The 980nm pump needs three energy level for radiation while 1480nm pumps can excite the ions directly to the metastable level .
Though pumping with 1480 nm is used and has an optical power conversion efficiency which is higher than that for 980 nm pumping, the latter is preferred because of the following advantages it has over 1480 nm pumping.
- It provides a wider separation between the laser wavelength and pump wavelength.
- 980 nm pumping gives less noise than 1480nm.
- Unlike 1480 nm pumping, 980 nm pumping cannot stimulate back transition to the ground state.
- 980 nm pumping also gives a higher signal gain, the maximum gain coefficient being 11 dB/mW against 6.3 dB/mW for the 1.48
- The reason for better performance of 980 nm pumping over the 1.48 m pumping is related to the fact that the former has a narrower absorption spectrum.
- The inversion factor almost becomes 1 in case of 980 nm pumping whereas for 1480 nm pumping the best one gets is about 1.6.
- Quantum mechanics puts a lower limit of 3 dB to the optical noise figure at high optical gain. 980 nm pimping provides a value of 3.1 dB, close to the quantum limit whereas 1.48 pumping gives a value of 4.2 dB.
- 1480nm pump needs more electrical power compare to 980nm.
- Typically, 980 nm pumping results in a noise figure 1 dB lower than that for 1480 nm pumping.
- The shorter wavelength results in less noise.
