1 Optical communications Components and enabling technologies Optical networking Evolution of optical networking: road map SDH = Synchronous Digital Hierarchy SONET = Synchronous Optical Network SDH SONET Bit rate STM-1 OC Mbit/s STM-4 OC Mbit/s STM-16 OC Gbit/s STM-64 OC Gbit/s STM-256 OC Gbit/s 40 Gbit/s corresponds to half a million simultaneous telephone conversations References S.V. Kartalopoulos, Introduction to DWDM Technology -- Data in a Rainbow, John Wiley & Sons, R. Ramaswami and K.N. Sivarajan, Optical Networks -- A Practical Perspective, Morgan Kaufmann Publishers, T. E. Stern and K. Bala, Multiwavelengts Optical Networking -- A Layered Approach, Addison-Wesley, R.J. Bates, Optical Switching and Networking Handbook, McGraw- Hill, 2001.
2 Components and enabling technologies Optical fiber Light sources, optical transmitters Photodetectors, optical receivers Optical amplifiers Wavelength converters Optical multiplexers and demultiplexers Optical add-drop multiplexers Optical cross connects WDM systems
3 Optical fiber Optical fiber is the most important transporting medium for highspeed communications in fixed networks. The optical fiber has many advantages: - immune to electromagnetic interference - does not corrode - huge bandwidth (25 Tbit/s) Some drawbacks: - connecting fibers requires special techniques (connectors, specialized personnel to splice and connect fibers) - does not allow tight bending An optical fiber consists of - ultrapure silica - mixed with dopants to adjust the refractive index A optical cable consists of several layers - silica core - cladding, a layer of silica with a different mix of dopants - buffer coating, which absorbs mechanical stresses - coating is covered by a strong material such as Kevlar - outermost there is a protective layer of plastic material
4 Fiber cable consists of a bundle of optical fibers, up to 432 fibers. The refractive index profile of a fiber is carefully controlled during the manufacturing phase Typical refractive index profiles are - step index profile - graded index profile The light rays are confined in the fiber by total reflection at the core.cladding interface in step-index fibers or by more gradual refraction in graded index fibers.
5 The fiber can be designed to support - several propagation modes: multimode fiber - just a single propagation mode: single-mode fiber Multimode graded index fiber - small delay spread - a 1% index difference between core and cladding amounts to a 1-5 ns/km delay spread - is easy to splice and to couple light into - bit rate is limited: up to 100 Mbit/s for lengths up to 40 km - fiber span without amplification is limited Single mode fiber - almost eliminates delay spread - is more difficult to splice and to exactly align two fibers together - is suitable for transmitting modulated signals at 40 Gbit/s or higher and up to 200 km without amplification
6 Dispersion is an undesirable phenomenon in optical fibers - causes an initially narrow light pulse spread out as it propagates along the fiber There are different causes for dispersion - modal dispersion - chromatic dispersion Modal dispersion - occurs in multimode fibers - due to different (lengths of) propagation paths of different modes Chromatic dispersion - the material properties of the fiber, such as dielectric constant and propagation constant depend on the frequency of the light - each individual wavelength of a pulse travels at different speed and arrives at the end of the fiber at different time - dispersion is measured in ps/(nm*km), i.e. delay per wavelength variation and fiber length Dispersion depends on the wavelength - at some wavelength the dispersion may be zero - in conventional single mode fiber this typically occurs at 1.3 µm - below, dispersion is negative, above its positive For long-haul transmission, single mode fibers with specialized index of refraction profiles have been manufactured - dispersion-shifted fiber (DSF) - the zero-dispersion point is shifted at 1.55 µm
7 Fiber attenuation is the most important transmission characteristic - limits the maximum span a light signal can be transmitted without amplification Fiber attenuation is due to light scattering on - fluctuations of the refractive index - imperfections of the fiber - impurites (metal ions and OH radicals have a particular effect) A conventional single-mode fiber has two low attenuation ranges - one at about 1.3 µm - another at about 1.55 µm Between these, there is a high attenuation range ( µm), with a peak at 1.39 µm, due to OH radical - special fibers that are almost free of OH radicals have been manufactured - such fibers increase the usable bandwidth by 50% - the whole range from µm to µm is usable, allowing about 500 WDM channels at 100 GHz channel spacing The attenuation is measured in db/km; typical values are db/km at 1.31 µm db/km at 1.55 µm - for comparison, the attenuation in ordinary clear glass is about 1 db/cm = 10 5 db/km
8 Light sources, optical transmitters One of the key components in optical communications is the monochromatic (narrow band) light source. Desirable properties - compact - monochromatic - stable - long lasting Light source may be one of the following types: - continuous wave (CW); emits at a constant power; needs an external modulator to carry information - modulated light; no external modulator is necessary Two most popular light sources are - light emitting diode (LED) - semiconductor laser
9 Light emitting diode (LED) An LED is a monolithically integrated p-n semiconductor diode. Emits light when voltage is applied across its two terminals. In the active junction area electrons in the conduction band and holes in the valence band are injected. Recombination of the electron with holes releases energy in the form of light. Can be used either as a CW light source or modulated light source (modulated by the injection current).
10 Characteristics of LEDs: Relatively slow: modulation rate < 1 Gbit/s Bandwidth depends on the material; relatively wide spectrum Amplitude and spectrum depend on temperature Inexpensive Transmits light in wide cone; suitable for multimode fibers
11 Semiconductor laser LASER (Light amplification by stimulated emission of radiation) Semiconductor laser is also known as - laser diode - injection laser Operation of a laser is the same as for any other oscillator: gain (amplification) and feedback. As a device semiconductor laser is similar to an LED: a p-n semiconductor diode. A difference is that the ends of the active junction area are carefully cleaved and act as partially reflecting mirrors - this provides feedback. The junction area acts as a resonating cavity for certain frequencies (those for which the roundtrip distance is a multiple of the wavelength in the material; constructive interference). The light fed back by the mirrors is amplified by stimulated emission. Lasing is achieved above a threshold current where the optical gain is sufficient to overcome losses (including the transmitted light) from the cavity. T. E. Stern and K. Bala, Multiwavelengts Optical Networking -- A Layered Approach, Addison-Wesley, 1999.
12 A Fabry-Perot laser the cavity can support many modes of oscillation; it is a multimode laser. In single frequency operation all but a single longitudinal mode must be suppressed. This can be achieved by different approaches: - cleaved-coupled cavity (C 3 ) lasers - external cavity lasers - distributed Bragg reflector (DBR) lasers - distributed feedback (DFB) lasers The most common light sources for high-bitrate, long-distance transmission are the DBR and DFB lasers. T. E. Stern and K. Bala, Multiwavelengts Optical Networking -- A Layered Approach, Addison-Wesley, Laser tunability is important for multiwavelength network applications. Slow tunability (on a ms time scale) is required for setting up connections in wavelength or waveband routed networks - achieved over a range of 1 nm via temperature control. Rapid tunability (on a ns-µs time scale) is required for TDM- WDM multiple access applications - achieved in DBR and DFB lasers by changing the refractive index, e.g. by changing the injected current in grating area. Another approach to rapid tunability is to use multiwavelength laser arrays - one or more lasers in the array can be activated at a time
13 Lasers are modulated either directly or externally - direct modulation by varying the injection current - external modulation by an external device, e.g. Mach-Zehnder interferometer T. E. Stern and K. Bala, Multiwavelengts Optical Networking -- A Layered Approach, Addison-Wesley, 1999.
14 Photodetectors, optical receivers A photodetector converts the optical signal to a photocurrent that is then electronically amplified (front-end amplifier). In a direct detection receiver, only the intensity of the incoming signal is detected - in contrast to coherent detection, where the phase of the optical signal is also relevant - coherent systems are still in research phase Photodetectors used in optical transmission systems are semiconductor photodiodes. The operation is essentially the reverse of a semiconductor optical amplifier: - the junction is reverse biased - in absence of optical signal only a small, minority carrier current, the dark current flows - a photon impinging on the surface of the device can be absorbed by an electron in the valence band, transferring the electron to the conduction band - each excited electron contributes to the photocurrent PIN photodiodes (p-type, intrinsic, n-type) An extra layer of intrinsic semiconductor material is sandwiched between the p and n regions Improves the responsivity of the device - captures most of the light in the depletion region
15 Avalanche photodiodes (APD) In a photodiode only one electron-hole pair is produced by an absorbed photon. This may not be sufficient when the optical power is very low. The APD resembles a PIN - an extra gain layer is inserted between the i and n layers - a large voltage is applied across the gain layer - photoelectrons are accelerated to sufficient speeds - produce additional electrons by collisions: avalanche effect - largely improved responsivity
16 Optical amplifiers Optical signal travelling in a fiber suffers attenuation. The optical power level of the signal must be periodically conditioned. Optical amplifiers are key components in long haul optical systems. An optical amplifier is characterized by - gain: ratio of output power to input power (in db) - gain efficiency: gain as a function of input power (db/mw) - gain bandwidth: range of frequencies over which the amplifier is effective - gain saturation: maximum output power, beyond which no amplification is reached - noise: undesired signal due to physical processes in the amplifier Types of amplifiers Electro-optic regenerators Semiconductor optical amplifiers (SOA) Erbium-doped fiber amplifiers (EDFA)
17 Electro-optic regenerators Optical signal is - received and transformed to an electronic signal - amplified electronically - converted back to optical signal at the same wavelength
18 Semiconductor optical amplifiers (SOA) The structure of an SOA is similar to that of a semiconductor laser. It consists of an active medium (p-n junction) in the form of waveguide; usually made from InGaAsP. Energy is provided by injecting electric current over the junction. SOAs are small, compact and able to be integrated with other semiconductor and optical components. They have large bandwidth and relatively high gain (20 db). Saturation power in the range of 5-10 dbm. SOAs are polarization dependent and thus require a polarization-maintaining fiber. Because of nonlinear phenomena SOAs have a high noise figure and high cross-talk level.
19 Erbium-doped fiber amplifiers (EDFA) EDFA is a very attractive amplifier type in optical communications systems. EDFA is a fiber segment, a few meters long, heavily doped with erbium (a rare earth metal). Energy is provided by a pump laser beam. Amplification is achieved by quantum mechanical phenomenon of stimulated emission - erbium atoms are excited to a high energy level by pump laser signal - they fall to a lower metastable (long-lived, 10 ms) state - an arriving photon triggers (stimulates) a transition to the ground level and another photon of the same wavelength is emitted
20 EDFAs have a high pump power utilization (> 50 %). Directly and simultaneously amplify a wide wavelength band (> 80 nm in the region 1550 nm) with a relatively flat gain. Flatness of the gain can be improved with gain-flattening optical filters. Gain in excess of 50 db Saturation power is as high as 37 dbm Low noise figure Transparent to optical modulation format Polarization independent Suitable for long-haul applications EDFAs are not small and cannot easily be integrated with other semiconductor devices.
21 Wavelength converters Enable optical channels to be relocated Achieved in optical domain by employing nonlinear phenomena Types of wavelength converters Optoelectronic approach Optical gating: cross-gain modulation Four-wave mixing Optoelectronic approach Simplest approach Input signal is - received - converted to electronic form - regenerated - transmitted using a laser at a different wavelength R. Ramaswami and K.N. Sivarajan, Optical Networks -- A Practical Perspective, Morgan Kaufmann Publishers, 1998.
22 Optical gating: cross-gain modulation Makes use of the dependence of the gain of an SOA (semiconductor optical amplifier) on its input power. Gain saturation occurs when high optical power is injected - carrier concentration is depleted - gain is reduced Fast - can handle rates 10 Gbit/s R. Ramaswami and K.N. Sivarajan, Optical Networks -- A Practical Perspective, Morgan Kaufmann Publishers, 1998.
23 Four-wave mixing Four-wave mixing is usually an undesirable phenomenon in fibers Can be exploited to achieve wavelength conversion In four-wave mixing, three waves at frequencies f 1, f 2 and f 3 produce a wave at the frequency f 1 +f 2 -f 3 When - f 1 = f s (signal) - f 2 = f 3 = f p (pump) a new wave is produces at 2f p -f s Four-wave mixing can be enhanced by using SOA to increase the power levels. Other wavelengths are filtered out. R. Ramaswami and K.N. Sivarajan, Optical Networks -- A Practical Perspective, Morgan Kaufmann Publishers, 1998.
24 Optical multiplexers and demultiplexers An optical multiplexers receives many wavelengths from many fibers and converges them into one beam that is coupled into a single fiber. An optical demultiplexer receives from a fiber a beam consisting of multiple optical frequencies and separates it into its frequency components, which are coupled in individual fibers (as many as there are frequencies).
25 Prisms and diffraction gratings Prisms and diffraction gratings can be used to achieve these functions in either direction (reciprocity) - in both of these devices a polychromatic parallel beam impinging on the surface is separated into frequency components leaving the device at different angles - based on different refraction (prism) or diffraction (diffraction grating) of different wavelengths.
26 Arrayed waveguide grating (AWG) AWGs are integrated devices based on the principle of interferometry - a multiplicity of wavelengths are coupled to an array of waveguides with different lengths - produces wavelength dependent phase shifts - in a second cavity the phase difference of each wavelengths interferes in such a manner that each wavelength contributes maximally at one of the output fibers Reported systems - SiO 2 AWG for 128 channels with 250 GHz channel spacing - InP AWG for 64 channels with 50 GHz channel spacing
27 Optical add-drop multiplexers (OADM) Optical multiplexers and demultiplexers are components designed for wavelength division (WDM) systems - multiplexer combines several optical signals at different wavelengths into a single fiber - demultiplexer separates a multiplicity of wavelengths in a fiber and directs them to many fibers The optical add-drop multiplexer - selectively removes (drops) a wavelength from the multiplex - then adds the same wavelength, but with different data An OADM may be realized by doing full demultiplexing and multiplexing of the wavelengths - a demultiplexed wavelength path can be terminated and a new one created
28 Optical cross connects Channel cross-connecting is a key function in communication systems. Optical cross-connection may be accomplished by - hybrid approach: converting optical signal to electronical domain, using electronic cross connects, and converting back to optical domain - all-optical switching: cross-connecting directly in the photonic domain Hybrid approach is currently more popular because the alloptical switching technology is not fully developed - all optical NxN cross-connects are feasible for N = large cross-connects, N up to 1000, are in experimental or planning phase All-optical cross-connecting can be achieved by - optical solid-state devices (couplers) - electromechanical mirror-based free space optical switching devices
29 Solid-state cross-connects Based on semiconductor directional couplers Directional coupler can change optical property of the path - polarization - propagation constant - absorption index - refraction The optical property may be change upon application of - heat - light - mechanical pressure - current injection - electric field The technology determines the switching speed, for instance - LiNbO 3 crystals: order of ns - SiO 2 crystals: order of ms
30 A multiport switch, also called star coupler, is constructed by employing several 2x2 directional couplers For instance, a 4x4 switch can be constructed from six 2x2 directional couplers Because losses are cumulative, the number of couplers in the path is limited and, therefore, also the number of ports is limited, perhaps to 32x32.
31 Microelectromechanical switches (MEMS) Tiny mirrors micromachined on a substrate - outgrowth of semiconductor processing technologies: deposition, etching, lithography - a highly polished flat plate (mirror) is connected with an electrical actuator - cab be tilted in different directions by applied voltage R.J. Bates, Optical switching and networking handbook, McGraw-Hill, 2001.
32 MEMS technology is still complex and expensive. Many MEMS devices may be manufactured on the same wafer - reduces cost per system Many mirrors can be integrated on the same chip - arranged in an array - experimental systems with 16x16=256 mirrors have been built - each mirror may be independently tilted Based on mirror arrays an all-optical space switch can be constructed. R.J. Bates, Optical switching and networking handbook, McGraw-Hill, 2001.
33 WDM systems WDM = Wavelength Division Multiplexing - equivalent of frequency division multiplexing in the optical domain. In WDM systems several optical signals at different wavelengths are carried on a single fiber - each wavelength may carry traffic at STM-4, STM-16 or STM- 64 rate (622 Mbit/s, 2.5 Gbit/s or 10 Gbit/s). Increases the capacity of a fiber by the number of wavelengths carried. Depending on the channel (wavelength) separation one speaks about - WDM - DWDM (Dense WDM) - CWDM (Coarse WDM) ITU-T recommendation G.692 defines - 43 wavelength channels from 1530 to 1565 nm - with a spacing of 100 GHz - each channel carrying an STM-64 signal at 10 Gbit/s - total capacity 430 Gbit/s (enough to transmit volumes of an encyclopedia in 1 second) Commercial systems with 16, 40, 80 and 128 channels per fiber have been announced - 40 channels with a channel spacing of 100 GHz - 80 channels with a channel spacing of 50 GHz Theoretically more than 1000 channels may be multiplexed in a fiber - aggregate bandwidths of 8 Tbit/s per fiber are feasible (corresponding to 100 million simultaneous telephone conversations) - maybe even 40 Tbit/s per fiber can be achieved - in an optical cable of 432 fibers with 50% utilization this amounts to the total aggregate bandwidth of 8000 Tbit/s
34 Initially, WDM and DWDM systems are used in point-ti-point configuration - to create a "big fat pipe" for long-haul transport OADM multiplexers may be added enabling the system to drop and add channels along its path.
35 DWDM networks with ring topology may also be created - covers a local or metropolitan area - spans a few tens of kilometers - one node serves as a hub where all wavelengths are sourced and terminated - each node has an OADM to drop off or add one or more wavelength channels - number of nodes is typically less than the number of wavelengths
36 Fault protection may be accomplished with dual counterrotating rings - when a fault is detected, the neighboring nodes reroute the traffic via a U-turn optical cross-connect - requires additional optical cross-connect devices that put an additional burden on the power and cost budget of the ring network
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