It is confirmed that the multiplexer and the semiconductor laser can be operated up to 4 Gbit/s with pseudorandom pulse pattern. The relaxation oscillation, pattern effect and tailing effect of the semiconductor laser are also studied. It is confirmed that eye degradation is predominantly due to fluctuation of initial time delay caused by pattern effect, and tailing effect and relaxation oscillation are not fatal factors of 4 Gbit/s modulation under proper bias conditions.