Period doubling is an irregular phenomenon exhibited by semiconductor lasers under high frequency modulation. When it occurs, the repetition frequency of the laser power becomes half of that of the modulation signals, which is undesirable to most applications. This paper reports the control of period doubling in modulated semiconductor lasers by external optical injection and demonstrates for the first time that such nonlinear dynamics can be used advantageously in realizing all-optical clock frequency division. We show that period doubling in modulated semiconductor lasers can be either suppressed or enhanced by providing external CW optical injection. The dependence on injection wavelength, and injection power has a been investigated systematically to establish an improved understanding towards the control of period doubling in modulated semiconductors lasers. The fact that period doubling can be enhanced by optical injection is further used to realize all-optical clock frequency division. To demonstrate this, an optical clock signal at 19.6 GHZ was injected into a semiconductor laser. By adjusting the resonance frequency of the laser to around 9.8 GHz through dc bias, strong period doubling was observed, which resulted in a frequency-halved optical clock signal at 9.8 GHz with a remarkably low level of phase noise. Our investigations have also shown that without changing the biasing condition of the laser this low level of phase nose can be maintained within 1-dB range over an input frequency range of 400 MHz, which is a distinct advantage over other techniques.