Diode Laser-Based Sensor for High Precision Measurements of Ambient CO 2 and Field-Worthy UV Backscatter Lidar for Cirrus Cloud Studies

Transcription

1 Physical Sciences Inc. VG Diode Laser-Based Sensor for High Precision Measurements of Ambient CO 2 and Field-Worthy UV Backscatter Lidar for Cirrus Cloud Studies D. Sonnenfroh and Mark Allen Prepared for: UITI2008 Meeting Park City, UT December 2008 Work supported by DoE Contract # s DE-FG02-05ER84296 and DE-FG02-06ER New England Business Center Andover, MA 01810

2 Diode Laser-Based Sensor for High Precision Measurements of Ambient CO 2 Program Goal VG Robust predictions from global climate change models require precise inputs including ambient concentrations, and sources and sinks of greenhouse gases Project goal is to develop the next generation network sensor for high precision measurements of ambient CO 2 Target sensor characteristics: Measure CO 2 mole fraction in dry air to 1:3000 with 1 minute averaging Low (calibration) gas usage Unattended operation for 6 months Low acquisition cost Insensitive to motion Non-expert user Low maintenance cost Work supported by DoE Contract DE-FG02-05ER84296

3 Technical Approach Tunable Diode Laser Absorption Spectrometer The TDL Absorption Spectrometer consists of: Long optical path (55 m in only 20 cm of physical space) using Integrated Cavity Output Spectroscopy (ICOS) Telecom-grade TDL operating at 2 µm for high reliability Compact packaging sensor controller on single printed circuit board VG Nafion Dryer Power Supply ICOS Cell Controller Board

4 Wavelength Modulation Spectroscopy (WMS) Laser temperature initially tuned to center of absorption line (ν o ) VG Laser wavelength then repeatedly scanned across absorption line (through current modulation) producing amplitude modulation at detector AM frequency of absorption dips is 2ω m Radio receiver technology used to demodulate AM signal to yield concentration Transmission δ δ Wavelength ν o Wavelength Detected Laser Power ν o +δ ν o ν o -δ π ω m Time 2π ω m 3π ω m ν(t) = ν o + δsin(ω m t) A(t) ~ A o [1-a cos 2 ω m t] C-5215c no absorption with absorption

5 Engineering Prototype Architecture Pressure Sensor Dryer Diaphragm Pump Detector Module System SWaP 20x43x56 cm 15 kg 40 W VG ICOS Cell AC-DC Power Supply DC Power and Signal Input Board WMS Board Laser & System Controller

6 CO 2 Measurement Precision Mixing Ratio VG Goal of 1:3000 measurement precision has been achieved

7 Sensor Intercomparison PSI TDL UNH NDIR VG CO2 [ppmv] EDT Time [hours] Comparison starting near noon on 16 July through noon on 17 July Excellent agreement between PSI TDL sensor and UNH NDIR sensor

8 Sensor Intercomparison 3 17 July VG Peaks arise overnight when winds are calm; mixing is minimized and CO 2 from plant respiration builds up Magnitude depends on many factors

9 Field-Worthy UV Backscatter Lidar for Cirrus Cloud Studies Program Goal VG The MicroPulseLidar (MPL) was developed at NASA in the early 90s to enable full-time autonomous operation for climate studies of clouds and aerosols. The MPL was designed to overcome lifetime, maintenance, and safety concerns associated with unattended, full time operation via low pulse energy, high repetition rate design. Under the DoE ARM program, the MPL was the cornerstone technology that enabled the creation of a world-wide lidar network focused on studies of the climatic effects of clouds and aerosols. Goal is to increase the MPL s capability for cirrus cloud retrievals Next generation autonomous backscatter lidar for cirrus clouds. Operation in UV (349 nm) using all solid state lasers Increase in SNR of 30x for returns at 18 km relative to MPL system at 523 nm. Capable of detecting sub-visual cirrus at ranges to 20 km with 30 m resolution Polarization sensitive detection of return signal (cloud phase) Work supported by DoE Contract DE-FG02-06ER84427

10 PSI/QPeak Technical Approach System Components: Diode-Pumped Solid State Laser (DPSSL) technology using QPeak s MultiPass Slab (MPS) technology Transmitter operates at 349 nm with ~3 mj/pulse at 2.5 khz PRF. Fiber optically-coupled, water-cooled, pump diodes Maintain all the advantageous characteristics of existing MPLs. VG UV laser transmits x1000 more power than MPL bistatic design mitigates increased scatter Water cooling used to manage increased waste heat Eventual remote field operation in Tropical Pacific requires minimal downtime from failure or maintenance Fiber-coupled diode laser pump modules can be installed in field in short time and with no impact on alignment. Detector modules that can be installed in field in short time and with no impact on alignment.

11 UVBL Prototype Design Features Biaxial configuration Transmit aperture 5 cm Eyesafety compliance with ANSI Z Receiver aperture 20 cm Portable unit (~1 m 3 ) Autonomous operation Detection optics design includes polarization separation optics Gated, channel plate detectors Detectors and data acquisition system can be included inside the frame Laser chiller and power supply housed in separate rack with umbilical lines $250K production cost target VG

12 UV Transmitter Laser Architecture VG Repetition Rate khz UV Pulse Energy - 3 mj Pulse Width - 19 ns MultiPass Slab (MPS) technology Cylindrical lenses focus output from fiber coupler into gain medium LIMO fiber coupled pump diode laser Very compact 20,000 hr lifetime at 40 W output power Internal volume Bragg grating aides field replacement Center wavelength tolerance: ± 0.2 nm

13 UV Transmitter CAD Model VG Dimensions: 12 x 71 x 74 cm Fibers drive the size of package

14 Transceiver Design PBS L P L NBF F LASER F BX D D Electronics Laser Driver Laser Controller DAQ/ Computer L: Lens F: Fiber P: Pinhole PBS: Pol. Beam Splitter BX: Beam Expander D: Detector NBF: Narrowband Filter VG Biaxial Geometry Transmit aperture 5 cm To meet eye safety Receiver aperture 20 cm Enhanced for uv performance Design rationale Minimize scattered UV and fluorescence reaching detectors. Need dual polarization analysis. Monostatic design has 2 independent optical axes in a single physical space. Alignment problem is no different from that in a bistatic design. Both axes see the same environment in monostatic Sigma Space is working with scope manufacturers on approaches to robust thermal and mechanical systems. Fiber coupled detectors enables field replacement of detectors without realignment Detector candidates Channel photomultipliers

15 From Lidar Transceiver: TTL inputs from up to 4 detectors System Interface From Lidar Transceiver: Analog inputs for temperature, energy monitor etc. VG Laser Power Supply Q-SW SYNC OUT Multi Channel Scaler 12-bit ADCs LIDAR DATA SYSTEM USB LINK DATA ACQUISITION, CONTROL AND DISPLAY Lidar Data System Up to 4 photon counting channels, 50 MHz max count rate per channel (TTL) 15, 30, 75 m resolution, customizable 12-bit ADC for analog measurements for housekeeping data Uses sync pulses from laser Q-switch (TTL) USB link to DAQ computer Windows application Display and program behavior can be customized using the configuration file