  We present a compact and lightweight 1.5 {\mu}m lidar using a free-running
single-photon detector (SPD) based on a multi-mode fiber (MMF) coupling
InGaAs/InP negative feedback avalanche diode. The ultimate light detection
sensitivity of SPD highly reduces the power requirement of laser, whilst the
enhanced collection efficiency due to MMF coupling significantly reduces the
volume and weight of telescopes. We develop a specific algorithm for the
corrections of errors caused by the SPD and erbium-doped fiber amplifier to
extract accurate backscattering signals. We also perform a comparison between
single-mode fiber (SMF) coupling and MMF coupling in the lidar receiver, and
the results show that the collection efficiency with MMF coupling is five times
higher than SMF coupling. In order to validate the functionality, we use the
lidar system for the application of cloud detection. The lidar system exhibits
the ability to detect both the cloud base height and the thickness of
multi-layer clouds to an altitude of 12 km with a temporal resolution of 1 s
and a spatial resolution of 15 m. Due to the advantages of compactness and
lightweight, our lidar system can be installed on unmanned aerial vehicles for
wide applications in practice.
