Laser Coherence Length Research Articles

Parallel indirect time-of-flight ranging using on-chip dual-frequency combs

The significant advancements in autonomous vehicle applications demand detection solutions capable of swiftly recognizing and classifying objects amidst rapidly changing and low-visibility conditions. Light detection and ranging (LiDAR) has emerged as a robust solution, overcoming challenges associated with camera imaging, particularly in adverse weather conditions or low illumination. Rapid object recognition is crucial in dynamic environments, but the speed of conventional LiDARs is often constrained by the 2D scanning of the laser beam across the entire scene. In this study, we introduce a parallelization approach for the indirect time-of-flight (iToF) ranging technique. This method enables efficient and high-speed formation of 1D clouds, offering the potential to have extended range capabilities without being constrained by the laser coherence length. The application potential spans mid-range autonomous vehicles ranging to high-resolution imaging. It utilizes dual-frequency combs with slightly different repetition rates. The method leverages the topology of the target object to influence the phase of the beating signal between the comb lines in the RF domain. This approach enables parallel ranging in one direction, confining the scanning process to a single dimension, and offers the potential for high-speed LiDAR systems. A tri-comb approach will be discussed that can provide an extended unambiguous range without compromising the resolution due to the range–resolution trade-off in iToF techniques. The study starts by explaining the technique for parallel detection of distance and velocity. It then presents a theoretical estimation of phase noise for dual combs, followed by an analysis of distance and velocity detection limits, illustrating their maximum and minimum extents. Finally, a study on the mutual interference conditions between two similar LiDAR systems is presented, demonstrating the feasibility of designing simultaneously operating LiDARs to avoid mutual interference.

Adaptive carrier-phase-noise-canceled LiDAR for range-Doppler imaging beyond hundreds of laser coherence length.

Carrier-phase noise limits both the performance and the maximum operation range of coherent LiDAR. To address this issue, we propose a carrier-phase-noise-canceled LiDAR based on an auxiliary interferometer and adaptive filters. Compared to previous methods, this approach is calibration-free and offers higher compensation accuracy, as well as applicability of dynamic target detection. Experiments of range-Doppler imaging for stationary targets and rotating extended targets have been performed, and the detection results close to the theoretical resolution were obtained at the round trip distance to the target beyond 981 times and 106 times coherence length, respectively.

Equal-intensity beam splitter realization by wire grid polarizers for passive laser speckle reduction.

A method to realize an equal-intensity beam splitter (EIBS) using wire grid polarizers (WGPs) is proposed. The EIBS consists of WGPs with predetermined orientations and high-reflectivity mirrors. We demonstrated the generation of three laser sub-beams (LSBs) with equivalent intensities using EIBS. The three LSBs were incoherent by introducing optical path differences larger than the laser coherence length. The LSBs were used to reduce speckle passively, where the objective speckle contrast was reduced from 0.82 to 0.5 when all three LSBs were used. The feasibility of EIBS in speckle reduction was studied using a simplified laser projection system. The structure of the EIBS implemented by WGPs is simpler than EIBSs obtained by other methods.

Large field Digital Image Plane Holography with a double cavity high speed laser

The three velocity components in a fluid plane can be measured by applying Digital Image Plane Holography. This technique is limited by the laser coherence length, which reduces its application with high speed lasers that, generally, have a very short coherence length. In addition, the use of a double cavity can also imply a small wavelength difference between the two laser beams. In this work, we present an improved Optical Path Length Enlarging Device that allows the velocity measurement, in a 2D field whose width is four times larger than the laser coherence length. The optical set-up and the procedure for measuring in a larger field (ten times the laser coherence length) were optimized, and the issues derived from the laser spatial and temporal coherence and wavelength changes were analyzed and solved. Digital Image Plane Holography with the Optical Path Length Enlarging Device and Particle Image Velocimetry were applied for measuring the whole velocity field in the central plane of a cylindrical cavity with a rotating lid, for two Reynolds numbers (800 and 2000), showing both of them a very good agreement with the numerical simulations.Graphical abstract

Fast diffuse correlation spectroscopy with a low-cost, fiber-less embedded diode laser.

Diffuse correlation spectroscopy (DCS), a popular optical technique for fast noninvasive measurement of blood flow, is commonly implemented using expensive fiber-coupled long coherence length laser systems. Here, we report the development of a portable and fiber-less approach that can be used as a low-cost alternative to illuminate tissue in DCS instruments. We validate the accuracy and noise characteristics of the fiber-less DCS laser source, by comparisons against traditional DCS light sources, with experiments on controlled tissue-simulating phantoms and in humans.

Large-scale range diffraction grating displacement sensor based on polarization phase-shifting.

A method is proposed and demonstrated to improve a diffraction grating displacement sensor to simultaneously achieve nanometer-level resolution and an extended range of operation. The method exploits the polarization phase-shifting optical path to extract two sinusoidal signals with a quadrature phase shift. The interpolation circuit is applied to nonlinearly convert two sinusoidal signals into a standard incremental AB quadrature digital signal, implementing an extended operation range with the magnitude of a laser coherence length. This work enables displacement measurement operated at large-scale range, and provides a significant guide for the design of a high performance micro-displacement sensor.

Phase-Noise-Compensated OFDR Realized Using Hardware-Adaptive Algorithm for Real-Time Processing

A hardware-adaptive algorithm is proposed for phase-noise-compensated optical frequency domain reflectometry (PNC-OFDR) and the corresponding hardware system is designed to realize the real-time processing. The system not only keeps a high measurement performance with spatial resolution and measurement range of traditional PNC-OFDR, but also improves the processing efficiency, which was the main drawback in traditional PNC-OFDR. In the proof-of-concept experiment, a 7-cm spatial resolution is achieved over 100-km fiber link, which is much beyond the laser coherence length. By theoretical analysis, even at the “worst compensation point,” the signal-to-niose ratio is almost 40 dB. In the hardware-processing based system, a 7-cm spatial resolution is achieved over 37.5-km fiber link, in which the limited range is restricted by the register capacity available in the hardware we used. We believe the system is suitable for practical applications because of the real-time processing capability.

Time domain diffuse correlation spectroscopy: modeling the effects of laser coherence length and instrument response function.

Diffuse correlation spectroscopy (DCS) is an optical technique that non-invasively quantifies an index of blood flow (BFi) by measuring the temporal autocorrelation function of the intensity fluctuations of light diffusely remitted from the tissue. Traditional DCS measurements use continuous wave (CW) lasers with coherence lengths longer than the photon path lengths in the sample to ensure that the diffusely remitted light is coherent and generates a speckle pattern. Recently, we proposed time domain DCS (TD-DCS) to allow measurements of the speckle fluctuations for specific path lengths of light through the tissue, which has the distinct advantage of permitting an analysis of selected long path lengths of light to improve the depth sensitivity of the measurement. However, compared to CW-DCS, factors including the instrument response function (IRF), the detection gate width, and the finite coherence length need to be considered in the model analysis of the experimental data. Here we present a TD-DCS model describing how the intensity autocorrelation functions measured for different path lengths of light depend on the coherence length, pulse width of the laser, detection gate width, IRF, BFi, and optical properties of the scattering sample. Predictions of the model are compared with experimental results using a homogeneous liquid phantom sample that mimics human tissue optical properties. The BFis obtained from the TD-DCS model for different path lengths of light agree with the BFi obtained from CW-DCS measurements, while the standard simplified model underestimates the BFi by a factor of ∼2. This Letter establishes the theoretical foundation of the TD-DCS technique and provides guidance for future BFi measurements in tissue.

Development and characterization of a multidistance and multiwavelength diffuse correlation spectroscopy system.

.This paper presents a multidistance and multiwavelength diffuse correlation spectroscopy (DCS) approach and its implementation to simultaneously measure the optical proprieties of deep tissue as well as the blood flow. The system consists of three long coherence length lasers at different wavelengths in the near-infrared, eight single-photon detectors, and a correlator board. With this approach, we collect both light intensity and DCS data at multiple distances and multiple wavelengths, which provide unique information to fit for all the parameters of interest: scattering, blood flow, and hemoglobin concentration. We present the characterization of the system and its validation with phantom measurements.

Step-by-step guide to reduce spatial coherence of laser light using a rotating ground glass diffuser.

Wide field-of-view imaging of fast processes in a microscope requires high light intensities motivating the use of lasers as light sources. However, due to their long spatial coherence length, lasers are inappropriate for such applications, as they produce coherent noise and parasitic reflections, such as speckle, degrading image quality. Therefore, we provide a step-by-step guide for constructing a speckle-free and high-contrast laser illumination setup using a rotating ground glass diffuser driven by a stepper motor. The setup is easy to build, cheap, and allows a significant light throughput of 48%, which is 40% higher in comparison to a single lens collector commonly used in reported setups. This is achieved by using only one objective to collect the scattered light from the ground glass diffuser. We validate our setup in terms of image quality, speckle contrast, motor-induced vibrations, and light throughput. To highlight the latter, we record Brownian motion of micro-particles using a 100× oil immersion objective and a high-speed camera operating at 2000 Hz with a laser output power of only 22 mW. Moreover, by reducing the objective magnification to 50×, sampling rates up to 10,000 Hz are realized. To help readers with basic or advanced optics knowledge realize this setup, we provide a full component list, 3D-printing CAD files, setup protocol, and the code for running the stepper motor.

In vivo photoacoustic imaging of blood vessels using a homodyne interferometer with zero-crossing triggering.

We demonstrate a quasinoncontact photoacoustic imaging method using a homodyne interferometer with a long coherence length laser. The generated photoacoustic signal is detected by a system that is locked at its maximum sensitivity through the use of balanced detection and zero-crossing triggering. The balanced detector is substantially equalized, so its output is zero when the system reaches the maximum sensitivity. The synchronization approach is used to trigger the excitation and detection of the photoacoustic signal. The system is immune to ambient vibrations. A thin water layer on the sample surface is used to reduce the effect of the rough tissue surface. The performance of the system is demonstrated by in vivo imaging of the microvasculature in mouse ears.

Long-range measurement of Rayleigh scatter signature beyond laser coherence length based on coherent optical frequency domain reflectometry

Long-range C-OFDR measurement of fiber Rayleigh scatter signature is described. The Rayleigh scatter signature, which is an interference pattern of backscatters from the random refractive indices in fibers, is known to be applicable to fiber identification and temperature or strain sensing by measuring its repeatability and its spectral shift. However, these applications have not been realized at ranges beyond the laser coherence length since laser phase noise degrades its repeatability. This paper proposes and demonstrates a method for analyzing the optical power spectrum of local Rayleigh backscatter to overcome the limitation imposed by laser phase noise. The measurable range and spatial performance are also investigated experimentally with respect to the remaining phase noise and noise reduction by signal averaging with the proposed method. The feasibility of Rayleigh scatter signature measurement for long-range applications is confirmed.

Spatial Resolution Improvement in OFDR Using Four Wave Mixing and DSB-SC Modulation

Spatial resolution in optical frequency domain reflectometry (OFDR) is a function of frequency sweep span. In this letter, a new method to enhance OFDR spatial resolution is proposed, by increasing the applied sweep span. The proposed method uses degenerated four wave mixing (FWM) with only a single light source, and this results in a preserved coherence length. The double sideband with suppress carrier (DSB-SC) modulation is used to create two sidebands, with each sideband treated as a separate light source. It is demonstrated by means of experiments that the proposed method can increase the spatial resolution by three times. Furthermore, it is also confirmed that DSB-SC modulation and FWM can be used to cancel the laser phase noise in OFDR and detect break point beyond laser coherence length with three times higher spatial resolution.

Fourier transform-limited optical frequency-modulated continuous-wave interferometry over several tens of laser coherence lengths.

We report on a versatile optical frequency-modulated continuous-wave interferometry technique that exploits wideband phase locking for generating highly coherent linear laser frequency chirps. This technique is based on an ultra-short delay-unbalanced interferometer, which leads to a large bandwidth, short lock time, and robust operation even in the absence of any isolation from environmental perturbations. In combination with a digital delay-matched phase error compensation, this permits the achievement of a range window about 60 times larger than the intrinsic laser coherence length with a 1.25 mm Fourier transform-limited spatial resolution. The demonstrated configuration can be easily applied to virtually any semiconductor laser.

Swept Source OCT Beyond the Coherence Length Limit

We present a simple modification to a swept source optical system that overcomes the limitations in the depth range associated with the short coherence length of fast swept lasers. It is based on a wavelength dependent delay induced by a diffraction grating pair with negative dispersion incorporated in a reference arm of a Mach–Zehnder optical coherence tomography interferometer. The experimental depth range in air obtained was as large as 1.6 cm for a source coherence length of 4 mm.

傅里叶望远镜激光发射系统性能分析

The performance of Fourier telescope transmitting system directly affects the image resolution and quality. The parameters and performance requirements of laser are analyzed. The design outline of fiber laser is presented. Meanwhile, the point is put forward that the laser coherence length need to be at least 1.6 times of the residual optical path. The stability of laser power and frequency is simulated. To ensure the stability of the laser, some methods are put forward from the points of design, algorithm and practice. Furthermore, combined with the results of simulation and experiment, the influences of the transmitter performance on image quality is analyzed, from the aspects of transmitting aperture layout, aperture amount, position precision and the beam pointing error. And point out that baseline redundancy, image resolution, target spectrum and image quality are the factors should be synthetically considered when arranging transmitter array. The formula for calculating the number of transmitting apertures is obtained based on the two-dimensional sampling theorem and the statistical analysis results. The transmitting aperture number is calculated for the Fourier telescope whose resolution would be 5 cm for objects in 1000 km low earth orbit. In addition, it is concluded that the transmitting aperture position error should be less than 5% of the minimum aperture spacing. ©, 2015, Chinese Optical Society. All right reserved.

Compensation of phase error in optical frequency-domain reflectometry using delay-matched sampling

We proposed and experimentally demonstrated a delay-match sampling method to measure and compensate the laser phase error in optical frequency-domain reflectometry system. By using the error signal extracted from a simple auxiliary Mach-Zehnder interferometer with only a 10-ns delay, the laser phase error is effectively compensated. Considerable improvement is achieved in spatial resolution from 200 m to 7 cm at a measurement distance over 10 times the round-trip laser coherence length.

Incoherent optical frequency domain reflectometry based on a Kerr phase-interrogator.

We present a novel approach for incoherent optical frequency-domain reflectometry based on a frequency-swept sinusoidal optical signal and a Kerr phase-interrogator. The novel approach eliminates dependence on the laser coherence-length allowing for long-range operation. Long-range detection of reflection points as far as 151 km at a spatial-resolution of 11.2 cm is experimentally demonstrated.

Quantitative, depth-resolved determination of particle motion using multi-exposure, spatial frequency domain laser speckle imaging

Laser Speckle Imaging (LSI) is a simple, noninvasive technique for rapid imaging of particle motion in scattering media such as biological tissue. LSI is generally used to derive a qualitative index of relative blood flow due to unknown impact from several variables that affect speckle contrast. These variables may include optical absorption and scattering coefficients, multi-layer dynamics including static, non-ergodic regions, and systematic effects such as laser coherence length. In order to account for these effects and move toward quantitative, depth-resolved LSI, we have developed a method that combines Monte Carlo modeling, multi-exposure speckle imaging (MESI), spatial frequency domain imaging (SFDI), and careful instrument calibration. Monte Carlo models were used to generate total and layer-specific fractional momentum transfer distributions. This information was used to predict speckle contrast as a function of exposure time, spatial frequency, layer thickness, and layer dynamics. To verify with experimental data, controlled phantom experiments with characteristic tissue optical properties were performed using a structured light speckle imaging system. Three main geometries were explored: 1) diffusive dynamic layer beneath a static layer, 2) static layer beneath a diffuse dynamic layer, and 3) directed flow (tube) submerged in a dynamic scattering layer. Data fits were performed using the Monte Carlo model, which accurately reconstructed the type of particle flow (diffusive or directed) in each layer, the layer thickness, and absolute flow speeds to within 15% or better.

Nanostructuring Thin Polymer Films with Optical Near Fields

In the present work, we report on the application of optical near fields to nanostructuring of poly(trimethylene terephthalate) (PTT) thin films. By exposure to a single ultraviolet nanosecond laser pulse, the spatial intensity modulation of the near-field distribution created by a silica microsphere is imprinted into the films. Setting different angles of incidence of the laser, elliptical or circular periodic ring patterns can be produced with periods as small as half the laser wavelength used. These highly complex patterns show optical and topographical contrast and can be characterized by optical microscopy (OM) and atomic force microscopy (AFM). We demonstrate the key role of the laser wavelength and coherence length in achieving smooth, extended patterns in PTT by using excimer laser (193 nm) and Nd:YAG laser (266 nm) pulses. Reference experiments performed in Ge2Sb2Te5 (GST) demonstrate that nanopatterning in PTT is triggered by ablation as opposed to GST, in which nanopatterning originates from laser-induced phase change, accompanied by a small topographical contrast. The experiments presented in this work demonstrate the suitability of optical near fields for structuring polymer films, opening up new possibilities for nanopatterning and paving the way for potential applications where optical near fields and polymer nanostructures are involved.