We propose a novel relative humidity (RH) sensor based on hollow core fiber using direct absorption spectroscopic
method in this paper. The wavelength scanning around water vapor absorption peak around 1368.59 nm is realized by
injecting saw-tooth modulated current to a DFB laser diode. A reference signal is used as a zero absorption baseline and
to help reduce the interference from DFB laser source and probed region. The humidity level is determined by the
normalized voltage difference between reference signal and sensor signal at the peak of water vapor absorption. We
demonstrate that a length of 5 cm hollow core fiber with a fixed small air gap between SMF and hollow core fiber as an
opening achieves a humidity detection resolution of around 0.02%RH over the range 0 to 50%RH which does not require
the use of any hygroscopic material.