Laijun Wang

Demonstration of almost octave-spanning cascaded four-wave mixing in optical microfibers

We demonstrate almost octave-spanning cascaded four-wave mixing (CFWM) in optical microfibers. Pumped by two synchronized picosecond lasers at about 850 nm, microfibers with a length of 10-20 cm can generate CFWM spanning from a few hundred nanometers to almost one octave, depending on the diameter of the microfibers and the detuning between the two pumps. CFWM in microfibers, which has the advantages of easy fabrication, highly efficient coupling, relatively short length, and easy integration with fiber systems, can be used for applications in widely tunable multiline phase-sensitive amplification, multiwavelength coherent sources, and ultrashort pulse synthesis.

Sub-kHz lasing of a CaF 2 whispering gallery mode resonator stabilized fiber ring laser

We utilize a high quality calcium fluoride whispering-gallery-mode resonator to passively stabilize a simple erbium doped fiber ring laser with an emission frequency of 196THz (wavelength 1530nm) to an instantaneous linewidth below 650Hz. This corresponds to a relative stability of 3.3 × 10(-12) over 16μs. In order to characterize the linewidth we use two identical self-built lasers and a commercial laser to determine the individual lasing linewidth via the three-cornered-hat method. We further show that the lasers are finely tunable throughout the erbium gain region.

Fiber-based multiple-access optical frequency dissemination.

We demonstrate a fiber-based, multiple-access optical frequency dissemination scheme. Without using any additional laser sources, we reproduce the stable disseminated frequency at an arbitrary point along the fiber link. Relative frequency stabilities of 3×10(-16)/s and 4×10(-18)/10(4) s are obtained. A branching fiber network for high-precision synchronization of optical frequency is made possible by this method, and its applications are discussed.

Purification of Sulfuric and Hydriodic Acids Phases in the Iodine-sulfur Process

Iodine-sulfur (IS) thermochemical water-splitting cycle is the most promising massive hydrogen production process. To avoid the undesirable side reactions between hydriodic acid (HI) and sulfuric acid (H2SO4), it is necessary to purify the two phases formed by the Bunsen reaction. The purification process could be achieved by reverse reaction of the Bunsen reaction. In this study, the purification of the H2SO4 and HI Phases was studied. The purification proceeded in both batches and the continuous mode, the influences of operational parameters, including the reaction temperature, the flow rate of nitrogen gas, and the composition of the raw material solutions, on the purification effect, were investigated. Results showed that the purification of the H2SO4 phase was dominantly affected by the reaction temperature, and iodine ion in the sulfuric acid phase could be removed completely when the temperature was above 130℃; although, the purification effect of the HI phase improved with increasing of both the flow rate of nitrogen gas and temperature.

High-accuracy measurement of the 113Cd+ ground-state hyperfine splitting at the milli-Hertz level.

We have developed a microwave frequency standard based on the 15.2 GHz ground-stated hyperfine transition of (113)C(+) ions. Using a laser-cooled ion cloud trapped in a linear quadrupole Paul trap, the clock transition frequency is measured to be 15 199 862 855.0125(87) Hz, with an accuracy at the 10(-13) level. The main errors are from the microwave frequency reference used in the experiment. The precision is improved by nearly two orders of magnitude than that reported before.

Ultra-stable radio frequency dissemination in free space

We demonstrate an ultra-stable radio frequency (RF) dissemination scheme over 80 m free space. The frequency dissemination stability is 3.2 × 10(-13)/s and 4.4 × 10(-17)/day, which can be applied to transfer frequency signal without compromising its stability in a global navigation satellite system (GNSS) or radio astronomy.

The three corner hat measurement of three hydrogen masers in remote locations via fiber based frequency synchronization network

As the first step of Beijing regional time and frequency network, Currently, Tsinghua University (THU) and the Changping site of National Institute of Metrology (NIM-Changping) have been linked and synchronized via the fiber network. The frequency stability of the hydrogen maser in NIM and the ultra-stable crystal oscillator steered by a cesium atomic clock in THU was measured. More atomic clocks from the Beijing Institute of Radio Metrology and Measurements, Hepingli site of NIM and Peking University (PKU) will join the synchronization network soon, with these clocks we can get the stability of each clock though three-corner-hat method.

REVIEW OF SIMULATION METHODS OF THE DISTILLATION COLUMN AND THERMODYNAMIC MODELS IN THE HI DECOMPOSITION SECTION OF THE IODINE-SULFUR PROCESS

The iodine-sulfur thermochemical water-splitting process (I-S process) is one of the most promising and efficient mass hydrogen production methods without greenhouse gas emissions. To a large extent, distillation in the HI decomposition section of the I-S process affects the efficiency of the entire process. This article summarizes the simulation methods for HI distillation in the iodine-sulfur process, including thermodynamic models, design schemes, and parameter analyses. The performance of the thermodynamic models and heat duties of different flow sheets were compared, and the preferable model and flow sheet are recommended. This study can provide a reference for the selection of thermodynamic models and it is also meaningful for the design and simulation of the HI distillation column.

Accurate and fast fiber transfer delay measurement based on phase discrimination and frequency measurement

An accurate and fast fiber transfer delay measurement method is demonstrated. As a key technique, a simple ambiguity resolving process based on phase discrimination and frequency measurement is used to overcome the contradiction between measurement accuracy and system complexity. The system achieves a high measurement accuracy of 0.2 ps with a 0.1 ps measurement resolution and a large dynamic range up to 50 km as well as no dead zone.

Improvement of the signal-to-noise ratio of the clock signal for the frequency standard based on 113 Cd + ions

Microwave frequency standard based on laser-cooled 113Cd+ ions has promising performance. The fluorescence decays during detection are one of main limits to the signal-to-noise ratio (SNR) of clock signal. This phenomena is assumed cause by detection laser. We try to verify this assumption by rate equations. Two methods to improve SNR are also proposed.