|
We present a frequency-domain method for measuring various types of optical fibers primarily using a vector network analyzer (VNA). Through proper E-O conversion to launch frequency sweeping signals into the fiber and O-E conversion at the receiving side, the VNA measures the complex transfer function (CTF) of the fiber transmission for a given launch condition. The group delay information can be calculated from the inverse Fourier transform of the CTF. Due to the long fiber length relative to the number of points used by VNA for frequency sampling, the measured CTF is under-sampled and aliased. With proper de-aliasing procedures, the aliased CTF can be transformed into a modified CTF in the local time frame. The output pulses can then be recovered with the full group delay information obtained. The group delay and dispersion of different modes or different cores can be obtained from the CTF over a range of wavelengths. For single mode fibers, we can obtain the group delay and chromatic dispersion from the wavelength dependence. For fibers with multiple modes, cores or polarization maintaining fibers with two distinct polarization modes, the group delay of each mode or core can be obtained simultaneously. For multimode fibers, we have developed a thorough procedure to conduct differential mode delay measurements and calculate modal bandwidth, equivalent to the time domain method defined by the standard. Overall, the VNA based frequency domain method can measure various types of optical fibers from short lengths of a few hundred meters to long lengths of many kilometers at different wavelengths. The technique is unique among fiber measurement techniques in that it can determine the group delay, dispersion, modal and bandwidth properties across different fiber types, not limited to just single mode fibers.
|
