The setup of this problem is basically to model a neuron that spikes randomly according to some Poisson distribution over some time interval.
Suppose I have a time interval $[0,T]$ for some fixed $T$ (we're treating $T$ as some large number). Suppose we have a random variable $X \sim \mathcal{Pois}(\lambda)$ over that time interval. Let $t_1,t_2,\dots, t_n$ be the times in $[0,T]$ that my event occurs. Define a function:
$$\phi(t) = 2\pi k + \frac{t-t_k}{t_{k+1}-t_{k}}$$
where $k$ enumerates the spikes. The numerator is basically the time since last spike, and the denominator is the time between spikes. The order parameter $R(t)$ is defined as $|e^{i \phi(t)}|$. Is there any way to compute $E\left[R(t) \right]$ as a function of $\lambda$, or at least get an estimate of it non-numerically?