Construction and characterization of a universally tunable modulator

Abstract

Arbitrary admixtures of amplitude and phase modulated light can be used to generate linear, null-crossing error signals for locking Fabry-Perot cavities that are detuned from resonance by arbitrary amounts. Unfortunately, no commercially available device is capable of producing the desired arbitrary combinations of amplitude and phase modulation. This work pertains to construction and characterization of a Universally Tunable Modulator (UTM), capable of producing just such combinations. The UTM was prototyped by modifying a New Focus 4104 amplitude modulator. With the application of feedback control of fabry-perot cavities in mind, the UTM was characterized using a test cavity to measure error signals. The resulting error signals were then fit to the calculated error signals to give the fractions of amplitude and phase modulation. It was shown that it is possible to produce nearly pure amplitude and phase modulation, as well as intermediate modulation states, using the UTM. The resulting error signals also indicate the UTM's suitability for both on- and off-resonance locking of Fabry-Perot cavities.