Sweep frequency modulator using voltage-controlled oscillator for development of incoherent FMCW optical ranging system

Generally, the incoherent frequency modulated continuous wave (I-FMCW) for optical ranging system uses a waveform generator as a sweep frequency modulator; However, it makes a system running non-stand alone and bulky. Therefore, a sweep frequency modulator developed by a voltage-controlled oscillator (VCO) is proposed to substitute a waveform generator. By using the proposed modulator, it can easily adjust a frequency bandwidth and repetition rate of a sweep frequency because both parameters play an important role in governing range resolution. In order to create the system, the VCO is supplied a triangle waveform, where the amplitude and repetition rate can be adjusted by LabView program through a digital-to-analog converter (DAC). In the experiment, a laser's intensity is internally modulated by a developed modulator from 200 MHz to 400 MHz. Then, a modulated light is launched into a device under test (DUT) and it is detected by an avalanched photodetector as an optical ranging signal. The system also employs a double balance mixer (DBM) to combine the optical ranging signal and a reference from the modulator, resulting in a beat frequency. Ultimately, our approach is experimentally demonstrated to range a DUT with two categories: variation of optical fiber length and object detection in free space. The results show that the system can identify the range of optical fiber up to 343 m with a spatial resolution of 0.52 m and an object in free space up to 7 m with a spatial resolution of 0.75 m by using the beat frequency. This system can potentially be applied in lidar, distributed fiber optic sensors, and object profiling.