A waveguide Bragg grating is apodized by varying the duty cycle of selected grating periods while fixing the pitch of the grating periods. The device allows for the user or operator’s hands to be coupled with a variety of coupling stiffnesses and in a variety of symmetry modes, leading to enhanced rehabilitation of the impaired arm. The grip housing has a depression button on one side and a threaded notched cover that can be removed for cell replacement on the other. The second polyvinyl acetal resin layer is comprised mainly of a coacetalized product of polyvinyl alcohol with aldehyde having 4-6 carbon atoms and aldehyde having 1-3 carbon atoms. A plurality of filters of varied light transmissivity may be provided from which to select a desired filter based on environmental lighting conditions and user preference.