Feedback Laser Research Articles

Detecting Genetic Variation in Plants by Mapping Cell Water Dynamics With Terahertz Laser Feedback Interferometry

Detecting Genetic Variation in Plants by Mapping Cell Water Dynamics With Terahertz Laser Feedback Interferometry

Collimated beam formation in 3D acoustic sonic crystals

We demonstrate strongly collimated beam formation, at audible frequencies, in a three-dimensional acoustic phononic crystal where the wavelength is commensurate with the crystal elements; the crystal is a seemingly simple rectangular cuboid constructed from closely-spaced spheres, and yet demonstrates rich wave phenomena acting as a canonical three-dimensional metamaterial. We employ theory, numerical simulation and experiments to design and interpret this collimated beam phenomenon and use a crystal consisting of a finite rectangular cuboid array of 4×10×10 polymer spheres 1.38 cm in diameter in air, arranged in a primitive cubic cell with the centre-to-centre spacing of the spheres, i.e. the pitch, as 1.5 cm. Collimation effects are observed in the time domain for chirps with central frequencies at 14.2 kHz and 18 kHz, and we deployed a laser feedback interferometer or Self-Mixing Interferometer – a recently proposed technique to observe complex acoustic fields—that enables experimental visualisation of the pressure field both within the crystal and outside of the crystal. Numerical exploration using a higher-order multi-scale finite element method designed for the rapid and detailed simulation of 3D wave physics further confirms these collimation effects and cross-validates with the experiments. Interpretation follows using High Frequency Homogenization and Bloch analysis whereby the different origin of the collimation at these two frequencies is revealed by markedly different isofrequency surfaces of the sonic crystal.

Self-injection-locked second-harmonic generation at 532 nm in high-Q Fabry-Perot micro-cavities

Based on the combination of DFB laser self-injection-locking (SIL) and external-cavity frequency-doubling, we have successfully demonstrated, for the first time to our knowledge, the self-injection-locked second-harmonic generation (SHG) at 532 nm by adopting the high-Q Fabry-Perot (FP) micro-cavities. With the DFB incident pump power of 79 mW at 1064 nm, the maximum output SHG power of 20.7 mW at 532 nm was achieved from the 11 mm long plane-concave FP external-cavity of 0.12 mL volume with the 10 mm long PPLN crystal. The corresponding output SHG efficiency of 26.2 % and the Q-factor of 56.5 M both exceeded all known SIL-SHG results reported, and the SIL SHG output power was one order of magnitude higher than the previous maximum of over 2 mW. The reliable SIL has been realized by utilizing the natural birefringence of the intracavity nonlinear crystal to provide laser feedback, and the intrinsic linewidth of the SIL pump laser at 1064 nm was about 30 Hz. Moreover, by replacing the PPLN crystal with the LN and KTP crystals, the output SHG power of 6.1 mW and 2.7 mW at 532 nm were respectively obtained with the Q-factors of 29.3 M and 27.6 M. Our new SIL-SHG design with the high-Q FP micro-cavity paves the way to a new type of the field-deployable ultra-narrow-linewidth visible and near-visible lasers of low size, weight, and power (SWaP).

Two-dimensional flow vector measurement based on all-fiber laser feedback frequency-shifted multiplexing technology

The decomposition and identification of signals are crucial for flow vector acquisition in a multi-dimensional measurement. Here, we proposed a two-dimensional (2D) flow vector measurement system based on all-fiber laser feedback frequency-shifted multiplexing technology. The reliable performance of the system is characterized by experimental verification and numerical simulation. An orthogonal dual-beam structure is employed to eliminate the impact of an unknown incident angle in the practical application. Meanwhile, the vector velocity signals in 2D can be decomposed into one-dimensional (1D) scalar signals by adopting the frequency-shifted multiplexing, which makes it easy to obtain the vector information and velocity distribution of fluid motion through the self-mixing interference frequency spectrum. Moreover, the measured flow rates present a high linearity with syringe pump speeds ranging from 200 to 2000 μL/min, and the velocity information of the different incidence angles is easily obtained with high precision. This work may pave the way for the acquisition and processing of multi-dimensional flow vector signals, with potential applications in biomedical monitoring and microflow velocity sensing.

Distributed Feedback Lasing in Thermally Imprinted Phase‐Stabilized CsPbI3 Thin Films

AbstractAll‐inorganic cesium lead halide perovskites (CsPbX3, with X = I, Br, Cl) are of great interest for light‐emitting diodes and lasers, as they promise improved thermal stability compared to their organic–inorganic analogues. However, among this family of materials, CsPbI3 shows a detrimental phase instability that causes the perovskite to convert to a thermodynamically preferred non‐perovskite phase (yellow phase) at room temperature. In fact, reports on lasers using thin films of CsPbI3 as gain medium are missing, as of yet. Here, the first distributed feedback (DFB) lasers based on CsPbI3 thin films are presented with a resonator directly patterned into the perovskite by thermal nanoimprint. This breakthrough is unlocked by the additive polyvinyl pyrrolidone (PVP), that affords the formation of perovskite layers consisting of phase stable γ‐CsPbI3 nanocrystals, that are even preserved during thermal imprint at 170 °C. The DFB lasers show a low lasing threshold of 45 µJ cm−2 at room temperature under optical pumping and a tunable emission in the deep red spectral region between 714.1 to 723.4 nm. It is anticipated that the findings of this work will have a broad relevance for future electrically driven perovskite lasers and for light‐emitting diodes based on CsPbI3 as active medium.

Impact of feedback time-distribution on laser dynamics

Time-distributed optical feedback in semiconductor lasers has gained attention for its ability to produce high-quality chaos and effectively suppress the time delay signature. However, the fundamental impact of the distribution of feedback in time on laser dynamics remains unexplored. In this paper, we investigate this topic by using fiber Bragg grating (FBG) feedback. We theoretically study the laser response using FBGs of different lengths but similar reflectivity, effectively stretching the impulse response over a longer period while maintaining its overall shape. We observe that there is a critical value of a grating length where fluctuations in laser stability emerge. We attribute this phenomenon to the damping of relaxation oscillations when the zeros of the FBG reflectivity spectrum align with the laser side lobes around the relaxation oscillation frequency. We also uncover an asymmetrical dynamical behavior of the laser for positive and negative frequency detuning. We deduce that this asymmetry is a characteristic feature of FBG feedback and delve into the specificities that trigger such behavior. Published by the American Physical Society 2024

Rotational Doppler effect of vortex beam with frequency-shifted laser feedback

We propose and demonstrate a rotational Doppler measurement system based on frequency-shifted laser feedback interferometry. The system utilizes a vortex beam as the probe, which is modulated by the target and then reflected back into the laser cavity. Compared with the conventional rotational Doppler methods, this approach effectively enhances the amplitude of the generated beat signal through intra-cavity interference between the echo signal and local oscillator. Consequently, it improves sensitivity in weak-signal detection and enables measurement with low probe-beam power. A signal to noise (SNR) enhancement of over 15 dB was observed compare to the Mach-Zehnder structure. This system inherits the advantages of rotational Doppler effect, possessing the capacity to get a frequency proportional to the topological charge of vortex. Experimental validation demonstrates accurate measurement of rotational angle and velocity, with the ability to distinguish rotational direction through heterodyne detection. For vortices with topological charge ranging from 18 to 21 and rotational speeds in range of 150 to 600 revolutions per minute (RPM), the measurement error is within ±3 RPM. Overall, the proposed system possesses high sensitivity to echo signal, low photon consumption, and direction information availability, which provides a new approach for optimizing the rotational Doppler effect in practical applications.

Terahertz microscopy using laser feedback interferometry based on a generalised phase-stepping algorithm

AbstractIn this paper we report an improved method of coherent sensing through the use of a generalized phase-stepping algorithm to extract magnitude and phase information from interferometric fringes acquired by laser feedback interferometry (LFI). Our approach allows for significantly reduced optical sampling and acquisition times whilst also avoiding the need for fitting to complex models of lasers under optical feedback in post-processing. We investigate theoretically the applicability of this method under different levels of optical feedback, different laser parameters, and for different sampling conditions. We furthermore validate its use experimentally for LFI-based sensing using a terahertz (THz)-frequency laser in both far-field and near-field sensing configurations. Finally we demonstrate our approach for two-dimensional nanoscale imaging of the out-of-plane field supported by individual micro-resonators at THz frequencies. Our results show that fully coherent sensing can be achieved reliably with as little as 4 sampling points per imaging pixel, opening up opportunities for fast coherent sensing not only at THz frequencies but across the visible and infra-red spectrum.

Integration by Selective Area Growth of Multiple Semi‐Insulating Buried Heterostructure‐Distributed Feedback Lasers with Optimized Performance Over 100 nm in the O‐Band

The introduction of selective area growth (SAG) in a mature semi‐insulating buried heterostructure (SIBH) platform for the realization of photonic integrated circuits on monolithic InP has been demonstrated. A thorough determination of the relations between quantum well thickness, transition energies, and mask geometries is performed on dedicated wafers by means of extensive micro‐X‐Ray diffraction and microphotoluminescence measurements. Based on those results, SAG is used to grow, with a single epitaxy step, AlGaInAs multiple quantum wells heterostructures, to tailor the active regions of Fabry–Pérot and distributed feedback (DFB) lasers emitting in the O‐Band. SIBH DFB lasers are realized, exhibiting threshold currents < 7.2 mA (at 25 °C), and emitting over 100‐nm spectral range in the O‐Band.

Highly sensitive interferometry with strong anti laser jamming capability based on frequency shift optical feedback

With the proliferation of diverse laser equipment, the optical environment in which highly sensitive optical systems operate has become harsh and intricate. Mitigating laser jamming originating from the operational waveband is a challenging problem encountered by laser interferometric measurement systems. In this paper, we set forth a robust anti-jamming optical interferometry with high sensitivity. This method is based on frequency shift optical feedback, namely anti-jamming laser feedback interferometry (AJLFI). A rear-end pumping Nd:YVO4 microchip laser with bidirectional output channels is employed as the interferometric system source, and the photodetector is posterior to the laser cavity. This particular laser structure and the unique detector placement method from optical feedback interferometry make the laser cavity operate as a high-stop-band filter to filter out laser jamming in the operational waveband. In the AJLFI-based system we established, the jamming laser with a wavelength equaling that of the interferometric system suffers a notable attenuation of 44.8 dB. Concurrently, jamming laser falling within a range of ±10 nm from the wavelength of interferometric system gets an attenuation exceeding 44.43 dB. In addition, we set up the standard laser feedback interferometry(LFI)-based system and the Michelson heterodyne interferometry(MHI)-based system with the same experimental parameters as the AJLFI-based system to perform anti-jamming capability comparative experiments. In the experiments, a pulsed laser beam with a peak power of 1780.9 W is employed as laser jamming in the operating wavebands, which is precisely directed toward the target in conjunction with the probe light of the interferometric systems and scattered into the interferometric systems. The experimental findings demonstrate that the AJLFI-based system exhibits superior anti-jamming capability in both phase and amplitude measurements compared to conventional LFI-based and MHI-based systems.

Frequency combs induced by optical feedback and harmonic order tunability in quantum cascade lasers

This study investigates the interaction between frequency combs and optical feedback effects in Quantum Cascade Lasers (QCLs). The theoretical analysis reveals new phenomena arising from the interplay between comb generation and feedback. By considering the bias current corresponding to free-running single mode emission, the introduction of optical feedback can trigger the generation of frequency combs, including both fundamental and harmonic combs. This presents opportunities to extend the comb region and generate harmonic frequency combs with different orders through optimization of external cavity parameters, such as losses and length. Furthermore, this study demonstrates that optical feedback can selectively tune the harmonic order of a pre-existing free-running comb by adjusting the external cavity length, particularly for feedback ratios around 1%, which are readily achievable in experimental setups. Under strong feedback conditions (Acket parameter C > 4.6), mixed states emerge, displaying the features of both laser and external cavity dynamics. While this study is predominantly centered on terahertz QCLs, we have also confirmed that the described phenomena occur when utilizing mid-infrared QCL parameters. This work establishes a connection between comb technology and the utilization of optical feedback, providing new avenues for exploration and advancement in the field. In fact, the novel reported phenomena open a pathway toward new methodologies across various domains, such as the design of tunable comb sources, hyperspectral imaging, multi-mode coherent sensing, and multi-channel communication.

Imaging elastic waves in solids: how to use laser feedback interferometry to visualize them.

The use of ultrasonic elastic waves is a well established technique for non-destructive testing of materials and structures, in particular to exploit the interaction of waves with structural features to detect and characterize defects. Optical methods offer the advantage of visualising the distribution of elastic waves in a non-contact manner without disturbing the elastic wave. In this work we propose a laser feedback interferometry (LFI) based system as a cost effective, non-contact, alternative to a well established laser Doppler vibrometer technique. We demonstrate the visualization of the elastic waves, using an example of an elastic wave propagating through a prismatic acrylic rod. We show that the ultra-compact and simple implementation of LFI enables accurate visualization of the elastic waves in solids, and opens the pathway to a range of new opportunities in ultrasonic non-destructive testing and evaluation.

Coherent terahertz laser feedback interferometry for hydration sensing in leaves.

The response of terahertz to the presence of water content makes it an ideal analytical tool for hydration monitoring in agricultural applications. This study reports on the feasibility of terahertz sensing for monitoring the hydration level of freshly harvested leaves of Celtis sinensis by employing a imaging platform based on quantum cascade lasers and laser feedback interferometry. The imaging platform produces wide angle high resolution terahertz amplitude and phase images of the leaves at high frame rates allowing monitoring of dynamic water transport and other changes across the whole leaf. The complementary information in the resulting images was fed to a machine learning model aiming to predict relative water content from a single frame. The model was used to predict the change in hydration level over time. Results of the study suggest that the technique could have substantial potential in agricultural applications.

All-Fiber Laser Feedback Interferometry for Sequential Sensing of In-Plane and Out-of-Plane Displacements

In this paper, an all-fiber laser feedback interferometer (LFI) with a diffraction grating was developed for sequential measurement of in-plane and out-of-plane displacements without changing the optical arrangement. When the light emitted from an erbium-doped fiber ring laser is incident on a reflection grating at the Littrow angle, the diffracted light will return into the laser cavity along the original path, thus generating laser feedback interference. Experimental results reveal that the all-fiber system could achieve a precision of 40 nm in both in-plane and out-of-plane displacements sensing. Compared with the traditional all-fiber LFI, the proposed sensing system transfers the measuring scale from laser wavelength to grating period, and it has the advantages of good anti-interference performance and reliability.

Static Laser Feedback Interferometry-Based Gaze Estimation for Wearable Glasses

Fast and robust gaze estimation is a key technology for wearable glasses, as it enables novel methods of user interaction as well as display enhancement applications, such as foveated rendering. State-of-the-art video-based systems lack a high update rate, integrateability, slippage robustness, and low power consumption. To overcome these limitations, we propose a model-based fusion algorithm to estimate gaze from multiple static laser feedback interferometry (LFI) sensors, which are capable of measuring distance toward the eye and the eye’s rotational velocity. The proposed system is ambient light robust and robust to glasses slippage. During evaluation, a gaze accuracy of 1.79° at an outstanding update rate of 1 kHz is achieved, while the sensors consume only a fraction of the power compared with the state-of-the-art video-based system.

Comparison of Physical and System Factors Impacting Hydration Sensing in Leaves Using Terahertz Time-Domain and Quantum Cascade Laser Feedback Interferometry Imaging.

To reduce the water footprint in agriculture, the recent push toward precision irrigation management has initiated a sharp rise in photonics-based hydration sensing in plants in a non-contact, non-invasive manner. Here, this aspect of sensing was employed in the terahertz (THz) range for mapping liquid water in the plucked leaves of Bambusa vulgaris and Celtis sinensis. Two complementary techniques, broadband THz time-domain spectroscopic imaging and THz quantum cascade laser-based imaging, were utilized. The resulting hydration maps capture the spatial variations within the leaves as well as the hydration dynamics in various time scales. Although both techniques employed raster scanning to acquire the THz image, the results provide very distinct and different information. Terahertz time-domain spectroscopy provides rich spectral and phase information detailing the dehydration effects on the leaf structure, while THz quantum cascade laser-based laser feedback interferometry gives insight into the fast dynamic variation in dehydration patterns.

Laser polarization imaging method based on frequency-shifted optical feedback

The traditional laser feedback imaging techniques ignore the polarization effect, and only a single scattering information is obtained. In this paper,polarization imaging method based on frequency-shifted optical feedback is proposed, which is simple and suitable for most laser feedback imaging configurations. Using the polarization effect of self-mixed heterodyne interference, the separation and further processing of the target information can be realized. Taking the laser confocal feedback imaging system as an example, the effectiveness in information acquisition and anti-specular reflection noise is verified by experiments. This method, which combines polarization imaging with laser feedback imaging techniques, demonstrates great potential in applications of the industrial and medical fields.

Random-modulated pulse lidar using a gain-switched semiconductor laser with a delayed self-homodyne interferometer.

We propose the generation of random-modulated pulses using a gain-switched semiconductor laser with a delayed self-homodyne interferometer (DSHI) for lidar applications. By emitting non-repetitive random-modulated pulses, ambiguity in ranging and interference in detection can be mitigated. When gain-switched, the wavelength of the laser fluctuates abruptly at the beginning of the pulse and then drops until it stabilizes toward its continuous-wave (CW) state. By beating the two pulses with instantaneous frequency detuning from the DSHI, pulses consisting of random and down-chirped modulations can be generated without any complex code generation and modulation. In this study, we investigate the waveforms and spectra of the random-modulated pulses generated under various homodyne delay lengths, switching currents, and pulsewidths. We characterize their signal-to-noise ratio (SNR), precision, and cross-correlation between consecutive pulses to evaluate their performance in lidar applications. For a good SNR of over 12 dB, the generated pulses have an optimal precision of approximately 1 mm in ranging, which is substantially better than the chaos-modulated pulses generated based on laser feedback dynamics. By establishing a random-modulated pulse lidar based on the proposed gain-switched homodyne scheme, we successfully demonstrate 3D imaging and profiling with good precision.

Laser Feedback Frequency-Modulated Continuous-Wave LiDAR and 3-D Imaging

We propose and demonstrate a novel light detection and ranging (LiDAR) system for 3D imaging, combined with laser feedback interferometry (LFI) and frequency-modulated continuous-wave (FMCW). Compared with the conventional FMCW method, the intra-cavity interference between the echo signal and local oscillator enhances the amplitude of the generated beat signal effectively, which improves the sensitivity in weak-signal detection with low photon consumption. The improvement is still effective even with frequency modulation nonlinearity. Besides, the system inherits the advantages of FMCW, possessing the capacity to measure position and velocity simultaneously. In experiments, the validity of distance and velocity measurement is demonstrated, and 3D imaging is realized by employing a pair of beam scanners. With a less than 200 μW probe beam, the system achieves imaging over 65 meters away in various scenes with good fidelity, and the distance resolution is better than 1 mm. A portable prototype is also designed and fabricated. Overall, the proposed system possesses high echo-signal sensitivity, simple structure, and dynamic information availability, which provides a new idea for FMCW-LiDAR optimization for practical applications.

Laser Feedback FMCW Ranging System Based on Multiple-Equal-Phase-Subdivision Resampling

In this article, we propose a laser feedback frequency-modulated continuous-wave (FMCW) ranging system based on multiple-equal-phase-subdivision (MEPS) resampling. An auxiliary interferometer is employed to generate clock signal for resampling to suppress the spectral broadening caused by the nonlinearity of frequency modulation. Different from the conventional FMCW system, interference occurs between the light returning from the target and the intra-cavity light, which enables the reference arm of the measuring interferometer to be removed. The length of the auxiliary fiber required by the Nyquist sampling theorem is compressed through MEPS resampling, which reduces the size of the auxiliary path and expands the detection range. The MEPS-resampling-based distance ranging by an auxiliary delay fiber (shorter than the target distance) is demonstrated through simulation and tests. And a series of experiments are conducted to evaluate the effectiveness of the proposed system. Experimental results show that the system has a resolution of 1.98 mm and linearity of 4.99×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> . The repeatability, characterized by the standard deviation, is tested to be less than 35.2 μm. The combination of the laser feedback interferometry (which has the advantages of compactness and auto-alignment), MEPS resampling, and the FMCW optical ranging enables the proposed system to maintain high resolution and precision with a simpler structure.