Near field characterization of multimode optical fibers

Author(s):
M. Oliviero - PHOTONLAB - POLITECNICO DI TORINO
G. Perrone - PHOTONLAB - POLITECNICO DI TORINO

Industry:
Research

Products:
LabVIEW, Machine Vision

The Challenge:
Setting up an automatic testbench for characterization, by near-field analysis, of the modal distribution in multimode optical fibers for telecommunication.

The Solution:
A camera for infrared imaging to grab the near field profile on the cleaved facet of the optical fiber under test. The images from the camera are acquired and digitized by the NI PCI 1409, hence processed with a NI LabVIEW program to compute some characteristic parameters i.e. Mode Transfer Function (MTF), Modal Power Distribution (MPD) and Encircled Flux (EF).

Short Summary
The new challenge of optical communications is the widespread of optical fibers in local area networks and short-range data transmissions. This is likely to be accomplished by use of multimode fibers and related devices. For correct data transmission, the power distribution propagating along the fiber must satisfy some requirements, and the aim of this work has been the development of an automated system for precise analysis of the power distribution through some characteristic parameters (i.e. modal power distribution and encircled flux). The setup, which includes a LED source, a camera and a frame grabber controlled by LabVIEW, provides a real-time analysis of the near-field pattern of a multimode fiber and enables a quick computation of the mentioned parameters. The system has become essential for quick inspection of fibers as well as for thorough and exhaustive characterization of devices such as mode controllers and mode scramblers.

Article

Introduction
Optical fibers have enabled the tremendous development of modern telecommunications and data exchange on a long-haul scale. While well established in transoceanic links, the use of optical fibers is still at an early stage for short-range data transmissions, the so called “last mile” connections. Different techniques and standards are entering the market of optical communication, however the general trend seems the use of multimode fibers (MMFs) for short links and local/metropolitan area networks. In a MMF the light propagating into the fiber is the superposition of several electromagnetic field configurations called “propagation modes”, each carrying a share of the total power transmitted through the fiber. In comparison to single-mode fibers (in which only one single field configuration can propagate), MMFs exhibit a number of advantages, including the large tolerance to misalignment, easy installation, availability of low- cost transmitters and receivers. The main problem of MMF is that the information carried by the optical signal may be corrupted during its propagation, if the optical power is incorrectly distributed among the propagation modes. The distribution of power among the various modes in a MMF is known as the modal power distribution (MPD). The MPD plays a particularly important role in the performance of fiber in Local Area Networks (LANs). The MPD in a LAN fiber system also has a critical effect on the bandwidth that is available, particularly for Gigabit Ethernet transmission. In addition to MPD, a quantitative measure of the light distribution in a MMF is given by Encircled Flux (EF). Both MPD and EF can be computed from the near field pattern .
With the aid of a NI PCI 1409 frame grabber and by exploiting LabVIEW features, including the libraries from the new VISION module, we have set up a test bench for measurement of these quantities in an easy and fast way.

Setup for measure of the mode profile
The setup for MMFs characterization is depicted in Figure 1. It consists of LED source centered at the fiber operating wavelength (usually in the near infrared, either 850nm or 1300nm), which can be directly connected to the fiber system under test or through a mode controller. In the latter case, the meaning of the measurement is the analysis of the performance of the mode controller, which should work to distribute the power among the modes in a controlled fashion. The near field image on the MMF facet is magnified by a 40 microscope objective designed to yield low attenuation in the infrared spectrum. The MMF and the microscope objective are mounted on piezo-driven micropositioners for precise alignment. A VIDICON camera is placed at fixed distance from the objective. The near field pattern, grabbed by the camera, is digitized by the card NI PCI-1409, recorded and processed through a LabVIEW program.

Analysis of the near field pattern
An example of a near field pattern acquired by the system is depicted in Figure 2a, along with the LabVIEW user interface to control the acquisition and to process the image.
In order to compute the MPD and EF it is necessary to locate the centroid of the near field pattern; this is done by evaluating the first-order moment (the built-in function of NI-vision enables the straightforward calculation of the centroid). The min/max levels of the analog-to-digital converter of the frame grabber are adjusted via software so that the intensity of the image utilizes all the 256 available levels. Starting from the centroid coordinates, the NF profiles along x/y axis are extracted and processed (Figure 2b). The non-linear response of the vidicon camera is taken into account by numerical compensation. If the source exciting the fiber is very noise, the NF pattern will suffer from intensity flickering; the latter can be compensated by multiple averaging of the NF profiles.
MPD and EF are calculated upon the guidelines of the Telecommunication Industries Association (TIA); the details of such calculations are here omitted for brevity. As an example, Figure 3 and 4 depict MPD and EF measures of a MMF excited with an LED source at 850nm through a commercial mode controller. Both measures are within the template recommended by the standard, hence the mode controller satisfies the specifications.
We have made several measurements with this setup, analyzing multimode fibers, mode controllers and mode scramblers and we have reckoned reliability and good reproducibility of the results. The testbench described in this paper is then expected to become a turnkey setup for characterization of optical fibers and optical fiber components.

Conclusion
A testbench for fast and quick characterization of the power distribution of multimode fibers and devices has been developed. The system acquires the near field pattern of an optical fiber/device through a NI-PCI 1409 frame grabber and computes some characteristics parameters i.e. modal power distribution and encircled flux. Thanks to some features of the LabVIEW control program (compensation of the camera non-linearity, calculation of the centroid), the system shows to be a useful and efficient tool for analysis of multimode fibers and devices.

Author Information:
For more information on this Case Study, contact:
M. Oliviero
PHOTONLAB - POLITECNICO DI TORINO
massimo.olivero@polito.it

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