OPTICAL NETWORKS Building Blocks A. Gençata İTÜ, Dept. Computer Engineering 2005
Introduction An introduction to WDM devices. optical fiber optical couplers optical receivers optical filters optical amplifiers optical switches. 2
Optical Fiber Excellent physical medium for high-speed networking Two low-attenuation windows: Centered approx. 1310 nm, a window of 200 nm Centered approx. 1550 nm, a window of 200 nm Combined, they provide a theoretical upper bound of 50 THz. 3
Optical Fiber By using low-attenuation windows for data transmission, the signal loss can be made very small. This reduces the number of amplifiers needed. 1550 nm window is preferred for long-haul wide-area networks. Standards set for industry: ITU grid. Optical signals have been sent over 80 km without amplification. Low error rates: Typically operate at BERs of less than 10-11. Fiber is also, flexible, light, reliable in corrosive environment. Immune to electromagnetic interference, and does not cause interference. 4
Optical Transmission in Fiber Fiber is a thin filament of glass which acts as a waveguide. Waveguide: A physical medium which allows the propagation of electromagnetic waves like light. Due to total internal reflection, light propagates the length of the fiber with little loss. Light speed in vacuum: c = 3 10 8 m/s. Light can also travel through any transparent material, but the speed is slower. The ratio of the speed of light in vacuum to that in a material is the material s refractive index n. n mat = c / c mat 5
Optical Transmission in Fiber When light travels from material of a given refractive index to a material of a different refractive index, the angle at which the light is transmitted in the second material depends on: the refractive indices and the angle at which light strikes the interface. Snell s Law: n a sin Θ a = n b sin Θ b If n a > n b and Θ a is greater than some critical value, the rays are reflected back into material a from its boundary with material b. 6
Optical Transmission in Fiber Fiber consists of a core completely surrounded by a cladding. Core and cladding consist of glass of different refractive indices. 7
Critical Angle Typical delay of light in optical fiber is 5 µs/km Single-mode vs. multimode fiber 8
Attenuation Attenuation in optical fiber leads to a reduction of the signal power as the signal propagates over some distance. When determining the maximum distance that a signal can propagate for a given transmitter power and receiver sensitivity, one must consider attenuation. Receiver sensitivity is the minimum power required by a receiver to detect the signal. Let P(L) be the power of the optical pulse at distance L km from the transmitter, and A be the attenuation constant of the fiber (in db/km). Attenuation is characterized by: P(L) = 10 A.L / 10 P(0) where P(0) is the power at the transmitter. 9
Dispersion Dispersion is the widening of a pulse duration as it travels through a fiber. As a pulse widens, it can broaden enough to interfere with neighboring pulses (bits) on the fiber, leading to inter-symbol interference. Dispersion thus limits the bit spacing and the maximum transmission rate on a fiber-optic channel. Several kinds: intermodal dispersion chromatic dispersion material, waveguide, profile dispersions polarization mode dispersion 10
Nonlinearities in Fiber Nonlinear effects in fiber may potentially have a significant impact on the performance of WDM optical communication systems. Nonlinearities in fiber may lead to attenuation, distortion, and cross-channel interference. In a WDM system, these effects place constraints on the spacing between adjacent wavelength channels, limit the maximum power on any channel, and may also limit the maximum bit rate. Types: Nonlinear refraction Stimulated Raman scattering Stimulated Brillouin Scattering Four-wave mixing 11
Nonlinearities in Fiber It is shown that, in a WDM system using channels spaced 10 GHz apart and a transmitter power of 0.1 mw per channel, a maximum of about 100 channels can be obtained in the 1550-nm low-attenuation region. The details of optical nonlinearities are very complex, and beyond our scope. However, it is important to understand that they are a major limiting factor in the available number of channels in a WDM system, especially those operating over distances greater than 30 km. The existence of these nonlinearities suggests that WDM protocols which limit the number of nodes to the number of channels do not scale well. 12
Optical Fiber Couplers Coupler is a general term that covers all devices that combine light into or split light out of a fiber. It can be either active or passive device. The splitting ratio, α, is the amount of power that goes to each output. For a two-port splitter, the most common splitting ratio is 50:50, though splitters with any ratio can be manufactured. Combiners are the reverse of splitters, and when turned around, a combiner can be used as a splitter. A 2 2 coupler is a 2 1 combiner followed immediately by a 1 2 splitter. 13
A 16 x 16 Passive Star Coupler The passive-star coupler (PSC) is a multi-port device in which light coming into any input port is broadcast to every output port. The PSC is attractive because the optical power that each output receives P out equals: P out = P in / N where P in is the optical power introduced into the star by one node. 14
Optical Transmitter Lasers are used as optical transmitter. The transmitters used in WDM networks often require the capability to tune to different wavelengths. Some primary characteristics of interest for tunable lasers are: the tuning range, the tuning time, whether the laser is continuously tunable (over its tuning range) or discretely tunable (only to selected wavelengths). 15
Types of Transmitters The table summarizes tuning range and time of different types of lasers. 16
Optical Receivers Optical filters transform the optical signal into electronic signal. There are different types of tunable optical filters. These filters are characterized primarily by their tuning range and tuning time. The tuning range specifies the range of wavelengths which can be accessed by a filter. A wide tuning range allows systems to utilize a greater number of channels. The tuning time of a filter specifies the time required to tune from one wavelength to another. 17
Tunable Filters The table shows various types of filters and their tuning range and time. 18
Alternate Receiving Devices An alternative to tunable filters is to use fixed filters or grating devices. Grating devices typically filter out one or more different wavelength signals from a fiber. Such devices may be used to implement optical multiplexers and demultiplexers or receiver arrays. 19
Optical Amplifiers All-optical amplification may differ from opto-electronic amplification in that it may act only to boost the power of a signal, not to restore the shape or timing of the signal. This type of amplification is known as 1R (regeneration). In 2R (regeneration and reshaping), the optical signal is converted to an electronic signal which is then used to directly modulate a laser. The optical signals may be amplified by first converting the information stream into an electronic data signal, and then retransmitting the signal optically. Such amplification is referred to as 3R (regeneration, reshaping, and reclocking). 20
Types of Amplifiers Semiconductor laser amplifier Fabry-Perot Traveling-wave Doped-fiber amplifier EDFA PDFFA Raman amplifier 21
Switching Elements According to the signal carriers, there are optical switching and electronic switching. In the switching granularity point of view, there are two basic classes, circuit switching corresponding to wavelength routing, cell switching corresponding to optical packet switching and optical burst switching. As far as the transparency of signal is considered, there are opaque switching and transparent switching. 22
Optical Cross-connect (OXC) An optical cross-connect (OXC) switches optical signals from input ports to output ports. A basic cross-connect element is the 2 2 crosspoint element. It routes optical signals from two input ports to two output ports and has two states: cross state and bar state. 23
1024x1024 Clos Fabric 24
MEMS Micro-electro mechanical systems. Widely believed to be the most promising for large-scale optical cross-connects. Based on mirrors, membranes, and planar moving waveguides. The two major approaches are 2-Dimensional and 3-Dimensional approaches. The 3-D Optical MEMS based on mirrors is popular because it is suitable for compact, large-scale switching fabrics. The optical signals passing through the optical fibers at the input port are switched independently by the MEMS mirrors with two-axis tilt control and then focused onto the optical fibers at the output ports. In the switch, any connection between input and output fibers can be accomplished by controlling the tilt angle of each mirror. The 3D MEMS-based OOO switches is in sizes ranging from 256 x 256 to 1000 x 1000 bi-directional port machines. Encouraging research show that 8000 x 8000 ports will be practical within the foreseeable future. 25
3D MEMS Optical Switch 26
Wavelength Routing Devices A wavelength-routing device can route signals arriving at different input fibers (ports) of the device to different output fibers (ports) based on the wavelengths of the signals. Wavelength routing is accomplished by: demultiplexing the different wavelengths from each input port, optionally switching each wavelength separately, and then multiplexing signals at each output port. The device can be either: non-reconfigurable, in which case there is no switching stage between the demultiplexers and the multiplexers, and the routes for different signals arriving at any input port are fixed (referred to as routers), or reconfigurable, in which case the routing function of the switch can be controlled electronically. 27
Non-reconfigurable Wavelength Router 28
Wavelength Routing Switch The WRS has P incoming fibers and P outgoing fibers. On each incoming fiber, there are M wavelength channels. Similar to the nonreconfigurable router, the wavelengths on each incoming fiber are separated using a grating demultiplexer. The outputs of the demultiplexers are directed to an array of M P P optical switches between the demultiplexer and the multiplexer stages. All signals on a given wavelength are directed to the same switch. The switched signals are then directed to multiplexers associated with the output ports. Finally, information streams from multiple WDM channels are multiplexed before launching them back onto an output fiber. 29
PxP WRS 30
Wavelength Conversion Consider the following network. 31
Wavelength Conversion Three lightpaths have been set up: C to A on wavelength λ1, C to B on λ2, and D to E on λ1. To establish a lightpath, we require that the same wavelength be allocated on all the links in the path. This requirement is known as the wavelengthcontinuity constraint. Can we establish a lightpath from A to B? 32
Wavelength Conversion It is easy to eliminate the wavelength-continuity constraint, if we were able to convert the data arriving on one wavelength along a link into another wavelength at an intermediate node and forward it along the next link. Such a technique is referred to as wavelength conversion. A single lightpath in such a wavelengthconvertible network can use a different wavelength along each of the links in its path. 33
Wavelength Converter The function of a wavelength converter is to convert data on an input wavelength onto a different output wavelength among the N wavelengths. 34