Chapter 8: Optical Fibers and Components TOPICS WDM optical networks Light transmitted through an optical fiber Types of optical fibers Impairments Components: Lasers, optical amplifiers, couplers, OXCs Connection-Oriented Networks - Harry Perros 1
A point-to-point WDM optical link Tx λ 1 λ 1 Rx Tx λ W Power amplifier optical fiber In-line amplification optical fiber Preamplifier λ W Rx Wavelength multiplexer Wavelength demultiplexer Typically, up to 300 km without the need for amplification. Speed (on a wavelength): 10 G, 40 G, 100 G (soon) No. of wavelengths: 40, 80, 160 Connection-Oriented Networks - Harry Perros 2
An example of an optical network Mesh network Ring 1 Ring 4 Ring 2 Ring 3 Connection-Oriented Networks - Harry Perros 3
How light is transmitted through an optical fiber Wave Electric field Source Light propagates in a series of electromagnetic spherical waves. On the front of each wave there is an electrical field and vertical to it there is a magnetic field Connection-Oriented Networks - Harry Perros 4
An optical fiber Cladding Core Cladding Core and cladding Cladding Core Cladding Core Refractive index n 1 Refractive index n 1 n 2 n 2 Radial distance Radial distance a) Step-index fiber b) graded-index fiber Connection-Oriented Networks - Harry Perros 5
Refraction and reflection of a light ray θ f Refracted ray n 2 n 1 θ ι θ r Incident ray Reflected ray Connection-Oriented Networks - Harry Perros 6
Angle of launching a ray into the fiber Cladding Cladding Core Core θ l θ ι θ r Cladding Cladding Cladding Optical transmitter Core Cladding Connection-Oriented Networks - Harry Perros 7
Multi-mode fibers It carries multiple light rays concurrently, each at a slightly different reflection angle within the optical fiber core. It is used for short distance (LANs) Core/diameter of a multi-mode fiber: 50/125 µm, 62.5/125 µm, 100/140 µm Connection-Oriented Networks - Harry Perros 8
Multi-mode fibers Light can enter the core at different angles, making it easier to connect the light source to broader light sources such as LEDs. Light rays travel down multiple reflective paths (modes) These reflective paths reinforce each other or cancel each other at specific points Connection-Oriented Networks - Harry Perros 9
Electric fields in multi-mode fibers 2 A Cladding Core 1 B Cladding The electric fields of the two rays are reinforced if both are going up or down. On the other hand, when one goes up and the other down, they cancel each other, and no light is formed Connection-Oriented Networks - Harry Perros 10
Electric field amplitudes for various fiber modes Cladding Core m=0 m=1 m=2 Cladding Connection-Oriented Networks - Harry Perros 11
Propagation of modes Cladding Cladding a) step-index fiber Cladding Cladding b) Graded-index fiber Connection-Oriented Networks - Harry Perros 12
Single-mode fiber Cladding Cladding It is designed for the transmission of a single ray or mode of light as a carrier. It is used for long-distance transmission. Core/diameter: 9 or 10 / 125 µm Connection-Oriented Networks - Harry Perros 13
Impairments The transmission of light through an optical fiber is subjected to optical effects, known as impairments. There are: linear impairments, and non-linear impairments. Connection-Oriented Networks - Harry Perros 14
Linear impairments These impairments are called linear because their effect is proportional to the length of the fiber. Attenuation: Attenuation is the decrease of the optical power along the length of the fiber. Dispersion Dispersion is the distortion of the shape of a pulse. Connection-Oriented Networks - Harry Perros 15
Attenuation 2.5 2.0 Attenuation, db 1.5 1.0 0.5 800 1000 1200 1400 1600 1800 Wavelength, nm Connection-Oriented Networks - Harry Perros 16
Dispersion Dispersion is due to a number of reasons, such as modal dispersion, chromatic dispersion, polarization mode dispersion. Connection-Oriented Networks - Harry Perros 17
Modal dispersion Power Power Power Time Time Time In multi-mode fibers some modes travel a longer distance to get to the end of the fiber than others In view of this, the modes have different delays, which causes a spreading of the output pulse Connection-Oriented Networks - Harry Perros 18
Chromatic dispersion It is due to the fact that the refractive index of silica is frequency dependent. In view of this, different frequencies travel at different speeds, and as a result they experience different delays. These delays cause spreading in the duration of the output pulse. Connection-Oriented Networks - Harry Perros 19
Chromatic dispersion can be corrected using a dispersion compensating fiber. The length of this fiber is proportional to the dispersion of the transmission fiber. Approximately, a spool of 15 km of dispersion compensating fiber is placed for every 80 km of transmission fiber. Dispersion compensating fiber introduces attenuation of about 0.5 db/km. Connection-Oriented Networks - Harry Perros 20
Polarization mode dispersion (PMD) It is due to the fact that the core of the fiber is not perfectly round. In an ideal circularly symmetric fiber the light gets polarized and it travels along two polarization planes which have the same speed. When the core of the fiber is not round, the light traveling along the two planes may travel at different speeds. This difference in speed will cause the pulse to break. Connection-Oriented Networks - Harry Perros 21
Non-linear impairments They are due to the dependency of the refractive index on the intensity of the applied electrical field. The most important non-linear effects in this category are: selfphase modulation and four-wave mixing. Another category of non-linear impairments includes the stimulated Raman scattering and stimulated Brillouin scattering. Connection-Oriented Networks - Harry Perros 22
Types of fibers Multi-mode fibers: They are used in LANs and more recently in 1 Gigabit Ethernet and 10 Gigabit Ethernet. Single-mode fiber is used for long-distance telephony, CATV, and packet-switched networks. Plastic optical fibers (POF) Connection-Oriented Networks - Harry Perros 23
Single-mode fibers: Standard single-mode fiber (SSMF): Most of the installed fiber falls in this category. It was designed to support early long-haul transmission systems, and it has zero dispersion at 1310 nm. Non-zero dispersion fiber (NZDF): This fiber has zero dispersion near 1450 nm. Connection-Oriented Networks - Harry Perros 24
Negative dispersion fiber (NDF): This type of fiber has a negative dispersion in the region 1300 to 1600 nm. Low water peak fiber (LWPF): The peak in the attenuation curve at 1385 nm is known as the water peak. With this new type of fiber this peak is eliminated, which allows the use of this region. Connection-Oriented Networks - Harry Perros 25
Plastic optical fibers (POF) Single-mode and multi-mode fibers have a high cost and they require a skilled technician to install them. POFs on the other hand, are very low-cost and they can be easily installed by an untrained person. The core has a very large diameter, and it is about 96% of the diameter of the cladding. Plastic optic fibers find use in digital home appliance interfaces, home networks, and cars Connection-Oriented Networks - Harry Perros 26
Components Lasers Photo-detectors and optical receivers Optical amplifiers The 2x2 coupler Optical cross connects (OXC) Connection-Oriented Networks - Harry Perros 27
Light amplification by stimulated emission of radiation (Laser) A laser is a device that produces a very strong and concentrated beam. It consists of an energy source which is applied to a lasing material, a substance that emits light in all directions and it can be of gas, solid, or semiconducting material. The light produced by the lasing material is enhanced using a device such as the Fabry-Perot resonator cavity. Connection-Oriented Networks - Harry Perros 28
Fabry-Perot resonator cavity. It consists of two partially reflecting parallel flat mirrors, known as facets, which create an optical feedback that causes the cavity to oscillate. Light hits the right facet and part of it leaves the cavity through the right facet and part of it is reflected. Left facet Right facet Connection-Oriented Networks - Harry Perros 29
Since there are many resonant wavelengths, the resulting output consists of many wavelengths spread over a few nm, with a gap between two adjacent wavelengths of 100 to 200 GHz. A single wavelength can be selected by using a filtering mechanism that selects the desired wavelength and provides loss to the other wavelengths. Connection-Oriented Networks - Harry Perros 30
Tunable lasers Tunable lasers are important to optical networks Also, it is more convenient to manufacture and stock tunable lasers, than make different lasers for specific wavelengths. Several different types of tunable lasers exist, varying from slow tunability to fast tunability. Connection-Oriented Networks - Harry Perros 31
Modulation Modulation is the addition of information on a light stream This can be realized using the on-off keying (OOK) scheme, whereby the light stream is turned on or off depending whether we want to modulate a 1 or a 0. Connection-Oriented Networks - Harry Perros 32
WDM and dense WDM (DWDM) WDM or dense WDM (DWDM) are terms used interchangeably. DWDM refers to the wavelength spacing proposed in the ITU-T G.692 standard in the 1550 nm window (which has the smallest amount of attenuation and it also lies in the band where the Erbium-doped fiber amplifier operates.) The ITU-T grid is not always followed, since there are many proprietary solutions. Connection-Oriented Networks - Harry Perros 33
The ITU-T DWDM grid Channel λ (nm) Channel λ (nm) Channel λ (nm) Channel λ (nm) code code code code 18 1563.05 30 1553.33 42 1543.73 54 1534.25 19 1562.23 31 1552.53 43 1542.94 55 1533.47 20 1561.42 32 1551.72 44 1542.14 56 1532.68 21 1560.61 33 1590.12 45 1541.35 57 1531.90 22 1559.80 34 1550.12 46 1540.56 58 1531.12 23 1558.98 35 1549.32 47 1539.77 59 1530.33 24 1558.17 36 1548.52 48 1538.98 60 1529.55 25 1557.36 37 1547.72 49 1538.19 61 1528.77 26 1556.56 38 1546.92 50 1537.40 62 1527.99 27 1555.75 39 1546.12 51 1536.61 28 1554.94 40 1545.32 52 1535.82 29 1554.13 41 1544.53 53 1535.04 Connection-Oriented Networks - Harry Perros 34
Photo-detectors and optical receivers The WDM optical signal is demultiplexed into the W different wavelengths, and each wavelength is directed to a receiver. Each receiver consists of a photodetector, an amplifier, and signal-processing circuit. Connection-Oriented Networks - Harry Perros 35
Optical amplifiers The optical signal looses its power as it propagates through an optical fiber, and after some distance it becomes too weak to be detected. Optical amplification is used to restore the strength of the signal Connection-Oriented Networks - Harry Perros 36
λ 1 Tx λ 1 Rx Tx λ W Power amplifier optical fiber In-line amplification optical fiber Preamplifier λ W Rx Wavelength multiplexer Wavelength demultiplexer Amplifiers: power amplifiers in-line amplifiers pre-amplifiers Connection-Oriented Networks - Harry Perros 37
1R, 2R, 3R Prior to optical amplifiers, the optical signal was regenerated by first converting it into an electrical signal, then apply 1R (re-amplification), or 2R (re-amplification and re-shaping) or 3R (re-amplification, re-shaping, and re-timing) and then converting the regenerated signal back into the optical domain. Connection-Oriented Networks - Harry Perros 38
The Erbium-doped fiber amplifier (EDFA) Signal to be amplified 1550 nm Coupler Isolator Erbium-doped fiber Isolator Laser 850 nm Connection-Oriented Networks - Harry Perros 39
Two-stage EDFA Signal to be amplified 1550 nm Isolator Coupler Erbium-doped fiber Coupler Isolator Laser 850 nm Laser 850 nm Connection-Oriented Networks - Harry Perros 40
Raman optical amplifiers It boosts the signal by transferring energy from a powerful pump signal to a weaker signal. It is typically stationed at the end of a fiber run and pumps backward several kilometers to amplify signals through that region Connection-Oriented Networks - Harry Perros 41
The 2x2 coupler Fiber 1 Input 1 Output 1 Input 2 Fiber 2 Output 2 Tapered region Coupling region Tapered region The 2x2 coupler is a basic device in optical networks, and it can be constructed in variety of different ways. A common construction is the fused-fiber coupler. Connection-Oriented Networks - Harry Perros 42
3-dB coupler A 2x2 coupler is called a 3-dB coupler when the optical power of an input light applied to, say input 1 of fiber 1, is evenly divided between output 1 and output 2. Connection-Oriented Networks - Harry Perros 43
If we only launch a light to the one of the two inputs of a 3-dB coupler, say input 1, then the coupler acts as a splitter. If we launch a light to input 1 and a light to input 2 of a 3-dB coupler, then the two lights will be coupled together and the resulting light will be evenly divided between outputs 1 and 2. In the above case, if we ignore output 2, the 3-dB coupler acts as a combiner. Connection-Oriented Networks - Harry Perros 44
A banyan network of 3-dB couplers λ 1 λ 1,λ 2..,λ 8 λ 2 λ 1,λ 2..,λ 8 λ 3 λ 4 λ 1,λ 2..,λ 8 λ 1,λ 2..,λ 8 λ 5 λ 1,λ 2..,λ 8 λ 6 λ 1,λ 2..,λ 8 λ 7 λ 8 λ 1,λ 2..,λ 8 λ 1,λ 2..,λ 8 Connection-Oriented Networks - Harry Perros 45
Optical cross connects (OXCs) Input fibers λ 1 CPU Output fibers λ 1 λ W Fiber 1 λ W Fiber 1 λ 1 λ 1 λ W Fiber N Switch fabric λ W Fiber N Connection-Oriented Networks - Harry Perros 46
OXC functionality It switches optically all the incoming wavelengths of the input fibers to the outgoing wavelengths of the output fibers. For instance, it can switch the optical signal on incoming wavelength λ i of input fiber k to the outgoing wavelength λ i of output fiber m. Connection-Oriented Networks - Harry Perros 47
Converters: If it is equipped with converters, it can switch the optical signal of the incoming wavelength λ i of input fiber k to another outgoing wavelength λ j of the output fiber m. This happens when the wavelength λ i of the output fiber m is in use. Converters typically have a limited range within they can convert a wavelength. Connection-Oriented Networks - Harry Perros 48
Optical add/drop multiplexer (OADM): An OXC can also be used as an OADM. That is, it can terminate the optical signal of a number of incoming wavelengths and insert new optical signals on the same wavelengths in an output port. The remaining incoming wavelengths are switched through as described above. Connection-Oriented Networks - Harry Perros 49
Transparent and Opaque Switches Transparent switch: The incoming wavelengths are switched to the output fibers optically, without having to convert them to the electrical domain. Opaque switch: The input optical signals are converted to electrical signals, from where the packets are extracted. Packets are switched using a packet switch, and then they are transmitted out of the switch in the optical domain. Connection-Oriented Networks - Harry Perros 50
Switch technologies Several different technologies exist: micro electronic mechanical systems (MEMS) semiconductor optical amplifiers (SOA) beam steering (Polatis) holograms Also, 2x2 directional coupler, such as the electro-optic switch, the thermo-optic switch, and the Mach-Zehnder interferometer, can be used to construct large OXC switch fabrics Connection-Oriented Networks - Harry Perros 51
2D MEMS switching fabric Input ports Up i Down Actuator Mirro r j Output ports Connection-Oriented Networks - Harry Perros 52
A 2D MEMS OADM Drop wavelengths λ 1,λ 2..,λ W λ 1,λ 2..,λ W λ 1,λ 2..,λ W i λ 1,λ 2..,λ W Add wavelengths Terminate wavelengths Logical design Add wavelengths 2D MEMS implementation Connection-Oriented Networks - Harry Perros 53
3D MEMS switching fabric Output wavelengths MEMS array Input wavelengths Inside ring y axis Mirro r x axis MEMS array Connection-Oriented Networks - Harry Perros 54
Semiconductor optical amplifier (SOA) A SOA is a pn-junction that acts as an amplifier and also as an on-off switch Current p-type n-type Optical signal Connection-Oriented Networks - Harry Perros 55
Α 2x2 SOA switch Wavelength λ 1 is split into two optical signals, and each signal is directed to a different SOA. One SOA amplifies the optical signal and permits it to go through, and the other one stops it. As a result λ 1 may leave from either the upper or the lower output port. Switching time is currently about 100 psec. Polymer waveguides SOAs Polymer waveguides λ 1 λ 2 Connection-Oriented Networks - Harry Perros 56
Beam steering (Polatis) Connection-Oriented Networks - Harry Perros 57