If we increase the power of input/modulating signal, the modulation index m (Vm/Vc i.e input signal power/carrier signal power) becomes greater than unity, which causes over modulation in case of Amplitude Modulation (AM), so it is advisable not to amplify the input signal power above certain level with respect to carrier signal power. That is why EDFA is preferred to amplify a modulated signal as a post-amplifier or booster.

Thank you very much it was really very helpful answers.

Best regards

Thanks

Handling of high power Laser is very cautious as it may hurt anyone in case of careless.

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Amit Kumar Garg. "Handling of high power Laser is very cautious as it may hurt anyone in case of careless." This is less of a problem with Erbium amplifiers as used in optical fibre syetems, as our eyes are opaque to 1.5 microns, so there is far less risk of retinal damage. Also in most fibre systems the mean power levels are not high, as signal distortion caused by fibre non-linearity limits the performance.

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Mr. Epworth: The thermal energy contained in infra red waves may damage the eye structure. Better consult an eye physician. 

In optical communication systems modules, the laser exposure hazards is not a problem, as the laser sources are pigtailed with fibers. The issue with high power laser in transmitter section involves some factors: one of them is the handling issue in terms of internal temperature control which creates operating wavelength changes. Higher cost in production process, Higher price at market level, bigger size, etc etc.

Faramarz E. Seraji You say " The thermal energy contained in infra red waves may damage the eye structure." The difference between IR and visible is that because the aqueous humor is apaque to IR, the lens (and curved corneal surface) cannot focus the optical power onto a tiny area on the retina.

The energy is spread sufficiently widely that the temperature rise in the tissue is negligible for light levels used in fibre communications. If you used an external lens to image the light from the fibre end directly onto the cornea surface you might achieve some superficial tissue damage, but this would be noticed no more than a tiny speck of dust of the eye, and would quickly disappear as the cornea tissue is our fastest healing tissue. Of course one should be sensible but 1.3 and 1.5 micron sources are far safer than visible and 0.85 microns.

"Better consult an eye physician " Faramarz E. Seraji. I did many times ;

Dear Epworth: I appreciate your explanation. The question is why not high power laser is not used in transmitter end of optical communication systems. It was one of the safety reasons of why high power lasers are not used. How much high the power should be, is not appeared in question either.  

Anyway, my suggestion of consulting a physician was meant for the questioner to be on safe side of the issue. I was not referring you Sir. From long time back I have known you having worked at STC, UK. 

Best Regards

F.E.Seraji

"The question is why not high power laser is not used in transmitter end of optical communication systems."

The answer: Optical nonlinearity in the fibre. This distorts the signals and creates cross-talk between multiple wavelengths. The intensity fluctuations modulate the refractive index and hence the optical phase. Single channel and lower bit rates can use higher launch powers.

Cheers

It's OK nonlinearity is also part of the issue, which may surface in optical fiber in either case whether using high power laser diode or midway amplifiers.