In the realm of cosmological ray studies employing plastic scintillation fibers, it is essential to conduct quantitative analyses of the photon number from the fiber's output pulse for the successful design of readout electronics.

Given the absence of a weak cursor setting device such as a single photon source. This study aims to quantitatively analyze the number of photons generated by photon incidents within the fiber calibrating without a weak photon source such as a single photon source.

Firstly, photon numbers within weak optical pulses induced by muons in optical fibers with diameters of 1 mm and 2 mm were determinated by the calibration method that making use of inherent non-photogenerated carrier characteristics of the silicon photomultiplier tube (SiPM). Then, the Geant4 software was employed to simulate the theoretical photon yield of muons in these optical fibers, and the simulation results were compared with experimental data for validation.

The verification results indicate that the anticipated photon count in the optical pulses within fibers with diameters of the 1 mm and 2 mm fibers are 44 and 85, respectively. The deviation from the simulation results is 4.55% and 10.59%, respectively.

The results validate the efficiency of the low photon number measurement method, demonstrating its ability to accurately measure the photon count generated by the incident fiber without the need for additional calibration equipment. This method may extend to other scenarios that require the measurement of photon numbers in weak light pulse situations.