Beam scanning technology is widely used in LIDAR, space optical communication, adaptive optics, and other fields. The microlens array scanner (MLAS) has the potential to realize large angle beam scanning with large beam size. In this paper, the current research status of MLAS and research progress are briefly introduced first. After that, the two-dimensional scanning mathematical model of the Kepler structured MLAS is analyzed and established using Fourier optics. Then, simulations are conducted to demonstrate the process of two-dimensional discrete addressing scanning. The analysis results show that the scanning angle that can be addressed by MLAS is determined by the parameters of the microlens array, and that the displacement error between the microlens arrays leads to scanning spot energy decrease. Finally, some thoughts about the future development of MLAS are given.
Segmented telescope is an effective way to realize high-resolution observations in astronomy. An important work for high-resolution observations using segmented telescopes is phasing the segmented primary mirror. Modified Shack-Hartmann sensor. Is proposed for piston error detection. The interference pattern created by a circular lens placed across two adjacent mirrors in exit pupil plane is used as the signal of the modified Shack-Hartmann sensor. Piston errors need to be extracted from the interference pattern. The offset of lens and gap error of adjacent mirrors causes the distortion of interference pattern, and leads to a reduction in the detection accuracy of existing piston error extraction techniques. In this paper, we propose to replace the circular lens with a square lens and the mathematical model of the corresponding interference pattern is modeled by Fourier optics, including the one-dimensional and two-dimensional analytical solution of the interference pattern. The simulation results show that the proposed analytical solution can effectively characterize the interference pattern in the ideal situation and in the presence of lateral offset of the lens and the gap error of the adjacent mirrors situation. The results presented here give a deeper insight into the interference pattern of modified Shack-Hartmann sensor, and are of great help for developing new piston error detection techniques based on modified Shack-Hartmann sensor.
Gas flow distribution of reaction chamber of reactive ion etching (RIE) etcher is usually considered to be a main factor in determining both the plasma distribution and etching uniformity. Based on the continuum fluid and heat transfer models of the commercial software, Fluent(Ansys), the gas flow distribution of the reaction chamber was simulated. And then the spatial distribution profiles of the pressures above the electrode surface under the different mass flow (50~250sccm) inlet conditions , and the influence of the different GAP (L = 0.03~0.06m) of the chambers on the gas flow uniformity were discussed. The result shows that the pressure distribution above the electrode has the character which the pressure is higher in the center of the electrode and lower at the edge and increases with the rise of the mass flow of inlet. And the uniformity of the gas flow distribution is enhanced with the rise of the GAP of the chamber.
Gemini-style protected-silver mirror (Sub / NiCrNx / Ag / NiCrNx / SiNx / Air) is a suitable choice for optical instruments requiring both long-term environmental durability and high broadband reflectance. Three Gemini-style protected-silver mirrors with NiCrNx interlayer thicknesses between 0.1 and 0.6 nm were prepared by magnetron sputtering, and the dependences of spectral properties and environmental durability of these protected-silver mirrors on the thickness of NiCrNx interlayer between the silver layer and SiNx layer were investigated in-depth. The reflectance, transmittance and total scattering loss measurements, optical microscope, and scanning electron microscope imaging were employed to characterize the spectral properties and surface morphology, and accelerated environmental tests, including humidity test and salt fog test, were applied to investigate the environmental durability. The experimental results showed that both optical and corrosion-resistant properties of protected-silver mirrors were NiCrNx interlayer thickness dependent, and an optimum NiCrNx interlayer thickness should be ∼0.3 nm for Gemini-style protected-silver mirrors to have reasonably both high reflectance in a broadband spectral range from visible to far infrared and good corrosion resistance for long-lifetime applications in harsh environments.
A combined cavity ring-down (CRD) and photometry technique is employed to measure the transmittance of optical laser components in a range extending from below 0.01% to over 99.99%. In this combined technique, the conventional photometric configuration is used to measure, by ratioing the transmitted light power to the input power, the transmittance ranging from below 0.01% to over 99% with a typical relative uncertainty below 0.3%, and the CRD configuration is used to measure the transmittance higher than 99% with an uncertainty below 0.01%. Eight test samples with transmittance in the range of nearly 99.99% to approximately 0.013% are experimentally measured. Uncertainties of approximately 0.0001% for the transmittance of 99.9877% and of 0.003% for the transmittance of 0.013% are achieved with respectively the CRD and photometric schemes of a simple experimental apparatus. The experimental results showed that the combined technique is capable of measuring the transmittance of any practically fabricated optical laser components.
A filtered optical feedback cavity ring down (FOF-CRD) technique employing a continuous wave Fabry-Perot diode
laser is employed to measure the total optical losses, i.e., absorption and scattering in optical components with arbitrary
thickness. The FOF from the ring down cavity (RDC) is re-injected into the oscillator cavity of the diode laser, and the
coupling efficiency of the laser into the RDC is significantly enhanced due to the FOF effect. An optical component
having parallel optical surfaces is inserted exactly normal to the light beam in the RDC. The optical losses of the
component are obtained from the change in the ring-down time of the RDC containing the component with respect to
that of the empty RDC. The measurement results for different samples are in good agreement with conventional laser
calorimetry data. The experimental results have demonstrated that the FOF-CRD technique is simple, inexpensive and
fast for measuring optical losses of optical components used in high-power laser system.
Cavity ring-down (CRD) techniques based on measuring the rate of decay of light intensity inside the optical cavity,
are widely used for trace gas analysis and high reflectivity measurement. In this presentation a filtered optical feedback
CRD (FOF-CRD) technique employing a multi-longitudinal-mode continuous-wave diode laser is investigated for
measuring high reflectivity of high reflective mirrors. The original spectrum of the diode laser without the effect of FOF
has two longitudinal modes covering tens of the free spectral ranges (FSR) of the ring down cavity (RDC). Due to the
relatively broadband spectrum, the theoretical efficiency of coupling the laser power into the RDC is less than 0.05%. In
the FOF-CRD scheme, on the other hand, the FOF induced overall spectrum broadening is experimentally observed,
with the diode laser running with several longitudinal modes. However the bandwidth of each longitudinal mode is
significantly reduced. The coupling efficiency of the laser power into the RDC is higher than 20% in FOF-CRD
technique. The enhancement of the coupling efficiency induced by the FOF effect is nearly three orders of magnitude.
High accuracy measurements of high reflectivity are achieved with this simple FOF-CRD scheme.
The influence of a predefined threshold value on high reflectivity measurement in an optical feedback cavity ring-down
technique (OF-CRD) is investigated. The strong optical feedback from the front cavity mirror of a linear ring-down
cavity is re-injected into the oscillator cavity of the diode laser and causes line-width reduction of diode laser. The
narrowed laser line is occasionally in resonance with one or more ring-down cavity modes and large resonant peaks are
observed in the ring-down cavity output signals. The diode laser is switched off at the negative step of the square-wave
modulation signal. If the amplitude at the negative step exceeds the pre-defined threshold value, an exponential decay
signal is recorded to fit the cavity decay time. The reflectivity of the cavity mirror is determined statistically to be
99.9907±0.0002% and 99.9899±0.0004% with threshold values of 350mV and 50mV, respectively. All reflectivities
measured at six cavity lengths with a higher threshold are more precise than that measured with the lower threshold. A
method to determine the threshold value is proposed for accurate high reflectivity measurement.
Highly reflective mirrors have been widely used in high power lasers, laser gyros, and gravitational-wave detection, etc.
However, reliable measurement of high reflectivity (R>99.99%) is difficult. In this paper a novel optical feedback cavity
ring-down technique (OF-CRD) by re-injecting the strong optical feedback from the ring-down cavity (RDC) into the
oscillator cavity of a Fabry-Perot diode laser is developed for the ultra-high reflectivity measurement. The laser line is
narrowed and occasionally in resonance with one or more ring-down cavity modes. The amplitude of the RDC output
signal is enhanced by a factor of over two orders of magnitude, compared with the conventional phase-shift CRD
technique. Four pairs of cavity mirrors with different reflectivity are used to investigate the sensitivity and
reproducibility of the OF-CRD technique. The accuracy is greatly enhanced from about 0.003% to 0.00003% as the
reflectivity of cavity mirrors increases from about 99.8% to 99.996%. A folded RDC with cavity length of 70cm is
constructed by inserting a planar test mirror into the linear RDC and the reflectivity of the test mirror is statistically
determined to be 99.9526±0.0004%. The OF-CRD is simple, reliable, highly-sensitive and cost efficient.
When a modulated laser beam irradiates an optical component, the laser-induced surface deformation has both direct
current (DC) and alternating current (AC) portions. Explicit surface deformation and surface thermal lens (STL) theory
models are developed to describe the DC and AC portions of the surface deformation and corresponding STL signals.
Experimentally, a setup combining laser calorimetry (LC) and STL technique is developed to measure the absolute
absorptance and laser-induced surface deformation of optical components. The absorptance measurement is implemented
by LC with excellent stability and repeatability. The surface deformation measurement is realized with STL amplitude
by defining an approximately linear relationship between the AC (or DC) STL amplitude and the maximum AC (or DC)
deformation. As an example, the deformation value of a BK7 substrate coated with a TiO2/SiO2 film stack of absolute
absorptance 1.32×10-3, irradiated by a 1064nm laser with 3.8W power is determined to be 34.3 nm with the experimental
STL amplitude, in good agreement with the theoretical value of 35.8 nm calculated by the explicit surface deformation
model. An indirect approach is proposed to determine accurately the irradiation beam radius by fitting the experimental
data of the radial AC intensity change at the detection plane to the explicit STL model. By performing a theoretical fit to
the experimental frequency dependence of the STL amplitude, the thermal properties of the optical component (i.e. the
thermal diffusivity) can also be determined.
Exponential-decay based cavity ring-down (CRD) techniques, such as the pulsed-CRD and continuous-wave (cw) CRD employing a fast switch to shut down the laser beam, are widely used for high reflectivity measurement. In this paper the influence of the response time of the experimental apparatus on the high reflectivity measurement is investigated theoretically and experimentally. Theoretical expressions taking into account the instrumental response time are given for both pulsed- and cw-CRD techniques, respectively. By establishing a simple cw-CRD setup employing detectors with different response time, the influence of the instrumental response time on the high reflectivity measurement is experimentally investigated. By applying a multi-parameter estimation technique to determine simultaneously the cavity decay time and the overall response time of the experimental apparatus via fitting the experimental CRD signal to the corresponding theoretical model, the influence of a long instrumental response time on the reflectivity determination is eliminated. The reflectivities of the cavity mirror measured with detectors with different rise time are in excellent agreement. On the other hand, the error of high reflectivity measurement increases with the increasing rise/fall time of the apparatus in cases that the CRD signals obtained by detectors with relatively slow rise time are simply treated with a single exponential decay fitting procedure.
A simple and sensitive photothermal technique-photothermal detuning (PTDT), which is based on the absorption-induced shift of reflectance or transmission spectrum of an optical coating, is developed to measure the absorption of coated optical components. A PTDT theory is developed to describe the signal's dependence on the structural parameters of the optical coatings and on the geometric parameters of the experimental configuration. An experiment is performed to measure the PTDT signal of a highly reflective multilayer coating used in 532nm by using a probe beam with a wavelength of 632.8nm. By optimizing the incident angle of the probe beam, the measurement sensitivity is maximized. Good agreements between the theoretical predictions and experimental results are obtained.
A theoretical model for the surface thermal lens (STL) signal with modulated top-hat and Gaussian beam excitations is
developed. For an optical coating sample, distributions of the temperature and surface deformation in both transient and
quasi-steady states are deduced, and the STL amplitudes, corresponding to the alternating current (AC) deformation and
direct current (DC) deformation, respectively, are defined. Numerical simulations and comparison results for the
temperature and deformation demonstrate that there exist large differences in radial distributions between the AC and
DC cases especially in the high modulation frequency range. The pulsed or AC STL amplitude under the top-hat beam
excitation is approximately two times of that under the Gaussian beam excitation at the optimum detection distance in
the high frequency, and correspondingly, the DC STL amplitude in quasi-steady state with top-hat beam excitation is
only ~1.1 times of that with Gaussian beam excitation at the optimum detection distance. Influences of the heating-beam
radius and modulation frequency on the STL amplitudes are also presented and compared. The application of the STL
technique to the deformation measurement of an optical component is discussed.
A continuous-wave cavity ring-down (CRD) technique, in which a diode laser is employed as light source, is developed to determine the high reflectivity of optical dielectric mirrors. Based on on-axis alignment, large resonant peaks are obtained for CRD signal. When the amplitude of signal exceeds the predefined threshold, laser beam is switched off and a single exponential decay signal is obtained to fit the cavity decay time. The reflectivity of the cavity mirror, measured at several cavity lengths, is statistically determined to be 0.99737 at 1064 nm, with an uncertainty of 6×10-5. On the other hand, with off-axis geometry, a steady signal is obtained to make a sensitive lock-in measurement and the steady signal itself fitted with appropriate theories to determine the cavity decay time. The reflection of the cavity mirror obtained by three methods and at different cavity lengths are in excellent agreement.
Cavity Ring-Down Spectroscopy (CRDS) is a highly sensitive loss measurement technique. It was used for quantitative diagnostics of molecular species. In our experiment CRDS has been applied for measuring the reflectivities of optical mirrors. An experimental system has been established. The reflectivities of a pair of cavity mirrors and of a 22.5° optical mirror have been measured. The reflectivities of the cavity mirrors are (99.925±0.001) % and that of the 22.5° mirror is (99.992±0.003) %° The measurement error was as low as 0.001%.
Pulsed surface thermal lens (TL) technique is used to investigate the laser conditioning and to measure the nonlinear absorption of LaF3/MgF2 dielectric multilayers deposited on CaF2 substrate at 193 nm. The conditioning effect is monitored starting from the first shot of irradiation and in a shot-by-shot basis. The LaF3/MgF2 multilayers show a very strong conditioning. The ratio of the absorptions before and after the laser irradiation is approximately 4 - 8 for a highly reflective (LH)20 LaF3/MgF2 multilayer, and approximately 4 for (1L3H)7 and (3L1H)7 multilayers. For comparison, a (LH)20 LaF3/AlF3 multilayer shows only weak laser conditioning effect, with an absorption ratio of approximately 1.4. Our experimental results suggest that the strong conditioning effect of the LaF3/MgF2 multilayer is due to the possible interaction between the LaF3 and MgF2 layers, which results in considerable increase and conditionability effect of the LaF3 absorption. The fluoride multilayers present non-negligible nonlinear absorption, and the two-photon absorption coefficient of the multilayers is estimated to be 5(DOT)10-7cm/W.
HR layer stacks with increasing number of HL pairs of fluoride material deposited on different substrates have been successfully investigated with respect to the laser radiation damage threshold at 248 nm and 193 nm. In this paper the investigation has been extended on resonant as well as non-resonant oxide coatings deposited by ion beam sputtering (IBS). Compared to conventional evaporation and plasma ion assisted evaporation technique, IBS coatings exhibit a higher packing density, thus preventing water to enter the film which woudl degrade the quality of the coatings. In contrast, the thermal stress is increased in IBS layer stacks. The measurements were carried out by use of the pulsed thermal lens technique enabling us to measure the advent of otpical breakdown induced by laser fluences in the subdamage range.
A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is used to measure in situ the thermoelastic response of UV dielectric coatings under excimer laser irradiation with fluences from below to above damage threshold (LIDT). At fluences below LIDT, the thermal lens amplitude is caused by the surface displacement of the optical coating samples, due to the coating absorption and the thermal expansion of the substrate. Measurements are made on highly reflective (HR) dielectric coating on quartz and copper substrates. For copper substrate HfO2/SiO2 HR mirror, the damage of the coating is caused by thermal stress due to the strong thermal expansion of the copper substrate. For quartz substrate sample, on the other hand, the thermal stress caused by the thermal expansion of the substrate is relatively weak and the coating is damaged by the defect- absorption induced melting and evaporation of the coating. The copper substrate sample therefore shows lower LIDT than the quartz substrate sample. For HfO4-2)/SiO2 HR coating on copper sample, we also observed a decrease of thermal diffusion rate with increasing the fluence form below to above LIDT, which indicates that delamination occurs at interface of the coating/substrate and/or between different coating layers.
A mode-mismatched surface thermal lens technique with pulsed top-hat beam excitation and near field detection scheme is developed to measure in situ the thermoelastic response of UV dielectric coatings and bulk materials under excimer laser irradiation. The thermal lens technique is demonstrated to be not only convenient for an accurate determination of the laser-induced damage threshold (LIDT), but also sensitive to measure the thermoelastic response of dielectric coatings irradiated with fluences much below the LIDT, and hence, to carry out time resolved predamage investigation. The minimum detectable surface displacement of approximately 0.002nm is achieved with a simple experimental configuration. Nonlinear absorption of UV dielectric materials and coatings are demonstrated. The surface thermal lens technique is also a convenient technique to distinguish different damage mechanisms: thermal stress induced damage or melting induced damage, depending on the thermo-elastic properties of the substrate. Hence, this technique allows to indicate qualitatively the relative contribution of linear and nonlinear absorption as possible causes for laser damage. Moreover, the nonlinear effect in laser conditioning of a LaF3/MgF2 highly reflective dielectric coating has been observed experimentally.
Photothermal techniques, such as probe beam deflection and thermal lens detection, have been widely used for low absorption measurement, thermal characterization, and laser- induced damage detection of optical coatings. In specially configured photothermal techniques, the probe beam either detects the photothermally induced refractive index change inside the sample via propagation through the interacting region in the measured sample, or detects the surface displacement via reflection from the deformed surface. Usually, due to the very low absorption of the sample or/and the short interaction length, a very high sensitivity is required for such applications. It is therefore of importance to maximize the sensitivity for each measurement, by selecting appropriate detection scheme and optimizing the performance of the selected scheme. In this paper, we first maximize the sensitivity of these photothermal techniques by configuration optimization, then compare their maximum sensitivity. The applicability of the pulsed photothermal techniques to optical coating characterization is also discussed.
The thermal decay of surface temperature has been calculated and measured on Al2O3/SiO2 coatings by using photothermal technique near damage threshold fluence. It could be shown that under certain conditions the decay times (tau) has been found to be in the order of some microseconds. This fact gives no explanation of the measured decrease of threshold fluence in some cases for higher repetition rates and shot numbers on Al2O3/SiO2 and HfO2/SiO2 multilayers for (lambda) equals 248nm. Furthermore, it could not be found any influence of the substrate materials. Thus, other than thermal accumulation is responsible for the lowered damage threshold by increasing repetition rate. Additionally, performed calculations of the thermal decay using 20ns, 248nm laser excitation confirm the experimental results. Even in the case of Al2O3/SiO2 coatings on copper no effect of the substrate as a heat sink could be measured. For HfO2/SiO2 coatings the behavior is in accordance with the fact that the thermal conductivity of HfO2 films is markedly lowered compared to the bulk value.
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