3:30 PM - 5:00 PM
[PCG19-P11] Development of mid-IR heterodyne spectrometer with fibers and a fiber coupler for the observation of planetary atmospheres
Keywords:heterodyne, mid-IR, spectrometer
Fiber optics enables significant improvements in optical measurement equipment, toward smaller, lighter, higher efficiency, lower cost, better stability and operability, stronger endurance, and lower failure rates. It also enables simultaneous and multiple measurements of targets that are hard to access directly. All of these are essential factors for onboard instruments for planetary orbiters, landers, rovers, aircraft, etc. This technology has been applied in many fields in the visible region and is beginning to be adopted in the near-IR region. On the other hand, in the mid-IR region, which has richer spectral bands of molecules and contains rich information on the atmosphere, geology, and life-related substances, the progress was slow and limited by transmission materials. This study is the development of the fiber-based mid-IR laser heterodyne spectrometer, which is a part of our research to realize fiber optics in the mid-IR region by hollow fibers.
Mid-IR laser heterodyne spectroscopy is a method in which the light from the target is mixed with the light from a local oscillator (LO), and its frequency is converted to the intermediate frequency, which is the frequency difference between the target and the LO. This method can achieve a high wavelength resolution of λ/dλ>106 (frequency resolution <30 MHz) in the mid-IR around the wavelength of 10 μm (frequency of 30 THz). This enables direct detection of planetary wind velocities with a resolution of several 10 m/s and has achieved unique results such as the wind changes associated with global dust storms on Mars and the super-rotation of Venus and Titan. However, because it requires precise convention of two optical paths, high-precision optical-axis adjustment with many mirrors and a beam splitter is essential. It makes the spectrometer large and complex, so it was unsuitable for instruments aboard spacecraft. We solve this problem by utilizing a hollow fiber that has high transmittance in the mid-IR region.
In this paper, we report the feasibility of laser heterodyne spectroscopy in the mid-IR region using a hollow fiber and a fiber coupler established through the following tests. (1) For the hollow fiber, transmission efficiencies of >85%/m were achieved for both CO2 lasers and quantum cascade lasers, and transmission efficiency of 89.6%/m was achieved for sunlight, i.e., incoherent natural light. It was also confirmed that heterodyne spectroscopy of a CO2 laser and a quantum cascade laser through a fiber using a beam splitter can achieve the same sensitivity as that without fibers. (2) For hollow fiber couplers, we have demonstrated that heterodyne spectroscopy of CO2 lasers and quantum cascade lasers with a fiber coupler is as sensitive as the system with a beam splitter (Nakagawa et al., Applied Optics, 2023). In addition, the heterodyne spectroscopy using a CO2 laser as the LO showed the system noise temperature was the same as the quantum noise limit.
With these results, we have shown for the first time that it is possible to construct a heterodyne spectroscopy system using fibers and a hollow fiber coupler. Although these results were obtained with a laboratory system, we are now assembling a portable fiber-based heterodyne spectrometer for the attachment to a telescope. This system will be used for the test observations of (a) wind velocity and trace gas compositions of terrestrial upper atmosphere using solar transmissions and (b) mesospheric wind velocity in the Venusian atmosphere.
Mid-IR laser heterodyne spectroscopy is a method in which the light from the target is mixed with the light from a local oscillator (LO), and its frequency is converted to the intermediate frequency, which is the frequency difference between the target and the LO. This method can achieve a high wavelength resolution of λ/dλ>106 (frequency resolution <30 MHz) in the mid-IR around the wavelength of 10 μm (frequency of 30 THz). This enables direct detection of planetary wind velocities with a resolution of several 10 m/s and has achieved unique results such as the wind changes associated with global dust storms on Mars and the super-rotation of Venus and Titan. However, because it requires precise convention of two optical paths, high-precision optical-axis adjustment with many mirrors and a beam splitter is essential. It makes the spectrometer large and complex, so it was unsuitable for instruments aboard spacecraft. We solve this problem by utilizing a hollow fiber that has high transmittance in the mid-IR region.
In this paper, we report the feasibility of laser heterodyne spectroscopy in the mid-IR region using a hollow fiber and a fiber coupler established through the following tests. (1) For the hollow fiber, transmission efficiencies of >85%/m were achieved for both CO2 lasers and quantum cascade lasers, and transmission efficiency of 89.6%/m was achieved for sunlight, i.e., incoherent natural light. It was also confirmed that heterodyne spectroscopy of a CO2 laser and a quantum cascade laser through a fiber using a beam splitter can achieve the same sensitivity as that without fibers. (2) For hollow fiber couplers, we have demonstrated that heterodyne spectroscopy of CO2 lasers and quantum cascade lasers with a fiber coupler is as sensitive as the system with a beam splitter (Nakagawa et al., Applied Optics, 2023). In addition, the heterodyne spectroscopy using a CO2 laser as the LO showed the system noise temperature was the same as the quantum noise limit.
With these results, we have shown for the first time that it is possible to construct a heterodyne spectroscopy system using fibers and a hollow fiber coupler. Although these results were obtained with a laboratory system, we are now assembling a portable fiber-based heterodyne spectrometer for the attachment to a telescope. This system will be used for the test observations of (a) wind velocity and trace gas compositions of terrestrial upper atmosphere using solar transmissions and (b) mesospheric wind velocity in the Venusian atmosphere.