A technique for fabricating ultra-high precision optics is presented. The technique employs a thin multiple aperture mask positioned in front of the substrate during sputter deposition to selectively occlude the beam. The apertures are small in regions where low material deposition is required and correspondingly larger in regions requiring more. During deposition, the substrate is slowly moved back and forth behind the mask over a distance equal to the pitch of the apertures (typically around 2 - 4 mm). This smoothes out any residual patterning of the substrate due to the aperture design of the mask. Using this technique, a transmission optic having an rms physical thickness uniformity less than 0.5 nm, or λ/1000 (measured at 632.8nm) has been produced from a lithium niobate substrate. We believe that this technique will enable the production of the next generation optics for semiconductor fabrication.
Narrow bandpass Fabry-Perot etalons are widely used in solar astronomy for spectroscopic imaging. Solid electro-optically tunable filters made of thin, single-crystal lithium niobate are presented in this article. The pass-band is typically ~0.02nm at 550nm. We describe customized corrective and high-reflectivity optical coatings designed and manufactured to tailor the filter for the specific application. Spectral reflectance is calculated to satisfy wavelength requirements and to achieve optimal optical performance. The measured optical thickness of the lithium niobate wafer is an important factor in determining the optimal design of the etalon mirrors. Out-of-band rejection and bandwidth requirements are also taken into account, as well as the influence of the spectral properties of a high-order filter which blocks adjacent etalon orders. Design customization is particularly important in the case of tandem and double-pass etalons.
The manufacture and testing of the 'core' optical substrates for the Laser Interferometer Gravitational-wave Observatory (LIGO) are described in this paper. These substrates are for use in long baseline Michelson interferometers with Fabry Perot cavities up to 4 km in length in each arm. The optical surfaces of the substrates (250 mm diameter by up to 100 mm thick) are specified either flat or curved, with radii of curvature varying between 7 and 15 km and tolerance bands on the radius equivalent to variations in the sag (over 200 mm) of twenty nanometers or so. Very strict tolerances were also placed on the astigmatism of the surfaces and the surface errors in two spatial frequency bands, one at low frequencies ('waviness') and another at high frequencies ('roughness'). In some cases the radius of the wavefront emerging from the substrate was also specified (for a collimated test beam).
The Laser Interferometer Gravitational-wave Observatory (LIGO) is a long baseline Michelson interferometer, with arms of up to 4 km in length each containing a Fabry Perot cavity. CSIRO has manufactured 32 core optical components for the LIGO interferometer consisting of five different groups of optical elements. Long radii of curvature (7 km - 15 km) and tolerances in the order of plus or minus 200 m in the radius are specified. Although the components are made of hyper pure fused silica there are some residual inhomogeneities in the material. The optics used in transmission must be figured so that the influence of these material inhomogeneities on the transmitted wave front is compensated for. This was done by correcting the surface figure on side 2 of the optics. The approach we took to manufacturing the transmission optics was to calculate the quadratic component of refractive index gradient (Delta) n of the substrate from the measurements of the transmitted wavefront and the surface profile of the two substrate surfaces, determine what shape had to be produced on side two of the substrates to compensate for this gradient and then produce this by optical polishing. The surfaces were polished on rigid solid laps of Zerodur coated with a thin layer of Teflon as the polishing matrix, a technique developed by CSIRO for super-polishing very flat surfaces.
CSIRO is manufacturing the `core' optical substrates for LIGO, a Michelson interferometer with arms up to 4 km in length each containing a Fabry Perot cavity. The beam splitter and input test mass mirrors (the entrance mirror to each cavity) have specifications not only for the optical surfaces but also for the radius of curvature of the wave front transmitted through the optical substrate. Our approach to manufacturing the substrates is to calculate the quadratic component of refractive index gradient (Delta) n from measurement of the transmitted wave front and the surface relief of the two substrate surfaces. After one of the surfaces (S1) is polished to specification, the radius on the second side required to achieve the specification on the transmitted wave front is calculated (using the measured value of (Delta) n, the actual value of S1 and the target value of the transmitted wave front). Results of this work and complications of the measurement procedure due to the thermal inertia and poor thermal conductivity of the silica substrates will be presented.
Core optical substrates for the Laser Interferometer Gravitational Wave Observatory are being manufactured and tested at CSIRO. These substrates are for use in long baseline Michelson interferometers with Fabry Perot cavities up to 4 km in length in each arm. The optics consist of 32 high quality fused silica substrates, comprising folding mirrors, end test masses, input test masses, recycling mirrors and beamsplitters. The dimensions of the substrates are 250 mm diameter by up to 100 mm thick. The optical surfaces are either flat or curved, with radii of curvature between 7 km and 15 km and tolerance bands on the radius equivalent to variations in sag (over 200 mm) of about 20 nm.
Conventional models of ESPI deformation measurement are valid only for planar objects, and then only in the paraxial region. If an object is non-planar, then accurate deformation values can only be recovered if the surface shape is also known. This is because the local sensitivity vectors of deformation are a function of both the illumination and observation directions, which vary with the 3D position of the object points. We present a compact ESPI system for measurement of both shape and deformation using a mutual optical configuration. We also introduce an efficient method of data analysis. The system is demonstrated on a vibrating cylinder, where the shape information is used to derive the true out-of-plane deformation on the cylinder surface.
An electronic speckle pattern interferometry system based on a frequency-doubled twin Nd:YAG laser emitting dual pulses at a TV camera field rate (50 Hz) was developed to analyze addition fringes generated by transient deformation of a test object. The main advance has been the automatic, quantitative analysis of dual-pulsed addition ESPI data by the introduction of carrier fringes and the application of Fourier methods. The carrier fringes are introduced between dual pulses by tilting the reference beam with a mirror driven by a galvanometer. The resulting deformation-modulated addition fringes are enhanced with a variance filter before evaluation of the phase distribution using a Fourier transform method with bandpass filtering. From the wrapped phase distribution, a continuous phase map is reconstructed using an iterative weighted least- squares unwrapper. The linear phase distribution associated with the carrier fringes is removed by evaluating it with a least-squares fitting algorithm. Preliminary results obtained for a thin plate submitted to an acoustic shock show the suitability of the system for the quantitative evaluation of transient deformation fields.
Metrology procedures for determining the power, astigmatism, low and high spatial frequency variations in the surface profile on flat and curved optical surfaces are described. The procedures are applied to the characterization of optics produced for the Pathfinder program of the laser interferometer gravitational observatory and demonstrate that in the case of low spatial frequency surface errors measured by optical interferometry, measurements to a resolution down to (lambda) /2000 are possible in the measurement of the standard deviation of surface variations.
To measure a number of components of displacement with electronic speckle pattern interferometry (ESPI), multiple interferograms are usually captured in succession for different sensitivity vectors. Simultaneous measurement of a number of orthogonal components of displacement using ESPI can be carried out by application of the Fourier transform method. An object is illuminated by several object beams, and the scattered light is combined with one reference beam to form a first speckle image. After providing different tilts to the object beams, a second speckle image is recorded. A third speckle image is then recorded with the object loaded. The difference of the first and second images contains a set of straight carrier fringes. The difference of the first and third images contains a corresponding separate set of modulated carrier fringes due to the loading. The Fourier transforms of these two difference images show multiple peaks which correspond to the carrier fringes of the different object beams. By appropriately masking the corresponding peaks in the two Fourier transforms, the different sets of carrier fringes for the loaded and unloaded object configurations, can be separated. The phase corresponding to the different orthogonal components of displacement can then be retrieved from the ratios of the real to imaginary parts of the two inverse Fourier transforms of each filtered peak. An example of the measurement is presented.
A stroboscopic holographic interferometry technique has been developed and used to measure the modal behaviour of ultrasonic transducers. Displacements of a few nanometres at frequencies into the megahertz region can be measured over the entire surface simultaneously. 1.