Active ODLs, on the other hand, are electronic devices that perform the conversion of an RF signal to optical and then output the signal again as RF. From a high-level perspective, the device first converts an incoming analog RF signal to an optical signal. The optical signal is then transmitted through a single-mode fiber coil of a specific length with a delay. After passing through the fiber optic coil, the signal is converted back to an RF signal and output by the device.

To complete this process, an active Optical Delay Line contains several components, including a laser, a photodiode, an optical modulator, and of course a single-mode fiber coil. For more complex applications and additional features such as variable delay, other components such as optical switches, optical and RF amplifiers, and dispersion compensators can be integrated into the device. It is important to note that the total delay includes the combined delay of the fiber optic coil and the delay time for the signal to travel through the electronic components at both ends.

In which applications can active optical delay lines be used? As mentioned in the introduction, RFoF communication systems frequently used in defense and military systems (eg, radar, GPS, SATCOM, and long-range antennas) often need to incorporate active ODL devices.

Finally, active Optical Delay Line equipment is often much more expensive than passive Optical Delay Line due to the inclusion of additional hardware to implement the necessary signal conversion and delay processes. However, the choice of whether to opt for active or passive Optical Delay Line should always be based on specific application and project requirements.