# Photonic Systems and Devices for Linear Cell Radar
## 4 Linear Cell Radar Based on RoF [6; 12]  
For the safe operation of airport runways, foreign-object debris detection (FODD) is an indispensable activity. This system should be used to cover runways whose lengths are longer than a few kilometers. A typical runway width is 60 m. To ensure safe take-off and landing, small objects should be detected by high-resolution imaging. A standard FOD whose diameter and height are 1 inch each is defined to measure the performance of such imaging technologies. For the detection of such FOD, we can use various techniques including high-resolution cameras and lidars. However, their performance would be degraded in severe weather conditions such as fog and snow. The time duration for detection is also an important issue. Time intervals of take-offs and landings in busy airports would be 90 s, so the time duration for imaging should be shorter than 30 s to save time for air traffic controllers to make decisions. Millimeter-wave radars which can provide high-resolution imaging with a short time duration would be candidates for the detection of FOD [6; 28; 29; 30]. In the millimeter-wave range, a wide spectrum may be used to achieve a range resolution of a few centimeters. Bands with a frequency of 60 GHz attract much attention for unlicensed high-speed short-distance telecommunications. However, due to their large attenuation by oxygen, 60 GHz bands are not suitable for moderate distance applications. On the other hand, as shown in Figure 9, atmospheric attenuation in the range from 70 GHz to 110 GHz (E- and W-bands) is much smaller than that in 60 GHz bands. Dry air attenuation is the lowest at 94 GHz in the frequency region higher than 60 GHz. In this range, 92-100 GHz is internationally allocated for radio location services. Even in the band from 92 GHz to 100 GHz, millimeter-wave attenuation in the air limits millimeter-wave radar coverage. High-power traveling wave tube amplifiers can be used to cover a runway by a radar unit. However, the high cost and short lifetime of the amplifiers are issues for reduction of operation cost. In order to solve this issue, a radar system consisting of many antenna units connected by radio-over-fiber has been developed, where information collected at each antenna unit would be complied coherently at a central unit. Waveforms for radar operation are transferred by optical fibers between the antenna units and the central unit. Figure 8: Current-voltage (I-V) curves for the dark current and photocurrent with avalanche multiplication [12]. The system described in Figure 10 is called the linear cell radar system, which provides linear-shaped coverage by the use of linearly located antenna units. It is rather difficult to provide two-dimensional wide-area coverage using RoF links, because a huge number of links would be required. However, for one-dimensional service coverage, RoF systems with antenna units located along with optical fibers to form linear-shaped coverage would be feasible for applications in important public infrastructure such as airport runways, railways, motorways, etc. N units could cover an area N times longer, while a system with a radar unit needs at least an N\({}^{4}\) times larger radiation power to achieve an N times longer range. A radar system consisting of many radar heads could largely reduce the total radiation power of the system and the required millimeter-wave output at each unit, so that the high-resolution imaging for long area coverage could be achieved by low-cost semiconductor devices. A liner cell radar system using radio-over-fibers has been developed for FOD detection on runways [6; 7]. The system uses FW-CW (frequency modulation continuous wave) radar technology, where the sweep range and center frequency of the transmitted FM signal were 7.9 GHz and 96 GHz, respectively. In Reference [28], the basic characteristics of a millimeter-wave FW-CW radar were estimated, where the range resolution was 8 cm.