Intelligent optical communications will be based on Planar Lightwave Circuit integrated optics

By MICHAEL LEIGH and KELLY WILLIAMS, Lynx Photonic Networks--PLC-based optical switches address the three major considerations that will drive market preference: volume pricing, high performance, and extreme reliability.

Jul 22nd, 2002

When the concept of all-optical switching first flourished, many alternative approaches emerged. Apart from Planar Lightwave Circuits (PLCs), these included micro-electromechanical systems (MEMS), liquid crystals, electro-holographic, electro-mechanical, electro-optic, index bubbles, and others. But as the many creators of these different all-optical switching technologies attempted to move from prototype to mature commercial product, it became clear that many of them were not able to offer a highly reliable, scalable, cost-effective, volume manufactured product.

One clear winner, however, is the PLC-based optical switch, which addresses the three major considerations that will drive market preference: volume pricing, high performance, and extreme reliability.

Recent design and technology innovations in PLC switch modules have increased its profile. Examples include:

• Addition and integration of wavelength selection, dynamic switching, energy management, and optical power monitoring functionality
• Scalability to 16, 32, 48 and more ports, for such applications as dynamically reconfigurable optical add/drop multiplexers
• "Stackable" pay as you grow architectures
• Integrated, intelligent 1 for N protection switching
• Double switch element design to dramatically reduce "optical leakage," achieving module crosstalk levels better than -40 dB and more
• Special design and fabrication of the switching elements to increase switching speed while minimizing power consumption
• Intelligent control planes for rapid system integration and time-to-market advantage.

All of this adds up to support for legacy networks going through gradual, "evolutionary" transition, such as ubiquitous SONET/SDH networks. Support for advanced network applications--including hybrid ring/mesh networks, inter-ring protection and bandwidth management, optical virtual private networks, and various forms of "distanced-tuned" multicast and broadcast video services--can be added seamlessly without affecting existing services.

True next-generation switching functionalities

The combination of PLC technology and advanced control plane software is ideal for these tough fiscal times. Telecommunications system vendors can now offer their service provider customers a compendium of next-generation functionalities to enable revenue-generating service offerings while reducing capital and operational expenditures.

Optical networks are very analog in nature. Like the light from your car headlights, they are highly distance-sensitive. The objective is to minimize "optical loss" (i.e. maximize network performance) by matching the light needed to the distance it has to travel and to the other network elements with which it interacts. Smart PLC switches can multicast an input signal to any combination of the output ports--at any required power ratio between the output ports. Multicasting may require calculation of power distribution among outputs according to each path's ideal power budget. This "distance tuning" is known as weighted multicasting, and it becomes essential in complex network optimization.

In this situation, output power can also be further controlled by "attenuating" the strong outputs, adding flexibility and advanced control to optimal distribution of all source power among the multiple outputs. This set of capabilities is just as applicable to high-value data multicasting/broadcasting services as it is to video.

Another function of dynamic output power control is to improve overall network performance by "tuning" other network elements--even those that may be located a long way from the switch. The new PLC switches can be specified with a dynamic range of 20 dB or more and can attenuate output power to support various demands, such as aggregate output equalization, for example.

When an EDFA, used as a WDM amplifier, is located after the switch module or multiplexer, there will likely be some demand for power control of the signals. If signals arrive at the EDFA with different power levels, the strongest signal will be dominant in determining the amplifier gain. The PLC switch can attenuate the stronger output signal to achieve the same (equalized) output power for all the output signals, before they enter the EDFA.

These characteristics give PLC switches the advantage over competing technologies. For example, MEMS switches are simply "on/off" devices; all of the previously described functions must be handled outside the switch (e.g., with external attenuators), increasing complexity that translates into overall higher cost, lower reliability and higher optical losses.

Another application in which PLC holds an advantage is fast switch reconfiguration. Whether the requirement is to provision a service connection, provide protection or instant network restoration or pre-provisioning, lost connectivity time must be kept to a minimum. As carrier networks move from a ring to a mesh topology, restoration time becomes a key element. Because reconfiguration at several nodes is likely to be required to reestablish a connection, minimizing the time requirement at each node is imperative. Total system recovery in a timely fashion (< 50 ms for SONET) demands that each nodal switch can reconfigure as fast as possible. The new switches complete connections in fewer than 2 ms, with no ringing or overshoot.

The new PLC switches were developed using advanced optics and customized electronics for sophisticated light path management, and it is all driven with intelligent software, with its own Graphical User Interface and protocol that gives the system integrator total control. All communication with the system management bus can be defined in a high-level language that speeds application development, enables debugging, and executes complex connection instructions. This results in accurate, rapid, dynamic provisioning/re-provisioning of light circuits. The software interface supports sudden changing demands on the network, adding intelligence to the switching, multicasting, and VOA capabilities, which results in a "network-aware" photonic communication subsystem. The software is also field-upgradeable for subsequent releases incorporating support for new services.

These thermo-optic switches are truly "photonic communication switching subsystems" and are rapidly becoming the cornerstone of next generation optical network systems that will deliver Intelligent Optical Networks (IONs).

Photonic switching as a whole remains in its infancy. There are many technologies that can simply switch light at the core of the network, but only PLC technology can elegantly and intelligently manage the service and system requirements at the core of the telecommunications network.

Michael Leigh is president and co-founder of Lynx Photonic Networks, and Kelly Williams serves as marketing communications director. They may be reached via the company's Web site at

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