Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1999-07-26
2001-04-03
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06211950
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical pulse reflectometer.
RELATED TECHNOLOGY
The ongoing establishment and expansion of optical fiber networks calls for the use of suitable optical measuring instruments during installation, for maintenance purposes and for the monitoring thereof. A particularly important measuring instrument in this connection is the optical pulse reflectometer, known also as the optical time-domain reflectometer (OTDR). Such a measuring instrument is used to measure and evaluate reflections in optical fiber transmission systems. Reflections occur in optical components, such as in a light-conducting fiber, principally because of points of sudden irregularity in the refractive index, for example at the end of the fiber, and as a result of Rayleigh scattering, caused by inhomogeneities of the fiber. Further examples of reflecting optical components are connectors, junctions and splices. In a known optical pulse reflectometer, the pulses emitted by a laser diode are supplied via a static or passive power splitter to the fiber to be measured. The static power splitter, known also as a beam splitter, may be in the form of a semi-opaque reflector, an optical fiber 2×2 coupler (known also as a directional coupler) or a 2:1 coupler, i.e., an inversely operated Y-junction. The function of the passive beam splitter is to change the propagation direction of the light pulse, i.e., to direct the light pulses spatially. The latter is necessary to ensure that the optical signal components, reflected by the fiber due to Rayleigh and/or Fresnel scattering, are directed onto a photodetector. However, the passive beam splitter has the disadvantage that approximately only half of the reflected signal component is returned to the photodetector. The other half of the reflected signal component is directed in an undesired manner towards the laser diode, with the consequence that an optical isolator must be additionally connected upstream from the laser diode to protect against the undesired reflections.
Furthermore, the reflected light component is additionally attenuated by inevitable imperfections in the fiber connections. If, instead of an inversely operated Y-branch, passive 2×2 couplers or semi-opaque reflectors are used, it is even so that only about one fourth of the light intensity emitted by the laser diode is utilized. In other words, the signal emitted by the laser diode is attenuated by a factor of 4 before it reaches the photodetector.
German Patent Application No. A-4 437 821 describes a shielding device for an optical time-domain reflectometer, also known as an OTDR, which uses a high-speed switch and a pumped light source for producing optical pulses. The essential function of the high-speed switch is to selectively block a frequency range of the light reflected by the fiber to be measured, the light also being described as Fresnel-reflected light. To achieve this, the optical high-speed switch is operated synchronously with the optical pulse produced by the light source.
U.S. Pat. No. 5,388,172, describes an optical switching device for an OTDR, where a pulsed laser and an optical receiver are connected to a fiber to be measured. The optical switching device performs the function of keeping the Fresnel reflections from the fiber away from the optical receiver.
European Patent Application No. A 0 502 422 describes an optical time-domain reflectometer, in which a Raman laser device and an optical detector are connected via an optical switch to the fiber to be measured. To be able to measure the loss properties of the optical fiber at any wavelength at all, within a broad spectrum, a Raman scattered-light pulse is used. For this, the Raman laser device uses a solid-state laser, which is pumped by a laser diode to produce a light pulse.
The International Patent Application having International Publication No. WO-A 91 12509 describes an OTDR which uses a pulsed laser to produce light pulses. The pulsed laser and an optical receiver are connected via a passive optical coupler to a fiber to be measured.
In the essay, “A Low Crosstalk and Polarization Independent Optical Waveguide Switch for OTDR”, NEC Research and Development, no. 99, October 1990, Tokyo, Japan, pp. 75-83, XP000178479, H. Kawashima et al. describe an optical waveguide switch for an OTDR that works with a wavelength of 1.3 &mgr;m, with a Ti:LiNbO
3
waveguide. As a light source, a laser diode is used, in turn, which produces light pulses.
SUMMARY OF THE INVENTION
An object of the present invention, therefore, is to further develop the optical pulse reflectometer described at the outset so as to enable the circuit expenditure and, thus, the costs to be reduced by eliminating the need for synchronization between the light source and the control device used for switching the optical switch.
The object of the invention, therefore, is to further develop the initially described optical pulse reflectometer in such a manner that its sensitivity can be improved without increasing the optical transmitting power of the light source and such that it is not necessary to employ an additional optical isolator to protect the light source from reflections.
The optical pulse reflectometer according to the present invention includes at least one—generally known—optical switch for selectively connecting of a light source, particularly a laser diode, to an associated test object and for the selectively connecting to the test object to an associated photodetector. The optical switch is switched, for example, by a control device which switches the optical switch at predetermined points in time. In this manner, the optical transmitting power of the light source is injected virtually without loss of power into the test object, in particularly an optical fiber, and that the light reflected in the test object is subsequently supplied virtually unattenuated to the photodetector. Thus, without having to actually increase the optical transmitting power of the light source, it is possible, inter alia, to test fibers over greater lengths than in known methods heretofore using static power splitters. In addition, for a measured distance of a given length, it is possible to employ shorter light pulses, thereby enhancing the spatial resolution of the measurement. Since, compared, for example, to passive 2×2 couplers or semi-opaque reflectors, an optical switch allows approximately four times the power utilization, the pulse duration for detecting Rayleigh scattering can be reduced to one fourth. (Note that the intensity of the Rayleigh scattering is a function of the energy (light intensity×pulse duration) of the pulse.) For OTDR measurements, this means that significant points on the fiber can be located with greater accuracy. Consequently, for example, faults on a fiber cable can be pinpointed with suitable accuracy. Due to the higher signal level, Fresnel reflections also produce correspondingly stronger echo signals in the photodetector, which are, therefore, also able to be detected with greater sensitivity. In contrast to a conventional optical pulse reflectometer, the optical switch virtually eliminates the need for an optical isolator for protecting the light source from reflections. This is due to the fact that virtually all of the reflected signal component is directed to the photodetector.
The optical switch used may, for example, be a 2×2 directional coupler or an acoustooptic modulator, both of which are in effect known. An example of a switchable optical 2×2 directional coupler is the thermooptically controlled coupler, described by Norbert Keil in the essay “Optische Schalter aus Kunststoff—Schlüsselkomponenten in den Telekom-Netzen der Zukunft” [
Optical switches made of plastic—Key components in the telecom networks of the future
], ntz, issue December 1995, pages 36-41. As a light source, the optical pulse reflectometer uses a light source operated in constant light mode, the switchable optical switch also being used to produce light pulses. In thi
Deutsche Telekom AG
Font Frank G.
Kenyon & Kenyon
Merlino Amanda
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