All-optical logic AND-NOR gate with three inputs based on cross-gain modulation in a semiconductor optical amplifier

We report the realization of a novel all-optical logic AND-NOR gate based on cross-gain modulation (XGM). The used scheme requires only one SOA to perform the logic gate with three input signals. A 8.5 dB dynamic extinction ratio with a switching time of about 650 ps for the rise time and 100 ps for the fall time.     

Experimental investigation of the impact of optical injection on vital parameters of a gain-switched pulse source

An analysis of optical injection on a gain-switched distributed feedback (DFB) laser and its impact on pulse parameters that influence the performance of the pulse source in high-speed optical communication systems is presented in this paper. A range of 10 GHz in detuning and 5 dB in injected power has been experimentally identified to attain pulses, from an optically injected gain-switched DFB laser, with durations below 10 ps and pedestal suppression higher than 35 dB. These pulse features are associated with a side mode suppression ratio of about 30 dB and a timing jitter of less than 1 ps. This demonstrates the feasibility of using optical injection in conjunction with appropriate pulse compression schemes for developing an optimized and cost-efficient pulse source, based on a gain-switched DFB laser, for high-speed photonic systems.     

SOA-based loop mirror for tunable pulse-width generation

We propose and experimentally demonstrate a system for the generation of pulses of tunable pulse-width as those required in high spectral efficiency optically routed networks. Pulse narrowing of 500 ps pulses by 90% is accomplished through a SOA based non-linear loop mirror. Optical switching through the SOA loop mirror is used to shape and carve these large pulses (e.g., 500 ps) generated by non-expensive low-frequency optoelectronic components to narrow pulses (e.g., 50 ps). We also calculate the minimum loop size and optimum repetition rate of the original pulse train for the generation of the shorter pulse-width pulse train.     

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.     

Design and analysis of novel highly nonlinear photonic crystal fibers with ultra-flattened chromatic dispersion

The present article describes novel highly nonlinear photonic crystal fibers (HN-PCFs) with flattened chromatic dispersion and low confinement losses. The proposed design has been simulated based on the finite-difference method with anisotropic perfectly matched layers absorbing boundary condition. It is proved that the design novel HN-PCFs is obtained a nonlinear coefficient greater than 45 W⁻¹ km⁻¹ and low dispersion slope −0.009 ps/(nm².km) at 1.55 μm wavelength. In addition, results from numerical simulation show that the ultra-flattened dispersion of 0 ± 0.65 ps/(nm.km) can be obtained in a 1.36-1.62 μm wavelength range with confinement losses lower than 10⁻⁷ dB/m in the same wavelength range. Another advantage of the proposed HN-PCFs is that it possessed modest number of design parameters.     

41 mW high average power picosecond 177.3 nm laser by second-harmonic generation in KBBF

We report on the generation of high average power, high repetition rate, and picosecond (ps) deep-ultraviolet (DUV) 177.3 nm laser. The DUV laser is produced by second-harmonic generation of a frequency-tripled mode-locked Nd: YVO₄ laser (<15 ps, 80 MHz) with KBBF nonlinear crystal. The influence of different fundamental beam diameters on DUV output power and KBBF-SHG conversion efficiency are investigated. Under the 355 nm pump power of 7.5 W with beam diameter of 145 μm, 41 mW DUV output at 177.3 nm is obtained. To our knowledge, this is the highest average power for the 177.3 nm laser. Our results provide a power scaling by three times with respect to previous best works.     

Growth and oxidation of Cr films on the W(1 0 0) surface

The growth and oxidation of Cr films on the W(1 0 0) surface have been studied with low energy electron microscopy (LEEM) and diffraction (LEED). Cr grows in a Stranski-Krastanov (SK) mode above about 550 K and in a kinetically limited layer-by-layer mode at lower temperature. Stress relief in the highly strained pseudomorphic (ps) Cr film appears to be achieved by the formation of (4 × 4) periodic inclusions during the growth of the third layer between 575 and 630 K and by growth morphological instabilities of the third layer at higher temperature. Kinetic or stress-induced roughening is observed at lower temperature. In the SK regime, three-dimensional (3D) Cr islands nucleate after the growth of three Cr layers. 3D island nucleation triggers dewetting of one layer from the surrounding Cr film. Thus, two ps Cr layers are thermodynamically stable. However, one and two layer ps Cr films are unstable during oxidation. 3D clusters, that produce complex diffraction features and are believed to be Cr₂O₃, are formed during oxidation of one Cr layer at elevated temperature, T ? 790 K. The single layer Cr film remains intact during oxidation at T ? 630 K. 3D bulk Cr clusters are formed predominantly during oxidation of two ps Cr layers.     

Numerical design of high nonlinear coefficient of microstructure optical fibers

This paper presents a theoretical design of highly nonlinear microstructure optical fiber with dispersion-flat characteristics. The APSS™ 2.3 software based on the finite difference method with perfectly matched boundary conditions is used to simulate the properties of the proposed microstructure optical fiber. According to simulation, the proposed fiber warrants a high nonlinear coefficient of the order 41 W⁻¹ km⁻¹ and a low dispersion of 0.25 ps/nm/km at 1550 nm wavelength. It assumes a dispersion-flat characteristic of 0 ± 0.50 ps/nm/km in a 1450-1620 nm wavelength range centering 1550 nm wavelength with a modest number of design parameters.     

Ablation mechanism study on metallic materials with a 10 ps laser under high fluence

Single shot ablation of metallic materials of aluminium, titanium alloy (Ti6Al4V) and gold has been studied with 10 picoseconds (ps) laser pulses experimentally and theoretically. The ablation rate variation at high fluence was explained by a simplified predictive model based on critical-point phase separation (CPPS) theory. A comparison between experimental and numerical results inferred that CPPS may well be the dominant ablation mechanism for high fluence laser ablation at 10 ps laser duration.     

Core-level shifts at the Pt/W(1 1 0) monolayer bimetallic interface

We have measured W and Pt 4f_7/2 core-level photoemission spectra from interfaces formed by ultrathin Pt layers on W(1 1 0), completing our core-level measurements of W(1 1 0)-based bimetallic interfaces involving the group-10 metals Ni, Pd, and Pt. With increasing Pt coverage the sequence of W spectra can be described using three interfacial core-level peaks with binding-energy (BE) shifts (compared to the bulk) of −0.220 ± 0.015, −0.060 ± 0.015, and +0.110 ± 0.010 eV. We assign these features to 1D, 2D pseudomorphic (ps), and 2D closed-packed (cp) Pt phases, respectively. For ∼1 ps ML the Pt 4f_7/2 BE is 71.40 ± 0.02 eV, a shift of +0.46 ± 0.09 eV with respect to the BE of bulk Pt metal. The W 4f_7/2 core-level shifts induced by all three adsorbates are semiquantitatively described by the Born-Haber-cycle based partial-shift model of Nilsson et al. [39]. As with Ni/W(1 1 0), the difference in W 4f_7/2 binding energies between ps and cp Pt phases has a large structural contribution. The Pt 4f lineshape is consistent with a small density of states at the Fermi level, reflective of the Pt monolayer having noble-metal-like electronic structure.     

Ultrahigh-channel-count fiber Bragg grating based on the triple sampling method

A triple sampling method to have enabled excellent channel uniformity and high in-band energy efficiency is firstly proposed for the design of an ultrahigh-channel-count fiber Bragg grating (FBG), which is based on the simultaneous utilization of two amplitude-assisted phase sampling (AAPS) functions and a phase-only sampling (POS) function. As an example, one linearly chirped FBG with consecutive 1215 channels enabling to cover all fiber telecom bands (O + E + S + C + L + U) is numerically demonstrated, which has a length of 9 cm, a dispersion of − 1360 ps/nm, and a channel spacing of 50 GHz. The maximum index-change required for 10 dB strength of the FBG is less than 6 × 10^− 3.     

Development of a nearly transform-limited, low-repetition-rate, picosecond optical parametric generator

A low-repetition-rate (10-Hz), picosecond (ps) optical parametric generator (OPG) seeded at the idler wavelength with a high-power diode laser is demonstrated. The output of the OPG at ∼566 nm is amplified in dye cells, resulting in signal enhancement by more than three orders of magnitude. The nearly transform-limited beam at ∼566 nm has a pulsewidth of ∼170 ps, with an overall output of ∼2.3 mJ/pulse. The laser is tuned either by tuning the nonlinear crystal or the seed-laser current. The applications of such a simple, compact, high-performance, tunable ps laser system for linear and nonlinear spectroscopies are outlined.     

Duty-cycle division multiplexing (DCDM)

A multiplexing technique, which is based on duty-cycle division, is proposed. The channel multiplexing and demultiplexing are performed electrically at the single user bit rate, which is very economic. In a three-user system (3×10 Gb/s), the simulation results show that the best receiver sensitivity value achieved is −30.1 dBm with an optical signal-to-noise ratio (OSNR) of 22.3 dB, while the chromatic dispersion tolerance ranges from 192 to 280 ps/nm. Migration from 30 to 120 Gb/s is achieved with the penalty of 6.4 and 5.2 dB in the receiver sensitivity and OSNR, respectively, for the worst user.     

Electronic structure and electron dynamics at the GaSb(0 0 1) surface studied by femtosecond pump-and-probe pulsed laser photoemission spectroscopy

Transiently excited electron states at the GaSb(0 0 1) surface have been studied by means of time- and angle-resolved photoemission spectroscopy based on a femtosecond laser system. A normally unpopulated surface electron state has been found at ∼250 meV above the valence band maximum with a strong confinement at the center of the surface Brillouin zone. The lifetime of transiently excited carriers at the intergap surface states has been found to be ∼11 ps, associated with rapid carrier diffusion.     

Characteristics of wave-breaking-free operation in a passively mode-locked all-fiber ring laser based on AS-Yb-PBGF

A passively mode-locked all-fiber ring laser is constructed based on all-solid Yb-doped photonic bandgap fibers (AS-Yb-PBGF), which provides the laser gain and anomalous dispersion simultaneously. Without any other dispersion compensators, the cavity is simple, compact, and maintenance- and alignment-free. The characteristics of wave-breaking-free operation in the laser are investigated by means of numerical simulations using the split-step Fourier method. The approximately parabolic pulses can be generated and evolved for maintaining the normal group-velocity dispersion intracavity, when the length of the single-mode fiber is between 2 and 4 m. The obtained highest energy of the output pulse is up to 2.27 nJ with the pulse duration of 8.61 ps, corresponding to the time-bandwidth product of 10.05. By dechirping out of cavity, the pulse duration is 0.70 ps.     

Passively mode-locked ytterbium fiber laser utilizing chirped-fiber-Bragg-gratings for dispersion control

A robust, self-starting picosecond pulse source based on ytterbium (Yb³⁺) doped fiber laser is described. Utilizing a chirped-fiber-Bragg-grating (C-FBG) for dispersion control, solitary mode-locking is obtained without bulk dispersion compensation elements. A semiconductor saturable absorber (SESAM) is used for stable self-starting. 3.6 ps pulses are produced, with 45 MHz basic repetition-rate and mW scale average output power at 1060 nm. Detailed numerical simulations based on the modified nonlinear Schrödinger equation agree well with the experimental results and are used as a design tool for the solitary mode-locked picosecond laser. The presented design can be simply employed in an all-fiber environmentally-stable system.     

Femtosecond laser ablation of silver foil with single and double pulses

The average ablation depth per pulse of silver foil by 130 fs laser pulses has been measured in vacuum over a range of three orders of magnitude of pulse fluence up to 900 J cm⁻². In addition, double pulses with separations up to 3.4 ns have been used to probe time scales of relevance for femtosecond ablation. The double pulse ablation depth, when each pulse fluence is 0.7 J cm⁻², falls to that of a single pulse as the pulse separation is increased from 0 ps to 700 ps. This time scale decreases to only 4 ps as the fluence is increased to 11 J cm⁻². It then jumps to 500 ps across a transition fluence where the slope of the ablation depth versus logarithmic fluence characteristic changes abruptly to a higher value. In addition, for pulse separations near 1000 ps, the second pulse can cause re-deposition of ejecta from the first pulse resulting in a double pulse ablation depth only 40% that of the first pulse alone. This has important implications for the interpretation of double pulse femto-LIBS intensities. Our results suggest that the optical properties of nano or mesoparticles play a significant role in double pulse ablation with large pulse separations.     

All-optical continuously tunable delay with a high linear-chirp-rate fiber Bragg grating based on four-wave mixing in a highly-nonlinear photonic crystal fiber

A scheme for hi-fi all-optical continuously tunable delay is proposed. The signal wavelength is converted to a desired idler wavelength and converted back after being delayed by a high linear-chirp-rate (HLCR) fiber Bragg grating (FBG) based on four-wave mixing (FWM) in a highly-nonlinear photonic crystal fiber (HN-PCF). In our experiment, 400 ps (more than 8 full width of half maximum, FWHM) tunable delay is achieved for a 10 GHz clock pulse with relative pulse width broaden ratio (RPWBR) of 2.08%. The power penalty is only 0.3 dB at 10⁻⁹ BER for a 10 Gb/s 2³¹−1 pseudo random bit sequence (PRBS) data.     

Performance comparison of pre-, post- and symmetrical-dispersion compensation techniques using DCF on 40 Gbps OTDM system for different fibre standards

Various factors like polarization mode dispersion, non-linear effects, Kerr effect, second and third order dispersion impose limit on the performance (transmission distance, pulse broadening) of Optical Time Division Multiplexed (OTDM) transmission system. In this paper, the performance comparison of 40 Gbps OTDM transmission system with pre-, post-, and symmetrical-dispersion compensation techniques for different fibre standards has been carried out. It has been observed that for pre compensation, ITU 655 [D = 3.78 ps/nm/km] came to be best suitable fibre with the dispersion compensating fibre length of 2.36 km for maximum reach of 3000 km. For post- and symmetrical-compensation, the behaviour of alcatel [D = 8 ps/nm/km] is almost similar with maximum transmission distance of 3200 and 3050 km respectively.     

Suppression of slow gain recovery in ultralong quantum-dash semiconductor optical amplifier emitting at 1.55 μm

Gain dynamics in two quantum-dash semiconductor optical amplifiers of different lengths emitting in the 1.55 μm region are investigated experimentally and compared. It is shown that slow gain recovery due to total carrier relaxation is totally suppressed in the ultralong amplifier. Consequently, the 10%-90% gain recovery time is drastically reduced from about 40 ps (short sample) to 10 ps (long one).