This paper describes a method to evaluate the modulated waveforms output by a high-speed external phase modulator over a wide wavelength range by using linear optical sampling (LOS) and a wavelength-swept light source. The phase-modulated waveform is sampled by LOS together with the reference signal before modulation, and the modulation waveform is observed by removing the phase noise of the light source extracted from the reference signal. In this process, the frequency offset caused by the optical-path length difference between the measurement and reference interferometers is removed by digital signal processing. A pseudo-random binary-sequence modulated signal is observed with a temporal resolution of 10 ps. We obtained a dynamic range of ∼40 dB for the measurement bandwidth of 10 nm. When the measurement bandwidth is expanded to entire C-Band (∼35 nm), the dynamic ranges of 37 ∼ 46 dB were observed, depending on the wavelengths. The measurement time was sub-seconds throughout the experiment.