This paper presents the development of a polymer optical fiber (POF) strain gauge based on the light coupling principle, which the power attenuation is created by the misalignment between two POFs. The misalignment in this case is related to the strain on the structure that the fibers are attached. This principle has the advantages of low cost, ease of implementation, temperature insensitive, electromagnetic fields immunity and simplicity on the signal processing. The advantages presented can overcome the limitations of electronic strain gauges. For this reason, an analytical model for the POF strain gauge is proposed and validated. Furthermore, the proposed POF sensor is applied on an active orthosis for knee rehabilitation exercises through flexion/extension cycles. The controller of the orthosis provides 10 different levels of robotic assistance on the flexion/extension movement. The POF strain gauge is tested at each one of these levels. Results show good correlation between the optical and electronic strain gauges with root mean squared error (RMSE) of 1.87 Nm when all cycles are analyzed, which represents an error of less than 8%. Furthermore, the proposed sensor presented higher stability than the electronic one, which can provide advantages on the rehabilitation exercises and on the inner controller of the device.