Characterization and application of triode-type EA modulator

Abstract

In this paper, we have characterized triode-type electroabsorption modulators (Tr-EAMs) which have two junction diodes with the same electrical polarity connection. When a reverse voltage is applied to the junction acting as the optical absorption waveguide (collector side junction), the curve of current and optical transmission versus the applied voltage characteristics follows almost the same trace as the curve for a conventional diode-type EAM, except for the dc-bias shift toward the reverse voltage direction. The voltage shift is theoretically estimated to be almost constant and just over the built-in voltage of the other junction (emitter-side junction). When the forward voltage is applied to the collector-side junction, the current does not how and the optical power level does not change, because the applied voltage is clamped to be the built-in voltage of the collector-side junction. The structure of the Tr-EAM was practically configured using two laterally arrayed PIN junctions. The current and transmission characteristics as a function of the applied voltage were as expected. The voltage shift between the Tr-EAM and conventional diode-type EA modulators with a PIN junction (PIN-EAM) was almost 0.7 V. There was no degradation in 3 dB-down bandwidth and improved electrical reflection was observed in the reverse voltage range. The Tr-EAM retains the values even in the forward voltage range due to its high impedance in contrast to the PIN-EAM. This structure leads to the realization of an extended applied voltage range and electrical robustness. We confirmed that the Tr-EAM resulted in the extension of the voltage range from -12 V to +12 V as compared to -12 V to +0.6 V in the PIN-EAM. Aging tests carried out under a bias voltage of +12 V for over 2000 h show no degradation in the leakage current in the present study. The Tr EAM has attractive features for application purposes, since it has a step-like optical transmission as a function of the applied voltage. In the nonreturn-to-zero (NRZ) modulation scheme, cross points of the eye pattern were adjusted with a de bias voltage without timing jitter in the trailing edges. Bit-error-rate experiments under the back-to-back condition show a wide bias voltage tolerance of 0.86 V giving the required received power of less than -33 dBm compared to 0.25 V for the PIN-EAM. In the return-to-zero (RZ) modulation scheme, a transmission gate with a 0-100% time slot window corresponding to the demultiplexed bit-rate was realized under sinusoidal modulation with a constant voltage swing; 20 GHz RZ pulses mere gated to 5 and 10 GHz with various gate windows by tuning the dc bias voltage.