An explanation of phenomena in frequency upconversion of radio over fiber system

In this paper, the principle of frequency upconversion without any radio frequency (RF) local oscillator (LO) for radio over fiber (RoF) system is described from both the conventional and novel perspectives. Under conventional optical injection locking (OIL) theory, the frequency upconversion is achieved when the slave laser (SL) after redshift is locked by one of the frequency components in the sideband of the modulated optical signal from master laser (ML). When the baseband modulation signal is with long sequence of consecutive bits `1´ or bits `0´, the loss-of-lock phenomenon occurs, i.e., the quality of frequency-upconverted signal is deteriorated, which could not be interpreted by the conventional OIL theory. A novel interpretation is proposed preliminarily to illustrate the loss-oflock phenomenon. The frequency component of the baseband modulation signal is assumed to be contributed by rising or falling edge solely. When the amplitude of the modulation signal is constant, only the direct current (DC) component will be observed, which leads to the missing of the modulated optical signal´s frequency component to lock SL. Therefore, the SL and ML run freely, and the locking condition no longer holds.