Mach-Zehnder interferometers comprise heater elements configured to have projections in the plane of optical waveguides positioned such that two adjacent sections of one optical waveguide arms are heated by a common heater element. The heater and at least a substantial section of the heated waveguide segments can be curved. Configurations of an optical waveguide arm can comprise an outer curved heated section, an inner curved heated section, and a loopback waveguide section connecting the outer curved heated section and the inner curved heated section, with average radius of curvature selected to form an open accessible space. Appropriate configurations of the two optical waveguide arms provide for nested configurations of the arms that provide for a compact structure for the interferometer.