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Ahmad Mustafa

Hi Damian and Tech Support,

I worked on your suggestions and now have better understanding of how multimode components work in OptiSystem. I have come up with some more questions though. Attached is my .osd simulation file for understanding the questions better.

I have a single channel transceiver where the Spatial CW Laser centered at 193.1 THZ has only one mode as can be seen from the single Value of Power Ratio Array and Pol. X m,n index array to represent the single mode laser that I have in the lab. It can be seen on the Spatial Visualizer where I only see one mode. The length and Attenuation of Parabolic-Index Multimode Fiber are set to 0.4m and 0.61dB/km respectively according to my lab setup. The Min. Signal Power is set to -19dBm. Finally, the signal is received in the Spatial Optical Receiver and I get perfect Eye Diagram and 0 BER on the BER Analyzer. Following are my queries that I want to understand:

1) The spatial CW Laser generates only one mode but I see multiple modes on Spatial Visualizer after Parabolic-Index Multimode Fiber. Should I not see only the fundamental mode as the length of the multimode fiber and the attenuation value is really low to generate multiple modes in the fiber?

2) How can I measure the power of the individual mode after the MM fiber? I assume the power in the modes besides the fundamental mode is really less that they can be ignored in this case.

3) The spot size after MM fiber is bigger than the spot size right after Spatial CW laser. I believe it is because of Modal dispersion. Yes?

4) I have increased the Min. Signal Power to -19dBm from the default value which is -100dBm to reduce the number of modes and I can further reduce the number of modes by further increasing this min. signal power value but in my lab, I cannot simply do this because of the hardware limits. Can you make any suggestion how to reduce the number of unwanted modes coming out of the Parabolic-Index Multimode fiber because by increasing the power of the input laser and length of the MM fiber along with the higher attenuation value, the generation of higher number of modes and with that mode coupling effects would make the BER and Eye Diagram go bad as per my understanding.

Please let me know if anything is not clear regarding the questions I posed.

Many thanks