100G/Lambda PAM4: Benefits of Integrated Drivers

By Toshiaki Mukoujima on January 18, 2019 | Leave a Comment

Hyper-scale data centers are generating strong demand for transceiver solutions with higher port densities and lower cost per bit, which is driving the technology shift to single-lambda 100Gbps with PAM4 technology. PAM4 means pulse amplitude modulation on four levelsand it is a simple modulation technique that allows us to put more data onto existing fiber. In other words, you don't have to reconfigure the data center with more fiber every time you want to improve the bandwidth, you just have to increase the data rate using advanced modulation PAM4 technology. These components for single-lambda 100G are scalable to 400Gbps transceivers with four channel drivers and CWDM4 wavelengths.

However, these advanced modulation techniques are not completely 'free', but come with additional requirements on the optical components used, and, in particular, consume higher amounts of electrical power. This problem is further exacerbated by the ongoing trend to smaller form factor transceiver modules to further increase faceplate density. Now OSFP and QSFP-DD 400G form factors are emerging that are roughly the same size as a current QSFP 100G module, which uses 4 wavelengths at 25 Gbps. The factor of four increase in data rate between the two form factors is the result of two changes: 1) the intrinsic d 'on-off' or baud rate is increased from 28 Gbaud to 53 Gbaud, giving one factor of 2, and 2) PAM4 modulation is implemented giving the second factor of 2. This requires the optical components used for 100G/lambda to be only slightly higher in electrical power consumption than 25 Gbps, which requires new designs.

One new innovation is to the 53 Gbaud Open Drain Driver (ODD) for PAM4. This ODD is used to drive linear operation of electro-absorbtively modulated (EML) lasers. With a typical 70mW of power consumption per channel and small size, this new high speed driver is well suited for space and power efficiency in small form factor pluggable modules such as OSFP and QSFP-DD.

The driver is what supplies the laser with the required voltage, and in the past the driver has not been closely coupled with the laser, but they have been two separate components. This separation reduces performance because the driver cannot respond fast enough to create a distinct signal. This is because of additional solder joints, versus wire bonding. In addition, a lot of power is lost with an external packaged driver, with the power having no place to go, but to generate excessive heat.

One solution is to go with a transmitter architecture that integrates the two, meaning an integrated driver directly beside the laser, which reduces size and as well as the electrical power (heat). This reduced size and heat enables all of these components to fit not only into an OSFP but also into a QSFP-DD form factor, which will be a requirement in the data center market.

The power consumption advantage per channel is 75% less: 300mW of power loss versus 70mW of power loss. Also the signal quality is greater by using an integrated driver which means a reduced cost per bit cost.