A system for decoding a signal of a rotating portion of a machine. A second cassette frame unit is interchangeable with the first cassette frame unit and contains a plurality of second levelling rollers with juxtaposed axes of rotation spaced by a second pitch substantially greater than the second diameter of the second rollers. The slider has a front end face and a rear end face, in which a readwrite element is formed. The apparatus provides low cost, high power, optically pumped edge emitting and surface emitting lasers which can be operated at elevated temperatures or power levels.