In recent years, passively mode-locked fiber lasers (MLLs) have rapidly developed and been widely used in many application fields, such as optical frequency comb generation, high-resolution optical sampling, precision optical metrology, low noise microwave signal extraction, photonics-based radar system.

In these systems, the noise property should be improved as much as possible in order to assure the required system performance in terms of sensitivity and precision.

Over the past decades, theoretical analysis models related to noise characterization have been built. However, no detailed study on timing jitter and phase noise at different harmonics of a passively MLL has been done so far as we know, which is important in generating low noise high frequency microwave signal from a MLL and some other applications.

The research team led by Prof. WANG Yishan from Xi’an Institute of Optics and Precision Mechanics (XIOPM) has experimentally investigated the noise properties of a homemade 586 MHz mode-locked laser (MLL).

The variation of the timing jitter versus the harmonic order is measured, which is consistent with the theoretical analyses. The work was published in IEEE PHOTONICS JOURNAL.

They found that the measured timing jitter decreases with the increasing harmonic order at first and then remains constant with a ±20 fs fluctuation. The experimental data is well consistent with the theoretical fitting curve.

Based on the analyses we infer that for low harmonics the higher timing jitters are dramatically affected by intensity noise of the MLL and RIN-coupled jitters, while at high harmonic frequencies the ASE noise is the dominant contributor to timing jitter.

The comparison between the homemade MLL and a state-of-art commercial SG show that the OGRFS shows a better noise performance at high offset frequencies, however the higher phase noise at low offset frequencies leads to a higher timing jitter than the commercial SG.

With the further optimization, the RF signal from MLLs with high frequencies and low timing jitters would have great potential applications in modern radar systems.

This work was supported by the National Key R&D Program of China and the Natural Science Foundation of China.

                  Comparison of timing jitters at different harmonics for laser output ratios of 5% and 10% ( Image by XIOPM )