Frequency-shifted Optical Feedback Research Articles

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.

Sensing by Dynamics of Lasers with External Optical Feedback: A Review

External optical feedback (EOF) has great impacts on the properties of lasers. It influences the stable operation of lasers. However, various applications based on lasers with EOF have been developed. One typical example is self-mixing interferometry technology, where modulated steady-state laser intensity is utilized for sensing and measurement. Other works show that laser dynamics can also be used for sensing, and the laser in this case is more sensitive to EOF. This paper reviews the sensing technology that uses the dynamics of lasers with EOF. We firstly introduce the basic operating principles of a laser with EOF and discuss the noise properties of and intensity modification in lasers induced by EOF. Then, sensing applications using laser dynamics are categorized and presented, including sensing by frequency-shifted optical feedback, relaxation oscillation frequency, and dynamics with self-mixing interferometry signals and laser optical chaos. Lastly, we present an analysis of the transient response waveform and spectrum of a laser with EOF, showing its potential for sensing.

Dynamic Evolution of Microcavity Raman Laser subjected to Frequency-shifted Feedback and its Application in Laser Spectrum Encoding Thermal Sensing

Dynamic evolution of microcavity Raman laser subjected to frequency-shifted optical feedback is studied. The relaxation peak of the microcavity Raman laser, which matches the oscillation mode of the feedback signal, can be amplified based on the optical feedback effect. Due to the stronger interaction between the optical field and the frequency-shifted feedback signal, narrower relaxation linewidth of microcavity Raman laser is realized, and multi-level resonance sidebands on the laser intensity spectrum will emerge. Taking the advantage of optical feedback technology, a miniature microcavity Raman laser encoding thermal sensor with ultrahigh resolution of 0.00238 C is demonstrated. The tunability of the measurement range can be obtained by adjusting the frequency shift of the feedback signal. Furthermore, temperature variation determines the overall pattern of the microlaser spectrum when subjected to optical feedback. Based on the spectrum patterns, a laser encoding mechanism that involves simultaneous monitoring of multiple information is illustrated, which serves as a fingerprint of the laser mode and shows potential to study transient thermal dynamics in microsystems. Besides thermal sensing, the proposed method is expected to find further promising applications including physical thermodynamics, environmental monitoring, and biomedical thermal research.

Coherent laser detection of the femtowatt-level frequency-shifted optical feedback based on a DFB fiber laser.

The theoretical basis and experimental realization of an all-fiber self-mixing laser Doppler velocimetry based on frequency-shifted feedback in a distributed feedback (DFB) fiber laser are presented, which employs a pair of fiber-coupled acousto-optic modulators to adjust the modulation intensity and frequency of the laser self-mixing effect. Moreover, the minimum optical feedback intensity for the velocity signal successfully measured by the interferometer is 5.12 fW, corresponding to 0.16 photons per Doppler cycle. The results demonstrate that the proposed scheme can adapt to the non-contact measurement requirements of the wide-range speed and weak feedback level in the complex environment.

Delay-induced instability in phase-locked dual-polarization distributed-feedback fiber lasers

We study experimentally and theoretically a dual-polarization fiber laser submitted to time-delayed frequency-shifted optical feedback. In addition to the usual frequency-locking regime that is expected when the frequency shift is close to the polarization beat frequency, we observe at high pumping rates a dynamical pulsing regime inside the locking range. This regime is experimentally evidenced in a full-fibered experiment based on an erbium-doped distributed-feedback fiber laser in which polarization beat frequency is about 1 GHz. A rate-equation model including the frequency-shifted feedback term reproduces well the experimental bifurcation maps, provided that both the time delay and a phase-amplitude coupling parameter ($\ensuremath{\alpha}$ factor) are taken into account. The impact on microwave-photonics applications is discussed.

Frequency-Shifted Optical Feedback Measurement Technologies Using a Solid-State Microchip Laser

Since its first application toward displacement measurements in the early-1960s, laser feedback interferometry has become a fast-developing precision measurement modality with many kinds of lasers. By employing the frequency-shifted optical feedback, microchip laser feedback interferometry has been widely researched due to its advantages of high sensitivity, simple structure, and easy alignment. More recently, the laser confocal feedback tomography has been proposed, which combines the high sensitivity of laser frequency-shifted feedback effect and the axial positioning ability of confocal microscopy. In this paper, the principles of a laser frequency-shifted optical feedback interferometer and laser confocal feedback tomography are briefly introduced. Then we describe their applications in various kinds of metrology regarding displacement measurement, vibration measurement, physical quantities measurement, imaging, profilometry, microstructure measurement, and so on. Finally, the existing challenges and promising future directions are discussed.

Ultrasound vibration measurements based on laser optical feedback imaging.

This paper examines the detection of ultrasound vibrations with nanometric amplitude by using a laser optical feedback imaging (LOFI) setup. By means of numerical simulations, we show typical examples of ultrasound vibrations having different temporal shapes (harmonic and transient), extracted from the laser output power modulation induced by the frequency-shifted optical feedback. Considering the laser quantum noise dynamic and the detection noise separately, we show that the simulated vibration noise is in good agreement with the theoretical prediction. Also, we demonstrate that ultra-high frequencies (in the gigahertz range) can be detected by using a usual LOFI setup with a low-power laser (few mW) and a conventional detection with a usual white noise level. Then we show how the noise of a short transient vibration can be reduced by the reconstruction of its wide vibration spectrum by concatenation. Finally, the experimental detection of transient-harmonics ultrasound vibrations propagating in water and detected at the air/water interface is presented.

Nonlinear modification of the laser noise power spectrum induced by frequency-shifted optical feedback

In this article, we study the non-linear coupling between the stationary (i.e. the beating modulation signal) and transient (i.e. the laser quantum noise) dynamics of a laser subjected to frequency shifted optical feedback. We show how the noise power spectrum and more specifically the relaxation oscillation frequency of the laser are modified under different optical feedback condition. Specifically we study the influence of (i) the amount of light returning to the laser cavity and (ii) the initial detuning between the frequency shift and intrinsic relaxation frequency. The present work shows how the relaxation frequency is related to the strength of the beating signal and the shape of the noise power spectrum gives an image of the Transfer Modulation Function (i.e. of the amplification gain) of the nonlinear-laser dynamics.The theoretical predictions, confirmed by numerical resolutions, are in good agreements with the experimental data.

Self-Stabilized Optoelectronic Oscillator Using Frequency-Shifted Feedback and a Delay Line

We report a new scheme of photonic microwave oscillator at telecommunication wavelength. It consists of two monolithic distributed feedback (DFB) laser diodes, whose beat-note is stabilized by means of frequency-shifted optical feedback. The feedback beam is modulated by the delayed beat-note itself, and this permits to get a stable optically carried microwave without the need of any external reference. Dual and single sideband modulation schemes were tested by using an intensity modulator and a frequency shifter, respectively. The single sideband scheme results in a better stability and a 10 dB improvement of the phase noise. A phase noise level of −65 dBc/Hz, detection limited, at 100 Hz from a 12 GHz carrier is obtained.

Photonic microwave oscillator based on monolithic DFB lasers with frequency‐shifted feedback

A photonic oscillator, locked to a master RF synthesiser, was built by using a monolithic dual-wavelength DFB semiconductor laser submitted to a frequency-shifted optical feedback. A [3; 10] GHz tuning range is reported, with a phase noise level lower than −70 dBrad2/Hz at a 10 Hz offset from the carrier.

Power spectral characteristic of a microchip Nd:YAG laser subjected to frequency-shifted optical feedback

We systematically investigated the power spectrum of a Nd:YAG laser with external frequency-shifted feedback and identified three factors dominating the spectrum, namely, the feedback level, the pumping level of the laser diode (LD), and the shifted frequency introduced in the external cavity. For very weak feedback, the laser power spectrum presents two peaks at frequencies of Ω and ωr, which are the shifted frequency and relaxation oscillation frequency, respectively. When the feedback level is increased to an intermediate level, the laser presents a series of nonlinear effects to cause harmonic and parametric oscillation; for strong feedback, only harmonic peaks are observed in the spectrum. The impact of the pumping levels and the amount of frequency shifting are also experimentally investigated. Especially, even at a weak feedback level, when the frequency Ω approaches ωr, strong nonlinear effects still appear in laser dynamics to make the laser power present only harmonic oscillation. A theoretical analysis is provided which agrees well with the experimental results.

Phase and intensity dynamics of a two-frequency laser submitted to resonant frequency-shifted feedback

We propose an experimental and theoretical study of the dynamical regimes observed when the two modes of a two-frequency solid-state laser are coupled by frequency-shifted optical feedback. The detuning associated to the feedback is close to the frequency of the laser relaxation oscillations. Special attention is devoted to the dynamics of the phase of the beat note between the modes relative to the phase of an external radio-frequency reference. In particular, we analyze in detail the transition from phase locking to the external reference to phase drift. This transition occurs through a window of frequency locking without phase locking [Th\'evenin, Romanelli, Vallet, Brunel, and Erneux, Phys. Rev. Lett. 107, 104101 (2011)]. Furthermore, a large variety of dynamical behaviors, such as weak intensity modulation, self-pulsing, and chaos, are experimentally found. These results are successfully reproduced by a theoretical model based on coupled rate equations. The extension of the model to the description of other active media such as semiconductor lasers is briefly outlined.

Coherent microscopy by laser optical feedback imaging (LOFI) technique

The application of the non-conventional imaging technique LOFI (laser optical feedback imaging) to coherent microscopy is presented. This simple and efficient technique using frequency-shifted optical feedback needs the sample to be scanned in order to obtain an image. The effects on magnitude and phase signals such as vignetting and field curvature occasioned by the scanning with galvanometric mirrors are discussed. A simple monitoring method based on phase images is proposed to find the optimal position of the scanner. Finally, some experimental results illustrating this technique are presented.

Beat-note locking in dual-polarization lasers submitted to frequency-shifted optical feedback

We derive a delay-differential equation model that describes continuous-wave (cw) or passively Q-switched (PQS) two-frequency solid-state lasers submitted to frequency-shifted feedback (FSF). The study focuses on the locking of the beat note between the two free-running laser frequencies to a reference external frequency. The locking domain is obtained analytically in the cw regime. The PQS regime is treated by adding a saturable absorber population in the model equations. In this case, numerical simulations permit us to evaluate a locking range that is smaller than in the cw case. We find good agreement between the theoretical predictions and experiments carried out with a cw diode-pumped dual-polarization Nd:YAG laser as well as with previously published experimental results obtained with cw Er:Yb:glass [ Opt. Lett. 32, 1099 (2007)] and PQS Nd:YAG [ Opt. Lett. 33, 2524 (2008)] lasers. Applications of the FSF locking technique include the lidar-radar technique, for which a highly coherent beat note is required.

Self-mixing thin-slice solid-state laser metrology.

This paper reviews the dynamic effect of thin-slice solid-state lasers subjected to frequency-shifted optical feedback, which led to the discovery of the self-mixing modulation effect, and its applications to quantum-noise-limited versatile laser metrology systems with extreme optical sensitivity.

Simple and Accurate Optical Frequency Domain Ranging Using Off-the-Shelf DFB Lasers Subject to Frequency-Shifted Optical Feedback

It is demonstrated that accurate distance measurement may be effected using a commercially available distributed feedback (DFB) semiconductor laser subject to frequency-shifted optical feedback. A simple experimental arrangement is employed wherein a chirped frequency comb is generated in an external cavity DFB laser using an intracavity acoustic-optic modulator. The frequency comb generates a beat frequency which is proportional to the path difference of a target and reference arm. An estimate of the accuracy and resolution of the measurement is given.

Cell imaging by coherent backscattering microscopy using frequency-shifted optical feedback in a microchip laser

In this work, we present a new development of the laser optical feedback imaging technique for imaging biological structures with a high resolution. The first results obtained on human red blood cells and mice cerebral and muscular tissues slices are shown. The performances of the system and its future developments are also discussed.

Beat-note jitter suppression in a dual-frequency laser using optical feedback

An efficient locking technique based on optical feedback is demonstrated to suppress jitter on the rf beat note between the two modes of a dual-frequency Yb:Er glass laser. The method consists of a self-injection process in which one selected mode serves as a master oscillator to lock and stabilize the second mode via a frequency-shifted optical feedback. The beat note adjusted near 170 MHz was stabilized with an accuracy of 250 mHz using an optical feedback loop with a double pass through an acousto-optic modulator. The beating note can be tuned over 300 kHz by controlling the reference oscillator. The extensions and limitations of the technique are discussed.

Synthetic aperture laser optical feedback imaging using galvanometric scanning

We describe a new one-dimensional synthetic aperture imaging laser radar (ladar) using the resonant sensitivity of a microchip laser to frequency-shifted optical feedback and galvanometric scanning of the target under investigation. In our experiment, the laser is both the source and the detector, providing optical amplification with self-aligned heterodyne detection. By using galvanometric scanning, we achieve an along-track spatial resolution better than the diffraction limit.

Self-mixing interferometry with a laser diode: experimental considerations for sensing applications

An investigation of the effects of external reflectance and pump current on the steady state current threshold and the output power of a laser diode are presented. Back-coupled external reflectance is achieved by placing a mirror in front of the laser diode cavity. Furthermore, by using a rotating wheel, the effect of external reflectance, pump current and combination of individual contributions to frequency shifting on the laser intensity power spectrum are explored. Feedback power ratios below 0.05 and pump currents up to 2Isth (Isth, solitary threshold current) are used in these experiments. The power emitted by the two laser facets are measured through an internal (built-in package) and an external photodiode; both observations are compared. Surprisingly, when evaluating the power–current (P–I) curve for the feedback and solitary cases, a cross over is noticed at ≈1.54Isth only through the external photodiode. Nonetheless, measurements obtained with the internal photodiode agree with the theoretical steady state analysis. Moreover, it is proved that a linear combination of the individual frequency shifts holds in the power spectrum; in addition, a modulation of the carrier density by frequency-shifted optical feedback is also observed. The results from these experiments provide a practical background that ensures a correct interpretation of measurements when using this system for sensing applications.