Radio-over-fiber transmission using semiconductor laser under weak external light injection

Optical injection locking is a promising method to overcome the limitations exhibited by directly modulated distributed feedback lasers in analog radio-over-fiber applications. Here the effects of optical injection locking are investigated experimentally for relatively low injection ratio, which is favored for practical applications. The measured responses of optical injection locking with low injection ratio still show distinctive modulation characteristics, controllable by the frequency detuning and the injection ratios. At the higher positive detuning the resonance peak can extend to 22 GHz (limited only by the analyzer instrument), which is 4.2 times greater than in the free running case. Reduction in the noise level becomes better as the system moves towards negative detuning. The reduction reaches to 5.97 dB at the resonance peak for −20 GHz detuning and a −2.9 dB injection ratio. Moreover, reduction also can be realized in the low frequency regime 0–10 GHz. We also achieved a 9.1 dB spur free dynamic range improvement. The error vector magnitude requirement specification of 3.1 % for 64-QAM OFDMA can be satisfied by an injection locked laser with 5–10 dB enhancements, comparing to a free running laser even after transmission over different transmission distances.