Since 1.7 μm lasers are located in the eye-safe wavelength band and also within the fingerprint absorption peaks of many important gas molecules, they have potentially important applications in biomedical, gas sensing, and other fields. Meanwhile, as a novel structured light field, vortex beams can have unique features like annular light intensity distribution, helical phase wavefront, and orbital angular momentum. Therefore, developing high-performance 1.7 μm vortex lasers and investigating involved technologies can further expand the application fields of the lasers, providing scientific significance and application prospects. It is generally difficult for traditional rare-earth-ion doped fibers and crystals to cover the 1.7 μm emission band, or they can only have very weak laser gain in this wavelength band. Additionally, the vortex beam generation usually relies on a free-space lasing structure. These factors ultimately result in a complex vortex lasing configuration operating in the 1.7 μm band with extremely poor integration and low output power. Thus, we employ a helical long-period fiber grating as a vortex mode converter, and propose a high-power all-fiber vortex laser based on a 1.7 μm random fiber laser (RFL) with half-opened cavity, producing a maximum output power of 2.09 W at 1690 nm. Benefiting from the all-fiber structure of the vortex RFL, the laser output shows excellent temporal stability with a short-term temporal fluctuation as low as 2.8%. The results can not only provide a feasible approach to achieve a compact 1.7 μm high-power vortex laser with excellent temporal stability, but also further expand its applications in laser medicine, gas detection, optical tweezers, biological imaging, and other fields.
First, a 1.7 μm high-power RFL is constructed based on the stimulated Raman scattering effect. Then, a helical long-period fiber grating is adopted as a vortex mode converter with the vortex mode conversion efficiency of about 97% corresponding to 16 dB, which can convert the 1.7 μm random lasing into a first-order vortex beam. In this sense, a 1.7 μm high-power vortex RFL with an all-fiber structure is achieved, with the maximum output power of 2.09 W and central wavelength of 1690 nm. Benefiting from the all-fiber structure of the vortex RFL, the whole lasing system has a compact configuration with sound integration and simple thermal management and thus can achieve high-power vortex beam output. Additionally, the vortex RFL shows excellent temporal stability (short-time temporal fluctuation as low as 2.8%), modeless resonant output, and low relative fluctuations. It is expected that the output power of the vortex RFL can be further enhanced by increasing the incident power of the 1.7 μm RFL and optimizing the performance of the helical long-period fiber grating.
The 1.7 μm high-power vortex random lasing is realized based on a 1.7 μm RFL and a helical long-period fiber grating. The maximum output power is 2.09 W and the central wavelength is 1690 nm (Fig. 3). Furthermore, Fig.3(b) shows the relationship between the output power and the slope efficiency of the 1.7 μm vortex RFL and the incident power. The output power of the vortex RFL increases almost linearly without obvious saturation signs for the whole power scaling range. By increasing the injection power of the 1.7 μm RFL and replacing the helical long-period fiber grating with better performance, the output power of vortex RFL can be further enhanced. Meanwhile, the topological charge of the vortex RFL is characterized based on a homemade Mach-Zender interferometer, where the vortex laser output is interfered with a reference beam (or the spherical wave). The topological charge is measured to be one, which means the first-order vortex beam (Fig. 4). Finally, the short-time lasing characteristics and the radio frequency (RF) spectrum of the 1.7 μm vortex RFL at the highest output power of 2.09 W are measured. Thanks to the inherent excellent temporal stability and modeless resonant output characteristics of random fiber lasing, the 1.7 μm vortex lasing output inherits the intrinsic advantages of RFL, exhibiting very low short-time temporal fluctuations of 2.8% without resonant cavity frequencies in the RF spectrum (Fig. 5).
We propose a 1.7 μm high-power vortex RFL with an all-fiber structure. The 1.7 μm high-power vortex RFL is realized based on a RFL with a half-open cavity and the helical long-period fiber grating. The maximum output power is 2.09 W and the central wavelength is 1690 nm. The vortex RFL shows excellent temporal stability, low relative intensity fluctuation, and modeless oscillation output. The short-time temporal fluctuations are as low as 2.8%. By increasing the injection power of the 1.7 μm RFL and replacing the helical long-period fiber grating with better performance, the output power of the vortex RFL can be further increased. The vortex RFL with higher topological charges can be realized by simply replacing the corresponding helical long-period fiber grating. This work provide a feasible scheme for the realization of high-performance 1.7 μm vortex lasers, which is expected to be applied to laser medicine, gas detection, optical tweezers, and bio-imaging fields.
A semiconductor saturable absorber mirror (SESAM) has the advantages of self-starting, easy integration, wide wavelength coverage, support for all-solid-state laser technology, fast saturation, compact structure, and flexible design. It has become a Q-switched and mode-locked element for various types of lasers such as solid-state, fiber, and semiconductor lasers. Recently, the rapid development of picosecond Yb-doped fiber lasers and their wide application in industrial processing have heightened interest in SESAM applied to Yb-doped fiber lasers. The technology of designing and epitaxial growing SESAM has been relatively mature abroad, and the development of SESAM has been carried out in China in recent years, but the research on SESAM devices in China mainly focuses on solid-state lasers, and there are few reports on the development and characterization of SESAM for fiber lasers. In the present study, we report the effects of the quantum well period numbers in the absorption region on the field distribution, modulation depth, and reflection spectrum of SESAM, and the key characteristic parameters of SESAM are characterized, which has important reference value for the further study of SESAM.
In order to improve the characteristic parameters of multi-quantum well semiconductor saturable absorption mirror (SESAM) for fiber lasers, the effects of different quantum well period numbers on the field distribution, modulation depth, and reflection spectrum of the device were analyzed. The epitaxial growth of three kinds of quantum well structures with different period numbers of 7, 15, and 30 quantum wells was carried out by metal-organic compound vapor deposition (MOCVD) method. The reflectance spectra of the samples were measured by spectrophotometer, and the nonlinear test and mode-locking experiments were carried out on the developed three kinds of SESAM structures. The dynamic response of SESAM structures was tested by pump detection technology.
The simulation calculates the electric field distribution of the semiconductor saturable absorption mirror at 1064 nm (Fig. 2). When the complete electric field wave is present in the saturable absorption region, there are always peaks and troughs in the absorption region. Reflectance is calculated for saturable absorption mirrors of different quantum well structures (Fig. 3). The results show that the lowest reflectivity of the three structures is at 1064 nm, and more periods of quantum wells indicates lower reflectivity of SESAM at 1064 nm and higher modulation depth. Nonlinear tests and mode locking experiments are performed on epitaxial sheets of the three structures after growth (Fig. 7). The test results show that the SESAM of the three structures realizes self-starting mode locking, and the pump interval of stable mode locking is 150-200 mW. Pump detection of the SESAM of 15 quantum well structures yields a recovery time of 5 ps (Fig. 8).
By simulating the calculation of the light field distribution of SESAM of different periods, it is found that when the number of quantum wells is large enough, there is a complete standing wave in the absorption zone generated by the incident light field, and the number of standing waves increases with the thickness of the absorption layer. The reflectance of the saturable mirror of the subtrap structure with different number of periods is calculated. The results show that the reflectance of SESAM decreases gradually at 1064 nm with the increase in the number of periods of the absorption layer quantum well, and the bandwidth at low reflectance becomes narrower, which also means that the tolerance of the growth error of SESAM is also smaller. By using MOCVD technology, epitaxial growth of three SESAM structure samples with different quantum well period numbers is carried out, and nonlinear testing and mode-locking experiments are carried out on the grown samples. The results show that the three SESAM structures tested all realize self-starting mode-locking, and the pump range of stable mode-locking is 150-200 mW. When the pump power is less than 150 mW, stable mode-locking cannot occur. When the pump power is more than 200 mW, the mode-locking pulse appears double pulse phenomenon. For resonant SESAM, although increasing the number of quantum wells can increase the modulation depth of the SESAM, too many quantum wells are more likely to deviate from the design value in the epitaxial growth process. The number of quantum wells has little effect on the saturation flux, and the improvement of saturation reflectance is very limited. The narrowest mode-locking pulse width of 7 quantum well structure samples is about 20 ps; the narrowest mode-locking pulse width of 15 quantum well structure samples is about 11 ps, and the narrowest mode-locking pulse width of 30 quantum well structure samples is about 8 ps. The dynamic response of 15 quantum-well SESAM structures is tested using pump detection technology, and the response recovery time is measured to be 5 ps.
Secure communication based on chaotic laser has received much attention in recent years because of its high speed, long distance, and compatibility with existing fiber-optic networks. Much effort has been devoted to improving the rate of chaotic secure communication by increasing chaos bandwidth or using higher-order modulation. Unfortunately, there still exists a rate gap between the chaotic secure communication and the current fiber-optic communication. Polarization division multiplexing of chaotic laser is a potential alternative to reduce the rate gap. The key to implementing the polarization division multiplexing-based chaotic secure communication is establishing high-quality chaos synchronization. However, the influences of polarization of chaotic laser, i.e., the degree of polarization (DOP), on the chaos synchronization are not ascertained clearly. In this paper, the effects of DOP of chaotic laser on the synchronization quality are investigated experimentally, and the optimization methods and conditions are achieved for yielding high-quality and stable chaos synchronization. This work underlies the high-speed chaotic secure communication using polarization division multiplexing.
Firstly, we generate a chaotic laser from the master laser subject to mirror optical feedback and use the polarization controller and polarization beam splitter to make the chaotic laser characterized with a single polarization. Then, we inject it unidirectionally into the slave laser over the fiber link to achieve the single-polarization master-slave open-loop chaos synchronization. The polarization controller can adjust the state of polarization of the chaotic laser, and the DOP can be analyzed quantitatively by detecting the power from the output ports of the polarization beam splitter. Based on this experimental system, we examine the evolution of DOP and analyze its effect on the synchronization quality over time for fiber links with different transmission distances, when the threshold point (0.90) and the critical saturation point of high-quality synchronization are selected as the initial states. By changing the DOP of the chaotic laser in an experiment, we ascertain the effects of DOP on the effective injection strength and the quality of master-slave chaos synchronization firstly; then we analyze the evolution trend of DOP and its effect on the effective injection intensity and the quality of chaos synchronization within 60 minutes. Finally, the trend of DOP of the chaotic laser as a function of distance and time, as well as its effect on the quality of master-slave chaos synchronization are studied.
We experimentally achieve master-slave chaos synchronization by injecting single-polarization chaotic laser from the master laser into the slave laser through a polarization beam splitter, and chaos synchronization with synchronization coefficients of 0.986 and 0.962 is achieved under back-to-back and 200 km scenarios, respectively (Figs. 2 and 3). By comparing the back-to-back and 200 km transmission scenarios, we find that the quality of master-slave synchronization degrades under 200 km transmission with the same injection strength (Fig. 4), which is due to the distortion of chaotic laser caused by chromatic dispersion and enhancement of nonlinear effects. It is also found that the DOP of chaotic laser changes with time after a long-distance transmission, which reduces the injection efficiency of the master laser to the slave laser (Figs. 5-7). As a result, the effective injection strength is decreased, and the quality of master-slave chaos synchronization is degraded. In addition, we select the threshold point and the critical saturation point of high-quality synchronization as the initial states and observe the evolution of DOP and synchronization quality over time after transmission with different distances. It is found that under a similar variation of DOP and the same transmission distance, the chaos synchronization degrades less and is more stable for the initial state under the critical saturation point, compared with the initial state of the threshold point. It is noted that the deterioration of DOP originates mostly from the shape defect of fiber, as well as the vibration and temperature variation in the environment. Optimizing the fabrication technology of fiber, reducing vibration, and stabilizing temperature will all help to mitigate the deterioration of DOP. In addition, a polarization tracker can also be used to optimize the DOP in real time.
In this paper, the evolution of DOP of chaotic laser and its effect on the chaos synchronization quality, as well as the corresponding optimization methods are explored experimentally in the master-slave open-loop configuration. Results show that the DOP of chaotic laser deteriorates gradually with the increase in transmission distance and time: the DOP is separately reduced by 0.253, 0.332, and 0.473 within 60 minutes when the chaotic laser is transmitted over 100 km, 200 km, and 280 km fiber links, respectively. The deterioration of DOP reduces the effective injection strength of the master laser to the slave laser and thus degrades the chaos synchronization quality. The enhancement of injection strength will increase the system tolerance to the variation of DOP and improve the robustness of chaos synchronization, affording a high-quality long-distance chaos synchronization. It is believed that this work paves the way for high-speed long-distance chaotic secure communication based on the polarization division multiplexing.
The X-ray free-electron lasers (FELs) have undergone a significant transformation in the fields of biology, chemistry, and material science. The capacity to produce femtosecond and nanoscale pulses with gigawatt peak power and tunable wavelengths down to less than 0.1 nm has stimulated the construction and operation of numerous FEL user facilities worldwide. Shanghai soft X-ray free-electron laser (SXFEL) is the first X-ray FEL user facility in China. Its daily operation requires precise control of the accelerator state to ensure laser quality and stability. This necessitates high-dimensional, high-frequency, and closed-loop control of beam parameters. Furthermore, the intricate demands of scientific experiments on FEL characteristics such as wavelength, bandwidth, and brightness make the control and optimization task of FEL devices even more challenging. This activity is usually carried out by proficient commissioning personnel and requires a significant investment of time. Therefore, the utilization of automated online optimization algorithms is crucial in enhancing the commissioning procedure.
A deep reinforcement learning method combined with a neural network is employed in this study. Reinforcement learning uses positive and negative rewards obtained from the interaction between agents and the environment to update parameters. It does not require input from the inherent nature of the environment and is not dependent on data sets. In theory, this methodology has the potential to be implemented in various scenarios to optimize any given parameter in online devices. We employ SAC, TD3, and DDPG algorithms to adjust multiple correction magnets and optimize the output power of the free electron laser in a simulation environment. To simulate non-ideal orbit conditions, the beam trajectory is deflected by a magnet at the entrance of the first undulator. In the optimization task, we set the current values of seven correction magnets in both horizontal and vertical directions as the agent's action. The position coordinates of the electron beam along the x and y directions of the undulator line after passing through the seven correction magnets are set as the environment's state. The intensity and roundness of the spot are used as evaluation criteria for laser quality. During the simulation, Python is used to modify the input file and magnetic structure file of Genesis 1.3 to execute the action. The status and reward are obtained by reading and analyzing the power output and radiation field of Genesis 1.3. For each step in the optimization process, the agent first performs an action and adjusts 14 magnet parameters to correct the orbit. At this time, the environment changes and returns a reward to the agent according to evaluation criteria for laser quality. The agent optimizes its action to maximize cumulative reward.
In the FEL simulation environment, we use SAC, TD3, and DDPG algorithms with parameters listed in Table 2 to optimize the beam orbit under different random number seeds. Figure 2 shows the training results of the proposed algorithm. As the learning process of SAC and TD3 algorithms progresses, the reward function converges, and the FEL power eventually reaches saturation. SAC and TD3 algorithms maximize FEL intensity at about 400 steps, with the convergence results of the SAC algorithm being better than those of the TD3 algorithm. This is because the TD3 algorithm, built on the DDPG algorithm, mitigates the impact of overestimation of action value on strategy updating and enhances the stability of the training process. The SAC algorithm maximizes the entropy while maximizing the expected reward, enhances the randomness of the strategy, and prevents the strategy from prematurely converging to the local optimal value. Furthermore, after convergence, the power mean of the SAC algorithm is noticeably more stable compared to that of the TD3 algorithm. Its confidence interval is also smaller, indicating better stability. The gain curve and initial curve of the three algorithms in the tuning task are shown in Fig. 3(a). The SAC algorithm approximately optimizes the output power from 0.08 GW to 0.77 GW, slightly higher than that of TD3 algorithm and significantly higher than that of DDPG algorithm. The optimized orbits and initial orbits of the three algorithms are shown in Fig. 3(b). Due to the deflection magnet applied at the entrance of the system and the drift section set, the beam is deflected and divergent in the first 2.115 m of the undulator structure, with the uncorrected orbits maintaining this state. The SAC, TD3, and DDPG algorithms all make adjustments to the orbits. Figure 3(b) shows that the orbits optimized by the SAC algorithm are closer to the center of the undulator, namely the ideal orbits, in both horizontal and vertical directions, which can also explain that the output power optimized by SAC is higher than that of TD3 and DDPG. To more directly reflect the results of orbit optimization, we compare the initial light spot at the outlet of the undulator with the optimized light spots of three algorithms (Fig. 4). The initial light spot is offset in both x and y directions and has weak intensity. However, the light spot optimized by SAC is completely centered in the undulator with the highest intensity, while it remains offset in the x direction for the other two algorithms.
We employ deep reinforcement learning techniques to simultaneously control multiple correction magnets to optimize the beam orbit within the undulator. The deep reinforcement learning approach acquires rules from past experiences, avoiding the need for training with a calibration dataset. In contrast to heuristic algorithms, this approach exhibits superior efficiency and less proneness to local optima. In this study, the SAC and TD3 algorithms have been shown to effectively optimize beam orbit and improve spot quality through the analysis of system state, reward balancing, and action optimization. Results of the simulation indicate that the TD3 algorithm effectively optimizes the laser power to 0.71 GW, thereby resolving the issue of bias that arises from overestimating the action value of DDPG. Furthermore, the SAC algorithm has been utilized to optimize laser power to a value of 0.77 GW, demonstrating a marked improvement in the learning efficiency and performance of DDPG. The SAC optimization is based on the maximum entropy principle and is indicative of improved training effectiveness and stability. Thus, the SAC algorithm exhibits strong robustness and holds the potential to be utilized for the automated light optimization of SXFEL.
In recent years, a novel InGaAs well-cluster composite (WCC) quantum-confined structure has been demonstrated that the special structure has excellent optical properties, which are important for the realization of ultra-wide tunable lasers and synchronous dual-wavelength lasers. The WCC structure is based on the self-fit migration of indium atoms caused by the indium-rich cluster (IRC) effect, which are typically regarded as defects to be avoided for the conventional InGaAs quantum-well structure. Therefore, its special optical characteristics remain neglected. The formation mechanism of this WCC structure is based on the migration of indium atoms under high strain background. The strain will gradually accumulate with the continuous deposition of InGaAs material thickness. In order to relax the high strain in the InGaAs layer, indium atoms would automatically migrate along the material growth direction and form IRCs after the InxGa1-xAs is grown to exceed the critical thickness on the GaAs. Therefore, how to effectively determine the critical thickness of indium atom migration is of great significance for the study of WCC structures. However, there is little research on the critical thickness of the WCC structure. The traditional measurement methods on quantum well thickness make it difficult to obtain the thickness fluctuations at different positions. Furthermore, it is not possible to accurately evaluate the critical thickness of indium atom migration in the asymmetric InxGa1-xAs WCC structure. Therefore, the critical thickness of indium atom migration is investigated by collecting spontaneous emission (SE) spectra from different positions in the WCC structure.
First, in order to study the critical thickness of indium atom self-fit migration in the IRC effect, an asymmetrical InGaAs WCC quantum confinement structure is grown on a GaAs substrate. Because IRCs generally occur in highly strained InGaAs/GaAs systems, the active layer used In0.17Ga0.83As/GaAs/GaAs0.92P0.08. The thickness of the In0.17Ga0.83As layer is designed to be 10 nm because an InGaAs layer thinner than 10 nm is insufficient to obtain the IRC effect. Second, in order to measure SE spectra, the sample is processed to obtain a 3.0 mm×1.5 mm configuration. The device is vertically pumped from a fiber-coupled 808 nm pulsed laser at room temperature. The pump beam is focused into a 0.2 mm diameter spot. The fiber coupler is used to collect the SE spectra emitted from the corresponding pumping region from the bottom of the WCC structure. The SE spectra from different positions of the WCC structure are measured by moving the sample. The SE spectra exhibit typical bimodal characteristics. The formation mechanism is that the self-fit migration of the indium atoms in the WCC structure would reduce the indium content in the corresponding InGaAs regions, consequently generating normal and indium-deficient InxGa1-xAs regions. The spectra with dual peaks come from the superposition of spectra emitted from the normal In0.17Ga0.83As layer and indium-deficient In0.12Ga0.88As layer with different band gaps. The intensity fluctuation of the dual peaks mainly depends on the thickness fluctuation of the two materials. Third, the critical thickness can be evaluated by comparing the intensity of dual peaks.
The self-fit migration of indium atoms leads to the formation of both normal In0.17Ga0.83As and indium-deficient In0.12Ga0.88As regions in the WCC structure. The bimodal configuration in the spontaneous emission spectra is a remarkable feature of the IRC effect taking place in the InGaAs-based WCC structure. The SE intensity mainly depends on the InxGa1-xAs material thickness L and the peak wavelength λ. Based on the dual peaks in SE spectra from different positions of the WCC structure, the intensity ratio of the dual peaks can be calculated, with a maximum intensity ratio of 1.2115 and a minimum value of 0.5968. The thickness of the In0.17Ga0.83As layer corresponds to 4.6 nm and 6.4 nm, respectively (Fig. 3). Due to the migration of indium atoms occurring after the thickness of the In0.17Ga0.83As layer reaches the critical thickness, the material within the critical thickness is normal In0.17Ga0.83As material. This means that as long as the growth thickness of the In0.17Ga0.83As layer does not exceed 4.6 nm, indium atoms will not migrate. This is because the strain accumulation is not sufficient to generate the IRC effect. In summary, the critical thickness for self-fit migration of indium atoms can be evaluated as approximately 4.6 nm. Finally, in order to illustrate the accuracy of this conclusion, the spontaneous emission spectrum of a 4 nm thick In0.17Ga0.83As/GaAs compressively strained quantum well is collected under the same injected carrier density. It is found that there is only one peak in the spectra (Fig. 4). The result indicates that indium atoms do not migrate to form IRCs in the 4 nm thick In0.17Ga0.83As/GaAs material. Although there is strain accumulation in the 4 nm thick In0.17Ga0.83As material, it is not enough to produce the IRC effect. Therefore, the bimodal configuration in spectra disappears. This is consistent with the experimental results, which demonstrate the relative accuracy of the conclusion.
In this paper, the critical thickness of indium atom migration in InGaAs asymmetric WCC quantum confinement structures is calculated by measuring the spontaneous emission spectra emitted from the different positions of the WCC structure. The SE spectra emitted from different pump regions are measured by focusing the pump beam on the local surface of the WCC sample. By analyzing the bimodal intensity and ratio of the SE spectra, the normal In0.17Ga0.83As layer thickness fluctuation of 4.6-6.4 nm is obtained. Furthermore, the critical thickness for the migration of indium atoms is determined to be approximately 4.6 nm. This research content has important value for the development and application of InxGa1-xAs asymmetric WCC quantum confinement structures.