APPARATUS FOR MEASURING THE PROPERTIES OF AN OPTICAL FIBER

Abstract: Disclosed is an apparatus for measuring the properties of an optical waveguide fiber. The apparatus is free of apertures, lenses, and mirrors usually required in the measurement of certain waveguide fiber properties. The apparatus employs an optical switch at the launch end of the optical fiber to be tested and another optical switch at the output end of the optical fiber to be tested. The switches preserve the mode power distribution, particularly the spot size, of light passing therethrough. The apparatus may be used to measure bandwidth or attenuation of a multimode waveguide fiber, both of which are affected by launched and detected mode power distribution.

Description: APPARATUS FOR MEASURING THE PROPERTIES OF AN OPTICAL FIBER Waveguide Fiber Measurement Apparatus Cross-Reference to Related Applications This application claims the benefit of U.S. Provisional Application No. 60/128,504, filed April 9, 1999 and U.S. Provisional Application No. 60/129,706, filed April 16, 1999. Background of the Invention 1. Field of the Invention The present invention relates generally to an apparatus for measuring optical properties of waveguide fiber, and particularly to an apparatus that employs optical switching of light sources or detectors. 2. Technical Background Waveguide fiber optical measurements have always been a costly part of the manufacturing process. This is particularly true of multimode fiber measurements that include bandwidth, attenuation, numerical aperture, core diameter, and differential mode delay. Traditional optical measurement systems have used optics benches and bulk optic components consisting of lenses and movable mirrors to fold optical paths and to combine signals for the various measurements. One connection to the test fiber is established using an XYZ translation stage in front of a final lens before the detector. Translation stages are known to be temperature sensitive and subject to backlash in their movable parts. The connection at the light launch end of the fiber is made to a source of light appropriate for the desired measurement. Because certain of the multimode fiber optical properties, viz., bandwidth and attenuation, are launch sensitive, typically, measurements using more than one launch condition are desired. In addition, measurements at more than one wavelength are usually desired so that the launch end connection may have to be made numerous times. Thus, these measurement benches are notoriously slow, difficult to align and maintain in alignment, and large in size in that they have a surface area on the order of a square meter. To maintain reliability, such a bench must be periodically calibrated against a standard bench using standardized fiber. Time consuming and costly repeat measurements are often required. At the present time, standard optical specifications for multimode fiber performance criteria include measurements made using a launch condition having a spot size and numerical aperture which excites all of the modes of the multimode waveguide fiber. This launch condition is called the overfilled condition and is defined in the industry standards Fiber Optic Test Procedure (FOTP) 54. Attenuation measurements are made using a limited or restricted launch, referred to as Limited Phase Space Launch (LPS) and defined in FOTP 50. The LPS launch is similar to the 30 urn spot size launch described below. More recently, a demand for multimode fiber optimized for laser sources has increased the number of different launch conditions for bandwidth measurements. This in turn increases the number of connections at the measurement bench that compounds the problems with such benches. Thus there is a need for a fiber optic measurement apparatus that provides ease of connection and alignment of the components of the apparatus and of the fiber to be tested. Switching the launch end of the waveguide fiber among sources having different wavelengths or launch conditions should be fast and reliable. The present invention meets this need for less costly, faster, and more repeatable waveguide fiber measurements. Summary of the Invention One aspect of the present invention is an apparatus for measuring optical waveguide fiber that makes use of an N X 1 optical switch at the launch end of the fiber under test and a 1 X M optical switch at the detector end of the fiber under test. The light sources having the desired wavelengths and launch conditions, i.e., spot size and numerical aperture, are each connected to one of the N ports of the N X 1 switch. The detectors are each connected to one of the M ports of the M X1 switch. The result is, the fiber may be connected between the two switches and remain connected while all of the desired measurements are made. The launch end switch is selected to preserve the launch conditions, i.e., the mode power distribution, of the sources. The detector end switch is selected to preserve the mode power distribution of the light exiting the fiber under test. For certain of the measurements, a reference fiber is first connected between the switches to establish, for example a baseline launch power or launch pulse width. Thus in the bandwidth measurement, the pulse width of a pulse passing through the fiber under test is compared to the reference pulse width. The same comparison is made for the attenuation measurements, except that in this measurement the power exiting the fiber under test is compared to the launched power. In an embodiment of the invention, the spot size or numerical aperture of the launched light varies from one light to another source. Also certain of the sources are single mode lasers. In a preferred embodiment, the single mode laser sources have a spot size in the range of about 8 μm to 30 μm. In another embodiment either the spot size or the numerical aperture of the launched light may be restricted so that not all modes of a multimode fiber carry power, that is, are excited. A further embodiment of the measurement apparatus includes an OTDR coupled to the switches via a 1 X 2 coupler so that a trace of reflected power can be made of light launched into each end of the fiber. The details of the OTDR connection are set forth below in the description of Fig. 1. Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawing. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawing is included to provide a further understanding of the invention, and is incorporated in and constitute a part of this specification. The drawing illustrate an embodiment of the invention, and together with the description serve to explain the principles and operation of the invention. Brief Description of the Drawings Figure 1 is a schematic of an embodiment of the invented waveguide fiber measurement apparatus. Detailed Description of the Preferred embodiments Reference will now be made in detail to the present preferred embodiments of the invention, an example of which is illustrated in the accompanying drawing. An exemplary embodiment of the measurement apparatus of the present invention is shown in Figure 1 , and is designated generally throughout by reference numeral 10. Description In accordance with the invention, the present invention for an apparatus to measure waveguide fiber includes an N X 1 switch 2 for launching power into the fiber under test. As embodied herein, and depicted in Figure 1 , each of light sources 4 are optically coupled through 1 X 2 connector 12 to one of the N input ports of the N X 1 switch. In the case of the OTDR, 6, a second optical connection is made through switch 12 to the output end of the 1 X M switch, 8. This arrangement allows one to obtain an OTDR trace from each end of the fiber under test. Also shown at the N input ports of switch 2 are sources 14 for measurement of differential mode dispersion (DMD) of the fiber under test at one or more wavelengths. The fiber may be optically connected into the measurement apparatus 10 by means of splices 18. These may be fusion splices or any one of the many mechanical splices known in the art. Variable attenuator 20 may be placed in the circuit for use in cases where the launched light power is too high for the detectors 22. Overdriving the detectors is most likely to occur when acquiring the reference light signal mentioned above. Switch 24 is positioned to send light power from the detector in use to data storage and analysis means 26. Typically the analysis and storage means include an oscilloscope and a computer having an analogue to digital interface. These analysis means, including the computer programs used to compute bandwidth and attenuation are known in the art (see FOTP's referenced above) and thus will not be discussed further here. Examples of the launch conditions or mode power distributions used in the apparatus of Fig. 1 are as follows. A very restricted launch condition of spot size about 9.3 μm and numerical aperture (NA) about 0.14 may be achieved using a standard step index single-mode fiber as optical fiber pigtail 28 at one input port of switch 2. A plurality of restricted launches may then be achieved by using the standard single mode fiber in conjunction with a multimode fiber under test and offsetting the single mode fiber core relative to the multimode fiber core. Moderately restricted launch conditions can be achieved using as pigtail 28 a 50 μm core multimode fiber wrapped about a mandrel. Five turns of such fiber wrapped around a 5 mm diameter mandrel provided a spot size (diameter) of 30 urn and a numerical aperture of 0.13. An over filled launch was achieved using as pigtail 28 a step index multimode fiber having a core diameter greater than about 100 urn and a numerical aperture greater than about 0.30. Example Measurements using the apparatus as embodied in Fig. 1 were carried out. Switch 2 was a JDS, DP8T switch PN: SC1618-D2SP SN: B6B0366. Testing was repeated using as switch 2 the respective JDS switches, 1x2 switch PN: SW12-Z000311 SN: JC034991 , and 1x8 switch PN: SB0108- Z000329 SN: GB029604. Variable attenuator 20 was a JDS, PN: HA9-Z046 SN: KC000660. Four different launch conditions were used to measure bandwidth of a 62.5 micron core, 125 μm outside diameter fiber. These were as described above: • a standard overfilled defined by TIA/EIA FOTP ; • a moderately restricted launch condition providing a 30 um spot, achieved using 5 turns of 50 um core fiber around a 5 mm diameter mandrel; • a restricted launch condition generated by offsetting the core of a standard step index single-mode fiber by 4 um relative to a 62.5 um core fiber; and, • an very restricted launch created by using a standard step index single-mode fiber. Results of the test are set forth in Table 1. The percent difference of the bandwidth measurement from that made on a reference bench are given for each launch condition and each switch type. The percent difference in bandwidth measurement caused by the variable attenuator is given in the last row of Table 1. The percent differences are presented as BW850 nm/BW1300 nm. Measurements at 1300 nm wavelength were not made using the single mode fiber (SMF) launch. Table 1 Launch Overfilled 30 um 4 um offset SMF DP8T 1%/5% -19%/-8% -11%/-6% -20%/ - 1X2 1%/1% -23%/-6% -6%/-3% -21 %/ 1x8 4%/2% -48%/-15% -31%/-15% -33%/ Variable 1%/5% 0%/1% -1%/0% 1%/ - Attenuator The impact of the attenuator at the end of the system was shown to be quite small, less than 5% in all cases. Most switches show a low percent difference, especially in the case of the overfilled launch. In summary, the invention provides a way of combining sources at multiple wavelengths and with multiple launch conditions through a fiber optic switch, thus eliminating the need for open air, bulk optical components. This provides the means for making a multimode fiber bandwidth measurement whereby a test fiber must undergo one connection to the test apparatus for a complete measurement under all permutations of these conditions. The invention also provides a means of combining multiple optical measurements using fiber optic switch technology. Thus, through one connection to the test apparatus, multiple measurements can be performed. For example, an optical time domain reflectometer (OTDR) or differential mode delay (DMD) measurement can be combined with bandwidth and attenuation by connecting them to additional ports of the switches. This design eliminates the optical bench and utilizes a single electronic equipment rack for the components. One connection then provides means to switch in the various launch conditions, various wavelengths and various measurements without the need for open air optics. There is also a significant boost in dynamic range of the measurement by elimination of the power that is lost in an open air optical circuit. The dynamic range is known in the art to be the amount of attenuation which can be placed in a measurement path while retaining a signal to noise ratio that allows a measurement to be made. Dynamic range of a measurement system thus translates directly into the length of fiber which can be measured. It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.

Claims: What is claimed is: 1. An apparatus for measuring optical properties of waveguide fiber comprising: a first optical switch having N input ports and at least one output port; a plurality of lasers or light emitting diode light sources, each member of the plurality being optically coupled to one of said N input ports; a second optical switch having at least one input port and M output ports; a plurality of light detectors, each said detector optically coupled to one of the M output ports of said second optical switch; and, light measuring means optically coupled to receive light from any one of said light detectors, a reference optical fiber length or an optical fiber length to be tested being optically coupled between the at least one output port of said first switch and the at least one input port of said second switch; wherein, said first and second switches serve to maintain the mode power distribution of light passing therethrough. 2. The apparatus of claim 1 wherein a multimode fiber is measured and the spot size and numerical aperture of the mode power distribution launched into the multimode fiber is sufficient to launch power into all of the allowed modes of the multimode fiber. 3. The apparatus of claim 1 wherein a multimode fiber is measured and the spot size or numerical aperture of the mode power distribution is restricted such that some allowed modes of the multimode do not carry power. 4. The apparatus of claim 3 wherein a pre-selected number of said lasers are single mode lasers which provide light having a spot size in the range of about 8 μm to 30 μm. 5. The apparatus of claim 1 further including a variable attenuator optically coupled into a light path beginning at one of said light sources and ending at one of said light detectors. 6. The apparatus of claim 1 wherein said light measuring means is configured to measure bandwidth of a multimode fiber to be tested. 7. The apparatus of claim 1 wherein said light measuring means is configured to measure attenuation of a multimode fiber to be tested. 8. The apparatus of claim 1 further including a third switch having at least one input port and at least two output ports; and, an OTDR optically coupled to the at least one input port of said third switch; wherein, one of the at least two output ports of said third switch is optically coupled to an input port of said first switch and one of the at least two output ports of said third switch is optically coupled to an output port of said second switch. 9. The apparatus of claim 1 wherein either of said first or second switches is a modular unit. 10. The apparatus of claim 1 further including means to automatically switch between any two of the N input ports of said first coupler. 11. The apparatus of either one of claims 1 or 10 furthering including means to automatically switch between any two of the M output ports of said second switch.

Inventors: HACKERT MICHAEL J, HACKERT, MICHAEL, J.
Applicants: CORNING INC, CORNING INCORPORATED
Application year: 2000
Main classification: G01M11
IPC codes: G01M11