Technology development for a compact rubidium optical frequency reference

The precision and accuracy of navigation and radar systems is typically limited by the stability of their internal frequency references. Currently, microwave atomic frequency references are close to the limits of what they can achieve in terms of short term stability. Optical atomic frequency references have demonstrated several orders of magnitude improvement in both short-term as well as long-term stability. In the past decade, improvements in optical frequency comb (OFC) technology have enabled the precise measurement of optical frequencies with much smaller form-factors, spurring research to build a portable optical atomic clock referenced to the 87Rb 5S1=2; F = 2 ! 5D5=2; F0 = 4 two-photon transition (TPT). Using a single laser source and simple Doppler-free spectroscopy in a heated Rb vapour cell one can generate an atomic reference signal with a linewidth approaching 334 kHz. The research presented here, compares the suitability of a telecoms (1550-1560 nm) CW laser with a narrow bandwidth OFC laser of 10-20 modes spaced apart at 3-6 GHz frep. The OFC laser achieves more than double the second harmonic conversion effciency compared with the CW laser, while delivering up to 30 mW of 778 nm light. The 778 nm OFC is then used to excite the reference transition and demonstrate coherent interaction of all OFC modes. Towards the aim of making the system compact, the research explores the use of micro-fabricated vapour cells, 3D printed oven designs and a chip-scale DFB (distributed feedback) laser with the prospects of integrating both the laser and spectroscopy on to a single micro-fabricated semiconductor platform. Pre-stabilising a noisy laser to an optical cavity is commonly required for optical atomic clocks, in order to resolve narrow-linewidth transitions. Towards this application, a low-drift, all-metal optical cavity is developed and characterised using Allvar metal, which possesses a negative coe�cient of thermal expansion (CTE). The overall cavity CTE can be temperature tuned to yield a CTE of < 0.001 ppm/°C at � 27 °C. The long-term cavity mode stability of the cavity was measured while referenced to one of the 87Rb 5S1=2 ! 5P3=2 780 nm transitions, residual drifts of 0.3 MHz/hr on time-scales up to 5 hrs (after subtracting o� pressure-correlated frequency shifts). The all-metal cavity should be less sensitive to thermal gradients as well as more responsive temperature stabilisation than ultra-low expansion cavities.The precision and accuracy of navigation and radar systems is typically limited by the stability of their internal frequency references. Currently, microwave atomic frequency references are close to the limits of what they can achieve in terms of short term stability. Optical atomic frequency references have demonstrated several orders of magnitude improvement in both short-term as well as long-term stability. In the past decade, improvements in optical frequency comb (OFC) technology have enabled the precise measurement of optical frequencies with much smaller form-factors, spurring research to build a portable optical atomic clock referenced to the 87Rb 5S1=2; F = 2 ! 5D5=2; F0 = 4 two-photon transition (TPT). Using a single laser source and simple Doppler-free spectroscopy in a heated Rb vapour cell one can generate an atomic reference signal with a linewidth approaching 334 kHz. The research presented here, compares the suitability of a telecoms (1550-1560 nm) CW laser with a narrow bandwidth OFC laser of 10-20 modes spaced apart at 3-6 GHz frep. The OFC laser achieves more than double the second harmonic conversion effciency compared with the CW laser, while delivering up to 30 mW of 778 nm light. The 778 nm OFC is then used to excite the reference transition and demonstrate coherent interaction of all OFC modes. Towards the aim of making the system compact, the research explores the use of micro-fabricated vapour cells, 3D printed oven designs and a chip-scale DFB (distributed feedback) laser with the prospects of integrating both the laser and spectroscopy on to a single micro-fabricated semiconductor platform. Pre-stabilising a noisy laser to an optical cavity is commonly required for optical atomic clocks, in order to resolve narrow-linewidth transitions. Towards this application, a low-drift, all-metal optical cavity is developed and characterised using Allvar metal, which possesses a negative coe�cient of thermal expansion (CTE). The overall cavity CTE can be temperature tuned to yield a CTE of < 0.001 ppm/°C at � 27 °C. The long-term cavity mode stability of the cavity was measured while referenced to one of the 87Rb 5S1=2 ! 5P3=2 780 nm transitions, residual drifts of 0.3 MHz/hr on time-scales up to 5 hrs (after subtracting o� pressure-correlated frequency shifts). The all-metal cavity should be less sensitive to thermal gradients as well as more responsive temperature stabilisation than ultra-low expansion cavities

Luminescent Transition Metal Complexes: Optical Characterization, Integration into Polymeric Nanoparticles and Sensing Applications

Photoluminescence is a fascinating phenomenon which has a huge impact on our daily life. Many important applications are based on this principle, such as imaging diagnostics, bioanalytic, photocatalysis, solar cells, or optoelectronic devices. The emerging demands for versatile photoluminescent materials have encouraged generations of scientists to develop different types of luminophores, ranging from molecular dyes to luminescent nanomaterials. Among them, luminescent transition metal complexes (TMCs), which consist of one (or more) metal center and several organic or inorganic ligands, are drawing increasing interest due to their unique photophysical and photochemical properties, such as large Stokes shift, long-lived triplet excited state, sharp emission band (f-block metal complexes), and multiple stale oxidation states (d-block metal complexes). These distinct optical properties are not only of great research interest, but also have led to commercial applications, such as imaging agents, optical sensors, light-harvesting materials, optical barcoding, and displays. The demands and desires of optoelectronic devices and higher requirements in bioanalytic make the development of new TMCs necessary, which needs to be examined in detail not only in terms of their chemical but much more importantly their photophysical properties. In this work, a series of new types of luminescent TMCs are involved, including Cr(III)-, Pt(II)-, and Pd(II) complexes. Based on their optical studies, a series of proof-of-concept applications were designed by introducing these metal complexes to different nanomatrix, such as polymeric nanoparticles and metal-organic frameworks (MOFs), resulting various luminescent nanosensors or energy-conversion materials. The major part of this work is based on the [Cr(ddpd)2]3+ complex (ddpd = N, N′‐dimethyl‐N, N´-dipyridine‐2‐ylpyridine‐2,6‐diamine) and his derivatives. Fundamental photophysical studies of these Cr(III) complexes showed that their photoluminescence properties can be significantly enhanced by ligand and solvent deuteration. Moreover, a choice of bulky counter anions can provide an enhancement in the photoluminescence properties as well as the oxygen sensitivity. In addition, based on the photophysical understanding of the [Cr(ddpd)2]3+ complex, a proof-of-concept study of photon upconversion in molecular chromium ytterbium salts was completed. Upon an excitation of the Yb3+ sensitizers at 976 nm, these solid-state salts produced upconverted luminescence of the Cr3+ activator at 780 nm at room temperature. Another proof-of-concept study based on the [Cr(ddpd)2]3+ complex was investigated by designing and developing multianalyte nanosensors for simultaneously measuring temperature (“T”), oxygen (“O”), and pH (“P”) in aqueous phase under one excitation wavelength. Apart from the [Cr(ddpd)2]3+ complexes, four novel Pt(II)- and Pd(II)-complexes bearing tetradentate ligands were also studied regarding their photophysical properties in solutions and in polystyrene nanoparticles (PS-NPs). In PS-NPs, the aggregation-induced Metal-Metal-to-Ligand Charge-Transfer (3MMLCT) state of the fluorinated Pt(II) complex is red-shifted compared to the monomeric emission and performs insensitive to oxygen, allowing the particles as self-referenced oxygen nanosensor in both the luminescence intensity and lifetime domains. Additionally, a triplet–triplet annihilation upconversion (TTA-UC) system was developed based on a crystalline MOF. A Pd(II) porphyrin complex acted as the sensitizer immobilized in the MOF walls, while a 9,10-diphenylanthracene annihilator was filled in the channels. Upon green light excitation at 532 nm, the resulting MOF crystalline showed an upconverted blue emission with delayed lifetime from 4 ns to 373 µs and a triplet–triplet energy transfer efficiency of 82%

NASA Tech Briefs, September 2011

Topics covered include: Fused Reality for Enhanced Flight Test Capabilities; Thermography to Inspect Insulation of Large Cryogenic Tanks; Crush Test Abuse Stand; Test Generator for MATLAB Simulations; Dynamic Monitoring of Cleanroom Fallout Using an Air Particle Counter; Enhancement to Non-Contacting Stress Measurement of Blade Vibration Frequency; Positively Verifying Mating of Previously Unverifiable Flight Connectors; Radiation-Tolerant Intelligent Memory Stack - RTIMS; Ultra-Low-Dropout Linear Regulator; Excitation of a Parallel Plate Waveguide by an Array of Rectangular Waveguides; FPGA for Power Control of MSL Avionics; UAVSAR Active Electronically Scanned Array; Lockout/Tagout (LOTO) Simulator; Silicon Carbide Mounts for Fabry-Perot Interferometers; Measuring the In-Process Figure, Final Prescription, and System Alignment of Large; Optics and Segmented Mirrors Using Lidar Metrology; Fiber-Reinforced Reactive Nano-Epoxy Composites; Polymerization Initiated at the Sidewalls of Carbon Nanotubes; Metal-Matrix/Hollow-Ceramic-Sphere Composites; Piezoelectrically Enhanced Photocathodes; Iridium-Doped Ruthenium Oxide Catalyst for Oxygen Evolution; Improved Mo-Re VPS Alloys for High-Temperature Uses; Data Service Provider Cost Estimation Tool; Hybrid Power Management-Based Vehicle Architecture; Force Limit System; Levitated Duct Fan (LDF) Aircraft Auxiliary Generator; Compact, Two-Sided Structural Cold Plate Configuration; AN Fitting Reconditioning Tool; Active Response Gravity Offload System; Method and Apparatus for Forming Nanodroplets; Rapid Detection of the Varicella Zoster Virus in Saliva; Improved Devices for Collecting Sweat for Chemical Analysis; Phase-Controlled Magnetic Mirror for Wavefront Correction; and Frame-Transfer Gating Raman Spectroscopy for Time-Resolved Multiscalar Combustion Diagnostics

Undulator design for Laser Plasma Based Free electron laser

The fourth generation of synchrotron radiation sources, commonly referred to as the Free Electron Laser (FEL), provides an intense source of brilliant X-ray beams enabling the investigation of matter at the atomic scale with unprecedented time resolution. These sources require the use of conventional linear accelerators providing high electron beam performance. The achievement of chirped pulse amplification allowing lasers to be operated at the Terawatt range, opened the way for the Laser Plasma Acceleration (LPA) technique where high energy electron bunches with high current can be produced within a very short centimeter-scale distance. Such an advanced acceleration concept is of great interest to be qualified by an FEL application for compact X-ray light sources. We explore in this paper what the LPA specificities imply on the design of the undulator, part of the gain medium. First, the LPA concept and state-of-art are presented showing the different operation regimes and what electron beam parameters are likely to be achieved. The LPA scaling laws are discussed afterwards to better understand what laser or plasma parameters have to be adjusted in order to improve electron beam quality. The FEL is secondly discussed starting with the spontaneous emission, followed by the different FEL configurations, the electron beam transport to the undulator and finally the scaling laws and correction terms in the high gain case. Then, the different types of compact undulators that can be implemented for an LPA based FEL application are analyzed. Finally, examples of relevant experiments are reported by describing the transport beamline, presenting the spontaneous emission characteristics achieved so far and the future prospects

An evaluation of selected estimation methods for the processing of differential absorption lidar data

This work examines the application of selected estimation methods to path integrated direct detection CO₂ lidar data, with the objective of improving the precision in the estimates of the log power, and log power ratios. Particular emphasis is given to the optimal estimation techniques of Kalman filtering theory, and to the consequent requirements for system and measurement model identification. A dual wavelength system was designed and constructed, employing two hybridised TEA lasers, a co-axial transceiver, and direct detection.Over a period of several months, a database of differential absorption measurements was accumulated, each consisting of 10,000 dual wavelength lidar returns. Various wavelength pairs were used, including those recommended for the monitoring of H₂O, CO₂, NH₃ and C₂H₄. A subset of this database is used to evaluate the above mentioned estimation methods. The results are compared with simulated data files in which it was possible to control precisely process models which are believed to form an approximation to the real processes latent in the actual lidar data

Airborne laser sensors and integrated systems

The underlying principles and technologies enabling the design and operation of airborne laser sensors are introduced and a detailed review of state-of-the-art avionic systems for civil and military applications is presented. Airborne lasers including Light Detection and Ranging (LIDAR), Laser Range Finders (LRF), and Laser Weapon Systems (LWS) are extensively used today and new promising technologies are being explored. Most laser systems are active devices that operate in a manner very similar to microwave radars but at much higher frequencies (e.g., LIDAR and LRF). Other devices (e.g., laser target designators and beam-riders) are used to precisely direct Laser Guided Weapons (LGW) against ground targets. The integration of both functions is often encountered in modern military avionics navigation-attack systems. The beneficial effects of airborne lasers including the use of smaller components and remarkable angular resolution have resulted in a host of manned and unmanned aircraft applications. On the other hand, laser sensors performance are much more sensitive to the vagaries of the atmosphere and are thus generally restricted to shorter ranges than microwave systems. Hence it is of paramount importance to analyse the performance of laser sensors and systems in various weather and environmental conditions. Additionally, it is important to define airborne laser safety criteria, since several systems currently in service operate in the near infrared with considerable risk for the naked human eye. Therefore, appropriate methods for predicting and evaluating the performance of infrared laser sensors/systems are presented, taking into account laser safety issues. For aircraft experimental activities with laser systems, it is essential to define test requirements taking into account the specific conditions for operational employment of the systems in the intended scenarios and to verify the performance in realistic environments at the test ranges. To support the development of such requirements, useful guidelines are provided for test and evaluation of airborne laser systems including laboratory, ground and flight test activities

Tomographic imaging of combustion zones using tunable diode laser absorption spectroscopy (TDLAS)

This work concentrates on enabling the usage of a specific variant of tunable diode laser absorption spectroscopy (abbr. TDLAS) for tomogaphically reconstructing spatially varying temperature and concentrations of gases with as few reconstruction artifacts as possible. The specific variant of TDLAS used here is known as wavelength modulation with second harmonic detection (abbr. WMS-2f) which uses the wavelength dependent absorbance information of two different spectroscopic transitions to determine temperature and concentration values. Traditionally, WMS-2f has generally been applied to domains where temperature although unknown, was spatially largely invariant while concentration was constant and known to a reasonable approximation (_x0006_+/- 10% ). In case of unknown temperatures and concentrations with large variations in space such techniques do not hold good since TDLAS is a “line-of-sight” (LOS) technique. To alleviate this problem, computer tomographic methods were developed and used to convert LOS projection data measured using WMS-2f TDLAS into spatially resolved local measurements. These locally reconstructed measurements have been used to determine temperature and concentration of points inside the flame following a new temperature and concentration determination strategy for WMS-2f that was also developed for this work. Specifically, the vibrational transitions (in the 1.39 microns to 1.44 microns range) of water vapor (H2O) in an axi-symmetric laminar flame issuing from a standard flat flame burner (McKenna burner) was probed using telecom grade diode lasers. The temperature and concentration of water vapor inside this flame was reconstructed using axi-symmetric Abel de-convolution method. The two different sources of errors in Abel’s deconvolution - regularization errors and perturbation errors, were analyzed and strategies for their mitigation were discussed. Numerical studies also revealed the existence of a third kind of error - tomographic TDLAS artifact. For 2D tomography, studies showing the required number of views, number of rays per view, orientation of the view and the best possible algorithm were conducted. Finally, data from 1D tomography was extrapolated to 2D and reconstructions were benchmarked with the results of 1D tomography

Semiconductor Nanocrystals: From Quantum Dots to Quantum Disks

The bottom-up colloidal synthesis opened up the possibility of finely tuning and tailoring the semiconductor nanocrystals. Numerous recipes were developed for the preparation of colloidal semiconductor nanocrystals, especially the traditional quantum dots. However, due to the lack of thorough understanding to those systems, the synthesis chemistry is still on the empirical level. CdS quantum dots synthesis in non-coordinating solvent were taken as a model system to investigate its molecular mechanism and formation process, ODE was identified as the reducing agent for the preparation of CdS nanocrystals, non-injection and low-temperature synthesis methods developed. In this model system, we not only proved it\u27s possible to systematically study the formation procedure of semiconductor nanocrystals, the insight learned during the research but also enhanced our understanding to this delicate system and promoted the development of synthetic chemistry. Although quantum dots could be routinely prepared in the lab with mature recipes, the colloidal semiconductor quantum well type materials are still hard to fathom. CdSe quantum disks structure was thoroughly analyzed with polar axes as the growth direction along the thickness direction, with both basal planes ended with Cd atom layer, which was coordinated with carboxylate ligands. Besides, four different thickness CdS quantum disks were prepared, its size-dependent lattice dilation, extremely sharp band-edge emission, and two-order of magnitude faster photoluminescence decay compared to quantum dots was investigated