1.

INTRODUCTION

The onset for the HIGS idea was brought about by the ever increasing requirements for the spectrometers in Earth observation satellites [1]. The spectral resolution requirements were getting more stringent as well as those for the spatial resolution. The amount of light captured per detector pixel will thereby reduce strongly, requiring a larger instrument to meet the SNR requirement. Since this will ultimately lead to large and costly instruments, we considered alternative approaches to space-borne observations for atmospheric chemistry and climate applications. Owing to the knowledge built up using spectrometers, the HIGS concepts could be developed.

The name HIGS is an acronym that stands for Huib’s Innovative Gas Sensor. The HIGS idea was conceived in the year 2013 around the time the HIGGS boson was first measured, and it was selected as name for this instrument concept to honour the inventor Huib Visser. HIGS can most easily be described as a static FTS (Fourier Transform Spectrometer), however, while a standard FTS is used to measure a spectrum, HIGS makes use of the knowledge about the spectrum thus enabling a new measurement method. The name CarbonHIGS will be used in the following in case we discuss the CO₂ measuring unit. For general discussions on the presented measuring method the name HIGS will be used, since HIGS is not limited to the detection of CO₂. In 2017 result on a breadboard version of HIGS, operating in the visible wavelength range was published [2]. That breadboard was designed to detect NO2 in the atmosphere.

The past two decades have seen an impressive evolution of space-borne instruments for atmospheric chemistry applications (SCIAMACHY, GOME, GOME-2, OMI, TropOmi, and TropOMI/Sentinel-5p). The instrument concept has evolved from scanning to push-broom imaging, enabled by the advent of large image detectors in the near- and short-wave infrared spectral regions. More recently, the concept of push-broom imaging spectrometers has been applied to measuring greenhouse gases carbon dioxide (CO2) and methane (CH₄) by JAXA’s GOSAT and NASA’s OCO-2 missions, as well as the Chinese TANSAT mission. While these missions, as well as the CNES’ future MicroCarb mission, are mainly dedicated to studying the global carbon cycle, which is dominated by biogenic sources and sinks, imaging of emission plumes from anthropogenic CO₂ sources, such as power plants, has been demonstrated by [3]. Due to its nature as a well-mixed gas, the enhanced concentration of CO₂ in emission plumes quickly decreases with distance from the source. Therefore, spatial resolution is of paramount importance for imaging anthropogenic CO₂ plumes. Detection and quantification of mid-size point sources (power pants) and cities will require spatial resolution below one kilometre, while still maintaining high precision. Such high resolution is difficult to obtain from conventional push-broom imaging spectrometers based on diffraction grating technology. This is because the dispersion of light for measuring an extended spectral band at high spectral resolution necessitates a large entrance pupil size to reach the required signal-to-noise ratio (SNR) of the measured radiance. This in turn leads to impractically large instruments and unfeasible grating sizes.

3.

HIGS EXPLAINED

The idea of HIGS is based on the fact that many gases in the Earth’s atmosphere show a characteristic absorption structure. As an example the absorption coefficient for CO₂ is shown in Fig. 1. This regular absorption feature shows a strong resemblance with the transmission of an interferometer having a optical path difference (OPD) between its two arms. This OPD creates a wavelength dependent throughput where the two interferometer outputs are of course mutually out of phase. If the OPD is matched to the spectral density of the absorption features, a highly selective and sensitive detector can be created. In this way one output of the interferometer will have the spectral throughput corresponding to the absorption features, while the other output receives light of the wavelengths that are not, or at least not so strongly, absorbed. The bandwidth of light reaching the detector is limited by a band-filter. The HIGS instrument records two images, the outputs of the interferometer, pertaining to one ground scene. The difference in intensity of those two images is a direct measure for the concentration of the gas being measured.

3.1

Modes of operation

Looking at Fig. 1 two HIGS implementation modes become apparent. The first is the Line-mode. In this mode a large OPD value is implemented such that a fast oscillating throughput function is obtained that matches the line structure in the absorption features. The second mode of operation is the Lobe-mode, in which a smaller OPD is created such that the oscillating throughput matches the envelope of the absorption features.

Figure 1.

Absorption coefficient of CO₂ showing two lobes, each consisting of many absorption lines.

3.1.1

Line mode

As stated above, in the Line mode the OPD value is tuned such that the transmission function equals the spectral density of the absorption features. In this mode the HIGS instrument is optimally sensitive for the gas to which the OPD is matched, and shows limited sensitivity for other absorption features in the spectral range being measured.

A drawback of the Line mode is that a large OPD values is often required in order to arrive at a high spectral density. In the implementation section, Sec.4, the two basic interferometer types will be explained with their limitations on OPD values. The overlap of the OPD filter and the absorption features is often only good over a limited wavelength range. This wavelength range has to be selected by the band filter in the system, resulting in less photons as compared to the Lobe mode, that can lead to a decreased sensitivity.

In Fig. 2 a possible Line mode OPD is shown. The lines being measured are in the shorter wavelength lobe as shown in Fig. 1. The required OPD is about 8 mm.

Figure 2.

Filter functions for the Line mode. In red and blue the OPD functions and in black the band-filter. In grey the CO₂ spectrum is shown.

3.1.2

Lobe mode

Figure 3 shows the filter functions for the Lobe mode. In the Lobe mode a smaller OPD value is required since the spectral density is far lower than in the Line mode: an OPD value of about 0.37 mm was used. The drawback of the Lobe mode is that it is less selective than the Line mode. The Lobe mode can be more sensitive for the gas being measured and the signal levels are higher than in the Line mode owing to the wider spectral range being measured.

Figure 3.

Filter functions for the Lobe mode. In red and blue the OPD functions and in black the band-filter. In grey the CO₂ spectrum is shown.

3.2

Detection limit

In order to get a value for how sensitive a given HIGS configuration is for measuring a certain gas, the first steps of the data analysis are discussed. The throughput spectra have to be integrated over the spectral range as defined by the band filter. A scaled version of the signal to be integrated in case of CO₂ measurements is shown in Fig. 4, both for the Line and Lobe mode. The signal per detector pixel can be described by

Figure 4.

In Red and Blue the in- and out-of-phase signals are shown for the two presented modes

where I(λ) stands for the Earth radiance (expressed in ph/cm².sr.s.nm) multiplied by the etendue (cm².sr), the efficiency of the optical system and detector, and the integration time. The integration interval λ_min to λ_max is defined via the band-filter. The subscripts in and out stand for in-phase and out-of-phase, respectively, i.e. the two images as recorded in the HIGS system.

The difference signal

is a measure for the concentration of the gas being measured. The noise, or inaccuracy in determining this difference signal is equal to

Signal noise is for HIGS often allowed owing to the high signal levels, therefor other noise sources are omitted here. Since the ΔS can become zero for certain concentration levels depending on spectral width of the band-filter, the ratio between ΔS and the Noise cannot be used to specify the Signal to Noise ratio. It is better to determine what concentration change can be measured with an SNR of unity. To determine this value the slope of ΔS as a function of concentration has to be determined, the ΔS’,. The detection limit DL (at unit SNR), expressed in ppm, is found by

For this analysis the Earth albedo is assumed to be known. The detection limit for the CarbonHIGS unit operating in Lobe mode (lhs), and Line mode (rhs) are shown in Fig. 5. For the shown detection limit a 3×3 binning is taken into account as well as an f/5 beam towards the detector. To arrive at the detection limits, an integration times of about, or even exceeding 60 s is required. A scanning mirror to freeze the scene enables these integration times.

Figure 5.

Detection limit for measuring CO₂, for the two presented modes.

4.

IMPLEMENTATIONS

The HIGS system is an interferometer with an OPD between its two arms. A sketch of a HIGS based system is shown in Fig. 6. The light path from the entrance aperture, through the telescope and band filter, into the interferometer can be seen. The interferometer has two outputs that can be guided to a single detector, or each to its own detector. In an interferometer the light is split into two arms. The light propagates through these two arms, after which the light recombines. Splitting can be done in two ways, power- or polarization splitting, see Fig. 7. A power splitting interferometer that is best suited for HIGS is the Mach-Zehnder, based on the accessible two output ports that are mutually in anti-phase. The OPD creating element for a Mach-Zehnder is a planar piece of glass with the proper thickness to create the desired spectral throughput function.

Figure 6.

Sketch of HIGS system.

Figure 7.

Layout of the interferometers in Fig. 6. Top: the power splitter based Mach-Zehnder, and bottom: the polarization based interferometer.

For a polarization splitting interferometer the most stable version is the one where the beams propagate common path. This can be achieved via birefringent crystals where the splitting of the polarized input is into the Ordinary- and Extraordinary beams in the crystals. The incoming light needs to be linearly polarized and the optical axis of the crystal has to be placed under 45 degrees with the polarization direction. A polarizing beam splitting cube in the light path after passage thought the birefringent crystal combines the two modes and creates the two images as required for the HIGS concept. The OPD for birefringent crystal is the thickness of the crystal multiplied by the difference between the ordinary and extraordinary refractive indices. Since this difference is often very small this automatically results in thick crystal. Due to this a birefringent based HIGS cannot be used in case a large OPD value is required.

5.

DISCUSSION AND CONCLUSIONS

The HIGS concepts has been explained and two modes of measuring have been presented, the Line- and Lobe-mode. Due to the fact that the Line-mode for many gasses will require a large OPD value, i.e. exceeding 1 mm, the Mach-Zehnder based HIGS is often most suited for that mode. The Lobe-mode, owing to the widely spaced spectral fringes, requires a low OPD value and can make use of the polarization based interferometer. This system is inherently stable owing to the common path propagation through the interferometer. Although the Lobe-mode will have a higher SNR than the Line-mode, owing to the larger spectral range, this does not automatically lead to a higher sensitivity for the gas being measured. This sensitivity is determined by the difference signal between the two images and is found to be higher for the Line-mode in certain situations. It should be noted that the Line-mode will have lower sensitivity to other gasses, e.g. in the case of measuring CO₂, water vapour and aerosols.

The schematically presented system will have a detection limit of down to 1 ppm of CO₂, for a ground pixel of 200 × 200 m, which makes it an ideal system to fly along a larger instrument and to operate in so-called zoom-mode. A scan mirror will allow the system to measure for a longer time at one place, thereby obtaining the indicated low detection limit. The larger instrument, although having a coarser ground sampling, can be used for some of the required corrections. For the corrections that need to be recorded using the same ground sampling, an additional HIGS or other 2D imager needs to be integrated in the same satellite. Ideas for a completely stand-alone mission have been developed and will be presented in the future.

In the course of 2021 the ESA study will be completed. At that time the sensitivity to stray-light and ghosting will be known, and a full tolerance analysis will have been made. A follow-up paper will be issued in which all these results will be presented.