Detachable interface toward a low-loss reflow-compatible fiber coupling for co-packaged optics (CPO)

High-density reflow-compatible fiber I/O is one of the challenges for co-packaged optics (CPO). This paper developed a detachable coupling interface based on expanded beam edge coupling, which can be applied for optical coupling between lasers, PICs, and fibers, seamlessly supporting many channels with high efficiency. It comprises a removable fiber connector and a permanent chip/device connector, in which microlens/lens arrays are used for waveguide mode expansion and MT-like connectors are used for position registration. An effective alignment scheme based on beam detection was developed and implemented in an assembly station for building the removable fiber connectors, while the permanent chip/device connector was assembled by active alignment to a pre-made fiber connector mated with a registration connector. Promising results were obtained from the proof-of-concept demonstrations of the coupling from SiP PIC and III/V lasers to fibers using the off-the-shelf lenses and modified MT registration connectors. In both cases, less than 1 dB coupling loss was achieved with an expanded beam size of 160 µm in diameter. Even with a relatively large lens offset of ∼35 µm, the detachable fiber array connectors showed good interchangeability. Such a coupling interface is expected to be solder-reflow compatible by replacing the plastic registration connectors with ceramic ones, making it a promising candidate for the solution to CPO fiber I/O.