Versatile, high-speed force transducer using a laser diode beam as an optical lever

A force transducer with variable sensitivity and speed is described. Its mo ving element is a cantilever beam that projects vertically into a muscle ba th. A brace constrains bending of the beam to a short, proximal "hinge." Ro tation of the beam about the hinge is amplified 30-fold bg an optical lever consisting of a laser diode beam reflected from a mirror on the cantilever to a photodiode pair. This design places the electrical components at a di stance from the damp environment of the muscle bath. Large changes in sensi tivity and speed can be obtained by substituting different cantilevers. Sma ller changes can be made by varying the length of the hinge. A transducer w ith a 6-mm cantilever optimized for the study of single, skinned skeletal m uscle fibers is described in detail. This device had a resonant frequency o f 22 kHz and sensitivity such that the total root-mean-square noise in the circuit was more than 500-fold smaller than the expected maximum force. Var iations of this device with orders of magnitude different sensitivities are also described.