Components startup General Photonics Inc. has just closed a second round of finance, Light Reading has learned.

The company raised $5 million from unnamed venture capitalists, which it says will be enough to see several new components through the design process and into production.

The startup doesn't require a great deal of extra cash at this point because "we're already running cash positive," according to Steve Yao, its founder and CTO. In fact, he says, it generated $1.2 million in revenue in the four months following its first round of funding.

General Photonics is developing components and modules that tackle polarization mode dispersion (PMD) -- a signal impairment resulting from the fact that some parts of an optical signal take longer than others to travel down a fiber.

Despite the fact that it's addressing one of the key obstacles to the deployment of 40-Gbit/s systems -- and doing it rather well, it appears -- the company has managed to stay below the radar.

Perhaps one reason for this is that General Photonics isn't setting itself up as a supplier of turnkey solutions, like Phaethon Communications and Yafo Networks, which offer subsystems that bolt onto existing DWDM equipment (see Can Yafo Lift Speed Limits?). Instead, it provides various key components needed for PMD compensation to these companies -- and also to network equipment vendors, such as Nortel Networks Corp. (NYSE/Toronto: NT), that have the knowledge to incorporate those components into their own system designs.

"We looked at the market and found that there are no key components out there that people can assemble to make a turnkey module or product," says Yao. "So we took one step back."

The first step was to identify what the building blocks of a PMD compensator are, he says. But before digging into the details, it's necessary to understand the basics of PMD.



As the name implies, PMD is caused by light of one polarization traveling faster than another. Polarization describes the direction of vibration of the electric component of light. (Light is an electromagnetic wave, remember?) Light can be split into two components at right angles, which travel independently: These are called polarization modes.

Fundamentally, PMD is caused by the core of the fiber not being perfectly round in cross-section. As a result, the two polarization modes "see" subtly different optical characteristics and travel at different speeds in the fiber. When they get to the receiving end, the delay between the arrivals of the two polarization modes can make it difficult or impossible to interpret the data (for more on PMD, see Nonlinear Effects).

To correct for PMD, the fast part of the signal needs slowing down, so that the slow part can catch up. This is done using a so-called differential delay line (DDL). Yao claims that General Photonics is the only company selling a standalone DDL, a product that it launched at the Optical Fiber Communications Conference in Anaheim earlier this year.

It sounds simple, but in practice it's more complicated, because, unlike other forms of dispersion, PMD is not fixed. On the contrary, it comes and goes in a completely unpredictable fashion as a result of environmental influences, such as fiber bends, splicing, temperature changes, vibration, and a variety of external forces that change the circularity of a fiber core.

Therefore, PMD has to be treated dynamically, which requires a couple more parts. One is a polarization controller -- a widget that converts any polarization state to any other. It's needed because the polarization state of the light coming out of the fiber is random and needs converting to a known axis before it can be processed by the DDL.

The final key component is a polarization detector, which can analyze the signal and decide what values to set on the polarization controller and the DDL.

General Photonics has developed all three components, as well as a module that integrates a polarization detector with a DDL, says Yao. There are innovations in all of the components, he says, but the design of the polarization controller resulted in the most significant improvement over existing technologies, and generates the biggest portion of the company's revenues.

"We squeeze the fiber to make it birefringent," explains Yao. When a fiber is squeezed, its refractive index increases at right angles to the direction of squeezing. "All you need is a piece of fiber being squeezed in four different places, and then it's possible to rotate the polarization by any desired angle. It's that simple. And it all fits in a three-inch-long package."There are lots of other ways of making a polarization controller, but they all have significant drawbacks, says Yao. JDS Uniphase Inc. (Nasdaq: JDSU; Toronto: JDU) has developed a device based on lithium niobate. E-Tek (now part of JDSU) has a liquid-crystal-based component, and Corning Inc. (NYSE: GLW) is using electro-optics.

Yao feels that all of these approaches have fundamental problems. Liquid crystals are too slow, with response times of about 50 milliseconds. The fastest PMD variation recorded in the field is just a few milliseconds, he says. That means a PMD compensator needs to respond in 100 microseconds -- 2,000 times faster than what's possible with a liquid crystal.

Lithium niobate is very expensive, has high insertion losses, and requires a high operating voltage of 120 volts (a figure quoted on JDSU's data sheets). Electro-optic technology runs into problems that require the system to be reset periodically; otherwise the voltage needed to turn the polarization gets bigger and bigger.

"Our approach has an insertion loss that's 10 times better than other devices, a polarization-dependent loss that's 100 times better than other devices -- and it costs one tenth as much as lithium niobate."

Yao is reluctant to name customers but does point out that his company has very close ties with Phaethon (see Phaethon Embarks on Tunability). Yao and Alan Willner, a founder of Phaethon, were both professors at the University of Southern California.

"The performance of General Photonics' products is outstanding," says Willner in a General Photonics' press release announcing the company's fiber-squeezer polarization controller.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com