Low-cost Laser Sources Research Articles

Exploring single-mode VCSEL wavelength tuning for low-cost 3D optical coherence tomography and OCT angiography.

Low-cost optical coherence tomography has recently emerged as a growing field due to the increased need for general availability of OCT devices outside of the clinics. One of the main obstacles in creating low-cost SS-OCT systems is the price of the laser. In this work, we study the influence of different tuning parameters (e.g., frequency, duty cycle, modulation curve, temperature) on the resulting bandwidth of the previously proposed low-cost single-mode thermally-tunable vertical-cavity surface-emitting laser (VCSEL) source at 850 nm. With optimal parameters, the laser achieves a tuning bandwidth of 10.2 nm at a 50 kHz A-scan rate. In addition, we show the first 3D rendered volume scans of both anterior and posterior segment using a custom VCSEL-based low-cost OCT setup. With the help of deep-learning-based denoising, it was possible to critically reduce the noise in single scans. Moreover, by investigating the phase stability, it became apparent that phase stability between sweeps increases with rising modulation frequencies, making the auxiliary interferometer obsolete. Thus, the system's 50 kHz tuning regimen is also suitable for functional extensions such as OCT angiography.

Tunable wide spacing all-fiber dual-wavelength holmium-doped fiber laser using a Lyot filter operating above 2 µm

In this work, we successfully demonstrated an all-fiber, dual-wavelength holmium-doped fiber laser (HDFL) with a tunable wavelength spacing range of 8 nm to 24 nm. The HDFL utilizes a Lyot filter to obtain a stable dual wavelength output, with the Lyot filter being constructed from a polarizer, a 20 m polarizing maintaining fiber (PMF) and two polarization controllers (PCs). The dual-wavelength HDFL has an operating wavelength range from 2072.1 nm to 2097.2 nm and a maximum spacing of 24 nm between the lasing wavelengths as well as a signal-to-noise ratio of up to 45 dB and a side mode suppression ratio as low as 0.15 dB. The HDFL is highly stable, with the lasing output have a low power fluctuation of only 2.3 dB over a continuous operation period of 75 min. The proposed HDFL would have substantial applications as a low cost laser source for sensing applications beyond the 2 µm region, particularly those that require a control signal.

High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes, enabling multi-dimensional quantum control of light fields. Until now, generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration, high power, high vortex order, and a low cost and compact laser source has been very challenging due to the limitations of available generation methods. Here, we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques. This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order, which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator. The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and <650 fs, respectively. In particular, 424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W, several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

60 nm Span Wavelength-Tunable Vortex Fiber Laser with Intracavity Plasmon Metasurfaces

Wavelength-tunable vortex fiber lasers that could generate beams carrying orbital angular momentum (OAM) hold great attention in large-capacity optical communications. The wavelength tunability of conventional vortex fiber lasers is, however, limited by a range of ∼35 nm due to narrow bandwidth and/or insertion loss of mode conversion components. Optical metasurfaces apart from being compact planar components can flexibly manipulate light with high efficiency in a broad wavelength range. Here, we propose and demonstrate for the first time, to the best of our knowledge, a metasurface-assisted vortex fiber laser that can directly generate OAM beams with customizable topological charges. Due to the designed broadband gap-surface plasmon metasurface, combined with an intracavity tunable filter, the laser enables an OAM beam with a center wavelength continuously tunable from 1015 to 1075 nm, nearly twice of other vortex fiber lasers ever reported. The metasurface can be designed at will to satisfy requirements for either low pump threshold or high slope efficiency of the laser. Furthermore, the cavity-metasurface configuration can be extended to generate higher-order OAM beams or more complex structured beams in different wavelength regions, which greatly broadens the possibilities for developing low-cost and high-quality structured-beam laser sources.

A novel fast-qualitative balance test method of screening for vestibular disorder patients

Body balance test is one of the methods of assessing vestibular level. However, the results are still qualitative, depending on the subjectivity of the doctor. This study proposes a new, low-cost method to quantitatively determine the degree of body imbalance. The proposal includes a low-cost laser source, a proposed rectangular paper frame, a camera, and a computer. The rectangular frame is mounted on the patient. The laser source is fixed and projected onto this rectangular frame. The laser projection point is taken as the origin point to evaluate the movement of the frame, which is also the movement of the patient’s body. This rectangular frame is pre-marked with points to get more accuracy of the position of the laser point. Therefore, this measurement is not affected by the position of the camera during recording. The video is then procecced by computer to determine the position of laser point, it is also presented the movement of the patient’s body. Initial trials were conducted on vestibular and normal patients. The results show that there is a clear difference in the balance of the vestibular and healthy people. The proposed method can be used to support quantitative screening for vestibular disease.

Generation of 64-fs L-band stretched pulses from an all-fibre Er-doped laser.

We demonstrate an L-band all-fibre erbium-doped laser mode locked by nonlinear polarisation rotation and working in the stretched-pulse regime. The use of a single segment of gain fibre with appropriate length and dispersion and a Brewster fibre grating optimised for the L band as an in-fibre polariser enables the generation of pulses at 1.59-μm central wavelength, which can be linearly compressed to 64-fs duration. Numerical simulations of the laser model support our experimental findings. Our laser design gives a route towards low-cost and low-complexity fibre-integrated laser sources for applications requiring L-band ultrashort pulses.

Short-Wave Infrared Confocal Fluorescence Imaging of Deep Mouse Brain with a Superconducting Nanowire Single-Photon Detector

Optical microscopy is a valuable tool for in vivo monitoring of biological structures and functions because of its noninvasiveness. However, imaging deep into biological tissues is challenging due to the scattering and absorption of light. Previous research has shown that the two optimal wavelength windows for high-resolution deep mouse brain imaging are around 1300 and 1700 nm. However, one-photon fluorescence imaging in the wavelength region has been highly challenging due to the poor detection efficiency of currently available detectors. To fully utilize this wavelength advantage, we demonstrated here one-photon confocal fluorescence imaging of deep mouse brains with an excitation wavelength of 1310 nm and an emission wavelength within the 1700 nm window. Fluorescence emission at 1700 nm was detected by a custom-built superconducting nanowire single-photon detector (SNSPD) optimized for detection between 1600 nm and 2000 nm with low detection noise and high detection efficiency. With the PEGylated quantum dots and SNSPD both positioned at the optimal imaging window for deep tissue penetration, we demonstrated in vivo one-photon confocal fluorescence imaging at approximately 1.7 mm below the surface of the mouse brain, through the entire cortical column and into the hippocampus region with a low-cost continuous-wave laser source and low excitation power. We further discussed the significance of the staining inhomogeneity in determining the depth limit of one-photon confocal fluorescence imaging. Our work may motivate the further development of long wavelength fluorescent probes, and inspire innovations in high-efficiency, high-gain, and low-noise long wavelength detectors for biological imaging.

Broadband Continuously Tunable All-Fiber Laser Based on OPG for CARS Imaging

A broadband continuously tunable SESAM all-fiber laser based on optical parametric generation (OPG) for coherent anti-stokes Raman scattering (CARS) imaging applications is presented in this article. This laser structure is built on all-polarization maintaining fibers yielding high environmental stability. The OPG is based on PM photonic crystal fiber as parametric gain pumped by a wavelength-tunable (1025 to 1055 nm) and repetition rate-tunable (3 MHz to 500 KHz) Yb-doped pulsed fiber laser. The all-fiber structure features free polarization adjustment in slow-axis tunable operation with a wavelength range from 770 to 940 nm for signal radiation and 1225 to 1510 nm for idler radiation. The pump and signal pulses (CARS-Stokes and CARS-pump respectively) are intrinsically overlapped, both spatially and temporarily, due to the four-wave mixing (FWM) generation and emitted from a single fiber end. The CARS-Stokes and CARS-pump pulse duration vary between 25 to 32 ps and 21 to 24 ps, respectively in the whole band, ensuring a good overlap of the generated signals, and both exhibit a Gaussian profile with high spatial beam quality. The frequency conversion allows to obtain a frequency difference between the generated signals from 1100 to 3300 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> - 1</sup> . The reported low-cost laser source is ideal for bio-imaging applications, especially in CARS imaging, due to its compactness, simplicity, robustness and ease of handling. A patent has been filed based on this technology.

Nonlinear Optical Properties of Ag Nanoplates Plasmon Resonance and Applications in Ultrafast Photonics

Metal nanomaterials have attracted increasing attention due to their outstanding nonlinear optical and photonic properties, making them as potential saturable absorber (SA) candidates for realizing ultrafast photonic devices. In this article, we demonstrate the generation of mode-locked dual-wavelength pulse trains in an Ag nanoplates (AgNPTs)-based Yb-doped all-fiber laser for the first time to the best of our knowledge. The AgNPTs are synthesized by seed-mediated growth and then integrated into a fiber ferrule by optical deposition, which serve as SA in the ring laser cavity. The plasmonic properties of such nanoplates are measured by absorption spectrophotometry and their nonlinear optical properties are characterized by Z-scan. The measured nonlinear saturable absorption of the AgNPTs-based SA is 6.4%. In our laser, the dual-wavelength synchronous mode-locking is achieved at the center wavelengths of 1031.92 and 1033.24 nm with 3-dB spectral bandwidth of 0.52 and 0.46 nm, respectively, where 293-ps pulse trains with a repetition rate of 11.43 MHz are obtained at the pump power of 350 mW. The results demonstrate that the solution-processed AgNPTs are promising SA candidates for achieving stable and low-cost pulsed laser sources which could be used for spectroscopy and ultrafast photonics.

Modelling Neural Dynamics with Optics: A New Approach to Simulate Spiking Neurons through an Asynchronous Laser

In this paper, we propose a novel approach for implementing spiking neurons through an optical system. Spiking neurons are a new approach to emulate the neural processes that occur in the brain, known as the third generation of artificial neural networks. They have been successfully used to build a new technology called neuromorphic engineering, which looks for a better performance than traditional computing in tasks usually performed by AI-based systems. Our optical system consists of a low-cost laser source, based on a microcontroller and a continuous-wave laser diode; the microcontroller allows producing synchronous or asynchronous pulses with complex time profiles. Here, through said system we have successfully reproduced most of the neural dynamics observed in biological neurons. These dynamics have been reproduced using a very simple optical array with a great potential for the development of neuromorphic systems. The optical system has been experimentally validated.

Compact actively Q-switched laser for sensing applications

Current active Q-switching is mainly generated by acousto-optical modulators (AOMs) or electro-optical modulators (EOMs). However, EOMs make the setup bulky, while AOMs require the radiofrequency (RF) signal and water-cooling. We present a novel approach to active Q-switching of an integrated waveguide chip laser by utilizing a deformed helix ferroelectric liquid crystal cell (6mm×6mm) with a thickness of 2 mm. This device can be directly integrated onto the Ytterbium-doped waveguide chip and controlled by the electrical signal. In experiments, with an electrical signal with a pulse duration of 2 μs and a magnitude from 10 V to 84 V, the Q-switched lasers can be observed at a repetition rate ranging from 0.1 kHz to 20 kHz. The shortest pulse duration and the maximum slope efficiency is 38 ns and 22% respectively. This novel integrated and low-cost laser source is a promising tool for a broad range of applications such as gas sensing and LIDAR.

Tailored wetting of copper using precise nanosecond direct laser interference patterning

Nowadays, functionalized surfaces become more and more important for industrial applications concerning wettability, ice-repellency, self-cleaning, or tribology. In this contribution, an innovative approach is presented using the direct laser interference patterning (DLIP) technique for tailoring the wettability of copper surfaces. The interference of the laser beams was conducted by using prisms for forming a like-like pattern with a spatial period Λ of 3.3 µm. Moreover, texture depths up to 1.01 µm were reached. The wettability could be set to static water contact angles between 120° and 150° This paper gives inside into the viability of structuring copper materials using a low-cost nanosecond laser source in combination with an innovative beam shaping setup.

Carbon nanotube-synchronized dual-color fiber laser for coherent anti-Stokes Raman scattering microscopy.

In this Letter, we present a passively synchronized dual-color fiber laser at 1.03 and 1.53 µm for coherent anti-Stokes Raman scattering (CARS) microscopy. This fiber laser consists of both Yb- and Er-doped laser cavities, combined by an ∼1.2m common branch with a single-walled carbon-nanotube-based saturable absorber. Stable passively synchronized mode-locked state is obtained within a total cavity length mismatch of 50 µm. We demonstrate the capability of the synchronized dual-color fiber laser for CARS spectroscopy in both low- and high-wavenumber regions, with a resolution of 6.6cm-1. Furthermore, a CARS image in the field of view 150×60µm2 of polystyrene at the Raman shift of 3041cm-1 has also been achieved. The results show the feasibility of our passively synchronized dual-color fiber laser to be employed as a stable and low-cost ultrafast laser source for CARS.

Optical-resolution photoacoustic microscopy with ultrafast dual-wavelength excitation.

Fast functional and molecular photoacoustic microscopy requires pulsed laser excitations at multiple wavelengths with enough pulse energy and short wavelength-switching time. Recent development of stimulated Raman scattering in optical fiber offers a low-cost laser source for multiwavelength photoacoustic imaging. In this approach, long fibers temporally separate different wavelengths via optical delay. The time delay between adjacent wavelengths may eventually limits the highest A-line rate. In addition, a long-time delay in fiber may limit the highest pulse energy, leading to poor image quality. In order to achieve high pulse energy and ultrafast dual-wavelength excitation, we present optical-resolution photoacoustic microscopy with ultrafast dual-wavelength excitation and a signal separation method. The signal separation method is validated in numerical simulation and phantom experiments. We show that when two photoacoustic signals are partially overlapped with a 50-ns delay, they can be recovered with 98% accuracy. We apply this ultrafast dual-wavelength excitation technique to in vivo OR-PAM. Results demonstrate that A-lines at two wavelengths can be successfully separated, and sO2 values can be reliably computed from the separated data. The ultrafast dual-wavelength excitation enables fast functional photoacoustic microscopy with negligible misalignment among different wavelengths and high pulse energy, which is important for in vivo imaging of microvascular dynamics.

An Integrated Germanium-Based THz Impulse Radiator with an Optical Waveguide Coupled Photoconductive Switch in Silicon

This paper presents an integrated germanium (Ge)-based THz impulse radiator with an optical waveguide coupled photoconductive switch in a low-cost silicon-on-insulator (SOI) process. This process provides a Ge thin film, which is used as photoconductive material. To generate short THz impulses, N++ implant is added to the Ge thin film to reduce its photo-carrier lifetime to sub-picosecond for faster transient response. A bow-tie antenna is designed and connected to the photoconductive switch for radiation. To improve radiation efficiency, a silicon lens is attached to the substrate-side of the chip. This design features an optical-waveguide-enabled “horizontal” coupling mechanism between the optical excitation signal and the photoconductive switch. The THz emitter prototype works with 1550 nm femtosecond lasers. The radiated THz impulses achieve a full-width at half maximum (FWHM) of 1.14 ps and a bandwidth of 1.5 THz. The average radiated power is 0.337 W. Compared with conventional THz photoconductive antennas (PCAs), this design exhibits several advantages: First, it uses silicon-based technology, which reduces the fabrication cost; second, the excitation wavelength is 1550 nm, at which various low-cost laser sources operate; and third, in this design, the monolithic excitation mechanism between the excitation laser and the photoconductive switch enables on-chip programmable control of excitation signals for THz beam-steering.

Numerical modelling and optimization of actively Q-switched waveguide lasers based on liquid crystal transducers.

We have recently experimentally demonstrated that a novel liquid crystal-based photonic transducer for sensing systems could be utilized as an active Q-switch in a miniaturised and integrated waveguide laser system. In this paper, we now present a comprehensive numerical modelling study of this novel laser architecture by deriving a set of equations that accurately describe the temporal optical response of the liquid crystal cell as a function of applied voltage and by combining this theoretical model with laser-rate equations. We validate the accuracy of this model by comparing the results with previously obtained data and find them in excellent agreement. This enables us to predict that under realistic conditions and moderate pump power levels of 500 mW, the laser system should be capable of generating peak power levels in excess of 1.1 kW with pulse widths of about 20 ns, corresponding to pulse energies > 20 μJ. We believe that such a low-cost and ultra-compact laser source could find applications ranging from trace gas sensing and LIDAR to material processing.

An extended-cavity diode laser at 497 nm for laser cooling and trapping of neutral strontium

We present the first extended-cavity diode laser in Littrow configuration operating in the cyan wavelength range around 497 nm. The gallium-nitride based diode laser features a free-space output with up to 60 mW, operates in a single frequency mode, is tunable over a range of more than 8 nm and has a Lorentzian linewidth of less than 90 kHz. A detailed characterization of the tuning capabilities of the diode laser and its emission spectrum is provided. This compact, simple and low cost laser source replaces more complex systems based on frequency doubling and therefore simplifies the development of future compact and mobile optical clocks based on neutral strontium. Applications include efficient repumping of strontium atoms from the 5s5p state and the 9.8 MHz broad 5s5p transition might be of interest for sub-Doppler cooling.

Deep surface relief grating in azobenzene-containing materials using a low-intensity 532 nm laser

The inscription of deep surface relief grating (SRG) in glassy monomeric azobenzene materials using a low intensity diode pumped solid state (DPSS) laser operating at 532 nm is demonstrated. Previously developed epoxy-based star-like azobenzene-containing material was applied. Both deep SRG and multi-dimensional structures can be fabricated using this cheap, effective material in combination with a low cost laser source. The dependencies of the SRG performance (amplitude, inscription rate, stability) on exposure conditions (time, intensity) and layer thickness are presented. The replication of the inscribed gratings using polydimethylsiloxane is demonstrated. The results show that effective all-optical fabrication of surface relief master-gratings can be realized using low cost chemistry and widely used low-intensity coherent light source operating at 532 nm.

Optical Spectroscopic Ultrasound Displacement Imaging: A Feasibility Study

Photoacoustic imaging has been intensively studied in recent years and many of the achievements have already been applied in clinical diagnosis and therapy, e.g., using spectroscopic photoacoustic imaging to extract functional and molecular information. However, spectroscopic photoacoustic imaging requires expensive and bulky tunable laser source, which severely hinders its further development toward a portable device. In this paper, we propose a novel imaging method, named optical spectroscopic ultrasound displacement (OSUD) imaging, which enables optical spectroscopic imaging in deep scattering tissue using multiple low-cost continuous-wave laser sources and ultrasound imaging equipment. The principle of the OSUD imaging method will be introduced, followed by preliminary experimental results. The OSUD imaging may exploit the deep penetration merit of ultrasound and provide another pathway to provide spectroscopic optical absorption contrast in deep scattering tissue beyond commonly used photoacoustic imaging, which will be further validated in the future work.

Review of Low-Cost Photoacoustic Sensing and Imaging Based on Laser Diode and Light-Emitting Diode.

Photoacoustic tomography (PAT), a promising medical imaging method that combines optical and ultrasound techniques, has been developing for decades mostly in preclinical application. A recent trend is to utilize the economical laser source to develop a low-cost sensing and imaging system, which aims at an affordable solution in clinical application. These low-cost laser sources have different modulation modes such as pulsed modulation, continuous modulation and coded modulation to generate different profiles of PA signals in photoacoustic (PA) imaging. In this paper, we review the recent development of the photoacoustic sensing and imaging based on the economical laser sources such as laser diode (LD) and light-emitting diode (LED) in different kinds of modulation types, and discuss several representative methods to improve the performance of such imaging systems based on low-cost laser sources. Finally, some perspectives regarding the future development of portable PAT systems are discussed, followed by the conclusion.