Standard stripe-length dependent optical-pumping measurements have been performed on AlGaN/AlGaN multiple quantum wells (MQWs) on an AlN buffer grown using two different kinds of technologies, i.e., “GaN interlayer” and “porous AlN buffer.” The net modal gains of the two samples along both m- and a-axis have been obtained, showing that the net modal gain of the MQWs on the AlN grown using “GaN interlayer” is higher than that on the AlN grown using “porous AlN buffer.” Reciprocal space mapping measurements have indicated that the MQW structure on the AlN structure grown using “GaN interlayer” is fully strained while that on the AlN grown using “porous AlN buffer” is partially strain-relaxed. The net modal gain along the m-axis is higher than that along the a-axis in both samples, highly reasonably indicating that the most favourable orientation for forming the cavity facets is not 〈11-20〉 direction of c-plane sapphire, along which III-nitride on c-plane sapphire is normally cleaved.