In 3D laser projectors, specific requirements are imposed on the projection line width, necessitating a dedicated focusing module designed to adjust this parameter through modulation of the projected laser beam's echo energy. However, environmental light interference, projection distance variations, and surface reflectivity discrepancies collectively attenuate the laser echo signals to such weak levels that automated line width adjustment becomes unfeasible due to compromised signal recognition. To address this challenge, a photovoltaic cell array is employed as the echo signal receiver, while the laser beam is modulated using a 50% duty cycle square wave to generate differential bright-dark signals. The return energy of the projected beam is amplified by the primary amplifier circuit, and then summed up one-to-one according to the cycle in the field editable logic gate array to realize the digital amplification of the differential signal. This method can effectively reduce the influence of background light interference and Gaussian noise and improve the signal quality by firstly amplifying the received signal by hardware and then amplifying the differential signal by software.