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

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.

I. Introduction

High-resolution optical spectroscopy has attracted considerable attentions due to its extensive applications, including sensitive and non-intrusive detection of gas molecule [1]–[4], characterization of physical and biochemical optical sensors [5], [6], and performance evaluation of narrow-linewidth laser sources and optical combs [7], [8], etc. Especially, for the measurements of transmission intensity and phase features of narrow-bandwidth microcavities [9], which are essential components for optical modulators and highly-sensitive sensors, an ultrahigh spectral resolution of sub-MHz is necessary, which puts forward strict requirements for optical spectroscopy.