The far infrared response of granular thin-film Bi2Sr2CaCu2O8 superconductor has been investigated using long (ap5 mgrs) but sharply truncated free electron laser pulses in the frequency range between 50 cm–1 and 125 cm–1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (ap 200 cm–1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model.