A fiber ring laser which implements hybrid mode locking technique has been proposed and experimentally demonstrated to generate pulse train at 20 GHz repetition rate with ultrashort pulse width. Graphene and charcoal nano-particles acting as passive mode lockers are inserted into a rational harmonic mode-locked fiber laser to improve the performance. With graphene saturable absorbers, the pulse duration is shortened from 5.3 ps to 2.8 ps, and with charcoal nano-particles, it is shortened to 3.2 ps. The RF spectra show that supermode noise can be removed in the presence of the saturable absorbers. Numerical simulation of the pulse transmission has also been carried out, which shows good agreement with the experimental results.
We numerically study the broadband mid-infrared supercontinuum generation in a non-uniform SF57 microstructured fiber. The dispersion of the fiber is tailored by linearly varying the air hole diameters along the propagation distance. The fiber has zero dispersion at two wavelengths. This allows a continuous shift of the higher zero dispersion wavelength to a longer wavelength. Results show this scheme can significantly broaden the generated spectrum.
An axially non-uniform tapered As<sub>2</sub>S<sub>3</sub> planar waveguide has been designed for mid-IR supercontinuum generation. The dispersion profile is varying along the propagation distance. Numerical results show this scheme significantly broadens the generated continuum, extending from ~1 μm to ~7 μm.
We have studied high speed optical logic utilizing ultrafast two-photon absorption (TPA) induced phase change in semiconductor optical amplifiers (SOA). Results show that this scheme can realize all-optical logic and encryption at data speeds to 250 Gb/s.
A scheme to generate return-to-zero on-off keying (RZ-OOK) high speed all-optical pseudo random bit sequence (PRBS)
using binary phase shift keyed (BPSK) signal based on quantum-dot semiconductor optical amplifiers (QD-SOA) has
been designed and studied. The PRBS is generated by a linear feedback shift register (LFSR) composed of all-optical
logic XOR and AND gates. The XOR gate is composed of a pair of QD SOA Mach-Zehnder interferometers, which can
generate BSPK signal to realize all-optical logic XOR gate. Results show that this scheme can mitigate the patterning
effects and increase the operation speed to ~250Gb/s.
We have designed a tapered rib waveguide and numerically studied the generation of supercontinuum using such
waveguides. The Air-SF57 glass-SiO<sub>2</sub> waveguide is 2 cm long, with a varying etched depth to manage the total
dispersion. Numerical simulations are conducted for input pulses at a wavelength of 1550 nm. The proposed waveguide
geometry greatly broadens the output spectrum extending from ~1000 nm to ~ 4600 nm at -30 dB level, caused by
continuous modification of the phase matching condition for dispersive wave emission. The coherence property has also
been investigated, demonstrating that fully coherent supercontinuum can be obtained with proper pumping conditions.
Optical latches are important for a wide range of applications including communication systems, optical logic systems, optical random access memory (RAM) and encryption. All optical logic operations using quantum dot (QD) based semiconductor optical amplifier (SOA) and Mach-Zehnder interferometer (MZI) have been studied. The building block of an optical latch such as NAND gate has been fabricated and their operation experimentally demonstrated at ~ 80 GHz. A rate equation model has been developed for the QD-SOA-MZI and it has been used to analyze the Boolean logic operation. The model has been used to analyze the Set-Reset (S-R) latch and the D-Flip-Flop (DFF) devices. The DFF is the basic device for building larger logic circuits. The results show that the latches would work to speeds of ~ 250 Gb/s.
We have studied the broadband mid-IR supercontinuum generation in a lead-silicate tapered microstructured fiber for
femto-second input pulses at 1550 nm. The supercontinuum generated extends from ~1000 to ~ 5000 nm for a suitably
designed tapered fiber. The coherence properties of the supercontinuum depends on the input pulse parameters. It is
possible to generate perfectly coherent supercontinuum with a flat broadened spectrum extending to ~5000 nm in this fiber taper.
In this paper we numerically study the coherence properties of the supercontinuum generated in a lead-silicate
microstructured fiber taper, with an increasing core radius along the propagation distance which tailors the
dispersion property. Simulations are conducted by adding quantum noise into the input pulse at 1.55 μm, and
the complex degree of first-order coherence function and the overall spectral coherence degree are both
calculated. Although the spectral broadening is comparable, the coherence degree is shown to vary with
different pumping conditions. It decreases with higher peak power and longer duration due to the significant
competition between the soliton-fission process and the noise-seeded modulation instability. By controlling the
input pulse parameters, it is possible to generate perfectly coherent supercontinuum with a flat broadened
spectrum extending to ~5μm in this fiber taper.
A fiber ring laser which generates ~ 570 ps wide pulse train at 40 GHz has been demonstrated using a photonic crystal
fiber as a nonlinear optical loop mirror (NOLM). Theoretical simulation of the NOLM transmission has been carried out
using the split-step Fourier method.
Semiconductor optical amplifiers are important for wide range of applications in optical networks, optical tomography
and optical logic systems. For many of these applications particularly for optical networks and optical logic, high speed
performance of the SOA is important. All optical Boolean operations such as XOR, OR, AND and NOR has been
demonstrated using SOA based Mach-Zhender interferometers (SOA-MZI). A rate equation model for SOA-MZI has
been developed. The model has been used to analyze the Set-Reset (S-R) latch, the gated S-R latch and the D-Flip-Flop
devices. The modeling results suggest that the Flip-Flop circuits should work at high speeds. An optical pseudo-random
bit stream (PRBS) generator is important for all-optical encryption circuits. A model of a PRBS generator using SOAMZI
based devices has been developed. We show that a PRBS generator can work @ 80 Gb/s using regular SOAs and
@ ~ 250 Gb/s or at higher speeds using two-photon absorption based processes in SOAs.