Tonghui Liu

Ultra-wide square pulses generation in a Yb-doped fiber laser based on nonlinear polarization rotation effect

An ultra-wide square pulse is demonstrated in a passively mode-locked Yb-doped fiber laser based on nonlinear polarization rotation effect. In our experiment, the pulse width can be tuned from 26 ns to 164 ns without wave-breaking when the pump power increases from 210 mW to 600 mW. And the peak power of the square pulse remains constant. The maximum output single pulse energy is 8.75 nJ at a pump power of 600 mW. To the best of our knowledge, this is the widest pulse in any mode-locked Yb-doped fiber laser.

An ultra-long cavity passively mode-locked fiber laser based on nonlinear polarization rotation in a semiconductor optical amplifier

In this paper we investigate an ultra-long cavity passively mode-locked fiber laser based on a semiconductor optical amplifier (SOA). Experimental results are presented which indicate that stable mode-locked pulses can be obtained by combining nonlinear polarization rotation (NPR) in the SOA with a polarization controller. By adding a 4 km single mode fiber into the ring cavity, a stable fundamental-order mode-locked pulse train with a repetition rate of 50.72 kHz is generated through the NPR effect in the SOA. The central wavelength, 3 dB bandwidth and single pulse energy of the output pulse are 1543.95 nm, 1.506 nm and 33.12 nJ, respectively. Harmonic mode-locked pulses are also observed in experiments when the parameters are chosen properly.

Numerical studies of supercontinuum generation based on quasi-rectangle wave pumping

Spectral broadening and the generation of supercontinuum are inherent features of nonlinear optics, and have been studied intensively for many years. Supercontinuum generation has been found numerous applications in such various fields as spectroscopy, pulse compression, and the design of tunable ultra-short femtosecond laser sources. Generally, supercontinuum is mainly generated by hyperbolic-secant soliton pulse. It is rarely investigated about the spectral broadening when pumping with pulse of a rectangle or quasi-rectangle shape, which is an interesting direction of supercontinuum generation. In this paper, supercontinuum generation based on soliton, quasi-rectangle, and rectangle pulses are investigated numerically. Firstly, we introduced the general nonlinear Schrödinger equation (GNLSE) to deduce the temporal and spectral evolution of pulse broadening in the photonic crystal fiber. Secondly, with different types of pulses, including hyperbolic-secant, ultra-Gaussian and rectangle pulses, we investigated the generation of new frequency and spectral broadening by numerical methods. Comparisons of characteristics of spectral and temporal evolution with different pulses are made in the following parts of paper. It is found that the generation of dispersive waves and Raman effect play main roles in the spectral broadening and generation of supercontinuum, as the broadband wave is generated by the coupling of Raman soliton and dispersive waves through cross-phase modulation. Apparent side-bands can be observed in our simulation under the condition of quasi-rectangle and rectangle pulses pumping, which affect the smooth of spectrum. Thus, to our acknowledge, a flat supercontinuum can be generated by changing the parameter of pulse to make a better coupling of the side-bands.

Numerical simulation of passively mode-locked fiber laser based on semiconductor optical amplifier

Passively mode-locked fiber laser (MLFL) has been widely used in many applications, such as optical communication system, industrial production, information processing, laser weapons and medical equipment. And many efforts have been done for obtaining lasers with small size, simple structure and shorter pulses. In recent years, nonlinear polarization rotation (NPR) in semiconductor optical amplifier (SOA) has been studied and applied as a mode-locking mechanism. This kind of passively MLFL has faster operating speed and makes it easier to realize all-optical integration. In this paper, we had a thorough analysis of NPR effect in SOA. And we explained the principle of mode-locking by SOA and set up a numerical model for this mode-locking process. Besides we conducted a Matlab simulation of the mode-locking mechanism. We also analyzed results under different working conditions and several features of this mode-locking process are presented. Our simulation shows that: Firstly, initial pulse with the peak power exceeding certain threshold may be amplified and compressed, and stable mode-locking may be established. After about 25 round-trips, stable mode-locked pulse can be obtained which has peak power of 850mW and pulse-width of 780fs.Secondly, when the initial pulse-width is greater, narrowing process of pulse is sharper and it needs more round-trips to be stable. Lastly, the bias currents of SOA affect obviously the shape of mode-locked pulse and the mode-locked pulse with high peak power and narrow width can be obtained through adjusting reasonably the bias currents of SOA.

Tunable optical delay line based on four-wave mixing wavelength conversion and dispersion in optical fibers

In recent years,tunable optical delay line has attracted much research interest for its applications in optical communications,optical signal processing and optical control of phased array antennas in microwave communications. Compared with optical delay line based on slow light techniques, all-optical wavelength conversion followed by signal propagation in a dispersive medium presents an alternative approach to control the delay time because it is simpler and more controllable. In this paper, a tunable optical delay line with a large time delay up to 650ps based on four-wave mixing (FWM) wavelength conversion in 420m high nonlinearity fiber (HNLF) and dispersion in 8km standard single mode fiber (SMF) is experimentally demonstrated.Our experimental results are in good agreements with the principle.