Heterodyne Experiments Research Articles

Multiplicative suppression of decoherence.

Combining different protocols extends the life of a quantum system

Application of multiple scattering theory to Doppler velocimetry of ejecta from shock-loaded samples

We address the actual problem of retrieving the physical parameters of ejecta from data of heterodyne Doppler velocimetry. Under the assumption that ejected particles are randomly spaced and their number within the probed volume is great, the noise-free component of the power spectrum of heterodyne beats is shown to be expressed in terms of a solution to the transport equation for the field correlation function which accounts for multiple scattering and absorption of the probing beam in the ejecta cloud. This provides a means for theoretical modeling of experimental Doppler data. The ejecta cloud is considered as a plane layer of particles moving in the air away from the free-surface of the shock-loaded sample. The spatial profile of the scattering coefficient and the cloud thickness are related directly to the distribution of ejected particles over velocities and coordinates. The slowing-down of ejected particles in the air leads to an ambiguous relation between the velocity of particles and their position, resulting in essential complication of the particle distribution. Using a multi-group description of the ejecta cloud, we reduce the transport equation to a system of linked Milne-like equations. The system is solved numerically with the discrete ordinate code. Varying the values of the cloud optical thickness and the parameters of the particle distribution over sizes and initial velocities, we fit the calculated spectrum to the time-resolved data of heterodyne Doppler measurements. Such an approach enables retrieving the primary ejecta characteristics (the total ejected mass, the density-velocity profile and the size distribution of ejected particles) directly from heterodyne experiments. Application of the proposed method to the velocimetry data on ejecta is presented.

A 490 GHz planar circuit balanced Nb-Al2O3-Nb quasiparticle mixer for radio astronomy: Application to quantitative local oscillator noise determination

This article presents a heterodyne experiment which uses a 380–520 GHz planar circuit balanced Nb-Al2O3-Nb superconductor-insulator-superconductor (SIS) quasiparticle mixer with 4–8 GHz instantaneous intermediate frequency (IF) bandwidth to quantitatively determine local oscillator (LO) noise. A balanced mixer is a unique tool to separate noise at the mixer's LO port from other noise sources. This is not possible in single-ended mixers. The antisymmetric IV characteristic of a SIS mixer further helps to simplify the measurements. The double-sideband receiver sensitivity of the balanced mixer is 2–4 times the quantum noise limit hν/kB over the measured frequencies with a maximum LO noise rejection of 15 dB. This work presents independent measurements with three different LO sources that produce the reference frequency but also an amount of near-carrier noise power which is quantified in the experiment as a function of the LO and IF frequency in terms of an equivalent noise temperature TLO. Two types of LO sources are used: a synthesizer driven amplifier/multiplier chain and a Gunn oscillator driven multiplier chain. With the first type of LO we find different near-carrier noise contributions using two different power pre-amplifiers of the LO system. For one of the two power pre-amplifiers we measure TLO=30±4 K at the LO frequency 380 GHz and TLO=38±10 K at 420 GHz. At the frequency band center 465 GHz of the Gunn driven LO we measure a comparable value of TLO=32±6 K. For the second power pre-amplifier, a significant higher TLO value of the synthesizer driven LO is found which is up to six times larger compared with the best values found for the Gunn driven LO. In a second experiment, we use only one of two SIS mixers of the balanced mixer chip in order to verify the influence of near-carrier LO noise power on a single-ended heterodyne mixer measurement. We find an IF frequency dependence of near-carrier LO noise power. The frequency-resolved IF noise temperature slope is flat or slightly negative for the single-ended mixer. This is in contrast to the IF slope of the balanced mixer itself which is positive due to the expected IF roll-off of the mixer. This indicates a higher noise level closer to the LO's carrier frequency. Our findings imply that near-carrier LO noise has the largest impact on the sensitivity of a receiver system which uses mixers with a low IF band, for example, superconducting hot-electron bolometer HEB mixers.

Heterodyne research on times delay of laser pulses based on active Fabry-Perot cavity

The delay theoretical analysis result of single-longitudinal-mode pulse injected in active Fabry-Perot cavity has been obtained. The gain in Fabry-Perot cavity has been calculated and the numerical simulation result of output pulse has been obtained. The results show that when the energy of the pulse decreases at a certain value, the gain produced by increased pump particle population can compensate the loss in the cavity and the delay pulse can achieve steady state. In experiment, the active Fabry-Perot cavity is a Nd: YAG laser sided pumped by laser diode. For the pulse inputed externally, we achieve 140 pulses within 2 ups. Finally, the reference light is delayed using the active Fabry-Perot cavity. And we show the heterodyne experiment of signal light whose frequency shift generated by an acousto-optic modulation (AOM), and the experiment results accord with the frequency shift of the AOM and the error is less than 4%.

Phase locking of a 15 Terahertz quantum cascade laser and use as a local oscillator in a heterodyne HEB receiver

We demonstrate for the first time the closure of an electronic phase lock loop for a continuous-wave quantum cascade laser (QCL) at 1.5 THz. The QCL is operated in a closed cycle cryo cooler. We achieved a frequency stability of better than 100 Hz, limited by the resolution bandwidth of the spectrum analyser. The PLL electronics make use of the intermediate frequency (IF) obtained from a hot electron bolometer (HEB) which is downconverted to a PLL IF of 125 MHz. The coarse selection of the longitudinal mode and the fine tuning is achieved via the bias voltage of the QCL. Within a QCL cavity mode, the free-running QCL shows frequency fluctuations of about 5 MHz, which the PLL circuit is able to control via the Stark-shift of the QCL gain material. Temperature dependent tuning is shown to be nonlinear, and of the order of -16 MHz/K. Additionally we have used the QCL as local oscillator (LO) to pump an HEB and perform, again for the first time at 1.5 THz, a heterodyne experiment, and obtain a receiver noise temperature of 1741 K.

Phase sensitive optical near-field mapping using frequency-shifted laser optical feedback interferometry

The use of laser optical feedback Imaging (LOFI) for scattering-type scanning near-field optical microscopy (sSNOM) is proposed and investigated. We implement this sensitive imaging method by combining a sSNOM with optical heterodyne interferometry and the dynamic properties of a B class laser source which is here used both as source and detector. Compared with previous near field optical heterodyne experiments, this detection scheme provides an optical amplification that is several orders of magnitude higher, while keeping a low noise phase-sensitive detection. Successful demonstration of this complex field imaging technique is done on Silicon on Insulator (SOI) optical waveguides revealing phase singularities and directional leakage.

Characterization of intensity-dependent optical rotation phenomena in chiral molecules in solution

The rotation of the plane of polarization of linearly polarized light by chiral molecules in solution is due to a forward scattering event. Ordinary optical rotation, a single-photon effect, is independent of intensity. As the light intensity is increased, other effects can appear, such as two-photon scattering or alignment of the molecule by one photon and scattering with a change of polarization by another. Both of these effects result in intensity-dependent (or nonlinear) optical rotation. A polarimeter was used to measure the nonlinear optical rotation of solutions in a heterodyne experiment. No nonlinear optical rotation was found in molecules lacking an absorption band near the laser frequency. In the three pyrimidine nucleosides studied, which do have such an absorption band, a nonlinear optical rotation was identified that was cumulative with each laser pulse. The effect persisted with a time constant that was on the order of seconds and characteristic of the molecule.

23 GHz operation of a room temperature photovoltaic quantum cascade detector at 5.35μm

We present a room temperature operated 5.35μm quantum cascade detector which was tested at high frequencies using an optical heterodyne experiment. Two slightly detuned continuous wave distributed feedback single mode quantum cascade lasers were used to generate a beating signal. The maximum frequency at which the resulting microwave signal could be detected was 23GHz. The cutoff behavior of our device was modeled with a simple RLC circuit and showed excellent agreement with the experimental data.

Dynamic Property of the Initial Process in the Dielectric Relaxation Phenomenon in KH2PO4

A new heterodyne experiment on the ferroelectric KH 2 PO 4 elucidates the extremely early stage of the excitation process of the ferroelectric relaxation mode. The time dependence of the response function G(t) shows a continuous rising from the time origin where the mode is excited by the ultra-short femtosecond pulses. The limit of the Debye model is overcome by a new phenomenological response function proposed here based on the experimental result.

An Integrated Acoustooptic Frequency Shifter Driven by Surface Acoustic Wave for 1.55 µm Optical Wavelength

An integrated acoustooptic frequency shifter in a crossed-channel waveguide is demonstrated at an optical wavelength of 1.55 µm. A novel tapered device was designed to make the acoustooptic interaction region longer and thus the driving surface acoustic wave (SAW) power lower, and the device was fabricated by the proton-exchange method and post-annealing in a 128° rotated Y-cut LiNbO3 substrate. At SAW power of 1.4 W, a 40% diffraction efficiency and a bandwidth of 5 MHz centered at 196 MHz were obtained. The frequency shift of Bragg diffracted light was confirmed in an optical heterodyne experiment.

Demonstrating optical beat notes through heterodyne experiments

The interference between two independent beams of light, often called optical heterodyne detection, is of considerable theoretical and practical importance. This note concentrates on considerations of instrumentation and technique that arise in demonstrating optical heterodyning, and describes three experimental demonstrations of the phenomenon.

A method to lower the detection limit for optical beat signals froma frequency-dithered stabilized laser

A narrow absorption feature in an atomic or molecular gas (such as iodineor methane) is used as the frequency reference in many stabilized lasers.As part of the stabilization scheme an optical frequency dither is appliedto the laser. In optical heterodyne experiments, this dither is transferredto the RF beat signal, reducing the spectral power density and hence thesignal to noise ratio over that in the absence of dither.We removed the dither by mixing the raw beat signal with a dithered localoscillator signal. When the dither waveform is matched to that of thereference laser the output signal from the mixer is rendered dither free.Application of this method to a Winters iodine-stabilized helium-neonlaser reduced the bandwidth of the beat signal from 6 MHz to 390 kHz,thereby lowering the detection threshold from 5 pW of laser power to 3 pW.In addition, a simple signal detection model is developed which predictssimilar threshold reductions.

Investigation of the spectral width of quantum cascade laser emission near 5.2 μm by a heterodyne experiment

We report line width measurements of a quantum cascade distributed feedback laser by a heterodyne experiment. At currents slightly above threshold and a laser output power higher than 1 mW, the full width at half maximum of the beat signal was narrower than 0.5 MHz, which gives us an upper limit of the laser line width. As reference laser we used a carbon monoxide laser. Both lasers were operating unstabilized in continuous wave mode emitting light at about 5.2 μm.

A visible extended cavity diode laser for the undergraduate laboratory

We demonstrate how to construct a simple single-frequency extended cavity diode laser (ECDL) for the undergraduate laboratory using mainly standard opto-mechanical components. This ECDL is operated with both 635 and 670 nm laser diodes. We present three experiments that can be performed using this ECDL, namely spectroscopic studies of iodine, second harmonic generation, and an optical heterodyne experiment using the ECDL with a helium–neon laser.

Heterodyne measurement of vibrational wave packets of diatomic molecules

A method is presented for reconstructing the quantum state of vibrational wave packets in diatomic molecules by a heterodyne experiment. After the preparation of a wave packet, a femtosecond laser pulse interferes with the induced polarization and the resulting signal is frequency filtered and measured by a photodetector. The conditions are analyzed under which the resulting time- and frequency-resolved photocurrent contains the information needed to recover the quantum state of the prepared wave packet. It is shown that the vibrational wave function in the excited electronic state can be measured directly without the need of involved numerical reconstruction. Simulations of the proposed experiment are performed for sodium dimers.

Heterodyne pump-probe and four-wave mixing in semiconductor optical amplifiers using balanced lock-in detection

We demonstrate a new detection scheme for pump-probe and four-wave mixing heterodyne experiments, using balanced detection and a dual-phase lock-in for spectral filtering. The technique allows the use of low repetition-rate laser systems, as is demonstrated on an InGaAsP/InP bulk optical amplifier at 1.53 μm. Ultrafast pump-induced changes in the amplitude and phase of the transmitted probe signal are simultaneously measured, going from small to large signal changes and with no need of an absolute phase calibration, showing the versatility and the sensitivity of this detection scheme. The results for small perturbations are consistent with previous pump-probe experiments reported in literature. Time-resolved four-wave mixing in the absorption regime of the device is measured, and compared with numerical simulations, indicating a 100 fs dephasing time.

Contactless measurements of the photocarrier lifetime in amorphous silicon by a heterodyne experiment

We describe a heterodyne detection scheme for an optical measurement of the photocarrier lifetime in semiconductors. The setup utilizes the Doppler shift of a laser beam that is diffracted from a periodic modulation of the optical constants of a sample, when it is illuminated with a moving interference grating. The advantage of heterodyne detection is its high sensitivity and the extraction of phase information of the diffracted beam. We present measurements on amorphous hydrogenated silicon.

Frequency shifted feedback lasers: continuous or stepwise frequency chirped output?

The temporal behavior of a frequency shifted feedback laser is studied on a time scale comparable to the round trip transit time. Two possible models are considered: a continuous chirp or a discrete chirp which increases by the frequency shift each transit time. The consequences of these two predictions on heterodyne and homodyne experiments are analyzed and it is shown that no experimental evidence exists which distinguishes between these models.

Chirped-frequency generation in a translated-grating-type frequency-shifted feedback laser

The properties of a frequency-shifted feedback (FSF) laser using a translated grating as the frequency-shifting element are investigated. FSF operation is attained by feedback of the first-order diffracted light from a grating coupler, which is translated in a direction perpendicular to the grating normal. A diode-pumped Nd:YVO(4) gain medium is used. Chirped-frequency components are periodically generated with a chirp rate of 1.8 THz / s. The unique spectral characteristics of this device are demonstrated with a Michelson interferometer and a heterodyne experiment.

A fiber optic frequency shifter

A fiber optic frequency shifter capable of shifting the frequency of light traveling through the fiber has been designed, constructed, and tested. The frequency of the transmitted light is shifted by stretching and contracting the optical fiber that is wound around two piezoelectric ceramic tubes. The frequency of the light is shifted by linearly changing the path length along which the light must travel. The diameters of two piezoelectric ceramic tubes are changed when a linearly increasing electric field is applied to the tubes. A servo control system is used to correct and compensate for nonlinearity of the piezoelectric material and assembly. A frequency shift of up to 305 Hz for up to 2.5 s with a linewidth of less than 1 Hz is achieved with the present setup. This frequency shifter has been successfully applied to a laser light scattering apparatus for which heterodyne experiments are made for particle electrophoretic mobility and zeta potential measurements.