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We report a detailed study of a miniature optical relative humidity (RH) sensor based on a polymer-infiltrated photonic crystal fiber interferometer. The sensor shows a high sensitivity to RH variations with a change in its reflected power of about 12 dB for a humidity change of 84% RH. The sensor has the advantages of a very compact length of 1 mm, and an end-type probe configuration makes it suitable for monitoring humidity in microenvironments. The repeatability, long-term stability, measurement accuracy, and temperature dependence of the sensor are studied in this paper. The observed low thermal sensitivity of the sensor suggests that temperature compensation may not be needed if it is used in normal environments. The response time of the sensor is found to be 400 ms for a change in RH of ~ 30% RH. The fast response time suggests that the sensor can potentially be used as a human breath rate monitor in a clinical situation.