Measurement Assurance Program for 

Wavelength Dependence of 

Polarization Dependent Loss in Fiber 

Optic Devices over the Wavelength 

Range from 1535 nm to 1560 nm

NIST

Special

Publication

250-60

Rex M. Craig

Chih-Ming Wang

 NIST Measurement Services:
NIST Special Publication 250-60

NIST MEASUREMENT SERVICES:

Measurement Assurance Program for Wavelength 

Dependence of Polarization Dependent Loss in Fiber 

Optic Devices over the Wavelength Range from

1535 nm to 1560 nm

 

Rex M. Craig

Optoelectronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

325 Broadway

Boulder, CO 80305

Chih-Ming Wang

Statistical Engineering Division

Information Technology Laboratory

National Institute of Standards and Technology

Boulder, CO 80305

February 2003

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director

 
National Institute of Standards and Technology Special Publication 250-60

Natl. Inst. Stand. Technol. Spec. Publ. 250-60, 44 pages (February 2003)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2003

 

For sale by the Superintendent of Documents, U.S. Government Printing Office

Internet: bookstore.gpo.gov — Phone: (202) 512-1800 — Fax: (202) 512-2250

Mail: Stop SSOP, Washington, DC 20402-0001
iii

Contents

1. Introduction..............................................................................1

2. MAP Process Outline.................................................................. 2

3. MAP Transfer Standard Description and Design................................ 2

4. MAP Transfer Standard Implementation.......................................... 4

5. Uncertainties Associated with a MAP Transfer Standard......................5

5.1. Uncertainty in PDLl at Fixed Temperature.......................................... 5

5.2. Uncertainty due to Temperature Dependence of PDLl............................ 6

5.3. Uncertainty Due to Large Thermal Variations During Shipping................8

5.4. Uncertainty Due to Connector/Photodetector PDL................................. 8

5.5. Uncertainties Associated with Source Characteristics..............................9

6. Summary..................................................................................10

7. References.................................................................................11

Appendix A, Assurance Procedure for the PDLl MAP.............................. 13

Appendix B, The PDLl Measurement System and Selected MAP Results ...... 15

Appendix C, Qualification of Artifacts and Measurement System................ 31

Appendix D, Uncertainties Associated with All-States Measurement............. 33

Appendix E, Sample PDLl Certificate...................................................35

Appendix F, Effect of Source Spectral Symmetry..................................... 43

Appendix G, Shipping and Handling Instructions.................................... 45

Appendix H, PDLl Artifact Tray/MM Depolarizer Mount.......................... 47
iv

Abstract

This paper provides the documentation to establish a Measurement Assurance Program (MAP) 

using an optical fiber diattenuator for the calibration of polarization-dependent loss (PDL) across 

the range of telecommunication wavelengths from 1535 to 1560 nm. This MAP transfer 

standard (or artifact) is based on an all fiber design with well-characterized PDL as a function of 

wavelength and temperature. This MAP is intended for the calibration of PDL measurement 

equipment, and is not intended for the simulation of PDL in actual systems. This document 

details the uncertainty analysis, certification procedure and suggested use for the MAP.

Keywords: polarization dependent loss, PDL; diattenuator, fiber optic; measurement assurance 

program, wavelength division, WDM, Mueller matrix, spectral dependence, wavelengthdependence

Important Note: The user of the MAP artifact is referred to Appendix E for specific details and 

instructions concerning handling and usage of the device.
1

Introduction

Polarization-dependent loss references are important to support high-bandwidth optical fiber 

networks. As network bandwidth is increased, either by increasing modulation rates or by 

multiplexing channels in the wavelength or time domains, performance begins to suffer as a 

result of systematic effects that include polarization dependence. Polarization dependence 

manifests itself in a variety of ways that are commonly interrelated and includes polarizationdependent (insertion) loss (PDL) of components, polarization-dependent gain (PDG) of 

amplifiers, polarization-dependent response of photoreceivers, polarization-dependent centerwavelength shift (PDW) of narrow filters, and polarization-mode dispersion (PMD) in 

components and fiber. Of these measurements, the first three can be calibrated by the PDL 

reference described here. 

PDL is defined as 10 log(Tmax/Tmin) (in dB), where Tmax and Tmin are the maximum and 

minimum transmittances over the entire polarization state space. With the advent of wavelengthdivision multiplexed (WDM) fiberoptic communication systems, we have the additional 

complexity of wavelength dependence of PDL (PDL(λ), hereafter referred to as PDLλ). In a 

WDM system, many wavelength channels are present in the same fiber, thereby increasing the 

bandwidth of the system by the number of channels. PDLλ manifests itself as a statistical noise 

source in each of these channels. PDLλ references are therefore needed to calibrate instruments 

that characterize WDM components.

This Measurement Assurance Program (MAP) transfer standard is an optical fiber diattenuator 

for the calibration of polarization-dependent loss across the 1535 – 1560 nm range of 

telecommunication wavelengths. The MAP artifact1 is an all-fiber design with PDL accurately 

measured as a function of wavelength and temperature. The artifact is intended for use in the 

calibration of PDL measurement equipment and is not intended for the simulation of PDL in 

actual systems, since the output is not single-mode.

Based on our experience with the telecommunications industry, we determined that 

wavelength-dependent PDL reference values between 0.05 dB and 0.3 dB (with an emphasis at 

0.1 dB) would be most useful, as this is the accuracy to which most components and network 

design models are specified. This document summarizes the design and implementation of the 

PDL MAP followed by the results of the uncertainty analysis and MAP certification procedure. 

 

1

The terms “artifact”, “standard artifact”, and “transfer standard” are used interchangeably.
2

Section 3 describes the nature and design of the MAP artifact, while Sec. 4 details 

implementation. Section 5 outlines the sources of uncertainty and associated analysis for both 

the MAP artifact and the PDLλ measurement systems. The results are summarized in Sec. 6.

2. MAP Process Outline

NIST measurement assurance programs can assist in the establishment of NIST traceability 

for those situations where a limited number of artifactual transfer standards exist. The difference 

between the MAP program and the SRM program is that the artifact standards are loaned rather 

than sold. This MAP program allows NIST to measure the standard artifact before and after it is 

measured by the customer. While there is a sequence of events associated with the use of a MAP 

artifact, to the user, the end result is similar to that obtained by the use of an SRM, with one 

exception. That is, the certificate is not given to the customer until after return of the standard 

artifact to NIST. The sequence of events is outlined in App. A.

3. MAP Transfer Standard Description and Design

A schematic of the MAP artifact is shown in Fig. 1. This all-fiber device provides a 

wavelength dependent PDL in the range of 0.05 - 0.3 dB (approximate) within a subset (1535 –

1560 nm) of the ITU "C-band" for wavelength multiplexing. The polarization dependence arises 

from a segment of single-polarization (polarizing or PZ) fiber [1,2] that provides relatively stable 

PDL over the range of 5 - 40 °C. Polarizing fiber is a special form of polarization-maintaining 

fiber that maintains a linearly polarized guided mode parallel to its so-called "slow" axis while 

the orthogonal linearly polarized mode is unguided. The input is a short single-mode section for 

accurate coupling to system leads. The output section is composed of approximately 40 meters of 

high-numerical-aperture (NA) multimode fiber that effectively depolarizes the signal, thereby 

reducing uncertainty due to the added PDL of connectors and detectors that follow. Hot-wire 

fusion splicing is used to produce clean joints between the dissimilar fibers. Our early 

experience with the large uncertainties of our own opto-mechanical fiber-pigtailed PDL artifact 

prototypes led us to conclude that a properly prepared all-fiber design can be more stable over 

the long-term. Severe thermal or mechanical shocks, however, can shift the value or cause 

temporary instability. 
3

Another reason for choosing fiber was the importance of simulating wavelength dependent 

PDL in order to allow calibration of measurement system linearity using broadband tunable 

sources. Because of its design, polarizing fiber is well suited as a source of wavelengthdependent PDL [3]. Due to limitations associated with multiple-reflection interference in this 

design, PDL measurements using high-coherence laser sources do not give reliable results. 

Rather, tunable filters following broadband sources such as amplified-spontaneous-emission 

(ASE) or light-emitting-diode (LED) devices are the preferred choice, as the coherence length is 

suitably short to prevent interference. 

An important aspect to the construction of a PDL reference is the fact that the total system 

PDL due to many small contributions is, to a good approximation, the sum of the individual 

contributions [4]. In addition, PDL can be introduced by virtually anything in the optical circuit 

including connectors and photoreceivers. Thus, the relationship of the transfer standard to the 

measurement system is altered by the means of its connection to the system. Connectors are 

always an issue in PDL measurements, and consequently angle-polish connectors (APC) are 

ruled out, despite their superior return loss, due to their inherently high PDL. For PDL 

repeatability on connection and disconnection, our best results are obtained with high-quality 

FC/UPC connectors. 

The 40 m multimode fiber section following the PDL element is used to mitigate the effect of 

extraneous PDL in the receiver end of the user’s measurement system. The multimode fiber 

depolarizes the output light, drastically reducing the amount of amplitude modulation (false 

PDL) induced by PDL in the detector, etc. This is a more practical solution than to merely 

measure the background PDL of the system because a single-mode output from the MAP artifact 

Figure 1. Construction schematic of the MAP artifact detailing the 

individual fiber components.

Input section

(singlemode)

<5 cm

PDL section

(polarizing fiber)

2 - 10 mm

Output section

(multimode depolarizer)

>40 m

FC/UPC
4

would serve to randomize the alignment between the MAP and the PDL of the detector making it 

impossible to compensate through a background correction.

4. MAP Transfer Standard Implementation 

 Photographs of the MAP transfer standard are shown in Fig. 2. The major components are the 

aluminum fiber tray, diattenuator section (short length of PZ fiber), FC/UPC input connector, the 

bulkhead mating sleeves, and the multimode-fiber depolarizer loop. The input section is a 5 cm 

length of common 9/125 μm single-mode (SM) fiber connectorized with a high-quality FC/UPC 

connector. This section is fusion-spliced to a section of PZ fiber (18 dB/m extinction) whose 

length (5 – 10 mm) is empirically determined to produce a PDL in the nominal range. The 

output section consists of a 40 m length of high-numerical-aperture (0.37 NA) multimode (MM) 

fiber (50/125 μm) that is fusion spliced to the diattenuator section. The diattenuator section is 

immobilized and protected by a splice protector that consists of soft silicone rubber tubing and a 

stainless steel rod inside shrinkable tubing. The splice protector is fixed to the tray with adhesive 

and the tray is filled with a shock-absorbing silicone gel. The MM depolarizing loop is wound 

around the base of the tray and terminated with another high-quality FC/UPC connector.

When first constructed, artifacts have only a short length (~25 cm) of MM fiber to simplify 

initial qualification. Following thermal and optical qualification, these prototypes are then 

spliced to the depolarizer loop. The completed artifact is then subjected to additional thermal 

cycling to relieve stresses.

Because there remains some susceptibility to thermal shock, a small battery powered 

temperature-logging device that monitors a thermocouple connected to the interior is attached to 

Figure 2. The completed MAP transfer standard showing both front and top views. 
5

the rear-facing panel of the enclosure. This logging device is capable of sampling the device 

temperature over several weeks. This covers time spent in shipment and measurement at a 

remote location. On return, the logged information is processed to ensure that there were no 

temperature excursions capable of invalidating the calibration.

5. Uncertainties Associated with a MAP Transfer Standard

 There are three primary issues of uncertainty associated with PDL measurements of this MAP. 

These are the internal uncertainty of the user’s measurement system, the external uncertainty 

introduced by the act of connecting a test specimen, and the uncertainty of the transfer standard 

itself. The dominant sources of uncertainty in the transfer standard are the calibration, 

temperature dependence and shipping offsets. We discuss each of these issues below.

5.1. Uncertainty in PDLl at Fixed Temperature

NIST characterizes the wavelength dependence of PDL in these artifacts using the 4-

state Mueller/Stokes measurement system designed for this purpose and described in 

App. B (also a separate publication). Included in the appendix is a detailed uncertainty 

analysis for measurements done at room temperature (23 °C). The 15 nm single-scan 

wavelength range is a temporary limitation of the present measurement system and is the 

region over which the scan wavelength is calibrated dynamically against a portion of an 

HCN reference cell absorption spectrum (NIST Standard Reference Material (SRM) 

2519). Consequently, the PDLλ data combine two scans that, together, span the total 

range. Figure 3 gives a typical PDLλ scan over the full 1535 – 1560 nm range along with 

a least-squares fit. Actual PDLλ data at ITU "C-band" grid wavelengths and associated 

uncertainties accompany each MAP artifact in its Certificate. The combined standard 

uncertainty [5] of the NIST system over any scan in the 1530 – 1560 nm region at a 0.12 

dB reference level2 is uMS(0.12) = 0.0079 dB (from App. B).
6

5.2. Uncertainty due to Temperature Dependence of PDLl

Temperature dependence of PDLλ (PDL(λ,T)) is characterized over wavelength by a 

different measurement system based on automated scans over all states of polarization. 

Wavelength tunability is supplied by a broadband source followed by a tunable filter. A 

schematic of the test system appears in Fig. 4. It consists of an ASE source followed by 

a fiber-Fabry-Perot tunable filter (FFP-TF) of 0.7 nm 3 dB bandwidth (Lorentzian 

profile), a fiber-loop polarization controller, an optical power meter with sampling and 

averaging capabilities, and the device under test (DUT) enclosed in an environmental 

test chamber. Because the measurement system is an all-states type, the uncertainties in 

 

2 A nominal PDL value of 0.12 dB is used throughout to establish a reference level for uncertainty calculations

Figure 3. Typical PDLλ data for the MAP standard. Error bars represent the measurement 

system's standard uncertainty.

0

0.05

0.1

0.15

0.2

0.25

1530 1535 1540 1545 1550 1555 1560 1565

Wavelength (nm)

PDL (dB)

Figure 4. Schematic of the all-states PDL system used for thermal characterization of the 

PDLλ artifact.

Pigtail Polarizer

90/10

Splitter

DUT with

Temp. Control

Broadband Source

with Tunable Filter

Fiber-Loop 

Polarization Scanner

Wavelength

Monitor

Intensity Ref.

Monitor

Power 

Measurement

OSA

Power meter ASE/

FFPTF

99/1

Splitter

90%

10%

99%

1%

Power meter
7

PDL at a particular temperature can be characterized in a manner similar to that in [6] 

with the uncertainty budget given in Table 1 of App. D. If T1 and T2 are the maximum 

and minimum measurement temperatures, the differential temperature coefficient's 

contribution to uncertainty at a given wavelength can be approximated by 

(PDL T (l l ,

1 ) − − PDL( , 2 T2 )) (T T 1 2 ). Figure 5 gives the temperature coefficient data over 

the 1525 – 1565 nm wavelength range along with a polynomial fit for three artifacts. 

Individual measurements at temperature T1 = 26 °C are completed in 10 s and averaged 

over a group of 20 measurements at each wavelength in 5 nm increments. The 

temperature is then cycled to T2 = 20 °C, and stabilized, and the artifact is measured 

again. The quoted uncertainty over the range 1535 – 1560 nm range is taken from Fig. 5 

to be the maximum absolute value observed in the data (0.0056 dB/°C) multiplied by the 

user's environmental temperature uncertainty ΔT. An environmental temperature 

uncertainty bound of ΔT = ±1°C produces a uT = = 0.0056/ 3 0.0032dB standard 

uncertainty in PDLλ [5]. Uncertainty estimates are included in the Certificate for 

temperature bounds of both ±1 °C and ±3 °C. The choice of bound is determined by 

the degree of temperature control in the user's measureme nt environment.

Figure 5. Typical temperature dependence of PDLλ data for 3 MAP artifacts.

-0.006

-0.005

-0.004

-0.003

-0.002

-0.001

0

0.001

0.002

1520 1530 1540 1550 1560 1570

Wavelength (nm)

Temperature Coefficient (dB/°C)

MAP 1

MAP 2

MAP 3
8

5.3. Uncertainty Due to Large Thermal Variations During Shipping

Because these devices may retain some sensitivity to large temperature cycles that can 

shift PDLλ, they must be well insulated during shipping and shipped by the most rapid 

means available. In addition, a logging device will constantly monitor the ambient 

temperature over the entire span of the trip. NIST will tabulate this data at the end of the 

calibration cycle to insure that the artifact has not been exposed to temperature extremes 

(outside the range 0 – 50 °C) that may invalidate the Certificate. The uncertainty has 

been calculated from observed shifts of wideband PDL values at room temperature 

following a cycling of the ambient temperature between two extremes that begin over 15 

– 35 °C and widen to 0 – 50 °C. Shifts are characterized by a ratio of the PDL standard 

deviation, σshift, over four thermal cycling sequences to the mean PDL , <PDL>, 

observed. Only devices that maintain a ratio of shift standard deviation to mean 

(σshift/<PDL>) of less than 10% are selected for MAP artifacts. The uncertainty will be 

taken as the maximum observed ratio. Based on a series of measurements, the 

maximum of (σshift/<PDL>) is 3.7% of wideband PDL, which gives a standard 

uncertainty of uship(0.12) = 0.0045 dB for a 0.12 dB reference PDL value.

5.4. Uncertainty Due to Connector/Photodetector PDL

Any connection between two singlemode optical fibers can introduce PDL, whether 

through Fresnel reflections, lateral core offset, or mode-field mismatch. Thus the artifact 

is subject to a PDL uncertainty at the input connection. While these uncertainties are 

generally small with high-quality FC/UPC connectors, only by averaging results over 

multiple measurement trials with various input leads and connectors can the user reduce 

input lead contributions. This can be seen by characterizing each PDL contribution as a 

"vector" (or axis) normal to propagation. This vector determines whether input connector 

PDL (PDL1) adds or detracts from the internal system PDL (PDL2). The result is 

approximated by PDLtot ≅ + PDL1 (q1 ) PDL2 (q2 )⋅ − cos 2( (q q 1 2 ))

uuuuuuur uuuuuuuuuur uuuuuuuuuuuru

dB where q1 and q2

represent the angles of PDL vector for the connector and system PDL. When vectors are 

aligned, the total PDL is the sum of the two. Likewise when the axes are orthogonal the 
9

total approximates the difference. There is no way to determine, a priori, the relative 

angle between the two axes. The user may estimate this uncertainty by sampling system 

PDL with a variety of input leads and connectors without a DUT in place. One is thus 

effectively averaging over several axis angles and connector types. The result is a 

random component to be added in quadrature to the MAP uncertainty rather than an 

offset to be added directly. At the output of the MAP artifact an accurate calibration 

requires that this random component be minimized through depolarization. Although 

the artifact's output depolarizer is not completely effective, the remaining degree-ofpolarization (DOP) is small (approximately 5 % for a source bandwidth of 0.7 nm or 

greater). Consequently any remaining PDL due to output connectors and to 

photodetectors contributes with only a 5 % weighting, and is negligible.

5.5. Uncertainties Associated with Source Characteristics

Fixed or tunable sources with 3-dB bandwidths between 0.7 and 20 nm are generally 

appropriate sources for use with the MAP artifact. Certification for larger bandwidths 

can be accommodated by special request. Examples of appropriate sources include 

filtered ASE sources, filtered LED sources, and multimode lasers. Narrowband fixed or 

tunable laser sources cannot be used, in general, due to the prevalence of interference 

effects generated by high coherence and back-reflection. Interference must be avoided 

in any precise measurement of PDL since the drift of the associated maxima and minima 

are often indistinguishable from the PDL signal. A useful alternative is the use of a 

broad optical source with a narrowband filter that is capable of wavelength dither at high 

frequencies, such as a fiber-Fabry/Perot design driven by a PZT transducer. The dither 

is an effective decorrelator if the dither period is low compared to photodetector 

integration time. 

 While broadband sources typically exhibit a low degree of polarization, this may not 

always be true. A common source of false PDL signals is a partially polarized source 

followed by singlemode fiber and a polarizer positioned prior to a polarization 

controller. Drift in fiber birefringence causes the polarized component of the source to 

wander over all states, producing erroneous intensity fluctuations on the output of the 

polarizer. Consequently, the user should recognize this and ensure that the source is 
10

well depolarized with degree-of-polarization generally less than 1% unless the source 

and polarizer are separated by only a very short, stable length of intervening fiber. The 

alternative is source-intensity normalization through a ratio incorporating a second 

power monitor prior to the polarization controller, as in Fig. 4.

 Finally, the user should be aware that the measured PDL, using a broadband source 

symmetric in wavelength, is the integrated PDLλ over the source bandwidth. For 

sufficiently narrow sources, the measured PDL is the source-center-wavelength value. 

The effect of source spectral asymmetry is discussed in App. F.

6. Summary

The standard uncertainties described in sections 5.1 to 5.4 above are added in quadrature and 

multiplied by a coverage factor of 2 [5] to yield the expanded uncertainty included in the 

Certificate. PDL uncertainties due to the customer’s connections should be estimated separately 

as outlined above and then added in quadrature to the expanded uncertainty.

A description of the NIST 4-state Mueller/Stokes measurement system appears in App. B. 

The qualification procedure for the MAP appears in App. C, and uncertainties associated with 

all-states measurement are given in App. D. A sample MAP certificate is presented in App. E. 

The certificate includes the certified PDLλ, along with the associated standard uncertainties for 

two different temperature uncertainties about room temperature (23 °C), and instructions for use 

of MAP transfer standard. The actual MAP certificate will be sent to the user following a MAP 

measurement. A cautionary description of the effect of asymmetric source spectra is given in 

App. F. Finally, shipping instructions are included in App. G, and a schematic diagram of the 

aluminum tray/spool assembly appears in App. H.
11

7. References

1. M. J. Messerly et al., "A broad-band single polarization optical fiber," J. Lightwave Technol., 

vol. 9, pp. 817 – 820, 1991.

2. K. Tajima et al., "A new single-polarization optical fiber," J. Lightwave Technol., vol. 7, 

pp.1499 - 1503, 1989.

3. M. J. Messerly, R. E. Budewitz and B. K. Nelson, "Characterization of single polarization 

fiber," Tech. Digest – Symposium on Optical Fiber Meas., NIST Special Publication 864, 

1994, pp. 185 – 188, 1994.

4. N. Gisin, "Statistics of PDL," Opt. Comm., vol. 114, pp. 399-405, 1995.

5. B. N. Taylor and C. E. Kuyatt, "Guidelines for evaluating and expressing the uncertainty of 

NIST measurement results", Tech. Note 1297, Nat. Inst. Stand. Technol.,, 1994.

6. Hewlett-Packard Product Note, "Polarization-dependent loss measurements using modular 

test system configurations", PN-11896-1, 4/95
Appendix A

13

Assurance Procedure for the PDLl MAP

1. NIST performs calibration of its Mueller/Stokes PDL system using the BK73 reference.

2. NIST performs calibration of its All-States PDL system using the BK7 reference.

3. NIST performs initial temperature coefficient measurements of MAP unit.

4. NIST measures MAP unit using Mueller Stokes technique.

5. NIST initializes temperature logger and packs unit prior to shipping.

6. NIST ships unit to customer.

7. Customer measures PDL of unit and ships it back to NIST within 4 – 6 weeks, if possible.

8. NIST receives unit.

9. NIST unpacks unit and analyzes temperature logger for exposure to temperatures outside 

0 °C – 50 °C range.

10. NIST runs final Mueller/Stokes measurement of MAP unit.

11. NIST runs final temperature-coefficient measurements.

12. On verification that all parameters are within specification, NIST mails certificate to 

customer.

 

3 Certain materials and equipment are identified in this paper to foster complete understanding. Such identification 

does not imply recommendation or endorsement by NIST, nor does it imply the materials or equipment are 

necessarily the best available.
Appendix B

(Preliminary)

15

The PDLl Measurement System and Selected MAP Results

Abstract – Building on previous work, a rapid, automated, non-mechanical measurement system 

for spectral characterization of polarization-dependent loss (PDL) has been developed. A 

deterministic fixed-states Mueller/Stokes method in conjunction with real-time calibrated 

spectral information is used to derive wavelength-dependent Mueller matrix elements. Voltagemodulated liquid-crystal variable retarders set the input polarization states. A narrow, voltagetuned filter provides a wavelength sweep following a broadband source; the sweep wavelength is 

calibrated in real-time by hydrogen-cyanide reference lines. This rapid measurement system can 

measure PDL over a wavelength range of 15 nm in 5 seconds. A complete uncertainty analysis 

has been conducted for PDL in the range of 0.05 to 0.3 dB with an expanded uncertainty of 

0.0098 dB over the range from 1535 to 1560 nm. Performance was verified with a Fresnel 

reference. Finally, design and performance results from all-fiber artifact calibration standards 

are presented.

1. Introduction

Until the advent of dense wavelength-division multiplexing (DWDM) systems, polarizationdependent loss (PDL) was usually characterized at only a specific wavelength [1]. More

recently, however, the wavelength dependence of PDL (PDL(λ), hereafter referred to as PDLλ) 

has assumed greater importance, particularly in component metrology. A modification of a

previously reported, deterministic fixed-states technique [2] has been implemented that allows 

PDL to be characterized as a function of wavelength. The goal is to establish a capability for 

rapid measurement of PDLλ to a resolution finer than 0.005 dB without sacrificing accuracy. 

This approach uses a non-mechanical matrix technique employing ratio detection and

simultaneous spectral calibration via a hydrogen-cyanide (HCN) reference that is faster than 

most tunable-laser systems (taking only 5 seconds for a 15 nm range) and more accurate than 

techniques based on optical spectrum analyzers [3]. In addition, this approach can be customized 

to the resolution requirements of both wideband and narrowband devices. Wideband devices

such as switches, commonly exhibit a small variation in PDLλ that doesn't require a fine 

wavelength resolution. In contrast, a narrow-channel device such as a filter may exhibit

relatively small values of PDLλ over the transmission (or reflection) band but large values at the 
Appendix B

(Preliminary)

16

band edge. This type of device requires higher wavelength resolution for accurate 

characterization. 

New results from an all-fiber PDLλ artifact reference are also presented. This artifact is 

intended to become a NIST Measurement Assurance Program (MAP) transfer standard with PDL 

values in the range of 0.05 – 0.3 dB for the calibration of PDL measurement instrumentation.

In Sec. 2 the concept of the measurement method is discussed, and Sec. 3 describes the 

experimental implementation. Section 4 outlines a comprehensive uncertainty analysis.

2. Concept

Our method is a further variation of a matrix technique, sometimes referred to as the

Mueller/Stokes technique, developed by Favin, Nyman and Wolter [4],[5] and modified by Craig 

et al. [2]. The method relies solely on measurements of power ratio at specific polarization 

states, but has now been extended to accurately cover a range of wavelengths. As in reference 

[2], four well-defined polarization states are necessary to determine the first-row Mueller matrix 

elements of a component. Diattenuation, the global polarization dependence of transmittance, 

can be determined from these (wavelength-dependent) matrix elements. Because the

measurement depends only on the relative coordinates of the four states, the sole requirements on 

the set are that they maintain relative angular separations of 90° (orthogonality) about the origin 

of the polarization Poincaré sphere, as shown in Fig. 1. Thus any constant retardance in the 

measurement path will not affect the measured PDL, since pure retardance does not alter the 

orthogonality of the states, but merely rotates them uniformly on the sphere. MeasurementFig.1a: Initial Poincaré sphere trajectories of the liquid-crystal variable retarder (LCVR) state generators. 

Fig. 1b: LCVR trajectories following birefringent displacement

Figure 1a Figure 1b 
Appendix B

(Preliminary)

17

system PDL in conjunction with retardance, however, modifies this scenario somewhat by

altering the relationship between the launched states and can potentially introduce significant

uncertainty. This uncertainty and its resolution will be discussed in detail in section 4.

In the Mueller/Stokes technique, PDL is measured by launching four orthogonal polarization 

states into the device-under-test (DUT) and for each state, measuring the ratio of the transmitted 

power (IT1(λ)...IT4(λ)) to the launched power (IL1(λ)...IL4(λ)). Including the wavelength 

dependence of these powers, the first row of Mueller matrix elements can be defined from 

Ref. [2] as

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

( )

1 2

1 2

1 2 11

1 2 12

13 1 2 3

14 1 2 3

1 2 4

1 2 4

1

2

1

2

1

2

1

2

T T

L L

T T

L L

T T T

L L L

T T T

L L L

I I

I I

m I I

m I I

m I I I

m I I I

I I I

I I I

l l

l l

l l l

l l l

l l l l

l l l l

l l l

l l l

        +

 

      −       =

      −   + +      

 

−   + +

   

 . (1)

The spectral transmittance extrema, Tmin(λ) and Tmax(λ) are obtained from the matrix elements 

above by optimization as

( ) ( ) ( ) ( ) ( )

222

max 11 12 13 14 Tmmmm l = llll + + +

( ) ( ) ( ) ( ) ( )

222

min 11 12 13 14 Tmmmm l = llll − + + , (2)

so that

PDLl≡ PDL (l) =10log   T T max (l l ) min ( )  

holds globally over the entire space of polarization states and wavelength. 

Errors due to PDLλ internal to the measurement system and the wavelength dependence of the 

liquid-crystal variable retarders (LCVR) are dramatically reduced through normalization of DUT 

input and output powers in two separate ratios. This arrangement virtually eliminates fluctuations 

in source power, common-mode drift, spectral dependence in the measurement system and 

system PDLλ. In the first normalization, four initial ratio measurements I I T T 1 4 (l l )K ( ) are 

taken that are in fact signal-to-reference ratios without the DUT in place, while the final four 
Appendix B

(Preliminary)

18

measurements I I L L 1 4 (l l )K ( ) are signal-to-reference ratios with the DUT in place. These 

ratios remove spectral dependence in various elements of the system as well as source 

fluctuations and common-mode drift. The second normalization is simply the ratio of IT and IL

as used in Eq. 1 above; this removes the inherent PDLλ of the system so that only the PDLλ of 

the DUT remains.

3. Implementation

Figure 2 shows a schematic of the measurement system. The system consists of three major 

sections: (1) an amplified, spontaneous-emission (ASE) source with a tunable, filtered output 

calibrated against a NIST wavelength reference, (2) the LCVR polarization state generator, and 

(3) a measurement section. 

Light source and wavelength calibration: The ASE source in Fig. 3 is of a spectrally flattened 

NIST design followed by a fiber-Fabry-Perot tunable filter (FFP-TF) tunable over the ASE range 

from 1535 – 1560 nm with a full-width-at-half-maximum (FWHM) bandwidth of 0.7 nm and a 

free-spectral-range of 85 nm. The FFP-TF is driven by a ramp-generator circuit in a repetitive 

sweep that is triggered by signals from the control computer. The FFP-TF output is tapped by 

two 10/90 wavelength-independent couplers in series. The first directs 10 % of the light through 

a NIST HCN wavelength reference (Standard Reference Material (SRM) 2519) [6] to one input 

ASE Sweep Source

1530-1560 nm nm

GRIN

Polarizer

0°

LCVR 1

45°

LCVR 2

0°

GRIN

Open Beam State Generator

DUT

Depolarizer

Out

Computer

Ref

80/20 Splitter

20% Ref. 80% Signal

Log Ratio 

Detector

DUT Baseline 

Switch

Measurement Section

Master Synch. Clock

Out

Figure 2: Simplified diagram of the measurement system
Appendix B

(Preliminary)

19

of a balanced differential InGaAs photoreceiver. The second directs 9 % of the light to the other 

input of the differential photoreceiver to act as a source-level reference. The differential 

photoreceiver produces an HCN absorption spectrum that acts as a calibration signal with 50 dB 

of common-mode rejection. This calibration signal is sampled to 16-bit resolution by the control 

computer and logged. The absorption peaks were fitted by a least-squares quadratic fit and 

compared to the known peak wavelengths following each scan, providing near real-time spectral 

calibration of each sample point. This calibration signal appears in Fig. 4. Presently, each sweep 

spans only 15 nm of the range from 1535 – 1560 nm because of speed and sampling-density 

constraints related to HCN calibration; a full span of 25 nm must be covered in at least two steps. 

The 0.7 nm Lorentzian bandwidth Δl of the source at l = 1550 nm gives a coherence length 

2

l

c = lplΔ ≅1.1mm . (3)

This coherence length is insignificant compared to the typical distances (of order 1 cm) 

between uncoated surfaces in the measurement system, so multiple reflection interference is 

minimized.

Polarization-state generation: The major portion of the swept-source output light is 

collimated by a GRIN lens and then passed through a Glan-Thompson polarizer followed by a 

series of two LCVR units used to generate the four polarization states. An LCVR is a liquid 

crystal element whose retardance can be controlled by an applied voltage. The first LCVR, with 

fast axis at 45° to the incident polarization,provides either zero or half-wave retardance. The 

second LCVR, with fast axis at 0°, provides either zero or quarter-wave retardance. Polarization 

modulation is synchronized with each source wavelength sweep in the sense that each state 

setting triggers a new spectral sweep.

Grating Tuned

Double-Pass ASE

FFP Tunable Filter

Ramp

Generator

10/90 Splitter 10/90 Splitter

HCN Gas Cell

Reference

Balanced

Photodetector

Sweep Output

Calibration

Signal

Master

Synch. Input

Ref

Signal

Figure 3: Diagram of swept wavelength source with HCN calibrator (Sweep 

Source)
Appendix B

(Preliminary)

20

The choice of sweep priority, (i.e., each polarization state having its own wavelength sweep 

rather than the launching of each of four states at every wavelength of a single spectral sweep) 

was made because it allows rapid spectral calibration. Each spectral sweep was sampled at a rate 

sufficient to resolve HCN absorption peaks to the required accuracy. The spectral data for each 

polarization state are aligned in wavelength. Voltages are generated by the control computer and 

synchronized to the source sweep by a master-clock-modulated LCVR retardance. Retardance 

modulation was checked periodically against a polarimeter to ensure the proper relationship 

among the four states. After the second LCVR, a pigtailed GRIN lens launches the polarizationmodulated light back into single-mode fiber connected to a spectrally flattened 80/20 splitter. 

Measurement and calibration: Transmitted and launched powers were measured respectively 

with and without the DUT in place, c.f. Fig. 2. The 80/20 splitter was used followed by a ratio 

detector to obtain the IT and IL ratios to source power (removing effects of drift). The major 

portion (80 %) of the splitter output can be switched between the DUT and a direct connection to 

the first input of a second balanced differential photoreceiver to establish a measurement 

baseline. The system calibration was checked by replacing the DUT with a fiber-pigtailed open0 100 200 300 400

Sample No.

-0.2

0.0

0.2

0.4

Relative Line Strength

1536

1541

1546

1551

Fit Wavelength (nm)

HCN Spectra Samples

Peak vs. Sample Fit

Figure 4: Typical P-branch HCN absorption signature from 1539 

to 1551 nm with a quadratic calibration fit
Appendix B

(Preliminary)

21

beam cavity containing a polished BK-7

4

 glass block. The block can be tilted to vary the Fresnel 

reflections in a predictable way to emulate a PDL. Calculated (Fresnel) and measured PDL 

values as a function of input angle are presented in Fig. 5 and agree to ±0.005 dB. The residual 

system PDL was measured at 0° (normal incidence) as a 0.0014 ± 0.0006 dB offset over all 

wavelengths. A residual slope indicates a small systematic uncertainty since BK7 has negligible 

dispersion over this range. 

Finally, the DUT PDLλ signal was sampled at 450 points for each 15 nm wavelength scan. A 

scan period of 285 ms was required for each polarization state, after which the state was changed 

to the next in the series. An entire PDLλ scan over the 15 nm range takes less than 5 seconds. 

4. Uncertainties

The design of this measurement system is both simpler and more sophisticated than that 

described in Ref. [2], in that taking appropriate ratios of key parameters now reduces most 

uncertainties. Items 1, 2, 3, and 4 below outline the four major systematic uncertainties in 

polarization and power that, together, comprise the total systematic contributions. The remaining 

random uncertainties are listed in order of decreasing impact. Values listed correspond to one 

standard deviation at 1545 nm. The uncertainty analysis is guided by [7]. 

 

4

 The identification is made to adequately describe the experimental procedure and is not an endorsement by the 

National Institute of Standards and Technology nor does it imply that the material is the best available for this use.

1538 1540 1542 1544 1546 1548 1550 1552

Wavelength (nm)

0.0

0.1

0.2

0.3

Calibrated Measurement

Calculated Ref. Value

30

25

20

10

5

WPDL (dB)

Incidence Angle (deg.)

0

15

Figure 5: Open-beam BK7 artifact data calibrated at 30° incidence. Centerline is the 

calculated PDLλ value. Residual wavelength slope falls within 2σ uncertainty.
Appendix B

(Preliminary)

22

• Type B Components [7]

1. Polarization-state accuracy. Launched polarization states were measured using a 

polarimeter. Polarization state accuracy depends on both the polarimeter and the 

LCVR bias calibration. In this case, the polarimeter has been calibrated (open beam) 

for three linear states and one circular state by means of a 60-dB extinction GlanThompson polarizer followed by a NIST quarter-wave retardance reference [8]. The 

retardance reference is certified to 0.1° uncertainty at 1300 nm and has 

0.2° uncertainty for low-coherence sources out to 1560 nm. The angular resolution 

for alignment of these two optical elements is 0.02°, a negligible contribution. The 

associated contribution to uncertainty is the total calibration uncertainty of all four 

states generated by the pair. Polarimeter/LCVR system measurements are repeatable 

to 0.06 % (0.1°) over linear state angles and to 0.6 % (0.5°) in ellipticity over the 

Poincaré sphere at 1545 nm. Following the polarimeter calibration, the LCVR bias 

was adjusted at 1545 nm to match the calibration states to within approximately 

2.0 %. However, a small systematic offset in the ellipticity of the LCVR circular 

state of 3.0 % remained. The resulting root-sum-of-squares (RSS) PDL standard 

uncertainty contribution was 1.2 % (in dB) for each of the linear states and 2.1 % for 

the circular state. 

2. Retarder wavelength dependence. The same drive voltage was used on each LCVR 

state regardless of wavelength, and this causes an uncertainty in the polarization 

states due to the LCVR wavelength dependence. Based on wavelength-dependent 

retardance measurements supplied by the vendor, the retardance exhibits only a 

simple inverse dependence on wavelength l as ,fVR = Φ 2p l f (V T, / ) where 

Φf (V T, ) is the LCVR phase delay (in nm) as a function of voltage V and 

temperature T. The associated uncertainty contribution is: 

, ( )

2

( ) 2 , / ΔfVR l f = pΦ Δ V T l l , where a wavelength scan of Δ = l 10 nm about 

l = 1550 nm produces a normalized standard uncertainty of Δ = f f VR/ VR 0.35%. This 

uncertainty is manifested primarily as an uncertainty in the accuracy of each Stokes 
Appendix B

(Preliminary)

23

state and its associated matrix element. This uncertainty could be reduced through 

the use of a look-up table.

3. Retarder temperature dependence The LCVR retardance has an uncertainty due to 

temperature of: 2 ( , )( ) , VR T

V T T

T

p f

f

l

∂Φ Δ

Δ =

∂

where the vendor's estimate of 

∂Φ ∂ f T is 0.004 nm/°C. A temperature variation of 0.5 °C (ΔT ) gives a normalized 

standard uncertainty contribution of (Δ = f f VR

)

T

/ VR 0.12% for our conditions.

4. System internal PDL. The LCVR elements contribute their own PDL (0.02 dB) to 

that of the DUT. This manifests itself as a change in intensity with a change in 

polarization state. This PDL was assumed constant during both phases of the 

measurement and is cancelled in the normalization ratios. However, any variation in 

system PDL over the course of a measureme nt appears as an uncertainty in signal 

power. Given the maximum observed drift of the internal PDL of 0.02 dB over 

intervals of 5 minutes, this yielded a PDL standard uncertainty due to LCVR elements 

of 0.015 %.

Propagated measurement system uncertainty. The element percentages above, inherent to 

the measurement system, are adjusted standard uncertainties that assume a rectangular 

distribution [7]. From these values, an error-propagation model based on the defining 

equations (Eq. 1) produces a Type B standard uncertainty of 0.0047 dB at a reference PDL 

value of 0.12 dB. 

• Type A Components [7]

1. PDL measurement repeatability. Effects that can be considered statistical in nature 

include possible coherent interference at the photoreceiver, calibration-spectral-peak 

registration uncertainty and resulting fit uncertainty, and photoreceiver noise. The 

combined effect is encompassed by a measurement of system repeatability. Ten 

undisturbed measurements of a 0.12 dB BK-7 PDLλ were made following the initial 

baseline measurement. The standard deviation of these measurements averaged over 

all wavelengths yielded 0.0012 dB as the effective standard uncertainty of single-
Appendix B

(Preliminary)

24

sweep system noise and incorporates all effects such as optical interference and 

detector noise that average down over multiple measurements. 

Combined standard uncertainty for the measurement system. The Type A and Type B 

components above are assumed uncorrelated, so their root-square-sum (RSS) at the 0.12 dB 

level is an uncertainty 2 2

sys u = (0.0012 + 0.0047 ) dB = 0.0049 dB for an open-beam PDL 

measurement. Using a coverage factor (k = 2), the expanded uncertainty [7] follows as 

2usys = 0.0098 dB.

• Type A Artifact Contributions

1. Birefringence-effect uncertainty. When measuring devices with significant 

birefringence in the path (fiber pigtails etc.), the uncertainty associated with 4-state 

measurements has been calculated to be a variation of 4.5% about the mean of 

measured PDLλ in dB. This value is derived from a computer model (to be discussed 

in a future publication) of four-state measurements in the presence of multiple PDL 

sources linked by birefringent fiber. At the 0.12 dB PDL reference value, this 

uncertainty is 0.0054 dB.

2. Connector uncertainty. As in reference [2], a significant source of uncertainty that is 

not inherent to the LCVR system is variations in measured PDL of DUT artifacts with 

connectorized pigtails following disconnection and reconnection. This could be 

explained by connector alignment (core offset) errors. We have measured this 

uncertainty to be 0.0029 dB for connectorized artifacts.

Combined standard uncertainty for the measurement artifact. Summing the uncorrelated 

artifact contributions above, the total RSS is 2 2

DUT u = (0.0054 + 0.0029 ) dB = 0.0061 dB at 

the 0.12 dB level. Note that this uncertainty is present in any Mueller/Stokes 4-state 

measurement of PDL.

Total Combined Standard Uncertainty. The RSS total of all previous uncertainty 

contributions is then DUT

2 2 2 2 uMS = ( u u sys + ) = (0.0049 + 0.0061 ) dB = 0.0079dB , which 
Appendix B

(Preliminary)

25

represents the uncertainty in a typical measurement. The expanded uncertainty is then 

2uMS = 0.016 dB.

• Wavelength Calibration Uncertainty. Based on sampling density, the wavelength 

uncertainty is estimated to be 0.025 nm for the 0.7 nm FWHM bandwidth of the tunable 

filter element. For wideband devices in which PDL is constant or varies slowly with 

wavelength, this uncertainty is not significant. For narrowband devices such as filters, 

however, this uncertainty may be significant. This value enters the artifact uncertainty in 

a product with the slope of PDLλ.

Stress and thermal drifts in optical fiber birefringence must be minimized over the course of a

measurement. The measurement method is sensitive to small errors in the matrix elements, such 

as those due to retardance drift between the baseline and DUT measurement. For this reason, 

one should use short, straight leads anchored to a fixed surface, and calibrate the system prior to 

each series of measurements.

5. All-Fiber PDL Artifact Results

A series of all-fiber PDL artifacts has been constructed (Fig. 6) to aid in the establishment of a 

NIST PDLλ transfer standard,. These artifacts consist of three sections: single-mode (SM), 

polarizing (PZ), and multimode (MM) fiber, fusion-spliced together to generate PDL in the range 

of 0.05 dB – 0.3 dB over the ITU C-band. The PDL section consists of 5 – 10 mm of polarizing 

fiber (18 dB/m extinction) that provides relatively stable PDL over the range of 15 – 35 °C. The 

input is a 5 cm section of single-mode (SMF-28) fiber for accurate coupling to system leads. The 

output section is composed of 40 m of 50/125 μm step-index multimode fiber. The high 

numerical aperture (0.37) of the multimode fiber is sufficient to essentially depolarize the signal, 

Input section

(singlemode)

<5 cm

PDL section

(polarizing fiber)

2 - 10 mm

Output section

(multimode depolarizer)

>40 m

FC/UPC

Figure 6: Simplified schematic of the all-fiber PDLλ artifact.
Appendix B

(Preliminary)

26

thereby reducing measurement uncertainty arising from output connector and detector PDL, and 

produces a more accurate calibration. Hot-wire splicing was found to produce the most reliable 

joints between the dissimilar fibers. The entire length of the splice region was gently 

immobilized with custom protectors. 

This design was chosen initially to provide a simple and relatively inexpensive fabrication 

method for multiple units. The all-fiber design avoids the usual complications associated with 

open-beam optics. In practice, one typically trades one set of problems for another. Given the 

highly stressed nature of polarizing fiber, for example, some sensitivity to variations in the 

thermal environment can be expected. It was surprising however, to see the magnitude of the 

variations in PDL with temperature immediately following construction. These variations can 

easily reach ± 80 % of the intended value due, presumably, to shifts in the splice protection 

assembly. With repeated thermal cycling over the temperature range of 0 – 50 °C, though, 

thermal movement usually begins to subside and measured PDL begins to stabilize. This 

behavior is illustrated in Fig. 7. Additional challenges include reliably splicing dissimilar fibers. 

Fusion splicers that employ electric arcs were found to be particularly troublesome by causing 

bubbling of the PZ fiber material. A different splicer using hot-wire heating was found to 

produce, with the right parameters, a more consistent and better quality splice. Another issue is 

that of back-reflections at the junctions of dissimilar fibers. When dissimilar fibers are joined, 

there will be back-reflections due to joint imperfections and to Fresnel reflections at the index 

boundaries, both of which lead to etalon interference when high-coherence sources are used to 

0.080

0.085

0.090

0.095

0.100

0.105

0.110

0.115

0.120

1 3 5 7 9 11 13 15 17 19 21 23 25 27

Initial Sequence Final Sequence

Broadband PDL (dB)

Figure 7: All-fiber artifact temperature variation. The "Initial Sequence" illustrates the thermal instability in 

PDL before annealing. The "Final Sequence" is the reduced thermal instability after annealing.
Appendix B

(Preliminary)

27

measure PDL. The level of these reflections can be inferred from the observed interference 

modulation of PDLλ as measured with a highly coherent source such as a tunable laser. The 

magnitude of the modulation, ΔPDLmod (Fig. 8), is approximately 0.25 dB, which can be equated 

to the fringe contrast of a simple etalon from:

2 2

mod 10log((1 ) /(1 ) ) ΔPDL = − + R R j j , (4)

where Rjis the reflection at each splice joint. Consequently, Rj can be solved to yield a reflection 

of 1.4 % across each joint. This effect precludes the use of these artifacts in measurement 

systems employing highly coherent sources. In fact, to prevent this effect, one should limit the 

coherence length Lc to 10 % of the PZ fiber length (or Lc < 1.1 mm). This coherence length 

corresponds to a source bandwidth of approximately 0.7 nm (for a source with Lorentzian 

profile) and is satisfied by our measurement system.

Some recent PDLλ measurement results from the present system are presented in Fig. 9 for 

three artifacts. Only selected data points are shown, along with the uncertainties calculated 

above and a least-squares fit to a fourth-order polynomial. These data span the PDL and 

wavelength range currently available for transfer via the NIST Measurement Assurance Program. 

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

1510 1530 1550 1570 1590 1610 1630

Wavelength (nm)

PDL (dB)

Figure 8: PDLλ data on a 0.1 dB (nominal) artifact taken using a tunable laser source.
Appendix B

(Preliminary)

28

6. Conclusions

An accurate, rapid, non-mechanical technique of characterizing the wavelength dependence 

of polarization-dependent loss for both single-mode and bulk-optic devices has been developed 

that offers advantages over more traditional methods. The technique has an expanded 

uncertainty of 0.0098 dB when measuring a device with 0.12 dB of PDL. A typical 

measurement scan requires 5 s. Overall accuracy depends on calibration to a primary artifact. 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

1530 1535 1540 1545 1550 1555 1560 1565 1570

Wavelength (nm)

PDLl (dB)

Figure 9: PDLλ for three artifact prototypes with corresponding polynomial least-squares fits.
Appendix B

(Preliminary)

29

References

[1] M. Gadonna and A. Mabrouki "Polarization Sensitivity Measurements Methods for Passive 

Optical Components," Conference on European Fibre Optic Communications and Networks, 

EFOC&N '93, The Hague, Netherlands, Technical Digest, pp. 65-67. (1993)

[2] R. M. Craig, S. L. Gilbert and P. D. Hale, "High Resolution, Nonmechanical Approach to 

Polarization Dependent Transmission Measurements," IEEE J. Lightwave Technol., vol. 16, 

pp. 1285-1294, (1998).

[3] D. Derickson, ed., Fiber Optic Test and Measurement. Prentice Hall PTR, 1998, chap 9.5.

[4] D. L. Favin, B. M. Nyman, and G. Wolter, "System and Method for Measuring Polarization 

Dependent Loss," U. S. Patent 5371597, Dec. 6 1994; B. M. Nyman, D. L. Favin, and G. 

Wolter, "Automated System for Measuring Polarization Dependent Loss," OFC '94, San 

Jose, California, U.S.A., Technical Digest, pp. 230-231. (1994)

[5] B. Nyman and G. Wolter, "High-resolution measurement of polarization dependent loss," 

IEEE Photon. Technol. Lett., vol. 5, pp. 817-818, July (1993).

[6] S. L. Gilbert, W. C. Swann, and C. Wang, "Hydrogen Cyanide H

13C14N Absorption

Reference for 1530 nm to 1560 nm Wavelength Calibration – SRM 2519," Natl. Inst. Stand. 

Technol. Special Pub. 260-137 (1998).

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

NIST measurement results,".Tech. Note 1297, Nat. Inst. Stand. Technol.,, (1994).

[8] K.B. Rochford, A.H. Rose, P.A. Williams, C.M. Wang, I.G. Clarke, P.D. Hale, and G.W. 

Day, "Design and performance of a stable linear retarder", Applied Optics, V36, pp. 6458-

6465, (1997).
Appendix C

31

Qualification of Artifacts and Measurement System

Prototype Qualification Prior to Final Assembly

Low yields in the assembly of the MAP artifact prompted preliminary testing of the 

artifacts after splicing but before final packaging. This testing is described here.

The polarizing section is approximately 10 mm of polarizing (PZ) single-mode fiber (18 dB/m 

extinction). The actual length of the polarizing section can vary somewhat since splices can 

modify transfer-standard behavior, possibly by introducing additional PDL or changing the 

rejected-mode extinction. The input is a 5 cm section of common single-mode (SM) fiber for 

accurate coupling to system leads. The output section is composed of approximately 40 meters 

of multimode (MM) fiber (50/125 μm, 0.37 NA) to depolarize the signal and reduce uncertainty 

due to external connector and detector PDL. The MM fiber is wound on a rectangular spool with 

chamfers of 1.27 cm radius-of-curvature. Hot-wire splicing is used to produce clean joints 

between the dissimilar fibers. The MM fiber is wound around an aluminum block that also 

provides a tray for the immobilized polarizing section to reside in. The samples are thermally 

cycled in an environmental chamber. 

All prototype samples are subjected to at least five thermal cycles over the range of 0 – 50 °C to 

stress-anneal the samples, which serves to either improve or degrade performance. This stressannealing has been found to be a good indicator of overall stability of the prototype sample. Only 

those that improve in stability are used as MAP artifacts and are subjected to an additional five 

thermal cycles over the same range following a 1 week interim resting period that allows the PZ 

fiber section to equilibrate. The PDL is monitored after each cycle group by an all-states 

measurement system with a broadband source that detects any instability within the band. 

Samples are also monitored for high insertion loss and rejected if insertion loss exceeds 3 dB.

Those prototypes that show less than ±10 % variation in PDL following each of the five thermal 

cycles described above go on to a second round of thermal cycling like that of the first stage with 

one exception. The testing is now performed with a 1 mW ASE source with a filter centered at 

1540 nm that provides a greater signal-to-noise ratio within the band for the all-states 

measurement. The second round of testing is aimed at isolating those prototypes that settle to 

within a ±5 % window about the nominal PDL range. 
Appendix C

32

Prototypes that pass this selection criteria are further immobilized through the use of a thermally 

conductive, shock-absorbing dielectric gel poured into the tray, covering the PZ fiber section and 

fiber splices. This gel moderates both temperature gradients and shock loads, both of which 

lead to potential stress on the active section. The output end of the MM depolarizer section is 

with a FC/UPC connector for output to a power-meter or photodetector.

At this point, the prototypes are measured for the temperature coefficient of PDL at several 

wavelengths with the system shown in Fig. 4. This test spans the temperature range 20 to 26 °C 

(23 ± 3 °C) and measurements consist of 20-point PDLλ averages at 5 nm wavelength intervals at 

both temperature extremes. The resulting slope is calculated to produce the thermal uncertainty 

as a function of wavelength.

Measurement System Qualification 

Specimens that pass all tests to this point are characterized for wavelength dependence of PDL 

using a 4-state (Mueller/Stokes) matrix method measurement system. This system measures 

PDLλ with a standard uncertainty of ±0.0079 dB over the 1535 – 1560 nm range.

The first test is designed to check the measurement system calibration. This is done with an 

assembly that permits the insertion and removal of a BK7 glass Fresnel reflection standard from 

a collimated beam cavity formed by a high precision, AR-coated asphere that produces a 6 mm 

beam waist. Without the Fresnel standard assembly in place, the beam is undisturbed and allows 

a precise baseline to be set. With the assembly in place, the collimated beam is intercepted by an 

identical asphere that launches the beam into the output MM fiber in with relatively low loss.

The baseline is first taken with source sweep set to the shorter-wavelength range (1535 – 1550 

nm). The measurement system is set to average over 20 complete 4-state sequences. The 

transfer standard assembly is then replaced by the MAP artifact and the system measures PDLλ

another 20 times. This procedure is then repeated five times for an ensemble average. 

The entire procedure is now repeated with source sweep set to the longer-wavelength range 

(1545 – 1560 nm). The region of overlap is checked to ensure results that fall within the 

expected uncertainty, and the values within the overlap range are averaged.
Appendix D

33

Uncertainties Associated with All-States Measurement

The uncertainties associated with the secondary all-states measurement system used for 

thermal characterization of PDLλ MAP artifact are given in Table 1 for a measurement time of 

10 s using Ref. [5]. RSS is defined as the root-sum-of-squares.

Table 1. Estimated expanded uncertainty of the

 all-states measurement system.

Element Description Standard Uncertainty

Type A

Source stability (over 10 s) 0.001 dB

Connector PDL 0.0029 dB

Power meter PDL 0.003 dB

Polarization controller PDL 0.0023 dB

Observed repeatability 0.003 dB

Type B

Coverage deficiency5 (5 % of 0.1 dB) 0.0029 dB

Power meter linearity 0.0006 dB

Total uAS = RSS (Type A + Type B) 0.0057 dB

Expanded uncertainty = 2× uAS = 0.013 dB

 

5 Attempts to cover "all-states" of the Poincaré sphere will be limited by the measurement time and the fraction of 

the sphere not covered is the "coverage deficiency."
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

National Institute of Standards & Technology

Measurement Assurance Program 

for

Photometric Polarization-Dependent Loss Measurement 

in the 1540 – 1560 nm Wavelength Range

This Measurement Assurance Program (MAP) [1] is intended for polarization-dependent loss (PDL) 

calibration over the spectral range of 1540 nm to 1560 nm. This transfer standard is intended to calibrate 

only measurement systems that employ photometric methods of PDL determination in transmittance. 

Photometric methods depend only on the relative variations in transmitted power as a function of 

polarization state. These methods include pseudo-random all-states [2][3], deterministic all-states [4][5], 

deterministic fixed-states (Mueller/Stokes) [3][6][7], and enhanced all-states/optical spectrum analyzers 

(OSA) [2]. The certified value of this transfer standard is not applicable to polarimetric methods such as 

the Jones matrix analysis of Heffner [8]. In addition, this transfer standard is not certified for PDL 

calibration in reflection.

This limitation in scope is necessary because the input to this transfer standard is singlemode optical fiber 

and the output is multimode optical fiber. Consequently, the output has been substantially depolarized in 

order to reduce the contribution of wavelength-dependent PDL in connectors, components, and detectors 

that follow the transfer standard in the measurement system. Since polarimetric methods require analysis 

of the output polarization state, the depolarized output of this transfer standard renders it unsuitable for 

use with polarimetric methods. This approach was chosen to allow a more accurate calibration of the 

common measurement systems and is not a judgement on the performance of any polarimetric method.

Description: This transfer standard is an all-optical-fiber device that consists of three major elements 

designed to simulate a wavelength-dependent PDL. The input port is singlemode fiber. The output port 

is 50/125 μm step-index multimode fiber of high numerical aperture (NA = 0.37), that depolarizes the 

signal to approximately 5 % degree-of-polarization (when the source bandwidth is ≥ 1 nm) to minimize 

further PDL contributions from connectors and detectors. The PDL is produced by a short section of 

polarizing fiber that is fusion-spliced between the input and output fiber. The unit is packaged in a small 

instrument box (approximately 30 cm x 12.5 cm x 9 cm) with FC/UPC fiber connectors for the input and 

output.

Limits of Certification: The certification of this transfer standard is issued when the MAP artifact is 

returned to NIST, provided the transfer standard meets the criteria outlined in this document throughout 

the measurement cycle and is stored, handled, and used in accordance with the instructions given. 

Certification will not be issued if the transfer standard is damaged or otherwise altered or if the transfer 

standard was subjected to temperatures outside the 0 °C to 50 °C range.

The research and development effort leading to certification was performed by R. M. Craig of the NIST 

Optoelectronics Division. Statistical consultation was supplied by C.M. Wang of the NIST Statistical 

Engineering Division. Additional information regarding calculation of uncertainties is found in [1].
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

Certified Values and Uncertainties: This transfer standard is certified for wavelength-dependent PDL 

(PDLλ), which is the maximum measured polarization-induced change in the insertion loss measured over 

1540 nm to 1560 nm. PDLλ is defined by:

max

min

( ) ( ) 10log

( )

T

PDL PDL

T

l

l

l

l

≡ = (dB), (1)

where max min T T (l l )and ( ) are the maximum and minimum transmittance over the entire polarizationstate space at wavelength λ. PDLλ is measured at NIST using a Mueller/Stokes technique.

A locally weighted regression (LOESS) fit [9] is performed on measured PDLλ data. The fit is used to 

yield interpolated values (and

associated uncertainties) at ITU

DWDM grid wavelengths as shown 

in Fig. 1 and Table 1. The PDLλ

data in Fig. 1 results from two

overlapping wavelength scans

joined in the overlap region by

linear interpolation. The transfer

standard is certified at 23 °C and 

has moderate temperature

dependence. This is accounted for 

as an uncertainty that increases with 

increasing deviation of

measurement temperature from the 

target 23 °C. The user should

choose the uncertainty (23 ± 1 °C or 

23 ± 3 °C) that corresponds to the 

temperature range experienced by

the transfer standard during the

measurement.

The measurements of wavelength dependence were performed using the NIST PDLλ measurement system 

[10] with crosschecks provided by the NIST all-states PDL measurement system using an amplified, 

spontaneous emission (ASE) source with a 0.7 nm bandwidth tunable filter. The expanded uncertainties

in Table 1 were calculated in accordance with NIST uncertainty policy [11] as

( ) ( )

( ) ( ) ( )

( ) ( )

2 2 2

2 2 2 2 2 rep

Tot MS ship fit

dPDL dPDL

dT N d l

l l s l l l l l

l

   

= + ⋅  + +   ⋅ + +

   

u u uT u u u (2)

where uMS (l)is the standard uncertainty of the NIST PDLλ measurement system calculated according to 

[10]. The second term of Eq. 2 is the variance due to thermal dependence. uT = ΔT 3 is the uncertainty 

associated with the maximum allowed difference (ΔT) of the transfer-standard temperature from the target 

23 °C (either 1 °C or 3 °C in Table 1), and dPDL( ) l dT is the temperature dependence (dB/°C) of the 

PDLλ at room temperature (as described below). The derivative term here is approximated by a

difference. The next quantity is the variance in the mean of repetitive measurements made on this transfer 

standard, which establishes the measurement system noise. s l rep ( ) is the maximum standard deviation 

of repetitive PDLλ scans over wavelength, and N is the total number of PDLλ scans used in the 

certification. The fourth term is the variance due to center wavelength uncertainty. uλ is the standard 

Figure 1. Certified wavelength dependent PDL for MAP transfer 

standard, including expanded uncertainties for 1 °C.

0.000

0.050

0.100

0.150

0.200

0.250

0.300

1535 1540 1545 1550 1555 1560 1565

Source Center Wavelength (nm)

PDL° (dB)
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

uncertainty in source center wavelength of the PDLλ scan range, and dPDL d (l l ) is the linearregression slope of the combined PDLλ scan (described below). The use of a linear-regression slope 

rather than an actual derivative is a reasonable approximation here given the relatively small contribution 

of center wavelength uncertainty. The fifth term, uship is the standard uncertainty assigned to offsets 

incurred during shipping (described below). Finally, ufit is the standard uncertainty associated with the 

LOESS fitting algorithm. The coverage factor of two is introduced to produce an expanded uncertainty 

for a confidence interval of approximately 95 %.

Table 1. Certified values and expanded uncertainties

of wavelength-dependent PDL (PDLλ) at ITU channel grid frequencies

Central Frequency PDLλ Expanded Uncertainty 2 ( ) Tot u dB

(THz) (Channel)

Nominal 

Wavelength 

λ0 (nm)

PDLλ (dB)

(LOESS fit)

(23 °C ±1 °C) (23 °C ±3 °C)

194.7 (16) 1539.77 XXXXX XXXXX XXXXX

194.6 (15) 1540.56 XXXXX XXXXX XXXXX

194.5 (14) 1541.35 XXXXX XXXXX XXXXX

194.4 (13) 1542.14 XXXXX XXXXX XXXXX

194.3 (12) 1542.94 XXXXX XXXXX XXXXX

194.2 (11) 1543.73 XXXXX XXXXX XXXXX

194.1 (10) 1544.53 XXXXX XXXXX XXXXX

194.0 (9) 1545.32 XXXXX XXXXX XXXXX

193.9 (8) 1546.12 XXXXX XXXXX XXXXX

193.8 (7) 1546.92 XXXXX XXXXX XXXXX

193.7 (6) 1547.72 XXXXX XXXXX XXXXX

193.6 (5) 1548.51 XXXXX XXXXX XXXXX

193.5 (4) 1549.32 XXXXX XXXXX XXXXX

193.4 (3) 1550.12 XXXXX XXXXX XXXXX

193.3 (2) 1550.92 XXXXX XXXXX XXXXX

193.2 (1) 1551.72 XXXXX XXXXX XXXXX

193.1 (0) 1552.52 XXXXX XXXXX XXXXX

193.0 (-1) 1553.33 XXXXX XXXXX XXXXX

192.9 (-2) 1554.13 XXXXX XXXXX XXXXX

192.8 (-3) 1554.94 XXXXX XXXXX XXXXX

192.7 (-4) 1555.75 XXXXX XXXXX XXXXX

192.6 (-5) 1556.55 XXXXX XXXXX XXXXX

192.5 (-6) 1557.36 XXXXX XXXXX XXXXX

192.4 (-7) 1558.17 XXXXX XXXXX XXXXX

192.3 (-8) 1558.98 XXXXX XXXXX XXXXX

192.4 (-9) 1559.79 XXXXX XXXXX XXXXX

192.5 (-10) 1560.61 XXXXX XXXXX XXXXX

NOTE: All wavelengths are referenced to vacuum. Measurement uncertainties on PDLλ assume 

uλ = 0.025 nm, ( ) 10 0.0012 s l rep

= and uT corresponds to either a 23 ± 1 °C or a 23 ± 3 °C 

temperature uncertainty.
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

Instructions for Use: Make calibrating measurements at a nominal temperature of 23 °C. As discussed 

in the section on Temperature Dependence (below), the measurement uncertainty increases as the

measurement temperature diverges from this 23 °C target. Two temperature tolerances are allowed in 

Table 1, 23 ± 1 °C and 23 ± 3 °C. For measurement stability, hold the transfer standard at the

measurement temperature for at least one hour before making measurements. Make the measurements at 

one or more ITU wavelengths within Table 1. As described in the section on Input PDL (below), 

connector variations can lead to fiber-core offsets that produce PDL. Input PDL can cause measurement 

error so it is recommended that PDLλ be measured as the average of multiple measurements done with a 

variety of input fiber leads and associated connectors. This helps to ensure random orientations of input 

PDL axes. As described in the Lead Birefringence section (below), both static and dynamic birefringence 

in the fiber leads of the measurement system can introduce measurement error in Mueller/Stokes systems. 

In this case, it is recommended that the pigtail leads be as short as possible and remain rigidly fixed 

during a measurement. Consequently, Mueller/Stokes systems require somewhat more care to properly 

calibrate. Finally, the output fiber is a 0.37 NA step-index multimode type with 50/125 μm dimensions. 

To avoid additional insertion losses, it is recommended that the output fiber lead to the user's detector or 

measurement system be multimode cable of the same NA. Because the output of the transfer standard is 

depolarized, calibration uncertainty due to subsequent small (<0.05 dB) connector/detector PDL will be 

negligible, permitting a more accurate measurement of the transfer standard's value. Calibrate the 

instrument to the corresponding value from Table 1 using the procedure specified by the instrument 

manufacturer, c.f. Fig’s. 2a and 2b. Wavelength linearity of the instrument can be checked by repeating 

the procedure at a substantially different ITU channel and comparing it to Table 1.

Care should be taken in making connections to the transfer standard. The bulkhead connectors on the 

transfer standard unit are the FC/UPC type. When making connections to the transfer standard, use high 

quality FC/UPC connectors. The cleanliness of the connectors is important. Use a dust-free and residuefree air source and a commercial fiber-endface cleaner before making each connection. If such a cleaner 

is not available, then use lens paper wetted with reagent-grade alcohol to wipe the ferrule endface and use 

a clean air source to dry the connector. To retain certification, the transfer standard should not be 

subjected to temperatures outside the range of 0 °C to 50 °C, and the unit should be stored in a dry 

environment with long-term temperatures within the range of 15 °C to 35 °C. 

The transfer standard should not be subjected to any mechanical or thermal shocks or strong 

vibrations and the lid should not be removed. Do not use FC/APC type connectors; otherwise 

erroneous results and possible connector damage will occur.

Input PDL: Any connection between two singlemode optical fibers can introduce PDL, whether through 

Fresnel reflections, lateral core offset, or mode-field mismatch. Thus the transfer standard is subject to a 

PDL uncertainty at the input connection. We have measured connector PDL values in the range of 0.002 

– 0.02 dB, but larger values may be possible. In addition, the axis of the input PDL relative to the axis of 

the internal PDL determines whether it adds or subtracts from the internal PDL. When the axes are 

aligned, the total PDL will approximate the sum of the two contributions for relatively small values 

[12][13], and likewise when the axes are orthogonal the total will approximate the difference. There is no 

way to determine, a priori, the relative angle between the two axes. Only by sampling with a variety of 

input leads and connectors can this uncertainty be reduced by effectively averaging over several axis 

angles. In addition, use only leads terminated with FC/UPC type connectors.

Temperature Dependence: Because the transfer standard uses polarizing fiber, it is sensitive to

temperature. The temperature coefficient is individually measured for each transfer standard. The

wavelength-dependent temperature coefficient is obtained from PDLλ measurements at 20 °C, 23 °C, and 
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

26 °C. This temperature dependence has been added in quadrature to the other uncertainties to give the 

final uncertainty listed in Table 1 for two possible temperature excursions. Also, since the artifact retains

 some sensitivity to large temperature variations during shipping, an estimate has been made of the 

uncertainty contribution of uship. Based on measured results of simulated shipping conditions, the 

estimate has been made from the uncertainty contribution as a percentage change in a 0.12 dB nominal 

reference value. This shift is taken as the adjusted maximum observed shift standard deviation of PDLλ

normalized to the wavelength mean of PDLλ, and was found to be 3.5 %. The result (at the 0.12 dB) 

reference value) is calculated to be 0.035 0.120 0.0045 ship s = ⋅ = dB dB . A small logging device attached 

PDL Measurement System

transfer 

standard

SM fiber lead

MM fiber lead

input output input output

Figure 2a Basic calibration arrangement for all systems.

PDL Measurement System

transfer 

standard

input output input output

Figure 2b Measurement system baseline.

PDL Measurement System

transfer 

standard

input output input output

MM fiber lead

SM fiber 

lead

Figure 2c. Mueller/Stokes retardance uncertainty measurement.
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

to each artifact will constantly monitor the temperature environment. Upon return, the thermal history 

will be analyzed for any temperature excursions outside of the 0 °C – 50 °C range of certification.

Lead Birefringence: In the majority of photometric methods, lead birefringence is not an issue as these 

methods are invariant under Poincaré sphere rotation(s). However, in the case of the four-state 

Mueller/Stokes measurement, both static and dynamic lead birefringence can impose an uncertainty on 

the measurement [1]. Large static lead birefringence destroys the required state-space symmetry between 

(a) initial baseline measurements without the transfer standard in place and (b) the final measurement 

with the transfer standard in place. The net effect of large static birefringence is to scale the PDL by 

factors both greater than and less than one. Dynamic lead birefringence in long interconnecting leads 

alters the baseline PDL level during the course of extended Mueller/Stokes measurements. To minimize 

this uncertainty, use short, straight leads anchored to a surface for interconnections and limit measurement 

time between baseline measurements. When short, rigidly mounted input leads are not possible, the 

orientation of the fiber leads should be randomized between measurements and the measurements should 

be averaged in order to minimize the lead birefringence effects. Note that the most complete 

randomization of the leads must include orientations where the fibers do not lie in a single plane as in the 

case of a fiber-loop polarization controller, c.f. Fig. 2c. 

Optical Source Type: Because the transfer standard depends on wavelength and because PDL

measurements are subject to optical interference arising from multiple reflections, the bandwidth of the 

source is an important consideration. Very broad sources such as light-emitting diodes (LED) will tend to 

average (integrate) the transfer standard PDLλ over the bandwidth. Conversely, very narrow sources such 

as external-cavity-tunable lasers can be precisely adjusted spectrally, but the very long coherence length 

may lead to very large variations due to interference. In addition, polarizers should not follow polarized 

sources with pigtail leads, otherwise large power fluctuations may occur. NIST strongly recommends 

using a stabilized source such an ASE or edge-emitting LED (EELED) followed by a filter with a 3-dB 

bandwidth in the range of 0.7 – 20 nm. Note that the filter bandwidth will naturally restrict the range over 

which the source may be tuned to ensure that no significant power falls outside of the certified

wavelength range of PDLλ. Finally, the user should be aware of the additional uncertainty introduced by 

asymmetry in the source spectra. Refer to App. F of Ref. 1 for a discussion. Source profiles that are 

significantly different from this simple model must be handled individually.

Multiple Reflections: The most probable cause of multiple reflections is poor connections. Use only

FC/UPC type connectors cleaned as described in the Instructions for Use section. Dust or dirt in the 

bulkhead adapter or on the connector ferrule endface can cause multiple reflections across the specimen, 

which will add noise to the measurement. Other sources of reflection in the measurement system are 

equally important. If the reflections cannot be reduced, multiple measurements can be made at slightly 

different wavelength sampling points or temperatures in order to average out the multiple reflection 

effects. 

Center Wavelength: Because the wavelength of each PDLλ scan at NIST is registered in real-time to a 

hydrogen-cyanide gas reference (SRM 2519), wavelength uncertainty is small and limited primarily by 

the accuracy of fit coefficients obtained during that registration. The standard uncertainty of

measurement wavelength for each certified PDL value is 0.025 nm. When multiplied by an observed 

slope in PDLλ of 0.006 dB/nm, we obtain a standard uncertainty of 0.0015 dB due to wavelength 

uncertainty.
Appendix E

 Sample Certificate. Actual certificate to be sent following MAP measurement

References

1. Craig, R. M., and Wang, C. M., " Measurement Assurance Program for Wavelength Dependence of 

Polarization Dependent Loss of Fiber Optic Devices in the 1540 – 1560 nm Wavelength Range," 

NIST Special Publication 250-60. (forthcoming)

2. D. Derickson, Fiber Optic Test and Measurement, Upper Saddle River, NJ: Prentice Hall, 1998.

3. R. M. Craig, S. L. Gilbert, and P. D. Hale, "High-resolution, nonmechanical approach to polarizationdependent transmission measurements," J. Lightwave Technol., vol. 16, pp. 1285-1294, July 1998.

4. N. Mekada, A. Al-Hamdan, T. Murakami, and M. Miyoshi, "New direct measurement technique of 

polarization dependent loss with high resolution and repeatability." Tech. Dig., Symp. Optic. Fiber 

Measurements, SOFM'94. Boulder, CO, pp. 189-192.

5. M. A. Bhatti and A. S. Siddiqui, "A new polarization dependent loss measurement system," Int. J. 

Optoelectronics, vol. 11, 1 pp. 39-41, 1997.

6. B. Nyman and G. Wolter, "High-resolution measurement of polarization dependent loss," IEEE 

Photon. Technol. Lett., vol. 5, pp. 817-818, July 1993.

7. D. L. Favin, B. M. Nyman, and G. Wolter, "System and method for measuring polarization dependent 

loss,". U.S. Patent 5371597, Dec. 6, 1994.

8. B. L. Heffner, "Deterministic, analytically complete measurement of polarization-dependent 

transmission through optical devices," IEEE Photon. Technol. Lett., vol. 4, pp. 451-454, May 1992.

9. Cleveland, W.S. and Devlin, S.J., "Locally Weighted Regression: An Approach to Regression 

Analysis by Local Fitting," Journal of the American Statistical Association, Vol. 83, pp. 596-610. 

1988.

10. R. M. Craig, "Accurate spectral characterization of polarization dependent loss." Tech. Dig., Symp. 

Optic. Fiber Measurements, SOFM'2000. Boulder, CO, pp. 113-116.

11. B. N. Taylor and C. E. Kuyatt, "Guidelines for evaluating and expressing the uncertainty of NIST 

measurement results, "Tech. Note 1297, Nat. Inst. Stand. Technol., 1994.

12. N. Gisin, "Statistics of PDL," Opt. Comm., vol. 114, pp. 399-405, 1995.

13. A. Elamari, N. Gisin, B. Perny, H. Zbinden and C. Zimmer, "Statistical prediction and experimental 

verification of concatenations of fiber optic components with polarization dependent loss," J. 

Lightwave Technol., vol. 16, pp. 332-339, 1998.
Appendix F

43

Effect of Source Spectral Symmetry 

Given the significant dependence of PDL on wavelength, the user may find it important to 

consider not only the source bandwidth but also the spectral symmetry across that bandwidth. It 

is possible for asymmetry of the source spectrum to cause PDL measurement errors. Refer to 

Fig. 1 for an example of a simple, triangular asymmetry in the source spectrum. Here one can 

clearly see that the spectral intensity peak, P0 (dB), is offset from the center and that a spectral 

slope, ms (dB/nm), can characterize the asymmetry. Given that an asymmetric (non-zero-slope) 

source spectrum will shift the integrated average PDLλ away from the midpoint of the λaveraging range, it is possible to establish a maximum permissible spectral slope. A negative 

spectral slope will weight the integrated average PDLλ towards shorter wavelengths, while a 

positive slope will weight the average towards longer wavelengths. The maximum spectral slope 

(over a 3 dB bandwidth) allowable to maintain a 1 % uncertainty is calculated to be ms ± 0.2 

dB/nm while the maximum slope to maintain a 3 % uncertainty is ms ± 0.6 (dB/nm). For a 0.12 

dB reference value, this leads to a standard uncertainty contribution of 0.0018 dB. There are, of 

course, more general cases that must be dealt with individually. These include source spectra for 

which the asymmetry is not well represented by a slope. To minimize this effect, the user 

should verify that the source spectrum is symmetric about the peak.

Figure 1. Illustration of the effect of a simple, triangular, source spectral asymmetry. Note 

that a symmetric filter will integrate the spectral PDL to the midpoint value and that 

the asymmetric component will contribute a small offset.

m1

m2

m=m1/m2 s

P0

Asymmetry

offset of WPDL

Symmetric

mean of WPDL

Sourc

e

B

W

Sy

m

m

etric

p

art As

y

m

m

etric

p

art

WPDL

2*FWHM
Appendix G

45

Shipping and Handling Instructions

Because the transfer-standard assembly retains some sensitivity to thermal and mechanical 

shock, it is shipped as rapidly as possible in a specially padded and insulated shipping container. 

We request and strongly recommend that this container be reused for the return shipment as it is 

very protective and helps to assure continued certification. The assembled units contain a 

temperature-logging device on the back panel that the user should be aware of but, in general, 

not concerned with. This logging device will monitor the temperature environment during 

shipment and use.
46

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Appendix H

47

PDLl Artifact Tray/MM Depolarizer Mount 

The following is a schematic of the aluminum artifact mounting tray and depolarizer spool. 

Units given are in inches rather than SI. The upper tray holds the fiber splices as well as the 

immobilizing gel. It is important to note the dimension of the spool's radii of curvature.

11.00"

10.75"

2.25"

2.00"

0.25"

0.38"

1.25"

0.75"

0.75"

0.75" 0.75"

0.25"

0.25"

0.25"

0.125" thru

0.50"

R 0.13"

R 0.50"

R 0.20"

1.625" open 0.75" open

R 0.25"

4.00" Open

0.13"

5.50"

R 0.2"

0.125" Blind

R 0.2"

1.13"

4.00" Open

0.25"

0.125" Thru 0.125" Thru