11:30 AM - 11:45 AM
[PCG18-04] Development of mid-IR laser heterodyne spectrometer with hollow optical fiber for planetary exploration
Keywords:mid-infrared, spectroscopy
Since in-situ observation of planetary atmosphere is difficult, spectroscopic remote-sensing observations have become a powerful tool in planetary atmospheric studies. The mid-infrared (MIR) region(λ = 5 – 20 μm) involves numerous absorption and emission lines of atmospheric molecules. The MIR laser heterodyne spectroscopy provides high spectral resolution > 106-7, which is much greater than other direct spectroscopic measurements. Such high-resolution spectroscopy enables to derive the vertical distribution of atmospheric components and temperature, and wind speed. This technique combines an IR source signal from the observing target and an IR laser (a quantum cascade laser (QCL) and/or a CO2 gas laser) as the local oscillator (LO). In the MIR region, three groups including Tohoku University have succeeded in developing laser heterodyne spectrometers for planetary atmospheric observations. They have achieved unique observations of the atmospheres of various planets and satellites such as Venus, Mars, Jupiter, and Titan (Kostiuk et al, 1983; Kostiuk et al., 2001; Sonnabend et al., 2010; Miyamoto et al., 2021).
In the current system, two beams are combined at a ZnSe beam splitter and then focused onto a HgCdTe photomixer. In this scheme, a precise optical alignment is highly required to combine two beams. The volume and weight reductions are also the issue. These issues constrain the transportability of ground-based measurements, and due to the lack of stability and miniaturization of the optics, space-born measurement has not yet been realized. In this study, we aim to solve these issues by simplifying those optics using MIR-transmissive hollow optical fibers.
There have been few optical fibers which have a high transmittance at the wavelengths longer than 2 μm. Recently, MIR-transmissive hollow optical fibers have been developed by Tohoku University (Matsuura et al., 1995). The fibers are made of glass tubing, covered by a conductive Ag layer covered by a dielectric AgI layer. The fiber couplers using this hollow optical fiber have also been developed by University of Toyama. Here, we have evaluated the characteristics of the hollow optical fibers and the fiber couplers, and verified their feasibility for laser heterodyne spectroscopy. Our results are summarized as follows:
(1) Transmission characteristics of the hollow optical fibers strongly depend on its incident angle. With our appropriate incident optics, the transmittance at 10.3 μm by CO2 gas laser and 9.6 μm by QCL (coherent sources) could achieve more than 85 %/m. We also succeeded the notable high-transmittance to be 89.6 %/m even for the sunlight (incoherent source), for the first time.
(2) We also confirmed the applicability of the hollow optical fibers and the fiber coupler to MIR laser heterodyne technique. The heterodyne spectroscopy with the hollow optical fibers and the fiber coupler resolved the spectral feature of the narrow laser emission line.
(3) Using the LO fed into the hollow optical fiber, the obtained system noise temperature (less than 3,000 K) at 10.3 μm is only ~100% above the quantum limit. This could provide the excellent minimum detectability in this regime. Also, such sensitivity is quite comparable with the system without the hollow optical fiber. This result suggests that the MIR laser heterodyne system using hollow optical fibers can provide the sensitivity necessary for observing the atmosphere of Mars, Venus, and other planets.
Our results demonstrate the feasibility of simplifying the optical system of the MIR laser heterodyne spectrometer using the hollow optical fibers and the fiber coupler. Such unique instrument could contribute to a deeper understanding of planetary systems, via orbiter, balloon, lander systems.
In the current system, two beams are combined at a ZnSe beam splitter and then focused onto a HgCdTe photomixer. In this scheme, a precise optical alignment is highly required to combine two beams. The volume and weight reductions are also the issue. These issues constrain the transportability of ground-based measurements, and due to the lack of stability and miniaturization of the optics, space-born measurement has not yet been realized. In this study, we aim to solve these issues by simplifying those optics using MIR-transmissive hollow optical fibers.
There have been few optical fibers which have a high transmittance at the wavelengths longer than 2 μm. Recently, MIR-transmissive hollow optical fibers have been developed by Tohoku University (Matsuura et al., 1995). The fibers are made of glass tubing, covered by a conductive Ag layer covered by a dielectric AgI layer. The fiber couplers using this hollow optical fiber have also been developed by University of Toyama. Here, we have evaluated the characteristics of the hollow optical fibers and the fiber couplers, and verified their feasibility for laser heterodyne spectroscopy. Our results are summarized as follows:
(1) Transmission characteristics of the hollow optical fibers strongly depend on its incident angle. With our appropriate incident optics, the transmittance at 10.3 μm by CO2 gas laser and 9.6 μm by QCL (coherent sources) could achieve more than 85 %/m. We also succeeded the notable high-transmittance to be 89.6 %/m even for the sunlight (incoherent source), for the first time.
(2) We also confirmed the applicability of the hollow optical fibers and the fiber coupler to MIR laser heterodyne technique. The heterodyne spectroscopy with the hollow optical fibers and the fiber coupler resolved the spectral feature of the narrow laser emission line.
(3) Using the LO fed into the hollow optical fiber, the obtained system noise temperature (less than 3,000 K) at 10.3 μm is only ~100% above the quantum limit. This could provide the excellent minimum detectability in this regime. Also, such sensitivity is quite comparable with the system without the hollow optical fiber. This result suggests that the MIR laser heterodyne system using hollow optical fibers can provide the sensitivity necessary for observing the atmosphere of Mars, Venus, and other planets.
Our results demonstrate the feasibility of simplifying the optical system of the MIR laser heterodyne spectrometer using the hollow optical fibers and the fiber coupler. Such unique instrument could contribute to a deeper understanding of planetary systems, via orbiter, balloon, lander systems.