1.

INTRODUCTION

For IR spectral measurements in the gas-phase, the absorption peaks tend to be much narrower than for the condensed phase. High-resolution measurements of the fine structure of the trace gas absorption bands is often desirable, both to minimize interferences and to provide additional pressure and temperature vertical data. Relevant previous instruments for atmospheric trace-gas studies include bulk-optic dispersive spectrometers such as the SCIAMACHY VIS/NIR spectrometer on ESA’s ENVISAT and the ACE FT-IR on CSA’s SCISAT-1.

Typically, the current high-resolution spectrometers are based on bulk-optic spectrometers employing a relatively long optical path to achieve the required spectral dispersion. The mass of the current bulk-optic spectrometers typically exceeds 15 kg. This necessitates large and costly space platforms. The long optical path also makes the instrument performance and optical alignment sensitive to thermal variations and microvibrations that can be encountered in space.

Various Fourier Transform Infrared (FT-IR) spectrometers based on Michelson interferometers have also flown, including ACE on CSA’s SCISAT-1 [3] and NASA’s Thermal Emission Spectrometer (TES [1]). The FT-IR performs the spectral measurements sequentially using a Michelson interferometer to determine an interferrogram or Fourier Transform of the desired spectral data using a scanning mirror to vary the optical path in one arm of the interferrometer. The spectral retrievals require that the optical signal be constant during the entire measurement of the interferrogram. To attain high spectral resolution requires a relatively long 10 to 20 cm precision movement of the scanning mirror.

Instrumentation for space has relatively severe restrictions on mass, power and data transmission rates due to the high launch cost per kg. Therefore, miniaturization of the instrumentation while maintaining or exceeding the performance of bulk instruments is important to maximize the attainable scientific benefit and facilitate more missions through lower costs. Also, extensive use of in situ data processing is needed to extend the amount of information that can be transferred back to Earth.

MPB has advanced its patented IOSPEC technology (US 7,034,935 B1) for miniature guided-wave IR spectrometers to provide high performance comparable to large bench-top spectrometer systems but in a very compact and ruggedized footprint [5,6,7]. The technology was originally developed in collaboration with DRDC Valcartier and INO.

MPB’s IOSPEC (Integrated Optical Spectrometer) uses an antireflection-coated, high-index, low loss (<0.05 db/cm) IR dielectric slab waveguide structure with advanced IR linear detector arrays and smart multichannel parallel signal processing. The collected light is focused directly into the waveguide spectrometer through a 60 by 2000 mm slit for the FP-IOSPEC and a programmable 16x16 element slit array for the line-imaging binary-coded, broad-band spectrometers. The high-index waveguide allows a large input acceptance angle (NAi>0.3). The spectrometer waveguide integration precisely defines the position of the diffracted signal at the output focal plane, providing robust long-term optical alignment. IOSPEC spectrometers assembled over 10 years ago are still maintaining their optical alignment.

The optical signal is guided within the slab waveguide onto a master blazed grating that also serves as a concave reflector. The precision master grating, formed using microfabrication techniques to yield atomically-flat blazed grating elements, provides diffraction efficiencies approaching theoretical limits (> 85% peak diffraction efficiency) with low background signal scattering (<0.05%). Additional integrated optics at the output plane provide a 30-40 mm wide linear output focal plane for efficient coupling to focal plane arrays; facilitating a 4000 nm spectral operating range in first order diffraction (m=1) with spectral resolution to Δλ/λ= 1/7500.

The current 1.2 to 5 μm IOSPEC integrated optical spectrometer has been packaged by MPB in a compact module, as shown in Fig. 1, that is only about 20 x 20 x 15 cm in size and under 2 kg in mass. A version for operation from 8 to 12 microns has also been recently prototyped in collaboration with DRDC Valcartier.

Fig. 1:

Photograph of current IOSPEC module weighing under 2 kg including input optics and TE-cooled IR detector array.

Key current IOSPEC features:

IOSPEC employs an electronically-scanned detector array with parallel-processing multiplexing that can provide spectral scan rates exceeding several hundred scans per second to facilitate a relatively high sample throughput. The elimination of moving components and integration of the optical system provides more reliable long-term performance in non-ideal environments.

Even with active cooling and nominal temperature stabilization, infrared detector arrays, such as PbSe, can exhibit some unwanted signal drift and instability that can be comparable in magnitude to weaker optical signals. Proprietary active smart optical signal processing algorithms have been developed by MPB that facilitate a significant increase in the attainable SNR for multiplexed linear detector arrays. Using the traditional signal averaging techniques, the ultimate signal detectivity achievable rapidly saturates due to the increasing contribution of systematic variations in the detector signal, significantly limiting the attainable SNR. However, with the active smart averaging, as shown in Fig. 2 for a 256 pixel PbSe array, no such saturation in noise reduction was observed even for extended measurement times. This provides the potential of over three orders of magnitude improvement in the attainable SNR relative to traditional signal processing techniques.

Fig. 2:

Variation of the net noise for 256 pixel multiplexed PbSe linear array at 260K with the number of scans in active smart average with a full scale of 10V.

Fig. 3 shows the diffuse reflectance spectra of alunite and gypsum, minerals that are relevant for planetary exploration, that were measured with the current waveguide spectrometer operating at a coarse 8 nm/pixel bandwidth in m=1 using a miniature 3 W light source. These trial spectra were comparable to the reference spectra as provided by the Univ. of Winnipeg using a large bulk-optic spectrometer with 50 W source, providing good SNR and excellent differentiation of various minerals.

Fig. 3:

Spectra of gypsum and alunite measured using 60 degree incidence by an IOSPEC spectrometer with 50 μm input slits, 3 W miniature source and PbSe detector array, relative to a Spectralon reference for R=100%.

While a 4 to 8 nm/pixels measurement bandwidth is adequate for space studies of ground-cover and aerosols, much higher spectral resolutions are needed for atmospheric trace gas studies to resolve the fine structure of the tras gas absorption and minimize various interferences.

The following paper discusses a ground-breaking miniature Fabry-Perot guided-wave spectrometer concept, FP-IOSPEC, that can provide simultaneous multichannel high spectral resolution measurement capabilities for trace gas analysis to below 0.03 nm FWHM with a mass under 3.5 kg and net power requirement under 5 W. The mass and size reduction enables multiple such spectrometers to be accommodated on a single microsat to provide complementary data synergy and enable new science. The technology development targets the instrument requirements for the potential CSA MEOS Miniature Earth Observation Satellite mission.

2.

MEOS Mission Overview

MEOS is an advanced low-cost microsat mission that is being developed for the Canadian Space Agency for leading-edge correlated atmospheric/ground-cover studies from a 700 km Sun-synchronous orbit. The MEOS mission fills a strategic need for data to assist understanding the dynamics behind the current climate changes by studying some of the fundamental processes associated with the global water and carbon cycles and their interactions with the land cover, especially the role of the Canadian Boreal forest.

The common thread in the climate-related effects is the interaction between the surface ecosystems and the carbon- and nitrogen-containing gases in the lower troposphere. Uptake of CO₂ by growing vegetation, release of CH₄ and N2_O by soil processes and the effects of carbon and water cycle chemistry all interact strongly in a system that is both extremely complex and poorly understood at the present time (see Fig 4). The natural sources and sinks currently help to moderate the atmospheric GHG concentrations.

Fig. 4:

Global; carbon cycle with pre-industrial ‘natural’ fluxes in black and “anthropogenic” fluxes in red (source: IPCC AR4 WG1, May 2007).

The short-term response of the surface ecosystems is sensitive to variations in the composition of the lower atmosphere on the diurnal or seasonal time scale. Fig. 5 shows the diurnal/seasonal near-surface CO₂ variations measured over an extended time period within the Canadian Boreal forest. The CO₂ samples were obtained from a ground monitoring station using the flask method by Fluxnet. As opposed to the steadily increasing 1 to 1.5 ppm/year trend in the well-mixed mean CO₂ levels higher up in the atmosphere, as observed currently from space, the near-surface seasonal and diurnal CO₂ variations over a natural source/sink can exceed 20-30%.

Fig. 5:

Annual cycle of CO₂ near-surface concentration in BOREAS region (Canada).

The prediction of future behaviour depends on the ecosystem response, so an accurate understanding of this response is essential to achieve the goal of good long-term predictions. In summary, it is essential to consider the lower troposphere and surface ecosystem together and determine the behaviour of this coupled system under varying GHG concentrations in order to get an accurate prediction of the future climate response to anthropogenic GHG. These variations and interactions with the natural sources/sinks must be known accurately before global CO₂ levels can be quantified for climate modelling. The CO₂ and CH₄ concentrations and seasonal variations also reflect the health of the forests and the surface ecosystem

In order to extract the lower tropospheric concentrations of CO₂, CH₄, CO, N₂O and H₂O over natural sources and sinks, MEOS will innovatively combine geocolocated limb vertical profiling with follow-on nadir column measurements, coupled with near reat-time modelling. to extend the attainable precision and coverage. The measurements will be accomplished using separate limb and nadir instrument suites to avoid risky satellite manouvers and to maximize the data measurement times for both views. Previous missions have either alternated between nadir and limb measurement modes, such as SCIAMACHY, or only had a single view such as the occultation measurements provided by Canada’s SCISAT-1, or the pure nadir measurements planned for NASA’s Optical Carbon Observatory (OCO).

The limb Earth tangent point measurements will consist of sixteen 1.5 km x 160 km vertical pixels. This requires high pointing stability that will be achieved using a miniature Star Tracker and momentum wheel system for +/-0.01° pointing stability, coupled with a GPS for spacecraft position knowledge.

The follow-on nadir view will track the same geolocation/air-space as limb view. The nadir measurements will provide a 160 km swath width with sixteen 5x10 km² spatial pixels. The selection of the MEOS nadir ground pixel size involves a trade-off between SNR, resolution of ground sinks/sources and minimization of signal contamination by clouds. Based on the proposed MEOS instrument characteristics, a 5x10 km² nadir FOV per spatial pixel can provide SNR>300 for the high resolution channels. Preliminary analysis by CCRS [4] using data over Canada indicates that this will provide MEOS with >20% clear sky probability, a significant improvement over the previous missions (<7% clear skies for SCHIAMACHY using 30x60 km² ground pixel size for the CO₂ and CH₄).

Each view features a dedicated instrument suite and telescope with 100 mm diameter scanning mirror. The collimated optical signal collected by the hybrid telescope will be distributed to the instruments. For each view, two complementary miniature line-imaging spectrometers will measure simultaneously the broadband solar-illuminated 1.22 to 2.45 μm spectrum at about 1.2 nm resolution for the land cover and aerosol data, and the FP-IOSPEC multiple microchannels at 0.03 nm Full-Width-half-Maximum (FWHM) for the relevant trace gases to improve on the trace gas retrievals relative to previous and planned missions. The collected optical signal will be alternated between the two spectrometers, such that one undergoes its dark cycle while the other undergoes its illuminated cycle. The nadir view also includes a scanning Fabry-Perot 0.02 nm FWHM high resolution spectrometer for the O2 A-band near 0.76 μm to provide estimations of the total column pressure to correct the GHG concentration estimations..

The limb and nadir instrument suites will leverage the advanced fault-tolerant, spacegrade microprocessor-based data acquisition and control electronics that have been developed in collaboration with Xiphos Technologies and CSA for the Fiber Sensor Demonstrator (FSD [7]) on ESA’s Proba-2. This is scheduled for launch in 2009. Fig. 8 shows a photograph of the flight CPU PCB. This features a spacegrade DC-DC converter from Interpoint and FPGA from Xilinx, as well as the Xiphos latch-up protection for the DC power lines. Data and command I/O is provided using dual, redundant RS422 differential receivers and drivers. The FSD CPU PCB also contains 768 kbytes of SRAM. This will be augumented with 2 Gbyte of flash memory with fault-tolerant operation.

Fig. 6:

MEOS dual dedicated suites of miniature high-performance instruments for simultaneous limb and nadir views, including high-resolution CMOS imager.

Fig. 7:

Schematic of nadir miniature instrument suite. The limb instrument suite is similar but omits the O₂ A-band Fabry-Perot imaging spectrometer.

Fig. 8:

Photograph of FSD flight CPU PCB designed with Xiphos.

The proposed measurement scenario for the nadir view is “Step and Track” consisting of a digitized step between viewpoints on the ground, as shown schematically in Fig. 9. This methodoogly allows an extended signal integration time of 2 to 5 seconds per geographic point for an SNR>400 in the high resolution channels over grasslands while minimizing the background albedo variations. The target tracking will be performed by tilting the primary mirror of the nadir telescope. Accurate geographical positioning information will be provided by the CMOS VIS imager, supplemented by the MEOS GPS and pointing knowledge.

Fig. 9:

Schematic of MEOS nadir “Step and Track” measurement methodology.

Fig. 9:

Photograph of prototype FP-IOSPEC lab breadboard.

A multidisciplinary international science/payload team has been assembled, as summarized in Table 2, that has demonstrated international expertise in all areas of the MEOS mission, instrument and science requirements to accomplish the program described above. The mission science is lead by Prof. James Sloan who has gained considerable relevant measurement and analysis experience, as well as scientific publications for the aerosol studies through participation in the ACE instrument on SciSat-1. The team science expertise extends to gas phase and cloud/aerosol measurements and retrievals; atmospheric and surface ecosystem modelling and distributed parallel computing. While the data processing, modelling and computational challenges are demanding, the requirements are within the capabilities of modern distributed computing systems as available to the MEOS science team.

Table 1:

Summary of MEOS Instruments.

Table 2:

Summary of Current MEOS ScienceTteam

3.

FP-IOSPEC Spectrometer

The Fabry-Perot guided-wave FP-IOSPEC spectrometer synergistically combines advanced tunable optical filtering technologies with subsequent guided-wave multichannle wavelength demultiplexing to overcome current performance limitations by innovatively using the two relatively mature technologies in new ways:

The proprietary mirrors for the tunable optical filter were custom fabricated by INO using multilayer vacuum deposition. They are designed to provide a constant transmitted FWHM with wavelength between 1500 and 2450 nm.

Ansys Multiphysics was employed to simulate the design of the tunable FP etalon mechanical stage. This proceeded through two iterations to yield a very stable, but compact monolithic flexure structure using piezo actuators to provide stable tuning of the FP transmission wavelength and stable operation (see Fig 9). An advanced FP Etalon mechanical stage, about 60 mm O.D. by 60 mm long, was fabricated and integrated with three vacuum-compatible piezo actuators to enable electronic tuning of the FP mirror alignment and gap spacing.

4.

Preliminary Bread Board Testing

The trial FP etalon provided an excellent measured net transmittance >80% at the FP cavity resonances. This includes about 1 dB of coupling losses. The FP spectral tuning was provided by the monolithic mechanical flexure stage using three piezo actuators with strain-gauge sensor closed-loop feedback. Full wavelength tuning of the FP Etalon was experimentally achieved over the selected 8 nm Free Spectral Range (FSR) to the next adjacent FP resonance (see Fig. 8b).

Fig. 10 shows the variation of the transmitted FWHM as a function of the FP Etalon FSR or mirror spacing. An FP etalon FWHM of 0.04 nm has already been obtained using the trial FP Etalon at a FSR of 1.2 nm, very close to the desired 0.03-0.04 nm FWHM for the FP-IOSPEC flight unit. At the current IOSPEC pixel spacing of 8 nm, the FSR is about 0.18 nm, still excellent resolution that is approaching the 0.1 nm spectral resolution of the relevant NASA OCO [9] and JAXA GOSat [10] instruments for CO₂ remote measurement.

Fig. 10:

Experimentally achieved FWHM using the trial monolithic FP Etalon equipped with INO-custom-fabricated dielectric mirrors.

Fig. 11 shows the MPBC laboratory test bed that combined the integrated FP-IOSPEC with a 36” long vacuum gas cell. The gas cell was evacuated to provide a T=100% reference spectra. The net transmittance through the CO₂ or CH₄ samples in the gas cell was referenced to the corresponding reference spectra measured with the gas cell evacuated. First, coarse resolution spectra of the gases were taken using the current IOSPEC native 8 nm/pixel bandwidth. These provided an excellent fit of the absorption peak positions with corresponding MODTRAN and Hitran simulations of the CO₂ or CH₄. The Modtran (Modest resolution atmospheric Transmittance) simulator was developed for the US Air Force and is used extensively for predicting remote sensing measurements at moderate resolutions to about 1 nm.

Fig. 11:

Photograph of lab test set-up for CO₂ and CH₄ transmittance measurements.

A high-resolution spectrum of the CO₂ absorption band near 1572 nm was measured using a fine sampling step of about 0.01 nm by the current IOSPEC spectrometer (see Fig. 12(a)). Through the combination of narrow-band fine-filtering of the input optical signal using a tunable optical filter with subsequent signal wavelength demultiplexing and measurement by the IOSPEC miniature guided-wave spectrometer, the FP-IOSPEC spectrometer is capable of providing resolution of the CO₂ fine-line spectral features. The experimental measurements and 0.28 nm line spacing provided a good fit to the corresponding Hitran simulations given in Fig. 12(b).

Fig. 12:

(a) Preliminary IOSPEC high-resolution spectrum of CO2 1573 nm band using tuned narrow-band source (12’ path length at 4 atm.) - 0.28 nm peak spacing and (b) Simulation of CO2 1573 and 1580 nm absorption bands for 1 standard atmosphere using the Hitran data base [8] - about 0.3 nm CO₂ fine peak spacing near 1573 nm.

5.

CONCLUSIONS

The MPBC guided-wave spectrometer miniaturization provides an order of magnitude reduction in the mass and volume relative to traditional bulk-optic spectrometers while also providing significant performance advantages; including mechanical simplicity, robust optical alignment of the spectrometer optics on a high-performance dielectric IR slab waveguide structure, and wide spectral range of operation in first-order diffraction (m=1).

The MEOS payload encompasses ground-breaking innovation in miniaturized high-performance infrared (IR) spectrometers to enable substantial new atmospheric science and Earth observation to be accomplished on a low-cost microsat platform. The MEOS instrumentation is based on the advancement of MPBC’s patented technologies [US 7,034,935 B1) for high performance miniature guided-wave IR spectrometers [5,6] that are currently being bread-boarded with the assistance of the Canadian Space Agency.

The spectrometer miniaturization is a significant and original advance that facilitates the use of multiple dedicated spectrometers on the same microsat platform to allow simultaneous and coordinated measurements to improve the achievable mission science. The net resources required in terms of power (<23 W) and mass (<22.5 kg) for the proposed baseline suite of MEOS instruments (see Table 1) is less than the mass and power typically required for a single bulk-optic IR spectrometer. The MEOS innovative instrument miniaturization provides the following significant capabilities:

The significant benefits of the FP-IOSPEC instrument include:

Table 3 compares key MEOS characteristics to several relevant current and planned missions. The previous missions have been based on large, bulk-optic instruments that require costly platforms. Through the innovations proposed for MEOS, a significant atmospheric science mission can be performed on a low-cost microsat at fraction of the previous costs. Relative to previous/planned missions, the MEOS instrument suite provides a self-sufficient, geographically collocated data set with improved spectral resolution of the trace gas fine-line features. The MEOS instrument development will have a tremendous impact on the cost/performance of future atmospheric and EO missions to meet the sustained long-term measurements required to study climate change.

Table 3:

Comparison of MEOS to previous/planned relevant EO missions.