4:45 PM - 5:00 PM
[PCG19-12] Feasibility study of the mid-infrared laser heterodyne spectrometer with the hollow optical fiber coupler for incoherent lights
Keywords:atmosphere, high dispersion spectroscopy, mid infrared, fiber optics
The mid-infrared region, which contains many molecular absorption lines, is important for observations of terrestrial and planetary atmospheres. In this region, heterodyne spectroscopy can achieve ultra-high wavelength resolution of λdλ ~107 with the sensitivity close to the quantum noise limit. This capability enables the observation of spectral shapes sufficient to obtain the abundance of trace gases and the altitude profiles of temperature and wind velocity in the upper atmosphere. Using this technique, there have been unique ground-based observations of the atmospheres of Venus, Mars, Jupiter, and Titan. However, conventional heterodyne spectrometers use many optical elements to achieve high-precision optical alignment and tend to be large and heavy configurations. There are some attempts to simplify the system by using hollow fibers or hollow waveguides, in order to be more stable, smaller, and lighter. It is required to enable long-term continuous ground-based observations or future small instruments aboard spacecraft.
With hollow fibers, we have developed a new heterodyne spectroscopy scheme in which lights from the source and LO were transmitted in hollow fibers and combined in a hollow fiber coupler. Compared to polycrystalline and chalcogenide fibers, the hollow fiber achieves high transmission efficiency over a wide bandwidth of 2-12 µm. However, hollow fibers transmit light while reflecting it internally, resulting in multi-mode transmission of light injected at different angles. Multi-mode transmission makes differences in optical path length, which reduces light coherence. Since heterodyne spectroscopy potentially requires coherent LO light, there has been concern about the reduction in sensitivity due to the loss of light coherence.
In our previous work, a heterodyne spectroscopy system was constructed with a hollow fiber in the optical path of the LO light, in which the source and LO lights were combined with a beam splitter as the conventional system. This fiber achieved a high transmission of ~87 %/m for coherent laser light and ~89 %/m for incoherent sunlight. The sensitivity to incoherent light in the system achieved the same level as the conventional system without fibers. It indicated that the transmission of the hollow fiber did not affect the sensitivity of the heterodyne system. We also verified a hollow fiber coupler applied for heterodyne spectroscopy. This can make it easy to combine two lights compared to a beam splitter used in conventional systems. Using coherent laser light as a source, heterodyne spectroscopy with the fiber coupler achieved high wavelength resolution (λdλ > ~6 × 106) comparable to the conventional system. On the other hand, the response characteristics for incoherent light have not been evaluated yet.
In this study, we investigated the feasibility of mid-infrared laser heterodyne spectroscopy system with the hollow optical fiber coupler for incoherent lights. With our test system, we showed the capability of the heterodyne spectrometer with a hollow fiber coupler for incoherent light. First, we succeeded in measuring an absorption line of C2H4 in a gas cell with the incoherent background light from the blackbody. The measured line profile agreed well with the model profile. In addition, we observed terrestrial atmospheric absorption in sunlight. The measured intensity and spectral profile were consistent with the model with simple assumption. These results demonstrated the applicability of the heterodyne spectrometer with the hollow fiber and the hollow fiber coupler to the observations of terrestrial and planetary atmospheres.
With hollow fibers, we have developed a new heterodyne spectroscopy scheme in which lights from the source and LO were transmitted in hollow fibers and combined in a hollow fiber coupler. Compared to polycrystalline and chalcogenide fibers, the hollow fiber achieves high transmission efficiency over a wide bandwidth of 2-12 µm. However, hollow fibers transmit light while reflecting it internally, resulting in multi-mode transmission of light injected at different angles. Multi-mode transmission makes differences in optical path length, which reduces light coherence. Since heterodyne spectroscopy potentially requires coherent LO light, there has been concern about the reduction in sensitivity due to the loss of light coherence.
In our previous work, a heterodyne spectroscopy system was constructed with a hollow fiber in the optical path of the LO light, in which the source and LO lights were combined with a beam splitter as the conventional system. This fiber achieved a high transmission of ~87 %/m for coherent laser light and ~89 %/m for incoherent sunlight. The sensitivity to incoherent light in the system achieved the same level as the conventional system without fibers. It indicated that the transmission of the hollow fiber did not affect the sensitivity of the heterodyne system. We also verified a hollow fiber coupler applied for heterodyne spectroscopy. This can make it easy to combine two lights compared to a beam splitter used in conventional systems. Using coherent laser light as a source, heterodyne spectroscopy with the fiber coupler achieved high wavelength resolution (λdλ > ~6 × 106) comparable to the conventional system. On the other hand, the response characteristics for incoherent light have not been evaluated yet.
In this study, we investigated the feasibility of mid-infrared laser heterodyne spectroscopy system with the hollow optical fiber coupler for incoherent lights. With our test system, we showed the capability of the heterodyne spectrometer with a hollow fiber coupler for incoherent light. First, we succeeded in measuring an absorption line of C2H4 in a gas cell with the incoherent background light from the blackbody. The measured line profile agreed well with the model profile. In addition, we observed terrestrial atmospheric absorption in sunlight. The measured intensity and spectral profile were consistent with the model with simple assumption. These results demonstrated the applicability of the heterodyne spectrometer with the hollow fiber and the hollow fiber coupler to the observations of terrestrial and planetary atmospheres.