We report a high-power all-fiber-integrated femtosecond chirped pulse amplification system operating at 1064 nm, which consists of a dispersive wave source, a fiber stretcher, a series of ytterbium-doped amplifiers and a chirped volume Bragg grating (CVBG) compressor. The dispersive wave is generated by an erbium-doped mode-locked fiber laser with frequency shifted to the 1 μm region in a highly nonlinear fiber. With three stages of ytterbium-doped amplification, the average output power is scaled up to 125 W. Through CVBG, the pulse duration is compressed from 525 ps to 566 fs, the average output power of 107 W with a high compression efficiency of 86% is achieved, and the measured repetition rate is 17.57 MHz, corresponding to the peak power of 10.8 MW.
© 2016 Optical Society of America
High peak-power and high average-power femtosecond laser systems are required in both fundamental research and industrial applications, including high order harmonic generation, attosecond science, high-precision material processing, waveguide writing, and so on [1–4]. Ti:sapphire laser systems are commonly employed for many of these applications but the emerging femtosecond fiber chirped pulse amplification (CPA) system provides an alternative solution with more compact, robust and environmentally-stable performance . The traditional fiber CPA system often consists of an all-fiber seed, a fiber stretcher, a series of fiber amplifiers and a free space compressor . To produce femtosecond pulses at 1 μm wavelength, dispersion-managed soliton mode-locking with broadband spectrum is a promising technique [7,8]. However, their development remains hindered by lack of dispersion compensating components.
Silica fiber usually provides positive dispersion at 1 μm region. For obtaining anomalous dispersion at this wavelength one needs to use special designed and expensive fibers or devices, such as chirped fiber Bragg grating (CFBG) , high order mode fiber (HOM)  and solid core photonic band gap fiber (SC-PBG) . Recently, a dispersive wave based femtosecond laser source at 1 μm has attracted more attentions. It is based on an erbium-doped mode-locked fiber laser, and the frequency is shifted to 1 μm through a short segment of highly nonlinear fiber (HNLF) . Compared with ytterbium-doped fiber seed, the erbium-doped fiber oscillator at 1.5 μm communication wavelength is technically matured and cheaper to build, since a large selection of different fibers involved is commercially available. In addition, it gives the dispersive wave spectrum with broad bandwidth (more than 30 nm), smooth profile and linearly chirp which renders great potential for further pulse stretch and compression .
A lot of achievements are made with free-space or free-space coupled laser structures, for example, an average output power of 72 W with pulse energy of 17 mJ is demonstrated by an Yb:YAG thin-disk amplifier seeded by a combination of a femtosecond Er-doped fiber oscillator with Yb-doped fiber preamplifier . In another case, through free-space coupling, an Yb:PCF amplifier seeded by an Er:fiber system delivers 6-μJ pulses with duration of 145 fs at a repetition rate of 10 MHz . Recently, more and more attentions are paid to all-fiber-integrated laser system. In 2010, Andrianov et al. reported a 200 mW and 85 fs laser pulse in a hybrid erbium-doped laser/ytterbium-doped amplifier system. It is based on nonlinear wavelength conversion of the erbium-doped fiber laser pulses from 1.56 μm to the 1 μm range in a 10 cm dispersion-shifted fiber . An average power of 5.7 W with wavelength of 1040 nm and pulse duration of 135 fs was demonstrated by Kieu et al., and its 1 μm seed was obtained from an octave-spanning supercontinuum . Moreover, a peak power of 2.5 MW was obtained by using an active tapered fiber in ytterbium-doped master oscillator power amplifier (MOPA) system .
The polarization independent chirped-volume Bragg grating (CVBG) is demonstrated as a competitive compression device with lots of advantages . Differ from grating pair or prism pair, CVBG is a monolithic device with small size of several centimeters only, and has a higher potential of compression efficiency [20,21]. And the CVBG can provide a large amount of dispersion which is beneficial to stretch or compression of the ultrafast laser pulses. Furthermore, as a bulk device, CVBG is widely used in high energy (milli-J level) and high average power (hundred-W level) laser systems.
Here we report a high power all-fiber CPA system by using a CVBG as the compressor. The whole system consists of an erbium-doped mode-locked fiber laser with frequency shifted to 1 μm region, a long fiber stretcher, a series of ytterbium-doped amplifiers and a CVBG compressor. All these make the fiber CPA system more compact, robust and feasible. By optimizing the spectrum shape and power efficiency, a femtosecond laser at 1064 nm is achieved. As the result, the final average output power is more than 100 W with compression efficiency up to 86%. The pulse duration is dechirped to 566 fs, corresponding to the peak power of 10.8 MW.
2. Experimental setup
The experimental setup of the high power femtosecond fiber CPA system consists of four parts, as shown in Fig. 1. The first part is an erbium-doped laser source, which is a dispersion-managed SESAM passively mode-locked fiber laser in a ring configuration. A 976 nm continuous wave laser diode with maximum output power of 450 mW is used as the pump source through a wavelength division multiplexer (WDM1). The gain medium is a 0.95 m erbium-doped fiber (LIEKKI Er80-8/125) with group velocity dispersion (GVD) of 19.6 ps/nm/km at 1550 nm, and the core-absorption coefficient is about 43 dB/m at 976 nm. A 0.9 m dispersion compensation fiber (DCF) with GVD of −146 ps/nm/km at 1550 nm is used to compensate the anomalous dispersion in the cavity. The SMF28 fiber with GVD of 18.0 ps/nm/km at 1550 nm is adopted as the pigtail fiber of all elements. The total cavity length is around ~11.77 m including the adding up pigtail fiber of 9.92 m. The corresponding repetition rate is 17.57 MHz, and the net dispersion of the single round-trip is about −83.2 fs2.
The function of the second part is to generate target dispersive wave. The 1.5 μm mode-locked laser is further amplified by an erbium-doped fiber (Nufern SM-ESF-7/125) amplifier. After that, the amplified femtosecond laser is injected to a short segment of 9.5 cm HNLF with second-order dispersion parameter of 2.19 ps/nm/km to generate dispersive wave around 1 μm. Then, the dispersive wave pulse is made to propagate in a long fiber stretcher, an 1120 m single mode fiber of HI1060 with GVD of −39.2 ps/nm/km at 1064 nm, to stretch the pulse duration.
The third part is a series of ytterbium-doped power amplifiers. All the amplifiers in this system are forward pumped, and all the gain fibers are non-PM ytterbium-doped fiber. Only the radiation wavelength from 0.97 to 1.2 μm can be amplified because of the limitation of the gain bandwidth of the ytterbium-doped fiber. The gain medium of the first-stage preamplifier is a 4 m single mode double-clad 7/128 μm ytterbium-doped fiber, and its cladding absorption coefficient is 5 dB/m at 976 nm. In the second-stage preamplifier, a 2 m multimode double-clad 20/130 μm ytterbium-doped fiber is used as the gain medium with the cladding absorption coefficient of 10.8 dB/m at 976 nm. After a band-pass filter, a (6 + 1) × 1 pump-signal combiner is used to deliver the pump light into the master amplifier which is a 1.7 m double-clad 30/250 μm ytterbium-doped fiber with cladding absorption coefficient of 16.3 dB/m at 976 nm. And finally, the fiber end is cut into an angle of 8 degrees to eliminate back reflection and prevent the end facet damage. Since all components are directly spliced, these all-fiber laser amplifiers are compact and alignment-free.
In the final part, the collimated output from the 30/250 μm ytterbium-doped fiber is injected into a 5 cm angle-tilted CVBG compressor (OptiGrate Corp.) with 5 × 5 mm aperture. It has a flat-top spectral profile, the central wavelength is 1063.9 nm with 10 dB spectral bandwidth of 11.8 nm, and its diffraction efficiency and dispersion are 86% and 42 ps/nm, respectively.
An ultrafast photodetector followed by a 25 GHz digital oscilloscope (Agilent DSO-X 92504A) and an optical spectrum analyzer (Yokogawa AQ6370C) with resolution of 0.02 nm are used to monitor the temporal and spectral profiles of the output pulses, respectively. And an autocorrelator (APE) is used to measure the pulse durations.
3. Results and discussion
A self-starting erbium-doped dispersion-managed mode-locked laser is built with the pulse width of 500 fs at the repetition rate of 17.57 MHz, and it exhibits a spectral bandwidth of 12.9 nm at the center wavelength of 1555.5 nm, as shown in Fig. 2(a). After that, the pulse is further amplified to 70 mW in an EDFA, and it is simultaneously compressed to 50 fs as a result of the nonlinear compression effect. Then the amplified laser is injected to a 9.5 cm HNLF with a mode field diameter of 2.23 μm. HNLF is a key component used for the nonlinear wavelength conversion in this system, the splicing efficiency between HNLF and SMF28 is carefully optimized to 80% or more. When the average output power of EDFA is increased, the supercontinuum is generated in the HNLF, and the obtained dispersive wave covers the wavelength from 1020 to 1100 nm with center wavelength of 1060 nm and 3 dB spectral bandwidth of 40 nm, as shown in Fig. 2(b). The average power of dispersive wave around 1 μm is about 2 mW. After optimization test, an 1120 m single mode fiber of HI1060 with normal dispersion is used as the stretcher. The pulse duration of the dispersive wave is stretched to 524 ps, as shown in Fig. 3(a).
A series of all-fiber ytterbium-doped amplifiers is used to boost the average output power. In the first-stage of ytterbium-doped preamplifier, the average output power of 250 mW around 1 μm is obtained. The measured peak-to-peak fluctuation of the pulse train is 2.5%. The radio-frequency spectrum of the pulse train is shown in Fig. 3(b), the fundamental peak located at the repetition rate of 17.57 MHz has a signal-to-background ratio of 60 dB, indicating a stable pulse train. And its 3 dB spectral bandwidth is narrowed to 12.2 nm (the 10 dB spectral bandwidth is 23.8 nm) in consequence of gain-narrowing effect, and the center wavelength is shifted to 1062.9 nm. In fact, the output power of the first-stage preamplifier is limited by the isolator. After the second-stage preamplifier, the center wavelength of 1063 nm with 10 dB spectral bandwidth of ~20 nm (full spectral bandwidth more than 50nm) is achieved, and its average output power is up to 1.5 W which is enough for further amplification. However, seeing that the spectral bandwidth almost remains unchanged in the process of master amplification (data not listed), the present spectrum is obviously too broad to be completely reflected in the compressor, because the reflection bandwidth of the CVBG is only 11.8 nm with flat-top spectral profile. The spectral filter effect of the CVBG will cut off the spectrum edges of the signal, and this will reduce the diffraction efficiency significantly as we previously reported in Ref .
In order to increase the compression efficiency, a band-pass filter is adopted and put ahead of the master amplifier. The 10 dB spectral bandwidth is narrowed to 11.1 nm with flat-top spectral profile. In the master amplifier, a 1.7 m 30/250 μm double-clad ytterbium-doped fiber is used as the gain medium, and six multimode laser diodes are used as the pump source which can provide 163 W pump power. After stripping pump by the dichroic mirror, the maximum average power of 125 W is achieved. The B-integral of the ytterbium-doped amplifiers is 3.44 rad simulated by commercial software. The beam quality M2 values are 1.49 (parallel) and 1.48 (perpendicular). The slope efficiency of the master amplifier is up to 77.9%. At this moment, the 10 dB spectral bandwidth maintains at 11 nm, which well matches the reflective spectrum of CVBG compressor, as shown in Fig. 4(a). The spectral modulations are come from the multimode interference in the large-mode-area fiber.
The polarization independent CVBG possesses the advantage of compact structure and higher compression efficiency. Compared with the traditional diffraction grating pair compressor with groove density of 1200 line/mm, CVBG is a monolithic device with the dispersion of 42 ps/nm, its compression distance needed could be 50 times reduced. In this experiment, the 125 W collimated output power of master amplifier is injected into a 5 cm CVBG compressor. The beam diameter of the incident beam on the CVBG is 1.36 mm. To reduce heat accumulation, the CVBG is put in an aluminum heat sink at room temperature. After compression, an average output power of 107 W is achieved. The slope efficiency of compression is 86%, as shown in Fig. 4(b), which is higher than that using diffraction grating pair, and also higher than that without the filter inserted as in our previous work. The center wavelength of the final output is 1064 nm with 10 dB spectral bandwidth of 11.14 nm. The beam quality M2 values are 1.53 (parallel) and 1.52 (perpendicular), as shown in Fig. 5(a). The autocorrelation trace measured by APE autocorrelator is fitted in with Gaussian profile, indicating the dechirped pulse duration to be 566 fs, as shown in Fig. 5(b), and corresponding peak power of 10.8 MW.
However, the compressed pulse has a time-bandwidth product (TBP) of 1.67, which is 3.8 times larger than the transform limited of Gaussian pulses. The reason is that the HI1060 fiber has the third-order dispersion (TOD) of 74 fs3/mm , and hence enormous TOD will accumulate in the fiber stretcher which is difficult to compensate and compress. Moreover, the CVBG dispersion is difficult to be adjusted, and the only way to modify the net dispersion is to change the fiber length. According to our experiments, for the shortest compressed pulse duration, the optimal fiber length is 1120 m, the relations between pulse duration after CVBG compression and length of HI1060 fiber stretcher is shown in Fig. 6. It clearly demonstrates the limitation of this system.
Our next effort is to further compress the pulse duration though reducing TOD accumulation. One promising approach is using CFBG as the stretcher instead of the long fiber of HI1060, because the CFBG can provide large linear chirp in a short section of optical fiber and offers dispersion that is continuously adjustable through changing the tension or temperature.
In conclusion, we introduced a CVBG based all-fiber CPA system operating at 1064 nm. This system is seeded by a dispersive wave source which is generated by an erbium-doped mode-locked fiber laser with frequency-shifted to the 1 μm region in a highly nonlinear fiber. Through a series of ytterbium-doped fiber amplifiers, the average power is scaled up to 125 W. After the CVBG compressor, a 107 W average power at 17.57 MHz repetition rate and 566 fs pulse duration is achieved, corresponding to the peak power of 10.8 MW. Future work will be focused on reducing TOD accumulation and increasing the peak power. This will lead to numerous applications, where robustness and compactness are desirable.
National Natural Science Foundation of China (NSFC) (61527822, 61235010).
The authors acknowledge the funding support from the National Natural Science Foundation of China.
References and Links
2. P. M. Paul, E. S. Toma, P. Breger, G. Mullot, F. Augé, P. Balcou, H. G. Muller, and P. Agostini, “Observation of a train of attosecond pulses from high harmonic generation,” Science 292(5522), 1689–1692 (2001). [CrossRef] [PubMed]
3. K. Sugioka and Y. Cheng, “Ultrafast lasers—reliable tools for advanced materials processing,” Light Sci. Appl. 3(4), e149 (2014). [CrossRef]
4. F. Chen and J. R. V. de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). [CrossRef]
6. K. Kim, X. Peng, W. Lee, S. Gee, M. Mielke, T. Luo, L. Pan, Q. Wang, and S. Jiang, “Monolithic polarization maintaining fiber chirped pulse amplification (CPA) system for high energy femtosecond pulse generation at 1.03 µm,” Opt. Express 23(4), 4766–4770 (2015). [CrossRef] [PubMed]
7. X. Liu, J. Laegsgaard, and D. Turchinovich, “Highly-stable monolithic femtosecond Yb-fiber laser system based on photonic crystal fibers,” Opt. Express 18(15), 15475–15483 (2010). [CrossRef] [PubMed]
8. J. W. Nicholson, S. Ramachandran, and S. Ghalmi, “A passively-modelocked, Yb-doped, figure-eight, fiber laser utilizing anomalous-dispersion higher-order-mode fiber,” Opt. Express 15(11), 6623–6628 (2007). [CrossRef] [PubMed]
9. A. Galvanauskas, M. E. Fermann, D. Harter, K. Sugden, and I. Bennion, “All-fiber femtosecond pulse amplification circuit using chirped Bragg gratings,” Appl. Phys. Lett. 66(9), 1053–1055 (1995). [CrossRef]
10. S. Ramachandran, S. Ghalmi, J. W. Nicholson, M. F. Yan, P. Wisk, E. Monberg, and F. V. Dimarcello, “Anomalous dispersion in a solid, silica-based fiber,” Opt. Lett. 31(17), 2532–2534 (2006). [CrossRef] [PubMed]
12. C. W. Freudiger, W. Yang, G. R. Holtom, N. Peyghambarian, X. S. Xie, and K. Q. Kieu, “Stimulated Raman scattering microscopy with a robust fibre laser source,” Nat. Photonics 8(2), 153–159 (2014). [CrossRef] [PubMed]
13. D. Kielpinski, M. G. Pullen, J. Canning, M. Stevenson, P. S. Westbrook, and K. S. Feder, “Mode-locked picosecond pulse generation from an octave-spanning supercontinuum,” Opt. Express 17(23), 20833–20839 (2009). [CrossRef] [PubMed]
14. J. Fischer, A. C. Heinrich, S. Maier, J. Jungwirth, D. Brida, and A. Leitenstorfer, “615 fs pulses with 17 mJ energy generated by an Yb:thin-disk amplifier at 3 kHz repetition rate,” Opt. Lett. 41(2), 246–249 (2016). [CrossRef] [PubMed]
15. M. Wunram, P. Storz, D. Brida, and A. Leitenstorfer, “Ultrastable fiber amplifier delivering 145-fs pulses with 6-μJ energy at 10-MHz repetition rate,” Opt. Lett. 40(5), 823–826 (2015). [CrossRef] [PubMed]
16. A. Andrianov, E. Anashkina, S. Muravyev, and A. Kim, “All-fiber design of hybrid Er-doped laser/Yb-doped amplifier system for high-power ultrashort pulse generation,” Opt. Lett. 35(22), 3805–3807 (2010). [CrossRef] [PubMed]
17. K. Kieu, R. J. Jones, and N. Peyghambarian, “High power femtosecond source near 1 micron based on an all-fiber Er-doped mode-locked laser,” Opt. Express 18(20), 21350–21355 (2010). [CrossRef] [PubMed]
18. M. Y. Koptev, E. A. Anashkina, K. K. Bobkov, M. E. Likhachev, A. E. Levchenko, S. S. Aleshkina, S. L. Semjonov, A. N. Denisov, M. M. Bubnov, D. S. Lipatov, A. Y. Laptev, A. N. Gur’yanov, A. V. Andrianov, S. V. Muravyev, and A. V. Kim, “Fibre amplifier based on an ytterbium-doped active tapered fibre for the generation of megawatt peak power ultrashort optical pulses,” Quantum Electron. 45(5), 443–450 (2015). [CrossRef]
19. G. Chang, M. Rever, V. Smirnov, L. Glebov, and A. Galvanauskas, “Femtosecond Yb-fiber chirped-pulse-amplification system based on chirped-volume Bragg gratings,” Opt. Lett. 34(19), 2952–2954 (2009). [CrossRef] [PubMed]
20. W. Liu, D. N. Schimpf, T. Eidam, J. Limpert, A. Tünnermann, F. X. Kärtner, and G. Chang, “Pre-chirp managed nonlinear amplification in fibers delivering 100 W, 60 fs pulses,” Opt. Lett. 40(2), 151–154 (2015). [CrossRef] [PubMed]
21. L. Glebov, V. Smirnov, E. Rotari, I. Cohanoschi, L. Glebova, O. Smolski, J. Lumeau, C. Lantigua, and A. Glebov, “Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses,” Opt. Eng. 53(5), 051514 (2014). [CrossRef]
22. R. Sun, D. Jin, F. Tan, and P. Wang, “High power femtosecond all-fiber chirped pulse amplification system based on Cherenkov radiation,” in Conference on Lasers and Electro-Optics (CLEO), OSA Technical Digest Series (Optical Society of America, 2016), paper STu1P. 7. [CrossRef]
23. H. Kalaycioglu, B. Oktem, C. Şenel, P. P. Paltani, and F. Ö. Ilday, “Microjoule-energy, 1 MHz repetition rate pulses from all-fiber-integrated nonlinear chirped-pulse amplifier,” Opt. Lett. 35(7), 959–961 (2010). [CrossRef] [PubMed]