Laser Diode / LED to Fiber Delivery System Questionnaire NOTE 2: Sometimes the customer does not know what type of fiber he has or wants. In that case we need to know the customer's application to help him pick the proper fiber. The following information might help you select the fiber type.
First, a multimode fiber is not a multi-strand or bundle of fibers. A multimode fiber is a single fiber with a 25 to 1000 micron core diameter. When coherent laser light is coupled into multimode fiber, the output shows speckles as shown in the following figure. This speckle pattern changes when you bend the fiber. We can make the beam more uniform by adding a special diffuser plate at an additional cost. This will also reduce the coupling efficiency. The term multimode means there is more than one path for light to travel inside a single fiber. It does not mean the unit consists of multiple fibers in a bundle.
The output from a singlemode fiber (SM) shows a nice, smooth gaussian profile as shown. Singlemode fiber does not maintain polarization under stress such as bending, for that you need polarization maintaining (PM) fiber. PM fiber is also singlemode.
One can couple higher power into multimode fiber than into singlemode fiber. The smallest usable size of the multimode fiber for high power application depends on the laser power and wavelength. If you want to collimate or refocus the output from multimode fibers then, the larger the fiber core size is, the larger the divergence angle of the collimated beam will be.
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