  We demonstrate that the mode number of Andreev bound states in bilayer
graphene Josephson junctions can be modulated by in situ control of the
superconducting coherence length. By exploiting the quadratic band dispersion
of bilayer graphene, we control the Fermi velocity and thus the coherence
length by the application of the electrostatic gating. Tunneling spectroscopy
of Andreev bound states reveals a crossover from short to long Josephson
junction regimes as the gate voltage is approached near the charge neutral
point of bilayer graphene. Furthermore, quantitative analysis of Andreev
spectrums for different mode numbers allows us to quantitatively estimate the
phase-dependent Josephson current. Our work paves a new way to study multi-mode
Andreev levels and to engineer Fermi velocity with bilayer graphene.
