光纤激光器与放大器设计案例

8.6 Bent Large Mode Area Fiber
File: Bent large mode area fiber.fpw Here, we investigate how bending influences the propagation of light in a large mode area fiber. We use a simple step-index design, but other index profiles could be investigated as well. Diagram 1 shows how light evolves if the bending becomes stronger and stronger along the fiber, until all light is lost into cladding modes. Diagram 2 plots the propagation losses as a function of curvature radius for different modes. This calculation takes quite a few minutes, but the diagram delivers a lot of information.
8.2 Step-index Fibers
File: Step-index fibers.cf.fpw This script contains a simple custom form and conveniently allows the calculation of basic properties of step-index fibers. (It also serves as a nice demonstration for custom forms.) The user enters fiber parameters like core diameter and numerical aperture into the form, and calculated values like mode radii, mode areas and effective indices are displayed. Also, a number of diagrams can be made, e.g. for displaying intensity profiles.
源自文库
germanium concentration is assumed. The refractive index is interpolated between that of silica and germania according to the local germania content. The refractive indices of silica and germania are calculated from Sellmeier formulas, so that the wavelength dependence is included, which we need for calculating the chromatic dispersion. The mode solver (section 2.5) provides functions which deliver effective refractive indices, group indices, group velocity dispersion, etc., for all modes.
8.4 Launching Light into a Multimode Fiber (Mode-based Simulation)
File: Fiber launch.fpw This script simulates the results of launching light into a fiber with several guided modes. The input beam is a Gaussian beam, which can be offset from the core center and tilted against the fiber axis. The script can calculate the resulting amplitudes of all modes, from which the intensity profile at the fiber output can be calculated efficiently (without numerical beam propagation!). Apart from a numerical output of the resulting mode powers, the following diagrams can be made: Diagram 1 shows the powers of all the guided modes as functions of the input beam position. Diagram 2 shows the powers of all the guided modes as functions of the input beam tilt. Diagram 3 shows the output intensity profile for a given beam offset. Diagram 4 shows the output intensity profile for a given beam tilt.
8.3 Modes of a Germanosilicate Fiber
File: Germanosilicate fiber.fpw (The form settings file Germanosilicate fiber.fpi allows one to do similar things in the forms mode.) This script is a more sophisticated case for the calculation of the properties of fiber modes. We consider a germanosilicate multimode fiber, where a supergaussian transverse profile of the
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光纤激光器与放大器设计软件案例
8.1 Calculating Fiber Modes
File: Fiber modes.fpw (The form settings file Fiber modes.fpi allows one to do similar things in the forms mode.) For simplicity, this script demonstrates only the calculation of fiber modes with the integrated mode solver. The use of such modes in a fiber amplifier model is demonstrated in section 8.12. The script works with a refractive index profile defined with tabulated values. With a few lines of script code, these are read into an array, and the index function n_f(r) uses interpolated values from that array. It is shown then how to produce various diagrams displaying the modal properties of the fiber: A first diagram shows all radial functions. Different colors are used for different l values. The refractive index profile and the effective indices of the modes are also shown. The second diagram shows the intensity pattern of a selected mode. Diagram 3 shows the number of modes as a function of the wavelength. At 1.96 µm, the single-mode regime starts. Diagram 4 shows how the effective mode indices depend on the wavelength. One sees that all these indices reach the cladding index at their cut-off wavelength. Diagram 5 shows the fraction of power in the core for all modes versus wavelength.
8.5 Launching Light into a Single-mode Fiber (with Beam Propagation)
File: Launching light into a single-mode fiber.fpw Here, we use numerical beam propagation to investigate what happens if light is launched into a single-mode fiber with imperfect launch conditions: the input beam profile is Gaussian (not perfectly fitting to the guided mode of the fiber) and can be offset in terms of position and incidence angle. Based on the calculated fiber mode, one can already calculate the launch efficiency, but only with numerical beam propagation one learns what exactly happens in the fiber. Diagram 1 shows the field amplitude profile in the yz plane. One can see how some of the launched light gets into the cladding. Diagram 2 shows the launch efficiency as a function of the initial beam radius.
8.7 Tapered Fiber
Files: Tapered fiber.fpw, Tapered fiber.cf.fpw We assume that light is into a guided mode of a fiber. After some distance of propagation, the fiber core gets continuously smaller due to tapering of the fiber. The simulation shows how the mode field adapts to the core size, unless the change of core size is too fast. Diagram 1 shows the field amplitudes in the yz plane. Diagram 2 shows how the beam parameters evolve along the fiber: the optical power, beam radius, mode area, etc. The version Tapered fiber.cf.fpw contains a custom form, with which the input parameters can be entered more conveniently.