Exceptional points (EPs) in whispering-gallery-mode microresonators systems have attracted substantial attention due to their intriguing and anomalous optical characteristics. Recently, EPs have been experimentally observed in silicon microrings with coupling manipulation elements, such as an S-shaped waveguide and notch. In this paper, the observation of EP in a nanocylinder-loaded silicon microring is experimentally demonstrated. The device consists of a 5- μm microring with two nanocylinders placed close to the outer edge of the microring. By tailoring the size and position of the two nanocylinders, the fully asymmetric coupling between the clockwise- and counterclockwise-propagating modes occurs, leading to the implementation of EP. Experimentally, the spectral response is investigated by single-side excitation from clockwise and counter-clockwise directions. The reciprocal transmission and nonreciprocal reflection spectra are observed, which confirms the proposed device works in the vicinity of EP. The construction of EP in silicon microring paves the way to basic science and applied technology in non-Hermitian physics.
Thulium-doped fiber lasers have been of interest in many fields including medical surgery, remote sensing and material processing. Because of the nonlinear effect and photodarkening effect, it is difficult to achieve high power laser output in the monolithic laser cavity under continuous wave (CW) mode. In a laser diodes (LDs)-pumped monolithic fiber laser, a pulse laser with a higher peak power than that in CW mode can be realized through the key technique of “overshoot pump modulation”. By applying overshoot pump modulation to 6 LDs at 793 nm to pump the thulium-doped fiber in the laser resonator, we obtained a pulse laser output with a frequency of 10 Hz and a pulse duration of 10 ms. Spectral data revealed that the laser wavelengths in CW mode and pulse mode are both around 1940 nm. On this basis, a peak output power of 515 W in the pulse mode and an average output power over 50 W in the CW mode can be experimentally reached. The output laser pulse waveform is basically consistent with the pump waveform of the diodes, and there is a small delay between them. In this work, the frequency and pulse duration of quasi-CW thulium-doped fiber laser around 1940 nm will be continuously improved anyway as the next step.
Traditional endoscopes use fiber bundles to transverse the images and have the disadvantages of large size and poor flexibility. Unlike the single mode fibers just guide the fundamental mode, multi-mode fibers (MMFs) accommodate a large number of guided modes. Accordingly, one MMF can be utilized to transverse imaging for endoscopy. This MMF based endoscope can effectively overcome the above shortcomings. However, there are effects such as modal dispersion and mode coupling in the transmission process of multimode fiber, which will cause the image transmitted in the fiber to be distorted or even unrecognizable. The randomness of fiber shape in fabrication detoriosate the distortion. By measuring the transmission matrix, it is possible to recover the image through MMF. But the overall shape and structure of MMF will change in the operation of endoscope. The transmission characteristics of multimode fiber are only closely related to the fiber geometry, but traditional MMF imaging techniques are applicable when the shape of the optical fiber during imaging is consistent with the shape of the optical fiber when the transmission characteristics are calibrated. This work will focus on solving these problems. This paper explores the imaging technology of single multimode optical fiber, proposes the use of neural network to fit the multimode optical fiber transmission matrix, and calculates the input light field through the reverse transmission matrix and the output speckle light field. The traditional real number network has limited fitting effect on the multimode fiber transmission matrix. Inspired by the physical process of multimode fiber modulation of the light field, the real number network is extended to the field of complex numbers, and complex number operations are implemented in the network in the form of real numbers. This article studies the imaging regularity and effectiveness of multimode fiber under the condition of slight deformation and severe deformation. The neural network fitting method of multi-mode optical fiber transmission matrix used in this paper can maintain its effectiveness within a certain deformation range, and has great application potential in some endoscopic applications.
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