Cataloged from PDF version of article.We demonstrate generation of pulses with up to 4 mu J energy at 1 MHz repetition rate through nonlinear chirped-pulse amplification in an entirely fiber-integrated amplifier, seeded by a fiber oscillator. The peak power and the estimated nonlinear phase shift of the amplified pulses are as much as 57 kW and 22 pi , respectively. The shortest compressed pulse duration of 140 fs is obtained for 3.1 mu J of uncompressed amplifier output energy at 18 pi of nonlinear phase shift. At 4 mu J of energy, the nonlinear phase shift is 22 pi and compression leads to 170-fs-long pulses. Numerical simulations are utilized to model the experiments and identify the limitations. Amplification is ultimately limited by the onset of Raman amplification of the longer edge of the spectrum with an uncompressible phase profile. (C) 2010 Optical Society of Americ

Ilday, F. O.

Kalaycioglu, H.

Oktem, B.

Paltani, P.

Senel, C.

Optics Letters

Bilkent University Institutional Repository

April 1, 2010 / Vol. 35, No. 7 / OPTICS LETTERS 959Microjoule-energy, 1 MHz repetition rate pulsesfrom all-fiber-integrated nonlinearchirped-pulse amplifierH. Kalaycioglu,1,* B. Oktem,2 Ç. S¸enel,1 P. P. Paltani,1 and F. Ö. Ilday11Physics Department, Bilkent University, Çankaya, Ankara 06800, Turkey2Material Science and Nanotechnology Graduate Program, Bilkent University, Ankara 06800, Turkey*Corresponding author: hamitkal@bilkent.edu.trReceived February 11, 2010; accepted February 16, 2010;posted March 2, 2010 (Doc. ID 124188); published March 23, 2010We demonstrate generation of pulses with up to 4 J energy at 1 MHz repetition rate through nonlinearchirped-pulse amplification in an entirely fiber-integrated amplifier, seeded by a fiber oscillator. The peakpower and the estimated nonlinear phase shift of the amplified pulses are as much as 57 kW and 22, re-spectively. The shortest compressed pulse duration of 140 fs is obtained for 3.1 J of uncompressed ampli-fier output energy at 18 of nonlinear phase shift. At 4 J of energy, the nonlinear phase shift is 22 andcompression leads to 170-fs-long pulses. Numerical simulations are utilized to model the experiments andidentify the limitations. Amplification is ultimately limited by the onset of Raman amplification of the longeredge of the spectrum with an uncompressible phase profile. © 2010 Optical Society of AmericaOCIS codes: 060.2320, 320.7090, 060.3510.There is much worldwide interest in the developmentof powerful, robust, and simple-to-operate ultrafastfiber lasers. Their biggest drawback is the limitationto peak power by nonlinear effects. While most ef-forts are focused on weakening these effects throughthe use of large core specialty fibers [1], another ap-proach is to manage the nonlinear effects. Examplesof this approach include proposals on direct compen-sation of nonlinear effects [2], mitigation by spectralpreshaping [3], and indirect compensation via third-order dispersion [4–6]. The latter, also known as non-linear chirped-pulse amplification (nonlinear CPA),has led to the generation of 30 J, 240 fs pulses [7]and 100 J, 650 fs pulses [5] using 40-m-core spe-cialty photonic crystal fibers, along with 100 J,270 fs pulses [8] using a 80-m-core rod-type ampli-fier. The main advantages of nonlinear CPA are thatit enables generation of short pulses from a fiber-stretcher, grating-compressor pair with unmatchedthird-order dispersion and allows the accumulationof significant nonlinear phase shift. The main draw-back is that pulse quality is significantly reduced.Nevertheless, this is acceptable for many applica-tions based on peak power, where pulse quality is ofsecondary concern.To date, all implementations of nonlinear CPA pro-ducing high-energy pulses have relied on the use ofspecialty Yb-doped fibers with extremely large corediameters requiring the use of bulk optics to couplesignal and pump light into the fiber. Although thesefibers allow higher energies to be generated, they re-quire special preparation of the fiber ends and muchcare in bulk coupling of pump light and maintainingsingle-mode operation. These aspects negate some ofthe primary advantages of fiber amplifiers. To makefull use of the advantages of the nonlinear CPA ap-proach, the entire amplifier should be implementedin fiber. Recently, we demonstrated a 10 W fiber am-plifier at 40 MHz repetition rate, where pulse propa-0146-9592/10/070959-3/$15.00 ©gation from fiber oscillator until the grating compres-sor was in fiber [9].Here, we demonstrate an all-fiber-integrated non-linear CPA system, seeded by a fiber oscillator, gen-erating pulses with energy up to 4 J at 1 MHz rep-etition rate. The integrated architecture allows trulyrobust long-term operation. The shortest dechirpedpulse duration of 140 fs is obtained for 3.1 J ampli-fier output, corresponding to a total nonlinear phaseshift of 18. Peak powers of the chirped pulses fromthe amplifier are up to 57 kW, which corresponds tothe generation of largest nonlinear phase shifts froma fiber amplifier while maintaining compressibility tosub-200 fs, in addition to being the highest peak pow-ers obtained from an all-fiber-integrated amplifier, tothe best of our knowledge. Numerical simulationsprovide better agreement with experiments com-pared with prior reports and give insight into thetrade-offs of nonlinear fiber CPA systems.The experimental arrangement of the laser systemis shown in Fig. 1. The seed pulses are generatedfrom a Yb-doped fiber laser oscillator similar to thatin [10]. The fiber section of the Yb laser oscillator con-sists of 6.6 m of single-mode fiber (SMF) (HI-1060)and 0.30 m of Yb-doped fiber, followed by another0.93 m of SMF. The total dispersion of the cavity iscalculated to be about 6150 fs2. The gain fiber ispumped in-core by a 976 nm fiber-coupled pump di-ode with an output power up to 550 mW. The pumplight is delivered via a 980/1030 nm wavelength di-vision multiplexer. An optical isolator ensures unidi-rectional operation, which facilitates self-starting op-eration. Mode locking is initiated and stabilized bythe nonlinear polarization evolution effect. The laseroutput is checked against multiple pulsing by way oflong-range autocorrelation and 12 GHz bandwidth rfspectral measurements.The all-fiber-integrated amplifier consists of afiber-pigtailed acousto-optic modulator (AOM), a fiber2010 Optical Society of America960 OPTICS LETTERS / Vol. 35, No. 7 / April 1, 2010stretcher, a core-pumped preamplifier, and acladding-pumped power amplifier. Finally, the ampli-fied pulses are compressed in a grating compressor.All pump light is delivered through fiber couplers.The preamplifier consists of a 0.30-m-long Yb-dopedfiber (of the same type as in the oscillator) pumpedbackward with a 300 mW pump diode. The poweramplifier is pumped codirectionally to protect thepump diodes from optical damage. Pump and signaldelivery to the gain fiber is accomplished with amultiport pump-signal combiner (MPC). Four975 nm diodes, each with up to 10 W power coupledto a 105-m-core multimode fiber, are used as pump.The amplifier consists of 2-m-long, large-mode-area(LMA), double-clad (DC) ytterbium-doped fiber witha 20 m core with NA of 0.06, and a 125 m claddingwith NA of 0.46. The output is taken through a fiber-pigtailed isolator-collimator device with a transmis-sion ratio of 90%.The fiber laser oscillator produces 3-ps-longchirped pulses with a bandwidth of 45 nm at a rep-etition rate of 28 MHz [Fig. 2(a)]. An output power of105 mW seeds the amplifier directly through a 50%output coupler placed after the gain fiber andtraverses the fiber-integrated AOM, reducing the rep-etition rate to 1 MHz. The pulses are then stretchedto 150 ps in a 100-m-long fiber stretcher [compris-ing HI-1060, with an estimated group velocity disper-sion of 22 fs2/mm and third-order dispersion (TOD)of 74 fs3/mm] and amplified to 104 nJ in the preamp-lifier. The FWHM bandwidth of the spectrum is mea-sured to be about 20 nm [Fig. 2(b)]. The temporalwidth, which is also reduced owing to gain narrow-ing, is measured to be 80 ps using a 20 GHz band-width sampling scope and a fast detector with a risetime of 35 ps [Fig. 2(d)]. The 50% reduction in thepulse duration is due to the dominance of the gain fil-tering in the preamplifier over self-phase modulation(SPM). The situation is reversed in the power ampli-Fig. 1. (Color online) Schematic diagram of the amplifiersystem. PC, polarization controller; PBS, polarization beamsplitter; AOM, acousto-optic modulator; WDM, wavelength-division multiplexer; LMA, large mode area; DC, double-clad; MPC, multiport pump-signal combiner.fier.Numerical simulations of the pulse generation andamplification based on the model described in [9]guide the experiments at all stages. As can be seen inFig. 3(a), experimentally the compressed pulse widthdecreases from 350 to 150 fs with increasing pulseenergy from 0.5 to 3 J as a result of the compensa-tion of TOD by SPM, and as supported by numericalsimulations [Figs. 3(a) and 3(b)]. The broadeningmechanism at the long wavelength edge of the spec-trum is attributed to the Raman effect, which occursmainly in the 75-cm-long undoped lead fiber of thecollimator, where more than half of the nonlinearphase shift is accumulated. This has been confirmedby observing reduction of the Raman effect by reduc-ing the undoped fiber length, while keeping the totalnonlinear phase shift fixed. The Raman amplificationis suppressed by the gain filtering during amplifica-tion. This likely explains the absence of Raman shiftsFig. 2. (Color online) Measured spectra of the pulse from(a) the oscillator, (b) the preamplifier, and (c) the power am-plifier at 3.1 W of average power (black curve). Retrievedspectral phase using the PICASO algorithm (jagged curve).(d) Measured pulse duration from the preamplifier (reddashed-dotted curve) and the power amplifier (black solidcurve).Fig. 3. (Color online) Variation of the compressed pulsewidth as a function of (a) pulse energy and (b) total nonlin-ear phase shift in the power amplifier. Triangles pointingup (down) are experimental (simulated) results.April 1, 2010 / Vol. 35, No. 7 / OPTICS LETTERS 961in reports on fiber amplifiers based on rod-type fibersand other photonic crystal fibers, where the output istaken directly out of the amplifier [7,8]. The Raman-amplified spectral components acquire an irregularphase shift [11], which is not straightforward to com-pensate, leading to accelerated deviation from thetransform limit at higher energies. While the use of afiber-coupled isolator–collimator component is essen-tial for robust operation and to prevent backcouplingof light during, for example, material processing, itenhances the nonlinear effects.The shortest compressed pulses are obtained in the2.5–3.1 J range with the corresponding nonlinearphase shift of 14–18. At 3.1 J of pulse energy, thespectral width is about 25 nm [Fig. 2(c)]. The pulsesare compressed with FWHM durations of 140 fs and170 fs at 3.1 J and 4.0 J, respectively, as inferredfrom the autocorrelation and optical spectrum mea-surements using the PICASO algorithm [12] (Fig. 4).The simulated and experimental time-bandwidthproducts are, respectively, 0.90 and 0.92 at 3.1 J, and1.14 and 1.25 at 4.0 J. The efficiency of our gratingcompressor is fairly low, at 33%, resulting in a com-pressed pulse energy of 1 J. The peak power is esti-mated to be 4.6 MW by integrating the portion ofthe energy inside the pedestal in the retrieved pulse.10 MW of peak power should be attainable with theuse of higher-quality gratings.In conclusion, we have demonstrated an all-fiber-integrated amplifier, implementing nonlinear CPA,Fig. 4. (Color online) Intensity autocorrelation (black solidcurve) and autocorrelation of the retrieved pulse (reddashed curve) of the compressed pulse at 3.1 W outputfrom the power amplifier. Inset, retrieved pulse shape us-ing the PICASO algorithm based on the experimental auto-curve), simulated pulse form (red dotted curve).with no free-space pump or signal-beam propagation.Thus the amplifier is misalignment free and com-pletely robust and is routinely being used for mate-rial processing in our laboratory. It produces70-ps-long pulses with up to 4 J of pulse energy,57 kW of peak power, and total nonlinear shift of22 at 1 MHz repetition rate. These are the highestpeak power, pulse energy, and nonlinear phase shiftsreported thus far from an all-fiber-integrated ampli-fier, to our knowledge. The chirped pulses from theamplifier are compressible to 140 fs and 170 fs dura-tion at 3.1 J and 4.0 J of amplifier output energy,respectively. Intrapulse Raman amplification in theundoped lead fiber of the collimator–isolator deviceafter the gain fiber limits compressibility and in-creases the pedestal at higher pulse energies. The ab-sence of Raman-shifted components in previous re-ports on nonlinear fiber CPA systems is attributed togain filtering during amplification and no subsequentpropagation in undoped fiber.This work was supported by TÜBI˙TAK undergrants 106T017 and 106G089, Marie Curie IRGFiberLaser, Bilkent University Research Funds, andby the Distinguished Young Scientist Award of theTÜBA.References1. S. Hädrich, J. Rothhardt, T. Eidam, J. Limpert, and A.Tünnermann, Opt. Express 17, 3913 (2009).2. F. Ö. Ilday and F. W. Wise, J. Opt. Soc. Am. B 19, 470(2002).3. T. Schreiber, D. Schimpf, D. Müller, F. Röser, J. Limp-ert, and A. Tünnermann, J. Opt. Soc. Am. B 24, 1809(2007).4. S. Zhou, L. Kuznetsova, A. Chong, and F. W. Wise, Opt.Express 13, 4869 (2005).5. L. Shah, Z. Liu, I. Hartl, G. Imeshev, G. C. Cho, and M.E. Fermann, Opt. Express 13, 4717 (2005).6. A. Chong, L. Kuznetsova, and F. W. Wise, J. Opt. Soc.Am. B 24, 1815 (2007).7. L. Kuznetsova and F. W. Wise, Opt. Lett. 32, 2671(2007).8. Y. Zaouter, J. Boullet, E. Mottay, and E. Cormier, Opt.Lett. 33, 1527 (2008).9. P. K. Mukhopadhyay, K. Özgören, I. L. Budunoglu, andF. Ö. Ilday, IEEE J. Sel. Top. Quant. 15, 145 (2009).10. F. O. Ilday, H. Lim, J. R. Buckley, F. W. Wise, and W. G.Clark, Opt. Lett. 28, 1365 (2003).11. C. Headley III and G. P. Agrawal, J. Opt. Soc. Am. B 13,2170 (1996).12. J. W. Nicholson, J. Jasapara, W. Rudolph, F. G. Omen-etto, and A. J. Taylor, Opt. Lett. 24, 1774 (1999).correlation and spectrum measurements (black solid

Microjoule-energy, 1 MHz repetition rate pulses from all-fiber-integrated nonlinear chirped-pulse amplifier

We demonstrate generation of pulses with up to 4 μJ energy at 1 MHz repetition rate through nonlinear chirped-pulse amplification in an entirely fiber-integrated amplifier, seeded by a fiber oscillator. The peak power and the estimated nonlinear phase shift of the amplified pulses are as much as 57 kW and 22π, respectively. The shortest compressed pulse duration of 140 fs is obtained for 3.1 μJ of uncompressed amplifier output energy at 18 π of nonlinear phase shift. At 4 μJ of energy, the nonlinear phase shift is 22 π and compression leads to 170-fs-long pulses. Numerical simulations are utilized to model the experiments and identify the limitations. Amplification is ultimately limited by the onset of Raman amplification of the longer edge of the spectrum with an uncompressible phase profile. © 2010 Optical Society of America

Kalaycioglu H.

Şenel Ç.

Paltani P.P.

Ilday F.Ö.

See	discussions,	stats,	and	author	profiles	for	this	publication	at:	https://www.researchgate.net/publication/42976031Microjoule-energy,	1	MHz	repetition	rate	pulsesfrom	all-fiber-integrated	nonlinear	chirped-pulse	amplifierArticle		in		Optics	Letters	·	April	2010DOI:	10.1364/OL.35.000959	·	Source:	PubMedCITATIONS20READS415	authors,	including:Some	of	the	authors	of	this	publication	are	also	working	on	these	related	projects:Laser-material	interactions	View	projectLaser	Characterization	View	projectHamit	KalayciogluBilkent	University49	PUBLICATIONS			337	CITATIONS			SEE	PROFILECagri	SenelBilkent	University17	PUBLICATIONS			55	CITATIONS			SEE	PROFILEPunya	Prasanna	PaltaniInternational	Institute	of	Information	Technolo…20	PUBLICATIONS			110	CITATIONS			SEE	PROFILEFatih	Ömer	IldayBilkent	University216	PUBLICATIONS			3,488	CITATIONS			SEE	PROFILEAll	content	following	this	page	was	uploaded	by	Hamit	Kalaycioglu	on	03	October	2017.The	user	has	requested	enhancement	of	the	downloaded	file.April 1, 2010 / Vol. 35, No. 7 / OPTICS LETTERS 959Microjoule-energy, 1 MHz repetition rate pulsesfrom all-fiber-integrated nonlinearchirped-pulse amplifierH. Kalaycioglu,1,* B. Oktem,2 Ç. Şenel,1 P. P. Paltani,1 and F. Ö. Ilday11Physics Department, Bilkent University, Çankaya, Ankara 06800, Turkey2Material Science and Nanotechnology Graduate Program, Bilkent University, Ankara 06800, Turkey*Corresponding author: hamitkal@bilkent.edu.trReceived February 11, 2010; accepted February 16, 2010;posted March 2, 2010 (Doc. ID 124188); published March 23, 2010We demonstrate generation of pulses with up to 4 J energy at 1 MHz repetition rate through nonlinearchirped-pulse amplification in an entirely fiber-integrated amplifier, seeded by a fiber oscillator. The peakpower and the estimated nonlinear phase shift of the amplified pulses are as much as 57 kW and 22, re-spectively. The shortest compressed pulse duration of 140 fs is obtained for 3.1 J of uncompressed ampli-fier output energy at 18 of nonlinear phase shift. At 4 J of energy, the nonlinear phase shift is 22 andcompression leads to 170-fs-long pulses. Numerical simulations are utilized to model the experiments andidentify the limitations. Amplification is ultimately limited by the onset of Raman amplification of the longeredge of the spectrum with an uncompressible phase profile. © 2010 Optical Society of AmericaOCIS codes: 060.2320, 320.7090, 060.3510.There is much worldwide interest in the developmentof powerful, robust, and simple-to-operate ultrafastfiber lasers. Their biggest drawback is the limitationto peak power by nonlinear effects. While most ef-forts are focused on weakening these effects throughthe use of large core specialty fibers [1], another ap-proach is to manage the nonlinear effects. Examplesof this approach include proposals on direct compen-sation of nonlinear effects [2], mitigation by spectralpreshaping [3], and indirect compensation via third-order dispersion [4–6]. The latter, also known as non-linear chirped-pulse amplification (nonlinear CPA),has led to the generation of 30 J, 240 fs pulses [7]and 100 J, 650 fs pulses [5] using 40-m-core spe-cialty photonic crystal fibers, along with 100 J,270 fs pulses [8] using a 80-m-core rod-type ampli-fier. The main advantages of nonlinear CPA are thatit enables generation of short pulses from a fiber-stretcher, grating-compressor pair with unmatchedthird-order dispersion and allows the accumulationof significant nonlinear phase shift. The main draw-back is that pulse quality is significantly reduced.Nevertheless, this is acceptable for many applica-tions based on peak power, where pulse quality is ofsecondary concern.To date, all implementations of nonlinear CPA pro-ducing high-energy pulses have relied on the use ofspecialty Yb-doped fibers with extremely large corediameters requiring the use of bulk optics to couplesignal and pump light into the fiber. Although thesefibers allow higher energies to be generated, they re-quire special preparation of the fiber ends and muchcare in bulk coupling of pump light and maintainingsingle-mode operation. These aspects negate some ofthe primary advantages of fiber amplifiers. To makefull use of the advantages of the nonlinear CPA ap-proach, the entire amplifier should be implementedin fiber. Recently, we demonstrated a 10 W fiber am-plifier at 40 MHz repetition rate, where pulse propa-0146-9592/10/070959-3/$15.00 ©gation from fiber oscillator until the grating compres-sor was in fiber [9].Here, we demonstrate an all-fiber-integrated non-linear CPA system, seeded by a fiber oscillator, gen-erating pulses with energy up to 4 J at 1 MHz rep-etition rate. The integrated architecture allows trulyrobust long-term operation. The shortest dechirpedpulse duration of 140 fs is obtained for 3.1 J ampli-fier output, corresponding to a total nonlinear phaseshift of 18. Peak powers of the chirped pulses fromthe amplifier are up to 57 kW, which corresponds tothe generation of largest nonlinear phase shifts froma fiber amplifier while maintaining compressibility tosub-200 fs, in addition to being the highest peak pow-ers obtained from an all-fiber-integrated amplifier, tothe best of our knowledge. Numerical simulationsprovide better agreement with experiments com-pared with prior reports and give insight into thetrade-offs of nonlinear fiber CPA systems.The experimental arrangement of the laser systemis shown in Fig. 1. The seed pulses are generatedfrom a Yb-doped fiber laser oscillator similar to thatin [10]. The fiber section of the Yb laser oscillator con-sists of 6.6 m of single-mode fiber (SMF) (HI-1060)and 0.30 m of Yb-doped fiber, followed by another0.93 m of SMF. The total dispersion of the cavity iscalculated to be about 6150 fs2. The gain fiber ispumped in-core by a 976 nm fiber-coupled pump di-ode with an output power up to 550 mW. The pumplight is delivered via a 980/1030 nm wavelength di-vision multiplexer. An optical isolator ensures unidi-rectional operation, which facilitates self-starting op-eration. Mode locking is initiated and stabilized bythe nonlinear polarization evolution effect. The laseroutput is checked against multiple pulsing by way oflong-range autocorrelation and 12 GHz bandwidth rfspectral measurements.The all-fiber-integrated amplifier consists of afiber-pigtailed acousto-optic modulator (AOM), a fiber2010 Optical Society of America960 OPTICS LETTERS / Vol. 35, No. 7 / April 1, 2010stretcher, a core-pumped preamplifier, and acladding-pumped power amplifier. Finally, the ampli-fied pulses are compressed in a grating compressor.All pump light is delivered through fiber couplers.The preamplifier consists of a 0.30-m-long Yb-dopedfiber (of the same type as in the oscillator) pumpedbackward with a 300 mW pump diode. The poweramplifier is pumped codirectionally to protect thepump diodes from optical damage. Pump and signaldelivery to the gain fiber is accomplished with amultiport pump-signal combiner (MPC). Four975 nm diodes, each with up to 10 W power coupledto a 105-m-core multimode fiber, are used as pump.The amplifier consists of 2-m-long, large-mode-area(LMA), double-clad (DC) ytterbium-doped fiber witha 20 m core with NA of 0.06, and a 125 m claddingwith NA of 0.46. The output is taken through a fiber-pigtailed isolator-collimator device with a transmis-sion ratio of 90%.The fiber laser oscillator produces 3-ps-longchirped pulses with a bandwidth of 45 nm at a rep-etition rate of 28 MHz [Fig. 2(a)]. An output power of105 mW seeds the amplifier directly through a 50%output coupler placed after the gain fiber andtraverses the fiber-integrated AOM, reducing the rep-etition rate to 1 MHz. The pulses are then stretchedto 150 ps in a 100-m-long fiber stretcher [compris-ing HI-1060, with an estimated group velocity disper-sion of 22 fs2/mm and third-order dispersion (TOD)of 74 fs3/mm] and amplified to 104 nJ in the preamp-lifier. The FWHM bandwidth of the spectrum is mea-sured to be about 20 nm [Fig. 2(b)]. The temporalwidth, which is also reduced owing to gain narrow-ing, is measured to be 80 ps using a 20 GHz band-width sampling scope and a fast detector with a risetime of 35 ps [Fig. 2(d)]. The 50% reduction in thepulse duration is due to the dominance of the gain fil-tering in the preamplifier over self-phase modulation(SPM). The situation is reversed in the power ampli-Fig. 1. (Color online) Schematic diagram of the amplifiersystem. PC, polarization controller; PBS, polarization beamsplitter; AOM, acousto-optic modulator; WDM, wavelength-division multiplexer; LMA, large mode area; DC, double-clad; MPC, multiport pump-signal combiner.fier.Numerical simulations of the pulse generation andamplification based on the model described in [9]guide the experiments at all stages. As can be seen inFig. 3(a), experimentally the compressed pulse widthdecreases from 350 to 150 fs with increasing pulseenergy from 0.5 to 3 J as a result of the compensa-tion of TOD by SPM, and as supported by numericalsimulations [Figs. 3(a) and 3(b)]. The broadeningmechanism at the long wavelength edge of the spec-trum is attributed to the Raman effect, which occursmainly in the 75-cm-long undoped lead fiber of thecollimator, where more than half of the nonlinearphase shift is accumulated. This has been confirmedby observing reduction of the Raman effect by reduc-ing the undoped fiber length, while keeping the totalnonlinear phase shift fixed. The Raman amplificationis suppressed by the gain filtering during amplifica-tion. This likely explains the absence of Raman shiftsFig. 2. (Color online) Measured spectra of the pulse from(a) the oscillator, (b) the preamplifier, and (c) the power am-plifier at 3.1 W of average power (black curve). Retrievedspectral phase using the PICASO algorithm (jagged curve).(d) Measured pulse duration from the preamplifier (reddashed-dotted curve) and the power amplifier (black solidcurve).Fig. 3. (Color online) Variation of the compressed pulsewidth as a function of (a) pulse energy and (b) total nonlin-ear phase shift in the power amplifier. Triangles pointingup (down) are experimental (simulated) results.April 1, 2010 / Vol. 35, No. 7 / OPTICS LETTERS 961in reports on fiber amplifiers based on rod-type fibersand other photonic crystal fibers, where the output istaken directly out of the amplifier [7,8]. The Raman-amplified spectral components acquire an irregularphase shift [11], which is not straightforward to com-pensate, leading to accelerated deviation from thetransform limit at higher energies. While the use of afiber-coupled isolator–collimator component is essen-tial for robust operation and to prevent backcouplingof light during, for example, material processing, itenhances the nonlinear effects.The shortest compressed pulses are obtained in the2.5–3.1 J range with the corresponding nonlinearphase shift of 14–18. At 3.1 J of pulse energy, thespectral width is about 25 nm [Fig. 2(c)]. The pulsesare compressed with FWHM durations of 140 fs and170 fs at 3.1 J and 4.0 J, respectively, as inferredfrom the autocorrelation and optical spectrum mea-surements using the PICASO algorithm [12] (Fig. 4).The simulated and experimental time-bandwidthproducts are, respectively, 0.90 and 0.92 at 3.1 J, and1.14 and 1.25 at 4.0 J. The efficiency of our gratingcompressor is fairly low, at 33%, resulting in a com-pressed pulse energy of 1 J. The peak power is esti-mated to be 4.6 MW by integrating the portion ofthe energy inside the pedestal in the retrieved pulse.10 MW of peak power should be attainable with theuse of higher-quality gratings.In conclusion, we have demonstrated an all-fiber-integrated amplifier, implementing nonlinear CPA,Fig. 4. (Color online) Intensity autocorrelation (black solidcurve) and autocorrelation of the retrieved pulse (reddashed curve) of the compressed pulse at 3.1 W outputfrom the power amplifier. Inset, retrieved pulse shape us-ing the PICASO algorithm based on the experimental auto-curve), simulated pulse form (red dotted curve).View publication statswith no free-space pump or signal-beam propagation.Thus the amplifier is misalignment free and com-pletely robust and is routinely being used for mate-rial processing in our laboratory. It produces70-ps-long pulses with up to 4 J of pulse energy,57 kW of peak power, and total nonlinear shift of22 at 1 MHz repetition rate. These are the highestpeak power, pulse energy, and nonlinear phase shiftsreported thus far from an all-fiber-integrated ampli-fier, to our knowledge. The chirped pulses from theamplifier are compressible to 140 fs and 170 fs dura-tion at 3.1 J and 4.0 J of amplifier output energy,respectively. Intrapulse Raman amplification in theundoped lead fiber of the collimator–isolator deviceafter the gain fiber limits compressibility and in-creases the pedestal at higher pulse energies. The ab-sence of Raman-shifted components in previous re-ports on nonlinear fiber CPA systems is attributed togain filtering during amplification and no subsequentpropagation in undoped fiber.This work was supported by TÜBİTAK undergrants 106T017 and 106G089, Marie Curie IRGFiberLaser, Bilkent University Research Funds, andby the Distinguished Young Scientist Award of theTÜBA.References1. S. Hädrich, J. Rothhardt, T. Eidam, J. Limpert, and A.Tünnermann, Opt. Express 17, 3913 (2009).2. F. Ö. Ilday and F. W. Wise, J. Opt. Soc. Am. B 19, 470(2002).3. T. Schreiber, D. Schimpf, D. Müller, F. Röser, J. Limp-ert, and A. Tünnermann, J. Opt. Soc. Am. B 24, 1809(2007).4. S. Zhou, L. Kuznetsova, A. Chong, and F. W. Wise, Opt.Express 13, 4869 (2005).5. L. Shah, Z. Liu, I. Hartl, G. Imeshev, G. C. Cho, and M.E. Fermann, Opt. Express 13, 4717 (2005).6. A. Chong, L. Kuznetsova, and F. W. Wise, J. Opt. Soc.Am. B 24, 1815 (2007).7. L. Kuznetsova and F. W. Wise, Opt. Lett. 32, 2671(2007).8. Y. Zaouter, J. Boullet, E. Mottay, and E. Cormier, Opt.Lett. 33, 1527 (2008).9. P. K. Mukhopadhyay, K. Özgören, I. L. Budunoglu, andF. Ö. Ilday, IEEE J. Sel. Top. Quant. 15, 145 (2009).10. F. O. Ilday, H. Lim, J. R. Buckley, F. W. Wise, and W. G.Clark, Opt. Lett. 28, 1365 (2003).11. C. Headley III and G. P. Agrawal, J. Opt. Soc. Am. B 13,2170 (1996).12. J. W. Nicholson, J. Jasapara, W. Rudolph, F. G. Omen-etto, and A. J. Taylor, Opt. Lett. 24, 1774 (1999).correlation and spectrum measurements (black solid

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Microjoule-energy, 1 MHz repetition rate pulses from all-fiber-integrated nonlinear chirped-pulse amplifier

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