Fiber optics devices based on selective modal excitation

Inside any wave-guiding structure, such as optical fibers, light propagates in a discrete set of modes. Each mode is defined by its direction of propagation, amplitude profile, phase distribution and polarization. Proper modal excitation and manipulation can yield to novel and low cost optical devices. This paper covers the concepts of designing low cost and functional optical fiber devices based on selective modal excitation. On-axis single mode to a multimode fiber coupling, for example, selectively excites radial modes only. Controlling the phase properties between these modes produces an all-fiber band-pass filter when connecting another single mode fiber at the output. A controllable band-rejection fiber is produced through off-axis excitation of large-mode area fiber with fiber Bragg grating (FBG) inscribed on it. Utilizing long period fiber grating, a simple all-fiber device that can transform a Gaussian-shaped laser beam into a uniform intensity beam is achieved. The device is based on the interaction of a core mode and a cladding mode partially coupled out by the grating.