arXiv subjects

Hongxin Zou

Publications and source records attributed to Hongxin Zou.

5 recordsLinked to original sources

arXiv2024

Superposition- and interference-induced optical spectrum distortion in the figure-9 fiber laser

The output pulse spectra of the figure-8 and figure-9 lasers typically exhibit more pronounced distortion than those from mode-locked lasers based on other saturable absorbers, as well as the spectra of their own intracavity pulses. Here, we demonstrate two figure-9 lasers with repetition rates of 190.6 MHz and 92.4 MHz and introduce a self-designed beam splitter that exhibits minimal spectral filtering into the fiber loop to output two interference-free pulses. By applying superposition and interference calculations to the experimental spectra of these two pulses, we obtained calculated spectra and found that their characteristics agree with the distortion features observed in the experimental spectra from the other two ports where superposition and interference occur. Therefore, we conclude that the severe spectral distortion is caused by spectral superposition and interference, rather than the commonly believed nonlinear effects. Furthermore, analysis based on the interference theory of the figure-9 laser reveals that the $p$-components of the two intracavity light beams usually interfere with non-equal intensity at the beam splitter where interference occurs, while the $s$-components always interferes with almost equal intensity. This mechanism results in a significant yet stable spectral difference between the intracavity and output pulses. Moreover, a change in the pump power can amplify the difference between the two $s$-components, thereby leading to the emergence of a minor peak at the optical spectrum center. These findings provide new perspectives for simulating spectra that closely resemble experimental results and deepen our understanding of spectral evolution and pulse dynamics of the figure-9 lasers.

physics.optics↗

arXiv2024

In-situ aligned all-polarization-maintaining Er-doped fiber laser mode-locked by a nonlinear amplifying loop mirror

Despite the wide applications for high-repetition-rate mode-locked fiber lasers, challenges persist in shortening the cavity length and coupling the fiber collimators for most existing techniques. Here, we introduce a novel collimator alignment method and demonstrate an all-polarization-maintaining erbium-doped fiber laser that contains a nonlinear amplifying loop mirror with a repetition rate of 213 MHz. Compared to the conventional method, we achieve in-situ alignment of the collimators in a simplified two-step process. Besides, through a comparison of the spectra from the output ports of the laser, we assess their quality and establish the spectral evolution relationships among these ports. It is found that, in addition to the widely believed large nonlinear effects, spectral interference also plays a significant role in spectral distortion. Moreover, a transition between different stability states is observed from the power variation of the single pulse.

physics.optics↗

arXiv2024

Observation of periodic optical spectra and soliton molecules in a novel passively mode-locked fiber laser

Due to the necessity of making a series of random adjustments after mode-locking in most experiments for preparing soliton molecules, the repeatability of the preparations remains a challenge. Here, we introduce a novel all-polarization-maintaining erbium-doped fiber laser that utilizes a nonlinear amplifying loop mirror for mode-locking and features a linear shape. This laser can stably output soliton molecules without any additional adjustment once the mode-locking self-starts. Moreover, it can achieve the transition from soliton molecule state to soliton state, and then to multi-pulse state by reducing the pumping power. The unconventional method of generating multi-pulses, combined with a wide pumping power range of 200--640 mW for maintaining mode-locking, allowed us to observe periodic optical spectra with two complete cycles for the first time. Based on the experimental facts, we develop a multistability model to explain this phenomenon. With its ability to switch between three stable states, this flexible laser can serve as a versatile toolbox for studying soliton dynamics.

physics.optics↗

arXiv2022

Inverse saturable absorption mechanism and design optimization of mode-locked fiber lasers with a nonlinear amplifying loop mirror

From the point of view of the differential phase delay experienced by the two counterpropagating optical fields, the self-starting of the mode-locked fiber laser with a nonlinear amplifying loop mirror (NALM) is theoretically studied. Although it is generally believed that NALM shows a saturable absorption effect on both continuous wave (CW) light and pulses, we find a counter-intuitive fact that cross-phase modulation (XPM) leads to opposite signs of differential nonlinear phase shifts (NPSs) in these two cases, resulting in inverse saturable absorption (ISA) during pulse formation process. The ISA is not helpful for the self-starting of laser mode-locking and can be alleviated by introducing a non-reciprocal phase shifter into the fiber loop. In addition, we analyze the influences of gain-fiber position, splitting ratio, and optical attenuator in the fiber loop, on the differential NPS and self-starting process. These results are helpful for optimizing the design of NALM and lowering the self-starting threshold of the high-repetition-rate mode-locked fiber laser.

physics.optics↗

arXiv2010

Experimental study on discretely modulated continuous-variable quantum key distribution

We present a discretely modulated continuous-variable quantum key distribution system in free space by using strong coherent states. The amplitude noise in the laser source is suppressed to the shot-noise limit by using a mode cleaner combined with a frequency shift technique. Also, it is proven that the phase noise in the source has no impact on the final secret key rate. In order to increase the encoding rate, we use broadband homodyne detectors and the no-switching protocol. In a realistic model, we establish a secret key rate of 46.8 kbits/s against collective attacks at an encoding rate of 10 MHz for a 90% channel loss when the modulation variance is optimal.

quant-ph↗