OPTICAL POWER TRANSMISSION MONITORING SYSTEM

Abstract: An optical power transmission monitoring system for monitoring a multiplicity of transmission lines includes a first plurality n of optical sources; a second plurality m of optical detectors; means for time multiplexing each of the n sources through m different lines to the m detectors; and means for spatially multiplexing each of the m detectors through n different lines to the n sources for simultaneously sensing radiation at each detector through a transmission line from a unique source.

Description: OPTICAL POWER TRANSMISSION MONITORING SYSTEM FIELD OF INVENTION This invention relates to an optical power transmission monitoring system for monitoring a multiplexity of transmission lines, and more particularly to such a system which time multiplexes the sources and spatially multiplexes the detectors to obtain high speed and reliability with economy of parts. BACKGROUND In the development and use of fiber optic communications systems, many of the components require optical performance verification, either as part of the production quality checking or as a type-approval activity. Typical components are fibers, fiber cables and their sub-assemblies, optical filters, splitters, and directional couplers. For example, in the type-approval of fiber cable, it is necessary to monitor the optical transmission loss when the cable is subjected to various environmental tests. Cable degradation, performance and quality can in virtually all cases be assessed (e.g. , to IEC standards) by detecting small changes in optical loss in the cable caused by environmental factors such as impact by a mass, bending, pressure (or crush force), heat or temperature cycling, or water immersion tests. Generally, the degree of loss change must be determined and compared to acceptance limits. Other applications might be to test longer term performance of cables in field-installed circumstances, or filter or splitter or discrete component durability, aging and environmental testing. Typically loss is measured by measuring the change in optical power output from the device under test (DUT) when the DUT is illuminated with a stable light source. A further characteristic of these devices is that a large number of fibers within the DUT/cable are required to be measured, usually en masse and often simultaneously (or almost so). For example, during an impact test, the loss in each fiber must be checked after each strike of the hammer or mass, before the next strike occurs. Therefore a larger number of optical power monitoring channels is required- up to 144 fibers. Similarly, in a fiber splitter device, many fiber outputs (e.g., up to 16) may be required. Alternatively, several devices may be tested together, meaning multiples of 16 fibers must be monitored. Therefore optical power/transmission loss monitoring applied to fiber cable and component design must be capable of large numbers of independent channels. To independently measure the optical transmission of several fibers, a number of prior art approaches exist. One method uses a single source and detector, combining optical switches or other switching devices to divert light to/from each fiber and monitor loss/optical power sequentially. This method requires an optical switching technology to access each fiber as required, allowing the light power to be transmitted through the fiber to the detector. Recording power levels for each channel in rotation, then repeating the process at timed intervals, allows all fibers to be monitored for degradation during the testing session. The optical switching may be accomplished by manually connecting fibers, by mechanical means using a moving common fiber channel which is aligned by motor to the required output fiber channel, or by moving an array of the fibers under test to allow a specific fiber in the array (or "bundle" of fibers) to move into the optical path from an optical input fiber or light beam. Other means to accomplish switching may include micro-optical assemblies capable of accomplishing the fiber movements of the larger purely mechanical assemblies mentioned above, or by integrated optical switching elements. The problem in all cases is the precision required: for single-mode fibers with a core diameter of lOμm an accuracy of alignment of lμm is desired. There are two primary drawbacks with this method: One is that the movement of the fiber within the switch and its alignment tolerances require sub-micron accuracy in mechanical moving parts, which even with elaborate mechanical designs can result in poor repeatability of the fiber-to-fiber alignment and therefore the switch transmission loss factor. Long term wear and reliability are also issues to be considered. In integrated optical or similar "solid state" devices, thermal drift can result in similar transmission loss variations. Therefore these switching technologies can suffer from poor optical loss measurement repeatability. The other is that the switching time of the switch (typically 200ms) and the sequential nature of the measurement dictate that the "cycle rate" to completely sample the entire set of fiber inputs/outputs is slow, and in some cases too slow to allow economic DUT test times, especially when large numbers of channels must be measured. For example, a cable of 256 fiber elements may require 1 sec per switching operation or 256 seconds, or 4 minutes 16 seconds for a complete cycle. A second method uses a source-detector combination for each fiber measured. This is a parallel detection method more suitable to smaller numbers of DUT fiber channels. The primary drawback is the cost of many sources and detectors especially when more than 8 channels are provided. SUMMARY OF INVENTION It is therefore an object of this invention to provide an improved optical power transmission monitoring system. It is a further object of this invention to provide such an improved optical power transmission monitoring system which has low cost high speed readout and uses fewer sources and detectors. It is a further object of this invention to provide such an improved optical power transmission monitoring system which has no moving optical parts, is low in cost, small and compact. It is a further object of this invention to provide such an improved optical power transmission monitoring system which produces reliable and repeatable results. The invention results from the realization that an improved optical power transmission monitoring system which has low cost compared to a parallel system and higher speed and reduced mechanical problems compared to a sequential system can be achieved by time multiplexing the sources driving the transmission lines being monitored and spatially multiplexing the detectors sensing the output from the lines so that each line can be uniquely monitored. More particularly, each detector is enabled to sense light from one line of a set of lines simultaneously energized by a single source, as each source and its defined set of lines are enabled. This invention features an optical power transmission monitoring system for monitoring a multiplicity of transmission lines. There is a first plurality n of optical sources and a second plurality m of optical detectors. There are means for time multiplexing each of the n sources through m different lines to the m detectors and means for spatially multiplexing each of the m detectors through n different lines to the n sources for simultaneously sensing radiation at each detector through a transmission line from a unique source. In a preferred embodiment the means for time multiplexing may include a timing circuit for enabling each source sequentially in successive time periods. The detector may include an analog to digital converter enabled by the timing circuit synchronously with enablement of each of the sources. Each source may include an LED and may also include a filter for narrowing the spectral band of the radiation emitted. The means for spatially multiplexing may include an optical splitter associated with each source for distributing the output from a source over a plurality of m transmission lines. The means for spatially multiplexing may include an optical combiner associated with each detector for collecting the output from each source over a plurality of n transmission lines. Each optical source may include light sources of a number of different wavelengths. The transmission lines may be fiber optic elements. DISCLOSURE OF PREFERRED EMBODIMENT Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: Fig. 1 is a schematic block diagram of an optical power transmission monitoring system according to this invention; Fig. 2 is a more detailed schematic diagram of the optical power transmission monitoring system of this invention; Fig. 3 is a timing diagram of the waveforms occurring in the system of Fig. 2; and Fig. 4 is an enlarged detailed view of multiple wavelength source and multiple splitter in accordance with another embodiment of this invention. There is shown in Fig. 1 an optical power transmission monitoring system 10 according to this invention including a plurality of light sources 12 which provide their light through splitters 14 to the device under test, DUT 16, such as, for example, a plurality of transmission lines. The outputs from DUT 16 are collected in combiners 18 and delivered to the detectors 20 which provide an analog voltage representative of the power of the radiant energy transmitted through DUT 16. That analog voltage may be converted such as by an analog to digital converter (ADC) 22 included in each of the detectors for delivery to a computer PC 24 for analysis and presentation. In accordance with this invention, light sources 12 are time multiplexed by a driver 26 under the control of clock 28 so that they provide their outputs through splitters 14 in successive time periods. Combiners 18 spatially multiplex the outputs from DUT 16 to detectors 20 so that the outputs from each light source are directed to the proper detectors 20. The signal from driver 26 enables each of the ADCs 22 only during the period when a light source is energized. This time multiplexing of the sources and spatial multiplexing of the detectors provides great advantages in terms of economics, speed and reliability, and can be understood much more readily with specific reference to the operation of a preferred embodiment as shown in Fig. 2, where optical power transmission monitoring system 10' includes a device under test 16' which is an optical transmission cable including, for example, 256 individual fiber optic elements. Light sources 12' include n LED sources 12a, 12b, 12c, 12n, and m detectors 20a, 20b, 20m. In this case, where DUT 16' contains 256 individual fiber optic elements, n may be sixteen and m may be sixteen so that their product equals 256, or they may be any other multiple of 256, for example, 8 and 32, 4 and 64, depending upon the economies and simplicity afforded by the balance of LEDs to detectors. Each LED 12a-n has associated with it a splitter 14a-n which receives at its input the radiation from its associated LED and provides m outputs where m is the number of detectors 20a-n, each one of the outputs going to a different detector. Thus for example, splitter 14a provides m outputs to fiber optic elements la-lm; splitter 14b provides outputs to fiber optic elements 2a-2m; splitter 14c provides outputs to fiber optic elements 3a-3m; and splitter 14m provides outputs to fiber optic elements na-nm. At the other end, each of the combiners 18a-18n, where n is the number of splitters and LED sources, receives n inputs, one input from each splitter/LED source. Thus combiner 18a receives an input from fiber optic element la, 2a, 3a-na; combiner 18b receives an input from fiber optic elements lb, 2b, 3b-nb; combiner 18n receives inputs from fiber optic elements lm, 2m, 3m-nm. In operation, LED pulse driver circuitry 26' provides an enabling pulse to energize LED 12a, then LED 12b, then LED 12c. . . and finally LED 12n, each for successive periods of one millisecond each. When LED 12a is energized splitter 14a propagates radiation over n optical fiber elements la, lb-lm so that each detector receives at this time only one input, and during this time when the input is being received the detector or its associated ADC converter is enabled to receive that input, convert it to digital form and pass it on to computer 24. While the time multiplexing of sources 12' causes each of the sources to be energized independently, successively, the detectors and their ADCs are energized all at the same time each time each of the LED sources is energized. This can be seen more clearly, graphically, in Fig. 3, where the source enabling signals from LED sources 12a, 12b, 12c. . .12n are depicted at 40a, 40b, 30c . . .40n. Thus only one LED source is energized in each of the time periods t 6 shown along the abscissa, where each time period is typically one millisecond. However, the detector ADC is enabled at every one of those time periods 42a, 42b, . . .42n where n again is chosen as sixteen. Since detectors 20' are enabled during each of the n periods they will have an output during each period, as shown by output signals 44a, 44b, 44m. However, each output is uniquely defined. For example, the output of detector 20a during the first time period, t,, signal 44a 1 can only have come through optical fiber element la from LED source 12a because it is only source 12a that is enabled during that period. Likewise, during the second period, t 2 , the input to detector 44a2 to detector 20a can only come through optical fiber 2a from source 12b since that is the only source enabled during that time period, and so on. For each of the time periods t,_ n , for each of the detectors a-m. Thus by time multiplexing the sources and spatially multiplexing the detectors the system provides high-speed monitoring which is fast enough to operate at many cycles per second so that the effect of hammer blows, bending, and other effects can be seen. While at the same time there is a marked economy in the number of components required and the need for moving mechanical and optical parts with all the attendant wear and alignment problems is eliminated. The individual components are generally available. For example, the LED sources may be off the shelf components obtained from Lasertron, Inc. (Massachusetts), HRV Communications, Inc. (California), Nortel Ltd. (UK), EG&G Optoelectronics (Canada); the splitters and combiners may be off the shelf components obtained from Gould Electronics, Safiam (UK), JDS (Canada) and Amphenol (Illinois); the detectors may be off the shelf components obtained from EG&G Optoelectronics (Canada), Nortel Ltd. (UK), Lasertron, Inc. (Massachusetts) and Epitaxx (New Jersey); and the ADCs may be off the shelf components obtained from National Electronics, Inc. (Texas) and Data Translation, Inc. (Massachusetts). Returning to Fig. 2, LED sources 12a-n may each include a filter 50a, 50b, 50c, . . ., 50n for narrowing the spectral band delivered to the splitters, for example from 70 nm to 10 nm to better control the monitoring procedure. In addition, as shown in Fig. 4, LED source 12' may include more than one wavelength source. For example, each may include an LED having a wavelength of 1310 nm 52, and one of 1550 nm 54, so that the DUT can be monitored at two different wavelengths. In that case each of the LED sources' time period would be split into two sub-periods and splitter 14' would be a two- sixteen splitter rather than a one-sixteen splitter; otherwise the system would operate the same. Although specific features of this invention are shown in some drawings and not others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. Other embodiments will occur to those skilled in the art and are within the following claims: What is claimed is:

Claims: CLAIMS 1. An optical power transmission monitoring system for monitoring a multiplicity of transmission lines, comprising: a first plurality n of optical sources; a second plurality m of optical detectors; means for time multiplexing each of said n sources through m different lines to said m detectors; and means for spatially multiplexing each of said m detectors through n different lines to said n sources for simultaneously sensing radiation at each detector through a transmission line from a unique said source. 2. The optical power transmission monitoring system of claim 1 in which said means for time multiplexing includes a timing circuit for enabling each said source sequentially in n successive time periods. 3. The optical power transmission monitoring system of claim 2 in which each said detector includes an analog to digital converter enabled by said timing circuit synchronously with enablement of each said source. 4. The optical power transmission monitoring system of claim 1 in which each said source includes an LED. 5. The optical power transmission monitoring system of claim 1 in which each said source includes a filter for narrowing the spectral band of radiation emitted. 6. The optical power transmission monitoring system of claim 1 in which said means for spatially multiplexing includes an optical splitter associated with each source for distributing the output from a source over a plurality of m transmission lines. 7. The optical power transmission monitoring system of claim 1 in which said means for spatially multiplexing includes an optical combiner associated with each detector for calibrating the output from each source over a plurality of n transmission lines. 8. The optical power transmission monitoring system of claim 1 in which each said optical source includes light sources of a number of different wavelengths. 9. The optical power transmission monitoring system of claim 1 in which said transmission lines are fiber optic elements. 10. The optical power transmission monitoring system of claim 1 in which each said source is a laser.

Inventors: BARLOW ARTHUR J, BARLOW, ARTHUR, J.
Applicants: BARLOW ARTHUR J, EG & G INSTR INC, BARLOW, ARTHUR, J., EG & G INSTRUMENTS, INC.
Application year: 1997
Main classification: H04L
IPC codes: H04B10