Figure 1 shows a schematic of the optical experiment. The output power of the laser (LSR980H-4W, Ningbo Yuan Ming Laser Technology Co., Ltd., Ningbo, China) was modulated by a current source controller (LSR-PS-FA, Ningbo Yuan Ming Laser Technology Co., Ltd., Ningbo, China), which is compatible with transistor–transistor logic (TTL) modulation. The pulse width was modulated by microcontroller unit (MCU) (MC9S12XS128, Freescale Semiconductor, Inc.). The laser output was coupled into a diameter optical fiber through a lens fiber coupler (APFC-3AT, Zolix, Beijing, China). The output power, which was measured at the distal end of the fiber, ranged from 0 to 2.2 W and the pulse durations could be as short as . An electrophysiograph (RM6240C, Chengdu Instrument Factory, Chengdu, China) was used to collect the signals, which were transmitted to the computer.