Good Optical Links

Below is the list of few good links for optical power budget calculations, design and troubleshooting.

Compressed Air Cleaning Issues for Fiber-Optic Connections

Inspection and Cleaning Procedures for Fiber-Optic Connections

Calculating the Maximum Attenuation for Optical Fiber Links

Introduction to Optical Fibers, dB, Attenuation and Measurements

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SeaMeWe-4 Undersea cable cut

Another undersea cable cut made Middle East limited access to internet. The main SeaMeWe-4 cable suffered a "shunt fault" and came into contact with sea water somewhere in the Mediterranean.
Fortunately, this dependence on SeaMeWe-4 will come to an end this year. The consultancy says that "five new cables are scheduled to enter service between Europe and Egypt in 2010. The first two, Telecom Egypt’s TE North cable and the IMEWE consortium cable, will enter service in May. Orascom’s MENA cable, the Europe India Gateway consortium cable, and Reliance’s FLAG Hawk cable are all expected to follow before year-end 2010." Capacity will skyrocket, and the new cable will also bring helpful redundancy to the region.
Source:- Arctechnica

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How to calculate end to end transmit power in fiber

Calculation of power budget in fiber deployent is one of the biggest challenge. If the power is more or less than the power budget, it will bring lot of problems like throughput, drops and many more. One must ensure that the end to end link is working up to the level and able to delivery according to the commitment and design. To ensure these parameters, one must know about how to calculate the end to end power budget. Power budget is nothing but it provides the margin that compensates for variations of transmitter power and receiver sensitivity levels.

Power Budget = Minimum Transmit Power - Minimum Receiver Sensitivity Level

Minimum and maximum receiver and transmit levels could be obtained from the product specifications.
With this we need to calculte the total loss caused by the equipments in end to end deployment. Click here to see the values of losses w.r.t to equipment.

End to End Loss Calculation = (Fiber attentuation*km) + (Connector attenuation *Number of connectors) + (Splices * Number of splices) + (Passive devices*Number)

The above mentioned calculations are basics but the question comes in mind that how much power should one point transmit and another point will receive. Whenever a starting point will transmit some power the same power doesn't receive by the another point; This is because of losses in the path. The another point is that the power received by the end point should not exceed its maximum receiver sensitivity. So we can use the given formula to calculate the input power to link.


Input Power = Maximum Tranmit Power - End to End Loss Calucation < Maximum Power Receiver Sensitivity
example will help to understand the whole calculations w.r.t to the live deployment:-

The minimum optical transmitter launch power is –12.5 dBm, and the maximum optical transmitter launch power is –2 dBm at 1310 nm. The minimum receiver sensitivity is –30 dBm, and the maximum receiver sensitivity is –3 dBm at 1310 nm. Total fiber stretch is of 3 Km with 3*fusion splices and 2*patch panels. We can ad the connectors also but in this example I am using that.

First calculate the total power budget which means how much maximum power we can allow for the link. As mentioned above in the formula we need to add the values.

Power Budget = Minimum Transmit Power(-12.5) - Minimum Receiver Sensitivity Level(-30)
= -12.5 + 30
= 17.5 dB


Now calculate the losses

This is the MMF, assume graded index 50/125 = 3 Km * 0.7 dB/KM => 2.1dB

2* Fusion Splices = 2* 0.02dB => 0.04dB
2* Patch Panels = 2* 2dB => 4.0 dB

Total Fiber Loss in 3 Km = 2.1 + 0.04 + 4 => 6.14dB


Now we need to calculate the input power:-

Maximum Transmit Power(-2) - Total Fiber Loss(6.14) < Maximum Receiver Sensitivity -2-6.14 => -8.14dB < -3dB

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Values which can affect power budget of fiber

See the previous posts of Fiber Splicing and Difference between Single and Multimode Fiber.

To calculate power budget we should know the fiber attentuation characterstics, Component loss values and reference margin values. Once all the values are known one can easily calculate te power budget.

Fiber Attenuation


Insertion Loss With Respective Components

Loss Margin

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Types Of Fiber Splicing

What does happens when fiber cut occur? Someone need to fine the location of cut and restore. The restroration technique of fiber or merging the two fiber ends in one to increase its length is known as splicing. It is done in two ways

1. Mechanical Splicing
2. Fusion Splicing

Every splicing add some looses in the end to end calculation. For fusion splicing a loss of 0.02 dB is measured and for mechanical loss of 0.75 dB is measured. If it is not done in good manner it could increase the losses consequence drop in the data rate. The best way to check the total number of splices in end to end circuit is to see the OTDR (Optical Time Domain Reflector) report which helps to find the problems in the fiber.


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Single Mode and Multimode Fiber

I am always bit confused about single mode and multimode fiber cables. The differentiation between them is not so tough to understand. The only difference is the core part.
The core is the highly refractive region of the optical fiber from which the light is trasmitted. According to the ITU the diameter of the single mode fiber should fall between 8 to 10 micron meter and 50 to 62.5 micro meter of multi mode fiber cable.
Now how to remember which cable is used small or large distance. It's also very easy. Think single mode fiber cable is having a very small core where in only a light can transmit, it means a monopoloy of light is there and it could travel upto to a large distance. The multimode fiber cable is having a big core which means more lights could travel in the big core, a quarrel always happen where more than one comes, so no one can survive for a long period of time. Hence MMF cables are used for short distance.

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