Calculating Optical Power Budgets - I'm confused!

[fvalpondi]

Hello everyone,

I have to calculate some optical budgets for a network (Single Mode) that I am designing. It's my first time doing this, and I am a little bit confused.

I know how to get the available optival power, and I know that I have to take into account the connector loss and the distance loss. According to what the constructor told me, the cables are not going to be spliced, but connected via patch pannels in each station. That is why I did not consider the splice loss.

As fas as I know, I am using the following formula:

Final_dB = Available_Power_Budget(db) - (Connector Loss * Connections Number) - (Distance loss(dB/km) * km)

Some information I found, tell me that the Final_dB has to be at least 2 dB. Other informations say it has to be 3 dB and other 5 dB.... Which is really the dB margin that has to remain? Is there a maximum dB margin which would lead me to use an attenuator?

Thanks a lot for your help!

best regards,

Fabio

PD: I made a couple calculations, and the result surprises me...With a 10 km Module (and 9 km connection) I have a higher dB margin than when I use a 70km Module for a 48km connection (Cable loss is 0,4dB/km and Connector Loss is 0,3dB)

Connection Endpoints	Central <-> Station 9 (bypassing Station 8)
Modules Used	Single Mode - 70 km
Available Budget (dB)	23
Distance (km)	48
Distance Loss (Cable Loss) (dB)	19,2
Number of Connections	4
Total Connector Loss (dB)	1,2
AVAILABLE BUDGET	2,6

Connection Endpoints	Station 1 - Station 2
Modules Used	Single Mode - 10 km
Available Budget (dB)	11,5
Distance (km)	9
Distance Loss (Cable Loss) (dB)	3,6
Number of Connections	2
Total Connector Loss (dB)	0,6
AVAILABLE BUDGET	7,3

[Tom Randstrom]

The 70km SMF optical transceiver is probably operating at 1550nm, versus 1310nm for the 10km link.  At 1550nm, the fiber has a loss value of ~0.25dB/km versus @1310nm, 0.25dB/km.

Fiber loss:

48km x 0.25dB/km (1550nm) = 12 dB

48km x 0.35dB/km (1310nm) = 16.8 dB

Connector loss:

I would check with the contractor to verify how many splices/connectors are in this path. I would suspect there will be more than 4 connectors/splices in a 48km link.  You will have a patch panel at each end to attach your equipment jumper to. This would leave only 2 connectors for the 48km fiber run; or 16km per reel of fiber cable (very large reel).

What optical transceivers are being used?

Designing with a 3dB margin is typical.  Most of the estimated loss values that you use will provide you with additional margin as the actual losses will typically be less.

You will install an attenuator on a fiber link when your Expected Receive Power is greater than the Optical Receiver's Maximum Input Threshold (without including the system margin).  This typically happens when you have a short fiber link, compared to the optical transceiver's reach.

Expected Receiver Power = Optical Transmitter Power (maximum) - Fiber Loss - Splice Loss - Connector Loss

If the Expected Receiver Power > Optical Receiver Input Threshold

Insert an optical  attenuator to bring the optical level within the Receiver's operating window (between the Receiver Threshold and Receiver Sensativity-Minimum).

Good Reference Site for the Basics of Fiber Optics

http://www.thefoa.org/

Good luck!

[fvalpondi]

Hi Tom,

thank you very very much for your answer! It was really helpful.

I am using Siemens Optical Transceivers for the long distance connections. And Cisco Optical Transceivers for the short distance connections.

MM992-2LD

(2 x 1000 Mbps, SC ports optical, single mode glass, up to max. 10km)

9/125 μm single mode fiber

0 -10 km (0.5 dB/km at 1310 nm; maximum insertion loss 0.5 dB; 6 dB max. permitted FO cable attenuation at 3 dB link power margin)

MM992-2LH

(2 x 1000 Mbps, SC ports optical, single mode glass, up to max. 40km)

9/125 μm single mode fiber

*) -40 km (0.4 dB/km at 1550 nm; maximum insertion loss 0.5 dB, 18 dB max. permitted FO cable attenuation at 2 dB link power margin,  ( *) minimum cable attenuation 3 dB) )

MM992-2LH+

(2 x 1000 Mbps, SC ports optical, single mode glass, up to max. 70km)

9/125 μm single mode fiber

*) -70 km (0.28 dB/km at 1550 nm; maximum insertion loss 0.5 dB, 21 dB max. permitted FO cable attenuation at 2 dB link power margin, ( *) minimum cable attenuation 8 dB )

Best regards.