Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
1999-04-09
2001-07-10
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
C250S227140
Reexamination Certificate
active
06259517
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to laser-based processes and, more particularly, to systems for determining light-path interruption when laser energy is carried over an optical fiber.
BACKGROUND OF THE INVENTION
Laser energy is now in some instances transported over relatively long distances via fiber optic cable. Such laser installations are increasingly used in industrial applications for materials processing, process monitoring, and process control.
As an example, industrial Raman spectroscopy for chemical process monitoring and control uses laser energy from a laser source installed in a central control room instrument. The instrument couples the laser energy into an optical fiber cable that is routed to a remote probe head. The remote probe head is typically installed into a pipeline that may be hundreds of meters away from the laser source.
The fiber cable connecting the remote probe head to the central instrument poses a potential hazard if it is accidentally severed at some point en route. Potential hazards caused by laser energy escaping from a severed fiber optic cable include 1) eye damage to facility personnel, and 2) heating/ignition of explosive gases.
There are many disclosed fiber-optic safety systems, but all present drawbacks in terms of complexity and/or fail-safe operation. U.S. Pat. No. 5,012,087 to Rockstroh et al, entitled FIBER OPTIC SAFETY SYSTEM, detects energy leakage from an optical fiber transmitting a high-power laser beam by providing a second fiber in the same jacket. The second fiber is connected at one end to a light source and at the other end to a photosensitive diode, such that leakage of energy from the power transmitting fiber causes the second fiber to fail, thereby reducing or terminating the transmission of light to the photosensitive diode. Although this system is simple in concept, in the event that the photosensitive diode manages to detect an appropriate amount of light even with failure of the primary fiber, fail-safe operation is not guaranteed. Furthermore, this concept requires electrical power at the remote end of the fiber to power either a light source or a photodetector, which adds the complexity of power cables and, for hazardous environments, the need for an explosion-proof enclosure.
U.S. Pat. No. 5,270,537 to Jacobs, entitled LASER INITIATED ORDINANCE SYSTEM OPTICAL FIBER CONTINUITY TEST, describes a system wherein a primary laser source used to detonate an explosive is switched out of the path of a light-carrying fiber to introduce light from a test source used to detect breaks or discontinuities. The secondary light source generates a series of light pulses which are transmitted through the length of the fiber and reflected by a dichroic coating at the opposite end. A photodetector is positioned near the test light source to detect the reflected pulses, such that if there is a break in the optical fiber, the pulse of light that is reflected will be of lower intensity than would be expected. This system is narrowly tailored for a specific purpose, requiring a large number of optical components and moving parts. This type of testing is directed to applications in which fiber continuity testing is required just prior to launching a single, short pulse of laser light on the fiber. Since the testing is not simultaneous with the transmission of the laser pulse, this approach does not meet the need of continuous monitoring of the continuity of a fiber carrying a cw laser beam.
U.S. Pat. No. 5,729,012 to Wood et al, entitled PHOTOLUMINESCENCE BUILT-IN-TEST FOR OPTICAL SYSTEMS, utilizes the same optical fiber to carry laser energy from a primary source, a test source, and wavelengths emanating and returning from the distal end of an optical fiber. In contrast to the '537 patent to Jacobs, the apparatus of Wood et al includes a photoluminescent material disposed at the junction of an optically-initiated device in the second or distal end of the optical fiber, such that, in a test mode, the photoluminescent material is exposed to light from the test source, resulting in photoluminescence at a third wavelength. The photoluminescent light travels through the optical fiber back to the detector at the source end which, when detected, is used to indicate optical fiber continuity. For applications involving spectroscopic monitoring, the photoluminescent material would produce unwanted spectral signals of its own, which might obscure or mask the desired spectral signals from the sample being monitored. Consequently, the photoluminescent approach is not suitable for applications involving the spectral analysis of various substances.
SUMMARY OF THE INVENTION
The subject invention resides in methods and apparatus for detecting the breakage of an optical fiber used to carry laser energy from a source location to a remote location. Importantly, the system requires no electrical components at the remote location, thereby avoiding electrically induced explosion hazards.
The system continuously monitors the integrity of the laser-guiding fiber, whether the laser itself is on or off. If a fiber cable interruption is detected at any point between the source and remote locations, the system may cut off, lock out, or reduce laser power, as appropriate. The invention is ideally suited to remote spectroscopic monitoring, such as industrial Raman spectroscopy, wherein optical fibers are used to interface a central instrument to a probe head.
A system according to the invention would include a laser outputting a beam of light having a wavelength at a first location, and a first optical fiber having a first end supported to receive the light from the laser and a second end to deliver the light to a remote location. An optical element is used to introduce the test signal into one end of the optical fiber so that the test signal is carried to the opposite end of the fiber, where it is detected and converted into an electrical signal. Electrical circuitry may be employed to perform a desired function, for example, to reduce or terminate power to the laser, or to prevent laser from entering the first end of the optical fiber, if the detected test signal does not meet a predetermined criterion, such as failing to reach, or falling below, a particular value.
To assist with detection, the optical test signal is preferably at a wavelength different from that of the laser. The test signal may also be modulated and/or encoded. In the preferred embodiment, the test signal, which originates at the first location, is carried by a second optical fiber to the remote location, where it is introduced into the second end of the first fiber. The test signal propagates through the first fiber to the first location, where it is separated from the path of the laser beam and detected by a suitable detector and associated electronics at the first location.
Alternatively, the test signal could be introduced to the first fiber at the first location, and separated from the laser beam path at the remote location. In this case, the second fiber is used to carry the test signal to the first location where it is sensed by the detector and electronics. Neither case requires electrical power at the remote location.
The invention is useful in a variety of applications that might benefit from fiber breakage detection, including systems of the type wherein the laser energy is used to induce a spectral emission from a sample disposed at the remote location, as in Raman or fluorescence detection. In such a configuration one or more additional fibers may be used to carry the spectral emission back to the first location, for example, for analysis purposes. Where possible, the preferred embodiment uses commercially available components, as might be manufactured for the telecommunications industry, enabling the system to be implemented at a relatively low cost.
REFERENCES:
patent: 4673291 (1987-06-01), Heckmann
patent: 5012087 (1991-04-01), Rockstroh et al.
patent: 5270537 (1993-12-01), Jacobs
patent: 5729012 (1998-03-01), Wood et al.
Barlow Dane D.
Slater Joseph B.
Tedesco James M.
Font Frank G.
Gifford Krass Groh Sprinkle Anderson & Citkowski PC
Kaiser Optical Systems, Inc.
Nguyen Tu T.
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