An experimental study was performed to determine the effects of vibration on the propagation of light through SMA- and ST-type fiber-optic connectors. A multimode, fiber-optic link was vibrated from 0 to 10,000 Hz at a constant peak acceleration along the connector transverse and longitudinal axes. All other environmental parameters were ambient. Transfer characteristics through the connection were examined as a function of vibrational frequency using both laser and light-emitting diode (LED) light to illuminate the system. Slight differences in operation between the SMA and ST connectors were observed with no appreciative attenuation as a result of vibration. Vibration did cause the constant-amplitude input light to be modulated in the connector; however, the amplitude of vibration-induced noise was less than 3 standard deviations from the mean

Madzsar, George C.

Sovie, Amy L.

English

NASA Technical Reports Server

NASA Technical Memorandum 1036_54
Vibrational Testing of Optical
Fiber Connector Joints
Amy L. Sovie and George C. Madzsar
Lewis Research Center
Cleveland, Ohio
Prepared for the
Second Annual Conference on Health Monitoring
for Space Propulsion Systems
sponsored by the University of Cincinnati
Cincinnati, Ohio, November 14-15, 1990
NASA
"-(NA_A-1"M-103654) VI_I_,ATIONAL T_STI,NG UF
OPTICAL FIBER CONNECTOR JnlNTS (NASA) I0 p
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VIBRATIONAL TESTING OF OPTICAL FIBER CONNECTOR JOINTS
Amy L. Sovie and George C. Madzsar
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
SUMMARY
An experimental study was performed to determine the effects of vibration
on the propagation of light through SMA- and ST-type fiber-optic connectors.
A multimode, fiber-optic link was vibrated from 0 to 10,000 Hz at a constant
peak acceleration along the connector transverse and longitudinal axes. All
other environmental parameters were ambient. Transfer characteristics through
the connection were examined as a function of vibrational frequency using both
laser and LED light to illuminate the system. Slight differences in operation
between the SMA and ST connectors were observed with no appreciative attenua-
tion as a result of vibration. Vibration did cause the constant-amplitude
input light to be modulated in the connector; however, the amplitude of
vibration-induced noise was less than 3 standard deviations from the mean.
INTRODUCTION
Interest is increasing in the use of fiber-optic sensors and harnesses
for rocket engine condition monitoring. Because of this interest, there is a
need to better understand the behavior of fiber systems in harsh environments.
As a result, studies are being conducted at the Lewis Research Center to
determine which fiber-optic technologies are feasible for incorporation in
rocket engines in the near term and which need further development. Using the
space shuttle main engine requirements to typify environmental conditions, we
anticipated that vibration would be particularly perturbing to the light trans-
fer characteristics through a fiber-optic system. Therefore, tests were
performed to qualitatively determine the effects of vibration on optical
throughput in a fiber-optic connector.
BACKGROUND
Vibration can induce several changes in an optical signal passing through
a fiber-optic connector. One such change is attenuation caused by axial or
transverse misalignment of the fibers. Connector feedthroughs (such as the
S/_- and ST-types tested in these experiments) mechanically hold two fibers
face-to-face, typically passing the signal with a 1-dB loss (ref. 1). Vibra-
tion may cause the fibers or connectors to move with respect to each other
within the feedthrough, thereby inducing increased losses due to misalignment.
This misalignment will cause less light to couple through the connector.
Since the misalignments occur at the frequencies of vibration, the detection
circuitry will see the frequency content.
Vibration may also cause signal attenuation due to microbending of the
fiber at the fiber-connector interface. Microbend losses occur when the fiber
is bent around a radius smaller than its "minimum bend radius," allowing the
propagating light waves to become radiating waves which escape the fiber clad-
ding. Such bending of the fiber not only introduces a loss mechanism but weak-
ens the fiber and may cause damage. Microbending is most probable at points in
a fiber-optic system where the fibers are attached to heavy components such as
a controIler or bulk-optic sensor.
TEST SETUP
Figure 1 shows the experimental setup for the vibration tests. Light
from an optical source illuminates the fiber and passes through the vibrating
fiber-optic connector interface to the second fiber. The intensity of this
light arriving at the detector is converted to a voltage and then amplified.
Signal attenuation is determined statistically from a time domain signal, and
the frequency components are determined with a digital spectrum analyzer.
Because of the significant differences in their mechanical structures,
both SMA- and ST-type connectors were tested. Both types are commercially
available and commonly used. SMA connectors fasten by a threaded screw mechan-
ism and ST connectors use a spring lock similar to coaxial cable connectors.
Schematic drawings of an ST and an SMA connector are shown in figure 2.
Illumination sources used include a laser beam from a single frequency
helium-neon laser and broadband light from a light-emitting diode (LED). The
laser was used since many sensing techniques, such as interferometry, require
a coherent source. The LED's may be used for sensors, such as intensity-
modulated sensors, that do not require a coherent source. Separate tests were
performed using each type of light source.
The connector feedthrough was attached to a mechanical shaker, which was
driven by a 0 to 10 kHz random-frequency generator. The shaker provided for
both horizontal and vertical mounting of the connector and an accelerometer to
measure the vibration.
The equipment used in these experiments were a Hewlett Packard model
HP3582A digital spectrum analyzer, a Hewlett Packard model HP54111D digital
storage oscilloscope, and a Hewlett Packard series 9000 model 360 computer.
In the 0 to 10 kHz frequency range, the spectrum analyzer had a resolution of
40 Hz. For signal average and standard deviation calculations, 8000 samples
were included in the population, sampled at a rate of 25,000 samples per
second.
TEST PRO(3ED_
Initial testing to verify the test setup was performed by disconnecting
the fiber-optic connector from the shaker, applying a random voltage to the
shaker, and monitoring the detector outputs. This confirmed that vibration
did not affect the light input or the fibers themselves and that effects occur-
ring with the shaker connected to the system were indeed due to vibration
effects in the connectors.
Tests were repeated several times to insure that connect-disconnect dis-
continuities did not cause significant changes in the experimental results.
No measurable differences were found.
To measure vibration-induced attenuation in the connector, a constant-
amplitude light signal was applied to the system and the detector output was
monitored and digitized by a digital storage oscilloscope. First, the average
light intensity and its corresponding standard deviation were calculated with-
out vibration being applied to the connector. The connector was subjected to
random-frequency, constant-force vibrations, and the detector output was com-
pared to the nominal average and standard deviations.
When the detector output was examined with a spectrum analyzer, modula-
tions of the signal at frequencies corresponding to the excitation frequencies
were observed. Measurements of light intensity versus frequency were recorded
without vibration applied to the system. These were then compared to the meas-
urements of intensity versus frequency during vibration, from which light
throughput as a function of vibration could be identified.
Both connectors were evaluated with light from an LED under longitudinal
and transverse vibration. Only the SMA connector was tested with laser
illumination.
RESULTS AND CONCLUSIONS
Figures 3 to 8 display the amplitude of light passing through the connec-
tor as a function of signal frequency. For the six test configurations, the
dashed lines indicate optical throughput without vibration, and the solid
lines indicate optical throughput with vibration. Figure 9 displays three
plots: each plot corresponds to light amplitude versus frequency at different
vibrational amplitudes using laser light and an SMA connector vibrating in the
longitudinal direction. Figures 10 and 11 display the time domain signal dur-
ing vibration for this setup, with lines corresponding to the signal average
and three standard deviations from the average calculated from the detector
output during no vibration. All tests were repeated several times to verify
the data with no significant variations. Vibrational effects on laser and LEB
systems with the two types of fiber-optic connections are now summarized,
including the dependence of these results on axis of vibration.
Axis of Vibration
Figures 7 and 8 show that transverse and longitudinal vibration of the
feedthrough produced similar results in the coherent laser system. The LED
systems in figures 3 to 6 showed sensitivity related to the axis of vibration.
SMA connectors exhibited modulation in the lower two-thirds of the frequency
range of testing for vibrations occurring along both the longitudinal and
transverse axes. ST connectors showed no effects of vibration past 2 kHz along
the connector's transverse axis; however, they showed significant modulation at
all frequencies tested when vibration occurred along the longitudinal axis.
Modulation: LED System
The digital spectrum analyzer showed that the LED signal was being modu-
lated at low frequencies but not at all frequencies of vibration. Broadband
systems have been shown to average out affects which appear in single-mode sys-
tems because of the wide range of propagating modes and velocities (ref. 2).
Therefore, it was expected that this system would be less affected by vibration
than by the laser system. Figures 3 to 6 show that vibration-induced modula-
tion is damped for certain frequencies and exhibits different characteristics
for the different types of connectors tested.
It is important to note that this modulation does not occur over the
entire range of frequencies tested, except through the ST connector under lon-
gitudinal vibration (fig. 6). Figure 6 suggests that the ST connector's
spring is not responding to the vibrations and could be damaged.
Modulation: Laser System
Modulation of the laser light inside the connectors did occur at all fre-
quencies of vibration. This modulation is shown in figures 7 and 8 by the
approximate 25-decibel greater throughput over the frequencies when the shaker
was activated. Peaks occurring in the plots are most likely due to the reso-
nant frequencies of the mechanical setup. Therefore, the laser system is
equally sensitive to vibrations at all frequencies, up to 10,000 kHz and
20 g's acceleration.
Level of Vibration
The frequency distribution of a fiber link under different levels of
vibration is plotted in figure 9. For the levels of vibration tested, the
amplitude of oscillation (frequency is held constant} did not have a signifi-
cant effect on signal modulation. At very small amplitudes, enough of the sig-
nal is chopped to cause a constant-amplitude signal to exhibit oscillatory
behavior, which will be seen by the detector circuitry as noise.
Signal-to-Noise Ratio
As a signal is modulated, the added frequency content can be seen by the
detection circuitry as noise. This noise becomes significant to the system if
the peak-to-peak amplitudes are greater than those of the original signal.
Figure 10 shows that the noise bandwidth with vibration applied to the system
is not greater than the nominal bandwidth for the broadband system. The laser
system in figure 11 shows a noise bandwidth exceeding the 3-sigma band, with a
corresponding increase in signal-to-noise ratio of 6 percent.
Attenuation
The calculated mean intensities in figures 10 and 11 show that there was
no significant attenuation of the system's total power output due to vibration.
The LED system in figure 10 had means for the nonvibrating and vibrating
setups, respectively, of 0.7813 and 0.7811V. The laser system means were
0.6908 V under no vibration and 0.6880 V with vibration.
It is therefore concluded that, at the levels of vibration tested in this
experiment, no appreciable change in the total power output level occurs as a
result of vibrations.
CONCLUD ING REMARKS
The tests performed show that vibration has an effect on the frequency
content of light propagating through a fiber-optic link. The single-mode
laser light exhibited modulation over the range of frequencies tested, while
broadband light from an LED exhibited different levels of modulation corre-
sponding to different vibrational frequencies. For sensors such as interferom-
eters and multiplexing schemes such as heterodyning, this behavior must be
studied carefully before application of such sensors and multiplexing schemes
to a vibrating environment may be considered practical (ref. 3). These
frequency-dependent effects must also be noted when optimizing the amount of
signal processing and active optical feedback required in a fiber-optic sensor
bus.
Though anticipated, no significant amounts of total power attenuation due
to microbending at the connector interface were found. Although intensity-mod-
ulated sensors will experience a reduction in signal-to-noise ratio as a
result of the oscillatory behavior, they will not be limited by severe reduc-
tions in total power throughput.
Neither of the two commonly used connectors tested could eliminate modula-
tion due to vibration along both axes. Further experimentation needs to be
performed with a statistically valid sample of connector types and quantities.
Other mechanical configurations are available and should be studied for resist-
ance to vibrational effects.
In order to determine the applicability of fiber optics to rocket engines,
many more environmental parameters need to be considered (ref. 4). Shock
levels up to 40 g's, temperature transients from cryogenic to hot gas tempera-
tures, and a harsh chemical environment are all expected to impose limits on
the fiber-optic technologies that can be applied to rocket engines.
REFERENCES
I. Cherin, A.H.: An Introduction to Optical Fibers. McGraw-Hill, 1983.
2. Personick, S.D.: Fiber Optics. Plenum Press, 1985.
3. Davis, C., et al.: Fiberoptic Sensor Technology Handbook. Optical Tech-
nologies Inc., McLean, VA, 1982.
4. Delcher, R., et al.: Determination of the Durability of the Fiberoptics
in Rocket Engine Environments. AIAA Paper 90-2229, July 1990.
I CONNECTOR AND DIGITAL I F
LIGHT | FEEDTHROUGH,7 SPECTRUM I I COMI
SOURCE _ ANALYZER I /
"-_OP'TI'CAL,--I'-V' - L'--"_?"_Z""SHAKERI _.2 _
FIBER I _ | ' "
T I OPTICAL DETECTION
I IRANDOMNOISE I
tC'RCUITRY
OSC ILLOSCOPE
FIGURE I. - SCHEMATIC OF THE TEST SETUP.
ST FEEDTHROUGH ST CONNECTOR
FEEDTHROUGH _ CONNECTOR
FIGURE 2. - SIDE VIEW SCHEMATfCS OF ST AND SMA CONNECTORS AND
FEEDTHROUGHS.
-80
-88
-104
-112
-120
-128
m
VIBRATION
WITH
..... WITHOUT
1 2 3 4 S 6 7 8 9 10
FREQUENCY, KHz
FIGURE 3. - AMPLITUDE VERSUS FREQUENCY OF THE DETECTOR
OUTPUT WHEN LIGHT FROM AN LED IS SENT THROUGH AN SMA
CONNECTOR UNDER TRANSVERSE VIBRATIONS.
-84 --
-92
• -100
i..-
- 108
-116
-124
VIBRATION
WITH
WITHOUT
1 I I I 1 I I l I I
1 2 3 q 5 6 7 8 9 10
FREQUENCY, KHz
FIGURE 4. - _PLITUDE VERSUS FREQUENCY OF THE DETECTOR
OUTPUT WHEN LIGHT FR_ AN LED IS SENT THROUGH AN
CONNECTOR UNDER LONGITUDINAL VIBRATIONS.
ORiGiNAL PAGE IS
OF POOR QUALrrY
6
-96 -- VIBRATION
-104 -- WITH
WITHOUT
-128 -- ..,....... . .
-136 I
0 _ 2 3 q 5 6 7 8 9 10
FREQUENCY,KHz
FIGURE 5. - AMPLITUDE VERSUSFREOUENCYOF THE DETECTOR
OUTPUTWHENLIGHT FRO_ AN LED IS SENT THROUGHAN ST
CONNECTOR UNDER TRANSVERSE VIBRATIONS.
-112
_=
- 120
-48
-56
.... -64
-72
_ -80
-88
-96
-I04
-112
-120
VIBRATION
. WITH
_ ..... WITHOUT
• • ,
,,--°,o.,'.,_
l lltllllll
I 2 3 q 5 6 7 8 9 10
FREQUENCY, KHz
FIGURE 6. - AMPLITUDE VERSUS FREQUENCY OF THE DETECTOR
OUTPUT WHEN LIGHT FROM AN LED IS SENT THROUGH AN ST
CONNECTOR UNDER LONGITUDINAL VIBRATIONS.
-lOq
-t12
-120
-56 T VIBRATIONWITH
-64_i ..... WITHOUT
_
-88 _ • :
_% -- "._,;...
"%,"..
• ','.I "'",- °o ",
," °,
- I I I I 1 l [ 1 I I
I 2 3 q 5 6 7 8 9 10
FREQUENCY, KHz
FIGURE 7. - AMPLITUDE VERSUS FREODENCY OF THE DETECTOR
OUTPUT WHEN LASER LIGHT IS SENT THROUGH AN _ CONNECTOR
UNDER TRANSVERSE VIBRATIONS.
-q8 VIBRATION
_- WITH
-56 .....
-64_72
-30 _,
-96
-I04
-112
- 120
B •
""',l"'., ,. , ,.,°, °"
o, - , ,,.,,,, ; ,
m
1 I l I I I I I l J
I 2 3 q S 6 7 8 9 10
FREQUENCY, KHz
FIGURE 8. - AMPLITUDE VERSUS FREQUENCY OF THE DETECTOR
OUTPUT WHEN LASER LIGHT IS SENT THROUGH AN _ CONNECTOR
UNDER LONGITUDINAL VIBRATIONS.
ORIGINAL PAGE IS
OF POOR QUALITY
-48
-56
-64
-72
-80
_ -88
-96
-10q
-112 --
-120
0
VIBRATION
HIGH
-I ..... LOW
----- NO
_,, ,e*-t ,
".. " ," ,- .,,., ."• ,. , -,• . • "
..]k___*° " " _%°"°° _° "'°;" •
_ --""'x.-,,_,,_,__r.,,,,,,.,¢.,
I I I t l t 1 I I t
I 2 3 q 5 6 7 8 9 I0
FREQUENCY, KHz
FIGURE 9. - N4PLITUDE VERSUS FREQUENCY FOR LASER LIGHT
THROUGH AN SMA CONNECTOR AT NO, LOW, AND HIGH LEVELS
OF VIBRATIONAL FORCE. THE FREQUENCIES OF VIBRATION
WERE HELD CONSTANT•
.789 --
.787
.785
> .78]
_ .78T
.779
.777
.775
.77_ I I I I
_ .695
:>
•690
,_ .685
•680
I 1 l ._7s
.04 .08 .12 .IG .20 .2_ .28 .32
TIME, sec
FIGURE 10. - AMPLITUDE VERSUS TIME OF THE DETECTOR OUT-
PUT FOR A VIBRATED LED SIGNAL• IIORIZONTALLINES COR-
RESPOND TO TIIEMEAN AND ONE-, 1340-AND THREE-SIGRA BANDS
FOR AN UNVIBRAIED SIGNAl.•
i
•700 --
I I I I I I I 11
0 .04 .08 .12 .IG .20 .2h .28 .32
TitlE,SEC
FIGURE 11. - /V4PLITUDEVERSUS TIME OF TIIEDETECIOR OUTPUT
FOR A VIBRATED LASER SIGNAL• IIORIZONIALLINES CORRESPOND
TO THE MEAN AND ONE-, TWO AND THREE-SIGMA BANDS FOR AN
UNVIBRATED SIGNAL.
ORIGINAL PAGE IS
OF POOR QUALrP¢
Report Documentation PageNational Aeronauticsand
Space Administration
1. Report No.
NASA TM- 103654
2. Government Accession No.
4. Title and Subtitle
Vibrational Testing of Optical Fiber Connector Joints
7. Author(s)
Amy L. Sovie and George C. Madzsar
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
3. Recipient's Catalog No.
.....5. Report Date
6, Performing Organization Code
8. Performing Organization Report No.
E-5845
10. Work Unit No.
553-13-00
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the Second Annual Conference on Health Monitoring for Space Propulsion Systems sponsored by
the University of Cincinnati, Cincinnati, Ohio, November 14-15, 1990.
16. Abstract
An experimental study was performed to determine the effects of vibration on the propagation of light through
SMA- and ST-type fiber-optic connectors. A multimode, fiber-optic link was vibrated from 0 to 10,000 Hz at a
constant peak acceleration along the connector transverse and longitudinal axes. All other environmental parameters
were ambient. Transfer characteristics through the connection were examined as a function of vibrational frequency
using both laser and LED light to illuminate the system. Slight differences in operation between the SMA and ST
connectors were observed with no appreciative attenuation as a result of vibration. Vibration did cause the
constant-amplitude input light to be modulated in the connector; however, the amplitude of vibration-induced
noise was less than 3 standard deviations from the mean.
17. Key Words (Suggested by Author(s))
Fiber optics; Vibration; Rocket engines; Connectors;
SMA; ST
18. Distribution Statement
Unclassified - Unlimited
Subject Category 74
19. Security Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
21. No. of pages
10
22. Price"
A02
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Vibrational testing of optical fiber connector joints

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