DESIGN OF DOUBLE-PASS ELECTROABSORPTION MODULATORS WITH LOW-VOLTAGE, HIGH-SPEED PROPERTIES FOR 40 GB S MODULATION/

We discuss device design for electroabsorption (EA) modulators using a double-pass (DP) configuration to attain low-voltage, high-speed oper ation for wide-wavelength 40 Gb/s optical intensity modulation. It is important to suppress residual antireflective-coated facet reflectivit y R-1 for the input-output port and on-state propagation loss A(bs, on ) in the waveguide for reducing level change variation Delta caused by the interference effect, As a result, R-1 < 0.5% and A(bs, on) < 1.5 dB are required to obtain Delta less than 1 dB, The most interesting f eatures-reduced drive voltage and improved a figure of merit (bandwidt h to drive voltage)-are found theoretically compared to those of conve ntional a single-pass (SP) configuration. Drive voltage reduction by i ntroducing a DP configuration is clarified: the degree of the reductio n is decided by nonlinearity ''n'' of absorption increase, assuming vo ltage dependence on increase of the absorption coefficient is proporti onal to the applied voltage to the n power (V-n). For the same device length, the drive voltage is decreased to 2(-1/n) leading up to an imp rovement of a figure of merit up to 2(1/n). We then present experiment al results for fabricated devices having an InGaAsP bulk absorption la yer showing small interference effect and large improvement of charact eristics. Drive voltages reduced to 52-60% are achieved over a SP conf iguration with the same waveguide length, which is in good agreement w ith the theoretical expectation of 2(-1/n). Design window for 40 Gb/s modulation appears by introducing a DP configuration even for devices with a bulk absorber, Proper waveguide length design provides high per formance with a low drive voltage similar to 1.2 V, a modulation bandw idth over 30 GHz (showing a figure of merit of similar to 25 GHz/N), a n insertion loss below 8 dB, and a small variation of on-state loss le ss than 1 dB for wide wavelength range of input light from 1.545 to 1. 558 mu m. An almost twice larger modulation index is confirmed even un der a dynamic condition.