The loop back waveguide structures 2598 in the embodiments allow for testing of the electrical and optical operation of the assembly without the need to fully complete the fabrication of the complete submount assembly, and without the need to connect the assembly to an external fiber. Connection and testing with the externally mounted optical fiber connections can be time consuming and costly, raising the costs associated with production and testing of the assemblies 2510. In the event that device assemblies are found to yield test data that are outside of a desired test range, further processing of the assembly can be discontinued, for example, in some embodiments to avoid expensive processing of device assemblies that would not be expected to meet targeted product operational specifications.

      In the drawing shown in FIG. 25 of an embodiment of the inventive ODI that includes a loop back waveguide feature 2598 for wafer level testing, the section of the wafer on which the loop back test feature resides can be discarded after singulation of the die. This sacrificial area of the die lies in a section of a wafer or substrate between two adjacent submount die for this embodiment. In other embodiments, the section of the substrate or die with the loop back waveguide is not discarded after fabrication. The die separation cut lines for an embodiment are shown in FIG. 25. In other embodiments, the loop back waveguide structures are part of the sub-mount assembly and remain part of the assembly upon completion of the sub-mount assembly fabrication. In FIG. 26, approximate die sizes for the submount assembly 2510 are shown within the dotted lines for an embodiment in which the sacrificial regions are singulated and for which these singulated parts can be discarded after testing and separation. Elimination of the loop back structures after fabrication and testing can assist in minimizing substrate area for the final assemblies 2510. Optical devices 2556 formed with waveguides 2257 and connected to optoelectronic devices 2554 are also shown on the sub-mounts or interposer assembly 2510 in FIG. 26.

      The loop back waveguide structure 2598 in the inventive ODI allows light to travel from a light sending device 2554, such as a laser, mounted on the submount assembly 2510, outside the submount package in some embodiments, and back into a receiving device 2554, such as a photodiode so that measures of functionality can be ascertained from the assembly at an intermediate stage of the production process. In an embodiment using the simple structure shown in FIG. 26, the loop back structure 2598 resides on a sacrificial section of the wafer that will be separated from the submount assembly upon completion of the production process.