System Power Penalty and Link Margin

[Sushank Chaudhary]

Hi Optiwave Team,
How to measure the system power penalty and link margin budget in OptiSystem ? Also in multi-mode transmission system, OptiSystem is unable to calculate the effect of modes after transmission. No doubt, spatial visualizer is there for visualizing the mode profile of system but it shows the same profile even after the transmission of multi-mode signal which does not make any sense. Is their any alternate option to visualize the effect of mode profile after the transmission.

Regards
Sushank Chaudhary

[Damian Marek]

To calculate the link margin budget of a system, first duplicate the layout. Then add an Attenuator component before your photodetector. While increasing the Attenuator’s loss monitor the BER of your system. Once the BER is below your desired threshold, the Attenuator’s loss should signify the link margin budget of your system.

For the system power penalty, you can perform the same protocol. For example, if you are interested in the dispersion power penalty. You duplicate your layout, remove the cause of dispersion and add an Attenuator before the photodetector. Then increase the loss until the BER matches that of your dispersion-including-layout.

For your multi-mode question, do you have a design layout I can look at? (OptiSystem 12)

[Sushank Chaudhary]

Hi Damian Marek,

Thanks for your reply. For multimode signal , can OptiSystem is capable of measuring the beam diameter ? If yes , what is the procedure ?

[Damian Marek]

Hi Sushank,

Calculating the beam diameter and mode measures of fibers is done very easily in OptiFiber. In OptiSystem, you can use the Encircled Flux Visualizer to find the FWHM, as well as other mode measures, by manually comparing radius to percentage of power encircled. It might even be simpler to export the data to Matlab and calculate the mode measures automatically.

[Sushank Chaudhary]

Hi Damian,

Thanks again for your reply. Can you send some example how to measure the modes with Matlab and OptiSystem.

Regards

Sushank

[Damian Marek]

I made a quick example that demonstrates using the Matlab component to calculate the effective area of a spatial mode. The effective area is the modal measure often used to characterize nonlinear effects in waveguides. To make it work, you need to change the Matlab file by choosing a new filename and directory in the save() command.

[Sushank Chaudhary]

Hello Damian,

Thanks for your quick response. The above optisytem file is unable to open in our OptiSystem software. Can you share the snap shot of the model designed in the OptiSystem for the calculation of modes with matlab. Sorry for the inconvenience.

[Damian Marek]

Sorry about that. I’ve included a picture of the Matlab component settings as well.

[Sushank Chaudhary]

Dear Danial,

I simulated as per your suggestions, but it didn’t work. I am attaching the snapshot of simulation window. Kindly have a look on that.

[Damian Marek]

Make sure the OptiSystem project file and .m file are in the same folder.

[Sushank Chaudhary]

Dear Damian,

Yes they are in same folder. It shows the warning of error “in line 1 undefined function of ModeMeasuredMatlab while calculating MAtlab compnent

[Damian Marek]

Hi Sushank,

I believe this error is because Matlab cannot find the .m file. The simulation is working for me (with both files on the Desktop). If both files are in the same folder than maybe the file name is wrong?

You will also want to explicitly include the search path that your files are in, especially if they are not included in the default Matlab search path folders. For example, if my .m file was in “My Downloads” folder, which is not included in the Matlab search path, I would write “C:\Users\dmarek\Downloads” into the Matlab search path parameter box of the Matlab component. I have attached a picture to make it clear.

This should make it work! As an aside don’t forget to change the save() command (line 43) in the .m file to correspond to your desired file name and directory.

Let me know if there are any more problems!

[Sushank Chaudhary]

Hello Damian,

Thanks for providing the information. Now that file is working. If i want to plot the different values for beam diameter corresponding to different transmission link then what is the procedure ? ? Suppose if there are 5 iterations for transmission link i.e 20, 40 , 60 , 80 & 100. Now i want to plot the 5 values of beam diameter against transmission link, what is the procedure for that. Thanks in advance 🙂

[Damian Marek]

Hi Sushank,

I think I understand the problem you are getting at in the original post here. Although what you suggested is possible in OptiSystem (measuring the beam diameters at different propagation lengths), the results will not be very interesting. OptiSystem does not model the interference of modes with different propagation constants, which can cause the spatial profile to change. It also assumes a fiber with no imperfections or bends, which can cause mode coupling. Although I think these effects are more important on smaller length scales. On the other hand, if by transmission link you are interested in free space propagation, then beam broadening will be observed.

If you are interested in these effects, you can use OptiFiber to determine the spatial profile change with propagation distance.

[Sushank Chaudhary]

Hi Damian,

Thanks for your reply. Yes, exactly that the thing i want to do with OptiSystem. I am quite interested to see the effect of broadening of multimode optical beam in free space propagation but unfortunately even with free space propagation, there is no change in spatial profile with the distance. If you have solution to this problem, kindly suggest.

[Damian Marek]

OK! Sorry for the mix up. To simulate free space effects on the spatial profile use the Spatial Connector component, which is found in the “Default/Passives Library/Optical/Connectors” folder. I’m going to guess you were using the FSO Channel or the OWC Channel to simulate your design and it wasn’t giving the results you were looking for. The reason being is these components include transmitting and receiving telescopes in their models and so the spatial profile is not changed.

[Sushank Chaudhary]

So it means there is no effect on spatial profile if transmitting through FSO/OWC channel which is not making the sense. If there is effect on spatial profile, spatial analyzer is not able to detect that effect. Any alternative method to measure the effect of beam through FSO/OWC channel. Thanks in advance.

[Damian Marek]

The FSO/OWC channel components can be thought of as a subsystem, which consists of a transmitting telescope and receiving telescope with the free space propagation occurring between them. The spatial analyzer does not detect a change in the profile, because it is detecting the output of the receiving telescope, which realistically has lenses to refocus the light. The beam broadening over large distances is modeled as a power loss. Take a look at the component help for the FSO and OWC components to see exactly how the loss is calculated.

If you want to observe the beam broadening over large distances (i.e. km’s) you would have to write your own Matlab model. If however you are interested in the um regime you can use the Spatial Connector.

[Sushank Chaudhary]

Dear Damian,
Does it that the telescopes in the FSO link just directly images the spatial profile ? Also do the spatial profiles and beam diameter take into account modal dispersion, laser beam divergence and power modal coupling?

Thanks in advance.

[Damian Marek]

You are correct, the spatial profile is not changed using the FSO and OWC channel. For your second question, it depends on the component you use. The multimode fibers are modeled as described in this other forum post:

https://optiwave.com/forums/topic/single-mode-light-coupling-into-multimode-fiber/

The Spatial Connector component will model laser beam divergence.

[Sushank Chaudhary]

So the conclusion is ” OptiSystem is unable to calculate the diameter of multimode beam when pssed through wireless channels.

[Damian Marek]

Over short distances you can by using the Spatial Connector, however multimode dispersion and power coupling is only included in the multimode guided medium such as the multimode fiber. The FSO and OWC components are assumed to be in the Fraunhofer region where waves are assumed to be planar, so you can’t measure the mode diameter.

[Sushank Chaudhary]

Hi Damian,
Thanks for your reply.
Okay let take a simple example ” I used CW multi-mode laser with LG 00 mode and send it directly over the 100 km of multimode parabolic indexed fiber. Now i compared the spatial profile after the multi-mode fiber and after the CW spatial laser by using spatial visualizer in OptiSystem. Now it shows the same spatial profile as after the laser as well as after multimode fiber. There is no effect in the spatial profile as it suppose to be attenuated when transmitted over such long distance. Can you justify this ??

Thanks in advance.

Regards
Sushank

[Damian Marek]

Take a look at the Help for this component. I’ll try to summarize it here for you too. The parabolic index measured fiber uses the input field’s spatial information to calculate the coupling coefficients to each of the modes that the fiber supports. It can also include different modal delays and attenuation in the final output time domain calculation.

Since this component is a waveguide I wouldn’t expect the mode field diameter to change. Of course you can have higher order modes excited by scattering (by imperfections in the fiber) or by mode coupling (which could be caused by bends), which unfortunately this model does not incorporate. However, I would not expect the field’s final spatial information to change that drastically.

Is there an article you are trying to replicate?

[Naazira Badar]

Hi Damian..
I wanted help for the same topic- link margin and you have mentioned making use of an attenuator component in reply #11229.
Actually, I am not able to access optisystem this time. So i am not able to check if there is an explicit attenuater component available or do we need to make use of any other component for this?

Regards
Naazira.

[Naazira Badar]

Hi Damian..
Could you please tell me if I can make use of the optical attenuator and get results for my link margin in FSO system?

Regards
Naazira Badar

[Rajguru M. Mohan]

Hi,
Inorder to compensate for the system degradation, the signal power has to be increased to
achieve the same SNR or BER performance as that of an ideal system. This increase in
power is called the Power Penalty.
There are two types of signal degradation which can contribute to the power penalty.
1) Degradation during propagation in the optical fiber
2)Degradation due to peripheral electronic and optic components in the system like the
lasers, photo-detectors, couplers etc.
Due the fluctuations the mean of the peak current is reduced decreasing the SNR.
The SNR can be restored by increasing the pulse amplitude. Hence there is power penalty
for timing jitter. It can be noted that the power penalty very rapidly increases with the timing jitter. When
the rms value of the timing jitter is about 0.15, the power penalty becomes infinite.
The power penalty may become significant if the semiconductor laser is biased above the
threshold. For lasers biased below threshold, the extinction ratio is typically 0.05 and the
power penalty is less than 0.4 dB.

Thanks

[Rajguru M. Mohan]

Hi,
I think the link margin, is the difference between the receiver’s sensitivity (i.e., the received power at which the receiver will stop working) and the actual received power.
Probably the single most significant parameter in describing the performance of an FSO link is the link margin.
Basically, this is the amount of light received by a terminal over and above what is required to keep the link active
(hence the term “margin”). The higher the LINK MARGIN is, the higher the quality of the received signal and the farther we are from the cliff. The closer to 0 dB, the worst the quality of the received signal and therefore the closer to the point where the system stops to operation.
When the LINK MARGIN is 0 dB, we are in the limit of the correction. The services can be displayed, although occasionally can be seen some artifact, but only a slight signal degradation can cause the drop through the cliff. The system will stop to work immediately.

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