U.S. patent application number 11/068661 was filed with the patent office on 2006-10-05 for methods and apparatus for optical modulation amplitude measurement.
This patent application is currently assigned to International Business Machines Corporation. Invention is credited to Casimer DeCusatis, Daniel M. Kuchta, Jeremy Daniel Schaub.
Application Number | 20060222370 11/068661 |
Document ID | / |
Family ID | 36581763 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060222370 |
Kind Code |
A1 |
DeCusatis; Casimer ; et
al. |
October 5, 2006 |
Methods and apparatus for optical modulation amplitude
measurement
Abstract
Techniques for measuring optical modulation amplitude (OMA) are
disclosed. For example, a technique for measuring an OMA value
associated with an input signal includes the following
steps/operations. The input signal is applied to a photodetector,
wherein the photodetector is calibrated to have a given
responsivity value R, and further wherein the photodetector
generates an output signal in response to the input signal. The
output signal from the photodetector is applied to a radio
frequency (RF) power meter, wherein the RF power meter measures the
root mean squared (RMS) power value of the output signal received
from the photodetector. The OMA value associated with the input
signal is determined in response to the root mean squared (RMS)
power value measured by the RF power meter. The OMA value may be
determined as a function of a factor F derived from a relationship
between an amplitude of a data signal and the RMS value of the data
signal.
Inventors: |
DeCusatis; Casimer;
(Poughkeepsie, NY) ; Kuchta; Daniel M.;
(Patterson, NY) ; Schaub; Jeremy Daniel; (Austin,
TX) |
Correspondence
Address: |
Ryan, Mason & Lewis, LLP
90 Forest Avenue
Locust Valley
NY
11560
US
|
Assignee: |
International Business Machines
Corporation
Armonk
NY
|
Family ID: |
36581763 |
Appl. No.: |
11/068661 |
Filed: |
February 28, 2005 |
Current U.S.
Class: |
398/135 ;
324/754.23 |
Current CPC
Class: |
G01M 11/333 20130101;
G01M 11/30 20130101 |
Class at
Publication: |
398/135 ;
324/765 |
International
Class: |
H04B 10/00 20060101
H04B010/00; G01R 31/26 20060101 G01R031/26 |
Claims
1. A method of measuring an optical modulation amplitude (OMA)
value associated with an input signal, comprising the steps of:
applying the input signal to a photodetector, wherein the
photodetector is calibrated to have a given responsivity value R,
and further wherein the photodetector generates an output signal in
response to the input signal; applying the output signal from the
photodetector to a radio frequency (RF) power meter, wherein the RF
power meter measures the root mean squared (RMS) power value of the
output signal received from the photodetector; and determining the
OMA value associated with the input signal in response to the root
mean squared (RMS) power value measured by the RF power meter.
2. The method of claim 1, wherein the input signal is applied to
the photodetector from a fiber optic link.
3. The method of claim 1, wherein the photodetector generates a
photocurrent in response to the input signal.
4. The method of claim 3, wherein the photocurrent is converted to
an average optical power value based on the given responsivity
value R.
5. The method of claim 1, wherein the determining step further
comprises converting the RMS power value to the OMA value.
6. The method of claim 5, wherein the RMS power value is converted
to the OMA value through an equation: OMA = 10 * log .times.
.times. 10 .times. ( F * ( 0.001 * 10 ( RF RMS 10 ) / 50 ) R *
0.001 ) . ##EQU6## wherein F represents a predetermined factor and
RF.sub.RMS represents the measured RF RMS power in units of
dBm.
7. The method of claim 6, wherein the portion of the equation under
the radical converts the measured RF RMS power, RF.sub.RMS, from
dBm into the square of the RMS signal current, I.sub.RMS.sup.2, the
RMS signal current is then converted to a peak current, I.sub.PEAK,
by multiplying by a factor F, the peak current is converted to a
linear OMA value by dividing by the responsivity R of the
photodetector, and the linear OMA is finally converted to the OMA
value in units of dBm by dividing by one milliWatt and taking
10-log.
8. The method of claim 7, wherein the factor F is derived from a
relationship between an amplitude of a data signal and the RMS
value of the data signal and a frequency response of the measuring
system.
9. The method of claim 1, wherein the input signal is compatible or
desired to be made or verified with at least one of a Fibre Channel
(FC) standard and a 10 Gigabit/second Ethernet standard.
10. The method of claim 1, wherein the OMA value is compatible or
desired to be made or verified with at least one of a Fibre Channel
(FC) standard and a 10 Gigabit/second Ethernet standard.
11. The method of claim 1, wherein the photodetector comprises a
photodiode.
12. The method of claim 4, further comprising the step of
determining an extinction ratio based on the average optical power
value and the OMA value.
13. The method of claim 1, wherein the step of determining the OMA
value is performed in accordance with a processor or a look-up
table.
14. Apparatus for measuring an optical modulation amplitude (OMA)
value associated with an input signal, comprising: a photodetector
for receiving the input signal, wherein the photodetector is
calibrated to have a given responsivity value R, and further
wherein the photodetector generates an output signal in response to
the input signal; a radio frequency (RF) power meter, operatively
coupled to the photodetector, wherein the RF power meter measures
the root mean squared (RMS) power value of the output signal
received from the photodetector; and means for determining the OMA
value associated with the input signal in response to the root mean
squared (RMS) power value measured by the RF power meter.
15. The apparatus of claim 14, wherein the determining means
comprises a processor.
16. The apparatus of claim 14, wherein the determining means
comprises a look-up table.
17. The apparatus of claim 14, further comprising a portable
housing for containing the photodetector, the RF power meter and
the determining means.
18. The apparatus of claim 14, wherein the photodetector comprises
a photodiode.
19. An article of manufacture for measuring an optical modulation
amplitude (OMA) value associated with an input signal, comprising a
machine readable medium containing one or more programs which when
executed implement the step of: in response to the application of
the input signal to a photodetector, wherein the photodetector is
calibrated to have a given responsivity value R, and further
wherein the photodetector generates an output signal in response to
the input signal, and application of the output signal from the
photodetector to a radio frequency (RF) power meter, wherein the RF
power meter measures the root mean squared (RMS) power value of the
output signal received from the photodetector; determining the OMA
value associated with the input signal in response to the root mean
squared (RMS) power value measured by the RF power meter.
20. A method of measuring an optical modulation amplitude (OMA)
value associated with an input signal, comprising the steps of:
obtaining the input signal; and determining the OMA value
associated with the input signal by converting a root mean squared
(RMS) power value measured from the input signal as a function of a
factor F, wherein an optimum value for factor F is a value which
when used to convert from RMS power to OMA gives an OMA value that
is in substantial agreement with an OMA value that would have been
obtained using a standards-specified method.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to optical network parameter
measurement techniques and, more particularly, to techniques for
measuring optical modulation amplitude.
BACKGROUND OF THE INVENTION
[0002] The Fibre Channel (FC) standards are an American National
Standards Institute (ANSI) standards set that defines a common
transport system for use with different protocols or applications.
The initial core FC standard is identified as X.3230-1994-Fibre
Channel Physical and Signaling Standard (FC-PH), the disclosure of
which is incorporated by reference herein.
[0003] Until recently, FC optical transceivers were specified with
an acceptable range of optical receiver sensitivity and transmitter
output measured in terms of average optical power at a minimum
extinction ratio level. When an optical link failure occurred,
fault determination procedures typically called for measurement of
the average optical power at both transmit and receive ends of the
link. If the transceiver power levels were within specified limits,
but the link continued to fail, then the problem was attributed to
the optical cable plant. It was possible to measure segments of the
link using an average optical power meter until the fault was
localized and corrected. On the other hand, a defective transceiver
could be identified by measuring its optical output and input power
levels. If the link was delivering acceptable power levels and
errors continued to occur, the failure was assumed to reside in the
optical receiver.
[0004] Recently, however, the FC standard changed its
specifications. The change is found in FC-PI-2, Revision 4,
Appendix A.5, the disclosure of which is incorporated by reference
herein. Rather than including receiver sensitivity at a given
extinction ratio, the Standard now defines a new parameter, Optical
Modulation Amplitude (OMA), which refers to the optical amplitude
of the signal, i.e., the difference in amplitude between a logic 1
and a logic 0. The OMA is a function of both received average
optical power, P.sub.AVE, and extinction ratio, E: OMA Linear = 2 *
P AVE * ( E - 1 E + 1 ) ##EQU1## in linear units or OMA log = 10 *
log .times. .times. 10 .times. ( 2 * P AVE * ( E - 1 E + 1 ) 0.001
) ##EQU2## in units of dBm with P.sub.AVE in Watts.
[0005] The extinction ratio is the linear ratio of optical power
between a logic level 1 and 0 measured under fully modulated
conditions. This Standards change means that it is no longer
possible to determine whether an optical transceiver is within
specification simply by measuring the average optical power input
to the receiver. The OMA, as specified in the Standard, requires
measurements of an extinction ratio that, in accordance with
existing measurement approaches, can only be made accurately in a
lab or manufacturing environment with an expensive digital
oscilloscope or similar equipment.
[0006] This situation currently affects all FC components, as well
as IBM Corporation's zSeries.TM. Fibre Connection (FICON) links
(which use the FC physical layer). Also, the practice of using
OMA-based specifications already exists in the 10 Gigabit/second
Ethernet (10 G Ethernet) Standard, the disclosure of which is
incorporated by reference herein, as will be explained below. The
OMA-based specifications are likely to extend into higher data rate
standards or higher fiber count links in the future, as well as
other protocols besides FC and 10 G Ethernet.
[0007] Since the two standards specify an OMA measurement technique
that is quite difficult and expensive to perform in the field by
installation/repair personnel, it would be desirable to have a low
cost portable OMA measurement tool which can be correlated with the
various standards.
[0008] The 10 G Ethernet Standard specifies a relative intensity
noise (RIN) OMA measurement using a photodetector and a power
meter. Such an approach is a ratio measurement of noise power to
signal power and thus does not calibrate the photodetector to
obtain a true OMA reading.
[0009] Hewlett Packard (Palo Alto, Calif.) produced the 8151A
Optical Pulse Power Meter which was capable of measuring OMA on
square wave signals up to 250 MegaHertz (MHz). This unit used
separate high and low peak detection circuits with variable ramps
to determine the high and low optical levels.
[0010] U.S. Pat. No. 5,850,409 and U.S. Patent Application No.
2003/0090289A1 both describe a method and circuit for "measuring
OMA." But, in actuality, these methods and circuits are configured
for maintaining the internal OMA of a transmitter over temperature
after the transmitter has been appropriately set up by a user.
These methods and circuits do not and can not report a calibrated
OMA.
SUMMARY OF THE INVENTION
[0011] Principles of the present invention provide techniques for
measuring optical modulation amplitude (OMA).
[0012] For example, in one aspect of the invention, a technique for
measuring an optical modulation amplitude (OMA) value associated
with an input signal includes the following steps/operations. The
input signal is applied to a photodetector, wherein the
photodetector is calibrated to have a given responsivity value R,
and further wherein the photodetector generates an output signal in
response to the input signal. The output signal from the
photodetector is applied to a radio frequency (RF) power meter,
wherein the RF power meter measures the root mean squared (RMS)
power value of the output signal received from the photodetector.
The OMA value associated with the input signal is determined in
response to the root mean squared (RMS) power value measured by the
RF power meter.
[0013] Further, the input signal may be applied to the
photodetector from a fiber optic link. The photodetector may
generate a photocurrent in response to the input signal. The
photocurrent may be converted to an average optical power value
based on the given responsivity value R. The determining
step/operation may further include converting the RMS power value
to the OMA value. Conversion may be performed in accordance with a
factor F which may be derived from a relationship between an
amplitude of a data signal and the RMS value of the data signal and
a frequency response of the measuring system.
[0014] Also, the input signal and the OMA value may be compatible
or desired to be made or verified with the FC Standard and/or the
10 G Ethernet Standard.
[0015] Still further, the photodetector may include a photodiode.
The step/operation of determining the OMA value may be performed in
accordance with a processor or a look-up table. The technique may
also include the step/operation of determining an extinction ratio
based on the average optical power value and the OMA value.
[0016] In another aspect of the invention, a technique for
measuring an OMA value associated with an input signal includes
obtaining the input signal, and determining the OMA value
associated with the input signal by converting a root mean squared.
(RMS) power value measured from the input signal as a function of a
factor F, wherein an optimum value for factor F is a value which
when used to convert from RMS power to OMA gives an OMA value that
is in substantial agreement with an OMA value that would have been
obtained using a standards-specified method.
[0017] Accordingly, principles of the present invention provide
techniques that enable measurement of OMA in a variety of settings
and applications. For example, the inventive techniques may be used
for field (e.g., customer location) measurement of OMA. Also, by
way of further example, the inventive techniques may be used for
optical component manufacturing measurements, which may lower costs
through lower equipment costs and faster testing time.
[0018] These and other objects, features and advantages of the
present invention will become apparent from the following detailed
description of illustrative embodiments thereof, which is to be
read in connection with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a diagram illustrating an optical modulation
amplitude meter, according to an embodiment of the present
invention;
[0020] FIG. 2 is a diagram illustrating a comparison of the 10 G
Ethernet Standard optical modulation amplitude measurement and an
optical modulation amplitude measurement, according to an
embodiment of the present invention;
[0021] FIG. 3 is a diagram illustrating an ideal square wave
pattern with definition of terms;
[0022] FIG. 4A is a diagram illustrating the frequency response of
various filters, which represent different system responses, used
in root mean square simulations;
[0023] FIG. 4B is a diagram illustrating a representative eye
pattern resulting from the use of one of the filters of FIG. 4A;
and
[0024] FIGS. 5A and 5B are diagrams showing the optimum value of
factor F as a function of signal bandwidth and filter shape using
two lengths of square wave patterns;
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0025] While illustrative embodiments will be described below in
the context of the FC standard, it is to be understood that
principles of the invention are not limited to use with the FC
standard and thus are more generally applicable for use with any
suitable high data rate transport systems, e.g., FICON (Fibre
Connection), 10 G Ethernet, etc. Furthermore, while principles of
the invention may be suitable for use in fault determination, they
are not limited to use therein. That is, OMA measurement techniques
of the invention may be suitable for data communications standards
conformance testing and verification, field applications,
manufacturing applications, etc.
[0026] As will be described in illustrative detail below,
principles of the invention provide a new, portable, low cost
measurement tool which can directly determine OMA levels on a fiber
optic link and correlate them with various standards.
[0027] In one embodiment, the portable OMA meter includes a
calibrated photodetector, a radio frequency (RF) power meter, and a
processor. The photodetector is connected directly to the RF power
meter in most situations, although amplifiers or attenuators could
be placed between the photodetector and the RF power meter to
handle situations of very weak or very strong signals respectively.
The photodetector has a responsivity, R, which is calibrated at
several wavelengths of interest, e.g., 850 nanometers (nm), 1310 nm
and 1550 nm. It is assumed that the direct current (DC) and
alternating current (AC) responsivity are the same; although, if
different, one skilled in the art would readily appreciate how to
adjust the calibration. Further, the AC responsivity difference
from DC could be accounted for in the processor. It is to be
appreciated that calibrated photodetectors may be purchased or one
ordinarily skilled in the art will realize how to calibrate a
photodetector using well-known calibration techniques, e.g.,
National Institute of Standards and Technology (NIST) calibration
services.
[0028] It is to be understood that the photodetector could be, for
example, a gallium arsenide (GaAs) or an indium gallium arsenide
(InGaAs) positive-intrinsic-negative (PIN) photodiode or a
metal-semiconductor-metal (MSM) photodetector. In fact, suitable
photodetectors other than a photodiode may be employed in
accordance with the techniques of the invention.
[0029] The direct current (DC) photocurrent of the photodetector,
I.sub.DC, is measured and converted to average optical power using
the simple relationship: P AVE = I DC R . ##EQU3## The RF power
meter measures the root mean squared (RMS) electrical power of the
signal from the photodetector into a 50 Ohm load which is converted
to OMA through the following relationship: OMA = 10 * log .times.
.times. 10 .times. ( I PEAK R * 0.001 ) = 10 * log .times. .times.
10 .times. ( F * I RMS 2 R * 0.001 ) = 10 * log .times. .times. 10
.times. ( F * ( 0.001 * 10 ( RF RMS 10 ) / 50 ) R * 0.001 )
##EQU4## This relationship is explained in the following way. The
portion of the equation under the radical converts the measured RF
RMS power, RF.sub.RMS, from dBm into the square of the RMS signal
current, I.sub.RMS.sup.2. The RMS signal current is then converted
to a peak photocurrent, I.sub.PEAK, by multiplying by a factor F.
This factor F comes from the relationship between the amplitude of
a data signal (e.g., a square wave signal) and the RMS value of the
data signal. This factor will be discussed in greater detail below.
However, in one embodiment, factor F may be equal to two or about
two, e.g., 2.10. However, other suitable factors may be
employed.
[0030] The peak current is converted to linear OMA by dividing by
the responsivity, R, of the photodetector. The linear OMA is
finally converted to OMA in units of dBm by dividing by 1 milliwatt
(0.001 Watt) and taking "10-log".
[0031] Furthermore, using the determined P.sub.ave and OMA values,
the extinction ratio can be calculated. Thus, principles of the
invention provide a portable extinction ratio measurement tool as
well.
[0032] Referring initially to FIG. 1, a diagram illustrates an
optical modulation amplitude meter, according to an embodiment of
the present invention. As shown, OMA meter 100 includes a
calibrated photodetector 102 which generates DC photocurrent 104
and AC photocurrent 105, RF power meter 106 which measures signal
RMS power 108, and processor 110 which, in response to outputs 104
and 108, performs average optical power calculation 112 and OMA
calculation 114. In addition, OMA meter 100 includes a memory 116,
input/output (I/O) device(s) 118, data interface(s) 120 and barcode
scanner 122, each operatively coupled to processor 110 via a
suitable computer bus 124. Further, as shown, the components of the
OMA meter are contained in a portable housing 126.
[0033] The term "processor" as used herein is intended to include
any processing device, such as, for example, one that includes a
CPU (central processing unit) and/or other processing circuitry. By
way of further example, the processor could include one or more
application-specific integrated circuits (ASICs) or one or more
field programmable gate arrays (FPGAs). It is also to be understood
that the term "processor" may refer to more than one processing
device and that various elements associated with a processing
device may be shared by other processing devices.
[0034] Processor 110 may also have associated therewith memory 116
such as, for example, RAM, ROM, a fixed memory device (e.g., hard
drive), a removable memory device (e.g., diskette), flash memory,
etc. Accordingly, software components including instructions or
code for performing the methodologies described herein may be
stored in one or more of the associated memory devices (e.g., ROM,
fixed or removable memory) and, when ready to be utilized, loaded
in part or in whole (e.g., into RAM) and executed by a CPU.
[0035] Also, processor 110 may also have associated therewith one
or more input devices (e.g., keypad, etc.) for entering data to the
processing unit, and/or one or more output devices (e.g., speaker,
display, etc.) for presenting results associated with the
processing unit. Such devices are referred to as I/O device(s) 118.
Also, photodetector 102 and RF power meter 106 may have their own
I/O devices (e.g., displays) that may be used in accordance with
OMA meter 100.
[0036] Further, processor 110 may also have associated therewith
one or more data interface(s) 120 for formatting data obtained in
accordance with the operations performed by OMA meter 100 into one
or more data formats for transfer (upload) to a source remote from
the meter. For example, results of computations such as the average
optical power calculation (112) and the OMA calculation (114) may
be formatted to a particular format for transfer to a computing
system to which OMA meter 100 may be operatively coupled.
[0037] Still further, processor 110 may also have associated
therewith bar scanner 122 for use in reading a uniform product code
(UPC) label affixed to a particular component that OMA meter 100 is
being used to measure. For example, the barcode scanner could scan
the UPC on an optical transceiver such that any readings taken for
that transceiver are stored in accordance with an index number
associated with the transceiver, e.g., a serial number. This would
make for easy retrieval and reference of the obtained
measurements.
[0038] In one illustrative embodiment, a portable OMA meter has
been realized using a New Focus (San Jose, Calif.) model 1481
photodetector, and a Hewlett Packard HP437B RF power meter with
HP8484A power head. The calculation of OMA in this case is
performed by using a programmed spreadsheet cell (e.g., Microsoft
Excel) when the power meter reading was recorded manually or with a
LabView (National Instruments Corporation, Austin, Tex.) program
when the power meter was read by a computer through the general
purpose interface bus (GPIB) port.
[0039] To verify the use of this technique, the OMA of each channel
of a 12 channel parallel optical transmitter was measured using the
method specified in section 52.9.5 of the physical layer
specification of 10 Gb/s Ethernet (the standard method) and
compared to the OMA calculated from the RMS RF power meter reading
of the same signal. The graph of FIG. 2 shows the measured results
with the standard method in diamonds and two measurements using the
RF power meter, in accordance with the present invention, in
triangles and squares. The difference between the Standard method
and this method is less than 0.5 dB or 11%. The difference between
the two curves using the RF power meter was an experiment to see if
the result was affected by adding some dwell time to the
measurement. In this case, dwell time did not appreciably affect
the results.
[0040] This particular implementation was capable of measuring OMA
down to the level of -15 dBm (noise floor) and up to +7 dBm
(saturation level).
[0041] In this particular implementation, the dc photocurrent was
not monitored nor verified against average optical power, as this
was considered too trivial since the New Focus model 1481 provides
a calibrated dc photocurrent output on its front panel.
[0042] In this particular implementation, the OMA meter was not
portable due to the RF power meter, however; Agilent Technologies
(Palo Alto, Calif.) offers this power meter with an optional
battery pack for field measurements. Aeroflex Inc. (Plainview,
N.Y.) also offers a portable RF power meter model no. 6970 that may
be employed.
[0043] One main advantage to measuring the OMA with an RF power
meter is the sheer simplicity. One does not have to set up and
trigger a high speed scope and then take histograms on separate one
and zero levels to get the OMA. The OMA meter of the invention
simply needs an optical signal. In practice, it has been found that
this method also works on full speed modulated signals, not only
square wave patterns. This has the advantage for field measurements
where the service technician may not be able to configure an I/O
port to generate a square wave pattern when servicing or diagnosing
a system that is in use.
[0044] Size is another advantage. This particular implementation
takes significantly less space and weight than a high speed
oscilloscope. It is anticipated that a unit designed specifically
for the field could be made even more compact.
[0045] Cost is another advantage. The current cost of the
components in the particular implementation was about 1/7.sup.th of
the cost of a high speed oscilloscope. It is anticipated that
further cost reductions could be realized when making a field
portable unit.
[0046] This technique provides a calibrated measure of the OMA
which can be used to check for compliance with various
standards.
[0047] This measurement technique does not depend on the data rate
of the signal. Therefore, it is extendable from existing 1 Gb/s and
2 Gb/s modules up to 10 Gb/s and beyond, limited only by the
bandwidth of the photodetector and RF power meter.
[0048] The factor F used in the calculation of OMA from the RMS
value of RF power comes from the relationship of the RMS value of a
data signal, in this case, a periodic square wave signal, to its
peak to peak amplitude. V RMS = 1 T .times. .intg. 0 T .times. v 2
.function. ( t ) .times. d t = 1 T .times. .intg. 0 T / 2 .times. (
V peak ) 2 .times. d t + 1 T .times. .intg. T / 2 T .times. ( - V
peak ) 2 .times. d t = 1 T .times. .intg. 0 T / 2 .times. ( V amp 2
) 2 .times. d t + 1 T .times. .intg. T / 2 T .times. ( - V amp 2 )
2 .times. d t = 1 T .times. T 2 .times. V amp 2 4 + 1 T .times. T 2
.times. V amp 2 4 = V amp 2 4 = V amp 2 ##EQU5## or
V.sub.amp=2*V.sub.RMS for an ideal square wave pattern, using the
terms illustrated in FIG. 3. For an ideal sine wave, V.sub.amp=2
{square root over (2)}*V.sub.RMS. This would suggest that for real
data patterns, which may have overshoot, undershoot and/or inter
symbol interference, the exact relationship between the RMS value
and the peak to peak signal amplitude might lie somewhere between 2
and 2 {square root over (2)}.
[0049] To investigate this, simulations were performed to calculate
the optimum factor F given realistic square wave patterns that had
been filtered with varying bandwidths and filter shapes. The
optimum value of F is the value which when used to convert from RMS
power to OMA in accordance with the present invention will give an
OMA value that is in agreement with the OMA value that would have
been obtained using one of the Standards-specified methods (e.g.,
FC or 10 G Ethernet). The filters are meant to represent the
variety of system responses that are likely to be encountered in
the field.
[0050] FIG. 4A shows an example set of filters and FIG. 4B shows a
simulated eye diagram using one of those filters. Note that there
is a significant amount of overshoot in this eye which is typical
of many vertical cavity surface-emitting laser (VCSEL) transmit
eyes at 10 Gb/s.
[0051] FIGS. 5A and 5B show the calculation of the optimum factor F
by comparing the RMS value of the signal to the method of
calculating OMA defined in the 10 G Ethernet. The 10 G Ethernet
Standard specifies that a variable length square wave, with periods
ranging from 8 to 22 bits, can be used to determine the OMA.
Therefore, both extremes were calculated in FIGS. 5A and 5B. In
these curves, the optimum value for F approaches two as the signal
bandwidth increases and more closely resembles an ideal square wave
pattern. For lower signal bandwidths, the optimum value for F goes
up as expected.
[0052] Note that since the 10 G Ethernet Standard specifies this
variable length square wave, the measurement of the OMA as defined
in the Standard will vary depending on the period of the square
wave that is chosen. This was investigated using the same set of
filters as described above. It was found that for system bandwidths
as low as 0.3 of the data rate, the range of square wave periods
results in a 5% variation in the measured OMA. That is, two people
could measure the OMA on the same signal using different extremes
of the Standard and arrive at values that are 5% apart.
[0053] The curves shown in FIGS. 5A and 5B can be used to select a
reasonable value for the factor F for a given implementation of
this invention. For example, if the photodetector 102 has a 10 GHz
bandwidth and is used in an instrument that measures 10 Gb/s
signals, then it is reasonable to assume that the overall signal
bandwidth, which will be filtered by the photodetector, would
likely have a bandwidth in the range of 7-10 GHz for most cases.
From the graph, a good choice for the factor F would be 2.1, since
this would induce a worst case error of +7% and -5% compared to an
OMA measurement using a square wave with a 22 bit period. The error
range reduces to +6% and -2% when using an 8 bit period square
wave.
[0054] A preferred OMA meter implementation of the invention has a
calibrated photodetector, an RF power sensor and a processor fully
contained in a battery operated hand held sized box.
[0055] Alternatively, an OMA meter implementation of the invention
can include a calibrated photodetector, an RF meter, and a look-up
table. That is, rather than using a processor to determine OMA
results via real-time calculations, the OMA results can be
previously calculated for given RMS values with the results
provided in accordance with a look-up table. Thus, a user of the
meter can obtain the RMS signal power from the RF power meter and
determine the OMA value by consulting the look-up table which has
an OMA value associated with that RMS value. Further, a user
familiar enough with repetitively-measured RMS values may simply
recall the corresponding OMA value without consulting the look-up
table. By way of example, the look-up table may be printed on the
meter or available via a display or some other output mechanism
associated with the OMA meter.
[0056] Furthermore, the OMA meter could have inputs for the
photodetector responsivity at several discrete wavelengths or could
have the entire responsivity curve stored in memory with inputs
only for wavelength.
[0057] As mentioned above, the dynamic range of the OMA meter could
be extended through the use of amplifiers and/or attenuators or
even a variable gain amplifier. The gain of any amplifier or loss
of any attenuator would be taken into account in the OMA
calculation.
[0058] Still further, since the OMA meter of the invention has the
capability of measuring the DC optical power, a separate optical
power meter would not be required in the field.
[0059] The OMA meter (processor) could store acceptable OMA ranges
for the various standards and provide a pass/fail indicator to the
field engineer. Such a unit would also possess the ability to be
updated as new standards are generated, older standards are revised
or custom (e.g., company or vendor-specific) limits are
adopted.
[0060] To further lower cost, the bandwidth of the photodetector
and power sensor could be deliberately set low, or chosen to be
low, and an appropriate factor relating RMS to OMA chosen using the
curves shown in FIGS. 5A and 5B.
[0061] A different factor could be applied to convert from RMS to
OMA when the data signal does not contain a nearly equal
distribution of ones and zeros or the data signal is not an NRZ
format such as return-to-zero (RZ).
[0062] The input to the photodetector does not have to be fiber
coupled. Free Space coupling using optical elements could be
employed.
[0063] Although illustrative embodiments of the present invention
have been described herein with reference to the accompanying
drawings, it is to be understood that the invention is not limited
to those precise embodiments, and that various other changes and
modifications may be made by one skilled in the art without
departing from the scope or spirit of the invention.
* * * * *