This study shows the operating principle of a new digital system for measuring optical fiber length. The system combines an exclusive-OR (XOR)-based fiber-optic length sensor with a dual-slope analog-to-digital converter (ADC). The sensor receives two input signals: a reference signal from the transmitting end and a regenerated signal from the receiving end. Both signals are square waves with a period of T and a 50% duty cycle. The DC level of the sensor output varies linearly with the actual fiber length. Digital length measurement is conducted by using the dual-slope ADC to perform analog arithmetic operations, including addition, subtraction, multiplication, and division.