Swept Laser Research Articles

Improvement of layer fabrication efficiency and dilution rate for nickel based super alloy by multibeam directed-energy-deposition with blue diode lasers

A bead of nickel based super alloy was deposited on an SS304 substrate by a multibeam directed energy deposition method using a blue diode laser. Previously, the fabrication of nickel based super alloy using a blue diode laser had not been performed. Therefore, the laser output power and sweep speed were varied to investigate the effects of these variables on the bead. The results showed that laser output power and sweep speed affected the bead height, width, fabrication efficiency, and dilution rate in the fabrication of nickel based super alloy beads. In addition, we were able to produce a lower dilution than previously reported.

Sub-kHz-linewidth broadband-swept fiber laser based on Rayleigh scattering-enabled Brillouin random lasing oscillation with dynamic tunning Brillouin gain spectrum.

A sub-kHz-linewidth broadband-swept fiber laser using Rayleigh scattering-based Brillouin random lasing oscillation is proposed and experimentally demonstrated. Benefiting from Brillouin-involved acoustic damping and arbitrary-wavelength distributed Rayleigh feedback, leveraging instantaneously tuning Brillouin gain spectrum induced by a frequency-sweeping pump, a highly coherent random lasing emission with cavity mode elimination as well as frequency noise suppression is achieved in a sweeping manner. Results show that the proposed sweeping Stokes laser with a two-order-magnitude compressed linewidth of 840 Hz and 20 dB frequency noise suppression can unprecedentedly operate over the maximum wavelength range of 16 nm. Dynamic characteristics of the sweeping laser frequency are experimentally investigated, indicating a minimum residual nonlinearity of 0.0001 within the frequency-sweeping range of 126.63 GHz. It is believed that the proposed swept fiber laser may have attractive potential in diverse applications, including sensing and imaging.

Real‐Time Measurements of Photonic Microchips with Femtometer‐Scale Spectral Precision and Ultrahigh Sensitivity

AbstractPhotonic integrated circuits (PICs) are enabling breakthroughs in several areas, including quantum computing, neuromorphic processors, wearable devices, and more. Nevertheless, existing PIC measurement methods lack the spectral precision, speed, and sensitivity required for refining current applications and exploring new frontiers such as point‐of‐care or wearable biosensors. Here, the “sweeping optical frequency mixing method (SOHO)” is presented, surpassing traditional PIC measurement methods with real‐time operation, 30 dB higher sensitivity, and over 100 times better spectral resolution. Leveraging the frequency mixing process with a sweeping laser, SOHO excels in simplicity, eliminating the need for advanced optical components and additional calibration procedures. Its superior performance is demonstrated on ultrahigh‐quality factor (Q) fiber‐loop resonators (Q = 46 × 106), as well as microresonators, realized on a new optical waveguide platform. An experimental spectral resolution of 19.1 femtometers is demonstrated using an 85‐meter‐long unbalanced fiber Mach Zehnder Interferometer, constrained by noise resulting from the extended fiber length, while the theoretical resolution is calculated to be 6.2 femtometers, limited by the linewidth of the reference laser. With its excellent performance metrics, SOHO has the potential to become a vital measurement tool in photonics, excelling in high‐speed and high‐resolution measurements of weak optical signals.

Distributed Temperature Sensing through Network Analysis Frequency-Domain Reflectometry.

In this paper, we propose and demonstrate a network analysis optical frequency domain reflectometer (NA-OFDR) for distributed temperature measurements at high spatial (down to ≈3 cm) and temperature resolution. The system makes use of a frequency-stepped, continuous-wave (cw) laser whose output light is modulated using a vector network analyzer. The latter is also used to demodulate the amplitude of the beat signal formed by coherently mixing the Rayleigh backscattered light with a local oscillator. The system is capable of attaining high measurand resolution (≈50 mK at 3-cm spatial resolution) thanks to the high sensitivity of coherent Rayleigh scattering to temperature. Furthermore, unlike the conventional optical-frequency domain reflectometry (OFDR), the proposed system does not rely on the use of a tunable laser and therefore is less prone to limitations related to the laser coherence or sweep nonlinearity. Two configurations are analyzed, both numerically and experimentally, based on either a double-sideband or single-sideband modulated probe light. The results confirm the validity of the proposed approach.

Mitigating fast thermal instability by engineered laser sweep in AlN soliton microcomb generation

Transient thermal instability represents a significant challenge in generating soliton microcombs. Fast laser sweep can be an efficient method to mitigate thermal instability, but it requires an ultrahigh laser sweep rate for crystalline microresonators with fast thermal relaxation. Here, we engineer a laser sweep waveform to generate AlN-on-sapphire soliton microcombs with an intermediate sweep speed (<30 GHz/μs). Two laser sweep methods with backward plus forward tuning or two-step backward tuning added after the fast forward laser sweep were demonstrated to stabilize solitons. Reducing the soliton number is found to be useful to stabilize solitons in fast laser sweep. The effectiveness of the methods was numerically verified. Our measurements and simulations also reveal the impacts of different thermal relaxation processes occurring at quite different time scales on thermal instability. The requirement of the laser sweep protocols is discussed.

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.

The feasibility of using several soybean oil-based derivatives for asphalt binder modification: A preliminary study

This paper examined the modification effects of three soybean oil-based derivatives for use in asphalt binder modification. Functional group grafting, free radical polymerization of monomers, and reversible addition-fragmentation chain transfer polymerization (RAFT) were used to prepare three soybean oil-based derivatives, respectively. The test results of proton nuclear magnetic resonance spectroscopy (1H NMR), gel permeation chromatography (GPC), and Fourier-Transform Infrared (FTIR) proved that the three bio-derivatives were successfully prepared. Performance grading (PG), multiple stress creep recovery (MSCR), linear amplitude sweep (LAS), temperature-frequency sweep test and laser scanning confocal microscope (LSCM) tests were conducted to evaluate the modification effects of three derivatives on a base asphalt. The results indicated that poly (styrene-AESO) (PS-PAESO) reacted physically and chemically with the base asphalt binder during modification, and the development of a polymer elastic network improved the elasticity of base asphalt and thereby increased the rutting resistance and aging resistance. The incorporation of acrylated epoxidized soybean oil (AESO)and poly (acrylated epoxidized soybean oil) (PAESO)were found to improve the fatigue resistance of base asphalt, however, adversely affected its elasticity and high temperature performance. Additionally, it was found that PAESO with a higher molecular weight and polymer conversion might have the potential to ease the negative effects on high temperature performance. In general, three soybean oil-based derivatives have different emphases on asphalt modification, which could be used in various applications of asphalt in the paving industry with economic and environmental benefits.

Measurement of capillary waves with a laser wave recorder

Objectives. Capillary waves on the sea surface play an important role in remote sensing, both in the optical and microwave wavelength ranges. However, processes of electromagnetic radiation scattering on a rough sea surface cannot be studied in the absence of reliable monitoring of the parameters of these capillary waves under natural conditions. Therefore, the aim of the present work was to develop methods for such monitoring purposes and test them under laboratory and field conditions.Methods. Novel laser-based methods for recording capillary waves at frequencies up to 100 Hz were developed in the laboratory. The proposed remote methods, which do not interfere with the sea surface, are based on the recording of scattered laser radiation using a video camera.Results. Under laboratory conditions, spatial profiles, time dependences of heights for all points of a laser sweep trajectory, and frequency power spectra were obtained. It is shown that slopes in capillary waves can reach 30° and that the amplitude of capillary waves at frequencies above 25 Hz does not exceed 0.5 mm. A new version of a scanning laser wave recorder was tested under natural conditions on an offshore platform. The measurements confirmed the possibility of measuring the parameters of sea waves on spatial scales covering 3 orders of magnitude: from units of millimeters to units of meters.Conclusions. The developed wave recorder can be used to carry out direct measurements of “instantaneous” sea surface profiles with a time synchronization precision of 10-4 s and a spatial accuracy of better than 0.5 mm. The method makes it possible to obtain large series (21000) of «instantaneous» wave profiles with a refresh rate of 60 Hz, which opens up opportunities for studying the physics of wave evolution and the influence of wave parameters on the scattering of electromagnetic waves. The advantage of the method is the direct nature of the measurement of applicates and other wave characteristics not only in time but also in space. The entirely remote method does not distort the properties of the surface and is not affected by wind, waves, or sea currents. The possibility of using the proposed method under natural conditions at any time of the day and in a wide range of weather conditions has been experimentally ascertained.

Ultra-broadband wavelength-swept Ti:sapphire crystal fiber laser.

An ultra-broadband wavelength-swept laser (WSL) was generated using glass-clad Ti:sapphire crystal fiber as the gain media. Due to the low signal propagation loss of the crystal fiber, the swept laser has a tuning bandwidth of 250 nm (i.e., 683 nm to 933 nm) at a repetition rate of 1200 Hz. The steady-state and pulsed dynamics of the WSL were analyzed. The 0.018-nm instantaneous linewidth corresponds to a 3-dB coherence roll-off of 7 mm. When using the laser for swept-source optical coherence tomography, an estimated axial resolution of 1.8 µm can be achieved.

Dynamic Demodulation of Low-Finesse Fabry-Perot Sensors Based on Instantaneous Frequency Analysis

The optical path length (OPL) of the Fabry-Perot cavity may change during the spectrum acquisition; it introduces demodulationerror or even causes demodulation failure based on traditional algorithms. To address this problem, the relationship between the dynamic OPL and instantaneous frequency is investigated. Demodulation of the dynamic OPL based on instantaneous frequency analysis is proposed. By combining spectrogram (SP) and Wigner-Ville Distribution (WVD), an effective time-frequency (TF) analysis method is investigated to calculate the dynamic OPL at each sampling point in a period of a sweeping laser. Experimental results show that the demodulation speed is 4.8 MHz at a 2 KHz sweeping rate, the max deviation of a sinusoidal variation of OPL is around 0.14 μm, and the max deviation of a triangular variation of OPL is around 0.13 μm.

Vector of motion measurements in the living cochlea using a 3D OCT vibrometry system.

Optical coherence tomography (OCT) has become an important tool for measuring the vibratory response of the living cochlea. It stands alone in its capacity to measure the intricate motion of the hearing organ through the surrounding otic capsule bone. Nevertheless, as an extension of phase-sensitive OCT, it is only capable of measuring motion along the optical axis. Hence, measurements are 1-D. To overcome this limitation and provide a measure of the 3-D vector of motion in the cochlea, we developed an OCT system with three sample arms in a single interferometer. Taking advantage of the long coherence length of our swept laser, we depth (frequency) encode the three channels. An algorithm to depth decode and coregister the three channels is followed by a coordinate transformation that takes the vibrational data from the experimental coordinate system to Cartesian or spherical polar coordinates. The system was validated using a piezo as a known vibrating element that could be positioned at various angles. The angular measurement on the piezo was shown to have an RMSE of ≤ 0.30° (5.2 mrad) with a standard deviation of the amplitude of ≤ 120 pm. Finally, we demonstrate the system for in vivo imaging by measuring the vector of motion over a volume image in the apex of the mouse cochlea.

Avalanche Photodiodes With Composite Charge-Layers for Low Dark Current, High-Speed, and High-Power Performance

In this work, a novel In_0.52Al_0.48As based top-illuminated avalanche photodiode (APD) is demonstrated. By combining the composite charge-layer design with a special p-side up etched mesa structure to zero the electric (E)-field at the periphery of this APD's multiplication (M-) layer, the edge breakdown phenomenon can be eliminated. This in turn leads to the simultaneous high-speed, high-saturation-power, high responsivity, and low-dark current performance characteristics of our APDs, which are essential for high-performance coherent receiver applications. The demonstrated device, with its simple top-illuminated structure exhibits a wide optical-to-electrical (O-E) bandwidth (21 GHz), high responsivity (5.5 A/W at 0.9 V_br), and saturation current as high as 8 mA with a large active diameter of 24 μm for easy optical alignment. Furthermore, the nonlinear driving of a wavelength sweeping laser in the self-heterodyne beating setup can generate an optical pulse train like waveform, providing an effective optical modulation depth of up to 158%, which leads to a maximum photo-generated RF power (at 10 GHz) from our APD as high as +5.5 dBm. The excellent performance of our demonstrated APDs opens up new possibilities for the next generation of coherent receivers.

Dual-Comb Swept-Wavelength Interferometry: Theory and Experiment

Much efforts have been put to elaborate and improve different high precision measurement schemes for characterization of advanced photonic devices and optical fibers with increasing bandwidth requirements. In light of this, swept-wavelength interferometry and dual-comb spectroscopy have been extensively applied in characterization procedures. In this paper we present in detail an experimental scheme that combines these two techniques and overcomes their limitations by using a tunable laser source in order to sweep over the frequency comb spacing and capture all intermediate frequencies. We demonstrate full-field broadband measurements over 1.25 THz comb bandwidth with increased frequency resolution, which can be performed in only 5 ms sweep. We also show that the nonlinearity of the laser sweep can be removed without an auxiliary interferometer in the setup.

Optical Coherence Tomography for Diagnosis and Treatment of Corneal Diseases

Optical coherence tomography (OCT) is a modern non-contact real-time imaging of anterior and posterior eye’s segments. Based on the principle of low-coherence interferometry, it provides the analysis of tissue structures. In this review, we discuss technical aspects of two different OCT platforms: Time-Domain OCT (TD-OCT), and Spectral-Domain (SD-OCT) with the use of Fourier transformation — Fourier-Domain (FD-OCT). Over the last several years, the development of OCT has increased the resolution of images by different ways of optical filtering. The usage of tunable swept laser instead of superluminescent diode in modern Swept-Source OCT (SS-OCT) provides the sufficient quality of image. Anterior segment OCT can detect the pathological changes in different ocular structures and may be a key tool for monitoring of their progression. In pterygium OCT shows the true extent of fibrovascular granulation tissue in stromal layers, whereas in keratitis it provides imaging of corneal infiltration. Frequently, OCT-pachymetry can be advantageous in the detection of subclinical keratectasias. In patients with bullous keratopathy OCT can help to distinguish corneal edema and fibrosis. The new era of OCT is characterized by application of this method in keratorefractive surgery. Microscope-mounted (portable) and microscopeintegrated OCT systems have been developed. During LASIK surgery intraoperative OCT (iOCT) helps to assess flap interface, measure flap and residual bed thickness. It is a useful tool to reveal the structural changes during corneal collagen crosslinking. At the beginning of keratoplasty, iOCT helps to determine the corneal thickness, extent and depth of opacity, Descemet membrane perforation. iOCT guides decision-making regarding keratoplasty modification, depth and diameter of trephination. Furthermore, a real-time visualization of ocular structures during keratoplasty decreases the frequency of intra- and postsurgical complications.

FMCW lidar multitarget detection based on skeleton tree waveform matching.

Frequency-modulated continuous-wave lidar realizes 4D (three-dimensional space and velocity) imaging of the scene by emitting positive and negative frequency sweep laser signals. The premise of it is to identify the frequency points corresponding to the same target in the positive and negative sweep echo signals. For dechirp receiving, there is usually one peak in the frequency spectrum of the positive and negative sweep signals, respectively. Therefore, it is easy to identify and match the peaks. But in a complex environment, the laser beam will irradiate multiple targets at the same time. In addition, beam scanning and target motion cause the echo spectrum to broaden. The above reasons make it extremely difficult to identify and match peaks in practice. To solve this problem, the waveform-matching algorithm based on the skeleton tree is first applied to multitarget echo pairing. The basic idea of the algorithm is to quantify the target echo hierarchically to generate a skeleton tree. The generation of nodes is based on the relative amplitude of waveform peaks and reflects the characteristics of wave crests nesting. Then the similarity of the signal is determined by comparing the distance between the two signal waveform feature trees. Finally, the waveforms are matched in terms of similarity. To further substantiate the role of the proposed algorithm, imaging experiments and related comparative data for different targets have been completed. The results show that the accuracy of matching processed by the algorithm exceeds 90%, which is improved by about 50% compared with not using the algorithm for the target whose overlapping part accounts for a large proportion of itself.

Phase stable swept-source optical coherence tomography with active mode-locking laser for contrast enhancements of retinal angiography

Swept-source optical coherence tomography (SS-OCT) is an attractive high-speed imaging technique for retinal angiography. However, conventional swept lasers vary the cavity length of the laser mechanically to tune the output wavelength. This causes sweep-timing jitter and hence low phase stability in OCT angiography. Here, we improve an earlier phase-stabilized, akinetic, SS-OCT angiography (OCTA) method by introducing coherent averaging. We develop an active mode-locking (AML) laser as a high phase-stable akinetic swept source for the OCTA system. The phase stability of the improved system was analyzed, and the effects of coherent averaging were validated using a retina phantom. The effectiveness of the coherent averaging method was further confirmed by comparing coherently and conventionally averaged en face images of human retinal vasculature for their contrast-to-noise ratio, signal-to-noise ratio, and vasculature connectivity. The contrast-to-noise ratio was approximately 1.3 times larger when applying the coherent averaging method in the human retinal experiment. Our coherent averaging method with the high phase-stability AML laser source for OCTA provides a valuable tool for studying healthy and diseased retinas.

Time Domain Discrete Fourier Domain Mode Locked Laser With k-Space Uniform Comb Lines

Frequency comb swept lasers or wavelength stepped swept lasers with comb filters have been proposed in optical domain subsampling or circular-ranging optical coherence tomography (OCT) to achieve high axial scan rate and extended imaging range with a given acquisition bandwidth. However, the comb filters lack flexibility in varying the free spectral range (FSR). In this article, we propose and demonstrate a novel discrete Fourier domain mode locked (FDML) laser by using intracavity optical intensity modulation. Swept signals with comb lines uniformly distributed in the frequency domain is generated with tailor-made pulse patterns according to the sweep trace of the swept filter. Discrete k -space uniform swept signals with different FSRs and pulse durations are generated in the same laser. The FDML laser discretized by temporal modulation is more stable than that with a comb filter. The discrete FDML lasers will be potential sources of OCT and the fast-developing swept source LiDAR.

Feedback motion control of a spatial double pendulum manipulator relying on swept laser based pose estimation

Spatial pose estimation devices for mobile and cable-suspended robots have been rapidly developed. The HTC Vive sensor, which operates with swept laser beams, has aroused many researchers’ interest. We present experiments with a double pendulum robot equipped with the HTC Vive Tracker. A linear feedback controller ensured the tracking of pre-defined end-effector trajectories of various speeds. The pose feedback of the controller was provided by the HTC Vive. As a reference, the realized trajectory was measured by the OptiTrack motion capture system. We report that the motion control of a spatial double pendulum robot can be achieved by using a linear feedback controller together with cable winch and fan actuators providing six independent inputs. The accuracy of the HTC Vive was proved to be sufficient for the feedback position control of indoor mobile robots. We report that the error strongly correlates with the linear/angular velocity, the acceleration and the jerk.

High-Power and High-Responsivity Avalanche Photodiodes for Self-Heterodyne FMCW Lidar System Applications

In this work, we demonstrate the high-power and high-responsivity performance of the dual multiplication (M-) layers in In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.52</sub> Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.48</sub> As based avalanche photodiode (APD). The dual M-layer design in our APD structure effectively constrains the multiplication process to a thin high-field region rather than the whole thick M-layer. It thus minimizes the space charge effect (SCE) within and avoids increasing the tunneling dark current for the case of directly shrinking M-layer thickness in APD. Furthermore, by combining the specially designed mesa shape with this dual M-layer structure, the edge breakdown can be well suppressed. These benefits lead to an ultra-high gain-bandwidth product (450 GHz; 1 A/W at unit gain) and a high saturation current (>12 mA) can be simultaneously achieved in our device. By nonlinearly driving a wavelength sweeping laser in the self-heterodyne lidar setup, it can generate an optical pulse train-like waveform, providing an effective optical modulation depth of 200% to feed into our demonstrated APD at the receiver-end. Under such scheme, the photo-generated RF (1 GHz) power from our APD with a 6.3 A/W responsivity can be as high as +6.95 dBm at a high (7 mA) output photocurrent. Such high-power and high-responsivity characteristics of our APD can further improve the signal-to-noise (S/N) ratio and dynamic range performances in each pixel of the lidar image. A high-quality 3-dimensional (D) FMCW lidar image is constructed based on our APD, without the integration of any electrical amplifier at the receiver end.

PID 제어를 이용한 파장 스위핑 레이저의 스위핑 선형화

PID 제어를 이용한 파장 스위핑 레이저의 스위핑 선형화