Laser Feedback Interferometry Research Articles

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.

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.

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 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.

A Highly Integrated Ambient Light Robust Eye-Tracking Sensor for Retinal Projection AR Glasses Based on Laser Feedback Interferometry

Robust and highly integrated eye-tracking is a key technology to improve resolution of near-eye-display technologies for augmented reality (AR) glasses such as focus-free retinal projection as it enables display enhancements like foveated rendering. Furthermore, eye-tracking sensors enables novel ways to interact with user interfaces of AR glasses, improving thus the user experience compared to other wearables. In this work, we present a novel approach to track the user's eye by scanned laser feedback interferometry sensing. The main advantages over modern video-oculography (VOG) systems are the seamless integration of the eye-tracking sensor and the excellent robustness to ambient light with significantly lower power consumption. We further present an algorithm to track the bright pupil signal captured by our sensor with a significantly lower computational effort compared to VOG systems. We evaluate a prototype to prove the high robustness against ambient light and achieve a gaze accuracy of 1.62\,$^\circ$, which is comparable to other state-of-the-art scanned laser eye-tracking sensors. The outstanding robustness and high integrability of the proposed sensor will pave the way for everyday eye-tracking in consumer AR glasses.

Improving the measurement accuracy of refractive index of GaAs and Sapphire Crystal by laser feedback interferometry

Improving the measurement accuracy of refractive index of GaAs and Sapphire Crystal by laser feedback interferometry

A CNN-based Human Activity Recognition System Combining a Laser Feedback Interferometry Eye Movement Sensor and an IMU for Context-aware Smart Glasses

Smart glasses are considered the next breakthrough in wearables. As the successor of smart watches and smart ear wear, they promise to extend reality by immersive embedding of content in the user's field of view. While advancements in display technology seems to fulfill this promises, interaction concepts are derived from established wearable concepts like touch interaction or voice interaction, preventing full immersion as they require the user to frequently interact with the glasses. To minimize interactions, we propose to add context-awareness to smart glasses through human activity recognition (HAR) by combining head- and eye movement features to recognize a wide range of activities. To measure eye movements in unobtrusive way, we propose laser feedback interferometry (LFI) sensors. These tiny low power sensors are highly robust to ambient light. We combine LFI sensors and an IMU to collect eye and head movement features from 15 participants performing 7 cognitive and physical activities, leading to a unique data set. To recognize activities we propose a 1D-CNN model and apply transfer learning to personalize the classification, leading to an outstanding macro-F1 score of 88.15 % which outperforms state of the art methods. Finally, we discuss the applicability of the proposed system in a smart glasses setup.

Terahertz quantum cascade laser under optical feedback: effects of laser self-pulsations on self-mixing signals.

In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal - well known in interband diode lasers - was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback (C ≤ 1) to fringes modulated by SPs under moderate feedback (1 < C ≤ 4.6), and then [under strong feedback (C > 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.

Terahertz near-field nanoscopy based on detectorless laser feedback interferometry under different feedback regimes

Near-field imaging techniques, at terahertz frequencies (1–10 THz), conventionally rely on bulky laser sources and detectors. Here, we employ a semiconductor heterostructure laser as a THz source and, simultaneously, as a phase-sensitive detector, exploiting optical feedback interferometry combined with scattering near-field nanoscopy. We analyze the amplitude and phase sensitivity of the proposed technique as a function of the laser driving current and of the feedback attenuation, discussing the operational conditions ideal to optimize the nano-imaging contrast and the phase sensitivity. As a targeted nanomaterial, we exploit a thin (39 nm) flake of Bi2Te2.2Se0.8, a topological insulator having infrared active optical phonon modes. The self-mixing interference fringes are analyzed within the Lang–Kobayashi formalism to rationalize the observed variations as a function of Acket’s parameter C in the full range of weak feedback (C &amp;lt; 1).

Observation of optical feedback dynamics in single-mode terahertz quantum cascade lasers: Transient instabilities

We provide an experimental evidence of transient instabilities (TIs) in a terahertz (THz) quantum cascade laser (QCL) under optical feedback, in contrast to the widely accepted claim that THz QCLs are ultrastable against feedback. The TIs appear as periodic oscillations in emitted power or terminal voltage of the laser with an increasing oscillation frequency as feedback increases. The absence of relaxation oscillations and low linewidth enhancement factor in THz QCLs makes them a platform uniquely suitable for exploring external-cavity-related dynamics in semiconductor lasers. This work opens a pathway to a THz sensing and imaging modality based on these TIs, which has much reduced complexity compared to existing approaches using laser feedback interferometry.

All-fiber laser feedback interferometry with 300 m transmission distance.

A novel, to the best of our knowledge, interferometry based on the laser feedback technique with long transmission distance is proposed. The system has the advantages of high sensitivity for uncooperative targets and a simple structure. Also, the quasi-common path orthogonally polarized light compensation method is designed to compensate for the drift in long-distance transmission and achieves more than 200 times reduction experimentally. Using a copper block as a target, the displacement resolution of 20 nm is demonstrated experimentally with 300 m transmission distance. The sub-microwatt power consumption of the measurement beam indicates the high sensitivity of the interferometry. The performance, in terms of linearity, is also evaluated. Although a lot needs to be improved, the proposed method is promising for further development toward practical applications, like sensors in remote, nuclear radiative, or other harsh conditions.

Remote eavesdropping at 200 meters distance based on laser feedback interferometry with single-photon sensitivity

A remote eavesdropping system based on the laser feedback interferometry is proposed. The diffuse objects neighboring the speaker can work as inconspicuous sensors. The modulated incident beam is scattered back to the laser cavity by the objects, forming the self-mixing interference. The huge amplification caused by the self-mixing interference provides the system with ultra-high sensitivity, even the feedback light whose intensity is -104dB lower than the laser output can still be sensed. The comprehensible voice reconstruction of the object at the distance of 200 m can be realized, although only 0.91 photon in each interference modulation cycle is scattered back into the laser cavity. Additionally, the range of the incident angle of the beam can be up to ±45 deg, and the optimal one is ±30 deg. This system can meet various requirements of acoustic signals detection and be potentially applied in fields as disaster relief and remote surveillance.

Phase-interrogated surface plasmon resonance sensor based on laser feedback interferometry

This work reports a phase-interrogated surface plasmon resonance sensor based on a laser feedback interferometer to detect a small refractive index change. The laser feedback interferometer technique is introduced to demonstrate the sensing technique's performance, which offers several inherent advantages, such as compact, optically self-aligned, and high accuracy measurement. Moreover, this technique doubles the interrogating light beam propagation to the solution under the test, leading to an improvement of intrinsic sensitivity measurement up to two-fold. The proposed technique was characterized, and the performance was compared to another phase-interrogated surface plasmon resonance (SPR) based on the Mach-Zehnder interferometer. Laser feedback interferometer SPR has shown significant improvement to the performance parameter under test, namely sensitivity, drift, repeatability, and resolution.

Laser feedback interferometry in multi-mode terahertz quantum cascade lasers.

The typical modal characteristics arising during laser feedback interferometry (LFI) in multi-mode terahertz (THz) quantum cascade lasers (QCLs) are investigated in this work. To this end, a set of multi-mode reduced rate equations with gain saturation for a general Fabry-Pérot multi-mode THz QCL under optical feedback is developed. Depending on gain bandwidth of the laser and optical feedback level, three different operating regimes are identified, namely a single-mode regime, a multi-mode regime, and a tuneable-mode regime. When the laser operates in the single-mode and multi-mode regimes, the self-mixing signal amplitude (peak to peak value of the self-mixing fringes) is proportional to the feedback coupling rate at each mode frequency. However, this rule no longer holds when the laser enters into the tuneable-mode regime, in which the feedback level becomes sufficiently strong (the boundary value of the feedback level depends on the gain bandwidth). The mapping of the identified feedback regimes of the multi-mode THz QCL in the space of the gain bandwidth and feedback level is investigated. In addition, the dependence of the aforementioned mapping of these three regimes on the linewidth enhancement factor of the laser is also explored, which provides a systematic picture of the potential of LFI in multi-mode THz QCLs for spectroscopic sensing applications.