Kazuya Nagashima

Sn–Ag–Cu Solder Reflow-Capable 28-Gb/s × 4-Channel High-Density Parallel-Optical Modules

This paper describes a unique solution to the next-generation high bit-rate and high channel-density parallel-optical links whereby the size of parallel-optical modules would have to be considerably reduced. Since the main problem concerning the size of optical-to-electrical conversion part is associated with the mounting of the parallel-optical module to the printed-circuit-board (PCB), this paper puts forward a remarkably compact parallel optical-module design that can be soldered directly onto the PCB. That is made possible by the high temperature tolerance of 250 °C required for Sn-Ag-Cu solder-reflow process. The miniature parallel-optical module measures only 10.5 mm in length, 6.5 mm in width, and 1.5 mm in height. The resulting parallel-optical modules are capable of four-channel error-free parallel pseudo-random binary sequence 231-1 transmission at 28 Gb/s without using clock and data recovery as well as feed-forward error correction (FEC); hence, these parallel-optical modules are applicable in next-generation high-density 100 Gb/s Ethernet applications that use FEC.

A Record 1-km MMF NRZ 25.78-Gb/s Error-Free Link Using a 1060-nm DIC VCSEL

We report successful achievement of a record long distance error-free transmission of 1000-m 50-μm-core multimode fiber (MMF) in a non-return-to-zero 25.78-Gb/s modulated vertical cavity surface emitting laser-based optical link using four-channel miniature parallel-optical modules. The optical power is centralized within the core of MMF by an active alignment assembly. The restricted launch condition gives a very high effective modal bandwidth (EMB) of >9 GHz · km based on the calculations and the required performance data specified in TIA-FOTP-220. An EMB of 11.5 GHz · km provides a very good agreement between the calculated and measured power penalty characteristics. Such a long distance low-cost MMF link, operating at 1060-nm wavelength is potentially an alternative solution to PSM4.

VCSEL-based parallel-optical modules for >100 Gb/s applications

We introduce solder reflow-capable high-density parallel-optical modules for >100 Gb/s optical interconnects. Polymer-waveguide-coupled parallel-optical modules are also introduced with a unique mounting technology. 1060-nm 28-Gb/s InGaAs/GaAs VCSEL realizes a good signal quality at high temperature and error free for MMF transmission beyond 500m.

1060-nm 28-Gb/s

To reduce the power consumption of 4-channel 28-Gb/s miniature optical modules, a 4-channel double intra-cavity 1060-nm InGaAs/GaAs quantum-well vertical-cavity surface-emitting laser (VCSEL) array with 5-μm-wide apertures, a high differential gain, and a low temperature-insensitive differential resistance in conjunction with a specially optimized thermo-electric cooler heat sink for a reduced thermal resistance are used. Most important is the excellent resistance compatibility between the VCSEL and a low-power SiGe BiCMOS driver IC. Consequently, a 4-channel 28-Gb/s NRZ PRBS 231 - 1 link operation at a bit error rate of 10-12 for an operational case temperature ranging from 25 °C to 70 °C was achieved at an unprecedentedly low VCSEL bias current of only 4 mA.

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We report the design and transmission characteristics of clock-data-recovery (CDR)-integrated 28-Gb/s × 4-channel parallel-optical modules for QSFP28 AOCs. The module keeps the same package size with the 28-Gb/s × 4-channel module that we reported in a previous work. The CDR circuitry was integrated to the new module. Owing to the CDR functionality, we achieved a link total jitter margin of >0.4 U.I. when operating at a bit stream of 28-Gb/s, 231-1 PRBS. A degraded electrical signal at the TX input can be compensated by the CDR circuitry to fully meet the specification of CEI-28G-VSR, where the total jitter margin was improved from 0.06 U.I. to 0.42 U.I. successfully together with remarkably low power dissipation of 390 mW/lane.

-Channel Modules Operated Over the Case Temperature Range

We report a solderable 4-channel VCSEL-based optical transceiver module perfect for high-volume production of QSFP AOCs. The 4-channel module can be simply mounted on a PCB together with other electronics components through a solder reflow process. We fabricated a PCBA in compliance with the standard of QSFP edge connector. The PCBA has an MCU written with a firmware included with a look-up table to control the optical module over the operating temperate range. Integrating the PCB assembly into a die-cast housing, we fabricated fully engineered QSFP AOC. As we tested a BER characteristic for 14-Gb/s ×4-channel operation, the QSFP AOC has a total jitter margin of 0.5 U.I., and achieve a BER of <;10-15.

CDR-integrated Sn-Ag-Cu-solder reflow-capable miniature 28-Gb/s × 4-channel optical modules

We demonstrate an error free 28-Gb/s × 4-channel parallel-optical link using Sn-Ag-Cu solder reflow-capable miniature 1060-nm VCSEL-based optical modules. As an alternative solution to PSM4, we achieved error free transmission in 50-micrometer-core MMF beyond 500 m, owing to a low chromatic dispersion at 1060-nm, when the modules are operated at a bit stream of 28-Gb/s 231-1 PRBS.

A fully engineered QSFP AOC using solderable 4-channel optical transceiver modules

In recent years, optical interconnects are attracting attention to achieve high-speed signal transmission in high-end routers, high-end servers, and high performance computing systems. In rack-to-rack applications, AOCs that have the same electrical interface as conventional electrical cables have been equipped in actual systems especially. On the other hand, board-to-board applications have been expected as the next-generation high-density optical interconnects supported by high-density mounting of parallel-optical modules nearby LSI. In order to meet both of demands, we developed 10-Gb/s × 4-channel parallel-optical modules that are capable to be mounted by Sn-Ag-Cu-solder-reflow process which is the most popular for handling other SMDs. The size of fabricated modules is as small as 12mm × 7.2mm × 1.5mm. We paid attention to minimize the thickness to realize a simple heat dissipation structure together with other SMDs. We achieved a very low height as same as 1.5mm in total. We tested transmission characteristics and achieved error-free transmission in back-to-back, OM2 of 100 m and OM3 of 100 m where a power-penalty is estimated to 1dB. Through experiments of transmitting all 4 channels simultaneously, we clarified that the modules have a sufficient jitter margin in all channel. And we verified Sn-Ag-Cu reflow temperature resistance of the modules based on JIS standard whose peak temperature is as high as 250°C (i.e., JIS C 60068-2-58). We evaluated output power variation in TX module and sensitivity variation in RX modules before and after test. Both optical power variation and sensitivity variation are less than ±10%. We also fabricated a small PCB whose edge is compliant with a standard QSFP+ connector and TX and RX modules are mounted a PCB through Sn-Ag-Cu-solder-reflow process. We performed primary reliability tests of temperature cycling, mechanical shock and vibration tests based on the specified conditions in Telcordia GR-468-CORE. As results, optical power and input sensitivity deviation are less than ±10% and transmission characteristics have no degradation in all tests.

1060-nm VCSEL-based 28-Gb/s × 4-channnel optical signal transmission beyond 500-m MMF using high-density parallel-optical modules

This paper presents recent development results of our 28-Gbps VCSELs featured with double intra-cavity structure and a lasing wavelength of 1060 nm. The double intra-cavity realizes very low cavity loss due to undoped semiconductor bottom DBR and dielectric top DBR layers. Compressively strained InGaAs MQW provides high differential gain that contributes to low power consumption and high reliability. Based on our 10-Gbps VCSEL structure, we carefully optimized MQW, selective oxide structure, cavity length, and doping profile in order to achieve high speed operation while maintaining high reliability and other laser performances. The developed VCSELs exhibit modulation 3 dB-bandwidth exceeding 20 GHz and D-factor of 10 GHz/(mA)1/2. Typical threshold current and slope efficiency are 0.5 mA and 0.5 W/A, respectively. The paper also discusses static and dynamic characteristics of VCSELs with various oxide aperture sizes simultaneously fabricated on the same wafer. For a longer transmission distance and better optical coupling to a multimode fiber, optical lateral confinement is precisely controlled to reduce spectral width as well as far-field pattern. Clearly opened eye diagrams are obtained at a bit rate of 28 Gbps. Bit error rate tests are also performed and 28 Gbps error free transmission has been confirmed over 300 meters of multimode-fiber optimized for 1060 nm with a PRBS pattern length of 231-1.

Sn-Ag-Cu-solder-reflow-capable 10-Gb/s × 4-channel very thin high-density parallel-optical modules

We demonstrate >1.3-Tb/s VCSEL-based on-board optical module for high-density optical interconnects. The optical module integrates 28-Gb/s × 24-channel transmitter and receiver into one package of 1-inch^2 footprint. Subsequently, the total data rate is as high as 1.34 Tb/s. As investigated the temperature distributions of an optical module in calculation and experiment, an operating case temperature of optical module is lower than the maximum case temperature of 70 degree C in a practical air-cooling environment with the total power consumption of 9.1 W when activating all CDR circuitries as the harshest condition. The module exhibits a total jitter margin of 0.48 U. I. at a BER of 10^-12 when operated by a 28.05-Gb/s NRZ PRBS bit stream for each channel. By bypassing CDR circuitries with a capable length of electrical transmission line of 30 mm, a jitter margin was degraded to 0.21 U. I. at a case temperature of 70 degree C. If a system accepts such a level of jitter margin, the total power consumption can be suppressed to 6.0 W and an operating case temperature can be decreased accordingly.