Jau-Ji Jou

Comparison of single-/few-/multi-mode 850 nm VCSELs for optical OFDM transmission

For high-speed optical OFDM transmission applications, a comprehensive comparison of the homemade multi-/few-/single-transverse mode (MM/FM/SM) vertical cavity surface emitting laser (VCSEL) chips is performed. With microwave probe, the direct encoding of pre-leveled 16-QAM OFDM data and transmission over 100-m-long OM4 multi-mode-fiber (MMF) are demonstrated for intra-datacenter applications. The MM VCSEL chip with the largest emission aperture of 11 μm reveals the highest differential quantum efficiency which provides the highest optical power of 8.67 mW but exhibits the lowest encodable bandwidth of 21 GHz. In contrast, the SM VCSEL chip fabricated with the smallest emission aperture of only 3 μm provides the highest 3-dB encoding bandwidth up to 23 GHz at a cost of slight heat accumulation. After optimization, with the trade-off set between the receiving signal-to-noise ratio (SNR) and bandwidth, the FM VCSEL chip guarantees the highest optical OFDM transmission bit rate of 96 Gbit/s under back-to-back case with its strongest throughput. Among three VCSEL chips, the SM VCSEL chip with nearly modal-dispersion free feature is treated as the best candidate for carrying the pre-leveled 16-QAM OFDM data over 100-m OM4-MMF with same material structure but exhibits different oxide-layer confined gain cross-sections with one another at 80-Gbit/s with the smallest receiving power penalty of 1.77 dB.

CWDM DFBLD Transmitter Module for 10-km Interdata Center With Single-Channel 50-Gbit/s PAM-4 and 62-Gbit/s QAM-OFDM

Single-channel 50-Gbit/s 4-level pulse amplitude modulation (PAM-4) and 62-Gbit/s orthogonal frequency division multiplexed quadrature amplitude modulation (QAM-OFDM) direct encoding of an uncooled coarse wavelength division multiplexer (CWDM) 100-Gbit/s distributed feedback laser diodes (DFBLD) transmitter module at 1310-nm transmitter interdata center transmission over 10-km single-mode fiber (SMF) are demonstrated. To enable preleveled 16-QAM OFDM and PAM-4 data transmissions at 62 and 50 Gbit/s, the 1310-nm channel of the DFBLD CWDM transmitter provides modulation bandwidth of 15xa0GHz, side-mode suppression ratio of 48 dB, and relative intensity noise of −126xa0dBc/Hz. For encoding the PAM-4 data, the 1310-nm channel of the CWDM DFBLD 100-Gbit/s transmitter achieves transmission capacities of 58 and 50 Gbit/s after BtB and 10-km SMF transmissions, respectively. Instead of supporting the back-to-back 16-QAM OFDM with the transmission at 62 Gbit/s with forward error correction (FEC) certified BER of 3.3 ×xa010−3, such a CWDM 100-Gbit/s DFBLD transmitter module can still maintain its highest data rate capacity to indicate a receiving power penalty of 0.35 dB even after 10-km SMF propagation. This is mainly attributed to the selected channel wavelength of the narrow-linewidth DFBLD, which suffers from extremely low chromatic dispersion and RF fading effect.

Efficient Heat Dissipation of Uncooled 400-Gbps (16×25-Gbps) Optical Transceiver Employing Multimode VCSEL and PD Arrays

An effective heat dissipation of uncooled 400-Gbps (16×25-Gbps) form-factor pluggable (CDFP) optical transceiver module employing chip-on-board multimode 25-Gbps vertical-surface-emitting-laser (VCSEL) and 25-Gbps photodiode (PD) arrays mounted on a brass metal core embedded within a printed circuit board (PCB) is proposed and demonstrated. This new scheme of the hollow PCB filling with thermally-dissipated brass metal core was simulated and used for high temperature and long term stability operation of the proposed 400-Gbps CDFP transceiver. During one-hour testing, a red-shift of central wavelength by 0.4-nm corresponding temperature increment of 6.7u2009°C was observed with the brass core assisted cooler module. Such a temperature change was significantly lower than that of 28.3u2009°C for the optical transceiver driven with conventional circuit board. After 100-m distance transmission over a multimode fiber (OM4), the 400-Gbps CDFP transceiver exhibited dispersion penalty of 2.6-dB, power budget of ≧ 3-dB, link loss of ≦ 0.63-dB, mask margin of 20%, and bit error rate (BER) of <10−12 with maintained stability more than one hour. The developed 400-Gbps CDFP transceiver module employing low-power consumption VCSEL and PD arrays, effective coupling lens arrays, and well thermal-dissipation brass metal core is suitable for use in the low-cost and high-performance data center applications.

Few-mode VCSEL chip for 100-Gb/s transmission over 100 m multimode fiber

A few-mode (FM) vertical cavity surface emitting laser (VCSEL) chip with heavily zinc-diffused contact layer and oxide-confined cross-section is demonstrated for carrying pre-leveled 16-quadrature amplitude modulation orthogonal frequency division multiplexing (QAM-OFDM) data in OM4 multi-mode fiber (MMF) over 100xa0m for intra-data-center applications. The FM VCSEL chip, which has an oxide-confined emission aperture of 5xa0μm, demonstrates high external quantum efficiency, provides an optical power of 2.2xa0mW at 38 times threshold condition, and exhibits 3xa0dB direct-modulation bandwidth beyond 22xa0GHz at a cost of slight heat accumulation. At a DC bias point of 5xa0mA (22.6Ith) the FM VCSEL chip, with sufficiently normalized modulation output, supports Baud and data rates of 25 and 100xa0Gb/s, respectively, with forward error correction (FEC) certifying receiving quality after back-to-back transmission. After passing through 100xa0m OM4 MMF with a receiving power penalty of 4xa0dB, the FM VCSEL chip demonstrates FEC-certified transmission of the pre-leveled 16-QAM OFDM data at 92xa0Gb/s.

Low-cost TO-CAN package combined with flexible and hard printed circuit boards for 25-Gb/s optical subassembly modules

Abstract. A low-cost transistor outline-CAN (TO-CAN) package, which is combined with flexible printed circuit board (PCB) and hard PCB, has been developed for a 25-Gb/s optical subassembly module. On the flexible PCB, the transmission line structure used top ground microstrip line, and the wider transmission bandwidth can be obtained. Using ground pads and ground notch technologies, the impedance of connection between flexible PCB and hard PCB was designed to match with the impedances of signal traces of the flexible and hard PCBs. In the TO-CAN package, a TO-46 header was used, and the header needs to closely connect with the flexible PCB. The bandwidth of TO-46 package combined with flexible and hard PCBs can achieve above 23 GHz. The clear 25-Gb/s transmission eye diagram was also measured, and the rise time, fall time, and Q-factor of the eye diagram are 13.78, 13.56 ps, and 8.76, respectively. The TO-46 package combined with flexible and hard PCBs has been verified to be suitable for application in 25-Gb/s optical subassembly modules.

Analysis and design of connection between flexible PCB and hard PCB for 25 Gb/s optical subassembly module

The rapid development of information and communication technologies has driven the rapid growth of data traffic and the large requirement of high-speed optical transceiver modules. Usually, optical subassemblies through flexible printed circuit boards (PCBs) connect with the hard PCBs to compose as optical transceiver modules. Because of the impedance mismatch of connection between flexible PCB and hard PCB, the operation bandwidth of the module can be limited. In this paper, the design of connection between flexible PCB and hard PCB in optical subassembly modules will be investigated for increasing the operation speed of the module. First, the transmission bandwidth of flexible PCB is analyzed. High-frequency performances of three transmission line structures, micro-strip line, grounded coplanar waveguide, and top ground micro-strip line, are investigated in flexible PCB. High-frequency performances of flexible PCB with different bending types are also investigated. The flexible PCB has been verified to be suitable to use in 25 Gb/s applications. Next, the high-speed transmission performance is analyzed through the connection between flexible PCB and hard PCB, because the impedance mismatch usually occurs at the connection. The ground pads are added near the signal line of flexible PCB, and the ground pads are connected to the ground layer in PCB through ground vias. The anti-pad in hard PCB is added under the signal solder pad. The dip of S21 response can be eliminated by the optimized ground pad. The impedance mismatch can be reduced by the optimized anti-pad, the impedance increases from 40-Ω to 49-Ω at the connection between flexible PCB and hard PCB, and the transmission bandwidth can increase from 16.5 GHz to 23.5 GHz. TO-46 header is a common and low-cost packaging solution for DFB laser and photodiodes. The connection between flexible PCB and TO-46 header is also investigated. While the connection has a gap, the dip of frequency response will occur at lower frequency. While the connection is very close, the bandwidth can be larger than 25 GHz. The flexible PCB, hard PCB, and TO-46 header are combined together, and the 23 GHz transmission bandwidth can be obtained. The 25 Gb/s transmission eye diagram is also measured, and the eye height, rise time, fall time, and Q-factor are 268.14 mV, 13.78 ps, 13.56 ps, and 8.76, respectively. The combined flexible PCB, hard PCB, and TO-46 header are verified to be suitable to apply in 25 Gb/s optical subassembly modules, and will be able to be used in 100G Ethernet.

Design of printed circuit board with separate heat metal block for high-speed optical transceiver module

Recently, the size of high-speed optical transceiver module is reduced gradually, but the channel number is increased. Because the densities of optic-electronic devices and driver chips are increased in the optical transceiver modules, the module temperature will raise. In electronic elements, the thermal noise is increased due to the higher temperature. The thermal noise will lead to the degeneration of signal transmission performance. Therefore, we use a separate heat metal block embedded in the high-speed optical transceiver module. In addition, the design of printed circuit board (PCB) in high-speed optical transceiver module will be also investigated. The coplanar waveguide is used and the impedance match of signal trace is also designed in our PCB. The coupling capacitors are added near the high-frequency signal input or output ends to block DC level. The coupling capacitor pad size is different with the width of signal trace, so the anti-pad is designed under the coupling capacitor. The anti-pad is used to improve the impedance mismatch of signal path through the coupling capacitor. The ground vias are also designed in our PCB. The ground of coplanar waveguide is connected with the other ground layer through many ground vias to achieve equal potential for all ground. While the impedance match and the equal potential for all ground can be carefully designed, the quality of high-speed signal transmission will become better in PCB. In high-speed optical modules, the heat sources are usually from the high-power electronic devices, such as laser diode driver chips, receiver amplifier chips, and clock and data recovery chips. The temperature of the high-power chips will influence the quality of signal transmission, and even lead the module failure. Therefore, the metal block is embedded at the location of high-power chips in the high-speed PCB, and the high-power chips are mounted on the separate heat metal block. The metal block has to be isolated with PCB to avoid the short circuit with signal traces. We used the six-layer PCB to design a 4×25-Gb/s optical transceiver module. The 1.2-cmx1-cm separate heat metal block was embedded in the 0.43-cm ×0.53-cm PCB, and the laser diode driver chip and receiver amplifier chip were mount on the metal block. The 25-Gb/s eye diagrams of transmitter and receiver were measured. For transmitter, the rise time, fall time, and jitter are 14.53-ps, 17.61-ps, and 10.854-ps, respectively. For receiver, the rise time, fall time, and jitter are 18.12-ps, 18.12-ps, and 12.051-ps, respectively. Using the PCB with separate heat metal block, the 4×25-Gb/s optical transceiver module has been designed and realized.

Optimized design of through-hole via in high-speed printed circuit board

In the printed circuit board (PCB) of high-speed optical transceiver module, the signal transmission speed is limited by many effects. In a multilayer PCB, a through-hole via is usually used to connect the signal path between the upper-layer and lower-layer. The impedance of signal path in PCB significantly influences on the signal performance. While the impedance of through-hole via mismatches with the impedance of signal path, the transmission bandwidth of the PCB can be limited. In this paper, the optimized design of impedance of through-hole via in high-speed PCB will be investigated for increasing the transmission speed in PCB. First, the Rogers 4350B high-speed PCB is used and the vertical K-type connectors are used to connect measure equipment and PCB. The vertical K-type connectors are connected with signal traces by through-hole vias. The impedance mismatch occurs at the through-hole via, so the design of the anti-pad is optimized. The anti-pad size is varied to compensate the impedance mismatch. The impedance of the through-hole via is increased from 44-Ω to 51-Ω. We have optimized the design of the anti-pad for the vertical K-type connectors connecting with signal traces. Next, the Rogers 3003 high-speed PCB is used, its transmission loss is lower and its transmission speed is higher than the Rogers 4350B PCB. The S-parameters of 5-cm signal traces in Rogers 3003 and 4350B PCBs are measured through a network analyzer. According to the S21 insertion loss, the 3-dB bandwidth is 21.3-GHz in Rogers 3003 PCB and the bandwidth is 9.58-GHz in Rogers 4350B PCB. The higher transmission speed in Rogers 3003 PCB can be verified. The design of the through-hole via between the upper-layer and lower-layer is also optimized. Using the Rogers 3003 PCB, the distance between the through-hole via and nearby ground vias and the size of anti-pad are varied to improve the impedance mismatch of signal path. The impedance of the through-hole via is increased from 47.6-Ω to 52.5-Ω, and the transmission bandwidth is improved and increased from 18.3-GHz to 24.5-GHz. The eye diagram of through-hole via transmission in Rogers 3003 PCB is also measured, and the clearer eye diagram can be obtained. We have optimized the characteristic impedances of the through-hole vias between the vertical K-type connector and signal trace, and between the upper-layer and lower-layer. The transmission bandwidths of the through-hole vias can be improved. The optimized deign can be applied in PCB of high-speed optical transceiver module.

High-frequency characteristics and equivalent circuit model of a TO-46 header for 25-Gb/s applications

The TO-46 header used for high speed VCSEL or PIN-TIA package is characterized through a specially designed K connector experimentally. An equivalent circuit model of the TO-46 header has been extracted. The TO-46 header is shown to be capable to be used in 25 Gb/s applications.

Electrical characterization of a 25 Gbit/s VCSEL module with TO-46 form factor packaging

A cost-effective differential-mode packaging of the TO-46 VCSEL module is proposed. The optimal bonding-wire and package solution have been demonstrated through frequency-domain and time-domain simulations. The differential-mode modulation bandwidth of the VCSEL module of above 21 GHz is achieved and a clearly opening eye diagram of 25 Gbit/s is observed. The electrical characterization of the VCSEL module under different driven source impedances is simulated and discussed.