The rate of vapor condensation on a solid surface depends on the ambient relative humidity (RH). Also, surface plasmon resonance (SPR) on a metal layer is sensitive to the refractive index change of its adjacent dielectric. The SPR effect appears as soon as a small amount of moisture forms on the sensor, resulting in a decrease in the amount of light transmitted due to plasmonic loss. Using this concept, we developed a fiber optic humidity sensor based on SPR. It can measure the ambient RH over a dynamic range from 10% to 85% with an accuracy of 3%.

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MPB has developed a 10W Polarization Maintaining Optical Fiber amplifier (1550 nm) for space applications. The prototype is based on three stages of optical amplification with photodiodes at each stage, monitoring the output power. It includes the control electronics and software with feedback loops to dynamically control and monitor the amplifier. The design had to overcome many challenges to comply with the mechanical, thermal, radiation, and vacuum requirements for the LEO satellite space environment, while at the same time meeting the price targets for LEO constellations by maximizing the use of commercial off the shelf (COTS) components. The following were the main challenges: a) to effectively dissipate the heat generated (75-90 W); b) to select radiationtolerant electronics to drive the needed electrical current; c) to source and effectively implement components, such as the combiners and isolators, in the high power optical path compatible with vacuum at 10W output. The major challenge with regard to heat management was to find an optimal method to dissipate the heat from the third stage (high power) Erbium Ytterbium Doped Fiber. Commonly, this fiber is spooled on an Aluminium spool. The difference in the Constant Temperature Extension (CTE) between the fiber (low) and Aluminium (high) leads to a detachment of the fiber at low temperature with a high risk of breaking the fiber when passing from OFF to ON. At high temperatures, the Aluminium extends much more than the fiber, leading to an over tension on the fiber with a high risk of mechanical breakage. Different designs of the spool, supports inside the box, selection of materials, and process implementations were tried. An innovative, proprietary method was developed to satisfy this requirement. The unit successfully passed performance testing between -20°C and +40°C in vacuum with 10W output, with a wall plug efficiency of 11%. The lower temperature limitation was due to the specification of the high-power laser diodes. The higher temperature was limited by the local heating and risk of mechanical breaking of the third-stage COTS combiner and isolator. Vibration and mechanical shock are not foreseen to be an issue. The simulation demonstrated the prototype is complying with these requirements. Moreover, MPB has built similar instruments at lower power levels that have successfully passed these qualification tests. The components used were available as COTS products, including the radiation-tolerant electronics. All the components were qualified individually for > 30 krad, in vacuum, and for the temperature range -35°C to +65°C except for the highpower laser diodes which were limited to -25°C. MPBC is continuing the qualification, implementing minor design changes, in order to satisfy the complete temperature range (-35°C to +65°C).

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A high-power Er/Yb polarization-maintaining fiber amplifier is described. The amplifier operates on a single wavelength of 1546 nm and consists of a pre-amplifier with a gain up to 64 dB followed by a double-clad 25/300-m Large Mode Area (LMA) Er/Yb fiber booster stage, counter-pumped through a (2+1):1 PM combiner by two 140-W 915-nm laser diodes. The design of the amplifier required developing new splicing and re-coating approaches for the LMA PM fibers and characterizing mode-field adaptors for single-mode (SM) and LMA fibers and low-loss high-power PM pump-signal combiners maintaining a high Polarization Extinction Ratio (PER). The amplifier provides a robust diffraction-limited output of more than 50 W from true single-mode PM1550 fiber with an M 2 < 1.03 and a long-term (~200 hours) stability better than 5% in constant current mode (i.e., no power control feedback loop). With low-loss, high-quality splices between fibers with different MFDs, the maximum local fiber temperatures were kept below 60 o C. The amplifier design ensured that, even when operating at the 50-W level, the parasitic ASE in the 1-micron region was less than 2 mW. The output PER of >15 dB was limited by the quality of the pump-signal combiner which has a high probability of being improved in the future.