Inphi adds a laser driver to its 100-gigabit PAM-4 DSP

Inphi has detailed its second-generation Porrima chip family for 100-gigabit single-wavelength optical module designs.

Source: Inphi

The Porrima family of devices is targeted at the 400G DR4 and 400G FR4 specifications as well as 100-gigabit module designs that use 100-gigabit 4-level pulse-amplitude modulation (PAM-4). Indeed, the two module types can be combined when a 400-gigabit pluggable such as a QSFP-DD or an OSFP is used in breakout mode to feed four 100-gigabit modules using such form factors as the QSFP, uQSFP or SFP-DD.

The Gen2 family has been launched a year after the company first announced the Porrima. The original 400-gigabit and 100-gigabit Porrima designs each have three ICs: a PAM-4 digital signal processor (DSP), a trans-impedance amplifier (TIA) and a laser-driver. 

“With Gen2, the DSP and laser driver are integrated into a single monolithic CMOS chip, and there is a separate amplifier chip,” says Siddharth Sheth, senior vice president, networking interconnect at Inphi. The benefit of integrating the laser driver with the DSP is lower cost, says Sheth, as well as a power consumption saving.

The second-generation Porrima family is now sampling with general availability expected in mid-2019.

PAM-4 families 

Inphi has three families of PAM-4 ICs targeting 400-gigabit interfaces: the Polaris, Vega and Porrima.

The Polaris, Inphi’s first product family, uses a 200-gigabit die and two are used within the same package for 400-gigabit module designs. As well as the PAM-4 DSP, the Polaris family also comprises two companion chips: a laser driver and an amplifier.

Siddharth ShethInphi’s second family is the Vega, a 8x50-gigabit PAM-4 400-gigabit DSP chip that sits on a platform’s line card.

“The chip is used to drive backplanes and copper cables and can be used as a retimer chip,” says Sheth.

“For the Porrima family, you have a variant that does 4x100-gigabit and a variant that does 1x100-gigabit,” says Sheth. The Porrima can interface to a switch chip that uses either 4x25-gigabit non-return-to-zero (NRZ) or 2x50-gigabit PAM-4 electrical signals.

There are going to be two or three hyperscalers coming online in 2020 but maybe not as aggressively as the first hyperscaler

Why come out with a Gen2 design only a year after the first Porrima? Sheth says there was already demand for 400-gigabit PAM-4 chips when the Porrima first became available in March 2018. Optical module makers needed such chips to come to market with 400-gigabit modules to meet the demand of an early hyperscale data centre operator. 

“Now, the Gen2 solution is for the second wave of customers,” says Sheth. “There are going to be two or three hyperscalers coming online in 2020 but maybe not as aggressively as the first hyperscaler.” These hyperscalers will be assessing the next generation of 400-gigabit PAM-4 silicon available, he says.

The latest design, like the first generation Porrima, is implemented using 16nm CMOS. The DSP itself has not been modified; what has been added is the laser-driver circuitry. Accordingly, it is the transmitter side that has been changed, not the receiver path where Inphi does the bulk of the signal processing. “We did not want to change a whole lot because that would require a change to the software,” he says.

A 400-gigabit optical module design using the first generation Porrima consumes under 10W but only 9W using the Gen2. The power saving is due to the CMOS-based laser driver consuming 400mW only compared to a gallium arsenide or silicon germanium-based driver IC that consumes between 1.6W to 2W, says Inphi.

The internal driver can achieve transmission distances of 500m while a standalone driver will still be needed for longer 2km spans.

Sheth says that the advent of mature low-swing-voltage lasers will mean that the DSP’s internal driver will also support 2km links.

PAM-4 DSP

The aim of the DSP chip is to recover the transmitted PAM-4 signal. Sheth says PAM-4 chip companies differ in how much signal processing they undertake at the transmitter and how much is performed at the receiver.

“It comes down to a tradeoff, we believe that we are better off putting the heavier signal processing on the receive side,” says Sheth.

Inphi performs some signal processing on the transit side where transmit equalisation circuits are used in the digital domain, prior to the digital-to-analogue converter.

The goal of the transmitter is to emit a signal with the right amplitude, pre-emphasis, and having a symmetrical rise and fall. But even generating such a signal, the PAM-4 signal recovered at the receiver may look nothing like the signal sent due to degradations introduced by the channel. “So we have to do all kind of tricks,” he says.

Inphi uses a hybrid approach at the receiver where some of the signal processing is performed in the analogue domain and the rest digitally. A variable-gain amplifier is used up front to make sure the received signal is at the right amplitude and then feed-forward equalisation is performed. After the analogue-to-digital stage, post equalisation is performed digitally.

Sheth says that depending on the state of the received signal - the distortion, jitter and loss characteristics it has - different functions of the DSP may be employed.

One such DSP function is a reflection canceller that is turned on, depending on how much signal reflection and crosstalk occur. Another functional block that can be employed is a maximum likelihood sequence estimator (MLSE) used to recover a signal sent over longer distances. In addition, forward-error correction blocks can also be used to achieve longer spans. 

“We have all sorts of knobs built into the chip to get an error-free link with really good performance,” says Sheth. “At the end of the day, it is about closing the optical link with plenty of margin.” 

What next?

Sheth says the next-generation PAM-4 design will likely use an improved DSP implemented using a more advanced CMOS process. 

“We will take the learning from Gen1 and Gen2 and roll it into a ‘Gen3’,” says Sheth. 

Such a design will also be implemented using a 7nm CMOS process. “We are now done with 16nm CMOS,” concludes Sheth.