wdm receiver and glass technologies sharpen light clarity and processing

June 1, 1997

wdm receiver and glass technologies sharpen light clarity and processing

A novel, intelligent wavelength-division multiplexing receiver design, coupled with a patented optical material, holds the potential for providing bandwidth, protocol, and flexibility benefits to cost-sensitive local area networks

HEINZ willebrand, Punit Kalra

eagle optoelectronics llc

Paul dempewolf, boyd hunter

Lightpath technologies inc.

The introduction of wavelength-division multiplexing (wdm) technology into local area networks (lans) offers a communications platform that can scale and yield high performance in a cost-effective manner. Just as in telecommunications systems, a large increase in bandwidth can result from using multiple wavelengths. However, more relevant for lan environments, wdm technology permits many lan protocols to run at various speeds simultaneously in the same fiber-optic network. To successfully migrate to these environments, however, wdm system costs must adapt to the lower price range of existing lan bandwidth platforms.

A novel wdm technology has emerged to meet these requirements. One key component of this system is a robust, intelligent receiver module that can lock automatically onto different transmission wavelengths and track the wavelength drift of transmitter sources. Another key component is a patented Gradium glass dispersive element that enables low-cost monolithic integration of the receiver onto a circuit board (see Fig. 1).

Wavelength operation

Contrary to standard telecommunications approaches, the wdm receiver module used in this approach does not need fixed and preassigned wavelength channels to perform wdm operations. However, if desired, it can work with such wavelengths. Wavelengths entering the receiver module from an optical fiber are demultiplexed across a linear array of photodetector elements. The photodetector array converts the photonic data into electronic signals.

After the optical-to-electrical (O/E) conversion, a position decoder circuit rapidly determines the detector elements with the strongest signals. To perform this wavelength calibration, a network controller places the receiver in "decode" mode. The position decoder finds the local detector position(s) of the transmitting laser(s) and sends this information to a selector/router switch.

During system initialization, all the lasers are switched on in sequence, and each node in the system creates and stores its own translation table for the wavelength-position conversion. This table is located in the selector/router switch.

The selector/router is basically a programmable crossbar switching network that obtains its routing information from the translation table. The selector/router also includes a data buffer that can be read out by a host computer.

During the multiwavelength data-transmission phase, only the currents from a single detector element or a narrow-wavelength band from a few neighboring elements--identified previously during setup for a particular transmitter--are decoded and sent to a quantizer for binary conversion.

This receiver design, which is similar to a universal asynchronous receiver-transmitter, is cost-efficient when compared to present methods that attempt to comply with the International Telecommunication Union (itu) standard for incorporating wdm technology into a lan system. Difficulty with the itu standard emerges because pc-lan systems do not need the stabilized environment required by the itu specifications for long-distance transmission.

Standard long-haul wdm telecommunications systems require highly stabilized environments to maintain the wavelength match of transmitting and receiving components. A change in laser frequency or transmitter temperature can cause a wavelength drift and, consequently, the transmission wavelengths would spatially drift to a nondesignated photodetector.

In a network environment, the receiver module would be switched periodically into "decode" mode. As a result, wavelength drifts of transmitter sources could be tracked at the receiver site by updating the wavelength translation table. This approach permits a low-cost software solution that overcomes the requirement for expensive hardware.

The receiver module can operate in the short-wavelength (0.8-micron) or long-wavelength (1.55 micron) range. Because many lan systems operate within distances of less than 1 km, wdm receiver operation in the short-wavelength range becomes cost-effective. Various inexpensive lasers for this range are commercially available at low cost. The high attenuation of optical fiber (2 to 3 dB/km) in this range can be tolerated because of the moderate lengths of lan networks.

To achieve the system goals of the wdm receiver module design, the Gradium dispersion component had to meet demanding operational criteria. The major requirement called for a small, inexpensive, high-performance, monolithic wavelength demultiplexer. In addition, the demultiplexer had to be integrated onto a flat circuit board and also permit an incoming optical fiber to be coupled to a flat surface and the receiver`s photodetector array without air space.

In meeting these requirements, the Gradium glass material offers a refractive index variation in one dimension, unlike the radial gradient index materials that have been available for years (see Fig. 2). The important differences between Gradium glasses and previous gradient materials are the result of manufacturing process stability and profile customization--the ability to design any desired profile shape. Profile repeatability is as good as or better than the normal batch-to-batch variation in optical glasses, in stark contrast to ion-exchange technologies.

Because so many base glasses are available to the Gradium glass process, a variety of refractive index profiles, dispersion profiles, and mechanical and thermal properties can be engineered into the material, as needed. These additional degrees of freedom allow the extraction of high performance from the optical system.

Although some practical limitations exist, the range of properties (refractive index [changes <0.5] and dispersion profiles [Abbe value changes <50]) embodied in homogeneous optical glasses sets the limits on Gradium glasses (see Fig. 3). These changes in optical properties can be effected over centimeter- to submillimeter-length scales.

The varying optical properties of Gradium glass are important because the incoming wavelengths of light are bent toward the high-index regions of the glass, much as a marble rolling on an inclined plane is pulled by gravity. The combination of curved path and varying properties of Gradium glass can separate and focus a multicolor laser beam (see Fig. 4).

wdm applications

For use in wdm transmission systems, various Gradium-based applications are being developed, including:

A Gradium biaxial lens, which offers the functionality of a cylindrical lens in a plano-plano-package, can be used in place of the curved surfaces in the first approach. The biaxial configuration allows the transverse refractive index gradient to do the focusing.

Gradium materials possess both a refractive index gradient and a dispersive gradient. The refractive-index gradient forces the light to bend as it transverses the glass. Therefore, different light colors experience a different delta n or delta v profile (a spatial variation of u [index] or v [Abbe number, a measure of dispersion])as they transverse the glass. These effects can be combined with the surface geometry to produce a dispersive element that also collimates and focuses light. These attributes eliminate grating efficiency issues.

Development efforts are under way to produce a low-cost lan transceiver module that allows 4-wavelength wdm operation for interconnection to lan routers and bridges. Further development steps are targeting 8 and 16 wavelengths. More intelligence is being added to transform the transceiver from a conventional multiwavelength device to one that can track and communicate over multiple wavelengths simultaneously. In this stage of development, the receiver can lock onto any wavelength in a preassigned band. This adaptive lightwave transceiver is expected to serve as a common communication device in both fiber-to-the-desk and fiber-to-the-home environments. u

Heinz Willebrand is chief technical officer and Punit Kalra is director of marketing and product development at Eagle Optoelectronics llc, Boulder, CO. Paul Dempewolf is director of optoelectronics product development and Boyd Hunter is manager of product development and technical services at LightPath Technologies Inc., Albuquerque, NM.

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