We propose and demonstrate the use of multimode fibers (MMF) inside a laser cavity as a new path to generate spatiotemporal modelocked pulses with high beam quality and high energy. Prior to our work, MMFs in optical cavities resulted in the generation of low-quality output beam profiles by spatiotemporal mode-locking. Here we present a versatile approach to reach high energy per pulse directly in the mode-locked MMF oscillator with a near single-mode output beam profile. Our approach relies on spatial beam self-cleaning via the nonlinear Kerr effect inside the cavity achieved by controlling spatiotemporal pulse propagation with a dispersion-managed design. We demonstrate the versatility of our approach with Yb-doped and Er-doped multimode laser cavities which generate pulse energies of 24 nJ and 16 nJ, respectively. The high peak power reached in the MMF within the cavity induced a Kerr self-beam cleaning which produced a near Gaussian mode output (M2<1.13).
KEYWORDS: Neural networks, Signal generators, Data modeling, Gold, Embedded systems, Diffusers, Detection and tracking algorithms, Control systems, Brain
We propose a general neural-network based learning framework to solve highly ill-posed problems to predict a system’s forward and backward response function. Such an approach has applications in target-oriented system’s control in fields such as, optics, neuroscience and robotics. The proposed method is able to find the appropriate continuous space input of a system that results in a desired output, despite the input-output relation being nonlinear, the system being time-variant and\or with incomplete measurements of the systems variables and lack of labeled data required for supervise learning.
We propose an imaging method for controlling the output of scattering media such as multimode fibers using machine learning. Arbitrary images can be projected with amplitude-only calibration (no phase measurement) and fidelities on par with conventional full-measurement methods.
We present a fiber-based 3D printing system with the highest printing resolution. Fabricated lines and microstructures are presented too. A microlens (NA 0.9) is printed in the Nanoscribe 3D-printer and attached to the fiber. A femtosecond laser (780nm, 80MHz) is used for two-photon polymerization and the focused spot on the tip of the lens is generated by wavefront shaping with the Transmission Matrix method. The spot size (lens with NA 0.9) is compared to the spot size of the fiber (NA 0.29) and to the spot size obtained when a commercially available microlens is attached (NA 0.54). Exposure tests are conducted by printing lines or voxels on a photosensitive resin and finally a prepared high aspect ratio structure is used for demonstrating the utility of a fiber-3D printing system to reach confined areas for microfabrication.
The performance of fiber mode-locked lasers is limited due to the high nonlinearity induced by the spatial confinement of the single-mode fiber core. To massively increase the pulse energy of the femtosecond pulses, amplification is performed outside the oscillator. Recently, spatiotemporal mode-locking has been proposed as a new path to fiber lasers. However, the beam quality was highly multimode, and the calculated threshold pulse energy (>100 nJ) for nonlinear beam self-cleaning was challenging to realize. We present an approach to reach high energy per pulse directly in the mode-locked multimode fiber oscillator with a near single-mode output beam. Our approach relies on spatial beam self-cleaning via the nonlinear Kerr effect, and we demonstrate a multimode fiber oscillator with M2 < 1.13 beam profile, up to 24 nJ energy, and sub-100 fs compressed duration. Nonlinear beam self-cleaning is verified both numerically and experimentally for the first time in a mode-locked multimode laser cavity. The reported approach is further power scalable with larger core sized fibers up to a certain level of modal dispersion and could benefit applications that require high-power ultrashort lasers with commercially available optical fibers.
Volume holography has been widely investigated for information storage and other applications. An increase in the number of multiplexed holograms leads to dynamic range loss when they are optically recorded. Computer generated holograms can achieve better performance if constructed voxel-by-voxel or as a multilayer structure. Advancements in 3D printing enabled the fabrication of multilayered diffractive elements in the micro-scale. To obtain an accurate design, we deploy the Learning Tomography (LT) method, which is an optimization algorithm for computationally imaging 3D distribution of the refractive index. Here, instead of imaging an object, we define a 3D structure that achieves a desired functionality.
We present the first spatiotemporally mode-locked fiber laser with self-similar pulse evolution, to the best of our knowledge. Our multimode fiber laser produces amplifier similaritons with near-Gaussian beam quality (M2<1.4) at the output. Ytterbium based laser generates 2.3 ps pulses at 1030 nm with 2.4 nJ energy. The output pulses are externally compressed to 192 fs with a grating compressor. Intracavity large spectral breathing (>6) and less chirped pulses than the cavity induced total dispersion are the verifications of the spatiotemporal self-similar pulse propagation.
Image delivery through multimode fibers (MMFs) suffers from modal scrambling which results in a speckle pattern at the fiber output. In this work, we use Deep Neural Networks (DNNs) for recovery and/or classification of the input image from the intensity-only images of the speckle patterns at the distal end of the fiber. We train the DNNs using 16,000 images of handwritten digits of the MNIST database and we test the accuracy of classification and reconstruction on another 2,000 new digits. Very positive results and robustness were observed for up to 1 km long MMF showing 90% reconstruction fidelity. The classification accuracy of the system for different inputs (phase-only, amplitude-only, hologram intensity etc.) to the DNN classifier was also tested.
We demonstrate multimode fiber probe that accomodates dual modality properties for high power ultrashort pulse delivery. A commercially available multimode graded-index (GRIN) fiber is used for two-photon imaging and/or femtosecond laser ablation of Cochlea hair cells. Lensless focusing and digital scanning of ultrashort pulses through the optical fiber is realized using the transmission matrix technique. We investigate the performance and the limitations of the GRIN probe in terms of focusing efficiency and peak power delivery. Selective laser ablation guided by the twophoton image obtained through the GRIN fiber is realized by proximally-only control of the femtosecond laser beam.
KEYWORDS: Additive manufacturing, Endoscopy, Manufacturing, Single photon, Multimode fibers, 3D printing, Graded index fibers, Energy transfer, Diffraction, Digital micromirror devices
We show that a multimode fiber which can be either a graded index fiber or fiber bundle can be used to deliver shaped light to build useful complex parts in areas difficult or impossible to reach with conventional manufacturing tools. We will show complex objects of micrometer scale that are made by additive manufacturing with either a single photon or a 2 photon process. The large effective core area of the multimode fiber allows two orders of magnitude higher pulsed energy transfer while maintaining a spatial and temporal diffraction limit. This enable both subtractive and additive manufacturing.
We demonstrate high power ultrashort pulse delivery through a commercially available multicore fiber (MCF) and a multimode graded-index fiber (GRINF) for imaging and laser ablation. Lensless focusing and digital scanning of ultrashort pulses through the optical fibers is realized using wavefront shaping. We compare the performance of the two systems in terms of focusing efficiency and peak power delivery. Furthermore, we investigate the limitations that nonlinearities induce when high peak power ultrashort pulses are launched in MCFs and GRIN fibers. Proximally-only controlled two-photon fluorescence imaging and laser ablation are demonstrated through both investigated systems.
The need for ultrathin fiber-based devices that can deliver light to confined places in order to perform imaging and/or laser ablation of a desired target has been a research area of significant interest. The current endoscopic devices are based on distal optics and scanning mechanisms to focus and scan the light in the end of the fiber. The distal components are limiting factors for decreasing the size of the device. However, using wavefront shaping techniques, lensless focusing and scanning of a laser focus spot through the fiber can be achieved, enabling a smaller endoscopic tool. In our case, a high power focus spot is created by wavefront shaping of the light through a multicore fiber (MCF), providing the possibility of two-photon fluorescence (TPF) imaging. Femtosecond laser ablation through the endoscopic device can be also a powerful tool for a range of applications. Therefore, we investigate limitations in the maximum peak power that can be delivered through the MCF due to nonlinear effects induced in the fiber cores in the ablation peak power regime. After characterizing the capabilities of our system, we demonstrate that femtosecond pulsed laser ablation can be performed through the MCF.
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