We propose and demonstrate optical true time delay using tapered SOI contradirectional couplers with single sidewallmodulated
Bragg gratings. The contradirectional couplers consist of two tapered rib waveguides with different width,
and the Bragg gratings are modulated in the inner sidewall of the wider one. The optical signal is launched from the wide
waveguide and coupled to the narrow waveguide through the Bragg gratings structure. Along the direction of light
propagation, the waveguide width varies linearly, so the reflection wavelength is different at different positions.
Therefore, linear delay line can be realized within the grating passband using the present structure. In the simulation,
grating period is 310nm and grating number is 2400, corresponding to the grating length of 744μm. Using 2.5D FDTD
simulation, the current structure can realize optical group delay of 20ps within bandwidth of 18nm. The proposed device
is fabricated on a 220nm SOI chip with Electron Beam Lithography (EBL) and Inductively Coupled Plasma (ICP)
etching. In the experiment, continuous light is modulated by 10GHz radio-frequency signal and travel through the chip,
which is finally detected by the oscilloscope. By adjusting the wavelength of input light, group delay of different
wavelength are recorded by the oscilloscope. The experimental results show that group delay of 28ps is realized within
the bandwidth of 20nm. In the end, the drift of the reflection spectrum and delay lines under different temperature are
analyzed. The reflection spectrum drifts 0.1nm/°C and causes redshift of the corresponding delay line.