We demonstrated a pulsed erbium-doped fiber laser (EDFL) based on a few-layer molybdenum disulfide saturable absorber (MoS2-SA). The MoS2-SA was fabricated into a film structure by evaporating the mixture of MoS2 nanosheets and polyvinyl alcohol. The Raman spectra and nonlinear optical characteristics were measured to confirm the quality of the as-prepared MoS2-SA. By inserting the MoS2-SA into an EDFL, the pulsed operation from the stable Q-switched to mode-locking regime could be achieved by simply increasing the pump power level. The Q-switched pulse duration and repetition rate could vary from 5.18 to 3.53 μs and from 72.74 to 86.39 kHz, respectively. The maximum pulse energy was 74.93 nJ. After the achievement of Q-switched operation, the ultrashort mode-locking pulse was obtained by further increasing the pump power. The results further demonstrate the excellent saturable absorption ability of the few-layer MoS2 at telecommunication waveband.
The depressed-index dual core PCF-based broadband directional coupler is designed by using semivectorial finite
difference method. We study the influence of the structural parameters on the coupling characteristics in the dual core
PCF. The results show that once the structural parameter of the cladding of the dual core PCF and the doping degree in
the dual core region are optimised, the broadband coupling can be realised: in the wavelength range of 1.22~1.65μm, the value of the coupling length is stabilized at 26637nm±235nm and when the couplers are designed as two kinds
of couple with coupling ratio of 50% and 10%, the coupling ratios of (50±0.702)% and (10±0.664)% are realized,
respectively.
The evolution properties of the self-similar parabolic pulse(similariton) with higher-order effects in microstructured fiber
amplifier with normal group-velocity dispersion are investigated in this paper. The numerical results show that the
higher-order effects greatly distort similariton's waveform, frequency and the linearity of chirp. The drift of pulse center
increases with distance. The influence of different higher-order effects on similariton is analyzed. High quality
propagation of similariton can be attained by manipulating the geometrical parameters of the microstructured fiber
amplifier. These results are significant in the further study of similariton propagation in high-power ultrashort fiber
amplifier, laser and transmission system.
Vector soliton is obtained in erbium-doped fiber laser via nonlinear polarization rotation techniques. In experiment, we
observe the every 4- and 7-pulse sinusoidal peak modulation. Temporal pulse sinusoidal peak modulation owes to
evolution behavior of vector solitons in multiple polarization states. The polarizer in the laser modulates the mode-locked
pulses with different polarization states into periodical pulse train intensities modulation. Moreover, the
increasing pumping power lead to the appearance of the harmonic pulses and change the equivalent beat length to
accelerate the polarization rotation. When the laser cavity length is the n-th multiple ratios to the beat length to maintain
the mode-locking, the mode-locked vector soliton is in n-th multiple polarization states, exhibiting every n-pulse
sinusoidal peak modulation.
Self-similar pulse propagation properties in Yb+3-doped fiber amplifiers with the effects of higher-order
dispersion and gain dispersion have been studied in this paper. The effects of initial pulse energy, third-order
dispersion and the doped ions dipole relaxation time on the pulse propagation quality are relatively expounded
with the analytical and simulative methods. It is shown that with nonlinear area of parabolic pulses being enlarged,
the center of chirp moves forward or backward, due to the difference of third-order dispersion coefficient. And the
figure of pulses is distorted with the pulse peak delaying to one side and the center location excursing. Gain
dispersion can be a filter, which can properly compress pulse-width. Results proved the possibility of the
self-similar pulse existing in gain medium with a certain relaxation time.
We propose a new method to enhance solitons' robustness to polarization-mode dispersion (PMD) in a long-haul conventional single-mode fiber (SMF). The method is based on the utilization of four-wave mixing (FWM) effects of two principal polarization states of optical pulses. An intensively bound state of solitons can be achieved when the FWM effect is taken into account. It is proved numerically that the solitons with FWM are more adaptive to polarization-mode dispersion than the ones without FWM in the standard fiber links.
The enhancement of adaptive abilities of nonlinear optical pulses to polarization mode dispersion (PMD) is obtained by modulating breath-depth of two polarization states of the pulses. The breath-depth modulation is realized based on the interaction among self-phase modulation, cross phase modulation, group velocity dispersion and degenerate four-wave mixing, which results in making two polarization states of the pulses into a tighter state when nonlinear pulses propagate in optical fibers with high birefringence. Numerical results show that the transmission stabilities of modulated breath-depth optical pulses are improved in conventional fibers with random birefringence. The pulse broadening trend of the modulated breath-depth pulses induced by PMD is effectively suppressed by our proposed method. The symmetric output pulse shape is also reshaped in conventional fibers.
The theoretical analysis of a flat-top AWG with multimode interference (MMI) power splitter at the input end is done. Several corrections to the former papers are made. For example, the field coupled into the arrayed waveguides is not treated with approximation, and domain of integration for coupling coefficient of an arbitrary waveguide in the array is chosen to be the waveguide spacing, which is coincident with the real optical propagation in AWG. Expressions for spatial distribution of field are obtained at the interfaces between waveguides, while an analytic expression for each channel response of AWG is derived. All of the expressions obtained are the functions of AWG configuration parameters. The output response of each channel is obtained by the analytic method much more quickly than by beam propagation method (BPM), and the period of optimization design of AWG is shortened greatly. Finally, a design example of flat-top AWG with good performance is presented, and its important specifications are calculated.
Nonlinear optical pulses with sub-picosecond width will exhibit the phenomena of timing jitter and pulse decay induced by the third-order dispersion and Raman self-frequency shift. The optical phase conjugation is proposed to compensate for the high-order dispersion and high-order nonlinear effect. However, the spectral phase conjugation can’t eliminate the time delay induced by Raman self-frequency shift and the temporal phase conjugation can’t avoid the pulse splitting into two pieces induced by the third order dispersion. Thus, the schemes of the combination of the spectral and temporal phase conjugation are supposed to use for recovering the distorted pulse and reducing the time delay induced by the third-order dispersion and Raman self-frequency shift. There exist the optimized schemes to deploy the relative position between the spectral phase conjugator and the temporal phase conjugator to obtain the optimization transmission qualities with the minimum penalty for timing jitter and pulse distortion. In addition, the spectral phase conjugator and temporal phase conjugator should be settled down after the midway and before the midway, respectively, in order to obtain the residual third-order dispersion to enhance the frequency red-shift trend of two polarization components after the temporal phase conjugator for reducing the time delay.
We proposed a new approach to enhance supercontinuum (SC) spectrum generation in dispersion-decreasing optical fiber with a new dispersion profile. The approach is based on the change of the chromatic dispersion D(λ,z) as a concave function of wavelengths that has no zero-dispersion wavelengths over the whole or part of the fiber as D(λ,z) is positive. A flatter and broader spectrum can be produced in our scheme. The general criteria parameters are obtained for characterizing SC shapes in the optical fibers.
A new nonlinear optical loop mirror (NOLM) constructed from a dispersion decreasing fiber (DDF) and a general fiber is proposed. Numerical simulation shows that a ultra-short pulse with pedestal suppression and small chirp generated by the new optical loop mirror can steadily evolutes over a very long fiber. When the pulse takes the best quality factor, the bigger compression factor is gained and the shorter optical fiber is used comparing to the use of the nonlinear optical loop mirror constructed from dispersion decreasing fiber (DDF-NOLM).
An optical phase conjugator is used to enhance the robustness of optical pulses to polarization dispersion. The
polarization components of optical pulses form a breather in birefringent fibers firstly and make a polarization
component along fast axis compressed through spectra inversion function of the optical phase conjugator. The two
polarization modes are compressed further due to the cross-phase modulation effect and trap each other fully in time
domain. The optical phase conjugator is required to settle down before higher birefringent fibers in order to suppress the
influence of birefringence effectively. The limit value of δ reaches 7.5 that can be suppressed by enhanced robustness of
the optical pulses by using the optical phase conjugator in the case that each polarization component is a fundamental
soliton. Moreover, the function of the optical phase conjugator will reduce the delay of optical pulses for a different
polarization angle.
We discuss the propagation characteristics of the electromagnetic field in the symmetric metal-clad planar uniaxial crystal waveguide. For the special waveguide structure that the optical axis of the crystal is on the x-z plane, where x is the normal direction of the waveguide plane and z is the propagation direction, we obtain the analytic accurate solution of the modal fields. It is shown that:(1) there are TE mode and TM mode in this special waveguide structure; 2) The direction of the wave vector and the Poynting vector of TM mode are not collinear; (3) The principal mode of the waveguide is the principal mode of TE mode for the negative uniaxial crystal, but the one of TM mode for the positive uniaxial crystal.
The nonlinear Schroedinger equation in erbium-doped fiber and the more generalized form of the propagation equation in the erbium-doped fiber amplifiers are obtained which have included the phase shift that the erbium ions induce. An analytical expression is given to the erbium ions induced phase shift and frequency chirp. It was found that the frequency chirp dose not change much with the wavelength except at the neighboring wavelengths (around 1.531 micrometer) where the absorption and the emission cross-sections of the erbium ions reach their maximum, and the frequency chirp has opposite sign on the two sides around 1.531 micrometer.
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