Diffraction gratings are know, and have been fabricated for more than one century. They are now making a come back for two reasons: first, because they are now better understood which leads to the efficient exploitation of what was then called their “anomalies”; secondly, because they are now fabricable by means of the modern manufacturing potential of planar technologies. Novel grating can now perform better than conventional gratings, and address new application fields which were not expected to be theirs. This is the case of spatial applications where they can offer multiple optical functions, low size, low weight and mechanical robustness.
The proposed contribution will briefly discuss the use of gratings for spatial applications. One of the most important applications is in the measurement of displacement. Usual translation and rotation sensors are bulky devices, which impose a system breakdown leading to cumbersome and heavy assemblies. We are proposing a miniaturized version of the traditional moving grating technique using submicron gratings and a specific OptoASIC which enables the measurement function to be non-obtrusively inserted into light and compact electro-mechanical systems. Nanometer resolution is possible with no compromise on the length of the measurement range.
Another family of spatial application is in the field of spectrometers where new grating types allow a more flexible processing of the optical spectrum.
Another family of applications addresses the question of inter-satellite communications: the introduction of gratings in laser cavities or in the laser mirrors enables the stabilization of the emitted polarization, the stabilization of the frequency as well as wide range frequency sweeping without mobile parts.
Laser polarization control is revisited at the light of the possibilities offered by resonant gratings associated with the multilayer mirror of the laser. As compared with classical Brewster elements, resonant grating mirrors have a richer functionality in that they can achieve transverse mode control. Furthermore, they are fully planar monolithic elements which can be fabricated by batch technologies and lead to the utmost miniaturization of the laser module. A number of designs and experimental demonstrations are presented.
Prism and grating coupling techniques can be used to retrieve the refractive index and thickness of a thin film or a stack of layers from the measurement of the effective index of the guided modes they propagate. These techniques are discussed as possible means to assess the wafer scale index and thickness uniformity in the prespective of the batch manufacturing of resonant gratings. The application example considered is a grating polarizer used as a microlaser polarizing mirror.
Polarization filtering in a laser mirror is achieved by means of a corrugation grating defined in the last high index layer of multilayer mirror based on a metal or metallized substrate. The grating couples the undesired TE polarization to a high order propagation mode of the metal based multilayer stack. The resulting dip in the TE spectral reflection curve is wide enough to cover the gain bandwidth of a Nd:YAG active medium. A technology trimming scheme is designed and demonstrated to shift the TE reflection dip to 1064 nm.
The operation of a polarizing grating laser mirror relies upon the coupling of the undesired polarization to one mode of the multilayer mirror. This is accompanied by an increased electric field in the mirror, thus to a decrease of the damage threshold by a factor 50. The damage threshold of the grating multilayer for the uncoupled lasing polarization is as much as 80% of that of the gratingless multilayer. The laser damage threshold tests have been made on a grating multilayer of SiO2/HfO2 deposited by sputtering.
A Fabry-Perot comprising a multilayer mirror and a mirror operating according to the resonant reflection of a grating slab waveguide exhibits the remarkable property of possibly being single order. The analysis of this new type of resonator is made and applied to the understanding of the operation of a known filtering laser mirror.
The present work relates to the design, the fabrication and the spectroscopic characterisation of an efficient grating polarising structure which uses a quasi extra-cavity resonant grating in association with a multilayer to dictate the polarisation emitted by a semiconductor pumped solid-state Nd:YAG laser at 1.064 μm wavelength.
The spatial coherence of optical gratings fabricated by means of a step & repeat camera is characterized by a diffractive interferometric displacement sensor using the grating under test as the grating scale. The displacement sensor head comprises two readout gratings at a definite distance from each other which allows the determination of the local deviation of the grating period with a resolution of 0.001 nanometer.
The mechanism of direct bonding at room temperature has been attributed to the short range inter-molecular and inter-atomic attraction forces, such as Van der Waals forces. Consequently, the wafer surface smoothness becomes one of the most critical parameters in this process. High surface roughness will result in small real area of contact, and therefore yield voids in the bonding interface. Usually, the root mean square roughness (RMS) or the mean roughness (Ra) are used as parameters to evaluate the wafer bondability. It was found from experience that for a bondable wafer surface the mean roughness must be in the subnanometer range, preferentially less than 0.5 nm. When the surface roughness exceeds a critical value, the wafers will not bond at all. However RMS and Ra were found to be not sufficient for evaluating the wafer bondability. Hence one tried to relate wafer bonding to the spatial spectrum of the wafer surface profile and indeed some empirical relations that have been found. The first, who proposed a theory on the problem of the closing gaps between contacted wafers was Stengl. This gap-closing theory was then further developed by Tong and Gosele. The elastomechanics theory was used to study the balance between the decrease of surface energy due to the bonding and the increase of elastic energy due to the distortion of the wafer. They considered the worst case by assuming that both wafers have a waviness, with a wavelength (lambda) and a height amplitude h, resulting in a gap height of 2h in a head to head position. This theory is simple and can be used in practice, for studying the formation of the voids, or for constructing design rules for the bonding of deliberately structured wafers. But it is insufficient to know what is the real area of contact in the wafer interface after contact at room temperature because the wafer surface always possesses a random distribution of the surface topography. Therefore Gui developed a continuous model on the influence of the surface roughness to wafer bonding, that is based on a statistical surface roughness model Pandraud demonstrated experimentally that direct bonding between processed glass wafers is possible. This result cannot be explained by considering the RMS value of the surfaces only, because the wafers used show a RMS value larger than 1 nm. Based on the approach exposed in reference six, a rigorous analysis of wafer bonding of these processed glass wafers is presented. We will discuss the relation between the bonding process and different waveguide technologies used for implementing optical waveguides into one or both glass wafers, and give examples of optical devices benefiting from such a bonding process.
A sub-micron grating can be the key miniaturized planar element in Micro-opto-electromechanical systems where it performs a number of optical functions such as light routing, beam splitting, spectral-analysis, polarization filtering, beam recombination, spatial resonance excitation, in the domains of displacement measurement, biochemical sensors, environment monitoring, laser emission control, WDM communications, to quote a few current applications. This paper will describe the main stream technology of sub-micron gratings and illustrate its application potential.
We propose a new fabrication method for monomode optical integrated devices. As an example, we show how a deeply buried planar waveguide can be obtained by assembling two half waveguides. These ones are realized by ion exchange and the assembling method is the wafer direct bonding (WDB). The optical properties are studied and compared with theory. The results prove that direct bonding, as in VLSI batch technology, is a low cost and high performance technology for optical devices fabrication.
The growing demand for complex components for integrated- optic requires fabrication methods allowing good reproducibility and a minimized number of processing steps. The ion-exchange technique is attractive because it can be used to make inexpensive and versatile waveguides in glass. To ensure reliable operation, the waveguides have to be buried beneath the surface. Various methods for making buried waveguides are based on field assisted ion exchange with one or more steps in the procedure. We propose a new assembling method in which a buried optical waveguide is obtained by direct bonding of two separate waveguides. We found that direct bonding of preprocessed wafers is a useful and versatile step in the fabrication of integrated optical components. Wafer direct bonding is compatible with VLSI batch technology and device miniaturization therefore low cost combined with high performance can be achieved. Furthermore, a large variety of symmetrical or asymmetrical index profiles is possible and the method allows simultaneous fabrication of many identical optical components. Optical properties of the component are studied and the advantages of this new process are summarized and compared with other techniques.