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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
High-peak-power, high-energy, high-average-power pulsed fiber laser system with versatile pulse duration and shape A. Malinowski,1,* P. Gorman,1,2 C. A. Codemard,1,2 F. Ghiringhelli,1 A. J. Boyland,1,2 A. Marshall,1 M. N. Zervas,1,2 and M. K. Durkin1 1 2
SPI Lasers UK Ltd., 6 Wellington Park, Tollbar Way, Hedge End, Southampton SO30 2QU, UK
Advanced Laser Laboratory, Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK *Corresponding author: [email protected] Received July 17, 2013; revised October 2, 2013; accepted October 3, 2013; posted October 8, 2013 (Doc. ID 194110); published November 8, 2013 We present a pulsed fiber laser system with average power up to 265 W, pulse energy up to 10.6 mJ, pulse duration adjustable in the range 500 ps–500 ns, repetition rate fully controllable from single-shot operation up to 1 MHz, and the ability to control peak power independently of pulse energy. The system has a compact, all-spliced construction. Such a versatile laser will have wide applications in materials processing. © 2013 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.2020) Diode lasers. http://dx.doi.org/10.1364/OL.38.004686
Pulsed fiber lasers can provide high average powers, high pulse energies, and high peak powers, as well as a wide choice of pulse durations [1], and they have a large range of possible industrial applications. The characteristic advantages of fiber lasers such as compactness, highwall-plug efficiency, reliability, and maintenance-free operation make those lasers very compelling. Indeed, commercial nanosecond-pulse-duration fiber lasers are available with multi-mJ pulse energy and 100 s of W average power. However, a typical fiber laser system operates within a limited performance range, which reduces the range of applications. In this Letter, we present an agile laser system that overcomes those limitations and can provide high average power, high pulse energy, high and adjustable peak power, and widely adjustable pulse duration and repetition rate and hence is suitable for a very wide range of applications while maintaining all the advantages inherent in fiber laser systems. Higher than 10 mJ pulse energies have been reported in fiber laser-type systems before. For example, 82 mJ pulse energy has been demonstrated in a M 2 6.5 system [2] using a 200 mm∕600 mm core/cladding diameter fiber with precisely controlled coiling with fairly large diameter to moderate the M2. Pulse energy of 26 mJ has been demonstrated with close to single-mode beam quality (M 2 < 1.3) in photonic crystal fibers [3], and nanosecond pulses have been demonstrated with peak powers >4 MW [4] in an inflexible rod-type fiber. All the above examples have utilized free-space launching and coupling into the power amplifiers. This inevitably results in multiple interfaces, and special care should be taken to avoid spurious backreflections. Also, such a topology requires careful alignments and makes the laser system prone to surface contamination. Finally, rod-type fiber amplifiers are shown to suffer from modal instabilities, which severely limit their power scaling [5]. In our system, we have used normal solid-core fibers with no more than 200 μm cladding size, which can be easily spliced and relatively tightly bent. No change in performance was observed at 5 cm bend radius. This makes packaging easy and means that 0146-9592/13/224686-04$15.00/0
the entire master-oscillator power amplifier (MOPA) system can be assembled using standard industrial splicing techniques. A fully spliced 10 mJ (300 ns) multimode laser using standard fiberized components has been reported [6]. This was achieved by amplifying μJ pulses from a fiber laser generated through relaxation oscillations. More than 200 W output power and 2.3 mJ pulses have been reported in a fully fiberized system by amplifying a Qswitched fiber laser [7]. These approaches are limited in the range of pulse widths they can produce and have no control of pulse shape independent of pulse amplitude. Our system utilizes a directly driven diode laser as the seed (master oscillator). By controlling the drive current to the seed diode, we are able to cover a much wider range of pulse durations than were achieved with the above systems and have the opportunity to shape the seed pulses to compensate the effects of saturation of our amplifiers. The ability to deliver pulses with very different characteristics from the same laser is very useful in a variety of applications. This is particularly beneficial when performing different operations during the process time, such as engraving and then cleaning the same area [8], or when processing composite or multilayer samples, where the different pulses are used for processing different layers. The relatively high M 2 of our system was chosen to match emerging fiber laser applications. While fiber laser development typically targets close to single-mode output and M 2 close to 1, there are many applications for which a more top-hat beam profile is preferred [9]. For example, color or black marking on stainless steel [10], texturing with fairly large (∼100 μm) features, paint removal, and anodized aluminium large area removal all require fairly flat-topped pulses to avoid localized melting or “gouging” from the center of the spot. In the case of marking or engraving, it is now recognized that high power and high energy output with optimized M 2 larger than 1 is required in order to minimize the processing time while keeping the marking and engraving quality high [11]. © 2013 Optical Society of America
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
We demonstrate a fiber MOPA system with an average power of up to 265 W, pulse energies up to 10.6 mJ, high (and adjustable) peak powers, and pulse duration adjustable in the range 500 ps–500 ns, with good spectral quality. Because of the ability to modulate the pumps, repetition rate is fully controllable from single-shot operation up 1 MHz (only limited by drive electronics and laser scanners). Figure 1 shows a simplified schematic of the system. The MOPA consists of a directly driven Fabry–Perot (F–P) diode seed laser operating at 1061 nm followed by three stages of amplification in nonphotodarkening Yb-doped fibers. All the amplification stages are based on SPI’s proprietary GT-Wave fiber coupling technology [12,13]. The system has a compact, all-fiber and fully spliced construction. The seed laser is directly electrically driven by a homemade laser driver. It delivers ∼1 W peak power and has a cw spectral bandwidth ∼3 nm. The first and second amplifier stages consist of singlemode Yb-doped fiber. An acousto-optic modulator (AOM), developed by Gooch and Housego in support of the development of this system, which can handle a maximum input power of 5 W, is placed between the second and third amplification stage. It serves as an optional pulse shaper, with a minimum rise/fall time of 30 ns. The final stage utilizes a 50 μm step-index core (NA 0.1) multimode Yb-doped fiber that is directly spliced to a 62.5 μm multimode delivery fiber of length ∼2.5 m. The fiber output is terminated with an angled end cap to avoid any spurious detrimental reflection. It is pumped with up to 405 W of diode laser power combined in a 200 μm delivery fiber. Power conversion efficiency of the final stage is ∼70% with respect to launch pump power (Fig. 2) and the output beam has a measured M 2 7.3. High energy pulses undergo significant gain reshaping [14] in fiber amplifiers, due to the relatively modest saturation energies [15]. This leads to reduced FWHM pulse duration and increased peak power. Because of the long interaction length, fiber amplifiers are also susceptible to a variety of nonlinear effects that can degrade the spectral quality of the output, in particular self-phase modulation and stimulated Raman scattering [16]. Hence, control over the pulse shape, and consequently of the peak power of the laser at particular pulse energy, is very important. Considerable control over the peak power can be achieved by pre-shaping the pulses before they enter the final amplifier [17–23]. In our system, this can be achieved in two ways. The first is by controlling the shape of the rising edge of the seed laser pulse [21]. This
Fig. 1. Schematic of fiber laser system. YFA, ytterbium fiber amplifier; SM, single mode; MM, multi-mode; AOM, acoustooptic modulator.
Fig. 2.
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Power performance (inset: beam profile at focus).
is limited by instability issues when operating the seed near threshold. The second is by shaping the leading edge of the output pulse from the second preamplifier using the AOM before launching to the final amplifier. The practical limitation here is the amount of pulse energy sacrificed at this stage and hence the requirement for greater gain in the final amplifier. Figure 3 shows the evolution through the laser system of various pulses, all with full duration about 500 ns, and all amplified to 10.6 mJ, but shaped in different ways. There is relatively modest reshaping in the preamplifiers, but reshaping in the final amplifier is substantial. Figure 3(a) shows two pulses generated by the seed laser. The pulse shown in green has a rise time of ∼2 ns, the pulse shown in red has a rise time of 70 ns. Figure 3(b) shows the results of shaping at the output of the AOM. The green trace shows the effect of reshaping on a seed pulse with a rising edge of ∼2 ns, the red trace the output with a seed pulse with rise time 70 ns, and the blue trace showing the 2 ns rise time after reshaping with the AOM. The waveform used with the AOM has approximately 13 dB loss at the start of the pulse (in addition to the standard insertion loss) and a rising edge of 100 ns. This was chosen because we wanted reshaping to cause no more than 20% additional loss before launching to the final amplifier. Figure 3(c) compares pulses with full width of 500 ns produced with different shaping methods and amplified to 10.6 mJ (265 W average power at 25 kHz): using the squarest available pulse from the seed, with a rising edge of ∼2 ns, yields a FWHM width of 5 ns and peak power of 456 kW. A seed pulse with a rising edge of 70 ns yields a FWHM width of 58 ns and peak power of 100 kW. Shaping the 2 ns rise time pulse from the seed with the AOM yields FWHM width of 118 ns and peak power 62 kW. The effect of pulse shaping on the spectrum is significant (Fig. 4). The 456 kW peak power pulse, while it still has a narrow (∼3.5 nm) peak (corresponding to the long low power tail of the pulse), also has a very spectrally broad component (corresponding to the peak of the pulse), containing ∼15% of the total energy. The seedshaped pulse by contrast is only slightly broadened, and while there is evidence of Raman amplification (peak at 1120 nm), it is at −40 dB compared to the main peak.
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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Fig. 5.
Fig. 3. (a) Seed laser output with 2 ns and 70 ns rise times, (b) pulse shapes out of AOM, and (c) corresponding pulse shapes out of final amplifier with 10.6 mJ pulse energy.
The AOM-shaped pulse shows even less broadening, with no sign of Raman amplification. We could also directly drive the seed to produce pulses as short as 0.3 ns. This involves cutting off the current during the rising edge of diode response, so the seed peak power is lower than for longer pulses (∼100 mW). With
Fig. 4.
Spectra of various pulses with 10.6 mJ pulse energy.
0.5 ns pulse amplified to 65 μJ.
the three available amplification stages, there is still sufficient gain in the system to amplify this to >100 kW. Figure 5(a) shows a 0.3 ns seed pulse (spectral bandwidth 6 nm) and the final amplified pulse with peak power 108 kW and pulse energy 65 μJ (65 W average power at 1 MHz). There is an increase of pulse duration to ∼0.5 ns FWHM. This broadening is not power dependent and is of the magnitude that might be expected due to modal dispersion in the multimode final amplifier and delivery fiber, but this has not been investigated in detail. The spectrum [Fig. 5(b)] shows some broadening; spectral width is 9 nm, but power in the Raman peak is still at −25 dB compared to the center wavelength and 96% of the power is within 20 nm of the spectral peak. Since we do not need to consider separately the contributions of a narrow peak and long low power tail, short pulses that are not significantly reshaped are more useful than long ones for investigating the relationship between peak power and spectral quality. We used 3 ns pulses with pulse energies up to 590 μJ (265 W average power at 450 kHz) to examine spectral broadening in our system [Fig. 6(a)]. Because of the very high gain available in fiber amplifiers, their nonlinear effective length is short compared to their actual length. Therefore in a system like the one described here, it may be pulse evolution in the delivery fiber that makes the main contribution to nonlinear effects on the pulse spectrum. The relative contribution of the final amplifier and delivery fiber to spectral broadening was investigated by changing the length of the delivery fiber and comparing the fraction of laser power within 20 nm of the peak wavelength as a function of pulse peak power. Figure 6(b) shows the spectrum of a 3 ns pulse [shown in Fig. 6(a)] with delivery fiber length of 1.7 and 0.3 m. With 1.7 m delivery fiber, the Raman peak is at −13 dB compared to the main peak, with 0.3 m it is completely suppressed. Figure 6(c) shows spectral quality as a function of peak power for each length of delivery fiber. Shortening the delivery fiber to ∼30 cm results in excellent spectral quality even at peak power levels of 180 kW. It is clear that nonlinear interactions in the delivery fiber dominate spectral broadening and that by reducing the delivery fiber to a short (but practical in the case of an amplifier mounted on machining head) very high peak powers can be achieved with good spectral quality. We have demonstrated a highly versatile high pulse energy fiber laser system. Compared to previous high pulse energy fiber lasers [1], the F–P diode seed allows a wide range of durations and built-in pulse shaping
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
Fig. 6. Dependence of spectral quality on bandwidth for different lengths of delivery fiber.
capability. Very high peak powers could be achieved: The practical limitation in application will be set by delivery fiber lengths and spectral quality requirements. The average power was limited by the available pump power and can be readily scaled further. SPI acknowledges support for this work under the European Commission 7th Framework Programme (FP7-NMP-2008- 4.0.4) project “Leadership in Fibre Laser Technology,” grant agreement no. CP-IP 228587-1. References 1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). 2. M. Cheng, Y. Chang, A. Galvanauskas, P. Mamidipudi, R. Changkakoti, and P. Gatchell, Opt. Lett. 30, 358 (2005). 3. F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Lett. 37, 1073 (2012). 4. C. D. Brooks and F. D. Teodoroa, Appl. Phys. Lett. 89, 111119 (2006). 5. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Express 19, 13218 (2011).
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6. S. Maryashin, A. Unt, and V. P. Gapontsev, Proc. SPIE 6102, 61020O (2006). 7. Q. Fang, Y. Qin, B. Wang, and W. Shi, Appl. Opt. 52, 6744 (2013). 8. SPI Lasers UK Ltd., http://www.spilasers.com/Applications/ redENERGY/Engraving_with_a_Fiber_Laser.aspx?. 9. G. Shannon, http://info.miyachiunitek.com/blog/bid/132560/ Fiber‑Laser‑Micro‑Welding‑Single‑Mode‑or‑Multi‑Mode. 10. P. Laakso, S. Ruotsalainen, H. Pantsar, and R. Penttilä, “Relation of laser parameters in color marking of stainless steel,” presented at the 12th Conference on Laser Processing of Materials in the Nordic Countries (NOLAMP), Copenhagen, August 24–26, 2009. 11. G. Piccino and J. Gold, Industrial Laser Solutions for Manufacturing, http://www.industrial‑lasers.com/ articles/print/volume‑18/issue‑5/features/marking‑and‑ engravingmdashwhen‑less‑is‑more.html. 12. A. B. Grudinin, D. N. Payne, P. W. Turner, J. Nilsson, M. N. Zervas, M. Ibsen, and M. K. Durkin, “Multi-fibre arrangements for high power fibre lasers and amplifiers”, U.S. patent 6,826,335 (April 28, 2000). 13. M. N. Zervas, A. Marshall, and J. Kim, Proc. SPIE 7914, 79141T (2011). 14. L. M. Frantz and J. S. Nodvik, J. Appl. Phys. 34, 2346 (1963). 15. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, IEEE J. Quantum Electron. 33, 1049 (1997). 16. J. W. Dawson, M. J. Messerly, R. J. Beach, M. Y. Shverdin, E. A. Stappaerts, A. K. Sridharan, P. H. Pax, J. E. Heebner, C. W. Siders, and C. P. J. Barty, Opt. Express 16, 13240 (2008). 17. W. Williams, C. Orth, R. Sacks, J. Lawson, K. Jancaitis, J. Trenholme, S. Haney, J. Auerbach, M. Henesian, and P. Renard, in Inertial Confinement Fusion Annual Report (Lawrence Livermore National Laboratory, 1996), p. 184. 18. G. Kulcsar, Y. Jaouen, E. Olmedo, and G. Canat, “Maximizing energy extraction from Er3+/Yb3+ doped fiber amplifier in shaped microsecond pulses,” presented at the Conference on Lasers, and Electro-Optics, 2003, paper CMK6. 19. M. Shaw, W. Williams, R. House, and C. Haynam, in Inertial Confinement Fusion Semiannual Report (Lawrence Livermore National Laboratory, 2004). 20. W. Shaikh, I. O. Musgrave, A. S. Bhamra, and C. HernandezGomez, in Central Laser Facility Annual Report (CCLRC Rutherford Appleton Laboratory, 2005/2006), p. 199. 21. K. T. Vu, A. Malinowski, D. J. Richardson, F. Ghiringhelli, L. M. B. Hickey, and M. N. Zervas, Opt. Express 14, 10996 (2006). 22. D. N. Schimpf, C. Ruchert, D. Nodop, J. Limpert, A. Tünnermann, and F. Salin, Opt. Express 16, 17637 (2008). 23. A. Malinowski, K. T. Vu, K. K. Chen, J. Nilsson, Y. Jeong, S. Alam, D. Lin, and D. J. Richardson, Opt. Express 17, 20927 (2009).
OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
High-peak-power, high-energy, high-average-power pulsed fiber laser system with versatile pulse duration and shape A. Malinowski,1,* P. Gorman,1,2 C. A. Codemard,1,2 F. Ghiringhelli,1 A. J. Boyland,1,2 A. Marshall,1 M. N. Zervas,1,2 and M. K. Durkin1 1 2
SPI Lasers UK Ltd., 6 Wellington Park, Tollbar Way, Hedge End, Southampton SO30 2QU, UK
Advanced Laser Laboratory, Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, UK *Corresponding author: [email protected] Received July 17, 2013; revised October 2, 2013; accepted October 3, 2013; posted October 8, 2013 (Doc. ID 194110); published November 8, 2013 We present a pulsed fiber laser system with average power up to 265 W, pulse energy up to 10.6 mJ, pulse duration adjustable in the range 500 ps–500 ns, repetition rate fully controllable from single-shot operation up to 1 MHz, and the ability to control peak power independently of pulse energy. The system has a compact, all-spliced construction. Such a versatile laser will have wide applications in materials processing. © 2013 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (140.2020) Diode lasers. http://dx.doi.org/10.1364/OL.38.004686
Pulsed fiber lasers can provide high average powers, high pulse energies, and high peak powers, as well as a wide choice of pulse durations [1], and they have a large range of possible industrial applications. The characteristic advantages of fiber lasers such as compactness, highwall-plug efficiency, reliability, and maintenance-free operation make those lasers very compelling. Indeed, commercial nanosecond-pulse-duration fiber lasers are available with multi-mJ pulse energy and 100 s of W average power. However, a typical fiber laser system operates within a limited performance range, which reduces the range of applications. In this Letter, we present an agile laser system that overcomes those limitations and can provide high average power, high pulse energy, high and adjustable peak power, and widely adjustable pulse duration and repetition rate and hence is suitable for a very wide range of applications while maintaining all the advantages inherent in fiber laser systems. Higher than 10 mJ pulse energies have been reported in fiber laser-type systems before. For example, 82 mJ pulse energy has been demonstrated in a M 2 6.5 system [2] using a 200 mm∕600 mm core/cladding diameter fiber with precisely controlled coiling with fairly large diameter to moderate the M2. Pulse energy of 26 mJ has been demonstrated with close to single-mode beam quality (M 2 < 1.3) in photonic crystal fibers [3], and nanosecond pulses have been demonstrated with peak powers >4 MW [4] in an inflexible rod-type fiber. All the above examples have utilized free-space launching and coupling into the power amplifiers. This inevitably results in multiple interfaces, and special care should be taken to avoid spurious backreflections. Also, such a topology requires careful alignments and makes the laser system prone to surface contamination. Finally, rod-type fiber amplifiers are shown to suffer from modal instabilities, which severely limit their power scaling [5]. In our system, we have used normal solid-core fibers with no more than 200 μm cladding size, which can be easily spliced and relatively tightly bent. No change in performance was observed at 5 cm bend radius. This makes packaging easy and means that 0146-9592/13/224686-04$15.00/0
the entire master-oscillator power amplifier (MOPA) system can be assembled using standard industrial splicing techniques. A fully spliced 10 mJ (300 ns) multimode laser using standard fiberized components has been reported [6]. This was achieved by amplifying μJ pulses from a fiber laser generated through relaxation oscillations. More than 200 W output power and 2.3 mJ pulses have been reported in a fully fiberized system by amplifying a Qswitched fiber laser [7]. These approaches are limited in the range of pulse widths they can produce and have no control of pulse shape independent of pulse amplitude. Our system utilizes a directly driven diode laser as the seed (master oscillator). By controlling the drive current to the seed diode, we are able to cover a much wider range of pulse durations than were achieved with the above systems and have the opportunity to shape the seed pulses to compensate the effects of saturation of our amplifiers. The ability to deliver pulses with very different characteristics from the same laser is very useful in a variety of applications. This is particularly beneficial when performing different operations during the process time, such as engraving and then cleaning the same area [8], or when processing composite or multilayer samples, where the different pulses are used for processing different layers. The relatively high M 2 of our system was chosen to match emerging fiber laser applications. While fiber laser development typically targets close to single-mode output and M 2 close to 1, there are many applications for which a more top-hat beam profile is preferred [9]. For example, color or black marking on stainless steel [10], texturing with fairly large (∼100 μm) features, paint removal, and anodized aluminium large area removal all require fairly flat-topped pulses to avoid localized melting or “gouging” from the center of the spot. In the case of marking or engraving, it is now recognized that high power and high energy output with optimized M 2 larger than 1 is required in order to minimize the processing time while keeping the marking and engraving quality high [11]. © 2013 Optical Society of America
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
We demonstrate a fiber MOPA system with an average power of up to 265 W, pulse energies up to 10.6 mJ, high (and adjustable) peak powers, and pulse duration adjustable in the range 500 ps–500 ns, with good spectral quality. Because of the ability to modulate the pumps, repetition rate is fully controllable from single-shot operation up 1 MHz (only limited by drive electronics and laser scanners). Figure 1 shows a simplified schematic of the system. The MOPA consists of a directly driven Fabry–Perot (F–P) diode seed laser operating at 1061 nm followed by three stages of amplification in nonphotodarkening Yb-doped fibers. All the amplification stages are based on SPI’s proprietary GT-Wave fiber coupling technology [12,13]. The system has a compact, all-fiber and fully spliced construction. The seed laser is directly electrically driven by a homemade laser driver. It delivers ∼1 W peak power and has a cw spectral bandwidth ∼3 nm. The first and second amplifier stages consist of singlemode Yb-doped fiber. An acousto-optic modulator (AOM), developed by Gooch and Housego in support of the development of this system, which can handle a maximum input power of 5 W, is placed between the second and third amplification stage. It serves as an optional pulse shaper, with a minimum rise/fall time of 30 ns. The final stage utilizes a 50 μm step-index core (NA 0.1) multimode Yb-doped fiber that is directly spliced to a 62.5 μm multimode delivery fiber of length ∼2.5 m. The fiber output is terminated with an angled end cap to avoid any spurious detrimental reflection. It is pumped with up to 405 W of diode laser power combined in a 200 μm delivery fiber. Power conversion efficiency of the final stage is ∼70% with respect to launch pump power (Fig. 2) and the output beam has a measured M 2 7.3. High energy pulses undergo significant gain reshaping [14] in fiber amplifiers, due to the relatively modest saturation energies [15]. This leads to reduced FWHM pulse duration and increased peak power. Because of the long interaction length, fiber amplifiers are also susceptible to a variety of nonlinear effects that can degrade the spectral quality of the output, in particular self-phase modulation and stimulated Raman scattering [16]. Hence, control over the pulse shape, and consequently of the peak power of the laser at particular pulse energy, is very important. Considerable control over the peak power can be achieved by pre-shaping the pulses before they enter the final amplifier [17–23]. In our system, this can be achieved in two ways. The first is by controlling the shape of the rising edge of the seed laser pulse [21]. This
Fig. 1. Schematic of fiber laser system. YFA, ytterbium fiber amplifier; SM, single mode; MM, multi-mode; AOM, acoustooptic modulator.
Fig. 2.
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Power performance (inset: beam profile at focus).
is limited by instability issues when operating the seed near threshold. The second is by shaping the leading edge of the output pulse from the second preamplifier using the AOM before launching to the final amplifier. The practical limitation here is the amount of pulse energy sacrificed at this stage and hence the requirement for greater gain in the final amplifier. Figure 3 shows the evolution through the laser system of various pulses, all with full duration about 500 ns, and all amplified to 10.6 mJ, but shaped in different ways. There is relatively modest reshaping in the preamplifiers, but reshaping in the final amplifier is substantial. Figure 3(a) shows two pulses generated by the seed laser. The pulse shown in green has a rise time of ∼2 ns, the pulse shown in red has a rise time of 70 ns. Figure 3(b) shows the results of shaping at the output of the AOM. The green trace shows the effect of reshaping on a seed pulse with a rising edge of ∼2 ns, the red trace the output with a seed pulse with rise time 70 ns, and the blue trace showing the 2 ns rise time after reshaping with the AOM. The waveform used with the AOM has approximately 13 dB loss at the start of the pulse (in addition to the standard insertion loss) and a rising edge of 100 ns. This was chosen because we wanted reshaping to cause no more than 20% additional loss before launching to the final amplifier. Figure 3(c) compares pulses with full width of 500 ns produced with different shaping methods and amplified to 10.6 mJ (265 W average power at 25 kHz): using the squarest available pulse from the seed, with a rising edge of ∼2 ns, yields a FWHM width of 5 ns and peak power of 456 kW. A seed pulse with a rising edge of 70 ns yields a FWHM width of 58 ns and peak power of 100 kW. Shaping the 2 ns rise time pulse from the seed with the AOM yields FWHM width of 118 ns and peak power 62 kW. The effect of pulse shaping on the spectrum is significant (Fig. 4). The 456 kW peak power pulse, while it still has a narrow (∼3.5 nm) peak (corresponding to the long low power tail of the pulse), also has a very spectrally broad component (corresponding to the peak of the pulse), containing ∼15% of the total energy. The seedshaped pulse by contrast is only slightly broadened, and while there is evidence of Raman amplification (peak at 1120 nm), it is at −40 dB compared to the main peak.
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OPTICS LETTERS / Vol. 38, No. 22 / November 15, 2013
Fig. 5.
Fig. 3. (a) Seed laser output with 2 ns and 70 ns rise times, (b) pulse shapes out of AOM, and (c) corresponding pulse shapes out of final amplifier with 10.6 mJ pulse energy.
The AOM-shaped pulse shows even less broadening, with no sign of Raman amplification. We could also directly drive the seed to produce pulses as short as 0.3 ns. This involves cutting off the current during the rising edge of diode response, so the seed peak power is lower than for longer pulses (∼100 mW). With
Fig. 4.
Spectra of various pulses with 10.6 mJ pulse energy.
0.5 ns pulse amplified to 65 μJ.
the three available amplification stages, there is still sufficient gain in the system to amplify this to >100 kW. Figure 5(a) shows a 0.3 ns seed pulse (spectral bandwidth 6 nm) and the final amplified pulse with peak power 108 kW and pulse energy 65 μJ (65 W average power at 1 MHz). There is an increase of pulse duration to ∼0.5 ns FWHM. This broadening is not power dependent and is of the magnitude that might be expected due to modal dispersion in the multimode final amplifier and delivery fiber, but this has not been investigated in detail. The spectrum [Fig. 5(b)] shows some broadening; spectral width is 9 nm, but power in the Raman peak is still at −25 dB compared to the center wavelength and 96% of the power is within 20 nm of the spectral peak. Since we do not need to consider separately the contributions of a narrow peak and long low power tail, short pulses that are not significantly reshaped are more useful than long ones for investigating the relationship between peak power and spectral quality. We used 3 ns pulses with pulse energies up to 590 μJ (265 W average power at 450 kHz) to examine spectral broadening in our system [Fig. 6(a)]. Because of the very high gain available in fiber amplifiers, their nonlinear effective length is short compared to their actual length. Therefore in a system like the one described here, it may be pulse evolution in the delivery fiber that makes the main contribution to nonlinear effects on the pulse spectrum. The relative contribution of the final amplifier and delivery fiber to spectral broadening was investigated by changing the length of the delivery fiber and comparing the fraction of laser power within 20 nm of the peak wavelength as a function of pulse peak power. Figure 6(b) shows the spectrum of a 3 ns pulse [shown in Fig. 6(a)] with delivery fiber length of 1.7 and 0.3 m. With 1.7 m delivery fiber, the Raman peak is at −13 dB compared to the main peak, with 0.3 m it is completely suppressed. Figure 6(c) shows spectral quality as a function of peak power for each length of delivery fiber. Shortening the delivery fiber to ∼30 cm results in excellent spectral quality even at peak power levels of 180 kW. It is clear that nonlinear interactions in the delivery fiber dominate spectral broadening and that by reducing the delivery fiber to a short (but practical in the case of an amplifier mounted on machining head) very high peak powers can be achieved with good spectral quality. We have demonstrated a highly versatile high pulse energy fiber laser system. Compared to previous high pulse energy fiber lasers [1], the F–P diode seed allows a wide range of durations and built-in pulse shaping
November 15, 2013 / Vol. 38, No. 22 / OPTICS LETTERS
Fig. 6. Dependence of spectral quality on bandwidth for different lengths of delivery fiber.
capability. Very high peak powers could be achieved: The practical limitation in application will be set by delivery fiber lengths and spectral quality requirements. The average power was limited by the available pump power and can be readily scaled further. SPI acknowledges support for this work under the European Commission 7th Framework Programme (FP7-NMP-2008- 4.0.4) project “Leadership in Fibre Laser Technology,” grant agreement no. CP-IP 228587-1. References 1. D. J. Richardson, J. Nilsson, and W. A. Clarkson, J. Opt. Soc. Am. B 27, B63 (2010). 2. M. Cheng, Y. Chang, A. Galvanauskas, P. Mamidipudi, R. Changkakoti, and P. Gatchell, Opt. Lett. 30, 358 (2005). 3. F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, Opt. Lett. 37, 1073 (2012). 4. C. D. Brooks and F. D. Teodoroa, Appl. Phys. Lett. 89, 111119 (2006). 5. T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann, Opt. Express 19, 13218 (2011).
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