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Simplifying what and why of Raman Amplifier.

August 23, 2017

 

It's always a wondering situation when we discuss Raman Amplifier;its benefits , requirement and application.I have tried to make it simpler to understand here.

Hope it will help the readers.

 

Introduction

  • The Raman amplifier is typically much more costly and has less gain than an EDFA amplifier. It, therefore, it is used only for specialty applications.
  • The main advantage this amplifier has over the EDFA is that it generates very little noise and hence does not degrade span OSNR as much as the EDFA.
  • Its typical application is in EDFA spans where additional gain is required but the OSNR limit has been reached.
  • Adding a Raman amplifier may not significantly affect OSNR, but can provide up to a 20 dB signal gain.
  • Another key attribute is the potential to amplify any fiber band, not just C band as is the case for the EDFA. This allows for Raman amplifiers to boost signals in O, E, and S bands (for CWDM amplification application).
  • The amplifier works on the principle of stimulated Raman scattering  (SRS), which is a nonlinear effect.
  • It consists of a high-power pump laser and fiber coupler (optical circulator).
  • The amplification medium is the span fiber in a distributed type Raman amplifier (DRA).

Common type of Raman amplifier 

  • The lumped or discrete type Raman amplifier internally contains a sufficiently long spool of fiber where the signal amplification occurs.
  • The DRA pump laser is connected to the fiber span in either a counter pump (reverse pump) or a co-pump (forward pump) or configuration.
  • The counter pump configuration is typically preferred since it does not result in excessively high signal powers at the beginning of the fiber span, which can result in nonlinear distortions,

  •  The advantage of the co-pump configurations is that it produces less noise.

Principle

As the pump laser photons propagate in the fiber, they collide and are absorbed by fiber molecules or atoms. This excites the molecules or atoms to higher energy levels. The higher energy levels are not stable states so they quickly decay to lower intermediate energy levels releasing energy as photons in any direction at lower frequencies. This is known as spontaneous Raman scattering or Stokes scattering and contributes to noise in the fiber. 

Since the molecules decay to an intermediate energy vibration level, the change in energy is less than the initial received energy during molecule excitation. This change in energy from excited level to intermediate level determines the photon frequency since Δ f = Δ E / h . This is referred to as the Stokes frequency shift and determines the Raman gain versus frequency curve shape and location. The remaining energy from the intermediate level to ground level is dissipated as molecular vibrations (phonons) in the fiber. Since there exists a wide range of higher energy levels, the gain curve has a broad spectral width of approximately 30 THz. 

During stimulated Raman scattering, signal photons co-propagate frequency gain curve spectrum, and acquire energy from the Stokes wave, resulting in signal amplification.

Theory of Raman Gain

The Raman gain curve’s FWHM width is about 6 THz (48 nm) with a peak at about 13.2 THz below the pump frequency. This is the useful signal amplification spectrum. Therefore, to amplify a signal in the 1550 nm range the pump laser frequency is required to be 13.2 THz below the signal frequency at about 1452 nm.

Multiple pump lasers with side-by-side gain curves are used to widen the total Raman gain curve. 

where fp = pump frequency, THz  fs = signal frequency, THz Δ f v = Raman Stokes frequency shift, THz 

Raman gain is the net signal gain distributed over the fiber’s effective length.It is a function of pump laser power, fiber effective length, and fiber area.

 

 

For fibers with a small effective area, such as in dispersion compensation fiber, Raman gain is higher. Gain is also dependent on the signal separation from the laser pump wavelength,Raman signal gain is also specified and field measured as on/off gain. This is defined as the ratio of the output signal power with the pump laser on and off.In most cases the Raman ASE noise has little effect on the measured signal value with the pump laser on. However, if there is considerable noise, which can be experienced when the measurement spectral width is large, then the noise power measured with the signal off  is subtracted from the pump on signal power to obtain an accurate on/off gain value.The Raman on/off gain is often referred to as the Raman gain.

Noise sources

Noise created in a DRA span consists :-

  • Amplified spontaneous emissions (ASE)
  • Double Rayleigh scattering (DRS)
  • Pump laser noise.

ASE noise is due to photon generation by spontaneous Raman scattering.

DRS noise occurs when twice reflected signal power due to Rayleigh scattering is amplified and interferes with the original signal as crosstalk noise.

The strongest reflections occur from connectors and bad splices.

Typically DRS noise is less than ASE noise, but for multiple Raman spans it can add up. To reduce this interference, ultra polish connectors (UPC) or angle polish (APC) connectors can be used. Optical isolators can be installed after the laser diodes to reduce reflections into the laser. Also span OTDR traces can help locate high-reflective events for repair.

Counter pump DRA configuration results in better OSNR performance for signal gains of 15 dB and greater. Pump laser noise is less of a concern because it usually is quite low with RIN of better than 160 dB/Hz.

Nonlinear Kerr effects can also contribute to noise due to the high laser pump power. For fibers with low DRS noise, the Raman noise figure due to ASE is much better than the EDFA noise figure. Typically the Raman noise figure is –2 to 0 dB, which is about 6 dB better than the EDFA noise figure.

Raman amplifier noise factor is defined as the OSNR at the input of the amplifier to the OSNR at the output of the amplifier.

Noise figure is the dB version of noise factor

The DRA noise and signal gain is distributed over the span fiber’s effective length.

 

Counter pump distributed Raman amplifiers are often combined with EDFA pre-amps to extend span distances. This hybrid configuration can provide 6 dB improvement in the OSNR, which can significantly extend span lengths or increase span loss budget. Counter pump DRA can also help reduce nonlinear effects by allowing for channel launch power reduction.

 

  Functional Block Diagram for CoPropagating and Counter Propagating Raman Amplifier

                                                         

                       Co-Propagating                                                                                                                                                Counter-Propagating

 

Field Deployment architecture of EDFA and RAMAN Amplifiers

 

Interesting to know

 

 

Reference:-

  1. Planning Fiber Optic Networks by Bob Chomycz
  2. https://www.cisco.com/c/en/us/products/collateral/optical-networking/ons-15454-series-multiservice-provisioning-platforms/data_sheet_c78-658538.html

 

Animated map shows the undersea cables that power the internet

July 19, 2017

 

Link

How Layer 0(zero) is so efficient with "Latency" -One of the key parameter for Operator.

July 19, 2017

Layer 0(zero) merely introduces any latency to the network.

 

  • It's mostly the electronics that imparts latency in the network.
  • The more we play with bit streams/signals ;more will be the latency.
  • Most of the latency optimization happens in layer 1-3.

Future Transport Network Requirements:


Optical Fiber Link Design requirements

June 10, 2017

The optical link design essentially is putting the various optical components, which we discussed earlier, so that information can be transmitted satisfactorily. The satisfactoriness of the transmission can be defined in terms of some characteristic parameters.

 The user generally specifies the distance over which the information is to be sent and the data rate to be transmitted. The Designer then has to find the specification of the system components.         

The designer generally has to define some additional criteria either as per the standards or as per the user specifications.

  The Design criteria are given in the following.

Primary Design Criteria

  • Data Rate
  • Link length             

Additional Design Parameters

  • Modulation format eg Analog/digital
    • Depends upon the type of signals user want to transmit. For example if it is a TV signal, then may be analog transmission is more suited as it requires less bandwidth and better linearity. On the other hand if data or sampled voice is to be transmitted, digital format may be more appropriate.
    • The digital signals have to be further coded to suite the transmission medium and for error correction.
  • System fidelity: BER, SNR
    • The system fidelity defines the correctness of the data received at the receiver.
    •  For digital transmission it is measured by the Bit Error Ratio (BER) . The BER is defined as:-
    • In optical system, the BER has to be less than 10E-9
    • For analog system, the quality parameter is the Signal-to-noise (SNR) ratio. In addition, there is a parameter called the inter-modulation distortion, which describes the linearity of the system.                          
  • Cost : Components, installation, maintenance            
    • Cost is one of the important issues of the link design.           
    • The cost has three components, components, installation and maintenance.        
    • The component and the installations cost are the initial costs. Generally, the installation cost is much higher than the component cost for long links. This is especially true for laying the optical cable. It is therefore appropriate to lay the cables keeping in view the future needs.
    • The optical link is suppose is supposed to work for at least 25years. The maintenance costs are as important as the initial cost. An initial cheaper system might end up into higher expenses in maintenance and therefore turn out to be more expensive as a whole.
  •   Upgradeability
    • The optical fiber technology is changing very rapidly and the data rates are increasing steadily.       
    • The system should be able to adopt new technology, as well should be able to accommodate higher data rates with least possible changes.
  •   Commercial availability          
    • Depending upon which part of the world one is, the availability of the components and the systems may be an issue.

ref:http://nptel.ac.in/courses/117101054/16

How and where do we get pre and post FEC BER

March 19, 2017

How and where do we get pre and post FEC BER

The first thing to note is that for each frame there are two sets of 20 parity bits. One set is associated with the end to end post FEC BER. The other is used to measure the span by span raw BER. The points at which these parity bits are terminated are illustrated below.

Processing point

Process description

A

Calculate and insert the post FEC parity bits (those over which FEC is calculated) over the frame up to and including the MS OH.

B

Encode FEC over the frame up to and including the MS OH.

C

Calculate and insert the pre FEC parity bits (those over which FEC is not calculated) over the frame up to and including the RS OH.

D

Terminate the raw BER based on the pre FEC parity bits.

E

Re-calculate the pre FEC parity bits over the frame up to, and including, the RS OH.

F

Decode FEC to produce the final data.

G

Terminate the post FEC BER based on the post FEC parity bits.

 

We can use the raw BER extracted at each RS terminating point (regens and LTEs) to estimate the post FEC BER. Note that this estimate is based on an assumption of a Poisson distribution of errors. In contrast the real post FEC BER can only be extracted at the MS terminating equipment (LTEs), and this is used to feed into the PM error counts.

Following are the terminologies you will come across when referring FEC Performance parameters:

PRE-FEC BER are the bit errors caused by attenuation, ageing, temperature changes of the optical fiber. PRE-FEC indicates that the signal on the optical fiber is FEC 
encoded. The FEC decoder will recover the original signal, but depending on the PRE_FEC BER it will succeed to recover the original signal completely without errors. 
Or, if the BER on the fiber is too high, the recovered signal will  contain bit errors. 

If the signal was FEC encoded the remaining bit errors after the decoder are called POST_FEC BER. 

The NO_FEC BER are the bit errors detected when no FEC coding is used on the optical fiber. 

Uncorrected words are the word that FEC is not able to corrects.It shows that the current FEC is not able to correct anymore and we need to look for more advance FEC.

MAXIMUM NUMBER OF ERBIUM-DOPED FIBER AMPLIFIERS IN A DWDM LINK

January 10, 2017

The maximum number of erbium-doped fiber amplifiers (EDFAs) in a fiber chain is about four to  six.

 

Explanation 

The rule is based on the following rationales:

1. About 80 km exists between each in-line EDFA, because this is the approximate distance at which the signal needs to be amplified.

2. One booster is used after the transmitter.

3. One preamplifier is used before the receiver.

4. Approximately 400 km is used before an amplified spontaneous emission (ASE) has approached the signal (resulting in a loss of optical signal-to-noise ratio [OSNR]) and regeneration needs to be used.

An EDFA amplifies all the wavelengths and modulated as well as unmodulated light. Thus, every time it is used, the noise floor from stimulated emissions rises. Since the amplification actually adds power to each band (rather than multiplying it), the signal-to-noise ratio is decreased at each amplification. EDFAs also work only on the C and L bands and are typically pumped with a 980- or 1480-nm laser to excite the erbium electrons. About 100 m of fiber is needed for a 30-dB gain, but the gain curve doesn’t have a flat distribution, so a filter is usually included to ensure equal gains across the C and L bands.

For example, assume that the modulated power was 0.5 mW, and the noise from stimulated emission was 0.01 mW. The signal-to-noise ratio is 0.5/0.01 or 50. If an EDFA adds a 0.5 mW to both the modulated signal and the noise, then the modulated signal becomes 1 mW, and the noise becomes 0.501 mW, and the SNR is reduced to 2. After many amplifications,even if the total power is high, the optical signal-to-noise ratio becomes too low. This typically occurs after four to six amplifications.

Another reason to limit the number of chained EDFAs is the nonuniform nature of the gain. Generally, the gain peaks at 1555 nm and falls off on each side, and it is a function of the inversion of Er+3. When a large number of EDFAs are cascaded, the sloped of the gain becomes multiplied and sharp, as indicated in Fig. 6.3. This results is too little gain-bandwidth for a system. To help alleviate this effect, a gain flattening device often is used, such as a Mach–Zehnder or a long-period grating filter.

 

 

Reference

1. A. Willner and Y. Xie, “Wavelength Domain Multiplexed (WDM) Fiber-optic Communications Networks,” in Handbook of Optics, Vol. 4., M. Bass, Ed.,McGraw-Hill, New York, pp. 13–19, 2001.

2.http://www.pandacomdirekt.com/en/technologies/wdm/optical-amplifiers.html

3.http://blog.cubeoptics.com/index.php/2015/03/what-edfa-a-noise-source

Source: Optical Communications Rules of Thumb

Note:I have heard many times among optical folks discussing  maximum number of amplifiers in a link;so thought of posting this.

Why is it preferable to put attenuator/pad at the Receive end of Optical Module?

December 23, 2016

 

Few analogies proving the subject:-

 

 

attenuators in fiber optic data link

 

  • If the distance is to short and the attenuator is too close to the transmitter, the reflected light off the attenuator will be directed back towards the Tx laser. Which will also blow your transmitter.so we place it at Rx.

 

  • Also keeping attenuator at Rx will attenuate the noise along with the signal.

 

  • The most important reason for putting them on the RX side is that you are protecting that which needs to be protected - the receiver in your optics. This way you know that you're not going to potentially blow the receiver in your optics by plugging in too large a signal because you assumed there was an attenuator on the TX at the far end, and there wasn't.
  • It's more convenient to test the receiver power before and after attenuation or while adjusting it with your power meter at the receiver, plus any reflectance will be attenuated on its path back to the source.

 

 

Keynote on Using Attenuators With Fiber Optic Data Links

 

BER

The ability of any fiber optic system to transmit data ultimately depends on the optical power at the receiver as shown above, which shows the data link bit error rate as a function of optical power at the receiver. (BER is the inverse of signal-to-noise ratio, e.g. high BER means poor signal to noise ratio.)  Either too little or too much power will cause high bit error rates. 

Too much power, and the receiver amplifier saturates, too little and noise becomes a problem as it interferes with the signal. This receiver power depends on two basic factors: how much power is launched into the fiber by the transmitter and how much is lost by attenuation in the optical fiber cable plant that connects the transmitter and receiver.

If the power is too high as it often is in short singlemode systems with laser transmitters, you can reduce receiver power with an attenuator. Attenuators can be made by introducing an end gap between two fibers (gap loss), angular or lateral misalignment, poor fusion splicing (deliberately), inserting a neutral density filter or even stressing the fiber (usually by a serpentine holder or a mandrel wrap). Attenuators are available in models with variable attenuation or with fixed values from a few dB to 20 dB or more.

gap loss attenuators 
Gap-loss attenuators for multimode fiber
 

serpentine attenuator
Serpentine attenuators for singlemode fiber
 

Generally, multimode systems do not need attenuators. Multimode sources, even VCSELs, rarely have enough power output to saturate receivers. Singlemode systems, especially short links, often have too much power and need attenuators.

For a singlemode applications, especially analog CATV systems, the most important specification, after the correct loss value, is return loss or reflectance! Many types of attenuators (especially gap loss types) suffer from high reflectance, so they can adversely affect transmitters just like highly reflective connectors. 

attenuators in fiber optic data link

Choose a type of attenuator with good reflectance specifications and always install the attenuator ( X in the drawing) as shown at the receiver end of the link. This is because it's more convenient to test the receiver power before and after attenuation or while adjusting it with your power meter at the receiver, plus any reflectance will be attenuated on its path back to the source. 

testing attenuated power at receiver
Test the system power with the transmitter turned on and the attenuator installed at the receiver using a fiber optic power meter set to the system operating wavelength. Check to see the power is within the specified range for the receiver.

If the appropriate attenuator is not available, simply coil some patchcord around a pencil while measuring power with your fiber optic power meter, adding turns until the power is in the right range. Tape the coil and your system should work. This type of attenuator has no reflectance and is very low cost! The fiber/cable manufacturers may worry about the relaibility of a cable subjected to such a small bend radius. You should probably replace it with another type of attenuator at some point, however.

singlemode wrap attenuator
Singlemode attenuator made by wrapping fiber or simplex cable around a small mandrel. This will not work well with bend-insensitive fiber.

 

 

ref:http://www.thefoa.org/tech/ref/appln/attenuators.html

Power Change during add/remove of channels on filters

August 24, 2016

 

The power change can be quantified as the ratio between the number of channels at the reference point after the channels are added or dropped and the number of channels at that reference point previously.We can consider composite power here and each channel at same optical power in dBm.

So whenever we add or delete number of channels from a MUX/DEMUX/FILTER/WSS following equations define the new changed power.

For the case when channels are added (as illustrated on the right side of Figure 1 ):

                                                     

where:

 A   is the number of added channels

 U   is the number of undisturbed channels

For the case when channels are dropped (as illustrated on the left side of Figure 1):

                                                     

where:

                            D   is the number of dropped channels

                            U   is the number of undisturbed channels

                                                                            Figure 1

For example:

-           adding 7 channels with one channel undisturbed gives a power change of +9 dB;

-           dropping 7 channels with one channel undisturbed gives a power change of –9 dB;

-           adding 31 channels with one channel undisturbed gives a power change of +15 dB;

-           dropping 31 channels with one channel undisturbed gives a power change of –15 dB;

 

refer ITU-T G.680 for further study.

HD-FEC & SD-FEC differences

August 11, 2016
Items HD-FEC SD-FEC
Definition Decoding based on hard-bits(the output is quantized only to two levels) is called the “HD(hard-decision) decoding”, where each bit is considered definitely one or zero.  Decoding based on soft-bits(the output is quantized to more than two levels) is called the “SD(soft-decision) decoding”, where not only one or zero decision but also confidence information for the decision are provided.
Application Generally for non-coherent detection optical systems, e.g.,  10 Gbit/s, 40 Gbit/s, also for some coherent detection optical systems with higher OSNR

coherent detection optical systems, e.g.,  100 Gbit/s,400 Gbit/s.

Electronics Requirement ADC(Analogue-to-Digital Converter) is not necessary in the receiver. ADC is required in the receiver to provide soft information, e.g.,  coherent detection optical systems.
specification general FEC per [ITU-T G.975];super FEC per [ITU-T G.975.1]. vendor specific
typical scheme Concatenated RS/BCH LDPC(Low density parity check),TPC(Turbo product code)
complexity medium high
redundancy ratio generally 7% around 20%
NCG about 5.6 dB for general FEC;>8.0 dB for super FEC. >10.0 dB
 Example(If you asked your friend about traffic jam status on roads and he replies) maybe fully jammed or free  50-50  but I found othe way free or less traffic

 

                                                               

 

 

 

 

Difference between OTDR and COTDR.

August 5, 2016

What is a OTDR ?

Optical Time Domain Reflectometer - also known as an OTDR, is a hardware device used for measurement of the elapsed time and intensity of light reflected on optical fiber.

How it works?

The reflectometer can compute the distance to problems on the fiber such as attenuation and breaks, making it a useful tool in optical network troubleshooting.

The intensity of the return pulses is measured and integrated as a function of time, and is plotted as a function of fiber length.

What is a COTDR?

Coherent Optical Time Domain Reflectometer - also known as a COTDR, An instrument that is used to perform out of service backscattered light measurements on optically amplified line systems.

How it works?

A fiber pair is tested by launching a test signal into the out going fiber and receiving the scattered light on the in-coming fiber.  Light scattered in the transmission fiber is coupled to the incoming fiber in the loop-back couplers in each amplifier pair in a repeater.

 

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Comments

Well Explained....

Hi G Majhi,

Actually only end nodes could be managed via GCC but you cannot manage a OLA/ILA site which doesn't have provision to drop GCC or terminate OTU;So we avoid GCC alone to manage nodes when we have multiple OLA sites .

In case you have two site locations with OTU(Transponders),you can use GCC for sure.

Hi,

Can you please explain, when GCC0 is required for DWDM/OTN networks. I believe if amplifiers can not be managed through GCC0. And if GCC0 is sufficient then why OSC ?

Thanks & best Regards

Hi Akhilesh,

Hope this will answer your query.

we are adding FS bytes to equalize the payload size with the standard available size..example

we know that OPU2 is 9.953Gbps which is the rate of standard STM64 signal........(1)

now lets consider mapping 4xODU1 into OPU2
=4xODU1=4X2.498=9.9951Gbps................(2)

Now if you compare both (1) and (2),you will find a difference in the rate and it will be around

9.953-9.995=41.82Mbps or 4182ppm.........(3) but we know that through available mapping procedures we cannot compensate difference of 4182ppm.

We can compensate for +/-20ppm as per standard.So we definitely need to add some stuffing byte so that justification could be achieved with fixed size JC bytes as we do stuffing in SDH frames.

so we add as much number of standard bytes so that we can atleast reach justification of +/-20ppm so if you add 16x4 FS then the
rate for OPU2 will be 9.9952Gbps...............(4)

now after adding FS bytes ;rated difference will be 9.9952-9.9951 ==177Kbps=18ppm.....................(5)

value of (5) i.e 18ppm is under limit and ofcourse we can compensate such difference with standarized +/-20ppm margin.

normally placing of FS bytes is in multiple of 16byte due to OTN frame reed soloman FEC capability.

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