Optical code division multiple access systems in AlGaInAs/InP

The rise of photonic integration makes optical code division multiple access (OCDMA) worth revisiting due to its promising role in future all-optical networks. OCDMA has the potential to exploit the surplus bandwidth of optical fibres and to carry over to the optical domain the benefits seen CDMA radio communication systems, such as the effective sharing of the spectrum for multiple network subscribers, and resistance to jamming and eavesdropping. One of the major requirements for the deployment of OCDMA in networks is integration. This thesis presents a research study of integrated OCDMA systems using the AlGaInAs/InP semiconductor material system. This material is considered due to its useful intrinsic properties such as thermal stability, strong electron confinement, and low threshold, making it suitable for fabricating optoelectronic devices. Two bespoke OCDMA systems are considered for integration: coherent temporal phase coding (TPC), and incoherent wavelength-hopping time-spreading (WHTS) OCDMA systems. TPC systems are excellent for high speed communications due to their static en/decoding enabling features. In this research, a 2×2 asymmetric Mach Zehnder interferometer (AMZI) is used to generate a 2-bit phase code, allowing multiplexing for up to four users. A semiconductor mode-locked ring laser is also embedded in the circuit, and using a synchronous mode-locking method, adequate signal en/decoding is achieved. WHTS systems on the other hand fully exploit the spectral and temporal space available in networks by assigning each user with a unique wavelength-time hop sequence for en/decoding data signals. Here, a mode-locked laser array is used with intracavity distributed Bragg reflectors (DBRs) for spectrally tuning each laser, and a 4:1 multimode interference coupler is used to combine the laser signals into a single channel for amplification, modulation and transmission. The integrated system is fully characterised and synchronisation experiments are performed to show the potential for its use in high speed multi-user networks. Mode-locked lasers play an important role in many OCDMA implementations due to their wide spectrum and discrete temporal properties, which can be easily exploited during data en/decoding. Various mode-locked laser devices have been studied during this research with additional embedded components such as intracavity DBRs and phase controllers for precise tuning of the wavelength and pulse repetition frequency. However, the noisy nature of passively operating mode-locked lasers make them prone to high jitter, which can result in high bit error rates. Synchronisation schemes are thereby explored in order to temporally stabilise the pulse oscillations to make them suitable for use in long haul transmission systems. This includes synchronous and hybrid mode-locking, as well as a passive technique using an optical fibre loop to provide phase feedback, which is shown to promote ultralow RF linewidths in mode-locked lasers

Laterally-Coupled Dual-Grating Distributed Feedback Lasers for Generating Mode-Beat Terahertz Signals

We present a laterally-coupled AlGaInAs/InP DFB laser emitting two longitudinal modes simultaneously within the same cavity and integrated with EAM. A stable 0.82 THz beating signal was observed over a wide range of bias parameters

Mode-locking and frequency mixing at THz pulse repetition rates in a sampled-grating DBR mode-locked laser

We report a sampled grating distributed Bragg reflector (SGDBR) laser with two different gratings which mode-lock independently at respective pulse repetition frequencies of 640 and 700 GHz. The device operates in distinct regimes depending on the bias conditions, with stable pulse trains observed at 640 GHz, 700 GHz, the mean repetition frequency of 666 GHz, and the sum frequency of 1.34 THz (due to nonlinear mixing). Performance is consistent and highly reproducible with exceptional stability observed over wide ranges of drive bias conditions. Furthermore, a monolithically integrated semiconductor optical amplifier is used to amplify the pulse trains, providing an average output power of 46 mW at 666 GHz

Generation of high speed polarization modulated data using a monolithically integrated device

We report on the generation of high speed polarization modulated data via direct electrical binary data injection to the phase shifter section of a monolithically integrated laser diode integrated with a polarization controller. The device is fabricated on standard InP/AlGaInAs multiple quantum-well material and consists of a semiconductor laser, a passive polarization mode convertor and an active differential phase-shifter section. We demonstrate the generation of 300 Mbit/s Polarization Shift Keyed data

Using Altman and Sherrod Z- Score Models to Detect Financial Failure for the Banks Listed on the Iraqi Stock Exchange (ISE) Between 2009 – 2013

Purpose: The Purpose of the study was to examine the validity of the Altman Z- Score and Sherrod Z- Score models in financial failure prediction. To achieve the study's goal, references from various authors who have reviewed this topic were used.   Theoretical framework: The study highlights the importance of analyzing and delving into the various notions of financial failure and distress. When it comes to potential effects on the wealth of creditors, stockholders, and society as a whole, academics and researchers consider a company's distress and bankruptcy to be the most important issue to be studied. In order to maintain the goal of company survival and continuity before the disaster happens, many academics started looking for a method to identify and forecast distress and failure.   Design/methodology/approach: Altman Z-score and Sherrod Z- score employed a multi-discriminant model to predict the financial position of ten ISE banks between 2009 - 2013. Z- Score models from Altman and Sherrod were used to determine whether the banks listed on the ISE are exposed to failing financially. Ten banks out of the forty - six banks listed on the ISE were selected. The study only used secondary data obtained from the chosen banks' financial statements in ISE.   Findings: Based on Altman's Z- score model, the study examines that certain banks are particularly exposed to failure. In contrast, the Sherrod Z- Score model indicates that the chosen banks have some issues, but they are minor, and the risk of bankruptcy is low.   Research, scientific and social implications: By using a failure prediction model, it is possible to determine the likelihood that banks will experience financial failure in the future. Investors could use this information to guide their decision-making going forward.   Originality/value: The value and importance of research related to the study of financial failure prediction models in Iraqi commercial banks. The research also seeks to explain financial failure models and the extent to which investors benefit from these models

Cross-sectional Nanoscale Resolution Mapping of Potential and Current Distribution in 3D Structure of Vertical Cavity Surface Emitting Laser iii-v Nanostructures

Vertical cavity surface emitting lasers (VCSELs) hold a major promise in the telecommunication and data interfacing due to their efficient manufacturing pathways and prospects of seamless integration with microelectronics components. VCSEL structures include multilayer Distributed Bragg Reflection (DBR) surrounding an active cavity that typically have multiple quantum wells (QWs) and in some devices quantum dots (QDs) layers [2]. The properties, morphology and quality of multiple buried layers and interfaces are crucial for the development of novel devices, improving device performance and optimization of production processes. Unfortunately, accessing these layers to explore these generally three-dimensional (3D) structures is often a laborious (e.g. via cross-sectional transmission or scanning electron microscopies, EMs) task. Significantly, the sample preparation can also change properties of the material and the device studied (e.g. Ga ion implantation during FIB milling) and usually allows to see only a very limited part of the wafer. Furthermore, the EM does not allow to access local physical properties of the device – such as local electric potential, current density and heat generation, all being extremely crucial to the device performance. Here we report for the first time the direct observation of local electric potential and conductance in the bulk of VCSEL stack by using combination of the Ar-ion beam exit cross-section polishing (BEXP) that creates an oblique section with sub-nm surface roughness through the VCSEL structure [2] combined with the material sensitive scanning probe microscopy (SPM). We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM), surface potential mapping via Kelvin Probe Force Microscopy (KPFM) [3] and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). These allowed to observe the resulting geometry of the device, including active cavity MQW, and to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. In conclusion, this approach opens unique novel possibility to directly explore the physical phenomena of operation of VCSELs and other iii-v devices, helping to advance the manufacturing of these these devices, as well as opening insight into the fundamental electronic and atomistic phenomena in these complex nanostructured materials [4]

Recent progress in distributed feedback InGaN/GaN laser diodes

Laser diodes based on Gallium Nitride (GaN) are useful devices in a wide range of applications including atomic spectroscopy, data storage and optical communications. To fully exploit some of these application areas there is a need for a GaN laser diode with high spectral purity, e.g. in atomic clocks, where a narrow linewidth blue laser source can be used to target the atomic cooling transition. We report on the continuous wave, room temperature operation of a distributed feedback laser diode (DFB-LD) with high-order notched gratings. The design, fabrication and characterization of DFB devices based on the (Al,In) GaN material system is described. A single peak emission at 408.6 nm with an optical power of 20 mW at 225 mA and a side mode suppression ratio (SMSR) of 35 dB was achieved. Additionally, we demonstrate the use of a GaN DFB-LD as a transmitter in visible optical communications system. We also present results from a DFB-LD optimized for laser cooling of Sr+

3D mapping of nanoscale physical properties of VCSEL devices

There is clear lack of methods that allows studies of the nanoscale structure of the VCSEL devices1 that are mainly focused on the roughness of the DBR, or using FIB cross-sectioning and TEM analysis of failed devices to observe the mechanism of the degradation. Here we present a recently developed advanced approach that combines Ar-ion nano-cross-sectioning with material sensitive SPM2 to reveal the internal structure of the VCSEL across the whole stack of top and bottom DBR including active area. We report for the first time the direct observation of local mechanical properties, electric potential and conductance through the 3D VCSEL stack. In order to achieve this, we use beam exit cross-section polishing that creates an oblique section with sub-nm surface roughness through the whole VCSEL structure that is fully suitable for the subsequent cross-sectional SPM (xSPM) studies. We used three different SPM measurement modes – nanomechanical local elastic moduli mapping via Ultrasonic Force Microscopy (UFM) 3, surface potential mapping via Kelvin Probe Force Microscopy (KPFM) and mapping of injected current (local conductivity) via Scanning Spreading Resistance Microscopy (SSRM). xSPM allowed to observe the resulting geometry of the whole device, including active cavity multiple quantum wells (MQW), to obtain profiles of differential doping of the DBR stack, profile of electric potential in the active cavity, and spatial variation of current injection in the individual QW in MQW area. Moreover, by applying forward bias to the VCSEL to initiate laser emission, we were able to observe distribution of the potential in the working regime, paving the way to understanding the 3D current flow in the complete device. Finally, we use finite element modelling (FEM) that confirm the experimental results that of the measurements of the local doping profiles and charge distribution in the active area of the VCSEL around the oxide current confinement aperture. While we show that the new xSPM methodology allowed advanced in-situ studies of VCSELs, it establishes a highly efficient characterisation platform for much broader area of compound semiconductor materials and devices. REFERENCES. 1. D. T. Mathes, R. Hull, K. Choquette, K. Geib, A. Allerman, J. Guenter, B. Hawkins and B. Hawthorne, in Vertical-Cavity Surface-Emitting Lasers Vii, edited by C. Lei and S. P. Kilcoyne (2003), Vol. 4994, pp. 67-82. 2. A. J. Robson, I. Grishin, R. J. Young, A. M. Sanchez, O. V. Kolosov and M. Hayne, Acs Applied Materials & Interfaces 5 (8), 3241-3245 (2013). 3. J. L. Bosse, P. D. Tovee, B. D. Huey and O. V. Kolosov, Journal of Applied Physics 115 (14), 144304 (2014)

Applications of Single Frequency Blue Lasers

Gallium nitride (GaN) sources are becoming a regular part of today's world and are now key devices for lighting infrastructures, communications systems and quantum applications, amongst others. In particular, many applications have seen the shift from LEDs to laser diodes to make use of higher powers, higher bandwidths and increased transmission distances. Laser communication systems are well established, however there are applications where the ability to select a single emitted wavelength is highly desirable, such as quantum atomic clocks or in filtered communication systems. Distributed feedback (DFB) lasers have been realised emitting at a single wavelength where the grating structure is etched into the sidewall of the ridge. The main motivation in developing these lasers is for the cooling of ions in atomic clocks; however their feasibility for optical communications is also explored. Narrow linewidth lasers are desirable and this paper will explore how this is achieved. Data rates in excess of 1 Gbit/s have also been achieved in a directly modulated, unfiltered system. These devices lend themselves towards wavelength division multiplexing and filtered optical communications systems and this will be analysed further in the work presented here