Phase-Dispersion Spectroscopy With High Spectral Resolution Using a Wideband Ultra-Linearly Swept Optical Source

Abstract

We propose a phase-dispersion spectroscopy with high spectral resolution by developing a wideband ultra-linearly swept optical source (ULSOS). Highly precise optical frequency sweeping is achieved by externally modulating a narrow-linewidth fiber laser with a linearly swept radio-frequency signal. By using the injection-locking technique and high-order four-wave-mixing process, the sweeping span is enlarged to be 98 GHz, while the sweeping nonlinear error is as low as 136 kHz. Benefiting from the high-performance of the ULSOS, a spectrometer with an ultrahigh spectral resolution (136 kHz, determined by the sweeping nonlinear error of the ULSOS) is developed. Besides, an unbalanced Mach–Zehnder interferometer is used in the system with sample under test (SUT) incorporated inside it, and a phase extraction algorithm is employed to characterize the phase-dispersion features of the SUT. Moreover, a sweeping nonlinearity compensation technique is utilized to remove the noise in the measured transmission phase caused by the sweeping nonlinear error of the ULSOS. As a proof of the concept, the transmission intensity and phase spectra of HCN gas and fiber resonators with MHz-level bandwidth are characterized. The proposed technique opens a new possibility for spectroscopy with sub-MHz spectral resolution and the capability of dispersion measurement, without the usage of costly mode-locked laser.