A multimode optical fiber is provides, which includes an optical core and an optical cladding surrounding the optical core. The optical core has a refractive graded-index profile. The optical cladding includes: an inner layer surrounding the optical core, an intermediate layer, called a “depressed trench”, surrounding the inner layer, and an outer layer surrounding the depressed trench and having a constant refractive index. The depressed trench has a width W and a negative refractive index difference Δnt with respect to the outer layer, and is designed so as to satisfy the following inequality: |0.585677−114.681×S+13.7287×S2+18.7343×S×W−4.61112×S×Δnt.103−0.913789×W×Δnt.103|+2×W×Δnt.103