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Abstract
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.
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1. Introduction
Datacenter IP traffic doubles every 2-3 years, driving the need to double the Ethernet switching and connectivity bandwidth with the same cadence. Pluggable optics have scaled and kept pace with the past bandwidth scaling demands. However, over the next 5-10 years, Ethernet switches are expected to support a throughput of 51.2 Tb/s or higher. A paradigm shift is likely needed for switch IOs due to the system power consumption limit and the front panel bandwidth bottleneck. Consequently, co-packaged optics (CPO) was proposed to replace traditional pluggable optics and has since been attracting more attention from academia and the industry [1,2].
The realization of CPO requires multi-dimensional innovations and breakthroughs, from materials, devices, and photonic chips to packaging and integration. One of the critical technologies is a multi-channel reflow-compatible fiber IO. For a switch ASIC with 51.2 T or larger I/O bandwidth, each CPO module needs to provide a bandwidth of 3.2 Tbps or more, considering the co-packaging MCM substrate size limit [3,4]. With the state-of-the-art 100 G optics successfully implemented in the optical transceivers [5], 32 or more optical lanes (input and output) would be needed. Counting the fibers needed for the laser sources that are likely to be off-package, a CPO module needs many I/O fibers. Even with the future 200 G optics, the number will still be on a few 10s. Hence the fiber IO for CPO modules has to support a large port count with high density. In addition, the CPO modules are to be attached to a shared ASIC substrate via solder bumps or other high-density bumps for the benefit of high-speed signal integrity, while the complete ASIC package with CPO modules may be assembled onto a mother-board BGA via solder reflow. The CPO modules may have to undergo one or more solder reflow steps. Therefore, its fiber IO has to be reflow-compatible. Unfortunately, most materials used for conventional fiber coupling and assembly, including fiber itself and epoxies, are not rated for high reflow temperature. Reflow-compatible large port-count low-loss fiber IO remains a challenge for the practical implementation of CPOs.
At the CPO integration scale, silicon photonics (SiP) is the most promising technology choice, as neither the discrete components nor the integrated PIC based on InP could achieve acceptable yields for mass production. However, optical coupling to a Silicon-based PIC chip with low loss in a manufacturable fashion has been challenging due to the large mode mismatch between the silicon waveguide and the fiber. Two coupling approaches are generally used, surface-normal coupling using grating couplers [6,7] and edge coupling using spot size converters (SSC) [8]. Grating couplers (GCs) with a proper taper and grating apodization design can provide output with a mode size matching the regular single-mode fiber (SMF). But they usually have a few drawbacks, including limited bandwidth with center wavelength susceptible to fabrication tolerances, ambient temperature change, and strong polarization dependence. In addition, it's hard to achieve a coupling loss of less than 1 dB even with an advanced 65 nm CMOS process due to the imperfect mode profile of a GC and the diffraction losses. On the other hand, edge couplers (ECs) can produce a more desirable output mode profile and achieve broadband coupling with a low polarization-dependent loss (PDL). However, it needs a fairly long structure plus substrate removal (undercut) to convert a sub-micron silicon waveguide mode to match the mode of an SMF. Edge coupling can indeed achieve a very low coupling loss of about 0.5 dB, but the end surface of a PIC with undercut is very fragile, making butt-coupling of fiber array to such a surface not very manufacturing friendly. Whether it's GC or EC coupling, fibers are commonly attached to a SiP PIC using permanent adhesive bonding, ‘pigtailed’ in other words. Such fiber pig-tailing precludes standard surface mounting and solder reflow assembly of an ASIC package with CPOs onto a system board. S. Mathai et.al [9]. reported a detachable optical fiber coupling interface for a SiP PIC with GCs for surface-normal output, in which a vision-aligned glass lens array is permanently attached to the PIC to collimate the GC outputs as expanded beams while a detachable part bends and focuses the collimated beams into fibers. Without a detachable fiber pigtail, this coupling interface is reflow-compatible, but the demonstrated performance was not ideal, with a loop-back coupling loss as high as 6.3 dB.
In this paper, we developed a low-loss optical coupling interface for SiP PICs and other edge-emitting waveguide devices and chips. It's a detachable coupling interface consisting of a microlens array and a registration connector with pin/pinhole positioning on the edge-emitting PIC or device side for waveguide output collimation and output beam registration, respectively, as the permanent part and a fiber collimator array with a mating connector on the fiber side as the removable part. An effective alignment scheme was developed and implemented in an assembly station for building the removable fiber array connectors. The permanent piece was assembled in three steps: 1) passively attaching a lens array to the sub-mount of the PIC/device using a pick-and-place machine; 2) mating a registration connector with a pre-made fiber collimator array; 3) aligning the collimated PIC/device outputs with the fiber collimator array and finally bonding the registration connector in situ onto the sub-mount of the PIC/device. Both the lens array and the registration connector are attached to a common substrate as the PIC/device using adhesives or bonding materials that can survive the solder-reflow process. Such coupling interfaces can work for lasers, SiP PIC chips, and even fiber arrays to achieve re-mateable optical coupling with high efficiency. Promising results were obtained from the proof-of-concept demonstrations. The remaining paper proceeds as follows: Section 2 provides a detailed description of this single-mode edge coupling interface, including the optical design, mechanical fixtures, and assembly processes. Section 3 presents the experimental setup and results. And finally, the work is summarized in Section 4.
2. Design
2.1 Overview of the detachable optical coupling interface
There are a couple of challenging problems to solve to achieve low loss coupling between waveguides on a PIC chip and fibers, mode mismatch, and alignment between the PIC waveguides and fibers. Especially for the case of SiP PIC, the mode size of the sub-micron silicon waveguide is almost a couple of orders of magnitude smaller than that of an SMF with a mode field diameter (MFD) of about 10 µm. Various SSCs were developed on the PIC side to tape the waveguide modes to SMF mode size or halfway to, e.g., 3 µm while having a large numerical aperture (NA) fiber fused on the SMF as an adaptor. Since the Rayleigh range of a 10 µm or smaller mode is very short, butt-coupling with sub-micron alignment accuracy is required to achieve high coupling efficiency. As neither the fiber nor the V-groove-based fiber array device is rated for the high reflow temperature, a solder reflow compatible coupling interface would require the fiber end to be detachable or attached after the reflow process. For a transceiver module with a single OE engine, attaching a fiber/fiber array as the last assembly step could be a reasonable solution but hardly acceptable for an ASIC package with multiple CPO modules integrated. A detachable fiber coupling interface is, therefore, necessary. However, a detachable fiber coupling interface with sub-micron position accuracy is hard to accomplish and could be too expensive for practical implementation. The solution is to use expanded beam coupling. As shown in Fig. 1, the small mode field of the waveguides at the two ends, the PIC and the fiber array, is collimated to a large mode-size beam by a microlens or microlens array. And the two ends are registered together using a pair of MT-like kinematic registration connectors, one on the PIC side along with a microlens/microlens array attached permanently to a common substrate using reflow-compatible bonding materials, and the other registered and fixed with the fiber collimator array as the removable part. As explained in detail in the following section, expanded beam coupling can relax the alignment tolerances dramatically and achieve a consistent remating coupling performance with a conventional fiber connector registration structure.
2.2 Optical modeling for expanded beam coupling
The schematic of the optical path from a PIC waveguide coupling to an optical fiber using collimating lenses is depicted in Fig. 2. Both the PIC waveguide and the fiber mode can be approximated using Gaussian mode with corresponding waist radius of ωT0 and ωR0. The lens focal length for the PIC and fiber sides are fT and fR, respectively. A Gaussian beam with a waist size of ω0 remains a Gaussian mode when propagating through a lens with a focal length of f. For a distance of l from the input Gaussian beam waist to the lens, the new waist location S and the waist size of the Gaussian output beam can be calculated using the following formulas [10,11],
Assuming a SiP PIC waveguide mode size of 1.5 µm in radius with the aid of an SSC, and a fiber mode radius of 5 µm at a wavelength of 1.3 µm, collimating lenses with a focal length of 0.35 mm and 1.0 mm are selected, respectively. Figure 3 plots the lens transform of the PIC waveguide mode (red lines) and the fiber mode (blue lines). The distances from the input modes to the lenses are normalized by the corresponding lens focal length. As the plot indicates, by controlling the mode distance to the lens, the PIC waveguide and fiber can have size-matched lens collimated output modes. For example, with a distance lT of 0.354 mm and an lR of 1.028 mm, the collimated PIC waveguide and fiber mode would have the same size of about 75 µm with a waist location of 11 mm and 7.3 mm, respectively.
Fig. 3. Lens transform of a PIC waveguide mode with a mode size of 1.5 µm (red lines) and a fiber mode with a mode radius of 5 µm (blue lines).
As discussed in [12–14], the optical coupling loss (in dB) of the PIC to the fiber for the system shown in Fig. 2 can be calculated using the overlapping integral of the Gaussian modes, ωT, and ωR, which can be further simplified to the following generalized formula [15]
Figure 4 plots the coupling loss of three different mode sizes, ωT=ωR = 1.5 µm, 5 µm, and 75 µm, for different types of mismatches between the two modes. As depicted in Fig. 4(a) and Fig. 4(c), when the mode size is small, e.g., 1.5 µm and 5 µm, the coupling is sensitive to both lateral offset and waist separation between the two modes. Sub-micron lateral alignment and waist separation of less than a few microns are needed to achieve low-loss coupling, which typically requires active alignment. However, if the mode size is increased, e.g., to 75 µm, the lateral alignment and waist separation tolerances can be relaxed significantly to a couple of 10’s of microns and a few millimeters, respectively.
Fig. 4. Calculated Gaussian modes coupling loss for different types of mismatches, (a) lateral misalignment, (b) angular misalignment, (c) waist separation, (d) mode size mismatch.
The opposite is true of angular alignment, as shown in Fig. 4(b). Small modes can tolerate an angular misalignment of up to a few degrees, while large modes become much more sensitive. For a 75 µm mode, the angular misalignment has to be less than 0.1 degrees to avoid significant coupling loss. The sensitivity of the coupling to the mode size mismatch is identical for different nominal mode sizes, as plotted in Fig. 4(d). A mode size mismatch of up to 30% is tolerable without a severe coupling loss penalty.
In summary, the expanded beam coupling of the PIC waveguide to fiber using lenses results in substantially relaxed translational alignment tolerances. With a lateral position tolerance of a couple of 10’s of microns, it becomes relatively easy to use a connector structure, as shown in Fig. 1, to make a detachable coupling interface where pins and pinholes are used to maintain the alignment for remating. While the angular alignment tolerance does become tighter for expanded modes, it can be managed seamlessly using simple techniques as described in the following section.
2.3 Connector structure and assembly
As shown in Fig. 1, the detachable coupling interface consists of two assemblies, an expanded-beam fiber connector (EBFAC) and an expanded-beam PIC connector (EBPC). As discussed earlier, the lateral position and propagation angle of the expanded beams from the two assemblies must be aligned within acceptable tolerances for a low-loss PIC to fiber coupling. It is accomplished by an MT-like connector structure using the following apparatus. As depicted in the alignment system schematic shown in Fig. 5(a), the registration connector has precision pin/pinholes for position registration and a flat front surface A as the angular fiducial. A hole opens in the connector's middle to allow optical beams to pass through. And a recess is created to house a microlens array. A reference connector is placed before a simple beam detection sensor having an imaging sensor installed at the focal plane of a lens. A reference beam normal to surface A calibrates the beam detection sensor. Its beam location on the imaging sensor is set as the origin representing the beam direction normal to the connector front surface A.
Fig. 5. Collimated fiber array connector: (a) Alignment system schematic; (b) Assembly conceptual view.
When making an EBFAC, a microlens array is first deposited into the recess of a registration connector by a precision die bonder using the pinholes on the connector as the position reference. A glass spacer is then attached to the back of the lens array to control the distance from the fiber to the lens. Next, this subassembly is mounted onto the alignment system by mating with the reference connector, as depicted in Fig. 5(b). And then, a 6-axis manipulator is used to move a fiber array to have its end surface butt-coupled with the glass spacer. Finally, the 6-axis manipulator adjusts the fiber array's lateral position and in-plane rotation until the output beams of the fiber array have their spots on the IR image sensor all moved to the origin. Once the alignment is done, the fiber array is epoxied to the glass spacer. The alignment station releases the complete assembly by detaching its connector from the reference connector. Since the lens array is registered with the pinholes on the connector via the precision die bonding process, the lateral position of the expanded beams is expected to be within a few microns. Hence, both accurate beam position and propagation angle can be achieved for an EBFAC.
For ease of manufacturing, a variation on the above procedure is used to construct the EBPC. As shown in Fig. 6, a common substrate is used to carry the PIC, lens array, and MT-like connector. It has a pre-made surface topology based on the PIC thickness and the lens array dimensions to have coarse alignment for the PIC waveguides and the lenses in height. The area for the connector attachment is also recessed to allow enough clearance for alignment. The PIC is first attached to the substrate. The lens array is then deposited onto the same substrate using a precision die bonder with accurate lateral position and distance relative to the PIC waveguides within a few microns. However, a relatively large offset may exist in the vertical direction because the lens array is not actively controlled vertically. There could be a vertical offset of up to 20 µm counting the PIC substrate thickness tolerances of ±10 µm and the lens array dicing tolerance of ±10 µm, which could result in a beam angle offset of up to a few degrees. Hence, active alignment is used to register the beams with a registration connector. The registration connector is mated with an EBFAC first and manipulated by a 6-axis alignment stage to have the output beams of the PIC waveguides aligned with the fibers. Once aligned, the registration connector is bonded in situ onto the substrate using epoxy, solder, or other means. And finally, a complete EBPC is obtained by de-mating the connector from the EBFAC. The EBFAC is a fiducial reference unit to locate the expanded PIC waveguide modes. And the mating registration connector locks the alignment position. As all EBFACs are made with beam location registered to its connector pin/pinholes and beam angle registered to the connector front surface, they are expected to be interchangeable when mating with an EBPC.
3. Experimental results
As shown in Fig. 7, an alignment station was built experimentally for making EBFACs. Standard MT ferrules, with the center portion drilled away for passing the beams and a recess machined on the back end, were used as registration connectors, as described in the previous section. One modified MT ferrule was used on the alignment station as a reference connector. The beam detection sensor was implemented using a lens with an aperture of 50 mm and a focal length of 200 mm, and an infrared imaging sensor with a pixel size of 15 µm placed on the lens focal plane. The output of the imaging sensor is sent simultaneously to a monitor for visual display and a video acquisition card for image processing. Aided by a spot detection algorithm, a laser beam focused onto the image sensor can be identified with its central location measured within a couple of pixels, resulting in a beam angle resolution of better than 1 mrad for the bean detection sensor.
3.1 Detachable expanded beam connector for PIC-to-fiber coupling
An O-band edge coupler test structure on a SiP chip we recently fabricated using AMF's Active 200 mm platform was used to demonstrate the detachable fiber coupling interface. As shown in Fig. 8(a), the PIC test structure has two waveguides spaced at 1.5 mm, each has an SSC on one end for edge coupling, and the other is connected to a shared GC through a multimode interferometer (MMI) splitter for light input. The SSC converts the sub-micron silicon waveguide mode to a 3 µm mode, while the GC is designed with a mode size matching the SMF.
Fig. 8. (a) Schematic of a SiP PIC for coupling demonstration; (b) A picture of a PIC assembly before connector attachment.
Using the setup shown in Fig. 7, we first built the EBFAC with an 8-fiber V-grooved array with a pitch of 250 µm. While in this experiment, we used an off-the-shelf silicon lens array with a pitch of 750 µm, a focal length of 1.6 mm, and a substrate thickness of 1 mm, in the future, a custom-made lens array can be produced to match the 250 µm pitch. A glass spacer with a thickness of 2.1 mm was chosen for a collimated beam waist size of about 80 µm in diameter. Following the alignment and assembly procedures described in the previous section, three EBFACs were made using manual lens array placement. Figure 9(a) shows a picture of one complete connector assembly.
Fig. 9. Pictures of the assembled connectors: (a) an 8-port expanded beam fiber array connector; (b) a detachable expanded beam PIC-to-fiber coupling demo
To build an EBPC, the PIC was attached to a common carrier substrate, and a V-grooved fiber was aligned with the GC and attached to the PIC for light input, as shown in Fig. 8(b). A precision bonder was used to mount a silicon lens array with an accurate lateral alignment with the PIC waveguides and a distance of 0.39 mm to the PIC edge. The off-the-shelf lens array has a pitch of 750 µm and a focal length of 0.49 mm, resulting in a collimated output with a waist size of about 80 µm. One of the EBFACs mated with a registration connector was then used to align with the two output beams from the PIC until a maximum optical power was received in fiber ports 1 (red) and 7(blue). The registration connector was finally fixed onto the common substrate using epoxy. The picture in Fig. 9(b) shows the complete detachable expanded beam PIC to fiber coupling demo. A good coupling efficiency of better than 85%, which is measured from a waveguide on the PIC to the fiber after the EBFAC connector, was achieved for both channels, as shown in Fig. 10. Both channels have a similar alignment tolerance trend, and a 1 dB lateral alignment tolerance of about ±35 µm was observed. Similar performances were obtained by swapping different EBFACs to connect with the same EBPC, as summarized in Table 1. The offset represents the deviation of the lens center position relative to the first EBFAC measured by a KEYENCE VHX-7000 microscope using the MT pinholes as the reference fiducial.
Table 1. Swapping test results of different EBFACs for detachable PIC coupling
As the results indicate, the EBFACs are indeed interchangeable. Even with a lens array lateral offset of up to 35 µm, the PIC to fiber coupling loss variation is less than 0.3 dB, which suggests that relatively low precision placement equipment can be used for volume production of EBFACs in the future.
3.2 Detachable expanded beam connector for laser-to-fiber coupling
The same coupling scheme was also tried for a detachable coupling from a laser diode (LD) to a fiber. Single-channel EBFACs were made using the same lens design. And an expanded beam laser connector (EBLC) was built using the same assembly procedures. The singlet silicon lens used in this case has a focal length of 0.5 mm and a substrate thickness of 0.5 mm to collimate the LD output with a far-field divergence angle of 26 ° [16]. The distance from the lens to the LD was 0.36 mm to achieve a collimated output beam with a waist size of about 80 µm.
A picture of the complete coupling assembly is shown in Fig. 11(a). A coupling efficiency of about 90% from the output facet of LD chip to the fiber after the EBFAC connector is obtained. Before the registration connector was epoxied to the metal-based common substrate, the EBFCA was moved around for tolerance characterization. A 1 dB lateral alignment tolerance of about ±35 µm was measured, as shown in Fig. 11(b). The difference in the horizontal (X) and vertical (Y) axis is due to the different divergence angles of the LD emission.
The repeatability of the coupling was tested by detaching and attaching the EBFAC from/to the EBLC. The insertion loss variation measured over 20 times was within 5%, indicating the MT connectors maintained an accurate position and angle alignment for remating.
A swapping test similar to the PIC coupling was also performed using three different EBFACs. The test results are summarized in Table 2. Again, one EBFAC is used as the reference. The lens offset of the other two EBFACs was measured using the MT pinholes as the fiducial, as shown in Fig. 12. Even for a relative offset of 35 µm, the coupling loss penalty is less than 0.6 dB. Interchangeability is achieved even with relatively low-precision lens placement equipment.
Fig. 11. Detachable expanded beam connector for a laser to fiber coupling: (a) a picture of a complete connector assembly (b) Measured lateral alignment tolerances.
Fig. 12. Microscope images of EBFAC #1, #2, and #3 for lens offset measurement using the MT guide hole as the fiducial. The design values for the two axial directions are 0 mm and 1.925 mm, respectively.
Table 2. Swapping test results of different EBFACs for detachable LD coupling
4. Summary
A detachable connector interface based on expanded beam coupling for edge-emitting/coupling photonic devices and components was successfully demonstrated. Microlens/lens arrays with different designs were used for waveguide mode expansion, and MT-like registration connectors were used to lock the alignment for remating. This interface supports multiple parallel ports and works for low-loss optical coupling of SiP PIC, III/V lasers, and fiber arrays. A simple yet effective alignment method and assembly setup based on beam detection were developed for building interchangeable expanded beam connectors. Promising results were obtained from the experimental demonstrations of both SiP PIC and LD to fiber coupling using off-the-shelf lenses and modified MT connectors. In both cases, less than 1 dB coupling loss was achieved. And the fiber array connectors made separately were proven to be interchangeable even with relatively large lens offsets due to manual placement.
The modified MT ferrules used as the registration connectors in this work for quick proof-of-concept demonstration are not rated for solder reflow temperature. However, since the position tolerances are relatively loose, as demonstrated, we have designed a ceramic registration connector with laser-drilled pinholes for the EBPCs to validate its solder-reflow compatibility in future work.
With low coupling loss, the capacity to support multiple parallel ports in high density, and the potential to be fully solder-reflow compatible, the detachable coupling interface developed is very promising as the optical I/O solution to the CPO applications.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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