The coupling of the optical signal from a multi-mode to a single-mode fiber is a critical issue in application fields that involve numerical aperture optical systems and high accuracy optical characterization tools, such as astronomy and laser testing.
Indeed, such multi-mode to single-mode coupling is virtually impossible without loss of spectral information while keeping an acceptable power output. These articles will be aimed at screening the possible setups for such coupling, and finding the ones that offer the best trade-off between spectral information losses and power losses.
At first sight, one could think that by directly plugging a multi-mode fiber (large core, large aperture) into a single-mode one (small core, small aperture), the signal would be fully transmitted into the single mode fiber.
Figure 1. Direct multi-mode to single-mode coupling
Given the random distribution of the light information in the fiber, the information is not uniformly spread in the fiber creating a speckle pattern, as shown in Figure 2.
Figure 2: Multimode fiber speckle pattern (http://scialert.net/fulltext/?doi=jas.2010.312.318&org=11)
For this reason, all the spectral modes in the multimode fiber are not necessarily coupled into the smaller core single-mode fiber inducing likely spectral information losses.
Because this speckle pattern results from the interference of the propagation modes in the fibers, it makes it tremendously sensitive to environmental variations (temperature, vibration…). For this reason, the speckle pattern is extremely unstable thus making transmitted signal in the single-mode fiber unstable.
As an example, this video shows a loss of spectral information that can occur in a direct multi-mode fiber to single-mode fiber coupling when the fibers are moved.
Likewise, the use of tapered optical fiber could be, at first, considered a suitable option to perform the multi-mode to single-mode fiber coupling. However, because of the conservation of étendue in fibers, the coupling from large core, large numerical aperture (N.A.) fiber to small core, small N.A. fiber is physically impossible, leading to obvious losses of spectral information as shown in the example below.
Figure 3. Light propagation in a tapered fiber
For the exact same reason, a system composed of lenses will obviously show the exact same limitations as the tapered fiber, as shown in figure 4 .
Figure 4. Light propagation in a lens based system
Besides, except using intermediate speckles reducers, this does not solve at all the speckle issue and the problem of spectral and power losses remains the same.
Nonetheless, there are other ways to decrease the speckles issue, like speckle reducers or use of diffuser lens, and even efficiently coupling a signal from a multi-mode to a single-mode fiber, such as using an integrating sphere or a scrambler, which will be presented in a next article.