There are some quantum dot and multiple quantum well structures along with the lasers that you mentioned. I am attaching some links of papers that you may find useful in that case. 

All the best for your research :) 

http://aip.scitation.org/doi/abs/10.1063/1.93786

http://aip.scitation.org/doi/abs/10.1063/1.92380

https://spie.org/Publications/Proceedings/Paper/10.1117/12.848398

http://ieeexplore.ieee.org/document/4266009/

Dear Monalisa,

The purpose of Yb doping in Er:Yb lasers and amplifiers is to enable efficient pumping with widely spread 980 nm diodes. You see, absorbtion cross section of an Er ion at this wavelength is rather small, which is not the case for an Yb ion letting it to effectively absorb pump radiation. Then Yb ions transfer the energy to Er ions through phonon interaction. Of course, as far as Yb medium is two-level in this case, the excitation and pumping rate will be limited, however, it is not the problem for a laser or an amplifier as a whole, and this is still beneficiary for short length media. If there is no restrictions on the length of the media, small absorbtion cross section of an Er ion can be compensated by increasing the length of interaction. But, as I mentioned, Yb lasing won't be a problem and in fact won't even happen for 1.55 um applications if there is no resonator at Yb generation wavelength (~1 um) which is mostly the case and will surely be the case for an amplifier. What can indeed happen is back-transfer of energy from Er ions to Yb ions which depends on various conditions including doping rate and pumping rate, and which can lead to the loss of energy in a laser or an amplifier but it is still acceptable.

Thank you Uday and Anton