Exploration of double clad fibers for increased stability of bidirectional free space optical links

Abstract

Bidirectional, high data rate, low size, weight, and power (SWaP), and low cost free space optical links are needed for space and aeronautic communication applications to send and receive large volumes of data. We are exploring design strategies for optical transceivers to reduce SWaP and cost through increased misalignment tolerance (pointing requirement reduction) and sharing the optical transmit and receive paths (imposing optical symmetry). In applications where the detector is fiber coupled, the receive fiber numerical aperture is the main driver of the pointing accuracy requirement. Increasing the numerical aperture of the receive fiber reduces the pointing requirement. In an optically symmetric design, the fibers both transmit and receive the light. Hence, increasing the receive fiber numerical aperture requires a similar increase of the transmit fiber. Unfortunately, increasing the transmit fiber numerical aperture causes instability in received power over small misalignments. Double clad fibers offer a solution. These fibers transmit from a single mode core and receive light in a larger numerical aperture. Results show that as transceiver fibers, double clad fibers have an improved misalignment tolerance and a higher stability for small changes in misalignment when compared to single mode fibers and multimode fibers. Also, double clad fibers are shown to match the performance of an asymmetrical link design with a single mode transmit fiber and a multimode receive fiber.

Beaconless pointing and tracking for bidirectional optical links using MEMS mirror nutation

“…Previously, researchers explored using double-clad fiber (DCF) to increase coupling stability in bidirectional free-space links. 15 Here, we propose using DCF with a secondary detector to help decrease L RX resulting from nutation tracking. DCF supports propagation both in its core and in its primary cladding.…”

“…In previous demonstrations, the core has been used to launch the TX beam, while a multimode receiver was used to capture RX power from both the core and the inner cladding. 15 However, for nutation tracking, it is necessary to be able to distinguish how much RX power coupled into the core alone, since position sensing depends on its coupling sensitivity given by g(r). Consequently, to take advantage of the extra power, a secondary detector coupled only to the cladding would be needed.…”