Working at the limit of optical transmission performance

• Expect to see new optical transmission records at the upcoming ECOC 2023 conference. 
• Keysight Technologies' chart plots the record-setting optical transmission systems of recent years.
• The chart reveals optical transmission performance issues and the importance of the high-speed converters between the analogue and digital domains for test equipment and, by implication, for coherent digital signal processors (DSPs).

Graphic explanation

Shown is the net bit rate plotted against the baud rate. Also shown are lines with the number of bits per symbol. These are not the bit resolution of the DAC but the bits for both polarisations. For example, 14bit/symbol refers to 7-bit per polarisation. The DACs making up the transmission systems plotted are either 6-bit or 8-bit. Source: Keysight

Engineers keep advancing optical systems to send more data across an optical fibre.

It requires advances in optical and electronic components that can process faster, higher-bandwidth signals, and that includes the most essential electronics part of all: the coherent DSP chip. 

Coherent DSPs use state-of-the-art 5nm and 3nm CMOS chip manufacturing processes. The chips support symbol rates from 130-200 gigabaud (GBd). At 200GBd, the coherent DSP's digital-to-analogue converters (DACs) and analogue-to-digital converters (ADCs) must operate at at least 200 giga samples-per-second (GSps) and likely closer to 250GSps. DACs drive the optical modulator in the optical transmission path while the ADCs are used at the optical receiver to recover the signal.

Spare a thought for the makers of test equipment used in labs that drive such coherent optical transmission systems. The designers must push their equipments' DACs and ADCs to the limit to generate and sample the waveforms of these prototype next-generation optical transmission systems.

Optical transmission records

The recent history of record-setting optical transmission systems reveals the design challenges of coherent components and how ADC and DAC designs are evolving.

It is helpful to see how test equipment designers tackle ADC and DAC design, given the devices are a critical element of the coherent DSP, and when vendors are reluctant to detail how they achieve 200GBd baud rates using on-chip CMOS-based ADCs and DACs.

Nokia and Keysight Technologies published a post-deadline paper at the ECOC 2022 conference detailing the transmission of a 260GBd single-wavelength signal over 100km of fibre.

The system achieved the high baud rate using a thin-film lithium niobate modulator driven by Keysight's M8199B arbitrary waveform generator. The M8199B uses a design consisting of two interleaved DACs to generate signals at 260GSps.

A second post-deadline ECOC 2022 paper, published by NTT, detailed the sending of over two terabits-per-second (Tbps) on a single wavelength. This, too, used Keysight's M8199B arbitrary waveform generator.

The chart above highlights optical transmission records since 2015, plotting the systems' net bit rate - from 800 gigabits to 2.2 Tbps - against a symbol rate measured in GBd.

As with commercial coherent optical transport systems, the goal is to keep increasing the symbol rate. A higher symbol rate sends more data over the same fibre spans. For example, the 400ZR coherent transmission standard uses a symbol rate of some 60GBd to send a 400Gbps wavelength, while 800ZR doubles the baud rate to some 120GBd to transmit 800Gbps over similar distances.

"With the 1600ZR project just started by the OIF, this trend will likely continue," says Fabio Pittalá, product planner, broadband and photonic center of excellence at Keysight.

The signal generator test equipment options include the use of different materials - CMOS and silicon germanium - and moving from one DAC to a parallel multiplexed DAC design.

Single DACs

In 2017, Nokia achieved a 1Tbps transmission using a 100GBd symbol rate. Nokia used a Micram 6-bit 100GSps DAC in silicon germanium for the modulation.

For its next advancement in transmission performance, in 2019, Nokia used the same DAC but a faster ADC at the receiver, moving from a Tektronix instrument using a 70GHz ADC to the Keysight UXR oscilloscope with a 110GHz bandwidth ADC. The resulting net bit rate was nearly 1.4 terabits.

Keysight also developed the M8194A arbitrary waveform generator based on a CMOS-based DAC. The higher sampling rate of this arbitrary waveform generator increased the baud rate to 105GBd, but because of the bandwidth limitation, the net bit rate was lower.

The bandwidth of CMOS DACs can be improved but it tops out in the region of 50-60GHz. "It's very difficult to scale to a higher baud rate using this technology," says Pittalá. Silicon germanium, by contrast, supports much higher bandwidths but has a higher power consumption.

In 2020, Nokia reached 1.6Tbps at 128GBd using the Micram DAC5, an 8-bit 128GSps DAC based on silicon germanium. A year later, Keysight released the M8199A arbitrary waveform generator. "This was also based on 8-bit silicon germanium DACs operating at 128GSps, but the signal-to-noise ratio was greatly improved, allowing to generate higher-order quadrature amplitude modulation formats with more than sixteen levels," says Pittalá.

This arbitrary waveform generator was used in systems that, coupled with advanced equalisation schemes, pushed the net bit rate to almost 2Tbps.

Going parallel

For the subsequent advances in baud rate, parallel DAC designs, multiplexing two or more DACs together, were implemented by different research labs.

In 2015, NTT multiplexed two DACs that advanced the symbol rate from 105GBd to 120GBd. In 2019, NTT moved to a different type of multiplexer, which, used with the same DAC, increased the baud rate to around 170GBd. Nokia also demonstrated a multiplexed design concept, which, together with a novel thin-film lithium niobate modulator, extended the symbol rate to 200GBd, achieving a 1.6Tbps net bit rate.

Last year, Keysight introduced its latest arbitrary waveform generator, the M8199B. The design also adopted a multiplexed DAC design.

Multiplexing two DACs. SR refers to sample rate, BW refers to bandwidth. Source: Keysight.

"There are two 128GSps 8-bit silicon germanium DACs that are time-interleaved to get a higher speed signal per dimension," says Pittalá. If the two DACs are shifted in time and added together, the result is a higher sampling rate overall. However, Pittalá points out that while the sample rate is effectively doubled, the overall bandwidth is defined by the individual DACs (see diagram above).

Pittalá also mentions another technique, based on active clocking, that does increase the bandwidth of the system. The multiplexer is clocked and acts like a fast switch between the two DAC channels. "In principle, you can double the bandwidth, " he says. (See diagram below.)

Using a clocking scheme for the multiplexing of two DACs. SR refers to sampling rate and BW refers to bandwidth. Source: Keysight.

The Keysight's M8199B's improved performance, combined with advances in components such as NTT's 130GHz indium phosphide amplifier, resulted in over 2Tbps transmission, as detailed in the ECOC 2022 paper. As the baud rate was increased, the modulation scheme used and the net bit rate decreased. (Shown by the red dots on the chart).

In parallel, Keysight worked with Nokia, which used a thin-film lithium niobate modulator for their set-up, a different modulator to NTT's. The test equipment directly drove the thin-film modulator; no external modulator driver was needed. The system was operated as high as 260GBd, achieving a net bit rate of 800Gbps.

Pittalà notes that while the NTT system differs from Nokia's, Nokia's two red points on the extreme right of the chart continue the trajectory of NTT's six red points as the baud rate increases.

OFC'23 O-band record

The post-deadline papers at the OFC 2023 conference earlier this year did not improve the transmission performances of the ECOC papers.

A post-deadline paper published at OFC 2023 showed a record of coherent transmission in the O-Band. Working with Keysight, McGill University showed 1.6Tbps coherent transmission over 10km using a thin-film lithium niobate modulator. The system operated at 167GBd, used a 64-QAM modulation scheme, and used the Keysight M8199B.

Pittalà expects that at ECOC 2023, to be held in Glasgow in October, new record-breaking transmissions will be announced.

His chart will need updating. 

Further information

Thin-film lithium niobate modulators, click here