Rakon (France)

16 -18 rue du Canal, 91160 Longjumeau, FranceAbstractThe development of Ultra violet (UV), along with near infrared (NIR) and infrared (IR)transmissive fibers, has motivated rapid evolution in technologies including communica-tions and analytical chemistry. In both cases the emitting source, whether it be a laserdiode or a participant in a chemical reaction, uses the fibre to transmit the light to adiffraction grating for dispersion. In the case of a communications application, thisdispersed light will either terminate at a detector or be directed into other fibres.Diffraction gratings that have been optimized using ion -etching techniques or classicalruling have been used to produce devices capable of handling up to 20 fibres with up to 40wavelengths simultaneously.IntroductionHolographic diffraction gratings (HDG) are now routinely used in areas as diverse asSynchrotron Radiation and laser scanning instrumentation.The one linking element is alwaysthe fact that a diffraction grating will diffract wavelengths of interest in useable form.Once this basic principle is established it is then necessary to consider the ability ofthe grating to do the job and to continue to function over an extended period of time andenvironmental circumstances. To this end gratings , which as masters areformed in a photo- resist on a glass substrate, both meet and fail to meet various usersconditions. For example, given that holographic masters can begenerated to efficiently diffract a wavelength of interest in reflection mode, a user whoalso wants to be able to remove the ubiquitous fingerprint may find some reason for dis-appointment following an organic solvent rinse. Insofar as holographic gratings neverreally lived up to their expectations of becoming inexpensive alternatives to classicalgratings the first step in the evolution of the HDG was the development of a system ofepoxy replication from a holographically generated master. Such gratings, in addition tobeing cleanable in most conventional organic solvents increased the grating's durability,consistency of performance and finally, but not least importantly, reduced their price.The dilemmas left to solve come from evolving technologies, such as the requirements ofworkers in the vacuum ultraviolet (VUV) and those who require, for example, gratings foruse with optical waveguides or fibre optics. For the VUV the answer to producing a dur-able grating suitable for a very high vacuum environment is solved by creating an organicfree grating; meaning either a classical master or an ion -etched holographic grating inwhich the grooves are to be found in the substrate material itself. Such a grating canalso be epoxy replicated but would be self defeating in a VUV instrument, but very usefulin a non- vacuum evironment.For applications requiring very high efficiency at discrete wavelengths in the visibleor IR, in either reflection or transmission mode, a fairly deep groove with respect tofrequency offers a first iteration on the way to the final solution. An obvious way ofattacking this requirement is to sculpt the groove profile to create a structure consis-tent with the requirements. Such a proposal is viable using ion -etching and has been thesubject of the attention of a number of workers in the field(1,2,3,4)Ion -Etching TechniqueTo produce an ion etched grating it is first necessary to produce a throughwhich the ion beam will mechanically remove surface atoms until the glass substrate be-neath is milled to the desired structure. This is achieved by forming interferencefringes at the intersection of two laser beams that alternately expose the photo resistfollowed by wet processing, thus leaving a relief profile sinusoidal in character. For amore detailed explanation see references (6,8,7) ,This, then, is the mask through whichthe etching process is allowed to proceed. Figure 1.Glass material was found to be capable of maintaining excellent surface flatness andsmoothness, and is significantly superior to many metalic materials. The choice of

A study was conducted using electromagnetic theory to determine the parameters necessary to optimize laminar groove gratings for use in the vacuum ultraviolet. This study predicted that the absolute efficiency of correctly optimized laminar and sinusoidal groove gratings should be about the same, and that triangular groove gratings should show higher efficiency. For the example calculated, it was shown that when compared to a triangular groove grating, laminar grooves should show far superior higher order attenuation at wavelengths around blaze and be better or equal up to about twice the blaze wavelength. From then on, the triangular profile is better. Actual results measured showed good correlation with predicted values.