A novel coherent laser radar architecture based on temporal-diversified optical orthogonal-frequency-division-multiplexing

Coherent Laser Radar Receiver (CLRR) typically employs high-quality bulk optics to collect and direct light at the photodetector plane. Collection of the back-scattered light from an optically-diffused target requires a large aperture area but it is limited by the atmospheric coherence length. Combining the local laser oscillator (LO) and the received optical signal at the photodetector requires careful optical alignment and good mechanical stability. Alternatively, the collected light can be coupled directly into an optical fiber connect to an optical hybrid that leads to the photodetector. The advantage is that the photodetector and the electronics can be located considerably away from the free-space collecting optics which may have a real-estate constraint. Furthermore, higher speed photodiodes of smaller active areas can be employed to process higher bandwidth signals. However, coupling into a single-mode fiber (SMF) requires high-quality bulk optics and the optical alignment is quite difficult. The coupling loss for SMF is substantially higher than say coupling into a multi-mode fiber (MMF). MMF provides better coupling efficiency and more flexibility in the collecting optics. Furthermore, coupling into MMF is less susceptible to power-fade due to speckle than SMF. Assuming a single focusing lens is used to image the optical beam at the target with a Lambertian surface to an optical fiber, the ratio of the optical power collected into a MMF to that into a SMF without turbulence is given by