Notice, it's DR4 not FR4 that Rockley "announced" in 2020, but never delivered to the JV (as far as I know).  It requires 4 channels transmit and 4 channels receive (8 fibers). FR4 is a multiplexed standard, 4 channels multiplexed into one fiber transmit and one fiber on the receive, which typically requires a separate demux, usually made of thin-film silica (glass) as a separate component to fit into the module. (Ours is in the waveguide.) In addition, the polarization and thermal issues are extremely difficult to deal with.  We will be demonstrating a 400G FR4 receive only engine at OFC next week, awaiting the completion of a new generation of lasers while we evaluate both a modulated version on the OI (either SiPh or TFLN) or a less costly and lower loss version using 56 Gbaud Directly Modulated Lasers (DMLs, which not so long ago were believed to be impossible to produce, but are now available from certain vendors) on an already proven TX and RX 400G OI platform.  The form factor of the 400G OE is small enough to be able to fit 2 into either of the standard 800G form factors.  And, we have experience with architecting both 2 OEs and 4 OEs into a single module.