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Blog posts : "General"

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.



  • The Raman amplifier is typically much more costly and has less gain th…

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Animated map shows the undersea cables that power the internet

July 19, 2017



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 R…

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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 dis…

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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…

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January 10, 2017

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



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 ampli…

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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 we place it at Rx.


  • Also keeping…

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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 when…

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HD-FEC & SD-FEC differences

August 11, 2016
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 quan…

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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 …

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A walk-through with non linear impairments in optical fiber

July 13, 2016

Non-linear interactions between the signal and the silica fibre transmission medium begin to appear as optical signal powers are increased to achieve longer span lengths at high bit rates. Consequently, non-linear fibre behaviour has emerged as an important consideration both in high capacity system…

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Understanding multipliers and divisors value in calculating OTN frame rates (255,239,238,237 etc) for OPUk,ODUk and OTUk

June 19, 2016

**Multiplicative factor is just a simple math :eg. for ODU1/OPU1=3824/3808={(239*16)/(238*16)}

Here value of multiplication factor will give the number of times for rise in the frame size after adding header/overhead.

Example:let consider y=(x+delta[x])/x; In terms of OTN frame here delta[x] is i…

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How to know if errors are due to linear or non-linear issue in an optical link?

June 2, 2016


When the bit error occurs to the system, generally the OSNR at the transmit end is well and the fault is well hidden.
Decrease the optical power at the transmit end at that time. If the number of bit errors decreases at the transmit end, the problem is non-linear problem.
If the number of bit …

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What is Optical Power Requirement and margin for a optics module's power?

June 2, 2016

Optical power tolerance: It refers to the tolerable limit of input optical power, which is the range from sensitivity to overload point.

Optical power requirement: If refers to the requirement on input optical power, realized by adjusting the system (such as adjustable attenuator, fix attenua…

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What are main advantages and drawbacks of EDFAs?

June 1, 2016

The main advantages and drawbacks of EDFAs are as follows.


  • Commercially available in C band (1,530 to 1,565 nm) and L band (1,560 to 1,605) and up to  84-nm range at the laboratory stage.
  • Excellent coupling: The amplifier medium is an SM fiber;
  • Insensitivity to light polariz…

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May 26, 2016

A Lower Order-ODU or LO-ODU is an ODUk, whose OPUk transports a inside mapped client.

An Higher Order-ODU or HO-ODU is an ODUk, whose OPUk transports an inside multiplexed ODUj.

LO-ODU and HO-ODU have the same structure but with different clients.
The LO-ODU is either mapped into the …

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Invalid Data Flag or Suspect Interval in PM(Performance Monitoring) counters.

May 18, 2016

The ITU standards define a “suspect internal flag” which should indicate if the data contained within a register is ‘suspect’ (conditions defined in Q.822). This is more frequently referred to as the IDF (Invalid Data Flag).

PM is bounded by strict data collection  rules as defined in stand…

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Why does coherent detection introduce pre-FEC bit errors?

February 19, 2016

Why does coherent detection introduce pre-FEC bit errors?

  • The DSP algorithm at the receive end detects and analyzes the phase and amplitude of a received signal in real time to calculate and compensate the distortion of the signal caused by factors such as CD, PMD, and nonlinearity. Because CD…

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FEC and Latency

February 19, 2016

Why does FEC introduce latency?

The addition of FEC overhead at the transmit end and FEC decoding at the receive end cause latency in signal transmission.

Relationship between latency and FEC scheme types

  • To improve the correction capability, more powerful and complex FEC codes must be u…

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Measuring Coherent System OSNR using integral method

February 19, 2016
For coherent signals with wide optical spectrum, the traditional scanning method using an OSA or inband polarization method (EXFO) cannot correctly measure system OSNR. Therefore, use the integral method to measure OSNR of coherent signals.

Perform the following operations to measure OSNR using t…

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20 Blog Posts