Photothermal technology provides sensitive detection of the optical absorption in bulk materials and coatings. To obtain the absolute absorption numbers it requires a proper calibration. In this work a new, “proxy pump” calibration approach is described. The proxy pump has a wavelength at which the material exhibits high enough optical absorption to be evaluated via direct loss measurements. The pump beam is shaped to have the same spot size as the main pump at which the optical absorption of the material is to be determined. Once the thermal field in the material has the same profile both with the proxy and main pump, the sample is self-referenced. Consecutive tests with proxy and main pumps provide absolute absorption numbers. LBO crystals are notorious objects for which the photothermal response is not easy to calibrate since the material has very low absorption in the UV, visible and near IR. In order to calibrate these materials using the above approach we used 2.3 nm laser as a proxy pump. At this wavelength LBO absorbs more than 15 % per cm length. For crystals oriented in XY and YZ planes the photothermal response was found to be 3 times weaker than for Schott glass NG12 with the same amount of absorbed power. With this correction, NG12 glass that has absorption more than 45 %/cm in the wide range of wavelengths can be used as a reference calibration material for LBO crystals.
Substrate-transferred crystalline coatings are a groundbreaking new concept for the fabrication of ultralow-loss mirrors. The single-crystal lattice structure of these substrate-transferred GaAs/AlGaAs Bragg mirrors exhibits the lowest mechanical losses and hence unmatched Brownian noise performance, which nowadays limits the stability of precision optical interferometers. Another outstanding feature of these coatings is the wide spectral coverage of the GaAs/AlGaAs material platform. Limited by interband absorption at short wavelengths and the reststrahlen band at long wavelengths, crystalline coatings can be employed as low-loss multilayers from approximately 900 nm up to 5 μm and beyond. Excellent optical performance has been demonstrated in the near-infrared with excess optical losses (scatter + absorption) as low as 3 parts per million (ppm), enabling cavity finesse values up to 360,000 at 1.55 μm. Our first attempts at applying crystalline coatings in the mid-infrared has resulted in mirrors with excess optical losses of 159 and 242 ppm at 3.3 and 3.7 μm, respectively. Remarkably, these results are already on par with current state-of-the-art amorphous mirror coatings. Absorption measurements based on photothermal common-path interferometry (PCI) reveal that the optical losses are largely dominated by optical scatter. Via, PCI, we have confirmed absorption losses below 10 ppm at 3.7 μm, showing the enormous potential of GaAs/AlGaAs Bragg mirrors at mid-infrared wavelengths. An optimized fabrication process, which is currently under development, can efficiently suppress optical scatter due to accumulated growth defects on the surface. Ultimately, we foresee excess losses significantly less than 50 ppm in the mid-infrared spectral region.
Substrate-transferred crystalline coatings have recently emerged as a groundbreaking new concept in optical
interference coatings. Building upon our initial demonstration of this technology, we have recently realized significant
improvements in the limiting optical performance of these novel single-crystal GaAs/AlGaAs multilayers. In the nearinfrared
(NIR), for center wavelengths spanning 1064 to 1560 nm, we have reduced the excess optical losses (scatter +
absorption) to less than 5 ppm, enabling the realization of a cavity finesse exceeding 300,000 at the telecom-relevant
wavelength range near 1550 nm. Moreover, we demonstrate the direct measurement of sub-ppm optical absorption at
1064 nm. Concurrently, we investigate the mid-IR (MIR) properties of these coatings and observe exceptional
performance for first attempts in this important wavelength region. Specifically, we verify excess losses at the hundred
ppm level for wavelengths of 3300 and 3700 nm. Taken together, our NIR optical losses are now fully competitive with
ion beam sputtered films, while our first prototype MIR optics have already reached state-of-the-art performance levels
for reflectors covering the important fingerprint region for optical gas sensing. Thus, mirrors fabricated via this
technique exhibit the lowest mechanical loss (and thus Brownian noise), the highest thermal conductivity, and,
potentially, the widest spectral coverage of any “supermirror” technology, owing to state-of-the art levels of scatter and
absorption losses in both the near and mid IR, all in a single material platform. Looking ahead, we see a bright future for
crystalline coatings in applications requiring the ultimate levels of optical, thermal, and optomechanical performance.
The PCI technique, a modification of photothermal spectroscopy, has become a powerful tool for testing various low absorptive optical materials and components. The current state of the technique and recent progress in extending its capabilities toward the mid-infrared region is presented. A 3.39 μm probe was used for testing and studying various semiconductor materials, such as p-doped GaAs, that can exhibit non-thermal response to the pump beam in addition to the thermal one. A simple theoretical model of the PCI method is shown to describe adequately the experimental data, making it possible to calibrate the setup without using a calibration standard.
This paper present the results of computer simulations of pattern recognition using LPCC filters, designed for various types of distortions of input images (rotation, change of scale and rotation + change of scale).
One of the widespread methods for distorted patterns is to use a distortion invariant correlation filters. Invariant filters have different properties that are quite good for different pattern recognition problems. This paper presents the results of computer simulations of pattern recognition using different modern approaches on distortion invariant correlation filters. The different types of correlation filters (MACE, GMACE, LPCCF, WBKF and others) are compared for input test sets of different examples of patterns. There are presented results of pattern recognition for different types of distortions. The output correlation peaks are compared by its characteristics. The obtained results of comparison provide that in some cases there are correspondences between the choused correlation filter, the variant of pattern and type of the distortion for optimal output peak characteristics.
One of the main problems of optical data processing is the problem of image recognition. There were given much attention to optoelectronic methods of recognition of distorted images nowadays. There are a number of different approaches for the solution of such problem. One of the most popular approaches is using of optical correlators for this field. The main problem of this approach is to select an object to provide a correlation of input image with it. One of the widespread methods is to use an effective object-an invariant correlation filter. The paper presents the results of investigations on image recognition with the help of Wavelet Basis Kernel Filters (WBKF). Both results of the theory and computer simulations are presented. Also computer simulations hold a comparison of image recognition results with the help of other different approaches (GMACE, SDF and so on). The obtained results seem to be better for WBKF recognition in some cases. There are presented authors suggestions about using of WBKF filters for different distortion invariant image recognition problems and results of image recognition in presence of white noise.
The LIGO project has completed the installation of large fused silica optical components in the vacuum systems of its observatories. Commissioning work on the Hanford 2 km interferometer has determined an upper limit to the optics losses, allowing comparison with design and pre-installation testing. Planning and development of sapphire optics for the next generation, advanced LIGO detector is now underway, including polishability, optical homogeneity, absorption, and birefringence. The advanced optics development also includes research aimed at lowering coating loss.
The effects of reduction, oxidation, Li-enrichment and impurity on LiTaO<SUB>3</SUB> crystals were studied. It is demonstrated that the best LiTaO<SUB>3</SUB> crystals show less absorption than LiNbO<SUB>3</SUB>, less photorefraction and no green-light-induced infrared absorption.