Optical Cross-correlation Research Articles

Absolute distance measurement to coaxial multi-targets based on optical balanced cross-correlation using a single femtosecond laser.

We describe a high-precision ranging method based on an optical balanced cross-correlation system with a scanning repetition rate using a single femtosecond laser. By scanning the repetition rate of a laser, measuring pulses can be overlapped with reference pulses. It is an effective method to make reference pulses overlap with coaxial multiple target pulses without additional mechanical devices. The overlapped pulses are launched to the optical balanced cross-correlation system, which improves the time resolution measurement to the attosecond level. Two nominal distances are measured, and an additional commercial laser interferometer is used as a comparison to evaluate the accuracy of our measurement system. Moreover, the thickness of three stacked glasses is measured by our measurement system to verify that this system can measure coaxial multiple targets more quickly than conventional optical balanced cross-correlation systems using a single optical frequency comb.

Dynamics of Ultrafast Phase Transitions in (001) Si on the Shock-Wave Front.

We demonstrate an ultrafast (<0.1 ps) reversible phase transition in silicon (Si) under ultrafast pressure loading using molecular dynamics. Si changes its structure from cubic diamond to β-Sn on the shock-wave front. The phase transition occurs when the shock-wave pressure exceeds 11 GPa. Atomic volume, centrosymmetry, and the X-ray-diffraction spectrum were revealed as effective indicators of phase-transition dynamics. The latter, being registered in actual experimental conditions, constitutes a breakthrough in the path towards simple X-ray optical cross-correlation and pump-probe experiments.

Assessment of Aeolian Activity in the Bodélé Depression, Chad: A Dense Spatiotemporal Time Series From Landsat-8 and Sentinel-2 Data

There are several hotspots of dust production in the central Sahara, the Bodélé Depression (BD) in northern Chad is considered the largest source of aerosol dust worldwide, with the fastest Barchan dunes that migrate southwesterly. Less is known about the complex patterns of dune movement in the BD, especially on a short time scale. Time-series inversion of optical image cross-correlation (TSI-OICC) proved to be a valuable method for monitoring historical movements with low uncertainties, high spatial coverage, and dense temporal coverage. We leveraged ∼8 years of Landsat-8 and ∼6 years of Sentinel-2 data to capture the dune migration patterns at BD. We used TSI-OICC, creating four independent networks of offset maps from Landsat-8 and Sentinel-2 images, and forming three networks by fusing data from the two sensors. We depended on the multi spatial coherence estimated from Sentinel-1 interferograms to automatically discriminate between the active and stagnant regions, which is important for the postprocessing steps. We combined the data from the two sensors in areas of overlap to assess the performance of the fusion between two sensors in increasing the temporal scale of the observations. Our results suggest that dune migration at BD is subject to seasonal and multiyear variations that differed spatially across the dune field. Seasonal variations were observed with migration slowing during the summer months. We estimated the median for velocities belonging to the same season and calculated the seasonal sliding coefficient (SSC) representing the ratio between seasonal velocities. The median SSC reached a maximum value of ∼2 for winter/summer, while the ratios were ∼1.10 and ∼1.35 for winter/spring and winter/autumn, respectively. The seasonal variability of the temporal patterns was strongly supported by the wind observations. Between (1984–1998 and 1998–2007) and (1998–2007 and 2013–2021), decelerations in dune velocities were observed with percentages of ∼4 and ∼28%, respectively, and these decelerations were supported by a deceleration in wind velocities. Inversion of time series provides dense spatiotemporal monitoring of the dune activity. The fusion between two sensors allows condensing the temporal sampling up to a weekly scale especially for locations exposed to contamination of high cloud cover or dust.

Modelling two-laser asynchronous optical sampling using a single 2-section semiconductor mode-locked laser diode.

We present a theoretical overview and a proposed methodology which demonstrates SLASOPS (single laser asynchronous optical sampling) as a single-laser alternative to the conventional two-laser ASOPS technique. We propose the optical and electronic setup in which SLASOPS may be achieved experimentally with a single 2-section mode-locked laser diode as the pulsed-laser source and simulate how asynchronous optical sampling is generated and detected theoretically. We highlight the technique's ability to provide customizable scan ranges, scan rates and scan resolutions through variation of the imbalance in the interferometer arms and by tuning the repetition rate of the pulsed-laser source, which we present as optical cross-correlations between pulse pairs. We incorporate jitter into the system mathematically to assess the limitations on resolving both intensity and interferometric cross-correlation traces and to investigate the effects of averaging such traces in real-time. Analysis is then carried out on cross-correlation trace amplitude, width, and temporal positioning in order to discuss the technique's ability for deployment in typical optical sampling applications. In particular we note SLASOPS' ability to conduct asynchronous optical sampling using only a single laser, halving both the expense and technical requirements, doing so at megahertz scan rates, and within a spatial precision of just a few microns.

Demonstration of 3D photon correlation spectroscopy in the hard X-ray regime.

Three-dimensional photon correlation spectroscopy (3D PCS) is a well-known technique developed to suppress multiple scattering contributions in correlation functions, which are inevitably involved when an optical laser is employed to investigate dynamics in a turbid system. Here, we demonstrate a proof-of-principle study of 3D PCS in the hard X-ray regime. We employ an X-ray optical cross-correlator to measure the dynamics of silica colloidal nanoparticles dispersed in polypropylene glycol. The obtained cross correlation functions show very good agreement with auto-correlation measurements. This demonstration provides the foundation for X-ray speckle-based studies of very densely packed soft matter systems.

Absolute distance measurement using cross correlation interferometer with a repetition rate multiplication frequency comb

In this paper, the Fabry-Perot etalon is used to multiply the repetition rate of the fiber optical frequency comb. The repetition rate is amplified from 250 MHz to 10 GHz, and the corresponding pulse interval is reduced from 1200 mm to 30 mm. For the pulse cross correlation ranging method, the repetition rate multiplication can greatly reduce the length requirement of the scanning reference arm. We analyze in detail the principle of cross correlation interferometry based on repetition rate multiplication frequency comb. A numerical mode of the function is comprehensively established. The basic parameters of optical source and Fabry-Perot cavity for the influence of filtered optical spectrum and cross correlation fringe are analyzed through the numerical simulation. The multiplied frequency comb is utilized for absolute ranging with the help of a pulse cross correlation method. By comparison, our result differs from the result obtained by a conventional counting interferometer only by 4 μm for distances up to 210 mm.

Sub-Femtosecond Timing Jitter From a SESAM Mode-Locked Yb-Fiber Laser

We demonstrate sub-femtosecond timing jitter of ultrashort pulses from a dispersion managed Yb-fiber laser mode-locked by a SESAM. Timing jitters of the mode-locked Yb-laser under different dispersion conditions (when the intra-cavity dispersions are −0.005 ps2, +0.001 ps2, +0.006 ps2) are characterized by the single-crystal balanced optical cross-correlator. In our experiment, an ultra-low timing jitter of 48 attoseconds is achieved when the jitter spectral density is integrated from 3 kHz to 1 MHz offset frequency. We think that the obtained timing jitter level is the lowest for fiber lasers mode-locked by real saturable absorbers. This ultra-low timing-jitter, stably mode-locked fiber laser source is expected to be used for high precision applications outside of the laboratory.

Timing Fluctuation Correction of A Femtosecond Regenerative Amplifier

We report on the long-term correction of a timing fluctuation between the femtosecond regenerative amplifier and the reference oscillator for the seed 100 PW laser system in the Station of Extreme Light (SEL). The timing fluctuation was characterized by a noncollinear balanced optical cross-correlator that maps the time difference to the sum frequency intensity of the amplifier and oscillator laser pulses. A feedback loop was employed to correct the timing jitter by adjusting the time delay line in the amplifier beam path. The timing fluctuation was reduced to 1.26 fs root-mean-square from hundreds of fs over 10 hours. Benefitting from excellent performance and long-term stability, this timing jitter correction scheme, as a component of optical synchronization in the 100 PW laser facility, will be integrated into SEL.

Quantum limited timing jitter of soliton molecules in a mode-locked fiber laser.

Soliton molecules in mode-locked lasers are expected to be ideal self-organization patterns, which warrant stability and robustness against perturbations. However, recent ultra-high resolution optical cross-correlation measurements uncover an intra-molecular timing jitter, even in stationary soliton molecules. In this work, we found that the intra-molecular timing jitter has a quantum origin. Numerical simulation indicates that amplified spontaneous emission (ASE) noise induces a random quantum diffusion for soliton pulse timing, which cannot be compensated by soliton binding mechanism. By suppressing indirectly coupled timing jitter at close-to-zero cavity dispersion, a record-low 350 as rms intra-soliton-molecular jittering is obtained from an Er-fiber laser in experiment. This work provides insight into the fundamental limits for the instability of multi-soliton patterns.

Two Color Balanced Optical Cross Correlator to Synchronize Distributed Lasers for SHINE Project

Two Color Balanced Optical Cross Correlator to Synchronize Distributed Lasers for SHINE Project

Large optical nonlinearity enhancement under electronic strong coupling

Nonlinear optical responses provide a powerful way to understand the microscopic interactions between laser fields and matter. They are critical for plenty of applications, such as in lasers, integrated photonic circuits, biosensing and medical tools. However, most materials exhibit weak optical nonlinearities or long response times when they interact with intense optical fields. Here, we strongly couple the exciton of cyanine dye J-aggregates to an optical mode of a Fabry-Perot (FP) cavity, and achieve an enhancement of the complex nonlinear refractive index by two orders of magnitude compared with that of the uncoupled condition. Moreover, the coupled system shows an ultrafast response of ~120 fs that we extract from optical cross-correlation measurements. The ultrafast and large enhancement of the optical nonlinar coefficients in this work paves the way for exploring strong coupling effects on various third-order nonlinear optical phenomena and for technological applications.

OpenOpticalFlow_PIV: An Open Source Program Integrating Optical Flow Method with Cross- Correlation Method for Particle Image Velocimetry

This paper describes OpenOpticalFlow_PIV, an open source Matlab program integrating the optical flow method with the cross-correlation method for extraction of high-resolution velocity fields from particle images with large displacements. This hybrid method provides an additional tool to process PIV images, which combines the advantages of the optical flow method and cross-correlation method and overcomes the intrinsic issues of the two methods. The principles of the hybrid method are concisely described, including the cross-correlation method for initial coarse-grained estimation and the optical flow method for refined high-resolution estimation. This paper gives more detailed descriptions of the main program, relevant subroutines and selection of the relevant parameters for computation. Examples are given to demonstrate applications of the hybrid method.

Atomic scale displacements detected by optical image cross-correlation analysis and 3D printed marker arrays

For analyzing displacement-vector fields in mechanics, for example to characterize the properties of 3D printed mechanical metamaterials, routine high-precision position measurements are indispensable. For this purpose, nanometer-scale localization errors have been achieved by wide-field optical-image cross-correlation analysis. Here, we bring this approach to atomic-scale accuracy by combining it with well-defined 3D printed marker arrays. By using an air-lens with a numerical aperture of 0.4 and a free working distance of 11.2, mathrm{mm}, and an 8times 8 array of markers with a diameter of 2, upmumathrm{m} and a period of 5,upmu mathrm{ m}, we obtain 2D localization errors as small as 0.9, AA in 12.5, mathrm{ms} measurement time (80, mathrm{frames}/mathrm{s}). The underlying experimental setup is simple, reliable, and inexpensive, and the marker arrays can easily be integrated onto and into complex architectures during their 3D printing process.

Attosecond timing jitter within a temporal soliton molecule

The interactions of optical solitons in passively mode-locked lasers result in abundant bound states that reflect intriguing nonlinear attractor behaviors in complex dissipative systems. In this study, we tested the strength limits of the fixed-point attractors that govern bound soliton molecule formation in passively mode-locked lasers. To this end, we probed the relative timing jitter in a closely separated stationary doublet soliton molecule produced by a Kerr-lens mode-locked Ti:sapphire laser. By adopting a balanced optical cross-correlation method with a 5 z s / 0 H z temporal resolution, we derive an upper estimate of 60 as intramolecular timing jitter, which is integrated from 100 Hz to the Nyquist frequency (60 MHz). To the best of our knowledge, this is the first evidence for attosecond timing jitter within robust bound solitons in dissipative systems.

Measurement of absolute timing jitter of SESAM mode-locked lasers with yoctosecond sensitivity

Absolute timing jitter from a mode-locked laser is characterized with a record low noise floor of 122ys/Hz (1.5×10-14s2/Hz). We develop a novel measurement technique using cross-spectrum methods in combination with a dual balanced optical cross-correlator system, with three independent low-noise mode-locked lasers.

Femtosecond synchronization of multiple mode-locked lasers and a microwave oscillator by multicolor electro-optic sampling.

Simple multicolor electro-optic sampling-based femtosecond synchronization of multiple mode-locked lasers is demonstrated. Parallel timing error detection between each laser and a common microwave is achieved by wavelength division multiplexing and demultiplexing. The parallel timing error detection enables simultaneous femtosecond synchronization of more than two mode-locked lasers to the microwave oscillator, even when the lasers have different repetition rates. The residual root-mean-square (rms) timing jitter of laser-laser synchronization measured by an optical cross correlator is 2.6 fs (integration bandwidth, 100 Hz-1 MHz), which is limited by the actuator bandwidth in the laser oscillator. The long-term rms timing drift and frequency instability of laser-microwave synchronization are 7.1 fs (over 10,000 s) and 5.5×10-18 (over 2000 s averaging time), respectively. As a versatile and reconfigurable tool for laser-laser and laser-microwave synchronization, the demonstrated method can be used for various applications ranging from ultrafast x-ray and electron science facilities to dual- and triple-comb spectroscopy.

Quantifying the spatio-temporal patterns of dune migration near Minqin Oasis in northwestern China with time series of Landsat-8 and Sentinel-2 observations

Investigation of the spatio-temporal patterns of dune migration in a large-scale area with optical remote sensing techniques can help us to better understand aeolian phenomena and mitigate sand-dust disasters. With the rapid growth in data volume, extracting more accurate dune displacement time series and rates from optical observations has become possible; however, the method is not yet fully fledged. To address this issue, we propose an extended algorithm for the mature optical imagery cross-correlation (OICC) technique based on Landsat-8 (L8) and Sentinel-2 (S2) acquisitions. The main innovative points of this algorithm are: 1) the proposed pairing strategy for the OICC processing; 2) the modularized post-processing procedures for noise removal; and 3) the introduction of singular value decomposition (SVD) time-series inversion of the redundant optical observations to quantify the dune migration. To test the effectiveness of this algorithm, it was applied in the study of dune migration near Minqin Oasis in northwestern China, using enriched L8 and S2 images collected between April 2013 and April 2018. Compared with the original OICC results in stable areas, the post-processing and inversion of the proposed algorithm reduce the uncertainty by around 22–35% and 3–5% for L8, 29–48% and 5–12% for S2, respectively. The cross-comparison between the L8- and S2-derived displacement time series shows high consistency, and presents a lower uncertainty than the result of the traditional no-inversion method. Furthermore, the derived displacement rates show spatial patterns that are similar to those of the manually digitized results obtained with historical Google™ Earth (GE) images. These comparisons show the advantage of the proposed algorithm in automatically and accurately quantifying dune migration. Taking into account these measurements, the spatio-temporal evolution patterns of dune migration in the study area were analyzed. From the spatial perspective, the sand dunes move along a northwest-southeast axis with four detected transport pathways. Our research also shows that around 1087.7 km2 of dune fields present an active status. The active sand dunes are currently encroaching on around 155.5 km2 and 4.4 km2 of land each year outside and inside the oasis, respectively, representing a problem of rapid desertification. Temporally, the displacement time series along the dominant migration direction appears as seasonal variations that are seemly consistent with the changes in local atmospheric conditions. The proposed algorithm provides a new perspective to investigate the spatio-temporal evolution of dune migration with medium-resolution L8 and S2 optical datasets.

Timing jitter reduction through relative intensity noise suppression in high-repetition-rate mode-locked fiber lasers.

We reported the timing jitter reduction of an 882 MHz mode-locked NPE Yb:fiber lasers through active relative intensity noise suppression. The timing jitter spectra measurements based on balanced optical cross-correlation (BOC) technique show a reduction of ~10 dB in the Fourier frequency range from ~3 kHz to ~30 kHz with a unity-gain crossing point of 80 kHz. The results verify the theoretical prediction that the relative intensity noise (RIN) induced timing jitter by self-steepening effect dominates the jitter performance below ~100 kHz. Further comparison with the analytic model shows that the effect of RIN decays below ~3 kHz. Thus, the timing jitter reduction is not obvious at low frequency. To the best of our knowledge, this is the first experimental report on the timing jitter reduction through active RIN suppression in high-repetition-rate mode-locked fiber lasers.

Development of ultrafast capabilities for X-ray free-electron lasers at the linac coherent light source.

The ability to produce ultrashort, high-brightness X-ray pulses is revolutionizing the field of ultrafast X-ray spectroscopy. Free-electron laser (FEL) facilities are driving this revolution, but unique aspects of the FEL process make the required characterization and use of the pulses challenging. In this paper, we describe a number of developments in the generation of ultrashort X-ray FEL pulses, and the concomitant progress in the experimental capabilities necessary for their characterization and use at the Linac Coherent Light Source. This includes the development of sub-femtosecond hard and soft X-ray pulses, along with ultrafast characterization techniques for these pulses. We also describe improved techniques for optical cross-correlation as needed to address the persistent challenge of external optical laser synchronization with these ultrashort X-ray pulses.This article is part of the theme issue ‘Measurement of ultrafast electronic and structural dynamics with X-rays’.

Single-mode fiber-optic Fabry–Perot interferometry sensor based on optical cross-correlation demodulation

A single-mode fiber-optic Fabry–Perot interferometry (FPI) sensor based on optical cross-correlation demodulation is presented, with advantages of both low cost and small size. The sensor mainly consists of a light-emitting diode, a glass wafer, a linear charge-coupled device, a cylindrical lens, a fiber-optic coupler, a sensing head, and a circuit board. The cavity length of the FPI sensor is demodulated by the optical cross-correlation relationship between the glass wafer and the Fabry–Perot cavity. When the cavity length changes, the pixel number corresponding to the peak value of the cross-correlation image shifts accordingly. To optimize the demodulation parameters, a simulation model is established. The experimental results show that the resolution and variation range of cavity length measurement are achieved to be 0.6 nm and ±4 μm, respectively. The proposed fiber-optic sensor is promising for the temperature and pressure measurements in circumstances of intense electromagnetic interference and long distance.