In results presented at the 2000 Optical Fiber Communication conference (OFC; March 5-10; Baltimore, MD), researchers from Agilent Technologies (Palo Alto, CA) outlined how they have taken a concept originally developed for inkjet printers and used it in an all-optical switch. In an inkjet printer, bubbles of gas created by electrical heating propel droplets of ink toward paper; after almost two decades of commercial development, the technology is consistent and reliable. The Agilent device creates bubbles in the same way, but uses them instead as a quickly appearing and disappearing gas-fluid optical interface.
At each switching point, two silica-based single-mode waveguides intersect at a fluid-filled trench such that the angle between each waveguide and the normal to the trench is greater than the angle at which total internal reflection begins for a gas-to-fluid interface. When an oblong bubble is created at the intersection, light reflects off it; when the bubble disappears, light passes straight through. Switching time is less than 1 ms; crosstalk was measured to be -70 dB. The researchers have fabricated an array of such elements into a 32 x 32 optical switch. To achieve the desired 20-year lifetime, the device is hermetically sealed and made of materials that are compatible with the working fluid.
The Agilent array device must compete with existing technology that includes thermo-optic and micro-electromechanical systems (MEMS) switches. Thermo-optic switches, many of them containing Mach-Zehnder waveguide interferometers, are large, making 32 x 32 arrays impractical; switches based on MEMS mirrors require precise alignment of optical fibers to the mirrors. The inkjet switch has the additional advantage that when no electricity is going to the device, light passes straight through the waveguides, according to Julie Fouquet, project manager at Agilent Laboratories. The 32 x 32 switch is "two dimes long," says Fouquet.
Preliminary calculations of the 32 x 32 switch show losses of 2.6-6.9 dB along the different paths, based on a 0.07-dB loss across each trench and a 2.1-dB reflection loss off the sidewall of an empty (gas-filled) trench. Maximum polarization-dependent loss is 0.05 dB. Arrays of 32 x 32 switches will be combined into devices as large as 512 x 512. As a straightforward extension of inkjet technology, the switches are extremely reliable, she notes. "Inkjet pens are well-proven; large inkjet arrays should be reliable for 5 x 108 shots," she says.
Agilent is working with Alcatel (Paris, France) and other customers to commercialize the switch and is building prototypes for certain partners. The company aims to have commercial prototypes available by the end of the year; such devices will meet the applicable Telcordia reliability specifications, according to Quata Ocano of Agilent's Optical Networking Division.
Also at OFC, researchers from NTT (Tokyo, Japan) described the development of an all-optical switch that, although not based on inkjet technology, makes use of a shifting gas-fluid interface. The NTT device contains a gas-filled trench that holds a small bead of index-matching oil wetted to the sides of the trench. A pair of microheaters produces a thermal gradient along the switch, drawing the bead of oil in or out of the optical path by thermocapillary action, causing either transmission or total internal reflection; the action is bistable. The researchers have built a 16 x 16 switch on a 23 x 23-mm chip. Insertion losses ranged from 4 to 10 dB.
