Abstract—We have experimentally demonstrated bright-dark pulses in a nonlinear polarization rotation (NPR) based mode-locked Erbium-doped fiber laser (EDFL) with a long cavity configuration. Bright–dark pulses could be achieved when the laser works in the passively mode-locking regime and the net group velocity dispersion is quite anomalous. The EDFL starts to generate a bright pulse train with degenerated dark pulse at the mode-locking threshold pump power of 35.09 mW by manipulating the polarization states of the laser oscillation modes using a polarization controller (PC). A split bright–dark pulse is generated when further increasing the pump power up to 37.95 mW. Stable bright pulses with no obvious evidence of a dark pulse can also be generated when further adjusting PC and increasing the pump power up to 52.19 mW. At higher pump power of 54.96 mW, a new form of bright-dark pulse emission was successfully identified with the repetition rate of 29 kHz. The bright and dark pulses have a duration of 795.5 ns and 640 ns, respectively. Keywords—Erbium-doped fiber laser, Nonlinear polarization rotation, bright-dark pulse. I. INTRODUCTION ULSE fiber lasers have gained tremendous interest in recent years due to their practical applications in laser communications, LIDAR, material processing, sensing, medical care, laser acceleration and nonlinear frequency conversion [1], [2]. Generally, there are two types of pulse in fiber lasers: the bright pulse, which is a sharp increment of laser intensity beyond a continuous laser background with lower or near zero intensity, and the dark pulse, which is, in contrast, a deep intensity dip below a continuous laser background with nonzero intensity [3]. There are various types of dark or bright pulse/soliton, such as group-velocity-locked vector solitons (GVLVSs) [4], phase (or polarization)-locked vector solitons [5], dissipative solitons [6], and vector dark polarization domain wall solitons [7]. The dynamics of pulses/solitons in a fiber laser cavity can be described by the nonlinear Schrödinger equation (NLSE) [8]. Moreover, the complex Ginzburg–Landau equation (CGLE) and the coupled higher-order NLSE have also been used to interpret the dynamics of pulses/solitons in fiber lasers [7]-[9] Compared with dark pulses, bright pulses are relatively more convenient to generate and have been widely investigated and applied in various practical applications. However, dark pulses have also attracted intense attention owing to their unconventionality in the generating process, which provides a powerful tool for investigating the mechanism of laser pulse evolution and R. Z. R. R. Rosdin, N. M. Ali, S. W. Harun and H. Arof are with the Department of Electrical Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia (e-mail: raja_zaimas@um.edu.my) oscillation [10], [11]. Moreover, dark pulses have some unique advantages such as being less sensitive to fiber loss than bright pulses, less affected by intra pulse-stimulated Raman scattering, and having better stability in long-distance communications with respect to the Gordon–Haus jitter [12]. In this paper, we report on a bright-dark pulse emission in EDFL based on nonlinear polarization rotation (NPR) technique. The bright and dark are two orthogonal linear polarization components that are coupled incoherently in the laser cavity. Bright–dark pulses could be achieved when the laser works in the passively mode-locking regime and the net group velocity dispersion is quite anomalous. We show experimentally that under appropriate operation conditions, an all-normal dispersion cavity fiber laser can emit a train of bright-dark pulses. II. EXPERIMENTAL ARRANGEMENT A schematic of the NPR-based mode-locked fiber laser cavity is shown in Fig. 1. The pump source was a laser diode (LD) with emission centered at 1480 nm. A 3m long erbium- doped fiber (EDF) was used as the laser gain medium, pumped by the LD via a 1480/1550 fused wavelength division multiplexer (WDM) coupler. To realize the bright-dark pulse at ~1550 nm, the total intra cavity group velocity dispersion (GVD) is set to be quite anomalous. The EDF used has an absorption coefficient of approximately 11 dBm-1 and dispersion of -21.64 ps/nm.km at 1550 nm. Other fibers in the cavity are a 7.0 km long of dispersion-shifted fiber (DSF) with dispersion of about 2.7 ps/nm.km and a 5.5 m long standard SMF (18 ps/nm.km), which constituted the rest of the ring. The cavity operates in anomalous where the net GVD and fundamental repetition rate are estimated as -24.47 ps2 and 29.0 kHz, respectively. A polarization dependent isolator (PDI) was used in the cavity to force the unidirectional operation of the ring, and eliminate undesired feedback from the output end facet. A 20/80 fused fiber optical coupler was used to extract 20% energy from the cavity. To match the polarization states from one round trip to the next, a polarization controller (PC), consisting of three spools of SMF-28 fiber, was placed in the ring cavity after the PDI. The pump power and average output power were measured by an optical power meter (OPM). The monitoring of the output spectra and pulse trains was performed using an optical spectrum analyzer (OSA, AQ6370B) with a minimum resolution of 0.02 nm and a 500 MHz digital phosphor oscilloscope (Tektronix TDS 3052C). The pulse width was measured by an autocorrelator (Alnair). The total length of the laser cavity is estimated to be around 7.01 km. R. Z. R. R. Rosdin, N. M. Ali, S. W. Harun, H. Arof Bright–Dark Pulses in Nonlinear Polarisation Rotation Based Erbium-Doped Fiber Laser P World Academy of Science, Engineering and Technology International Journal of Physical and Mathematical Sciences Vol:8, No:12, 2014 1461 International Scholarly and Scientific Research & Innovation 8(12) 2014 scholar.waset.org/1307-6892/10000029 International Science Index, Physical and Mathematical Sciences Vol:8, No:12, 2014 waset.org/Publication/10000029