High throughput signals with high spectral efficiency (SE) are attracting interest for their application to data center interconnection application. For such application, a high baud rate and high-order modulation are desirable. This article describes each of the three steps in designing a high SE signal with a net rate of 1-Tbps/λ in detail. First, the baud rate dependency of the signal-to-noise ratio (SNR) is obtained by applying methods such as precise digital calibration technique. Test signals of PDM -16 quadrature amplitude modulation (QAM), PDM-64QAM, and PDM-256QAM were compared to illustrate the modulation format dependence on signal SNR. Clipping ratio optimization and joint digital equalization and optical equalization were performed to maximize the SNR. Second, the achievable capacity is calculated from the obtained baud rate dependency of the SNR. Symbol rate is selected to maximize the achievable capacity. WDM-grid and the Nyquist pulse shaping roll-off factor is selected to achieve a high SE. Third, the rate parameter of probabilistically shaped (PS) signal are determined to generate 1-Tbps/λ net rate signal. A high fiber capacity of 41-Tbps is successfully demonstrated with C-band WDM transmission over 100 km with 41-WDM 96-Gbaud PDM-PS-256QAM signals in a 100-GHz grid.