Abstract
(English)
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The MOSIS project is aimed at the development of industrial applications for the technology of silicon micro machined adaptive mirrors. The main advantage of this technology consists in the possibility of low-cost high-quality adaptive optics, which can be used on a much wider scale than the traditional high-cost adaptive optics, which is used in astronomy and military applications. The mirrors have apertures ranging from 10 mm to 50 mm; the number of control electrodes ranges from 1 to 37; they can be deformed by up to 10 microns from their original shape; and their response speed is several hundred Hertz. They are applicable in a wide variety of electro-optic systems, including displays, vision systems, medical equipment, laser optics and optical information processing. The research is divided into four work packages: Technology; Beam Shaping; Imaging; and Spatial Light Modulators. The IMT is involved in the second and fourth work packages.
During the last year of the project, we finalized the development of the high power laser dynamic coupling for pumping Neodymium doped fibbers and of the 1xN single mode fiber switch, and we demonstrated the coding of holographic memories using the deformable mirror to generate the phase codes
The details of these three systems are described in the report for the European Commission (annex 1).
The MOSIS project has also found world-wide publicity by an article published by Michael Hatcher in Opto & Laser Europe (OLE) May 2001, p. 37-39 (annex 2).
The three breadboards which have been developed and finalized during the last year of the project are described below:
High power laser dynamic coupling for pumping Neodymium doped fibbers
The development of this breadboard started during the second year of the project. A high-power Nd-doped fiber amplifier for coherent intersatellite links had been developed at the IMT. Its weakness is the low coupling efficiency of the high power diode laser pump into the fiber amplifier. This is due to the particular output pattern of the diode laser module, which produces a low coupling efficiency of around 30%. We propose to increase the coupling efficiency of the complex high power diode laser module into the double clad doped fiber of the amplifier. The diode pump laser module has a maximum power output of 23 W. A membrane mirror specifically designed to withstand this high power was used to optimize the coupling. We proved the possibility to increase the coupling efficiency of high power laser diode arrays into double clad Nd-doped fibers, but we met a technological limitation imposed by the maximum deflection of the membrane mirror. TU Delft recently developed a membrane mirror of 25 mm diameter, which could satisfy the needs of this experiment.
1xN single mode Fiber switch
As proposed in the last intermediate report, we did extend the fiber switch to a larger number of connections. We result is a 1xN switch for single mode fiber optical communication systems, which is composed of an array of fibers, an achromatic lens, and an adaptive membrane mirror. The working principle of the optical switch is as follows: the center fiber of the array delivers the input signal, this signal is collimated by the lens, back reflected on the membrane mirror and refocused by the lens to an other fiber. The addressing of the receiving fiber is made by lateral displacement of the lens. However, using the achromatic lens under off-axis conditions introduces aberrations, which cause coupling losses to the receiving single-mode fibers. The deformable membrane mirror is used to adaptively correct these aberrations. The optimization of the coupling efficiency is made with the help of a genetic algorithm. For each position of the lens, the optimized voltages on the electrodes of the membrane mirror can be stored during the calibration procedure and afterwards recalled during operation of the switch. A demonstrator has been set up with a commercially available linear array of 32 single-mode fibers disposed in V-grooves, an achromatic lens mounted on a two-dimensional translation stage, and a membrane mirror made of silicon nitride coated with aluminum and electro-statically activated by 37 electrodes. To demonstrate the capabilities of the aberration correction we used the first fiber in the array as input fiber and optimized the coupling efficiency to all the other fibers in the array. We obtained insertion losses of less than 3 dB and a cross talk below 30 dB. These results prove the feasibility to build a switch with a two-dimensional array of more than 1000 addressable fibers.
Phase coding for holographic memory
Phase multiplexing for recording holograms is usually made with the help of digital Hadamard matrices, which have the particularity to be mutually orthogonal. We propose a phase coding for multiplexed holographic memories where the reference uses orthogonal Zernike polynomials generated with the help of a deformable membrane mirror. This membrane mirror is made of silicon nitride and coated with aluminum. It is fifteen millimeters large, less than one micrometer thick and electrostatically activated by 37 hexagonally disposed electrodes. The first fifteen Zernike modes can be efficiently generated by this configuration of electrodes, which allows writing holograms for each of these modes without cross talk. The Zernike modes are measured with the help of a Shack-Hartmann sensor. The pages of data are composed by an LCD spatial light modulator (LCTV with 640x480 pixels) and undergo an optical Fourier transform before being holographically recorded in a photorefractive crystal (Iron doped lithium-niobate) with the phase coded reference beam. The deformable membrane mirror has a frequency response of up to 1 kHz, and therefore the pages of data can be retrieved up to this speed, if the receiving CCD camera is fast enough. We have been able to write about 10 independent holograms using independent Zernike modes and 8 independent holograms using the same Zernike mode at different amplitudes. Thus, we have demonstrated the capability of the membrane mirror to be used instead of Hadamard matrices for multiplexing phase holograms. The limitation which we met for the maximum number of recorded holograms without cross-talk is due to the fact that the higher order membrane modes can only be produced with small amplitudes and that the Zernike modes produced by the membrane mirror are not perfectly orthogonal. An improvement is possible with larger membrane mirrors activated by more electrodes.
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