J. Eur. Opt. Society-Rapid Publ. 21, 2( 2025) 17
during propagation of the optical signal, and Dk is the wavelength change of the transmitted optical signal.
The three terms on the right side of equation( 25) correspond to three time-domain processing methods for optical true delay. The first term means that variable delay can be achieved by changing the total path length L of optical signal transmission, making an optical path difference between each path. This is the most direct method to achieve variable delay. However, as soon as the length of each fiber is determined, it cannot be changed. The second term meas that different delays between different optical channels in the system can be achieved by changing Dn( k), which is the refractive index of the dielectric material in the optical path. This delay method is generally used for systems with additional control, such as delay systems with temperature control. However, this method has low reliability and poor operability, making it difficult to achieve high delay accuracy. The third term is that the dispersion of dielectric materials D( k) can be used to achieve variable delay, that is, different wavelengths of optical signals will produce different delays when propagating in the same medium.
The fourth type of scheme for achieving true optical delay is spectral domain processing, which is a true delay scheme based on spatial optical modulation technology. The core of this type of scheme is to convert the optical signal in the time domain into a spectrum in the frequency domain for processing. By using Fourier transform and optical modulation techniques in the frequency domain to adjust the phase and intensity of the signal, to make sure that optical true time delay can be achieved. The advantage of this method is that it can weaken the influence of highorder dispersion in the transmission medium and compensate for the filtering characteristics that may occur in the system [ 26 ].
5 Research status of OTTD technology
Continuously improving delay accuracy and achieving as much continuous and adjustable delay as possible are two important aspects of studying fiber optic delay lines. At present, research on optical true delay lines mainly focuses on four aspects: micro-ring resonators, grating delay lines, multi-path switchable optical switches, and wavelength selective optical delay lines.
5.1 Micro-ring resonant cavity array and its principle
The micro ring or micro disk resonant cavity array is an important component of the resonant tunable integrated fiber delay line. Essentially, the slow light effect is realized by generating resonance, thus slowing down the propagation speed of light. Finally, the amount of delay can be adjusted by adjusting the deceleration factor.
5.1.1 Principles and research status of micro-ring resonant cavity array
The slow light effect refers to an important physical effect that enhances the interaction between the optical field and matter by increasing the group refractive index of the waveguide to reduce the group velocity of the optical signal propagating in it. Although the effective refractive index of the waveguide itself changes very little, the group refractive index varies greatly due to the slow light effect. The change in group refractive index can affect the dispersion characteristics of optical signals, thereby affecting the optical bandwidth. By precisely controlling the dispersion characteristics of the medium and the parameters of the optical pulse, it is ensured that the change in group refractive index leads to a reduction in dispersion, which reduces the degree of waveform distortion of the optical signal and is beneficial for reducing the optical bandwidth. On the other hand, due to the slower propagation speed of light in slow light media, the propagation time of light pulses through the medium becomes longer, allowing the system more time to precisely control the delay of light pulses. Therefore, the slow light effect can be used to achieve high-precision time delay. Finally, the slow light effect can significantly increase the delay range. Due to the slower propagation speed of light in slow light media, media of the same length can provide greater time delay. This means that shorter media can be used to achieve larger time delays, significantly reducing the length of optical delay lines and tuning power consumption [ 27 ].
In 2013, Burla et al. [ 28 ] proposed and implemented an integrated photon beamforming network based on a programmable optical ring resonator, which can achieve continuous adjustable optical delay of 236 ps in the Ku band( 10.7 – 12.75 GHz). However, the adjustment accuracy of the optical true delay line designed using this method is not high. In order to further improve the adjustment accuracy and maximum adjustable range, and enhance the adjustment accuracy of the optical true delay line. Xiang et al. [ 29 ] proposed a low loss continuously tunable optical true delay line based on Si 3 N 4 ring resonator in 2018, creatively using Side Coupled Integrated Spaced Sequence of Resonators( SCISSOR). The maximum delay of this optical true delay line can reach 500 ps. To expand the working bandwidth, Shan et al. [ 30 ] proposed a broadband continuously tunable microwave photon delay line based on cascaded silicon micro rings in 2021. The working bandwidth of the delay line can reach 16 GHz, with a minimum adjustable step size of 20 ps and a maximum adjustable range of 160 ps. Xue et al. [ 31 ]( Tab. 1) demonstrated an optically controlled beamforming network based on micro microresonator frequency comb source and dispersion delay. The working bandwidth of this network is 12 GHz, and the adjustable accuracy and range have been further improved compared to Shan et al.’ s research.
5.1.2 Brief summary
The advantages of the optical true delay line based on the micro ring resonant cavity array are easy to manufacture, a large adjustable range of optical delay, and the ability to achieve continuous delay. The disadvantage is that the change in deceleration factor is usually thermal regulation, which limits the beam switching time for beamforming using micro ring resonators as delay lines or phase shifters.