As described in a previous article, achieving an efficient light coupling from multimode fiber (MMF) to single mode fiber (SMF) is a difficult task, especially when trying to avoid any spectral losses while keeping an acceptable power conversion. The fiber to fiber coupling option, as well as the tapered fiber solution and the lenses system being discarded, other alternatives coupling systems have to be investigated.
In the following article, the most common coupling setup, the integrating sphere as well as two innovative systems, which are the photonic lantern and a multimode fiber scrambler, will be screened.
The integrating sphere
An integrating sphere consists of a hollow sphere with its inner surface covered with a highly reflective coating, with two small holes for input (MMF) and output (SMF), as shown in Figure 1. Because of the many scattering reflections of the light beam into the sphere, the light is equally distributed in every direction mode causing a mixing of the spatial information. As a result, the speckle pattern of the input MMF is mixed up, and the output SMF shows a uniform speckle pattern containing all the spectral information of the input MMF.
Figure 1: Integrating sphere
However, the integrating sphere has a major drawback when it comes to the MMF to SMF coupling: the power losses of the system are huge. For instance, using the theoretical integrating sphere power conversion ratio (see Figure 2), one can notice that in a system with a MMF input and a SMF (5/125, N.A. 0.12) output going through a 2” standard integrating sphere (reflectiveness 0.99), the power conversion ratio will be 0,000024%, or 65.8 dB, which makes it usable only for really high power systems.
Figure 2: Theoretical power conversion in an integrating sphere
The photonic lantern
The photonic lantern is an innovative device, which was first introduced by Leon-Saval’s team in 2005. It consists of a MMF which splits into several SMF via a tapered fiber used to decrease the numerical aperture of the system, as shown in Figure 3. Such a coupling is possible only if the number of SMF output fibers is equal or greater than the number of propagating modes in the multimode fiber. If this condition is not respected, efficient coupling cannot take place, consistently with the brightness theorem.
However, when this condition is fulfilled, the system shows a great efficiency (0.56 dB loss), and no spectral information loss.
Figure 3: Photonic lantern working principle
To conclude, the photonic lantern is the most efficient existing MMF to SMF coupling system in terms of spectral and power losses, but it also has major flaws. Indeed, the number of system outputs can be problematic: for instance, a 100 µm MMF has several hundred modes, therefore hundreds of SMF outputs, that is to say as many characterization instruments.
In addition to this, as of now, the photonic lantern is only available as prototype and is difficult to fabricate.
The multimode scrambler
The multimode scramblers are fiber systems designed in order to mix up the modes in MMF. They can have many different configurations but are generally made of a really long MMF (typically more than 10 meters) which is mechanically stretched and constrained in order to mix up the modes of the fiber. This mixing of the modes of the fiber makes the speckle pattern uniform at the output of the instrument such as in the following example.
The spatial information now being homogeneously spread all over the fiber head, it allows a direct MMF to SMF coupling without any loss of spectral information. Major power losses can be expected because of the surface area ratio, though no loss of spectral information can be observed, and the signal level remains steady even when the fiber is moved around (see next article).
Following this study, one can summarize the main characteristics of each setup, as in the following chart:
|Integrating Sphere||Photonic lantern||Multimode scrambler|
|Spatial mode losses||No||No||No|
|Typical power losses (dB)||66 (into a 5/125, N.A 0.12 SMF)||0.56||33 (for a 100 µm MMF)|
|Number of outputs/interrogators||1||As many as the number of modes in the MM fiber||1|
To conclude, one can observe that the photonic lantern is the most efficient device to perform a MMF to SMF coupling. However, because of the many SMF outputs, it is not suitable for characterization devices requiring SMF inputs. On the other hand, the multimode scrambler performs the same operation as the integrating sphere but offers a much better power conversion ratio, thus it should be preferred over an integrating sphere to be used with a characterization tool.
To illustrate this with a practical case, a SMF input instrument (ZOOM Spectra) will be shown in use with a scrambler (Multimode Scrambler – SpeckleFreeTM MMS-201), a standard integrating sphere and with a direct fiber to fiber coupling in a further article.