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We propose a novel non-hygroscopic humidity sensor based on a photonic bandgap fiber (PBF) and a digital wavelength modulation spectroscopy (WMS). PBF is employed as the sensing element due to the large overlap of transmitted light with humid air inside the core of the PBF. High sensitivity detection is achieved by using WMS-this shifts the detection bandwidth to a higher frequency. We use the ratio between second harmonic (2f) and the first harmonic (f) of the WMS signals at the water vapor absorption peak around 1368.59 nm for detection of relative humidity. In addition, a PBF fiber reference cell is introduced to lock the DFB laser source at 1368.59 nm. With the WMS and reference cell, various disturbances, such as laser power fluctuations, attenuation variations of the sensing element and fiber can be suppressed. We experimentally implement the scheme with a PBF sensing head of 5 cm and a PBF reference cell of 7 cm. High humidity resolution of ~0.2% Relative humidity (RH) over the range from 0 to 95% RH is achieved.