Integrated optics evanescent wave sensors suffer from attenuation losses and reduced contrast ratio when the sensing layer index is too close to the waveguide index. This is attributed to an increase in mode mismatch at inner interfaces of the devices. By properly balancing the losses on each arm of an integrated Mach–Zehnder the contrast ratio is increased to nearly 100% on the entire index range, furthermore, a new technique used to make a smooth transition between sensing and nonsensing waveguides reduced the losses by 5 dB. ©1995 American Institute of Physics.