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A five-wavelength-switchable thulium-doped fiber laser with a fiber Bragg grating (FBG)-based five-channel transmitting Fabry-Pérot (F-P) filter and a multi-coupler based triple-ring cavity (CTRC) filter, is experimentally proposed. For the first time, a multi-channel F-P filter and specially-designed coupler-based compound cavity filter were combined to realize multi-wavelength-switchable operation, with single-longitudinal-mode (SLM) lasing for each wavelength channel in a 2 µm band fiber laser. The theoretical analysis of the CTRC filter and the principle for achieving SLM operation by corporately using the F-P filter and CTRC filter were presented in detail. The experimental results demonstrated the good performance of the proposed filters. Five lasing wavelengths (1941.13 nm, 1941.22 nm, 1941.30 nm, 1941.39 nm, and 1941.47 nm) were obtained with good switchability, and each operated in a stable SLM state. The OSNR of each lasing wavelength was higher than 50 dB, and the linewidths of all obtained wavelengths were less than 8 kHz.

We present AlGaAs-based monolithically integrated dual-wavelength lasers and master-oscillator power-amplifiers emitting around 785 nm which combine a near diffraction limited beam quality with single longitudinal mode emission, both achieved over a wide range of output powers. This holds for alternating dual wavelength operation which is of interest for applications in spectroscopy as well as for simultaneous dual wavelength operation which can be used to generate THz-radiation. Compared to previous designs the lateral optical confinement is provided by two differently deep etched ridge waveguides. Shallow etching is used in lasing and amplifying sections to ensure single-lateral mode operation, whereas deep etching is employed to form more compact S-bends and to enable the use of multi-mode interference (MMI) couplers instead of Y-couplers.

Miniaturized solid-state lasers with a single longitudinal mode are vital for various photonic applications. Here we prepare red-emissive carbon dots (CDs) with a photoluminescence quantum yield (PLQY) of 65.5 % by combining graphitic nitrogen doping and surface modification. High-concentration doping alters the CDs' emission from blue to red, while the electron-donating groups and polymer coating on their surfaces improve the PLQY and photostability. The CDs exhibit excellent stimulated emission characteristics, with a low threshold of amplified spontaneous emission (ASE) and long gain lifetime. A planar microcavity with only one resonant mode, which fitted with the CDs' ASE peak, was constructed. Combining the CDs and microcavity produced a solid-state laser with a single longitudinal mode, a linewidth of 0.14 nm and a signal-to-noise ratio of 14.8 dB (Q∼4600). Our results will aid the development of colorful solid-state micro/nano lasers with potential use in practical photonics.

The existing yoke magnetizers can be used for large diameter bridge cable detection, but cannot provide the optimal bias magnetic field due to the repulsion of the polarities arranged in the circumferential direction, which reduces the energy conversion efficiency of the magnetostrictive longitudinal mode guided wave transducer. A magnetostrictive longitudinal mode guided wave transducer with the ring permanent biased magnet is provided to improve energy conversion efficiency for large diameter bridge cables. Firstly, a ring magnetizer for the axial magnetization is proposed, which inspired by the structure of the yoke magnetizer and the adjustable bias magnetic field generated by the solenoid coil. Then, the specific transducer with the ring magnetizers is designed for PES(C) 7–127 cable. Based on the magnetostrictive coupling coefficient curves and magnetization curve of the cable steel wire, the finite element models verify that the magnetization effect of the ring magnetizer is better than the yoke magnetizer for PES(C) 7–127 cables. Furthermore, the structural parameters of the ring magnetizers are optimized based on magnetization strength and magnetization uniformity. Finally, the improved efficiency of the transducer is verified by experiments. The energy conversion efficiency of the transducer is increased by 43.65% at the transmitting side, 4.76% at the receiving side, 48.41% at the both sides. Besides, the weight of the ring magnetizers is 19.85% lower than the yoke magnetizers.

A distributed feedback (DFB) laser for high-speed modulation with asymmetric corrugation-pitch-modulated (ACPM) grating structure and high-reflection and anti-reflection coating (HR-AR) coating is demonstrated theoretically and experimentally in this paper. The ACPM gating structure is fabricated by the reconstruction equivalent chirp (REC) technology, which is with low cost. The impact of the longitudinal spatial hole burning (LSHB) effect and the facet phase on the characteristics of the ACPM DFB laser is discussed in detail. The numerical simulation results show that the proposed ACPM DFB laser can suppress the LSHB effect significantly. Comparing with the conventional HR-AR DFB laser, the ACPM DFB laser can achieve higher output efficiency, better single-mode yield, and wider modulation bandwidth. The modulation eye diagrams of HR-AR DFB lasers with different grating structures are also presented. The experimental results show that the LSHB effect in the HR-AR DFB laser with ACPM structure is suppressed effectively. The laser with ACPM structure fabricated by the REC technique has high slope efficiency, stable single longitudinal mode operation, and wide modulation bandwidth.

In this letter, we experimentally demonstrate multi-band linear frequency modulation (LFM) signal generation subjected to multi-input optical injection in an external cavity-based Fabry-Perot laser diode (ECFP-LD). The ECFP-LD is a specially designed FP-LD with a self-injected single longitudinal mode tuned up to 10 nm. The non-linear dynamics in period-one oscillation in ECFP-LD subjected to multi-input optical injection shows unique characteristics for multi-band linear frequency modulation. In the proposed scheme, four external input beams are injected to ECFP-LD, among which one is power modulated with the saw-tooth like waveform, and others are with the constant power. The varying power beam red-shifts the self-injected mode of ECFP-LD and mode-hopping on attaining a particular power level. Hence, multi-band LFM signals are generated by beating self-injected and multi-input injected beams with a large time-bandwidth product (TBWP). The total bandwidths of 12GHz (LFM1: 33–36 GHz, LFM2: 29–32 GHz, LFM3: 25–28 GHz, LFM4: 22–25 GHz) for four-band linear frequency modulation signals with a TBWP of 12000 is measured. The generated multi-band LFM signal provides flexibility on central frequency tunability and dynamic chirp rate for a multifunctional radar system.

On the basis of air heater characteristics, a new computational model was developed in this paper, which was aimed at investigating acoustics and instabilities in air heaters. This model included the effects of mean flow, viscosity, entropy waves, non-linear acoustics and realistic boundary conditions. In addition, it was practical for air heaters with hundreds of injectors, complex configurations and geometries. Analytical solutions of acoustics in a closed rectangular cavity were used to verify and validate the computational model. It was shown that the predicted critical parameters of air heater agreed well with the experimental data or design values. This model predicted the self-excited spinning tangential modes without any preliminary assumptions about them. Traditional combustion response function assumes that combustion mainly takes place in a so-called rapid combustion zone, and this zone is usually modelled as a disc in the combustor near the injection head. However, in practice, the flame has a spatial distribution. This paper described the effect of flame spatial distribution on predictions of oscillation frequency and mode. It was found that frequency and mode shape of oscillations closely depended on the length of the heat release zone. Comparison of different heat release zones indicated that the increment of heat release length exhibited an increased tendency toward lower-order longitudinal modes, when the heat release zone was located near the faceplate where it was the pressure antinode of the longitudinal mode. Modulation of heat release length might cause bifurcations between standing and turning modes. A noticeable tendency was toward higher-order standing tangential mode with increasing heat release length. Finally, theoretical analysis of the modal behaviour, i.e. standing or spinning waves, was performed.

We demonstrate a distributed feedback (DFB) semiconductor laser based on i-line lithography and surface periodic current injection technologies with a lasing wavelength of 795 nm, which corresponds to the D1 spectrum of Rb and has many applications with respect to alkali vapor lasers and atomic clocks. The maximum output power, side mode suppression ratio (SMSR), and single-longitudinal-mode tunable range of our device are 50.89 mW (at 20 °C), 34.56 dB, and 12.792 nm, respectively. The rate of change in wavelength with temperature of the laser is 0.401 nm/°C, which is equivalent to that of a Fabry-Perot (FP) laser and much better than that of a general DFB device. This device has a cavity the same as an FP laser without any etching gratings, meanwhile the lasing spectrum behaves a single longitudinal mode characteristic as a DFB laser.

In this paper, a static multi-longitudinal mode laser self-mixing absolute angle measurement system is developed to accurately monitor the deflection angle. The experimental results show that this multi-longitudinal mode laser angle sensor is capable of effectively measuring the deflection angle of the vertical dihedral mirror. The angle-measurement range and resolution of the self-mixing experimental sensing system can be obtained as ±22.0° and 0.008°, respectively. Furthermore, in-depth analysis of the angle-measurement error and related error sources are performed, of which the absolute error is less than 0.37° that demonstrates the good reliability and practicality of the self-mixing angle sensor.

Speckle contrast of single, dual, and triple emitter broad area laser diodes (BA-LDs) with a wavelength of 638 nm was experimentally investigated. Speckle noise was relatively small near the threshold current probably due to the longitudinal mode competition. Higher output power led to smaller speckle noise owing to the spectrum widening. A wide stripe presented an advantage in terms of speckle reduction in the single emitter LDs. It is also found that wider emitter width leads to the smaller the speckle noise. A larger number of emitters decreased speckle noise due to the beam superposition effect, even for multi-emitter LDs in which the emitters are built into one chipa.

There are fewer studies on combustion instability (CI) characteristics of non-premixed methane-oxygen flames compared to those of premixed flames. Therefore, this study examined the effect of the total delay time ( τ t o t a l ), which was calculated using the flame structure and speed, on the CI characteristics of a non-premixed methane-oxygen flame using a coaxial injector. The experiments were repeated by varying the O/F ratio, which is a key parameter of τ t o t a l , and combustor length. Increasing τ t o t a l shifted the CI mode from a high harmonic to the fundamental longitudinal mode. Further, increasing the combustor length shifted the CI mode from the fundamental longitudinal to the third harmonic mode resulted in the pressure perturbation occurring only in particular frequency ranges. The CI strength also changed with varying the combustor length, despite a constant τ t o t a l . In conclusion, this study showed that the time-lag analysis based on τ t o t a l and combustor length might be important for determining the CI characteristics of a non-premixed methane-oxygen flame formed by a single coaxial injector, and thus, τ t o t a l could be the main parameter for determining CI characteristics. Further, the relationship between the flame transfer function and CI frequency was investigated. This study is expected to advance our collective understanding of CI mechanisms and provide a means of avoiding CI by applying time-lag analysis to a non-premixed flame.

The paper presents the design and fabrication of lateral-field-excited (LFE) resonators based on 42° Y-cut single-crystal LiTaO3 (LT) on silicon dioxide (SiO2). A simple coplanar top electrode is defined to excite the bulk acoustic wave modes in the suspended LT/SiO2 structure, and the fabrication process that only involves two lithography steps is more simplified compared to that of commercial film bulk acoustic wave resonators. For a model structure consisting of LT(670 nm)/SiO2 (1500 nm) thin film, two types of acoustic modes are both piezoelectrically active in the LT film: the first one is the thickness-shear mode with a resonance frequency of 2.46 GHz, an electromechanical coupling ( $k_{eff}^{2}$ ) of 1.4%, a high quality factor ( ${Q}$ ) of 1690, and the second one corresponds to longitudinal mode with a resonance frequency of 4.39 GHz, ${k} _{eff}^{2}$ of 1.2%, a high ${Q}$ of 1590, which is among the highest reported for piezoelectric MEMS resonators operating at this frequency range. The excellent performance would enable application scenarios including high-resolution sensors, low-phase-noise oscillators, and low-loss, high selectivity filters in the sub-6 GHz range for the fifth-generation (5G) wireless communication.

A wavelength-switchable single-longitudinal-mode thulium-doped fiber laser using a sampled fiber Bragg grating is proposed in this study. The laser can be switched between three single-wavelength and three dual-wavelength modes of operation. For the single-wavelength mode of operation, the maximum power and wavelength fluctuations in 50 min were ±0.404 dB and ±0.01 nm, and the linewidths at three wavelengths were 0.39, 1.88, and 0.89 kHz, respectively. For the dual-wavelength mode of operation, the maximum power and wavelength fluctuations in 20 min were ±0.926 dB and ±0.03 nm, respectively. The proposed thulium-doped fiber laser will find useful application in optical communication and free-space optical detection and sensing.

Within the coupled wave method complemented by the quasi-optical approach, the feasibility of diffraction-mode selection over the transverse index in lasers with planar Bragg resonators is studied. Acceptable Fresnel parameters at which the diffraction losses from the Bragg-structure edges provide steady-state single-mode lasing with a narrow angular spectrum are determined. It is shown that the such stable lasing at high Fresnel parameters is achieved by shifting the operating transition frequency to the frequency of the longitudinal mode of a Bragg resonator with the highest Q-factor.

In the near future, lean-premixed hydrogen combustion technology for low-emission gas turbine engines is expected to be crucial in the effort to mitigate greenhouse gas emissions, in association with energy system decarbonization. In this study, we examine combustion instabilities in lean fully-premixed hydrogen-air flames in a mesoscale multinozzle array; little is currently known about how these flames respond to acoustic perturbations. Several measurement techniques, including phase-synchronized OH* chemiluminescence imaging, OH Planar Laser Induced Fluorescence, acoustic pressure, and the two microphone method, are used, together with reduced-order acoustic modeling, to identify the key physical properties of lean-premixed hydrogen-air flames, in comparison with lean-premixed methane-air flames. We show that extremely compact lean-premixed hydrogen flames are preferentially coupled to higher eigenmodes of a given system, L3 – L6, including approximately 1 kHz high-frequency instabilities, while the instabilities of methane-air flames are predominantly limited to the first longitudinal mode under the same range of operating conditions. This is attributed to the fact that the dynamics of the methane flames are governed by the collective motion of constituent flames, involving a complex process of the emergence, convection, and interaction of large-scale structures. By contrast, the hydrogen flames in the multinozzle configuration oscillate in isolation within a very short distance and without strong flame-to-flame interactions. This is particularly suitable for accommodating high-frequency heat release modulations. The triggering of intense sound generation from lean-premixed mesoscale hydrogen flames is correlated strongly with a combination of flame surface destruction due to front merging and the flames’ close proximity to a pressure antinode. These results, for the first time, highlight the key features of self-excited combustion instabilities of mesoscale multinozzle hydrogen-air flames in a well-controlled laboratory-scale experiment, and could pave the way for future carbon-neutral gas turbine combustion technology.

A kHz-linewidth single-frequency laser at 1070 nm was demonstrated based on homemade heavily Yb3+-doped Yb:YAG-derived silica fiber (YDSF), which was fabricated by molten-core fabrication method. Stable single-longitudinal-mode (SLM) lasing operation was achieved by adopting ring cavity configuration, where a piece of un-pumped polarization maintaining Yb3+-doped fiber (PM-YDF) was utilized as a saturable absorber (SA). A maximum output power of the stable single longitudinal mode continuous wave laser up to 40 mW was obtained with a pump power of 542 mW at 976 nm. The optical signal-to-noise ratio (OSNR) was ~60 dB, and the laser linewidth was measured to be less than 4.3 kHz, which is limited by the measurement setup. The fluctuation of center wavelength of the laser was less than 0.1 pm within 10 min. Wavelength tuning from 1069.99 to 1070.42 nm was achieved by controlling the temperature of the fiber Bragg grating (FBG).

It is an important issue to ensure the long-term reliable service of the reinforced concrete structures by developing effective online monitoring techniques. In this paper, three different accelerated corrosion experiments were conducted by using the electrochemical method, and the ultrasonic pitch-catch and the electrochemical impedance spectroscopy methods were employed to discuss the corrosion phenomena by using the embedded annular piezoelectric ultrasonic transducer. The results show that the capacitance arc length in the high frequency zone of the electrochemical impedance spectra increases gradually with increasing the corrosion rate. When the corrosion rate is larger than 3%, obvious corrosion products can be found on surface of the reinforced concrete, and there exist two capacitance arcs in the corresponding electrochemical impedance spectra. The ultrasonic analysis results show that there is an obvious longitudinal mode wave packet in the time domain spectra, and the peak-peak value shows a linear decreasing trend with increasing the corrosion time. The dominant frequency peak value also decreases with increasing the corrosion time accompanying with the appearances of the new peaks in the low frequency zone. The wavelet transform method was also employed for the ultrasonic time-frequency domain analysis. Based on variations of the acoustic energy and dominant frequency components of the ultrasonic wave, the corrosion extent of the reinforced concrete can be qualitatively evaluated.

We propose a new kind of distributed feedback (DFB) semiconductor laser based on the sampled Moire grating (SMG) integrated with a grating reflector (GR). Coupling coefficient along the laser cavity can be controlled, and $\pi $ phase shift ( $\pi $ -PS) can be introduced by SMG. The GR, which is a uniform sampled grating, can provide light reflection instead of high reflection (HR) facet coating. Thus, the front and rear facets are both coated with anti-reflection coatings (AR/AR) to avoid the influence of random grating phase at the HR facet. Therefore, more output power, better single longitudinal mode (SLM) operation, and higher wavelength precision can be simultaneously achieved, compared with traditional DFB lasers with AR/HR facet coatings. As an example, an 8-channel DFB laser array based on the proposed structure has been simulated. The proposed laser may benefit the applications requiring both high output power and wavelength precision, such as wavelength division multiplexing (WDM) transmission systems and gas sensors.

Metal pipes having an inner diameter of about 25 mm or less are frequently used as heat exchangers for power plants, gas pipes, and water pipes. However, erosion and corrosion due to long-term use may cause serious accidents, such as steam leaks, resulting in economic loss and environmental pollution. Therefore, inspection of the entire length and thickness before shipping or monitoring during operation are important technologies. However, no inspection technology including the inside of the wall thickness has been developed. The purpose was to develop an ultrasonic probe that can inspect the inner and outer surfaces from the inside of the pipe at the same time. The developed ultrasonic probe is based on an electromagnetic ultrasonic transducer (EMAT) that does not require a couplant and is then easy to install in a pipe. The EMAT for the longitudinal and for the transverse vibration mode guided wave are connected in series in order to take into account the variety of defects. First, the EMAT was successfully developed for each mode. That is, it was conducted by using the magnetostrictive effect for the longitudinal mode type and by using the Lorentz force for the transverse mode type. and evaluated to improve the performance. The reflected signal from a notch defect was then evaluated in the state that each EMAT was connected in series using any artificial defects and found to be able to detect any notches with about 10% depth or about 15% circumferential length.

In this investigation, a wavelength-selected single-longitudinal-mode (SLM) erbium-doped fiber (EDF) ring laser utilizing C-band EDF based gain medium over a tuning bandwidth of 1524.0 to 1573.0 nm is demonstrated experimentally. A self-injection Rayleigh backscattering (RB) is designed in the EDF ring laser configuration to suppress the multi-longitudinal-mode (MLM) oscillation and narrow the obtained linewidth. Here, the physical output features of the proposed self-injected RB EDF laser are also studied and discussed, such as the optical signal to noise ratio (OSNR), output power, and laser linewidth, respectively. Therefore, the demonstrated EDF laser not only can reach SLM output, but also can produce a flat output spectrum of 1524.0 to 1554.0 nm with 0.2 dB power variation. Moreover, the measured fluctuation of each wavelength can be kept at ±0.05 nm based on the presented self-injected RB EDF laser, while the uncertainty issue is also considered.

Effective properties of a polyethylene matrix and $$0.965\left({K}_{0.48}N{a}_{0.52}\right)\left(N{b}_{0.96}S{b}_{0.04}\right){O}_{3}-0.035B{i}_{0.5}N{a}_{0.5}Z{r}_{0.15}H{f}_{0.75}{O}_{3}$$ (KNNS-BNZH) inclusion piezocomposite were calculated using finite element method. Auxetic [negative Poisson's ratio (NPR); $$\nu =-0.32$$ ] and non-auxetic (positive Poisson’s ratio $$\nu =0.2$$ ) polyethylene matrix was considered. Sensing, actuation, and energy harvesting performance of the piezocomposite are investigated at six different volume fractions using the effective properties. Performance was found to improve with an increase in volume fractions of the KNNS-BNZH piezoelectric inclusions in the polyethylene matrix. Significant improvement in voltage and harvested power was observed in the case of longitudinal mode when auxetic polyethylene matrix was used compared to non-auxetic. Poisson’s ratios effect was studied by varying it between $$\nu =-0.9$$ and $$\nu =0.4$$ at 30% piezoelectric inclusion by volume fraction. Sensing, actuation, and energy harvesting performance were studied at each Poisson’s ratio both in transverse and longitudinal modes. Auxetic piezocomposite at $$\nu =-0.9$$ performed better compared to non-auxetic at $$\nu =0.4$$ . Voltage, actuation, and power all increased with the use of auxetic polyethylene matrix as compared to the non-auxetic one.

The existing magnetostrictive guided wave transducers are usually circumferential symmetric and cannot be installed on semi-exposed pipes, such as water wall pipes. To test the semi-exposed pipes, we proposed a magnetostrictive transducer with semicircular structure, which consists of a semi-ring magnet and a semicircular coil. Firstly, energy coupling mechanism of magnetostrictive guided wave in the semi-exposed pipe is analyzed. The finite element models of the semi-ring magnet and the semicircular coil are established to study the magnetic field distributions in the pipes. From the simulation results, the semi-ring magnet can provide an axial static magnetic field that is uniform in circumferential direction and the axial direction of the pipe. The semicircular coil can generate the asymmetry axial alternating magnetic field. The magnetic field strength of the pipe surface covered by the semicircular coil is about two times than the uncovered pipe surface. The asymmetry magnetic field causes the induced guided waves include not only axisymmetric modes, but also non-axisymmetric modes. A prototype magnetostrictive guided wave transducer for semi-exposed pipe is provided. Experiments are carried out on two pipes with notch and wear defects respectively. The results show that the transducer can generate the longitudinal mode guided waves as the mainstay and the flexural mode as the auxiliary. The flexural mode is removed by the chirplet transform and the proposed transducer can be used to detect the flaw in the semi-exposed pipe.

We present results on the first demonstration of self-sweeping effect in a ring Er-doped fiber laser. Reverse self-sweeping (with decreasing wavelength) in a range of ~0.1 nm was obtained. Typical sweeping rate was 4 pm/s, which is sufficiently slower as compared with earlier reported fiber lasers. Dual longitudinal mode self-sweeping (similar to reported earlier in linear cavity Er-doped fiber laser) is obtained. The laser generates sequentially long overlapping pulses. Frequency of each subsequent pulse increases by one inter-mode spacing. However, the typical duration of dual-mode operation is ~10 ms, which is one order longer than in previous works. We demonstrated that increase in pump power causes deceleration of sweeping and elongation of dual-mode pulses up to 500 ms. The developed source can be used for fiber sensor interrogation.

Here we demonstrated an efficient high-power single-frequency thulium-doped fiber ring laser operating at 1720 nm. Three cascaded sub-rings were inserted into the main cavity to significantly enlarge the effective free spectral range. By incorporating a fiber Bragg grating, the single longitudinal mode operation was achieved. The maximum single-frequency output power reached up to 1.11 W under 3.75-W launched pump power, while the slope efficiency with respect to the absorbed pump power was 46.4%. The laser linewidth at maximum single-frequency power was measured of 1.9 kHz. Potential power scaling of the single-frequency output power with different quantity and lengths of the sub-rings was also theoretically investigated.

A compact high power extended interaction oscillator (EIO) is studied with an aim to increase its output power. The cavity is designed, so that its transverse mode is TM 11 and the longitudinal mode is 2π and the operating frequency is 38 GHz. The modulating E z field is optimized in such a way that it varies ascendingly in the growing z-direction. This gradually growing E z -field has been found, to have a positive impact on sheet-beam EIO output performance in terms of its output power P out and beam-wave interaction efficiency. To incorporate the negative influence of skin effect and surface roughness on the output performance the conductivity of background material is taken one-fourth of that of pure copper σ Cu . After field optimization the final design output power P out is predicted to improve from 20.19 kW to 24.98 kW and efficiency is improved from 33% to 40.83%.

We proposed a new kind of distributed feedback (DFB) laser based on sampled grating with non-central phase shift. The output facet of the laser is coated with antireflection (AR) film. Another facet is coated with high-reflection (HR) film to improve output power. Non-central phase shift is introduced for better single longitudinal mode (SLM) operation. Precise wavelength is achieved by changing sampling periods. A 16-channel DFB laser array is experimentally fabricated and shows high output power over 150 mW, high side mode suppression ratio (SMSR) above 45 dB, and low wavelength deviation less than 0.3 nm.

We proposed a monolithically integrated narrow linewidth semiconductor laser composing of an active section and a passive narrow band reflector (NBR) based on waveguide Bragg gratings. The π phase shifted Bragg grating is used in the active section to obtain single longitudinal mode output. The NBR is consist of a taper waveguide, π phase shifted antisymmetric Bragg grating (π-PS-ASBG) and uniform Bragg grating. Two sections are integrated on an identical InP wafer and share one continuous waveguide. Because of the hybrid mode resonance in the passive section, it has a relatively long effective length to narrow the linewidth of laser. Narrow linewidth about 16 kHz is obtained when the laser's length is 800 µm and the coupling coefficient of the π-PS-ASBG is 180 cm−1.

This Letter presents, to the best of our knowledge, the first report of a narrow-linewidth ∼ 790 − 800 n m edge-emitting semiconductor distributed feedback Bragg reflection waveguide diode laser ( D F B 2 R L ). The D F B 2 R L s were fabricated using a ridge waveguide structure with 5th order, surface-etched grating forming the wavelength selective element. Unbonded devices with a 500 µm cavity length exhibited continuous wave threshold currents in the region of 25 mA with an output power of 2.5 mW per (uncoated) facet at 100 mA drive current. The devices operated in a single longitudinal mode, with side-mode suppression ratio (SMSR) as high as 49 dB and linewidths as low as 207 kHz. Devices maintained single mode operation with high SMSR over a 9 nm wavelength range as the temperature was swept from 15°C to 50°C.

Single longitudinal mode and single polarization are basic requirements of high performance fiber lasers, while their realizations are nontrivial, owing to the long laser cavity and lack of polarization selection of ordinary optical fibers. Here, we demonstrate an all-fiber narrow-linewidth laser realized on an external high-Q fiber ring, with combined functions of single-longitude-mode selection and linewidth reduction. A single-longitude-mode laser with a high polarization extinction ratio of ∼ 40 d B and low white frequency noise at 0.3 H z 2 / H z is achieved, corresponding to a fundamental linewidth of ∼ 0.92 H z . Using all non-polarization fiber components and ordinary gain fiber, our scheme shows the realization of narrow-linewidth single-polarization fiber lasers in a simple and cost-effective way, promising for broadband applications.

We present a design methodology for hybrid lasers to realise mode-hop free operation by controlling the cavity mode spacing. In this study, a compact hybrid photonic crystal laser (H-PhCL) was employed which allowed a reduction of the Fabry–Perot length of the laser cavity and eliminated the need for an active mode stabilisation mechanism in order to realise mode-hop free operation. The H-PhCL was formed by butt-coupling a reflective semiconductor optical amplifier (RSOA) with a two-dimensional silicon (Si) photonic crystal (PhC) cavity. Continuous stable single frequency operation with &gt, 40 dB side-mode suppression ratio (SMSR) of the laser was achieved for gain currents of up to 100 mA, i.e., up to four times the threshold current. The shorter length of the laser cavity enabled single frequency operation due to the selection of a single longitudinal mode by the PhC narrowband reflector. Various longitudinal mode spacing regimes were studied to explain the mode-hop free characteristics of the H-PhCL. The proposed hybrid laser design methodologies can be adapted to eliminate mode-hopping in laser wavelength.

An evaluation model for optimizing system parameters was established in this study using longitudinal-mode pre-lase selection technology. By solving the rate equation and calculating the evaluation function, we determined the evaluation efficiency of a single-longitudinal-mode operation using this scheme. The optimal condition for the characteristics of the single-longitudinal Q-switched laser was also analytically calculated in detail. We observed that the evaluation function derived using the two-step signal of the Q-switch modulation time and other resonator parameters significantly influenced the output characteristics of the single-longitudinal-mode Q-switched laser operation. Furthermore, the experimental results were consistent with the simulations.

The single-frequency operation of a thulium fiber laser at a short wavelength of 1720 nm is investigated in a ring resonator. Powerful single-longitudinal-mode operation was realized by utilizing an unpumped thulium-doped fiber as the saturable absorber. The fiber laser delivered 407 mW single-frequency output with a spectral linewidth of 4.4 kHz under 2.7-W launched pump power at 1570 nm, which turned to multi-longitudinal-mode operation at higher pump powers. Additionally, optical bistability of both output power and longitudinal mode behavior, originating from the saturable absorption effect, were observed and discussed. To the best of our knowledge, this is the first efficient 1.7-μm single-frequency fiber laser as well as the first demonstration of optical bistability in thulium-doped fiber lasers.

An L-band single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL) is proposed and experimentally demonstrated to exhibit an ultranarrow linewidth and a high optical signal-to-noise ratio. A multistage filter, which is composed of a thin film filter, a fiber Bragg grating, and a saturation absorption, is utilized as an effective mode-selecting filter to achieve an SLM EDFL. The SLM EDFL with an ultranarrow linewidth of 205 Hz is obtained at the measurement resolution bandwidth of 100 Hz. In addition, the stability performance of the EDFL is measured during 30 min. The maximum fluctuations of the center wavelength and the output power of the proposed EDFL are 0.04 nm and 0.01 dB.

In recent years diamond Raman lasers have become of interest for high power, single longitudinal mode (SLM) lasing. These systems benefit from the wavelength agility provided by the Raman mechanism, the lack of spatial hole burning in Raman gain medium and the high power handling ability of diamond. While single-frequency operation has been reported at infrared and visible wavelengths [1] , [2] , the detailed characteristics of the output frequency spectrum has not been formally addressed. Here we study the factors influencing the stability of the centre wavelength in diamond Raman lasers, and investigate the potential for "fast" thermo-optic actuation enabled by diamond’s supreme thermal conductivity. Such devices have the potential to supplant piezoelectric actuators which are inherently speed-limited by mechanical resonances.

We present design and performance data of two diode-pumped Alexandrite lasers developed explicitly as laser emitters in mobile potassium resonance lidar systems. The lasers yield an output power of up to 1.75 mJ at a repetition rate of 500 Hz with a beam quality of M 2 < 1.1 in both spatial directions. Reliable single longitudinal mode operation with a unrivaled narrow linewidth of 3.3 MHz at a potassium resonance line at 769.898 nm is achieved. The wavelength can be switched from pulse to pulse in a range of several gigahertz so the potassium line can be scanned. The lasers are finally integrated in highly efficient lidar systems with a power consumption of 500 W for the whole lidar system. The extremely high spectral requirements are investigated and the performance for different working points regarding repetition rates and pump durations is investigated. Several weeks of remotely controlled operation of the prototype in a field campaign were conducted without changes of the output parameters. Approximately 1000 h of reliable single longitudinal mode operation was achieved during the campaign and measurements of Doppler–Mie wind observations in the stratosphere and of the potassium layer in the mesopause were conducted simultaneously even at daytime.

A high-repetition pulsed Nd : YVO4 laser based on the multi-longitudinal-mode beat note is presented. The mechanism of multi-mode oscillating in the Nd : YVO4 laser with FP cavity is analyzed. With the multi-mode Nd : YVO4 laser in experiments, it is found that the more longitudinal modes beating in the laser cavity, the narrower time duration of the laser pulse is obtained. An adjustable aperture is employed in the laser cavity to weaken the transverse mode interference and improve the laser pulse train stability. At last, when the FP laser cavity length is 88 mm, by slight tuning the Nd : YVO4 crystal temperature, a 1.56 GHz high-repetition laser pulse with 0.18-ns time duration is achieved. The average output power of the pulse train is 1605 mW with the pump power at 6788 mW. The slope efficiency and the optical-to-optical efficiency are calculated as 31% and 24%, respectively.

In the study, ultrasonic longitudinal mode guided waves were employed to detect defects in elbowed tubes (without welds) with a diameter of 10 mm. Finite element simulation results highlighted that the emitted L(0,1) mode guided waves experienced strong reflection and mode conversion at the elbow region to generate F(1,1) mode, followed by slow and weak F(2,1) mode. The guided wave reflected from the elbow with a through-wall defect was manifested as two overlapped wave packets, which were good indicators of a defective elbow. To conduct L(0,1) mode guided waves inspection on the small-diameter elbowed tubes, a novel tailored squirrel-cage magnetostrictive sensor was employed in the experiment. The new sensor employed the configuration of segmental iron-cobalt strips and small-size permanent magnet arrays. The entire sensor is composed of two identical C-shaped sensor elements and can be recycled and installed conveniently. Experimental results obtained from healthy and defective tubes were consistent with the conclusions obtained from finite element simulations. An artificial through-wall defect at the elbow and a notch defect at the straight part of the tube could be simultaneously detected by L(0,1) mode guided waves through comparing experimental signals with simulation results.

Currently, the obtention of nano-structures based on III-V materials is expensive. This calls for novel and inexpensive nanostructure manufacturing approaches. In this work we report on the manufacture of a nanostructures consisting of alternating layers of In and GaAs on a silicon substrate by magnetron sputtering. Furthermore, we characterized the produced nanostructures using secondary ion mass spectroscopy (SIMS), X-ray diffraction analysis, and Raman spectroscopy. SIMS revealed variation in the concentration of In atoms across In/GaAs/In interphases, and X-ray diffraction revealed planes corresponding to phases associated with GaAs and InAs due to In interfacial diffusion across GaAs layers. Finally, in order to study the composition and crystalquality of the manufactured nanostaructures, Raman spectra were taken using laser excitation lines of 452 nm, 532 nm, and 562 nm at different points across the nanostructures.This allowed to determine the transverse and longitudinal optical modes of GaAs and InAs,characteristic of a two-mode behavior. An acoustic longitudinal vibrational mode LA(Γ) of GaAs and an acoustic longitudinal mode activated by disorder (DALA) were observed. These resulted from the substitution of Ga atoms for In atoms in high concentrations due to the generation of Ga(VGa) and/or Arsenic(VAs) vacancies.This set of analyses show that magnetron sputtering can be aviable and relatively low-cost technique to obtain this type of semiconductors.

We demonstrate InP-based buried grating coupled surface-emitting distributed feedback (DFB) lasers designed to operate at a wavelength of 2004 nm. The laser structure consists of three InGaAsSb/InGaAs QWs, with a 5 $\mu \text{m}$ wide double-channel ridge waveguide. A second-order semiconductor/semiconductor grating is used for in-plane feedback and vertical out-coupling. The single longitudinal mode emission wavelength of the fabricated laser can be adjusted from 2002.7 to 2006 nm without any mode hopping. High side-mode suppression ratio (SMSR) of at least 35 dB is achieved under all injection currents and temperature conditions. The edge output power reaches 19 mW, measured in continuous-wave (CW) mode at 10 °C. Simultaneously, the output power of surface emission reaches 8 mW.

A passively Q-switched single-frequency fiber laser with stable multi-wavelength operation is presented in this report. The laser cavity is constructed by using a 13-mm Er 3+ /Yb 3+ co-doped phosphate fiber with a multi-channel fiber Bragg grating and a semiconductor saturable absorber mirror attached to each fiber end, respectively. Quad-wavelength pulsed lasers are obtained and all of them maintain single-frequency operation. By controlling the pump power and the temperature of laser cavity, the Q-switched pulse operation is realized with repetition rates range of 126.6-350.9 kHz, a narrowest pulse duration of 104 ns, and a maximum pulse energy of 55.4 nJ. To the best of our knowledge, this is the first study to realize a Q-switched fiber laser operating at multiple wavelengths with single longitudinal mode. Furthermore, the evolutions of the pulse energy and repetition rates for different wavelength numbers are further analysis theoretically. This type of single-frequency pulsed fiber laser has extensive application prospect in fiber sensing and lidar systems.

A single longitudinal mode (SLM) fiber laser with a ring cavity based on Sagnac interferometer (SI) for temperature sensing in the 2-μm band is proposed and demonstrated. The simulation and experimental results show that the transmission spectrum of the SI filter is sensitive to temperature change. Based on this distinguishing feature, the correlation between the laser wavelength drift and temperature variation can be established. Experimental results show that the proposed laser operates stably at the resonant wavelength of 1988.21 nm and the optical signal-to-noise ratio is approximately 55 dB at ambient temperature. Through continuous measurement for 100 min, the fluctuation of the output laser power is lower than 0.74 dB, and the resonant wavelength shift is less than the minimum resolution of the spectrometer of 0.05 nm. This indicates that it has good stability for a certain period of time. At the same time, the laser operates stably in SLM state. A temperature sensitivity of 2.09 nm/°C is obtained in experiment. Notably, the proposed fiber laser sensor possesses the merits including high sensitivity, low cost and simple structure, which are beneficial to its practical application.

The flame dynamics of ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) composite propellant subject to the acoustic oscillation in a solid rocket motor (SRM) is investigated numerically in this article. The sandwich model is suitably modified to carry out the simulations of propellant combustion under unsteady conditions and the flame response to the acoustic oscillations with different amplitudes and frequencies is investigated. Attention is focused on the heat flux to the propellant surface and the Flame Describing Function (FDF) is utilized to capture the characteristics of the heat feedback under different forcing amplitudes and frequencies. The results indicate the flame responses are non-linear for the first longitudinal mode of acoustic oscillation and the gain of the fluctuating heat feedback reaches the point saturation at the high forcing amplitude. For high forcing frequency (the third acoustic mode), the gain of the fluctuating heat feedback shows insensitivity to the acoustic oscillations with different amplitudes and varies little. The full FDF confirms the nonlinearity of the gain response and shows the trend of fluctuating heat feedback as a function of different forcing frequencies and amplitudes.

A diode-pumped solid-state laser directly emitting continuous-wave (cw), single-longitudinal mode (SLM) output at 640-nm with a c-cut praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal was demonstrated for the first time, to the best of our knowledge. Combining two quarter-wave plates (QWPs) and one Brewster plate to form a twisted-mode cavity, this SLM solid-state laser achieved a maximum cw output power of 403 mW with a threshold of 600 mW and a slope efficiency of 26.8%. The emission spectrum had a linewidth of 150 MHz. The single-mode beam quality (M2) was 1.10 and 1.07 along the x and y direction, respectively.

We present a 445-nm InGaN blue diode laser in wavelength-tunable Littrow-type external cavity with a single longitudinal mode output, and a linewidth of 4.7 MHz. Its tuning range was estimated at 4 nm. The maximum output power was up to 20 mW, and the slope efficiency was approximately 0.36 W/A. The external cavity-diode laser linewidth was measured as 4.7 MHz with a lasing threshold to 8.1 MHz at higher applied current. The slope of the linewidth broadening with applied currents was calculated as 57 kHz/mA. And the wavelength tuning range was approximately 4.3 nm at low applied current and narrowed to approximately 2.6 nm at high applied current. A blue external cavity diode laser can deliver a single frequency beam within entire operation range.

In this paper, we demonstrate a regrowth-free double-tapered gain-coupled distributed feedback semiconductor laser. It is designed based on periodic surface current injection to reach a high-power and single-longitudinal mode. A continuous-wave output power of over 1.2 W/facet is achieved at 4 A. High single-longitudinal-mode output power reaches up to 0.9 W/facet at 3 A at each uncoated facet. The side mode suppression ratio is nearly 30 dB at 980 nm. The 3 dB spectral width is less than 2.7 pm. The lateral far field divergence angle is only 14.5°, the beam quality factor M2 is 1.7, achieving a near-diffraction-limit emission. Our device, produced by standard i-line lithography, enhances the output power while obtaining the pleasurable spectral and spatial properties. It has great potential in widespread commercial applications such as high efficiency pumping sources for its low-cost, easy fabrication technique and excellent performance.

A method based on the self-mixing vibration sensing signal of a multi-longitudinal mode laser is found to measure the temperature of the laser cavity. In the experimental system, the temperature variation of the laser cavity can be extracted from tracking the undistorted waveforms of the superimposed multi-longitudinal mode laser self-mixing vibration sensing signal that are highly susceptible to the compensating length of the external cavity. The temperature measurement resolution of the laser cavity temperature monitoring system is 0.253 K, which corresponds to $39.49~ {\mu }\text{m}$ /K of the external cavity sensitivity. It has also been shown that the experimental results are well consistent with the measurement of using the thermal imaging camera.