lumerical varfdtd扫描弯曲半径r

Question

采用lumerical varfdtd扫描弯曲损耗，半径从5um到10um，如图：

可以观察到，在r=8.6是透射率骤降，观察此时的光强分布，可以发现，光在弯曲波导中损耗严重，如图：

请问为什么只在r=8.6时候损耗如此严重，而其他半径时候透射率相对正常

Answer 1

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Lumerical Variational Fast Decoding Time (VFDT) is a technique used in optical fiber communication to estimate the loss of coherence due to spontaneous emission and signal processing errors during an optical fiber scan. The efficiency of the scan results depends on various factors, including the size of the bending radius r and the wavelength used for scanning.

In your case, when the bending radius r = 8.6 mm, we observe a significant drop in the transmitted power compared to other radii. This decrease can be attributed to several reasons:

1. Beam splitting: In bendable fibers, light splits into multiple beams that propagate at different angles. When light propagates through the fiber, it encounters surface roughness, imperfections in the waveguide geometry, or scattering from the material causing refraction and scatter. These imperfections cause constructive interference between these beams, leading to an increase in the refractive index distribution near the bend point. At larger radii, the increased refractive index allows more light to escape into the surroundings, which reduces the effective transmittance of the beam.

2. Spatial non-linearity: Optical signals are inherently non-linear, meaning they depend on the phase and polarization of the incident light. In a bent fiber, spatial variations in the phase and polarization due to surface roughness or micro-roughness create an additional contribution to the optical path length. As the distance from the center of the fiber increases, this spatial variation becomes more pronounced, leading to a higher loss of light.

3. Loss of modes: Light does not propagate through a single mode at a constant speed, but rather through an ensemble of modes with different speeds and phases. In the case of bendable fibers, the modes within the beam do not have the same phase relationship, causing inter-mode mixing and increased losses due to pathloss and mode conversion.

To illustrate these effects, consider the following steps:

1. Generate a laser source with a linewidth of 1 kHz at a wavelength of 1550 nm (in micrometers). For the low-radii region (5 um to 10 um), a continuous wave (CW) laser with a central wavelength of 1550 nm would work well.
2. Optically pump a fiber sample into resonance with the laser source by placing a reflector near the sample end and controlling the intensity of the CW laser. The pump laser should be centered near the fiber core with a slightly larger radius than the fiber core (e.g., 10 um) to prevent mode conversion.
3. Perform a series of scans across the entire弯曲 region, starting from 5 um and ending at 10 um. To achieve full-scale transmission throughout the range, use a multimode fiber with a high number of modes (e.g., >1000) and with different propagation lengths. This will ensure that light from each mode reaches the sample simultaneously.
4. Measure the collected power as a function of scan position and wavelength using an optical detection system, such as a photodiode array or a time-domain spectroscopy setup.
5. Analyze the data to identify any specific peaks corresponding to the start and stop positions of the scans, as well as the locations where the measured powers decline significantly.
6. Investigate the impact of these regions on the remaining sections of the fiber. At large radii, where the refractive index changes more significantly, there may be more refractive index variation within the sample and slower convergence of the decay rate. As a result, the observed transmission might appear more irregular and have a higher loss of light.
7. If necessary, perform simulations to predict the optimal scan parameters (e.g., resolution, scan angle, and mode selection) based on the characteristics of the sample and the available equipment. A thorough understanding of the underlying mechanisms can help optimize the VFDT technique for more efficient loss estimation in bent fibers.

Once you have understood the causes of the observed loss in the bend radius 8.6 mm, you can propose a solution based on the simulation results and practical considerations:

1. Optimize the scan settings: Adjust the scan angle, resolution, and mode selection to balance the trade-off between minimizing pathloss and preserving the effective transparency of the fiber. This may involve selecting a smaller scan angle (less than 30°), increasing the scan resolution to reduce noise and improve visibility of the signal, and choosing a mode that has a lower loss profile at small radii.

2. Use adaptive optics techniques: Adaptive optics systems can compensate for the varying refractive index within the fiber and help maintain a stable dispersion compensation. By continuously monitoring the fiber's dispersion profile and adjusting the defocus lens accordingly, adaptive optics can maintain a consistent background signal and minimize pathloss-induced distortions in the transmitted light.

3. Implement temporal averaging: By averaging the collected power over several consecutive scans, the decay rate can be smoother and more accurately estimated. This approach helps account for the inherent inter-channel broadening due to the random sampling of signal arrival times.

4. Utilize frequency domain analysis: Techniques like the Fast Fourier Transform (FFT) can provide information about the time-domain behavior of the transmitted light at different frequencies. By analyzing the FFT spectrum, you can identify key spectral features that contribute to the measured power loss, such as energy-level transitions and diffraction patterns, providing valuable insights into the decay dynamics.

By considering these strategies, you can improve the performance of the VFDT technique in bent fibers and more accurately estimate the loss due to scattered light and other imperfections in the propagation environment. Remember to validate the simulated results against experimental measurements and continually refine the optimization process to enhance the overall efficiency of the technique.