Dispersion Mismatch Research Articles

Two fast multiturn injection models for synchrotron injection efficiency optimization

A multiturn injection scheme is used in the XiPAF-Upgrading synchrotron, with beam loss mainly occurring at the injection septum. The paper presents two theoretical models for calculating the efficiency of multiturn injection in compact synchrotrons, with a focus on the situation of dispersion mismatch at the injection point. The first model is the beam phase ellipse rotation model, which is based on the beam projection distribution. The second model is the injection septum rotation model, which is based on the particle coordinate criterion. The deviation in efficiency between both injection models and the simulation results is less than 5%, and the calculation speed has increased by a factor of 1000 compared to PyORBIT simulations. The models are used to optimize the multiturn injection efficiency of the XiPAF-Upgrading synchrotron, resulting in an optimal injection efficiency of 92%.

Dispersion mismatch correction for evident chromatic anomaly in low coherence interferometry.

The applications of ultrafast optics to biomedical microscopy have expanded rapidly in recent years, including interferometric techniques like optical coherence tomography and microscopy (OCT/OCM). The advances of ultra-high resolution OCT and the inclusion of OCT/OCM in multimodal systems combined with multiphoton microscopy have marked a transition from using pseudo-continuous broadband sources, such as superluminescent diodes, to ultrafast supercontinuum optical sources. We report anomalies in the dispersion profiles of low-coherence ultrafast pulses through long and non-identical arms of a Michelson interferometer that are well beyond group delay or third-order dispersions. This chromatic anomaly worsens the observed axial resolution and causes fringe artifacts in the reconstructed tomograms in OCT/OCM using traditional algorithms. We present DISpersion COmpensation Techniques for Evident Chromatic Anomalies (DISCOTECA) as a universal solution to address the problem of chromatic dispersion mismatch in interferometry, especially with ultrafast sources. First, we demonstrate the origin of these artifacts through the self-phase modulation of ultrafast pulses due to focusing elements in the beam path. Next, we present three solution paradigms for DISCOTECA: optical, optoelectronic, and computational, along with quantitative comparisons to traditional methods to highlight the improvements to the dynamic range and axial profile. We explain the piecewise reconstruction of the phase mismatch between the arms of the spectral-domain interferometer using a modified short-term Fourier transform algorithm inspired by spectroscopic OCT. Finally, we present a decision-making guide for evaluating the utility of DISCOTECA in interferometry and for the artifact-free reconstruction of OCT images using an ultrafast supercontinuum source for biomedical applications.

Four-wave mixing based spectral Talbot amplifier for programmable purification of optical frequency combs

Optical frequency combs (OFCs) with programmable free spectral range and high optical carrier-to-noise ratio (CNR) play a crucial role in diverse research fields, including telecommunications, spectroscopy, quantum information, astronomy, sensing, and imaging. Unfortunately, the presence of stochastic noise often results in degraded optical CNR, leading to limited communication performance and measurement accuracy in comb-based systems. There is a lack of effective and flexible methods to improve the CNR of OFCs contaminated by broadband noise, hampering their widespread utilization. To address this challenge, we propose a four-wave mixing based spectral Talbot amplifier to purify OFCs flexibly. Our approach employs programmable spectral phase filters followed by a nonlinear Kerr medium to regenerate an OFC with superior CNR. In our experimental demonstration, we regenerated a 165-GHz spaced CNR enhanced OFC from a noise-dominated comb source spaced at 11 GHz, achieving up to ∼11-dB CNR improvement. The technique allows for a user-defined purification factor m to range from 7 to 15. Furthermore, our scheme demonstrates flexibility in adjusting the wavelengths of the regenerated comb lines via a tunable optical delay line without the need for a tunable seed laser. We also investigated the impact of the pump and signal on the regenerated comb experimentally and studied the influence of dispersion mismatch on the suppression of undesired sidebands numerically. Our proposed scheme presents a powerful alternative for programmable purification, manipulation, and detection of noise-dominated spectral waveforms.

Study of dispersion extraction for optical coherence tomography imaging.

Dispersion mismatch of optical coherence tomography (OCT), caused by inherent dispersion of biological sample, leads to axial resolution degradation and image blurring. However, the dispersion of biological samples may carry characteristic information of itself. Therefore, it is useful and important to extract dispersion as an additional parameter of biological samples. Here, we study and compare five categories of conventional OCT dispersion extraction methods first. The results show that the iterative method has the best accuracy and stability. Furthermore, we propose an improved iterative method, which achieves higher accuracy and stability in simulation compared with conventional methods. Then, the dispersion values of optical glasses are extracted experimentally utilizing the proposed method. The extracted values are consistent well with reference values. Furthermore, we apply the proposed method for extracting the dispersion characteristics of zebrafish eye. With the proposed method, OCT-based analysis of dispersion characteristics could be a powerful tool in clinical practice.

Handheld common-path swept-source optical coherence tomography angiography.

This study develops a handheld optical coherence tomography angiography (OCTA) system that uses a high-speed (200 kHz) swept laser with a dual-reference common-path configuration for stable and fast imaging. The common-path design automatically avoids polarization and dispersion mismatches by using one circulator as the primary system element, ensuring a cost-effective and compact design for handheld probe use. With its stable envelope (i.e., sub-µm shifts) and phase variation (corresponding to nm changes in axial displacement), the minimum detectable flow velocity is ∼ 0.08 mm/s in our experiment, which gives the common-path setup a high potential for application in a handheld OCTA system for clinical skin screening. In vivo skin structures and microvasculature networks on the dorsum of the hand and cheek of a healthy human are imaged successfully.

Dynamic Transmissive Metasurface for Broadband Phase‐Only Modulation Based on Phase‐Change Materials

AbstractDynamic metasurfaces provide exciting opportunities for spatial light modulation with ultra‐compact footprint and high resolution. Designs reported so far typically suffer from efficiency problem due to the strong coupling between amplitude and phase modulation. Huygens metasurface design eliminates the back propagation with interference of dual resonances, offering a potential solution for phase‐only modulation. However, the intrinsic dispersion mismatch between the dual modes restricts this design to a highly limited bandwidth. Here, a dynamic phase‐change metasurface design that enables transmissive phase modulation covering over 240° with high‐level transmisstance maintained over 0.8 and a large operation bandwidth of 80 nm ranging from 1516 to 1596 nm, is presented. To verify the potential usage of this design, a multi‐state phase‐change control scheme that enables quasi‐continuous phase modulation is further provided. The key feature of this design is the leverage of congener Huygens concept. Judicious incorporation of lossless phase‐change material (PCM) with congener Huygens metasurface allows wideband holding of Huygens condition, which provides a perfect platform to simultaneously address the efficiency and bandwidth problems. This novel feature represents a meaningful step forward to integrate spatial light modulation.

FMCW LiDAR with an FM nonlinear kernel function for dynamic-distance measurement.

Frequency-modulated continuous-wave (FMCW) LiDAR is an absolute-distance measurement technology with the advantages of high-precision, non-cooperative target measurement capabilities and the ability to measure distance and speed simultaneously. However, the existing range extraction method for FMCW LiDAR is associated with problems, such as requiring a high sample rate and dispersion mismatch. Here, we propose and demonstrate a dynamic range extraction method based on an FM nonlinear kernel function, which improves measurement accuracy without the use of a long auxiliary interferometer (as is required for the traditional method), reduces the influence of dispersion mismatch and the Doppler effect caused by target movement and can simultaneously measure the target motion information dynamically, with a lower measurement error than that of the existing range extraction method under the same conditions.

Dispersion compensation for spectral domain optical coherence tomography by time-frequency analysis and iterative optimization

We report a new numeric dispersion compensation method for the device’s dispersion mismatch in a spectral-domain optical coherence tomography (SD-OCT) for imaging the iridocorneal angle of human cadaver eyes. The dispersion compensation term is calculated by an automated iterative process that minimizes the wavenumber-dependent variance of the ridge extracted from the spatial-spectral distribution of a mirror’s spectral interferogram using short-time Fourier transform (STFT). Our method can extract different amounts of dispersion robustly, and the algorithm can work in a wide range of combinations of window sizes and overlaps when using an STFT. Comparable point spread functions (PSFs) are shown to a traditional polynomial fitting method. Lastly, we verified that our imaging system is able to visualize the iridocorneal angle details, such as trabecular meshwork (TM), Schlemm’s canal (SC), and collector channels (CCs), which are important ocular outflow structures associated with glaucoma management.

Generation of entangled photons via parametric down-conversion in semiconductor lasers and integrated quantum photonic systems

We propose and design a high-brightness, ultra-compact electrically pumped GaSb-based laser source of entangled photons generated by mode-matched intracavity parametric down-conversion of lasing modes. To describe the nonlinear mixing in highly dispersive and dissipative waveguides, we develop a nonperturbative quantum theory of parametric down-conversion of waveguide modes which takes into account the effects of modal dispersion, group and phase mismatch, propagation, dissipation, and coupling to noisy reservoirs. We extend our theory to the regime of quantized pump fields with a new approach based on the propagation equation for the state vector which solves the nonperturbative boundary-value problem of the parametric decay of a quantized single-photon pump mode and can be generalized to include the effects of dissipation and noise. Our formalism is applicable to a wide variety of three-wave mixing propagation problems. It provides convenient analytic expressions for interpreting experimental results and predicting the performance of monolithic quantum photonic systems.

Single A-Line Method for Fast Sample-Structure-Nondependent Dispersion Compensation of FD-OCT

Here we proposed a single A-line sample-structure-nondependent (SSNd) dispersion detection and compensation method for Fourier-domain optical coherence tomography (FD-OCT), without need for acquiring and processing B-scan data. A new FD-OCT dispersion mismatch index, based on the line slope of the bright line in the A-line spectrogram, has been presented in the proposed method. With the new dispersion mismatch index, the proposed single A-line method can fast and visually detect the dispersion mismatch states of FD-OCT setup and perform the SSNd dispersion compensation, just using a single A-line. Experimental results of multiple samples demonstrated the advantages and convenience of the proposed method, and also proved that the proposed method can be used to analyze the relation of OCT imaging and optical path difference between the sample and reference arms.

Efficient method for the compensation of dispersion mismatch in the frequency-scanning interferometry

Absolute distance measurements could be achieved by the technique of frequency scanning interferometry (FSI), which could provide a micron-level accuracy over several tens of meters. However, as the scanning range and distance increasing, the measurement accuracy and resolution would be seriously influenced by the dispersion mismatch effect, which is mainly caused by the dispersion difference between optical fiber and air. A dispersion compensation method was proposed in this paper, and the mismatch effect would be suppressed effectively without complicated calculations for each single measurement. The chirp in resampling clock would be corrected with a pre-calibration procedure, and the compensation factor is demonstrated for different distances. A calibration method of dispersion compensation factor is also proposed, and the linear regression residual is employed as the criterion. Then, the compensation method is evaluated by experiments with different spatial distances and scanning ranges. Finally, a 60-m experiment is carried out for accuracy comparison, and the measurement accuracy is better than 15 µm by comparing with commercial interferometers.

A high-precision range extraction method using an FM nonlinear kernel function for DFB-array-based FMCW lidar

Frequency modulated continuous wave (FMCW) lidar is a high-precision absolute distance measurement technology in which the resolution is proportionate to the frequency modulation (FM) bandwidth of the light source. The distributed feedback (DFB) array exhibits advantages that meet FMCW lidar, such as a wide FM range, narrow linewidth, high FM speed, and low cost. However, the existing range extraction method for DFB array-based FMCW lidar has problems such as signal splicing error, residual FM nonlinearity, high sample rate requirement, and dispersion mismatch. Here, we propose and demonstrate an FM nonlinear kernel function-based range extraction method, which improves the measurement precision of DFB array-based FMCW lidar. The distance to a target located at 5 m is experimentally measured using this method, and a precision of 1.9 μm is obtained, which is much better than the existing range extraction method under the same conditions.

Refractive index characteristics of edible oils based on spectrometry and effects of oil dispersion on OCT

It is necessary to investigate the wavelength-dependent variation rules of the refractive index of edible oils so as to explore the specificity of the dispersion in light propagation, imaging, and interference processes among different types of edible oil products. In this study, by deriving the refractive index equations of the double glass sheet holding device and oil, the reflectance spectra of three different types of oil samples, namely, peanut oil, colza oil, and kitchen waste oil, were measured via a spectrometer. Furthermore, the refractive index model of these different types of oil samples was investigated. Additionally, based on the oil dispersion characteristics, the dispersion of oil in optical coherence tomography (OCT) was compensated via deconvolution. In the wavelength range of [Formula: see text] (380, 1500)[Formula: see text]nm, the analytical expressions of the double glass sheet holding device and oils are featured by practical reliability. The refractive indexes of three different types of oils [Formula: see text] (1.38, 1.52) show normal dispersion characteristics. The Cauchy coefficient matrix of the oil refractive index can be used for oil identification; in particular, the healthy oil and waste oil differ significantly in terms of the Cauchy coefficient matrix in the infrared band. Oil dispersion has almost no influence on the phase spectra of oils but can enhance their amplitude spectra. The dispersion mismatch can be eliminated by calculating the convolution kernel. The envelope broadening factors of OCT interference signals of oil products are 0.84, 0.64, and 0.91, respectively. According to the present research results, the refractive index model of oil can effectively remove the influence of the holding device. The refractive indexes of three different types of oil samples show similar wavelength-dependent variation characteristics, which confirms the existence of many correlated components in these oil samples. The established refractive index model of oil in a wide spectral range, from the ultraviolet to the infrared band, can be adequately employed for identifying different types of oils. The numerical dispersion compensation based on the established refractive index model can enhance the axial resolution in OCT imaging.

A theoretical model for the micron resolution spectral domain optical coherence tomography

The resolution and effective imaging depth (EID) of spectral domain optical coherence tomography (OCT) are of great importance for visualizing the fine details of biological tissues. To this end, a theoretical model for the micron resolution spectral domain OCT was proposed with both the temporal coherence and the longitudinal spatial coherence considered. Based on the model, the parameters that influence the axial resolution, the signal intensity and the EID can be quantitatively analyzed. Among all the parameters the focusing effect of objective lens affects the signal intensity most due to the fast broadening of the effective point spread function (PSF) incident on the photosensitive surface of detector. It limits the EID defined by the signal fall-off less than 500μm. Similar to the dispersion mismatch between the sample arm and the reference arm, the NA of objective lens does influence the axial resolution in the form of convolution. A large NA, such as 0.4, would lead to multiple split peaks and severe side lobes in the axial PSF. The axial resolution could even be reduced to 11.6μm, which is about ten times of the micron level resolution. For the EID defined by the micron axial resolution NA < 0.1 should be recommended, otherwise, the method to eliminate the effect of NA must be taken. Finally, the experimental results are presented to demonstrate the accuracy and effectiveness of our model.

Fourier domain quantum optical coherence tomography.

Quantum optical coherence tomography (Q-OCT) is the non-classical counterpart of optical coherence tomography (OCT), a high-resolution 3D imaging technique based on white-light interferometry. Because Q-OCT uses a source of frequency-entangled photon pairs, not only is the axial resolution not affected by dispersion mismatch in the interferometer but is also inherently improved by a factor of two. Unfortunately, practical applications of Q-OCT are hindered by image-scrambling artefacts and slow acquisition times. Here, we present a theoretical analysis of a novel approach that is free of these problems: Fourier domain Q-OCT (Fd-Q-OCT). Based on a photon pair coincidence detection as in the standard Q-OCT configuration, it also discerns each photon pair by their wavelength. We show that all the information about the internal structures of the object is encoded in the joint spectrum and can be easily retrieved through Fourier transformation. No depth scanning is required, making our technique potentially faster than standard Q-OCT. Finally, we show that the data available in the joint spectrum enables artefact removal and discuss prospective algorithms for doing so.

Detection and compensation of dispersion mismatch for frequency-domain optical coherence tomography based on A-scan's spectrogram.

Balanced dispersion between reference and sample arms is critical in frequency-domain optical coherence tomography (FD-OCT) to perform imaging with the optimal axial resolution, and the spectroscopic analysis of each voxel in FD-OCT can provide the metric of the spectrogram. Here we revisited dispersion mismatch in the spectrogram view using the spectroscopic analysis of voxels in FD-OCT and uncovered that the dispersion mismatch disturbs the A-scan's spectrogram and reshapes the depth-resolved spectra in the spectrogram. Based on this spectroscopic effect of dispersion mismatch on A-scan's spectrogram, we proposed a numerical method to detect dispersion mismatch and perform dispersion compensation for FD-OCT. The proposed method can visually and quantitatively detect and compensate for dispersion mismatch in FD-OCT, with visualization, high sensitivity, and independence from sample structures. Experimental results of tape and mouse eye suggest that this technique can be an effective method for the detection and compensation of dispersion mismatch in FD-OCT.

Temperature gradient engineered to maintain pulse shape and efficiency of a 532 nm picosecond second-harmonic generation

We introduced a theoretical model to preserve the quality and the efficiency of a picosecond second-harmonic generation (SHG) in a 30-mm-long LBO crystal which is surrounded by a thermal oven. It is proposed to arrange a temperature gradient throughout the crystal length for mitigating the undesirable consequences of dispersion and group velocity mismatch. To optimize the pulse shape, a distance parameter is introduced and in addition, the SHG efficiency and pulsewidth are included in a merit function. By setting the temperature gradient as the variable the merit function is optimized. The results show that for temperature gradient of 60 °C, an SHG pulse with mostly Gaussian shape with 80.7% efficiency and with 4.47 ps pulsewidth is attainable. Furthermore, the effect of the chirping on the SHG pulse is investigated. Final results have turned the proposed model into an excellent candidate for obtaining an efficient SHG conversion with an appreciable pulse quality.

Ultrahigh-sensitive optical coherence elastography

The phase stability of an optical coherence elastography (OCE) system is the key determining factor for achieving a precise elasticity measurement, and it can be affected by the signal-to-noise ratio (SNR), timing jitters in the signal acquisition process, and fluctuations in the optical path difference (OPD) between the sample and reference arms. In this study, we developed an OCE system based on swept-source optical coherence tomography (SS-OCT) with a common-path configuration (SS-OCECP). Our system has a phase stability of 4.2 mrad without external stabilization or extensive post-processing, such as averaging. This phase stability allows us to detect a displacement as small as ~300 pm. A common-path interferometer was incorporated by integrating a 3-mm wedged window into the SS-OCT system to provide intrinsic compensation for polarization and dispersion mismatch, as well as to minimize phase fluctuations caused by the OPD variation. The wedged window generates two reference signals that produce two OCT images, allowing for averaging to improve the SNR. Furthermore, the electrical components are optimized to minimize the timing jitters and prevent edge collisions by adjusting the delays between the trigger, k-clock, and signal, utilizing a high-speed waveform digitizer, and incorporating a high-bandwidth balanced photodetector. We validated the SS-OCECP performance in a tissue-mimicking phantom and an in vivo rabbit model, and the results demonstrated a significantly improved phase stability compared to that of the conventional SS-OCE. To the best of our knowledge, we demonstrated the first SS-OCECP system, which possesses high-phase stability and can be utilized to significantly improve the sensitivity of elastography.

Analysis of performance improvement in a swept source for optical coherence tomography applications

This study explores the development of high-speed swept source with a wide imaging range and instantaneous line width. For the swept source in the 1.3 μm band, the sweep rate is 193.25 kHz through the sampling buffer structure, the maximum tuning range is 142.34 nm. Meanwhile, the numerical model of the SS–OCT system is established and the signal in the OCT imaging process is numerically simulated. The influence of the dispersion mismatch point diffusion function on the imaging system is studied. Simulation results provide valuable guidance for the selection of optical components in the SS–OCT systems.

High-precision frequency sweeping interferometry for absolute distance measurement using a tunable laser with sweeping range of 88 GHz

Distance measurement using frequency sweeping interferometry is an absolute distance measurement technique that allows for high accuracy over long distances. Notwithstanding, the measurement accuracy is affected by laser sweeping nonlinearity and limited sweeping range. In this work, an optimized post-processing linearization method is demonstrated to realize high-accuracy arbitrary distance measurement using a laser with small modulation range. The interference signal is sparsely resampled to eliminate the influence of the sweeping nonlinearity, and the absolute distance is obtained by analyzing the phase of the resampled signal. In the measurement system, a high-finesse Fabry–Pérot cavity placed in vacuum is used as the measurement reference, so the effect of dispersion mismatch is negligible. Moreover, the distance measurement result is determined by the linear fit of the phase of each resampled point. Therefore, the influence of target vibration and other external random noise can be partially eliminated, and the reliability of the result is high. In the experiment, the sweeping range of the laser source is only 88 GHz. Comparing with a fringe-counting interferometer, the standard deviation of the residual errors is 34 µm within a distance of 6.7 m.