Technique for measuring modal power distribution between an optical source and a multimode fiber

#The present invention relates to a method for characterizing the modal power distribution of an optical signal launched from a laser source into a multimode optical fiber.

#There is currently a great deal of interest in optical local area networks operating at speeds of a Gbps or more. An Ethernet standard for such transmission is now in place and will inevitably function to accelerate the use of high speeds optical LANs. In order to achieve these high bit rates, semiconductor lasers (for example, vertical cavity surface emitting lasers (VCSELs) or Fabry-Perot (FP) lasers) will be used as transmission sources. In the case of most optical LANs, however, multimode fiber will be used, due to both its ease of installation compared to single mode fiber and the fact that there exists a significantly large embedded base of multimode fiber.

#Historically, multimode fiber has been used with LEDs as sources, not lasers; where LEDs are known to launch nearly equal power into every mode (defined as an “overfilled” launch). In the current application for use with lasers, however, the optical launches will be very far from overfilled, based on the characteristics of laser emission. In order to develop fast, reliable, low-cost systems, it is crucial to understand the behavior of multimode fiber under these restricted, laser-based launched conditions.

#Referring now to the drawings,

#FIG. 1 illustrates an exemplary test arrangement for characterizing the differential mode delay (DMD) of a test fiber;

#FIG. 2 contains a graph illustrating simulated DMD data from characterizing the test fiber of FIG. 1;

#Prior to describing the modal power distribution measurement technique of the present invention, it is useful to understand the pertinent properties of the type of multimode fiber that will be used in optical LAN systems. In general, the standard multimode fiber used in local area network applications has a core diameter of 62.5 μm and a maximum fractional index difference (ncore−ncladding)/ncore of 0.02. This type of multimode fiber is typically used at one of two operating wavelengths, λ=850 nm or 1300 nm. The profiles are chosen to maximize the modal bandwidth, that is, to minimize the spread in modal group velocities. The optimal profile shape depends on the exact dispersion characteristics of the doped silica comprising the fiber, but is very close to a parabola

#The propagating modes can be divided into degenerate mode groups (DMGs) defined by the property that all modes in a DMG share a common phase velocity. In a standard multimode fiber at λ=1300 nm, there are roughly 22 such mode groups, while there are approximately 33 such groups at λ=850 nm. Random imperfections in the fiber will couple power between modes. In standard multimode fiber, the modes within a specific degenerate mode group are believed to be completely coupled within a few hundred meters, while the coupling between different mode groups is negligible.

#A pulse launched into a given mode propagates at the group velocity of the mode, and spreads due to chromatic dispersion. When a group of modes are coupled together, as happens within a mode group, the pulses continually share power and ultimately merge into a single pulse, possibly broadened by modal, as well as chromatic, dispersion. Therefore, when a pulse of light is launched into modes of several mode groups of a standard multimode fiber, the impulse response of the fiber is expected to be the superposition of 22 or 33 pulses, depending on the wavelength. In a high bandwidth fiber, the group delays of the different mode groups will be very nearly the same, while in a low bandwidth fiber, the pulses will be spread out in time. This modal dispersion is generally considered to be the limiting factor in the performance of multimode fiber.