Single Mode to Multi-mode back to Single Mode

In summary, the conversation discusses the design of an interferometer that couples a laser into a single mode fiber (SMF) which is fusion spliced to a multi-mode fiber (MMF). The modes in the fiber can be perturbed by changes in the index of refraction, resulting in an interference pattern. The speaker is seeking to measure this interference pattern and its changes, possibly using software or a spatial correlator. Additional information and resources are provided regarding speckle patterns and modes of optical fiber propagation.
  • #1
Warr
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Alright. I'm having a little trouble getting started on this design of an interferometer. Essentially what I want is to couple a laser into the single mode fiber (SMF), which is fusion spliced to a multi-mode fiber (MMF). Assuming the fiber is unperturbed, the modes will not mix and you will no interference pattern at the end of the MMF (or perhaps you will have an interference pattern characteristic of the 'unperturbed' state, I'm not really sure) . However, if the fiber is say, stretched, the index of refraction will change, causing an interference pattern. I'm wondering how I could measure this. If this were to be used to measure an oscillatory deformation of the MMF (lets pretend its change in length delta L is a sine function of some angular frequency omega), what would I see in terms of the output of the SMF. I'm pretty confused here...

Some more information. From wikipedia http://en.wikipedia.org/wiki/Speckle_pattern

"In the output of a multimode optical fiber, a speckle pattern results from a superposition of mode field patterns. If the relative modal group velocities change with time, the speckle pattern will also change with time. If differential mode attenuation occurs, modal noise results."
 
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  • #2
Your 2nd MMF fiber will not support the single mode that is injected, IMO.
 
  • #3
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I think if you splice the SMF fibre to the MMF fibre, when you apply the laser the light entering the MMF will develop many modes. This will result in a speckle pattern on a screen near the output. If you now deform the MMF I think the speckle pattern will change.
So it is a matter of how to measure the changes in speckle pattern, which in these times I suppose will be done by software, subtracting one image from the other say. Or using some sort of spatial correlator.
 
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1. What is the difference between single mode and multi-mode fiber?

Single mode fiber has a smaller core size and allows only one mode of light to propagate through, resulting in a narrower beam of light. Multi-mode fiber has a larger core size and allows multiple modes of light to propagate through, resulting in a wider beam of light.

2. Why would someone need to convert from single mode to multi-mode and back to single mode?

This conversion may be necessary when connecting two different types of fiber optic equipment that operate on different modes. For example, a single mode device may need to connect to a multi-mode device, and then back to a single mode device.

3. Is there a loss of signal or quality when converting between single mode and multi-mode?

Yes, there can be a loss of signal or quality when converting between single mode and multi-mode. This is because the different modes of light have different propagation characteristics, and the conversion process may not be able to perfectly match them.

4. What are some common methods of converting between single mode and multi-mode fiber?

There are several methods of converting between single mode and multi-mode fiber, including using mode conditioning patch cables, using mode converters, and using fiber optic media converters.

5. What factors should be considered when choosing a single mode to multi-mode conversion method?

Factors that should be considered include the type of equipment being connected, the distance between the devices, the required bandwidth and data rates, and the potential for signal loss or degradation.

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