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Characterization and equalization of nonlinearities in directly modulated resonant cavity light-emitting diodes

We present a detailed analysis of the nonlinearities occurring in the electro-optical (E/O) conversion of resonant cavity light-emitting diodes (RC-LEDs), which are typically used in optical short-range transmission. The analysis is focussed on the transfer characteristic between electrical modulation frequency at the RC-LED's input and the nonlinearities occurring in the optical output of the device. Nonlinearities are assessed by evaluating 2nd-order and 3rd-order harmonics, as well as 2nd-order intermodulation products (IM2) at the device output. The IM2 results are visualized as the Fourier transform of the RC-LED's 2nd-order Volterra kernel. The analysis is completed by investigation of suitable equalizers to increase the power budget of LED-based Gbps transmission. It is shown that 2nd-order nonlinearities dominate in the LED's E/O conversion and are strongly frequency dependent. The observed nonlinearities exhibit a clear low-pass behaviour, which implies that low input frequencies induce the highest nonlinearities at the RC-LED's optical output. Our investigations show further that power budget maximization requires the capability of equalizing dynamic transmitter nonlinearities, where the equalizer complexity may be minimized by covering only a short nonlinear memory.