A tunable optical oscillator with a period based on the speed of light is used to produce a periodic pulsed light signal with low jitter, low drift and high spectral purity. The pulsed light signal is generated through the use of a light source such as a laser, an optical switch, and two variable optical paths acting as delay elements. The variable delay elements form the basis of the system timing. The propagation time of the light signals passing through the delay elements determine the frequency of operation. The output frequency is based on the speed of light through these delay elements. The delay elements can be produced with materials that have a stable index of refraction over a temperature range and therefore the speed of light remains constant over a wide temperature range.