Low-Power Optical Technology Yields Energy-Harvesting Opportunities in High Data-Speed Applications

Reducing the run-away power consumption associated with data networking has become almost as important as increasing speed and bandwidth to cope with growing demand. Optical technology not only delivers performance, but also has potential for reducing energy requirements both at the backbone level through passive optical networks (PONs), and at the chip level, with devices such as optical memory and low-power optical transceivers.

This article will outline some of the ideas and techniques under development in the optical-networking sector that will result in reduced power consumption. Further, it will speculate on the opportunities that may then arise for energy-harvesting techniques in this domain. An obvious contender is the optical wireless system that may be used to extend the fiber backbone into the home, office, or enterprise, both in new and retrofit applications.

Thermal energy given off by these high-speed optical devices, modules, and subsystems is the most likely source for harvesting. A review of components will highlight the Laird eTEC thin-film modules, ideal for optical applications, plus power-conversion and management devices designed to operate with thermoelectric generators (TEGs), such as DC/DC converters, available from companies including Linear Technology, and step-up low-voltage booster devices from vendors including Advanced Linear Devices and Touchstone Semiconductors.

Network capacity race

Fiber optic cable is the enabling technology for very-high-speed data networks. In data centers, optical networks running at 1 and 10 Gbps are running out of steam. Upgrade paths to 40 Gbps systems are already in place. Enterprise systems are upgrading to 10 Gbps. But for the internet backbone, metro and long-haul networks, the race is on to reach 100 Gbps in the near future.

Demand is being driven on multiple fronts. It is led by video-based multimedia applications delivered not only by cable to homes and businesses, but also by the hugely growing number of internet-enabled ‘smart’ wireless devices. In addition, the ICT infrastructure of developing countries is expanding rapidly, and expected to continue for at least the next decade.

However, there is growing concern over the industry’s global carbon footprint. Although currently, the ICT industry is responsible for a relatively small proportion of global greenhouse gas emissions (2 to 2.5% according to the ITU), growth is projected to double this figure by 2020. Optical networking technology is regarded as the primary means of achieving the speed and capacity improvements demanded of datacom and telecom networks. It is less well known that fiber optics can also help reduce power consumption.

Meanwhile, many national and international initiatives have been launched in an attempt to improve efficiency and keep energy demands for the ICT sector under control. The GreenTouch¹ consortium is one example. Founded in 2010, it includes carriers, service providers and equipment manufacturers, as well as a significant number of the world’s leading universities and research institutes specializing in telecommunications technology. Its mission is to deliver by 2015, the architecture, specifications and roadmap to increase network efficiency by a factor of 1000 compared to 2010 levels. Several projects are underway that are focused on reducing power consumption.

Fiber efficiency

Passive optical networks (PONs) have been steadily emerging as a key technology in the battle to keep costs and power consumption down. This point-to-multipoint architecture, using shared optical fibers and unpowered optical splitters, has become popular in the US and Europe with telecom operators and growth is accelerating in Asia. Gigabit PON (GPON) is giving way to 10 GPON, with 40 GPON systems under development. An Ethernet compliant version (GEPON) helps migration.

A year ago (2012), the GreenTouch consortium announced a PON technology, aimed at reducing energy consumption in fiber to the home networks. Called Bit Interleaved PON (BiPON), the protocol is expected to enable a power reduction of 30 times over current technologies while improving performance and reducing cost.

Silicon photonics

At the chip level, efforts continue to improve communication links between processors and the optical backbone. IBM has used optical interconnects in servers since 2008 and exploits optical transceivers to deliver the performance it needs in the Power7 supercomputer.

But with the drive towards ever-faster, high-bandwidth data transfers in today’s more complex devices and systems, research into optical interconnects is beginning to pay off. Both monolithic-integration and stacked-chip architectures are showing potential. Called silicon photonics, the key to this technology is to integrate as much as possible into the silicon. A critical hurdle has been to integrate the lasers onto the chip, which is only just beginning to happen.

IBM has developed a method of integrating optical and electrical components onto a single 90 nm geometry, silicon-chip substrate. Dubbed ‘silicon nanophotonics²’, integrated features include modulators, germanium photodetectors, and highly-compact wavelength division multiplexers (WDM), together with conventional, high-performance digital and analog CMOS circuitry.

Figure 1: IBM’s silicon photonics technology will enable fully-integrated optical and electrical circuitry on a single chip. Blue optical waveguides transmit high-speed optical signals, while yellow copper wires carry high-speed electrical signals.

High transmit and receive at data rates (in excess of 25 Gbps), reduce data traffic congestion. Parallel optical data streams feed into a single fiber using WDM techniques. Scalability is an inherent feature, with the future vision of optical communications delivering terabytes of data between distant parts of computer systems, according to IBM. Power savings are likely to be a feature of this highly-integrated technology.

Other silicon photonics companies, such as Kotura, have made significant breakthroughs in terms of integrating optical components using silicon and low-power silicon germanium processes. Although the laser is not monolithically integrated, it can be assembled into the chip package. The company’s optical transceivers are said to dramatically reduce the size and power demands of servers and supercomputers.

In data centers, small, low-power optical devices convert high-speed electrical signals to optical signals, and transmit them via optical fiber (chip to board/rack to server) over any distance. Data is converted back from optical to electrical at the receiver end. In the race to 100 Gbps, Kotura is packing four 25 Gbps transceivers into a QSFP package. The device supports WDM, and consumes 3.5 W. However, this type of integration eliminates the manual assembly of hundreds of small parts, saving cost and space, as well as reducing overall power consumption.

Thanks for the memory

Research continues both in Europe and Japan into optical random access memory (RAM) devices that are not only ultra-low power themselves, but which can eliminate the need for electro-optic conversions and the associated energy required to power the switches and converters. This could result in larger-scale optical RAM systems that handle high bit-rate optical signals. Optical packet switching could, in turn, significantly reduce overall power consumption in high-speed, high-data-rate optical telecommunications systems.

Replacing electronic circuitry with optical RAM is regarded as a tough challenge, but ultimately the key to super-efficient optical routing and processing. A team from NTT in Japan has realized all-optical RAMs with a power consumption of only 30 nW, claimed to be 300 times lower than previously achieved.

Researchers from Belgian research establishment, Imec and Ghent University, together with other European institutes, have developed extremely-small, fast optical rams with minimal power consumption. The devices incorporate micro-disk lasers with a diameter of 7.5 µm, made from indium phosphide membranes. Not only can the devices be interconnected with other types of memory cells using silicon wires, but also the devices can be manufactured using well-proven, cost-effective silicon processes.

Thermal management

Harvesting energy from the heat dissipated by optoelectronic components has long been considered feasible. Scientists have noted that when high optical intensities are present, silicon soaks up photons in a process called two-photon absorption (TPA). The optical energy lost to TPA can be converted into electrical power that can be used to drive the chip.

A team from University of Central Florida developed the two photon photovoltaic (TPPV) effect which can harvest up to 40% of the carriers lost through TPA. The process can also be used in optically powered sensors for fiber-optic networks and other devices that use III-V materials, which demonstrate the same absorption effect as silicon.

But while energy harvesting at the microchip level currently remains in the realms of the researchers, there are techniques and devices commercially available for thermal management and harvesting thermal energy for use in optical and photonic applications.

The eTEC series of thin-film thermoelectric modules from Laird Thermal Products are designed specifically to stabilize the temperature in optoelectronic telecom and photonics applications. Thin-film technology allows for smaller packages, yet the devices are said to have ten times the heat pumping density per unit area than conventional devices. Aimed at high-powered devices such as optical transceivers, laser diodes and photodiodes, the eTEC devices are ideal for lower current applications with restricted physical space constraints. Heat pumping densities range from 75 to 90 W/cm².

Figure 2: Thin-film thermoelectric eTEC modules from Laird Thermal Products are ideal for optical telecom applications.

The Laird eTEC HV37 module measures just 3.39 x 2.05 x 0.62 mm (6.9 mm²), with a Qmax of 3.7 W at 25°C. The slightly larger HV56 module comes in at 10.2 mm² with a Qmax of 4.8 W at 25°C. The modules also feature precise temperature control, better than 2 ms response time, and are RoHS compliant. The company has a wide range of traditional thermoelectric modules in its OptoTEC series.

Energy management

Harvesting thermal energy from optoelectronic devices typically requires a thermoelectric generator (TEG). Laird offers a selection in its eTEG series. However, once harvested, the energy needs to be managed. One of the most popular devices for this type of application is the LTC3108 from Linear Technology. This highly-integrated DC/DC converter operates with TEGs as well as thermopiles and small solar cells. The step-up topology operates from input voltages down to 20 mV.

Figure 3: Linear Technology’s LTC3108 DC/DC converter is ideal for managing the surplus energy harvested by thermoelectric generators (TEGs).

The device incorporates a small step-up transformer, and provides a self-contained power management solution. LDO is 2.2 V at 3 mA, enough to power an external microprocessor. The main output is programmed to one of four fixed voltages and the host can enable a second output. A storage capacitor provides power when the input voltage source is unavailable.

Another popular option is the EH4205 micropower step-up low-voltage booster module from Advanced Linear Devices. It converts a low DC voltage (75 mV minimum) to a higher AC or DC voltage. The input voltage may come from photodiodes, TEGs or electromagnetic generators. It powers itself from the input voltage, starting as low as 230 µW. Nominal input impedance is 50 ohms (a 950 ohm version, EH4295, is also available). An on-board transformer couples to a dedicated MOSFET array, specifically designed for this application. An input decoupling capacitor integrates and then filters the input signal to drive the transformer.

Figure 4: Advanced Linear Devices’ micropower low-voltage booster modules.

It can be combined with the company’s energy harvesting modules, EH300/301, and can be used to trickle charge batteries or super capacitors. A bridge rectifier can be added by the user to produce a full-wave rectified DC output. This DC output can be used to power a current-compatible electronic circuit directly.

For energy harvesting applications where the minimum voltage available is higher, the TS3300 power management IC from Touchstone Semiconductor is worth considering. It combines a high-efficiency boost regulator and a low drop-out linear regulator in a single package. From a supply voltage of 0.6 V, the device can deliver at least 75 mA at 1.2 to 3 V. With the LDO input connected to the output of the boost regulator, serving as a post regulator for the boost and additional functions, such as buck-boost, can be provided.

The device can be used to post-regulate voltage buffered in a supercapacitor. The LDO can deliver up to 100 mA output current at a dropout voltage of 255 mV, and reduce the ripple voltage out of the boost regulator by a factor of three.

Harvesting opportunities

Emerging high-speed optical-network technology is likely to yield some opportunities for energy harvesting in the future. At the microchip level, it may well be possible to start powering, or partially powering, electro-optical circuitry or lasers by exploiting the heat generated by optical components.

Such techniques could be extended to trickle charge batteries or supercapacitors, which themselves could power remote units. Optical wireless-network systems are beginning to emerge, designed to be installed in enterprises or on a campus, to extend existing networks where additional high-capacity connections between offices or buildings are required. Such network extensions could be located in places where there is no main power. Battery-powered optical-network units or terminals may be feasible, and energy harvesting techniques could be employed to provide the power or charge batteries, as they are currently in conventional wireless sensor networks.

To provide the higher power needed for larger units and even small installations, solar energy may be a more viable solution. See DigiKey article on energy harvesting for mobile phone cells for a more in depth look at this subject.

Additional applications for optical networks are envisaged, to provide a safer means of remote monitoring and data collection in extreme conditions, such as in mines, where any danger of a spark from electrical circuitry has to be avoided.

Finally, indoor wireless optical-networking products are becoming available to provide high-speed, high-bandwidth networks in new and retrofit applications. The aim is to avoid the installation or replacement of cumbersome cabling networks and switching panels. It may be highly desirable to have remote units that are autonomously powered, and ideal for energy-harvesting technology.

Summary

Optical-networking technology will become prevalent in order to serve the high-speed, high-bandwidth demands of the ICT sector. An increasing focus on reducing power consumption at all levels, from chip to server farm, together with the potential for optical wireless networks, is likely to ensure a continued focus on the potential of energy harvesting. Thermal energy emitted directly by optoelectronic devices is the most likely source of power that can be scavenged, managed, stored, and re-used.

References:

1. GreenTouch consortium
2. IBM’s silicon nanophotonics