This paper presents design and measurement results of a fully integrated optical sensor for phase and amplitude detection of RF modulated optical signals up to 110 MHz in the near-infrared (NIR) region (650-850 nm) for use in frequency-domain spectroscopy instruments. The sensor consists of an NIR-sensitive photodetector monolithically integrated with a front-end analog amplifier and signal processing circuitry for amplitude and phase detection in an unmodified complementary metal oxide semiconductor (CMOS) process. A high-gain, low-noise differential transimpedance amplifier (TIA) is implemented to amplify the photocurrent signal. Amplitude and phase resolution are evaluated with a 690 nm laser diode modulated at 100 MHz. The amplitude response exhibits 2.2 mV/mu W resolution with 0.4% linearity. The measured amplitude output noise is 72 mu V. The proposed phase detector detects 0 degrees-360 degrees phase difference with a measured average phase resolution of 4.8 mV/degree and 255 mu V output noise. The sensor is implemented in a 180 nm CMOS technology and consumes 23.4 mW from a 1.8 V supply voltage.