Heterodyne System Research Articles

DC readout experiment in Enhanced LIGO

The two 4 km long gravitational wave detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector. As part of the upgrade to Enhanced LIGO, these modifications replaced the radio-frequency (RF) heterodyne system used previously. We describe the motivations for and the implementation of DC readout and the output mode cleaner in Enhanced LIGO. We present characterizations of the system, including measurements and models of the couplings of the noises from the laser source to the gravitational wave readout channel. We show that noise couplings using DC readout are improved over those for RF readout, and we find that the achieved shot-noise-limited sensitivity is consistent with modeled results.

Measurements of laser phase fluctuations induced by atmospheric turbulence over 2 kmand 175 km distances

A laser heterodyne system was used to measure the phase fluctuations imposed on a 1.5 μm wavelength laser beam when double-passed over long atmospheric paths. Two distances were used: 2 and 17.5 km. Results are given for intensity scintillation, phase fluctuation time series and spectra, and phase structure function. The results are found to agree well with theory: the spectrum of phase fluctuations follows the 8/3 power law predicted for Kolmogorov turbulence over 3 orders of magnitude in frequency. The methods reported here could be used to investigate large-scale temperature variations in the atmosphere.

Demonstration of a room temperature 2.48–2.75 THz coherent spectroscopy source

We report the first demonstration of a continuous wave coherent source covering 2.48-2.75 THz, with greater than 10% instantaneous tuning bandwidth and having 1-14 μW of output power at room temperature. This source is based on a 91.8-101.8 GHz synthesizer followed by a power amplifier and three cascaded frequency triplers. It demonstrates for the first time that purely electronic solid-state sources can generate a useful amount of power in a region of the electromagnetic spectrum where lasers (solid state or gas) were previously the only available coherent sources. The bandwidth, agility, and operability of this THz source have enabled wideband, high resolution spectroscopic measurements of water, methanol, and carbon monoxide with a resolution and signal-to-noise ratio unmatched by any other existing system, providing new insight in the physics of these molecules. Furthermore, the power and optical beam quality are high enough to observe the Lamb-dip effect in water. The source frequency has an absolute accuracy better than 1 part in 10(12) and the spectrometer achieves sub-Doppler frequency resolution better than 1 part in 10(8). The harmonic purity is better than 25 dB. This source can serve as a coherent signal for absorption spectroscopy, a local oscillator for a variety of heterodyne systems and can be used as a method for precision control of more powerful but much less frequency agile quantum mechanical terahertz sources.

Study on Heterodyne Stochastic Resonance System for Weak Signal Detection

The method based on stochastic resonance is a newly developed signal processing technology. Different from other methods by restraining the noise, it takes full advantage of the noises to strengthen the weak signal to improve SNR of the system. Based on stochastic resonance theory, a heterodyne system is presented in the paper to detect the frequency of the weak periodic signal. The simulation results manifest that the method is effective and the system parameters have extensive influence for frequency measurement. The parameters of the system are investigated for optimizing system design and weak signal detection.

Monolithically Integrated Photonic Heterodyne System

This paper presents the results from the first monolithically integrated photonic heterodyne system that allows the two optical sources to be mutually phase locked by locking to an external optical reference. High-spectral-purity signals of up to 50 GHz have been demonstrated from this first fabricated device, where the tuning range was limited by losses in the input waveguide. Successful phase locking was accomplished through short signal propagation delay of less than 2 ns achieved by monolithic integration and custom-made fast loop electronics. The approach can be extended to generate signals at >; 1 THz.

ALMA Band 10 tertiary optics

The ALMA Band 10 (787–950 GHz) receiver is a dual-polarization heterodyne system based on NbTiN superconducting technology. The coupling of energy from the secondary mirror of the ALMA Cassegrain antenna to the Superconductor–Insulator–Superconductor (SIS) mixers used for down-conversion is achieved by a frequency-independent optical system composed of two elliptical mirrors to focus and redirect the incoming radiation, a wire-grid to separate orthogonal linear polarizations and two corrugated horns, one for each polarization and SIS mixer. In this paper, we present the ALMA Band 10 tertiary optics design and evaluate its performance by quasi-optical techniques, Physical Optics simulations and measurements. Detailed results of secondary aperture efficiency and beam-squint are provided. The characterization procedure described in this paper can be used for any optical system at around 1 THz.

Coherent optical binary polarisation shift keying heterodyne system in the free-space optical turbulence channel

In this study, analytical and simulation results for the bit error rate (BER) performance and fading penalty of a coherent optical binary polarisation shift keying (2PolSK) heterodyne system adopted for a free-space optical communication link modelled as the log-normal and the negative exponential atmospheric turbulence channels are presented. The conditional and unconditional BER expressions are derived, demonstrating the comprehensive similarity between the 2PolSK and the binary frequency shift keying schemes with regard to the system sensitivity. The power penalty owing to the non-ideal polarisation beam splitter is also analysed. The receiver sensitivity employing 2PolSK is compared with other modulation schemes in the presence of turbulence and the phase noise. The results show that 2PolSK offers improved signal-to-noise ratio performance compared to the binary amplitude shift keying.

Atmospheric observations of multiple molecular species using ultra-high-resolution external cavity quantum cascade laser heterodyne radiometry

We demonstrate a widely tunable laser heterodyne radiometer operating in the thermal IR during an atmospheric observation campaign in the solar occultation viewing mode. An external cavity quantum cascade laser tunable within a range of 1120 to 1238 cm(-1) is used as the local oscillator (LO) of the instrument. Ultra-high-resolution (60 MHz or 0.002 cm(-1) transmission spectroscopy of several atmospheric species (water vapor, ozone, nitrous oxide, methane, and dichlorodifluoromethane) has been demonstrated within four precisely selected molecule-specific narrow spectral windows (∼1 cm(-1). Atmospheric transmission lines within each selected window were fully resolved through mode-hop-free continuous tuning of the LO frequency. Comparison measurements were made simultaneously with a high-resolution Fourier transform spectrometer to demonstrate the advantages of the laser heterodyne system for atmospheric sounding at high spectral and spatial resolutions.

Measuring the photodetector frequency response for ultrasonic applications by a heterodyne system with difference- frequency servo control

A technique for the calibration of photodiodes in ultrasonic measurement systems using standard and cost-effective optical and electronic components is presented. A heterodyne system was realized using two commercially available distributed feedback lasers, and the required frequency stability and resolution were ensured by a difference-frequency servo control scheme. The frequency-sensitive element generating the error signal for the servo loop comprised a delay-line discriminator constructed from electronic elements. Measurements were carried out at up to 450 MHz, and the uncertainties of about 5% (k = 2) can be further reduced by improved radio frequency power measurement without losing the feature of using only simple elements. The technique initially dedicated to the determination of the frequency response of photodetectors applied in ultrasonic applications can be transferred to other application fields of optical measurements.

High-Resolution Frequency Measurements of Far-Infrared Laser Lines

The frequency of four previously reported far-infrared laser lines have been measured to an accuracy of 100 kHz. These laser lines were measured using a heterodyne system which allowed for more accurate measurement. The four far-infrared laser lines which originated from Formic Acid (HCOOH), O-Deutero-Formic Acid (HCOOD), C-Deutero-Formic Acid (DCOOH), and Methyl Chloride (CH3Cl) were optically pumped by a highly stable, grating-tunable, CO2 laser. Three of the far-infrared laser lines have been measured previously using Fabry-Perot cavities not typicaly known for their high accuracy. The fourth far-infrared laser line had been measured previously by heterodyne methods but under nonoptimal conditions. The difference between the frequencies measured here and the listed frequencies for these laser lines ranged from 0.9 MHz to 3.9 GHz.

Density Regimes of Low-Aspect-Ratio RFP Plasmas in RELAX

The line-averaged electron density has been measured over a wide range of discharge conditions in a low-aspect-ratio reversed field pinch (RFP) machine RELAX. A 104 GHz heterodyne interferometer was mainly used throughout the experiments. The measured electron density ne varied from ∼0.3 × 1019 m−3 to ∼3 × 1019 m−3, depending upon the discharge conditions. A new 60 GHz homodyne interferometer, characterized by simplicity and low cost, has been developed for the density measurement particularly in low density regime. The results from the new homodyne interferometer have shown good agreement with those from the heterodyne system in low-density regime of ne up to ∼1019 m−3.

Calibration of a Michelson-type laser wavemeter and evaluation of its accuracy

A calibration procedure for a scanning Michelson-type wavemeter is presented. The investigation includes calibration of the internal laser, which acts as a reference for the instrument using the optical heterodyne system in order to determine its absolute frequency and its frequency stability, which enables us to improve the measurement accuracy of the wavemeter. The resolving power of the instrument was also evaluated exploiting the strongest ten hyperfine components of the R(127) 11–5 transition of 127I2 emitted from an iodine-stabilised He–Ne laser. In our way to perform the previous tasks, we found the necessity to test the sensitivity and frequency uncertainty of the optical heterodyne system due to unequal powers of the mixed lasers. The obtained test results revealed insignificant consequences on the combined uncertainty for frequency measurements and verified that the wavemeter could be used for accurate wavelength measurements for the laser sources in the wavelength range 600–1800 nm with an accuracy of ±0.2 pm.

Quasi-optical assessment of the ALMA band 9 front-end

The ALMA band 9 (600–720 GHz) receiver is a dual channel heterodyne system which is capable of detecting orthogonally polarised signals utilising a wire grid beam splitter. Two Superconductor–Insulator–Superconductor (SIS) mixers mounted behind hybrid mode corrugated horns are coupled to the 12 m Cassegrain antenna via a wavelength independent configuration of two off-axis elliptical mirrors. We outline an approach involving accurate physical optics simulations in conjunction with precise experimental measurements of the complete optical front-end which guarantees the highest performances. This practical verification approach can be generalised to all quasi-optical receivers to validate system performance. In this paper, we verify the optical design and estimate antenna system efficiency. Comparison between measurement and simulation indicates precise information is achievable in estimating system performance allowing potential improvements in ALMA instrument calibration accuracy.

Frequencies and Wavelengths From a New Far-Infrared Lasing Gas: $^{13}$CHD$_{2}$OH

The first laser action produced by the partially deuterated isotopic form of methanol, 13CHD2OH, has been observed. With this laser medium, eight far-infrared laser emissions were discovered having wavelengths ranging from 33.8 to 80.9 mum. A three-laser heterodyne system was used to measure the frequencies for these newly discovered laser lines and are reported with fractional uncertainties of plusmn3 times 10-7. The offset frequency of the CO2 pump laser was measured with respect to its center frequency for each 13CHD2OH laser emission.

Multichannel microwave interferometer for the levitated dipole experiment

A four-channel microwave interferometer (center frequency: 60 GHz) has been constructed to measure plasma density profiles in the levitated dipole experiment (LDX). The LDX interferometer has a unique design owing to the unique geometry of LDX. The main design features of the interferometer are: (1) the transmitted beam traverses the plasma entirely in O-mode; (2) the interferometer is a heterodyne system employing two free-running oscillators; (3) four signals of data are received from just on transmitted beam; (4) phase shifts are detected in quadrature. Calibration tests demonstrate that the interferometer measures phase shifts with an uncertainty of approximately 5 degrees . Plasma densities in LDX corresponding to phase shifts of up to 5pi are routinely and successfully measured.

Nanoscale defect detection by heterodyne interferometry

We construct an instrument that facilitates the measurement of nanoscale defects. It is based on heterodyne interferometry with phase measurement that utilizes a polarizing beam splitter to form a measuring signal and an oscillating cantilever tip that acts as a scanning probe to get the measurement values of sample topography. The dependence of the tip displacement on the variation of tip-sample distance and the comb scanning of the sample topography are investigated by experiments. The results prove that the tip displacement increases and is enough to be discriminated in various positions where the sample is approached. The system has been successfully utilized to measure the defect characterization by measuring the pitch of the standard sample. The results also show that the heterodyne system has good repeatability, a large measurement range, and high accuracy, with a measurement stability of 0.5 nm.

Measuring optical characteristics of the atmosphere with a laser heterodyne system

A laser heterodyne system for measuring optical characteristics of a turbulent atmosphere is described. Continuous readout data on the fluctuations in the phase of the radiation propagating in a channel in the atmosphere are presented. These data can be used to determine the major parameters of the turbulent state.

Real-time digital heterodyne interferometer for high resolution plasma density measurements at ISTTOK

With the implementation of alternating discharges (ac) at the ISTTOK tokamak, the typical duration of the discharges increased from 35 to 250 ms. This time increase created the need for a real-time electron density measurement in order to control the plasma fueling. The diagnostic chosen for the real-time calculation was the microwave interferometer. The ISTTOK microwave interferometer is a heterodyne system with quadrature detection and a probing frequency of 100 GHz (lambda(0)=3 mm). In this paper, a low-cost approach for real-time diagnostic using a digital signal programmable intelligent computer embedded system is presented, which allows the measurement of the phase with a 1% fringe accuracy in less than 6 micros. The system increases its accuracy by digitally correcting the offsets of the input signals and making use of a judicious lookup table optimized to improve the nonlinear behavior of the transfer curve. The electron density is determined at a rate of 82 kHz (limited by the analog to digital converter), and the data are transmitted for each millisecond although this last parameter could be much lower (around 12 micros--each value calculated is transmitted). In the future, this same system is expected to control plasma actuators, such as the piezoelectric valve of the hydrogen injection system responsible for the plasma fueling.

FIRI—A far-infrared interferometer

Half of the energy ever emitted by stars and accreting objects comes to us in the far-infrared (FIR) waveband and has yet to be properly explored. We propose a powerful Far-InfraRed Interferometer mission, FIRI, to carry out high-resolution imaging spectroscopy in the FIR. This key observational capability is essential to reveal how gas and dust evolve into stars and planets, how the first luminous objects in the Universe ignited, how galaxies formed, and when super-massive black holes grew. FIRI will disentangle the cosmic histories of star formation and accretion onto black holes and will trace the assembly and evolution of quiescent galaxies like our Milky Way. Perhaps most importantly, FIRI will observe all stages of planetary system formation and recognise the birth of planets via its ability to image the dust structures in planetary systems. FIRI is an observatory-class mission concept: three cold, 3.5-m apertures, orbiting a beam-combining module, with separations of up to 1 km, free-flying or tethered, operating between 25 and 385 μm, using the interferometric direct-detection technique to ensure μJy sensitivity and 0.02” resolution at 100 μm, across an arcmin2 instantaneous field of view, with a spectral resolution, R ~ 5,000 and a heterodyne system with R ~ 1 million. Although FIRI is an ambitious mission, we note that FIR interferometry is appreciably less demanding than at shorter wavelengths.

Performance of heterodyne wireless optical communication systems over gamma-gamma atmospheric turbulence channels

Closed-form expressions for the outage probability and the average bit error rate of wireless optical heterodyne communication systems over strong atmospheric turbulence are derived. The turbulence-induced fading is modelled as a multiplicative random process following the gamma-gamma distribution.