Long-wave IR sensing using spatial heterodyne spectroscopy

High-resolution, passive, long-wave IR (LWIR) spectroscopy has numerous civilian and military applications. Many of these take advantage of the unique spectral emission or absorption signatures of solids, liquids, and gases in the thermal IR regime. The spectral region between ~7 and 20μm is known as the ‘fingerprint region,’ since it contains many of these features, including rotational/vibrational molecular lines. For example, remote detection of the unique absorption and emission lines of many gases in the atmosphere can be done with high sensitivity and specificity.

Many measurement techniques are employed in the LWIR range using grating spectrometers,¹ imaging and nonimaging Fourier-transform spectrometers (FTSs),^2–6 Fabry-Perot interferometers,⁷ and prism spectrographs.⁸ Since the emerging optical technique of spatial heterodyne spectroscopy⁹ (SHS) has many unique properties that are beneficial for several LWIR sensing applications, we designed, built, and tested an SHS instrument that could operate in this wavelength range. Compared to conventional techniques, the main advantages of SHS are its high interferometric throughput and spectral resolution, the compact, rugged package, relaxed alignment and fabrication tolerances, and the technique's insensitivity to spectral errors caused by changing scenes.

The SHS concept is related to that of FTS, since it also features a beamsplitter that divides the incoming signal into two interferometer arms (see Figure 1). However, in SHS the latter terminate at fixed, tilted gratings that impose a wavelength-dependent tilt onto the diffracted wavefronts. After recombination at the beamsplitter and imaging onto a detector array, a complete interferogram can be recorded without using any moving parts. SHS-based instruments have previously been used in the UV and visible ranges, including in spectrometers for satellite remote sensing of the Earth's atmosphere,¹⁰ planetary astronomy,¹¹ and laboratory spectroscopy.¹² However, broad-band spectra have never been recorded successfully using LWIR SHS.

Figure 1. Basic concept of a non-field-widened SHS interferometer. Field widening can be achieved by placing a fixed prism in each interferometer arm, thus significantly increasing the device's sensitivity. FTS: Fourier-transform spectrometer. L_i(i=1, 2, 3): Lenses. θ_L: Grating angle. γ: Wavefront tilt angle.

We demonstrated that an LWIR SHS interferometer can be constructed by holding all optical components in compression,¹³ and solely relying on the machine tolerances of the structural elements for alignment. Since the resulting interferometer has no alignment mechanism, it is exceptionally robust. The beam splitter at the center of the fully assembled device (see Figure 2) is made of zinc selenide, the field-widening prisms (not visible) are antireflection-coated germanium, and the structure is molded from aluminum. The interferometer's design passband covers the wavelength interval between 8.4 and 11.6μm, with a resolving power of approximately 500.¹⁴

Figure 2. LWIR SHS interferometer. The green surface (center) is the beamsplitter (50mm diameter). The field-widening prism and grating of one interferometer arm are contained under the cap on the right.

The complete spectrometer—see Figures 3(a) and 3(b)—consists of a telescope, the interferometer, exit optics, and a cooled detector array. The telescope features a cooled passband filter and telecentric optics, which allow 1D imaging. The exit optics images the fringe pattern created by the interferometer, recording a complete interferogram for each detector row. The focal plane of the mercury cadmium telluride array detector and the passband filter are the only cooled spectrometer parts.¹⁴

Figure 3. (a) Scale drawing and (b) photograph of the spectrometer components. MCT: Mercury cadmium telluride.

To test the spectrometer's performance, we conducted transmittance measurements of methanol gas and a polyimide foil sample in a controlled laboratory environment. For the methanol measurements, we used a short gas cell filled with pure methanol. Figure 4 shows a representative result after conversion into the spectral domain and additional processing. We also show the good agreement of the data with a theoretical transmittance spectrum. The polyimide results match the characteristic absorption features of this polymer material to a similar extent.¹⁴

Figure 4. (black) Methanol transmittance spectrum obtained from our SHS instrument. (red) Best-fitting theoretical transmittance spectrum.¹⁴

In summary, we have demonstrated that the emerging technique of SHS is viable for LWIR applications, especially when they benefit from some of the unique SHS properties. For example, observations from moving vehicles are of potential interest since they require rugged instrumentation without moving parts. They usually also involve rapidly changing scenes. Future LWIR work will likely address the instrument's field hardening and miniaturization, as well as improving sensitivity and development of field-calibration techniques. In parallel, we are pursuing other novel uses of SHS, including measurements of methane on Mars¹⁵ and observation of atmospheric winds from space.¹⁶

This work was supported by the Office of Naval Research.