Startups pursue wafer-level integration
With component and subsystem developers now focused on cost as much as performance, startup companies continue to pursue various avenues toward wafer-level integration of active and passive functions. Planar-lightwave-circuit (PLC) technology has captured the majority of attention in this area in recent years; however, the recent closure or acquisition of several PLC startups indicates that this path to integration may be longer than originally envisioned. A new round of emerging companies has touted alternative approaches to combining active and passive functions on indium phosphide (InP) wafers.
ASIP (Somerset, NJ) represents one such company. Founded in July 2000 based on technology developed at Princeton University, the company's Asymmetric TwinGuide (ATG) approach enables the integration of passive and active functions within a single InP wafer growth step, according to ASIP marketing vice president Yassi Moghaddam. Whereas typical InP wafer processes rely on selective regrowth and selective-area growth—thereby adding complexity and cost while lowering yield—ATG creates asymmetry in the refractive index of different layers of the first wafer growth, thus forcing propagation in the desired layers. Lateral tapers (3 to 0.8 mm) are etched where needed to enable adiabatic transformation of the propagating mode between vertically separated layers—from an active layer near the surface of the wafer to a passive layer beneath it, for example.
Multiple layers, each optimized for its particular function, can be grown for more complex integration, according to Moghaddam; thus, functions would be integrated "vertically" rather than "horizontally," as is the case with PLC approaches. ATG devices can be created by semiconductor processing on standardized wafers, which would enable the production of such devices to be outsourced to standard wafer houses.
Moghaddam says the result is low-cost, highly integrated devices that can be customized rapidly. The company is demonstrating the capabilities of its technology through its first product, an uncooled 10-Gbit/sec electro-absorption modulated laser (EML) operating at 1310 nm. The EMLs, which carry the model number 10T101 and are delivered in chip-on-submount format, are targeted at the upcoming generation of 10-Gigabit Ethernet small-form-factor transceivers and transponders, particularly the X2, XPAK, and XFP. They do not require thermoelectric coolers, she says.
The new EMLs outperform the directly modulated lasers (DMLs) common to such applications, according to Moghaddam. They provide an electrical eye margin of 64% and a similar optical eye margin at 10°C; the optical eye margin grows to 67% at 50°C and is 54% at 85°C. The devices are also priced to be "competitive" with DMLs. Sampling of the laser began this summer, and production quantities are expected to be available in the first quarter of next year.
The initial version of the laser will support rates up to 10.75 Gbits/sec and transmission distances of 10 km. Moghaddam reports that a 1550-nm version is on the drawing board.
Meanwhile, Phosistor Technologies (Pleasanton, CA) also is pursuing monolithic integration of active and passive functions in InP. However, the key to Phosistor's approach is the ability to change bandgaps across the same wafer. The two-year-old company can link active regions using very short passive waveguides, according to Yan Zhou and Yuan Shi, both of whom are Phosistor vice presidents. The company can also passively align multiple fibers to its devices, thus lowering costs and increasing efficiency, they said. The process is capable of accommodating 40 channels across 100 nm in one growth.
The company will focus on photodetectors and optical performance monitors initially. Phosistor's optical-amplifier photodetector integrates a waveguide PIN photodetector with a semiconductor optical amplifier. It can also be packaged with an optional transimpedance amplifier in an RF butterfly package to create a device that is less than half the price of conventional avalanche photodiode devices but provides better sensitivity, according to the Phosistor spokesmen.
Meanwhile, the integrated optical performance monitors combine a passive input waveguide, a grating to separate the channels, and an array photodetector to measure power. It can be used to monitor channel wavelength, power, and optical signal-to-noise ratio in real time. The company claims its device outperforms monitors based on tunable Fabry-Perot filters or fiber Bragg gratings as well as arrayed-waveguide and diffraction gratings at a lower price.
The optical performance monitors are scheduled to sample this quarter. The company plans to use its process in the laser area next year.