linear amplification regime. To confirm the linear character of amplification and the lack of accumulated nonlinear phase, we verified the compression of the pulse. We numerically checked that the Fourier transform of such steep-edge spectra yields a time-bandwidth product around 0.7. Using a similar grating pair as in [3] (i.e., 600 lines ∕ mm, blazed at 1600 nm and 60% efficiency per pass) pulses have been dechirped to 1.45 ps duration [see Fig. 2(b)], that is 1.2 times the Fourier limit. Then we decreased the pulse duration (i.e., increased the spectral bandwidth) by varying the length of the DCF inside the CPO cavity. We first reduced the DCF length to

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Fig. 3. (Color online) Linear amplification of 14 ps seed pulse at the maximum average power of 1 W. (a) Input and output spectra; (b) autocorrelation traces of the output pulse before (blue) and after (red) compression.

0.4 m. This resulted in 14 ps seed pulse generation with 5.7 nm spectral bandwidth at 32 MHz repetition rate. A seed pulse of 100 mW was then amplified up to 1 W of average power again in the linear regime. Here, we can notice some pre-compression from 14 ps to 11.2 ps (E pulse  31 nJ) of the seed pulse inside the MOPA during amplification. Corresponding input and output spectra with, again, similar shapes are presented in Fig. 3(a). The output FWHM bandwidth of 7 nm leads to Fourier limit dechirped pulses of 876 fs as shown in Fig. 3(b) corresponding to a maximum peak power of 21 kW. Finally, we completely removed the DCF from the CPO cavity. Now the oscillator delivers 3.5 ps seed pulses with 14.8 nm spectral FWHM at 39 MHz repetition rate. Inside the amplifier this quite short seed pulse rapidly acquires a high peak power inducing a pulse break-up. This is illustrated in Fig. 4 where a strong pedestal appearing and developing is shown. Theoretical modelling of our MOPA reproducing also the real seed pulse parameters allows us to estimate a peak-power limit for such pulse distortion [see black curves in Fig. 4(a) and 4(b)]. Experimentally we obtained 0.3 W of output average power at the pump power P1  5 W while simulations predict slightly higher value ∼0.4 W at Esat  0.55 nJ. Taking into account numerically retrieved temporal pulse profiles (pulse. FWHM  170 fs) we could deduce a peak power limit of P peak ∼ 60 kW for the appearance threshold of pulse distortion. Our simulations do not match fully with experimental ones in particular in terms of temporal broadening of the pulse. Improvement would require to couple GinzburgLandau type equation with rate equations in order to take into account the shape of the gain and its z-dependence (a)

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Fig. 2. (Color online) Linear amplification of 20 ps seed pulses at the maximum output average signal power of 3.3 W. (a) Topx, right-y axes: slope efficiency. Bottom-x, left-y axes: input and output spectra. (b) Autocorrelation trace of the output pulse before (blue) and after (red) compression.

Fig. 4. (Color online) Pulse distortion in case of 3.5 ps seed pulse. (a) Experimental autocorrelation traces measured at the output of the amplifier for different amplifier pump powers (P1  5 W, P2  7 W, P3  10 W, P4  13 W). (b) Autocorrelation traces retrieved from numerical modelling with different saturation energies Esat of the amplifier gain (Esat is proportional to the pump power).

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Fig. 5. (Color online) Amplification of 9.3 ps externally stretched seed pulse with broad 14.77 nm spectrum. (a) Input and output spectra. (b) Autocorrelation traces of the externally stretched seed pulse (black) and amplified pulse at 1 W output average power before (blue) and after (red) compression.

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Fig. 6. (Color online) Nonlinear compression in 1 m of SMF28 fiber of 480 fs pulse. (a) Pulse spectra with different injected pulse energy (in cyan—maximum P peak  3.5 kW). (b) Corresponding autocorrelation traces. Autocorrelation of 145 fs pulse is in cyan.

with pump distribution [10]. This goes beyond the scope of this Letter and shall be devoted to further work. In the final part of the work we were interested in a proper amplification of the seed pulse with the broad spectrum of 14.8 nm. To solve the problem, we decided to simply add the stretcher outside the cavity. Note that re-designing the whole CPO to deliver 10 ps pulses with 15 nm FWHM spectrum would be another feasible solution in order to get rid of any external stretcher. With the external stretcher and after amplification we obtained at the MOPA output 1 W average power with 4.65 ps pulse duration (instead of 9.3 ps, again due to some amount of pre-compression). This corresponds to E pulse  26 nJ and P peak  5.5 kW. This pulse was further compressed using bulk gratings to 480 fs corresponding to a peak power of 32 kW (60% compression efficiency). The input and output spectra of the pulses are presented in Fig. 5(a). Here we can note that the output spectrum is not symmetric. In our double-clad amplifier the maximum of the gain band is shifted from 1560 nm to longer wavelengths. As a consequence, the short-wavelength part of such a broad spectrum is less amplified (this effect was not apparent for the narrower spectra in Fig. 2 and 3). Autocorrelation traces of the amplified pulse before and after compression are presented in Fig. 5(b). Further nonlinear compression was possible through injection of these 480 fs pulses in a 1 m long piece of standard SMF28 fiber having anomalous dispersion at 1560 nm. We succeeded to decrease the pulse duration down to 145 fs. The spectra and autocorrelation traces are presented in Fig. 6(a) and 6(b), respectively. We

suggest that using a passive fiber with anomalous dispersion at 1560 nm, but with an adapted larger core will yield higher average output power with similar or even shorter pulse durations (i.e., down to ∼100 fs). Conclusion. In this Letter, we investigated a high power MOPA with dissipative solitonic pulses. The simple and efficient all-fibered scheme was realized thanks to the exploitation of the very new LMA Er-doped fiber and a chirped pulse master oscillator. Three different amplification regimes were investigated by varying the seed pulse duration (20 ps, 14 ps, and 3.5 ps) along with corresponding spectra. In the linear amplification regime of 20 ps pulses, 3.3 W of average output power (118 nJ pulse energy) was obtained with 15% of slope efficiency. This is, to the best of our knowledge, the highest output average power obtained in ultrashort pulsed single stage erbium amplifiers. This record result confirms the great potential of such Er-doped LMA fibers in ultrafast high power systems. Further compression to 1.5 ps with 60% efficiency bulk reflective gratings gave 47 kW of peak power. With 3.5 ps seed pulses proper amplification was achieved by stretching them to 9.3 ps outside of the cavity with a 4 m long piece of normal dispersion fiber. After compression we obtained 480 fs pulse with 32 kW in peak power. Further nonlinear compression with a 1 m long piece of standard SMF28 fiber resulted in pulses of 145 fs duration and 3.5 kW of peak power (70% of pulse energy inside the main peak) limited by the power injected in the SM fiber. By appropriately enlarging the fiber core, pulses with duration around or below 100 fs would even be achievable without loss in energy. This work was partially supported by the Program “Extreme light fields and their applications” of the Russian Academy of Sciences, the grant MK-1459.2011.2 of the President of the Russian Federation and the ESP Carnot grant from CORIA Lab. The authors acknowledge the French Embassy for the travel grant of L.V. Kotov. References 1. B. Ortaç, M. Baumgartl, J. Limpert, and A. Tünnermann, Opt. Lett. 34, 1585 (2009). 2. T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Express 19, 255 (2011). 3. A. Cabasse, D. Gaponov, K. Ndao, A. Khadour, J.-L. Oudar, and G. Martel, Opt. Lett. 36, 2620 (2011). 4. F. Morin, F. Druon, M. Hanna, and P. Georges, Opt. Lett. 34, 1991 (2009). 5. J. C. Jasapara, A. DeSantolo, J. W. Nicholson, A. D. Yablon, and Z. Várallyay, Opt. Express 16, 18869 (2008). 6. J. C. Jasapara, M. J. Andrejco, A. D. Yablon, J. W. Nicholson, C. Headley, and D. DiGiovanni, Opt. Lett. 32, 2429 (2007). 7. L. V. Kotov, M. E. Likhachev, M. M. Bubnov, O. I. Medvedkov, D. S. Lipatov, N. N. Vechkanov, and A. N. Guryanov, Quantum Electron. 42, 432 (2012). 8. W. H. Renninger, A. Chong, and F. W. Wise, Opt. Lett. 33, 3025 (2008). 9. M. E. Likhachev, M. M. Bubnov, K. V. Zotov, D. S. Lipatov, M. V. Yashkov, and A. N. Guryanov, Opt. Lett. 34, 3355 (2009). 10. Z. Huang, J. Wang, H. Lin, D. Xu, R. Zhang, Y. Deng, and X. Wei, J. Opt. Soc. Am. B 29, 1418 (2012).