The idea of adopting the space domain as the next frontier for optical communication has received increasing attention in the last few years. Optical devices are the integral parts of a mode division multiplexing (MDM) transmission. Therefore, conducting an intensive study on the devices is paramount to the successful realization of the overall system. This paper presents a review of the recent advances in the inline components of an MDM system, consisting of mode converters, spatial (de) multiplexers, optical amplifiers, and few-mode fibers (FMFs). Also presented are different mode conversion and multiplexing schemes. Recent techniques of minimizing differential mode gain (DMG) in the optical amplifiers are also reviewed. The review covers other types of amplification schemes and their current standing in the MDM system. These include optical semiconductor amplifiers (OSAs), and the Raman amplifiers (RAs). Finally, the review also highlights the role of FMF, multicore fiber and their relationship with fan-in/fan-out devices.

A novel all optical measurement scheme is proposed to measure wideband microwave frequencies up to 30 GHz. The proposed method is based on a four-wave mixing (FWM) approach in a semiconductor optical amplifier (SOA) of both even order side-bands generated by an unknown microwave frequency modulating an optical carrier. The optical power of a generated FWM signal depends on frequency spacing between extracted side-bands. A mathematical relation is established between FWM power and frequency of an unknown signal. A calibration curve is drawn based on the mathematical relation which predicts the unknown frequency from power withdrawn after FWM