When applying phase-stepping interferometry to measure wavefront with aberration, distortion is
caused by various factors and the main one is phase-shifting error from phase shifter. In this paper,
the calculation theory and consequence in measurement error of the phase shift algorithm are
discussed. These algorithms include traditional three-phase algorithm, traditional four-phase
algorithm, traditional five-phase algorithm, carre algorithm, new compensating four-phase
algorithm and new compensating five-phase algorithm. According to the theory analysis and
computer stimulation, these methods are compared through introduce phase-shifting error,
wavefront aberration and Gauss noise, the results show the new compensating five-phase
algorithm is immune to phase-shifting error, and its result is insensitive to Gauss noise, it can
improve measurement accuracy effectively.
For a segmented primary mirror consisting of 7 segments organized in 1 concentric hexagonal band around the
central segment, of which the point spread function (PSF) is represented by the product of the grid function
(GF) and the PSFj of an individual segment. The system PSF and Strehl ratio given for the cases of the
gaps or piston errors are present.
At present, the large telescopes use segmented primary mirror. Segmentation and edge effects are closely related to
system image quality. Generally there are four kinds of edge effects including gaps between segments, turned down/up segment edges, piston and random tip-tilt errors. This paper concentrates on a primary mirror structure with a central octagonal segment and eight marginal approximately annular-sector segments. We will give a Strehl Ratio formula that contains edge effect parameters. In addition, both the derivation and simulation are based on the assumption that edge effects exist uniquely.
The design of segmented mirror is an effective way to increase the resolution of space optical remote sensor. We analyze
the gaps and piston that affecting the system imaging performance. The corresponding Point Spread Function (PSF),
modulation transfer functions (MTF) and SR (Strehl Ratio) are presented. The General Image Quality Equation (GIQE)
is used to Linking NIIRS to those effects to provide engineering the criteria for system performance. We show an example of how NIIRS is applied to measure the influence of these effects on system performance. In the example, the Δ NIIRS is computed by GIQE. Δ NIIRS is performed to measure small loss in image quality. When the Δ NIIRS is more than 0.2, the loss of image quality are non-negligible. The experiment show the NIIRS can provide a simple, yet powerful, tool for assessing and communicating image quality and sensor system requirements.
In multiconjugate Adaptive Optics (MCAO), the phase screens are used to simulate atmospheric turbulence layers to
study the optimal turbulence delamination and the determination of layer boundary position. In this paper, the method of
power spectrum inversion and sub-harmonic compensation were used to simulate atmospheric turbulence layers and
results can be shown by grey map. The simulation results showed that, with the increase of turbulence layers, the RMS
of adaptive system decreased, but the amplitude diminished. So the atmospheric turbulence can be split into 2-3 layers
and be modeled by phase screens. Otherwise, a small simulation atmospheric turbulence delamination system was
realized by JAVA.
A new type of space telescope program is developing technology which is to be used in the design of a 4-meter segmented mirror telescope. One acceptable configuration being considered for the primary mirror is a lightweight honeycomb sandwich structure segmented mirror. There can be optically-significant distortions when the primary mirror is exposed to typical thermal environment. This paper discusses the surface distortions subjected to thermal distributions.
Finite element analyses were performed to predict the optical surface distortions of the 4-meter primary mirror due to the effects of thermal variations. The temperature patterns were described by a least-squares fit to a polynomial expression, and the polynomial was then used to predict temperature patterns. The finite softwares were also used to analyze cases of axial gradients, radial gradients and the local thermal gradients etc. The result from the finite element analysis is analyzed, and is also presented.
A simple and high precision method to measure the phase modulation characteristics of Liquid Crystal Spatial Light Modulator, namely, the relationship between phase and voltage(gray), is proposed. Using the Digital Wave Front Phase-shifting Interferometer, the phase difference from different voltage (gray) can be obtained directly from the interferometer, so it is easy to get the phase modulating characters of Liquid Crystal Spatial Light Modulator. The wavefront
correction has been realized by using the Liquid Crystal Spatial Light Modulator in adaptive Optics. The distorted wavefront can be tested in the Interferometer and be expressed by Zernike Polynomial, using the phase modulation character of Liquid Crystal Spatial Light Modulator, the corresponding gray picture can be set up. The conjugate wavefront can be obtained, and thus the correcting of the static distorted wavefront is completed and the effect is displayed as proved by the improvement of correlative parameters such as PV value, rms value and strehl ratio.
The white-light continuum generation (WLCG) has been widely used as a probe beam for the ultrafast time-resolved difference absorbance spectrum or the seeding beam in an optical parametric amplifier (OPA). Therefore reducing the chirp effect in WLCG would be important. We report the study on WLCG in different medium by focusing 800nm laser pulse with a pulse duration of 150fs into 2mm thick CaF2, 2.4 and 0.5mm sapphire plates, try to find any medium as well as the optical pathlength effect on the chirp of the WLCG. The chirped dispersion of WLCG was determined by optical Kerr effect. The results show that the temporal span of the WLCG is almost independent on the optical pathlength of the medium in the observed spectral region, while it has an obvious dependence on the different media used.
The time-dependent optical reflectivity of HgCdTe detector at 0.63μm has been measured during irradiating by a 1.06-μm Q-switched Nd:YAG laser of 50-ns duration. The reflectivity was observed to increase abruptly to a value and to remain at that value for a period of time, which ranged from several nanoseconds to several hundreds of nanoseconds, depending on the irradiating pulse intensity. The duration of the “flat-top” portion of the reflectivity waveform represents the total time that the surface is melting. Subsequently, the reflectivity dropped abruptly to a value less than the initial one. The difference shows that the damage takes place in the incidence point. By measuring the time-resolved reflectivity and simulating physical structures of PbS and HgCdTe detector, whose dynamic mathematical model was created. By solving the equations of energy transport and thermal diffusion, the temperature rises of PbS and HgCdTe detector irradiated by pulse laser beam were studied, the relationship of power and temperature rises were discussed and the numerical solutions of dynamic temperature field were obtained. The experimental result of the reflectivity of HgCdTe detector was compared with calculated result. The results show finite element analysis is effective in solving the temperature field.
The samples of semiconductor’s surface temperature increased abruptly, when illuminated by laser pulse. The sample’s surface melted and remained liquid phase for a few hundreds ns. That caused reflectivity enhancement of the sample surface. In this article, numerical calculation was carried out on HgCdTe and PbS. And a 1mm-thick HgCdTe was used as the sample in the experiments. In the experiments, the sample was illuminated by laser pulse of 60ns duration from a Q-switched Nd:YAG laser. A beam from He-Ne laser was used as the monitor beam to illustrated the reflectivity changes of the sample. The results of the experiments were the conclusive evidences of our numerical calculation of the dynamic behavior in the sample.
Laser induced damage in the visible window glass, such as K9 glass, irradiated with Nd:YAG laser operating at 1.064μm wavelength and approximately 10ns pulse duration was investigated in the present experiments. The pulse energy, duration, and beam spot size were taken as changeable factors to study the laser damage results. The experimental arrangement consists of a Q-switched Nd:YAG laser with pulse duration of about 10ns operating at 1.064μm, a lens of 40cm focal length which focus the laser beam onto the sample, a diaphragm to adjust the incident laser energy, a power stabilized He-Ne laser at 0.6328μm providing a diagnostic beam onto the Nd:YAG spot size, a detector which can measure the 0.6328μm reflected from the spot during the pulses beating. Images analysis was conducted at different laser induced damage level. At last, the dynamic damage threshold measuring method was discussed.
The optimal turbulence delamination in MCAO for HeFei region are studied. Because of the delamination structure of atmospheric turbulence, the atmospheric can be split into several layers and be modeled by phase screens. It is shown that MCAO system considering 2- layer atmospheric turbulence has a high SR.
The influences of atmospheric turbulence on image resolution of space-borne optical remote sensing system are discussed. The coherence length of wavefront r0 and the variance of angle-of-arrival fluctuation σ2α of sensing system are the main basis to evaluate the atmospheric influence on image resolution. In this paper, different atmospheric turbulence models are chose to calculate r0 and σ2α in order to analyze the influence of atmospheric turbulence, and the results show that the influence of atmospheric turbulence should be weak and in most cases could be ignored for space-borne optical sensing system.
It is necessary for an adaptive optics system to be excepted to achieve established signal-noise ratio that enough signal photon fluxes are there in every subaperture. This requests that either the object imaged is bright enough or there is a bright guide star within the field of view. However, it is unfortunately as often as not in the case in nature and the artificial guide stars have need for.
There are two kinds of artificial beacons up to now. They are sodium laser guide star and Rayleigh guide star. The latter among which is generated by the Rayleigh scatter of laser beam from stratosphere. More specially, the laser beam transmitted by ground transmitter propagates upward and undergoes the absorption and the scatter of the atmosphere along the way. It is obvious that only the scattering of the atmosphere within specified altitude range is beneficial. So we always hope that the Rayleigh scattering in this range would be strong and the absorption would be weak to the greatest extent. Under stated altitude, both the scattering and the absorption are the weaker the better.
Either the absorbance or the scatterance is dependents on the wavelength of the laser beam and relates to the atmospheric constitution. In this paper, the optimum wavelength for Rayleigh laser beacons is presented by striving for relative extreme values of the transmissivity and the scattering.
Recently, the progress in adaptive optics makes the cone effect of Rayleigh laser guide stars be solved fairly. So that the applications of this kind of beacon in adaptive optics, especially in multiconjugate adaptive optics (MCAO) presents a bright future once again. The photon flux density required by the wavefront sensor of MCAO with certain performance is proportional to the number of layers. This results in that the laser energy required by guide stars for "classical" MCAO is very high. In the "new concept" MCAO, however, the Rayleigh beacons are formed by light scattered from whole higher atmosphere but not only a selected thin layer. Then the required laser energy degrades greatly.
Laser-induced fluorescence (LIF) is a promising method for pre-malignant lung tissue diagnosis and it is very important to find effective common diagnosis criteria needed by clinic. In this paper, some differences between normal and cancer lung tissue, such as I444/I414 ratio, main peak width, and integral area were exhibited and discussed. By using the fourth harmonic at 266nm of a Q-switched Nd:YAG laser as an excitation source, Optical Multi-channel Analyzer (OMA) as a detector, laser-induced fluorescence spectra of normal and cancer lung tissue in-vitro were measured. According to the spectra characteristics, three independent diagnosis criteria were obtained and analyzed. The total specificity is above 95%.
A new method of remote sensor of micro amount oil in water by laser induced fluorescence is presented. In the detecting system, a MODEL YAG571C pulsed triplicated laser (Continuum Corporation of U.S.A.) With wavelength of 355nm, pulse duration of 35ps, pulse energy of 3mJ was used as exciting source. Using multi-track spectrograph as the detector, a computer as the data processor, 355nm laser as the master trigger which contact the SRS-DG535 four channel digital delay/pulse generator, we have obtained fluorescence lifetime of some aromatic pollution materials in water and relevant 3D (time-wavelength-fluorescence intensity) fluorescent diagrams. By analyzing the spectra property, it is possible to discriminate the oil pollution types.
A laboratory unit is described that was designed to detect and identify the oil pollutants in the environment by means of the laser-induced fluorescence spectrum analyzer. UV laser beam from third harmonics of the Nd:YAG laser is used to excite target oil. The multichannel detection system consists of a spectrometer, a scientific-grade gated ICCD camera and a digital delay generator. A personal computer is responsible for the control for the whole system as well as for the data processing. In the experiment, the laser beam is directed toward the samples while the return fluorescence signals is collected by a Cassegrainian reflecting telescope. The received signals pass through a bunch of fibers, and go into the input slit of the spectrometer. The fiber bunch includes 19 fibers, one end of which take shape of disc to collect the signals from the telescope while the other arrange in a row to match the spectrometer slit. As the results, the fluorescence emitting spectrum of sample oil under the illumination of laser beam wavelength of 355nm is in the range 400nm to 700nm and center around 520nm. The laser and the receive system are both at a distance 25m from the target since the restriction of the working place. The system has the detection capacity for more distance.
When a laser beam is used to produce a sodium guide star, a Rayleigh guide star can be got as a 'by-product' with the temporal-gating method. In the case of the atmospheric phase distortion is confined to two thin turbulent layers, the sodium guide star is used to measure both the low-altitude and the high-altitude phase distortion (phi) b while the Rayleigh guide star provides a suitable probe signal for the contribution (phi) l of low-altitude turbulence layer. The wavefront distortion (phi) l of low-altitude turbulence layer. The wavefront distortion (phi) h causing by the high-altitude turbulence layer alone is then easy obtainable from (phi) l and (phi) b. The multiconjugate correction system could be founded upon these.
The relationship between the performance of a partial correction adaptive telescope using laser guide stars and the star brightness is discussed. The laser energy requirements for Rayleigh guide star as well as sodium guide star are presented in cases where the diameter d of a subaperture of the telescope is larger than the atmospheric coherence length r0 and the residual rms phase error across the subaperture is larger than 0.63.
Multiconjugate adaptive optics is an attractive tentative plan for increasing the isoplanatic patch of turbulent atmosphere. Some basic problems relating to this plan have been discussed and corresponding formulas have been given.
The bandwidth requirements for an adaptive optics system with R-C filter as servo response has been derived. The relationship of this bandwidth to Strehl Ratio has been presented in two cases of the constant wind speed and the constant pseudo-wind speed which predominates over the natural winds at all altitudes. It has been found that a bandwidth of hundreds hertz is more than enough for the former while thousands of hertz may be needed for the latter.
By taking the atmosphere as an N-layer model and keeping the isoplanatic angle invariable for all the layers, the gain of the isoplanatic patch compared to the conventional adaptive optical system has been derived.
The gains of an isoplanatic patch, both in diameter and in area, are presently derived for a Multiconjugate Adaptive Optics system through comparison with a conventional system for height-dependent refractive index structure coefficient. By treating the atmosphere as an N-layer model and keeping the isoplanatic patch size invariable for all layers, the patch gain is obtained.