Enhanced digital modulation of coherently coupled vertical cavity laser arrays: theory and application

Dynamic properties of coherently coupled 2x1 vertical-cavity surface-emitting laser (VCSEL) arrays have been studied theoretically and experimentally. Directly modulated VCSELs are the dominant digital light source in short-haul data communication links due to their low cost and low operating power. Due to the demand for faster internet speeds and high-performance data centers, there is an ever-increasing need for high-bandwidth, low-power, and low-cost laser sources. An ideal laser source would have data-rates exceeding 100 Gb/s, energy-per-bit ratios reaching less than 1 pJ/bit, and be fabricated/integrated using conventional methods. In this dissertation, monolithic 2x1 mutually phase-locked 850 nm VCSEL arrays, based on ion-implantation and photonic crystal design, are studied for high-speed datacom applications. From the theoretical perspective, coupled-mode rate equations are used to describe carrier-photon interactions in optically coupled semiconductor laser arrays. Bandwidth enhancement in phased laser arrays is explained using small-signal frequency-domain pole-zero analysis on coupled-mode rate equations. The effects of complex coupling coefficient, associated with index-antiguided and gain-guiding, on coupled laser dynamics, are explored. We study multiple modulation techniques, including single-, in-sync, and out-of-sync modulation using pole-zero analysis. We discover that bandwidth enhancement can be practically implemented using in-sync modulation of asymmetric arrays, which is experimentally verified. Dynamics surrounding PT-symmetry breaking and exceptional points are also studied using pole-zero analysis. By tuning the coupled lasers to an exceptional point, the array supermodes become indistinguishable, and the coupled-laser array forces the input signals into synchrony. Experimentally, we show progress on phased VCSEL arrays that can be designed to operate reliably through control of individual injection current to each laser element. In this manner, nearly all fabricated arrays can be biased to be mutually phase-locked. This dissertation attempts to visualize and characterize the locking region between two neighboring lasers by measuring far-field visibility to create a two-dimensional visibility map. Experimental data show that the locking region can be shifted or changed by ambient temperature, asymmetric array design, fabrication imperfections, and multimode operation. Additionally, the differential resistance, relative intensity noise, and harmonic distortion are measured to characterize the behavior of coherently coupled laser arrays in the locking region. Two-dimensional maps of total relative intensity noise and total harmonic distortion are created to visualize laser array dynamics in the locking region. We report high-speed digital modulation of 36 Gb/s by supplying the signal to both laser elements simultaneously. The resulting eye diagram is shown to be improved under coherent-coupling conditions. The analysis in this dissertation shows that the laser array must be carefully designed, biased, and modulated within the locking region to achieve an output signal with low noise, low distortion, and improved modulation performance