Photonic Implementation of an Instantaneous Frequency Measurement

With the rapid and ongoing developments in telecommunication and electronic warfare technology, faster and more flexible systems are in demand. Wideband signal processing is thus needed to implement such systems. Microwave photonics has been introduced as a tool for achieving such ultra broadband signal processing. Instantaneous Frequency Measurement (IFM) receivers play an important role in electronic warfare. They have been developed as a means of obtaining a rapid indication of the presence of a threat and to roughly identify the frequency of the threat signals. They also have the advantages of low-cost, compactness and moderate to good sorting capability in an interference-free environment. The main limitation of the traditional RF IFM receivers is constrained bandwidth. Microwave Photonic IFMs have been considered, but the main disadvantages of photonic realization of the recent IFM receiver is cost. This work aims to propose and demonstrate low-cost photonic IFM receivers with a broad frequency measurement range. The proposed methods are based on the use of photonic mixing to down-convert the RF modulated optical signals to DC. In a RADAR warning receiver, usually a bank of IFMs is required. Increasing the numbers of IFMs requires an increase in the number of photo-detectors. Thus if low-frequency, low-cost detectors can be used, then the net system cost will be reduced significantly. The concept is proven and the issues arising are analyzed. In the proof of concept system, measurement of the RF frequency required advance knowledge of the RF power. Secondly, the use of co-axial RF cables as delay elements, limited the bandwidth and increased bulk. Using a photonic hybrid approach to achieve orthogonal measurements was demonstrated as a means of identifying both RF frequency and power simultaneously and independently. Employing all optical mixing removed the need for co-axial RF cables delays using non-linear optical devices such as Semiconductor Optical Amplifier (SOA) and Highly Non-Linear Fiber (HLNF). The last investigation is to improve the sensitivity of the implemented IFM system. The sensitivity of the implemented system is characterized first and a lock-in technique is employed to improve the sensitivity of the system. The final system achieves a sensitivity of -41 dBm which is comparable with the traditional RF IFM receivers.