Brillouin spectroscopy is increasingly employed for biomedical research and recent advances such as line-scanning configurations further widen the scope of possible measurements. Current spectrometer technologies have limitations which are prohibitive for certain applications. We propose a novel hybrid VIPA-etalon cascade scheme compatible with both confocal and line-scanning geometries. A single VIPA is followed immediately with one or multiple essentially parallel-oriented standard Fabry-Perot etalons of approximately matching thickness. The cascaded etalons preserve near-unity throughput while increasing the contrast by more than 20 dB per stage. Other advantages include simplicity, compactness, and wavelength flexibility. We present simulations of the hybrid VIPA-etalon scheme and an experimental proof-of-concept measurement.
Quantum communication networks based on fiber optics are restricted in length since efficient quantum repeaters are not yet available. A free-space channel between a satellite in orbit and Earth can circumvent this problem. We have constructed a system to demonstrate the feasibility of quantum communication between space and earth using photons hyperentangled in their polarization and time-bin degrees of freedom. With this system, we have implemented superdense teleportation (SDT) with a fidelity of 0.94±0.02. To increase the efficiency of SDT, we have developed an active, polarization-independent switch compatible with SDT. We characterized the performance of its switching efficiency. Finally, we have constructed a novel two-level interferometer for time-bin qubit creation and analysis in orbit, and bounded its stability.
A wide array of general astrophysics studies including detecting and characterizing habitable exoplanets could be enabled by a future large segmented telescope with sensitivity in the UV, optical, and infrared bands. When paired with a starshade or coronagraph, such an observatory could enable direct imaging and detailed spectroscopic observations of nearby Earth-like habitable zone planets. Over the past several years, a laboratory-based Visible Nulling Coronagraph (VNC) has evolved to reach requisite contrasts over a ~ 1 nm bandwidth at narrow source angle separation using a segmented deformable mirror in one arm of a Mach-Zehnder layout. More recent efforts targeted broadband performance following the addition of two sets of half-wave Fresnel rhomb achromatic phase shifters (APS) with the goal of reaching 10-9 contrast, at a separation of 2λ/D, using a 40 nm (6%) bandwidth single mode fiber source. Here we present updates on the VNC broadband nulling effort, including approaches to addressing system contrast limitations.
The key to broadband operation of the Visible Nulling Coronagraph (VNC) is achieving a condition of quasi-achromatic destructive interference between combined beams. Here we present efforts towards meeting this goal using Fresnel rhombs in each interferometric arm as orthogonally aligned half wave phase retarders. The milestone goal of the demonstration is to achieve 1 × 10−9 contrast at 2λ/D over a 40 nm bandpass centered at 633 nm. Rhombs have been designed and fabricated, and a multi-step approach to alignment using coarse positioners for each rhomb and pair has been developed to get within range of piezo stages used for fine positioning. The previously demonstrated narrowband VNC sensing and control approach that uses a segmented deformable mirror is being adapted to broadband to include fine positioning of the piezo-mounted rhombs, all demonstrated in a low-pressure environment.
High Spectral Resolution Lidar (HSRL) is typically realized using an absorption filter to separate molecular returns from particulate returns. NASA Langley Research Center (LaRC) has designed and built a Pressure-Tuned Wide-Angle Michelson Interferometer (PTWAMI) as an alternate means to separate the two types of atmospheric returns. While absorption filters only work at certain wavelengths and suffer from low photon efficiency due to light absorption, an interferometric spectral filter can be designed for any wavelength and transmits nearly all incident photons. The interferometers developed at LaRC employ an air spacer in one arm, and a solid glass spacer in the other. Field widening is achieved by specific design and selection of the lengths and refractive indices of these two arms. The principal challenge in using such an interferometer as a spectral filter for HSRL aboard aircraft is that variations in glass temperature and air pressure cause changes in the interferometer’s optical path difference. Therefore, a tuning mechanism is needed to actively accommodate for these changes. The pressure-tuning mechanism employed here relies on changing the pressure in an enclosed, air-filled arm of the interferometer to change the arm’s optical path length. However, tuning using pressure will not adjust for tilt, mirror warpage, or thermally induced wavefront error, so the structural, thermal, and optical behavior of the device must be well understood and optimized in the design and manufacturing process. The PTWAMI has been characterized for particulate transmission ratio, wavefront error, and tilt, and shows acceptable performance for use in an HSRL instrument.
An integrated Structural-Thermal-Optical-Performance (STOP) model was developed for a field-widened Michelson interferometer which is being built and tested for the High Spectral Resolution Lidar (HSRL) project at NASA Langley Research Center (LaRC). The performance of the interferometer is highly sensitive to thermal expansion, changes in refractive index with temperature, temperature gradients, and deformation due to mounting stresses. Hand calculations can only predict system performance for uniform temperature changes, under the assumption that coefficient of thermal expansion (CTE) mismatch effects are negligible. An integrated STOP model was developed to investigate the effects of design modifications on the performance of the interferometer in detail, including CTE mismatch, and other threedimensional effects. The model will be used to improve the design for a future spaceflight version of the interferometer. The STOP model was developed using the Comet SimApp™ Authoring Workspace which performs automated integration between Pro-Engineer®, Thermal Desktop®, MSC Nastran™, SigFit™, Code V™, and MATLAB®. This is the first flight project for which LaRC has utilized Comet, and it allows a larger trade space to be studied in a shorter time than would be possible in a traditional STOP analysis. This paper describes the development of the STOP model, presents a comparison of STOP results for simple cases with hand calculations, and presents results of the correlation effort to bench-top testing of the interferometer. A trade study conducted with the STOP model which demonstrates a few simple design changes that can improve the performance seen in the lab is also presented.
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.
We describe the performance of a Nd:YAG laser which produces 4.1 mJ, 12 ns FWHM Q- switched pulses when side-pumped with 180 W, 250 microsecond(s) pulses from a stacked AlGaAs laser diode array (SDL-3230 TZB). We have studied the effects of varying gain and cavity length on the Nd:YAG laser performance. We have found that the properties of diode lasers require design compromises different than for lamp-pumped lasers.
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