By STEPHEN HARDY
All-optical switching, which some have touted as the road to the big-bandwidth promised land, has begun to move off the drawing board and into the field. Judging by the technology directions of a number of new entrants in the arena, a micro-electromechanical systems (MEMS) approach appears to be the most popular choice for large-scale fabrics. However, liquid-crystal techniques and other avenues will also lead to the all-optical future.
As previously reported, MEMS have emerged as the leading candidate to provide the underpinnings for optical crossconnects and other systems that require switching fabrics capable of accommodating hundreds or thousands of connections (see Lightwave, January 2000, page 1; September 1999, page 27; and June 1999, page 1). Lucent Technologies (Murray Hill, NJ) has already announced its intention to unveil an all-optical system it calls the LambdaRouter by the middle of this year. Company researchers discussed their progress in developing MEMS-based switching arrays in a post-deadline paper at the recent Conference on Optical Fiber Communications (OFC 2000) in Baltimore.
As reported by presenter D.T. Neil son, the Lucent group has demonstrated a fully provisioned 112x112 MEMS crossconnect in which all 12,544 connections have been verified. The mirror-based MEMS components are two-axis devices, mounted in a mechanical housing, that are electrostatically actuated. The characteristic dimension of the fabric is 10 cm.
Optical Micro-Machines has supplied a MEMS-based switching module such as the one on this card to Siemens for use in an optical crossconnect. (Photo courtesy of Optical Micro-Machines Inc.)
In the context of the crossconnect, the MEMS components are in a 2-D array, along with a fiber array with a collimating microlens array and a fold mirror to allow the MEMS and fiber/lens arrays to handle input and output. A light beam enters the fiber/lens array, where it is collimated and projected onto one of the MEMS micro-mirrors by a microlens. The micro-mirror tilts to reflect the incoming beam onto the fold mirror, which in turn reflects the light onto a second micro-mirror on the MEMS fabric. The second micro-mirror reflects the beam toward the appropriate output point on the fiber/lens array, from which it is coupled to an outgoing singlemode fiber.
The Lucent team reports that this system provides a connection switching time of less than 10 msec, including the drive-voltage rise time. Insertion loss of the optical fabric is 7.5 dB (±2.5 dB) in the minimum loss region around 1,550 nm. The diffractive microlenses on the fiber/lens array provide a connection loss variation of ±1 dB within the 1,525- to 1,565-nm band for which they were designed. Optical crosstalk into any output port is below -50 dB. The company researchers say the demonstrated crossconnect is optically equivalent to a system unfolded about the fold mirror; if constructed, such a system would use a second MEMS mirror and lens/fiber array to achieve a 224x224 optical crossconnect.
While Lucent appeared well ahead of its potential competition when it announced its all-optical switching platform last year, recent events indicate that the LambdaRouter won't be the only MEMS-based system on the market for long. For example, startup Xros Inc. (Sunnyvale, CA) has announced a crossconnect system based on its own MEMS technology. As is the case with Lucent's technology, Xros's silicon-based MEMS mirrors can rotate along two axes. Xros uses torsional flexures to achieve this performance; sensors at the flexures perform positional control. The X-1000 scales to 1,152x1,152 in three NEBS-compliant bays. The unit uses two 6x6-inch MEMS mirror arrays to provide connections in less than 50 msec.
According to information provided by Xros to Lih Y. Lin and Evan L. Goldstein of AT&T Labs-Research for their OFC tutorial session on lightwave micromachines, Xros plans to achieve typical switching speeds of <2.5 msec, with a maximum speed of <5 msec. Insertion loss targets range from a typical figure of <1.8 dB to a maximum of <3.0 dB. Crosstalk targets are <60 dB typical to <55 dB maximum. All of these figures pertain to device-level performance.
Xros impressed at least one company with its technology. Nortel Networks (Brampton, ON, Canada) announced shortly after the end of OFC its intention to buy Xros with common stock worth $3.25 billion. In a press release, Nortel revealed it expects Xros to begin providing switches for customer field trials late this summer, with deliveries slated for next year.
MEMs-based switching modules are now reaching the market. (Photo courtesy of Optical Micro-Machines Inc.)
The switch architectures used by Lucent and Xros have come to be known as "analog," for the variety of positions the mirrors can assume. While this technology provides superior scalability, it is extremely complex. Other MEMS devices potentially trade ease of scalability for simpler, two-position operation and thus are called "digital." An example of a switch that will incorporate this approach comes from Siemens (Munich, Germany), which unveiled its TransXpress Optical Service Node at the CeBIT show in Germany this past February. The Optical Service Node includes the MODIF/OXC, a managed optical distribution frame with a MEMS-based switching fabric at its core.
Optical Micro-Machines Inc. (San Diego) supplied the MEMS modules for the demonstration. Conrad Burke, the module manufacturer's new vice president of marketing and sales, says the company is already shipping 4x4 and 8x8 MEMS-based switching modules under the brand name Cross-Guard and expects to have 16x16 and 32x32 modules ready by the end of the year. The Siemens system currently uses the 4x4 and 8x8 modules and will incorporate the 32x32 modules when they become available, Burke says. The company also has touted an upcoming 256x256 building block for optical crossconnects called the Opto-Cop. This module will be in prototype this year, according to Burke.
Burke says that the ability to mass produce MEMS-based modules will become a critical skill-and market differentiator-in the near future. To this end, Optical Micro-Machines has invested heavily in automated manufacturing capabilities as well as improved packaging. The latter initiative includes the addition of electronics that will support "plug-and-play" operation.
Meanwhile, Nortel is not alone in linking with another company for MEMS technology. Corning Inc. (Corning, NY) has entered a joint development agreement with IntelliSense Corp. (Wilmington, MA) for the development of optical-communications products based on MEMS. IntelliSense has experience in MEMS-based design-tool development, process and product engineering, and MEMS manufacturing. Corning has not revealed whether it plans to take a digital or analog approach to MEMS-based switching fabrics. The new activity will complement Corning's previously discussed work in liquid-crystal switching technology (see Lightwave, December 1999, page 1).
Alliances such as this one provide hope for startups in the field. For example, Onyx Microsystems Inc. (Richmond, CA) has demonstrated its digital MEMS technology in hopes of forging a manufacturing and marketing alliance with a larger telecommunications company. According to Dr. Meng-Hsiung Kiang, director of technology at Onyx, the company expects to ship products in the 4x4 to 32x32 range in the fourth quarter of this year. Onyx specializes in creating extremely flat micro-mirrors that offer consistent performance in terms of maintaining the same angle position through repeated switch operations. Manufacturability also represents a potential competitive advantage, says Dr. Kiang.
While she says that digital MEMS-based switches can be cascaded in Clos configurations to create larger switching systems, Dr. Kiang reveals that Onyx is already at work on a second-generation MEMS technology that will support an analog architecture.
Mirror-based systems represent the most popular of the MEMS technologies. However, not all MEMS use mirrors. Agilent Technologies (Palo Alto, CA) has taken the wraps off of what it calls the "Agilent Photonic Switching Platform." As previously described in Lightwave (see Lightwave, March 2000, page 92), Agilent has combined the technology behind its inkjet printers with silica planar lightwave circuits to create a two-position switching device. The device consists of intersecting silica waveguides, with a trench etched diagonally at each point of intersection. The trenches contain an index-matching fluid that allows transmission in the default condition. To switch an incoming light beam, a thermal silicon chip creates a bubble in the fluid, which reflects the light from the input waveguide to the output waveguide via total internal reflection. Agilent claims switching speeds below 10 msec and a fiber-to-fiber loss of 7.5 dB in a 32x32 switch configuration.
Agilent has announced two products based on the technology, a 32x32 switch and a 16x32 device. Commercial prototypes of the two products should be available by the end of the year.
As MEMS technology continues to improve because of research across a wide range of companies, liquid-crystal technology also has reached the marketplace, albeit from a smaller number of vendors. As indicated previously, Corning has already unveiled its PurePath offering (see Lightwave, December 1999, page 1). Two other companies, SpectraSwitch Inc. (Santa Rosa, CA) and Chorum Technologies (Richardson, TX), launched their initial products at OFC.
SpectraSwitch debuted its WaveWalker N-1x2 and 1xN product lines. The former includes 2, 4, and 8 1x2 modules, while 1x2 and 1x4 modules currently make up the latter. As with all liquid-crystal devices, the WaveWalker products are solid-state devices and therefore do not have the moving parts associated with MEMS technology. Customer evaluations of the products should begin by the end of the year, if not sooner. For the 1x4 switch, the company is targeting a typical unconnectorized insertion loss of 2 dB and an unconnectorized return loss of 55 dB, with crosstalk at 40 dB. Switching speeds will be less than 4 msec. In the case of the N-1x2 devices, SpectraSwitch expects a typical unconnectorized insertion loss of 1 dB, with unconnectorized return loss, crosstalk, and switching speeds similar to the 1x4 device.
In a conversation a few weeks before OFC, Cindana Turkette, vice president of marketing and business development at SpectraSwitch, offered that these devices should prove most popular in such applications as optical add/drop multiplexers (OADMs), test equipment, and fiber ring-protection applications, rather than in large crossconnects. Along these lines, the company touted the 1xN portfolio for inline or remote fiber-optic system testing, field test-equipment multiplexing, and network reconfiguration and restoration in their discussions at OFC. Meanwhile, the N-1x2 products were described as applicable to dual-ring protection, field test-equipment multiplexing, and add/drop network reconfiguration.
While Corning sources in previous discussions with Lightwave had agreed with Turkette that liquid-crystal devices are best suited to applications requiring smaller fabrics, not everyone in the liquid-crystal camp is willing to concede the optical-crossconnect field to MEMS. Scott Grout, CEO at Chorum Technologies, says his company is developing architectures that would take advantage of liquid-crystal technology for such systems. Meanwhile, the company has released its first PolarWave liquid-crystal products, including its Fast Add/Drop Switch and 1x2 Switch devices. Whether for smaller applications in the near future or large-port-count uses down the road, Grout believes that Chorum's designs-which offer switching speeds in the microseconds, provide 40-50 dB of channel isolation and do not require temperature control-will provide an attractive alternative to MEMS-based devices and other switching technologies.
While MEMS and liquid crystal have attracted most of the industry's attention in recent months, other all-optical technologies remain in the wings. For example, Optical Switch Corp. (Richardson, TX) has continued its work in the area of frustrated total internal reflection (as well as total internal reflection), according to the company's marketing manager, Tom Hazelton. The company hopes to have demonstration models of a switch based on this technology in the first half of this year for applications requiring small port counts. The company displayed a switch for test applications at OFC, but declined to reveal the technology on which it was based, other than to say it was not based on frustrated total internal reflection. A 32x32 version of this switch should be available in the second half of this year, Hazelton reports.
The amount of investment poured into optical switching pales in comparison to the potential market size. For example, Xros quotes Pivotal Research as estimating the North American market for optical crossconnects at $427 million this year but at more than $10 billion in 2004. In its announcement of the Xros acquisition, Nortel quoted $543 million as the world market this year for "optical bandwidth-management systems," a figure that will rise to $15 billion in 2004. With a market this lucrative at stake, the industry can expect further technology announcements-and undoubtedly more company acquisitions-throughout the year.
While both MEMS and liquid-crystal technology have received significant attention for their application to optical switching, their uses extend to other areas, as well.
For example, CoreTek Inc. (Wilmington, MA) described its work on a MEMS-based tunable vertical-cavity surface-emitting laser (VCSEL) in a post-deadline paper at OFC. The optically pumped MEMS-VCSEL can couple more than 6 mW of optical power into a singlemode fiber, while offering a tuning range across the C-band. CoreTek has also developed a MEMS-based tunable filter. The company's expertise in both MEMS and VCSELs was cited as the primary reason for its acquisition by Nortel Networks, which agreed to pay $1.43 billion for the company.
Lucent, meanwhile, has researched MEMS for such applications as modulation, data transmission, gain equalization, and dispersion compensation. Most of this work revolves around a device the company calls the Mechanical Anti-Reflection Switch. For its part, Optical Micro-Machines has an attenuator module under development.
Meanwhile, both SpectraSwitch and Chorum Technologies have targeted nonswitching applications for their liquid-crystal technology. SpectraSwitch expects the vast majority of its sales to come from such applications as wavelength-selective devices, attenuators, and polarization compensators, rather than optical switches. Along similar lines, Chorum Technologies debuted its Dynamic Variable Attenuator alongside its switching products at OFC. The attenuator is a voltage-controlled solid-state device for broadband or single-channel applications.
As Agilent Technologies demonstrated with its new switching device, planar lightwave circuits (PLCs) can also form the basis of optical-switching systems. PLCs-the result of etching waveguides into substrates in much the same way that semiconductor wafers are created-have been the focus of work at such industry giants as Lucent and Nortel Networks for some time. Now, several startups are positioning themselves as alternative suppliers.
For example, Lightwave Microsystems Corp. (San Jose, CA) announced that its LightWeaver 2x2 Optical Switch Array should be available for sampling "within the next few months," in the words of a company statement. The company expects the device to find use in optical add/drop multiplexing, line protection, and bypass applications.
Meanwhile, Kymata Ltd. (Livingston, Scotland) has its production facilities up and running. In fact, company president Brendan Hyland says the company expects to have an additional facility online by the end of the year. While the company has started with PLC-based arrayed waveguide gratings (AWGs) and variable optical attenuators (Lightwave Microsystems supplies both products, as well), Hyland says work is underway on a switching product.
AWGs will undoubtedly prove a popular application for PLC technology as interest in the devices grows. E-TEK Dynamics (San Jose) has tapped Bookham Technology (Oxford, UK) to collaborate on an AWG dense wavelength-division multiplexing (DWDM) component. The multiyear agreement aims to develop AWG-based DWDM products for long-haul, metro, and cable-TV applications. The two companies displayed a preproduction 100-GHz, 40-channel device at OFC.
Finally, another young firm is preparing to join the parade. Optical Micro Devices Ltd. (Swindon, UK) will initially focus on providing micro-benches-a device that aids fiber alignment- according to the company's managing director, Kevin Ford. However, other PLC-based devices will shortly follow, he predicts. The company hopes to have its initial samples as early as the first or second quarter of next year. The company will focus on silica-on-silicon devices created on 8-inch wafers.