hp-pn8504-1_measurements of lightwave component reflections
TRANSCRIPT
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Measurements of lightwavecomponent re flec tions wi th theHP 8504B precision refle ctometer
Pr oduct Note 8504-1
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2
The precis ionref lectometer
A new d evelopment in optical
reflectom etry, th e HP 8504B pre-
cision reflectometer, significan tly
extend s m easurement capabilities.The tw o-event resolution is better
than 25 m icrom eters, while the
dyn am ic range exceeds 80 dB.
A m easurement tool now exists
specifically for the designer and
m anu facturer of precision light -
wave components and connectors.
The sensitivity exceeds th at of the
best power m eter solutions, wh ile
the resolution is sufficient to iso-
late each ind ividu al reflection
within a small, complex optical
assembly.
Table of Conten ts
HP 8504B operation su mmary 3
Return loss concepts and measurements 4
Basic concepts of reflect ion 4
Problems that resu lt from reflected ligh t 4
S urvey of return loss measurem ent m ethods: 5
Power meters 5
Opt ica l t ime-domain reflectometers 6
Opt ica l frequency-domain reflectometers 6
The precision reflectometer technique 6
The general measurement process 8
Inst rument warm-up 8
Reference extension cable select ion 8
Cleaning connectors 8
Select opera t ing wavelength 8Measurement calibrat ion: 9
Balance receiver 9
Magnitude ca libra t ion 9
Measure the test device 10
Opt imize the instrument setup 10
Measurement example:Connector pair 10
Measurement procedure 10
Increasing the measurement ra te 11
Measurement example:Char acterizing a photodiode assem bly 11
Measurement procedure 11
Opt imizing the inst rument setup 11
Applications 13
Measuring the reflect ions from an opt ical isola tor 13Character izing reflect ions within a laser assembly 13
Char acterizing devices pigtailed with mult imode fiber 14
Pr ecision measu rem ents of differential length. A 1XN coupler 15
Char acterizing high retu rn loss term inat ions: Index ma tching gel 16
Common quest ions and answers 17
Understanding measurement accuracy 18
Bibliography 19
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HP 8504Boperat ion summary
The following one page operating
summa ry is intended as a brief ref-
erence. Detailed inform at ion is dis-
cussed within the pr oduct note.1. Warm-up the instrum ent: For
maximum dynamic range and mea-
sur ement s tability, allow the inst ru -
ment to warm up for two hours.
2. Clean al l conn ection s: Clean
all fiber connectors a nd instr um ent
test port s used in both t he calibra-
tion a nd m easurement process.
At th is point you can follow th e
operating procedure below, or use
th e instr umen t’s “Guided Setup”
featu re. Press SYSTEM, [Guided
Setup].
3. Se lect wave length: Press
PRESET, MENU , and select th e
appropriate wavelength (under
[SOURCE MENU]).
4. Determine reference exten-
s ion length: Measure the length
(L2) of fiber cable leading to th e
device under test (DUT). Attachextension cable L1 between t he r ef-
erence extension ports with length
equal to or slight ly less th an L2. If
the extension cable length L1 is
greater tha n L2, the device may
not be seen in the instr umen t’s
measurement range.
5. Pe rform a cal ibration:
This r emoves DC offsets an d polar-
ization s ensitivity, an d sets a cali-
brat ed r eference level. Select CAL,
[Guided Cal]: Termina te the instr u-
ment test port with the high return
loss load (>40 dB) supplied with th einstrum ent. Adjust the polarization
balance as directed.
Note: Once the polarization calibra-
tion has been performed, the reference
extension cable L1 and the polariza-
tion adjustment knobs must remain
stationary. Attach a fiber cable of
length L2 or slightly longer, with
known return loss (typically a Fresnel
reflection) to the instrum ent t est port.Measure the sta ndard a s directed.
If the system is operating correctly,
ther e should be a single response seen,
similar to th e display sh own below.
6. Connect the test de vice:
Attach th e device under test (DUT)
to the instr ument t est port. Let the
instr umen t complete a full sweep.
Locate the reflections of interest
and reduce the measurement spa n
as m uch as possible using the MKR
FCTN and span keys.
7. Increase sensitivity: Increased
sensitivity can be achieved thr ough
averaging. Pr ess AVG, [AVERAGIN G
ON]. The nu mber of averages is set
using t he [Averaging Factor] function.
The num ber of avera ged traces is
displayed at th e left border of th e
display.
L1 L2
DUT
27 cm (in fiber)measurement
range
0<L2–L1<25 cm
HP 8504B Measurement Setup
Typical Display
0 dB return loss
Reflectiondisplayed indB return loss
50 dBreferencelevel
Instrumentnoise floor
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Return loss conceptsand meas urementtechniques
Th e reflection coefficient ‘ρ’ of this
interface is th e ra tio of the r eflected
electric field to the incident electric
field and is given by th e following:
ρ=n
1–n
2
n1+n
2
where n1
is the index of refraction
for th e mat erial the light is propa-
gating from and n 2 is th e index of refraction of the mat erial th e light
is tr aveling to.
The reflectance ‘R’ is a s imilar ter m
and is defined as the r at io of the
reflected beam inten sity to the inci-
dent beam int ensity and is given
by:
R= ρ2
= (n1–n
2)2
n1+n
2
(In the a bove cases, it is ass umed
that the light is tr aveling norma l
to the inter face plane.)
An exam ple of th is phenomenon is
an a ir-to-glass int erface. Air h as a n
index of refra ction of 1, while glass
has an index of approxima tely 1.5.
The r eflecta nce of this int erface is
then 0.04 or 4% while the reflection
coefficient is –0.2. A n egat ive re flec-
tion coefficient is an indication that
the reflected field has experienced a
180-degree phas e shift relat ive to
the incident field.
The reflective properties at an
interface can also be described loga-
rith mically in decibels. This par ame-
ter is called retu rn loss (RL) an d isgiven by:
RL= –10 log10 (power reflected)power incident
–10 log10
R
The air-glass int erface would ha ve
a r etur n loss of 14 dB.
Notice tha t th e smaller th e reflec-
tion, the lar ger the ret ur n loss. This
shows the ut ility of using ret ur n loss
to describe very small reflections.
Note: In measu ring reflections,
intu itively you would expect low
reflections to be displayed lower t han
high reflections. Consequently, the
HP 8504B uses a different conven-
tion in displaying retur n loss. Retur n
loss is computed a s:
10 log (power reflected)power incident
as opposed to:
–10 log (power incident )power reflected
Thus a retu rn loss of 20 dB is dis-
played as –20 dB and a retur n loss
of 50 dB is displa yed as –50 dB.
The m ajority of this documen t dea ls
with components and devices used
in systems tha t u se optical fiber.
Another phenomenon th at m ust be
consider ed is th e coupling of the
reflected light back into the fiber. In
general, th e light from a r eflective
inter face is not collima ted a nd only
a portion will ret ur n back thr ough
th e fiber. Thus, th e definition of
return loss must be modified again.
Return loss is then defined in terms
of what actua lly propagates ba ck thr ough the fiber rat her tha n what
would be predicted solely from the
differences in t he refractive indices
at a bounda ry.
Problems that resultfrom re f lecte d l ight
The m ost obvious pr oblem th at
occurs when light is reflected is t ha t
the t ran smitted signal is reduced.
In t he pr evious glass-to-air inter face
example, since 4% of the light was
reflected, only 96% of the or iginal
signal was transmitted. This corre-
sponds to a (–10 log10
(0.96)) 0.2 dB
loss in power.
Sometimes more important
th an power loss is the effect t ha t
reflected light ha s on the per for-
ma nce of lightwave component s.
Today’s h igh-speed lightwa ve com-
mu nication system s typically usenarrow linewidth lasers. The rela-
tive intensit y noise (RIN) and m od-
ulat ion char acteristics of such laser s
can be significantly degraded by
very sma ll amount s of backreflected
light. Reflected light can also cause
“bit errors” in digital communication
systems and distortion in analog
comm unication systems.
As light reflects off one int erfa ce,
the reverse traveling waveform may
be re-reflected off another inter face
(closer to the source). This results
in two forwar d tr aveling waves. Thema gnitude of the composite forwar d
tr aveling signa l is the vector sum of
these two waveform s. Depending on
the ph ase r elationship of the t wo
signals, which is in tur n dependent
on wavelength and t he path length
tha t th e reflected wave tra veled,
th e two waves may add eith er con-
str uctively or dest ru ctively. Su btle
changes in source wavelength a nd
environmenta l chan ges (such as
temperat ure) can cause this phase
relat ionship to var y significantly.
Thus, the total power seen at t he
detector will vary with time.
For these above mentioned reasons,
the components used in lightwa ve
systems such as isolators, connectors,
an d ph otodiodes t ypically have very
high return loss (low reflections)
an d as few reflections a s possible.
Bas ic conceptsof reflectio n
When light t ra vels across the
boundary between materials with
differen t in dices of refra ction or
densities, some portion of the light
will be reflected. The figure below
shows th e simplest form of this con-
cept, a plan e wave tra veling per-
pendicular to the boundar y between
the two materials.
Boundary
N1 N2
Incident
Reflected
Transmitted
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Survey o f returnloss measureme nttechniques
Contemporary methods
Virtually all return loss measure-
ment techniques employ some t ype
of optical r eflectometr y. This consist s
of illumina ting th e device un der t est
and measur ing the light th at is
reflected back. Optical reflectometers
include power m eter/coupler based
systems (sometim es called optical
contin uous wa ve reflectometers or
OCWR), optical tim e-doma in r eflec-
tometers (OTDR), optical frequency-
domain r eflectometer s (OFDR), an d
th e HP 8504B precision reflectome-
ter. Each of the a bove ment ioned
measu remen t techniques offer
unique advanta ges and benefits.
Pow er me ter/couplerbased measurements
Return loss measurements can be
made using a power m eter such as
th e HP 8153A light wave multime-
ter a nd th e HP 81534A return loss
module. The ret urn loss module
conta ins a sen sitive detector a nd
a dir ectiona l coupler. The sour cemodule of the HP 8153A illumina tes
the test device, while the dir ectional
coupler a nd detector sense only the
power th at is tr aveling in the reverse
direction.
This system is best suited for accu-
rat e measu rement of devices with a
single reflection su ch as conn ectors,
splices and att enua tors. It is also
used to measur e the aggregate or
“tota l” ret ur n loss of a device with
mu ltiple r eflections.
Return loss when thereare multiple reflections
While the power meter system is
both economical and easy to use, itdoes not pr ovide any spat ial inform a-
tion for resolving mu ltiple reflections.
There ar e two important implica-
tions t o consider. Fir st, ident ifying
and quan tifying each r eflection is
importa nt in the design and ma nu-
factu ring of high ret ur n loss compo-
nent s. Second, th e total retur n loss
of a component can vary significan tly
when t wo reflections creat e a
Fabry-Perot resonator.
When m ultiple reflections exist, the
spectral char acteristics of th e light
source must be considered. If th e
light s ource has a very nar row line-
width, it will then ha ve a very long
coherence length. In brief, this
implies that the phase chara cteris-
tics of the light a re very st able.
When light reflects off two discrete
inter faces th ere will be two reverse
tr aveling waves. The total reverse
tr aveling power will be relat ed to the
vector su m of th e two waves. The
phas e relat ionsh ip of these t wo sig-
nals is dependent u pon the source
wavelength an d path length betweenthe two reflecting interfaces. The
relative phase between the two
waves will then det ermin e whether
th ey will add const ru ctively or
destru ctively. This is r elated t o the
Fabry-Perot effect. An example of
th e phenomenon can be demon-
str at ed with a simple connector
pair with an air gap of 1 mm.
Forwar d-traveling light will first
encounter the glass-air interface
at th e end of the first connector.
Approximately 3.5% of the light
will be reflected. Th e ma jorit y of
the light will continu e to th e air-
glass in ter face of the second connec-
tor. Again, t her e will be a 3.5%
reflection.
At several discrete wa velength s
(such as 1300 nm), the air “cavity”
length is such th at t he two reflected
waves will be precisely in pha se, andthe reflected power will be at a maxi-
mum . However, at other wavelengths
(such as 1300.4 nm), the t wo wave-
form s will be out of phas e, and t he
two signals will add destructively.
The r eflected power will be at a
minimum 1.
Return loss for two equal reflections1 mm air gap
R e t u r n L o s s ( d B )
Distance (mm)
1 mm
Return loss for two equal reflections1 mm air gap
N1N2
N1L
Er2Er1
ErTotalEr1
Er2 Er2
θ=?Er1
1 In t heory, if th ere were n o coupling losses, the
stimulus wa s monochromatic, and a ll of the
signals r e-reflected in t he cavity were consid-
ered in addition to th e primar y reflections, the
ret urn loss can go to infinity, implying a reso-
nant condition.
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The HP 8504Bprecis ion ref lectometer
A new development in optical
reflectometry, the HP 8504B pre-
cision reflectometer, significan tly
extends measurement capabilities.
The t wo-event resolution is better
tha n 25 micrometers, while the
dyna mic ra nge exceeds 80 dB. A
measu remen t t ool now exists spe-
cifically for the designer and man-
ufacturer of precision lightwave
components and connectors. The
sensitivity exceeds th at of the best
power m eter solutions, while the
res olut ion is sufficient to isolat e
each individual reflection within a
sm all, complex optical as sem bly.
The HP 8504B precision reflectome-
ter is bas ed on th e Michelson inter-
ferometer an d uses t he techniques
of “white light” inter ferometr y. A
1300 nm or 1550 nm low-coherence
light source is sent t o a power split-
ter. One pat h leads t o the DUT, while
the other pa th leads to a reference
mirror. Light reflected off both the
reference mirror and th e DUT is
recombined an d detected. If the pa th
length from t he source to th e reflec-
tion in t he DUT is the sam e as fromth e source to the mirr or, a coheren t
interference signal appears a t t he
dete ctor. By moving th e position of
the reference mirror, the instrument
can t hen “scan” the t est d evice for
reflections over a 400 mm r ange
(equivalent a ir dista nce). (The
It is worth ment ioning again that
th is effect will only occur when a
highly coherent source is used. This
has some important implications.
The ret ur n loss of a component can
vary significan tly depending upon
the stimulus. Return loss testing of
a component m ay not accurat ely pre-
dict its behavior in a working system
if the source char acteristics ar e not
similar in both situa tions. In addi-
tion, when component s ar e used in
a system with a h igh coherence laser
(such a s a DFB or N d.YAG), the com-
ponent retu rn loss char acteristics
ma y chan ge drama tically as wave-
length an d temperatur e are even
slight ly varied.
To more completely under sta nd
the reflection chara cteristics of
components a nd su bassemblies,
the individual r eflections mus t be
isolated an d char acterized. Othermeasur ement techniques are used
to both locat e an d qua nt ify individ-
ua l r eflections.
The OTDR
Optical time-doma in r eflectometr y
is the most familiar a nd comm only
used reflectometry measurementfor the installation and maintenan ce
of both long- an d short -hau l fiber
links. OTDRs locate faults by prob-
ing a fiber with a n optical pulse tr ain
and measuring the reflected and
backscat ter ed light . OTDR’s are typi-
cally not used for component -level
measurem ents due t o limitations
in r esolving sm all or closely spaced
reflections, unless very short
impulses an d very high-speed,
sensitive detectors ar e employed.
The OFDR
The two-event resolution and dea d-
zone problems inher ent with OTDR’s
can be impr oved by using a s wept
modulated lightwave instead of
pulses of light . The amplitu de and
envelope pha se r esponse is r ecorded
over a wide frequency span . An
inverse Four ier tra nsform is per-
form ed on t his dat a t o yield th e time-
domain r esponse. Depending upon
the m odulation bandwidth of th e
instr umen t, the t wo-event r esolution
can be better t han 10 mm (20 GHzbandwidth). The r eceiver is syn-
chronously tun ed to th e source,
thu s decreasing the susceptibility to
noise and increas ing the dynamic
ra nge to levels near 40 dB. (See
product specific litera tur e for t he
HP 8702 and H P 8703).
Precision lightwave reflectometerblock diagram
Detector Display
DUT
1300
1550
WDM
Coupler
Reference Mirror
ReferenceExtension
Return loss, two equal reflections vs. Wavelength
R e t u r n L o s s ( d B )
Wavelength
∆
Lambda0.5 mm
1300
A plot of retur n loss versus wa ve-
length sh ows th is result graph ically.
This effect is repetitive as a function
of wavelength so tha t m aximumsand m inimum s will occur a t several
wavelengths.
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400 mm m easurement window can
be offset by simply adding length to
the reference path at the instrum ent
front pan el).Note that in this measurement
scheme th ere ar e no pulses of light
tra nsmitted as there are for the
OTDR, an d th e light is n ot modu-
lated as it is for th e OFDR. It is the
short coheren ce length of th e LED
source in the precision reflectome-
ter which leads to very high r esolu-
tion measur ements.
To demonst ra te t he u tility of such
an inst ru ment , consider a high-speed
photodiode.
The ph ysical dimen sions of
th e device are mu ch less tha n a
centimeter, yet there are several
inter faces within th e device, each
potentially generat ing a reflection
an d contribut ing to the overall
ret ur n loss of th e device. Very h igh
resolution is requir ed to locate,
identify and qu ant ify each r eflec-
tion. The precision reflectometer
easily performs th is task, as seen
in the following measu rem ent.
Lens
Diode chip
Heat sink
In this measur ement, return loss is
displayed versu s th e one-way path
distance. The measurement span
is 6 mm so the horizontal scale is
0.6 mm per division. Retur n loss is
displayed in decibels. The top of th e
display is 0 dB retur n loss, the cen-
ter of th e display is –50 dB. Higher
retu rn loss levels are displayed as
a lower value on th e display since
they correspond t o lower va lues of
reflection. (Intuitively, you would
expect low reflections t o be displayed
at the bottom of the screen). Thus a
51.9 dB ret ur n loss value is dis-
played as –51.9 dB as noted by the
measurem ent m arker, which corre-
sponds to the r eflection a t t he front
face of th e diode chip.
The first r eflection is t he end of
the fiber connector. It appr oaches
a F resn el reflection since the fiber
does not physically contact the
device. The r eflections off the front
and ba ck of the lens, and th e front
and back of the diode chip are clear ly
seen. From this m easurement, you
see tha t th e retu rn loss of this device
is dominated by t he front face of the
diode chip. However, a power met er
measurem ent would be dominated
by the r eflection off th e end of the
fiber, making it difficult to extractany inform at ion about t he photodi-
ode itself.
It is also import ant to note the high
dynamic ran ge that t he interferom-
eter technique offers, several orders
of magnitude beyond th e capabilitiesof tr aditional meth ods.
The measurement ran ge of the
precision reflectometer is determined
by the length that the r eference mir-
ror can t ra vel and is 400 mm (equiv-
alent a ir distance). If measurement s
beyond t he 400 mm window are
required, the window is simply off-
set with t he appropriate reference
extension pat ch cord.
Knowing the ma gnitude and spa cing
of th e reflections yields informat ion
th at is useful in determining com-
ponent performan ce in systems with
narrow linewidth lasers. Specifically,
how much th e retur n loss may vary
for a given chan ge in wavelength.
In sum mar y, there ar e several tech-
niques for measuring r eturn loss.
The easiest m ethod is to use a power
meter. When reflections need t o be
spat ially resolved, the OTDR is used
for coar se mea sur ements of long
lines. The OFDR can be configured
for long-line m easur ement s or close-
in measur ements, depending on t he
modulation bandwidth u sed. Forthe highest resolution and dynamic
ra nge, the H P 8504B precision
reflectometer technique is optimum.
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Generalmeasureme nt processw ith the HP 8504B
8
measur ements. This will ensure
tha t var ious DC offsets within t he
instrum ent ha ve stabilized and can
be effectively removed from subs e-quent measurements.
Once the ins t ru ment h as been
warm ed up, the “Guided Setup”
featur e can be used.
Clean f iber ends a ndinstrumen t test ports
It is a good meas ur ement pr actice
to clean fiber int erfaces before per-
forming calibra tions and m aking
measu remen ts. Clean fiber ends
and connector ports a re essential
for good mea sur ement s. All thr eeinstrum ent ports and a ny fiber ends
(including the reference extension)
should be kept clean. Dirt y connec-
tors can result in spur ious responses
and reduced dynam ic range. To clean
the instrument ports, the connector
adapters are removed from the
instrument front panel, exposing
the cable ferrule.
Caution: Ex treme care shou ld
be taken to avoid damaging the
instrument con nector ferrules .
Dama ged connectors r educe mea-surement integrity and ar e not user
serviceable.
Please r efer t o the connector care
docum ent in the m anu al for connec-
tor cleaning an d care.
Select operat ingwavelength
The sta ndar d configurat ion for th e
HP 8504B includes both 1300 nm an d
1550 nm measurement capability.
When not using Guided Setup, to
select between 1300 or 1550 nm oper-ation, press MENU , [Sour ce Menu],
and either [1300] or [1550].
Select referenceextens ion
The first step in making a m easure-
ment is to select t he appr opriate
length of reference extens ion cable.
In gener al, the r eference extension
cable length s hould be equal to
th e “pigtail” or path length t o the
device to be tested. The length of
the r eference extension m ay be
slightly shorter than the pigtail,
but should not be longer. If the ref-
erence extension is slightly longer
th an the DUT cable, some of the
events to be exam ined may not be
in the 400 mm (air) measurement
span of the instrum ent.
The HP 8504B is designed so
tha t if the r eference extension
length is identical to that of the
DUT fiber, the first response will
appear appr oximat ely one division
to th e right of th e left edge of th e
display (in a 400 mm span). This
means t hat under n ominal condi-
tions, the reference extension cable
should be less tha n 10 millimeter s
equivalent air length (7 mm actual
fiber length) longer th an t he DUT
fiber. Due to the 400 mm allowable
measur ement span , the extensionshould be no more t ha n 360 milli-
meter s (250 mm actua l fiber length)
shorter t han t he DUT fiber. In math -
emat ical term s, this is given by:
–7 mm <L2–L1<250 mm
where: L2 is the DUT fiber length
an d L1 is the r eference extension
length.
The ideal condition is to have t he
two cables be of equal length.
The H P 8504B option 001 cont ains
refer ence exten sion cables of 40, 50,
75, 100, 125, 150, and 175 cm lengths.Optimum reference extension length
depends upon th e path length to
th e DUT.
If the DUT ha s no pigta il, L1 and
L2 are any two cables of equal length.
A condensed summ ar y of the mea-
surement procedure is found in the
front of th is document. In operating
the instrum ent there ar e “har dkeys”an d “softkeys”. Har dkeys ar e th ose
keys whose function is print ed dir-
ectly on t he ph ysical keypad. Th ese
keys ar e noted with bold type such
as PRESET. Softkeys are those tha t
ar e located to the right of the inst ru -
ment display and whose function is
displayed on th e instru ment display.
They are noted in brackets such as
[MKR ZOOM].
There ar e seven steps to perform ing
a measurement.
1 Warm up the instrument
2 Clean fiber ends andinstrument test ports
3 Select operating wavelength
4 Select reference extension
5 Perform a calibration
6 Connect the DUT
7 Optimize the measurement
The HP 8504B ha s a “Guided
Setu p” featur e. Guided Setup is a
powerful user inter face tha t leads
users th rough th e steps required to
make measu rements. Guided Setup
is implemented by pressing SYSTEM,
[Guided Setup]. The inst ru ment willthen display each step required to
setup and calibrate the instrum ent.
The following text describes the
steps u sed in Guided Setup, as well
as some discussion on wh y each
step is perform ed.
Warm-upthe ins trumen t
The HP 8504B is capable of making
measur ements of extremely small
reflections. Even very sma ll spuri-
ous signals within t he instru mentcan degrade dyna mic range. To
ensure ma ximum dynam ic ran ge
and measurement stability, turn on
the H P 8504B and a llow it to warm
up for two hours pr ior to ma king
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Option 001 Cable connection guide
DUT path Ref ext
L2 (cm ) cable s L140<L2<65 40
50<L2<75 50
75<L2<100 75
90<L2<115 40+50
100<L2<125 100
115<L2<140 40+75
125<L2<150 125
140<L2<165 40+100
150<L2<175 150
165<L2<190 40+125
175<L2<200 175
190<L2<215 40+150
200<L2<225 50+150
215<L2<240 40+175225<L2<250 50+175
240<L2<265 40+50+150
250<L2<275 75+175
265<L2<290 40+50+175
275<L2<300 100+175
290<L2<315 40+75+175
300<L2<325 175+125
315<L2<340 40+175+100
325<L2<350 175+150
340<L2<365 40+175+125
350<L2<375 75+100+175
365<L2<390 40+150+175
375<L2<400 50+150+175
Measurementcal ibrat ion
Once the reference extension h as
been selected an d att ached to the
instrument, the instrument must
go thr ough a measurem ent calibra-
tion. The measurement calibration
consists of balancing the polarization-
diversity receiver an d calibra ting
th e instr umen t for a ccur ate r eflec-
tion ma gnitude measurements.
Balance receiver
As light t ra vels th rough single mode
fiber, its polarization characteristics
vary. The m agnitu de of the detector
response is potent ially a function
of the polarization of the reflected
waveform r elative to the light in
the reference arm . Ideally, the detec-
tor r esponse is only a function of
th e reflection ma gnitude. To ensure
that the r eflection m easurement is
insensitive to polarization tr ans for-
mation, a m easurement calibrat ion
process is used. The instr um entreceiver consist s of two photodiodes
which r espond to orthogonal st at es
of polarization. During the Balance
Receiver calibration, the instrument
test port is terminat ed with a high
retu rn loss optical load (great er th an
40 dB) which is supplied with the
instrument. Therefore the light t hat
hits t he two detector diodes is only
from th e r eference mirror. The polar-
ization of this light is adjus ted with
the polarization adjustment knobs
at the instrument front panel in
such a way t hat the r esponses fromeach detector a re equal or balan ced.
Note: Polarization of the light in the
reference path must not be a l tered
once the Balance Receiver calibration
has been perform ed. T he reference
extension cable and polarization
adjustment k nobs mu s t no t be
moved to ensu re optim um perfor-
m ance. If th e reference extension is
m oved, t he receiver will no longer be
balanced an d su bsequent m easure-
m ents m ay be in error.
To perform the Receiver Balance
step, simply follow th e inst ru ctionsgiven by the instrum ent.
Magnitude cal ibration
The magnit ude calibrat ion is a sim-
ple process consisting of measur ing
a kn own reflection. The instr umen t
then automatically scales th e mea-
sur ed response so the t ru e value is
displayed.
For every reference extension cable
supplied in th e HP 8504B option 001,
there is a corr esponding fiber of equal
length which may be used as a cali-bration standar d. The return loss of
th e fiber end (a super P C ferr ule) is
15 dB or 3.16% reflection a t 1300 nm
an d 14.7 dB or 3.37% at 1550 nm .
As you continue with the “Guided
Setup” procedure, the instrum ent
will display the inst ru ctions t o per-
form the m agnitu de calibrat ion. The
HP 8504B scans t hrough the entire
measurement span and determines
the value of the peak response, which
should be the r eflection generat ed
at th e end of the cable. If you u se a
reflection st andar d other tha n aFr esnel reflection, it mu st ha ve a
retur n loss greater tha n 14 dB to
prevent sa tur at ion of th e receiver.
The HP 8504B compares t he mea-
sur ed value to the value ent ered by
th e user. Any differences ar e due to
systematic errors in th e measure-
ment system. This error term is sub-
sequent ly removed from all furth er
measurement s unt il another calibra-
tion is performed or t he instr ument
is preset to the default settings.
As a check, the calibrat ed measu re-ment of the r eflection sta nda rd can
be compared t o the known value.
It is importa nt to note that th ere
is no length calibra tion process an d
consequent ly the H P 8504B does
not make a bsolute length m easure-
ment s directly. It is a comm on mis-
conception t hat the m easurement
calibration process will offset the
position of the reflection sta ndar d
to the 0 length position. Recall tha t
when t he length of the DUT fiber
is identical to that of the r eference
extension, the first event appears
at a bout t he 40 mm point a nd not
0 mm. The instrument has n o know-
ledge of the length of the reference
extension cable used, nor th e length
of fiber t o the device un der t est.
Therefore, all distan ce accur acy is
in ter ms of relative distance to other
reflections in the measurement span.
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Connect and measurethe tes t device
The calibrated instrum ent can n ow
be used t o measur e devices. Once
th e device is connected, it mu st be
locat ed on th e screen . If a device is
connected while th e instru ment is
in a m easurement sweep, a response
ma y be genera ted in th e process of
ma king the connection tha t is not
related t o the a ctual device response.
Consequen tly, it is a good measu re-
ment pr actice to restart the mea-
sur ement as soon as t he device is
atta ched. Press MENU , [Full span],
MEAS , [Meas restar t]. The instru -
ment will then scan over its full
measur ement ran ge and display
all detectable r eflections within a
400 mm (air) span .
Optimizing th emeas urement
For measurement s at 1300 nm, the
reference mirror in the HP 8504B
tr avels at a constan t velocity of
18 mm /sec (21 mm/sec at 1550 nm ).
A 0 to 400 mm sweep will tak e over
22 seconds . The sweep-to-sweep time
can only be reduced by reducing th e
measurement span. Reducing the
measurement span will also increase
the spatia l resolution of multiple
reflections. It is recomm ended th at
once th e DUT responses h ave been
located, to reduce the mea sur ement
span to the narr owest range that
includes th e events of inter est.
Nar rowing of the s pan can be
achieved using th e MKR FCTN
men u a nd [MKR ZOOM] key, or by
using the SPAN , CENTER, STOP ,
and START keys.
Once the measurement span ha s
been optimized to include a ll events
of int erest , the n oise floor can be
reduced through data averaging.
Avera ging redu ces the effects of
ra ndom noise, and can increase th e
measur ement ra nge by 6 or 7 dB.
(Note: the nar rower th e measur e-
ment span , the more effective aver-
aging becomes). Press AVG and
[Avg on]. The default aver aging fac-
tor is 16, meaning that each new
meas urem ent is weighted by a fac-
tor of 1/16 and will contribute this
value to the current m easurement
tr ace. The averaging factor can beset to any integer value between 2
and 999.
Measurement example:connector pair
In t his measur ement example, you
will measur e the r etur n loss of a
simple connector pa ir. In a ddition,
you will see the connector charac-
teristics as it m akes the t ran sition
to a fully torqued connection.
Measurement procedureUsing the same general measure-
ment process described above, select
the reference extension cable. The
device to be measur ed is a simple
conn ector pa ir. One of th e conn ec-
tors is at the en d of a 75 cm pat ch
cord. Since the pat h length t o the
DUT is simply the length of this
pat ch cord, the ideal length for t he
referen ce exten sion is 75 cm. The
reference extension could be as short
as 51 cm, which would place the
reflection at th e end of the m easur e-
ment r ange, but sh ould not be anylonger th an 75 cm.
To calibra te, follow th e Guided S etup
or press CA L, [GUIDED CAL],
atta ch the supplied high retur n loss
terminat ion at the test port, adjust
the polarization adjustment knobs
for a ba lanced display as inst ru cted
by the instrum ent an d press [DONE].
For the magnitude calibration, the
calibrat ion sta nda rd can be one of
the 75 cm cables supplied with th e
instrum ent, or the DUT patchcord
connector end (if the r etur n loss isknown). Connect t he calibrat ion
standa rd to the test port . Pr ess
[FRESNE L 3.16%] when u sing the
supplied cable or [USER STD] and
enter th e value for t he reflection.
Pr ess [MEASURE]. The ana lyzer
will then measure the standar d and
adjust its m easur ement t o coincide
with th e actua l reflection value.The pat ch cord to be tested can
now be connected to the t est port .
Once it is connected, press MENU ,
[FULL SPAN]. The instr um ent will
then bring the reference mirror to
its sta rting position an d begin a
new measur ement t race. The posi-
tion of the mirr or is indicat ed by a
sma ll red dot a t th e bottom of the
instr umen t display. The location of
the dot on th e instru ment display is
proportional to the location of the
mirror on the tran slation st age. For
insta nce if the st ar t position of th emeasurement is set at 0 mm and
th e stop position is set at 100 mm,
the mirror will then be traveling
back and fort h over th e first 25%
of the a vailable mirr or movement .
The m irror position indicator dot
will th en m ove back an d fort h from
th e left edge of th e screen t o a point
25% or 2.5 divisions away from the
left screen edge. The actual dat a
meas ur ed will be displayed across
the ent ire screen.
As the mirr or tra vels across its full400 mm ra nge, the r eflection from
the end of the 75 cm pat ch cord
should be seen at appr oximately
th e 40 mm point. Becaus e the con-
nector is not ter minated, a retu rn
loss of about –15 dB should be seen.
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In this measurement, there are
several r eflections, some which ar e
very small. Through averaging, the
measurement sensitivity can be
increased. Press AVG, [Averaging
On], and let the instru ment ta ke
several sweeps. This allows us to
see reflections th at m ay not have
been visible before. In this mea-
sur ement , a fifth reflection is now
clearly visible.
Measureme nt exam ple: Highreturn loss air-gap co nnec tor
An example of this is a very high
return loss beveled-edge connector.The connector r esponse is easily seen.
For t his pa rt icular connector, you
not only measure th e retur n loss,
by decreasing the measurement
span t o 1 mm, you measu re th e gap
between th e connector ferru les.
Press MKR FCTN , and use the
knob to move the ma rker slightly
to the left of the first event on the
screen. Press [Mkr–> Star t]. Thenuse the kn ob to move the mar ker to
the r ight of the last event on the
display. Press [Mkr–> Stop].
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Applicat ions
The following sections demon-
str at e how th e HP 8504B precision
reflectometer can be used t o mea-
sure a variety of lightwave compo-nents. Although the measu rement
examples are just a small sample
of potential measurements, they
do provide a diverse survey of the
instrum ents capabilities and t he
types of measurements t hat can
be made.
In virt ua lly all cases, th e processes
for performing t he m easurements
are t he sam e as t hose described in
the pr evious sections for measu ring
the ph otodiode assembly and t he
connector pair. When m easur ement
techniques n ot previously intr o-duced are en count ered, these will
be described in appropriat e detail.
Measu ring theref lect ions froman op t ical i solator
Isolators are us ed to minimize the
impact of reflected light on other
components, in particular narrow
linewidth high-speed lasers. Not
only mus t th e isolator perform the
function of att enua ting backreflec-
ted light, it mu st do so without gen-
era ting r eflections of its own.
In gen era l, isolators consist of a
variety of components including
lenses, crysta ls and Fa ra day ele-
ments. Each individual component
ma y genera te a r eflection.
The procedure to measur e the
isolator is similar to tha t u sed for
th e photodiode in t he pr evious s ec-
tion under “Measurement example:
Char acterizing a photodiode assem -
bly.” Once the instr um ent ha s been
configured an d calibrated, t he isolatoris connected an d t he display opti-
mized.
This device ha s several int erfaces,
so there a re severa l reflections. The
measurem ent span is 30 mm. The
largest reflection, generat ed from
the angled fiber end, is approxi-
mat ely 60 dB. Beyond th e Far aday
element, th ere ar e no reflections,
indicatin g that t he isolator is per-
form ing its int ended function.
Characterizingref lect ions w ithina laser assembly
As ment ioned earlier, lightwa ve
source perform an ce can be degradedby backreflected light. Measuring
th e reflections by probing back into
the laser can give us insight into
how light r eflects inter na lly as well
as how back-reflected light ma y be
re-reflected from th e laser.
The d evice sh own below will first
be measur ed according to the pre-
vious procedures for measu ring
components.
This par ticular laser m odule con-
sist s of a pr otective window, a ball
lens, and th e laser chip. During
norma l opera tion of th e laser, lightpropagates from the chip and is
focused and a ligned th rough th e
ball lens before leaving th rough th e
window into a n att ached fiber. Light
tr aveling back into th e laser will
follow a similar pat h.
The resulting measurement shows
the r eflections generated a t each
inter face. The lar gest r eflection is
th e fiber end. Mar ker 1 shows the
reflection from th e front of the win-
dow, 2, the back of the window, 3,
th e front of th e ball lens, an d 4, the
back of the laser chip.
The response between m arker 3
an d 4 indicat es where th e back of
th e ball lens an d front of the laser
chip meet . Zooming in, we will
examine th is region.
LensLaser chip
Window
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Here we see that t here are actu-
ally two reflections since th ere is
a sma ll gap between the lens and
the laser chip. The size of the gapis 43 microns.
Par t of the tran smission pat h of this
device is the semiconductor m ate-
rial of the las er. This ma terial is
more transparent at 1550 nm than
at 1300 nm. By making the mea-
suremen t at 1550 nm (this requires
a new measurement calibration),
the r eflected energy from the end
of th e laser chip experiences less
at tenu at ion an d is then effectively
larger.
Typically, the transparency of
the semiconductor ma terial willvary with t he bias curr ent th rough
th e laser. However, if the laser is
biased above threshold, the trans-
mitt ed energy ma y be sufficient
enough t o satur ate t he r eceiver of
the HP 8504B. Therefore, care
must be taken in setting the laser
curren t t o optimize the t ra deoffs
between material tr ansparency and
instrum ent sat ura tion level.
Characterizingdevices pigtai ledwith mu lt imode f iber
The H P 8504B pr ecision reflectome-
ter measur ement system detects
reflections when the pa th length
to the reflection is identical to the
pat h length to the reference mirror.
Because the light traveling through
mu ltimode fiber will propagate over
man y different pat hs, the measur e-
ment of reflected ener gy thr ough
mu ltimode fiber will be different
tha n if the device were pigtailed
with single mode fiber.
In essence, the r eflection responses
tend to be broadened. An exam pleof this would be to measure t he
Fr esnel reflection at the en d of a
1.25m length of 62.5/125 multi-
mode fiber compared to a similar
measu remen t with 9/125 single
mode fiber.
Two things are appar ent in th e
above plot. Fir st, th e response is
significant ly broadened. The 3 dB
width is about 75 microns, compared
to less than 15 microns for the sin-
gle mode case. Second, the amplitude
of the reflection is reduced. This is
due t o two factors. The t otal energy
of the reflection is distributed over
a wider ran ge, which decrea ses the
peak a mplitude. In a ddition, a sig-
nificant a mount of th e reflected
energy is lost at th e multimode to
single mode core mismat ch at th e
instrum ent test port.
The above meas ur ement is for a
specific length of mu ltimode fiber.
As the length of fiber increas es, so
does the effective pulse sprea ding.In addition, for multimode fiber
lengths m uch beyond one met er,
th e mult imode effect will genera te
multiple r esponses for a single
event.
It is interest ing to examine the
impact that this spreading has on
measur ements. Consider the photo-
diode measu rem ent int roduced on
page 11.
The a bove plot is a composite
meas ur ement of the same device.
One measur ement is with single
mode fiber, while th e other is witha 1.25m length of multimode fiber.
Each reflection is still visible, and
the relative magnitude informa tion
is still valid.
Although t he measur ement capa-
bility of the HP 8504B degrades
when u sing multim ode fiber, the
dynamic range and two-event reso-
lution provide very useful inform a-
tion in locating and identifying
reflections.
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The function of the coupler is to
divide the inpu t power into two
pat hs. In s ome applications, it is
desirable to have the two path
lengths a s closely mat ched as pos-
sible. By monitoring the positions
of the reflections at the coupler out-
put s, we can determ ine the differ-
ential path length.
The measurement is straightfor-
ward. After set up an d calibrat ion,
th e coupler is connected t o the sys-tem. The outpu t ports of the coupler
are left unterm inated, which gener-
at es two large r eflections.
Determining the differential
path length is a simple matt er of
placing a m ar ker on ea ch reflection.
Press MKR FCTN , [Max Search],
[Mkr–>F ixed Mkr], [MKR 2],
MKR FCTN , [Peak search], [Next
highest pea k]. The analyzer will
th en display the one-way distan ce
between mark er 1 and mark er 2.
The measur ed distance value is the
equivalent distan ce tr aveled in air,
which in this case is t he different ial
path length to the t wo output ports.
To determine t he physical path
length differen ce, th e index of refra c-
tion value mu st be entered into the
instr umen t. In the case of glass
fiber, th e index of refraction is 1.46.
Press MEAS , [N VALUE], and ent er
1.46 x1. The displayed distan ce is
now the tr ue one-way physical
length difference.
In order to identify each pat h, we
simply termin ate one of the ports .
As expected, one of the m easur ed
reflections will decrea se in ma gni-tude th us indicat ing which response
coincides with the ter mina ted port.
This pr ocedure can be u sed for
virtua lly any 1xN coupler. The mea-
surement limitations are sensitivity
and two-event r esolution. As t he
power is divided into more and
more paths, th e magnitude of the
reflected ener gy from each outpu t
port will decreas e. However, the
sensitivity of the H P 8504B is such
tha t t he power could be divided into
over 1000 ports and the reflections
still be detectable.
Thus t he HP 8504B is sensitive
enough to measu re virtually any
1xN coupler. Obviously, th e pr acti-
cal measurement limitation then
becomes th e t wo-event resolution.
As the differential length of two
paths get smaller a nd sma ller, the
reflected signa ls displayed by the
instr umen t will eventua lly over-
lap. The two-event r esolution is
25 microns at 1300 nm an d can be
65 microns a t 1550 nm. Ther efore,
differential path lengths as sm allas 25 m icrons (air dista nce) can be
measured.
Precis ion measurementsof di f feren t ial length:1xN coupler
The HP 8504B is capable of making
relative length measu rements with
very high r esolution. This capability
can be used to measu re relat ive dif-
ferences in pat h length. An exam-
ple of this would be to measu re a
simp le 1x2 coupler.
Out 1
Out 2
In
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The differential distance measure-
ments can also be interpreted in a
“time” mode format . This is u sed t o
indicate t he differentia l time of flightor propagation time between events.
Time format can be activated by
pressing FORMAT , [TIME].
Using the t ime form at, we can now
determ ine the effective delay that
one signa l path experiences relat ive
to others. With 25 micron two-event
resolution, pr opagat ion delays as
sma ll as 80 femtoseconds can be
characterized.
Characterizing highreturn loss terminations:index matching gel
Index m atching gel is comm only
used to term inat e an optical fiber
in an a ttem pt to minimize reflected
light genera ted at t he fiber end.
Once the gel has been applied to
the fiber end, the comm on assum p-
tion is tha t virtu ally all of the light
is scattered or “absorbed” and essen-
tially no light is reflected back.
The high sensitivity of the HP 8504B
allows us t o see the r eal perform ance
of ma tching gel. The r esults can be
surprising.
The measur ement is quite simple.The Fr esnel reflection at t he end of
a cable is located and t he instr ument
span is r educed to 1 mm. Then th e
end of th e fiber is dipped in th e
matching gel. Typically, the result
is tha t th e single reflection is reduced
to a retur n loss value better than
50 dB.
However, there ar e cases when t wo
reflections exist, one at th e glass/gel
inter face and a second r eflection at
th e gel/air inter face.
In m ost applications, the gel termi-na tion provides an adequa te fiber
ter mina tion. However, it is not sa fe
to assum e tha t it completely elimi-
na tes ba ckreflections.
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Common ques t ionsand ans wers
The following section deals wit h a
variety of commonly asked questions
about the HP 8504B precision reflect-
ometer a nd th e techniques used tomake m easurements.
Q. Can th e HP 8504B detect fiber
backscatter?
The coherence length of the
HP 8504B sour ce is appr oximat ely
10 to 15 um . The ener gy reflected
from the DUT must h ave been gen-
erated within t his length in order
to be detected. Alth ough ther e will
be backscatt er genera ted over th is
length, the am ount of reflected
energy is t oo sma ll to be detected
by the HP 8504B.Q. Can the HP 8504B measure
fusion sp lices?
The measu rement sensitivity of
HP 8504B allows reflections with
retu rn loss values up t o 80 dB to be
measu red. In general, this is not
sufficient to det ect good fusion
splices.
Q. Th e HP 8504B d isplays 401
m easurem ent points in any m ea-
surement span. Is it possible for an
event to be between th ese points an d
therefore not be detected?The HP 8504B is designed to mak e
a measurement every 2.5 microns
along the measu rement pa th, inde-
pendent of the measurement spa n.
The coheren ce length of the inter-
nal source is 10 to 15 microns. Thus
several points will be measu red
within t his coheren ce length. Con-
sequent ly a r eflection will always
be detected, even if it falls between
th e actua l points of detection.
For wide measurement spa ns,
there are still only 401 data points
displayed, yet measurements arema de every 2.5 microns. For a 10 cm
measur ement span, th ere will be
40,000 measurements made. A
point will be displayed every
250 microns (401 point s over a
10 cm s pan). Thus each displayed
point will represen t th e largestresponse detected over 100 actua l
measurements. This also indicates
why the h ighest two-event resolu-
tion is achieved in th e nar rowest
measurement spans.
Q. Can th e HP 8504B be used to
measure devices pigtailed with
m ultim ode fiber?
The HP 8504B can be us ed to make
measurements in multimode fiber.
However, amplitude accur acy and
spat ial resolution will be degraded.
See “Characterizing devices pig-
tailed with multimode fiber” on
page 14.
Q. What happens wh en devices
with polarization maint aining fiber
(PMF) are measured?
Depending upon how light is
launched int o PMF, it will tr avel at
different velocities. Light traveling
on one principal axis will travel at
a different r ate t han light tr aveling
on the other principal axis. The light
emitt ed from th e HP 8504B sour ce
is only partially polarized, thu s
light will typically be tra veling onboth axes. Thus for a single reflection,
two extra responses may appear.
Q. Will the moving m echanical
components wear out?
The gears a nd motors in t he
HP 8504B are all designed to work
with much heavier loads an d stresses
tha n what is actually required. How-
ever, to avoid un necessa ry wear,
the HP 8504B will aut omatically
go into t rigger hold mode and stop
sweeping whenever t he instr ument
ma kes 500 sweeps (or th e specifiednum ber of averages, whichever is
greater) without the operator press-
ing a front pa nel key. Simply press
MEAS , [Contin uous], to resum e
sweeping.
Q. Wh y are the interferom eter
fringe patterns n ot seen?
An envelope detector is us ed to
smooth out the fringe pattern s, soonly one r esponse with in th e source
coheren ce length is displayed.
Q. Wh at is t he difference between
m easurem ents made with a power
m eter versus the HP 8504B and can
I determine total return loss using
the HP 8504B?
Total retur n loss and spa tially
resolved retu rn loss were discussed
in “Sur vey of ret ur n loss measur e-
men t t echniques” on pa ge 5. Total
retu rn loss is a function of the source
chara cteristics as well as t he r eflec-
tion m agnitudes and their spacing.
The 8504A can yield insight into
the component s of total ret ur n loss,
but n ot give an actua l value.
Q. What effects do connectors or
other losses ha ve when placed in
front of the DUT ?
Losses will reduce the m easu re-
ment sensitivity of the HP 8504. If
these losses are n ot included in the
ma gnitude calibration, they will
also decrease the m easured value
of reflections.
Q. What h appens when I m easure a
narrowband device such as a W DM
filter?
The high spat ial resolution of the
HP 8504B is based upon the wide
spectral width and subsequent
short coher ence length of its LED
sour ce. If a t est d evice’s res ponse
varies versus wavelength over th e
spectrum of the source, amplitude
accur acy and spat ial resolut ion
ma y be degraded.
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18
Unders tandingMeasurement Accuracy
There ar e severa l element s which
affect t he a ccura cy of any m easur e-
ment m ade with the HP 8504B
Pr ecision Reflectometer. In general,various contribut ions to measu re-
ment error include systematic
errors tha t are inherent in the
Pr ecision Reflectometer measu re-
ment technique and i tems tha t
degrade measurement accuracy,
yet can be minimized by the user
thr ough good m easurement tech-
niques.
Measurement errors can also be
classified into those that affect
amplitude measur ement a ccuracy
an d those th at affect positional or
spat ial a ccur acy.
Amplitudeaccuracy i s sues
The uncertainty in th e HP 8504B
measur ement of retur n loss magni-
tu de comes from s everal different
factors including:
• Dynamic accuracy
• Connector repeatability
• Transmission path losses
• Measurement flatness vs. position
• Sweep-to-sweep repeatability
• Polarization sensitivity
• Calibration accuracy
• Chromatic dispersion effects
• Source spectral width
Dynamic accuracy
Dynamic accur acy refers to the
ability of the HP 8504B to accu-
rat ely measur e over a wide ran ge
of retu rn loss values. The HP 8504B
is typically calibrat ed at the level
of a F resn el reflection. Ideally the
response of the r eceiver block in th e
HP 8504B will be linear versus returnloss. However, as the reflected sig-
nals from t he DUT become very
sma ll, th e response from t he detec-
tor can deviate from th is ideal lin-
ear relat ionsh ip. Optimizat ion of
the receiver has minimized this
effect. Th e r esulting contribut ion
to amplitude measur ement a ccuracy
is dependent upon th e reflection
ma gnitude. It is typically less tha n
±1.5 dB, but degrades a s th e reflec-
tion magnitu de approaches 80 dB.
Refer t o the cur ves in th e specifica-tion tables of the H P 8504B operat -
ing man ual for detailed informat ion.
For mea sur ement s of sma ll reflec-
tions, dynamic accur acy can be a
significant uncertainty term.
Connector repeatabil i ty
There will always be some insert ion
loss in a ny fiber-optic connection.
When the HP 8504B ma gnitude cal-
ibrat ion is executed, th e loss at the
test port connection is effectively
removed from the measurement of
the calibration stan dar d. However,
when t he DUT is connected to th e
test port, the inser tion loss of this
connection may be different tha n
the connection made during calibra-
tion. If the insert ion loss is higher
by 0.5 dB, subsequent ret ur n loss
meas urem ents will be twice this or
1 dB larger th an t he actual value.
This is because both t he stimu lus
signal an d th e reflected signal will
experience a 0.5 dB atten uat ion.
Similar ly, when t he DUT connection
to the test port is better tha n theconnection at calibrat ion, ret urn
loss values will be worse t han actual
by two times the insertion loss
improvement.
It is th erefore importa nt that proper
connector care and usage be observed
to min imize th e effects of conn ector
repeatability.
Transmiss ion path losses
Transmission path losses in the
DUT affect m easurements in a way
similar to connector ins ertion loss
repeatability.
Large r eflections can degra de the
meas urem ent of other r eflections
furth er int o the DUT. An exam ple
would be if a simple Fr esnel air ga p
existed in front of other reflections.
3.5% of th e ener gy is reflected a t
the glass-to-air interface, 96.5%
tr avels to the air-to-glass int erface
where again 3.5% of th e ener gy is
reflected. The net r esult is tha tonly 93% of the stimulus signal
(an 0.3 dB loss) reaches reflections
beyond t he a ir gap. Sim ilarly,
only 93% of th e reflected ener gy
will reach the instrument detector.
The resu lt is similar t o ha ving an
0.3 dB lossy splice, which would
lead to 0.6 dB measur ement err ors.
Measurementaccu racy vs. posit ion
A port ion of the int erna l light
path to the reference mirror in t he
HP 8504B is an open beam en viron-ment . As the r eference mirror is
moved and th e length of th e open
beam pa th is increased, ther e will
be some beam divergence. This beam
divergence can resu lt in power var i-
ation in the reference beam at the
detector. This phenomenon is very
repeatable. Consequently, most of
th is effect is removed from th e mea-
suremen t a s par t of the factory cali-
bra tion process.
The HP 8504B detection scheme
requires that the r eference mirrorideally moves at a constant velocity.
However, ther e will be some velocity
jitter.
The err or due to the combination
of the above two err or sources is
appr oximat ely ±1 dB.
Sweep-to-sweeprepeatabil i ty
Sweep-to-sweep repeat ability is the
sweep-to-sweep am plitude va riat ion
seen when measur ing a known stable
reflection. This err or source is also
relat ed to the mechanical movement
of the int erferometer m irror. It does
not include t he effects of noise when
measur ing reflections nea r t he instru -
ment noise noise floor. It is specified
to be less th an ±0.5 dB.
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Polarization sensit ivity
The stimulus signal from t he
HP 8504B is ra ndomly or only par-
tially polarized. As th e light tr avelsto and from th e DUT, its sta te of
polar ization will vary. Alth ough th e
HP 8504B receiver is designed to be
insensitive to the polarization char-
acterist ics of the r eflected light,
th ere will be some un certaint y in
amplitude measur ement accura cy
due t o polarizat ion, even if the source
stimulus is ra ndomly polarized. This
uncert aint y is typically less th an
±0.75 dB.
Calibration unce rtainty
When a ma gnitude calibra tion is
perform ed with t he HP 8504B, the
instrum ent uses the known return
loss value of a calibrat ion st an dar d
to remove man y system at ic err ors
in subsequent measu rements. The
cables supplied with t he HP 8504B
can be used as a reflection sta ndar d.
The retur n loss value at the open
end is consist ent with in ±0.1 dB. This
uncertainty in the a ctual retur n
loss of th e calibra tion sta nda rd will
result in uncertainty in DUT mea-
sur ement s of appr oximat ely ±0.1 dB.
To mainta in th is low uncerta inty,
the calibration stan dar ds (typically
the end of a fiber pa tch cord) mu st
be clean and undama ged.
Chromatic dispersion effects
Par t of the propagation path of the
reference signal is in a n open beam.
In most cases, light traveling in the
DUT pat h is all in fiber. When m ak-
ing measur ements at 1550 nm, the
amount of chr omatic dispersion
experienced by the light t ra veling
in th e reference path will be less
than that in the DUT path. This
mismatch in dispersion results in
a broadening and su bsequent drop
in th e peak value of the displayedreflection “impu lse”.
The peak value will decreas e mono-
tonically as a function of the length
of dispersion mismatch. This effect
is consistent and has been corrected
out by the H P 8504B. The instrument
assumes a dispersion coefficient of
17 ps/(nm*km). The result of this
corr ection leaves a r esidual err or
on th e order of ±0.3 dB.
The pr oblem becomes d ifficult when
the pa th t o the DUT is both in fiber
and a n open beam. The effects a rethen very difficult to remove from
the measur ement, and subsequent
uncert aint ies due to chromatic dis-
persion can approach 5 dB. The user
has the option of disabling the inter -
nal dispersion correction to facilitate
his own correction meth ods.
Effects of sourcespectral width
The spectral width of the HP 8504B
source is approximately 55 nm.
Another uncertaint y component will
exist if th e DUT r eflection charac-
teristics vary over th is spectral ra nge.
The level of uncerta inty is dependent
on the DUT characteristics.
Bibl iography
S. Newton, “Technology tren ds in
optical reflectom etry”, Photonics
Spectr a, November 1991, pp. 118-126
H. Booster, H . Chou, “High er R eso-
ution for Backscatter Measurements”,
Laser s an d Optr onics, October 1991,
pp. 27-30
Pos i t ionalAccuracy Issue s
The a ccur acy of the H P 8504B in
determ ining the r elative location
of reflections is based on its ability
to cont rol and monitor th e position
an d velocity of the reference mirr or.
This uncert aint y is less tha n 2% of
the measu rement spa n. To have the
highest a ccuracy, the nar rowest span
th at includes th e two events of
interest should be used.
Summary
In genera l, the individual error
components a re un corr elated. The
total measurement u ncerta inty is
determ ined with an "RSS" (Root
Sum Square) analysis, and n ot a
linear su mma tion.
Return loss (dB)
14.614.6514.714.7514.814.8514.914.95
6
5
4
3
2
1 R e l a t i v e o c c u r a n c e s
λ =1550 nmAvg =14.73 dBStd =±0.085 dB
Calibration Standard Repeatability
Return loss (dB)
14.8514.914.9515.015.0515.115.1515.2
6
5
4
3
2
1 R e l a t i v e o c c u r a n c e s
λ =1300 nmAvg =15.003 dBStd =±0.065 dB
19
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For more information, callyour local HP sale s officel i sted in your te lephonedirectory or an HP regional
office l isted below for thelocation of your nearest salesoffice.
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(305) 267 4245/4220
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