An optical pick up, including: a laser diode which outputs a light in response to an input current; a photo diode which receives the light and outputs a voltage; a digital potentiometer which adjusts the voltage output by the photo diode according to a voltage adjusting signal; and a controller which outputs the input current, sets a first reference voltage to be outputted by the photo diode according to the light, and outputs the voltage adjusting signal to the digital potentiometer according to a difference between the output voltage and the reference voltage.