An Interview with Robert Shore, SVP Global Marketing, Infinera

The inception of coherent based transmission was a watershed moment in optical networking. It dramatically increased the amount of capacity that could be transmitted on a fiber over a given distance. It enabled a number of different transmission techniques that were not possible with traditional direct detect technology. 

Direct detect technology is exactly how it sounds - you have a laser on one end and a photodiode on the other end and the photodiode detects the power level of the incoming optical signal. In the end, it’s either a one or a zero and that’s about it. As a result, the only way to increase capacity is to increase baud rate, which is how fast it flashes the light. You get to a point where you simply can’t flash the light faster without distortions making the signal unresolvable. 

There was a significant transition about 10 years ago to coherent based optical technology. While coherent based optical technology dramatically increases the complexity at the optical received level, it also provides more information about the nature of the optical signal. For example, a coherent receiver can detect the phase of the optical signal in addition to the power level. This enables the transmission of greater amounts of information at the same baud rate by adjusting the phase of the optical signal in addition to the power level. These new transmission techniques enable both greater effective transmission rates as well as optical signals that are much less susceptible to optical impairments enabling more information to be transmitted over greater distances.

Another capacity increasing technique used by coherent transmission technology is polarization mode multiplexing.  Polarization mode multiplexing involves taking a single optical signal and splitting it into two copies and filtering out one polarization mode from each signal. The phase of each optical signal can then be modified independently, effectively doubling the throughput of a single laser using a single wavelength.  The net result of all of this is that coherent enables us to transmit vastly more information over significantly longer distances because it uses a radically new type of receiver technology. 

Since the inception of coherent based transmission solutions, the industry has been working to leverage all of the different ways that we can manipulate an optical signal to get more and more capacity and enable transmissions over longer distances. We’ve now had several generations of coherent technology. Starting with 100 Gigabits per second (Gbps or 100G) was what we call the first generation of coherent, then came 200G (2nd generation), 400G (3rd gen), 600G (4th gen) and now we’re at 800G, which is 5th gen coherent technology. 

One of the biggest benefits we have is that the technology has become increasingly flexible. 100G could transmit at only 100G and typically with only a few configurations. When 200G was introduced and increasingly with 400G and 600G, the number of configuration options has increased by several orders of magnitude. While 5th gen coherent technology is often referred to as 800G, the reality is that it can be tuned to virtually any bit rate from 100G to 800G based on application needs. Decreasing the effective bit rate results in improved optical performance and greater unregenerated transmission distances. This is the beauty of this current generation of coherent technology. You can tailor transmission speed based on the required transmission distance. In many instances, these configuration adjustments can be done automatically reducing the amount of manual provisioning required. The most advanced coherent transceivers can sense what needs to happen and make the necessary adjustments. 

Coherent optics have allowed the industry to leverage subsea cable assets to get as much information over each individual cable as possible and as much information out of every laser as possible. There are two ways to increase the efficiency of a transceiver. The first is to increase the modulation rate, which is the complexity of the signal. If you can transmit a more complex signal, you can transmit more information at the same speed. 

This is the best way to get the most value. Increasing modulation not only increases the data rate, but it does it within the same amount of spectrum. You get much lower cost per bit and much better spectral efficiency – or capacity per fiber – by increasing modulation.

The downside of increasing modulation is that you sacrifice performance. As you ramp up modulation, you decrease the distance you can go. What you want is the highest modulation rate over a given distance.

The other method of increasing network efficiency it increases the baud rate – the speed at which you’re transmitting (i.e. the number of times per second the signal phase is being changed). This has a nice benefit because you can increase the baud rate with minimal impact on optical performance. The problem is that as you increase the baud rate, the signal gets wider – it occupies more and more spectrum.  Hence, while increasing baud rate improves the cost per bit as each transceiver is capable of transmitting more information, it does not improve spectral efficiency or increase fiber capacity. Optimal optical networking solutions effectively balance modulation rate and baud rate depending on each application.

With 5th gen, the peak modulation rate is 64 QAM and the baud rate is typically about 96 Gigabaud. The baud rate has been ramped up to get more capacity out of each laser, ultimately reducing the total cost of ownership, but it doesn’t actually improve spectral efficiency. 

One characteristic of each generation of coherent technology, is that the peak rates typically support similar maximum transmission distances of around 100-200km. Longer transmission distances are possible with each generation by decreasing the transmission rate. For example, 3rd gen technology was capable of transmitting its peak rate of 400G roughly 100-200km. With 4th gen, it was capable of transmitting 600G roughly 100-200km, but when dialed back to a 400G transmission rate it could achieve distance of 2000+ km.

While other solutions maintained that trend with their 5th gen (800G) coherent solutions, Infinera broke the mold. With Infinera’s 5th gen coherent solution (ICE6), Infinera was able to achieve transmission distances of more than 700km in live networks with deployable margins. This is essentially 2-3 times better than what was possible with peak rate of previous generations. This superior performance also propagates when ICE6 is configured for lower data rates enabling transmission distances of more than 1400km at 700G and more than 2400km at 600G. Also, with Infinera’s 5th gen coherent solution, it now has the theoretical capability at 100G of transmitting a 100G signal from any point on earth to any other point on earth without regeneration…if such a fiber network existed. 

What does 5th gen bring to subsea?  It enables 400G per wavelength transmissions at subsea distances, which is the first time the industry’s been able to do that. 

The whole idea is about, over a given distance, how much information can you get on a single wavelength and how much information can you get on that fiber. For some parts of the network, getting more information on each wavelength is the focus; and in other parts of the network, getting more information on each fiber is the focus. At the edge of the network, fiber has virtually little value and hence the cost of the laser is the critical factor. As you move closer to the core of the network – regional, long-haul and subsea – the equation flips and the cost of the laser becomes significantly less important because the value of the fiber and the amplifiers and all the plan in-between dwarfs anything an operator would spend on a laser. The worst thing that can happen to any subsea operator is that they run out of capacity on a fiber. The key focus for subsea operators is optical solutions that provide the greatest spectral efficiency, enabling the greatest amount of information on each fiber and providing the ability to extract more value out of every fiber. While high-performance optical engines can provide value across the network, the greatest value will be derived by regional, long-haul, and subsea network operators.

Rob Shore

Robert Shore, SVP Global Marketing, Infinera