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Abstract
We design and implement a cost-effective and compact 100-Gb/s (2 × 50 Gb/s) PAM-4 receiver optical sub-assembly (ROSA) by using a TO-can package instead of an expensive box-type package. It consists of an optical demultiplexer, two PIN-PDs and a 2-channel linear transimpedance amplifier. The components are passively aligned and assembled using alignment marks engraved on each part. With a real-time PAM-4 DSP chip, we measured the back-to-back receiver sensitivities of the 100-Gb/s ROSA based on TO-56 to be less than -13.2 dBm for both channels at a bit error rate of 2.4e-4. The crosstalk penalty due to the adjacent channel interference was observed around 0.1 dB.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
As the data traffic of the data center explosively increases due to social networking services (SNS), Video, Internet of Things (IoT), and cloud services, it requires to develop the cost-effective and high-speed optical modules. To meet these demands, PAM-4 modulation format, which is a higher order modulation scheme, has been extensively applied to the optical module. This is because the PAM-4 signaling doubles the spectral efficiency compared to the traditional NRZ signaling, although the required bandwidth of the constituent components are similar to that of the components for the NRZ. Therefore, the PAM-4 modulation scheme has been adopted as the baseline of Ethernet standardization [1,2] and also referred to as the baseline of 100-G Ethernet passive optical network (EPON) [3]. Furthermore, there have been many studies on optical transmitting and receiving technologies by using off-line processing method and the commercial digital signal processing (DSP) chip for PAM-4 signal [4–8].
In the 100-Gb/s optical module market for data center, it has been expected that the current 4 × 25-Gb/s NRZ optical module would be replaced with the 2 × 50-Gb/s PAM-4 optical module [9,10]. In the optical module, a transmitter optical sub-assembly (TOSA) and a receiver optical sub-assembly (ROSA) have been implemented with an expensive box-shaped package using a ceramic feedthrough [11–14]. To further lower module prices, the well-matured TO-based optical module technology can be an attractive solution because the TO-packaged optical module provides a low manufacturing cost by a simple optoelectronic assembly process. Most of TO packages has been only utilized in single-channel TOSA and ROSA for 25 Gb/s and 40 Gb/s [15–17]. It is difficult to increase the number of channel in TO package because of spatial and mechanical manufacturing process conditions.
In this paper, we present and demonstrate a low-cost and miniaturized 100-Gb/s (2 × 50 Gb/s) PAM-4 ROSA using a 2-channel TO-56 package. PAM-4 signals are very sensitive to noise, optical modules using a space-constrained multi-channel TO package should be carefully considered for suppressing reflection and crosstalk. Therefore, we have designed the new 2-channel TO package and high-speed signal path for good signal integrity through the 3-D EM simulation. We have newly fabricated the 2-channel TO-56 package and implemented the ROSA by passive alignment assembly. It is integrated with an optical demultiplexer (DMUX), two photodiodes (PDs) and 2-channel linear transimpedance amplifier (TIA). We have measured the receiving performance at back-to-back operation using real-time 100-Gb/s PAM-4 DSP chip. In addition, we have investigated the adjacent channel crosstalk of the TO-packaged ROSA.
2. Design and fabrication
2.1 Structure and design
Figure 1 shows the proposed TO package-based 100-Gb/s (2 × 50 Gb/s) PAM-4 ROSA structure. The ROSA converts the multiplexed 2-channel optical PAM-4 signal into two electrical PAM-4 signals. It consists of an optical DMUX based on thin-film filters, two PIN-type PDs and a 2-channel linear TIA. The center wavelengths of the optical DMUX are 1271 nm and 1311 nm. The 1-dB bandwidth and the channel isolation of the optical DMUX have been measured as ~13 nm and ~42 dB, respectively. The optical DMUX has a zig-zag scheme that optically separates wavelengths by attaching thin-film filters and a mirror plane (i.e. high-reflection coating) onto a glass block [13]. According to the typical TO assembly process, it is vertically arranged from the PD/TIA to the optical DMUX, as shown in Fig. 1. The ROSA is hermetically sealed with a window cap, as shown in Fig. 1. It is connected to a flexible printed circuit board (FPCB) after the final assembly process.
An optical receiver in which a plurality of channels are integrated should be structured so as to reduce the electrical noises (i.e. return loss and crosstalk). So, the differential impedance of the high-speed signal lead pins of the 2-channel TO package used in the ROSA are designed to be approximately 90 Ω. And a ground pin is added between the channels to reduce the crosstalk between channels, as shown in Fig. 2(a). It has verified through three-dimensional (3-D) electromagnetic (EM) simulation that the electrical crosstalk has been improved by ~30 dB compared to the case where there is no ground pin between channels. As shown in Fig. 2(a), the ground lead pin between the channels is connected to the ground of the aluminum nitride (AlN) substrate and the ground of the TO package by wire bonding. In the proposed ROSA structure, the high-speed signal flow is from port 1 (P1 and P2) to port 2 (P3 and P4): TIA output → AlN substrate → lead pin (in TO) → FPCB. Figure 2(a) shows that the aforementioned high-speed signal path is modeled for 3-D EM simulation. The stem thickness of the TO package used for modeling is 1.1 mm and the length of the FPCB is 12 mm. In order to make the lengths of the two complementary signals forming the differential signal equal to each other, the signal line pattern should be routed considering the wire bonding position, as shown in Fig. 2(a). Figure 2(b) shows the calculated results through the 3-D EM simulation. The insertion loss (SDD21) and return loss (SDD11) were calculated below –0.9 dB and below –20 dB up to 25 GHz, respectively. The far-end crosstalk between channels was calculated to be less than –37 dB up to 25 GHz.
As shown in Figs. 3(a) and 3(b), the TO package and FPCB are electrically connected in an orthogonal manner through the thru-hole via and the lead pin. There are two undesired parasitics from this connection structure. One is the inductance (L) resulting from the exposed lead pin to air between the TO package and the FPCB. The other is the parasitic capacitance (C) generated by the protruded lead pin acting like a stub after FPCB penetration. In order to compensate these undesired LC components, we have utilized a capacitive component resulting from antipad (i.e. void) between the signal via pad and the ground plane. It should be optimized to compensate the unwanted LC components considering the practical design values that may occur in actual module manufacture. In this design, when the air gap between the TO package and the FPCB is 150 μm and the length of the stub (protruded lead pin) is 400 μm, the spacing (antipad) between the signal via pad formed on the FPCB and the ground plane was derived as ~200 μm through 3-D EM simulation. Figure 3(b) shows the calculated return loss (SDD22) from port 2 (P3 and P4) as a function of the stub length. It can be seen that the return loss is less than –15 dB up to 25 GHz for the stub length (i.e. 0.4 mm) that can occur in the actual manufacturing process.
2.2 Fabrication
The inset of Fig. 4 shows a receptacle-collimator in which an LC receptacle and a collimating lens are integrated. We employ a zirconia sleeve to precisely align the optical axis between the fiber stub and the collimating lens. The offset due to misalignment of the optical axis can be managed as less than ± 2.5 μm. Both sides of the fiber stub and the input/output surfaces of the collimating lens are anti-reflection coated. The optical reflectance of the fabricated receptacle-collimator was measured to be less than ~28 dB [14]. Figure 4 plots the measured diameter of the emitted parallel beam as function of the propagation distance, and the beam diameter shows the value measured at 1/e2. In the proposed ROSA, the expected longest propagation distance from the receptacle-collimator to the PD chip through the optical DMUX is ~5 mm and the effective diameter of the lens formed in the PD is ~100 μm [14]. Thus the collimated beam diameter should be less than 100 μm in front of the PD’s lens. The fabricated receptacle-collimator has ~90-μm beam diameter at 5-mm propagation distance, as shown in Fig. 4. It indicates that the one-body receptacle-collimator is suitable for our proposed ROSA scheme.
Fig. 4 Measured diameter of the collimated beam diameter of the receptacle-collimator. Inset is a schematic of receptacle-collimator.
Figure 5(a) shows the fabrication process of the building components onto the TO package. The 3-dB bandwidth and responsivity of the used PD are more than ~20 GHz and 0.8 A/W at 1.3 μm wavelength, respectively. A coplanar-waveguide transmission line is formed on the PD carrier. The PD is back-illuminated with a 100-μm diameter lens and accurately mounted on the PD carrier by utilizing the flip chip bonding, and the center-to-center distance of the PDs is 500 μm. The bonding accuracy is approximately ± 5 μm. An alumina substrate with high thermal resistance is used to the PD carrier in order to reduce the heat input from the TIA. The dielectric constant and thermal conductivity of the alumina used are 9.5 and 15 W/m·K, respectively. The PD carrier on which the PD is mounted is attached to a predetermined position on the TIA. The used linear TIA has 3-dB bandwidth of more than 30 GHz and operates at automatic gain control (AGC) mode.
Fig. 5 Fabrication process: (a) integration of key components by alignment marks on each part, (b) optical alignment schematic between a receptacle-collimator and a TO block and (c) measured photocurrents on x-axis positions of the receptacle-collimator.
The optical DMUX is mounted by using alignment marks (dotted-line box) on the DMUX carrier, and the DMUX block is attached to the support by using another alignment marks (solid-line box) engraved on both sides of the carrier, as shown in Fig. 5(a). The center-to-center distance of the optical channels emitted from the optical DMUX is 500 μm. In the assembly processes up to now, the building components have been integrated by passive alignment method.
Figure 5(b) shows the final process of assembly of the receptacle-collimator and a TO block. This assembly process is performed by monitoring the optical coupling of each channel as the active alignment process. Figure 5(c) plots the PD output current for each channel as a function of x-axis positon of the receptacle-collimator. In order to equalize the receiving performance of each channel, the receptacle-collimator and the TO block are fixed by laser welding by shifting + 20 μm at channel 2. When the optical input power is –2.5 dBm, the PD output currents of the two channels were measured to be about 375 μA and 363 μA, respectively. Both channels showed an optical coupling efficiency of more than 80%. In the fabrication process by passive alignment, there is the difference in the maximum optical coupling point for each channel due to the accumulated assembly errors occurring during mounting of each component [13,14].
Figure 6 shows the fabricated 100-Gb/s (2 × 50 Gb/s) PAM-4 ROSA. The diameter of the TO package is 5.6 mm and it is electrically interfaced with FPCB. The ROSA is mounted on a heat-dissipating evaluation board for performance verification. The FPCB and the evaluation board are electrically press-connected by using a contact jig. The ROSA has optical signal input monitoring function and TIA output amplitude adjustment function for each channel.
3. Experimental results and discussions
Figures 7(a) and 7(b) show the experimental setup and electrical PAM-4 waveforms converted from both channels of the ROSA at 26.56-Gb/s data rate, respectively. An optical PAM-4 transmitter consists of a pulse pattern generator (PPG, SHF12104A) creating two 26.56-Gb/s NRZ signals (LSB and MSB), a 6-bit digital-to-analog converter (DAC, SHF604B) for 26.56-Gbaud electrical PAM-4 signal, an O-band tunable laser source, a linear driver amplifier and LiNbO3 Mach-Zehnder (LN) modulator, as shown in Fig. 7(a). The NRZ has 231-1 pseudo-random bit sequence (PRBS). 26.56-Gbaud optical PAM-4 signal has an outer extinction ratio (ER) of ~6.5 dB and is applied to the ROSA and then is converted to electrical PAM-4 signal, as shown in Fig. 7(b). We could obtain clearly opened eye diagrams from both channels at the optical input power of –5 dBm.
Fig. 7 (a) Experimental setup for observing output waveform of ROSA and (b) converted electrical waveforms from ROSA at 26.56-Gbaud data rate.
Figure 8(a) shows the measured optical spectra of the ROSA with optical coupling loss. The optical coupling loss is obtained from the ratio of the PD current monitored through the TIA to the current converted by the PD for incident light. As aforementioned, the responsivity of the PD is 0.8 A/W. The insertion loss and 1-dB passband of the optical DMUX was measured as ~0.97 dB and ~ ± 6.35 nm, as shown in Fig. 8(a). The measured center wavelengths of channels 1 and 2 were measured as 1270 nm and 1310 nm, respectively. The gray-colored area indicates the allowable wavelength range of CWDM mentioned in IEEE 802.3bs standard [2]. In Fig. 8(a), the center wavelength of each channel slightly shifted toward the short wavelength by about 1 nm according to the spectral characteristics of the thin-film filter due to the incident angle. It can be seen that the optical signal is incident with a larger incident angle than the designed incident angle (13.5°) of the filters due to the assembly error (~0.6°) of the optical DMUX. Figure 8(b) shows the measured frequency responses of the ROSA by using a lightwave component analyzer (Keysight’s N4373D). The 3-dB O/E bandwidth was observed to be more than 18.1 GHz and the return loss (S11) was measured to be less than –14 dB up to 25 GHz. It indicates the measurement including the evaluation board.
Fig. 8 (a) Measured wavelength transmission spectra of ROSA and (b) O/E bandwidth and electrical return loss of ROSA.
Figure 9(a) shows the test configuration for the receiving performance measurement of the ROSA using commercial real-time PAM-4 PHY chip (Inphi's IN015025-CA01) and Ethernet signal analyzer (Viavi’s ONT-603). The test set-up additionally includes linear driver amplifiers, modulators, Tunable laser sources, and an optical multiplexer (MUX) for O-band CWDM.
100-Gb/s (4 × 25.78 Gb/s) NRZ signals from the optical network tester are applied to the PHY chip board to generate 2 × 26.56-Gbaud PAM-4 electrical signals. Then, it is converted into optical PAM-4 signals through linear driver amplifiers and optical modulators connected to light sources. The optical PAM-4 signals are multiplexed through the optical CWDM MUX. The optical signals are applied to the implemented ROSA and it has converted to electrical PAM-4 signals. The input optical power of the ROSA is controlled by a variable optical attenuator (VOA). The electrical signals are applied to the PHY chip board and the optical network tester to measure the receiver performance of the ROSA.
Figure 9(b) plots the measured bit error ratio (BER) performance of the 100-Gb/s PAM-4 ROSA using real-time DSP in PAM-4 PHY chip. The sensitivities of channels 1 and 2 at back-to-back transmission were measured as –13.8 dBm and –13.3 dBm at BER of 2.4e-4, respectively. For the simultaneous channel operations, the sensitivities of channels 1 and 2 were measured as –13.7 dBm and –13.2 dBm at BER of 2.4e-4, respectively. Furthermore, we have successfully achieved error-free transmission at back-to-back link when enabling a forward error correction (FEC) function in PAM-4 DSP chip. The channel crosstalk penalty during simultaneous channel operation was measured as ~0.1 dB at BER of 2.4e-4 and is negligible. These results show that the proposed ROSA has similar receiver sensivities compared to the box-type ROSA [5]. It is expected to be an attractive solution for the cost-effective optical transceiver using DMLs or VCSELs.
4. Summary
We have implemented the cost-effective and compact 100-Gb/s (2 × 50 Gb/s) PAM-4 ROSA by employing the 2-channel TO package and successfully demonstrated the performance by using real-time PAM-4 DSP chip. The most of key building components (such as the optical DMUX, two PDs, and 2-channel TIA) were passively integrated on the TO-56 package. The ROSA has been designed by using 3-D EM simulation. For the implemented ROSA, the 3-dB O/E bandwidth and return loss were measured to be 18.1 GHz and –14 dB up to 25 GHz, respectively. The optical coupling efficiencies of both channels were more than 80%. The receiver sensitivity and crosstalk penalty were measured to be below –13.2 dBm and around 0.1 dB for both channels at BER of 2.4e-4, respectively. To our knowledge, we have firstly implemented the 100-Gb/s PAM-4 ROSA using the 2-channel TO package, and verified the error-free operation of the ROSA when enabling the FEC in PAM-4 DSP chip. From these results, we expect that this module can contribute to reduce the module price and power consumption for 100-Gb/s Ethernet applications.
Funding
Institute for Information & Communications Technology Promotion (IITP) grant funded by the Korea government (MSIT) (B0717-17-0041, Development of PAM-4 optical transceiver for short-reach Ethernet interface).
References and links
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