Aims. This paper describes the Heterodyne Instrument for the Far-Infrared (HIFI) that was launched onboard ESA's Herschel Space Observatory in May 2009. Methods. The instrument is a set of 7 heterodyne receivers that are electronically tuneable, covering 480−1250 GHz with SIS mixers and the 1410−1910 GHz range with hot electron bolometer (HEB) mixers. The local oscillator (LO) subsystem comprises a Ka-band synthesizer followed by 14 chains of frequency multipliers and 2 chains for each frequency band. A pair of auto-correlators and a pair of acousto-optical spectrometers process the two IF signals from the dual-polarization, single-pixel front-ends to provide instantaneous frequency coverage of 2 × 4 GHz, with a set of resolutions (125 kHz to 1 MHz) that are better than 0.1 km s −1 . Results. After a successful qualification and a pre-launch TB/TV test program, the flight instrument is now in-orbit and completed successfully the commissioning and performance verification phase. The in-orbit performance of the receivers matches the pre-launch sensitivities. We also report on the in-orbit performance of the receivers and some first results of HIFI's operations.
Measurements in the infrared wavelength domain allow us to assess directly the physical state and energy balance of cool matter in space, thus enabling the detailed study of the various processes that govern the formation and early evolution of stars and planetary systems in the Milky Way and of galaxies over cosmic time. Previous infrared missions, from IRAS to Herschel, have revealed a great deal about the obscured Universe, but sensitivity has been limited because up to now it has not been possible to fly a telescope that is both large and cold. Such a facility is essential to address key astrophysical questions, especially concerning galaxy evolution and the development of planetary systems.SPICA is a mission concept aimed at taking the next step in mid-and far-infrared observational capability by combining a large and cold telescope with instruments employing state-of-the-art ultrasensitive detectors. The mission concept foresees a 2.5-meter diameter telescope cooled to below 8 K. Rather than using liquid cryogen, a combination of passive cooling and mechanical coolers will be used to cool both the telescope and the instruments. With cooling not dependent on a limited cryogen supply, the mission lifetime can extend significantly beyond the required three years. The combination of low telescope background and instruments with state-of-the-art detectors means that SPICA can provide a huge advance on the capabilities of previous missions.The SPICA instrument complement offers spectral resolving power ranging from R ∼50 through 11000 in the 17-230 µm domain as well as R ∼28.000 spectroscopy between 12 and 18 µm. Additionally SPICA will be capable of efficient 30-37 µm broad band mapping, and small field spectroscopic and polarimetric imaging in the 100-350 µm range. SPICA will enable far infrared spectroscopy with an unprecedented sensitivity of ∼ 5 × 10 −20 W/m 2 (5σ/1hr) -at least two orders of magnitude improvement over what has been attained to date. With this exceptional leap in performance, new domains in infrared astronomy will become accessible, allowing us, for example, to unravel definitively galaxy evolution and metal production over cosmic time, to study dust formation and evolution from very early epochs onwards, and to trace the formation history of planetary systems.
We demonstrate the phase locking of a 2.7 THz metal-metal waveguide quantum cascade laser (QCL) to an external microwave signal. The reference is the 15th harmonic, generated by a semiconductor superlattice nonlinear device, of a signal at 182 GHz, which itself is generated by a multiplier chain ͑ϫ12͒ from a microwave synthesizer at ϳ15 GHz. Both laser and reference radiations are coupled into a bolometer mixer, resulting in a beat signal, which is fed into a phase-lock loop. The spectral analysis of the beat signal confirms that the QCL is phase locked. This result opens the possibility to extend heterodyne interferometers into the far-infrared range. To proceed with the applications of terahertz QCLs as LOs in a heterodyne spectrometer, the stabilization of the frequency or phase is required to either eliminate the frequency jitter or to reduce the phase noise. For a heterodyne interferometer either on the earth [4] or in space [5], the phase locking of multiple LOs to a common reference at low frequency is essential. Phase locking [6] a laser to a reference means to control the phase of the laser radiation field precisely. This serves not only to stabilize the frequency but also to transfer the line profile of the reference to the laser. In the case of frequency locking, the laser's average frequency is fixed, but its linewidth remains equal to the laser's intrinsic linewidth. Several experiments to stabilize a terahertz QCL have been published. They are the frequency locking of a 3.1 THz QCL to a far-infrared (FIR) gas laser [7], the phase locking of the beat signal of two lateral modes of a terahertz QCL to a microwave reference [8], and the phase locking of a 1.5 THz QCL to a multiplier chain LO source [9]. Since the multiplier chain source has been demonstrated only up to about 2 THz, for a practical solution for the use of a QCL as LO beyond this frequency, the phase needs to be locked to an external reference that can be generated conveniently and should preferably be far below the QCL frequency. Therefore, an important challenge is the demonstration of the phase locking of a singlemode terahertz QCL to a microwave reference signal (MRS), which is the scheme commonly used in existing solid-state LOs. The MRS should be multiplied to a terahertz frequency in the vicinity of the laser frequency to obtain a beat note or an intermediate frequency (IF). As demonstrated in the measurements of FIR gas laser frequency, the harmonics of MRS at 3.8 THz [10] and at 4.3 THz [11] can be generated by Josephson junction harmonic mixers, resulting in a beat between the laser and the upconverted frequencies. Another commonly used harmonic generator is a Schottky diode [12].In this Letter, we report the phase locking of a 2.7 THz QCL to a harmonic generated from an MRS by a semiconductor superlattice (SL) nonlinear device in combination with a multiplier chain. We demonstrate a practical phase-locking scheme that is extendable to a much higher frequency and quantify the phaselocked power.The QCL used is based on the doubl...
Aims. In this paper the calibration and in-orbit performance of the Heterodyne Instrument for the Far-Infrared (HIFI) is described. Methods. The calibration of HIFI is based on a combination of ground and in-flight tests. Dedicated ground tests to determine those instrument parameters that can only be measured accurately using controlled laboratory stimuli were carried out in the instrument level test (ILT) campaign. Special in-flight tests during the commissioning phase (CoP) and performance verification (PV) allowed the determination of the remaining instrument parameters. The various instrument observing modes, as specified in astronomical observation templates (AOTs), were validated in parallel during PV by observing selected celestial sources. Results. The initial calibration and in-orbit performance of HIFI has been established. A first estimate of the calibration budget is given. The overall in-flight instrument performance agrees with the original specification. Issues remain at only a few frequencies.
Context. Hydrides of the most abundant heavier elements are fundamental molecules in cosmic chemistry. Some of them trace gas irradiated by UV or X-rays. Aims. We explore the abundances of major hydrides in W3 IRS5, a prototypical region of high-mass star formation. Methods. W3 IRS5 was observed by HIFI on the Herschel Space Observatory with deep integration ( 2500 s) in 8 spectral regions. Results. The target lines including CH, NH, H 3 O + , and the new molecules SH + , H 2 O + , and OH + are detected. The H 2 O + and OH + J = 1−0 lines are found mostly in absorption, but also appear to exhibit weak emission (P-Cyg-like). Emission requires high density, thus originates most likely near the protostar. This is corroborated by the absence of line shifts relative to the young stellar object (YSO). In addition, H 2 O + and OH + also contain strong absorption components at a velocity shifted relative to W3 IRS5, which are attributed to foreground clouds. Conclusions. The molecular column densities derived from observations correlate well with the predictions of a model that assumes the main emission region is in outflow walls, heated and irradiated by protostellar UV radiation.
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