Industry works to solve the inefficiencies that have stymied the creation of a reliable and widely accessible all optical telecommunications network for the home.
This past March, the Optical Fiber Communication and National Fiber Optic Engineers Conferences (OFC/NFOEC) were held together for the first time. More than 650 companies showcased their technologies—offering a glimpse into the future of optical communications. Also evident from this event was that, for the first time, the promise of ubiquitous high-bandwidth fiber-to-the-home (FTTH) was more than a distant dream.
Component vendors, system suppliers, and service providers all demonstrated their technological readiness. Although the technology is ready, cost is still a barrier to FTTH implementation. Each company in the supply chain is working to reduce costs, but a few are following a unique path by transitioning integrated optics from the lab to the field.
 In this 10-Gb/sec laser source,a PLC Bragg grating produces an external cavity laser with a small, directly mounted gain chip. Because the gain chip is small, the frequency is stable and the laser can transmit high data rates over long distances (Photo: RIO) |
Adding integrated opticsOptical fibers can support a much higher modulation frequency than electrical wires, and they can also transmit many non-interfering signals at the same time. A complete fiber communication system can include a laser source that converts electronic signals into light; mirrors, beamsplitters, polarizers to combine optical signals; lenses to couple light into a fiber; intermediate amplification stages; then another network of mirrors, beamsplitters, polarizers to separate the different signal paths; and finally a detector that converts the optical signal to an electronic signal.
Like any system with mirrors and beamsplitters, these elements must be aligned, adjusted, and then maintained in that alignment. This is the step that integrated optics offers to eliminate.
One company bringing integrated optics into the field is Redfern Integrated Optics (RIO), Santa Clara, Calif. RIO has developed a 10-Gb/sec directly modulated external cavity laser source. The device consists of a gain chip coupled to a planar lightwave circuit (PLC). The PLC is both a waveguide and a wavelength stabilizer, constructed in silica on silicon using standard deposition and photolithographic manufacturing techniques borrowed from semiconductor production.
The gain chip can produce radiation over wavelengths from 1530 to 1565 nm. When coupled to a Bragg grating PLC, the combination forms an external cavity laser with a spectral width of less than a tenth of a nanometer. The small physical size of the directly-modulated gain chip makes for clean signal modulation, with very little “chirp,” or frequency shift within an optical pulse. This is especially important for transmission distances of 80 km or more, where chirp can degrade the quality of 10- Gb/sec modulation and increase the error rate.
RIO is developing a 10-Gb/sec laser on what they call a “silicon optical bench”—a PLC that incorporates additional functions. Radu Barsan, RIO’s President and CEO, believes their device “replaces obsolete/expensive/large/power-hungry technologies, much like the transistor-based portable radios replaced the vacuum tube radios, therefore enabling their widespread deployment.”
A 10-Gb/sec laser source is impressive, but, considering high-speed home datalines today are about ten thousand times slower, it’s going to take more than a high speed laser source to bring the next increment of telecommunication speed to the home.
That said, difficulties with installing the infrastructure are being overcome, but even when fiber is brought to a location the question remains, what next?
 ColorChip uses an ion-exchange process to produce waveguides embedded in a glass substrate. This SystemOnGlass is particularly well suited
for fiber-to-the-home applications, where transceivers like this would be
at every end-user’s location. |
Transceiver techThe answer is that the critical element at the home is going to be the optical transceiver, the element that converts the incoming data format to a format usable in the home. Here, integrated optics are also offering potential. ColorChip, based in Or-Akiva, Israel, is developing a set of transceivers using planar light circuits.
In general, PLCs work by confining light to regions of a material that are engineered to have a higher index of refraction than the surroundings. ColorChip starts with an inexpensive, omnipresent substrate: glass. A metallic film is coated on the glass, then etched away to expose the waveguide region. The glass chip is placed in a bath of molten silver salt, and the silver ions migrate into the glass. An electric field is then applied to the glass, and the ions migrate to create a waveguide buried within the glass. By patterning the waveguide it is possible to engineer couplers, splitters, wavelength filters, and other components.
The next step is to place the electronic die components onto the optical path along the glass. Photodiodes, laser diodes, and amplifiers are all directly mounted to the glass using surface mount technology. As Ari Mizrachi, ColorChip’s director of marketing says, “we’re trying to bring the capabilities of semiconductors to the photonics industry.” The cost advantages of the ColorChip approach make it an attractive technology for many elements in optical telecommunications networks, but, because each end-user needs a transceiver, FTTH may be most sensitive to cost and most open to the PLC transceivers. “We are enabling a different level of pricing,” says Mizrachi.
Flexibility question
Even more important than cost is the flexibility offered at these nodes. Demand in any telecommunications network is variable, both in the short-term and long-term. There are essentially two ways to address variable demand: build in enough capacity to exceed the highest demand, or build in flexibility to add capacity as demand grows. In practice, elements of both approaches are incorporated in existing network design, but reconfiguration is typically done by a technician in the field to reconnect optical fibers at a hub. Infinera, Sunnyvale, Calif., is deploying photonic integrated circuit (PIC) technology that aims to provide maximum flexibility at a minimum cost.
Infinera’s PIC is built on a substrate of indium phosphide (InP), a material that can directly generate and detect wavelengths typically used in long-distance optical fiber communications. “We’ve integrated on InP so we can integrate the components that cost the most. We’ve put over fifty components on one chip all interconnected with optical waveguides,” says Serge Melle, Infinera’s VP of Technical Marketing.
Because photodetectors, lasers, modulators, and other components are on a single chip, the cost of converting optical signals to electronic signals is significantly lowered. And because PIC technology eliminates the cost penalty associated with optical to electronic conversion (OEC), it becomes possible to electronically manage the data at each node, adding functionality and flexibility.
For example, if additional capacity is needed between two cities all the intermediate reconfiguration can be done remotely with software, with a technician only required to plug in fiber at each end of the link. That kind of enhanced signal management capability is why Freenet, the second largest Internet service provider in Germany, chose to deploy Infinera’s digital transport network as the backbone of their service.
“PICs have crossed the threshold for R&D into real-world, commercial deployment,” says Dave Welch, Infinera’s Chief Development Officer, in a session taped at the OFC/NFOEC Conference and broadcast at Optical Keyhole.
Enabling strategy
The ultimate promise of optical telecommunications is a high-bandwidth connection to every user, and that vision has been touted by the industry almost since the first practical telecommunications fiber was invented at Corning in 1970. The realization of that vision may now be in sight. According to Infinera’s Melle, “Photonic integration is one of the technologies that you want to look at to make high-capacity bandwidth widespread.”
—Richard Gaughan
Resources
ColorChip, +972-4-610-1330, www.color-chip.com
Infinera, 408-572-5427, www.infinera.com
RIO Inc., 408-970-3500, www.rio1.com
