Tamer F. Refaat

Twenty years of Tm:Ho:YLF and LuLiF laser development for global wind and carbon dioxide active remote sensing

NASA Langley Research Center (LaRC) has a long history of developing pulsed 2-μm lasers. From fundamental spectroscopy research, theoretical prediction of new materials, laser demonstration and engineering of lidar systems, it has been a very successful progress spanning around two decades. This article covers the 2-μm laser development from early research to current state-of-the-art instrumentation and projected future space missions. This applies to both global wind and carbon dioxide active remote sensing. A brief historical perspective of Tm:Ho work by early researchers is also given.

Backscatter 2-

A 2-μm backscatter lidar system has been developed by utilizing tunable pulsed laser and infrared phototransistor for the transmitter and the receiver, respectively. To validate the system, the 2-μm atmospheric backscatter profiles were compared to profiles obtained at 1 and 0.5 μm using avalanche photodiode and photomultiplier tube, respectively. Consequently, a methodology is proposed to compare the performance of different lidar systems operating at different wavelengths through various detection technologies. The methodology is based on extracting the system equivalent detectivity and comparing it to that of the detectors, as well as the ideal background detectivity. Besides, the 2-μm system capability for atmospheric CO2 temporal profiling using the differential absorption lidar (DIAL) technique was demonstrated. This was achieved by tuning the laser at slightly different wavelengths around the CO2 R22 absorption line in the 2.05-μm band. CO2 temporal profiles were also compared to in situ measurements. Preliminary results indicated average mixing ratios close to 390 ppm in the atmospheric boundary layer with 3.0% precision. The development of this system is an initial step for developing a high-resolution, high-precision direct-detection atmospheric CO2 DIAL system. A successful development of this system would be a valuable tool in obtaining and validating global atmospheric CO2 measurements.

\mu\hbox{m}

Water vapor and carbon dioxide are the most dominant greenhouse gases directly contributing to the Earths radiation budget and global warming. A performance evaluation of an airborne triple-pulsed integrated path differential absorption (IPDA) lidar system for simultaneous and independent monitoring of atmospheric water vapor and carbon dioxide column amounts is presented. This system leverages a state-of-the-art Ho:Tm:YLF triple-pulse laser transmitter operating at 2.05 μm wavelength. The transmitter provides wavelength tuning and locking capabilities for each pulse. The IPDA lidar system leverages a low risk and technologically mature receiver system based on InGaAs pin detectors. Measurement methodology and wavelength setting are discussed. The IPDA lidar return signals and error budget are analyzed for airborne operation on-board the NASA B-200. Results indicate that the IPDA lidar system is capable of measuring water vapor and carbon dioxide differential optical depth with 0.5% and 0.2% accuracy, respectively, from an altitude of 8 km to the surface and with 10 s averaging. Provided availability of meteorological data, in terms of temperature, pressure, and relative humidity vertical profiles, the differential optical depth conversion into weighted-average column dry-air volume-mixing ratio is also presented.

Lidar Validation for Atmospheric

A model of the spectral responsivity of In1–GaSb p-n junction infrared photodetectors is developed. This model is based on calculations of the photogenerated and diffusion currents in the device. Expressions for the carrier mobilities, absorption coefficient, and normal-incidence reflectivity as a function of temperature are derived from extensions made to Adachi and Caughey-Thomas models. Contributions from the Auger recombination mechanism, which increase with a rise in temperature, are also considered. The responsivity is evaluated for different doping levels, diffusion depths, operating temperatures, and photon energies. Parameters calculated from the model are compared with available experimental data, and good agreement is obtained. These theoretical calculations help us to better understand the electro-optical behavior of In1–GaSb photodetectors, and can be utilized for performance enhancement through optimization of the device structure.

\hbox{CO}_{2}

Novel heterojunction phototransistors based on AlGaAsSb-InGaAsSb material systems are fabricated and their characteristics are demonstrated. Responsivity of a phototransistor is measured with applied bias voltage at four different wavelengths. The maximum responsivity around 1400 A/W and minimum noise equivalent power of 1.83/spl times/10/sup -14/ W/Hz/sup 1/2/ from this phototransistor with bias of 4.0 V at a wavelength of 2.05 /spl mu/m were measured at 20/spl deg/C and -20/spl deg/C, respectively. Noise equivalent power of the phototransistor is considerably lower compared with commercially available InGaAs p-i-n photodiodes. Collector current measurements with applied incident power are performed for two phototransistors. Currents of 400 nA at low intensity of 0.425 /spl mu/W/cm/sup 2/ and of 30 mA at high intensity of 100 mW/cm/sup 2/ are determined. Collector current increases nearly by five orders of magnitude between these two input intensities. High and constant optical gain of 500 in the 0.46-nW to 40-/spl mu/W input power range is achieved, which demonstrates high dynamic range for such devices at these power levels.

Differential Absorption Lidar Applications

Detectors operating in the 2 mm regime are critical for several applications such as atmospheric remote sensing of CO2 and optical communication systems. Current state-ofthe-art detectors based on InGaAs and HgCdTe materials epitaxially grown on binary substrates suffer fabrication complexity and performance deterioration when tuned to the 2 mm wavelength. Beside the existing technology, InGaSb ternary alloy systems show a promising performance for near-infrared detectors. By varying the indium composition, one can tune the wavelength of detection from 1.7 to 5 mm, and specifically to 2 mm by growing In1˛xGaxSb with x value of 0.8. Epitaxial growth of InGaSb layers on different binary substrates has been reported using different techniques. 1‐3 Beside the complexity of these techniques, tuning to the 2 mm regime usually involves performance deterioration mainly due to the lattice mismatch problems. Lattice mismatch in the grown layers increases dark current and noise, limiting both dynamic range and sensitivity of a detector. The availability of bulk ternary substrates may significantly simplify the fabrication process by using simpler and lower cost techniques. In this letter the fabrication and characterization of InGaSb p‐n photodetectors using InGaSb substrates are presented. The device fabrication was carried out at Rensselaer Polytechnic Institute, while the characterization was carried out at NASA Langley Research Center. Tellurium doped n-type In0.17Ga0.83Sb substrates were grown from a high temperature melt using the vertical Bridgman technique. 4 The synthesis of InGaSb was carried out in silica crucible under argon ambient using pre-synthesized GaSb and InSb polycrystals. The compositionally graded InGaSb crystal was grown using a ,100. GaSb seed. The growth rate varied in the range of 0.1‐ 0.5 mm/hr during the course of the experiment. The temperature gradient of the furnace near the melt-solid interface was in the range of 10‐ 15° C/cm. No melt stirring was employed during the growth. After the entire melt solidified, the furnace was cooled down gradually to room temperature (at a rate of 15‐ 20° C/hr ) to avoid thermal cracking of the crystal. Wafers were sliced using a diamond wheel saw (Southbay Technology). The wafers were then polished to mirror shining using a three-step optimized lapping and polishing process. Lapping was done using boron carbide 14 micron size abrasives on a PanW pad and the two step polishing was done using alumina 1 and 0.3 micron slurries on nylon and velvet pads, respectively. To form the p-type layers, Zn diffusion was carried out using the leaky box technique. Zn pellets were used as the source and the diffusion was done at 450° C for 2 h under flowing nitrogen gas. A schematic for the photodiode structure is shown in Fig. 1. Metallization was carried out using electron-beam evaporator by depositing 200 A tin followed by 1000 A gold for the back side contact and 400 A tin followed by 800 A gold for the front side metal pads. The p-layer thickness is about 0.4 mm with 10 19 cm ˛3 doping concentration, while the bulk substrate is 750 mm thick with about 10 17 cm ˛3 doping concentration as determined by C‐V measurements. No antireflective coating or surface passivation was applied. Several diodes and photodiodes with different areas were fabricated. The device characterization was

Evaluation of an airborne triple-pulsed 2 μm IPDA lidar for simultaneous and independent atmospheric water vapor and carbon dioxide measurements

Two-micron detectors are critical for atmospheric CO2 profiling using the lidar technique. InGaAs and HgCdTe detectors are commercially available for this wavelength but they lack sufficient gain, which limits their detectivity. The characterization results of a novel AlGaAsSb/InGaAsSb phototransistor for 2-μm application are reported. The device was developed by AstroPower, Inc. for NASA Langley Research Center. Spectral response measurements showed the highest responsivity in a 1.9- to 2.1-μm region with a maximum value of 2650 A/W at 2 μm. A 2-μm detectivity of 3.9×10 11 cm Hz 1/2 /W was obtained, which corresponds to noise equivalent power of 4.6×10 –14 W/Hz 1/2 .

Modeling of the temperature-dependent spectral response of In1−χGaχSb infrared photodetectors

Field experiments were conducted to test and evaluate the initial atmospheric carbon dioxide (CO2) measurement capability of airborne, high-energy, double-pulsed, 2-μm integrated path differential absorption (IPDA) lidar. This IPDA was designed, integrated, and operated at the NASA Langley Research Center on-board the NASA B-200 aircraft. The IPDA was tuned to the CO2 strong absorption line at 2050.9670 nm, which is the optimum for lower tropospheric weighted column measurements. Flights were conducted over land and ocean under different conditions. The first validation experiments of the IPDA for atmospheric CO2 remote sensing, focusing on low surface reflectivity oceanic surface returns during full day background conditions, are presented. In these experiments, the IPDA measurements were validated by comparison to airborne flask air-sampling measurements conducted by the NOAA Earth System Research Laboratory. IPDA performance modeling was conducted to evaluate measurement sensitivity and bias errors. The IPDA signals and their variation with altitude compare well with predicted model results. In addition, off-off-line testing was conducted, with fixed instrument settings, to evaluate the IPDA systematic and random errors. Analysis shows an altitude-independent differential optical depth offset of 0.0769. Optical depth measurement uncertainty of 0.0918 compares well with the predicted value of 0.0761. IPDA CO2 column measurement compares well with model-driven, near-simultaneous air-sampling measurements from the NOAA aircraft at different altitudes. With a 10-s shot average, CO2 differential optical depth measurement of 1.0054±0.0103 was retrieved from a 6-km altitude and a 4-GHz on-line operation. As compared to CO2 weighted-average column dry-air volume mixing ratio of 404.08 ppm, derived from air sampling, IPDA measurement resulted in a value of 405.22±4.15  ppm with 1.02% uncertainty and 0.28% additional bias. Sensitivity analysis of environmental systematic errors correlates the additional bias to water vapor. IPDA ranging resulted in a measurement uncertainty of <3  m.

AlGaAsSb-InGaAsSb HPTs with high optical gain and wide dynamic range

A detailed analysis is presented on the temperature and alloy composition dependence of the optical properties of III-V alloys AlxGa1−xAsySb1−y and GaxIn1−xAsySb1−y in the energy range 0.5–6 eV. Expressions for the complex dielectric function are based on a semiempirical phenomenological model, which takes under consideration indirect and direct transitions below and above the fundamental absorption edge. Dielectric function and absorption coefficient calculations are in satisfactory agreement with available experimental data. Other dielectric related optical data, such as the refractive index, extinction, and reflection coefficients, can also be obtained from the model.

InGaSb photodetectors using an InGaSb substrate for 2μm applications

Double-pulsed 2-μm integrated path differential absorption (IPDA) lidar is well suited for atmospheric CO2 remote sensing. The IPDA lidar technique relies on wavelength differentiation between strong and weak absorbing features of the gas normalized to the transmitted energy. In the double-pulse case, each shot of the transmitter produces two successive laser pulses separated by a short interval. Calibration of the transmitted pulse energies is required for accurate CO2 measurement. Design and calibration of a 2-μm double-pulse laser energy monitor is presented. The design is based on an InGaAs pin quantum detector. A high-speed photoelectromagnetic quantum detector was used for laser-pulse profile verification. Both quantum detectors were calibrated using a reference pyroelectric thermal detector. Calibration included comparing the three detection technologies in the single-pulsed mode, then comparing the quantum detectors in the double-pulsed mode. In addition, a self-calibration feature of the 2-μm IPDA lidar is presented. This feature allows one to monitor the transmitted laser energy, through residual scattering, with a single detection channel. This reduces the CO2 measurement uncertainty. IPDA lidar ground validation for CO2 measurement is presented for both calibrated energy monitor and self-calibration options. The calibrated energy monitor resulted in a lower CO2 measurement bias, while self-calibration resulted in a better CO2 temporal profiling when compared to the in situ sensor.