Optical chip technology lowers fabrication costs

March 1, 1997

Optical chip technology lowers fabrication costs

george kotelly

Striving to overcome optical product cost barriers, Lightwave Microsystems in Santa Clara, CA, has developed a HiBit technology platform that is projected to lower the cost of building high-speed optical integrated circuits (ICs) by two-thirds. Using its patented materials, process, and design technology, the company plans to mass-produce customized optical circuits that integrate several discrete communications functions onto a single chip at less cost. This leading-edge technology and resulting cost reductions, claims the manufacturer, would enable telecommunications, cable-TV, and data communications providers to expand and extend their high-capacity fiber-optic communications links into regional and local-area backbone networks and, eventually, to local-loop networks.

Optoelectronics is the last area of fiber-optic technology where the company believes dramatic cost reductions can be acheived, for products such as photonic switches, optical amplifiers, and wavelength-division multiplexers (WDMs). Consequently, the company is pursuing the design and production of economical and affordable low-cost optical chips to accelerate the widespread deployment of high-speed fiber-optic networks.

According to Michael Hess, Lightwave Microsystems` president and chief executive, "By shrinking and integrating several optical transmission and processing functions into a single, mass-produced optical IC, the HiBit technology platform breaks through the cost bottleneck."

Adds George Ballog, vice president of marketing, "The company is modeling its HiBit technology in the optical domain after the evolution that has taken place in the microelectronics field in the form of asics [application-specific integrated circuits]. In microelectronics, as the capability of ICs went up, the costs went down. We hope to go down that same path for optical-circuit functions."

In duplicating the manufacturing process used successfully for microelectronic ICs, the HiBit platform combines active and passive optical functions on a single circuit bed. The optical analog to a microelectronic IC silicon switch is the company`s electro-optical switch, which is capable of operating in excess of 40 GHz. Unlike silicon switches, in which the speed of optical-to-electrical signal conversion limits throughput, the company`s optical devices directly switch the optical path. In this method, throughput is limited only by the speed of the transmitters.

The HiBit technology comprises three major segments: polymer materials, software tools, and automated IC manufacturing processes.

The company`s versatile and thermally stable Optimer nonlinear optical materials exceed optical industry requirements for thermal stability and signal attenuation while providing a medium for building optical circuits using standard wafer-processing technology. The basic optical building-block material is polyimide, which is doped with chromophore to make it optically active so that it can be used as an optical switching material.

Pushing the limit of available test equipment, the material can switch to 40 GHz (25 psec) and operates in a binary on/off switching mode. During switching, the material essentially changes the light beam`s index of refraction or angular direction, similar to the way lithium-niobate modulation works.

Polyimide is a dielectric material that has been used for more than 15 years in high-stress, high-performance electronics circuits. It is a high-temperature material that is processed at more than 300°C. Therefore, it easily withstands solder reflow temperatures to 260°C for integrating active devices, such as laser dies and detectors, onto a passive substrate. Low-limit temperatures cause no physical or chemical problems.

Software suite

A software suite of tools designed in-house is used to select and combine discrete active and passive optical device elements into single-chip solutions. Customized optical circuits can be produced in just a few months.

The individual design elements in the software suite allow the implementation of bending, splitting, combining, modulating, and filtering functions. These pre-designed elements are available for mixing and matching in prescribed ways to generate a specific optical function or behavior. The production cost advantage, as in asic microelectronics technology, is that after the initial investment, the individual elements can be reused to design other optical functions quickly, easily, and economically.

Presently, the suite contains 12 device elements, and new devices are planned. For example, when a new product is needed, such as a 1 ¥ 16 dense WDM, the suite uses splitting and wavelength filtering elements as the two basic cells. Then, for the multiplexer function, it cascades these functions to place 16 different wavelengths into 16 different ports and guides them down to one port for exiting over a fiber. Similarly, at the other endpoint for the demultiplexer function, the single optical path is reverse-cascaded to split and separate into 16 individual wavelengths again.

Automated manufacturing

Proven and well-established automated IC manufacturing and assembly processes are used by the company to achieve high reliability and low cost in production volumes. According to Ballog, "The company`s background strength is in semiconductor processing techniques."

The integrated IC fabrication process is laid down on a silicon substrate--just as a wafer is used to make microelectronic IC circuits--using a precise, high-density photo lithography process. Trenches or river beds are created for the waveguides. Their linewidths are approximately 3 microns wide and 8 microns long. Device counts can run to several hundred optical devices, depending on the specific design, and to hundreds of devices per wafer.

To demonstrate the capabilities of its optical integrated technology, Lightwave Microsystems is producing the LightWeaver MDM-16, a dense WDM and demultiplexer for high-speed telecommunications fiber-optic networks (see figure). The product can handle as many as 16 high-speed channels over a single fiber. Operating at the OC-48 rate of 2.4 Gbits/sec, the product can deliver a 40-Gbit/sec throughput over a single fiber. At 10 Gbits/sec (OC-192), it can accommodate 160 Gbits/sec. Moreover, all International Telecommunication Union (ITU) standards are satisfied for dense WDM multiplexers, such as 100- or 200-GHz channel spacing and greater than 25-dB isolation between channels.

The company claims that the multiplexer, priced $12,000 in volume quantities, costs less than one-third the price of competitive devices that use interference filters, fiber Bragg grating technologies, or phased-array waveguides. "This is the breakthrough in economics that`s needed to propel fiber optics deeper into the network," says Ballog.

The company has a 6-inch wafer fabrication line in operation. The active chip size of the 1 ¥ 16 multiplexer is 2.5 mm wide by 6 mm long (see photo, page 1). The overall package size is 1.5 inches wide by 4 to 5 inches long plus one fiber pigtail in and 16 fibers out. If needed, a built-in thermoelectric cooler is available.

The LightWeaver MDM-16 is currently in alpha or laboratory testing by two major transmission equipment suppliers. Beta testing is planned for next month, when qualified products will be shipping. q

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