Hi everybody!
I am working on my diploma thesis regarding eye endoscope. I would like to know more about the speckles in the multimode fiber. I would like to reduce speckles in MM fiber using vibration. I do not know why the vibration reduce the speckles and what happens with modes that are in the optical fiber.
Thank you for your answer!
Regards
Barbora Spurná
Jörn Bonse provides a very clear explanation of speckle motion in MM fiber resulting from fiber flexion. The next question is how to move the fiber to reduce the speckles adequately to meet you needs. Presumably, your aim is to create a smooth illumination field for your endoscope. So, how to vibrate the fiber, in terms of direction of motion, amplitude, frequency, etc? Some of these questions can be answered through experiment -- you will be time averaging (integrating) the moving speckle field over the frame time or exposure of your camera. Therefore, the vibration period must be less than 1/10 of the exposure and preferably much shorter to enable speckle patterns to average out. The amplitude of the vibration only needs to be enough to 'shake' the speckle pattern by a few characteristic speckle dimensions or so. Once you have a mechanism to vibrate the fiber, you can ensure that the amplitude of vibration is large enough. Finally, you need to create a diverse set of speckle patterns during an exposure time. Some schemes use two vibrators in orthogonal directions at different frequencies (that are not low integer multiples). It is probably best to build up your system with vibration frequency set first, then experiment to ensure that the speckle pattern averages out enough for your purposes. Add complexity if it is needed. Finally, there are other ways to smooth speckle patterns that use spinning diffusers in the light path. However, in your endoscope, your idea of vibration seems practical. Here is an interesting reference: https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-28-9-13662&id=431120.
I agree with Jörn and David. It is critical to choose proper vibration frequency according with the sensing camera (or detector) averaging exposure time. You can use a piezoelectric device connected to an electronic generator. The vibration amplitude can be sufficient if you glue or clamp a section of the fiber to the surface of the piezoelectric actuator.
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