Mixer-based time domain reflectometer and method

An apparatus to measure optical characteristics of a fiber optic transmission line or other optical medium may include a source to generate a bipolar pulse signal waveform. The apparatus may also include a mixer to mix the bipolar pulse signal waveform and an optical pulse and reflected signal waveform from the fiber optic transmission line or other optical medium to form a mixed product waveform, wherein the reflected signal is responsive to the optical pulse being transmitted into the fiber optic transmission line or optical medium.

BACKGROUND OF THE INVENTION

The present invention relates to optical signal transmission systems or the like and more particularly to a mixer-based time domain reflectometer and method for detecting any reflections, anomalies or defects in a fiber optic transmission line or other optical medium.

Transmission lines are commonly employed to communicate signals between various portions of an electronic system. For example, coaxial transmission lines, waveguides, and even parallel arrangements of metallic conductors are typically employed as transmission lines in such systems. Increasingly, fiber-optic transmission lines or other optical media are being used instead of conventional metallic transmission lines to communicate signals in electronic systems due to the generally higher noise immunity and lower signal attenuation properties obtainable in such lines. Additionally, fiber-optic transmission lines are generally thinner and lighter than metallic conductors of comparable capacity.

In systems employing fiber optic transmission lines or the like, difficulties may arise due to degradation of the line resulting from physical damage, aging, poorly matched and/or damaged connectors, or for other reasons. In practice, difficulties with transmission lines are frequently difficult to detect and diagnose, particularly in electronic systems where only a single terminal end of the transmission line may be accessible. Although a number of different methods are available to detect and diagnose transmission line difficulties, one useful and commonly employed method is time domain reflectometry. In time domain reflectometry, an optical pulse or pulses may be transmitted into a fiber optic transmission line or medium. Any anomalies or defects may result in a reflected signal which may be detected by a time domain reflectometer. Such reflectometers are usually formed from standard components as opposed to custom parts to keep costs reasonable. These standard components, such as mixers or the like, may require appropriate input or drive signals and modulation signals for optimum operation and ability to effectively measure and analyze input pulses and reflected waveforms and mixed or modulated waveforms to detect any reflections at selected segments along a fiber optic transmission line or other optical medium.

BRIEF SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, an apparatus to measure optical characteristics of a fiber optic transmission line or other optical medium may include a source to generate a bipolar pulse signal waveform. The apparatus may also include a mixer to mix the bipolar pulse signal waveform and an optical pulse and reflected signal waveform from the fiber optic transmission line or other optical medium to form a mixed product waveform, wherein the reflected signal is responsive to the optical pulse being transmitted into the fiber optic transmission line or optical medium.

In accordance with another embodiment of the present invention, an optical system may include a fiber optic transmission line or optical medium. The system may also include a mixer-based optical time domain reflectometer with a bipolar local oscillator to measure optical characteristics of the transmission line or optical medium.

In accordance with another embodiment of the present invention, an aerospace vehicle may include a fuselage and other components. The aerospace vehicle may also include a fiber optic transmission line or optical medium disposed in the fuselage, other components, or both. The aerospace vehicle may further include a mixer-based optical time domain reflectometer with a bipolar local oscillator to measure optical characteristics of the transmission line or optical medium.

In accordance with another embodiment of the present invention, a method to measure optical characteristics from a selected segment of a fiber optic transmission line or other optical medium divided into a predetermined number of segments may include determining an average value or voltage of an optical pulse and reflected signal waveform without any modulation on a local oscillator. The method may also include applying a bipolar pulse on the local oscillator at a time delay corresponding to the selected segment and mixing the bipolar pulse and the optical pulse and reflected signal waveform to form a mixed product waveform. The method may also include determining an average value or voltage of the mixed product waveform.

Other aspects and features of the present invention, as defined solely by the claims, will become apparent to those ordinarily skilled in the art upon review of the following non-limited detailed description of the invention in conjunction with the accompanying figures.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention.

FIG. 1is a block diagram of an exemplary optical system100including a mixer-based time domain reflectometer102with a bipolar local oscillator104in accordance with an embodiment of the present invention. The bipolar local oscillator104may be part of the mixer106as described in more detail below.

The optical system100may include a fiber optic transmission line108or other optical medium to transmit optical energy in the form of optical signals. The fiber optic transmission line108may be formed by multiple line portions110that may each be joined together by suitable optical connectors112to minimize signal reflections. The reflectometer102may measure optical characteristics of the line108. Measuring optical characteristics may include, but is not necessarily limited to detecting reflections that may be caused by anomalies in the line108or optical medium, such as discontinuities, defects, degradation or the like.

The system100may also include an optical signal source114to transmit optical signals. The optical signal source114may be a laser signal source or other optical source. The optical source114may emit one or more relatively short-duration pulses of optical energy towards a partial mirror118in response to an input signal120or other stimulus. The partial mirror118may transmit at least a portion of the optical pulse116into a terminal end122of the fiber optic transmission line108. The optical pulse may then propagate along the length of the line108. When the optical pulse116encounters an anomaly in the line108or optical medium, optical energy or a reflected signal pulse124may be reflected back toward the terminal end122of the line108or medium. The reflected optical pulse124or signal is a function of the difference in indices of refraction between the fiber material and the other material (usually air) at the break. The reflected optical pulse energy124may be substantially reflected by the partial mirror118into an optical receiver126. The optical receiver126may also detect the original optical signal pulse or energy116along with any reflected signals or reflected optical energy124responsive to the optical pulse116. The detected original optical pulse116and reflected optical energy124or signals may be converted to electrical signals by the optical receiver126forming a waveform that may be transmitted to the mixer106. The mixer106may be a commercially available mixer rather than a custom component to maintain reasonable costs. For example, the mixer106may be an Analog Devices AD8343 active mixer, available from Analog Devices, Inc. of Norwood, Mass., or a similar device.

The system100may also include a bipolar pulse generator128or similar signal source to generate a bipolar pulse. The bipolar pulse may include a predetermined characteristic for proper or effective operation of the mixer106. Many active mixer devices, such as the Analog Devices AD8343, expect a local oscillator input of either about +1 or −1, that is, the device may operate most optimally or efficiently when either substantially heavily turned on in the positive direction or substantially heavily turned on in the negative direction. Accordingly, a negative going bipolar pulse may serve to provide optimal or effective operation of the reflectometer102as described in more detail herein.

The bipolar pulse generator128may generate the bipolar pulse or pulse waveform in response to the input signal120or other stimulus that may also cause the optical energy or pulse116to be generated by the optical source114. The bipolar pulse may be delayed by a variable delay module130. As described in more detail herein, the variable delay module130may delay the bipolar pulse of a bipolar pulse signal waveform by a selected time duration corresponding to any reflection of a signal or pulse from a selected segment of the fiber optic transmission line108to measure optical characteristics from the selected segment of the line108.

The bipolar pulse signal waveform from the variable delay130may be applied to the local oscillator104. The bipolar pulse signal waveform and the optical pulse and reflected signal waveform from the optical receiver126may be mixed in the mixer106to form a mixed product waveform. Expressed in other terms, the optical pulse and reflected signal waveform may be modulated in the mixer106by the bipolar pulse signal waveform. An output of the mixer106may be coupled to an integrator132. The integrator132may time average the product signals or mixer output signals to provide a time-averaged output.

The fiber optic transmission line108may be divided or segmented into a predetermined number of segments (N) or intervals for purposes of analysis and identifying a location of an anomaly. Because the variable time delay module130performs a gating function, the reflected energy signals may be generated only from the segment of the line108or medium that corresponds to the selected time delay. The reflected energy signal waveform124may then be time averaged by the integrator132over the selected time interval and successively repeated for each of the predetermined number of segments (N) to generate an integrated value for the reflected energy or signal waveform over all of the segments of the line108or medium.

The system100may also include a microcontroller136to facilitate determination of the optical characteristics of the line108or medium or to detect reflections resulting from anomalies in the line108. The microcontroller136may include an analog-to-digital converter (A/D)138to receive the time averaged output signals or waveforms from the integrator132and to convert the signals to a corresponding digital signal or waveform.

The microcontroller136may also include a microprocessor140. The microprocessor140may perform various control functions and analysis of the waveforms as described in more detail herein. The microprocessor140may be coupled to an output device or system142. In one embodiment of the present invention, the system142may perform additional analysis of the waveforms or data generated by the microprocessor140. In another embodiment of the present invention the device or system142may be a display or other output device that may present the waveforms and other data to a user for analysis or evaluation. In a further embodiment, the output device or system142may be a buffer or similar storage device to store the data for access by other external systems (not shown).

The microcontroller140may also control operation of the variable delay module130to selectively delay the bipolar pulse signal to correspond to different segments along the line108or medium for measuring optical characteristics or detecting any anomalies or defects along the line108or medium.

In another embodiment of the present invention, the integrator132may be a radio frequency (RF) power detection unit or the like. The power detection unit132may receive the waveforms from the mixer106and generate a DC voltage corresponding to the power level of the waveforms. Accordingly, the power detection unit132may provide a continuous and generally constant DC voltage corresponding to the power level of the signals from the mixer106to the A/D converter138, which may transfer the power to the microprocessor140in digital form. The power detection unit132may include an Analog Devices AD8362 TRU-PWR Power Detector, or similar device.

FIGS. 2A and 2B(collectivelyFIG. 2) are a flow chart of an exemplary method200to measure optical characteristics or to detect any anomalies from a selected segment of a fiber optic transmission line or other optical medium in accordance with an embodiment of the present invention. The method200may be embodied in the optical system100ofFIG. 1or a similar system and may be performed thereby. In block202, an optical pulse signal may be transmitted into an optical medium, such as a fiber optic transmission line, similar to line108ofFIG. 1, or other optical medium. The optical pulse signal may be a laser pulse or similar optical pulse.

In block204, a predetermined constant positive or negative local oscillator (LO) signal may be generated. A local oscillator of a mixer, such as local oscillator104of mixer106ofFIG. 1, or a similar device, may operate optimally if driven either substantially positive or negative hard enough so that a radio frequency (RF) input signal is multiplied by either about a +1 or about a −1.

In block206, the optical pulse (VP) and the reflected signal may be mixed with the constant LO signal. A waveform may be generated containing the optical pulse and reflected signals or pulses. Referring also toFIG. 3,FIG. 3is a graph300of exemplary waveforms302-306to measure optical characteristics of a fiber optic transmission line or other optical medium in accordance with an embodiment of the present invention. The waveforms302-306may be generated in the mixer-based time domain reflectometer102ofFIG. 1and may be representative of the output signals of the A/D converter138. The waveform302inFIG. 3is an example of a waveform containing a detected optical pulse (VP) transmitted into a fiber optic transmission line or medium and reflected pulses (V1-V4) resulting from portions of the optical pulse (VP) energy being reflected by anomalies, such as connectors similar to connectors112ofFIG. 1or the like.

In block208, an average value or voltage (VCAL) of the detected optical pulse (VP) and reflected signal waveform for a fiber optic transmission line or medium segmented into a predetermined number of segments (N) or intervals may be determined. The average value or voltage may be determined without any modulation or signal being applied to a local oscillator of a mixer, such as mixer106(FIG. 1). The average value or voltage may be represented by equation 1:

In block210, a bipolar signal or waveform having predetermined characteristics for proper or effective operation of a mixer associated with a reflectometer may be generated. As previously discussed, many active mixer devices, such as the Analog Devices AD8343, expect a local oscillator input of either about +1 or −1, that is, for optimum operation the device is preferably either substantially heavily turned on in the positive direction or substantially heavily turned on in the negative direction. Accordingly, the bipolar signal or waveform may include a negative-going pulse or the like for substantially optimal or effective operation of the reflectometer mixer. Waveform304inFIG. 3is an example of a negative-going pulse in accordance with an embodiment of the present invention.

In block212, the bipolar pulse of the bipolar signal or waveform may be time delayed by a selected duration corresponding to a selected segment (ith segment) or interval along the fiber optic line or medium to measure optical characteristics or reflections from the selected segment. The negative-going pulse may be thought of as a window that is moveable along the fiber optic line or medium in response to the selected time delay to measure optical characteristics or detect anomalies at the location of the window corresponding to a selected segment of the line or medium.

In block214, the detected original optical pulse and reflected signal waveform may be mixed, multiplied or modulated, such as in mixer106ofFIG. 1, by the bipolar pulse waveform to form a mixed product waveform. In block216, an average value or voltage of the output of the mixer or mixed product waveform (VDISP) for the fiber optic transmission line segmented in the predetermined number of segments (N) may be determined. The average value or voltage of the mixed product waveform (VDSIP) may be represented by equation 2:

The average value or voltage may be determined by means, such as the integrator132or power detector unit inFIG. 1, and converted to a digital form, such as by A/D converter138. Referring toFIG. 3, the optical pulse and reflected signal waveform302may be mixed with the bipolar pulse waveform306to provide the mixed product waveform306after integration, such as by integrator132, and conversion to digital form by A/D converter138.

In block218, a value or voltage (Vi) at the selected segment (ith segment) may be determined. The value or voltage at the selected segment may include determining the difference between the average value or voltage of the detected optical pulse signal and reflected signal waveform (VCAL) and the average value or voltage of the mixed product waveform (VDISP). Accordingly, the value or voltage at a selected segment may be represent by equation 3:

In another embodiment of the present invention, the difference (VCAL-VDISP) may be amplified by a predetermined factor. Under some circumstances this may be deemed appropriate to take full advantage of the range of the A/D converter. The predetermined factor may be a function of the predetermined number of segments (N) or may be more or less for practical purposes.

In practice, the bipolar pulse or local oscillator window pulse to select a segment as described above may not always align exactly with a reflection pulse from the selected segment of the fiber optic transmission line or other optical medium. In the event of such non-alignment, part of the reflection pulse value will in one segment and the other part will be in an adjacent segment. Adjacent segments with significant values may be assumed to be associated and numerically combined, although this assumption may slightly reduce the temporal resolution of the measurement.

In block220, optical characteristics, such as any anomalies defects, discontinuities or the like, may be may be measured or detected in the fiber optic transmission line or other optical medium at the selected segment based on the value or voltage associated with the segment. In block222, anomalies along the fiber optic transmission line or other optical medium may be detected by selectively delaying the bipolar pulse or window pulse to correspond to other segments along the line or medium as previously discussed.

FIG. 4is an illustration of an exemplary aerospace vehicle400including an optical system402and mixer-based time domain reflectometer device404in accordance with an embodiment of the present invention. The aerospace vehicle400may be a commercial passenger aircraft as provide by the Boeing Company of Chicago, Ill. or other type of aircraft. The optical systems402may be similar to the optical system100ofFIG. 1. Various embodiments of an optical system, similar to optical system100ofFIG. 1may be used in association with various system and sub-systems of the aircraft400, such as flight control systems, communications systems within the aircraft400, such as telecommunications systems, in flight entertainment systems, Internet access systems and the like distributed to passenger seating, as well as other aircraft systems. The various embodiments of the optical system402and reflectometer device404may be used to perform fault-checking and/or operational monitoring of the fiber optic transmission lines or other optical medium that may be included in these various systems.

AlthoughFIG. 4illustrated the reflectometer devices404as possibly being an integral component of the aircraft400, those skilled in the art will readily understand that one or more embodiments of the reflectometer device404may also be incorporated into a portable test device, such as device406that may be separately coupled to the various systems and sub-systems of the aircraft400to perform any ground-based or other diagnostic analysis on selected optical systems.