  Modern LiDAR sensors find increasing use in safety-critical applications.
Therefore, highly accurate modeling of the system's behavior under demanding
environmental conditions is necessary. In this paper, we present a modular
structure to accurately simulate the amplified raw detector signal of a direct
time-of-flight LiDAR system for coaxial transmitter-receiver optics. Our model
describes, a measurement system based on standard optical components and a
detector able of converting single photons to an electrical signal. To verify
the model's predictions, single-point measurements for targets of different
reflectivity at defined distances were performed. Statistical analysis shows an
R-squared value greater than 0.990 for simulated and measured signal amplitude
levels. Noise modeling shows good accordance with the performed measurements
for different target irradiance levels. The presented results have a guiding
significance in the modeling of the complex signal processing chain of LiDAR
systems, as it enables the prediction of key parameters of the system early in
the development process. Hence, unnecessary costs by design flaws can be
mitigated. The modular structure allows easy adaption for arbitrary LiDAR
systems.
