Martin-Puplett Interferometer Research Articles

Martin-Puplett interferometer for studying THz and subTHz radiation

Abstract The design of a portable terahertz and subterahertz interferometer, manufactured according to the Martin-Puplett scheme, is presented. The possibility of using film polypropylene polarizers in interferometers of this type has been demonstrated. The provided possibility of using various types of radiation detectors makes it possible to perform measurements in high-performance or high-sensitivity modes. The presented interferometer is proposed to be used to measure the spectra of terahertz and subterahertz radiation produced during the interaction of bunched and continuous beams of charged particles with solid targets.

Design of a Fourier-transform spectrometer for detector characterization on CMB polarization telescope

We present the design of a Fourier-transform spectrometer (FTS) tailored for operation within the frequency range of 75 GHz to 300 GHz, offering a spectral resolution surpassing 2 GHz. This FTS can be used for characterizing detectors on a Cosmic Microwave Background (CMB) Polarization Telescope for the observation of CMB B-mode polarization. The FTS instrument is divided into the primary part, housing a Martin-Puplett interferometer (MPI) module, and the coupled part, which incorporates a beam expanding module and a beam steering module. An in-depth performance analysis focusing on the MPI module, involving considerations such as the orientation of the beam splitter, dihedral angle of the roof-top mirrors, and positioning uncertainty of the scanning mirror, has been conducted by using a simulation tool. Results indicate that the spectral characteristic remains unaffected by variations in the Reflection/Transmission ratio of the beam splitter, with any deviation from the 1:1 ratio uniformly decreasing intensity. The impact of dihedral angle errors of roof-top mirrors (below 0.02 degrees) and positioning uncertainty in the scanning mirror (lower than 3 × 10-3 mm) on the system performance is acceptable for the design of the FTS. The beam expanding module is capable of enlarging the output beam of the MPI module to roughly a 150 mm-radius. The beam steering module allows for directional adjustments within the range of +17 degrees to -17 degrees relative to the normal of the target plane. Preliminary testing on the MPI prototype reveals a spectral resolution of 1.5 GHz for the measured frequency.

Probing the peripheral self-generated magnetic field distribution in laser-plasma magnetic reconnection with Martin–Puplett interferometer polarimeter

Magnetic reconnection of the self-generated magnetic fields in laser-plasma interaction is an important laboratory method for modeling high-energy density astronomical and astrophysical phenomena. We use the Martin–Puplett interferometer (MPI) polarimeter to probe the peripheral magnetic fields generated in the common magnetic reconnection configuration, two separated coplanar plane targets, in laser-target interaction. We introduce a new method that can obtain polarization information from the interference pattern instead of the sinusoidal function fitting of the intensity. A bidirectional magnetic field is observed from the side view, which is consistent with the magneto-hydro-dynamical (MHD) simulation results of self-generated magnetic field reconnection. We find that the cancellation of reverse magnetic fields after averaging and integration along the observing direction could reduce the magnetic field strength by one to two orders of magnitude. It indicates that imaging resolution can significantly affect the accuracy of measured magnetic field strength.

Real-time control applications of JET heterodyne radiometer electron temperature profiles using ICRH

The Real Time ECE data processing has been recently implemented at JET, by using a cross calibration between the Martin-Puplett interferometer (also known as Michelson) measurements and the Heterodyne faster data. The applicability of RT control with ICRH has been discussed in recent works. In particular, in case of a hollow electron temperature profile, central ICRF heating could be applied to restore temperature peaking. In this work a study about the possibility of controlling the electron temperature profile by using ICRHas an actuator, with inputs from ECE measurements, is described. The ICRH is here used for the optimization of the plasma ramp-down, in order to correct the end of the discharge and avoid the plasma disruption, though a similar approach could also be proposed to improve the plasma performances during the flat top. Preliminary results have been obtained on the physical and logical feasibility of using ICRH in a real-time control loop. A database of good pulses in identifiable conditions has been compiled and validated by covering the main experiments both in the baseline and hybrid scenarios. Such scenarios have been simulated, for a selected numbers of discharges, using the full-wave propagation solver TORIC-SSQLFP, in order to establish the ICRH power deposition profiles to be used to locally restore a peaked electron temperature profile.

A Martin-Puplett interferometer (MPI) optical polarimeter: Design and laboratory tests.

Spontaneous and external magnetic fields interacting with plasmas are essential in high-energy-density and magnetic confinement fusion physics. Measuring these magnetic fields, especially their topologies, is crucial. This paper develops a new type of optical polarimeter based on the Martin-Puplett interferometer (MPI), which can probe magnetic fields with the Faraday rotation method. We introduce the design and working principle of an MPI polarimeter. With the laboratory tests, we demonstrate the measurement process and compare the results with the measurement result of a Gauss meter. These very close results verify the polarization detection capability of the MPI polarimeter and show the potential for its application in magnetic field measurement.

Control and Manipulation of Microwave Polarization and Power of a Frequency-Agile 198 GHz Gyrotron for Magnetic Resonance

The measurement and manipulation of the microwave polarization emitted from a frequency-agile 198 GHz gyrotron for dynamic nuclear polarization (DNP) are demonstrated. In general, gyrotrons emit linearly polarized radiation, yet in this case elliptical polarization is observed from the 198 GHz gyrotron window. Indeed, half of the microwave power is circularly polarized while the other half is linearly polarized with a polarization of 60∘ with respect to the horizontal plane. For optimal use of microwave power for DNP experiments, the elliptical polarization from the gyrotron is converted into circular polarization with a Martin-Puplett interferometer (MPI). The dependence of the DNP enhancement on the microwave polarization was investigated by modifying the microwave polarization with the MPI. In addition, the MPI can generate a linearly polarized beam, which holds promise for future development of induction-mode electron spin detected experiments.

A New Readout Electronic for Kinetic Inductance Detectors

With this contribution we show the readout electronics for kinetic inductance detectors (KIDs) that we are developing based on commercial IQ transceivers from National Instruments and using a Virtex 5 class FPGA. It will be the readout electronics of the COSmic Monopole Observer (COSMO) experiment, a ground based cryogenic Martin–Puplett Interferometer searching for the cosmic microwave background spectral distortions. The readout electronics require a sampling rate in the range of tens of kHz, which is both due to a fast rotating mirror modulating the signal and the time constant of the COSMO KIDs. In this contribution we show the capabilities of our readout electronics using Niobium KIDs developed by Paris Observatory for our 5 K cryogenic system. In particular, we demonstrate the capability to detect 23 resonators from frequency sweeps and to readout the state of each resonator with a sampling rate of about 8 kHz. The readout is based on a finite-state machine where the first two states look for the resonances and generate the comb of tones, while the third one performs the acquisition of phase and amplitude of each detector in free running. Our electronics are based on commercial modules, which brings two key advantages: they can be acquired easily and it is relative simple to write and modify the firmware within the LabView environment in order to meet the needs of the experiment.

Accurate sky signal reconstruction for ground-based spectroscopy with kinetic inductance detectors

Context.Wide-field spectrometers are needed to deal with current astrophysical challenges that require multiband observations at millimeter wavelengths. An example of these is the KIDs Interferometer Spectrum Survey (KISS), which uses two arrays of kinetic inductance detectors (KIDs) coupled to a Martin-Puplett interferometer (MPI). KISS has a wide instantaneous field of view (1 deg in diameter) and a spectral resolution of up to 1.45 GHz in the 120–180 GHz electromagnetic band. The instrument is installed on the 2.25 m Q-U-I JOint TEnerife telescope at the Teide Observatory (Tenerife, Canary Islands), at an altitude of 2395 m above sea level.Aims.This work presents an original readout modulation method developed to improve the sky signal reconstruction accuracy for types of instruments for which a fast sampling frequency is required, both to remove atmospheric fluctuations and to perform full spectroscopic measurements on each sampled sky position.Methods.We first demonstrate the feasibility of this technique using simulations. We then apply such a scheme to on-sky calibration.Results.We show that the sky signal can be reconstructed to better than 0.5% for astrophysical sources, and to better than 2% for large background variations such as in “skydip”, in an ideal noiseless scenario. The readout modulation method is validated by observations on-sky during the KISS commissioning campaign.Conclusions.We conclude that accurate photometry can be obtained for future KID-based interferometry using the MPI.

Polarizing interferometer for the unambiguous determination of the ellipsometric parameters.

We report on a polarizing interferometer-ellipsometer arrangement that overcomes the need for additional measurements with a retarder for the unambiguous determination of the ellipsometric parameters in the far infrared spectral range. It consists of a Martin-Puplett interferometer and a wire-grid polarizer as an analyzer. The application of such interferometer-ellipsometer is experimentally demonstrated on a polyethylene sample deriving the refractive index and the thickness in the spectral range between 15 and 35cm-1. Based on these results, a similar solution without a retarder for the mid-infrared spectral region is additionally proposed.

Bayesian inference using JET’s microwave diagnostic system

At the JET tokamak, three electron cyclotron emission (ECE) diagnostics (two Martin-Puplett interferometers and a heterodyne radiometer) and a reflectometer form the basic microwave diagnostic system. The standard analysis approaches deduce electron density and temperature profiles independently of each diagnostic measurement. Via the Bayesian framework Minerva, the microwave diagnostic system is modelled, and electron temperature and density profiles are inferred jointly for an Ohmic JET plasma. Furthermore, profile length-scales for different plasma domains, a magnetic field correction, distinct reflection properties of the high-field side and low-field side walls, and radiometer sensitivities are estimated together. This inference scheme can use one of two models to predict broadband ECE spectra; one is less accurate but fast for a single prediction, and a more accurate model, relying on the ray-tracer SPECE parallelised via web services. The faster model allows the investigation of correlations between parameters and the execution of a numerical marginalisation, i.e. an uncertainty propagation.

Demonstration of variable polarized coherent terahertz source

We are developing an accelerator-based terahertz source that can produce arbitrary polarization states from a linearly polarized coherent undulator radiation (CUR). The polarization manipulation of the CUR can be realized using the Martin–Puplett interferometer employed as an optical phase shifter. This study also demonstrates a variable polarization manipulator by using the terahertz CUR (THz-CUR) source driven by a compact photocathode RF-gun at the Institute of Advanced Energy, Kyoto University. The vertically polarized CUR with a frequency of 0.16 THz was manipulated into a horizontal linear polarization or circular polarization, and Stokes parameters were measured to derive the degree of polarization.

The POLARBEAR Fourier transform spectrometer calibrator and spectroscopic characterization of the POLARBEAR instrument.

We describe the Fourier Transform Spectrometer (FTS) used for in-field testing of the POLARBEAR receiver, an experiment located in the Atacama Desert of Chile which measures the cosmic microwave background (CMB) polarization. The POLARBEAR-FTS (PB-FTS) is a Martin-Puplett interferometer designed to couple to the Huan Tran Telescope (HTT) on which the POLARBEAR receiver is installed. The PB-FTS measured the spectral response of the POLARBEAR receiver with signal-to-noise ratio >20 for ∼69% of the focal plane detectors due to three features: a high throughput of 15.1 sr cm2, optimized optical coupling to the POLARBEAR optics using a custom designed output parabolic mirror, and a continuously modulated output polarizer. The PB-FTS parabolic mirror is designed to mimic the shape of the 2.5 m-diameter HTT primary reflector, which allows for optimum optical coupling to the POLARBEAR receiver, reducing aberrations and systematics. One polarizing grid is placed at the output of the PB-FTS and modulated via continuous rotation. This modulation allows for decomposition of the signal into different harmonics that can be used to probe potentially pernicious sources of systematic error in a polarization-sensitive instrument. The high throughput and continuous output polarizer modulation features are unique compared to other FTS calibrators used in the CMB field. In-field characterization of the POLARBEAR receiver was accomplished using the PB-FTS in April 2014. We discuss the design, construction, and operation of the PB-FTS and present the spectral characterization of the POLARBEAR receiver. We introduce future applications for the PB-FTS in the next-generation CMB experiment, the Simons Array.

Terahertz Spectrum Analyzer Based on Fourier Transform Interferometry

A Fourier transform terahertz (THz) spectrum analyzer is demonstrated based on a Martin–Puplett interferometer in the frequency range of 0.1–1.5 THz. High dynamic ranges are achieved at room temperature using a Fermi level-managed barrier diode as the detector. Correction for the frequency dependence of power measurements is achieved using a high-pressure mercury lamp by assuming blackbody radiation. The absolute reference level is determined to be 1 mW by comparing with a calibrated power meter at 0.11 THz using a Gunn oscillator as a reference source. Moreover, the power linearities are evaluated by measuring the reference attenuator at different power levels. Consequently, dynamic ranges are quantitatively verified up to 45 dB within the linearity of 0.2–4.9 dB at frequencies of 0.11, 0.22, and 0.33 THz. Harmonics measurements of the Gunn oscillator are also demonstrated.

Compact millimeter-wavelength Fourier-transform spectrometer.

We have constructed a Fourier-transform spectrometer (FTS) operating between 50 GHz and 330 GHz with minimum volume (355×260×64 mm) and weight (5.9 kg) while maximizing optical throughput (100 mm2 sr) and optimizing the spectral resolution (4 GHz). This FTS is designed as a polarizing Martin-Puplett interferometer with unobstructed input and output in which both input polarizations undergo interference. The instrument construction is simple with mirrors milled on the box walls and one motorized stage as the single moving element. We characterize the performance of the FTS, compare the measurements to an optical simulation, and discuss features that relate to details of the FTS design. The simulation is also used to determine the tolerance of optical alignments for the required specifications. We detail the FTS mechanical design and provide the control software as well as the analysis code online.

Monochromatic coherent transition and diffraction radiation from a relativistic electron bunch train

Electron beams of most accelerators have a bunched structure and are synchronized with the accelerating RF field. Due to modulation of the electron beam with frequency ν RF one can expect to observe resonances with frequencies ν k=k⋅ ν RF in radiation spectrum generated via any spontaneous emission mechanism (k is an integer and the resonance order). In this paper we present the results of spectral measurements of coherent transition radiation (CTR) generated by an electron bunch train from the Tomsk microtron with ν RF=2.63GHz in the spectral frequency range from 8 to 35 GHz. We also measured the spectrum of coherent diffraction radiation and demonstrated that the observed spectra in both cases consist of monochromatic lines. For spectral measurements the Martin-Puplett interferometer with spectral resolution of 800 MHz (FWMH) was employed. Using a waveguide frequency cut-off we were able to exclude several spectral lines to observe higher resonance orders of up to k =7.

Spectrum of coherent transition radiation generated by a modulated electron beam

The spectrum of coherent transition radiation has been recorded with the use of a Martin–Puplett interferometer. It has been shown that the spectrum includes monochromatic lines that are caused by the modulation of an electron beam with the frequency of an accelerating radio-frequency field νRF and correspond to resonances at ν k = kνRF k ≤ 10. To determine the length of an electron bunch from the measurement of the spectrum from a single bunch, it is necessary to use a spectrometer with the resolution Δνsp > νRF.

Polarizing beam-splitter rotation in Martin-Puplett interferometers for spectroscopic measurements at millimeter wavelengths

The spectroscopic measurement of the Cosmic Microwave Background at mm and sub-mm wavelengths received significant attention recently, aimed at measuring tiny spectral distortions of the Cosmic Microwave Background (CMB) relevant for cosmology. Several experiments, including OLIMPO (Masi et al. 2003), PRISM (André et al., 2014), MILLIMETRON (Smirnov and Baryshev, 2012), PIXIE (Kogut and Fixsen, 2011) are based on a Martin-Puplett Fourier-transform spectrometer. Its differential capabilities are the key to success in these difficult measurements. The polarizing beam splitter is the optical core of a Martin-Puplett interferometer. In this paper we analyze, analytically and experimentally, the systematic effects induced by a beam splitter orientation different from the canonical 45°. These effects are potenitally important for the delicate measurements of CMB spectral distortions. We find an analytical formula describing the effect, and verify experimentally, in the range 150–600GHz, that our formula correctly describes the results (with a C.L. of 88%). We also demonstrate that the rotation of the beam splitter does not induce distortions in the measured spectra.

Electron cyclotron emission spectra in X- and O-mode polarisation at JET: Martin-Puplett interferometer, absolute calibration, revised uncertainties, inboard/outboard temperature profile, and wall properties.

At the tokamak Joint European Torus (JET), the electron cyclotron emission spectra in O-mode and X-mode polarisations are diagnosed simultaneous in absolute terms for several harmonics with two Martin-Puplett interferometers. From the second harmonic range in X-mode polarisation, the electron temperature profile can be deduced for the outboard side (low magnetic field strength) of JET but only for some parts of the inboard side (high magnetic field strength). This spatial restriction can be bypassed, if a cutoff is not present inside the plasma for O-mode waves in the first harmonic range. Then, from this spectral domain, the profile on the entire inboard side is accessible. The profile determination relies on the new absolute and independent calibration for both interferometers. During the calibration procedure, the antenna pattern was investigated as well, and, potentially, an increase in the diagnostic responsivity of about 5% was found for the domain 100-300 GHz. This increase and other uncertainty sources are taken into account in the thorough revision of the uncertainty for the diagnostic absolute calibration. The uncertainty deduced and the convolution inherent for Fourier spectroscopy diagnostics have implications for the temperature profile inferred. Having probed the electron cyclotron emission spectra in orthogonal polarisation directions for the first harmonic range, a condition is derived for the reflection and polarisation-scrambling coefficients of the first wall on the outboard side of JET.

Field transients of coherent terahertz synchrotron radiation accessed via time-resolving and correlation techniques

Decaying oscillations of the electric field in repetitive pulses of coherent synchrotron radiation in the terahertz frequency range was evaluated by means of time-resolving and correlation techniques. Comparative analysis of real-time voltage transients of the electrical response and interferograms, which were obtained with an ultrafast zero-bias Schottky diode detector and a Martin-Puplett interferometer, delivers close values of the pulse duration. Consistent results were obtained via the correlation technique with a pair of Golay Cell detectors and a pair of resonant polarisation-sensitive superconducting detectors integrated on one chip. The duration of terahertz synchrotron pulses does not closely correlate with the duration of single-cycle electric field expected for the varying size of electron bunches. We largely attribute the difference to the charge density oscillations in electron bunches and to the low-frequency spectral cut-off imposed by both the synchrotron beamline and the coupling optics of our detectors.

Common-mode rejection in Martin-Puplett spectrometers for astronomical observations at millimeter wavelengths.

The Martin-Puplett interferometer (MPI) is a differential Fourier transform spectrometer that measures the difference between spectral brightness at two input ports. This unique feature makes the MPI an optimal zero instrument, able to detect small brightness gradients embedded in a large common background. In this paper, we experimentally investigate the common-mode rejection achievable in the MPI at millimeter wavelengths, and discuss the use of the instrument to measure the spectrum of cosmic microwave background anisotropy.