Round-trip laser threshold condition
= Round-trip laser threshold condition
If $g$ and $\alpha_i$ are intensity gain and loss per length, a linear cavity has round-trip intensity multiplier $R_1R_2e^{2(g-\alpha_i)L}$. Threshold occurs when it equals one, giving $g_{\rm th}=\alpha_i+\log[1/(R_1R_2)]/(2L)$. Phase resonance and the gain bandwidth determine which modes can oscillate.