Optical power meter for detecting loss factors in fiber optic communications

An optical power meter for a fiber optic link includes a photodetector that generates a current proportional to the energy of the light transmission in the fiber optic, a current-to-voltage converter that converts the current to a proportional voltage, and a voltage-to-frequency converter that converts the voltage to a digital pulse train proportional to the amplitude of the voltage. The digital pulse train increments a counter, which is read periodically by a microprocessor. The value read by the microprocessor corresponds to the average power in the light transmission. This digital value is converted to a power value in dBm and stored on a mass storage device and displayed on a liquid crystal display. The liquid crystal display shows the power level over time so that losses due to intrinsic and extrinsic loss factors can be quickly and easily identified. The power meter also includes software executed by the microprocessor that allows the user to input a range within which the power can fluctuate. If the measured power falls outside of that range, however, the power meter sets off an audible or visual warning signal to indicate a perturbation to the fiber optic link such as a mechanical stress, connector misalignment, or a security breach.

BACKGROUND OF THE INVENTION 
This invention relates generally to fiber optics and more particularly to 
detecting and identifying perturbations to the same. 
Fiber optic cable has become the predominant transmission media for 
telecommunications. The advantages of fiber optics are well known, most 
notably high bandwidth. There are, however, several disadvantages of using 
fiber optics over conventional communication paths. These disadvantages 
stem in large part from the manner in which data is communicated over the 
fiber optic cable. As is well known in the art, data is encoded in light 
pulses which are transmitted through the fiber optic cable. These 
lightwaves reflect within the fiber optic itself because of the difference 
in refractive indices of the fiber optic and the surrounding cladding. The 
cladding is a lossy glass that surrounds the conducting core. Because of 
this arrangement, the fiber optic cable is sensitive to slight 
perturbations to the cable, including mechanical stresses such as bending, 
twisting and pinching of the fiber itself. These perturbations cause the 
light in the fiber to be dispersed into the cable cladding, within a short 
distance, thereby increasing the bit error rate (BER) of the system. 
Fiber optic cable is particularly sensitive to interconnect problems 
between adjacent fibers. These problems can be classified into two 
categories: intrinsic and extrinsic. The intrinsic problems include 
numerical aperture (NA) mismatch, which occurs when the NA of the 
transmitting fiber is larger than that of the receiving fiber. Another 
intrinsic problem, core diameter mismatch, occurs when the core or 
diameter of the transmitting fiber is larger than that of the receiving 
fiber. Cladding diameter mismatch, on the other hand, occurs when the 
cladding of two different fibers differ, since the core is no longer 
aligned. The core may also not be perfectly centered within the cladding. 
Ideally, the geometric axis of the core and cladding should coincide. 
However, this is not always the case. When they do not coincide, the fiber 
is said to suffer from concentricity problems. The core cladding may be 
elliptical rather than circular. This causes problems when two fibers are 
joined together because the ends do not perfectly align. All of these 
intrinsic problems with the fiber itself cause certain of the light to be 
lost or dispersed. This produces a corresponding reduction in the 
intensity of the light, which increases the BER of the system. 
The extrinsic problems are those contributed by the connector that is used 
to join adjacent fibers. The four main intrinsic problems that cause loss 
in a fiber optic cable are lateral displacement of the connectors, end 
separation of the fibers, angular misalignment of the fibers, and surface 
roughness of the fibers. Lateral connector displacement occurs when the 
axis of one fiber is shifted laterally with respect to the adjacent fiber. 
When one fiber's axis does not coincide with that of the other, a loss 
occurs. This loss is approximately linear in decibels as a function of the 
lateral misalignment ratio L/D, where L is the displacement and D is the 
diameter of the fiber. When two fibers are brought together by a 
connector, the two opposing ends may be separated by a small gap. This gap 
produces two types of loss. The first is Fresnel reflection loss, which is 
caused by the difference in the refractive indices of the two fibers in 
the intervening gap, usually air. The second type of loss for multi-mode 
fibers results from the failure of high-order modes to cross the gap and 
enter the core of the second fiber. The two losses combined produce a 
so-called gap loss which is both a function of the end separation ratio 
S/D, where S is the separation gap and D is the diameter of the cable, and 
also of the numerical aperture (NA). The gap loss is generally linear with 
respect to the end separation ratio for a given NA. Further losses can 
occur from angular misalignment of two adjoining fibers. Ideally, the ends 
of the mated fibers should be perpendicular to the fiber axis and 
perpendicular to each other during engagement. If the fiber axis of one 
fiber is angularly offset with respect to the fiber axis of the adjacent 
fiber, a loss will occur. All of these extrinsic factors produce signal 
losses in the fiber. 
There are several instruments that allow a user to measure the signal level 
in a fiber optic cable. One such instrument is the FDDI Network Interface, 
Model No. J2173C, designed and manufactured by Hewlett-Packard Company of 
Palo Alto, Calif. This FDDI Network Interface can be interposed between a 
light transmission source and a light transmission destination to measure 
the average power level received through the fiber optic cable. This 
interface receives the light signal transmitted through the fiber optic 
cable from the light transmission source, measures the average energy, and 
displays the measured energy on a crude bar graph consisting of a linear 
array of light-emitting diodes (LED). 
A block diagram of the J2173C system is shown in FIG. 1. The system 10 
includes an optical power meter 12 which receives the optical energy 
transmitted over the fiber cable, as indicated by the downward slanting 
arrows. The optical power meter 12 is coupled to a microcontroller system 
14 via conductor 16. The optical power meter, as described further below, 
produces a digital pulse train whose frequency is proportional to the 
average optical power measured by the optical power meter 12. 
The microcontroller system 14 includes a microprocessor 18 coupled to a 
memory 20 over bus 22. Memory 20 can include both read only memory (ROM) 
as well as dynamic random access memory (DRAM). Coupled to the 
microprocessor 18 is a mass storage device 24, which communicates to the 
microprocessor over bus 26 in a conventional manner. The microcontroller 
system 14 also includes a counter 28 that has a clock input 30 coupled to 
bus 16 to receive the digital pulse train from the optical power meter. 
The counter 28 is also coupled to the microprocessor 18, which reads the 
digital count out of the counter over bus 32. This count is a digital 
representation of the average power measured by the optical power meter. 
The counter is reset or gated at predetermined time intervals by the 
microprocessor 18 according to the time stored in time base 34. The 
microprocessor 18 stores a value in time base 34 that represents the time 
period over which the pulses are to be counted. The microprocessor can 
change the value in the time base in order to change the resolution of the 
power measurement. The time base is coupled to a gate input 36 of the 
counter in order to reset the counter as well as latch the output of the 
counter in order to be read by the microprocessor 18. The microprocessor 
18 then reads this latched value out of the counter over bus 32 by 
enabling the output of the counter 28 by applying an output enable signal 
to output enable (OE) input 38 of the counter 28. 
The output of the counter 28 is a digital representation of the received 
optical energy measured by the optical power meter. The actual optical 
power is then determined by the microprocessor by either looking up the 
corresponding power level in a look-up table in memory 20 or calculating 
the corresponding power using a logarithmic equation, which is known to 
those skilled in the art. This optical power value is expressed in dBm and 
stored in the mass storage device 24. The optical power value is also 
displayed on display 40. The display, as mention above, consists of a 
plurality of LEDs arranged as a linear array to form a bar graph. The 
linear array corresponds to a power range of -30 dBm to -13 dBm, in 3 dBm 
increments. The microprocessor 18 illuminates the LEDs to provide a 
graphical indication of the current power level through the fiber optic 
cable. Although this is useful for many applications such as determining 
whether the FDDI specifications are being met, the resolution is 
insufficient to detect many of the loss factors described above. Many of 
these loss factors produce losses on the order of 0.1 db, which would not 
even be registered by the J2173C Network Interface. Moreover, even 
assuming the loss did produce a change in the graphical representation, 
this loss might be easily missed because the network interface has no way 
of indicating what the power level was prior to the loss. Thus, unless the 
technician was watching the graphical display when the loss occurred or 
remembered what the power level was prior to the loss, the loss may go 
undetected. 
Referring now to FIG. 2, a schematic diagram of the optical power meter 12 
is shown. The optical power meter 12 includes an energy to current 
converter 42 that is juxtaposed to a fiber optic cable to receive the 
incoming optical energy. The optical energy is received by a photodiode 44 
that converts the incoming optical energy to a current I. Converter 42 
also includes an amplifier 46 that forwards the received light data onto 
the destination coupled to terminal 48. 
The current I produced by the photodiode 44 is coupled to a 
current-to-frequency converter 50. The current-to-frequency converter 50 
includes a standard current-to-frequency converter 52 manufactured by 
Analog Devices and sold under the Part No. AD654JR. The 
voltage-to-frequency converter 52 is biased in a conventional manner as 
described in the data sheets accompanying the AD654JR and is therefore not 
described further. The current-to-frequency converter 60 produces a 
digital pulse train FREQ--OUT whose frequency is proportional to the 
amplitude of the current generated by the photodiode 44, which in turn, is 
proportional to the average power of the received light transmission. This 
relationship holds because of the unique coding scheme used in fiber optic 
communication. As is known in the art, the lowest level coding uses an 
equal number of ones and zeros over a predetermined number of bits. 
Accordingly, the frequency of the digital pulse train FREQ--OUT 
corresponds to the average power level in the received light transmission. 
Hewlett-Packard also makes another communication instrument with an 
integrated power meter Model No. 156MTS that overcomes one of the 
limitations of the HP J2173C. The HP 156MTS displays a numerical value 
corresponding to the average power level in the fiber optic cable. The 
numerical value has one digit to the right of the decimal point so 
presumably the power meter can resolve losses of .+-.0.1 dB. Thus, the HP 
156MTS should be able to resolve relatively small losses due to the 
above-described factors. The problem, however, is that this product does 
not include the ability to notify the technician of any such losses. As 
with the HP J2173C, a loss would likely go undetected unless the 
technician was observing the numerical display when the loss occurred or 
happened to remember what the numerical value was prior to the loss. 
Moreover, the 156MTS is not designed to stay in the signal path of an 
active fiber link. Instead, it is a test set designed solely for setting 
up and initially testing the link, not for full time monitoring. In either 
case, these products do not provide a reliable way to identify problems in 
the cable. 
Accordingly, a need remains for a method of informing a technician or other 
concerned personnel of a problem in the fiber optic cable. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the invention to reliably and accurately 
inform the user of loss factors in the fiber optic cable due to intrinsic 
and extrinsic factors. 
In order to accomplish this objective, the applicant has invented an 
optical power meter that includes a graphical display for displaying 
historical power levels of the power in the fiber optic link. The power 
meter can resolve very small losses in order to detect both intrinsic and 
extrinsic loss factors in the fiber optic link. The optical power meter, 
in the preferred embodiment, includes an optical energy to current 
converter that receives the optical energy transmitted through the fiber 
and converts it to a current proportional to the average power in the 
light transmission. This converter also amplifies the received light and 
forwards it onto its destination so as to not interrupt the transmission. 
This allows the optical power meter to be placed continuously in the 
system so as to record historical power level data. Power meter also 
includes a current-to-voltage converter, which in the preferred 
embodiment, is a transimpedance amplifier. The transimpedance amplifier 
converts the current to a negative voltage proportional to the amplitude 
of the current. The output of the transimpedance amplifier is coupled to a 
voltage-to-frequency converter that converts the voltage output of the 
transimpedance amplifier to a pulse train having a frequency proportional 
to the amplitude of the voltage signal. This pulse train increments a 
counter that is read in periodic intervals by a microprocessor. The 
microprocessor reads the digital output of the counter and resets the 
counter to begin counting the number of pulses in the next time period. 
This digital value provided by the counter then is a digital 
representation of the optical power level. The microprocessor can either 
then look up the power level corresponding to this digital representation 
in a look-up table or calculate the average power level on the fly using a 
logarithmic equation. 
The power meter is part of a protocol analyzer system, which is preferably 
portable, that includes an LCD display and a mass storage device. The 
microprocessor displays the power level on a graph whose vertical axis is 
the measured power level in dBm and whose horizontal axis is time. The 
microprocessor also stores the power data on the mass storage device so 
that it later can be retrieved and analyzed. This system also allows the 
user to change the time scale of the horizontal axis so that either long 
periods of time can be displayed to identify problems that may have 
occurred over a long period of time or a short time frame to analyze 
individual losses to determine the cause thereof. 
In another aspect of the invention, the system allows the user to set 
limits within which the power level should operate. If the measured power 
level falls outside of this user-defined range, the system will generate a 
warning signal either visually or audibly. This limit can be either above 
or below, or both, a baseline power level that is measured by the power 
meter. 
The foregoing and other objects, features and advantages of the invention 
will become more readily apparent from the following detailed description 
of a preferred embodiment of the invention which proceeds with reference 
to the accompanying drawings.

DETAILED DESCRIPTION 
Referring now to FIG. 3, a fiber optic communication system according to 
the invention is shown generally at 60. The system includes a light 
transmission source 62, a light transmission destination 64, and a fiber 
optic link shown generally at 66. The source 62 includes a computer 68 
located in a secured environment, as indicated by the dashed lines, 
although it is not required to be so secured by the invention. The 
destination similarly includes a computer 70 also preferably located in a 
secured environment. The secured environment is shown because one 
application of the power meter according to the invention is to detect 
security breaches to the fiber. The invention is not limited, however, to 
secured environments, but instead can be used in unsecured environments. 
Coupled between the source and destinations is a fiber optic link 66 that 
is both above ground and below ground, as is known in the art. The 
particulars of the fiber optic link are not important to the invention. 
Instead, the power meter according to the invention can be used with any 
fiber optic cable either now known or developed in the future. The fiber 
optic link will be referred to herein as cable 72, although it is 
understood that the cable may include a multiplicity of individual 
segments as well as having repeaters (not shown) located therein. 
Interposed between the source 62 and destination 64 is the optical power 
meter system 74 according to the invention. The system 74 includes an 
input jack 76 coupled to cable 72 that receives the light transmission 
conducted over cable 72. An additional fiber optic cable 78 is coupled 
between an output jack 80 of the system and an input jack 86 of computer 
70. The system, as mentioned above, measures the optical power and the 
light transmission while at the same time passing the data through to the 
destination. As shown in FIG. 1, the system 74 is preferably based on a 
portable personal computer. As with such computers, the system 74 includes 
a keyboard as well as a mouse input device (not shown) and a display 84. 
The display 84, in the preferred embodiment, is a liquid crystal display 
(LCD), but the invention is not limited thereto. The portable computer 
allows the optical power meter system to be easily transported and used in 
different environments. Alternatively, a desktop computer could be used or 
a stand-alone optical power meter system including the components 
hereinafter described. 
Referring now to FIG. 4, a block diagram of the system 74 is shown. The 
system is similar to that shown in FIG. 1, with several notable 
exceptions. Common elements between FIGS. 1 and 4 retain common reference 
numerals. The description above applies to those common elements and is 
therefore not repeated. 
The first difference is a different optical power meter 88 is used. This 
power meter 88 is described further below with reference to FIGS. 5 and 6. 
Another difference is that the system 74 includes an LCD display 84 and 
associated software to display historical power data, as discussed further 
below. Part of this software allows the microprocessor to change the value 
in the time base in order to change the bandwidth or sample time of the 
power measurement. In the preferred embodiment, the value is within the 
range of 0.01 to 1.0 seconds, but is not limited thereto. The system 
software also allows the user to specify the timeframe over which the 
measured power levels are displayed. This allows the user to view the 
power level over a long period of time to identify changes in the power 
level during that time or to focus in on short time intervals to analyze 
individual events to identify the root cause thereof. 
Referring now to FIG. 5, a schematic diagram of a first embodiment of the 
optical power meter 85 is shown. The optical power meter 88 according to 
the invention includes an energy to current converter 90 that is 
juxtaposed to the cable 72 to receive the incoming optical energy. The 
optical energy is received by a photodiode 92 that converts the incoming 
optical energy to a current I. The photodiode 92, in the preferred 
embodiment, is an indium gallium arsenide photodiode having a wavelength 
of 1310 nanometers (nm). Converter 90 also includes an amplifier 94 that 
forwards the received light data onto the destination. 
The current I produced by the photodiode 92 is passed through a high 
frequency filter comprised of multi-layered ferrites 96 and 98. The output 
of the high frequency filter is a DC current. The output of the filter is 
provided to a current-to-voltage converter 100. At the core of 
current-to-voltage converter is a transimpedance amplifier 102. The 
amplifier 102 has a noninverting input connected to ground and an 
inverting input connected to the output of the high frequency filter. By 
grounding the noninverting input, a zero bias voltage is provided to the 
anode of the photodiode 92. The transimpedance amplifier 102 includes a 
parallel capacitor C1 and resistor RI combination in the feedback loop to 
create a low frequency pole (e.g., 159 Hz). The lowpass filter in the 
feedback path is optional, however, because of the bandwidth of the 
transimpedance amplifier, which cannot respond to the high frequency 
components in any case. The transimpedance amplifier produces a negative 
voltage that is proportional to the amplitude of the photo current I. 
The output of the transimpedance amplifier 102 is coupled to a negative 
voltage-to-frequency converter 70. The converter 70 includes a standard 
voltage-to-frequency converter 78 manufactured by Analog Devices and sold 
under the Part No. AD654JR. The voltage-to-frequency converter 70 is 
connected in a conventional manner as described in the data sheets 
accompanying the AD654JR and is therefore not described further. One 
notable point, however, is that highly precise components are used such as 
0.1% resistors to achieve the desired level of resolution. In addition, a 
poly-styrene capacitor C2 is used instead of conventional tantalum 
capacitors. The negative voltage-to-frequency converter 70 produces a 
digital pulse train FREQ.sub.-- OUT whose frequency is proportional to the 
amplitude of the negative voltage generated by the current-to-voltage 
converter 100, which in turn is proportional to the average power of the 
received light transmission. This relationship holds because of the unique 
coding scheme used in fiber optic communication. As is known in the art, 
the lowest level coding uses an equal number of ones and zeros over a 
predetermined number of bits. Accordingly, the frequency of the digital 
pulse train FREQ.sub.-- OUT precisely corresponds to the average power 
level in the received light transmission. 
Another version of the optical power meter 88 is shown in FIG. 6. This 
embodiment is used where a negative bias is required for the photodiode. 
In this case, the current-to-voltage converter employs a differential 
buffer 106 having a resistor R2 coupled across its inverting and 
noninverting terminals to sense the current produced by the photodiode 92. 
The current is passed through resistor R2 thereby generating a voltage 
thereacross, which is detected by the differential buffer 82 and amplified 
to its output. The converters 70 is identical to that used in the first 
embodiment and is therefore not discussed further. 
Referring now to FIG. 7, a flow chart of the system software is shown. The 
software is executed by the microprocessor 18 shown in FIG. 5 and stored 
in the memory 20 and/or the mass storage unit 24. In addition to 
displaying the optical power level as it is measured, the system also 
monitors the level to detect slight variations that could be caused by 
internal and/or external loss factors. This operation is shown in FIG. 7. 
The system begins by initializing itself in step 112. As part of this step, 
the system gathers a baseline of information against which subsequent 
measurements can be compared. It is assumed that this baseline level 
corresponds to a unperturbed state of the fiber optic link. Accordingly, 
the initialization should be done as soon after the installation of the 
link as possible so that there are no intervening perturbations to the 
link. The system can also prompt the user during the initialization step 
to enter an upper and/or lower bound within which the user wants the power 
level to remain. If the measured power level falls outside of one of these 
bounds, the system will set an alarm condition or generate a warning, as 
described further below. 
After the initialization is complete, the system monitors and records the 
received optical power level in step 114. In this step, as described 
above, the microprocessor 18 reads the counter value and looks up or 
computes the corresponding power level. Once the power level is known, the 
value is stored to the mass storage device 24 and displayed on the LCD 
display 84 according to user-defined display parameters. 
The system then compares the received optical power level determined in 
step 114 to the user-defined limits set in the initialization step 112. If 
the power level is within these user-defined limits, the microprocessor 18 
transitions back to step 114 and the next sample is taken after the 
predetermined time period has elapsed. If, on the other hand, the received 
optical power falls outside one of the user-defined limits, the system 
sets an alarm condition in step 118 to notify the technician or user of 
the loss. This alarm condition can either be audible or visual. The alarm 
condition therefore notifies the user that the fiber optic link has been 
perturbed in some manner. The user can then examine the display to 
determine what may have been the root cause of the loss and therefore 
identify and isolate the problem. Thus, the system is a powerful tool to 
monitor and troubleshoot fiber optic networks. 
Several exemplary displays are shown in FIGS. 8-11. These displays show the 
resulting displays produced by the system shown in FIG. 4 under various 
conditions. In FIG. 8, the displayed power level shows a downward step. 
This step could be caused by a connector becoming misaligned or possibly 
due to a security breach, i.e., a fiber tap on the optical fiber itself 
This information could prompt the user to perform an inspection of the 
fiber link in order to identify the misaligned connector or possibly even 
the fiber tap. 
In FIG. 9, the horizontal axis shows a much shorter timeframe than that in 
FIG. 8. The system allows the user to specify the time scale by inputting 
the desired time scale using the keyboard or mouse, or alternatively, 
selecting between predefined time scales. The power loss shown in FIG. 9 
may be caused by a fiber being temporarily bent, twisted or pinched. 
Although the display shows a loss during a short time period, the power 
level rises back to substantially the same power level as before the 
perturbation. Thus, the system informs the user that the condition no 
longer exists that caused the loss. Nonetheless, the user may still want 
to know that such a perturbation existed because it may suggest an ongoing 
problem or may result in a weakening of the fiber link that could 
eventually cause a failure of the system. 
Another scenario is shown in FIGS. 10 and 11 using two different time 
scales. FIG. 10 shows the display generated by a mechanical disturbance of 
the fiber such as caused by an earthquake. The display in FIG. 10 uses a 
short time scale, which allows a user to examine the particulars of the 
power level during the disturbance. This level of resolution may allow the 
user to identify where, how and certainly when, the disturbance occurred. 
The same event is shown in a longer time scale in FIG. 11. This scale is 
useful where the user may be gone for several days and wants to display 
the entire length of his absence to identify any losses that may have 
occurred during that time. The user could then focus in on the event by 
changing the time scale to produce the display shown in FIG. 10. From 
this, the user could better ascertain the cause and potentially identify 
the source of the problem. 
Having described and illustrated the principles of the invention in a 
preferred embodiment thereof, it should be apparent that the invention can 
be modified in arrangement and detail without departing from such 
principles. I claim all modifications and variation coming within the 
spirit and scope of the following claims.