  Comb-mode resolved spectral domain interferometry (CORE-SDI), which is
capable of measuring length of kilometers or more with precision on the order
of nanometers, is considered to be a promising technology for next-generation
length standards, replacing laser displacement interferometers. In this study,
we aim to improve the measurement precision of CORE-SDI using programmable
spectral shaping. We report the generation of effectively broad and symmetric
light sources through the programmable spectral shaping. The light source used
here was generated by the spectrally-broadened electro-optic comb with a
repetition rate of 17.5 GHz. Through the programmable spectral shaping, the
optical spectrum was flattened within 1 dB, resulting in a square-shaped
optical spectrum. As a result, the 3-dB spectral width was extended from 1.15
THz to 6.7 THz. We performed a comparison between the measurement results of
various spectrum shapes. We confirmed an improvement in the measurement
precision from 69 nm to 6 nm, which was also corroborated by numerical
simulations. We believe that this study on enhancing the measurement precision
of CORE-SDI through the proposed spectral shaping will make a significant
contribution to reducing the measurement uncertainty of future CORE-SDI
systems, thereby advancing the development of next-generation length standards.
