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id100 series id150 series id201 series id300 series id400 series |
Singlemode or multimode input ? Applications of photon counting are not all compatible with singlemode optical fibers. In certain cases, the photons one wishes to detect are emitted by an extended source or in a broad solid angle and are thus difficult to collect in a single-mode fiber. In order to simplify the coupling of such photons id Quantique is introducing a new version of its single photon detector module with multimode optical fiber input. The fraction of the light emitted by a Lambertian source (extended source emitting uniformly in all directions) that one can couple into an optical fiber is proportional to the product of its numerical aperture squared and the area of the core: I µ NA²d² where I is the fraction of light coupled into the fiber, NA is the numerical aperture of the fiber and d is the diameter of the core of the fiber. For singlemode optical fibers, the numerical aperture is 0.13 and the core diameter about 9 microns. In the case of multimode fibers, the numerical aperture is 0.22 and the core diameter is 50 microns.
When comparing the fraction of light coupled into these two types of fibers, one obtains: I(MMF) / I(SMF) = 0.22² x 50² / 0.13² x 9² = 88. It is thus possible to couple about 88 times more light into a multimode fiber than into a singlemode fiber. However, the coupling from the multimode fiber to the sensitive area of the APD is reduced by about 20% with respect to the monomode fiber. Hence, if your photons are already coupled into singlemode fibers, choose the singlemode input. Nevertheless, if you are detecting photons from a Lambertian source, the total coupling efficiency is increased by a factor of 70 using the multimode input. This translates directly into a gain of the same factor in signal to noise ratio. |
The only commercial photon counter for NIR detection on the market! Related links Order  |
id Quantique SA | Chemin de la Marbrerie 3 |
1227 Carouge | Geneva | CH Phone: + 41 22 301 83 71 | Fax: + 41 22 301 83 79 |  |
