Acacia heralds the era of terabit-plus optical channels

Acacia Communications has unveiled the AC1200-SC2 that delivers 1.2 terabits over a single optical channel. 

The SC2 (single chip, single channel) is an upgrade of Acacia’s high-end AC1200 module. The AC1200 too is a 1.2-terabit module but uses two optical channels, each transmitting a 600-gigabit wavelength. The SC2 sends 1.2 terabits using two sub-carriers that fit within a single 150GHz-wide channel.

Each line is a data rate. Shown is the scope of how the baud rate and the modulation scheme can be varied and its impact on channel width, reach and data rate. Source: ADVA.

“In the SC2, we take care of everything so the user configures a single channel that is easier to manage in their network,” says Tom Williams, vice president of marketing at Acacia.

1.2-terabit channel

Acacia unveiled the AC1200 at the ECOC show in 2017. With its introduction, Acacia gained an advantage over its system-vendor rivals in bringing a 1.2-terabit coherent module to market using 600-gigabit wavelengths. The module supports up to 64-ary quadrature amplitude modulation (64-QAM) and a symbol rate of 69 gigabaud (GBd).

Systems vendors such as Ciena, with its WaveLogic 5, and Infinera, with its Infinite Coherent Engine 6 (ICE6), responded with their next-generation coherent designs that use symbol rates approaching 100GBd and support an 800-gigabit wavelength.

Sell-side research analysts interpreted the coherent developments as Acacia having a window of opportunity to exploit the AC1200 until the systems vendors’ coherent designs come to market in the coming year. The analysts also noted how 400 Gigabit Ethernet client signals better fit in an 800-gigabit wavelength compared to a 600-gigabit wavelength.

Then, in July, Acacia’s status as a merchant coherent technology supplier changed with the announcement that Cisco Systems is to acquire the company for $2.6 billion. Now, Acacia has detailed the SC2 as its acquisition awaits completion.

AC1200-SC2 

The SC2 uses the same form factor and electrical connector as the AC1200 module, simplifying the upgrading of system designs using the AC1200. However, the SC2 module uses a single fibre pair for its optical output whereas the AC1200 uses two pairs, one for each channel.

The SC2 module shares the same Pico coherent digital signal processor (DSP) and baud rates as the AC1200. The Pico DSP uses fractional quadrature amplitude modulation (QAM) and an adjustable baud rate.

Fractional QAM allows the tuning of the transmitted data rate by using a mix of adjacent modulation formats. For example, 8-QAM and 16-QAM are alternated, and the percentage of time each is used determining the resulting data rate. In turn, the baud rate can be increased to widen the signal’s spectrum, if the optical channel permits, such that using a lower modulation scheme may become possible, improving the reach (see diagram above).  

The AC1200 uses 50GHz- and 75GHz-wide channels while the SC2 uses 50-150GHz channels. For 600-gigabit and 1.2-terabit transmissions, the widest channels are used: 75GHz for the AC1200, and 150GHz for the SC2. “But as you go down in data rate, you can address the transmission in multiple ways,” says Williams. “You can run a higher modulation scheme in a narrow channel or, with a wider channel, run a lower modulation scheme to go further.” 

The result optical performance means that the SC2 can be used for multiple applications: from short-span data centre interconnect where the full 1.2-terabit capacity is sent using 64-QAM, to metro-regional and long-haul distances using 800-gigabit and 16-QAM, all the way to ultra-long-haul terrestrial and subsea links with 400-gigabitand quadrature phase-shift keying (QPSK) modulation.

The AC1200 and the SC2 have comparable optical performance in terms of spectral efficiency and reach. This is unsurprising given how both modules use the same Pico DSP, baud rates and modulation schemes.

The AC1200 uses two 75GHz channels, each carrying 600 gigabits, to send 1.2 terabits, while the SC2 uses two sub-carriers in a 150GHz channel. However, the SC2 has a slight advantage since no guard band is needed between the two channels as is required with the AC1200 (unless the AC1200 is sending a two-channel ‘superchannel’ whereby no dead zone is needed between the channels).

Acacia is not detailing how it generates the optical sub-carriers besides saying the change stems from the interface between the Pico DSP and its silicon photonics-based photonic integrated circuit (PIC). The company will also not say if the SC2 uses a new PIC design.

Operational benefits 

The fact that the SC2 and AC1200 deliver the same reach and capacity may explain why Acacia downplays the argument that the company has again leapfrogged its rivals with the advent of a module that sends 1.2 terabits over a single channel.

Instead, Acacia stresses the system and operational benefits resulting from doubling the data transmitted per channel.

“The SC2 module allows the entire capacity to be managed as a single channel,” says Williams. “The original [AC1200] module is well-suited to brownfield networks operating with 50GHz or 75GHz spacing, while the SC2 offers advantages in greenfield network architectures that can use channel plans up to 150GHz.”

Using a higher-capacity channel requires fewer optical components and reconfigurable optical add/ drop multiplexer (ROADM) ports thereby reducing networking costs, says Williams.

Using 150GHz-wide channels also aligns with an emerging consensus among network operators regarding wavelength roadmaps. “Network operators want to operate on some standardised grid based on regular multiples [50GHz, 75GHz] because it avoids fragmentation,” says Williams. 

Availability 

Acacia is already providing the SC2 module to certain customers that are undertaking validation testing. The firm is ready to ramp production based on particular customer demand.  

Acacia will also be demonstrating its latest module at this week’s ECOC show being held in Dublin.