Genxiang Chen

Design strategies for mitigating the influence of polarization effects on GaN-based multiple quantum well light-emitting diodes

The optoelectronic properties of GaN-based multiple quantum well (MQW) light-emitting diodes (LEDs) are investigated using a detailed theoretical model, in which the effects of strain, well coupling, valence band mixing, and polarization effects are fully considered. By solving the conduction and valence band effective mass equations together with Poisson’s equation self-consistently, the influence of various major design parameters, such as the well width, the barrier components, and the barrier thickness, on the electronic and optical properties of GaN-based MQW LEDs is studied. Numerical results show that the emission spectra of the LEDs are very sensitive to the above design parameters due to the polarization effect that is unique for GaN-based devices. Further analysis and simulations reveal that this sensitivity can be obviously suppressed by choosing InGaN as the barrier material.

Time and spectral domain properties of distributed-feedback-type gain-clamped semiconductor optical amplifiers

A comprehensive model for the analysis of distributed-feedback (DFB)-type semiconductor optical amplifiers (SOAs) both in time- and frequency-domain is developed by combination of a wideband field-based traveling-wave rate equation model and an extended transfer matrix formalism for corrugated structures derived in spectral domain. The numerical implementation of the model is successfully fulfilled by dividing the device into many short segments along the cavity length. Based on this unique model, some time- and spectral-domain properties, such as amplified spontaneous emission, laser action, and signal amplification of DFB-type gain-clamped SOAs are simulated and analyzed.

Photon barrier and photon tunneling in optical waveguide structure

Based on the well-known electron coherent tunneling phenomenon, a photonic tunneling Alter fabricated on an optical waveguide is proposed. The Bragg grating structure is applied as the photonic barrier. Two identical photonic barriers form a coherent resonance cavity or photon confinement quantum well so that at a specific wavelength in the forbidden band of the barrier, the photon can be tunneling through the waveguide with 100% transmission and very narrow bandwidth. Additionally, to facilitate the engineering design of this kind of photonic tunneling filter, a phase-shifted asymmetric-barrier structure is implemented. It is demonstrated that in this asymmetric-barrier structure, the tunneling wavelength is exactly the same as the Bragg wavelength of the grating, which is at the center of the barrier stop band. Some key parameters such as cavity/well and barrier lengths are investigated in order to control the filter bandwidth. Therefore the photon filter proposed should be easily designed for a specific wavelength and bandwidth and will have great applications in wavelength division multiplexing optic communication system and optical sensing. Finally it is pointed out that this tunneling filter is clearly related to the distributed-Bragg-grating-reflection laser diode.

Active Mode Locking: Quantum Oscillator vs. Classical Coupled Oscillators

Mode expansion approach is proposed to simulate the active mode locked laser. In this method, electric fields of optical modes in the laser cavity are treated as free classical oscillators. The optical modulator provides the coupling among the modes or oscillators. It is found that the eigenvalue of this coupled system is corresponding to the threshold optical gain and normal mode or eigenfunction is corresponding to optical field of each mode. The simulation results obtained by mode expansion method agree with those calculated by the well- known Master equation, which provides an analytical solution with a function similar to the quantum harmonic oscillator. However, since the proposed method focuses on the individual mode and its coupling with the other modes instead of mode profile in general, it gives more information than Master equation. To show the capability of the new method, several applications are explored where Master equation fails to solve the problems. It is believed that the proposed method helps design the devices such as optical pulse generator, multi-wavelength laser and therefore have great application in optical signal generation and processing

Classical coupled oscillators model of the rational harmonic mode locked laser

A coupled modes method is proposed to simulate the active mode locked laser. In this method, the electric fields of optical modes in the laser cavity are treated as free classical oscillators. The optical modulator provides the coupling among them. It is found that the eigenvalue of this coupled system is corresponding to the threshold optical gain, and the eigenfunction is corresponding to the optical field of each mode. The simulation results agree with the Master equation and experiments. Especially the model can be applied to the rational harmonic mode locking case by introducing the concept of ghost modes. The multitrip photons form a set of ghost modes or very weak oscillators. These ghost oscillators play the crucial rule working as the bridge to transfer energy and couple the real cavity modes/oscillators together. The model demonstrates both time domain pulse repetition rate multiplication and the frequency domain mode distribution. The proposed method will help understand the physics of rational...

Fabrication of high quality UV-written fiber bragg gratings

ConclusionIn conclusion, we have reported that 98.5 % peak power reflectivity fiber Bragg gratings were fabricated in the core of B/Ge co-doped optical fiber using 248 nm KrF excimer laser pulses to irradiate a phase mask. The fiber gratings fabricated by the method described above have negligible sideband and good wavelength selectivity. They can find applications in WDM optical fiber transmission systems, fiber grating external cavity semiconductor lasers, and rearearth-doped fiber lasers. But more efforts are needed to further improve the transmission spectra of the gratings, because the ripple in the short wavelength side is harmful to their use in WDM systems.

Fabrication of high-reflectivity fiber Bragg gratings by excimer laser irradiated phase masks

This paper reports the experimental results that near 100% peak reflectivity fiber Bragg grating have been fabricated by 248 nm KrF excimer laser irradiated a B/Ge co-doped fiber through a zero-order mulled phase mask and that the growth property of fiber gratings has been monitored using white light source and optical spectrum analyzer during the gratings formation. And we find that the evolution of the reflectivity of fiber gratings with exposure fluence can be perfectly fitted by power-law function of the form R equals cF(beta ).

Analysis of dispersion compensation using linearly chirped fiber gratings with uniform coupling profile

The use of linearly chirped fiber Bragg gratings with uniform coupling profile for compensating the dispersion of 100 km standard optical fiber link at 1550 nm are investigated theoretically for practical grating fabrication consideration. Numerical results show that a 40 mm long linearly chirped fiber Bragg grating with uniform coupling profile and optimal coupling strength can almost perfectly compensate the dispersion of 100 km standard communication fiber link up to 20 GHz. The dispersion delay curve of the fiber grating exhibits certain oscillation feature, and this does not significantly affect its dispersion compensation ability according to our calculations.

Analysis of near-field holographic pattern formation using phase masks

The near-field holographic interference patterns of phase masks irradiated by excimer lasers is analyzed in details theoretically in this paper. The interference intensity patterns produced by periodic phase masks are calculated using scalar diffraction modeling with various assumptions of mask parameters.

Fabrication of fused silica phase masks by reactive ion etching

A novel method for producing durable fused silica self- interference phase grating photomasks is described in this paper. The gratin pattern is formed into AZ-1350 photoresist by 325nm He-Cd laser optical holographic exposure and is transferred to a thin metal layer via focused ion beam lithography, then reactive ion etching in CHF3/O2 plasma is used to etch the fused silica substrate. Inspection performed by scanning electron microscopy shows that fabricated silica phase masks are perfect and no reactive deposition is formed int he grating grooves.