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Cw Fiber Laser Absorption Spectrometer for O2 Column Measurements in Support of the Ascends Mission

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Cw Fiber Laser Absorption Spectrometer for O2 Column Measurements in Support of the Ascends Mission 6.5 CW FIBER LASER ABSORPTION SPECTROMETER FOR O2 COLUMN MEASUREMENTS IN SUPPORT OF THE ASCENDS MISSION Jeremy Dobler*, Michael E. Dobbs, T. Scott Zaccheo, James Nagel, Wallace Harrison, Edward V. Browell, Berrien Moore III Cooler Dual Phase 1. INTRODUCTION Synchronous Detection Rx Optical Coupling Detector Pre Amp. BPF ADC (x4) ITT, along with our partners at NASA LaRC, Tele BPF Optics Digital Processing Return & Reference Signals for On & Off Atmospheric and Environmental Research Inc. (AER), & Ratios Tx Power Pre Amp. BPF ADC and the University of New Hampshire, are developing Detector an O2 Laser Absorption Spectroscopy (LAS) instrument Tx Tele. EDFA n Tx Tele. EDFA 2 Modulator f1 Seeder DFB ?1 to satisfy the requirements of the Active Sensing of 1 x n Wavelength Tx Tele. EDFA 1 Modulator f2 Seeder DFB ?2 Control Carbon Dioxide (CO2) over Nights, Days and Seasons Reference DFB EDFA Fiber Splitter (ASCENDS) mission as set forth by the National Controler Gas Wavelength Locking Research Council’s Decadal Survey; NRC (2007). Fiber Coupler Cell Laser Power Elec.Wire Detector Wavelength Monitor Amplifiers The purpose of the O2 lidar is to provide co- Optical Fiber located surface pressure measurements to enable Light Figure 1: ITT’s CO2 LAS has been operationally conversion of CO column measurements to CO mixing 2 2 validated via extensive ground and aircraft ratio. The team has chosen the 1.27um band of O to 2 campaigns and is TRL 6 make these measurements in order to take advantage of reduced atmospheric backscatter, and low sensitivity This operationally validated and mature architecture to relative humidity and temperature. offers several critical advantages in terms of ability to During 2007 and 2008, ITT succeeded in retrieve CO Mixing Ratio, while simultaneously demonstrating a set of low power horizontal ground- 2 minimizing program risk and cost: based O measurements using the same receiver used 2 for our CO system. The existing instrument has been 2 • Co-Located measurements of CO , O and developed and operationally validated for CO 2 2 2 Altimeter, reduces significant sources of bias and measurements through multiple airborne field errors in retrieval of CO mixing ratio. campaigns since 2004, using a combination of 2 • Simultaneous transmission of all wavelengths investments developed by ITT and NASA. Since the eliminates a significant source of error from fast initial measurements of O ITT has been internally 2, changes in target reflectivity; converts most natural funding the development of an advanced Fiber Raman and man-made noise terms to common mode noise Amplifier (FRA). This effort is in collaboration with the which is removed when the ratio’s are taken; and University of Arizona, and aims to produce the powers relaxes spacecraft pointing and jitter requirements. required to demonstrate this measurement from an • Robust all fiber laser transmitters operating in Aircraft and to scale it to space. continuous-wave mode increases laser transmitter This paper will review the instrument reliability an order of magnitude, while reducing architecture used for CO and O measurements and 2 2 cost. discuss the measurements to date. Evolution of the first • A single common detection chain for CO and O generation FRA and future plans will also be discussed. 2 2 (using RF modulation to separate wavelengths) eliminates sources of bias and/or drift in retrieval of 2. OVERVIEW OF ITT’S CW LASER ABSORPTION mixing ratio, which arise in other instruments that SPECTROSCOPY INSTRUMENT use separate optical paths, detectors, and The architecture for the CO2 LAS instrument electronics. for ASCENDS is illustrated in Figure 1. The all fiber- • A large-area HgCdTe APD provides gain of ~1000 coupled transmitter consists of N distributed feedback with a unity excess noise factor. lasers (DFB’s), N modulators or semi conductor • Non-diffraction limited, incoherent receiver which amplifiers (SOA’s), and N erbium doped fiber amplifiers. enables multiple low-cost fiber-coupled telescopes; Each continuous wave (CW) laser wavelength is enabling several meters of equivalent area at low modulated with its own RF frequency and transmitted cost. simultaneously. A lock-in method is used to separate • Multiple fiber amplifiers for CO2, O2 and Laser the wavelengths in the return measurement signal as Altimeter with independent, but co-aligned outputs. well as the transmitted energy reference signal. Enables functional redundancy at low cost. • Lock-in detection meets the mission requirements without the complexity and cost of coherent detection. Lock-in is significantly less sensitive to *Corresponding author address: background noise than Direct Detection. Jeremy Dobler, ITT, Fort Wayne, Indiana 46801, USA, email: [email protected] The additional components for the O2 function for ASCENDS are shown in Figure 2. The architecture for the transmitter subsystem for the O2 LAS instrument for 0.30 ASCENDS is the same as the CO instrument and composite OD 1.6km 2, H2O OD 1.6km shares the complete receiver including detector, O2 OD 1.6km 0.25 electronics and signal processing. 0.20 Online Position 0.15 OD 0.10 Offline position 0.05 0.00 1.2705 1.2710 1.2715 1.2720 1.2725 1.2730 1.2735 1.2740 1.2745 1.2750 1.2755 Figure 2: O2 specific components that will be added Vacuum Wavelength [um] to the existing CO2 LAS instrument Figure 3: HITRAN (04) spectra for O2 and H2O for a 1.6 km path with standard atmospheric conditions Note that Figure 2 shows all of the components required for adding the O2 module. However, many of the components are identical in function and form to those used for the validated CO2 instrument, the only difference being the operating wavelength. The main variation is that the O2 module will use a fiber Raman amplifier which is pumped by an Yb fiber amplifier, rather than the Erbium Doped Fiber Amplifier (EDFA) used for the CO2 system. More detailed information regarding the instrument and validation efforts can be found in; Browell (2008), Dobbs (2007, 2008), Harrison (2008). 3. FIELD MEASUREMENTS OF O2 The online absorption feature used in the initial O2 LAS experiment was at a vacuum wavelength of 1271.7074 nm at the peak. This transition was chosen Figure 4: Trough near 1262.5 nm is excellent choice to have sufficient line strength for ground or aircraft for space based measurements and decreases the based measurements. It also has minimal interference sensitivity to laser frequency jitter and drift by an from other atmospheric species, and it has a weak order of magnitude dependence on temperature. The offline wavelength used was at a vacuum wavelength of 1271.817nm. A 3.1 Experimental Setup plot from the HITRAN database, showing the absorption The O2 lidar was operated at ITT’s field test features of both water vapor and O2 in this region is site located in New Haven, IN. The instrument is shown in Figure 3. Several other lines more suitable for housed in a trailer, which is located 680 m from a space applications have been identified by our team for stationary target. The target consists of both light and example the trough shown in Figure 4. As described by dark surfaces, each has been shown to have fairly Korb (1983), a measurement using the trough between Lambertian reflectance profiles. The two targets allow two strong lines increases the sensitivity to pressure both high and low power measurements to be changes while decreasing sensitivity to laser frequency conducted at this site. These facilities are illustrated in jitter and drift. An O2 LAS using this trough is identical Figure 5. This test site is also equipped with in implementation to those carried out in this study. The instrumentation for monitoring atmospheric pressure, only difference is the wavelength of the seed laser and temperature, humidity, wind speed and direction, CO in the necessary components for this have already been 2 ppm and H2O in ppt. identified or obtained by ITT. for the narrow linewidth as the density of O2 atoms increases at the same time the line for each individual atom is broadened due to increased pressure. Based on analysis done by AER, an optimal position for sensitivity to pressure, for the line we are using on a horizontal path length, was determined to be TX/RX centered at a vacuum wavelength of 1271.716 nm. Further measurements were conducted in October and November using the new “online” (or sideline) position. These measurements showed agreements for all cases to better than 2%, and were better than 1% for the broad linewidth cases. The majority of the error in these measurements is the result of frequency jitter and laser Weather drift of only a couple of tenths of a pm. Since these 680 m Path measurements were made on the steep slope of the absorption feature they are very sensitive to these drifts. The improved results using the broader linewidths are Figure 5 ITT's test facility in New Haven, Indiana due to an effective reduction of the slope of the convolved absorption feature with the broader laser The transmitter used for the initial experiments spectrum. The measurements made in 2008 with the was assembled using all commercial off the shelf integrated transmitter were also made using the side (COTS) components. The first iteration of the line position and showed similar agreement. An transmitter produced only about 10mW per line (on and example of modeled versus measured data is shown in Figure 6. off). The only modification required to the CO2 receiver was to change the optical bandpass filter used for solar rejection to include a pass-band for the 1.27μm channel. The same telescope, detector, receiver electronics and signal processing lock-in system was used.
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