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NIST Technical Note 1556

QC

100

.U5753

|#1556

2010

NIST Technical Note 1556

Multimode Fiber Launch Study: Estimating the

Impact on Measurements of Propagation Delay

Tasshi Dennis

Electromagnetics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Boulder, CO 80305-3328

August 2010

U.S. Department of Commerce

Gary Locke, Secretary

National Institute of Standards and Technology

Patrick Gallagher, Director
National Institute of Standards U.S. Government Printing Office For Sale by the

and Technology Washington: 2010 Superintendent of Documents

Technical Note 1556 U.S. Government Printing Office

Natl. Inst. Stand. Technol. Stop SSOP, Washington, DC 20402-0001

Tech. Note 1556 Phone: (202) 512-1800

14 pages (August 2010) Fax:(202)512-2250

CODEN: NTNOEF Internet: bookstore.gpo.gov
Multimode Fiber Launch Study: Estimating the Impact on

Measurements of Propagation Delay

Tasshi Dennis

National Institute of Standards and Technology

Boulder, Colorado 80305

Executive Summary

The purpose of this study was to evaluate measurement uncertainties introduced into propagation

time delay calibrations of multimode fiber reference spools, as well as their use as transfer

standards. While the principles described are generally applicable, the results could be different

for other types of available multimode fiber and should be evaluated on an individual basis. The

study was conducted by use of a high-accuracy NIST technique for measuring delay, and again

the results obtained could be different, and with higher uncertainty, for more conventional

measurement methods. To that end, we also considered some limiting cases that could be

encountered with conventional measurement equipment, and examined the performance of one commercial instrument. The results of this study are presented along with suggested guidance

and areas for future study.

Introduction

The measured time delay of a multimode (MM) optical fiber can depend on the spatial optical

modes launched into the fiber to measure the delay. This dependence on the launch conditions is caused by modal dispersion, which is a specific characteristic of a MM fiber design [1,2].

Illustrated in Fig. 1(a) is a simplistic ray view of why different modes, represented by different

launch angles, have different path lengths and therefore different time delays in what is known as a step-index fiber. The square-top index profile is shown to the right. Typically, a steeper

launch angle results in a longer delay because the light ray experiences more internal reflections

at the core-cladding boundary. Therefore, an optical pulse launched into two different modes of

a fiber may experience broadening in time or it may even separate into two distinct pulses [1, 2].

One way to counter this modal dispersion is to make the refractive index profile of the fiber

higher at the center of the core than it is at the boundary with the cladding [1,2]. A common

index profile is parabolic in shape, as shown in Fig. 1(b), and is described as a graded-index

fiber. The profile equalizes the path lengths despite different launch angles. In the latest, highbandwidth multimode optical networks, the engineering assumption is that the graded-index

profile can be tailored so precisely to the application that modal dispersion is negligible.

However, the effect of dispersion may not be negligible relative to the low uncertainties required

for the delay calibration of multimode fiber reference spools. Also, this assumption is general 1\
(a) Step Index

Short Path

v. ^^ Core

Long Path Cladding

Position

i R

J

Index Profile

(b) Graded Index Position

Equal

Paths

Index Profile

Fig. 1 : Multimode ray propagation and index profiles for (a) step-index and (b) graded-index fiber.

not true for legacy MM optical fiber, due to alternate design criteria and variations at the time of

manufacture. Such legacy fiber was typically designed for a launch that filled as many modes as possible by use of a broadband source and not for high-bandwidth applications employing the

launch of at most a few modes with a narrow linewidth laser. When used in high-bandwidth

communications links, legacy fiber can introduce significant optical delay line artifacts. It is important to characterize the dependence of the delay on the mode launch conditions for the

given type of fiber. Such a study requires that one have a method for both controlling and

measuring the launch conditions, and measuring the fiber delay.

Conventional Fiber Delay Measurement Instrumentation

The most common field instrument for measuring the time delay of an optical fiber is the optical

time-domain reflectometer (OTDR). In an OTDR, short optical pulses are launched into the

fiber under test and the back reflection of the pulses is monitored by the instrument as a function

of time. The return light is generated throughout the fiber span both continuously from Rayleigh

scattering and abruptly from Fresnel reflections occurring at fiber terminations, splices, faults,

and optical components. The time delay is measured as half the round-trip period for the pulse to

reflect and return from the far end of the fiber being measured. The physical length of the fiber can be calculated from the time delay by use of the group refractive index, but only to the level

of uncertainty that the value of the index is known for that fiber.

For the measurement of multimode fiber, the optical pulse source inside the OTDR can be a laser or a light-emitting diode (LED). Legacy OTDRs have typically used lasers to achieve high

output power and short pulses, but typically do not consider the modal launch conditions.
NIST Hydrogen

Maser 10 MHzReference

Vector

Network

Analyzer

PorM

Photodiode D

Port 2

JP

"Line"

Connection

Platinum

Resistance

Thermometer

Bias

Control

MachZehnder

Modulator

;. "Thru"

Connection

SM

Mode Launch

Conditioner

Laser Source

SM

Wavelength

Meter

Pre-characterized

Fiber Jumper Pair

JAand JB

Temperaturecontrolled

Chamber

Fig. 2: Schematic of fiber group delay measurement apparatus. Thick black lines represent

multi-mode optical paths; thin black lines represent single-mode (SM) optical paths; gray

lines represent electrical paths. The gray "X" symbols represent FC/PC connections that are changed manually during the calibration process. The fiber cable JP connected to the

photodiode can be connected directly to the Mach-Zehnder modulator jumper JM for a "thru" connection, or to jumper JB from the fiber spool for the "line" connection. The

temperature of the spool is stabilized in one of three ovens. Each oven has a corresponding

jumper pair (denoted JA and JB) used to connect the fiber spool to the measurement system.
Modern OTDRs intended for Ethernet testing typically make use of high-power LEDs, with the

intent of controlling the modal launch conditions relative to industry standards. As we will

show, the type of source utilized in an OTDR and the particular launch conditions it offers will

have an influence on the delay that is observed for a particular multimode optical fiber.

NIST Fiber Delay Measurement Technique

We measured the group delay of multimode optical fibers with a frequency-domain "phase-shift"

technique similar to those described in references [3-5]. A schematic diagram of the apparatus

used is shown in Fig. 2. The laser source generated a stable continuous emission with

wavelength at 850 nm. Output port 2 of a vector network analyzer (VNA) drove the MachZehnder modulator with sine waves, modulating the laser source. The sine wave frequency was

stepped from 30 kHz to 1 GHz in increments of about 1.25 MHz. The optical signal traveled to the photodiode via the "thru" connection (via JM and JP in Fig. 2, excluding the fiber spool) or via the "line" connection (via the pre-characterized jumper pair JA and JB connected to JM and

JP, respectively, and the fiber spool being calibrated). The photodiode converted the modulation on the optical signal back to an electrical signal, and the VNA calculated the complex ratio of the

received electrical signal to the generated electrical signal. Measurements of this ratio were

acquired for each frequency^ (j = 1,..., 801) via an external computer for later processing. The

connection to the photodiode was changed manually from the "thru" connection to the "line"

connection, with measurements collected from both configurations. The phase delay of the

modulation envelope after traveling through the fiber spool and jumper pair was found, modulo

2k, from the phase of the ratio of the "line" measurement to the "thru" measurement:

Zlme If \Mfi

^thru where zline and r[hm are the complex ratios measured by the VNA for the "line" and "thru"

connections, respectively. Both of these measurements included effects due to the frequency

dependent mismatch and loss of the Mach-Zehnder modulator and the couplers in the VNA.

Because these effects were constant for the two measurements, they canceled in Eq. ( 1 ), and

what remained were contributions that were different in the two measurements. These included

the loss and insertion delay of the spool and jumper pair that were to be calculated, as well as some small effects due to multi-path interference between the fiber connectors. These

interference effects were kept to a minimum by use of physical contact connections of high

return loss (> 30 dB) throughout the system.

Equation (1) gives the phase </>(//) of the modulation envelope modulo 2k and the magnitude of

the modulation envelope, which was not needed for these calculations. The actual phase delay

Q>(fj) due to the fiber spool is related to the phase in Eq. (1) by

0(/.) = 2a:m + <*(/,), (2)
where m is an unknown integer and a function of the frequency /y. The group delay r of the

fiber is

r = aw

(3)

Because m is unknown, an iterative method was used to estimate r as follows:

1Pick an initial guess r* for r. For a 600 meter fiber, a reasonable guess would be r* =

Lng/c = 2964 ns, where L is the length of the fiber, ng = 1.481 is the group index of the

fiber [6], and c is the speed of light in vacuum.

2. Calculate the deviation of the measured phase from the estimated phase by use of the

estimate in step 1

A0j = arg

"line exp (ilTCfjT*)

3. At the lowest frequency, assume the error Ar = . Then

"<-7 2x

+ integer ( A rH/y )

(4)

(5)

They'th estimate of the group delay is r. =r*-Ar. . The algorithm typically converges as shown by the gray dots in Fig 3. Several visual and analytic cross checks are used to verify convergence to the correct value.

To average over ripples caused by multi-path interference and speckle, the average group

delay f over the frequencies between 250 MHz and 1000 MHz was calculated:

f = Tnean{T2 soMHz' ••• > tiooomhz)- ^ '

In Fig. 3, t is shown as the dashed horizontal line.

3049.84 ^

3049.82 -

3049.80 -

S 3049.78

toE

3049.76-

3049.74

*-75.r:s^^i'

—I

750 1000 Frequency (MHz)

Fig. 3: Typical convergence of the estimated group delay with increasing frequency, from data

analysis steps 3 and 4. The dashed line is the average group delay from step 5.
Control of Modal Launch

The modal launch system we constructed is illustrated in Fig. 4. In our frequency-domain

"phase-shift" method for measuring the fiber time delay, we minimized the impact due to chromatic dispersion by using a laser precisely tuned to 850.00 nm. This choice then required

that we externally modulate the source with a Mach-Zehnder modulator, optimized for 850 nm. The output fiber of the modulator was single-mode (SM) at 850 nm (5/125 urn core/cladding).

This created the very challenging task of forming a multimode launch out of a monochromatic

single-mode launch. We were able to do this through a series of processing steps that incurred a

total optical loss of 10 to 13 dB. The first step was a radial over-filled launch [7] between the

SM fiber and a graded-index MM fiber. This device was essentially a variable air-gap between

the optical connectors of the two types of fiber, which allowed the spatial cone of the SM launch

to spread out over the end face of the MM fiber. As the gap between the connectors was

increased, the number of modes excited in the MM fiber increased. The end of the graded-index

MM fiber was connected to a 3 meter step-index fiber that was looped and physically shaken to scramble the modal structure of the light in time. This speckle-averaging step reduced the spatial

coherence of the light over observation periods that were much longer than the scrambling

period. Static modal scrambling of the light was introduced by a serpentine wrapping of the

step-index fiber. Finally, the light was launched back into a 1 meter graded-index fiber, which

could be wrapped around a mandrel to strip off unwanted higher-order-modes. The 10 meter

fiber also allowed for the natural spatial filtering of lossy cladding modes.

Measurement of Modal Launch

We used a commercially available modal launch analyzer designed for the characterization of

modal launch in high-bandwidth optical networks. The instrument precisely images the emitting

end face of the multimode fiber onto a CCD camera. Image processing software is then used to calculate beam profile quantities from the image of the lit fiber, generally as a function of radial

distance from the center of the fiber core. The mode power distribution, encircled flux, mode

transfer function, and intensity profile are reported.

IN' SM

Radial

Overfill

<0-

Shaker

1mGI

/'""N Scrambler

3m SI

Stripper

noon

10m Gl

-OUT

Fig. 4: Mode launch control system that converts a single mode (SM) input to a multimode

output. The multimode fiber is either step-index (SI) or graded-index (GI) type.
Mode Launch Standards

The mode launch analyzer was designed to comply with the measurement procedure for

encircled flux specified by TIA/EIA standard FOTP-203: Launched Power Distribution

Measurement Procedure for Graded-Index Multimode Fiber Transmitters [8]. The instrument

also complies with the very recent standard specified in IEC 61280-4-1 for Fibre-Optic

Communications Subsystem Test Procedures - Part 4-1 : Installed Cable Plant - Multimode

Attenuation Measurement [9]. In the IEC document, the encircled flux plays a critical role in

achieving higher-accuracy attenuation measurements in 10G Ethernet networks. In that

application, the fiber is assumed to have a perfect graded-index profile, resulting in no modal

dispersion. However, the mechanical tolerance of connectors in these networks causes small

misalignments of the fiber cores, which in turn create offset launch conditions that increase the

uncertainty of optical loss measurements using optical time-domain reflectometers (OTDR's).

Modeling has shown that measurement uncertainty can be reduced to an acceptable level by

precisely specifying the encircled flux of test light launched by the OTDR. The IEC 61280-4-1

specification for encircled flux is a range given by a radial template; however it is very

restrictive: at 20 um radius the tolerance is 1.90 % and at 22 um radius it is only 1.22 %.

While the IEC standard was developed with fiber loss measurements in mind, it does provide a convenient and fairly precise definition of a modal launch condition that will be found in future

OTDR systems. As we have discussed, the modal launch condition can significantly influence

the measurement of time delay, particularly in legacy fibers. Conforming to the restrictive

launch condition specified by the IEC standard allows a level of control over the measured

variation in time delay. However, because the standard specifies a template with upper and

lower bounds, it is possible that various conforming launch conditions could still result in

measurable variations in time delay. During a particular delay calibration, only a small subset of

the infinite number of possible mode launch conditions passing the IEC encircled-flux criteria are generated. A secondary (user) laboratory may generate a different subset of passing IEC

conditions, perhaps with little commonality to the launches used during the calibration. For this reason, we studied the impact of encircled flux mode launch conditions on the measured time

delay, and used the results to estimate an uncertainty for our calibration system.

NIST Mode Launch Study

To test the dependence of delay on modal launch, we generated different encircled-flux launch

conditions, all of which conformed to the IEC 61280-4-1 specification. The bounds of the IEC

encircled flux template for 50/125 um core/cladding fiber at 850 nm are shown by the black

vertical lines in Fig. 5. We varied as many of the settings of our modal launch control system as possible to try to sample as much of the template range. We paid particular attention to the

encircled-flux bounds at large radii (20 um and 22 um), which are the most difficult criteria to meet in practice. These bounds are tight because the impact on measured attenuation accuracy is

greatest: light propagating at large radii is most affected by connector misalignment and cladding

loss.
U. 0.6' a

300m EF Launch Conditions

-

1

-

20 -

1

25 Radius (microns) Radius (microns)

Fig. 5: IEC encircled-flux template specifications (four black lines in each plot) and the measured

launch conditions generated for this study (gray curves).

We assumed that there is predominately a one-to-one mapping between encircled flux and

measured delay, although it does seem likely that two different launch conditions could result in

the same measured delay. We attempted to sample all of the range in delay variation by

uniformly covering the encircled-flux bounds at large radii. Figure 5 shows the encircled-flux curves for different launch conditions that we created for measuring the delay of a 300 m and a 600 m length of Corning InfiniCor 50 |um fiber [6]. In creating at least ten different conditions

for each length of fiber, we often pushed our modal launch control system to the limits of its operating range. Some of the measured delay variation could have been caused by low launch

powers; some conditions required a lot of radial over-filled launch and mandrel wrapping, both

of which are lossy. Figure 6 shows histograms of the measured delays for both lengths of fiber,

normalized to the nearest integer delay in nanoseconds. The histograms are coarse because of

the finite number of measurements. However, despite differences in appearance, the standard

deviations of the two sets of measurements are nearly equal.

Standard deviation of 10 measurements of 300 meter length: 0.00271 ns

Standard deviation of 1 1 measurements of 600 meter length: 0.00263 ns

The delay variation does not appear to depend on fiber length, which is consistent with the

notion that the launch dependence is governed by propagation within the first few tens of meters.

The time delay of a 300 m fiber is about 1500 ns, so the deviation we measured was on the order

of 5 parts in 1 . We took the standard deviation as an uncertainty estimate for the measurement

of delay caused by the mode launch conditions. In our method for calibration of time delay in

multimode fibers, the uncertainty due to mode launch appeared twice: from measuring the fiber

itself, and a separate measurement of the connecting jumper cables JA and JB. We combined
600 meter

0.812 0.814 0.816 0.818 0.820 0.822 0.824 0.826 Delay (ns) - 1492 ns

0.862 0.864 0.866 0.868 0.870 0.872 0.874 Delay (ns) - 3029 ns

Fig. 6: Histograms of the measured time delay for varying modal launch conditions into 300

and 600 meter fibers. The horizontal axes have been normalized to the nearest integer delay in nanoseconds.

the two uncertainty contributions in quadrature [10], namely V2 x 0.00271 ns, giving a total mode launch uncertainty of 0.0038 ns. The only other uncertainty contribution in our delay

measurement method that is comparable to the total mode launch is the uncertainty due to the

measurement of phase (0.0044 ns). Therefore, when constrained to the IEC launch specification,

the uncertainty due to mode launch for this type of multimode fiber is significant but not the

largest source of uncertainty.

We note that the results obtained here for variation in delay with launch conditions are specific to the fiber we measured. Substantially different results might have been obtained for other

multimode fibers, including such variations as the manufacturer, core size, operating wavelength,

and vintage.

Some Limiting Cases

It is informative to consider some limiting cases for both encircled flux and variations in delay,

in order to get a better sense for the magnitude of the effects in other situations. Outside of the

encircled flux template are two distinct limiting cases: under-filled launch (UFL) and over-filled

launch (OFL). While not strictly defined in terms of encircled flux, it is fairly easy to generate

launch conditions that clearly fall within or near these two categories. In an OFL all or most of

the modes of a multimode fiber are excited equally. This launch condition is typical of LED

sources, which have a wide angular divergence, broad spectral bandwidth, and short temporal

coherence. In a UFL only a few modes, perhaps just one, are excited. A UFL is easily generated

from a narrow-band laser having a collimated beam with a narrow angular divergence and a long

temporal coherence.
1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

•

Laser OFLv TT^V

. V /

. . / /

- .

' ' / . .

' / /

Laser UFL ^^ y [•••

/ /

. / "V/

-

, /

/ / \/ ^ LED OFL

/

/ y>>A>• Perfect / Square Top

" y^>^

2?r

5

—> 1 r_ 10 15

-i 1

1 1

i 20 25 Radius (microns)

Fig. 7: The encircled-flux curves for some limiting mode launch conditions. The vertical bars

indicate the IEC template bounds, as before. The "perfect square-top" profile is a calculated

curve.

Figure 7 shows some limiting encircled-flux curves we generated for comparison to the IEC

template (vertical bars). We created an OFL from an LED that had a singlemode fiber pigtail by

using a combination of radial over-filled launch, speckle averaging, and a serpentine mode

scrambler. The final output launch was delivered to the mode analyzer by use of a step index

fiber, which gave an approximate square-top intensity profile. The encircled-flux curve for this

LED OFL is well below the bounds of the IEC template. Shown for comparison is the calculated

encircled flux for the limiting case of an ideal square-top intensity profile. The LED OFL launch

lacked sufficient optical power to allow a fiber delay measurement under these conditions.

To have an OFL condition with sufficient light available for a delay measurement, we used a narrow-band laser source with our mode launch conditioner. An excessive degree of radial overfilled launch was required in order to generate the encircled-flux curve labeled "Laser OFL,"

which is notable for the large deviation from the IEC template at 20 urn and 22 urn radii. This

result represents our best effort at generating an OFL from a highly coherent source. Using this

launch, we measured about a 12 ps difference in delay for a 600 m fiber compared to a launch

that conforms to the IEC template. While this delay variation is significant, it represents a

limiting case obtained from extreme conditions relative to how fiber delay is calibrated at NIST,

and may be more indicative of older OTDRs with modulated LEDs and designed for legacy

networks that employ LED sources.

As a final point of reference, we generated a UFL condition by using a narrowband laser

launched from a singlemode fiber (5/125 urn core/cladding). As shown in Fig. 7, the encircled

flux curve is well above the IEC template bounds at all radii. While we made no rigorous delay

measurements, we have observed that the delay in a 600 m fiber can vary by tens of picoseconds

10
compared to a qualified IEC launch. This launch condition is probably representative of legacy

OTDRs based on lasers that pre-date multimode Ethernet testing with encircled-flux criteria.

Conventional OTDR: A Specific Example

We characterized the mode launch conditions of a legacy OTDR of an early 1990's vintage [11],

suitable for making conventional delay measurements using a source module designed for

multimode fiber at 850 nm. The internal launch fiber of the OTDR had a 62.5 um core, so we

attached additional launch fibers with 50 um cores and up to 1 1 m in length to condition the

launch to match the fibers being measured for delay [6]. The launch conditions generated by the

OTDR and measured by the mode analyzer are presented in Fig. 8, along with the IEC template

bounds. The condition labeled "(a)" was generated from the OTDR with a 1 m launch fiber of

50 um core connected to the mode analyzer. This launch state was in the over-filled domain,

having clearly missed the template at the 20 um and 22 um lower bounds. However, a single

mandrel wrap (1" diameter) was sufficient to tune the launch to within the encircled-flux

template. The three launch conditions labeled as "(b)" were generated from an 1 1 meter launch

fiber and had OTDR pulse durations of 1 ns, 20 ns, and 100 ns. In each case, a single mandrel

wrap was necessary to obtain a launch within the template bounds, as shown. The encircled flux

appears to be largely independent of the pulse duration. Finally, the launch condition labeled

"(c)" was generated with an 1 1 meter launch cord with six mandrel wraps, and clearly represents an under-filled launch by missing the upper template bounds at both 20 urn and 22 um.

1-<H

15 Radius (microns)

Fig. 8: Measured encircled-flux launch conditions of a conventional OTDR arc presented.

Launch (a) is from a 1 meter launch fiber attached to the OTDR. The three launches indicated

by (b) are for pulses with 1 ns, 20 ns, and 100 ns duration and an 1 1 meter launch fiber with one mandrel wrap. Launch (c) includes 6 mandrel wraps and is above the template bounds.

11
The optical spectrum of the OTDR source module was measured with a grating-based optical

spectrum analyzer. For pulse durations of 1 ns, 20 ns, and 1 00 ns the source spectrum had a full- width half-maximum bandwidth of about 3 nm, with an approximate Gaussian profile. This

moderately broad bandwidth is more indicative of a multi-mode laser or superluminescent diode

(SLED) rather than a highly-coherent DFB or highly-incoherent traditional LED. The spatial

profile measured on the mode analyzer showed no strong signs of speckle noise, and as a result

the need for speckle averaging did not appear critical.

We attempted to quantify the impact on delay as measured by the OTDR with two widely

different launch conditions: an over-filled launch with no mandrel wraps, and an under-filled

launch with 10 mandrel wraps. In its highest resolution mode using a 1 ns pulse, the OTDR

provides a display resolution of 0.01 m or 1 cm. This corresponds to a temporal display

resolution of only about 50 ps and is indicated by a reflection peak defined by multiple display

points. As a result, we were unable to observe a difference in delay for the two launch

conditions generated by this particular OTDR when measuring a 600 m fiber.

Nevertheless, the results are encouraging, because they indicate that this particular legacy OTDR

is easily capable of generating qualified encircled-flux launch conditions, and in addition does

not appear to require speckle averaging. This situation is a direct consequence of the source module utilized in this particular model of OTDR and is not representative of OTDRs in general,

both new and old. That the impact of modal launch could not be observed here is both a result of

the resolution offered by this particular OTDR and the type of fiber being measured. Measured

in exactly this way, the launch conditions can be considered negligible.

Recommendations and Future Study

The results of this study have estimated the impact of modal launch specific to the NIST

measurement technique as well as a particular model of OTDR. The activities and results of this

investigation provide some guidance and suggested areas for future study on the use of other

conventional laser- and LED-OTDRs. At this time we offer the following recommendations:

1Newly constructed reference spools should use modern fiber designed to minimize modal

dispersion. Doing so will minimize the modal uncertainty contribution, although not

necessarily eliminate it.

2. Reference spools containing legacy types of fiber should be characterized on a case-by- case basis for modal dispersion.

3. Use of an encircled-flux criterion to quantify and control the launch conditions of OTDR

delay measurements will help minimize the modal uncertainty. The practicality of this

approach depends largely on the nature of the OTDR light source.

4. The I EC encircled-flux standard is likely to be met with no modification and testing only

by modern LED-OTDRs designed specifically for this standard.

12
5. It may be practical to convert the launch of some legacy OTDRs to the IEC standard,

provided that a mode analyzer is used for quantification. Doing so would require further

study to ensure compliance.

6. Laser-OTDRs will by nature generate an under-filled launch such that under a variety of

conditions the modal launch uncertainty will dominate the total uncertainty budget.

7. For loss reasons, it is generally impractical to convert a laser-OTDR under-filled launch

into an IEC encircled-flux launch.

8. The unit-to-unit variability in the launch conditions of existing laser-OTDRs should be

studied and the variation in delay quantified.

Acknowledgement

Funding for this study was provided by the US Air Force METCAL and US Army Aviation and

Missile Command.

References

[1] J. Hecht, Understanding Fiber Optics, Prentice-Hall, 4th edition, chapter 4, 2002.

[2] D. Cunningham, B. Lane, and W. Lane, Gigabit Ethernet Networking, Indianapolis:

Macmillan Publishing Co. Inc, chapter 6, 1999.

[3] S. E. Mechels, J. B. Schlager, and D. L. Franzen, "Accurate measurements of the zero- dispersion wavelength in optical fibers,"/. Res. Nat. Inst. Standards Technol, vol. 102,

no. 3, 1997.

[4] Optical fibers, Part 1-22: Measurement methods and test procedures-Length

measurement, IEC International Standard 60793-1-22, IEC, 2001.

[5] J. C. Bermudez and W. Schmid, "Characterization of an optical fiber spool to be used as reference standard for OTDRs distance scale calibration," Proc. SPIE, vol. 5776, pp. 102-

108, SPIE, Bellingham, WA, 2005.

[6] Corning InfiniCor 50 fim Optical Fibers - Product Information, Corning Inc. product

information sheet PI 1457, available at www.corning.com, February 2010. NIST does not

endorse product or trade names, which are stated here only to specify the experiment

completely. Other products may be suitable yet produce different results.

[7] D. Cunningham, B. Lane, and W. Lane, Gigabit Ethernet Networking, Indianapolis:

Macmillan Publishing Co. Inc, p. 206, 1999.

[8] FOTP-203: Launched Power Distribution Measurement Procedure for Graded-Index

Multimode Fiber Transmitters, TIA/EIA Standard TIA/EIA-455-203, TIA/EIA. June

2001.

13
[9] Fibre-optic communication subsystem test procedures — Part 4-1: Installed cable plant -

Multimode attenuation measurement, IEC International Standard 61280-4-1, IEC, June

2009.

[10] B. N. Taylor and C. E. Kuyatt, "Guidelines for evaluating and expressing the uncertainty

of NIST measurement results," NIST Tech. Note 1297, NIST, Boulder, CO, Sept. 1994.

[11] Tektronix FiberMaster TFP2 with FM8500 multimode optical module. NIST does not

endorse product or trade names, which are stated here only to specify the experiment

completely. Other products may be suitable yet produce different results.

14
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National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of

materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program

coordinated by NIST under the authority of the National Standard Data Act (Public Law 90-396). NOTE: The

Journal of Physical and Chemical Reference Data (JPCRD) is published bimonthly for NIST by the American

Institute of Physics (A1P). Subscription orders and renewals are available from AIP, P.O. Box 503284, St. Louis. MO 63150-3284.

Building Science Series—Disseminates technical information developed at the Institute on building materials. components, systems, and whole structures. The series presents research results, test methods, and performance

criteria related to the structural and environmental functions and the durability and safety characteristics of building

elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject.

Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often

serve as a vehicle for final reports of work performed at NIST under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce in Part 10. Title 15, of the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of

the characteristics of the products. NIST administers this program in support of the efforts of private-sector

standardizing organizations.

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Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series collecti\el\ constitute the Federal Information Processing Standards Register. The Register serves as the official source of

information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and

Administrative Services Act of 1949 as amended. Public Law 89-306 (79 Stat. 1 127). and as implemented b\

Executive Order 11717(38 FR 12315, dated May 11,1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NIST Interagency or Internal Reports (NISTIR)—The series includes interim or final reports on work performed

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