Frequency-shifted Feedback Research Articles

Ultrafast visible fiber laser mode-locked by frequency-shifted feedback of an α-BaTeMo2O9 crystal acousto-optic modulator.

We report the first demonstration, to the best of our knowledge, of visible mode-locked fiber laser using frequency-shifted feedback (FSF) with a visible α-BaTeMo2O9 (α-BTM) crystal acousto-optic modulator (AOM). First, an α-BTM crystal is used as the visible high-quality AOM with a high diffraction efficiency of 85%, a fast rise/fall time of 79/98 ns, and a low insertion loss of 0.2 dB at 635 nm. Then, the 635 nm FSF mode-locked fiber laser is achieved using a Pr3+:doped ZBLAN double-clad fiber as a visible gain medium and the α-BTM AOM as a frequency-shifting element. Self-starting mode-locking at 635 nm directly generates red laser pulses with a pulse duration of 45 ps and a repetition frequency of 31.8 MHz. Furthermore, we investigate the evolution of mode-locking dynamics as the α-BTW-AOM feedback-frequency, which helps further understand the FSF dynamics by directly generating visible ultrashort pulses.

Ultrasensitive detection of remote acoustic vibrations at 300 m distance by optical feedback enhancement

Sensitive detection of remote vibrations at nanometer scale owns promising potential applications such as geological exploration, architecture, and public security. Nevertheless, how to detect remote vibration information with high sensitivity and anti-disturbance has become a major challenge. Reported current non-contact measurement methods are difficult to simultaneously possess characteristics of high light intensity sensitivity, long working distance, high vibration response sensitivity, and anti-disturbance of ambient light. Here, we propose a polarization-modulated laser frequency-shifted feedback interferometry method with the above characteristics, to obtain remote vibration information. The method can directly measure non-cooperative targets without the need for any cooperative markers. In each interference cycle, the energy as low as 2.3 photons can be effectively responded to, and the vibration amplitude sensitivity at 300 m can reach 0.72 nm/Hz1/2 at 1 kHz. This approach provides a strategy for the ultrasensitive detection of remote vibration that is immune to electromagnetic interference.

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.

Achieving shot noise limit imaging with a polarization-modulated optical loop in a laser frequency-shifted feedback system

This paper introduces a novel approach to attain the shot-noise limit in laser frequency-shift feedback (LFSF) imaging systems by employing a polarization-modulated optical loop. Compared to the traditional LFSF imaging system, the proposed system effectively eliminates detrimental parasitic feedback signals, enhancing both the signal-to-noise ratio and the response range of the LFSF imaging system. Specifically, in the polarization-modulated optical loop, only the measurement light propagating through the loop undergoes efficient modulation, while the parasitic feedback light experiences high-frequency modulation beyond the bandwidth of the photodetector. Experimental findings demonstrate the superiority of our proposed system over traditional LFSF imaging systems in the insensitivity of parasitic feedback, showcasing a higher signal-to-noise ratio and response range. Moreover, the proposed system achieved a distinctive polarization imaging technique, which exhibits the capability to discern and analyze the distinct specular reflection and depolarized scattering components in the measurement light, thus providing more comprehensive target information.

Generation of dual and quad-optical frequency combs in the injected radiation free mode-locked frequency-shifted feedback laser

The results of an optoelectronic system—frequency-shifted feedback (FSF) laser experimental examination are presented. The considered FSF laser is seeded only with optical amplifier spontaneous emission (ASE) and operates in the mode-locked regime, whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit. In the frequency domain, the spectrum of such a pulse sequence is an optical frequency comb (OFC). These OFCs we call initial. We consider the possibility of tunable acousto-optic (AO) dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime. The examined system applies a single frequency shifting loop with single AO tunable filter as the frequency shifter that is fed with several radio frequency signals simultaneously. The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width, envelope shape and position in the optical spectrum may be tuned. The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable filter (AOTF). The dual-combs frequency spacing is determined by the frequency difference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply. The quad-combs are obtained with three initial OFCs, forming a pair of dual-combs, appearing when three ultrasound frequencies feed the AOTF transducer. The quad-combs frequency spacing is defined by the difference between the frequency spacing of dual-combs. Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed. The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.Graphical abstract

A Polarization-Modulated Laser Frequency-Shifted Feedback System Reducing the Parasitic Noise Intensity by About Three Orders of Magnitude

We propose a parasitic noise suppression laser frequency-shifted feedback system based on polarization modulation, which can reduce the parasitic noise intensity by about 30 dB without affecting the signal of interest by applying different polarization modulations to the parasitic noise and the signal to make them converted to system-insensitive and sensitive states, respectively. Therefore, the proposed system can still get correct information which is originally drowned in parasitic noise. Based on the proposed principle, the displacement of the aluminum block 2 m away from the system is successfully recovered. Furthermore, the vibration of an ordinary carton which is 80 m away from the system is demodulated, and the voice signal near the carton is restored according to the vibration information with much better performance than there is no parasitic noise suppression. During the vibration recovery process, the effective response energy as low as 5.5 photons is achieved in each feedback modulation cycle. The successful implementation of the proposed parasitic noise suppression laser frequency-shifted feedback system based on polarization modulation provides a practical anti-interference and sensitive coherent detection method in weak light signals detection.

Revealing the buildup of dynamic mode-switchable frequency-shifted feedback laser based on photon–phonon interaction

Photon-phonon interaction (PPI) is an intriguing physical phenomenon in which the dynamic photon-phonon energy transfer allows the manipulation of acousto-optic properties in the media. Recently, there has been significant interest of PPI as it accompanies the conservations of energy and momentum processes. Mode-locking is a process in which temporal and spatial resonant modes establish stable synchronization through non-linear interactions. Here, we propose and reveal the buildup of a dynamic mode-switchable frequency-shifted feedback laser in which frequency shift and mode conversion are achieved based on PPI. Moreover, dynamic switching between LP11a/b mode and stable donut mode output is achieved and precisely controlled by PPI. Finally, by using the time-stretch dispersive Fourier transform (TS-DFT) detective system, we observe the transient dynamics established in frequency-shifted feedback (FSF) fiber laser, such as the direct evolution from continuous wave (CW) state to mode-locked state, CW state to multipulse state and subsequently to stable mode-locked state. All these findings would unveil the buildup of mode-locking mechanisms of dynamic mode-switchable FSF fiber lasers based on PPI with more degrees of freedom and unlock new possibilities in ultrafast laser-based technology, optical communication and light control.

Intracavity dynamics-based gain-assisted sensing with microtubule Raman microlaser

Microcavity lasers show excellent performance as a miniaturized microsensor in various applications. However, their relatively weak power may be easily submerged in system noises and disturbed by environmental fluctuations, rendering them ineffective at detecting small signals for precise sensing. To solve this problem, the laser differential frequency-shift feedback technique is demonstrated in a microtubule Raman laser to achieve the optical gain assistance. When the microlaser is frequency-shift-modulated and returns back to the resonator, the measurement signal can resonate with the laser relaxation oscillation and be significantly enhanced. The intracavity dynamics-based enhancement makes it effective for increasing intensity changes caused by analytes. Small signals that would otherwise be buried in system noises and go undetected can be more easily resolved. In addition, the microsensor reduces the spectral measurement range and offers a way to observe the fast dynamic response. Based on that, a measurement resolution of 50 nm nanoparticle detection limit and a refractive index noise-limited resolution of 8.18 × 10−7 refractive index unit (RIU) are demonstrated. The dynamic phase transition of thermosensitive hydrogel is further investigated as a validation of its rapid detection capability. Integrated with an inherent microfluidic channel, the proposed microsensor provides a direct interaction between analytes and probe light with ultrasmall sample consumption down to 50 pl. It is expected to boost the detection of weak signals in microlasers and enlighten the development of optofluidic microsensors in exploring diverse biochemical processes.

Transverse dynamical response of laser frequency-shifted feedback with mode mismatch

The transverse mode mismatch between the echo wave and optical fields in the laser cavity substantially impacts the responses of the laser frequency-shifted feedback (LFSF) system. This mismatch widely exists in experiments, which limits the detection sensitivity and degrades the signal-to-noise ratio. However, current research on the dynamic response of LFSF has predominantly concentrated on longitudinal modes and intensity modulation, with a lack of quantitative analysis regarding the dynamic response of the transverse modes. In this paper, we establish a transverse dynamical response model of LFSF considering the mode-mismatch. A calculation method of the effective reflectivity (ER) and mode-matching factor (MMF) is presented by decomposing the echo wave to the eigenmodes of the laser. The theoretical model is highly consistent with the experimental data. Finally, with this transverse dynamical response model, we develop a simple scheme to calibrate the effective reflectivity of scattering surfaces by employing a cat's eye retroreflector. Overall, this paper provides a detailed demonstration of the mismatch, which promises an effective theoretical basis to make full use of the high sensitivity of LFSF and apply it over a long distance.

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.

Influence of acousto-optic frequency shifter's thermal-induced birefringence on laser frequency-shifted feedback system

The acousto-optic frequency shifter (AOFS) is the key component of the laser frequency-shifted feedback (LFSF) system, and its thermal-induced birefringence (TIB) seriously affects the stability of the system. In this paper, the influence of the AOFS's TIB on LFSF system is analyzed, and the method of suppressing this influence is proposed. The theoretical model of LFSF considering the AOFS's TIB is established, and the influence on both amplitude and phase of frequency-shifted signal is analyzed. When the AOFS starts working, its TIB effect is obvious due to the accumulation of working heat, which makes the polarization state of its diffraction light change dynamically in most cases. The laser feedback imaging system and laser feedback displacement measurement system are built separately to evaluate the amplitude and phase stability. By principal axis alignment or quasi-common-path compensation, the amplitude variation, phase fluctuation, phase jump and phase temperature drift caused by the AOFS's TIB can be eliminated or suppressed. This study improves the startup stability and anti-ambient temperature interference ability of the LFSF system, which also has guiding significance to other heterodyne interference systems.

Spectro-temporal evolution of mode-locked lasing in fiber frequency-shifted feedback laser.

As a type of mode-locked fiber laser, fiber frequency-shifted feedback lasers (FSFLs) have been rarely studied regarding the buildup process of mode locking. We carried out an experimental investigation to show the unique characteristics of the process, such as the direct evolution of the Q switched mode-locked state to the mode-locked state, the unique phenomenon of spectral center frequency oscillation, evolution to a double-pulse state in harmonic mode locking, millisecond stabilization time, and exponentially decreasing change of relative phase per round trip. For the first time, to the best of our knowledge, the double-beam heterodyne method is applied to measure the evolution of laser spectra with pulse width in the nanosecond range. Understanding the unique buildup mechanism may aid in the engineering and application of FSFLs.

Simultaneous range and velocity measurement using a frequency shifted feedback laser

Laser Doppler velocimeters (LDVs) are non-contact sensors for measuring velocity, and generally used for measuring length of steel products. However, the distance range of the measurement target is restricted to a certain region in principle, making it difficult to measure the velocity of steel products with large distance fluctuations. To remove this restriction, a frequency-shifted feedback laser range and velocity meter (FSFL-RVM) was developed with an FSFL whose frequency changes proportionally with time at ultra-high speeds. This new sensor can measure the velocity without this restriction and additionally measure the distance without contact. The measurement accuracy was evaluated experimentally, and its measurement principle was verified. As a result, it was confirmed that its ability to measure velocities with the same accuracy as the LDV in a distance range five times that of the LDV and simultaneously measure distances within an accuracy of 0.1 mm.

Ranging with a frequency-shifted feedback laser using frequency-comb driven phase modulation of injected radiation

Theoretical and experimental data is presented for the application of an injection-seeded frequency-shifted feedback (FSF) laser for high accuracy ranging. Previous work discussed such a ranging scheme with a phase-modulated single-frequency laser where the phase modulation is done by an electro-optical modulator driven by a single frequency Ω which is swept over a certain bandwidth depending on the given experimental situation. In the present theoretical and experimental work, the phase modulation of the injection laser is done by a frequency comb with temporally fixed frequency components at intervals Ω d spanning a bandwidth adapted to the geometry of the object to be measured. It is shown that the superposition of such FSF radiation returning from the object and a reference surface on a detector leads to a train of sinusoidal pulses with an instantaneous frequency Ωinst in the radio-frequency range. The repetition rate of these pulses is Ω d and their duration is . The central result of this work is the observation that the path length difference between reference and object surface can be deduced from Ωinst, e.g. by frequency counting. The benefit of this approach lies in the fact that active frequency variation is not needed; all features of the entire system (FSF laser plus phase-modulated injected radiation) are constant in time. Proof-of-concept results using an FSF-laser ranging scheme based on a semiconductor laser are presented.

A Coherent Detection Method With 106 Higher Intensity Response Sensitivity Than Normal Heterodyne Interferometry

Based on the Nd: YVO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> microchip laser frequency-shifted feedback system, we carry out the detection of ultra-weak light signals, and verify that the response limit of the proposed system can reach 0.5 photon per second (corresponding to 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−7</sup> photon's energy per frequency modulation cycle of 0.42 μs), with approximately linear response to weak light. Then, the system is compared with the traditional Mach-Zehnder heterodyne interference system, the comparison results confirm the higher intensity response sensitivity of the laser feedback technology (about six orders of magnitude) when detecting both cooperative and non-cooperative targets. On this basis, a verification experiment is carried out on long-distance weak light imaging, and the “THU” pattern made of unpolished aluminum blocks is successfully reconstruct at 60 m distance (with an additional 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−7</sup> attenuation provided by the neutral density filter added to the optical path). During the imaging process, the response energy as low as 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> photon is achieved in each frequency modulation cycle of 0.42 μs. The successful implementation of the laser frequency-shifted feedback detection enables an anti-disturbance and sensitive coherent method in weak light signals detection.

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.

Distance Measurement of a Frequency-Shifted Sub-Terahertz Wave Source

In this paper, we report the development of a frequency-shifted (FS) terahertz (THz) wave source for the non-destructive inspection of buildings. Currently, terahertz-time domain spectroscopy (THz-TDS) is the mainstream method for non-destructive inspection using THz waves. However, THz-TDS is limited by its measurement range and difficulties encountered when there is a strong frequency dependence in the absorption characteristics and refractive index of the measurement target. To address these issues, we developed a novel non-destructive approach for inspection applications using frequency-shifted THz waves. Our system uses a frequency-shifted feedback (FSF) laser as the pump light source to generate FS-THz waves; this allowed us to obtain precise distance measurements of objects over a broad range of distances. We tested a prototype FS-THz system and confirmed successful measurement of spatial distances inside a building material.

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.

Picosecond pulse formation in the presence of atmospheric absorption.

Mode-locked mid-infrared (MIR) fiber laser research has been dominated by the generation of pulses in the picosecond regime using saturable absorbers (SAs) and more recently frequency shifted feedback (FSF). Despite the significant emphasis placed on the development of materials to serve as the SAs for the MIR, published pulse durations have been substantially longer than what has been reported in the near-infrared (NIR). In this report we present experimental data supporting the view that the majority of demonstrations involving SAs and FSF have been limited by the presence of molecular gas absorption in the free-space sections of their cavities. We show that the pulse duration is directly linked to the width of an absorption-free region of the gaseous absorption profile and that the resulting optical spectrum is nearly always bounded by strong absorption features.

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.