DE4009160C2 - Optical measuring device for a multimode fiber - Google Patents

Description

The invention relates to an optical measuring device for a Multi-mode fiber according to the preamble of claim 1. Ein such a measuring device is known from EP-A1 1 50 434.

Measuring devices for fiber optic technology have generally an optical plug connection as an input, if the device is a receiver, or as an output if the device represents a transmitter. In optical Communication links are also used Plug connections used, here the coupling of two Optical fibers have the lowest possible attenuation to keep losses on the line small. Changing couplings are rare, so that Reproducibility of such an optical connector none within the message transmission path represents an essential criterion.

On the other hand, the situation is in optical measurement technology different. A low optical transition loss is admittedly desirable because it increases measurement dynamics, however here is on the best possible reproducibility special value. Firstly, because that is external to the Optical fiber cable often connected to measuring device is changed, on the other hand, because different optical fibers connected to the same measuring device will. The fibers can, for example, with respect to their Core diameter and their numerical aperture vary.  

Even in the area of the predominantly used in the future Single-mode fibers have different fiber standards for example, field diameters of 9 µm and 10 µm.

Now the device-internal fiber gives way, for example their diameter from the externally connected fiber or misalignments occur in the connector of the two optical fibers meeting one another, so there are losses in the range of some 0.1 dB, so that a precisely defined one, in the external fiber optic cable coupled power is no longer available. Both known optical measuring devices for Communication links with optical fibers the connector transition errors outweigh all others Device error proportions.

DE 36 16 841 A1 describes a device for measuring the Through loss of an optical fiber (LWL) known with an optical transmitter and an optical fiber, the transmission light in a to be tested and arranged coaxially to the transmission fiber Test-LWL is guided, and with an optical Detector through which the output light from the test fiber is received becomes. The accuracy and reproducibility of Damping measurements are improved in that between the Output surface of the transmission fiber and input surface of the test fiber an air flow or a jet expanding element is arranged.

From GB 20 82 342 A is a connecting element for optical fibers known with beam expanders.

DE 32 18 014 A1 describes a method for expanding the numerical aperture of a light source known. Doing so the light from the light source z. B. successively through a gradient profile fiber, one step profile fiber and a second Gradient profile fiber sent. For full illumination of the To achieve the second gradient profile fiber, the step profile fiber should a larger core diameter and a larger one have numerical aperture as the second gradient profile fiber.

In ntz vol. 39 (1986) issue 1, pp. 28 to 35, the influence of Core diameter and the numerical aperture on the forwarding of light described by splice connections.

From telcom report 6 (1983) Supplement "News transmission with light "April 1983, pp. 102 to 108, is with end face coupling in connectors known how the relationship from exit surface to reception surface on the light transmission affects.

Starting from a measuring device according to the preamble of Claim 1, it is an object of the invention, this measuring device to be further developed so that the measuring device connection is as possible has low sensitivity to mismatches.

This object is achieved by the features specified in claim 1. The deliberate mismatch in the area of the measuring output or measuring input of the measuring device results in a higher basic attenuation of the fiber transition at the respective measuring device connection, but a considerably better reproducibility is achieved. This means that when different external fiber optic cables with different tolerances are connected, only relatively small changes in the measurement signal occur. In principle, the following two cases can occur in the area of the measurement inputs or measurement outputs:
Case A) The internal optical waveguide is designed as a multimode fiber (MMF), the external optical waveguide also being designed as a multimode fiber. In this case, the field sizes can be selected internally smaller than the external field sizes.
Case B) The internal optical waveguide is designed as a single-mode fiber, the external optical waveguide being designed as a multimode fiber. In this case, the internal field sizes are by definition smaller than the external field sizes.

In case A) there is a deliberate mismatch performed. Takes the core diameter in the direction of transmission by 30%,  so from a "70% coupling" be spoken, giving you the extra Advantage of an approximate equilibrium excitation wins. In this case, the influence of Coupling uncertainty from 0.29 dB to 0.02 dB reduced.

The invention is based on one in the Drawing shown embodiment closer explained. It shows

The only figure shows a measuring arrangement with a measuring transmitter and a Test receiver.

The measuring arrangement shown in the single figure consists of a measuring transmitter 1 , a measuring receiver 2 and an external optical fiber 3 , which is connected to internal optical fibers 6, 7 via a measuring output 4 and a measuring input 5 . The measuring transmitter 1 with optical waveguide 6 and the measuring receiver 2 with optical waveguide 7 are combined in the exemplary embodiment shown to form a measuring device 8 , however measuring transmitters and measuring receivers are often used as separate measuring devices at different locations.

The external optical waveguide 3 belongs to an optical communication system 9 (not shown in more detail), which can include amplifiers and other devices not shown here.

With the measuring device 8 , a measuring signal can be fed into the optical waveguide 3 from the measuring transmitter 1 via the measuring output 4 designed as a plug connection. The measuring receiver 2 receives this measuring signal via the measuring input 5, which is also designed as a plug connection. The attenuation of the entire transmission path is determined, for example, on the basis of level deviations between the transmitted and received measurement signal. So that these measurements can be carried out reproducibly even when connecting other external optical fibers, the same transition loss should always occur at the two plug connections of the measuring output 4 and the measuring input 5 . It is of secondary importance whether this basic attenuation is smaller or larger, but it should always be the same if possible, regardless of the external fiber optic cable connected.

Coupling to a multimode fiber

As an example, the coupling with a common one Telecom fiber examines a numerical aperture of 0.2 and a gradient index profile (50/125 µm) having.

The coupling efficiency is approximately at such a multimode fiber with adapted core radii a:

With:

n _x = coupling efficiency.

From this one obtains for the relative change in damping

The preferred mismatch solution works with reduced values of the internal fiber parameters, d. H. reduced area in the phase space diagram. A possible, but not limited to, dimensioning results from the application of 70% coupling:

With:

NA₁ = numerical aperture of the internal fiber.

The device's internal fiber is a step index fiber educated. So you get for the Coupling efficiency:

The following results for the relative change in damping:

With a radial offset of 10% of the core radius, accordingly 2.5 µm for a 50 µm fiber, the change Damping as follows:

0.29 dB in the adapted case (state of the art),
0.02 dB in the case of deliberate mismatch in accordance with the present invention.

The formation of the internal fiber as a step index fiber brings additional advantages in connection  to an external single-mode fiber, but then is overexcited. Use of the interior coating process manufactured fibers is however not desirable because of the central slump in refractive index exactly in the area of the coupling transition to Single-mode fiber lies and thus has a disruptive effect. Therefore, when coupled to a single mode fiber Internal step index fiber without the central drop in refractive index intended.

Claims (4)

1. Optical measuring device for a multimode fiber with a given core radius r₂, the measuring output and measuring input are each connected internally via an internal optical fiber to a measuring transmitter or measuring receiver, to the measuring output and measuring input of which the multimode fiber is connected and in which the core diameter at the measuring output and measuring input is in change the optical transmission direction from a smaller to a larger value, characterized in that the two internal optical fibers ( 6, 7 ) are step index fibers and that at the measuring output ( 4 ) and at the measuring input ( 5 ) in the optical transmission direction the core diameter by 20% to 50%, preferably 30%, decreases. 2. Measuring arrangement according to claim 1, characterized in that the internal optical waveguide ( 6, 7 ) is manufactured by the known outer coating method and thus has no central drop in refractive index. 3. Measuring arrangement according to one of claims 1 or 2, characterized in that a distance between the fiber ends of the internal and external optical waveguide ( 6, 7; 3 ) is provided for beam expansion of the transmitted light. 4. Measuring arrangement according to one of the preceding claims, characterized in that an imaging optical element for beam expansion is arranged at the transition between internal and external optical waveguides ( 6, 7; 3 ).