The Implantable Medical Devices (IMDs) are playing an increasing role for its benefit in monitoring and detecting a patient's disease symptoms. One of the challenges faced by the IMD developer is the need for a long lifetime of the devices. Along with the implementation of wireless power transfer (WPT) technology, a low-power communication between the implants and the receiver outside the patient's body is needed to ensure a low medical cost and risk from the need of battery replacements through surgery. Adopting optical wireless communication, in this article, a novel approach to solve the problem is introduced. Using an unsynchronized pulse-interval modulation (UPIM) technique, a system working principle is addressed, and a prototype is also built within a 10mm x 10mm x 2.3mm form factor. Experiments are conducted to test the system design, ensuring its ability to carry out low-power wireless communication with µW-level power consumption. The result shows that the proposed approach can achieve as low as 392uW power consumption. Finally, the technical challenges along with the possibility for further development is discussed.