Frequency-shifted Feedback Laser Research Articles

Modelocking of a frequency-shifted feedback laser triggered by amplitude modulation.

We report an experimental technique to trigger modelocking (ML) emission in frequency-shifted feedback (FSF) lasers. These lasers feature an intracavity modulator driven by a radio frequency tone, which shifts the light spectrum every cavity round trip. The technique consists of the drive of the modulator with a second tone at the cavity free spectral range (FSR) frequency. So, in addition to the frequency shift, a weak amplitude modulation (AM) appears synchronous with the cavity round trip time. The approach is successful as FSF cavities support chirped modes evenly spaced by the FSR, whose AM coupling produces convenient seed pulses for the ML onset. This results in ML emission at arbitrary frequency shifts and initiation thresholds lower than in standard, spontaneous FSF laser ML. Simulations indicate that the role of AM is to trigger the formation of ML pulses, but the primary mechanism of pulse buildup is the Kerr effect. The technique opens a new, to the best of our knowledge, practical route to initiate ML emission in FSF lasers.

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

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.

ASE recirculation effects in pulsed frequency shifted feedback lasers at large frequency shifts.

An analysis of the different emission regimes (continuous wave, Q-switched, and different forms of modelocking) of a C-band Er:fiber frequency shifted feedback laser at large frequency shifts is presented. We clarify the role of amplified spontaneous emission (ASE) recirculation in the origin of various spectral and dynamical properties of this type of laser. Specifically, we show that Q-switched pulses are supported by a noisy, quasiperiodic ASE recirculation pattern that univocally identifies the pulses within the sequence, and that these Q-switched pulses are chirped as a consequence of the frequency shift. A specific pattern of ASE recirculation, in the form of a periodic stream of pulses, is identified in resonant cavities, namely, those where the free spectral range and the shifting frequency are commensurable. The phenomenology associated with this pattern is explained through the moving comb model of ASE recirculation. Modelocked emission is induced from both integer and fractional resonant conditions. It is shown that ASE recirculation coexists with modelocked pulses, originates a secondary peak in the optical spectrum, and also drives Q-switched modelocking near resonant conditions. Harmonic modelocking with variable harmonic index is also observed in non-resonant cavities.

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.

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.

An All-Fiber Mode-Locked Pulse Laser by Fiber Bragg Grating-Based Acousto-Optic Frequency Shifter

An all-fiber frequency shifted feedback laser is developed to export mode-locked pulses with a narrow spectral bandwidth (238 MHz). A simple cavity structure is established by employing a fiber Bragg grating-based acousto-optic frequency shifter. An exotic reflection peak in a large spectral separation from the Bragg reflection peak is generated by applying flexural acoustic waves, and is utilized for narrow-band filtering and frequency shifting in the laser cavity. Stable mode-locked pulse output can be obtained with the cavity length as short as 16.5 m, whereas increasing the cavity length can reduce the pump power to 37 mW for mode-locked pulse output. A maximum power conversion efficiency of 17% with an average output power of 103 mW were obtained experimentally at a pump power of 600 mW, with corresponding pulse width, pulse energy and peak power of 2 ns, 88 nJ and 4.5 W, respectively. A time-bandwidth product of mode locked pulses of 0.476 denotes nearly Fourier-transform-limited performance. This work provides a new technique to implement efficient, compact and low-cost all-fiber FSFLs for narrow spectral bandwidth mode-locked pulse output.

Pulse doublets generated by a frequency-shifting loop containing an electro-optic amplitude modulator.

We investigate theoretically and experimentally an all-fibered frequency-shifting loop which includes an electro-optic amplitude modulator (EOM) and an optical amplifier, and is seeded by a continuous-wave laser. At variance with frequency-shifted feedback lasers, or Talbot lasers, that contain an acousto-optic frequency shifter, the EOM creates at each round-trip two side-bands that recirculate inside the loop. Benefiting from the high modulation frequency of the EOM, a wide optical frequency comb up to 40 GHz is generated. We demonstrate an original double-pulse regime when the loop length is a multiple of the RF modulation wavelength applied to the modulator. The inter-pulse interval is governed by both the bias voltage and modulation depth of the EOM. Besides, some typical waveforms such as saw-tooth and rectangle are experimentally obtained by properly setting operating frequency, bias voltage and the RF power. The system is modeled by a linear interference model that takes the amplitude modulation function and loop delay into account. The model explains the formation of pulse doublets and reproduces well all the experimental waveforms. Furthermore, the un-seeded loop driven above threshold also generates mode-locked picosecond pulse doublets with a continuously adjustable delay up to the modulation period.

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.

High Spatial Resolution Survey Using Frequency-Shifted Feedback Laser for Transport Infrastructure Maintenance

We propose a remote surface measurement system that uses a laser to inspect tunnel walls.To prevent accidents caused by aging parts of the transportation infrastructure, such as tunnels and bridges, the maintenance of such structures has grown in importance. Although these structures are checked by human inspectors, it is hoped that the process can be further automated through the development of remote sensing technologies.In this article, we focus on the detection of cracks on tunnel surfaces. As the concrete surfaces of tunnels can have many discolored areas, the precision of conventional remote inspection methods based on digital cameras is limited.Employing a frequency-shifted feedback (FSF) laser to overcome this difficulty, we adopt three measurement principles: reflectance imaging, 3D measurement, and spectroscopy. We have realized high spatial resolution, which is essential to our purpose. Using reflectance imaging, we have detected cracks of more than 200μm in width on a concrete surface. Using 3D measurement with an FSF laser, we have detected as 3D shape a 0.35 mm crack on an actual concrete surface. We also have detected the presence of water on a concrete surface using 2.95μm mid-infrared light in the laboratory.We discuss the use of our system to reliably detect 0.2-mm-wide cracks on the basis of experimental results. The measurement results for the reference targets and real concrete are described.

Ranging with frequency-shifted feedback lasers: from $$\upmu$$ μ m-range accuracy to MHz-range measurement rate

We report results on ranging based on frequency shifted feedback (FSF) lasers with two different implementations: (1) An Ytterbium-fiber system for measurements in an industrial environment with accuracy of the order of 1 $\mu$m, achievable over a distance of the order of meters with potential to reach an accuracy of better than 100 nm; (2) A semiconductor laser system for a high rate of measurements with an accuracy of 2 mm @ 1 MHz or 75 $\mu$m @ 1 kHz and a limit of the accuracy of $\geq $ 10 $\mu$m. In both implementations, the distances information is derived from a frequency measurement. The method is therefore insensitive to detrimental influence of ambient light. For the Ytterbium-fiber system a key feature is the injection of a single frequency laser, phase modulated at variable frequency $\Omega$, into the FSF-laser cavity. The frequency $\Omega_{max}$ at which the detector signal is maximal yields the distance. The semiconductor FSF laser system operates without external injection seeding. In this case the key feature is frequency counting that allows convenient choice of either accuracy or speed of measurements simply by changing the duration of the interval during which the frequency is measured by counting.

Tapered amplifier laser with frequency-shifted feedback

We present a frequency-shifted feedback (FSF) laser based on a tapered amplifier. The laser operates as a coherent broadband source with up to 370 GHz spectral width and 2.3 \muμs coherence time. If the FSF laser is seeded by a continuous-wave laser a frequency comb spanning the output spectrum appears in addition to the broadband emission. The laser has an output power of 280 mW and a center wavelength of 780 nm. The ease and flexibility of use of tapered amplifiers makes our FSF laser attractive for a wide range of applications, especially in metrology.

Optical real-time Fourier transformation with kilohertz resolutions

Real-time Fourier transformation (RTFT) of signals is a fundamental concept that enables Fourier analysis at speeds beyond the limitations of conventional digital signal processing engines. In the optical domain, RTFT is commonly performed by inducing large amounts of group-velocity dispersion on the signal (time-varying light wave) of interest to map the signal’s frequency spectrum along the time domain. However, the optical frequency resolution of this method is typically restricted above the gigahertz range, which represents a critical limitation for applications in real-time spectroscopy, ultrafast detection, imaging and sensing, and, more particularly, for photonic-assisted generation and processing of radio-frequency (RF) signals. Here we propose a new concept for realization of RTFT that involves superposition of multiple signal replicas that are shifted simultaneously along the temporal and frequency domains, leading to an output temporal waveform that effectively maps the input optical spectrum. This configuration overcomes the frequency-resolution limitations of dispersion-based RTFT schemes, while providing the desired signal’s spectrum with minimal latency, equal to the inverse of the frequency resolution. We experimentally demonstrate a practical implementation of the concept on optical signals using a frequency shifted feedback (FSF) laser, achieving a frequency resolution of ≃30 kHz and a time–bandwidth product exceeding 400, while the predicted linearity of the frequency-to-time mapping process is shown over a 20 GHz bandwidth. The introduced concept should be of general interest for high-speed, real-time Fourier analysis, beyond the optical-domain implementation reported here; moreover, this work also paves the way for novel applications of FSF lasers in several areas, including high-precision metrology and optical or RF waveform synthesis and processing.

Laser confocal feedback tomography and nano-step height measurement

A promising method for tomography and step height measurement is proposed, which combines the high sensitivity of the frequency-shifted feedback laser and the axial positioning ability of confocal microscopy. By demodulating the feedback-induced intensity modulation signals, the obtained amplitude and phase information are used to respectively determine the coarse and fine measurement of the samples. Imaging the micro devices and biological samples by the demodulated amplitude, this approach is proved to be able to achieve the cross-sectional image in highly scattered mediums. And then the successful height measurement of nano-step on a glass-substrate grating by combination of both amplitude and phase information indicates its axial high resolution (better than 2 nm) in a non-ambiguous range of about ten microns.

Theory of Talbot lasers

We provide a theoretical study of frequency-shifted feedback (FSF) lasers, i.e., lasers with an internal frequency shifter, seeded with a monochromatic wave. The resulting spectrum consists in a set of equidistant modes, labeled by $n$, whose phases vary quadratically with $n$. We prove the emergence of a temporal fractional Talbot effect, leading to generation of Fourier-transform-limited pulses at a repetition rate tunable by the parameters of the FSF cavity (cavity length and frequency shift per round trip), and limited by the spectral bandwidth of the laser. We characterize in detail the output field of this so-called ``Talbot laser'' and emphasize its specific intensity fluctuations. We evidence connections with some aspects of number theory by the appearance of Gauss sums and theta series in the expression of the laser field. Our predictions are in full agreement with the experimental results published in Guillet de Chatellus et al. [Opt. Express 21, 15065 (2013)]. Practical applications and limitations are discussed.

Generation of ultrahigh and tunable repetition rates in CW injection-seeded frequency-shifted feedback lasers

We show both theoretically and experimentally that frequency-shifted feedback (FSF) lasers seeded with a single frequency laser can generate Fourier transform-limited pulses with a repetition rate tunable and limited by the spectral bandwidth of the laser. We demonstrate experimentally in a FSF laser with a 150 GHz spectral bandwidth, the generation of 6 ps-duration pulses at repetition rates tunable over more than two orders of magnitude between 0.24 and 37 GHz, by steps of 80 MHz. A simple linear analytical model i.e. ignoring both dynamic and non-linear effects, is sufficient to account for the experimental results. This possibility opens new perspectives for various applications where lasers with ultra-high repetition rates are required, from THz generation to ultrafast data processing systems.

Pulsed frequency shifted feedback laser for accurate long distance measurements: Beat order determination

Long-distance measurements (10–1000m) with an accuracy of 10−7 is a challenge for many applications. We show that it is achievable with Frequency Shifted Feedback (FSF) laser interferometry technique, provided that the determination of the radio frequency beat order be made without ambiguity and on a time scale compatible with atmospheric applications. Using the pulsed-FSF laser that we developed for laser guide star application, we propose and test, up to 240m, a simple method for measuring the beat order in real time. The moving-comb and Yatsenko models are also discussed. The first of these models fails to interpret our long-distance interferometry results. We show that the accuracy of long-distance measurements depends primarily on the stabilization of the acoustic frequency of the modulator.

Heterodyne beatings between frequency-shifted feedback lasers

Frequency-shifted feedback (FSF) lasers are potential candidates for long distance telemetry due to the appearance of beatings in the noise spectrum at the output of a homodyne interferometer: the frequencies of these beatings vary linearly with the path delay. In this Letter we demonstrate that these beatings also occur in the heterodyne mixing of two identical, but distinct, FSF lasers. This phenomenon is explained by the passive cavity model and is exploited to characterize the time-spectrum properties of FSF lasers. Consequences on telemetry with FSF lasers are presented.