Christopher J. O'Brien

Effect of multiple transverse modes in self-mixing sensors based on vertical-cavity surface-emitting lasers

We investigate the effect of coexisting transverse modes on the operation of self-mixing sensors based on vertical-cavity surface-emitting lasers (VCSELs). The effect of multiple transverse modes on the measurement of displacement and distance were examined by simulation and in laboratory experiment. The simulation model shows that the periodic change in the shape and magnitude of the self-mixing signal with modulation current can be properly explained by the different frequency-modulation coefficients of the respective transverse modes in VCSELs. The simulation results are in excellent agreement with measurements performed on single-mode and multimode VCSELs and on self-mixing sensors based on these VCSELs.

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.

A Critical Comparison of High-Speed VCSEL Characterization Techniques

This paper critically compares, for the first time, common microwave and optical procedures used for the high-speed characterization of vertical-cavity surface-emitting lasers (VCSELs). The intrinsic small-signal modulation characteristics of a VCSEL are measured, and the related rate equation parameters are extracted. Observed trends show excellent agreement with theory. The modulation characteristics of the VCSEL are determined by examining three different responses: relative intensity noise, S21 response, and high-resolution optical spectra. The various experimental techniques yielded consistent results. The relative strengths and weaknesses of each measurement are investigated

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.

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.

The Effect of Multiple Transverse Modes in Self-Mixing Sensors Based on Vertical-Cavity Surface-Emitting Lasers

In this work we investigate the effect of multiple transverse modes, such as those found in vertical-cavity surface-emitting lasers, in self-mixing sensors. We show that the sensitivity of the system and the accuracy of the measurement changes periodically with target distance

Comparison of the RF characteristics of inversion channel and depletion channel SOS MOSFETs

Measurement of small-signal equivalent circuit parameters was carried out to estimate the RF performance of conventional inversion channel and depletion channel SOS MOSFETs. It was shown that the latter has significantly higher cutoff frequency fT attributed to electron mobility of the depletion channel.

A comparison of small signal modulation parameter extraction techniques for vertical-cavity, surface-emitting lasers

The small signal modulation characteristics of a vertical-cavity, surface-emitting laser (VCSEL) are determined using three different measurements: relative intensity noise, frequency response, and high resolution optical spectra. The resonant and damping frequencies were measured, and related rate equation parameters were extracted; excellent agreement was found both between experiment and theory, and amongst the different measurement techniques. The results and procedures are compared, and the findings are presented.

Investigation of the Effect of Array Geometry on the Performance of Free-Space Optical Interconnects

The effect of transmitter and receiver array configurations on the stray-light and diffraction-caused crosstalk in free-space optical interconnects was investigated. 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. In addition, we have included the electrical and optical noise in our analysis to give more accurate overall performance of the FSOI system. Our results show that by changing the square lattice geometry to a hexagonal configuration, we obtain an overall signal-to-noise ratio improvement of 3 dB. Furthermore, system density is increased by up to 4 channels/mm2.