Vacuum surface and gamma-ray irradiation effects on photodiodes and LED's play a significant role in the optical and electronic properties of these diodes, particularly in a space environment, which involves both ionizing radiation as well as vacuum conditions. Exposure of GaAs light emitting diodes (LED's) to vacuum gives rise to desorption of surface-adsorbed gases and subsequent free charge diffusion and redistribution that significantly alter diode properties. Changes in photodiode properties due to vacuum operation and y-radiation have also been observed. These changes are given, and a model explaining the changes is presented.
The effect of 30 MeV electron irradiWon op InGaA LEDs and InGaAs photodiodes was studied. Electron fluxes ranged from 1012 e/cm2 to 1015 e/cm2. The beam profile was measured with an improved scanning wire technique. During irradiation, light output, total current, and temperature were monitored for the LEDs. Responsivity and temperature were monitored for the photodiodes. Spectral characteristics and current-voltage curves were measured before and after irradiations. Changes in photodiode dark current were observed and LED lifetime-damage constant products were computed.
The application of fiber optic links in both commercial and military space systems, as well as other hostile environments, will mean that these links must be able to withstand exposure to radiation of various types. In this paper, we summarize our work on the effects of radiation on typical emitters and detectors employed in fiber optic links.
While delayed fracture, or static fatigue, of optical fibers is well known, it is not well understood, and the prediction of the time to failure under a given set of conditions can be problematic. Unlike short term fracture, which is quite well understood and quantified in terms of the theory of linear elastic fracture mechanics, the long term strength remains empirical. The goal of this study is to determine the design criteria for optical fibers subjected to long term applied mechanical loads. One difficulty in making lifetime predictions, as pointed out by Matthewson (Reference 1) and others, is that predictions made from data taken in tension and in bending do not agree. Another difficulty is the statistical nature of the fracture of glass. In making lifetime predictions it becomes important therefore that one (a) have ample data for statistical analysis and (b) have data for the loading configuration of interest. This is the purpose of our work. Since there is less data available in bending, and since several applications (such as wiring in aircraft and missiles) require bending, the data are taken in that configuration. The most significant finding in our work so far is the very large difference in static fatigue behavior between buffer coatings. Chandan and Kalish (Reference 2) and others have reported static fatigue curves, log (time to failure) versus log (applied stress), which are not linear, but rather bimodal. Our study confirms this result, but so far only for acrylate coated fibers. Silicone coated fibers show unimodal behavior. That is, the log (time to failure) versus log (applied stress) curve is linear, at least on the time scale studied so far. Data for acrylate coated fibers at 80°C in water are linear only for time scales of about one day, where a pronounced "knee" is observed. Data for silicone coated fibers under the same conditions are linear up to at least 6 months. Longer time scale tests and tests on fibers with other buffer materials are in progress.
We have treated optical fibers in different hydrogen environments to examine the effect of hydrogen treatment on radiation hardness of optical fibers. Both radiation-induced loss and hydrogen-induced loss are related to defect center formation in optical fibers. We have investigated the effects of treatment conditions on defects produced by drawing and 7-irradiation using the techniques of spectral attenuation, Raman scattering, and photoluminescence. Using Ge/P-codoped multimode optical fibers, we observed considerable reduction in the radiation-induced absorption at long wavelengths (X > 1µm) for hydrogen-treated fibers. Post-treatment of irradiated fibers by hydrogen can also reduce existing radiation-induced absorption significantly and increase the hardness of the fibers under sub-sequent irradiation. For Ge /F-codoped multimode fibers, hydrogen treatment has also shown its effectiveness in enhancing radiation hardness. The pretreated fibers show s5.15 % improvement at 850 nm in radiation resistance over untreated fibers. Varying the hydrogen treatment and irradiation history of the fiber had little effect upon the Raman spectrum. However, significant trends were found in the behavior of the photoluminescence. Emission bands in the 600-900 nm spectral region have been found to have intensities which depend upon fiber history. We attribute the improvement in radiation resistance to the passivation of precursors or defects in the optical fiber. Treatment with hydrogen converts certain types of defects into types which do not form deleterious color centers after irradiation. This defect-conversion interpretation is supported by the photoluminescence measurements which reveal variations in color center concentrations with hydrogen treatment, y-irradiation, and optical excitation.
Radiation resistance for several fiber samples is explored, with emphasis on variations in draw temperatures for both low- and high-OH content preforms. For a given prqorm type, draw temperature had minimal effect on radiation resistance for transient conditions below 10-7s. Dosimetry techniques were inter compared for pulsed electron accelerators. A high purity, high-RH content, fiber was tested for radiation response to determine transient attenuation from 4 ns to 104s.
The radiation resistivity of multi-mode and single-mode fibers was evaluated. Multi-mode fibers were investigated in terms of the core materials. It was found that the pure silica core fiber comprising pure silica glass produced by the plasma deposition process has excellent radiation resistivity. Further the photobleaching effect on pure silica core fibers was evaluated at room temperature and a low temperature. In investigating single-mode fibers, due attention was paid to the cladding materials. Several pure silica core single-mode fibers with varied BF contents in the cladding were prepared and evaluated. The results revealed that the single-mode fiber with large refractive index difference has better radiation resistivity. Further, a pure silica core and a Ge-doped core single-mode fibers were evaluated for the radiation resistivity at 1.55 μm, which has been rarely reported.
A program is underway to derive a model for quantitatively predicting the response of single mode optical fiber waveguides to ionizing radiation. The study is based on the extensive body of investigative work on radiation effects in bulk glass and optical materials. Inputs to the model include fiber fabrication history and cross-sectional analysis of fiber composition. Contributions to induced loss by each variable is determined by empirically fitting data from in-house generated samples and use of an orthogonal matrix data reduction technique.
Results of pulsed gamma radiation tests are reported for Raychem and Dainichi-Nippon silica core fibres, and for an STC doped core fibre. Measurement conditions included temperatures from -55°C to +70°C, absorbed doses between 170 and 4200 rad(Si), photobleaching at optical powers from 0.04 μW to 360 μW, and a range of launch conditions and sample lengths. The selection of suitable test conditions is discussed, and results are compared with those for other radiation hard doped core fibres.
We have made light-transmission measurements of high-purity, radiation-resistant, Raychem and Heraeus optical-fiber samples that had been exposed to 10- and 100-krad doses from 0.5-MeV-electron pulses. An electron beam from a Febetron 706 x-ray machine produced a radiation source that emitted the above doses in single pulses with a full-wave half maximum (FWHM) of approximately 1.5 ns. Light sources for the transmission measurements (a mercury/krypton-vapor flashlamp and a laser diode) provided spectral data for a continuous-wavelength band from 420 to 760 nm and a single wavelength at 820 nm. The light beam was dispersed by a grating and recorded by a streak camera. We gathered data at times ranging from approximately 3 ns to 1 μs after the radiation pulse.
Increasing interest in the use of high bandwidth optical diagnostic measurement systems at the Nevada Test Site (NTS) has prompted the use of single mode fiber (SMF). In an ef-fort to improve the time response of experimental systems designed for use in the 800- to 900-nm spectrum, SMF optimized for 850-nm operation has been obtained. Experimental systems to characterize this fiber have been developed. Measurements to determine the fibers' bandwidth, material dispersion, mode field diameter (MFD), cut-off wavelength, numerical aperture (NA), and spectral attenuation have been completed. These experimental data are presented in the text.
The recovery of radiation induced excess loss in optical fibers is dependent on the material composition and purity on the one hand and on environmental and test parameters on the other hand. We already described the general recovery characteristics and its dependence on the preform and fiber properties . Here, we present our results on the effects of the following test parameters on the recovery rates: bremsstrahlung dose, electron dose, electron energy, light wavelength, light power and fiber temperature. Pure-silica-core op-tical fibers were irradiated with bremsstrahlung or electrons from a Febetron 705 and the recovery was measured in the time regime 10-6s to 104s. The experimentally determined de-pendencies should be helpful in developing a consistent theoretical understanding of the transient radiation effects in optical fibers.
Optical fiber waveguides may be subjected to unique adverse environments onboard spacecraft, including wide temperature ranges and low dose rate radiation exposures. Since fiber reliability is essential, an accelerated lifetest has been designed to simulate deployment on the Space Station. The initial induced losses following exposure at -150 C are much lower in the fibers with pure than in those with doped silica cores. Good long term recovery is evident at this low temperature in fibers which do not contain P provided light is being transmitted in the waveguide since photobleaching is the dominant recovery mechanism in both types of fiber at -150 C. Except for the P-doped waveguides, the worst-case incremental losses are extrapolated to be < 10 dB/km for a 10 year, 1 rad/day exposure at -150 C with a -20 dBm signal in the fiber. Thus, optical fibers are attractive for use in spacecraft exposed to low dose rate natural space radiation environments.
Optical fiber communications systems must use repeaters for very longhaul applications. Optical fiber sensor systems need both optical power to activate the sensor and electrical power for analog to digital conversion, data processing and data communications. Motivated by the development of ultra-low loss fibers, the remote optical powering of devices such as sensors and relays has been investigated. Such a scheme should be useful in enhancing conventional system service lifetime, and for powering devices in constrained or hazardous environments. The remote powering of small devices using optical fibers has now been demonstrated in our laboratory. This is the first demonstration of an optical power delivery system including source, fiber, loss mechanisms, power conversion and device powering. The demonstration system will be discussed along with the damage thresholds for various silica and fluoride fibers, state of the art photovoltaic devices and infrared optical power sources.
Fiber optic cables have found widespread use in the ocean, but to fulfill its mission the cable must survive deployment and operation scenarios. Preservation of the manufactured strength and the optical transmission capacity of the optical fiber over its application lifetime are the primary system objectives. Other secondary objectives defined by system requirements include: fiber count, flexibility, weight, strength, lifetime, manufacturability, diameter, specific gravity, torsion stiffness, temperature influence, pressure effects, abrasion, cyclic flexure resistance, to name a few. This paper will discuss fiber optic cables, causes of failures, and their materials for use in the ocean for three general classes: a. Low cost, disposable cables. This requires the replacement rather than the repair of subject cables. Several applications fall into this category namely torpedo guidance, rapidly dispensible acoustic systems, and sonobuoy links. Life expectancy is months. b. Moderate cost, replaceable cables. This class of cables also requires the replacement of the cable; however, the system lifetime requires the cable be manufactured with the best materials for ocean service so once it is deployed, survival is ensured for many months or even years. Once the cable has failed, there will be no attempt to repair or recover the product, only replace it. Again, this type of cable would see an environment that spans the ocean depths. c. High cost, repairable cables. This class of cables constitutes the family of cables that generally require many years of service, extremely high cable bandwidth (high fiber count), span the major ocean depths and service continents or nations for their communications needs. The starting point for any cable design is defining the requirements the cable must meet. A systems approach is used to derive and impose on the design those requirements that influence cable function, life, cost and transmission capacity. A system analyses defines the hazards a cable will see over its lifespan. Each cable must be designed with the needs of the customer in mind so that it can be produced at a reasonable cost.
The lightguide technology has several unique fetures that open up new possibilities of its applications in alverse environments. These factors make this technology attractive in comparizon with classical metal cable based and radio technologies. In the paper we will review relevant works carried out in this country, namely: techno-logical specialization processess for industrial application oriented optical fibres and cables, classification of adverse environments and design rules for cables and optoelectronics, description of our technological facility, fiber optic health protection systems in adverse environments, example of real life applications of ruggedized fibre optic systems in HV/HP environments, ionizing radiation environments, mining, railway and industrial robotics.
Optical fibers pulled from silica glass prepared by OVD or MCVD technique are not optimum for most industrial applications such as computer interlinks, robotics or image guiders (regid or flexible). Advantages of multicomponent glasses for optical fibers, which have similar properties to silica glass but are much cheaper have been described in the paper. A special attention has been paid to materials for starting rods and determined optical properties in correlation to fibers obtained from them.