Stuck in a construction zone after picking up my son from the metro station, I watched patiently as the flagger carefully directed cars to take turns passing through a single-lane road. His coordination allowed vehicles to flow smoothly in both directions, despite the limited space. It was our turn to wait until they let clear all the vehicles coming from the opposite direction, and this idle time got me thinking.
As I have spent more than half of my life working in the optical networking industry, this scenario naturally made me reflect on how data streams are transmitted and received over a single strand of optical fiber.
Most metro and long-haul optical transmission networks require a pair of fibers, one to transmit the optical signal (Tx) and the other to receive it (Rx). However, duplex transmission (both Tx and Rx) over the same fiber is widely used in access networks where optical connectivity to many end sites is required, such as residential areas, business parks, enterprise locations, and wireless towers. In fact, optical transmission over a single fiber, also known as single-fiber working (SFW) or bidirectional (bidi), is the dominant architecture in passive optical networks (PONs) delivering residential and enterprise services. SFW is also widely used in 4G/5G wireless backhaul applications. The use of a single fiber helps network operators with limited fiber resources maximize their return on assets while avoiding the massive capital expenditure related to the installation and maintenance of new fibers (cost of the fiber optic cable, labor cost, permits/rights of way, etc.). It is estimated that it costs between $36,000 and $78,000 to install one kilometer of fiber (source: Fiber Broadband Association 2023 Annual Report).
Today, to perform a lower-bit-rate 10 Gb/s transmission over a single fiber, two wavelengths are used to carry information in opposite directions. Wavelength-division multiplexing (WDM) optical devices, known as diplexers or circulators, are used to combine and separate the two wavelengths at each end, as depicted in Figure 1.
While this solution works well for wavelengths up to 10 Gb/s using direct-detect optics, it falls short for higher-bit-rate wavelengths leveraging coherent optical technology. Coherent optics use a local reference signal in the receiver to lock the transmitter and receiver wavelengths to the same frequency. As a result, the two-wavelengths technique using diplexers/circulators cannot be used.
However, a small number of coherent optics are available today that use two lasers, one for transmit and one for receive, which enables them to be used over single-fiber networks. Such devices are more expensive and power hungry. When implemented in a pluggable form factor, this two-wavelengths solution requires a larger footprint, so it can only fit in a CFP2 form factor and not a QSFP-DD, which is a common form factor in routers, switches, and other third-party devices. Moreover, this two-wavelengths solution is limited to much lower optical launch power, as a micro EDFA (amplifier) cannot be used due to the limited space.
Can Coherent Optical Transmission Be Enabled in a QSFP-DD Form Factor?
The latest innovations in digital signal processors and high-speed optics have led to the creation of intelligent coherent pluggables. They provide deployment flexibility over a single fiber or fiber pairs, advanced optical performance, and dynamic capacity allocation, where capacity can be added or shifted in the network on the fly, all in a compact form factor (QSFP-DD and CFP2). At the heart of these intelligent coherent pluggables is XR optics technology based on digital subcarriers and driven by the Open XR Optics Forum. XR optics utilize digital signal processing to subdivide the transmission and reception of a single laser and wavelength into a series of smaller-frequency channels called digital subcarriers. These digital subcarriers can be independently managed and assigned to different destinations, enabling the industry’s first scalable point-to-multipoint connectivity and unlocking high-performance coherent transmission over single-fiber networks.
A single 400G XR optics pluggable generates 16 x 25 Gb/s digital subcarriers, where up to eight digital subcarriers (8 x 25 Gb/s) can be used in one direction (e.g., downstream) for a total of 200 Gb/s of capacity, and the remaining eight digital subcarriers (8 x 25 Gb/s) can be used for transmission from the far end (upstream). The DSP inside the intelligent coherent pluggable at each end “turns off” the transmission of the digital subcarriers that it is supposed to receive from the other end. Hence, up to 200 Gb/s of bidirectional traffic can be delivered on a single fiber. A pair of passive circulators is used at each site to segregate upstream from downstream traffic before connecting to the transmit and receive ports of the ICE-X 400G pluggable, as depicted in Figure 2. Given that the amount of bandwidth is a function of the number of allocated digital subcarriers, traffic flows can be symmetrical or asymmetrical in each direction. For example, 4 x 25 Gb/s (100 Gb/s) can be sent in one direction and 1 x 25 Gb/s (25 Gb/s) can be sent in the opposite direction. This flexibility is key in broadcast applications where the bandwidth is often much larger in one direction versus the other.
This breakthrough architectural approach has major implications across access, aggregation, and metro optical network applications. Benefits include a significant reduction in total cost of ownership, dramatic network simplification, and unprecedented network flexibility.
Single-fiber Applications
Enabling coherent optical transmission over single-fiber networks can be a game-changer in a wide scope of applications, as described below.
- Optical extension and transport: This application consists of providing high-bandwidth connectivity between two endpoints using point-to-point configuration over a single fiber, such as in wireless backhaul, OLT extension, cable fiber deep, router interconnect (IPoDWDM), data center interconnect ,and many others.
- 100G business services over existing PON infrastructure: While PON networks are highly suitable for residential services, the technology falls short in keeping up with the ever-increasing demand for capacity triggered by next-generation business services and 5G mobile transport. XR optics’ support for SFW enables PON network operators to take a phased and business-paced approach to deploying coherent-grade capacity over their existing GPON/XGS-PON/NG-PON2 infrastructures without network or service disruption, as depicted in the video below. This approach delivers an order-of-magnitude increase in capacity to support larger enterprises, research parks, medical centers, school campuses, and 5G towers with up to 200G shared between customers, versus sharing the 10G/25G supported by XGS-PON or newer 25G PON networks. Moreover, it elevates service agility by providing dynamic/automated service operations, including capacity increases in minutes and remote diagnosis and troubleshooting, all while leveraging existing PON building blocks. This enables PON operators to maximize ROI and accelerate time to market for next-generation business services.
Infinera’s ICE-X 400G XR intelligent coherent pluggables leverage XR optics technology to offer efficient and highly flexible coherent optical transmission over a single fiber. The ability to provide an order-of-magnitude increase in capacity and achieve an unprecedented level of network flexibility helps network operators:
- Maximize ROI and defer CapEx through efficient utilization of existing single-fiber spans
- Reduce transport costs by leveraging higher coherent capacity (200 Gb/s bidirectional) and reach
- Enhance network flexibility by leveraging symmetrical and asymmetrical traffic flows
Finally, it’s our turn now to move – the flagger waves to us with one hand while holding an octagonal sign that says “Slowly” in the other. I went back home still thinking how cars can go in opposite directions simultaneously over the same single road – just like ICE-X over a single fiber. Hovering cars, travelling at different altitudes? OK. I’ll stop, I think my imagination is going wild.
