WDM Glossary
Following are some of the frequent used DWDM terminologies.
TERMS |
DEFINITION |
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Arrayed Waveguide Grating (AWG) |
An arrayed waveguide grating (AWG) is a passive optical device that is constructed of an array of waveguides, each of slightly different length. With a AWG, you can take a multi-wavelength input and separate the component wavelengths on to different output ports. The reverse operation can also be performed, combining several input ports on to a single output port of multiple wavelengths. An advantage of AWGs is their ability to operate bidirectionally. AWGs are used to perform wavelength multiplexing and demultiplexing, as well as wavelength add/drop operations. |
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Bit Error Rate/Q-Factor (BER) |
Bit error rate (BER) is the measure of the transmission quality of a digital signal. It is an expression of errored bits vs. total transmitted bits, presented in a ratio. Whereas a BER performance of 10-9 (one bit in one billion is an error) is acceptable in DS1 or DS3 transmission, the expected performance for high speed optical signals is on the order of 10-15. Bit error rate is a measurement integrated over a period of time, with the time interval required being longer for lower BERs. One way of making a prediction of the BER of a signal is with a Q-factor measurement. |
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C Band |
The C-band is the “center” DWDM transmission band, occupying the 1530 to 1562nm wavelength range. All DWDM systems deployed prior to 2000 operated in the C-band. The ITU has defined channel plans for 50GHz, 100GHz, and 200GHz channel spacing. Advertised channel counts for the C-band vary from 16 channels to 96 channels. The C-Band advantages are:
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Chromatic Dispersion (CD) |
The distortion of a signal pulse during transport due to the spreading out of the wavelengths making up the spectrum of the pulse. The refractive index of the fiber material varies with the wavelength, causing wavelengths to travel at different velocities. Since signal pulses consist of a range of wavelengths, they will spread out during transport. |
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Circulator |
A passive multiport device, typically 3 or 4 ports, where the signal entering at one port travels around the circulator and exits at the next port. In asymmetrical configurations, there is no routing of traffic between the port 3 and port 1. Due to their low loss characteristics, circulators are useful in wavelength demux and add/drop applications. |
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Coupler |
A coupler is a passive device that combines and/or splits optical signals. The power loss in the output signals depends on the number of ports. In a two port device with equal outputs, each output signal has a 3 dB loss (50% power of the input signal). Most couplers used in single mode optics operate on the principle of resonant coupling. Common technologies used in passive couplers are fused-fiber and planar waveguides. WAVELENGTH SELECTIVE COUPLERS Couplers can be “tuned” to operate only on specific wavelengths (or wavelength ranges). These wavelength selective couplers are useful in coupling amplifier pump lasers with the DWDM signal. |
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Cross-Phase Modulation (XPM) |
The refractive index of the fiber varies with respect to the optical signal intensity. This is known as the “Kerr Effect”. When multiple channels are transmitted on the same fiber, refractive index variations induced by one channel can produce time variable phase shifts in co-propagating channels. Time varying phase shifts are the same as frequency shifts, thus the “color” changes in the pulses of the affected channels. |
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DCU |
A dispersion compensation unit removes the effects of dispersion accumulated during transmission, thus repairing a signal pulse distorted by chromatic dispersion. If a signal suffers from the effects of positive dispersion during transmission, then the DCU will repair the signal using negative dispersion. TRANSMISSION FIBER
DISPERSION COMPENSATION UNIT (DCU)
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Dispersion Shifted Fiber (DSF) |
In an attempt to optimize long haul transport on optical fiber, DSF was developed. DSF has its zero dispersion wavelength shifted from the 1310nm wavelength to a minimal attenuation region near the 1550nm wavelength. This fiber, designated ITU-T G.653, was recognized for its ability to transport a single optical signal a great distance before regeneration. However, in DWDM transmission, signal impairments from four-wave mixing are greatest around the fiber’s zero-dispersion point. Therefore, with DSF’s zero-dispersion point falling within the C-Band, DSF fiber is not suitable for C-band DWDM transmission. DSF makes up a small percentage of the US deployed fiber plant, and is no longer being deployed. DSF has been deployed in significant amounts in Japan, Mexico, and Italy. |
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Erbium Doped Fiber Amplifier (EDFA) |
PUMP LASER The power source for amplifying the signal, typically a 980nm or 1480nm laser. ERBIUM DOPED FIBER Single mode fiber, doped with erbium ions, acts as the gain fiber, transferring the power from the pump laser to the target wavelengths. WAVELENGTH SELECTIVE COUPLER Couples the pump laser wavelength to the gain fiber while filtering out any extraneous wavelengths from the laser output. ISOLATOR Prevents any back-reflected light from entering the amplifier. EDFA Advantages are:
EDFA Disadvantages are:
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Fiber Bragg Grating (FBG) |
A fiber Bragg grating (FBG) is a piece of optical fiber that has its internal refractive index varied in such a way that it acts as a grating. In its basic operation, a FBG is constructed to reflect a single wavelength, and pass the remaining wavelengths. The reflected wavelength is determined by the period of the fiber grating. If the pattern of the grating is periodic, a FBG can be used in wavelength mux / demux applications, as well as wavelength add / drop applications. If the grating is chirped (non-periodic), then a FBG can be used as a chromatic dispersion compensator. |
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Four Wave Mixing (FWM) |
The interaction of adjacent channels in WDM systems produces sidebands (like harmonics), thus creating coherent crosstalk in neighboring channels. Channels mix to produce sidebands at intervals dependent on the frequencies of the interacting channels. The effect becomes greater as channel spacing is decreased. Also, as signal power increases, the effects of FWM increase. The presence of chromatic dispersion in a signal reduces the effects of FWM. Thus the effects of FWM are greatest near the zero dispersion point of the fiber. |
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Gain Flattening |
The gain from an amplifier is not distributed evenly among all of the amplified channels. A gain flattening filter is used to achieve constant gain levels on all channels in the amplified region. The idea is to have the loss curve of the filter be a “mirror” of the gain curve of the amplifier. Therefore, the product of the amplifier gain and the gain flattening filter loss equals an amplified region with flat gain. The effects of uneven gain are compounded for each amplified span. For example, if one wavelength has a gain imbalance of +4 dB over another channel, this imbalance will become +20 dB after five amplified spans. This compounding effect means that the weaker signals may become indistinguishable from the noise floor. Also, over-amplified channels are vulnerable to increase non-linear effects. |
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Isolator |
An isolator is a passive device that allows light to pass through unimpeded in one direction, while blocking light in the opposite direction. An isolator is constructed with two polarizers (45o difference in orientation), separated by a Faraday rotator (rotates light polarization by 45o). One important use for isolators is to prevent back-reflected light from reaching lasers. Another important use for isolators is to prevent light from counter propagating pump lasers from exiting the amplifier system on to the transmission fiber. |
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L Band |
The L-band is the “long” DWDM transmission band, occupying the 1570 to 1610nm wavelength range. The L-band has comparable bandwidth to the C-band, thus comparable total capacity. The L-Band advantages are:
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Lasers |
A LASER (Light Amplification by the Stimulated Emission of Radiation) produces high power, single wavelength, coherent light via stimulated emission of light. Semiconductor Laser (General View) Semiconductor laser diodes are constructed of p and n semiconductor layers, with the junction of these layers being the active layer where the light is produced. Also, the lasing effect is induced by placing partially reflective surfaces on the active layer. The most common laser type used in DWDM transmission is the distributed feedback (DFB) laser. A DFB laser has a grating layer next to the active layer. This grating layer enables DFB lasers to emit precision wavelengths across a narrow band. |
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Mach-Zehnder Interferometer (MZI) |
A Mach-Zehnder interferometer is a device that splits an optical signal into two components, directs each component through its own waveguide, then recombines the two components. Based on any phase delay between the two waveguides, the two re-combined signal components will interfere with each other, creating a signal with an intensity determined by the interference. The interference of the two signal components can be either constructive or destructive, based on the delay between the waveguides as related to the wavelength of the signal. The delay can be induced either by a difference in waveguide length, or by manipulating the refractive index of one or both waveguides (usually by applying a bias voltage). A common use for Mach-Zehnder interferometer in DWDM systems is in external modulation of optical signals. |
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Multiplexer (MUX) |
DWDM Mux
DWDM Demux
Mux/Demux Technology
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Non-Zero Dispersion Shifted Fiber (NZ-DSF) |
After DSF, it became evident that some chromatic dispersion was needed to minimize non-linear effects, such as four wave mixing. Through new designs, λ0 was now shifted to outside the C-Band region with a decreased dispersion slope. This served to provide for dispersion values within the C-Band that were non-zero in value yet still far below those of standard single mode fiber. The NZ-DSF designation includes a group of fibers that all meet the ITU-T G.655 standard, but can vary greatly with regard to their dispersion characteristics. First available around 1996, NZ-DSF now makes up about 60% of the US long-haul fiber plant. It is growing in popularity, and now accounts for approximately 80% of new fiber deployments in the long-haul market. (Source: derived from KMI data) |
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Optical Add Drop Multiplexing (OADM) |
An optical add/drop multiplexer (OADM) adds or drops individual wavelengths to/from the DWDM aggregate at an in-line site, performing the add/drop function at the optical level. Before OADMs, back to back DWDM terminals were required to access individual wavelengths at an in-line site. Initial OADMs added and dropped fixed wavelengths (via filters), whereas emerging OADMs will allow selective wavelength add/drop (via software). |
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Optical Amplifier (OA) |
POSTAMPLIFIER Placed immediately after a transmitter to increase the strength on the signal. IN-LINE AMPLIFIER (ILA) Placed in-line, approximately every 80 to 100km, to amplify an attenuated signal sufficiently to reach the next ILA or terminal site. An ILA functions solely in the optical domain, performing the 1R function. PREAMPLIFIER Placed immediately before a receiver to increase the strength of a signal. The preamplifier boosts the signal to a power level within the receiver’s sensitivity range. |
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Optical Bandwidth |
Optical bandwidth is the total data carrying capacity of an optical fiber. It is equal to the sum of the bit rates of each of the channels. Optical bandwidth can be increased by improving DWDM systems in three areas: channel spacing, channel bit rate, and fiber bandwidth. The current benchmark for channel spacing is 50GHz. A 2X bandwidth improvement can be achieved with 25GHz spacing. CHANNEL SPACING Current benchmark is 50GHz spacing. A 2X bandwidth improvement can be achieved with 25GHz spacing. Challenges:
CHANNEL BIT RATE Current benchmark is 10Gb/s. A 4X bandwidth improvement can be achieved with 40Gb/s channels. However, 40Gb/s will initially require 100GHz spacing, thus reducing the benefit to 2X. Challenges:
FIBER BANDWIDTH Current benchmark is C-Band Transmission. A 3X bandwidth improvement can be achieved by utilizing the “S” & “L” bands. Challenges:
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Optical Fiber |
Optical fiber used in DWDM transmission is single mode fiber composed of a silica glass core, cladding, and a plastic coating or jacket. In single mode fiber, the core is small enough to limit the transmission of the light to a single propagation mode. The core has a slightly higher refractive index than the cladding, thus the core/cladding boundary acts as a mirror. The core of single mode fiber is typically 8 or 9 microns, and the cladding extends the diameter to 125 microns. The effective core of the fiber, or mode field diameter (MFD), is actually larger than the core itself since transmission extends into the cladding. The MFD can be 10 to 15% larger than the actual fiber core. The fiber is coated with a protective layer of plastic that extends the diameter of standard fiber to 250 microns. |
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Optical Signal to Noise Ratio (OSNR) |
Optical signal to noise ratio (OSNR) is a measurement relating the peak power of an optical signal to the noise floor. In DWDM transmission, each amplifier in a link adds noise to the signal via amplified spontaneous emission (ASE), thus degrading the OSNR. A minimum OSNR is required to maintain good transmission performance. Therefore, a high OSNR at the beginning of an optical link is critical to achieving good transmission performance over multiple spans. OSNR is measured with an optical signal analyzer (OSA). OSNR is a good indicator of overall transmission quality and system health. Therefore OSNR is an important measurement during installation, routine maintenance, and troubleshooting activities. |
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Optical Supervisory Channel |
The optical supervisory channel (OSC) is a dedicated communications channel used for the remote management of optical network elements. Similar in principal to the DCC channel in SONET networks, the OSC inhabits its own dedicated wavelength. The industry typically uses the 1510nm or 1625nm wavelengths for the OSC. |
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Polarization Mode Dispersion (PMD) |
Single mode fiber is actually bimodal, with the two modes having orthogonal polarization. The principal states of polarization (PSPs, referred to as the fast and slow axis) are determined by the symmetry of the fiber section. Dispersion caused by this property of fiber is referred to as polarization mode dispersion (PMD). |
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Raman |
Raman fiber amplifiers use the Raman effect to transfer power from the pump lasers to the amplified wavelengths. Raman Advantages are:
Raman Disadvantages are:
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Regenerator (Regen) |
An optical amplifier performs a 1R function (re-amplification), where the signal noise is amplified along with the signal. For each amplified span, signal noise accumulates, thus impacting the signal’s optical signal to noise ratio (OSNR) and overall signal quality. After traversing a number of amplified spans (this number is dependent on the engineering of the specific link), a regenerator is required to rebaseline the signal. A regenerator performs the 3R function on a signal. The three R’s are: re-shaping, re-timing, and re-amplification. The 3R function, with current technology, is an optical to electrical to optical operation (O-E-O). In the future, this may be done all optically. |
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S Band |
The S-band is the “short” DWDM transmission band, occupying the 1485 to 1520nm wavelength range. With the “S+” region, the window is extended below 1485nm. The S-band has comparable bandwidth to the C-band, thus comparable total capacity. The S-Band advantages are:
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Self Phase Modulation (SPM) |
The refractive index of the fiber varies with respect to the optical signal intensity. This is known as the “Kerr Effect”. Due to this effect, the instantaneous intensity of the signal itself can modulate its own phase. This effect can cause optical frequency shifts at the rising edge and trailing edge of the signal pulse. |
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SemiConductor Optical Amplifier (SOA) |
What is it? Similar to a laser, a SOA uses current injection through the junction layer in a semiconductor to stimulate photon emission. In a SOA (as opposed to a laser), anti-reflective coating is used to prevent lasing. SOA Advantages are:
SOA Disadvantages are:
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Span Engineering |
Engineering a DWDM link to achieve the performance and distance requirements of the application. The factors of Span Engineering are: Amplifier Power – Higher power allows greater in-line amplifier (ILA) spacing, but at the risk of increased non-linear effects, thus fewer spans before generation. Amplifier Spacing – Closer spacing of ILAs reduces the required amplifier power, thus lowering the susceptibility to non-linear effects. Fiber Type – Newer generation fiber has less attenuation than older generation fiber, thus longer spans can be achieved on the newer fiber without additional amplifier power. Channel Count – Since power per channel must be balanced, a higher channel count increases the total required amplifier power. Channel Bit Rate – DWDM impairments such as PMD have greater impacts at higher channel bit rates. |
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SSMF |
Standard single-mode fiber, or ITU-T G.652, has its zero dispersion point at approximately the 1310nm wavelength, thus creating a significant dispersion value in the DWDM window. To effectively transport today’s wavelength counts (40 – 80 channels and beyond) and bit rates (2.5Gbps and beyond) within the DWDM window, management of the chromatic dispersion effects has to be undertaken through extensive use of dispersion compensating units, or DCUs. SSMF makes up about one-third of the deployed US terrestrial long-haul fiber plant. Approximately 20% of the new fiber deployment in the US long-haul market is SSMF. (Source: derived from KMI data) |
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Stimulated Raman Scattering (SRS) |
The transfer of power from a signal at a lower wavelength to a signal at a higher wavelength. SRS is the interaction of lightwaves with vibrating molecules within the silica fiber has the effect of scattering light, thus transferring power between the two wavelengths. The effects of SRS become greater as the signals are moved further apart, and as power increases. The maximum SRS effect is experienced at two signals separated by 13.2 THz. |
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Thin Film Filter |
A thin film filter is a passive device that reflects some wavelengths while transmitting others. This device is composed of alternating layers of different substances, each with a different refractive index. These different layers create interference patterns that perform the filtering function. Which wavelengths are reflected and which wavelengths are transmitted is a function of the following parameters:
Thin film filters are used for performing wavelength mux and demux. Thin film filters are best suited for low to moderate channel count muxing / demuxing (less than 40 channels). |
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WLA |
Optical networking often requires that wavelengths from one network element (NE) be adapted in order to interface a second NE. This function is typically performed in one of three ways:
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dBm or decibel-milliwatt is an electrical power unit in decibel (dB), referenced to one milliwatt (mW).
dBm:- A mathematical Interpretation.
dBm definition
dBm or decibel-milliwatt is an electrical power unit in decibels (dB), referenced to 1 milliwatt (mW).
The power in decibel-milliwatts (P(dBm)) is equal to 10 times base 10 logarithm of the power in milliwatts (P(mW)):
P(dBm) = 10 · log10( P(mW) / 1mW )
The power in milliwatts (P(mW)) is equal to 1mW times 10 raised by the power in decibel-milliwatts (P(dBm)) divided by 10:
P(mW) = 1mW · 10(P(dBm) / 10)
1 milliwatt is equal to 0 dBm:
1mW = 0dBm
1 watt is equal to 30dBm:
1W = 1000mW = 30dBm
How to convert mW to dBm
How to convert power in milliwatts (mW) to dBm.
The power in dBm is equal to the base 10 logarithm of the power in milliwatts (mW):
P(dBm) = 10 · log10( P(mW) / 1mW )
For example: what is the power in dBm for power consumption of 100mW?
Solution:
P(dBm) = 10 · log10( 100mW / 1mW ) = 20dBm
How to convert dBm to mW
How to convert power in dBm to milliwatts (mW).
The power in milliwatts (P(mW)) is equal to 10 raised by the power in dBm (P(dBm)) divided by 10?
P(mW) = 1mW · 10(P(dBm) / 10)
For example: what is the power in milliwatts for power consumption of 20dBm?
Solution:
P(mW) = 1mW · 10(20dBm / 10) = 100mW
How to convert Watt to dBm
How to convert power in watts (W) to dBm.
The power in dBm is equal to the base 10 logarithm of the power in watts (W) plus 30dB:
P(dBm) = 10 · log10( P(W) / 1W ) + 30
For example: what is the power in dBm for power consumption of 100W?
Solution:
P(dBm) = 10 · log10( 100W / 1W ) + 30 = 50dBm
How to convert dBm to Watt
How to convert power in dBm to watts (W).
The power in watts (P(W)) is equal to 10 raised by the power in dBm (P(dBm)) minus 30dB divided by 10:
P(W) = 1W · 10( (P(dBm) – 30) / 10)
For example: what is the power in watts for power consumption of 40dBm?
Solution:
P(W) = 1W · 10( (40dBm – 30) / 10) = 10W
How to convert dBW to dBm
How to convert power in dBW to dBm.
The power in dBm is equal to the base 10 logarithm of the power in watts (W):
P(dBm) = P(dBW) + 30
For example: what is the power in dBm for power consumption of 20dBW?
Solution:
P(dBm) = 20dBW + 30 = 50dBm
How to convert dBm to dBW
How to convert power in dBm to dBW.
The power in dBW (P(dBW)) is equal to 10 raised by the power in dBm (P(dBm)) divided by 10:
P(dBW) = P(dBm) – 30
For example: what is the power in watts for power consumption of 40dBm?
Solution:
P(dBW) = 40dBm – 30 = 10dBW
dBm to Watt, mW, dBW conversion table
| Power (dBm) | Power (dBW) | Power (watt) | Power (mW) |
|---|---|---|---|
| -100 dBm | -130 dBW | 0.1 pW | 0.0000000001 mW |
| -90 dBm | -120 dBW | 1 pW | 0.000000001 mW |
| -80 dBm | -110 dBW | 10 pW | 0.00000001 mW |
| -70 dBm | -100 dBW | 100 pW | 0.0000001 mW |
| -60 dBm | -90 dBW | 1 nW | 0.000001 mW |
| -50 dBm | -80 dBW | 10 nW | 0.00001 mW |
| -40 dBm | -70 dBW | 100 nW | 0.0001 mW |
| -30 dBm | -60 dBW | 1 μW | 0.001 mW |
| -20 dBm | -50 dBW | 10 μW | 0.01 mW |
| -10 dBm | -40 dBW | 100 μW | 0.1 mW |
| -1 dBm | -31 dBW | 794 μW | 0.794 mW |
| 0 dBm | -30 dBW | 1.000 mW | 1.000 mW |
| 1 dBm | -29 dBW | 1.259 mW | 1.259 mW |
| 10 dBm | -20 dBW | 10 mW | 10 mW |
| 20 dBm | -10 dBW | 100 mW | 100 mW |
| 30 dBm | 0 dBW | 1 W | 1000 mW |
| 40 dBm | 10 dBW | 10 W | 10000 mW |
| 50 dBm | 20 dBW | 100 W | 100000 mW |
| 60 dBm | 30 dBW | 1 kW | 1000000 mW |
| 70 dBm | 40 dBW | 10 kW | 10000000 mW |
| 80 dBm | 50 dBW | 100 kW | 100000000 mW |
| 90 dBm | 60 dBW | 1 MW | 1000000000 mW |
| 100 dBm | 70 dBW | 10 MW | 10000000000 mW |
As we know that to improve correction capability, more powerful and complex FEC codes must be used. However, the more complex the FEC codes are, the more time FEC decoding will take. This term “baud” originates from the French engineer Emile Baudot, who was the inventor of 5-bit teletype code. The Baud rate actually refers to the number of signal or symbol changes that occurs per second. A symbol is one of the several voltage, frequency, or phase changes.
Baudrate = bitrate/number of bits per symbol ;
signal bandwidth = baud rate;
Baud rate:
It is the rate symbols which are generated at the source and, to a first approximation, equals to the electronic bandwidth of the transmission system. The baud rate is an important technology-dependent system performance parameter. This parameter defines the optical bandwidth of the transceiver, and it specifies the minimum slot width required for the corresponding flow(s).
Baud rate/symbol rate/transmission rate for a physical layer protocol is the maximum possible number of times a signal can change its state from a logical 1 to logical 0 or or vice-versa per second. These states are usually voltage, frequency, optical intensity or phase. This can also be described as the number of symbols that can be transmitted in 1 second. The relationship between baud rate and bitrate is given as.
Bit rate = baud rate * number of bits / baud
The number of bits per baud is deduced from the existing modulation scheme. Here, we are assuming that the number of bits per baud is one, so, the baud rate is the exactly same as the bit rate.
The spectral-width of the wavelength in GHz is equal to the symbol rate in Gbaud measured at the 3 dB point or the point where the power is half of the peak. As the baud rate increases, the spectral-width of the channels will increases proportionally. The higher baud rates, therefore, are unable to increase spectral efficiency, though there can be exceptions to this rule where a higher baud rate better aligns with the available spectrum. Increasing wavelength capacity with the baud rate, has far less impact on reach than increasing it with higher-order modulation.
Higher baud rates, offer the best potential for reducing the cost per bit in Flexi-grid DWDM networks and also in point-to-point fixed grid networks, even though higher baud rates are not significant in 50 GHz fixed grid ROADM networks. Higher baud rates also requires all the components of the optical interface, including the DSP, photodetector and A/D converters and modulators, to support the higher bandwidth. This places a limit on the maximum baud rate that is achievable with a given set of technology and may increase the cost of the interfaces if more expensive components are required.
Following are the few options that can help increase capacity of an Optical System
- Increasing the signal’s frequency
- Increasing the number of fibers
- Increasing the number of channels
- Increasing the modulation complexity.
The first option would require a proportional increase in bandwidth, while the other options would require the inclusion or replacement of equipment, resulting in higher cost, complexity, and power consumption.
Following are the major parameters associated with optical light receivers:-
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- Minimum threshold optical power, minimum sensitivity
- Responsiveness per wavelength
- Wavelength discrimination
- Receiver bit rate (max-min)
- Min-max threshold level (one-zero)
- Dependency on one’s density
- Dependence on polarisation
- Demodulation
- Receiver noise
- Dependency on bias
- Dependency on temperature
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Following are the major parameters associated with optical light sources:
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- OCh output power
- OCh wavelength AQ
- Line spacing
- Cutoff A (tunable sources)
- Tunability speed (tunable sources)
- Spectral width (tunable sources)
- Line width (light sources)
- Modulation depth (modulated sources)
- Bit rate (max-min) (modulated sources)
- Source noise
- Dependency on bias
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Following are the major parameters associated with fibers:
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- Forward attenuation/km
- Backward attenuation/km (if asymmetric)
- Polarization mode dispersion
- Polarization-dependent loss (PDL)
- Dispersion (chromatic and material)
- Zero-dispersion wavelength
- Dispersion flatness over spectrum range
- Birefringence
- Cutoff wavelength.
Main issues associated with EDFA designs are as follows:
1. The first issue is the flat gain. As EDFAs do not amplify all wavelengths through them the same; thus, the gain is not exactly flat.
2. The second issue is the pump power-sharing. The pump power is shared by all wavelengths in the link. Therefore the more the wavelengths, the lesser power per wavelength will be available. However, as wavelengths can get drop but not added, or some wavelengths get lost due to failures, EDFAs will amplify few wavelengths more.
These two issues can be mitigated by properly engineering the WDM system and by dynamic gain control.
3. The third issue is not as simple and is addressed differently. When engineering a fiber-optic path, it should be remembered that optical noise sources are cumulative and that the ASE of EDFAs introduces noise that degrades the signal to noise ratio (SIN). Although a strong optical signal launched into the fiber could overcome this, near the zero-dispersion wavelength region, four-wave mixing would become dominant, and it would degrade the SIN ratio.
This is because of the reduction of the minimal distance between two points of the constellation, which reduces the resilience to channel impairments. For instance, going from a PDM-QPSK up to a PDM-16QAM transmission doubles the data rate at the cost of an optical reach divided by a factor of 5.
Modulation is the process of encoding information onto a carrier signal, which is then transmitted over a communication channel. The choice of modulation scheme can have a significant impact on the reach of a system, which refers to the maximum distance over which a signal can be transmitted and still be reliably received.
There are several ways in which changing modulation can improve the reach of a system:
Increased spectral efficiency:
Modulation schemes that allow for higher data rates within the same bandwidth can improve the reach of a system by enabling more data to be transmitted over the same distance.
This is achieved by using more complex modulation schemes that allow for more bits to be transmitted per symbol.
Improved resistance to noise:
Some modulation schemes, such as frequency-shift keying (FSK) and phase-shift keying (PSK), are more robust to noise and interference than others.
By using a modulation scheme that is more resistant to noise, a system can improve its reach by reducing the probability of errors in the received signal.
Better use of available power:
Modulation schemes that are more efficient in their use of available power can improve the reach of a system by allowing for longer distances to be covered with the same power output.
For example, amplitude modulation (AM) is less power-efficient than frequency modulation (FM), which means that FM can be used to transmit signals over longer distances with the same power output.
Overall, changing modulation can improve the reach of a system by enabling more data to be transmitted over longer distances with greater reliability and efficiency.
SE is defined as the information capacity of a single channel (in bit/s) divided by the frequency spacing Δf (in Hz) between the carriers of the WDM comb:
SE = Rs log2(M) /Δf (1+r)
where Rs is the symbol rate, M is the number of constellation points of the modulation format, and r is the redundancy of the forward error correction (FEC) code, for example, r = 0.07 for an FEC with overhead (OH) equal to 7%
The following effects are the main sources of Q factor fluctuations:
PDL(polarization-dependent loss):
This corresponds to the dependence of the insertion loss of passive components to the signal state of polarization (SOP).
PHB (polarization hole burning):
This corresponds to the dependence of the optical amplifier gain to the signal SOP. The PHB is an effect that is significant in single-wavelength transmission since the degree of polarization (DOP) of a laser source is close to 100% unless a polarization scrambler is used. In WDM transmission systems, including a large number of wavelengths, however, the DOP of the optical stream is close to 0% due to the random distribution of the different wavelengths SOP. This effect becomes, therefore, negligible in a WDM transmission system.
The signal transmission quality is not stable over a long period of time because of the polarization effects occurring along the propagation path. The Time-varying system performance (TVSP) is deduced from testbed experiments where the fluctuations of the Q-factor are measured over a prolonged period of time. From this measurement, a Gaussian distribution is fitted to the measurements in order to deduce the standard deviation (s) and the average (mean Q) of the Q-factor distribution.
The optical spectrum analyzer (OSA) is the device typically used to measure OSNR. Signal and noise measurements are made over a specific spectral bandwidth Br , which is referred to as the OSA’s resolution bandwidth (RBW). The RBW filter acts as a bandpass filter allowing only the set amount of light spectrum to strike the OSA’s photodetector. The photodetector measures the average optical power in the spectral width. It cannot discriminate between two separate signals in the RBW spectrum. If there is more than one signal in RBW, it will treat and display them as one. Therefore, the ability of an OSA to display two closely spaced signals as two distinct signals is determined by the RBW setting. Typically, an OSA’s RBW range is adjustable between 10 and 0.01 nm with common settings of 1.0, 0.5, 0.1, and 0.05 nm.
Noise sources can be categorised as an active and passive.
Active sources such as optical plugs,lasers, receivers, and amplifiers generate noise in the fiber link.
Passive sources such as connectors, fiber, splices, and WDMs cause interference by distorting or reflecting the propagating signal power.
Below are ten major noise sources:
Compared with requirements for EDFAs for terrestrial applications and for Submarine applications, there are major important differences making the two types of amplifiers definitely two different components.
- High Chromatic Dispersion (CD) Robustness
- Can avoid Dispersion Compensation Units (DCUs)
- No need to have precise Fiber Characterization
- Simpler Network Design
- Latency improvement due to no DCUs
- High Polarization Mode Dispersion (PMD) Robustness
- High Bit Rate Wavelengths deployable on all Fiber types
- No need for “fancy”PMD Compensator devices
- No need to have precise Fiber Characterization
- Low Optical Signal-to-Noise Ratio (OSNR) Needed
- More capacity at greater distances w/o OEO Regeneration
- Possibility to launch lower per-channel Power
- Higher tolerance to Channels Interferences
Electronic Dispersion Compensation (EDC)
EDC is a technology that can help overcome the power losses that occur in optical link budgets. These losses can be caused by various factors such as inter-symbol interference (ISI) due to fiber chromatic and polarization mode dispersion, transmitter impairments, and limitations in the bandwidth of the optical or electronic components of the transmitter or receiver.
Two types of DCM are used in the DWDM link and are called post-compensation and pre-compensation. Since DCMs are considered part of the transmission line, the prefixes “post-” (after) and “pre-” (before) refers to the section of the transmission line that requires the compensation.
For the post-compensation DCM deployment, DCMs are placed after the fiber span that needs compensation. For G.652 fiber compensation, dispersion remains positive throughout the link.
Chromatic dispersion compensation modules (DCM), also known as dispersion compensation units (DCU) or Dispersion slope compensation module (DSCM), can be added to an existing fiber link to compensate for high link dispersion totals.
Following are factors contributing in DWDM design to increasing chromatic dispersion signal distortion
1. Laser spectral width, modulation method, and frequency chirp. Lasers with wider spectral widths and chirp have shorter dispersion limits. It is important to refer to manufacturer specifications to determine the total amount of dispersion that can be tolerated by the light wave equipment.
Introduction
Chromatic dispersion (CD) is a phenomenon in optical communication where different wavelengths of light travel at different speeds, causing the light pulses to spread and overlap. This dispersion can lead to signal distortion and degradation. In the realm of optical communication, both multi-channel Dense Wavelength Division Multiplexing (DWDM) systems and single-channel systems like Synchronous Digital Hierarchy (SDH) and Ethernet links on fiber are affected by chromatic dispersion. In this blog, we’ll delve into how CD impacts these systems differently and the measures taken to mitigate its effects.
Understanding Chromatic Dispersion
Chromatic dispersion occurs due to the varying refractive indices of different wavelengths of light in an optical fiber. This dispersion effect is a challenge in high-capacity optical networks, where accurate transmission of data is crucial.
Impact on Multi-Channel DWDM Systems
Multi-channel DWDM systems use multiple wavelengths to transmit data concurrently over a single optical fiber. In these systems, each channel experiences its own level of chromatic dispersion, leading to inter-symbol interference. The impact of CD becomes more pronounced as the number of channels increases. To counter this, DWDM systems employ techniques like dispersion compensating fibers (DCF) and advanced modulation formats to manage dispersion and enhance signal quality.
Effects on Single-Channel Systems
Single-channel systems, such as SDH and Ethernet links on fiber, transmit data using a single wavelength. While these systems are less susceptible to the complexities of multi-channel CD, they are not immune to dispersion-related issues. CD can still lead to signal distortion and limit transmission distances. To address this, SDH and Ethernet systems incorporate forward error correction (FEC) techniques, signal regeneration, and proper design considerations to ensure reliable data transmission.
Mitigation Strategies
In multi-channel DWDM systems, compensating for CD involves careful engineering and deployment of dispersion compensation modules and other advanced techniques. Single-channel systems typically use FEC algorithms to correct errors introduced by CD. Additionally, hybrid systems may utilize a combination of dispersion compensation and FEC techniques for optimal performance.
Conclusion
Chromatic dispersion is a challenge faced by both multi-channel DWDM systems and single-channel systems like SDH and Ethernet links on fiber. While the complexities of CD are more pronounced in multi-channel systems due to the varying wavelengths, single-channel systems are not exempt from its effects. Both types of systems rely on sophisticated mitigation strategies, such as dispersion compensation modules and FEC algorithms, to ensure reliable and high-quality data transmission. As optical communication technology advances, effective management of chromatic dispersion continues to be a critical consideration for network engineers and designers.
Introduction
Optical amplifiers play a crucial role in modern communication networks by boosting optical signals without converting them into electrical signals. To ensure optimal performance, it’s essential to understand the various performance parameters that define an optical amplifier’s capabilities.
Operating Wavelength Range
The operating wavelength range refers to the range of wavelengths within which the optical amplifier can effectively amplify signals. This parameter is determined by the amplifier’s design and the properties of the gain medium. The amplifier’s performance can degrade if signals fall outside this range, emphasizing the need to choose an amplifier suitable for the specific wavelength range of your network.
Nominal Input Power Range
The nominal input power range represents the power levels at which the optical amplifier operates optimally. If the input power exceeds this range, it can lead to signal distortion, nonlinear effects, or even damage to the amplifier components. Keeping input power within the specified range is essential for maintaining signal quality and amplifier longevity.
Input Range per Channel
In wavelength-division multiplexing (WDM) systems, different channels carry signals at varying wavelengths. The input range per channel defines the range of power levels for each individual channel. This parameter ensures that channels remain isolated from each other to prevent interference and crosstalk.
Nominal Single Wavelength Input Optical Power
For a single wavelength channel, the nominal input optical power indicates the ideal power level for optimal amplification. Operating too far below or above this power level can result in suboptimal performance, affecting signal quality and efficiency.
Nominal Single Wavelength Output Optical Power
Similar to the input power, the nominal single wavelength output optical power signifies the desired output power level for a single wavelength channel. This parameter ensures that the amplified signal has sufficient power for further transmission without introducing excessive noise or distortion.
Noise Figure
Noise figure characterizes the amount of noise added to the signal during the amplification process. A lower noise figure indicates better signal quality. Minimizing noise figure is vital to maintaining a high signal-to-noise ratio (SNR) and overall system performance.
Nominal Gain
Amplifier gain represents the factor by which the input signal’s power is increased. It’s a measure of amplification efficiency. Properly controlling and optimizing gain levels is crucial for achieving the desired signal strength while avoiding signal saturation or distortion.
Gain Response Time on Adding/Dropping Channels
In dynamic networks, channels may be added or dropped frequently. The gain response time defines how quickly the amplifier adjusts to these changes without causing signal disruptions. A faster gain response time enhances network flexibility and efficiency.
Channel Gain
Channels in a WDM system may experience different levels of gain due to variations in amplifier characteristics. Maintaining uniform channel gain is essential to ensure consistent signal quality across all channels.
Gain Flatness
Gain flatness refers to the consistency of gain across the amplifier’s operating wavelength range. Fluctuations in gain can lead to signal distortions, impacting network performance. Techniques such as gain equalization are used to achieve a flat gain profile.
Input Reflectance
Input reflectance is the portion of the incident signal that is reflected back into the amplifier. High input reflectance can lead to signal degradation and instability. Implementing anti-reflective coatings and proper fiber connectors helps minimize input reflectance.
Output Reflectance
Output reflectance refers to the amount of signal reflected back from the output of the amplifier. Excessive output reflectance can lead to signal feedback and instability. Output isolators and terminations are used to manage and reduce output reflectance.
Maximum Reflectance Tolerance at Input/Output
To maintain signal integrity, the maximum acceptable levels of reflectance at both input and output ports must be defined. Exceeding these tolerance levels can result in signal degradation and network disruptions.
Multi-channel Gain Slope
In multi-channel systems, variations in gain levels across different wavelengths can lead to unequal channel performance. Proper management of multi-channel gain slope ensures uniform amplification across all channels.
Polarization Dependent Loss
Polarization dependent loss (PDL) occurs when the amplifier’s performance varies with the polarization state of the incoming signal. Minimizing PDL is crucial to prevent signal quality discrepancies based on polarization.
Gain Tilt
Gain tilt refers to the non-uniform gain across the amplifier’s wavelength range. This can impact signal quality and transmission efficiency. Techniques such as using gain-flattening filters help achieve a more balanced gain distribution.
Gain Ripple
Gain ripple represents small fluctuations in gain across the amplifier’s operating range. Excessive gain ripple can cause signal distortions and affect network performance. Implementing gain equalization techniques minimizes gain ripple.
Conclusion
Understanding and optimizing these performance parameters is essential for ensuring the efficiency, reliability, and overall performance of optical amplifiers in complex communication networks. By carefully managing these parameters, network operators can achieve seamless transmission of data and maximize the potential of optical amplifier technology.
What is CD?
Chromatic dispersion (CD) is a property of optical fiber (or optical component) that causes different wavelengths of a light source to propagate at different velocities, means if transmitting signal, from a LASER source, this LASER source having spectral width and emit different wavelengths apart from its center wavelength. Since all light sources consist of a narrow spectrum of light (comprising of many wavelengths), all fiber transmissions are affected by chromatic dispersion to some degree. In addition, any signal modulating a light source results in its spectral broadening and hence exacerbating the chromatic dispersion effect. Since each wavelength of a signal pulse propagates in a fiber at a slightly different velocity, each wavelength arrives at the fiber end at a different time. This results in signal pulse spreading, which leads two inter-symbol Interference between pulses and increases bit errors
What is cause of CD?
Chromatic dispersion is due to an inherent property of silica optical fiber. The speed of a light wave depends on the refractive index, n, of the medium within which it is traversing. In silica optical fiber, as well as many other materials, n changes as a function of wavelength. Thus, different wavelengths travel at slightly different speeds along the optical fiber. A wavelength pulse is composed of several wavelength components or spectra. Each of its spectral constituents travel at slightly different speeds within the optical fiber. The result is a spreading of the transmission pulse as it travels through the optical fiber.
What is unit of CD and CD Coefficient?
The chromatic dispersion (CD) parameter is a measure of signal pulse spread in a fiber due to this effect. It is expressed with ps/nm units, where the picoseconds refer to the Signal pulse spread in time and the nanometers refer to the signal’s spectral width. Chromatic dispersion can also be expressed as fiber length multiplied by proportionality
Coefficient. This coefficient is referred to as the chromatic dispersion coefficient and is measured in units of picoseconds per nanometer times kilometer, ps/(nm ⋅ km). It is
Typically specified by the fiber the cable manufacturer and represents the chromatic dispersion characteristic for a 1 km length of fiber.
Which are main factors for CD?
Chromatic dispersion affects all optical transmissions to some degree. These effects become more pronounced as the transmission rate increases and fiber length increases.
Factors contributing to increasing chromatic dispersion signal distortion include the following:
1. Laser spectral width, modulation method, and frequency chirp. Lasers with wider spectral widths and chirp have shorter dispersion limits. It is important to refer to manufacturer specifications to determine the total amount of dispersion that can be tolerated by the lightwave equipment.
2. The wavelength of the optical signal. Chromatic dispersion varies with wavelength in a fiber. In a standard non-dispersion shifted fiber (NDSF G.652), chromatic dispersion is near or at zero at 1310 nm. It increases positively with increasing wavelength and increases negatively for wavelengths less than 1310 nm.
3. The optical bit rate of the transmission laser. The higher the fiber bit rate, the greater the signal distortion effect.
4. The chromatic dispersion characteristics of fiber used in the link. Different types of fiber have different dispersion characteristics.
5. The total fiber link length, since the effect is cumulative along the length of the fiber.
6. Any other devices in the link that can change the link’s total chromatic dispersion including chromatic dispersion compensation modules.
7. Temperature changes of the fiber or fiber cable can cause small changes to chromatic dispersion. Refer to the manufacturer’s fiber cable specifications for values.
How to mitigate CD in a link?
1. Change the equipment laser with a laser that has a specified longer dispersion limit. This is typically a laser with a narrower spectral width or a laser that has some form of pre-compensation. As laser spectral width decreases, chromatic dispersion limit increases.
2. For new construction, deploy NZ-DSF instead of SSMF fiber.NZ-DSF has a lower chromatic dispersion specification.
3. Insert chromatic dispersion compensation modules (DCM) into the fiber link to compensate for the excessive dispersion. The optical loss of the DCM must be added to the link optical loss budget and optical amplifiers may be required to compensate.
4. Deploy a 3R optical repeater (re-amplify, reshape, and retime the signal) once a link reaches chromatic dispersion equipment limit.
5. For long haul undersea fiber deployment, splicing in alternating lengths of dispersion compensating fiber can be considered.
6. To reduce chromatic dispersion variance due to temperature, buried cable is preferred over exposed aerial cable.
Introduction:
In the realm of optical communication, precision and reliability are paramount. Amidst the intricate components and techniques that ensure seamless data transmission, Differential Group Delay (DGD) emerges as a critical factor. This article takes you on a journey through the intricacies of DGD, unraveling its definition, implications, measurement techniques, and strategies for effective management. By the end, you’ll not only understand the concept of DGD but also appreciate its significance in maintaining the integrity of optical signals.
What is Differential Group Delay (DGD)?
Differential Group Delay (DGD) refers to the time difference between two orthogonal polarization states of an optical signal as it traverses a medium, such as a fiber-optic cable. This phenomenon arises due to various factors, including birefringence within the optical components and environmental conditions. DGD has the potential to degrade signal quality, leading to signal distortion, reduced data rates, and increased bit error rates.
The Role of DGD in Optical Communication:
DGD’s impact on optical communication is profound, influencing the overall system performance in multiple ways:
1. Signal Distortion:
DGD can cause pulse spreading and overlapping, leading to signal distortion and compromised data integrity.
2. Dispersion Compensation Challenges:
In high-speed optical systems, DGD poses challenges to dispersion compensation techniques, affecting data transmission over long distances.
3. Bit Error Rate (BER) Increase:
As DGD grows, the probability of bit errors occurring within the signal rises, directly impacting the reliability of the communication link.
4. System Robustness:
Managing DGD is crucial for ensuring the robustness of optical systems against external factors, such as temperature variations and mechanical stress.
Measuring Differential Group Delay: Techniques and Insights
Measuring DGD accurately is essential for identifying potential signal degradation and implementing effective mitigation strategies. Several techniques are employed for DGD measurement:
1. Interferometric Methods:
Interferometric techniques exploit interference between two orthogonal polarization states to determine DGD with high precision.
2. Time-Domain Methods:
Time-domain methods involve introducing a known time delay between polarization states and measuring the resultant phase difference.
3. Spectral Analysis:
Spectral analysis techniques utilize the spectral characteristics of the signal to calculate DGD based on phase variations.
Managing DGD: Strategies for Enhanced Signal Integrity
Efficiently managing DGD is imperative for maintaining optimal signal quality and system performance:
1. Polarization Mode Dispersion (PMD) Compensation:
Sophisticated PMD compensation techniques, such as adaptive compensation, can mitigate the effects of DGD.
2. Dispersion Compensation Modules:
Incorporating dispersion compensation modules helps counteract signal distortion caused by DGD.
3. Modulation Formats:
Selecting appropriate modulation formats can enhance the system’s tolerance to DGD-induced signal degradation.
4. Monitoring and Feedback:
Implementing real-time monitoring and feedback mechanisms enables dynamic adjustments to mitigate DGD-related issues.
Frequently Asked Questions about Differential Group Delay (DGD):
- Why does DGD occur in optical communication? DGD arises from the time delay difference between orthogonal polarization states caused by factors like fiber birefringence.
- How does DGD affect data transmission? DGD can lead to signal distortion, dispersion challenges, increased BER, and reduced system performance.
- What are the consequences of high DGD values? High DGD values can result in significant signal degradation, making reliable communication challenging.
- Can DGD be entirely eliminated? While complete elimination is challenging, effective mitigation strategies can minimize DGD’s impact on signal quality.
Polarization mode dispersion (PMD) is a property of a single-mode fiber or an optical component where pulse spreading is caused by different propagation velocities of the signal’s two orthogonal polarizations. Optical fibers or optical components can be modeled with two orthogonal polarization axes called principal states of polarization (PSP).
An optical signal propagating in a fiber is resolved into these two PSP axes. Each polarization axis (fast and slow axis) has a different propagation velocity. This is due to different refractive indexes in each axis caused by the birefringence of the material. The different velocities lead to pulse spreading at the receiver end.
PMD can be expressed as the square root of the fiber length multiplied by a proportionality coefficient. This coefficient is referred to as the PMD coefficient and is measured in units of picoseconds per square root kilometer (ps/√km). The PMD coefficient is typically specified by fiber cable manufacturers and represents the PMD characteristic for a particular length of that fiber.
Fiber characterization can be defined as the field measurement and recording of fiber span parameters that affect signal transmission over all or selected operating wavelengths. These measured parameters provide a true picture of the fiber span’s transmission limitations. They are used in network planning to ensure transmission links are designed within transceiver operating budgets and limits. Full fiber characterization is often necessary in modern high-speed link designs, where optical budgets are stretched to their maximum with little or no margin for error. Fiber quality can also be assessed with these parameters. Fiber characterization is performed after new fiber cable link construction, dark fiber purchase, or lease. This helps to ensure the fiber quality meets or exceeds required specifications and expectations. It also documents fiber parameters at the time of construction or acquisition for comparison with future measurements to determine fiber degradation due to aging, damage, and repair.
Macro bending: Macrobending is the attenuation associated with bending or wrapping the fiber. Light can “leak out” of a fiber when it is bent. As the bend becomes tighter, more light escapes. Macrobending loss, measured in decibels, increases at longer wavelengths where the optical confinement of the light is weaker. It also increases linearly with the number of turns. Traditionally, macrobending was not a limiting effect when cables were mostly of loose-tube or ribbon design and installed into ducts. The tightest bends incurred by fibers were in splice trays, where excess fiber would be stored in loops after jointing. This was reflected in the macrobending specification of ITU-T Recommendation G.652, where a minimum bend radius of 30 mm was defined to reflect typical splice tray dimensions and 100 turns were agreed upon to simulate the total excess fiber from all the splice sites between repeaters. But macrobending effects become more pronounced in networks installed close-to and within the building. Prevalent in this segment of the network are low-diameter mini-cables that are stripped-back designs, compared to the traditional sheathed loose-tube and ribbon cables. Lightweight and highly flexible, these new designs are preferred for their space efficiency (when installed into commensurately small micro-ducts) and ease of handling and routing (when installed on the inside and outside of buildings along tortuous paths). Bend radii of much less than 30 mm therefore have become commonplace.
Micro bending :Microbending attenuation of an optical fiber relates to the light signal loss associated with lateral stresses along the length of the fiber. The loss is due to the coupling from the fiber’s guided fundamental mode to lossy, higher-order radiation modes. Mode coupling occurs when fibers suffer small random bends along
the fiber axes. This random bending is usually caused by external mechanical stresses against the cable material that compress the fiber. The result is random, high-frequency perturbations to the fiber. Lateral stresses can be caused by pressure induced by manufacturing or installation or by temperature-induced dimensional changes in cabling materials that cause undesirable fiber/fiber or fiber/cable material interactions. These interactions can give rise to random microscopic bends or curvatures of <1-mm radius that create very small displacements of the fiber core from the fiber axis. Microbending effects can be seen at all the commonly used wavelengths in single mode fibers (1310, 1550, and 1625 nm), whereas macrobending effects are seen predominantly at 1550 and 1625 nm.
Latency is a time delay experienced in system and it describes how long it takes for data to get from transmission side to receiver side. In a fiber optical communication system, it is essentially the length of optical fiber divided by the speed of light in fiber core, supplemented with delay induced by optical and electro optical elements plus any extra processing time required by system, also called overhead. Signal processing delay can be reduced by using parallel processing based on large scale integration CMOS technologies.
Added to the latency due to propagation in the fiber, there are other path building blocks that affect the total data transport time. These elements include
- opto-electrical conversion,
- switching and routing,
- signal regeneration,
- Amplification,
- chromatic dispersion (CD) compensation,
- polarization mode dispersion (PMD) compensation,
- data packing, digital signal processing (DSP),
- protocols and addition forward error correction (FEC)
|
Band Description |
Wavelength Range (nm) |
|
850 Multimode Window |
800-910 |
|
O Original |
1260-1360 |
|
E Extended |
1360-1460 |
|
S Short |
1460-1530 |
|
C Conventional (EDFA Window) |
1530-1565 |
|
L Long (Extended EDFA) |
1565-1625 |
|
U Ultra Long Haul |
1625-1675 |