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J. Eur. Opt. Society-Rapid Publ. 21, 2( 2025)
Figure 8. Wavelength selective turel-time delay line based on AWG [ 38 ].( a) Working principle explanation( b) AWG layout.
5.2.2 Brief summary
Generally, the advantage of optical true delay lines based on grating spatial structures is that they are easy to integrate, but the disadvantage is that they cannot achieve continuous adjustment or require changing the wavelength of light for adjustment, which will increase the cost of the system.
5.3 Multi-path switchable OTTDL
5.3.1 Principles and research status of multi-path switchable OTTDL
Another way to achieve adjustable delay lines is to adjust the length of the optical path. The adjustable TTD can be achieved by designing optical waveguides with different lengths. The advantage of optical switch technology is that it provides a large delay selection, allowing the module to operate in ultra wide radar frequency bands.
In 2018, Pérez-López et al. proposed a programmable photonic integrated circuit, in which the architecture of the waveguide meshes used can be configured to achieve optical true time delay lines( OTTDLs) [ 36 ]. In this article, another independent adjustable unit structure was demonstrated in the experiment, which can achieve independent beam amplitude allocation and phase control with this coupler. Each path corresponds to a latency of 13.5 ps, with a maximum achievable latency of 148.5 ps. Figure 7 demonstrates the hexagonal network configuration with 5 paths and 11 paths.
In 2020, Zhu et al. designed an integrated optoelectronic chip based on a silicon on insulator platform, called a 1 8 microwave photon beamformer [ 37 ]. The chip consists of a
modulator, an eight channel optical beamforming network, and eight photodetectors, containing hundreds of active and passive components. Among them, the phase shifting part adopts eights 5-bit adjustable true time delay lines, with an adjustable range of 2 – 16 ps and a total adjustable range of 0 – 496 ps.
In 2023, Sun developed a 4-channel 5-bit programmable optical controlled beamforming network based on a highprecision optical switch delay line measurement and production platform, and conducted experimental tests on the network. The experimental results showed that the delay accuracy was better than ± 0.5 ps, which was higher than similar delay lines reported domestically, and the insertion loss consistency was better than ± 1 dB [ 24 ].
5.3.2 Brief summary
The most obvious advantage of the optical true delay line based on multi-path switchable switches is its fast switching speed, which can reach ns level speeds. This is because the adjustment method of the optical switch is mainly based on the photoelectric effect, which can achieve a speed of ns level. This type of delay line has good accuracy, but the optical delay line of the multi-path switch cannot achieve continuous adjustment.
5.4 Wavelength selective OTTDL
5.4.1 Principles and research status of multi-path switchable true time delay line
Array waveguide grating is a good wavelength selective device. Figure 8 shows the structure and working principle of the wavelength selective optical delay line of the array