A Novel Nanotube-Based Fiber Laser for Ultrashort Pulse Generation and Fast Measurements

We propose a nanotube-based erbium-doped fiber laser that can deliver conventional soliton (CS) and stretched pulse (SP) based on D-shaped fiber saturable absorber (DF-SA) where evanescent-field interaction works. The novel Nanotube-based Fiber Laser can generate SP or CS by tuning pump power and polarization controller (PC) properly. The net cavity dispersion of laser is slightly negative. In our experiment, by optimizing the PC in the cavity, CS and SP can be obtained at the central wavelengths of 1530.6 nm and 1530.3 nm due to on carbon nanotubes and the spectral filtering effect induced by nonlinear polarization rotation. Although the acquired CS and SP nearly have the same central wavelengths, they show distinct optical spectra, 3-dB bandwidths. The proposed fiber laser with switchable CS and SP is attractive for ultrashort pulse generation and fast measurements in practical applications.

A long interaction length for the guided light and the nanotubes is introduced by the D-shaped fiber saturable absorber, which guarantees an efficient nonlinear effect so that it facilitates laser mode locking even though the laser is under various conditions [36].Higher intracavity power can be introduced for higher energy pulse formation, because only a part of the optical power of the propagating mode interacts with the SWNTs for mode locking.Lots of researchers have focused on evanescent-field interaction with SWNTs due to the outstanding performance of higher energy pulse formation and low CNT-density threshold [37].The first experimental observation of four-wave mixing in SWNTs deposited on a DF has been reported by Chow et al. [38].By using the evanescent-field interaction of propagating light with the nanotubes, a mode locker that is immunized to the high optical power induced damage have been demonstrated by Song et al. [36] [39].
Different types of mode-locked pulses have been obtained due to different designs of fiber laser cavity, such as CS [29] [40] [41], SP [42], self-similar pulse [43] [44], and dissipative soliton (DS) [5] [17] [45].Because of the interaction between the fiber anomalous dispersion and nonlinear optical Kerr effect, CSs can be formed in negative dispersive regime [5] [41] [46].When the frequencies of dispersive waves are phase-matched, discrete sidebands whose positions are determined by the cavity length, net dispersion, and pulse duration will generate and distribute on both sides of the spectrum [46] [47] [48].When the net cavity dispersion approaches zero, SPs propagating in such a cavity are stretched and compressed periodically [21] [43] [49] [50].Self-similar pluses operating in normal-dispersion regime are the parabolic pulses with self-similar evolution [43] [51] [52].Moreover, the linear-chirped self-similar pulses can avoid pulse breaking when they propagate in the routes [53].The multiple interactions between fiber nonlinearity, normal dispersion, saturable absorption effect, and spectral filtering effect result in the formation of DS [54].DS has higher energy and wilder pulse width than that of CS, and its characteristics change dramati-Modern Instrumentation cally when it travels along the laser cavity [55].
Although as a lot of progress has been made, fiber laser on many key physical parameters (such as pulse energy, pulse width and peak power, etc.) still lags behind that of the solid laser.

Results and Discussions
With the pump power of ~50 mW, the continuous wave (CW) is observed from the outputted fiber laser.When the total pump power is increased to ~70 mW, CS with its typical spectrum is shown in Figure 2(a).the round-trip time of the cavity is ~144 ns.
In our experiment, the spectral filtering effect plays an important role on the pulse formation.The polarization sensitive DF-SA and PC can be considered as a NPR-induced spectral filter and the intensity transmission of light through the filter, T, can be expressed as [19] ( ) ( ) ( ) ( ) ( ) ( ) ( ) where ϕ 1 is the phase delay caused by the PC and ϕ 2 is the phase delay resulting from the fiber including both the linear phase delay and the nonlinear phase delay.The polarizer and analyzer have an orientation of angles θ and φ with respect to the fast axis of the fiber, respectively.From Equation (1), one knows that the bandwidth of NPR-induced filter varies with PC states.Therefore, the different PC' settings will generate pulses with different bandwidths, which in turn lead to various stretching factors [52].When the stretching factor is small enough and the interplay between anomalous cavity dispersion and positive Kerr nonlinearity of the fiber is balanced, the laser will operate in the soliton regime with spectral sidebands.As the stretching factor increases, spectral sideband generation will be reduced and there will be less dispersive radiation between pulses.Thus, stretched pulses will be generated with cleaner and broader spectra in the cavity when the stretching factor is large enough.Moreover, the solitons can be maintained as the pump power changes from the self-starting threshold to the maximum available power (550 mW), so the SA displays an ultra-high optical damage threshold.

Conclusion
By employing a DF-SA, we have proposed a simple and compact fiber laser system that can deliver CS and SP.With optimizing the PC properly and adjusting the pump power appropriately, we can achieve the CS and SP that have the sim-

Figure 1 Figure 1 .
Figure1shows the schematic diagram of our experimental setup.In the fiber laser cavity, a 980 nm laser diode (LD) is used as pump source via a 980/1550 nm wavelength-division multiplexer (WDM) coupler; an 18-m erbium-doped fiber (EDF) with the group velocity dispersion (GVD) parameter of -9 ps/nm/km and the absorption of 3 dB/m at 980 nm acts as the gain media; a PC in the cavity is used to adjust the state of polarization; the 10% port of a 90/10 coupler is adopted to extract the laser for measurement; the DF-SA integrated into the
ilar central wavelengths and fundamental repetition rates.Because of the different pump powers and operations on PC, the stretching factor which is tunable due to the spectral filtering effect induced by NPR varied dramatically, they have different pulse durations, 3-dB bandwidths and optical spectra.The pulse duration and 3-dB bandwidth of CS centered at 1530.6 nm are ~0.7 ps and 4.46 nm while that of SP with central wavelength of 1530.3 nm are ~1.0 ps and 9.59 nm, respectively.Our experimental setup of the nanotube-based Fiber Laser can realize the ultrashort pulse generation and fast measurements.But the key physical parameters (such as pulse energy, pulse width and peak power) still lag behind that of the solid laser.It is attractive and convenient for practical research applications of ultrashort pulse field because of the proposed fiber laser delivering two types of pulses.
et al.
DOI: 10.4236/mi.2018.72003 25 Modern Instrumentation rotation (NPR) [16] [17] [18] Therefore, development of ultrashort pulse fiber laser has important practical significance and broad market prospect.In this paper, we propose a fiber laser that can deliver switchable CS and SP based on a SWNT mode-locker deposited on D-shaped fiber (DF).The proposed fiber laser can generate SP or CS with central wavelength of 1530 nm by tuning pump power and polarization controller (PC) properly.The spectral bandwidth and pulse duration of the CS are 4.46 nm and ~0.7 ps, and these of the SP are 9.59 nm and ~1.0 ps.A NPR-induced filter is formed by the polarization sensitive DF-SA and PC.The stretching factor plays a key role on the switchable mode-locking operation, which can be tuned by the NPR-induced filter.The saturable absorber (SA) can withstand pump power up to 550 mW and deliver stable SP or CS without damage.It is attractive for practical applications of ultrashort pulse generation and fast measurements to implement our fiber laser with switchable CS and SP.