X %

i'^Jh NBS TECHNICAL NOTE 1032

*^fAU 0* *
NATIONAL BUREAU OF STANDARDS

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.

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Measurement of Far-Field and

Near-Field Radiation Patterns

from Optical Fibers

ATlOKAl. BtJMAO

or stakdabm

UBltABY

JUN 1 5 1981

no. (o>^.-

, *>

Ernest M. Kim

Douglas L. Franzen

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

*** \

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued February 1981
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1032

Nat. Bur. Stand. (U.S.), Tech. Note 1032, 48 pages (Feb. 1981)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON 1981

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

Price $2.50 (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

1

.

INTRODUCTION 1

2. RADIATION PATTERN CONSIDERATIONS IN OPTICAL FIBERS 1 2.1 Measurements Utilizing Far-Field Patterns 5 2.1.1 Radiation Angle (Numerical Aperture) 5 2.1.2 Attenuation with Restricted Launch via a Mode Filter 6 2.2 Measurements Utilizing Near-Field Patterns 6 2.2.1 Index Profile.'. 6 2.2.2 Core Di ameter 7 2.3 Mode Volume Transfer Function--A Measurement Utilizinq

Both Far- and Near-Field Patterns 7

3. DESCRIPTION OF FAR-FIELD MEASUREMENT SYSTEM 8 3.1 Source 8 3.2 Detector 10

4. PERFORMANCE OF FAR-FIELD SYSTEM 11 4.1 Precision 11 4.2 Accuracy 11 4.3 Dynamic Range IS

5. TYPICAL RESULTS FROM FAR-FIELD MEASUREMENTS IS 5.1 Radiation Angle (Numerical Aperture) 15 5.2 Attenuation with Restricted Launch via a Mode Filter 15 5.3 Symmetry of Radiation Patterns 18

5.4 Mode Dependence of OH" Absorption 18

6. DESCRIPTION OF NEAR-FIELD MEASUREMENT SYSTEM 21

7. PERFORMANCE OF NEAR-FIELD SYSTEM 23

7 .

1

Prec i s i on 23 7.2 Resolution and Accuracy 23

7.3 Dynamic Range 31

8. TYPICAL RESULTS FROM NEAR-FIELD MEASUREMENTS 31 8.1 Claddinq Liqht 31 8.2 Step-Index Fibers 31 8.3 Symmetry of Radiation Patterns 31 8.4 Index Dip Pathologies 31

9. CONCLUSION 38

10. REFERENCES 38

m

MEASUREMENT OF FAR-FIELD AND NEAR-FIELD

RADIATION PATTERNS FROM OPTICAL FIBERS

Ernest. M Kim* and Douglas L. Franzen

Systems are described for measuring the far- and near-field radiation patterns from optical fibers. Parameters which affect measurement precision, accuracy, resolution, and signal-to-noise ratio are discussed. Measurements utilizing radiation patterns are covered; this includes radiation angle (numerical

aperture), attenuation using mode filters, index profile, core diameter, and mode

volume transfer function. Experimental examples are given in many instances.

Key words: Attenuation: core diameter; far field; index profile; mode filter;

numerical aperture; radiation angle; radiation patterns.

1. INTRODUCTION

Radiation patterns exiting optical fibers yield important information used in fiber

specification. For example, in some instances core diameter and numerical aperture are obtained from radiation patterns. Measurement practices with regard to these parameters

continue to evolve as accumulated practical experience and the efforts of standards groups

lead toward uniformly accepted procedures. This Technical Note is one of a series intended

to describe the present design and capabilities of fiber measurement systems used at the

National Bureau of Standards. These systems are perhaps representative of current practice

in tne industry ana many of the present techniques and methods will be relevant to future

systems.

2. RADIATION PATTERN CONSIDERATIONS IN OPTICAL FIBERS

The radiation propagating in an optical fiber can take on various spatial and angular

distributions depending upon the relative excitation of fiber modes. Useful information

about the fiber can be ootained oy analyzing the radiation exiting an end. This analysis

can take place in either the far- or near-field region of trie end. A far-field measurement

determines the angular dependence of intensity ( irradiarice) sufficiently far from the end of tne fiber (fig. 2-1); wnereas, a near-field measurement determines the spatial intensity

distribution in the plane of the end face.

In the far-field, tne normal i zed, angular intensity distribution no longer depends on distance from the fiber. While tne intensity at any given angular coordinate decreases as the square of the distance from the fiber end, trie normalized pattern remains constant. If the far-field pattern is circularly symmetric about the fiber axis, then a single angular

coordinate is sufficient to describe the pattern.

The far-field, which is synonymous with the Fraunnoffer diffraction reqion, is usually

said to start at a distance

e NBS Coop graduate student on leave from New Mexico State University, Las Cruces, NM.
INTENSITY, I *-

(a) FAR FIELD

FIBER

(b) NEAR FIELD

igure 2-1. Radiation pattern measurements from optical fibers, (a) far field is qiven by

1(8) at z>>z , (b) near field is given by I(r) in xy plane.

AXIS

RADIUS

L

INDEX

igure 2-2. The meridional ray in a step index fiber makinq the maximum possible angle, e^,

with the fiber axis.
2a)

(1)

from the fiber end, where X is the wavelength of liqht and a the core radius [1]. When

z » zQ

, the amplitude (electric field) distribution in the far-field is closely related to the spatial Fourier transform of the amplitude distribution existing at z = 0, [1]. In practice, far-field measurements are made at distances greater than 10 z from the fiber

end. For the standard graded index core diameter of 50 \m and a wavelenqth of 1 ym, 10 z Q

is 2.5 cm. For the standard step index core diameter of 100 iim, the distance becomes

10 cm. Both of these lenqths represent convenient laboratory workinq distances.

Radiation patterns are measured on either short or lonq lenqths of fiber, dependinq

upon the application. Frequently the short lenqth is a 2 m cutback lenqth left over from an attenuation or bandwidth measurement. In this Technical Note, a lonq lenqth refers to the

entire lenqth of fiber under test and, for telecommunications fibers, qenerally exceeds

1 km. Radiation patterns depend upon launchinq conditions and contain contributions from both

quided and tunneling leaky modes of the fiber [2,3]. In short lenqths, patterns result

mainly from guided modes with a significant contribution from leaky modes. The relative

importance of the leaky mode contribution depends upon the index profile and launchinq con- ditions. In lonq lenqths, leaky modes have qenerally decayed and the patterns result from

guided modes modified by differential mode attenuation and mode coupling. Far- and nearfield patterns resultinq from overfilled launching conditions on graded index fibers generally become restricted with further propagation along the fiber. This occurs because differential mode attenuation is generally higher for higher order modes. Eventually an equilibrium is reached between mode couplinq and differential mode attenuation to yield a steady

state pattern. However, steady state is seldom observed in practice since mode couplinq

lenqths in good fibers are several kilometers T41 if not tens of kilometers [5].

The remainder of this section briefly discusses the measurement applications of farand near-field patterns. Various applications are summarized in table I. For some of these

measurements, procedures are pending before standards groups [61. In general, more attention has been given to standardizing procedures for graded index fibers than for step index

fibers.
Table I Measurement Applications of Radiation Patterns from Multimode Fibers

Appl ication Fiber length

employed

Short Long

(2 m)

Radiation

pattern

Far Near

field field

Launching

condition

General

comments

Radiation Angle

(Numerical aperture)

Overfill Meridional ly defined NA approximated for near parabolic graded index

fibers. Special considerations necessary for

step index.

2 Attenuation using

restricted launch

via a mode filter

Overfill Used to qualify mode

filters; applies to graded index fibers over

1 km in length.

3 Index profile Overfill Little or no leaky mode

correction reguired for near parabolic qraded

index fibers; large cor- rection for step index.

Limited resolution,

information on guiding

regions of fiber only.

4 Core diameter Overfill Possible uncertainty due

to low index "barrier"

layer, poor resolution

near core-cladding

boundary, may have

problems conforming to an index profile definition

of core diameter, is easy

to implement.

Mode volume

transfer function

Restricted Applies to graded index

fibers, measurement sys- tem must have ability to control launch mode volume and therefore beam

optics systems capable of

independently variable

launch spot and NA dre

usually required.
2.1 Measurements Utilizing Far-Field Patterns

2.1.1 Radiation Anqle (Numerical Aperture)

The cone of liqht rays accepted by a fiber is of practical importance. Launching efficiency of sources like LEDs and loss performance of joined sections of dissimilar fiber depend upon this parameter. "Radiation anqle" is a measure of the cone anqle transmitted by

the fiber [7]. This measurement is usually made in the far field of a short, 2 m lenqth ex- cited at a specified wavelenqth using overfilled launchinq conditions; i.e., constant irradiance over the core with a launch anqle greatly exceedinq the fiber acceptance anqle.

In early fiber work, the cone anqle containinq 90 percent of the transmitted power was determined by either inteqratinq a far-field intensity pattern or by translatinq an aperture

in the far field. The sine of the half anqle of this cone was frequently termed "90 percent

numerical aperture."

More recent procedures pendinq before standards qroups define radiation anqle directly

from a far-field intensity pattern [8]. In this instance, radiation anqle is defined as the

half anqle where the far-field intensity has decreased to 5 percent of the peak value. The

sine of this angle, for near parabolic index multimode fibers, is close to the index defined

numerical aperture.

Historically, the term "numerical aperture" (NA) has been used to describe the larqest

angle meridional ray accepted by a fiber, figure 2-2, with

NA = sin 9 = AV - n 2

2 (2)

n !-n 2 = u x /2E with A «- (3)

where m is the core index of refraction, m the cladding index, and A is defined by the

above equations. While figure 2-2 applies to step fibers, analysis shows that the numerical

aperture of graded index fibers is also given by eg (2) with ni being the on-axis index of

refraction [9].

The sine of the radiation angle based on the 5 percent intensity points is close to the

index defined NA of eg (2) for near oarabolic index fibers. Theoretical work shows the ac- ceptance angle of skew rays is always less than that of meridional rays in near parabolic

index fibers [10]. Adams, et al., show that leaky modes in a near parabolic index fiber are

all contained within the meridional ly defined numerical aperture [31. Thus, when all modes

are excited, the largest observed angle in a far-field pattern would be given by 6 Q in eq

(2). At the 5 percent level, patterns are close to the maximum observable angle.

For step fibers, the situation is quite different. Here the acceptance anqles for skew

rays can exceed 6 . When all modes are excited, radiation would appear at anqles qreater

than 8 . Thus, to obtain the numerical aperture based on eq (2) for short step index

fibers, special launchinq conditions would be necessary. This would probably mean launchinq

only meridional rays. This could be accomplished by small spot excitation at core center.
2.1.2 Attenuation with Restricted Launch Via a Mode Filter

In measuring the attenuation of lonq telecommunications type fibers, it is useful to launch with a distribution which qives good concatenation predictions [111. This is presently accomplished by restricting the launch, to avoid the excitation of certain high loss

modes which do not propagate far in the fiber. In general, this can be accomplished by one

of two approaches. The beam optics approach avoids the initial excitation of certain high

loss modes by restricting the launch spot size and numerical aperture at the fiber input

end. (70 percent of the core diameter at 70 percent of the fiber NA is the choice pending

before standards groups.) In the mode filter approach, the fiber is initially overfilled

and a mode filter is applied to strip out certain high loss modes.

Far-field radiation pattern measurements on lonq and short fiber lenqths are used to qualify a mode filter for an attenuation measurement. Current proposals call for overfilling the input of the fiber under test in both spot size and numerical aperture, then measuring the radiation angle (5 percent intensity) exiting the long length. An appropriate

mode filter is applied so the radiation angle exiting a 2 m length is less than that of the

long length by 3 + 3 percent. Now the mode filter has stripped away enough of the hi qh

angle-high loss modes to simulate, in a 2 m cutback length, the radiation pattern resulting

from a kilometer or more of propagation. Common mode filters include dummy fibers and

mandrel wraps [12], [13].

2.2 Measurements Utilizinq Near-Field Patterns

2.2.1 Index Profile

When a multimode optical fiber havinq a large number of modes is uniformly illuminated

to equally excite all bound modes, the near-field pattern approximates the refractive index

profile of the core [14]. In short lengths where differential mode attenuation and mode

coupling are negligible, the deviation from the index profile is due mainly to the presence

of tunneling leaky modes. Sladen, et a! . , have derived a leaky mode correction factor

allowing one to obtain the index profile from transmitted near-field data [2]. The radial

index profile difference, n(r) - n->, is given by

n(o)-n 2 D{ r \ n(r) - n 2 * W- (4)

C(r,z) P(o)

where n(r) is the index in the core, no the cladding index, P(r) the near-field intensity as

a function of radius, and C(r,z) the correction factor. C(r,z) depends on the nominal profile shape, fiber lenqth, and radial position. C(r,z) is zero at the core center and gradually rises to a maximum f\ear 0.9a, where a is the core radius. As an example [14], for a

1 m length of fiber havinq a near parabolic index profile, a core diameter of 80 pm, an NA

of 0.18, and at a wavelenqth of 0.9 vm, the correction factor is 8 percent at 0.6 a, rising

to 20 percent at 0.85 a. Whether a leaky mode correction should be made for near parabolic profile, graded index

fibers is still open to debate. Calculations by Petermann [15] indicate a slight core ellipticity can cause leaky modes to attenuate more rapidly than indicated in references [2]
and [14] and corrections would not be necessary for practical near parabolic fibers. Leaky

mode correction factors have been derived for elliptical core fibers [16].

Regardless of the appropriateness of a correction, the near field is a close approximation to the index profile for near parabolic fibers. Also, corrections mainly affect the

0.5a to 0.9a region and the apparent width of the pattern near the baseline is not affected.

This is of importance in determining core diameter from near-field measurements.

For step index fibers, leaky modes are definitely significant and for short lengths the near field profile differs substantially from the index profile (see sect. 8.2).

It should be pointed out that the above comments apply only to multimode fibers with

large numbers of modes. In fibers with single or few propagating modes, the near-field intensity distribution is given by the mode patterns themselves which can differ substantially

from the index profile shape.

The near-field intensity distribution gives information on the spatial location of

power in the fiber. Thus, one is measuring only the guidinq regions of the f iber. What

happens at the core cladding interface or in the cladding is not indicated.

2.2.2 Core Diameter

Core diameter is usually defined from the fiber refractive index profile. In one re- commended definition, the core diameter is that diameter on the index profile where the

refractive index of the core exceeds that of the cladding by k times the difference between

the maximum refractive index in the core and the minimum index in the cladding, where k is a specified constant (0 < k < 1) [7]. Near-field measurements can be used to determine core diameter in some circumstances because of the close relationship between near field and re- fractive index profile. Leaky mode corrections, if they are even necessary for near parabolic index fibers, do not affect the measured width of the near-field pattern near the

baseline. A core diameter measurement based on the full width at the 2 to 3 percent intensity points may be appropriate. A more serious uncertainty in determining core diameter

results from the use of low index "barrier" layers between the core and cladding. How the

near field is affected by such fluctuations near the cladding has not been established.

Also, in graded index fibers, there is a loss of resolution near the core claddinq boundary

due to the decrease in local numerical aperture [17]. Some of these problems may be alleviated in a recently described "modified near-field" technique [18]. Despite some limitations, near-field measurements do indicate where power is spatially located in the fiber

and, in some practical situations, this may be as important as the core diameter determined

from an index profile.

2.3 Mode Volume Transfer Function--A Measurement

Utilizing Both Far- and Near-Field Patterns

Holmes, et al . , have introduced a concept describing the effect of propagation on spatial and angular intensity distributions in graded index fibers [4]. They define an "effective mode volume", EMV, based on far- and near-field intensity distributions. Specifically, the EMV at a qiven point along the fiber is the square of the product of the FWHM
(Full Width at Half Maximum) of the near field and the sine of the HWHM (Half Width at Half

Maximum) of the far field.

EMV is related to the number of modes excited in the fiber. This can be intuitively

seen when applied to the expression for the total number of quided modes, N, in a fully

excited qraded index fiber having a power law profile a [9].

2 TMi a

N aA2+a

where a is the core radius, X the wavelenqth, and other terms have been previously

defined. After arranging terms,

N

= 2+a —) C(NA) • a]

2

X

2

a / 2tt 2

^ 2+a X

2 EMV

(6)

(7)

Of interest is the EMV transfer characteristic for a fiber. This describes how various

EMVs propagate in the fiber and is a plot of output EMV versus input EMV. By controlling

launch spot and NA, various input EMVs can be generated. Input EMVs are determined from

radiation patterns exiting a 2 m length at the input. The corresponding output EMVs are obtained from radiation patterns exiting the lonq lenqth.

The EMV transfer characteristic qives information on mode couplinq, differential mode

attenuation, and quasi-steady state EMV. A fiber without mode couplinq or differential mode

attenuation appears as a 45° line on the transfer characteristic plot; i.e., the output EMV

is equal to the input EMV.

A principle use of the EMV concept is in the prediction of concatenation. EMV transfer

characteristics and attenuation versus EMV for all the fibers in the link allows one to predict accurately the concatenated attenuation for any input EMV [41.

3. DESCRIPTION OF FAR-FIELD MEASUREMENT SYSTEM

3.1 Source

The radiation source used with both far- and near-field systems is capable of producing

independently variable spot sizes and launch numerical apertures, fiqure 3-1. Aperture 1, which controls launch spot size, is illuminated by either a strip filament lamp or a flat,

close-coiled filament, quartz haloqen lamp havinq an etched envelope. The lamp should have

qood spatial uniformity and, from this viewpoint, the strip lamp is preferred.

A broadband interference filter selects the nominal wavelenqth. Since these measurements are relatively insensitive to wavelenqth [191, a broadband filter with 80 nm linewidth

gives improved signal-to-noise over narrowband filters. Angles of incidence on the filter

are so small that angular dependent transmission effects are avoided.

The launch lens produces a demagnified image of aperture 1 on the fiber end; a demagnification ratio of 22 is typical and spot sizes down to 18 wn have been obtained. Aperture 2 controls the launch numerical aperture from a maximum of 0.36 to a minimum of 0.03.
CHOPPER

WHEEL

RADIATION

FROM

INCANDESCENLAMP

LAUNCH

LENS

1

BROAD BAND

INTERFERENCE

FILTER

A vUTRANSLATION

FIBER ?

APERTURE

NO. 1 APERTURE

NO. 2

Figure 3-1. Launch optics used with far- and near-field systems.

FIBER

CLADDING

MODE STRIPPER

PIN SILICON

DETECTOR

APERTURE

TO LOCK-IN

AMPLIFIER

STEPPER MOTOR

CONTROLLED ROTARY TABLE

Fiqure 3-2. Far-field measurement system usinq a fixed fiber end and a rotating detector.
Overfilled launchinq conditions are used for most of the measurements, Table I. Therefore, additional optics to optically center the aperture image on the fiber end are not usually employed. Peak power transmitted throuqh the fiber has been the aliqnment criterion

when overfilled launchinq conditions are used.

A chopper wheel is located in the beam to modulate the liqht at 900 Hz. This provides

a reference siqnal for narrowband, phase sensitive detection with a lock-in amplifier.

3.2 Detector

The far field may be scanned by a number of different techniques: ('1) fixed fiber end,

rotating detector, (2) fixed detector, rotatinq fiber end, (3) fixed fiber end, fixed detector, rotating mirror, and (4) fixed fiber end, detector array.

The system described in this Technical Note uses a fixed fiber end and a rotatinq

detector, fiqure 3-2. This approach represents perhaps the simplest construction of

available choices.

The fiber first passes throuqh a claddinq mode stripper consistinq of two 10 cm lonq

felt pads wetted with index matchinq fluid. Buffer coatinqs are removed from the fiber

where it contacts the mode stripper. Near-field scans show this type of mode stripper

effectively removes light from the cladding.

A vee groove positions the fiber so the end is coincident with the axis of rotation.

Some measurements, especially near fields, are affected by the amount of force used to hold

the fiber in the vee groove. These distortion effects are eliminated when a small, felt

padded weiqht holds the fiber in the qroove. Before makinq a measurement, the fiber end is visually inspected for flatness and perpendicularity. The vee qroove position can be

slightly adjusted by a two dimensional translation stage to assure that the detector scan passes through the maximum intensity part of the pattern.

Scanning is accomplished with a stepper motor controlled rotary table which swings the

detector through a 12 cm radius arc. At this distance the far-field criteria is satisfied

for core diameters of 100 m or less. Angular motion (V? arc min. per step) is fine enouqh

to qive smooth far-field curves. Detector aperture size is chosen with a trade-off between resolution and signal-tonoise ratio. Presently, a 0.8 mm diameter aperture is used qivinq a resolution of 0.38°.

Reducing aperture size to improve resolution, and hence accuracy, would not be important for most measurements which are relative comparisons (Table I). Radiation anqle (NA)

is the only far-field measurement which is absolute. A simple one-dimensional model based

on a parabolic shaped far-field pattern with an NA of 0.2 predicts an error of 1.6 percent

in the determination of radiation angle when the curve is acquired with 0.38° resolution.

Measurement precision for determining radiation angle is in the ranqe of 1 to 2 percent when

a new output end is prepared and realigned (Sect. 4.1); therefore, a significant reduction

in aperture size would not result in much improvement.

A silicon PIN diode operating in the photovoltaic mode is used as the detector [201.

This detector with an active area of 5.1 mrrr has a built-in operational amplifier and is mounted directly behind the aperture. To improve signal-to-noise ratio, a time constant of

10
0.4 s is used on the lock-in amplifier. The scanning rate is chosen so a resolution element

(0.38°) is scanned in approximately three time constants. At this rate a fiber far-field

pattern is obtained in three to four minutes.

Data may be acquired by several methods. For curve fitting or numerical analysis,

digital acquisition is necessary; however, for most applications, a good, wide paper strip

chart recorder is adequate.

4. PERFORMANCE OF FAR-FIELO SYSTEM

4.1 Precision

Precision is a statement of measurement repeatability. It says nothinq about a "systematic" error which is the offset of the averaqe of a larqe number of measurements from the

true value. Best precision is obtained when a measurement is repeated without disturbinq

any parameter. A typical example of this precision is qiven in fiqure 4-1. Here a farfield measurement was repeated seven times on graded index fiber A usinq overfilled launchinq conditions at a wavelength of 860 nm. Standard deviations for determining the full

widths at the 80, 50, 20, 10, and 5 percent intensity points are shown. Error bars represent + one standard deviation (2 standard deviations in lenqth) and are qiven as a percentage of the full width at that particular point. A typical value is in the 0.5 percent

ranqe. It should be noted that the width of an ink line is approximately 0.3 percent of the curve FWHM and therefore represents a limitation in determininq actual precision. The main

conclusion drawn from fiqure 4-1 is that the system has no appreciable dc drift problems.

For many measurements, the above precision is not applicable. In practice, new ends must be prepared and the system aliqned. To determine this precision, far-field measurements were repeated seven times, for each measurement a new output end was prepared and the

system slightly realigned to the maximum intensity point. Three different fibers representing two different manufacturers were measured, figures 4-2 and 4-3 [21]. Standard deviations for graded index fiber B (not shown) were about 1 percent on lower parts of the curve.

For graded index fiber A the correspondinq values are a little hiqher--in the 1 to 2 percent

ranqe. Fiber C, with a step index, has a much flatter top in qeneral aqreement with theory

[22]. Here the precision is also in the 1 to 2 percent ranqe. From these tests, a precision of 1 to 2 percent is indicated for a radiation angle measurement.

4.2 Accuracy

Accuracy is determined by the absolute value of the angular calibration factor, angular

resolution, and system linearity. The angular calibration factor was determined by reflecting a low divergence He-Ne laser beam off a mirror attached to the rotary table and onto the

laboratory wall. The table was rotated through 360° and the beam returned, within the

nearest step, to the original position. This procedure determines the angular rotation per step more accurately than is practically needed.

As previously mentioned, the finite angular resolution introduces an error when determininq the radiation angle. This error is estimated to be less than 1.6 percent.

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14
Detector non-linearity could affect the shape of the measured pattern. A calibrated

neutral density filter was used to confirm detector linearity up to the highest power levels

used with the system.

4.3 Dynamic Ranqe

Signal -to-noise ratio was determined by attenuating the source until the noise level was observed. Figure 4-4(A) shows the far field from a fiber having a loss of 5 dB when

excited with overfilled launch conditions. In figure 4-4(B), the source is attenuated by

approximately 17 dB to display the noise level. Also shown is the noise level on the baseline and at the peak value; in both cases the noise is about 4 pV oeak-to-peak. Signal-tonoise ratio for a short length of fiber, having negligible attenuation, is 34 dB at 860 nm using an 80 nm bandpass interference filter, overfilled launching conditions, and a time

constant of 0.4 s on the lock-in amplifier. In figure 4-4(B) the siqnal-to-noise ratio is

12 dB and represents the lowest level for making a reasonable measurement. Based on this

criteria, the system has a dynamic range of 22 dB. There are measurements, especially with

very restricted launching conditions, where more signal-to-noise would be desirable. The

easiest way to increase signal is to increase source linewidth. For some measurements this

may be acceptable. An additional 7 dB improvement in signal can be obtained by replacing

the broadband interference filter with a long pass filter having a cut-on wavelength of 0.78

urn. In this case the long wavelength cutoff, 1.1 un, is determined by the spectral response

of the silicon detector.

5. TYPICAL RESULTS FROM FAR-FIELD MEASUREMENTS

5.1 Radiation Angle (Numerical Aperture)

Radiation angle was determined for a number of commercially available multimode fibers.

Measurement precision was consistent with the results of section 4.1. Fiqure 5-l(A) is a typical result from a graded index fiber with a rather low NA of 0.16; while figure 5-1 ( B) shows a fiber with a rather high NA of 0.23. Results of figure 5-1 were obtained using

overfilled launching conditions on a 2 m length of fiber. In almost all cases, far-field

patterns from commercial graded index fibers have been smooth, bell shaped, and exhibit a single central maximum.

5.2 Attenuation with Restricted Launch via a Mode Filter

Radiation pattern measurements on both long (1 km or more) and short (2 m) lengths of

the same fiber are necessary to qualify mode filters for restricted launch attenuation measurements. A successful mode filter produces an output radiation anqle from a 2 m length

equivalent to the output radiation angle from the full test length excited by overfilled

launching conditions. Restrictions in radiation angle, due to fiber propagation, are a function of differential mode attenuation and mode coupling. For many typical graded index

fibers, the radiation anqle from an overfilled launch restricts by 4 to 8 percent after a kilometer of propagation.

15
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One mode filter possibility is a series of serpentine, macroscopic bends, fiqure 5-2.

This geometry in a step index fiber has been previously used as a mode-scrambler for bandwidth measurements [23] [24]. The specific geometry used here consists of eleven, 9 mm diameter nylon posts--six fixed and ffve' moveable. A fiber is placed in the mode filter and

the moveable posts are translated to produce sinusoidal bends having a period of 22 mm with

variable amplitude. Maximum far-field restrictions are obtained when the posts are evenly

aliqned (9 mm peak-to-peak amplitude).

The effect of this mode filter on the far-field pattern from a short length of graded

index fiber is shown in figure 5-3. The solid curve was obtained without the mode filter

using overfilled launching conditions to the 2 m length. The dotted curve is the same but

with the mode filter set to maximum amplitude. A restriction of 16 percent in radiation

angle is achieved at maximum amplitude. This particular fiber, when excited by overfilled

launching conditions, exhibits a 6 percent restriction in radiation angle after 0.9 km of

propagation. Thus, the serpentine mode filter in this case would be more than adequate to achieve the necessary restriction (9 percent) for an attenuation measurement.

It should be emphasized that mode filters affect different fibers in different ways.

The exact nature of the mode reduction is a function of: mode filter, buffer coating, cladding diameter, core diameter, numerical aperture, etc. Fibers have been found where the

serpentine mode filter has produced significantly different results than figure 5-3. It is therefore necessary to "qualify" a mode filter for the particular class of fibers beinq measured.

5.3 Symmetry of Radiation Patterns

In general, far-field measurements on sinqle core fibers qive smooth, symmetric, bell

shaped curves havinq a single maximum. Fine structure from the core would be diffracted

into relatively large angles. For example, a plane wave illuminating a circular aperture of

diameter d has a diffraction angle of 1.22 X/d. This is 1.4° when applied to a 50 urn diameter core at a wavelenqth of 1 vm. Pattern symmetry is indicated in fiqure 5-4. Here far-field patterns are folded about

a vertical axis passinq throuqh the midpoint of the width at half maximum. These examples

were selected from measurements on a large number of commercial fibers and are typical of

good and poor symmetries. Even in the poor case, the symmetry is fairly good.

5.4 Mode Dependance of OH" Absorption

The OH" radical causes spectrally localized absorption in optical fibers. One of the

stronger lines occurs near 950 nm. Previous backscatter measurements show that OH" is not necessarily distributed uniformly along the fiber length [25]. This section describes

far-field measurements to qualitatively determine the mode dependence of OH" absorption in a particular fiber. For these measurements, a 0.9 km fiber was chosen with a large OH" absorption peak and

little mode coupling. Spectral attenuation measurements indicate a loss of 5 dB/km at 850 nm which increases to 40 dB/km at 950 nm. By analyzing far-field patterns at 850 nm

18
NYLON POST

9mm DIA.

Figure 5-2. One type of mode filter consistinq of a series of serpentine, macroscopic

bends. Moveable posts can be translated into alignment with fixed adjacent

posts.

WITHOUT MODE FILTER

ANGLE, DEGREES

Figure 5-3. Effect of mode filter on 2 m length of graded index fiber. Solid curve is without mode filter, dotted curve is with mode filter set at maximum amplitude.

19
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from overfilled and restricted mode volume launches, this fiber was found to have little

mode coupling.

Qualitatively, if more OH" were located near the core-cladding interface, higher order

(large radius-large angle) modes would be attenuated more and a narrower pattern would

result at the output from an overfilled input; the converse would be true if the OH" were concentrated near the axis.

Figure 5-5 is the superposition of normalized far-field patterns at 857 nm and 957 nm using overfilled launching conditions and 70 nm bandpass filters. Precision in determining

these differences should be quite good since only interference filters were changed between

the two measurements. As shown, the presence of OH" absorption at 957 nm does little to alter the shape from the pattern at 857 nm where no OH" absorption is present. Therefore,

the OH" in this fiber appears to be uniformly distributed with a slight indication of more

OH" near the axis (957 nm curve is wider). The distribution of OH" would depend upon the

preform fabrication process so the above results apply only to the particular fiber.

6. DESCRIPTION OF NEAR-FIELO MEASUREMENT SYSTEM

The near-field system produces a magnified image of the fiber end face. A single

microscope objective gives a magnification of about 50, fiqure 6-1. By scanning a detector

across a diameter of the image, the near-field intensity distribution is obtained. In figure 6-1, the detector aperture is located at a fixed distance from the objective so the

magnification ratio remains constant independent of other system adjustments.

The detector is an apertured silicon PIN photodiode identical to the one used with the

far-field system. This detector is scanned across the image, which is nominally a few mm in diameter, by a stepper motor controlled translation stage having a movement of 0.4 pm per step. The detector aperture size, 80 un diameter, was chosen to be consistent with theoretical resolution limits. A smaller size does not improve resolution but results in lower

signal-to-noise ratio. An 80 urn aperture gives a resolution of 1.5 urn when referenced to the fiber endface.

Care should be used in selection of the fiber holder, which is positioned just after

the cladding mode stripper. The present system uses a vee groove with a small, felt padded

weight to hold the fiber in the groove. If the fiber is held with too much force, stress

will distort the near-field intensity distribution and scatter light into the cladding. The

vee groove is mounted on an xyz translation stage so the image can be brouqht into focus on the detector aperture.

Alignment is accomplished using the movable mirror and eyepiece shown in figure 6-1.

First, a fiber is imaged in the system at the measurement wavelength (typically 850 nm)

;

good sensitivity is obtained by imaging the on-axis index dip. Then, using deep red visible

light, the movable mirror is positioned to illuminate the eyepiece. The eyepiece is translated until the image appears in sharp focus. Now, if a fiber end is brouqht into focus

using the eyepiece, it will be in proper focus at the detector aperture when the mirror is noved out of position. This method of alignment was checked using a number of fibers.

21
ANGLE. DEGREES

Figure 5-5. Experiment to qualitatively determine mode dependence of OH" absorption,

Little mode dependence is indicated for this particular fiber.

CLADDING

MODE STRIPPER

FIBER IU

FIXED DISTANCE

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Figure 6-1. Near-field measurement system based on a radial scan of a magnified near-field

image.

22
Fibers were first aligned visibly with the eyepiece; no further improvement in the focus of

index dips could be obtained at the detector using 850 nm light.

The source used with the near-field system was previously described in section 3.1. Scan rate and lock-in amplifier time constant were chosen with a trade-off between mechanical drift and signal-to-noise ratio. Presently, the system scans a 50 urn core in 5 minutes

with a detection time constant of 0.13 s. These issues are addressed more fully in sections

7.2 and 7.3.

7. PERFORMANCE OF NEAR-FIELD SYSTEM

7.1 Precision

Best precision is obtained when the measurement is repeated without disturbing any

parameter except refocus. Figure 7-1 shows the results of four repeated measurements on a 1.1 km length of graded index fiber D. The standard deviations for determining the full

widths at the 80, 50, 20, 10, and 3 percent intensity points are indicated along the ordinate in the figure. A typical value is in the 0.3 percent range for the lower part of the curve. It should be noted that the width of an ink line is approximately 0.2 percent of the FWHM and therefore represents a limitation in determining actual precision. The main con- clusion from figure 7-1 is that the system has no appreciable dc drift during a 10 minute

period.

In practice, new ends must be prepared and the system refocused. To determine this

precision, near-field measurements were repeated four times with new end preparation and

alignment for each measurement. Three fibers, two graded and one step, 0, E, and C, were

used for this test; results on E and C are given in figures 7-2, and 7-3. For fibers E and

C, standard deviations for determining full-widths on the lower parts of the curves range

from 0.8 to 1 percent. For fiber D (not shown), the corresponding values are slightly

higher, 1 to 2 percent. This precision data contains effects of core ellipticity since

fibers were remeasured without regard to angular orientation. A precision of about 1.5

percent would apply to a core diameter measurement based on a determination of the nearfield full width close to the baseline on a short length of fiber.

7.2 Resolution and Accuracy

When all light emitted by the fiber is collected by the objective, the near-field

resolution is determined by fiber numerical aperture. This is a consequence of the expression for the resolution, Y, of a microscope objective with

Y ~M* . (8) NA

Where x is the wavelength and NA refers to the collected numerical aperture [26]. At a \ of

0.85 un and an NA of 0.2, Y is 2.6 ym.

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In a more detailed treatment where mode amplitudes are summed, Adams et al . , show that

near-field resolution is related to the number of propagating modes and, for a near parabolic fiber is approximately 2 a/V, where V is the normalized frequency [27]. V is given by

[9] as

V =ll a (NA) . (9)

X

Thus, the resolution becomes X/ttNA, which is about a factor of two less than eq (8). Theoretical limits therefore are approximately 2 un for a 0.2 NA graded index fiber at a wavelength of 0.85 un. Experimentally, upper limits to resolution were obtained by observing abrupt features

in near-field patterns. This includes index dips and core-cladding boundaries. Upper

limits are obtained since the actual shapes of the features are unknown. Figure 7-4(A)

shows the narrowest index dip, 2.5 m FWHM, observed with the system. Figure 7-4(B) is a 1.9 urn 10 to 90 percent edge response to a core cladding boundary in a step index fiber

after l.l km of propagation (leaky modes have died out). Both of these cases suqqest a resolution near 2 urn consistent with theoretical expectations.

The scanned dimension must be calibrated to make an absolute measurement. At present,

this is accomplished with a microscope stage micrometer having rulings every 10 urn. These

rulings are illuminated from behind the stage substrate with light, at the actual measurement wavelength, from a large core, large NA fiber. A typical calibration scan is shown in figure 7-5. Here the imaged rulings transmit more light than the background and appear as a series of peaks separated by 10 urn. A close inspection of figure 7-5 shows the peaks are

not exactly evenly spaced. We believe this is caused by uneven illumination across the

widths of the rulings. Such behavior can be observed by eye using visible light. Presently

a centrally located pair of lines spaced by 150 un is used to determine the calibration factor. In this way, the uncertainty in calibration due to uneven ruling illumination is reduced so resolution and stage micrometer accuracy become the limiting factors.

Preliminary results have been obtained to verify the calibration of commercial stage

micrometers. In this measurement the spacings between rulings are compared to transmission

fringes from a Fabry Perot interferometer, figure 7-5. Rulings which have higher transmission are located by a HeNe laser beam focused to 2 un. A motorized differential micrometer drives the stage and translation is measured by counting fringes. With an HeNe laser,

transmission fringes occur every X/2 or 0.3164 un. The Fabry Perot free spectral range is large enough so cavity modes from a common HeNe alignment laser are not observed. Figure

7-7 shows a typical result; in this case ruling centers are separated by 316 fringes (nearest integer fringe) or 99.98 urn. The average of 9 measurements over various ruling pairs

was 100.4 un with a standard deviation of 0.5 )jm--in reasonable agreement with the stated

spacing of 100 un. Further improvement would be expected by focusing the HeNe beam to a smaller spot and using a stage micrometer with sub-un ruling widths.

27
\- 90%

1.9 MICROMETERS

CORE CLADDING

U 10%

(B)

(A)

Figure 7-4. Experimental upper limits to near-field resolution at 860 nm. (A) narrowest

on-axis index dip observed, (B) in to 90 percent edqe response to core cladding

boundary on Fiber C.

28
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FABRV PEROT FRINGES

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30
7.3 Dynamic Ranqe

The source was attenuated until noise was observed on the near-field pattern from a 2 m length of graded index fiber, figure 7-8. In this instance the source (850 nm wavelength,

80 nm linewidth) was attenuated by 22 dB, thus indicating a signal-to-noise ratio of 39 dB

with an unattenuated source. Figure 7-8 with a signal-to-noise ratio of 17 dB represents

the lowest signal-to-noise ratio for extracting useful information; thus, fiber losses of up to 22 dB can be accomodated by the system.

Further improvements in signal-to-noise ratio could be achieved by increased source

linewidth or lock-in amplifier time constant. Figure 7-8 was scanned in about 10 minutes,

substantially longer scan times would cause the system to drift out of focus. Mechanical

drift is minimized by mounting all components securely to a 5 cm thick aluminum plate. With

a 10 minute total scan time, the lock-in amplifier time constant could be increased about a factor of five over figure 7-8 to pick up an additional 4 dB in signal-to-noise ratio.

8. TYPICAL RESULTS FROM NEAR-FIELD MEASUREMENTS

8.1 Cladding Light

Signal-to-noise ratio is sufficient to observe light scattered into the cladding due to stress on the fiber, incomplete mode stripping, etc. Figure 8-1 shows a scan over an entire

fiber diameter to detect light in the cladding; the cladding-air boundary is clearly shown.

Cladding light can be measured with a signal-to-noise ratio consistent with section 7.3.

8.2 Step Index Fibers

Several examples have already been given in this Technical Note of near fields from

graded index fibers. In contrast, short step index fibers are greatly affected by the

presence of leaky modes. Figure 8-2 is the superposition of near-field patterns from the

same step index fiber at a length of 1 m and 1.1 km. In 1.1 km, the leaky modes have been

attenuated so the near field closely resembles the index profile of a step fiber.

8.3 Symmetry of Radiation Patterns

From the very nature of the preform fabrication process, fibers should exhibit good

symmetry across a diameter. However, at times, cores can be out of round and not neces- sarily concentric with the outside cladding boundary. Figure 8-3 shows typical near-field

measurements from fibers. To illustrate radial symmetry the patterns have been folded about

a vertical axis passing through the midpoint of the width at half maximum. As shown, the

deviations Are slight and lines nearly overlap. The more asymmetric behavior that has been

observed has usually occurred within the index dip on-axis.

8.4 Index Dip Pathologies

Fibers made by chemical vapor deposition processes generally show an on-axis dip in the

index profile. This happens when the preform is either consolidated or collapsed and dopants evaporate out of the axial region [9]. Attempts have been made to compensate this process, giving rise to index peaks in cases of overcompensation [28]. This section is

31
30 30 RADIAL POSITION, MICROMETERS

Figure 7-8. Near-field from a 2 m length of graded index fiber with source attenuated by

22 dB. Signal-to-noise ratio of this figure is 17 dB.

32
60^ lio

RADIAL POSITION, MICROMETERS

Figure 8-1. Near-field scan across a fiber diameter including the core and cladding regions

to observe light in the cladding region.

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37
intended to show the diverse behavior of index dips found in commercially available graded

index fibers.

The outside chemical vapor deposition process qenerally gives smaller dips than the

inside process. Figure 8-4 is an example of a fiber made by an outside process and exhibits

the smallest dip we have observed.

Figure 8-5 shows the largest dip observed with the system to date. The depth is about

half of the peak height of the curve. In many of the index dips measured, the widths are close to the system resolution. Consequently, the dips are deeper than the near-field mea- surements indicate. Other methods for determining index profile such as refracted near field offer better resolution for these kinds of measurement.

9. CONCLUSION

The determination of radiation angle (numerical aperture) from far-field data at the 5 percent intensity points should result in good measurement precision and accuracy when

applied to near-parabolic, index fibers. In these fibers, leaky modes are contained within

the meridional ly defined numerical aperture so they should not affect the intensity at the

largest observed angles. However, in step index fibers the situation is different. Here leaky modes cause uncertainty if meridional ly defined numerical aperture is desired. At present, standards groups have not made recommendations for step index fibers.

More work is needed if core diameter is to be determined from a near-field measurement. Correlation between the near field and index profile at the core-claddinq boundary

must be established. The use of a low index "barrier layer" may cause differences between core diameter determined by near-field and index profile measurements. By judiciously

choosing definitions, it should be possible to reduce systematic errors between these two measurements. Technically, near-field measurements are much easier to impliment than index

profile measurements which, for the most part, rely on interferometry over small dimensions.

10. REFERENCES

[1] Cathey, W. T. Optical information processing and holoqraphy. New York: John Wiley;

1974.

[2] Sladen, F. M. E., et al . Determination of optical fiber refractive index profiles by

a near-field scanning technique. A.P.L. 28(5) :255-258: 1976 March.

[3] Adams, M. J., et al . Leaky rays on optical fibers of arbitrary (circularly symmetric)

index profiles. Electronics Letters, 11(11) :239-240; 1975 May.

[4] Holmes, G. T. Propagation parameter measurements of optical waveguides. Proceedings

of the SPIE, Fiber Optics for Communication and Control, Vol. 224: 1980.

[5] Kitayama, K., et al . Impulse response prediction based on experimental mode couplinq

coefficient in a 10-km long graded-index fiber. IEEE J. Ouant. Elect.

QE-16(3):356-362; 1980 March.

38
[6] Procedures for determining radiation angle (numerical aperture) and core diameter from

far and near-field patterns respectively are under consideration by Committee P6.6 of

the Electronic Industries Association.

[7] Hanson, A. G., et al . Optical waveguide communications glossary. NTIA Special

publication, NTIA-SP-79-4; 1979 September.

[8] Electronics Industries Association Committee P6.6 is currently recommending that

radiation angle be based on the far-field pattern half width at the 5 percent

intensity points.

[9] Miller, S. E.; Chynoweth, A. G. Optical Fiber Telecommunications. New York:Academic

Press; 1979.

[10] Matsumura, H. The light acceptance angle of a graded index fibre. Optical and

Ouantum Electronics 7:81-86; 1975.

[11] Eriksrud, M., et al . Comparison between measured and predicted transmission

characteristics of 12 km spliced graded-index fibres. Optical and Ouantum

Electronics. 11:517-523; 1979.

[12] Tateda, M. Optical loss measurements in graded-index fiber using a dummy fiber.

Applied Optics. 18(19) :3272-3275: 1979 October.

[13] Cherin, A. H.; Gardner, W. B. Standardization of optical fiber transmission

measurements. Laser Focus. 16(8):60-65; 1980 August.

[14] Adams, M. J., et al . Length-dependent effects due to leaky modes on multimode gradedindex optical fibres. Optics Communications. 17(2) :204-209; 1976 May.

[15] Petermann, K. Uncertainties of the leaky mode correction for near-square-law optical

fibres. Electronics Letters. 13(17) :513-514; 1977 August.

[16] Barrell, K. F.; Pask, C. Leaky ray correction factors for elliptical multimode

fibres. Electronics Letters. 16(14) :532-533; 1980 July.

[17] Hazan, J. P. Intensity profile distortion due to resolution limitation in fibre index

profile determination by near field. Electronics Letters. 14(5) : 158-160; 1978 March.

[18] Sabine, P. V. H., et al . Fibre refractive-index profiling by modified near-field

scanning. Electronics Letters. 16(23) :882-883; 1980 November.

[19] Sladen, F. M. E., et al . Measurement of profile dispersion in optical fibers: a direct technique. Electronics Letters. 13(7) :212-213; 1977 February.

[20] PIN photodiodes with built-in operational amplifiers are available from a number of sources. A 22 megohm feedback resistor is used with the operational amplifier.

[21] The alphabetical fiber designations are consistent within this Technical Note but do not apply to any other NBS publications.

"22] Gloge, D.; Marcatili, E. A. J. Multimode theory of graded-core fibers. B. S.T.J.

52(9):1563-1578; 1973 November.

.23] Tokuda, M. , et al . Measurement of baseband frequency response of multimode fibre by

usinq a new type of mode scrambler. Electronic Letters. 13( 5) : 146-147; 1977 March.

[24] Franzen, D. L.; Day, G. W. Measurement of optical fiber bandwidth in the time

domain. Nat. Bur. Stand. (U.S.) Tech. Note 1019; 1980 February.

39
[25] Conduit, A. J., et al . Spectral and length-dependent losses in optical fibres

investigated by a two-channel backscatter technique. Electronics Letters. 16(3): 77-

78; 1980 January.

[26] Born, M.; Wolf, E. Principles of Optics. New YorkrPergamon Press; 1964.

[27] Adams, M. .]., et al . Resolution limit of the near-field scanning technique.

Proceedings of the Third European Conf. Opt. Comm.; 1977; Munich.

[28] Eickhoff, W.; Krumpholtz, 0. Pulse response and microbending losses of an optical

glass-fibre waveguide with a bump in its index profile. Electronics Letters.

13(9):256-257; 1977 April.

40
JBS-114A ' REV. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN-1032

2. Performing Organ. Report No 3. Publication Date.

March 19>i

4. TITLE AND SUBTITLE

Measurement of Far-Field and Near-Field Radiation Patterns from

Optical Fiber s

5. AUTHOR(S)

Ernest M. Kim and Douglas L. Franzen

,'. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

). SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Supported in part by the Department of Defense, Calibration

Coordination Group (CCG).

0. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

1. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography or literature survey, mention it here)

Systems are described for measuring the far- and near-field radiation

patterns from optical fibers. Parameters which affect measurement precision,

accuracy, resolution, and signal-to-noise ratio are discussed. Measurements

utilizing radiation patterns are covered; this includes radiation angle

(numerical aperture), attenuation using mode filters, index profile, core diameter, and mode volume transfer function. Experimental examples are given in

many instances.

' KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

Attenuation; core diameter; far field; index profile; mode filter;

numerical aperture; radiation angle; radiation patterns.

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