For a plastic optical fiber based refractometer system the influence of the directivity of the fiber coupler on the
achievable resolution is analysed. It is also shown that provided the fiber length between the sensing tip and receiver is
less that 2 m that interference due to Rayleigh backscatter will not comprise operation of the refractometer. A novel
coupler based on a hybrid silica fiber-plastic fiber design is used experimentally to provide a comparison to the modelled
results. It is shown that the high directivity (>35 dB) of this coupler can significantly enhance the resolution of the
An analysis is presented of the sources and levels of crosstalk between the two signals
propagating in opposite directions on a bi-directional plastic fibre link. Contributions to crosstalk
from limits on the directivity of a new POF directional coupler and Rayleigh backscattering from
the POF fiber are considered individually. The measured directivity of the coupler is 37.3 dB,
better than any previous POF directional coupler. It is shown both theoretically and
experimentally that Rayleigh generated crosstalk decreases with the increasing fiber length and
that the level reaches a limit value circa 28.5 dB (below the input fiber level) when fiber length
exceeds 36.5 meters.
A new plastic optical fiber directional coupler has been developed. It is fabricated by cutting a shallow notch on the side of the 1 mm diameter cladding fiber, within which is placed a silica 125 um cladding diameter optical fiber for uplink coupling. This novel directional coupler features high efficiency in the uplink and downlink directions because of its advanced geometrical structure and the proposed coupling technique. Compared with the physical size of the notch, the physical size of the silica fiber is relatively small which in turn permits a very high uplink coupling efficiency. Furthermore the physical size of the plastic optical fiber is large relative to the notch which provides a high downlink coupling efficiency. In the uplink direction, we recently reported that with an optimized angle of coupling, the silica fiber can couple light into the plastic fiber with a measured efficiency up to 88% (equivalent to 0.56 dB insertion loss). In the downlink direction, because of the relatively small physical size of the notch, the loss of the power through the notch is low and is estimated to be less than 1.5 dB, given a notch width of 1 mm. In this paper, the variation in the downlink coupling losses with the notch dimensions are measured and compared to those from a theoretical model.
With the increasing information rates demanded in consumer, automotive and aeronautical applications, a low cost and high performance physical transmission medium is required. Compared with Silica Optical Fiber, Plastic Optical Fiber (POF) offers an economic solution for a range of high-capacity, short-haul applications in industrial and military environments. Recently, a new type of POF, the perfluorinated graded-index plastic optical fiber (PF GI-POF), has been introduced that has low losses and high bandwidth at the communication wavelengths 850 nm and 1300nm. POF is normally terminated perpendicular to the fiber axis. We propose an angle-ended POF, which is terminated at non-perpendicular angles to the fiber axis. The aim of the research is to investigate the numerical aperture (NA) characteristics of angle-ended POF along the major axis of the elliptical endface. A theoretical model indicates that the NA of the angle-ended POF will increase nonlinearly with tilt-angle and the acceptance cone will be deflected with the angle of the deflection increasing nonlinearly with tilt-angle. We present results for the measured NA and the measured deflection angle using the far-field radiation method. Results are presented for 13 angle-ended SI-POF tilt-angles. We also present results for theoretical value of NA and deflection angle as a function of tilt-angle. The agreement between the measured and theoretical value is good up to tilt-angles of about 15 degrees, beyond which deviation occurs.