Resource-Efficient Direct Characterization of General Density Matrix.

Sequential weak measurements allow for the direct extraction of individual density-matrix elements, rather than relying on global reconstruction of the entire density matrix, which opens a new avenue for the characterization of quantum systems. Nevertheless, extending the sequential scheme to multiqudit quantum systems is challenging due to the requirement of multiple coupling processes for each qudit and the lack of appropriate precision evaluation. To address these issues, we propose a resource-efficient scheme (RES) that directly characterizes the density matrix of general multiqudit systems while optimizing measurements and establishing a feasible estimation analysis. In the RES, an efficient observable of the quantum system is constructed such that a single meter state coupled to each qudit is sufficient to extract the corresponding density-matrix element. An appropriate model based on the statistical distribution of errors is utilized to evaluate the precision and feasibility of the scheme. We have experimentally applied the RES to the direct characterization of general single-photon qutrit states and two-photon entangled states. The results show that the RES outperforms sequential schemes in terms of efficiency and precision in both weak- and strong-coupling scenarios. This Letter sheds new light on the practical characterization of large-scale quantum systems and the investigation of their nonclassical properties.

Few-mode random fiber laser with a switchable oscillating spatial mode.

Random fiber lasers are of tremendous interest to diverse applications for optical fiber sensing, speckle-free imaging. To date, random fiber lasers with fundamental mode oscillation have been well developed. However, controllable oscillating spatial mode in random fiber lasers have not been reported yet. Here, we propose and demonstrate a few-mode random fiber laser with a switchable oscillating spatial mode based on mode injection locking. An external signal light is injected to realize the locking of transverse mode in this random fiber laser and the direct oscillations of the fundamental mode, hybrid mode, and high order mode can be realized, respectively. This random fiber laser operates in the high-order LP11 mode stably with a threshold of as low as 88 mW. High efficiency and high purity cylindrical vector beams can be obtained by removing the degeneracy of the LP11 mode. This work may pave a path towards random fiber lasers with controllable spatial modes for specific applications in mode division multiplexing, imaging, and laser material processing.

Stable generation of cylindrical vector beams with an all-fiber laser using polarization-maintaining and ring-core fibers.

An all-fiber laser using polarization-maintaining and ring-core fibers that are capable of automatically generating stable TE01 and TM01 modes is proposed and demonstrated experimentally. Two vector-mode coupling long-period fiber gratings (LPFGs) fabricated by a high-frequency CO2 laser are used in the fiber laser to realize efficient coupling between HE11 mode and TE01/TM01 mode. The polarization dependence of the LPFGs is simulated using the coupled-mode theory and verified by experiments. A ring-core fiber is employed to support the stable propagation of TE01 and TM01 modes. By carefully aligning the polarization direction of the input light, the mode coupling ratios of both LPFGs exceed 15 dB. The mode purities of TE01 and TM01 modes are 92.4% and 97.3%, respectively. Owing to the all-polarization-maintaining structure, the laser output is highly stable under environmental disturbance. This laser can be used as a stable cylindrical vector beam source for a wide range of applications, including surface plasmon excitation, optical tweezers, high-resolution metrology and so on.

Generation of stable orbital angular momentum beams with an all-polarization-maintaining fiber structure.

In this paper, we propose a stable orbital angular momentum (OAM) mode fiber laser with an all-polarization-maintaining fiber (PMF) structure based on a combination of two linearly polarized modes. The mode intensity ratio between the two linearly polarized modes can be adjusted by adopting a double-pump structure. A pair of polarization-maintaining long-period fiber gratings (PM-LPFGs) are used as a mode converter. The number of topological charges of the OAM mode beam can be tuned between +1 and -1 by stretching the fiber. By adopting an all-PMF structure, we can build an OAM mode fiber laser without a polarization controller and that is resistant to environmental disturbances. The purity of the OAM mode was approximately 93.6%. This stable and compact OAM mode fiber laser can be used as a laser source in practical applications and scientific research.

Adaptive modal gain controlling for a high-efficiency cylindrical vector beam fiber laser.

We propose and experimentally demonstrate an ytterbium-doped fiber laser emitting the single high-order cylindrical vector beams with a high efficiency and a high modal purity based on adaptive modal gain control. By the combination of a high-order pump with a self-designed ytterbium-ring doped fiber, modal dependent gain was tailored and specific transverse mode can be selected in the laser cavity. A model based on multimode propagation-rate equations is built up to demonstrate the behaviors of transverse mode competition in the fiber laser. Modal dependent gain of high-order mode pump are simulated numerically, which agree well with our experiment results. The slope efficiency of the fiber laser reaches 79.61% with a low threshold of 47.73mw. The purity of the generated high-order CVBs are in excess of 95%. Such a strategy enables the controllability of modal gain in a fiber laser and reveals the potential to offer a new and promising way to achieve a high-power fiber laser with an arbitrary single high-order transverse modes output.

Generation of mode-locked square-shaped and chair-like pulse based on reverse saturable absorption effect of nonlinear multimode interference.

In this paper, the phenomenon of reverse saturable absorption of offset-spliced graded index multimode fibers (OS-GIMF) is revealed. And based on that, the stable square-shaped and chair-like mode-locked pulses are demonstrated with the maximum pulse energy of 0.14 µJ and 23.8 nJ respectively, while the OS-GIMF acts as a saturable absorber (SA) in fiber laser. By adjusting polarization controller (PC) and the pump power, square-shaped and chair-like pulse can be switched to each other. This multimode SA could sever as high power light source owing to its high damage threshold, compact structure and low cost.