Means and their limitations, for calculating the potential induced loss in an optical fibre, when subjected to long term exposure to radiation where dose rates and fibre temperature can vary, are considered.

Optical systems operating in space must maintain their performance over long mission times. Glasses are well known to darken upon exposure to radiation, due to formation of color centers, resulting in performance degradation. Despite increasing demand, an approach which would allow accurate prediction of the end-of-life performance characteristics of space optical instruments, has not been elaborated yet. We propose here a phenomenological methodology that should help to solve the problem. In our model, functional dependencies describing defect generation and annealing are derived from mathematical models, taking into account dose levels and irradiation times relevant for space missions. Numerical values for the parameters come from experimental data. Our experimental methodology is also described. We apply this model to analyze the results obtained for BK7 glass subject to Co⁶⁰ gamma-radiation.

It seems like just yesterday that 10 Gb/s (OC-192) transmission was the norm in data rates with slower data rates disappearing to 40 Gb/s (OC-768) is not a trivial task. Electronic designers now face serious complexities as they push the technology to the limit. Issues such as chromatic dispersion and polarization mode dispersion are placing more stringent requirements on fiber optics and associated components. The requirement to gather and transfer data at faster and faster rates has spurned an evolution in the thought processes of Photonic engineers. It appears that to handle the ever-increasing bandwidths, Photonic techniques are the way to go analog-to-digital converters and polymer-based modulator systems are now being designed and tested in Air Force government laboratories. Only time will tell if these designs can handle the voluminous amounts of data.

Space radiation-induced effects and degradation in inorganic optical and electronic materials are well known and can severely limit mission effectiveness. The major radiation induced effects in commercially available optical materials are a reduction in the optical transmittance and changes to the refractive index. To avoid or mitigate the deleterious effects of radiation, new hardening approaches for developing new polymer materials is based on an electrostatic assembly (ESA) growth process. The ESA process is very amenable for allowing ease in the molecular engineering of photonic, optoelectronic, electronic and electro optic materials and devices. This paper reports the first observed data on the spectral transmission behavior of three new ESA grown electro optic modulator materials in the presence of gamma-rays. Pre- and post-irradiation data supports the contention that devices fabricated by the ESA growth processes are viable candidates for application in the natural space radiation environment.

After a 5-year mission, the Near-Earth Asteroid Rendezvous-Shoemaker (NEAR) spacecraft made a controlled landing 12 February 2001 onto the asteroid, 433 Eros. Onboard the spacecraft, the NEAR Laser Rangefinder (NLR), a laser altimeter, gathered over 11 million measurements throughout 2000 and 2001, providing a spatially dense, high-resolution, topographical map of Eros. This instrument, launched in February 1996, was subjected to a constant, albeit, low radiation background predicted during the mission design phase to be 3 krad, cumulative, from solar protons at a shield depth of 1.8 mm aluminum. Using the onboard NLR calibration capability, and through extended observation of NLR measurement performance, the instrument exceeded requirements for this particular radiation environment. Electronic parts for the altimeter had been reviewed, assessed and screened, as necessary, for space quality and radiation hardness during its development. The NEAR mission included an excursion beyond Mars' orbit during its 4-year transit, followed by a one-year mission orbiting the near-Earth asteroid, 433 Eros, continuously collecting altimetry data. The majority of the data collection occurred during solar maximum and, in particular, operated without interruption through the events on Bastille Day, 14 July 2000 (comparable to the large October 1989 events of the previous solar maximum) and 10 November 2000. At Earth, the July 2000 proton level provided in a few days over half of the expected cumulative radiation, predicted through use of Feynman's model. Based on uneventful operation of the NEAR, including the absence of any degradation in solar array currents due to proton displacement damage and the nominal performance of the altimeter, it appears that the 14 July event did not intersect the NEAR location. The NLR-derived topographic data successfully enabled determination of Eros' shape, mass, and density contributing to the understanding the internal structure and collisional evolution of Eros.

The future thermonuclear fusion reactor ITER will require remote-handled equipment to monitor its operation and to allow hazard-free manipulations during its frequent maintenance periods. Heavy shielded umbilicals will be required to connect the sensors and the actuators with their instrumentation. Multiplexing sensor signals turns out to be essential to ease the umbilical management. We are considering fibre optic technology, with its intrinsic wavelength multiplexing (WDM) capabilities, to handle these ITER multiplexing issues. We propose a new analog data link design for low-bandwidth sensors and actuators based on commercial-off-the-shelf (COTS) fiber optic components. We rely on passive components such as WDM couplers and fibre Bragg gratings (FBG) to build a radiation-resistant analog data link. WDM couplers remain operational up to a 13 MGy gamma total dose. A radiation-induced channel drift is observed. The refractive index change under ionizing radiation is proposed as the degradation mechanism. FBG filters continue to operate satisfactorily up to a 150 MGy total gamma dose and a neutron fluence of about 10¹⁵ n/cm². Our results on these COTS all-fibre passive components open perspectives to build a radiation-tolerant analog optical data link compatible with the ITER requirements.

Over the past years, various attempts have been made to accurately model the radiation response of the optical attenuation in optical fibres for nuclear environments. In this paper we present the results of a study on pure silica fibres where both the spectral and kinetic behaviour are explored during and after irradiation in spent fuel gamma facilities. Basic first- and second-order kinetic models are considered, as they provide insight into fundamental dependencies on temperature and dose-rate. Other popular models, like the power-law and stretched exponential forms of the basic kinetic paradigms, are investigated with emphasis on the spectral dependence of the constituting parameters. Since the radiation induced attenuation is in general the sum of contributions from different absorption bands, related to underlying radiation induced defects, the spectral dependencies are also tackled by Gaussian resolution. With this method, the recorded absorption spectra in the range from 450 to 1600 nm are decomposed into individual absorption bands, with fixed positions and widths in the photon-energy domain for the course of the experiments. The resulting amplitudes from the non-linear estimation process are then evaluated with respect to the same models used as for the radiation induced attenuation at single wavelengths.

Aluminum-coated optical fibers are loaded with molecular hydrogen by keeping the fibers in an H₂ atmosphere at high pressure and temperature. A high H₂ concentration in the fiber glass (up to 1 at. %) has been obtained by this technique. A drastic reduction of radiation-induced absorption in the visible spectral region in H₂- containing Al-coated fibers is demonstrated. The formation mechanism is the H(I)-center is analyzed.

The light emission intensity at the wavelength of 400-750 nm in the KU-1 silica core (OH content 1000 ppm) fiber waveguide under irradiation at BARS-6 pulsed fission reactor (pulse duration 80 microsecond(s) , dose per pulse <5.5x10¹² n/cm² (9 Gy), dose rate <7x10¹⁶ n/cm²s (1.1x10⁵ Gy/s) have been measured. The intensity of radiation-induced light emission has been found to depend on intensity of probing light. Lower intensity of the light emission has been observed for higher intensity of probing light (lasers, wavelength 532 and 632 nm). The light emission quenching occurs at the wavelengths shorter and longer than the wavelength of the probing light, and also at the equal wavelengths.

GLAS is a satellite laser altimeter designed to measure ice-sheet topography and cloud and atmospheric properties. The onboard Nd:YAG laser emits 1064nm and (frequency doubled) 532nm light, which is reflected from the Earth and atmosphere and collected by a telescope. A small portion of the light emitted toward the Earth is picked off by a free space to fiber optic coupling system and routed around the instrument by connectorized fiber optic cables. There is also a 2 km fiber optic delay line to provide for onboard timing calibration, a fiber optic pigtailed diode laser onboard test source and a fiber optic connectorized LED onboard test source. The stability of the signals through the fiber optic system must be maintained to 10% in amplitude and 20ps in timing for certain critical signal paths. It has been challenging to meet these requirements with the spacecraft resources available. Components from many commercial vendors have been used to satisfy these requirements. The system design, components selected and testing performed will be discussed in this paper.

Radiation tolerant fiber optic transmission links are desired for their potential use in future thermonuclear fusion reactors and other harsh radiation fields. As part of it, we design and test a radiation opto-electronic transmitter based on vertical-cavity surface-emitting lasers (VCSELs) and dedicated driver electronics consisting of discrete components. VCSELs have already demonstrated their high radiation acceptance level. We confirm this by on-line irradiation experiments on such devices up to 10 MGy total dose. For the design of the driver circuit, we rely on discrete commercial-off-the-shelf (COTS) bipolar transistors. When the radiation induced degradation of these components is considered within the time design of the circuits, total dose levels larger than 1 MGy can be tolerated. The driver uses standard TTL input signals and delivers a forward current of 12 mA to a pigtailed 840 nm VCSEL. SPICE simulations show that the driver still delivers a sufficient forward current to the VCSEL, in spite of the radiation induced degradation of the H_fe and V_CEsat values of the transistors. These simulations are verified by our experiments. At a total dose of 1 MGy, the measured decrease of the forward current is only about 8%. This induces an optical output power decrease that can still be tolerated with irradiated VCSELs, as shown by our experiments. We conclude that a high total dose hardened optical transmitter for use in nuclear instrumentation systems can be fabricated using discrete COTS bipolar transistors, COTS vertical-cavity surface-emitting lasers and COTS optical fiber.

To develop more cost-effective future satellites and spacecraft systems, instruments and avionics are evolving into smaller/lighter-weight and more power efficient modules. NASA's future missions will require that these lighter weight systems have smaller shielding mass margins and operate at cryogenics temperatures in stressing radiation. JPL has been exploring several approaches to improving the radiation performance of CMOS Active Pixel Sensor (APS) imagers for ultra-low power,

This paper is focused on transient effects produced by neutrons and gamma on a CCD sensor. The main effect encountered, due to neutron interaction in the silicon is random excitation of individual pixel or pixel clusters. We have investigated the different image degradations with a 14 MeV pulsed neutron source as a function of angle of incidence and CCD camera parameters. A complete analysis has been achieved for all fluences between 10⁴ and 10⁷ c/cm² including spectra of neutron induced noise. We have found a linear dependence of the number of bright and saturated pixels with fluence. The number and size of pixel clusters increase with fluence particularly for grazing angle. In a second time we have investigated gamma effects which increase the background noise of the CCD. We have compared their effects to the previous results obtained with 14 MeV neutrons to discuss on image processing algorithms to correct these phenomena.

Erbium-doped fiber lasers (EDFLs) may soon find applications in space as high bit rate optical communication systems and photonic analog-to-digital converters (ADCs). The rapid advancement in digital signal processing systems has led to an increased interest in the direct digitization of high- frequency analog signals. The potential high bandwidth, reduced weight, and reduced power requirements makes photonics an attractive technology for wide-band signal conversion as well as for use in space-based platforms. It is anticipated that photonic ADCs will be able to operate at sampling rates and resolutions far greater than current electronic ADCs. The high repetition rates and narrow pulse widths produced by EDFLs allow for high-speed impulse sampling of analog signals thus making it a vital component of a photonic ADC. In this paper we report on the in situ gamma-ray irradiation of an actively mode-locked EDFL operating at 1530 nm. The onset, growth and extent of ionization induced damage under time-resolved operational conditions is presented. The laser consisted of approximately 3 meters of erbium-doped fiber pumped by a laser diode operating at 980 nm. The picosecond pulses produced by the laser were initiated and controlled by a Mach-Zehnder lithium niobate electro-optic modulator. The active mode-locking element allowed for the precise timing control of the laser repetition rate which is critical in high-speed optical networking systems as well as in photonic ADCs.

This paper provides a comprehensive understanding of a tunable wavelength-based photonic high-speed analog-to- digital converter (ADC) approach and its anticipated performance. Analytic models, experimental data, and simulation results will provide the fundamental limits on the expected conversion speed and bit resolution. A thorough in situ study of the photonic signal conversion system under simulated space radiation conditions is required to precisely determine its performance in either a natural or man-made space environment. Although this study has not yet been performed on the entire system, preliminary performance predictions will be made based on previously published studies on the individual components within the system. The photonic components of particular interest that will be discussed in the context of radiation hardness includes the edge emitting laser diode with a MQW tuning element, the processor filters, the delay equalizers and the photodetectors.

The nature of the response of phosphorus doped fibres is reviews and related to their possible use in dosimetry. Attempts to model the behaviour, aiming to be able to predict behaviour where experimental data is scarce, are described. Suitable fibres for dosimetry are identified and the limits on their performance assessed.