  Tuning fork experiments at the undergraduate level usually only demonstrate a
tuning fork's linear resonance. In this paper, we introduce an experiment that
can be used to measure the nonlinear tuning curve of a regular tuning fork.
Using double-grating Doppler interferometry, we achieve measurement accuracy
within ten microns. With this experiment setup, we observe typical nonlinear
behaviors of the tuning fork such as the softening tuning curve and jump
phenomena. Our experiment is inexpensive and easy to operate. It provides an
integrated experiment for intermediate-level students and a basis for senior
research projects.
