High spectral resolution lidars (HSRLs) designed for aerosol and cloud remote sensing are increasingly being deployed
on aircraft and called for on future space-based missions. The HSRL technique relies on spectral discrimination of the
atmospheric backscatter signals to enable independent, unambiguous retrieval of aerosol extinction and backscatter.
NASA Langley Research Center is developing a tilted pressure-tuned field-widened Michelson interferometer (FWMI)
to achieve the spectral discrimination for an HSRL system. The FWMI consists of a cubic beam splitter, a solid glass
arm, and a sealed air arm. The spacer that connects the air arm mirror to the main part of the interferometer is designed
to minimize thermal sensitivity. The pressure of the sealed air-arm air can be accurately controlled such that the
frequency of maximum interference can be tuned with great precision to the transmitted laser wavelength. In this paper,
the principle of the tilted pressure-tuned FWMI for HSRL is presented. The pressure tuning rate, the tilted angle
requirement and challenges in building the real instrument are discussed.
High spectral resolution lidars (HSRLs) are increasingly being deployed on aircraft and called for on future space-based
missions. The HSRL technique relies on spectral discrimination of the atmospheric backscatter signals to enable
independent, unambiguous retrieval of aerosol extinction and backscatter. A compact, monolithic field-widened
Michelson interferometer is being developed as the spectral discrimination filter for an HSRL system at NASA Langley
Research Center. The interferometer consists of a cubic beam splitter, a solid glass arm, and an air arm. The spacer that
connects the air arm mirror to the main part of the interferometer is designed to optimize thermal compensation such that
the maximum interference can be tuned with great precision to the transmitted laser wavelength. In this paper, a
comprehensive radiometric model for the field-widened Michelson interferometeric spectral filter is presented. The
model incorporates the angular distribution and finite cross sectional area of the light source, reflectance of all surfaces,
loss of absorption, and lack of parallelism between the air-arm and solid arm, etc. The model can be used to assess the
performance of the interferometer and thus it is a useful tool to evaluate performance budgets and to set optical
specifications for new designs of the same basic interferometer type.
High spectral resolution lidars (HSRLs) designed for aerosol and cloud remote sensing are increasingly being deployed
on aircraft and called for on future space-based missions. The HSRL technique relies on spectral discrimination of the
atmospheric backscatter signals to enable independent, unambiguous retrieval of aerosol extinction and backscatter. A
compact, monolithic field-widened Michelson interferometer is being developed as the spectral discrimination filter for
an HSRL system at NASA Langley Research Center. The Michelson interferometer consists of a cubic beam splitter, a
solid glass arm, and an air arm. The spacer that connects the air arm mirror to the main part of the interferometer is
designed to optimize thermal compensation such that the frequency of maximum interference can be tuned with great
precision to the transmitted laser wavelength. In this paper, a comprehensive radiometric model for the field-widened
Michelson interferometeric spectral filter is presented. The model incorporates the angular distribution and finite cross
sectional area of the light source, reflectance of all surfaces, loss of absorption, and lack of parallelism between the airarm
and solid arm, etc. The model can be used to assess the performance of the interferometer and thus it is a useful tool
to evaluate performance budgets and to set optical specifications for new designs of the same basic interferometer type.
High spectral resolution lidars (HSRLs) have recently shown great value in aerosol measurements form
aircraft and are being called for in future space-based aerosol remote sensing applications. A quasi-monolithic
field-widened, off-axis Michelson interferometer had been developed as the spectral discrimination filter for
an HSRL currently under development at NASA Langley Research Center (LaRC). The Michelson filter
consists of a cubic beam splitter, a solid arm and an air arm. The input light is injected at 1.5° off-axis to
provide two output channels: standard Michelson output and the reflected complementary signal. Piezo packs
connect the air arm mirror to the main part of the filter that allows it to be tuned within a small range. In this
paper, analyses of the throughput wavephase, locking error, AR coating, and tilt angle of the interferometer are
described. The transmission ratio for monochromatic light at the transmitted wavelength is used as a figure of merit for
assessing each of these parameters.
KEYWORDS: Aspheric lenses, Wavefronts, Interferometry, Sensors, Computer simulations, Beam splitters, Ray tracing, Digital cameras, Systems modeling, Nano opto mechanical systems
With respect to null test, non-null test is more flexible and can provide fast, general test with acceptable accuracy. A
non-null interferometric aspheric testing system, which employs partial null lens and reverse optimization
reconstruction, is proposed. The partial null lens compensates most of the longitude aberration of the aspheric under test
and keeps the slope of the non-null wavefront within the resolution of the detector. The reverse optimization
reconstruction procedure reduces the retrace error of the non-null test and reconstructs the figure of the test aspheric. The
characteristic, design process of the partial null lens and especially the implement of the reverse optimization
reconstruction are discussed in detail. Computer simulation shows the reverse optimization reconstruction procedure can
reconstruct the aspheric figure error with an accuracy better than 1/200wave within 5 mins. The error analysis is also
considered and some conclusions are given. This research is of great importance for general aspheric surfacing and testing.
Based on the difference between theoretical with real interferogram images the figure of original aspheric surface can be
obtained using an algorithm of Reverse iterate Optimization Reconstruction (ROR) calculating technique. Because the
procedure of ray tracing path needed an accurate geometry of optical structure size so the aspheric and compensator LC must be located in the optical path. To avoid the compensator LC resulted in bigger spherical aberration a smart located method is proposed in this paper. Before measurement an aplanatic lens consists of compensator LC and another
removable lens LM that the last surface is a standard one. So Fiezau interference is formed by the standard one with reflected ray from the vertex of aspheric that the aspheric surface detected can be accurately located. At testing the
lenses LM will be removed and the aspheric is moved to an adapted position. The experiments show the displacement locating accuracy is an amount of micron. The RMS for aspheric testing of ROR calculating technique is better than 1/200 wavelength.
Aspheric design and fabrication have obtained great achievements with the fast developments of modern science and technology, especially computer science, while the test of aspherics has become a chief limitation of aspheric applications. Due to the arbitrary nature of aspherics, test of all aspherics with only one instrument seems impossible. This paper presents a non-null interferometric system that can be applied for general aspheric test. The systematic error of non-null aspheric test system is studied, according to which an error separating and correcting method is proposed then. Computer simulation shows the error correcting method can correct the systematic error of non-null aspheric test system effectively and efficiently. Experiments have been carried out with the proposed interferometric non-null aspheric test system and the results show the system can greatly increase the accuracy of the non-null aspheric test.
In the measurement of aspheric surfaces, the vertex sphere and the best fit sphere are often used as reference sphere to
calculate the non-null compensation deviation. In traditional interferometry, the detected wavefront is equal to twice of
the deviation; but it is true only in the null condition or with a certain tolerance in the near null condition. In the non-null
condition, when reference spherical wavefront (the best fit sphere in this paper) incidences to the aspheric surface, the
rays will not return in the same path but deviate certain angles which cause normal longitudinal aberration. If the normal
longitudinal aberration is small enough, for example, much smaller than one wavelength, the wavefront aberration can be
equalized to twice of the deviation between the aspheric surface and the reference sphere. However, if the normal
longitudinal aberration can not be negligible, the wavefront aberration should not be equalized to twice of the deviation.
In this paper, the distribution of the deviation between the aspheric surface and the reference sphere is modeled, and the
relationship between the wavefront aberration and the normal longitudinal aberration is discussed. Two paraboloids, one
with small asphericity and the other large, are analyzed respectively to compare the different result when whether
considering the influence of the normal longitudinal aberration. Computer simulation is also carried out in optical tracing
software.
Optical systems benefit from the use of aspheric surfaces because aspherics provide additional degrees of freedom for
aberration control, yielding higher performance while reducing system weight and complexity. But optical designers
always avoid to use aspheres with larger asphericity in optical systems mainly for the reason that the test of the steep
aspherics requires extraordinary skill of the experimenter and the technical means to reduce the asphericity of the
wavefronts that detected by the digital camera. Even though null optics can be adopted to compensate the asphericity of
the test wavefront, they are not suitable for many cases. Each aspheric surface requires a unique null optics, which
greatly increases a project's complexity, cost and time delay. In this paper, a type of novel lens with large longitude
aberration and simple structure is proposed to reduce the asphericity of the wavefronts. The characteristic and the design
process of the simple lens are discussed in detail. A comparison between the simple lens and the aplanat with different
focus setting is given. Computer simulation shows the simple lens has much more power in reducing asphericity.
The large slope of the aspheric departure presents great difficulty for optical testing researchers to test aspheric surfaces
and wavefronts. The instrument dynamic range of traditional interferometers does not support the high number of fringes
due to the steep slopes commonly found in aspherics. The radial shearing interferometer, which can greatly reduce the
slope of the aspheric wavefront under test, is always adopted to test aspherics. In this paper, the two evaluating
parameters of a radial shearing interferometer, the wavefront slope tolerance and the wave phase sensitivity, are
proposed. The effective radial shear plays important role in reduction of the wavefront slope and that of the wave phase
sensitivity. How the two parameters effect the radial shearing interferometer is discussed in detail. Computer simulation
shows that by properly choosing the effective radial shear, the radial shearing interferometer can obtain the best
performance when testing an arbitrary aspheric wavefront. This research is of greatly importance to the design of the
radial shearing interferometer for aspheric testing.
This paper describes a novel synchronous control system of high speed imaging, which combines a common path
interferometer system modulated by the space phase. The system can continuously grab multiple frame interferograms,
which contain transient flow field distortion. The study of this system will provide a fire-new means for the research of
aerodynamics. The light source of the system is Nd: YLF semiconductor pump solid pulsed laser of which wavelength is
1053 nanometers. The laser pulse width is less than 30 nanoseconds, far less than the exposure time of the camera
shutter. Thus the laser pulse can freeze the flow field within several dozen nanoseconds and catch the biggish change of
turbulent flow. The pulsed laser beam containing the information of turbulent flow enters a cyclical radial shearing
interferometer. The emergent lights, being respectively contracted and expanded, re-combine and form fringe pattern in
high space frequency, modulated with a definite carrier frequency. The fringe pattern is formed on the high speed CMOS
camera at last. An accurate short time delay circuit is provided for synchronization matching of the pulsed laser and
camera exposure. The speed of image acquisition in full pixels with 1280×1024 can reach 450 frames per second. This
interferogram acquisition system with compact configuration and strong anti-disturbance capability, has successfully
grabbed clear transient interferograms that provided reliable image information for follow-up image processing and flow
field density calculating.
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