SMF and MMF

make sure that MFD of one fiber is near about equal to another one
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFggjMAI&url=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMode_field_diameter&usg=AFQjCNHFbTkxqUDzTKyQlAdjO24QN3zInw&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFggqMAM&url=http%3A%2F%2Foptiwave.com%2Foptifiber-manuals%2Foptical-fiber-mode-field-diameter-and-mode-area-definitions%2F&usg=AFQjCNGnvUOWi2jbxDbGtrrJRDInDl4CAg&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFgg_MAg&url=http%3A%2F%2Ffibercore.com%2Ffiberpaedia%2Fmode-field-diameter-mfd&usg=AFQjCNGGQiR3u5e2a_683IDEhXRsbrv9XA&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFghLMAo&url=https%3A%2F%2Fwww.ophiropt.com%2Fuser_files%2Flaser%2Fbeam_profilers%2FMFDandSpotSize.pdf&usg=AFQjCNEgL1pGXN7vh3wdiWfSZ5bit7eJyA&bvm=bv.121421273,d.c2E
https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=13&cad=rja&uact=8&ved=0ahUKEwjVltrSicfMAhXYC44KHfNYBmUQFghaMAw&url=http%3A%2F%2Ffobasics.blogspot.com%2F2012%2F07%2Fmode-field-diameter.html&usg=AFQjCNFMU8VoyGFsBfZm-MRp3tXf7TmXqg&bvm=bv.121421273,d.c2E

Hello Sasha,
We know that in fiber optics , the mode field diameter (MFD) is an expression of distribution of the irradiance, i.e., the optical power per unit area, across the end face of a single- mode fiber.
For a Gaussian intensity (i.e., power density, W/m 2 ) distribution in a single-mode optical fiber , the mode field diameter is that at which the
electric and magnetic field strengths are reduced to 1/e of their maximum values, i.e., the diameter at which power density is reduced to 1/e 2 of the maximum power density, because the power density is proportional to the square of the field strength.

I will suggest you to go through this tutorial link of which I am posting below. I hope this would help to some extent.

Mode Field Diameter and Mode Area Definitions

Regards

Hi sasha

The Mode Field Diameter (MFD) is an important parameter related to the optical field distribution in the fiber. It has been shown that MFD provides useful information about the cabling performances, such as possible joint, macrobending, and microbending losses. The effective area of the fibers has a direct relation to the nonlinear distortions in long fiber links. Currently OptiFiber calculates the following spatial measures of the modes. I will provide you with a link which is very useful to understand it. I hope this will help you.
https://www.ophiropt.com/user_files/laser/beam_profilers/MFDandSpotSize.pdf

Mode Field Diameter and Mode Area Definitions

Thanks

Hello Karan,
The first link you have shared is quite helpful. Thanks for sharing it . It is indeed very helpful as far as Single mode fiber and Multi mode fiber profile settings are concerned. You are right that the effective area of the fibers has a direct relation to the nonlinear distortions in long fiber links. Thanking you.
Regards

Hello Sasha,

As far as your query is concerned i would suggest you take a point which umer has made in his comment. Try making the mode field diameter nearly equal to each other may be that would help. Also the link provided by Karan Ahuja seems to be useful because it is related to setting fiber profiles of single mode and multi mode. I hope your issue will be resolved.

Best Wishes

Burhan