Lock-in Detection Method Research Articles

Application of Lock-In Detection Method to HTS SQUID Contaminant Detection

Application of Lock-In Detection Method to HTS SQUID Contaminant Detection

Noise Suppression of Nitrogen-Vacancy Magnetometer in Lock-In Detection Method by Using Common Mode Rejection.

Nitrogen-vacancy (NV) centers in diamonds are promising solid-state magnetic sensors with potential applications in power systems, geomagnetic navigation, and diamond NV color center current transformers, in which both high bandwidth and high magnetic field resolution are required. The wide bandwidth requirement often necessitates high laser power, but this induces significant laser fluctuation noise that affects the detection magnetic field resolution severely. Therefore, enhancement of the magnetic field resolution of wide-bandwidth NV center magnetic sensors is highly important because of the reciprocal effects of the bandwidth and magnetic field resolution. In this article, we develop a common mode rejection (CMR) model to eliminate the laser noise effectively. The simulation results show that the noise level of the light-detected magnetic resonance signal is significantly reduced by a factor of 6.2 after applying the CMR technique. After optimization of the laser power and modulation frequency parameters, the optimal system bandwidth was found to be 75 Hz. Simultaneously, the system's detection magnetic field resolution was enhanced significantly, increasing from 4.49 nT/Hz1/2 to 790.8 pT/Hz1/2, which represents an improvement of nearly 5.7 times. This wide-bandwidth, high-magnetic field resolution NV color center magnetic sensor will have applications including power systems, geomagnetic navigation, and diamond NV color center current transformers.

Harmonic dependence of thermal magnetic particle imaging

Advances in instrumentation and tracer materials are still required to enable sensitive, accurate, and localized in situ 3D temperature monitoring by magnetic particle imaging (MPI). We have developed a high-resolution magnetic particle imaging instrument and implemented a low-noise multi-harmonic lock-in detection method to observe and quantify temperature variations in iron oxide nanoparticle tracers using the harmonic ratio method for determining temperature. Using isolated harmonics for MPI and temperature imaging revealed an apparent dependence of imaging resolution on harmonic number. Thus, we present experimental and simulation studies to quantify the imaging resolution dependence on temperature and harmonic number, and directly validate the fundamental origin of MPI imaging resolution on harmonic number based on the concept of a harmonic point-spread-function.

Photoacoustic spectrometry and resonant frequency tracking based dual-mode gas sensor

Photoacoustic spectrometry has excellent advantages in trace gas detection. However, due to the influence of gas saturation absorption, when the gas concentration spikes to a certain level, this technology is difficult to achieve accurate detection. Therefore, a dual-mode gas sensor based on photoacoustic spectroscopy and resonant frequency tracking is developed by the features of both detection technologies. A tunable DFB diode laser in the 1654 nm wavelength range and a loudspeaker are used for the photoacoustic detection, and the resonant frequency tracking, respectively. A resonant photoacoustic cell is used as the sensing element to work in dual-mode by time-division multiplexing technology. The wavelength-modulated photoacoustic detection is enabled in the methane concentration below 30%. When the gas concentration exceeds 30%, it will be automatically switched to the measurement-mode of resonant frequency tracking by the fast Fourier transform. The double-range gas detection is realized by a single sensor with dual-mode detecting functionality. With the wavelength modulation and lock-in harmonic detection method, the minimum detectable limit (1σ) of 3.5 ppm CH4 was demonstrated under the photoacoustic detection mode. The response time of the sensor is about 10 s within the measurement range of double-range.

Pseudoheterodyne near-field imaging at kHz repetition rates via quadrature-assisted discrete demodulation

Scattering-type scanning near-field optical microscopy enables the measurement of optical constants of a surface beyond the diffraction limit. Its compatibility with pulsed sources is hampered by the requirement of a high-repetition rate imposed by lock-in detection. We describe a sampling method, called quadrature-assisted discrete (quad) demodulation, which circumvents this constraint. Quad demodulation operates by measuring the optical signal and the modulation phases for each individual light pulse. This method retrieves the near-field signal in the pseudoheterodyne mode, as proven by retraction curves and near-field images. Measurement of the near-field using a pulsed femtosecond amplifier and quad demodulation is in agreement with results obtained using a CW laser and the standard lock-in detection method.

Magnetically Modulated Fluorescence of Nitrogen-Vacancy Centers in Nanodiamonds for Ultrasensitive Biomedical Analysis.

The negatively charged nitrogen-vacancy center in fluorescent nanodiamonds (FNDs) is a point defect with unique magneto-optical properties. It emits far-red fluorescence at ∼700 nm, and its intensity can be magnetically modulated with a depth of more than 10% at a field strength of 30 mT. We have closely examined this property and illustrated its practical use in biomedicine by applying a periodic, time-varying magnetic field to FNDs deposited on a surface or dispersed in a solution with a lock-in detection method. We achieved selective and sensitive detection of 100 nm FNDs on a nitrocellulose membrane at a particle density of 0.04 ng/mm2 (or ∼2 × 104 particles/mm2) and in an aqueous solution with a particle concentration of 1 ng/mL (or ∼1 fM) in 10 s as the detection limits. The utility and versatility of the technique were demonstrated with an application to background-free detection of FNDs as reporters for FND-based lateral flow immunoassays as well as selective quantification of FNDs in tissue digests for in vivo studies.

Fluorescence-detected Fourier transform electronic spectroscopy by phase-tagged photon counting.

Fluorescence-detected Fourier transform (FT) spectroscopy is a technique in which the relative paths of an optical interferometer are controlled to excite a material sample, and the ensuing fluorescence is detected as a function of the interferometer path delay and relative phase. A common approach to enhance the signal-to-noise ratio in these experiments is to apply a continuous phase sweep to the relative optical path, and to detect the resulting modulated fluorescence using a phase-sensitive lock-in amplifier. In many important situations, the fluorescence signal is too weak to be measured using a lock-in amplifier, so that photon counting techniques are preferred. Here we introduce an approach to low-signal fluorescence-detected FT spectroscopy, in which individual photon counts are assigned to a modulated interferometer phase ('phase-tagged photon counting,' or PTPC), and the resulting data are processed to construct optical spectra. We studied the fluorescence signals of a molecular sample excited resonantly by a pulsed coherent laser over a range of photon flux and visibility levels. We compare the performance of PTPC to standard lock-in detection methods and establish the range of signal parameters over which meaningful measurements can be carried out. We find that PTPC generally outperforms the lock-in detection method, with the dominant source of measurement uncertainty being associated with the statistics of the finite number of samples of the photon detection rate.

Active loss compensation in the resonant optical gyroscope with a "reflector": towards a cost-effective alternative for the navigational grade.

The performance of passive fiber optic gyroscopes involving ring resonators is limited mainly by the loss and finesse of the cavity. In this work, we show performance enhancement of the recently studied resonant fiber optic gyroscope with a "reflector" using active loss compensation. Our gyroscope does not require expensive ultra-narrow linewidth lasers, expensive lock-in detection methods, or polarization-maintaining fibers, which are mandatory for all standard resonant fiber optic gyroscopes. The performance of this gyroscope shows four-fold enhancement in the $Q$Q factor $ (2.2 \times {10^8}) $(2.2×108), compared to an earlier experimental setup involving losses. Enhanced sensitivity to rotation is experimentally demonstrated using loss compensation as well as tuning the embedded reflector in the resonator. A shot-noise-limited sensitivity of 0.03 deg/h is possible with this experimental realization. This work demonstrates that our gyroscope can provide a cost-effective alternative, even in the navigational grade.

Non-reciprocal biasing for performance enhancement of the resonant fiber gyroscope with ‘Reflector’ using In-line Faraday rotators: Design, analysis and characterization

Abstract Resonant fiber optic gyroscopes use highly expensive narrow linewidth lasers and complex lock-in detection methods since they measure changes in the beat frequency due to angular rotations. In our recent work, a novel resonant gyroscope with an embedded “reflector” was proposed along with a proof-of-principle demonstration. Our resonant gyroscope unconventionally utilizes the changes in optical intensity to measure angular velocity. In this work, a novel method of providing nonreciprocal bias has been designed and implemented using fiber pigtailed Faraday rotators to increase the gyroscope’s sensitivity. We have also created an in-line reflector within the resonator using thin film deposition, in order to stabilize and enhance the reflectance. Unlike earlier, we now use standard single mode fibers and a custom designed, feedback stabilized polarization controller to obtain a bias stability of 10.8 deg/h. Our all-fiber configuration shows enhanced performance and a shot noise limited sensitivity of 0.2 deg/h, indicating the potential for tactical grade performance (0.1–30 deg/h) at much reduced costs. The gyroscope’s performance in terms of sensitivity to Kerr nonlinear effects, ambient thermal changes and backscattering have also been analyzed and discussed in detail.

Accurate Estimation of Brillouin Frequency Shift in Brillouin Optical Correlation Domain Analysis

Estimation of the accurate value of Brillouin frequency shift (BFS) in a Brillouin optical correlation domain analysis (BOCDA) is challenging due to the contributions to the Brillouin gain spectrum (BGS) from the locations other than corresponding to the correlation between the pump and the probe. In this scenario, we demonstrate optimal post-processing algorithms to retrieve the BFS accurately. We first demonstrate a linear approximation based approach to estimate the BFS with an accuracy of $ 1 MHz. This approach needs to be modified for situations where simultaneous sensing is carried out from two or more locations, or with lock-in detection methods. A gradient descent method is proposed and demonstrated in such cases where the component corresponding to the second harmonic of the modulation frequency is used to optimally recover BFS. The method is tested for its performance at different locations of correlation peaks and for perturbation frequency range of $>$ 500 MHz, under different SNR conditions. The error in estimation is found to be less than 1.7 MHz across the entire frequency range for an SNR as low as 5 dB. The algorithm is also validated by comparison with experimental data. The proposed algorithm effectively increases the range and sensitivity of measurement for BFS estimation even when multiple locations are monitored simultaneously.

Detection of fluorescence from a mixed specimen consisting of micro particles attached to different fluorescent substances using a light waveguide incorporated optical TAS

A “ubiquitous human health care system” will require a monolithic optical total analysis system (TAS) consisting of waveguides and microfluidic channels based on a transparent resin chip. Together with the rapid development of the fluorescent marking method, fluorescence analysis by TAS of mixed-microparticle specimen attached to different fluorescent substances will be necessary. Towards realization of this, we here propose a novel method for using a part of the fluorescence acquired by irradiating microparticles with AC-modulated laser power as light dedicated to the discrimination of fluorescent substances. Since the light power for discrimination was extremely weak, we extracted effective signal components using a lock-in detection method. Then, by comparison with the signal of the original fluorescence, we could determine whether the fluorescence signal was from the microparticles attached to the fluorescent substance to be discriminated. Using a mixed specimen composed of microparticle-attached fluorescent substances with emission peaks of 520 nm and 600 nm, we found that 10% of the acquired fluorescence could successfully determine the specified fluorescent substance as a discrimination signal. The peak value of the discrimination signal was approximately double the amplitude of the stationary noise in the discrimination signal.

Terahertz wire-grid circular polarizer tuned by lock-in detection method.

We report the design, construction, and testing of a broadband-tunable terahertz circular polarizer, which we developed for our magneto-optical measurements using the far-infrared/THz laser source in the range of 0.25-7.5 THz. We present a thorough analysis of the lock-in amplifier signal generated by using a rotating analyzer with regard to setting the desired state of polarization. The phase-sensitive detection method is applied to a combination of a wire-grid polarizer and a parallel translation mirror providing a tunable retardance. The proposed technique is appropriate not only to free-standing grids but also to those deposited on transparent substrates which may introduce additional non-linear effects. The method is preferred when the distance between the mirror and the grid cannot be exactly determined, but the relative displacements are measured. The device enables switching between left- and right-handed polarization states on a time scale of a few seconds. Practical use of the circular polarizer is demonstrated by directly probing the far infra-red magneto-optical properties of the two-dimensional electron gas on the 458 μm laser line.

Co-axis digital holography based on sinusoidal phase modulation using generalized lock-in detection.

Sinusoidal phase modulating (SPM) interferometers are used to precisely measure complex light fields with simple interferometric setups. Recently, a generalized lock-in technique has been proposed for optimizing the signal extraction in phase-modulated interferometers. This article shows its applicability in digital holography as well as digital holographic interferometry. A compact homemade setup based on the Michelson interferometer was used to observe the scattering sample surface with a spatial resolution of 32 LP/mm; out-of-plane rotation was measured with a theoretical detection limit of 0.004°. The comparison between the generalized lock-in detection and traditional SPM interferometer method was done theoretically and experimentally.

Subfemtonewton Force Spectroscopy at the Thermal Limit in Liquids.

We demonstrate thermally limited force spectroscopy using a probe formed by a dielectric microsphere optically trapped in water near a dielectric surface. We achieve force resolution below 1fN in 100s, corresponding to a 2Å rms displacement of the probe. Our measurement combines a calibrated evanescent wave particle tracking technique and a lock-in detection method. We demonstrate the accuracy of our method by measurement of the height-dependent force exerted on the probe by an evanescent wave, the results of which are in agreement with Mie theory calculations.

Generalized lock-in detection for interferometry: application to phase sensitive spectroscopy and near-field nanoscopy.

A generalized lock-in detection method is proposed to extract amplitude and phase from optical interferometers when an arbitrary periodic phase or frequency modulation is used. The actual modulation function is used to create the reference signals providing an optimal extraction of the useful information, notably for sinusoidal phase modulation. This simple and efficient approach has been tested and applied to phase sensitive spectroscopy and near-field optical measurements. We analyze the case where the signal amplitude is modulated and we show how to suppress the contribution of unmodulated background field.

Variable-frequency lock-in detection for the suppression of beat noise in Brillouin optical correlation domain analysis

We propose and experimentally demonstrate a novel lock-in detection method to avoid a beat noise in Brillouin optical correlation domain analysis (BOCDA) which appears in the sweep of the sensing position and deteriorates the measurement accuracy by distorting the acquired Brillouin gain spectrum. In our analysis, the origin of the beat noise is shown to be the fluctuation of the Brillouin gain induced by the chopping of the intensity-modulated pump wave, and the optimal relation between the modulation and the lock-in frequencies is developed as an effective solution to circumvent the beat noise.

An all-optical photoacoustic spectrometer for trace gas detection

An all-optical photoacoustic spectrometer based on the diaphragm-based extrinsic Fabry–Perot interferometer (EFPI) fiber acoustic sensor for trace gas detection at atmospheric pressure and room temperature is first developed. The diaphragm-based EFPI fiber acoustic sensor is used to replace the conventional acoustic sensor in this system. Tunable erbium-doped fiber ring laser (TEDFRL) with an erbium-doped fiber amplifier (EDFA) is used as the excitation light source for the generation of the photoacoustic (PA) signal. A first longitudinal resonant PA cell with double absorption optical path is used to improve the sensitivity of the system. The advantages of all-optical photoacoustic spectrometer are immunity to electromagnetic interference, safely in flammable and explosive situations, and long distance sensing. This system is developed for the continuous and real-time measurement of acetylene gas at room temperature and atmospheric pressure. With the wavelength modulation and lock-in harmonic detection method, the minimum detectable limit of 1.56 parts-per-billion (ppb) volume acetylene (signal-to-noise ratio, SNR = 1) at the 1530.37 nm transition line was achieved.

High-Contrast Fluorescence Imaging Using new Optical Switches and Optical Lock-in Detection Imaging Microscopy

We have developed a new microscope imaging technique to isolate specific fluorescence signals from background signals by using lock-in detection of the modulated fluorescence of a unique class of optical probe. This optical lock-in detection (OLID) approach involves modulating the fluorescence emission of an optical switch probe through defined optical manipulation of its fluorescent and non-fluorescent states - a digital lock-in detection method is employed to isolate the modulated signal of interest from non-modulated signals in the sample, such as conventional fluorescent probes and natural fluorophores. I will discuss the spectroscopic and photochemical properties of several new synthetic and genetically-encoded optical switches and illustrate their applications in high contrast (OLID) imaging studies of specific structures and proteins in cultured cells and in living organisms.

Optical Lock-in Detection (OLID) and OLID-FRET Imaging Microscopy

The success of imaging fluorescent proteins within living cells is limited by the presence of background signals - while some improvements to image contrast can be achieved through background subtraction, these benefits diminish when the background signal varies in time and space. Further this approach does not allow imaging below the background signal, a regime of interest for single molecule biology. We have developed an optical lock-in detection (OLID) approach and associated signal analysis that involve modulating the fluorescence emission of the probe through deterministic, optical control of its fluorescent and non-fluorescent states, and subsequently applying a lock-in detection method to isolate the modulated signal of interest from non-modulated background signals. The lock-in detection method involves pixel-by-pixel based cross-correlation analysis that maps the correlation between the total fluorescence emission within single pixels of an image detected over several cycles of optical switching, and a reference waveform that represents the defined response of the switch probe to the switching cycle. The fundamentally new approach to imaging, which is similar in principle to radar, allows for the selective detection of emission from optical switch probes even in the presence of a larger population of conventional fluorescent probes. We have developed new synthetic and genetically-encoded optical switches for OLID and OLID-FRET imaging in living cells and tissues including nitrospirobenzopyran and naphthozaxine based probes as well as fusion proteins of Dronpa and a new optically switchable mcherry. OLID imaging using these probes is remarkably efficient at imaging specific structures and proteins in high and time varying background signal environments such as in living cells, in culture, in brain slices and in live Xenopus embryos and zebrafish larvae.

Optical lock-in detection imaging microscopy for contrast-enhanced imaging in living cells

One of the limitations on imaging fluorescent proteins within living cells is that they are usually present in small numbers and need to be detected over a large background. We have developed the means to isolate specific fluorescence signals from background by using lock-in detection of the modulated fluorescence of a class of optical probe termed "optical switches." This optical lock-in detection (OLID) approach involves modulating the fluorescence emission of the probe through deterministic, optical control of its fluorescent and nonfluorescent states, and subsequently applying a lock-in detection method to isolate the modulated signal of interest from nonmodulated background signals. Cross-correlation analysis provides a measure of correlation between the total fluorescence emission within single pixels of an image detected over several cycles of optical switching and a reference waveform detected within the same image over the same switching cycles. This approach to imaging provides a means to selectively detect the emission from optical switch probes among a larger population of conventional fluorescent probes and is compatible with conventional microscopes. OLID using nitrospirobenzopyran-based probes and the genetically encoded Dronpa fluorescent protein are shown to generate high-contrast images of specific structures and proteins in labeled cells in cultured and explanted neurons and in live Xenopus embryos and zebrafish larvae.