Furukawa Electric Co Ltd of Tokyo, Japan has developed a vertical-cavity surface-emitting laser (VCSEL) that operates at 25Gbps and has realized transmission of up to 500m over prototype multimode fibers developed by optical fiber product maker OFS of Norcross (near Atlanta), GA, USA (which is owned by Furukawa Electric) and Corning Inc.
“Both longer reaches and higher speeds are required for warehouse-scale data centers,” says Dr Haruki Ogoshi, senior fellow at Furukawa. “Our customers are raising expectations as they implement next-generation data centers,” he adds.
The rapid growth of cloud computing and expansion of ‘big data’ is causing a dramatic increase in the volume of data handled in data centers, notes Furukawa Electric. This has generated demand for greater throughput in servers, switches and routers in data centers. Increasing the serial speed of transmission of lasers and photo-detectors is one solution. In addition, there is a need to increase transmission distances between devices as data centers become larger. In mega data centers, for example, transmission distances of 300m or longer can be required. Currently, VCSELs emitting at a wavelength of 850nm are widely used for the lowest-cost optical interconnects. However, it is known that, when the transmission speed is changed from 10Gbps to higher-speed 25Gbps, the transmission distance is limited to less than 200m due to the influence of chromatic dispersion in multimode fibers. To solve these issues, Furukawa has developed a VCSEL with a wavelength of 1060nm that operates at 25Gbps.
“Increasing the VCSEL wavelength from 850nm to 1060nm reduces chromatic dispersion of fibers by approximately two thirds,” says Dr Durgesh Vaidya, senior manager Research and Development for OFS. “Accordingly, use of a multimode fiber with the modal dispersion minimized at 1060nm reduces transmission impairments and allows transmission distances to be extended,” he adds. “It is widely expected that it will be easier to increase VCSEL speeds in the future beyond 25Gbps by moving from 850nm to higher wavelengths.”
Furukawa created a prototype optical module integrating the newly developed 1060nm VCSEL in a small package with a driver integrated circuit and then conducted a transmission experiment close to actual operating conditions. When a multimode fiber from OFS or Corning was used with the modal dispersion optimized for a wavelength of 1060nm, a long reach of 300m was achieved. In a joint experiment, a short length of modal-dispersion-compensating fiber developed by Corning was used in conjunction with standard OM4 multimode fiber, and a transmission distance of 500m was achieved. These results were obtained without the use of electrical compensation technologies such as a clock data recovery (CDR) or an error correction. In this way, the new VCSEL technology can enable an increase in speed and distance without complicating future systems, reckons Furukawa Electric.
“This collaboration shows that long-wavelength VCSEL technology can meet the demands of next-generation data centers when combined with new, wavelength-optimized fiber, or even standard OM4 fiber with new modal-dispersion-compensation fiber,” says Dr Alan F. Evans, research director, Optical Physics and Transmission Technology, Corning.
