A weakly-coupled few mode fiber (FMF) with a simple double-layer core is designed and fabricated that support five (seven) weakly coupled mode groups with average attenuation of 0.21 dB/km (0.39 dB/km) at the C + L (O) optical wavelength bands. Two data transmission experiments are demonstrated utilizing the fiber. A 2-km orbital angular momentum (OAM) mode-group multiplexing (MGM) experiment in the O band achieves error-free transmission for all five multiplexed mode groups without multiple multiple-input multiple-output (MIMO) processing. A 40-km OAM mode division multiplexing (MDM) experiment supporting 14 mode channels in the C + L bands achieves bit error rates (BER) of below 2.4 × 10-2 (20% soft-decision forward-error-correction threshold) for all channels, based on low-complexity 4 × 4 or 2 × 2 MIMO equalization. These demonstrations prove the capability of the fiber to support weakly coupled MDM/MGM transmission across O + C + L optical wavelength bands.