Mode-by-mode impulse responses, or spectral transfer matrix (STM) of birefringent fibers are measured by using linear optical sampling, with assist of polarization multiplexed probe pulse. By using the eigenvalue analysis of the STM, the differential mode delay and PMD vector of polarization-maintaining fiber are analyzed as a function of optical frequency over 1THz. We show that the amplitude averaging of the complex impulse responses is effective for enhancing the signal-to-noise ratio of the measurement, resulting in improving the accuracy and expanding the bandwidth of the measurement.

We propose a technique based on Brillouin optical time domain analysis for measuring loss and crosstalk in few-mode fibers (FMFs). The proposed technique extracts the loss and crosstalk of a specific mode in FMFs from the Brillouin gains and Brillouin gain coefficients measured under two different conditions in terms of the frequency difference between the pump and probe lights. The technique yields the maximum loss and crosstalk at a splice point by changing the electrical field injected into an FMF as the pump light. Experiments demonstrate that the proposed technique can measure the maximum loss and crosstalk of the LP11 mode at a splice point in a two-mode fiber.

Terahertz (THz) band is an attractive candidate for future broadband (> 10 Gb/s) wireless backhaul and fronthaul. THz transmitter employing optical frequency comb can provide high quality THz carrier, and is useful to the future broadband THz communication systems based on coherent transmission technique. To realize coherent transmission, high quality carrier generation is essential and it is important to evaluate the signal quality of a THz transmitter. In this paper, we derive error vector magnitude (EVM) including optical impairments (optical amplifier noise, laser phase noise, optical crosstalk and IQ imbalance of optical modulator) of the optical frequency comb based transmitter. The calculated EVM is in good agreement with simulated one, and practical requirements for optical impairment are indicated. The analysis will be useful in the design of THz transmission systems employing an optical frequency comb.