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Abstract
We have successfully demonstrated a hybrid-integrated 400-Gb/s (4 × 100 Gb/s) CWDM4 PAM-4 receiver optical sub-assembly (ROSA) with a bandwidth-improved multilayer evaluation board. The proposed ROSA offers packaging simplification through passive optical alignment assembly of main components. In addition, we have proposed a structure to mitigate the bandwidth limitation issue caused by the typical edge connector mounting on the multilayer board, when needed bandwidth exceeds ∼20 GHz. With the bandwidth-improved multilayer board, the 3-dB bandwidth of the ROSA was observed to be >35.7 GHz and its receiver sensitivity was successfully measured to be <−10 dBm at FEC limit, bit error rate of 2.4e-4, for all channels.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In order to accommodate the ever-increasing data traffic, Ethernet standards including IEEE 802.3ba and 802.cd for 400G, 200G, 100G and 50G [1,2] and the industry consortiums including 100G Lambda Multi-Source Agreement (MSA) for 100G and 400G [3], Open Eye MSA for 50G/100G/200G/400G [4], and QSFP-DD800 MSA for 800G [5], have been very active to commercialize the related products. The strategies to reduce the manufacturing cost of optical module, while increasing transmission speed, are as follows: (1) adoption of a higher-order modulation scheme (i.e. PAM-4 modulation transmitting 2 bits per symbol) reduces the cost since relatively low frequency bandwidth is required and fewer devices are needed due to the decrease in the number of lanes [6–8]; (2) change in optical device implementation technology from III-V compound-base to silicon photonics-base allows room for accelerating miniaturization, packaging simplification, and mass production [9–11]. Hence, various silicon photonics technologies for optical components including modulators, WDM filters, photodetectors and polarization elements have been developed in recent years [12–14]. These silicon-based optical devices offer the aforementioned advantages, but technical issues such as insertion loss, temperature dependence, polarization dependence, and optical fiber coupling remain challenges for commercialization.
Practically, the performance of high-speed optical module could be verified with an evaluation board, which usually adopts multilayers to enable wiring for high-speed data signals, power supplies, and control/management functions. It should be noted that if the RF connector for high-speed signal is connected in a conventional way to the edge side of the multilayer evaluation board, the operating frequency bandwidth in the transmission characteristic becomes limited due to severe dip caused by the distortion of the return current at the connector connection [15].
We describe the implementation of 400 Gb/s CWDM4 receiver optical sub-assembly (ROSA) module with PAM-4 modulation scheme using commercial off-the-shelf (COTS) optical and electronic devices. The ROSA was verified through a low-cost and small-edge connector-based evaluation board that guarantees the signal integrity by incorporating the proposed connector launching structure.
In this paper, the design focus and fabrication process of the proposed ROSA are described in Section 2. The ROSA packaging process was greatly simplified by adopting passive alignment for some optical components. In addition, the electrical elements were arranged to have a straight high-speed signal flow. In Section 3, we investigate the return-current problem in the multilayer board with edge-mounted RF connectors and propose a new approach to mitigate the problem. The proposed connector launching structure has significantly improved the signal integrity of the evaluation board that is required to verify the stand-alone performance of the ROSA. In Section 4, with the newly-designed evaluation board, we have successfully measured the optical-to-electrical (O-E) response and observed the bit error rate (BER) performance of the ROSA. Summaries are followed in Section 5.
2. Implementation of 400 Gb/s CWDM4 ROSA
2.1 Design of ROSA
Figure 1 shows the block diagram and conceptual design of the proposed 400-Gb/s (4 × 100 Gb/s) CWDM4 ROSA [16]. It converts the multiplexed 4-channel optical PAM-4 signal into four electrical PAM-4 signals. It consists of an optical demultiplexer (ODMUX), 45° mirror plane (prism), four p-i-n photodiodes (PDs), and a 4-channel linear transimpedance amplifier (TIA). The ODMUX has zig-zag scheme [17,18] with thin-film filter operating at CWDM4 wavelengths (1271 nm, 1291 nm, 1311 nm, and 1331 nm). After the ODMUX, the optically separated signals are reflected 90° upward by the mirror plane (prism) and then coupled onto PD chips through an array of focusing lenses. The PD chips are precisely flip-chip bonded onto a PD carrier having through holes for the propagation of the optical signals. The PD chips and the TIA were mounted on different fixtures and electrically connected by wire bonding. The electrical signals are finally passing through the module package for being connected to the subsequent block. The main optical components, a prism, focusing lens, and four PD chips, are passively assembled. Furthermore, instead of bending the electrical signal path [17], the main electrical components that make up the ROSA’s high-speed signal path (PD-TIA-package) are horizontally placed in order to ensure better signal quality and the height difference was compensated by bending the optical signal path by 90° upward.
2.2 Fabrication of ROSA
Figure 2 shows the design of the main components and an inside view of the assembled ROSA module. The PD carrier to where the PD chips were bonded is shown in Fig. 2(a). The PD carrier was made of silicon, and through holes were formed to offer a path for the optical signal incident on the PD. The diameter of the formed holes was 0.2 mm and the distance between the centers of the holes was 0.75 mm. The solder (AuSn) was formed on top of the PD carrier for the flip-chip bonding of the PD chips. The thickness of the PD carrier was designed to be 0.45 mm, when added to epoxy thickness (∼20 μm), to meet the focal length of the lens, 0.47 mm, as shown in Fig. 2(b). In addition, dummy holes with a diameter of 0.28 mm were formed on both ends of the PD carrier for the passive alignment of the focusing lens and the PD chips. The size of the fabricated PD carrier was 4 mm × 1 mm × 0.45 mm. The displacement of the PD chips bonded onto the PD carrier was measured by the precision measuring system: 0.75 mm ± 2 μm at channel pitch and ±2.5-μm deviation from the central baseline (pink line) shown in Fig. 2(c). These bonding errors can be tolerated in the passive alignment assembly of main optical components [17,18].
Fig. 2. Parts of the ROSA: (a) PD carrier, (b) assembled prism and focusing lens and (c) inside view of the assembled ROSA.
Figure 2(b) shows prism, its support, and focusing lens (lens array) on the base substrate. The prism is to reflect the optical path 90° upward for the optical signal output from the ODMUX shown in Fig. 1. The mirror plane of the prism was formed by gold coating. The height and width of the prism was 0.82 mm × 0.82 mm. The support was placed on both ends of the prism and the focusing lens was attached on it. The focal length of the focusing lens was 0.47 mm and pitch was 0.75 mm. The effective diameter of the focusing lens was designed to accommodate collimated light beam with diameter up to 0.42 mm. Dummy lenses with diameter of 0.25 mm were placed on both ends of the lens array for the passive alignment with the PD chips.
Figure 2(c) shows the assembled ROSA. At 1.3-μm wavelength, the 3-dB bandwidth and responsivity of the COTS PD (Albis’ PD40X1) were >35 GHz and ∼0.7 A/W, respectively. The PD chip was a back-illuminated type and it was integrated with a 100-μm diameter spherical lens at its backside. The COTS linear TIA (Semtech’s GN1810) had 3-dB bandwidth of >35 GHz and it was interfaced with I2C communication that controls the operation mode (gain control mode) and monitors the photocurrent coming from the PD chips. The TIA and PD chips were electrically connected by gold wire bonding with minimum distance allowed. The PD chips bonded onto the PD carrier were underfilled to withstand external mechanical shock such as wire bonding process. The epoxy used for underfilling was EPO-TEK OG142-95, which is optically transparent and has a refractive index of ∼1.51. The close-up view inserted in Fig. 2(c) shows the alignment status of the focusing lens and the PD chips. The deviation between the center of the dummy hole and the center of the focusing lens was measured to be 6.7 μm on the x-axis direction and 4.2 μm on the y-axis direction. Single-layer capacitors (SLC1 and SLC2) were decoupling capacitors for the cathode bias of the PD chips and the power supply of the TIA, respectively.
Figure 3 shows the measured optical coupling efficiencies along horizontal (x-axis) and vertical (y-axis) displacement and tilting of the LC receptacle to the ODMUX. We have proposed the design of the LC receptacle with collimator [19] and its performance has been verified in previous works [8,17]. Figure 3(a) plots the coupling efficiency as a function of the displacement of LC receptacle in the x- and y-axis directions. During the measurement, there was no displacement in the direction of the z-axis (beam propagation direction). As aforementioned, the focusing lens (in the lens array) had an effective diameter of 0.42 mm and the PD was integrated with a spherical lens of 100-μm diameter. Alignment tolerances at 90% of peak coupling efficiency for each channel along x-axis displacement were as follows: +42 μm/−40 μm for Lane 0 (L0), +43 μm/−39 μm for Lane 1 (L1), +40 μm/−30 μm for Lane 2 (L2), and +37 μm/−34 μm for Lane 3 (L3). Likewise, alignment tolerances for y-axis displacement were as follows: +80 μm/−60 μm for L0, +67 μm/−60 μm for L1, +81 μm/−61 μm for L2, and +77 μm/−40 μm for L3. The displacement tolerance anisotropy between the x- and y-axes directions may be partly contributed by offsets (xoffset = 6.8 μm and yoffset = 4.2 μm shown in Fig. 2(c)).
Fig. 3. Optical alignment tolerances: (a) as x- and y-axis displacement and (b) as tilted angles of x and y axes.
Figure 3(b) shows the optical coupling efficiency as a function of the tilted angles in the direction of the x- and y-axis. Alignment tolerances at 90% of peak coupling efficiency on the tiled angle of the x- and y-axis were +2.6°/−1.2° and +4.3°/−3.8°, respectively. The asymmetry could also be attributed to the misalignment between the focusing lens and the PD chips.
It can be said that the alignment tolerance was large enough for normal passive alignment procedure [17,18], not to mention more advanced assembly process (e.g. precision pick & place equipment).
After the passive alignment process, to assemble the LC receptacle and the ODMUX, an active alignment process was adopted in such a way that the photocurrent of each channel of the TIA was monitored while aligning them.
Figure 4(a) shows the fabricated ROSA module, whose dimension (L × W × H) was 11.65 mm × 6.6 mm × 5.4 mm. It is suitable for the QSFP-DD form factor. In Fig. 4(b), the module’s coupling efficiency was obtained by normalizing the monitored photocurrent from each channel of the TIA with the responsivity (∼0.7 A/W) of the PDs. The optical coupling efficiency of the ROSA was >55% for all channels. It indicates that the insertion loss of the ROSA was <2.6 dB for all channels. The gray-colored region indicates the passband of each lane specified in 400G-FR4 [3]. The shift of center wavelength in each channel and the ripple in flat-top passband were negligible. The adjacent channel isolations were <−25 dB.
3. Implementation of the multilayer evaluation board
Figure 5(a) shows a typical 25-mm microstrip line on the multilayer test board connecting edge-mount GPPO connectors. The edge-mount GPPO (Molex’s 733000020) is a low-cost and compact connector solution with high performance that operates up to 65 GHz. The multilayer test board usually has a layer stacking configuration of Rogers’ RO4350B (0.167-mm thick) and two FR-4 layers (0.4-mm thick for each layer) as shown in Fig. 5(a).
Fig. 5. Multilayer board test with edge-mount GPPO connector: (a) multilayer test board, (b) conventional and (c) proposed connector launching structures, and (d) measured insertion and return losses.
Figure 5(b) shows the details of typical edge-mount GPPO connector attachment to the multilayer test board. The structure connected to the outer shell (ground) of the GPPO connector and the ground plane of the bottom side of the multilayer test board has a relatively long multilayer return-current path (MRCP), where the return current to the high-speed signal flows through the multiple ground planes of the multilayer test board. This configuration is fine when the bandwidth requirement of a device user test (DUT) is <20 GHz. However, the long return-current path causes severe return-current distortion and increases the characteristic impedance experienced by high-speed signals [15]. As a result, the edge-mount GPPO connector with the MRCP gave rise to severe dips (like wide resonance) at around 30 GHz in the insertion loss (red curve) as shown in Fig. 5(d) and 3-dB bandwidth reduced to <21.3 GHz.
To remedy the problem, we removed the two FR-4 layers underneath the GPPO attaching part to expose the first ground layer (1st ground plane). Then the exposed ground surface and the outer shell (ground) of the GPPO connector were directly connected by soldering as shown in Fig. 5(c). It provides the shortest return-current path, or a single-layer return-current path (SRCP). With the proposed SRCP structure, the insertion loss (S21, blue curve) was improved and the 3-dB bandwidth increased up to 44.2 GHz as shown in Fig. 5(d). Also, the return loss (S11, navy blue curve) was improved at ∼30 GHz region.
Figure 6 shows the fabricated evaluation board with 400-Gb/s (4 × 100 Gb/s) CWDM4 ROSA. The ROSA was electrically interfaced with a flexible printed-circuit board (FPCB) on the evaluation board. The ROSA attached on the multilayer evaluation board was mounted on a heat-spreading metal jig for performance verification. Optical input of the ROSA had the LC receptacle integrated with a collimating lens [8,17,19]. The I2C interface circuit implemented on the evaluation board allows the control of the operating mode (gain control mode) of the TIA in the ROSA and the monitoring of photocurrent at each channel. The evaluation board for ROSA performance verification employed Rogers RO4350B material on the upper layer for high-speed signal transmission and had a laminated structure consisting of two layers of FR-4 material for power and control signals. The evaluation board with multilayer PCB configuration was implemented by applying the edge-mount GPPO connection structure with the SRCP to prevent signal distortion.
4. Experimental results and discussions
We have observed the implemented ROSA’s inter-channel crosstalk and the result is shown in Fig. 7. The optical signal was applied to channel 1 (L1, 1291 nm) of the ROSA as an aggressor and the converted electrical outputs of other channels (L0, L2, L3) as victims were measured. Adjacent channel crosstalk (L1 to L0 and L1 to L2) was measured to be <−19.2 dB up to 50 GHz. L1 to L3 crosstalk, which is inter-channel crosstalk with the farthest channel in this configuration, was observed to be <−24.3 dB up to 50 GHz. The measured crosstalk was contributed from all the components in the signal path, including TIA, module package, and FPCB of the ROSA. Among them, the contribution of the TIA seems the largest, since it is the only active device [20]. The inter-channel crosstalk level shown in Fig. 7 is low enough not to significantly affect the ROSA performance.
Fig. 7. Measured inter-channel crosstalk of ROSA. Aggressor channel: L1, Victim channels: L0, L2 and L3.
Figure 8 shows the O-E responses (S21) and electrical return losses (S11) of the ROSA in the configuration of single-ended (optical) and differential (electrical) modes measured by the lightwave component analyzer (Keysight N4373D). With evaluation board using edge-mount GPPO connector with the MRCP, severe dips in the O-E response (red curve, S21) were observed in the range of 23 GHz to 30 GHz, and return loss (green curve, S11) was also seriously degraded at ∼27 GHz.
Fig. 8. Measured O-E response and return loss of the ROSA attached on the multilayer evaluation board of MRCP and SRCP connections.
On the other hand, the ROSA on evaluation board using edge-mount GPPO connector with the aforementioned SRCP had no dips in the O-E response (blue curve, S21) and good return loss (navy blue curve, S11) up to 50 GHz as shown in Fig. 8. The 3-dB O-E bandwidth (S21, blue curve) and electrical return loss (S11, navy blue curve) of the ROSA on the improved evaluation board were measured to be >35.7 GHz and <−10.7 dB up to 50 GHz, respectively.
Figure 9 shows the measurement setup and measured BER performance of the 400-Gb/s (4 × 100 Gb/s) CWDM4 ROSA using off-line processing. The experimental setup for an optical transmitter consists of a pulse pattern generator (PPG, SHF12104A) creating four 26.5625-Gb/s NRZ signals, a PAM-4 multiplexer (SHF616A), for generating 53.125-Gbaud electrical PAM-4 signal with the four NRZ signals, an O-band tunable laser source (Tunable LD), a linear driving amplifier and a LiNbO3 Mach-Zehnder modulator (MZM). The data pattern from the PPG had a pseudo-random binary sequence (PRBS). The PRBS was set to 215-1 pattern length due to the memory limits of the sampling oscilloscope used to acquire data in off-line processing. The generated 106.25-Gb/s PAM-4 optical signal had the transmitter and dispersion eye closure quaternary (TDECQ) of <3 dB and was applied to the ROSA. After that, the converted electrical PAM-4 signals from the ROSA was acquired using a real-time sampling oscilloscope (Tektronix DSA 73304D) with a bandwidth of 33 GHz and a sampling rate of 100 GS/s. The acquired signals were processed offline as shown in Fig. 9(a).
Figure 9(b) plots the BER performance of the ROSA measured on the multilayer evaluation board for two different GPPO edge-mounting cases. While the results with MRCP-connection evaluation board showed error floor, those with SRCP-connection evaluation board gave the receiver sensitivity of <−10 dBm at FEC limit, BER of 2.4e-4, for all channels. The SRCP-connection based evaluation board mitigated the signal distortion in itself and allowed to measure the performance of the ROSA.
5. Summary
We have implemented the hybrid-integrated 400-Gb/s (4 × 100 Gb/s) CWDM4 ROSA. Its performance was verified with a bandwidth-improved multilayer evaluation board using low-cost and compact edge-mount connectors. The key building components of the ROSA, prism, focusing lens array, and PD chips, were passively integrated. Flip-chip bonding (i.e. PD chip attachment onto PD carrier) and passive assembly (i.e. PD carrier and focusing lens) led to overall misalignment of <10 μm. Large alignment tolerance of >±30 μm at 90% coupling efficiency (i.e. 0.5-dB power penalty) made it easy to align the LC receptacle as a final assembly step. The optical coupling efficiency of the ROSA was >55% for all channels. With these proposed scheme and assembly process, the unit price of the CWDM4 ROSA is expected to be around US$250 in mass production. According to LightCounting’s market forecast report, the cost will be even lower to approximately US$100 or less [21]. The evaluation board having edge connectors mounted on a multilayer circuit board ensured signal integrity by shortening the return-current path at the edge-mount connector. With the proposed evaluation board design, its operation bandwidth was significantly increased by suppressing severe dips in O-E frequency response. The 3-dB O-E bandwidth and return loss of the ROSA were measured to be >35.7 GHz and <−10.7 dB up to 50 GHz, respectively. In addition, the back-to-back receiver sensitivity of the ROSA was measured to be <−10 dBm at FEC limit, BER of 2.4e-4, for all channels. It is expected that the method of mounting the RF connector on the multilayer circuit board demonstrated in this paper is to be utilized in the field of very wide bandwidth electrical signal connection.
Funding
Ministry of Science and ICT, South Korea (2019-0-00002).
Acknowledgments
This work was supported by Institute of Information & Communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No. 2019-0-00002, Development of On-Time•On-Rate Wireless Access and Optical Edge Cloud Networking Technologies for High-Precision Services).
Disclosures
The authors declare no conflicts of interest.
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