Pump-limited kW-class operation in a multimode fiber amplifier using adaptive mode control was achieved. A photonic lantern front end was used to inject an arbitrary superposition of modes on the input to a kW-class fiber amplifier to achieve a nearly diffraction-limited output. We report on the adaptive spatial mode control architecture which allows for compensating transverse-mode disturbances at high power. We also describe the advantages of adaptive spatial mode control for optical phased array systems. In particular, we show that the additional degrees of freedom allow for broader steering and improved atmospheric turbulence compensation relative to piston-only optical phased arrays.
The Reflection Grating Spectrometer (RGS) on Constellation-X is designed to supply astronomers with high spectral resolution in the soft x-ray band from 0.25 to 2 keV. High resolution, large collecting area and low mass at grazing incidence require very flat and thin grating substrates, or thin-foil optics.
Thin foils typically have a diameter-to-thickness ratio of 200 or higher and as a result very low stiffness. This poses a number of technological challenges in the areas of shaping, handling, positioning, and mounting of such optics. The most minute forces (gravity sag, friction, thermal mismatch with optic mount, etc.) can lead to intolerable deformations and limit figure metrology repeatability. We present results of our efforts in the manipulation and metrology of suitable grating substrates, utilizing a novel low-stress foil holder with friction-reducing flexures.
A large number of reflection gratings is needed to achieve the required collecting area. We have employed nanoimprint lithography (NIL) - which uses imprint films as thin as 100 nm or less - for the high-fidelity and low-stress replication from 100 mm diameter saw-tooth grating masters.
The Reflection Grating Spectrometer (RGS) on Constellation-X will require thousands of large gratings with very exacting tolerances. Two types of grating geometries have been proposed. In-plane gratings have low ruling densities (~500 l/mm) and very tight flatness and assembly tolerances. Off-plane gratings require much higher ruling densities (~5000 l/mm), but have somewhat relaxed flatness and assembly tolerances and offer the potential of higher resolution and efficiency. The trade-offs between these designs are complex and are currently being studied. To help address critical issues of manufacturability we are developing a number of novel technologies for shaping, assembling, and patterning large-area reflection gratings that are amenable to low-cost manufacturing. In particular, we report results of improved methods for patterning the sawtooth grating lines that are required for efficient blazing, including the use of anisotropic etching of specially-cut silicon wafers to pattern atomically smooth grating facets. We also report on the results of using nanoimprint lithography as a potential means for replicating sawtooth grating masters. Our Nanoruler scanning beam interference lithography tool allows us to pattern large area gratings up to 300 mm in diameter. We also report on developments in grating assembly technology utilizing lithographically patterned and micromachined silicon metrology structures ("microcombs") that have achieved submicron assembly repeatability.