We prefer 1300nm optical fiber over 850nm optical fiber because later one is more efficient. So am selecting 1300nm optical fiber. But I want to know the possibility of using same cable to transmit 850nm signal. Please help.
Most 1300-nm optical fiber systems use single-mode fiber. Most 850-nm optical fiber systems use multimode fiber. Therefore, typical 850-nm transmitters are designed to couple into multimode fiber.
Coupling into multimode fiber only requires focussing the light down onto a spot about 50 um in diameter. Single-mode fiber, on the other hand, typically has a core diameter of about 9 um.
Because of this, practically, if you take an off-the-shelf 850-nm transmitter and attach it to a typical fiber meant to be used for 1300 nm, the coupling efficiency will be very poor and only a small fraction of the tranmitter's power will reach the receiver.
Of course it's possible to design an 850 nm transmitter for coupling to 9-um fiber. If you had such a specialized device, then the main limitation would be that Rayleigh scattering losses increase for shorter wavelengths, so the fiber will have more attenuation for the 850-nm light.
If you tried to use an even shorter wavelength, like 680 nm, you could have an additional difficulty because the fiber designed to be single mode for 1300 nm would actually be multimode at the shorter wavelength, and this would increase the system dispersion.