JEOS RP ISSN01 | Page 23

18

J. Eur. Opt. Society-Rapid Publ. 21, 2( 2025)

Table 1. Comparison of research literature on micro-ring resonant cavity array. Structure

Delay accuracy / ps

Adjustable delay range / ps

Footprint( mm 2)

Note: MRR stands for Miniature Resonant Ring.

Table 2. Comparison of research literature on optical true delay lines.

5.2 Grating true time delay line

5.2.1 Principles and research status of grating true time delay line

Fiber Bragg Grating( FBG) plays a crucial role in fiber delay lines by reflecting or transmitting light of specific wavelengths to achieve delay effects. By adjusting the parameters of the fiber Bragg grating( such as refractive index modulation depth, grating length, etc.), the propagation time and delay of light can be controlled. Common types include true delay lines based on fiber Bragg gratings( FBGs) and linearly chirped fiber Bragg gratings( LCFBGs). In addition, in photonic integrated circuits, silicon-based waveguide gratings can be realized by periodically modulating the effective refractive index of the waveguide, which lays the foundation for the implementation of compact resonant delay lines.

Delay lines based on LCFBGs have achieved significant results in practical applications. For example, a 5-bit delay line based on LCFBG has achieved an average step of 5.39 ps, a delay accuracy of 2.46 ps, and a time delay range of 0 – 168.6 ps [ 32 ].

5.2.1.1 Principles and research status of grating delay line

Sun et al. [ 33 ] used the stepped chirped subwavelength grating waveguide Bragg grating technology in 2020, which achieved discrete tunable optical delay lines on silicon on insulator. The minimum adjustable step size in this technology is 6.6 ps, and the adjustable range of 10 subwavelength gratings is 60 ps. Sun et al. [ 33 ] used 10 subwavelength gratings and Bragg gratings in series to achieve a step structure, and conducted beamforming simulation experiments using FDTD. The simulation used PML as the boundary condition, under which the optical radiation in the propagation simulation area had no effect on the internal field. PML absorbs incident electromagnetic waves, which is equivalent to propagating to infinity. The simulation results indicate that the beams are pointing at 7.56 °, 15.13 °, and 23.2 °, respectively. The limitation of the above research work is that the working bandwidth is relatively narrow.

In order to further expand the work bandwidth, Wang et al. [ 34 ] developed an exponentially variable optical realtime delay line based on a 40 sub-wavelength grating waveguide array in silicon on insulator. The delay line achieves a phase shift step size of 4.7 ps and a maximum adjustable time of 181.9 ps within a bandwidth of 10 GHz. Srivastava et al. [ 35 ]( Tab. 2) proposed a photon based broadband TTD beamforming network, as shown in Figure 4.

In Figure 4, the lightwave from the tunable laser source is externally modulated by Mach-Zehnder modulator( MZM), the other two input terminals being powered by the RF signal produced by the RF signal generator and the DC signal produced by the modulator driver. The amplified modulated optical signal from the erbium-doped fiber amplifier is then divided equally into four arms by a 1 4 optical splitter, and each arm consists of an optical circulator and a raised cosine apodized linearly CFBGs( RCFBG) of distinct lengths and a chirp parameter as a TTD module. The lightwave, whose wavelength corresponds to the chirped grating period, is reflected and reaches the photo-detector( PD) by means of an optical circulator.

In Figure 6, the network uses different lengths ofraised cosine apodized linearly chirped Bragg gratings( RCFBG) and chirp rates as variable TTDs to control the maximum radiation direction of phased array antennas. The experiment shows that the real-time delay line of the light has an adjustable delay accuracy of 0.2 ps and a maximum adjustable delay range of 200 ps. The phased array antenna using this real-time delay line can achieve continuous beamforming in the Ku band, and the main lobe of its directional pattern can be scanned without squint between ± 36.8 °.