The continued acceleration of switching capacity and link transmission bandwidth is driving the need for new connectors in next generation optical networks. With 25.6Tb switch ASICs available in 2020 , only two years after the introduction of 12.8Tb switching, the industry is now looking to radical new architectures to achieve 51.2Tb switching, including the advent of optics integrated or co-packaged with ASIC technology. The standard 250μm pitch used in multi-fiber ferrules with 125μm cladding diameter optical fiber is physically too large to support the quantity of optical lanes that will be coupled inside the coming switching platforms. This paper describes a next generation MT-style ferrule designed for fibers with 80μm cladding diameter on a pitch of 165μm. By decreasing the pitch from 250μm to 165μm, up to 24 fibers can be placed in a single row between the pins of the MT-16 alignment structure. This tighter pitch enables higher fiber densities coupled directly to or in the proximity optical Tx/Rx photonic tiles. Endface geometry models combined with connector mating normal forces are based on the traditional 250μm pitch of MT-style ferrules. Fiber tip radii, fiber tip coplanarity, ferrule surface endface angles relative to alignment pin bores were measured empirically and documented on the new design. Varied topologies were combined with different mating forces demonstrating effective physical contact for the new ferrule. Mated pairs were monitored for attenuation changes during exposure to industry standard uncontrolled environment temperature cycling. Subsequent specifications for future end face geometry and mating spring force requirements are proposed.
Hybrid injection-molded ferrules are presented which consist of a polymer body and an over-molded glass insert. The average coefficient of thermal expansion observed at the front face of the ferrules is 8 ppm/C from room temperature to 100 C. This design could be applicable for direct heterogeneous re-matable connections between fiber ribbons and photonic integrated circuits which exhibit low thermal expansion and operate at elevated temperatures.
We are developing a 1x8 single mode (SM) optical interface to facilitate the adoption of dense wavelength division multiplexing (DWDM) silicon photonic (SiPh) optical interconnects in exascale computing systems. A common method for fiber attachment to SiPh transceivers is ‘pigtailing’- the permanent adhesive bonding of fiber/v-groove arrays to onchip grating couplers (GC). This approach precludes standard high throughput surface mounting and solder reflow assembly of the transceiver onto system printed circuit boards. Our approach replaces the fixed pigtail with a low profile, small form factor, detachable expanded beam optical connector which consists of four essential parts: a GC array, a surface mount glass microlens array chip, an injection molded solder reflowable optical socket, and an injection molded SM light turn ferrule. The optical socket and ferrule are supplied by US Conec Ltd. To design the GC, we developed an optical simulator that considers CMOS foundry constraints in the optimization process. On-wafer measurements of the GC coupling loss to SMF28 fiber at 1310nm is ~1.4dB with a 1dB bandwidth of ~22nm. This ensures a wide low loss spectral window for at least 16 DWDM channels. The geometry of the optical system is arranged so that only a simple spherical lens is required for efficient mode matching in the expanded beam space. The fiber to fiber insertion loss through the light turn ferrule, two microlenses and GCs, and a looped back SOI waveguide ranged from 4.1-6.3dB, with insertion loss repeatability of 0.2dB after multiple mating cycles.
Structured surfaces composed of subwavelength-sized features offer multifunctional properties including antireflective characteristics that are increasingly important for the development of micro-optical components. Here, three-dimensional (3-D) direct laser writing, via two-photon polymerization, is used to fabricate planoconvex spherical microlenses with antireflective structured surfaces. The surfaces are composed of subwavelength-sized conicoid structures, which are arranged fully conformal to the convex surface of the microlenses. The dimensions of the conicoid structures are optimized to effectively reduce Fresnel reflection loss over a wide band in the near-infrared spectral range from 1.4 to 2.2 μm, with a maximum reduction at 1.55 μm. Infrared reflection and transmission measurements are used, in combination with 3-D finite element calculations, to investigate the performance of the microlenses. The experimental results reveal that in the spectral range from 1.4 to 2.2 μm an effective suppression of the Fresnel reflection loss at the convex surface of spherical microlenses can be achieved. The transmittance enhancement is ranging from 1% to 3% for spherical microlenses with antireflective structured surfaces, in comparison to an uncoated reference.
The packaging of photonic devices remains a hindering challenge to the deployment of integrated photonic modules. This is never as true as for silicon photonic modules where the cost efficiency and scalability of chip fabrication in microelectronic production facilities is far ahead of current photonic packaging technology. More often than not, photonic modules are still packaged today with legacy manual processes and high-precision active alignment. Automation of these manual processes can provide gains in yield and scalability. Thus, specialized automated equipment has been developed for photonic packaging, is now commercially available, and is providing an incremental improvement in cost and scalability. However, to bring the cost and scalability of photonic packaging on par with silicon chip fabrication, we feel a more disruptive approach is required. Hence, in recent years, we have developed photonic packaging in standard, highthroughput microelectronic packaging facilities. This approach relies on the concepts already responsible for the attractiveness of silicon photonic chip fabrication: (1) moving complexity from die-level packaging processes to waferlevel planar fabrication, and (2) leveraging the scale of existing microelectronic facilities for photonic fabrication. We have demonstrated such direction with peak coupling performance of 1.3 dB from standard cleaved fiber to chip and 1.1 dB from chip to chip.
The impact of integrated photonics on optical interconnects is currently muted by challenges in photonic packaging and in the dense integration of photonic modules with microelectronic components on printed circuit boards. Single mode optics requires tight alignment tolerance for optical coupling and maintaining this alignment in a cost-efficient package can be challenging during thermal excursions arising from downstream microelectronic assembly processes. In addition, the form factor of typical fiber connectors is incompatible with the dense module integration expected on printed circuit boards. We have implemented novel approaches to interfacing photonic chips to standard optical fibers. These leverage standard high throughput microelectronic assembly tooling and self-alignment techniques resulting in photonic packaging that is scalable in manufacturing volume and in the number of optical IOs per chip. In addition, using dense optical fiber connectors with space-efficient latching of fiber patch cables results in compact module size and efficient board integration, bringing the optics closer to the logic chip to alleviate bandwidth bottlenecks. This packaging direction is also well suited for embedding optics in multi-chip modules, including both photonic and microelectronic chips. We discuss the challenges and rewards in this type of configuration such as thermal management and signal integrity.
The need for additional IO bandwidth for data center device interconnection is well established. Optical interconnects can deliver required bandwidth along with energy and space efficiency at a cost that encourages adoption. To this end, we are developing an optical transceiver incorporating multimode VCSEL emitters in a coarse wavelength division multiplex (CWDM) system capable of transmission at 25Gbps per channel, 100Gbps/fiber, and a maximum aggregate bidirectional data rate of 1.2Tbps. Electrical connection to the transceiver can be made by solder reflow or LGA connector, and optical connection is made by means of a custom optical connector supporting CWDM transmission.
This paper describes the design and performance of next generation, single-mode, multi-fiber, debris insensitive, expanded beam, interconnect components. This low cost, dense optical interconnect technology combined with recent advances next generation, high bandwidth, SM, silicon photonic based Tx/Rx devices is enabling unprecedented bandwidth densities for extended distances at reduced costs. A monolithic, multi-fiber ferule with integrated collimating lenses was designed with the same overall footprint as a traditional MT-type, multi-fiber rectangular ferrule. The new optical ferrule was designed with precision micro holes for alignment to the lens array allowing for future incorporation of multiple rows of fibers into a single ferrule unit. The monolithic, lensed based ferule design enables a low-cost, no-polish fiber termination methodology. The ferrule tested was manufactured with an array of 16 fibers in the footprint associated with traditional, 12 fiber, physical contact MT ferules via use of novel, molded in, end-face alignment features. Multiple optical models were built with ray tracing methodology to predict the insertion loss and return loss with varying refraction index, transmissivity and surface reflection properties of the ferrule. Empirical optical performance results closely match the optical modeling predictions. Insertion losses of <1.5dB were measured along with return loss values <=-30dB. Further analysis was done to characterize the robustness of the new interconnect with regard to debris insensitivity. Do to the nature of the expanded beam, free-space optical design, the impact of debris on the optical mating surface of the interconnect was significantly reduced when compared to traditional, physical contact single-mode interconnects
This paper describes the development, termination and performance of next generation
optical backplane interconnect components. This low cost, dense optical interconnect
technology combined with recent advances in 10G/lane and beyond, miniature imbedded
Tx/Rx devices is driving bandwidth density to unprecedented levels.
A monolithic, multi-fiber ferule with integrated collimating lenses was designed with the
same overall footprint as a traditional MT-type, multi-fiber rectangular ferrule. The new
optical ferrule was designed with precision micro holes for alignment to the lens array
allowing for incorporation of multiple rows of fibers into single ferrule unit. The design
supports up to four rows with as many as 16 fibers per row for a total potential lane count
of up to 64 within in a single ferrule.
A low cost termination is achieved by securing precision-cleaved fiber arrays into the
rear of the ferrule with a quick-cure, index matched, UV light activated epoxy. The
elimination of a polished fiber array greatly reduces the cost and complexity associated
with physical contact based multi-fiber interconnects. With the same overall footprint as
an MT ferrule, the new, lens-based ferrule can be used in conjunction with MPO and
other MT based connectors. However, by eliminating the need for physical contact via
the use of collimated light beams, the connection force per ferrule required is greatly
reduced, paving the way for high ferrule counts and mass insertion of dense optical
Mated pairs of the new ferrule were tested for insertion loss with the substitution method
and all channels were <1dB.