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amma radiation effects on the DPB SFS in space FOGs applications
in Li ∗, Ningfang Song, Jing Jin, Xueqin Wang, Rui kangchool of Instrument Science and Opto-electric Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China
r t i c l e i n f o
rticle history:eceived 22 March 2011ccepted 17 August 2011
a b s t r a c t
Superfluorescent fiber source (SFS) is generally considered to be promising in optical sensing. In thispaper, the gamma radiation effect on the double pass backward (DPB) SFS was investigated. Firstly, asample of DPB SFS together with a 9 m Er-doped fiber (EDF) was irradiated by 60Co source with radiation
eywords:PB SFSamma radiationOG
dose rate of 3.6 Gy/h and total dose of 200 Gy. And the results were analyzed to find that at the very startof radiation the RIL of the sampled Er-doped fiber was greater than that of sampled SFS while at about100 Gy the RIL slope of the EDF became less than that of the SFS. Above all, the loss from EDF was dominantto that from the SFS in gamma radiation environment. Mean wavelength of the SFS drifted about 4 nmmainly caused by the radiation loss. In the end, the potential influence to FOGs in space applications wassimulated.
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pace application
. Introduction
FOGs (fiber optic gyroscopes) have developed sufficiently toegin finding a place in a number of media to high performancepace applications, particularly satellite determination attitudeetermination and appendage control. They have shown excep-ionally advantages, such as high reliability, long life, low weight,ow angular random walk, good bias stability, good scale factor sta-ility and high resolution [1,2]. Although much of the interest forsing a FOG in space stems from the potential long life and high reli-bility, some problems still remain. An orbiting FOG has to face theenign environment, depending on the application scenarios. Radi-tion will unavoidably pose threat on the FOGs, for the influencen the key optical and electrical components.
The performances of FOGs, random noise and scale factor, areirectly influenced by light sources. Hence it is quite worthy toxplore the radiation effects of light sources. Er-doped super-uorescent fiber source (SFS) has been studied extensively forheir applications in fiber sensors, especially in the fiber opti-al gyroscopes (FOGs) [2–5]. The sources at 1.5 �m exhibit anique combination of high efficiency, high spatial coherence,road spectral emission, and excellent long-term stability of theean wavelength. In addition to that, the state-of-the-art thermal
ensitivity coefficients of SFSs are only several ppm/◦C, much betterhan SLD whose thermal coefficient is about 300 ppm/◦C.
As one of the key components in FOGs, research and experi-ents have been done to analyze the radiation effects and modeling
or the SFSs [6–8]. However, many of them are mainly concentrated
on the Er-doped fiber, a critical part in the SFS. The overall impacton the SFSs in radiation circumstance has not been researched indetail yet.
In this paper, the degradation characteristics of the SFS underthe gamma radiation environment are presented, including thepower loss and mean-wavelength drift. Particularly the Er-dopedfiber (EDF), the key part of the SFS, was also irradiated under thesame environment. Comparisons are made between their irradi-ated results. In the end the influence on the SFS-based FOGs was alsosimulated and discussed according to the degradation performanceof the SFS.
2. SFS configuration
Generally, there are four configurations for SFSs [3]. They aresingle pass forward (SPF), single pass backward (SPB), double passforward (DPF) and double pass backward (DPB). Of all these SFSarchitectures, the DPB configuration is typically preferred for FOGapplications due to its higher conversion efficiency, more opticalpower with shorter Er-doped fiber length, and maximum spectrumwidth with parameter optimizations [9,10].
Fig. 1 shows the conventional structure of a DPB SFS [10]. Itconsists of a 980 nm pump laser coupled by a wavelength divi-sion multiplexer (WDM) into a length of Er-doped fiber. The pumpfiber emits 1550 nm light. The wavelength-selected reflector (WSR)is located at the end face of the erbium fiber, which has specialreflection spectrum, increases the output power, and improves the
spectrum characteristics. The gain flattening filter can compensatefor nonuniformity and improve the flatness in the gain spectrum. Apolarization-insensitive isolator is taken to reduce optical feedbackat the signal output end.
The aims of the experiment are three folds: the first is theerification that the loss induced by EFF is the main term in theadiation-induced light power drop in SFSs. The second is theomparison between the radiation-induced mean-wavelength driftrom the SFS and that from the EDF. The last is the evaluation of theadiation effect to the fiber optic gyros.
.2. Experiment setup
60Co radiation source was used, with a constant radiation doseate of 3.6 Gy/h measured by a dosimeter. The total dose was accu-ulated up to 200 Gy. The experiment setup was shown in Fig. 2.As shown in Fig. 2, a DPB SFS and a 9 m Er-doped fiber were
ocated in the radiation circumstances. A light power driver wasut of the gamma radiation room to provide the current for theump diode in the SFS. At the same time, a 1.5 �m SFS light sourceas used to provide light power to the tested Er-doped fiber. As to
Fig. 2. Experiment setup of the D
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the measurements, one optical light power meter was responsibleto measure the power situation from radiated Er-doped fiber, andanother optical light power meter was used to calibrate the powerfrom tested SFS. The Agilent optical spectrum analyzer AQ6319recorded the mean-wavelength drift from the radiated SFS.
Especially stated here is that the length of Er-doped fiber forboth tested fiber and the SFS was 9 m. And the SFS was encapsulatedwith aluminum case. And the case was 1.5 mm thick and describedin Fig. 2 as the grey box.
4. Experiment results and discussion
4.1. Experiment results analysis
As described in Section 3, the test data included output powerfor both the Er-doped fiber and the SFS, and the mean-wavelengthdrift from the SFS. The radiation induced loss (RIL) was defined asdB and RIL, both samplings illustrated in Fig. 3.
As shown above, RIL for EDF could be divided into two parts, parta and b. While RIL for SFS was also portioned into part c and d. Wefound that at the very start of the radiation experiment, RIL sensi-tivity of the EDF was higher than that of the SFS. While at the partsof b and d, we could see that SFS’s output power became more sen-sitive to EDF. Both the EDF and the SFS had a turning point at about110 Gy, where the RIL slopes changed. Before the turning point, theEDF had the fitted RIL slope of 0.1095 dB/Gy, while the slope of theSFS was 0.06399 dB/Gy. After the turning point, the fitted slopes ofthe EDF and the SFS were 0.08203 dB/Gy and 0.09191 dB/Gy respec-tively. At the line part c and d, RIL included not only that from thetested EDF but also that from the SFS.
The turning point was reported to be a characteristic for irradi-
ated EDFs [7]. Before the turning point, the attenuation from EDFwas obviously stronger than that from SFS. And after the turningpoint, situations changed to the opposite direction. At the very startof irradiation, it was inferred that the effect of irradiation protection
Fig. 3. RIL during irradiation for both SFS and Er-doped fiber.
rom aluminum was remarkable. But as the radiation accumulated,he protective effectiveness decreased. After the turning point, thettenuation speed of the SFS exceeded that of the EDF. And it coulde seen that with the radiation dose cumulated, RIL for both the SFSnd the EDF would be very close to each other.
It is known that the radio-induced absorption was a conse-uence of the generation of color centers with the absorption bands
ying in the ultraviolet region. Hence in the band between 980 and550 nm RIL can be modeled [8]
IL = c˚1−f Df (1)
here ̊ is the radiation dose rate, D is the total radiation dose, cnd f are the temperature-dependant parameters.
The RILs of both the EDF and the SFS complied with the model.For the SFS the fitted model was
IL s = 0.0877 × (3.6)−0.194 × D1.194 (2)
For the EDF the fitted model was
IL e = 0.3591 × (3.6)0.1358 × D0.8642 (3)
The drift of the mean wavelength during the irradiation washown in Fig. 4. It could be seen that the mean wavelength fluctu-ted remarkably with the radiation dose.
At the end of the irradiation, the mean-wavelength � driftedbout 4 nm. The drifting could be modeled as
= 1563 + 2.501 × 10−6 × D2.697 (4)
For fiber optic gyroscopes, the performance index such as detec-ive SNR are directly influenced by the light sources. And below weill discuss how the SNR will change in the gamma ray radiation
nvironment due to SFSs’ degradation.
Fig. 4. Irradiation induced mean wavelength drift of the DPB SFS.
Fig. 5. Simulated SNR with degraded SFS parameters.
For the square-wave modulation, the relationship among detec-tive SNR, light power and mean-wavelength can be expressed as[11],
where ��s is residual phase, BW is the detection bandwidth, Pinis the detection light power, R is the responsivity of the photode-tector, ��FWHM is the half-power linewidth of the light source, � isthe mean wavelength, 〈iD〉 is the mean dark current, T is the tem-perature in Calvin, RL is the load resistor, e is the electron charge1.6 × 10−19, k is the Boltzman constant 1.38 × 10−23 J K−1, and c islight speed in the air 3.0 × 108 m/s.
To analyze the effect of the SFS to the fiber gyros’ performance,the parameters exception to the Pin and � are all considered to beconstant shown in Table 1.
Based on the above defined parameters and fitted models in Eqs.(2) and (4), the detective SNR can be simulated as following.
As the simulation result indicates that the detective SNR offiber gyros attenuates evidently with the cumulated radiation dosefrom the original 1.3645 × 10−9 to 1.3575 × 10−9 with about 0.5%decrease. And for high performance fiber gyros the effect could maynot be negligible (see Fig. 5).
5. Conclusion
In this paper, an SFS and an EDF were irradiated and onlinetested under a total accumulation dose of 200 Gy. The results haverevealed that in the radiation process the attenuation speed of theSFS was lower than that of the EDF possibly due to the protectionfrom the 1.5 mm aluminum encapsulation. As the radiation doseaccumulated, RILs in both of them approached to each other grad-ually. It was obvious that RIL in EDF was a principal portion for RILin the SFS under gamma irradiation environment. The mean-wave
length drifted with the radiation accumulation. Based on the simu-lation, for fiber gyros the detective SNR decreased with degradationof the SFS.
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cknowledgement
This paper is sponsored by the National Science Foundation ofhina (Grant No. 61007040).
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