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Blog posts : "General"


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?


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


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

DefinitionDecoding 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.

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

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

**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?


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?

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?

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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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.

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?

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

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

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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Method to Resolve the Adjacent-Channel Interference in 100G Transmission.

In a non-coherent WDM system, each optical channel on the line side uses only one binary channel to carry service information. The service transmission rate on each optical channel is called bit rate while the binary channel rate is called baud rate. In this sense, the baud rate was equal to the bit…

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Basics of Coherent System.

What Is Coherent Communication?

Definition of coherent light

A coherent light consists of two light waves that:

1) Have the same oscillation direction.

2) Have the same oscillation frequency.

3) Have the same phase or maintain a constant phase relationship with each other. Two coheren…

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Why heterodyne detection technique is used in Coherent technology receivers rather homodyne?

As we know that either homodyne or heterodyne detection can be used to convert the received optical signal into an electrical form. In the case of homodyne detection, the optical signal is demodulated directly to the baseband. Although simple in concept, homodyne detection is difficult to implement …

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Basic understanding on Tap ratio for Splitter/Coupler

Fiber splitters/couplers divide optical power from one common port to two or more split ports and combine all optical power from the split ports to one common port (1 × N coupler). They operate across the entire band or bands such as C, L, or O bands. The three port 1 × 2 tap is a splitter commonly…

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Why it is Good to have ORL >30dB for general fiber links?

Optical return loss (ORL) is the logarithmic ratio of the launch (incident) power divided by the total reflected power seen at the launch point. The total reflected power is the total accumulated reflected optical power measured at the launch caused by fiber Rayleigh scattering and Fresnel reflectio…

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