Lumentum talks switching & signalling developments

In detailing some of its recent product announcements and the associated optical networking trends, Lumentum provides useful pointers to watch out for at the upcoming OFC virtual conference and exhibition event in June.

Brandon Collings

Lumentum detailed recently its high-bandwidth coherent driver modulator (HB-CDM) that operates at a symbol rate up to 96 gigabaud (GBd).

"Lumentum is working with a decent number of network equipment makers (NEMs) on their high-performance coherent offerings using the HB-CDM component," says Brandon Collings, CTO of Lumentum.

The 96GBd device supports modulation formats from dual polarisation quadrature phase-shift keying (DP-QPSK) to 64-ary quadrature amplitude modulation (DP-64QAM) and when used with an appropriate coherent digital signal processor (DSP), the device supports up to 800-gigabit wavelengths.

Moving to a higher baud rate extends the transmission reach for each modulation format used. But the overall reach diminishes as the modulation scheme used becomes more complex.

"We talk about 800 gigabits but that is still a limited reach format even at 96 gigabaud," says Collings. "How far and how useful it is, comes down to what modulation format you use."

Achieving a 96GBd design is seen as foundational for the next symbol rate that will be 128GBd. "As you can imagine, we are engaging the population of NEMs at that rate," he says.

Such technology is at the early stages of development but a 128Gbd symbol rate will boost further the transmission distance for a given modulation and enable 1 terabit and even 1.2-terabit wavelengths.

The challenge to move to a higher baud rate is extending the bandwidth of a coherent design's key elements: the modulator, the DSP outputs, and the driver between the DSP and the photonics.

"It is optimising the capabilities of these three elements to provide enough bandwidth to generate the signal at the right baud rate," says Collings.

Other challenges include the packaging, getting the radio frequency (RF) signals into the package and onto the indium phosphide chip, and developing a sufficient fast driver chip.

8x24 wavelength-selective switch

Lumentum revealed late last year that its latest 8x24 wavelength-selective switch (WSS) was shipping in volume. The WSS supports 8-degree colourless, directionless and contention-less (CDC) reconfigurable optical add/drop multiplexers (ROADMs).

"Nothing in ROADMs moves quickly; there are no hockey sticks in ROADM land," says Collings.

But the announcement of volume manufacturing indicated the onset of a second-generation colourless, directionless and contention-less ROADM architecture.

First-generation CDC ROADMs use multicast switches. These are capable devices, says Collings, but have high optical loss and limited filtering capabilities. Erbium-doped fibre amplifier (EDFA) arrays are needed to compensate for the optical loss.

"The MxN switch has filtering and significantly less loss such that you can build a better multiplexer-demultiplexer for CDC architectures," says Collings.

The second-generation ROADMs also better suit advanced modulation-based optical wavelengths, given the ROADM's filtering and lower loss optical performance.

ROADM developments

The next step is to support 16-degree ROADMs that has become the mainstay of the larger-scale optical networks.

"That is no small undertaking," says Collings. "These are complicated devices."

Next-generation ROADM-based networks must also enable cost-efficient capacity expansion. That's because a fibre's C-band is already full in terms of how the information it can convey. "This is the Shannon limit," says Collings.

"How do we do that more cost-effectively than simply adding more and more of the same ROADMs and WSSes that we do today?" says Collings. 

This, says Lumentum, is the coming hurdle in ROADM design.

Growing capacity requires more amplified bandwidth and that means either adding the L-band spectrum or more C-bands in the form of multiple fibre-pairs along the same route.

"For each case, it is simply adding more WSSes so the cost-per-capacity is linear," he says. "We need to find WSS and transport solutions that enable more capacity and more amplified bandwidth at a lower cost-per-bandwidth."

This is possible by combining the C- and L-bands or multiple C-bands within one device.

69GBd TROSA for CFP2-DCO modules

Lumentum has developed a 69GBd transmitter and receiver optical subassembly (TROSA) for the CFP2-DCO coherent module market.

The TROSA combines in one package the optics, drivers and trans-impedance amplifiers (TIAs) used by a coherent transceiver.

The TROSA supports data rates from 100-400 gigabits-per-second (Gbps) and a range from data centre interconnect to long-haul distances.

"At 100Gbps, the distance becomes many thousands of kilometres," says Collings.

Using indium phosphide technology, Lumentum can include semiconductor optical amplifiers. Other solutions such as a silicon photonics design come without a laser or they require EDFA-based amplification that requires fibre handling.

"The result is a compact transmitter and receiver plus amplification in a single box," says Collings.

The CFP2-DCO TROSA is slightly larger and produces more output power than those used for the OIF 400ZR 120km coherent standard.

400ZR is implemented using smaller client-side form factors such as the QSFP-DD and OSFP and these use a smaller TROSA design.

Lumentum's CFP2-DCO TROSA is compliant with the 400-gigabit coherent interface used as part of the OpenROADM multi-source agreement (MSA).

DML for 100-gigabit PAM-4

Lumentum also now has a directly modulated laser (DML) for 100-gigabit 4-level pulse amplitude modulation (PAM-4) interface.

The DML is a cheaper and simpler laser design than an electro-absorption modulated laser (EML) for 100-gigabit single-lambda modules. Such modules also interface to 400-gigabit client-side optics when used in a breakout mode.

"The [DML laser] chip is smaller because it doesn't have the added modulation stage [of an EML] and you don't need as many biases and drive sources to control it," says Collings. "So the cost reduction comes from the chip being smaller and the much simpler drive circuitry."

This evolution in laser technology is common in the optical industry. Leading-edge interfaces start with EMLs which are then following with lower-cost same-rate DMLs technologies.

"This is just following that same trajectory of now being able to support 100 gigabit-per-lane with a simpler technology," says Collings.

Lumentum is working on EMLs that support 200-gigabit and higher performance per lane.

"Once you have a chip [supporting 200 gigabit-per-lane] you could enable solutions that hyperscalers would use, adapted to their needs and they don't always follow a standard," says Collings. "It is the obvious next step."