  The capacity of a discrete-time model of optical fiber described by the
split-step Fourier method (SSFM) as a function of the signal-to-noise ratio
$\text{SNR}$ and the number of segments in distance $K$ is considered. It is
shown that if $K\geq \text{SNR}^{2/3}$ and $\text{SNR} \rightarrow \infty$, the
capacity of the resulting continuous-space lossless model is lower bounded by
$\frac{1}{2}\log_2(1+\text{SNR}) - \frac{1}{2}+ o(1)$, where $o(1)$ tends to
zero with $\text{SNR}$. As $K\rightarrow \infty$, the inter-symbol interference
(ISI) averages out to zero due to the law of large numbers and the SSFM model
tends to a diagonal phase noise model. It follows that, in contrast to the
discrete-space model where there is only one signal degree-of-freedom (DoF) at
high powers, the number of DoFs in the continuous-space model is at least half
of the input dimension $n$. Intensity-modulation and direct detection achieves
this rate. The pre-log in the lower bound when $K= \sqrt[\delta]{\text{SNR}}$
is generally characterized in terms of $\delta$.
  It is shown that if the nonlinearity parameter $\gamma\rightarrow \infty$,
the capacity of the continuous-space model is $\frac{1}{2}\log_2(1+\text{SNR})+
o(1)$.
  The SSFM model when the dispersion matrix does not depend on $K$ is
considered. It is shown that the capacity of this model when $K=
\sqrt[\delta]{\text{SNR}}$, $\delta>3$, and $\text{SNR} \rightarrow \infty$ is
$\frac{1}{2n}\log_2(1+\text{SNR})+ O(1)$. Thus, there is only one DoF in this
model.
  Finally, it is found that the maximum achievable information rates (AIRs) of
the SSFM model with back-propagation equalization obtained using numerical
simulation follows a double-ascent curve.
