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发表于 2009-7-17 07:56:47
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以我的经验，从阶跃型多模光纤直接出来的激光，只要光纤的长度足够，近场光版一定是均匀的，而要直接在远场得到均匀的光斑，基本上是不现实的，真的有必要的话，可以采取其他的方法。
以下结果供你参考：
Waveguide Modes
Waveguide structures are spatially inhomogeneous structures which can guide waves. For light propagating in a waveguide, the self-consistency condition for a mode is more strict than for free-space modes: the shape of the complex amplitude profile in the transverse dimensions must remain exactly constant. Any re-scaling is not allowed, only an overall phase change (described by the propagation constant) and a loss or gain of total optical power. The phase change per unit length is called the propagation constant, and is generally different for different modes.
A waveguide has only a finite number of guided propagation modes, the intensity distributions of which have a finite extent around the waveguide core. The number of guided modes, their transverse amplitude profiles and their propagation constants depend on the details of the waveguide structure and on the optical frequency. A single-mode waveguide (e.g. a single-mode fiber) has only a single guided mode per polarization direction. As an example of a multimode waveguide, Figure 3 shows the transverse profiles of all the modes of a multimode fiber.
modes of a fiber
Figure 3: Electric field amplitude profiles for all the guided modes of a fiber with a top-hat refractive index profile (→ step index fiber). The two colors indicate different signs of electric field values.
A waveguide also has cladding modes, the intensity distributions of which essentially fill the whole cladding (and core) region. Optical fibers (even single-mode fibers) have a large number of cladding modes, which often exhibit substantial propagation losses at the outer interface of the cladding. |
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