In this paper, we performed a comprehensive evaluation on the performances of this improved technique. For this evaluation, we first implemented a highly nonlinear transmission link consisted of 640-km long standard single-mode fiber (SMF) and dispersion-compensating fiber (DCF) modules, and measured the OSNR of eight WDM channels for over 10 hours. The results showed that the improved polarization-nulling technique could measure the OSNR with accuracy better than ±0.5 dB even when we intentionally increased the signal powers incident on the SMF and DCF to 0 dBm/channel (to invoke large nonlinearities). We also measured the performance of the improved technique in an ultra long-haul transmission link (~2000 km) by using a re-circulating loop. Even after the transmission over such a long distance, there was no observable error in the measured OSNR. The performance of the improved polarization-nulling technique was not affected by PMD as long as the total PMD was less than 60 ps.