We demonstrate a distributed feedback laser based on a laterally coupled grating with a saturable absorber (LCDFB-SA) as a photonic spiking neuron. So far, the conventional time domain traveling wave (TDTW) method is unable to simulate the nonlinear dynamics of the laser when the SA is under reverse bias. This problem is solved by a modified TDTW method introducing a relationship between the SA carrier lifetime and the reverse bias, by which, the neuronal characteristics and tonic spiking repetition frequency tunable (TSRFT) characteristics of the device are simulated. Moreover, the side-mode suppression ratio (SMSR) of LCDFB-SA surpass 40 dB in tonic spiking state and is over 50 dB in continuous laser state. Threshold, refractory period and TSRFT characterization of the device are verified through experiment settings. Finally frequency encoding and reconstruction of RGB images based on TSRFT is performed. After multiple rounds of verification, the maximum normalized mean square error was 2.8808×10-4 and the average was 1.0101×10-4, demonstrating good frequency coding capability. LCDFB-SA is fabricated by the standard I-line lithography process, which provides a low-cost and mass-producible photonic spiking neuronal solution.