The present invention relates to an electromagnetic ultrasonic transducer for generating horizontally polarized transverse waves. In an embodiment, the distance from the center of the rotating shaft to an outer surface of the first claws making contact with the clamping region of the disk is longer than the distance from the center of the rotating shaft to an outer surface of the second claw making contact with the clamping region of the disk. This self-referencing optical sensor enables an optical interrogation system to detect a bio-chemical interaction independent of the effect of temperature by measuring a reference signal associated with the lower waveguide grating structure and measuring a sensing signal associated with the upper waveguide grating structure. The array is coupled to a panel.