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.
| Identifer | oai:union.ndltd.org:ADTP/253981 |
| Creators | Mr Feng-chuan Tsai |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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