Journal of Modern Optics Vol. 58, No. 7, 10 April 2011, 625 630 70-femtosecond Gaussian pulse generation in a dispersion-managed erbium-doped fibre laser Dinghuan Deng, Li Zhan*, Zhaochang Gu, Xiao Hu, Shouyu Luo and Yuxing Xia Department of Physics, State Key Laboratory of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai 200240, China (Received 16 September 2010; final version received 30 January 2011) We experimentally demonstrated the generation of transform-limit Gaussian ultrashort pulses as short as 70 fs from a dispersion-managed mode-locked erbium-doped fibre (EDF) laser. The output spectrum is close to the Gaussian profile with a full-width half-maximum (FWHM) output of 49 nm, and the measured autocorrelation trace exhibits the Gaussian profile. The shortest pulse duration of 70 fs was observed after external recompression. The time-band product is 0.44, showing the best transform limit pulse. Keywords: fibre laser; Gaussian pulse; femtosecond pulse; ultrashort pulse 1. Introduction The generation of ultrashort pulses with perfect Gaussian profile is easily demonstrated in a solidstate laser, because the laser light propagates in the air and short gain media with available low nonlinearity, which minimises the total nonlinear effect of the pulses. However, for a fibre laser, laser light propagates in the fibre, in which the group velocity dispersion (GVD) and the nonlinear effects must be considered. Thus, it is difficult to generate exactly Gaussian pulses in a fibre laser for the spectral and temporal wings. In an ultrashort pulse fibre laser, sidelobes are often seen in the output spectrum, which produces poor-quality pulse output. In a mode-locked fibre laser, the intracavity dispersion can be adjusted for a negative dispersion soliton [1,2], positive dispersion similariton [3 5], or dispersion-managed soliton regime by properly designing the cavity GVD structure, since a fibre can be made in positive or negative waveguide dispersion. Soliton operation can provide clean sech 2 -shape pulses with low single pulse energy, but it has severe limitations on pulse width and peak power. Further increasing pulse energy suffers from sidebands generation in the spectrum with period perturbation. When the cavity dispersion is controlled to be positive with short gain media, parabolic shape pulses with large nonlinear phase shift can be generated in the similariton fibre lasers, which scales up pulse energy. Therefore, the different GVD structures make different pulse profiles output from a fibre laser. The technique of dispersion management is often used in the design of modern fibre-optics communication systems. Also, the pulses with Gaussian intensity profiles are commonly seen in these periodic systems that contain fibres of differing dispersion [6,7]. In dispersion managed fibre laser, the Gaussian pulses have been theoretically predicted and experimentally recommended. However, for the large nonlinearity in fibre and poor dispersion management, it is difficult experimentally to generate an exact Gaussian pulse profile from an erbium-doped fibre (EDF) ring laser. Especially, when the pulse duration is less than 100 fs, temporal and spectral distortion is easily generated and deviated from Gaussian pulses for their high peak power. Furthermore, it is hard precisely to estimate the parameter values from real data. But when we carefully optimise experimental parameters such as GVD, output ratio, polarisation and pump power, Gaussian pulses with minimise nonlinear effect could be produced, which can greatly suppress temporal wings and eliminate spectral sidebands. We report the observation of exactly Gaussian pulse generation in an optimised dispersion-erbium fibre laser. The 49 nm full-width half-maximum (FWHM) output spectrum of the laser is well fitted with a Gaussian profile, and the measured autocorrelation trace is also fitted with Gaussian profile, and the pulse duration can be as short as 70 fs after external recompression with a time-band product of 0.44, showing the best transform limit Gaussian pulse. *Corresponding author. Email: lizhan@sjtu.edu.cn ISSN 0950 0340 print/issn 1362 3044 online ß 2011 Taylor & Francis DOI: 10.1080/09500340.2011.560398 http://www.informaworld.com
626 D. Deng et al. 980 nm Pump WDM PC1 PDI PC2 320 cm SMF 70 cm Nufern 980 fiber 10% OC Output So we optimised the laser cavity net dispersion to be slightly positive and injected proper pump power to avoid multiple pulses operation [9]. 135 cm EDF Figure 1. Experimental configuration of the erbium fibre ring oscillator. (The colour version of this figure is included in the online version of the journal.) While maintaining a good profile of pulse shape, a Gaussian pulse laser could greatly improve output pulse quality and maximise available peak power at the same time. 2. Experiment set-up Figure 1 shows the configuration of our erbium-doped fibre ring laser [8]. The all-fibre ring cavity is made of a 135 cm EDF (80dB/m peak absorption ratio at 1535 nm), forward pumped by a 976 nm laser diode through a 980/1550 wavelength division multiplexer (WDM), a 320 cm single-mode fibre (SMF-28), and a 70 cm Nufern 980 fibre. The GVD parameters of the fibres are, respectively, 51, 18, and 4.5 ps/(nm km) at around 1550 nm. A 10% optical coupler (OC) is located after the EDF to output the signal. This output location may reduce suffering the nonlinear effect of the fibre. A polarisation-dependent isolator (PDI) sandwiched with two polarisation controllers (PC1 and PC2) is used as the mode-locking component in the cavity. Modelocking was initiated by the nonlinear polarisation rotation (NPR) in the cavity [8].The output port is connected to a commercial second-order autocorrelator, an optical spectrum analyser (OSA) and a fast photodetector to monitor the characteristics of output pulses. Here, the net cavity dispersion can be changed by cutting the length of positive or negative dispersion fibres. In order to generate Gaussian pulse without spectrum sidebands, nonlinear effect should be controlled to be minimal, in our laser cavity, the use of positive and negative GVD fibres make the pulse stretched and compressed periodically along the fibre, for a dispersion balanced laser cavity, pulse could reproduce itself in one round trip, but when the laser net dispersion was set to be a bit positive, pulse reproduce itself in several round trips, the stretching factors (ratio of maximum to minimum pulse width within the loop) increase and the maximum pulse width is increased, which would eliminate the nonlinearity. 3. Results and discussion In the laser, the polarisation controllers are adjusted to provide stable pulses that build from noise when the pump power is around 330 mw; the unidirectional operation was forced by the PDI. When the cavity is too long with large negatdispersion, fibre induced nonlinearity would cause pulses distortions, where sidebands are always observed in both autocorrelation trace and spectrum. If the net dispersion is carefully optimised, mostly when the net dispersion is slightly positive [7], a shorter pulse with higher energy can be generated, and the typical output pulse width is about 100 fs with a repetition rate of 41.6 MHz. The modelocked condition was sensitive to a different set of the PCs and pump power, while the minimum pulse duration was affected by total net dispersion. Here, we further gradually increased the net dispersion to be slightly positive (was calculated to be þ0.014 ps 2 ), which means the ring cavity length was cut to be shorter, and carefully optimised the nonlinearity management of the pulses propagating in the fibres. In this way, the pulse could not reproduce itself in one round trip, but reproduced itself in several trips; the accumulated positive GVD linearised the chirp and stretched the width of the pulse, which minimised the nonlinearity. For proper set of the PC state, a smooth pulse profile with Gaussian shape could be generated in the cavity. Figure 2 shows the typical experimental result of spectrum A with a FWHM of 41 nm, which exactly fits with Gaussian profile. We further increased the spectrum width to 49 nm by tuning the PCs, which is the spectrum B in Figure 3. Note that the difference mode-locked condition of spectrum B from spectrum A was just the set of PCs, but it would affect the polarisation state along the cavity and influence the saturation mode-locking condition. The spectrum B deviated from the Gaussian profile near 1535 nm and the available Gaussian spectrum profile was limited to 49 nm. In the laser, in order to obtain a better dispersion compensation, a 135 cm high-doped EDF was used in the cavity, which has a peak absorption ratio of 80 db/m near 1535 nm, and the gain media could not be fully pumped under this pump level. Limitation of gain media at the peak absorption near 1535 nm makes the spectrum deviate from the Gaussian profile. Furthermore, the gain variation and bandwidth of erbium ions limit the available Gaussian spectrum profile, which results in poorquality pulse output. Our results show that the
Journal of Modern Optics 627 Figure 2. The Gaussian spectrum profile output from the laser. Note that spectral intensity in (a) is plotted on a linear scale and fit with a sech 2 pulse profile (dotted) and a Gaussian pulse profile (dashed), for comparison, (b) is plotted on a logarithmic scale. (The colour version of this figure is included in the online version of the journal.) orientation of the fibre at each side of the polariser must be performed carefully because it has a great importance on the resulting pulse shape and energy. We carefully optimised the output external recompression by cutting a pigtail of the output coupler and measuring the pulse width by the autocorrelator. While the output spectrum was spectrum B in Figure 3, we obtained the output autocorrelation trace signal as short as 99 fs, which is shown in Figure 4. While maintaining the Gaussian spectrum profile, the smooth autocorrelation trace could be also fitted with the Gaussian profile. As the pulse was confirmed to be Gaussian shape, its duration should be 70 fs. Considering the width of spectrum B was 49 nm, the pulse shows a time-band product of 0.44, which fits the Gaussian pulse transform limit properly besides a small deviation on the two wings. Deviations of the two wings are attributed to uncompensated positive third-order dispersion (TOD) and background noise, the accumulated TOD causes the pulses to be slightly asymmetric, which developed a pedestal on the autocorrelation trace. The wing deviations are also
628 D. Deng et al. Figure 3. The deviations of the two spectrum from the Gaussian pulse profile, (a) is plotted on linear and (b) on logarithmic. (The colour version of this figure is included in the online version of the journal.) conformed in the spectrum in Figure 3. As the average output power was measured to be 45.6 mw, the single pulse energy reaches 1.1 nj with a peak power of 15.7 kw. A clean Gaussian spectrum without any sidelobes showing a good pulse shape was generated in the laser; the laser has greatly improved output pulse quality and maximized available peak power. As the experiment [2,6] and theory [7] predicted, the fibre laser operating in soliton regime can provide clean sech 2 -shape pulses with a relatively large timeband product. In a dispersion-managed cavity, not only the amplitude and the width oscillate in a periodic manner, but also the frequency varies across the pulse. The pulse in the cavity should be chirped, and the nonlinear distortion effect of the pulse minimised. Thus, the Gaussian shape spectrum could easily form in a well-managed chirped laser cavity, and the pulse shape is also close to being Gaussian, although it contains considerable structure in the pulse wing. The pulses with Gaussian intensity profiles can be formed in periodic systems that contain fibres of differing dispersion [10,11]. In our experiment, laser net dispersion was set to be slightly positive (about þ0.014 ps 2 ). The pulse could not reproduce itself in one round trip
Journal of Modern Optics 629 Figure 4. Output pulse autocorrelation trace of the laser. (The colour version of this figure is included in the online version of the journal.) Figure 5. Output pulses train of the laser. but reproduced itself in several round trips, which can be confirmed in Figure 5 as the amplitude of the pulses train from the oscilloscope was slightly modulated by period reproduction. The several round trip accumulated positive GVD linearised the chirp and stretched the width of the pulse, which further minimised the nonlinearity, the spectrum flattens the top and pulses with greatly suppressed temporal wings could be produced. At the same time, the linear chirp means that the pulses could be efficiently recompressed to the transform limit. 4. Conclusion We have described the experiment demonstration of a 70 fs Gaussian pulse generation in a dispersionmanaged erbium fibre ring laser working on normal
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