  We present a stable and high-performance fiber-coupled NbTiN superconducting
nanowire avalanche photon detector (SNAP). We demonstrate afterpulse-free
operation in serially connected two SNAPs (SC-2SNAP), even in the absence of a
choke inductor, achieving a 7.7 times faster response speed than standard
SSPDs. The SC-2SNAP device showed a system detection efficiency (SDE) of 81.0%
with wide bias current margin, a dark count rate of 6.8 counts/s, and full
width at half maximum timing jitter of 68 ps, operating at 2.3 K.
