The invention relates to a probe for an implantable medical device. The system includes a housing for holding the magazines with an open region defined in the housing; a spring-loaded follower imparting a force against the magazines to move a first magazine into a dispensing position for removal from the housing through the open region. Consequently, the external connection will not interfere with the pivoting movement nor will it complicate handling and holding of the device. Variations in the dispersion of the transmitted excident light spot for the at least four plane-polarized light beams are calculated, and the fiber orientation is estimated from the variations.