We demonstrate a compact silicon nitride interferometer which uses waveguides with the same length and different effective indices instead of similar effective indices and different lengths. In such structures there is no need to have waveguide bends. This not only reduces losses but also results in an order of magnitude smaller footprint and thus enables much higher integration densities. We also study the tunability of this interferometer using thermo-optical effects induced by a simple aluminum heater and show that thermal tuning can compensate for the effects of fabrication variations on the spectral response. The application of the proposed design in a tunable mirror is also briefly discussed.