Antenna Remoting

Antenna Remoting Using RF over Fiber

Recent developments in lasers, modulators and detectors have enabled analog optical links that can operate at frequencies beyond 10GHz. In the link shown above, laser light is modulated by RF signals from an antenna directly connected to the optical modulator. Even though there is no amplification between the antenna and modulator, the system itself provides modulation so that the RF output from the balanced detectors is stronger than the RF signal at the modulator input. One complication to this system is a type of interference called third order intermodulation distortion (IMD) which is the result of non-linearity in the modulator. The diagram below shows a technique we are developing to eliminate 3rd Order IMD.

The left trace shows the output from a standard modulator. The two large peaks are the signals, the smaller peaks (38dB below the signals) are the unwanted results of IMD. If we had two modulators with different ratios of IMD to signal and we combined their signals with the correct phase and amplitude the IMD could be made to cancel out. It turns out that the two axes (labeled x & z in the figure above) do act like two different modulators. By carefully adjusting the angle of the input polarization, the condition for cancelling the 3rd order IMD can be achieved. This cancellation is demonstrated in the data on the right which shows the IMD peaks eliminated. This results in very high Dynamic Range, ~17dB greater than the standard link. When this technique is used with new modulators and detectors very low Noise Figures and positive RF Gain can be achieved as well.