Novak S. Petrovic

Analysis of optical channel cross talk for free-space optical interconnects in the presence of higher-order transverse modes

We analyze the effect of cross-talk noise on the performance of free-space optical interconnects (FSOIs). In addition to diffraction-caused cross talk, we consider the effect of stray-light cross-talk noise, an issue that, to the best of our knowledge, was not addressed previously. Simulations were performed on a microlens-based FSOI system using the modal composition and beam profiles experimentally extracted from a commercial vertical-cavity surface-emitting laser. We demonstrate that this cross-talk noise introduces significant degradation to interconnect performance, particularly for multitransverse-mode laser sources. A simple behavioral model is also developed that accurately approximates the cross talk noise for a range of optical sources and interconnect configurations.

Modeling diffraction in free-space optical interconnects by the mode expansion method

Free-space optical interconnects (FSOIs), made up of dense arrays of vertical-cavity surface-emitting lasers, photodetectors and microlenses can be used for implementing high-speed and high-density communication links, and hence replace the inferior electrical interconnects. A major concern in the design of FSOIs is minimization of the optical channel cross talk arising from laser beam diffraction. In this article we introduce modifications to the mode expansion method of Tanaka et al. [IEEE Trans. Microwave Theory Tech. MTT-20, 749 (1972)] to make it an efficient tool for modelling and design of FSOIs in the presence of diffraction. We demonstrate that our modified mode expansion method has accuracy similar to the exact solution of the Huygens-Kirchhoff diffraction integral in cases of both weak and strong beam clipping, and that it is much more accurate than the existing approximations. The strength of the method is twofold: first, it is applicable in the region of pronounced diffraction (strong beam clipping) where all other approximations fail and, second, unlike the exact-solution method, it can be efficiently used for modelling diffraction on multiple apertures. These features make the mode expansion method useful for design and optimization of free-space architectures containing multiple optical elements inclusive of optical interconnects and optical clock distribution systems.

Experimental and theoretical investigation of the structural, chemical, electronic, and high frequency dielectric properties of barium cadmium tantalate–based ceramics

Single-phase Ba(Cd1∕3Ta2∕3)O3 powder was produced using conventional solid state reaction methods. Ba(Cd1∕3Ta2∕3)O3 ceramics with 2wt% ZnO as sintering additive sintered at 1550°C exhibited a dielectric constant of ∼32 and loss tangent of 5×10−5 at 2GHz. X-ray diffraction and thermogravimetric measurements were used to characterize the structural and thermodynamic properties of the material. Ab initio electronic structure calculations were used to give insight into the unusual properties of Ba(Cd1∕3Ta2∕3)O3, as well as a similar and more widely used material Ba(Zn1∕3Ta2∕3)O3. While both compounds have a hexagonal Bravais lattice, the P321 space group of Ba(Cd1∕3Ta2∕3)O3 is reduced from P3m1 of Ba(Zn1∕3Ta2∕3)O3 as a result of a distortion of oxygen away from the symmetric position between the Ta and Cd ions. Both of the compounds have a conduction band minimum and valence band maximum composed of mostly weakly itinerant Ta5d and Zn3d∕Cd4d levels, respectively. The covalent nature of the directional d-elec...

Analysis of hexagonal array geometry for free-space optical interconnects with improved signal-to-noise ratio

The effect of transmitter and receiver array configurations on the performance of free-space optical interconnects (FSOIs) was investigated. Experimentally measured, spectrally resolved, near-field images of vertical-cavity surface-emitting laser (VCSEL) transverse modes were used as extended sources in our simulation model and combined with laser relative intensity noise and the receiver noise to determine the optimal array geometry. Our results demonstrate the importance of stray-light cross talk in both square and hexagonal configurations. By changing the array lattice geometry from square to hexagonal, we obtained an overall optical signal-to-noise ratio improvement of 3 dB. We demonstrated that the optical signal-to-noise ratio is optimal for the hexagonal channel arrangement regardless of the transverse mode structure of the VCSEL beam. We also determined the VCSEL drive current required for the best performance of the FSOI system.

Modeling diffraction and imaging of laser beams by the mode-expansion method

We present a new method of modeling imaging of laser beams in the presence of diffraction. Our method is based on the concept of first orthogonally expanding the resultant diffraction field (that would have otherwise been obtained by the laborious application of the Huygens diffraction principle) and then representing it by an effective multimodal laser beam with different beam parameters. We show not only that the process of obtaining the new beam parameters is straightforward but also that it permits a different interpretation of the diffraction-caused focal shift in laser beams. All of the criteria that we have used to determine the minimum number of higher-order modes needed to accurately represent the diffraction field show that the mode-expansion method is numerically efficient. Finally, the characteristics of the mode-expansion method are such that it allows modeling of a vast array of diffraction problems, regardless of the characteristics of the incident laser beam, the diffracting element, or the observation plane.

Novel array geometries for free-space optical interconnects with improved signal-to-noise ratio

We investigate the effect of transmitter and receiver array configurations on the stray-light and diffraction-caused crosstalk in free-space optical interconnects. The optical system simulation software (Code V) is used to simulate both the stray-light and diffraction-caused crosstalk. Experimentally measured, spectrally-resolved, near-field images of VCSEL higher order modes were used as extended sources in our simulation model. Our results show that by changing the square lattice geometry to a hexagonal configuration, we obtain the reduction in the stray-light crosstalk of up to 9 dB and an overall signal-to-noise ratio improvement of 3 dB.

Analysis of free-space optical interconnect misalignment tolerance in the presence of multimode VCSEL beams

Successful deployment of free-space optical interconnects (FSOIs) primarily depends on their tolerance to misalignment. In most misalignment studies done so far, laser beams were assumed to have purely Gaussian intensity distributions. We experimentally show that this is not the case, and that FSOI misalignment tolerance halves if less than 80% of the optical power is in the fundamental Gaussian mode.

Channel density in free-space optical interconnects

Free-space optical interconnects have been identified as the immediate successors to the troubled electrical wiring in most short-distance and high-speed digital applications. We apply the modified mode expansion method to calculate the maximum channel density (the number of channels per unit area) possible in a typical free-space optical interconnect configuration. We proceed to show that the density can be increased by departing from the traditional confocal optical design, by optimal positioning of the laser array relative to the transmitter microlens array. We also show that the interconnect density can be increased by using laser beams of higher frequencies and larger beam waists.

Modelling diffraction effects by the mode expansion method

An improvement in the information transfer rate (the amount of information communicated per unit area) in VCSEL-based free-space optical interconnects can be achieved by reducing the spacing between the microchannels. However, reducing the physical size of optical components results in an increase in diffractive spreading of laser beams and a decrease in the signal-to-noise ratio. The traditional methods used for describing diffraction phenomena tend to be rather cumbersome in most practical situations. However, the mode expansion method of Tanaka et al. (K. Tanaka, M. Shibukawa, O. Fukumitsu, IEEE Trans. Microwave Theory Tech. MTT-20, p749), applied in an improved form derived by the authors, is shown to be accurate and well suited for analysing the FSOI performance. It is also shown to consistently outperform the other approximate methods frequently used in the literature.

Analysis of the effect of transverse modes on free-space optical interconnect performance

Vertical-cavity surface-emitting lasers (VCSELs) and microlenses can be used to implement free space optical interconnects (FSOIs) which do not suffer from the bandwidth limitations inherent in metallic interconnects. A comprehensive link equation describing the effects of both optical and electrical noise is introduced. We have evaluated FSOI performance by examining the following metrics: the space–bandwidth product (SBP), describing the density of channels and aggregate bandwidth that can be achieved, and the carrier-to-noise ratio (CNR), which represents the relative strength of the carrier signal. The mode expansion method (MEM) was used to account for the primary cause of optical noise: laser beam diffraction. While the literature commonly assumes an ideal single-mode laser beam, we consider the experimentally determined multimodal structure of a VCSEL beam in our calculations. It was found that maximum achievable interconnect length and density for a given CNR was significantly reduced when the higher order transverse modes were present in simulations. However, the simulations demonstrate that free-space optical interconnects are still a suitable solution for the communications bottleneck, despite the adverse effects introduced by transverse modes.