1.3. Multiplexing, Grooming, and Switching

Transmission systems alone are not enough to build an optical network. Optical signals need to be multiplexed and demultiplexed at the end points. They also need to be groomed and switched at intermediate nodes. Grooming is the function of dropping a lower rate signal from one rate speed signal and adding it to another. Switching allows a signal received on one input port and channel to be transmitted on a different output port and channel.

Based on the switch fabric technology, optical switches can be broadly classified into two categories—opaque (or OEO) and transparent (or OOO) [Mouftah+98, Hinton93]. Opaque optical switches, also called optical cross-connects or OXCs, convert optical signals received at the input to electric signal, switch the electrical signals using an electronic switching fabric, and finally convert the electrical signal back to optical signal at the output. The name OEO captures the operational principle of the switch in the sense that it converts the incoming optical signal into electrical signal and then converts it back to optical signal. Transparent switches, also called photonic cross-connects or PXCs, on the other hand, do not perform this optical to electrical translation; they switch the incoming optical signal from the input port to the output port in the optical form (hence, OOO). Optically transparent switches operate over a range of wavelengths called the passband. For any given steady state, optical transparency allows a device to function independent of the type (e.g., analog, digital), format (e.g., SCM, SONET, GbE), or rate (e.g., 155 Mbps, 10 Gbps, 10 GHz) of the information on the optical signal being conveyed.

One of the problems with the OEO switches is that they need to perform multiple opto-electrical translations that can be both complex and expensive. On the positive side, as a by-product of opto-electrical translation, the optical signal undergoes regeneration and wavelength translation comes for free. OOO switches on the other hand, do not perform opto-electrical translation. As a result, they have the potential of being cheaper. OOO switches, however, are incapable of signal regeneration and wavelength translation. They also lack some of the performance monitoring and fault management capabilities that OEO switches offer. In the following we discuss different types of OEO and OOO switching elements.

1.3.1. Digital Cross-Connects and Add/Drop Multiplexers

Digital cross-connects, although not purely optical elements, play an important role in today's optical networks [Ramaswamy+02, Stern+99, Mukherjee97]. They can operate on either optical or electrical signals. Their switching fabric, however, is purely electrical.

Digital cross-connects are used for sub-wavelength level grooming and switching, that is, they work on time division multiplexed signals that may be carried as a wavelength in a WDM signal. Depending on switching granularity, they can be categorized as wideband, broadband, or ultraband cross-connects. Wideband cross-connects switch signals at the granularity of 1.5Mbps (DS1) while broadband cross-connects operate at 50 Mbps (STS-1) granularity. Ultraband is the latest addition to the digital cross-connect family. This type of cross-connect operates on optical signals and uses a 2.5Gbps (STS-48) electrical switching fabric. It is similar in functionality to an optical wavelength switch except that it is an OEO switch as opposed to an OOO switch. Advances in integrated circuit technology have allowed the creation of “ultra high” capacity broadband cross-connects with raw switching capacity similar to that of Ultraband cross-connects but with 50 Mbps switching granularity.

Digital cross-connects are typically located in Telco Central Offices, as shown in Figure 1-4. Wideband cross-connects are used for grooming, multiplexing, and interconnecting traffic between access and collector rings. Broadband cross-connects are used for the same function, but typically in metro-core and wide-area rings. The ultraband cross-connects are primarily concerned with interconnecting DWDM systems in wide area mesh and ring structured optical transport networks.

Today's optical networks also use add-drop multiplexers (ADM) quite extensively. ADMs are typically arranged in a ring topology connecting multiple service provider PoPs (points-of-presence). As the name suggests, they are used to add/drop traffic at a PoP to/from the ring. ADM rings operate at different speeds, and traffic can be added/dropped at different granularities. For example, metro-access rings typically operate at 150 Mbps (OC-3) and 622 Mbps (OC-12), and ADMs in these rings can add/drop traffic at 1.5 Mbps (DS1) or higher granularity. Interoffice metro rings and wide-area rings operate at 2.5Gbps (OC-48) and 10 Gbps (OC-192) speeds and typically traffic is added/dropped at 50Mbps (DS3) or higher speeds.

Standards currently exist that specify the exact characteristics and structure of various signals that digital cross-connects and ADMs operate on (see Chapters 2and 3). This is in sharp contrast to the OOO case where few standards yet exist and intervendor interoperability is very uncommon.

1.3.2. OOO Switch Fabrics

Research, development, and commercialization of OOO (photonic) switches encompasses a variety of switching technologies, including opto-mechanical, electro-optic, acousto-optic, thermal, micro-mechanical, liquid crystal, and semiconductor switch technologies [Mouftah+98, Hinton93]. The performance characteristics and hence possible application of photonic switches may depend on the technology used. It is difficult to predict breakthroughs, but should they occur, they may well revolutionize switching in telecommunications networks. Some technologies utilized in commercially available photonic switches are described in this section.

In order to appreciate the relative merits and shortcomings of different switching technologies, it is important to understand the different metrics used to characterize the performance of a photonic switch fabric. With an ideal photonic switch, all the optical power applied at any input port can be completely transferred to any output port, that is, the switch has zero insertion loss. Also, the optical power does not leak from any input port to any other input port or any undesired output port, that is, it has infinite directivity and zero cross talk. In addition, switch connections can be reconfigured instantaneously, that is, the switching time is zero, and any new connection can be made without rearranging existing connections, that is, the switch is nonblocking. Unfortunately, no switch is ideal.

In practice, the following characteristics of photonic switch elements and fabrics affect their performance:

• Switching speed: The switching speed of photonic switches ranges from sub-nanosecond to hundreds of milliseconds. Depending on the application, any of these can be acceptable, but some applications require higher speed than others.

• Switching efficiency: In an ideal switch, all the power is transferred from a given input port to the desired output port, and no power is transferred to any other port, in either forward or backward direction. How a switch performs these functions is measured using four primary metrics: insertion loss, cross talk, return, and directivity. Insertion loss is the power lost due to the presence of the switching device: the ratio of output power to input power. The cross talk of a switch is defined in terms of worst case power transfer from the input port into an unintended output port. The return of a switch is defined in terms of power transferred from the input port back into the same fiber. The directivity of a switch is defined in terms of the power transferred from the input port to a different input port. Insertion loss and cross talk should be as low as possible. Return loss and directivity should be as high as possible, indicating very little reflection back into the input.

• Wavelength dependence: Ideally, the performance of a photonic switch should be independent of the specific wavelength being switched. In practice, certain types of switches, such as opto-mechanical switches, are nearly wavelength independent. Waveguide based switches, on the other hand, are frequently optimized for narrower wavelength region. All the properties of the photonic switch must meet optical specifications across the wavelength range over which the switch is intended to be used. The main wavelength bands of interest for optical communication are those centered on the fiber loss minima at 1310 nm and 1550 nm.

• Polarization dependence: Switches need to have low polarization dependent loss (PDL), low polarization mode dispersion (PMD), and low polarization dependence in switching efficiency. If the signal passes through many switches, PDL can accumulate and lead to large variations in power levels. PMD is a problem primarily at high data rates. PMD in switches can be corrected, since it is a constant, unlike time-varying PMD in optical fiber. Polarization dependent switching efficiency can be problem if it leads to unacceptable cross talk for one of the polarizations.

In the following we discuss different switching technologies. Specifically, we describe the basic principle of switching under different technologies and discuss the intrinsic performance limitations and possible reliability concerns.

1.3.2.1. OPTO-MECHANICAL

This broad category of optical switching technology can be identified based on the use of motion to realize optical switching. They typically have very low loss, and extremely low cross talk. Switching speed of these switches vary from tens of milliseconds to hundreds of milliseconds. Opto-mechanical switches are the most commonly used optical switches today. This broad category can be further divided based on implementation specifics.

There are two types of opto-mechanical switches: moving fiber type and moving deflector type. Moving fiber technology uses direct moving fiber-to-fiber, or collimator-to-collimator alignment to reconfigure optical connections.^[1] Moving deflector technology uses a moving mirror or prism that moves in and out of the optical path of fixed collimators to reconfigure optical connections. Fiber or deflector motion may be created with a relay, a solenoid, or a stepper motor.

^[1] A collimator is a device to which a fiber is attached. The collimator then outputs a fixed beam of limited cross-section.

The most popular opto-mechanical switches are based on Micro-Electro-Mechanical Systems (MEMS). A MEMS is a small device that has both electrical and mechanical components. It is fabricated using the tools of the semiconductor manufacturing industry: thin film deposition, photolithography, and selective etching. Frequently, MEMS devices involve the use of semiconductor materials, such as silicon wafers, as well. MEMS devices offer the possibility of reducing the size, cost, and switching time of optical switches, and the ability to manufacture large arrays and complex networks of switching elements.

The switching element in a MEMS optical switch can be a moving fiber, or a moving optical component such as a mirror, lens, prism, or waveguide. The actuation principle for moving the switching element is typically electromagnetism, electrostatic attraction, or thermal expansion. One of the most popular forms of MEMS switches is based on arrays of tiny tilting mirrors, which are either two-dimensional (2D) or three-dimensional (3D). In a typical 2D array, the mirrors simply flap up and down in the optical equivalent of a crossbar switch. When they are down, light beams pass over them. When they are up, they deflect the beam to a different output port. With 3D arrays (see Figure 1-5), the mirrors can be tilted in any direction. The arrays are typically arranged in pairs, facing each other and at an angle of 90 degrees to each other. Incoming light is directed onto a mirror in the first array that deflects it onto a predetermined mirror in the second array. This in turn deflects the light to the predetermined output port. The position of the mirrors has to be controlled very precisely, for example, to millionths of degrees.

1.3.2.2. ELECTRO-OPTIC SWITCHES

Electro-optic switches are based on directional couplers. A 2 × 2 coupler consists of two input ports and two output ports, as shown in Figure 1-6. It takes a fraction of the power, α, from input 1 and places it on output 1. The remaining power, 1-α, is placed on output 2. Similarly, a fraction, 1-α of the power from input 2 is distributed to output 1 and the remaining power to output 2. A 2 × 2 coupler can be used as a 2 × 2 switch by changing the coupling ratio α. In electro-optic switches, the coupling ratio is changed by changing the refractive index of the material in the coupling region. One commonly used material for this purpose is lithium niobate (LiNbO₃). Switching is performed by applying the appropriate voltage to the electrodes. Electro-optic switches tend to be fast with switching times in the nanosecond range. Since the electro-optic effect is sensitive to polarization, electro-optic switches are inherently polarization sensitive, and tend to have relatively high loss.

1.3.2.3. ACOUSTO-OPTIC SWITCHES

In an acousto-optic device, a light beam interacts with traveling acoustic waves in a transparent material such as glass. Acoustic waves are generated with a transducer that converts electromagnetic signals into mechanical vibrations. The spatially periodic density variations in the material, corresponding to compressions and rarefactions of the traveling acoustic wave, are accompanied by corresponding changes in the medium's index of refraction. These periodic refractive index variations diffract light. Sufficiently powerful acoustic waves can diffract most of the incident light and therefore deflect it from its incident direction, thus creating an optical switching device. Acousto-optic switches are wavelength dependent and are more suitable for wavelength selective switches.

1.3.2.4. THERMO-OPTIC SWITCHES

These switches are based on Mach-Zehnder interferometers [Green92, Ramaswamy+02]. A Mach-Zehnder interferometer is constructed out of two directional couplers interconnected through two paths of differing lengths as shown in Figure 1-7. By varying the refractive index in one arm of the interferometer, the relative phase difference between two arms can be changed, resulting in switching an input signal from one input port to another. These switches are called thermo-optic switches because the change in the refractive index is thermally induced. Thermo-optic switches suffer from poor cross talk performance and are relatively slow in terms of switching speed.

1.3.2.5. MAGNETO-OPTIC SWITCHES

The magneto-optic effect refers to a phenomenon in which an electromagnetic wave interacts with a magnetic field. The Faraday effect is an important magneto-optic effect whereby the plane of polarization of an optical signal is rotated under the influence of a magnetic field. Magneto-optic switches use Faraday effect to switch optical signal. These switches are typically characterized with low loss and slow switching speed. They are somewhat wavelength dependent.

1.3.2.6. LIQUID CRYSTAL OPTICAL SWITCHES

A liquid crystal is a phase between solid and liquid. Liquid crystal-based optical switches also utilize polarization diversity and polarization rotation to achieve optical switching. Switches of this type are typically quite wavelength dependent, since the amount of polarization rotation depends on wavelength. Liquid crystal polarization rotation is also intrinsically temperature dependent. Switching speed is relatively slow, usually between 10–30 ms range, since the switching mechanism requires reorientation of rather large molecules.