Embodiments are disclosed of methods performed at an optical transmitter, devices and computer-readable storage media. For example, the method includes performing a process to select a PAM value from a set of candidate PAM values based on currently transmitted training data bits, transmitted Pulse Amplitude Modulation (PAM) values corresponding to previously transmitted training data bits, and training data bits to be transmitted subsequently, the selected PAM value corresponding to the currently transmitted training data bits. The selected PAM value is transmitted to an optical receiver, and an indication as to whether the optical receiver correctly detects the currently transmitted training data bits is received from the optical receiver. Then, the process is updated at least in part based on the indication. The manner of performing neural network learning at an optical transmitter side to obtain an encoding strategy can maximize the channel allowable capacity.