Linear Cavity Research Articles

Tunable noise-like pulse and Q-switched erbium-doped fiber laser.

A switchable, widely wavelength-tunable noise-like pulse (NLP) and Q-switched Er-doped fiber (EDF) laser with a linear cavity structure is proposed and experimentally demonstrated in this work. The net-normal-dispersion mode-locked NLP operation based on a semiconductor saturable mirror (SESAM) is realized in a 57 nm continuous tuning range from 1528 to 1585 nm by using a tunable filter (TF). When the pump power is 500 mW, the NLPs produce a maximum average output power of about 16 mW with a 3-dB spectral bandwidth of about 17 nm at the central wavelength of 1555 nm, while the average peak power is about 58.8 W. The measured characteristics of the output NLPs at 1555 nm are consistent with the numerical results under the condition of Δβ2, net = 0.095 ps2, and Esat = 0.77 nJ. In addition, stable Q-switched pulses with a 67 nm wavelength tuning range from 1518 to 1585 nm are obtained by adjusting the central wavelength of the filter. The maximum pulse energy reaches 231.4 nJ at the center wavelength of 1555 nm, corresponding to a peak power of about 278.8 mW. The proposed wavelength-tunable fiber laser is simple and versatile, demonstrating significant potential for numerous practical applications.

Optical Performance Comparison of Different Shapes of Cavity Receiver in the Fixed Line-Focus Solar Concentrating System

To optimize the fixed-focus solar concentrating system (FLSCS) and linear cavity receiver of better optical performance, the effects of receiver parameters (geometric shape, receiver position f, receiver internal surface absorptivity αab, and end reflection plane reflectivity ρr) on the relative optical efficiency loss ηre-opt,loss, the maximum value of the local concentration ratio Xmax, and the non-uniformity factor σnon were studied in the present study. The results showed that the increases of sun declination angle δ in the range of 0–8° have a weak effect on the ηre-opt,loss. The ηre-opt,loss are 2.25%, 2.72%, 12.69% and 2.62%, 3.26%, 12.85%, respectively, when the solar hour angle ω is 0°, 30°, 60° as δ = 0° and 8° for linear rectangular cavity receiver. The Xmax mainly depends on the energy flux distribution of first intercepted sunlight on the cavity absorber inner wall. Increasing the distance between the cavity absorber inner wall and the focal line Δf can affect the Xmax. The smaller the Δf, the greater the Xmax, and vice versa. The changing trend of σnon is basically consistent with that of the Xmax. When the f is 600, 625, 650, 675, 700 mm and the ω = 0°, the σnon are 0.832, 0.828, 0.801, 0.747, and 0.671, respectively, for linear rectangular cavity receiver. This work could establish the foundation for further research on the optical to thermal energy conversion in the FLSCS.

Machine learning method for calculating mode-locking performance of linear cavity fiber lasers

As a commonly used pulse light source, fiber mode-locked lasers are widely used in communication, frequency comb, nonlinear optics and other fields. Although people can accurately predict and analyze the performance of mode-locked pulse with given parameters of fiber laser by existing theories and calculation methods, it is difficult to analyze the mode-locked performance of fiber laser with multi-parameter changes by traditional methods. In this paper, we use machine learning technology to put forward a new view on this problem. Firstly, we use an artificial neural network (ANN) to judge whether a small noise pulse can evolve into a stable mode-locked state in the linear cavity fiber laser. Then, the time domain function of sample pulse is expanded by Fourier series. We use the Fourier coefficients as the network output and train another ANN that can predict the pulse shape quickly and accurately. Finally, we demonstrate how to use the genetic algorithm and the trained network to compute the parameters of the fiber laser when the given pulse width is known. The authors believe that the theoretical ideas and computational models presented in this work have great potential in the dynamics research and fabrication of fiber lasers.

Resonant generation of electromagnetic modes in nonlinear electrodynamics: Classical approach

The paper explores a theoretical possibility of resonant amplification of electromagnetic modes generated by a nonlinear effect in Euler-Heisenberg electrodynamics. Precisely, we examine the possibility of the amplification for the third harmonics induced by a single electromagnetic mode in radiofrequency cavity, as well as the generation of signal mode of combined frequencies induced by two pump modes ($\omega_1$ and $\omega_2$) in the cavity. Solving inhomogeneous wave equations for the signal mode, we formulate two resonant conditions for a cavity of arbitrary shape, and apply the obtained formalism to linear and rectangular cavities. We explicitly show that the third harmonics as well as the mode of combined frequency $2\omega_1 + \omega_2$ are not resonantly amplified while the signal mode with frequency $2\omega_1 - \omega_2$ is amplified for a certain cavity geometry.

High-efficiency mode-locked fiber laser with a switchable oscillating transverse mode in an all few-mode fiber linear cavity.

In this paper, an oscillating transverse mode switchable mode-locked fiber laser with a few-mode fiber linear cavity is proposed and demonstrated. An artificial filter is used to realize the mode gain modulation of the laser. The stable mode-locked pulsed operation with switchable wavelength is easily achieved and the oscillating transverse mode can be flexibly switched between the fundamental mode and high-order mode by adjusting the polarization controller. The mode-locked fiber laser directly oscillates in the high-order mode stably with a slope efficiency of as high as 12%, and the corresponding operating wavelength, repetition rate as well as pulse duration are 1054.07 nm, 22.662 MHz, 31.5 ps, respectively. Besides, a cylindrical vector beam with a high mode purity of 98.6% is obtained by removing the degeneracy of the LP11 mode. This compact and high-efficiency mode-locked fiber laser operating in switchable transverse mode has the potential application for laser processing, particle trapping, bioimaging, and mode division multiplexing system.

Research on uncertainty of optical feedback linear cavity ringdown spectroscopy

Cavity ring-down spectroscopy (CRDS) is a highly sensitive molecular absorption spectroscopic technology, which has been widely used in mirror reflectance measurement, atmospheric trace gas detection, molecular precision spectroscopy and other fields. It deduces the intracavity absorption by measuring the rapid variation of the ringdown signal. As a result, detector with high linearity, broad bandwidth and low electrical noise is indispensable. Additionally, due to the large noise in laser frequency, low laser to cavity coupling efficiency is resulted. Consequently, the cavity transmission is faint, which deteriorates the detection sensitivity. Optical feedback could address this problem by locking the laser to the cavity longitudinal mode. Then, the laser frequency noise is suppressed and hence better detection sensitivity is expected. Optical feedback CRDS has been widely studied with V-shape cavity. Compared to Fabry-Perot cavity, this cavity geometry is more sensitive to mechanical vibration and possesses lower finesse with an additional mirror. In this paper, optical feedback linear cavity ring-down spectroscopy based on a Fabry-Perot cavity with a finesse of 7800 is presented. The principle of the combination of optical feedback and linear cavity is explained from the perspective of the light phase, which shows the reflection would not generate efficient optical feedback if the feedback phase is properly controlled and laser to cavity locking could be therefore realized. And then, the factors influencing the stability of ring-down signal is analyzed, including the feedback ratio, the trigger voltage for the ringdown event and the distance between the light spot and the detector center. The experimental results show a superior fractional uncertainty of the empty ringdown time of 0.026% could be attained with a low feedback rate (3% FSR), a high ringdown signal trigger threshold (90% cavity mode amplitude) and superposition of the light spot with the detector center. With Allan variance analysis, the white noise response of 1.56×10<sup>-9</sup> cm<sup>-1</sup>/ HZ<sup>-1/2</sup> and the detection sensitivity of .29×10<sup>-10</sup> cm<sup>-1</sup> for trace gas detection could be achieved at the integration time of 180 s, corresponding to the lowest CH<sub>4</sub>concentration detection of 0.35 ppb at 6046.9cm<sup>-1</sup>. This robust spectroscopic technique paves the way for the construction of high sensitive and stable cavity based instrument for trace gas detection.

Uncertainty of optical feedback linear cavity ringdown spectroscopy

Cavity ring-down spectroscopy (CRDS) is a highly sensitive molecular absorption spectroscopic technology, which has been widely used in mirror reflectance measurement, atmospheric trace gas detection, molecular precision spectroscopy and other fields. It deduces the intracavity absorption by measuring the rapid variation of the ringdown signal. As a result, detector with high linearity, broad bandwidth and low electrical noise is indispensable. Additionally, owing to the large noise in laser frequency, low laser-to-cavity coupling efficiency is obtained. Consequently, the cavity transmission is faint, which deteriorates the detection sensitivity. Optical feedback can address this problem by locking the laser to the cavity longitudinal mode. Then, the laser frequency noise is suppressed and hence better detection sensitivity is expected. Optical feedback CRDS with V-shape cavity has been widely studied. Compared with Fabry-Perot cavity, this cavity geometry is very sensitive to mechanical vibration and possesses low degree of fineness due to an additional mirror. In this paper, optical feedback linear cavity ring-down spectroscopy based on a Fabry-Perot cavity with a degree of fineness of 7800 is presented. The principle of the combination of optical feedback and linear cavity is explained from the perspective of the light phase, which shows that the reflection will not generate efficient optical feedback if the feedback phase is appropriately controlled and laser to cavity locking can be therefore realized. And then, the factors influencing the stability of ring-down signal are analyzed, including the feedback ratio, the trigger voltage for the ringdown event, and the distance between the light spot and the detector center. The experimental results show that a superior fractional uncertainty of the empty ringdown time of 0.026% can be obtained with a low feedback rate (3% FSR), a high ringdown signal trigger threshold (90% cavity mode amplitude) and superposition of the light spot with the detector center. With Allan variance analysis, the white noise response of 1.6 × 10<sup>–9</sup> cm<sup>–1</sup>·Hz<sup>–1/2</sup> and the detection sensitivity of 1.3 × 10<sup>–10</sup> cm<sup>–1</sup> for trace gas detection can be achieved in an integration time of 180 s, corresponding to the lowest CH<sub>4</sub> concentration detection of 0.35 × 10<sup>–9</sup> at 6046.9 cm<sup>–1</sup>. This robust spectroscopic technique paves the way for constructing high-sensitive and stable-cavity based instrument for trace gas detection.

Peculiaritie of Polarization Waves Behavior under Excitation of an Extended Resonant Medium by Overlapping Extremely Short Light Pulses

With the coherent interaction of a sequence of extremely short light pulses with a resonant medium (when the duration of the pulses and the delays between them are shorter than the relaxation times of the population difference T1 and polarization T2 of the medium), it is possible to create spatial electromagnetically induced gratings (EMIG) of polarization and population difference in the medium, as well as their ultrafast control. The dynamics of such gratings is of interest in connection with the possibility of ultrafast control over the properties of a medium at times of the order of the pulse duration and their influence on the stability of generation of extremely short pulses in a laser with a short linear cavity. In this paper, based on the numerical solution of the system of Maxwell-Bloch equations, an unusual and interesting possibility of the appearance of narrow and alternating sections in the medium is shown and studied in detail, in which harmonic polarization waves of the medium are formed, propagating in opposite directions relative to neighboring sections. This situation occurs when two extremely short attosecond pulses collide in a two-level resonant medium. Keywords: attosecond pulses, electromagnetically induced gratings, polarization waves.

Spatially-multiplexed tunable dual-comb optical parametric oscillator at 250 MHz

We demonstrate a spatially-multiplexed dual-comb 250-MHz OPO from a single linear cavity. The adjustable repetition-rate difference is 4.1 kHz. Each idler comb has >200-mW average power at 3.5 μm with 30 nm bandwidth. The OPO is wavelength-tunable from 1.36 μm to 1.7 μm and 2.9 μm to 4.17 μm. © 2022 The Author(s)

Multiple fiber Bragg grating sensing system based on active mode-locking fiber laser

In this paper, a new multiple fiber Bragg grating (FBG) sensing system based on active mode-locking fiber laser is proposed. The active mode-locking fiber laser is composed of a linear cavity with an Fabry–Perot laser diode (F-P LD) as a reflective cavity mirror, and the erbium-doped fiber as the gain medium. Meanwhile, the F-P LD is also used as the modulating element of the fiber laser. Multiple FBGs cascaded in a long fiber are used as both the sensors in the system and the components for wavelength selection in the active mode-locking fiber laser. The capacity of the proposed sensing system to interrogate multiple FBGs in wavelength and spatial domain is investigated. The proposed sensing system has the characteristics of low cost, good stability, good compatibility, and can be used in quasi-distributed multi-point sensing.

Особенности поведения волн поляризации при возбуждении протяженной резонансной среды перекрывающимися предельно короткими световыми импульсами

With the coherent interaction of a sequence of extremely short light pulses with a resonant medium (when the duration of the pulses and the delays between them are shorter than the relaxation times of the population difference T1 and polarization T2 of the medium), it is possible to create spatial electromagnetically induced gratings (EMIG) of polarization and population difference in the medium, as well as their ultrafast control. The dynamics of such gratings is of interest in connection with the possibility of ultrafast control over the properties of a medium at times of the order of the pulse duration and their influence on the stability of generation of extremely short pulses in a laser with a short linear cavity. In this paper, based on the numerical solution of the system of Maxwell-Bloch equations, an unusual and interesting possibility of the appearance of narrow and alternating sections in the medium is shown and studied in detail, in which harmonic polarization waves of the medium are formed, propagating in opposite directions relative to neighboring sections. This situation occurs when two extremely short attosecond pulses collide in a two-level resonant medium.

Parametric Experiments of Volumetric Measurement Based on Linear Cavity Resonance

Propellant volumetric measurement based on the cavity acoustic resonance may be a promising alternative in the microgravity environment. This article experimentally analyzes the features of Helmholtz-resonance dynamics under some conditions, which may be confronted in on-orbit applications. Instead of the traditional Helmholtz model, an analytical 1-D model, validated by experimental data, is proposed to measure the propellant volume in the tank based on cavity resonant mechanisms. Experimental data show that influences of liquid–gas interfaces and droplets in the tank have limited influence on the Helmholtz-resonance mechanism and the corresponding volumetric measurement. However, droplets partly or fully penetrating in the neck segment significantly change the feature of cavity resonance and thus degrade the corresponding measurement performance.

Tunable narrow linewidth linear cavity fiber laser based on a one-way round-trip structure

A tunable single-longitude-mode (SLM) linear cavity fiber laser based on a one-way round-trip structure with a saturable absorber (SA) is demonstrated. A one-way round-trip structure, which is combined by two circulators, is introduced to the linear cavity to suppress the backward amplified spontaneous emission noise and isolate the interference of pump light to SA. The SA is implemented by using a section of unpumped erbium-doped fiber. As the temperature increases from 10 °C to 45 °C, the laser wavelength shifts linearly form 1549.948 nm to 1550.336 nm with the recorded sensitivity of ∼11.01 pm/°C. A SLM erbium-doped fiber laser (EDFL) with a high OSNR of 48 dB and an ultra-narrow linewidth of 585 Hz is obtained at the pump power of 100 mW. In accordance with the designed SLM operation, the EDFL has excellent stability in the center wavelength and the output power. The maximum fluctuations of the center wavelength and output power are 0.016 nm and 0.01 dB respectively.

A Method for WWTP Sludge Valorization through Hygienization by Electron Beam Treatment

This work reports on municipal sludge hygienization using electron beams. Three types of sewage sludge from two municipal wastewater treatment plants were tested: preliminary sludge with 4% TS, postflotation sludge with 2.5% TS and thickened preliminary sludge with 10% TS. The analysis of reference samples demonstrated the presence of bacteria and helminths ova in all examined samples. For the study of hygienization, electron beams from two types of accelerators, linear (Elektronika 10/10) and single cavity (ILU-6), were applied. For each type of accelerator, different irradiation methods were used: irradiation in sealed polyethylene bags using conveyor and flow irradiation installation. Experiments showed that the doses necessary for the elimination of mentioned pathogens were 4 kGy for preliminary sludge, 4 kGy for postflotation sludge and 5.5 kGy for preliminary sludge. The differences between the amounts of initial pathogens in preliminary and thickened preliminary sludge were marginal. It is possible that the higher irradiation dose required to hygienize thickened sludge resulted from higher TS concentration.

Eight-wavelength EDF laser combining tilted fiber grating with sampled fiber grating

A new eight-wavelength erbium-doped fiber laser, which utilizes linear cavity structure, was designed and experimentally authenticated. The laser was composed of two Sagnac loops: a left loop containing a fiber coupler (FC) with power ratio of 50:50 and a tilted fiber grating (TFG), and a right loop consisting of another FC, a sampled fiber grating (SFG) and three-ring-type polarization controller (PC). The TFG had many inhomogeneous peaks with maximum power differences of 3 dB and maximum modulation depths of 16 dB. Two adjacent transmission peaks in the SFG had approximately 0.3 nm spacings and maximum modulation depths of 8 dB. The two loops with TFG and SFG were jointly utilized to motivate multi-wavelength. When the PC was tuned to cause the PHB effect, 21 sorts of diverse lasing lines were achieved. All lasing lines with the OSNRs of more than 20 dB had fewer than 0.21 nm line widths.

Switchable and tunable linear cavity erbium-doped fiber laser based on FBGs embedded in Sagnac rings

A switchable and tunable erbium-doped fiber laser with a linear cavity based on fiber Bragg gratings embedded in Sagnac rings is proposed and experimentally verified. Due to the stress birefringence effect and the polarized hole burning effect, which are introduced into the single-mode fiber in the polarization controllers (PCs) by the PCs, the designed laser can achieve seven kinds of laser-states output including three kinds of single-wavelength laser states, three kinds of dual-wavelength laser states and one kind of triple-wavelength laser state. The optical signal-to-noise ratios of the output wavelengths are all higher than 52 dB, and the wavelength shifts are all less than 0.04 nm. Furthermore, the temperature tuning of the wavelength range is also researched, which is about 1.2 nm. Due to advantages, such as low cost, simple structure, easy switching and multiple laser states, the designed laser has great application potential in laser radar, optical fiber sensing and so on.

Compact passively Q-switched single-frequency distributed Bragg reflector fiber laser at 2.0 µm

Based mainly on the distributed Bragg reflector (DBR) short linear cavity with a 1.6-cm-long heavily Tm3+-doped germanate glass fiber and semiconductor saturable absorber mirror (SESAM), a compact passively Q-switched single-frequency fiber laser at around 1950 nm is demonstrated experimentally. By comparing pulse characters of Q-switched operations fulfilled via SESAMs with different parameters, a stable output pulse is optimized to deliver a maximum average power of 22.2 mW, a peak power of 0.67 W, and an optical signal-to-noise ratio over 61 dB. Moreover, the repetition rate of the output pulse can be tuned from 92 to 520 kHz with a narrowest pulse width of 64 ns. To the best of our knowledge, this is the first time a 2.0 µm passively Q-switched single-frequency DBR Tm3+-doped fiber laser has been realized, and it shows great potential application in remote sensing, biomedical science, and nonlinear optics.

Switchable and tunable dual-wavelength passively mode-locked fiber laser

We demonstrate a switchable and tunable dual-wavelength passively mode-locked fiber laser in a compact linear cavity. Two cascaded polarization-maintaining fiber Bragg gratings (PM-FBGs) are used as wavelength-selective components, and a semiconductor saturable absorber mirror (SESAM) is used as a mode-locked component. Four kinds of switchable single-wavelength and two kinds of switchable dual-wavelength mode-locked operations are achieved in 1.0-μm band under lower pump power condition. Through temperature control, we obtain the tunable single-wavelength and dual-wavelength mode-locked operations, and the tunable wavelength range is about 0.5 nm when increase the temperature from 25 °C to 80 °C.

Fabrication of nanoporous silica rods from curable nanocomposites and their application in Yb-doped fiber lasers

Rare-earth (RE) doped silica glass is essential for constructing high-power fiber lasers. However, the achievement of high doping concentrations with a uniform RE ion dispersion in the SiO2 glass matrix is hindered by clustering and inhomogeneous distribution. Herein, a novel technique—UV-curable nanocomposite technology (UV-CNCT)—is demonstrated for fabricating a nanoporous silica rod with a high doping concentration and uniform dispersion of RE ions in a SiO2 glass matrix. A preform with an Yb-doped P2O5-Al2O3-SiO2 core and pure SiO2 cladding, and a single cladding fiber with a doping concentration of 2.1 wt% (for Yb2O3) are fabricated. A linear all-fiber resonant cavity is constructed to analyze the laser performance. Experimental results reveal the highly uniform dispersion of dopants at a relatively high concentration for both the preform and fiber without aggregation, center dip. The proposed technology is expected to be an effective approach for the fabrication of high-quality RE-doped silica glass for advanced fiber lasers.

Passively mode-locked composite erbium fiber laser with a pulse repetition rate of 150 MHz

We demonstrate a short cavity passively mode-locked composite erbium-doped fiber laser with saturable absorber based on aerosol-synthesized single-walled carbon nanotubes. Optical pulses had a duration of 973 fs while an average output power – 6 mW. Using a shortened laser cavity, a stable soliton generation with a pulse repetition rate of 150 MHz at the output of the all-fiber linear cavity was obtained.