6. The parabola $C_{1}: x^{2}=2 p y$ has a focus at $F$, and the hyperbola $C_{2}: \frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ has foci at $F_{1}$ and $F_{2}$. Point $P$ is a common point of the two curves in the first quadrant. If $P F F_{1}$ are collinear, and $C_{1}$ and $C_{2}$ have a common tangent line at $P$, then the eccentricity of $C_{2}$ is