Passively aligned fiber-optic transmitter integrated into LTCC module

We studied the possibilities of LTCC (low temperature cofired ceramics) technology to fabricate transmitter arrays equipped with vertical-mounted multimode fiber pigtails. The developed LTCC module can be mounted vertically on a printed-circuit-board (PCB), thus providing small, essentially one-dimensional PCB footprint. The fiber is aligned and supported using a hole structure through the layers, and the surface-emitting source, such as VCSEL, is flip-chipped on the other side of the substrate. Thus, this kind of module can be used as a detachable electrical interface between the fiber-optic and electronic media. To evaluate the feasibility of the system, a 5-channel transmitter based on a 4-layer LTCC substrate was designed and realized, and mounted vertically on a test board. Each transmitter, sized 5 × 5 mm², included VCSEL and laser-driver chips as well as discrete passives. 62.5/125-micron fibers were used with metallic tubes as strain relieves. Before implementation, the optical alignment tolerances were examined by measurements and simulations, and the tolerances of fiber-mounting holes were evaluated by preparing test structures. The fiber alignment accuracy seemed adequate even for the first transmitter prototypes. Nevertheless, stringent requirements for the LTCC process are necessary to achieve the needed accuracy.