IEEE Transactions on Nuclear Science, Vol. NS-26, No. 6, December 1979
EFFECTS OF LOW-DOSE-RATE RADIATION ON OPTO-ELECTRONIC COMPONENTSAND THE CONSEQUENCES UPON FIBER OPTIC DATA LINK PERFORMANCE*
W. H. Hardwickt and A. H. KalmattIRT Corporation, San Diego, California 92138
ABSTRACT
Tests have been performed to examine the effects of low-dose-rate radiation exposure on opto-electronic compo-nents that can be used in the implementation of 20-Mbpsdigital fiber optic data links. Three important effectswere studied:
1. Exposure of PIN photodetectors to a flux of ion-izing particles produces current pulses whichwould cause false events or increased rms noisein the link.
2. The accumulation of total dose by the fibersproduces increased attenuation. When exposed ata low dose rate for a period of time, a compli-cated recovery process occurs at the same timeas the damage production. The result is thatthe amount of attenuation produced depends onthe dose rate.
3. Exposure of GaAQAs light-emitting diodes to aproton fluence causes an unexpectedly large andpermanent degradation in light output.
To determine the consequences of these effects on datalink performance, a link was designed for spaceborneapplications and the failure thresholds examined.
INTRODUCTION
Fiber optics is a very important technology, especiallyin the area of data communication. As a result, variousaspects of the technology are receiving attention, in-cluding radiation-testing of the components. Much ofthe testing has examined the effects of short pulses ofionization, ionization total dose, or neutron fluence.There have been some studies of low-dose-rate effectssuch as ones that would be produced by a space electronor proton environment. However, all aspects of low-dose-rate degradation mechanisms have not been examined.This paper reports on low-dose-rate effects upon light-emitting diodes (LEDs), fiber optic cables, and PINphotodetectors. The results discussed deal with pre-viously unreported areas or effects that differ somewhatfrom ones reported previously. The data to be presentedwere gathered as portions of a program whose overallobjective was to determine radiation susceptibilities offiber optic data links designed using commerciallyavailable opto-electronic components chosen for theirexpected radiation tolerance. Therefore, consequencesof the measured radiation effects in the components on
data link performance will also be discussed.
EXPERIMENTAL DETAILS
The opto-electronic components chosen for testing were
selected from commercially available components. Theselection criterion was a combination of projected radi-ation hardness and ability to meet non-radiationrequirements of the applications.The LED selected was the Bell Northern BNR-40-3-15-3GaAQAs device. Both "pigtailed" and "unpigtailed" ver-
sions were tested to separate effects in the semiconduc-tor devices and those associated with the pigtailing.The pigtails used were CG-02-8 fibers, manufactured byITT Electro-Optics. They were graded index, were all-glass fiber, and were chosen primarily because of theiravailability within a reasonable time.
*Work supported by AFWL under contract F29601-77-C-0106.
tCurrent address: JAYCOR, Del Mar, California.ttCurrent address: Northrop Research & Technology Cen-
ter, Palos Verdes Peninsula, California.
Two fiber cables were chosen for testing. Both weresingle-fiber cables with a fused silica core which wasselected for radiation hardness. One cable was the S-10-PS-1 manufactured by ITT Electro-Optics. The core inthis cable had a polymer clad, and thus, the fiber wasexpected to have as high a radiation tolerance as anyfiber then available. The second cable was the HT1/SA7-90 manufactured by Times Fiber Corporation. The core inthis cable had a borosilicate clad, and the cable wasselected because it would meet military temperaturerequirements (-54 to +95°C) for some applications whileretaining a reasonable amount of radiation tolerance.
The PIN photodetector selected was the 5082-4207 manu-factured by Hewlett-Packard. This device was chosenbecause it possessed the smallest ionization-sensitivevolume while still being optically compatible with thefibers. The glass windows in the device packages werealso tested separately so that the effects produced inthem could be separated from the semiconductor deviceeffects.
The radiation results to be reported are from electron,proton, or gamma exposures. The electron irradiationswere performed at the RADC Dynamitron using I-MeV elec-trons; the dose rate ranged from 25 to 23 x 103 rad(Si)/sec. The proton irradiations were performed at the U.C.Davis isochronous cyclotron using 30-tIeV protons; thedose rate ranged from 50 to 400 rad(Si)/sec. The gammairradiations were performed using the IRT 60Co source;the dose rate ranged from 0.1 to 10 rad(Si)/sec. Mea-surements on the fibers were made both during and afterexposure in all cases. Terminated 10-m lengths of cablewere used for testing, with no attempt made to mode-strip the light input. The attenuation was measured at907 nm using a LED source. Various lengths of cable,ranging from 1 to 10 m, were exposed. The PIN photo-detectors were measured both during and after the gammaand electron irradiations, but only after the protonirradiations. The LEDs were measured only after expo-sure in all cases.
EFFECTS IN FIBERS
There have been quite a few studies of effects in fiberoptics (Refs. 1-12). Some of the recent studies (Refs.9-11) have examined low-dose-rate effects with measure-ments made during exposure. One of the conclusions ofthese papers is that there is no dose-rate dependence ofthe radiation-induced attenuation except for the effectof annealing during irradiation. However, they reportedusing only one dose rate ("300 rad(Si)/sec) for steady-state exposures when doses greater than 100 rad(Si) wereaccumulated. Another finding of these studies was thatthe radiation-induced attenuation in the polymer-cladfibers tended to vaturate and perhaps even decrease at atotal dose of "10 rad(Si). The non-polymer-cladfibers, on the other hand, showed no such effect, andthe radiation-induced attenuation continued to increaseto total doses approaching 106 rad(Si).In the current study, the region above 100 rad(Si) andup to 107 rad(Si) was examined using varying dose ratesranging from 20 to 2.3 x 103 rad(Si)/sec. Severaleffects not previously reported were found. The resultswere independent of irradiating particle if the irradia-tion is expressed in terms of rads.
For dose rates between 20 and 2300 rad(Si)/sec, theradiation-induced attenuation was found to behave as
shown in Figures 1 and 2. The attenuation increasesuntil a peak occurs at a few thousand rads, and thendecreases as shown in Figure 1. The attenuation in
these figures is normalized such that the value of theinitial peak is 1.0. The amount of attenuation in thepeak (or at any other dose, for that matter) depends ondose rate; however, the shape of the curve is indepen-dent of dose rate. The peak attenuation versus doserate is shown in Figure 2. At total doses above 3 x104 rad(Si), the qualitative behavior of the two typesof fibers was found to differ. For the borosilicate-clad fiber, the attenuation stops decreasing and beginsa monotonic increase (at least to the maximum total-doselevel achieved in these experiments). The polymer-cladfiber, on the other hand, showed a continued decrease inattenuation with increasing total dose.
For these experiments, once the attenuation peak at afew thousand rads had been passed, the radiation-inducedattenuation level could be changed relatively rapidly bychanging the dose rate. The effect of step decreases inthe dose rate on the borosilicate-clad fiber attenuationis shown in Figure 3 (the zero of the time scale shownis arbitrary and is well beyond the beginning of expo-sure). The attenuation could also be increased by in-creasing the dose rate. If the intervening time (and,thus, the accumulated dose) is not too great, the pro-cess can be cycled and the same attenuation achieved ifthe same dose rate is re-established.
The behavior of the attenuation versus dose curvescannot be explained by any simple single process. Thepeaking and subsequent decrease in attenuation cannotoccur without both the cessation of production of theentity producing the attenuation and the disappearanceof this entity.
In the borosilicate-clad fiber, the increased attenua-tion at high doses requires an irradiation-induceddefect to be produced that does not exist in the PCSfiber. A logical candidate location would be in the
0 100 200 300TIME (sec)
RE-02372
Figure 3. Attenuation level in borosilicate-cladfibers at high doses as a function of dose rate
borosilicate clad, since borosilicate glass is much moreaffected by irradiation than are polymers used for fibercladding.An interesting feature of fiber behavior is observedduring recovery of the polymer-clad fibers followingexposure. If the fiber is irradiated to well beyond thepeak attenuation, the attenuation recovers to slightlyless than the original attenuation upon cessation ofexposure. This effect is shown in Figure 4 for thepolymer-clad fiber. The borosilicate-clad fiber doesnot behave in this manner, and the attenuation neverrecovers beyond the originally present attenuation, atleast for periods on the order of 300 sec, which is aslong as the recovery was followed. It is quite possiblethat the same mechanism occurs here, but the effect ofthe additional defect results in a residual amount ofattenuation.
00.2..-'i V. _-
5
co
< 0.1z
Cl 0
0m= 01
DOSE [rad(Si )]100 200 300 400
0 50 100 1 50 200 250TIME (sec)
Figure 4. Recovery of the polymer-clad fiberfollowing exposure
In both types of fibers, once the total dose exceeds-10 krad(Si), none of the initial peaking at the fewthousand rads was observed upon re-exposure, and thecurves pick up essentially where they left off at theend of the first exposure. This effect is shown in Fig-ure 5 for the polymer-clad fiber. If the first exposureis kept short, below 1 krad(Si) total dose, the peakingis still observed in the second exposure.
The hardening effect in the fibers - that is, decreaseor elimination of the initial peak in the radiation-induced attenuation - is stable for at least one day.
4809
BOROSILICATE-CLAD FIBER
POLYMER-CLAD FIBER
II1111111 1[11111 t i,,|,1 , ,,,
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DOSE [krad(Si)]
n.n° 100 200 200 300 400
POLYMER-CLAD FIBER
2.3 x 103 rad(Si)/sec0.15
0.10
0MACH I NEO
O. 05 MACHINE ON OFF MACHINE ON
n ,,I,,I,,I,,00 50 100 1 50 200
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Figure 5. Multiple exposure of polymer-clad fiber
Exposure of an irradiated fiber segment one day afterthe initial exposure showed that the hardening was stillpresent. However, exposure with electrons of fiber seg-ments that had previously been exposed with protonsshowed that the hardening had disappeared. Sixteen dayshad elapsed between these exposures. This indicatesthat the hardening is stable only for days but not forweeks (with the fibers stored at room temperature).
EFFECTS IN PIN PHOTODETECTORS
Photodetectors, which are a key element in optical sub-systems, have one drawback from a radiation point ofview: they have a relatively large active volume com-pared with other semiconductor devices. The large vol-ume results in a high sensitivity to ionization-inducedtransients. Compounding this problem in optical subsys-tems is the fact that receivers are often operated in avery high-gain condition to provide the sensitivity thatmany of the optical subsystems require. These two fea-tures of optical subsystems combine to produce ionization-induced transient photocurrents which, while qualitativelysimilar to those produced in other semiconductor compo-nents, cause an upset in optical subsystems at rela-tively low pulsed-radiation levels. They also produce adifferent vulnerability mechanism not often encounteredin non-optical applications. This different mechanismis the result of the response of the photodetectors to a'steady-state" flux of ionizing particles such as wouldbe encountered in a space electron environment similarto the environments examined in this study. Every par-ticle that interacts with the detector produces a pulseof photocurrent that can have two effects on the opticalsubsystem. First, the pulses can be large enough to bedetected as false bits in some digital applications.Second, the random occurrence of these pulses producesan increased rms noise. Both of these effects can
degrade system performance.
There have been a few studies that have examined theproduction of pulses in silicon photoconductive detec-tors (Refs. 13-15) and memory devices (Refs. 16-18).However, except for a small amount of experimentalinformation obtained in a study of a fiber optics datalink (Ref. 7) and an analytical examination (Ref. 19),the rms noise produced by particle fluxes has not beenstudied. This program undertook to experimentally exam-
ine more fully the response of photodetectors to fluxesof ionizing particles. liP 5082-4207 PIN photodiodeswere irradiated with 6OCo gammas and I-MeV electrons.The pulses were counted using a counter with a combina-tion of thresholding and input attenuation that allowedthe counting threshold to be set in 1-dB steps. The rmsnoise current and radiation-induced dark current were
ineasured using standard techniquies.The differential pulse amplitude distributions (numberof counts per unit current normalized by the particleflux) produced in a typical detector by electrons or
gammas are shown in Figure 6. The total count rate perunit electron flux gives a direct measure of the pro-jected area of the detector. For the gamma fluxresults, the probability of interaction and the produc-tion of secondaries in surrounding material must betaken into account (Refs. 13-15). Because of unavoid-able system noise at low amplitudes, measurements couldnot extend to zero amplitude. Therefore, the totalcount rate must be estimated by extrapolating the datato zero. This was done using an exponential distri-bution of pulse amplitudes as the data seem to indi-cate. This results in an effective area for electronsof 1.2 x 10-2 cm2. The devices were exposed from thefront, so this should equal the junction area on thechip. The agreement is good (within about 10%) when thearea masked by contacts and diffusion from the regionadjacent to the junction are considered. The effectivearea for gammas is 1.1 x 10O4 cm2, which is also reason-able when the interaction probability and secondaryelectron production are taken into account.
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RE-02375
Figure 6. Differential pulse amplitude distribu-tions for HP 5082-4207 photodetectors
The radiation-induced dark current in the detectors isshown in Figure 7. The dose rate dependence is 5.2 x010l0 and is the same for electrons and gammas if the
dose rate (D) is expressed in terms of rad(Si)/sec.From the definition of a rad, the generation rate insilicon can be calculated as 6.9 x 10-6 amp/cm3/[rad(Si)/sec]. The data are again in reasonableagreement with the known device area if masking by thecontacts and diffusion lengths are taken into account.
The radiation-induced rms noise current (in a 107 Hzbandwidth) produced in the detectors is presented inFigure 8. The noise is likely to be shot noise andshould be calculable from the normal shot noise equation
1/2i = (2q I Af)rms
where Af is the bandwidth, I is the radiation-inducedleakage current (Figure 7), and q is the average chargeper event. The average charge per event can be foundfroTm Figure 6 and is a current of 0.12 PA. This equatesto a charge of 2.5 x 10 15 C (1.6 x 104 electrons) if
Figure 7. Flux dependence of the dark currentin HP 5082-4207 photodetectors
DOSE RATE frad(Si)/secl
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the most important material property for LEDs, the re-combination lifetime. Therefore, predicting LED behav-ior from these results would be difficult.The proton irradiations caused the optical response ofthe LEDs to degrade significantly, as shown in Figure 9.There was a lot of scatter in the responses of the var-ious devices. The curves drawn on the figure indicatethe largest, smallest, and average amounts of change.Within the accuracy limits set by the data scatter, thepigtailed and unpigtailed devices indicated the sameamount of damage. The amount of degradation noted wasmuch larger than expected. Taking into account the rel-ative damaging power of protons and electrons and neu-trons (Ref. 28) and the prior electron test results onsimilar devices, and also the proton tests on bulk mate-rials, led to the expectation that the devices would de-grade <10% with a fluence of at least 1012 p/cm2, andperhaps as high as 1013 p/cm2. Instead, 50% degradationoccurred by a fluence on the order of 2 x 1011 p/cm2.The devices themselves were not inordinately sensitiveto irradiation. They were exposed to an electron flu-ence of 3 x 10l4 e/cm2, and the damage noted (shown inFigure 10) was attributable entirely to the fiber pig-tails. In addition, exposure of the devices to 3 x 1012n/cm2 or 105 rad(Si) of 60Co gammas produced no LEDdegradation.Theoretically, the damage produced in the LEDs by protonirradiation is not a dose-rate effect, although whetherdifferent dose rates would change the amount of damageproduced was not determined experimentally. It is, how-ever, not out of place to mention the permanent degrada-tion caused by protons here because it is a space radia-tion effect and it is in space radiation environmentswhere the low-dose-rate effects have a large impact.
ELECTRON FLUX (e/cm2)RE-02377
Figure 8. Radiation-induced rms noise increasein HP 5085-4207 photodetectors
the 107-Hz bandwidth sets the pulse decay time. Usingthe experimental values for q and I cives a calculatedrms noise current of 8.1 x 10 13 4 1/, where i is theelectron flux in e/cm2/sec. This is about 2.5 timeslarger than th experimentally measured dependence of3.2 x 10 13 $1 2, It is possible that some of thisdiscrepancy could be due to infinite-amplitude pulses.However, the distributions must actually stop at afinite energy. This eliminates the higher-energy pulsesthat were included in the extrapolation, which woulddecrease the calculated noise. Further, the exponentialextrapolation may itself be incorrect. One other possi-bility is that some part of the analytical treatment ofthe rms noise may be incorrect.
EFFECTS IN LIGHT-EMITTING DIODESAnother key element in a fiber optic link is the lightsource, and the most common light source currenitly inuse is the LED. The devices used are generally made ofGaAs or GaA2As (the one chosen for the program wasGaAQAs). There have been a number of recent studies ofelectron irradiation effects in LEDs (Ref. 20), but pro-ton testing of the devices has not been performed.Studies have been made of proton effects in GaAs (Refs.21-27) or GaA2As (Ref. 27) which showed degradationthresholds ranging from about 1012 to 1015 p/cm2,depending on irradiation energy (0.2 to 30 MeV) and thestarting material properties. The sole GaAZAs test alsoexamined GaAs, and the two materials behaved relativelysimilarly. Higher-energy irradiations (akin to thatused in this study) and higher carrier concentrations(such as are used in LEDs) produced thresholds at thiehigh end of the range. These studies did not examine
o L-
1010RE-02378
10 1PROTON FLUENCE (p/cm2
Figure 9. Proton fluence degradation of LEDs
RE-02379
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The electron, neutron, and gamma results indicate thatthe tested devices had the hardness that could be ex-pected of the device type. Therefore, the proton damageis the result of some mechanism unique to protons. Theresponsible mechanism was not uncovered in this study.It is not even known whether the mechanism is unique toGaAtAs devices, or whether it would occur in GaAsdevices as well. The main purpose of this portion ofthe paper is merely to point out the effect and to holdup a warning flag that LEDs may have difficulty surviv-ing in space radiation environments.
CONSEQUENCES OF LOW-DOSE-RATE RADIATIONUPON PERFORMANCE
To evaluate the consequences of these effects on datalink operation, one must deal with link design. In theoverall program two links were designed - one for space-borne and one for airborne applications. The genericdesigns and qualitative conclusions are similar in bothcases. Since the spaceborne link is more likely toencounter these low-dose-rate environments, it will bethe only one discussed here. This link was designedwith maximum radiation tolerance in mind, so the compo-nent choices and design techniques resulted in a linkdesign that is representative of the hardened level thatcan be achieved. The power budget for this spacebornelink is given in Table 1. The link design margin (DM)is 17.1 dB for the bit error rate (BER) to be increasedabove 10 8.Three mechanisms were considered that would cause degra-dation in the link design margin. First, in a particleflux environment, the interactions of ionizing particleswith the PIN photodetector will result in false eventsbeing detected by the receiver. Some false events arealso produced in the preamplifier but, because of thesmall active volume of the Texas Instruments TIEF-151,the number of these pulses is insignificant compared tothose produced in the detector. These false eventswould be produced as long as the link is exposed to theflux of ionizing particles. They disappear when theflux is no longer present. In an analog system, thefailure mechanism would not be detection of false eventsbut an increase in the rms noise level of the system,which was presented in Figure 8. Second, the PCS fiberused in the spaceborne link design has a radiationresponse with an attenuation peak at a few thousandrad(Si), and then the attenuation decreases at higherdoses. This peak in attenuation would result in linkfailure at some dose rate level, but recovery wouldoccur after an outage time that would depend upon themagnitude of the dose rate.
Finally, an allowance of 1 dB was made for the radi-ation-induced loss in components other than the LEDs,fibers, and PIN photodetectors. The overall test pro-gram confirmed that this 1-dB allowance was more than
adequate for the radiation levels to which the compo-nents were tested.
Using Figures 2 and 8, it is possible to write an empir-ical equation that relates the degradation in the linkdesign margin (DM) as a function of dose rate. The rad-iation-induced system losses are the sum of the lossesin the cable (in dB), plus the increase in the ratio (indB) of the radiation-induced noise current to the pre-irradiation noise current at the input of the amplifier,plus the loss in the other system components (1 dB forthis example).
i (radiation-induiced)DM = 1 + i x F(D) + 10 log n ( i - + 11,
L in (initial) J
where:
D = dose rate in rad(Si)/sec,
F(D) = a function of D found using Figure 2,
in = the equivalent noise current at the input of thereceiver (for this particular design, in = 47.5nA rms with no irradiation),
t = length of fiber optic cable in meters (Q = 10 mfor this link).
For this particular link, the equation is
DM = I + 10 x 1.2 x 10-2 6(0.44)-9
+ 10 log (2.02 x 10 ) (D) + 1(47.5 x 109 -a
By using this equation and substituting in values forthe dose rate, it was determined that the system losseswill be equal to the design margin, DM, at about 5.9 x103 krad(Si)/sec. Thus, the BER of the system would ex-ceed the design limits of 10 8 at this dose rate level.This, of course, assumes no previous radiation exposureand exposure to only a single rate.
Adding proton-induced degradation in LEDs would againlower the link failure level. Using the worst-caseresponse of the test device, a proton fluence approach-ing I x 101l p/cm2 (3 dB decrease in LED optical outputpower) would not cause the link failure level, in doserate environments, to decrease by more than a factor of2. Above a proton fluence of about 4 x 1011 p/cm2,link failure would occur independently of any othereffect. Thus, the interaction of LED damage with theother effects occurs only over a relatively narrowfluence region, and its relative importance would dependupon when the proton fluence was accumulated.
In an actual space application, the link would not beexposed to a truly steady-state dose rate. In fact, thelink would pass through various levels of radiationenvironments. This would be akin to the multiple-expo-sure testing shown in Figure 5, where less attenuation
Table 1. Spaceborne Link Power Budget
Power launched into pigtail (I = 100 mA, T = 250C)
Power into cable (-1.5 dB at pigtail to cable interface)
Allowance for LED (temperature, -1.5 dB for time)
Loss through bulkheads (-1.5 dB/bulkhead)
Fiber attenuation
Received power (-2.3 dB at fiber-to-photodetector interface)
Preamp S/N (R = 0.5 amp/W, I = 42.7 nA)
Allowance for bit error rate <10-8
375 )iW265 PW
169 PW
85 PW
83 uiW49 PW
27.6 dB
10.5 dB
Power margin before radiation 17.1 dB
4812
was produced in second and subsequent exposures. Thismakes predicting a failure level in a space radiationenvironment difficult because the complexity of theprocess requires that the radiation history as well aslevel be known.
SUMMARY
The testing reported here examined the effects of low-dose-rate radiation exposure of opto-electronic compo-nents. The application involved is the design and eval-uation of fiber optic data links, with data on low-dose-rate effects being particularly applicable to spaceborneapplications. Three effects were found to dominate.The first was the result of the interaction of a flux ofionizing particles with the PIN photodetectors. Eachparticle that interacts produces a pulse of current thatcould be detected as a false bit in a digital link. Inan analog link, the combined effect of a series ofpulses would be an increase in the rms noise level. Thesecond effect was the additional absorption in fibercables produced by ionization. The generation and re-covery processes, which occur simultaneously under low-dose-rate, long-time (at least several seconds) expo-sure, are complex. As a result, the amount of attenua-tion produced depends on the dose rate and on the priorradiation history. The third effect was an unexpectedlylarge degradation in the light output of GaAQAs LEDscaused by proton fluence exposure. The mechanism in-volved must be unique to proton damage, but the resultswere insufficient to determine any details.
The consequences of these radiation effects on data linkperformance were then examined by designing a link anddetermining its failure levels. The production ofpulses in the PIN photodetectors and the radiation-induced attenuation in fibers were found to contributeto the link failure (BER < 10-8) by degrding the signal-to-noise ratio in the receiver when exposed to particleflux environments in which the ionization dose rate is>6.2 x 103 rad(Si)/sec. The relative importance of LEDdegradation depends on when the proton fluence wasaccumulated.
ACKNOWLEDGMENT
The detailed design of the fiber optic data links forthis program was accomplished by Larry Stewart and TomBunch of Spectronics, Inc., under a separate subcon-tract. Permanent degradation measurements on all com-ponents irradiated in this program were performed byKeith Adams and Mike Yonimitsu.
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