# Integrated Microwave Photonics for Wideband Signal Processing
## 3 Integrated MWP Delay Line  
Optical delay lines, which delay an optical signal by a selected increment of time, play a key role in the implementation of a variety of microwave photonic signal processing functionalities [30; 31]. For example, one important application is optically controlled beamforming for phased array antennas, where the delay element is an essential component for compensating the frequency dependency of beam direction, or beam squint effect. Non-resonant delay lines are of great interest due to their ability to form a large time delay as well as a wide operating bandwidth. Thanks to the CMOS-compatible fabrication technology which promises a more cost effective, robust and compact solution, delay elements based on various chip-scale platforms have been intensively investigated. In particular, low propagation loss has been demonstrated in optical waveguides of wedge geometry, high-aspect-ratio Si\({}_{3}\)N\({}_{4}\), and high-quality silicon oxynitride films [32; 33; 34; 35; 36]. In [32], an optical delay line fabricated on a silicon chip has been presented, exhibiting an average measured waveguide loss of 0.08 \(\pm\) 0.01 dB/m in long spirals. Employing resonance behavior of the whispering-gallery-mode [37; 38] and microring cavities [39; 40; 41; 42; 43] forms another class to realize time delay functions. To extend the operating bandwidth of the delay lines, multiple microring resonators cascaded in versatile topologies such as coupled resonators optical waveguide (CROWs) [39; 40; 41; 42], and side-coupled integrated spaced sequence of resonators (SCISSORS) [40] have been proposed. In [43], an optical delay with high bandwidth and continuous tunability was achieved by employing microring resonators in a balanced SCISSORS configuration. A 10 GHz operating bandwidth using thermally tuned silicon microring resonators was experimentally demonstrated. Microring resonators in an eye-like configuration as a continuously tunable delay line has also been reported [44]. The eye-like delay line unit consists of the outer (Ring 1) and inner (Ring 2) racetrack resonators as shown in Figure 2a, where the group delay exists at the through port of the unit. The optical power in Ring 1 is partly coupled between the bus waveguides by the temperature-controlled Mach Zehnder Interferometer (MZI) power splitters in Block 1 and 2, and also Ring 2 by the static directional couplers in Block 3 and 4. The temperature-controlled  
Figure 1: Measured responses of the microwave photonics (MWP) phase shifter where the different colour lines show the phase shifter operation as the carrier is tuned from 1554.30 nm to 1554.55 nm (**a**) continuous radio frequency (RF) phase tuning (**b**) superimposed RF power variations at various RF phase shifts. Inset: Optical response of the phase shifter. MZI power splitters are regarded as the tunable coupling devices such that the tunable coupling ratios \(\kappa_{1}\) and \(\kappa_{2}\) are obtained in Block 1 and 2, respectively. The dependences of group delays on coupling ratios are plotted in Figure 2b,c. The analysis is conducted on the eye-like resonator with the outer ring circumference of 397 \(\upmu\)m which occupies the space smaller than 0.019 mm\({}^{2}\). With the temperature-controlled MZI power splitter between the resonator and the bus waveguides, the eye-like resonator based delay line can be operated in over-coupling and under-coupling regimes. It should be noted that a considerable delay of 627 ps can be achieved in the over-coupling condition while maintaining the optical loss less than 25 dB. Optical delay lines can also be realized via adopting a series of Mach Zehnder Interferometer (MZI) or microring resonator-based optical switches, thus generating a tunable time delay controlled by the optical path change [45; 46; 47; 48].