Adaptive optics (AO) is widely used in the fields of ground-based telescopes, biological imaging, human eye aberration correction and laser communication to correct wavefront distortion. One of the key components of an AO system is a wavefront sensor (WFS). We demonstrate a 1×19 photonic lantern to be used as a single-aperture wavefront WFS. By detecting the power and the phase difference of 19 fundamental mode output at the single mode end of the photonic lantern, the lowest 19 order Zernike coefficients of the wavefront to be measured is determined. Simulation results show that when the wavefront distortion RMS< 1.5 π, the residual RMS between the reconstructed wavefront and the wavefront to be measured is always lower than 3.5 × 10−3 π, which provides a reliable technical means for realizing high-speed high accuracy and perfect common path wavefront sensing in AO system.
Adaptive mode control system based on photonic lanterns has been made great progress in achieving stable fundamental mode in large mode area fiber, but there are still difficulties in generating specific spatial mode, such as orbital angular momentum mode. In this paper, purity of the target mode which is obtained by optical correlation filtering method based on computational holograms is chosen to be the evaluation function of the stochastic parallel gradient descent algorithm. The schematic diagram of the adaptive mode control system is improved, which consists of the photonic lantern, phase modulators, optical correlation filtering, detector, controller, etc. A simulation model combining optical correlation filtering method based on computational hologram and stochastic parallel gradient descent algorithm is established, and the numerical simulation results show that the orbital angular momentum mode with topological charge of ±1 and ± 2 is realized, and the corresponding mode purity is above 95% by using this adaptive mode control method based on a 5×1 photonic lantern.
We propose a method to measure the mode coefficient in a few-mode fiber with OAM modes. In this method, the eigenmodes are calculated through the optical fiber parameters, including LP modes and orbital angular momentum (OAM) modes. Using a single spatial light modulator (SLM) to load multi-channel computer-generated hologram (CGH), multiple matched filters can be generated at the same time to realize the real-time and accurate measurement of each mode coefficient (mode content and relative phase) and reconstruct the optical field. We validate the method on multimode beams, and the accuracy of mode coefficients measurement is up to 99%. It can be used as adaptive control coefficient to provide solutions for the expansion of photon lantern and other technologies.
Photonic lanterns can control beam modes by tuning phase, amplitude and polarization of multiple beam inputs, providing new methods to overcome mode instability in high-power large-mode area fiber lasers. The initial amplitude input of photonic lanterns are usually preset to be equal or random. The beam mode is then adaptively controlled by evaluating the beam output profile. This method can easily realize stable light output, but it sometimes runs into local optimum solutions. In this work, we propose a novel method to preset the initial amplitudes to realize better beam mode control. Based on the waveguide characteristics of the photonic lantern structure, we inversely work out the initial amplitude requirement of the input channels for controlling a certain mode using its transmission matrix. Taking 3×1 photonic lantern as an example, our simulation results show that this method can realize any mode control of LP01, LP11e, LP11o with their mode power ratios all above 99%, better than those preset methods to be equal or random. Our simulation method can be utilized in beam control experiments to achieve better beam control in photonic lantern-based fiber lasers
We have made simulations for a 3*1 photonic lantern and the latter experiments were carried out to test characters of the lantern we designed. The three core lantern with 0.004 rad taper angle has a 15 mm taper length which begins with a 125 μm outer diameter and ends with a 10.4 μm outer diameter(core diameter to outer diameter ratio is 2/25), while the NA of single mode fiber is 0.08 and for 25/400 μm core/cladding diameter few mode fiber it is 0.1. In order to obtain preferable LP01 mode shape, we used a phase control system to lock the phase of each channel staying consistent. The consequence demonstrated that the whole phase distribution of single mode fibers was controlled to an extent. Compared to amplitude, photonic lantern is more sensitive to phase fluctuation. We made simulations using Rsoft to test different variables of light propagating in 3*1 photonic lantern, and the results were in agreement with the conclusion. Thus, coherent beam combination based on dithering phase locked technique was studied systematically and we applied it to 3*1 photonic lantern. As a result, the beam received by light spot analyzer showed a stable mode shape but still existing some differences compared to expected LP01 mode or LP11 mode, which put forward reflection about the basic theory inside photonic lanterns (the beam propagating through tapered region didn’t make sense if using coherent beam combination calculation). At last, we attempted to explore the beam propagation through the lantern and to give any better evaluation function in phase control algorithm so as to improve the phase locked process.
The photonic lantern is an all-fiber-based linear optical element that couples light efficiently and evolves modes functionally between a set of single-mode waveguide and a multimode waveguide. Our study is based on different core photonic lanterns, which are fabricated by using the ‘ferrule technique’ method, with the single-mode array in a certain geometric arrangement. The process of the light propagation through the lanterns is simulated and the experiments according to the simulation results are conducted. The mode combining and evolution in the 3-core lanterns is simulated to study the necessary conditions for achieving the fundamental mode with good beam quality in large-mode area (LMA) fiber. Appropriate input (the amplitude ratio of each channel is 1:1:1, the phase and polarization state are the same) is injected at the single-mode (SM) end with some random disturbance on amplitude and polarization (the relative change is 20%). The M2 factor at the multimode (MM) end has a standard deviation of 0.0001 orders of magnitude. However, the M2 factor varies from 1.05 to 2 or even more with the changes of the phase difference at the SM end. Thus, the adaptive optics (AO) technique is used in our experiment, which can adaptively determine the appropriate phase to be applied to the input fibers. In addition, the modes behavior in 6- and 7-core photonic lanterns is simulated to obtain the ideal input (including the amplitude, phase and polarization of each SM channel) for achieving the six lowest order modes output. And these two kinds of photonic lanterns are compared from the aspects of drawing difficulty, mode coupling efficiency, transmission loss, and application feasibility in module multiplexing.
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