14. Given $A\left(x_{1}, y_{1}\right) 、 B\left(x_{2}, y_{2}\right)$ are two moving points on the ellipse $\frac{x^{2}}{a^{2}}+\frac{y^{2}}{b^{2}}=$ $1(a>b>0)$, $O$ is the origin, and $O A \perp$ $O B$. Find the minimum length of segment $A B$.