This paper describes a signal quality monitoring technique for wavelength division multiplexing (WDM) signals that uses an ultrafast field sampling approach, where the observed signal is parallelly sampled with N channel coherent mixers that include a delay of nT , where n is the channel number and T is the unit delay. Previously, we demonstrated that the technique was able to monitor ON-OFF Keying WDM signals with the aid of a digital finite impulse response filter. In this paper we apply the technique to a dual-channel sampling configuration to monitor differential phase-shift keying (DPSK) WDM signals. We show that the dual 16-branch serial sampling of the total field allows us to observe an 8-ch 25 Gb/s DPSK signal with a total data rate of 200 Gb/s. In addition, the measurement result is comparable to the result obtained by using an optical filter. We also confirm that the technique is capable of monitoring the optical signal to noise ratio (OSNR) up to over 20 dB. Moreover, by adaptively changing the filter profile, we simulate the dependence of the OSNR on the filter profile. In this paper, the technique uses an equivalent sampling approach with a sampling pulse that has a repetition frequency (sampling rate) as low as 10 MHz, so the use of the technology is limited to monitoring the signal. The technique can be used to receive an actual data transmission if the sampling rate is 1/NT, where N is the number of channels.