Because of the need to move large amounts of data effienciently, optical based communications are a critical component of modern telecommunications. And as a key enabler of optical communications, electrical components play a critical role in optical data links. Optoelectronic integrated circuits provide the bridge between the optical and electrical realms. Electronic integrated circuits are also integral parts of the optical link, interfacing with post processing circuitry and compensating for any limitations along the link. In this investigation, three circuits for optical data link applications are studied. Two optoelectronic integrated circuit front-ends for freespace and long haul applications, respectively and an active filter for near end cross talk cancellation associated with high data rate transmission. The first circuit is an 8x8 monolithic receiver array for a Spatial Division Multiplexing optical link. A compact and low power 8x8 array was designed and demonstrated a channel that received data at rates of 1Gb/s. It is the first completely monolithic demonstration of a 2D receiver array within a conventional ion implanted GaAs MESFET process. The second circuit demonstrated a long wavelength (1.55 m) optoelectronic receiver for long haul applications. The circuit utilized a TWA topology, which maximizes the available bandwidth from the GaAs MESFET process. It incorporated a thin-film inverted MSM photodetector to achieve nearly monolithic integration. The final circuit is a tunable high pass active filter in 0.18 m CMOS technology. As part of a NEXT noise canceller architecture, it will provide the means to extend data transmission in FR-4 legacy backplanes into the tens of Gb/s datarate.

Thesis (M.S.)--Naval Postgraduate School, 2004. / Title from title screen (viewed Aug. 13, 2004). "March 2004." Includes bibliographical references (p. 71-73). Also issued in paper format.

Performance requirements of short-distance digital communication links have increased considerably with the escalating demand for high speed and high density data links. The high aggregate bandwidth and channel density achievable by free-space optical interconnects (FSOIs) make them ideal replacement for electrical interconnection schemes. Optical interconnects potentially have low power consumption, and can facilitate the development of radically novel designs for VLSI architectures including heterogeneous multiprocessor systems, and highly parallel computing systems. Recent developments in the integration of Vertical-Cavity Surface-Emitting Laser (VCSEL) arrays and photodetector arrays with CMOS electronic circuitry have increased the practical potential of optical interconnects. However, VCSELs tend to operate in several transverse modes simultaneously, which will degrade the performance of FSOIs. Experimental investigation was performed to evaluate the operation characteristics and the intensity noise in VCSELs. The measurement results were later combined with optical simulations to analyse the effect of optical crosstalk in free-space optical interconnects. The VCSEL characterization included light-current-voltage relationships, relative intensity noise, modal spectral composition and modal beam profiles. The optical system simulation software (Code V) was used to simulate optical crosstalks in the FSOI system. Experimentally measured spectrally-resolved near-field images of VCSEL higher order modes were used as extended sources in the proposed simulation model. The simulation was performed using a combination of exact ray-tracing and the beam propagation method. A new type of crosstalk referred to as the stray-light crosstalk (SLC) was introduced. This type of crosstalk is caused by the overfill of the transmitter microlens by the VCSEL beam. It was discovered that part of the signal was imaged by the adjacent microlens to another channel, possibly far from the intended one. The simulation showed that the SLC is strongly dependent on the fill factor of the microlens, array pitch, and the channel density of the system. When comparing the diffraction-caused crosstalk (DCC) to SLC, an increase in the interconnection distance has little influence on the SLC. A simple behavioural model was developed which accurately approximates the crosstalk noise for a range of optical sources and interconnect configurations. The effect of transmitter and receiver array configurations on the performance of FSOIs was investigated. Our results demonstrate the importance of SLC in both square and hexagonal configuration. By changing the array lattice geometry from square to a hexagonal, we obtained an overall optical signal-to-noise ratio (SNR) improvement of 3 dB. The optical SNR is optimal for the hexagonal channel arrangement regardless of the transverse mode structure of the VCSEL beam was shown. Furthermore, the VCSEL drive current required for the best performance of the FSOI system was determined. The optimal focal length of the transmitter microlens array which maximises the SNR by minimising the combined effects of DCC and SLC was determined. Our results show that shorter focal length needs to be used for higher order modes to obtain optimal SNR in an FSOI system.

Performance requirements of short-distance digital communication links have increased considerably with the escalating demand for high speed and high density data links. The high aggregate bandwidth and channel density achievable by free-space optical interconnects (FSOIs) make them ideal replacement for electrical interconnection schemes. Optical interconnects potentially have low power consumption, and can facilitate the development of radically novel designs for VLSI architectures including heterogeneous multiprocessor systems, and highly parallel computing systems. Recent developments in the integration of Vertical-Cavity Surface-Emitting Laser (VCSEL) arrays and photodetector arrays with CMOS electronic circuitry have increased the practical potential of optical interconnects. However, VCSELs tend to operate in several transverse modes simultaneously, which will degrade the performance of FSOIs. Experimental investigation was performed to evaluate the operation characteristics and the intensity noise in VCSELs. The measurement results were later combined with optical simulations to analyse the effect of optical crosstalk in free-space optical interconnects. The VCSEL characterization included light-current-voltage relationships, relative intensity noise, modal spectral composition and modal beam profiles. The optical system simulation software (Code V) was used to simulate optical crosstalks in the FSOI system. Experimentally measured spectrally-resolved near-field images of VCSEL higher order modes were used as extended sources in the proposed simulation model. The simulation was performed using a combination of exact ray-tracing and the beam propagation method. A new type of crosstalk referred to as the stray-light crosstalk (SLC) was introduced. This type of crosstalk is caused by the overfill of the transmitter microlens by the VCSEL beam. It was discovered that part of the signal was imaged by the adjacent microlens to another channel, possibly far from the intended one. The simulation showed that the SLC is strongly dependent on the fill factor of the microlens, array pitch, and the channel density of the system. When comparing the diffraction-caused crosstalk (DCC) to SLC, an increase in the interconnection distance has little influence on the SLC. A simple behavioural model was developed which accurately approximates the crosstalk noise for a range of optical sources and interconnect configurations. The effect of transmitter and receiver array configurations on the performance of FSOIs was investigated. Our results demonstrate the importance of SLC in both square and hexagonal configuration. By changing the array lattice geometry from square to a hexagonal, we obtained an overall optical signal-to-noise ratio (SNR) improvement of 3 dB. The optical SNR is optimal for the hexagonal channel arrangement regardless of the transverse mode structure of the VCSEL beam was shown. Furthermore, the VCSEL drive current required for the best performance of the FSOI system was determined. The optimal focal length of the transmitter microlens array which maximises the SNR by minimising the combined effects of DCC and SLC was determined. Our results show that shorter focal length needs to be used for higher order modes to obtain optimal SNR in an FSOI system.

The aim of this project was to produce an integrate-able free space optical transceiver for Specks. Specks are tiny computing units that together can form a powerful network called a SpeckNet. The SpeckNet platform is developed by the SpeckNet consortium, which consists of five Scottish Universities and combines computer science, electrical engineering and digital signal processing groups. The principal goal of creating an optical transceiver was achieved by integrating in-house fabricated VCSELs (with lasing thresholds below 400 uA) and custom designed detectors on the SpeckNet platform. The transceiver has a very low power consumption (approximately 100 uW), which removes the need for synchronous communication through the SpeckNet thus making the network more efficient. I describe both static and dynamic beam control techniques. For static control, I used micro-lenses. I fabricated the lenses by greyscale electron beam lithography and integrated them directly on VCSEL arrays. I achieved a steering angle of 10 degrees with this design. I also looked at integrated gratings etched straight into a VCSEL and observed beam steering with an efficiency of 60% For dynamic control, I implemented a liquid crystal (LC) design. I built a LC cell with 30 individually controlled pixels, but I only achieved a steering angle of 1 degree. Furthermore, I investigated two different techniques for achieving beam steering by interference, using coupled VCSELs (a phased array approach). Firstly, using photonic crystals etched into the surface of the VCSEL, I built coupled laser cavities. Secondly, I designed and built bow-tie type VCSELs that were optically coupled but electrically isolated. These designs work by differential current injection causing an interference effect in the VCSELs far field. This technique is the first stepping stone towards realising a phased optical array. Finally, I considered signal detection. Using the same VCSEL material, I built a resonant-cavity detector. This detector had a better background rejection ratio than commercially available silicon devices.