Heterodyne Spectrometer Research Articles

HerschelSpace Observatory

Herschel was launched on 14 May 2009, and is now an operational ESA space observatory offering unprecedented observational capabilities in the far-infrared and submillimetre spectral range 55-671 {\mu}m. Herschel carries a 3.5 metre diameter passively cooled Cassegrain telescope, which is the largest of its kind and utilises a novel silicon carbide technology. The science payload comprises three instruments: two direct detection cameras/medium resolution spectrometers, PACS and SPIRE, and a very high-resolution heterodyne spectrometer, HIFI, whose focal plane units are housed inside a superfluid helium cryostat. Herschel is an observatory facility operated in partnership among ESA, the instrument consortia, and NASA. The mission lifetime is determined by the cryostat hold time. Nominally approximately 20,000 hours will be available for astronomy, 32% is guaranteed time and the remainder is open to the worldwide general astronomical community through a standard competitive proposal procedure.

Evaluation of payload performance for a sounding rocket vacuum ultraviolet spatial heterodyne spectrometer to observe C IV λλ1550 emissions from the Cygnus Loop

The results of a project to develop a spatial heterodyne spectrometer (SHS) for a sounding rocket mission to study the Cygnus Loop, a prototypical middle-aged supernova remnant, are discussed. The goal was to obtain a radial velocity-resolved spectrum of the C IV lambdalambda1550 emission line from bright features of the Cygnus Loop, as a test for mapping the diffuse hot interstellar medium (ISM). A full Fourier-transform analysis of Cygnus Loop emission data is presented, showing lack of velocity-resolved C IV emission detection. Optics contamination is shown to be the most likely problem, and ways to eliminate this contamination for future SHS sounding rocket and satellite missions are discussed.

First results from an all-reflection spatial heterodyne spectrometer with broad spectral coverage

Operation of an all-reflection, broadband, spatial heterodyne spectrometer (SHS) is reported. This Mark 2 SHS is constructed using a custom diffraction grating and other standard optical components. The custom grating is coarse (18 grooves/mm), with a symmetric blaze that allows its simultaneous use as dispersing element and beam splitter and combiner. The grating is combined with a plane mirror and a roof mirror to form a very stable ring interferometer which has been used successfully in earlier narrowband SHS designs. Fringes from the extra grating orders in the main blaze envelope are unexpectedly found to combine constructively with the desired primary fringes of the interferometer. Elimination of ambiguity between wavelengths above and below blaze in a given order, and order separation are demonstrated using a small tilt of the plane mirror about an axis in the plane of the figure. Coverage of a factor of four in wavelength in a single CCD frame is demonstrated.

Development and characterization of the superconducting integrated receiver channel of theTELIS atmospheric sounder

The balloon-borne instrument TELIS (TErahertz and submillimetre LImb Sounder) is athree-channel superconducting heterodyne spectrometer for atmospheric researchuse. It detects spectral emission lines of stratospheric trace gases that have theirrotational transitions at THz frequencies. One of the channels is based on thesuperconducting integrated receiver (SIR) technology. We demonstrate for thefirst time the capabilities of the SIR technology for heterodyne spectroscopy ingeneral, and atmospheric limb sounding in particular. We also show that theapplication of SIR technology is not limited to laboratory environments, butthat it is well suited for remote operation under harsh environmental conditions.Within a SIR the main components needed for a superconducting heterodynereceiver such as a superconductor–insulator–superconductor (SIS) mixer with aquasi-optical antenna, a flux-flow oscillator (FFO) as the local oscillator, and aharmonic mixer to phase lock the FFO are integrated on a single chip. Lightweight and low power consumption combined with broadband operation andnearly quantum limited sensitivity make the SIR a perfect candidate for use infuture airborne and space-borne missions. The noise temperature of the SIR wasmeasured to be as low as 120 K, with an intermediate frequency band of 4–8 GHz indouble-sideband operation. The spectral resolution is well below 1 MHz, confirmed byour measurements. Remote control of the SIR under flight conditions has beendemonstrated in a successful balloon flight in Kiruna, Sweden. The sensor and instrumentdesign are presented, as well as the preliminary science results from the first flight.

近红外空间外差光谱仪及水汽探测研究

近红外空间外差光谱仪及水汽探测研究

Complex-Fourier-transform integrated-optic spatial heterodyne spectrometer using phase shift technique

Proposed is a phase shift method that employs a planar waveguide spatial heterodyne spectrometer (SHS) to reveal the input light spectrum. A complex Fourier transformation derives the spectrum directly without any mathematical assumptions, thus eliminating the need for the Lagrange interpolation or deconvolution techniques. The available spectral range is one free spectral range of the SHS and is twice as wide as that available with the conventional method based on a Fourier cosine transformation.

Tunable, all-reflective spatial heterodyne spectrometer for broadband spectral line studies in the visible and near-ultraviolet

A spatial heterodyne spectrometer (SHS) is an interferometric device that combines high resolving power and a large input acceptance angle in a compact format suitable for use at small telescope focal planes and spacecraft for observations of faint, angularly extended emission-line sources. The primary limitation of SHS instruments has come from the spatial sampling of their output interference pattern image, which limits their use to a narrow bandpass. We describe the first-light results from a broadband all-reflective SHS that can be mechanically aligned to any heterodyne wavelength from 300 to 700 nm. Such an instrument can be extremely useful for the near simultaneous study of the brightness and line shapes of major atomic and molecular diagnostic emissions from extended astrophysical targets such as comets or planetary atmospheres. We discuss the results of this validation program and the potential improvements that could be used to expand and/or improve the broadband spectral response of the instrument.

Analysis of nonlinear effects in microwave spectrometers

At millimeter wavelengths, rotational spectra of trace gases in planetary atmospheres yield essential characteristics such as vertical profiles of abundance, temperature, and winds. The high resolution provided by heterodyne spectroscopy allows to retrieve the chemical composition and thermal structure of the atmosphere with very high accuracy. On several observation campaigns, however, small unexpected differences have been observed in the power spectral density (PSD) response among heterodyne spectrometer backends. In this article, we analyze what the impact of nonlinearity is on these observations. This investigation covers two major topics. The first one seeks to theoretically demonstrate the importance of PSD accuracy for retrieving the correct geophysical parameters under observation. This study is performed by means of radiative transfer and retrieval simulations, which show how deviations in the spectra might produce considerable changes in the vertical profile of trace gases retrieved in the Earth's atmosphere. Once the importance of PSD accuracy has been addressed, the second topic focuses on the detection of nonlinearity in spectrometer backends. On the basis of a differential approach, an improved experiment with more than one order of magnitude higher accuracy than before was conducted in order to detect and study nonlinear behavior in chirp transform spectrometers and, consequently, evaluate possible mechanisms to optimize their response. As a result, it is confirmed that the overall deviations introduced by nonlinearity in spectrometers have been decreased, and thus the performance of the backend instrument has been improved.

Comparison of HIPWAC and Mars Express SPICAM observations of ozone on Mars 2006–2008 and variation from 1993 IRHS observations

Ozone is a tracer of photochemistry in the atmosphere of Mars and an observable used to test predictions of photochemical models. We present a comparison of retrieved ozone abundances on Mars using ground-based infrared heterodyne measurements by NASA Goddard Space Flight Center’s Heterodyne Instrument for Planetary Wind And Composition (HIPWAC) and space-based Mars Express Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) ultraviolet measurements. Ozone retrievals from simultaneous measurements in February 2008 were very consistent (0.8 μm-atm), as were measurements made close in time (ranging from 8 μm-atm) during this period and during opportunities in October 2006 and February 2007. The consistency of retrievals from the two different observational techniques supports combining the measurements for testing photochemistry-coupled general circulation models and for investigating variability over the long-term between spacecraft missions. Quantitative comparison with ground-based measurements by NASA/GSFC’s Infrared Heterodyne Spectrometer (IRHS) in 1993 reveals 2–4 times more ozone at low latitudes than in 2008 at the same season, and such variability was not evident over the shorter period of the Mars Express mission. This variability may be due to cloud activity.

A broadband, high-resolution spatial heterodyne spectrometer

Design and performance parameters of a broadband, high-resolution spatial heterodyne spectrometer (SHS) with a transmitting beamsplitter are described. This Mark 1 SHS achieves more than a factor of five in continuous wavenumber coverage with a resolving power in hundreds of thousands. Progress toward an all reflection, broadband, high-resolution Mark 2 SHS is reported.

The Herschel mission and observing opportunities

Herschel is the fourth cornerstone mission in the European Space Agency (ESA) science programme. It will perform imaging photometry and spectroscopy in the far infrared and submillimetre part of the spectrum, covering approximately the 55–672 µmm range and thus bridging the traditional space infrared range with the groundbased capabilities. The key science objectives emphasize fundamental issues connected to the formation and evolution of galaxies and stars and stellar systems. However, Herschel will be an observatory facility and its unique capabilities will be available to the entire astronomical community for a wide range of observations. Herschel is equipped with a passively cooled 3.5 m diameter classical Cassegrain telescope. The science payload complement – two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution heterodyne spectrometer (HIFI) – is housed in a superfluid helium cryostat. The ground segment is jointly developed by the ESA, the three instrument consortia, and NASA/IPAC. Herschel is scheduled to be launched into a transfer trajectory towards its operational orbit around the Earth-Sun L2 point by an Ariane 5 ECA (shared with the ESA cosmic background mapping mission Planck) in 2009. Once operational about half a year after launch, Herschel will offer 3 years of routine science operations. Almost 20 000 hours of observing time will nominally be made available for astronomy, 32% is guaranteed time, the remainder is open time which is offered to the worldwide general astronomical community through a standard competitive proposal procedure.

Broadband, high-resolution spatial heterodyne spectrometer

Design and performance parameters for a broadband, high-resolution spatial heterodyne spectrometer (SHS) are reported. The Mark 1 SHS achieves more than a factor of 5 in continuous wavenumber coverage with a design resolving power in the hundreds of thousands.

Fully reflective external-cavity setup for quantum-cascade lasers as a local oscillator in mid-infrared wavelength heterodyne spectroscopy

To our knowledge we present the first experiments with a fully reflective external-cavity quantum-cascade laser system at mid-infrared wavelengths for use as a local oscillator in a heterodyne receiver. The performance of the presented setup was investigated using absorption spectroscopy as well as heterodyne techniques. Tunability over approximately 30 cm(-1) at 1130 cm(-1) was demonstrated using a grating spectrometer. A continuous tuning range of 0.28 cm(-1) was verified by observing the spectra of an internally coupled confocal Fabry-Pérot interferometer and the absorption lines of gas phase SO(2). In a second step the output from the system was used as a local oscillator signal for a heterodyne setup. We show that spectral stability and side mode suppression are excellent and that a compact external-cavity quantum-cascade laser system is well suited to be used as a local oscillator in infrared heterodyne spectrometers.

Hot-electron bolometer terahertz mixers for the Herschel Space Observatory

We report on low noise terahertz mixers (1.4-1.9 THz) developed for the heterodyne spectrometer onboard the Herschel Space Observatory. The mixers employ double slot antenna integrated superconducting hot-electron bolometers (HEBs) made of thin NbN films. The mixer performance was characterized in terms of detection sensitivity across the entire rf band by using a Fourier transform spectrometer (from 0.5 to 2.5 THz, with 30 GHz resolution) and also by measuring the mixer noise temperature at a limited number of discrete frequencies. The lowest mixer noise temperature recorded was 750 K [double sideband (DSB)] at 1.6 THz and 950 K DSB at 1.9 THz local oscillator (LO) frequencies. Averaged across the intermediate frequency band of 2.4-4.8 GHz, the mixer noise temperature was 1100 K DSB at 1.6 THz and 1450 K DSB at 1.9 THz LO frequencies. The HEB heterodyne receiver stability has been analyzed and compared to the HEB stability in the direct detection mode. The optimal local oscillator power was determined and found to be in a 200-500 nW range.

Applications for quantum cascade lasers and detectors in mid-infrared high-resolution heterodyne astronomy

Quantum cascade (QC) structures, for both emitting and detecting mid-infrared radiation, are powerful devices for spectroscopy. QC lasers (QCLs), which have been built for nearly 15 years, already play the leading role in certain wavelength regions. QC detectors (QCDs) are a fairly new development, which has been evolving from the QCL research. In high-resolution heterodyne spectrometers for astronomy, such as the Cologne tuneable heterodyne infrared spectrometer (THIS), QC devices help to open new windows to space as discussed in this paper. We will briefly review the use of QC devices in THIS, show recent results in measuring planetary atmospheric dynamics and give an outlook to astronomical goals for the future.

Retrieval of minor constituents in a cloudy atmosphere with remote-sensing millimetre-wave measurements

The presence of clouds in remote-sensing measurements is often the cause of information loss about the minor atmospheric constituent profiles that are the goal of the measurement. Millimetre- and submillimetre-wave sounding can be used to abate the problems of cloud contamination. A quantitative assessment of this possibility is provided with the systematic simulation of the retrieval of minor atmospheric constituents in the presence of ice cirrus clouds of different density and particle size. The simulations are made for the particular case of the MARSCHALS instrument which is a heterodyne limb-sounding spectrometer that measures the atmospheric thermal emission in the millimetre- and submillimetre-wave regions operating on board a stratospheric aircraft. Four types of measuring conditions are identified: clouds that do not modify the observation, clouds that are adequately described with an atmospheric continuum absorption model, clouds for which an error is introduced when the limited vertical resolution of the sounding does not resolve the sharp discontinuity of the cloud, and clouds which require the modelling of scattering effects for a correct retrieval. When a rigorous modelling of the cloud is performed, the retrieval of minor atmospheric constituent is marginally affected by the presence of the cloud. We find that in the worst case the cloud causes a loss, with respect to clear-sky conditions, of two degrees of freedom from about 90 and of 7% of information content relative to climatological information. Copyright © 2007 Royal Meteorological Society

Increased Modal Overlap for Improved Sensitivity in a Monolithic Intracavity Chemical Sensor

We discuss the design and fabrication of an InP-based single-chip chemical sensor with increased modal overlap with a chemical analyte. The fabricated devices use a sensor design with frequency tunable lasers and heterodyne spectrometers. By reducing the waveguide ridge width in one section of the laser, the transverse modal overlap with the analyte increases by 17 times, increasing the sensitivity by the same ratio. A frequency shift of 72 GHz/refractive index unit was measured with temperature effects removed. The frequency stability of this sensor is as low as 436 kHz leading to a minimum detectable index difference of 6 times 10-6.

Modeling of the generic spatial heterodyne spectrometer and comparison with conventional spectrometer

We describe the modeling of the generic spatial heterodyne spectrometer. This instrument resembles a somewhat modified Michelson interferometer, in which the power spectrum of the input source is determined by performing a one-dimensional Fourier transform on the output intensity profile. Code has been developed to analyze the performance of this type of spectrometer by determining the dependence of both spectral resolution and throughput on parameters such as aperture and field of view. An example of a heterodyne spectrometer is developed to illustrate the techniques employed in the modeling and a comparison undertaken between its performance and that of a conventional spectrometer. Unlike the traditional Fourier transform infrared system, the heterodyne spectrometer has the very desirable feature of having no moving components.

Detection of Diffuse Interstellar [O ii ] Emission from the Milky Way Using Spatial Heterodyne Spectroscopy

Using a newly developed spatial heterodyne spectrometer (SHS), we have obtained the first radial velocity resolved emission-line profiles of diffuse [O II] 3726 and 3729 ? emission lines from the warm (104 K) ionized component of our Galaxy's interstellar medium. These [O II] lines are a principal coolant for this widespread, photoionized gas and are a potential tracer of variations in the gas temperature resulting from unidentified heating processes that appear to be acting within the Galaxy's disk and halo. By spectrally isolating for the first time Galactic [O II] from atmospheric [O II] emission, we were able to detect interstellar [O II] out to 20? from the Galactic equator with intensities that range from tens of rayleighs near the Galactic plane to less than 1 rayleigh at high Galactic latitudes. The [O II] line profiles clearly show structure indicating emission along the lines of sight from both local and more distant interstellar gas. Comparisons of the [O II] intensities with the intensities of [N II] 6584 ? and H? 6563 ? observed with WHAM indicate that the observed variations in [N II]/H? and [O II]/H? in the diffuse interstellar gas are consistent with variations in temperature and confirm the value of the [O II] observations as a temperature diagnostic for the WIM.

Ozone abundance on Mars from infrared heterodyne spectra: II. Validating photochemical models

Ozone is an important observable tracer of martian photochemistry, including odd hydrogen (HO x ) species important to the chemistry and stability of the martian atmosphere. Infrared heterodyne spectroscopy with spectral resolution ⩾ 10 6 provides the only ground-based direct access to ozone absorption features in the martian atmosphere. Ozone abundances were measured with the Goddard Infrared Heterodyne Spectrometer and the Heterodyne Instrument for Planetary Wind and Composition at the NASA Infrared Telescope Facility on Mauna Kea, Hawai'i. Retrieved total ozone column abundances from various latitudes and orbital positions ( L S = 40 ° , 74°, 102°, 115°, 202°, 208°, 291°) are compared to those predicted by the first three-dimensional gas phase photochemical model of the martian atmosphere [Lefèvre, F., Lebonnois, S., Montmessin, F., Forget, F., 2004. J. Geophys. Res. 109, doi:10.1029/2004JE002268. E07004]. Observed and modeled ozone abundances show good agreement at all latitudes at perihelion orbital positions ( L S = 202 ° , 208°, 291°). Observed low-latitude ozone abundances are significantly higher than those predicted by the model at aphelion orbital positions ( L S = 40 ° , 74°, 115°). Heterogeneous loss of odd hydrogen onto water ice cloud particles would explain the discrepancy, as clouds are observed at low latitudes around aphelion on Mars.