We report on an all-fiber femtosecond ytterbium laser without dispersion compensation consisting of all-normal dispersion fibers. Mode-locking was achieved by nonlinear polarization evolution in combination with additional amplitude modulation generated by a fiber-based spectral filter. The generated pulses were highly chirped and had a maximum pulse energy of 1.8 nJ. The output pulse duration was 7.6 ps and could be dechirped to 179 fs.
©2008 Optical Society of America
Passively mode-locked ultrafast ytterbium fiber lasers operating in the wavelength range around 1 µm are subject of intensive investigations during the past years [1, 2, 3]. This is owing to their great potential of realizing cost-effective high pulse energy systems and more recently by developing simple all-fiber based setups which are operated in a highly normal dispersion regime [4, 5]. In the latter oscillator, anomalous dispersive components, like grating arrangements , photonic crystal fibers , photonic bandgap fibers , tapered fibers , fiber Bragg gratings , and higher-order mode fibers  formerly used in those laser systems in order to compensate the normal dispersion of fused silica fibers in the wavelength region of 1 µm have been omitted. This developmentwas triggered by the studies of Adel et al. in 2001 and by Herda and co-workers in 2004 with their results in ytterbium fiber lasers without dispersion compensation but still applying discrete optical components inside the cavity [11, 12]. However, owing to the highly uncompensated chirp in these fiber laser the pulse durations were in excess of several picoseconds. Prochnow et al. then reported a similar but compact all-fiber based system providing an alignment free and stable single pulse operation with an output energy of almost 1 nJ and a pulse duration of 629 fs . In all these ultrafast fiber oscillators operating without dispersion compensation stable mode-locking is realized by nonlinear polarization evolution (NPE) and by saturable absorber mirrors (SAM), respectively [13, 14]. The generation of stable pulses is possible owing to spectral filtering effects either by gain bandwidth filtering and/or by temporal filtering, which is equivalent for chirped pulses.
A sophisticated enhancement of this concept has been realized by Herda and Chong who implemented a further spectral filter inside the normal dispersion cavity providing an additional amplitude modulation and thus stabilizing the mode-locking mechanism [15, 16]. By using this method, Chong and co-workers achieved pulse energies above 20 nJ and dechirped pulse duration of 165 fs showing the enormous potential of this fiber laser configuration . A further development was shown by Kieu et al. by using a wavelength division multiplexer (WDM) as spectral filter inside a real all-fiber cavity . In this paper the authors had to use a saturable absorber based on carbon nanotubes to mode-lock the laser but the absorber degenerates after a few days.
In this paper we report to the best of our knowledge for the first time on an all-fiber ytterbium laser with additional amplitude modulation produced by a fiber-based spectral filter, which is mode-locked by nonlinear polarization evolution. The laser consists only of normal dispersion fibers without any dispersion compensation. The maximum pulse energy was 1.8 nJ at a pulse duration of 7.6 ps. The pulses could be dechirped to a pulse durations of 179 fs.
2. Experimental setup
A sketch of the experimental setup of the all-fiber ring oscillator is shown in Fig. 1. The cavity consists of all normal dispersion fibers. A WDM is used for delivering the maximum pump power of 488mW at 976 nm. 23 cm of ytterbium-doped fiber with a dopant concentration of about 1.15wt% is utilized, followed by 1.34m of single-mode fiber (SMF). For an adjustment of the polarization state a polarization controller (PC) is used here and passing the isolator. The fiber-based isolator has the same compact dimensions as the used WDMs. Inside the isolator the polarization state was split up and both polarizations were coupled into two polarization maintaining fibers. Here the intensity dependent polarization modulation is converted into an amplitude modulation forming the virtuell saturable absorber. The rejection port of the NPE is used as output port. The insertion loss of the isolator was about 4 dB. Following another 1.67m of SMF a customized designedWDM was spliced. This WDM was used as fiber-based spectral filter. A 5% fiber-based linear output coupler was used to monitor the optical spectrum and the intra cavity pulse energy. The total cavity length was 6.21m corresponding to a repetition rate of 33.3 MHz. Due to the all-normal dispersion fibers the laser operates at a cavity dispersion of 0.155 ps2.
The transmission curve of the implemented WDM for spectral filtering was measured with a white light source and is shown in Fig. 2. The transmission maxima were separated by 21nm and the full with at half maximum (FWHM) width was 11 nm. The transmission maxima were centered at 1021nm and 1042 nm.
3. Experimental results
After assembling the laser self-starting operation was obtained by increasing the pump power and slightly adjusting the polarization controller. As can be seen in Fig. 3(a) the mode-locking threshold was achieved at a pump power of 200mW. The slope efficiency was 9% and a maximum output power of 59mW at a pump power of 488mW, corresponding to a pulse energy of 1.8 nJ could be achieved. The emitted pulse train, measured with a photodiode (rise time of 70 ps), is shown in Fig. 3(b). It exhibits a pulse spacing of 30 ns, resulting in a pulse repetition rate of 33.3 MHz. In Fig. 4(a) the corresponding radio-frequency (rf) spectrum of the first intermode beat is shown, measured with a photodiode with a rise time of 35 ps and a high resolution rf-spectrum analyzer (Agilent E4440A). There is no indication of residual sidebands caused, e.g. by Q-switched mode-locking. These sidebands are at least 90dB below the carrier. Single-pulse operation was verified by using a long-range autocorrelator with a scanning range of 150 ps in combination with a photodiode (rise time of 70 ps) and a 70GHz sampling oscilloscope. Additionally, the optical spectrum was monitored regarding modulations revealing multiple pulsing.
A typical output spectrum of the NPE port is shown in Fig. 4(b). The spectra had a rmswidth of 13.2nm at a center wavelength of 1040nm and exhibit the typical characteristics of pulses shaped due to spectral filtering in an all-normal dispersion fiber laser. The steep edges and peaks on both sides of the power spectrum are evolved under the influence of self-phase modulation in a normal dispersion fiber. For comparison the optical spectrum taken at the 5% fiber coupler is also shown in Fig. 4(b). Here the rms width is 8.6 nm. Comparing both spectra emphasizes on one hand the effect of the spectral filtering by the filter WDM and on the other hand the pulse shaping in the fiber section. The asymmetric shape results from the reduced loss of the isolator at longer wavelengths and self steepening.
In Fig. 5(a) the autocorrelation function is shown, measured with a long-range autocorrelator. The autocorrelation width of 10.3 ps corresponds to a pulse duration of 7.6 ps. The output pulses were externally dechirped using a grating compressor with a group delay dispersion of -0.199 ps2. The resulting autocorrelation function had a width of 245 fs is shown in Fig. 5(b). By using the ratio of the widths of the bandwidth-limited autocorrelation function and the temporal pulse profile, both calculated by zero-phase Fourier transform of the power spectrum, the pulse duration can be determined to 179 fs. This is to the best of our knowledge the shortest pulse duration of a ytterbium all-fiber laser without dispersion compensation. The small pedestal in that function arised due to the nonlinear chirp and uncompensated third order dispersion of the pulse and cannot be dechirped by a grating compressor. However, this pedestal contains less than 10% of the total energy of the pulse, which is a reasonable value for an all-normal dispersion fiber laser and an extremely good value for an all-normal all-fiber laser set-up.
Without any readjustment of the polarization controller the laser operates for weeks without degradation of all pulse parameters. During this time the laser was switched off and on several times indicating the easy handling and turn key operation. Each time the laser returns in the same mode-locked state with the same output characteristics. In addition, environmental mechanical vibrations, for example owing to unintentionally hitting the optical table by lab tools, did not affect the laser operation. This setup is a major improvement of further setups like the one of Kieu et al.  because an additional saturable absorber is not required. Thus degrading effects of such absorbers does not appear. In the presented laser all advantages of an all-fiber laser come to the fore.
We presented a ytterbium femtosecond all-fiber laser without dispersion compensation inside the cavity. The laser was self-starting and mode-locked by nonlinear polarization evolution in combination with additional amplitude modulation generated by a fiber-based spectral filter. The output pulse duration was 7.6 ps and could be dechirped to 179 fs, which is to the best of our knowledge the shortest pulse duration of an all-fiber ytterbium laser without dispersion compensation. A maximum pulse energy of 1.8 nJ was obtained for the chirped pulses. With this setup we achieved turn-key operation and the laser operated for weeks in the same mode-locked state without any readjustment and degradation in the pulse parameters.
This work was partially supported by Federal Ministry of Education and Research under contract number 13N8553.
References and links
2. F. Ö. Ilday, J. R. Buckley, H. Lim, F. W. Wise, and W. G. Clark, “Generation of 50-fs, 5-nJ pulses at 1.03 µm from a wave-breaking-free fiber laser,” Opt. Lett. 28, 1365–1367 (2003). [CrossRef] [PubMed]
3. B. Ortaç, O. Schmidt, T. Schreiber, J. Limpert, A. Tünnermann, and A. Hideur, “High-energy femtosecond Yb-doped dispersion compensation free fiber laser,” Opt. Express15, 10725–10732 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-17-10725. [CrossRef] [PubMed]
4. O. Prochnow, A. Ruehl, M. Schultz, D. Wandt, and D. Kracht, “All-fiber similariton laser at 1 µm without dispersion compensation,” Opt. Express15, 6889–6893 (2007), http://www.opticsinfobase.org/abstract.cfm?URI=oe-15-11-6889. [CrossRef] [PubMed]
5. K. Kieu and F. W. Wise, “All-fiber normal-dispersion femtosecond laser,” Opt. Express16, 11453–11458 (2008), http://www.opticsinfobase.org/abstract.cfm?URI=oe-16-15-11453. [CrossRef] [PubMed]
6. H. Lim, F. Ö. Ilday, and F. W. Wise, “Femtosecond ytterbium fiber laser with photonic crystal fiber for dispersion control,” Opt. Express10, 1497–1502 (2002), http://www.opticsinfobase.org/abstract.cfm?URI=oe-10-25-1497. [PubMed]
9. I. Hartl, G. Imeshev, L. Dong, C. C. Cho, and M. E. Fermann, “Ultra-compact dispersion compensated femtosecond fiber oscillators and amplifiers,” in Conference on Lasers and Electro-Optics (CLEO), OSA Technical Digest Series (Optical Society of America, 2005), paper CThG1.
10. M. Schultz, O. Prochnow, A. Ruehl, D. Wandt, D. Kracht, S. Ramachandran, and S. Ghalmi “Sub-60-fs ytterbium-doped fiber laser with a fiber-based dispersion compensation,” Opt. Lett. , 32, 2372–2374 (2007). [CrossRef] [PubMed]
11. P. Adel, M. Auerbach, C. Fallnich, and H. Welling, “Super-stretched mode-locked Yb3+-fiber laser with 33 nm bandwidth and 56 nJ pulse energy” in Advanced Solid State Lasers (ASSL), OSA Trends in Optics and Photonics Vol. 50, 221–223 (2001), paper TuA4-1.
12. R. Herda and O. G. Okhotnikov, “Dispersion Compensation-Free Fiber Laser Mode-Locked and Stabilized by High-Contrast Saturable Absorber Mirror” IEEE J. Quantum Electron. 40, 893–899 (2004). [CrossRef]
13. V. J. Matsas, T. P. Newson, D. J. Richardson, and D. N. Payne, “Selfstarting passively mode-locked fibre ring soliton laser exploiting non linear polarisation rotation,” Electron. Lett. , 28, 2226–2228 (1992). [CrossRef]
14. U. Keller, D. A. B. Miller, G. D. Boyd, T. H. Chiu, J. F. Ferguson, and M. T. Asom, “Solid-state low-loss intracavity saturable absorber for Nd:YLF lasers: an antiresonant semiconductor FabryPerot saturable absorber,” Opt. Lett. 17, 505–507 (1992). [CrossRef] [PubMed]
15. R. Herda, O. G. Okhotnikov, E. U. Rafailov, W. Sibbett, P. Crittenden, and A. Starodumov, “Semiconductor Quantum-Dot Saturable Absorber Mode-Locked Fiber Laser” IEEE Photon. Technol. Lett. 18, 157–159 (2006). [CrossRef]
16. A. Chong, J. Buckley, W. Renninger, and F. Wise, “All-normal-dispersion femtosecond fiber laser,” Opt. Express14, 10095–10100 (2006), http://www.opticsinfobase.org/abstract.cfm?URI=oe-14-21-10095. [CrossRef] [PubMed]