The recently proposed 2-channel optical-time-division-multiplexing (OTDM) technique utilizing a sinusoidally modulated input light source provides an attractive solution to double the per-wavelength data rate of the short-reach intensity-modulation/direct-detection system. However, to maximize the transmission speed of such an OTDM system, it is necessary to generate the sinusoidal input light operating at the frequency much faster than the modulator's bandwidth. As a result, the extinction ratio (ER) of the generated sinusoidal input light can be significantly reduced, which, in turn, causes a large crosstalk in this OTDM system. To solve this problem, we propose to enhance the ER of the sinusoidal input light simply by decreasing the bias voltage applied to the Mach-Zehnder modulator (MZM) used for its generation (from the quadrature point toward the null point). We estimate that, by using this technique, it is possible to increase the ER from 3.1 dB to 14.9 dB when the required sinusoidal frequency is three times faster than the MZM's bandwidth. To verify the effectiveness of this ER enhancement technique, we realize the 2-channel OTDM system with a sinusoidally modulated input light source by using the MZMs having a 3-dB bandwidth of 17.2 GHz and demonstrate the transmission of the 300-Gb/s PAM8 signal. Thus, the ratio between the achieved data rate and the 3-dB bandwidth of the MZM is as high as similar to 17.4.