Recent advances in far-infrared detector technology have led to increases in raw sensitivity of more than an order of magnitude over previous state-of-the-art detectors. With such sensitivity, photon noise becomes the dominant noise component, even when using cryogenically cooled optics, unless a method of restricting the spectral bandpass is employed. The leading instrument concept features reflecting diffraction gratings, which post-disperse the light that has been modulated by a polarizing Fourier transform spectrometer (FTS) onto a detector array, thereby reducing the photon noise on each detector. This paper discusses the development of a cryogenic (4 K) diffraction grating spectrometer that operates over the wavelength range of 285 to 500 μm and was used to post-disperse the output from a room-temperature polarizing FTS. Measurements of the grating spectral response and diffraction efficiency are presented as a function of both wavelength and polarization to characterize the instrumental performance.

A range-resolved laser interferometer is the leading candidate for providing displacement metrology that can meet the stringent precision and low power dissipation requirements of cryogenic space astronomy missions. In prior work, a three-phase homodyne laser interferometer was developed using a simple optical and signal processing scheme that achieved the desired dynamic performance required for state-of-the-art cryogenic far-infrared spectrometers, however, the system required three detectors and amplifiers, and exhibited poor performance at low speeds due to 1/f noise associated with the detector and electronics.(1, 2) A frequency modulation continuous wave (FMCW) heterodyne approach was subsequently adopted to address this shortcoming by shifting the signal of interest well away from the 1/f region.(1, 3) This technique provides the additional advantage of enabling simultaneous multiaxis measurements all using a single laser and a single detector. This paper discusses several applications of the multiaxis FMCW design that as currently implemented can provide simultaneous measurements of up to 8 axes. The need for a precise and intrinsically low power 1-D displacement metrology system to measure the optical path difference of a cryogenic Fourier transform spectrometer was the initial driver for this research; applications in N -dimensions are currently being explored. Several applications of the technique at both high and low speeds are considered, including multiaxis precision linear positioning, the simultaneous measurement of the cryogenic coefficients of thermal expansion for multiple materials, and cryogenic accelerometry. Results from these applications will be presented and used to discuss limitations of the technique.

Optical fibers are commonly used for data transmission and sensing in industrial, geophysical, and aerospace markets, where they may be employed in high vacuum and cryogenic environments. The performance and integrity of optical fibers and their coatings is well understood over temperatures of ≈40 to 300 °C and pressures up to 100 atm, but their characteristics at cryogenic temperatures under high vacuum remain relatively unexplored. This study investigates the optical and mechanical reliability of selected fibers operating at cryogenic temperatures. The fiber samples under investigation were prepared with either an acrylate or polyimide coating. Several properties of the fibers were assessed, including optical loss, mechanical strength, and coating integrity. Optical loss was monitored continuously over a single temperature cycle from 300K to 4K and back. Additional samples were subjected to either one or three temperature cycles and held at 4K for extended periods. Mechanical strength of the thermally cycled fibers was determined via a 2-point bend method, and the coating material was characterized using Fourier transform infrared spectroscopy and thermogravimetric analysis.

The engineering development unit performance of a novel cryogenic scan mechanism suitable for post-dispersed polarizing FTS instruments is characterized at 4 K. Compliance to the stringent SPICA mission requirements is discussed through test results analysis.

The high spectral resolution mode of the SpicA FAR-infrared Instrument (SAFARI) is enabled by inserting a Fourier Transform Spectrometer (FTS), based on a Martin-Puplett interferometer, into the signal path of the instrument. The cryogenic FTS mechanism (FTSM) enables linear scans of two back-to-back rooftop mirrors sharing a common apex. ABB Inc. is under contract with the Canadian Space Agency to develop and test at 4 K an FTSM Engineering Demonstration Unit (EDU) for TRL-5 demonstration. The main SAFARI FTSM performance drivers are the stringent mechatronic demands (position stability of roof-top mirrors in step scan mode <10 nm RMS, velocity jitter in constant velocity mode <1% RMS at 85 µm/s, linear stroke <34 mm, error on position feedback laser metrology <10 nm RMS) and ultra-low photon leakage level from laser metrology (<1 pW), severely constrained by a tight thermal budget (heat dissipation <1.5 mW) under a specific micro-vibration environment (30 µg/√Hz external), all at cryogenic temperatures (4 K). In this paper, we describe the FTSM EDU developed and built at ABB with its laser metrology sensor. The FTSM EDU is based on a novel cryogenic design presented in 2020 using a reactionless and long-stroke flexure-based 4-bar linkage with stiffness compensation and a custom moving magnet actuator. Results from FTSM EDU room-temperature characterization and cryogenic proof-of-concept tests conducted at University of Lethbridge as a key step towards TRL-5 demonstration are discussed to assess the compliance of this novel design to the challenging cryogenic SAFARI FTSM performance requirements.

The continually increasing sensitivity required for the advancement of far-infrared astronomy dictates that the next generation of space-based observatories must employ cryogenically cooled telescopes and instrumentation. Operating cryogenic instrumentation in orbit poses several challenges, including the need for extremely low power dissipation and precise position measurement and control. In prior work, we reported on the development of a homodyne three-phase range-resolved laser interferometer, which demonstrated a displacement measurement uncertainty of 2:3nmrms at <4K.1 Effects of low frequency noise (1=f) in the electrical signals at low velocities were performance limiting due to the interpretation of noise as interference fringes. To avoid 1=f noise, a frequency-modulated continuous-wave (FMCW) heterodyne approach was adopted. An FMCW prototype was developed, and the preliminary results yielded an uncertainty of 29nmrms at <4K.2 In this paper we present the design of an integrated cryogenic FMCW range-resolved laser interferometer which features real time data processing for simultaneous displacement measurements of up to 8 axes. The performance of fibers and their coupling under ultra-high vacuum at cryogenic temperatures is largely unexplored, and we present the cryogenic characterization results of several key variables, including fiber type, termination, and mating, along with alignment effects due to the thermal contraction of fiber components. These results have been incorporated into the design of our cryogenic FMCW interferometer. Applications of cryogenic range resolved interferometry are discussed, with a focus on the integration of the FMCW interferometer with a custom 3-axis cryogenic accelerometer.

Recent advances in far-infrared detector technology have led to increases in raw sensitivity of more than an order of magnitude over previous state-of-the-art detectors. With such sensitivity, photon noise becomes the dominant noise component, even when using cryogenically cooled optics, unless a method of restricting the spectral bandpass is employed. The leading instrument concept features reflecting grating spectrometers to post-disperse the light that has been modulated by a polarizing Fourier transform spectrometer (FTS) onto a detector array, thereby reducing the photon noise on each detector. This paper describes the development of a cryogenic (4 K) reflection grating spectrometer which operates over the wavelength range from 285 µm–500 µm and was used to post-disperse the output from a room-temperature polarizing FTS. Measurements of the grating resolving power and diffraction efficiency are presented as a function of both wavelength and polarization to characterize the instrument performance

This paper describes the development and status of a cryogenic, far-infrared, post- dispersed, polarizing FTS (PDPFTS), a candidate instrument concept for the next generation of far- infrared astronomical space telescopes.

We describe the engineering development unit of a novel cryogenic scan mechanism suitable for post-dispersed polarizing FTS instruments. Compliance to the stringent SPICA mission requirements is demonstrated through analyses and results from proof-of- concept activities.

This paper discusses the development of a cryogenic grating spectrometer that has been used to evaluate the performance of a post-dispersed polarizing FTS over the range of 285-500 µm.

The performance of a prototype far-infrared post-dispersed polarizing FTS has been measured using a source module consisting of unresolved emission and absorption lines and the results are compared with theoretical simulations.

The sensitivity of state-of-the-art superconducting far-infrared detectors used in conjunction with cryogenically cooled space telescopes and instrumentation is such that spectroscopic observations are generally limited by photon noise from the astronomical source or by galactic foreground or zodiacal emission within the field-of-view. Therefore, an instrument design that restricts the spectral bandpass viewed by the detector must be employed. One method of achieving background limited, high resolution spectroscopy is to combine a high resolution component such as a Fabry-Pérot interferometer (FPI) with a lower resolution, post-dispersing system, such as a grating spectrometer, the latter serving to restrict the spectral bandpass. The resonant wavelength of an FPI is most often tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. In this paper, we present a novel design for an FPI in which the wavelength is tuned by scanning the angle of incidence on a high refractive index etalon. This concept simplifies the cryomechanical design, actuation, and metrology. The first results from the realized instrument are presented and compared with theory. The effects on the spectral response as a function of the incident angle have been simulated and shown to agree well with the observation.

The sensitivity of state-of-the-art superconducting far-infrared (FIR) detectors is such that wideband spectroscopic observations, particularly those employing Fourier transform spectrometers (FTS), will require techniques to reduce the spectral bandwidth of a detector to limit the photon noise from an astronomical source. The proposed SPICA SAFARI instrument employs grating spectrometers to post-disperse the light that has been modulated by a polarizing FTS onto a detector array, thereby reducing the photon noise on each detector. While the principles of this method are understood, to date an integrated system has not been realized in the laboratory. We present the development of a FIR post-dispersed polarizing FTS (PDPFTS) consisting of a warm FTS and a 4 K grating spectrometer as a first step to a fully cryogenic PDPFTS demonstrator. Realistic astronomical spectra are generated by combining line emission from a tunable THz photomixer source with continuum emission from a variable blackbody source.

Recent advances in far-infrared detector technology have led to increases in raw sensitivity of more than an order of magnitude over previous state-of-the-art detectors. With such sensitivity, photon noise becomes the dominant noise component, even when using cryogenically cooled optics, unless a method of restricting the spectral bandpass is employed. One method is to use a low-resolution diffraction grating spectrometer to post-disperse the signal from a high-resolution instrument, such as a Fourier transform spectrometer (FTS). This concept has been adopted for the SAFARI instrument on the SPICA mission. This paper discusses the development of a prototype cryogenic grating spectrometer that has been used to evaluate the concept of a post-dispersed polarizing FTS over the range from 285-500 μm.

The high spectral resolution mode of the SpicA FAR-infrared Instrument (SAFARI) is enabled by inserting a Fourier Transform Spectrometer (FTS), based on a Martin-Puplett interferometer, into the signal path of the instrument. The cryogenic mechanism (FTSM) enables linear scans of two back-to-back rooftop mirrors sharing a common apex. ABB Inc. is under contract with the Canadian Space Agency to develop and test at 4 K an FTSM Engineering Demonstration Unit (EDU) for TRL-5 demonstration. The main SAFARI FTSM performance drivers are the stringent mechatronic demands (position stability of roof-top mirrors <10 nm RMS, <34 mm linear stroke), severely constrained by a tight thermal budget (heat dissipation <1.5 mW) under a specific micro-vibrations environment (30 μg/√Hz external), all at cryogenic temperatures (4 K). In this paper, we describe a novel cryogenic FTSM design using a reactionless and longstroke flexure-based 4-bar linkage with stiffness compensation. This 1-DOF mechanism passively controls the guiding of the roof-top mirrors with flex pivots while the axial scanning is actuated and controlled with a custom moving magnet actuator (MMA). Static and dynamic balancing of the FTSM ensures that low vibration levels are transferred to/from the FTSM baseplate, and compensation of the mechanism stiffness reduces the force and drive current required from the MMA by a factor <10. Both features lead to MMA power consumption/dissipation <1.5 mW. Results from an engineering analysis of a dynamic model developed for the FTSM EDU are discussed to assess the compliance of this design to the challenging cryogenic SAFARI FTSM performance requirements.

The continued improvement in the sensitivity of superconducting far-infrared bolometers necessitates improved designs of cryogenically cooled broadband spectrometers in order to fully exploit the potential of such detectors. While Fourier transform spectrometers (FTS) have an illustrious history in astronomical research, the sensitivity of state-of-the-art detectors is such that the multiplex disadvantage of FTS is prohibitive unless the spectral bandpass can be restricted to less than 1%. One method of achieving this goal, and the one that has been adopted for the SPICA SAFARI instrument, is to use a diffraction grating as the post-dispersing component. Unlike a typical FTS, in which a single detector simultaneously measures a broad spectral band, a post-dispersed detection system requires multiple detectors, each with their own unique spectral, spatial, and temporal responses. Moreover, the narrow spectral band viewed by each detector results in an interferogram having a large coherence length; the signal is heavily modulated, yet truncated. While simulations play a useful role in modeling instrumental performance, there is no substitute for data obtained from a real implementation of an instrument concept. In this paper we describe the development and current status of a cryogenic, far-infrared, postdispersed, polarizing FTS (PDPFTS): a demonstrator for the SPICA SAFARI instrument.

The sensitivity of state-of-the-art superconducting far-infrared detectors is such that astronomical observations at these wavelengths are limited by photon noise from the astronomical source unless a method of restricting the spectral bandpass is employed. One such method is to use a high resolution Fabry-Perot interferometer (FPI) in conjunction with a lower resolution, post-dispersing system, such as a grating spectrometer. The resonant wavelength of an FPI is typically tuned by changing the spacing or medium between the parallel reflecting plates of the etalon. We previously reported on a novel design in which the wavelength is tuned by scanning the angle of incidence, which simplifies the cryo-mechanical design, actuation and metrology. Here we present first light results from the realized instrument.

The continually increasing sensitivity required for advancement of far-infrared astronomy dictates that the next generation of space-based observatories must employ cryogenically cooled telescopes and instruments. Cryogenic operation of interferometers such as those proposed for future space missions poses particular challenges, including the need for robust low power dissipation cryogenic position metrology. Instrumentation must be cooled to <4 K to avoid a noise contribution from self-emission and often contain moving components whose position must be measured precisely at cryogenic temperatures. In 2018, we reported on the development of a three-phase fiber-fed laser homodyne interferometer for optical position metrology that achieved a displacement uncertainty of 2.3 nm RMS at 4 K. In that design, one arm of the interferometer had an additional 2 m of optical fiber to carry the probe signal to the 4 K work space. Subsequently, a 2 m, armored, differential fiber pair was developed to balance the lengths of the probe and reference interferometric beams that were subject to thermal gradients. Although this led to an improved dynamic performance in the measurement of an oscillating target, low velocity performance was limited by 1/f noise in the photodetector circuit. Building on that work, we present the design and review the performance of a new frequency-modulated laser interferometer system we have developed that improves upon the three-phase system by eliminating the need for a differential fiber pair in cryogenic applications and achieves 29 nm RMS uncertainty for mechanical displacement velocities from 0 to ~4 mm/s.

Nature is such that observations at far-infrared wavelengths are optimal for exploring both the nearby and distant Universe. The minute amount of energy carried by far-infrared photons, however, requires extremely sensitive instrumentation for their detection. Moreover, the instrumentation itself must be cooled to <4 K to avoid an unwanted photon noise component from self-emission, and often requires precision metrology at these temperatures. A variety of cryogenic metrology techniques have been used successfully on previous space astronomy missions, each having its own limitations. In this paper we present a fiber-based laser metrology system, designed for optical position metrology at cryogenic temperatures.