# Photonic Systems and Devices for Linear Cell Radar
## 3 Photonic Devices for RoF Systems  
Frequency multiplication by optical modulation can easily generate and distribute millimeter-waves over optical fibers. For radio astronomy and RoF-based radars, frequency tunable sinusoidal signal generation is required for the synchronization of signal detection or LO signal distribution. As an example of such signal generation, a setup for frequency-modulated continuous-wave (FM-CW) signal generation with this optical modulation technique is shown in Figure 5[21; 22; 23; 24; 25; 26; 27]. This was designed for W-band radar application [24]. A digital-to-analogue converter (DAC) generated a swept-frequency electric signal, whose start and stop frequencies were 359.375 MHz and 390.625 MHz, respectively. An FM-CW signal whose center frequency and frequency width were 24 GHz and 2 GHz was generated by a series of frequency multipliers, where the sweep time was 1 \(\upmu\)s. A lightwave-generated tunable laser diode (TLD) was fed to a Mach-Zehnder modulator (MZM) to obtain sidebands. The lightwave wavelength was 1550.03 nm. The DC (direct current) bias of the MZM was set to be a maximum transmission condition to maximize the carrier and second-order sidebands and to minimize the first-order sidebands. To suppress the carrier, we used an optical band eliminating filter (OBF). The suppression ratio of the first-order sidebands depended on the extinction ratio (ER) of the MZM. We used a high-ER optical MZM to generate an optical signal, which had the upper and lower second-order sidebands as shown in Figure 6. The frequency separation between these two spectral components was 92-100 GHz. We also used an erbium-doped fiber amplifier (EDFA) and an optical band pass filter (OBPF) to boost the signal. The optical signal was fed to a photodetector to generate a W-band (92-100 GHz) frequency chirp signal, whose sweep rate was 8 \(\times\) 10\({}^{15}\) Hz/s. Figure 4: Configuration of RoF optical-to-radio interface for radio-wave reception. Figure 3: Configuration of RoF optical-to-radio interface for radio-wave radiation. Figure 2: Configuration of conventional optical-to-radio interface for radio-wave radiation. Quantum dots (QDs) semiconductor devices can be applied to light sources and photodetectors for RoF systems. Due to quantum confinement, QD light source devices can provide high-speed and wide-wavelength operation. In addition to light source devices including QD lasers and QD semiconductor optical amplifiers (QD-SOAs), QD photodetectors (QD-PD) have been reported to investigate particular functions such as an avalanche effect inside QD layers [12; 25; 26; 27]. In Reference [26], QD-PD with 20 stacked QD absorption layers was reported, where four InAs QDs monolayers and 20-nm-thick InGaAs spacer layers were grown on an InP substrate, by the use of molecular beam epitaxy via the strain-compensation technique. A PIN (p-i-n) structure with a 400-nm-thick active region was fabricated, where the QD density was 1.3 \(\times\) 10\({}^{12}\) cm\({}^{-2}\). A QD-PD device with a large active area was fabricated to estimate responsivities, where the size of the area was 0.5 \(\times\) 1.0 mm\({}^{2}\). For current-voltage (I-V) curves and frequency, responses were measured using a fabricated device with a small active area whose diameter was 14 \(\upmu\)m.  
The absorption coefficient and responsivity of a fabricated InAs QD absorption layer was estimated with surface illumination, where an InGaAs bulk absorption layer was used for reference. Firstly, the estimated absorption coefficient of InGaAs was 1 \(\times\) 10\({}^{4}\) cm\({}^{-1}\), which was obtained from the measured responsivity of a 0.4-\(\upmu\)m InGaAs layer. Calculated responsivity of a 24-nm InGaAs absorption layer was 0.015 A/W at 1550 nm. If the InAs QDs had the same absorption coefficient as InGaAs, that is, 1 \(\times\) 10\({}^{4}\) cm\({}^{-1}\), the expected measured responsivity would be approximately 0.015 A/W at a wavelength of 1550 nm.