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 operation for wide-wavelength 40 Gb/s optical intensity modulation. It is important to suppress residual antireflective-coated facet reflectivity R₁ for the input-output port and on-state propagation loss A_{bs, on} in the waveguide for reducing level change variation Δ caused by the interference effect. As a result, R₁<0.5% and A_{bs, on}<1.5 dB are required to obtain Δ less than 1 dB. The most interesting features-reduced drive voltage and improved a figure of merit (bandwidth to drive voltage)-are found theoretically compared to those of conventional a single-pass (SP) configuration. Drive voltage reduction by introducing a DP configuration is clarified: the degree of the reduction is decided by nonlinearity “n” of absorption increase, assuming voltage dependence on increase of the absorption coefficient is proportional to the applied voltage to the n power (V¹¹). For the same device length, the drive voltage is decreased to 2^-1n/ leading up to an improvement of a figure of merit up to 2¹n/. We then present experimental results for fabricated devices having an InGaAsP bulk absorption layer showing small interference effect and large improvement of characteristics. Drive voltages reduced to 52-60% are achieved over a SP configuration with the same waveguide length, which is in good agreement with the theoretical expectation of 2^-1n/. 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 performance with a low drive voltage ~1.2 V, a modulation bandwidth over 30 GHz (showing a figure of merit of ~25 GHz/V), an insertion loss below 8 dB, and a small variation of on-state loss less than 1 dB for wide wavelength range of input light from 1.545 to 1.558 μm. An almost twice larger modulation index is confirmed even under a dynamic condition