80-Gb/s OTDM system analysis of a vertical microcavity-based saturable absorber for the enhancement of pulse pedestal suppression

In future high-speed optical time-division-multiplexed (OTDM) systems, an important factor that needs to be considered for optical pulse generation schemes is the impact of pulse pedestals on the overall system performance. The results presented in this letter are two-fold; first, the impact due to the height of pulse pedestals in an 80-Gb/s OTDM system are established. Second, a solution is provided to overcome these high pedestal levels through the use of a vertical microcavity saturable absorber, which can significantly reduce the pulse pedestal level and give enhanced system performanc

Fabrication and characterisation of microscale air bridges in conductive gallium nitride

Abstract Fabrication and electrical characterisation of microscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN-AlInNGaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ∼17% InN fraction. The start of the fabrication sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ∼70% of the etch rate of GaN. Processing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported between anchor posts at both ends. Deposition of Ti-Au contact pads onto the anchor posts allowed study of the electrical characteristics. At low applied voltages, vertical conduction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear currentvoltage characteristics at low applied voltages, and had resistances of 1050 . The observed resistance values are compared with the predicted value based on materials parameters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2 × 10 5 A cm −2

Nanofabrication of gallium nitride photonic crystal light-emitting diodes

We describe a comparison of nanofabrication technologies for the fabrication of 2D photonic crystal structures on GaN/InGaN blue LEDs. Such devices exhibit enhanced brightness and the possibility of controlling the angular emission profile of emitted light. This paper describes three nano lithography techniques for patterning photonic crystal structures on the emitting faces of LEDs: direct-write electron beam lithography, hard stamp nanoimprint lithography and soft-stamp nanoimprint lithography with disposable embossing masters. In each case we describe variations on the technique as well as its advantages and disadvantages. Complete process details have been given for all three techniques. In addition, we show how high performance GaN dry etch techniques, coupled with optical process monitoring can transfer resist patterns into underlying GaN material with high fidelit

Thermal conductance of laterally-wet-oxidised GaAs/AlxOy Bragg reflectors

Thermal resistivity measurements were carried out on GaAs-based monolithic saturable absorber microcavities. Two-types of microcavity mirrors were compared: GaAs/AlAs against GaAs/AlxOy Bragg reflectors processed by lateral-wet-oxidation of Al(Ga)As layers. It is found that GaAs/AlxOy mirrors are not efficient heat dissipators, the GaAs/AlxOy microcavity structure showing a thermal resistivity more than ten times higher than the GaAs/AlAs structure. Using modelling to fit the experimental data, thermal conductivity of the 250 nm AlxOy layers is estimated to be approximately 0.007 WK–1cm–1. These results illustrate a significant drawback related to the use of thick wet-oxidised Al(Ga)As/GaAs layer

Cascadability assessment of a 2R regenerator based on a saturable absorber and a semiconductor optical amplifier in a path switchable recirculating loop

We assess a new 2R regenerator based on a microcavity saturable absorber and a semiconductor optical amplifier. Cascadability is demonstrated and the impact of regeneration span is studied in a 10-Gb/s two-path recirculating loop. A wavelength study demonstrates the tunability of the device over 13 nm

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Analysis of thermal limitations in high-speed microcavity saturable absorber all-optical switching gates

The limitations owing to device heating and thermo-optic effects in high-speed quantum-well microcavity saturable absorber devices are investigated both theoretically and experimentally. A simplified theoretical description of the device electronic, thermal, and optical properties is developed and applied to the modeling of the device switching characteristics for reamplification + reshaping step (2R) all-optical regeneration. These predictions are compared to nonlinear optical measurements performed with switching pulses of fixed duration and variable repetition rate on two devices with significantly different thermal properties. It is shown that proper optimization of the device thermal properties is crucial to avoid the degradation of device performance at high bit rate. It is also shown that the negative effects of optically induced heating on the switching contrast may be compensated to some extent by operating the device on the long wavelength side of the microcavity resonanc

Individually-addressed planar nanoscale InGaN-based light emitters

We report on a new fabrication approach to create individually-addressable InGaN-based nanoscale-LEDs. It is based on the creation by LEEBI of a spatially confined sub-micron-size charge injection path within the p-GaN of an LED structure

All-optical sampling and spectrographic pulse measurement using cross-absorption modulation in multiple-quantum-well-devices

The application of cross absorption modulation for optical sampling and phase sensitive pulse measurement is presented. Both a commercial 40 GHz electroabsorption modulator and a vertical microcavity saturable absorber are used to sample a 2 ps pulse train at 40 GHz. The modulator was then used to make cross-absorption modulation based frequency-resolved optical gating (FROG) measurements. The results were verified by comparison with a commercial optical sampling oscilloscope and the more standard second harmonic generation FROG technique