The collaboration of two companies to produce highly reliable, grating-stabilized 980-nm pump modules will enable submarine systems to increase wavelength channel counts to 32 and beyond. SDL Inc. (San Jose, CA), and Alcatel Optronics, headquartered in France, initiated an extensive reliability testing project of such modules in 1998 and recently announced successful results at the Topical Meeting on Optical Amplifiers and their Applications in Nara, Japan.
Alcatel now plans to incorporate SDL's latest 980-nm chips and fiber Bragg gratings to build high-power, high-reliability, wavelength-stabilized undersea pump modules.
Until its development partnership with Alcatel, SDL had never developed a product for undersea applications. The fundamental difference in qualifying a component for undersea application lies in the difficulty of replacing faulty components. In terrestrial systems, faulty components can be replaced in a few hours. The situation becomes much grimmer in a submarine application, where it may take days to repair and resume transmissions. Lost revenues could be disastrous.
"The main cost involved is not in replacing a failed component but in the loss of traffic you have on a fiber that is now dead," says Jo Major, director of communications, laser and device products at SDL. "A 980-nm pump for submarine use must have an order-of-magnitude better reliability than required for a terrestrial application. To prove that very, very low failure rate with confidence, you have to have extensive long-term testing of thousands of devices."
An example of the higher testing demands for a submarine product can be seen in a simplified model. A terrestrial test that may be successful enough for deployment of 980-nm pump chips in terrestrial applications might require 100 lasers. By contrast, to qualify for submarine applications, the same chips would be tested using 1000 or more lasers.
In the initial fabrication of the diode chip, the processes are identical for both terrestrial and undersea products. Sampling techniques are used to separate materials suitable for undersea applications based on both performance and reliability screening. The wafers suitable for submarine applications are processed through the line and become eligible for submarine applications.
Following all manufacturing and testing, fully specification-compliant product moves on to further analysis, known as "pedigree review," which eliminates any diodes with unusual qualities or parameters. If a particular characteristic is outside the normal parameters, higher or lower than the norm, the component is rejected.
To date, undersea systems have been deployed with 16 channels, says Major. But by achieving higher power with the same reliability required for undersea applications, a new breed of pump modules will now increase the use of dense wavelength-division multiplexing (DWDM) for submarine fiber deployment. Power is key to using DWDM technology under the sea.
"In a simple model for DWDM," says Major, "each 'on' state requires a certain number of photons so the detector accurately reads an 'on' state. As the data stream moves to higher bit rates, more power is required to detect an 'on' state, because you have less time to deliver those photons. In addition, each channel requires power. As channel counts and bit rates go up, the power requirement for a DWDM system also rises. That forces the amplification systems to have higher output power.
"So if we want to double the capacity on a network from eight to 16 or 16 to 32, we have to increase the output power of each amplifier by a factor of two. In simple terms, anytime you want more channels or higher bit rates, you need to get more optical power flowing through the system, either by using amplifiers with higher output powers or by spacing the modules closer together in the network."
Increasing the power poses a traditional problem--the tradeoff of power for reliability. As the output power is increased in a 980-nm chip, the reliability rate decreases. Terrestrial customers will normally run at higher power and slightly lower reliability. The submarine system community, without the luxury of relatively quick repairs, must turn the power down in favor of higher reliability.
While the advances offered by the SDL/Alcatel partnership promise to improve the performance of undersea systems, even more powerful pump lasers are on the horizon, according to Major. SDL is on the verge of qualification testing of its latest pump chip, the SDL 6540, which could double the reliable operational output power for pump modules.
SDL's current chip has a "kink" power (the maximum power for maintaining good optical qualities) of 310 mW. That equates to an actual operating power between 200 and 250 mW. SDL expects its 6540 chip will increase the optical kink power to beyond 700 mW, removing a major limitation in the manufacture of pump modules. The reliability work accomplished thus far, says Major, suggests that terrestrial operation could exceed 400 mW. SDL expects to have enough qualification data on the SDL 6540 by the first quarter of next year to bring the chip to market.