This paper reports the radiation performance results of several new product types designed for high radiation environments. The products tested include radiation hardened highly birefringent (HiBi) passive products for polarised applications and radiation tolerant active erbium doped fiber products for amplifiers. <p> </p>Radiation hardened, short beatlength HiBi fiber products have been developed for high accuracy polarisation maintaining (PM) gyros and sensors at both 1310nm and 1550nm operation in the space environment. The fibers have been tested up to 5kGy (500krad) – levels which could be expected in extreme, extra-terrestrial space environments. Results show a consistently low Radiation Induced Attenuation (RIA) of <7dB/km at 5kGy, giving a RIA value of 1.37×10<sup>-2</sup> dB/km/krad at 1550nm for this product range. <p> </p>Radiation tolerant EDF AstroGain™ fibers are intended for use in multichannel amplifiers in optical intersatellite communications. The structure of the fibers have been designed to deliver an accelerated recovery of radiation damage through photo-annealing using only the residual energy already available in an amplifier using a 980nm pumping regime. These products have been tested up to 200Gy (20krad) – levels which can be expected in Earth orbit environments over a 20-30 mission lifetime. Results show up to 100% recovery under continuous use for dose rates of 0.11rad/hr. It has also been demonstrated through analysis of the optical spectral output that this effect reverses the gain tilt, or spectral narrowing, induced by radiation damage through the C and L band. These combined fiber characteristics allow performance stability of the amplifier over the lifetime of the space mission.
An ytterbium (Yb) doped polarizing fiber is demonstrated. The fiber offers the opportunity to build all-fiber lasers with single polarization output and without the need for free-space polarizing components. Traditional single polarization fiber lasers utilize polarization-maintaining (PM) gain fiber with a single polarization stimulation signal. Whilst this results in an approximation to a single polarization laser, the spontaneous emission from the unstimulated polarization state limits the polarization extinction ratio (PER). The PER is further limited as the stimulated signal is prone to crosstalk. Furthermore, controlling amplitude modulation of the stimulated signal is critical for maximizing the peak power of an optical pulse, particularly for high energy lasers. If light is allowed to leak in to the unstimulated axis it will travel at a different velocity to the stimulated axis and can cross-couple back into the signal axis, creating an interference effect which leads to amplitude modulation on the signal pulse. Single-polarization Yb-doped fiber ensures that light on the fast axis is constantly attenuated; ensuring that light on the unstimulated axis cannot propagate and thus cannot degrade the PER or create amplitude modulation. In this paper we report on, to the best of our knowledge, the first demonstration of a single polarization Yb-doped bowtie optical fiber manufactured using a combination of Modified Chemical Vapor Deposition (MCVD) and rare-earth solution doping technology. The fiber has a single-polarization window of 80nm at the operating wavelength of 1060nm and a PER of >18dB. The fabrication and characterization of the fiber is reported.
Fiber optic gyroscopes (FOGs) are being used within increasingly severe environments, requiring operational temperatures in excess of the standard operating range for FOGs. Applications requiring these higher temperatures include: directional drilling of wells in oil and gas fields, space applications and military FOG applications. This paper will describe the relative merits of two high temperature acrylate coatings for an optical fiber designed for a FOG in such operating environments. Results for two high temperature acrylates are presented, tested in a 200m length of loose wound fiber, coiled and supported at 75mm diameter, in line with TIA/EIA-455-192 (FOTP-192). It can be seen that both coating types give very good polarization extinction ratio (PER) performance at high temperature up to 180oC, with better performance shown by one coating type on the low temperature side, since it does not harden to the same extent below 0oC. The long term thermal exposure effects will be discussed and experimental results presented which include testing the PER performance over temperature both before and after an extended period of high temperature endurance. This will demonstrate the relative merits of different styles of coatings. From the PER performance, the h-parameter of the fiber can be calculated and hence the preferred coating type selected and recommended for the customer operating environment.
We report on the rapid prototyping platform, developed at Fibercore, for producing spun multicore fiber (MCF) which maintains the high-specification and quality of a large-scale manufacturing process adding the versatility to fully customize fiber for specific applications. Such MCF has been produced by using an ultrasonic drill to accurately position the core holes in the cladding glass, achieving <0.4µm accuracy in fiber. Cross-talk between cores has been minimized by implementing high numerical aperture cores of 0.20, with levels less than -55dB over 400m. Additionally, the high level of germanium doping also allows fiber Bragg gratings (FBGs) to be written into each core without the need for hydrogen loading. Finally, in order to enable distinction between any potential twist and strain in the fiber from the bend under measurement, a permanent twist has been introduced in the fiber by spinning the preform whilst it is being drawn. The manufacturing cycle time for the fiber is 8 days, allowing rapid prototyping and repeat development cycles to be tested over a short period of time when creating new fiber designs.
The design of an optical fiber to give optimized sensing and lifetime performance for downhole fiber optic seismic sensors is presented. The SM1500SC(7/80)P is designed with an 80μm cladding diameter, pure silica core, high numerical aperture, high cut off wavelength and a polyimide coating to achieve outstanding performance when used in a coiled deployment state and operating in high temperature and hydrogen rich environments.
Fibercore have developed AstroGain<sup>TM</sup> fiber optimized for multichannel amplifiers used in optical satellite
communications and control. The fiber has been designed to take full advantage of the photo-annealing effect that results
from pumping in the 980nm region. The proprietary trivalent structure of the core matrix allows optimum recovery
following radiation damage to the fiber, whilst also providing a market leading Erbium Doped Fiber Amplifier (EDFA)
efficiency. Direct measurements have been taken of amplifier efficiency in a multichannel assembly, which show an
effective photo-annealing recovery of up to 100% of the radiation induced attenuation through excitation of point
Acrylate and polyimide coatings are found to have a suitable modulus for micro-seismic sensors whilst carbon coatings are too hard and inelastic for reliable use in this application. Fiber cladding designs can be optimized for mechanical reliability by using 80μm or 50μm cladding diameters and the numerical aperture (NA) increased to give low bend losses. To reduce splice losses, a bridging fiber has been developed, capable of reducing splice losses between telecoms fibers and reduced cladding diameter high NA sensor fibers by <50%.