Higher order mode propagation in an optical nanofiber

The propagation of higher modes, such as the LP11 mode, in optical nanofibers using the exponentially tapered optical fiber as a basic model is investigated. In order to preserve the LP11 mode in the downtaper as far as the nanofiber waist, the effect of varying the cladding-core radius ratio on the LP11 adiabatic criterion is modeled. A Laguerre–Gaussian beam is created in free space using a spatial light modulator (SLM) and coupled to a few-mode fiber. This device allows convenient switching between the fundamental and LP11 fiber modes. By selecting a few-mode fiber with a relatively low cladding-core ratio, the propagation of the LP11 mode down to a submicron waist has been maintained. Furthermore, by observing the transmission profile during tapering, it is possible to decisively terminate the pulling process in order to eliminate the two degenerate HE21 modes of the LP11 mode. As a result, a nanofiber can be fabricated through which only the TE01 and TM01 components of the LP11 mode propagate. Such a nanofiber has promising applications in the area of mode interference for controlled particle trapping sites.