Digital Planar Holography (DPH) has arrived due to progress in microlithography, planar waveguide fabrication, and theoretical physics. A computer-generated hologram can be written by microlithography means on the surface of a planar waveguide. DPH combines flexibility of digital holograms, superposition property of volume (thick) holograms, and convenience of microlithographic mass production. DPH is a powerful passive light processor, and could be used to connect multiple optical devices in planar lightwave circuits (PLCs), and if combined with active elements on the same chip, may perform not only analog operations but also logical ones. A DPH implementation of a multiplexer/demultiplexer with discrete dispersion is proposed and demonstrated, avoiding communication signal distortion inherent in multiplexers/demultiplexers with continuous dispersion. The concept of discrete dispersion leads to a device with a flat top transfer function without a loss penalty. The dispersion is created with custom-designed bandgaps for specific directions. A DPH hologram resembles a poly-crystal with long-range correlations, and it exhibits the properties of a quasi-crystal. Unlike photonic crystals, light in quasi-crystal may propagate in almost any direction. Single mode planar waveguides are specially designed to suppress parasitic reflections that appear due to mixture of TE-modes, TM-modes, and cladding modes. Demultiplexers with 2-32 channels were demonstrated on planar waveguides with binary single-layer lithography.
A novel concept of Photonic Bandgap Quasi-Crystal (PBQC) as a platform for planar integrated WDM optical devices is proposed. The PBQC can be lithographically fabricated in a planar waveguide as a computer-generated two-dimensional hologram. In this approach the spectral selectivity of Bragg gratings, focusing properties of elliptical mirrors, superposition properties of thick holograms, photonic bandgaps of periodic structures, and flexibility of lithography on planar waveguides are combined. In distinction to conventional combination of independent planar Bragg gratings, in PBQC we create multiple bandgaps by synthesizing a synergetic super-grating of a number of individual sub-gratings. The device spectral selectivity is determined by those of the sub-gratings. The super-grating comprises million(s) of dashes etched on an interface of a planar waveguide. Each dash is a binary feature placed by a computer program to serve simultaneously many channels. For realization of PBQC devices the software for generating super-gratings (GDS-II format) and 2-D simulation of its transfer function was developed. Direct e-beam writing and photolithography were used for manufacturing PBQC structures. For verification of the ideas behind the concept a number of multichannel MUX/DEMUX devices have been manufactured and experimentally tested. The results of detailed experimental study of 4- and 16-channel devices will be presented. Channel isolation ~30 dB was achieved in the 4-channel devices. The applications of PBQC platform for integrated light wave circuits are discussed.
We present recent work on the development of the compact ('table top') soft x-ray lasers (SXLs) using two approaches. In one approach we use low energy Nd/Glass laser (pulse duration, (tau) equals 1.5 nsec, beam energy E equals 3 - 5 J), and in the second approach we use powerful subpicosecond (PSP) laser ((tau) equals 200 fsec, E equals 0.1 - 0.3 J, maximum power density P approximately equal to 2 multiplied by 10<SUP>18</SUP> W/cm<SUP>2</SUP>). We discus generation high gain at 18.2 nm and 13.5 nm in CVI line focussed Nd/Glass laser on a carbon target and the unexplained difficulty to obtain gain-length GL greater than 4.5 in such a configuration. The time evolution of intensities of spectral lines and their correlation with time averaged spectra are analyzed. We also present measurement of plasma refraction as a possible explanation of the limitation of GL. More recently we changed the configuration of this experiment using as a target polyethylene microcapillary with focussing Nd/Glass laser at the entrance of the microcapillary. Very encouraging results were obtained for CVI 18.2 nm line and the possibilities of generation high GL in the near future are discussed.
We present recent results on the development of a small scale soft X-ray laser with low pumping laser energy using a multi-fin target in a two-target chamber. With only 4 J (2 ns) laser beam energy a maximum gain of 7.1 cm<SUP>-1</SUP> was measured for the CVI 18.2 nm line for a single 6 mm long target. Similar gain (6.5 cm<SUP>-1</SUP>) was also measured for the CVI 13.5 nm line. Control of the gain region in the plasma was demonstrated by changing the influx of iron into the plasma. We also present results from our attempt to generate gain on the 2-1 transition in LiIII at 13.5 nm using a powerful sub-picosecond KrF laser system following work by the RIKEN (Japan) group.
The high brightness and short pulse duration of soft x-ray lasers provide unique advantages for x-ray microscopy. We briefly review soft x-ray laser development at Princeton University and present results from the development of novel soft x-ray microscopes. The Princeton soft x- ray laser at 18.2 nm has been used to record high resolution contact images of biological specimens. More recently we have demonstrated proof-of-principle of reflection imaging in the soft x-ray wavelength range with the first results from a soft x-ray reflection imaging microscope. The microscope used a Schwarzschild objective with Mo/Si multilayer mirrors (normal incidence reflectivity of approximately 20% per surface) to form an image in reflected 18.2 nm soft x rays. In a separate experiment a novel `diffraction plate,' designed as an alternative to conventional condenser optics, has been tested.