Optical soliton molecules are common pulse states in dissipative system, and they can combine with each other to form soliton molecular complexes which exhibit cumbersome spectral characteristics. As there is an intrinsic connection between the spectra of the soliton molecules and soliton molecular complexes, we demonstrate that the spectra of soliton molecular complexes are the results of quadratic modulation between component solitons, through experimental verification and simulation analysis. Specifically, two strongly bound solitons modulate to form soliton molecules, and then the soliton molecules modulate with weakly bound soliton pairs to form soliton molecular complexes, which is called convolutional modulation. The results obtained have vital significance to better understanding of the intrinsic nature and interaction of the solitons. The formation of quadratic modulated spectra plays an inspiring role in the fields like modern fiber-optic communication systems, optical frequency combs and its related precision measurements and various practical applications.