Ever since the technology emerged during the first portion of the 20th century, radar has relied on a succession of sources, including magnetrons, klystrons, or traveling-wave tubes (TWTs), to generate the Radio Frequency (RF) energy it must transmit. On the receiver side, radars typically quartz crystal oscillators, to generate stable reference frequencies to compare the returns.

The energy leaves the radar’s antenna, speeds through the ether at the speed of light, collides with a target and is reflected to the antenna. By measuring the properties of this reflection, a radar detects a target and determines specifics like that target’s velocity, altitude, azimuth, vector and possibly identity.

Photonic radars do away with the likes of magnetrons, quartz crystals and instead use light to generate radio frequency (RF) energy and provide stable reference frequencies. As David Stupples, professor of electronic and radio engineering at City St. Georges University in London, noted “There is sometimes confusion in the public domain about the term photonic radar. Microwave-photonic radar uses optical photonic techniques within the radar architecture, but the radar signals transmitted to and received from the target remains RF/microwave electromagnetic radiation.”

An example of a microwave-photonic radar is one that may employ a laser generating two beams of light, each of which has a slightly different frequency. Laser light is found in the optical region of the electromagnetic (EM) spectrum, at frequencies between 300 GHz up to 30 petahertz (PHz). Two laser beams at slightly different frequencies will produce an oscillation and are merged into a fibre optic cable which feeds into a photodetector. The photodetector responds to the oscillation produced by the two merged laser beams, converting this into an electrical RF/microwave signal with the same oscillation as the merged beams. The electrical RF/microwave signal is then fed to the antenna and radiated as an electromagnetic wave.

Some simple arithmetic explains this process: Suppose the first laser beam has a frequency of 193.100 THz and the second has a frequency of 193.110 THz. The resulting electrical current will have a frequency of 10 GHz. This frequency falls within the X-band waveband of 8.5 GHz to 10.68 GHz. X-band is a popular choice for radar engineers when designing systems which must process air targets like ballistic missiles with a high degree of accuracy.

Why microwave-photonic radar?

RF waves oscillate through 360° from a peak to a trough and back to a peak, and vice versa, with this full motion known as a cycle. Frequency is a measurement of how many cycles are performed per second, and one cycle is equal to 1 Hz. Measuring phase is a measurement of exactly where that oscillation is in its cycle at a specific moment. A 10 GHz signal will perform ten billion oscillations per second, or one oscillation every 0.0000000001 seconds or 0.1 nanoseconds (ns).

Radars contain sophisticated clocks to measure exactly where a wave is in its cycle at any given moment. For example, at 0 ns, the oscillation will be at 0°, at 2.5 ns it will be at 90°, 180° at 5 ns, 270° at 7.5 ns and 10 ns at 360°. The radar will log the time of the oscillations as the signal is transmitted. If the signal goes out into the ether, hits a target and echoes back to the radar and the timings, or phase, of the oscillations remain the same, then the radar determines that the target is stationary. If the echoed oscillations are out of phase then the target is in motion. Returning to the above example, the echoed signal may be at 0° at 0.1 ns, at 90° at 2.6 ns, 180° at 5.1 ns, 270° at 7.6 ns, and completing the cycle at 360° at 10.1 ns. This mismatch regarding the phases of the outgoing signal and the echo indicates that the target is moving.

The problem with conventional radars is that the generated RF may have tiny errors in the purity of the signal’s oscillations, for example some oscillations may be slightly quicker than others. This can have implications for a radar’s accuracy which in turn can impact the radar’s ability to accurately process a small, fast target like a hypersonic missile, and accuracy can count for everything in air defence. Microwave-photonic radars will offer better signal purity and hence more accurate target measurement.

Waveband agility is another key benefit of the microwave-photonic radar. Generally, radars work with specific frequencies according to the role they must perform. Other constraints include the size of antenna the radar can accommodate and the amount of power the radar can consume, handle and transmit. Ultimately, frequency choice is always a compromise. A photonic radar, on the other hand, can move between different frequencies with relative ease. The light source can be programmed to change rapidly with minimum fuss by varying the frequency and amplitude of the laser beams: Changing the gap in frequency for the two split beams from 193.100 THz for the first, and 193.1058 THz for the second will yield a 5.8 GHz signal. Suddenly, the radar goes from being an X-band system to a C-band (5.25 GHz to 5.925 GHz) radar. The latter waveband is well suited for processing targets at range, but less suited to provide the sharp target resolution commonly associated with X-band.

Although microwave-photonic radar architectures potentially expand the frequencies a radar can transmit, and hence the targets it can process, some constraints remain: A radar’s antenna is generally tailored to the frequencies the system will process. Antennas typically have a length that is either one-half or one-quarter of the frequency wavelength they transmit or receive. While it may be possible to generate a raft of frequencies, it may not be possible to equip the radar with a single antenna design that accommodates all of these.

From an electronic protection the frequency agility promised by microwave-photonic radar have clear benefits. A sufficiently powerful electronic attack system performing barrage or sweep jamming across the entirety of the X-band could reduce the performance of any X-band radar within range, to such an extent that the latter can no longer process targets as desired. Being able to move the radar’s transmissions out of X-band entirely while the jamming occurs avoids the attack. The aggressor is forced to detect the targeted radar’s new signals, reconfigure their jammers and commence the attack afresh.

Other benefits of microwave-photonic radar approaches include the use of fibre optic cabling inside the radar to carry the light sources which cause less signal loss than traditional cabling carrying electrical signals. Less signal loss means less amplification has to be performed at the antenna to ensure the signal has the requisite power to perform its task as it heads into the ether. Reducing amplification levels also helps to reduce the radar’s overall power consumption.

The story so far

It is important to remember that microwave-photonic radar approaches do not replace existing radars but complement them. As Prof Stupples noted, the digital processors radars rely on to process targets will remain essential, as will the power amplifiers that increase the strength of the outgoing and incoming signals. He foresees hybrid systems where microwave-photonic technology will be embedded within conventional radar architectures to provide the improvements discussed above.

Furthermore, microwave-photonic technology does have disadvantages. These architectures are not noise-free as the lasers and their accompanying components can cause some internal electromagnetic disturbance. Laser components require continuous calibration and active control, Prof Stupples explained, which requires the radar to have additional software for this task. Moreover, a loss of signal strength can be experienced when electrical signals are being converted to an optical output and vice versa. Signal strength loss can risk reducing the radar’s efficiency. Although microwave-photonic technology promises to reduce signal amplification, some may still be needed: Laser components may be limited by the power levels they can handle, possibly requiring additional amplification of the outgoing signal.

Prof Stupples also notes that microwave-photonic architectures will need to have Size, Weight and Power (SWaP) attributes, and be sufficiently robust, to be accommodated in military radars inhabiting space-constrained platforms like combat aircraft or warships. These potential disadvantages can be addressed but will have to be done so at a price point which makes them attractive for defence procurement. Continued research and development in the microwave protonic radar sector is likely to advance SWaP reduction at which point the technology’s adoption could become attractive. Once this adoption becomes routine, it could help to reduce the cost of adopting microwave-photonic processes in military radars. Consequently, the proliferation of the technology could yield improvements in sensor performance across the entire military radar domain.