This paper reports an interesting phenomenon of optical modulation enhancement produced by CW optical sideband injection locking of a directly modulated Fabry-Perot Laser Diode (FPLD) lasing at 1557.1 nm wavelength, the modulation being sinusoidal in nature and effected at 300 kHz and 3 MHz. This investigation is carried out for direct modulation of the FPLD effected in the RF region at 300 kHz and 3 MHz in contradistinction to earlier studies being conducted with the modulation in the microwave frequency band. A nonlinear analysis has been carried out which corroborates experimental observation on modulation enhancement. At low level of optical Intensity Modulation (IM) index, not exceeding 20 %, typical modulation enhancement of 2 % at 300 kHz and 11 % at 3 MHz modulation frequency have been observed. Larger modulation enhancement is expected at higher values of IM indices of the slave FPLD and for medium to high power laser diodes. This circuit can have direct application in Wavelength Division Multiplexed-Passive Optical Network (WDM-PON).