An interposer (support substrate) for an opto-electronic assembly is formed to include a thermally-isolated region where temperature-sensitive devices (such as, for example, laser diodes) may be positioned and operate independent of temperature fluctuations in other areas of the assembly. The thermal isolation is achieved by forming a boundary of dielectric material through the thickness of the interposer, the periphery of the dielectric defining the boundary between the thermally isolated region and the remainder of the assembly. A thermo-electric cooler can be used in conjunction with the temperature-sensitive device(s) to stabilize the operation of these devices.