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We firstly propose a roadmap for the design of high repetition rate (~100 MHz) Tm-doped ultrafast fiber laser with an emission wavelength above 2000 nm. This roadmap is confirmed by both theoretical simulation and experimental verification. The designed Tm-doped fiber oscillator not only operates in continuous wave regime with a broad tunable wavelength range of ~ 100 nm, but also in mode-locking regime with an emission wavelength > 2000 nm (up to 2020 nm). A 470-fs laser pulse with a pulse repetition rate of ~96 MHz and the maximum average power of 50 mW are achieved at the output wavelength of 2007 nm, matching well with the theoretical prediction. In a further step, the seed laser pulse is amplified to 3.3 W, accompanied by a Raman frequency shift of the output wavelength to 2346 nm. The homogeneous intense broadband spectrum with a bandwidth of 240-nm centered at 2200 nm is also observed.

In this work, we demonstrate a novel In0.52Al0.48As based top-illuminated avalanche photodiode (APD), designed to circumvent the problem of serious bandwidth degradation under high gain (>100) and high power operation and significantly enhance the dynamic range in the established frequency modulated continuous wave (FMCW) lidar system. In our APD design, the carriers transiting through the dual multiplication (M-)layers are subjected to a stepped-up electric field profile, so they can be energized by the first step and propagate to the second step to trigger the avalanche processes. Such a cascade avalanche process leads to an ultra-high gain bandwidth product (460 GHz) with a 1 A/W responsivity at unit gain. Compared to the high-performance and commercial p-i-n PD and photo-receiver (PD + trans-impedance amplifier (TIA)) installed in the same lidar test bed, our demonstrated APD receiver (without TIA) has a larger S/N ratio under high operation gain (33 A/W) with less optical local-oscillator (LO) power required (0.25 vs. 0.5 mW), while exhibiting a wider dynamic range in each pixel. These advantages in turn lead to the construction of a better quality of 3-D lidar image by using the demonstrated APD.

This report presents the first demonstration of passive RF comb filters made using epitaxial GaN/NbN/SiC high overtone bulk acoustic resonators (epi-HBARs). The two-port device is fabricated on electronic-grade GaN, electrically transduced, and acoustically coupled. The multi-mode epi-HBAR comb filter demonstrated here has 158 sharp filter passbands periodically distributed between 1 and 4 GHz (L–S-bands) with a free spectral range (FSR) of 17 MHz. The individual passbands of the epi-HBAR comb filter demonstrate transmission bandwidths (BWs) up to 800 kHz, ${f} {\times } {Q}$ values of up to $7\times 10^{{14}}$ Hz, and an average ${k}_{ {\text {eff}}}^{ {{2}}} {\times } {Q}$ figure of merit of 41.2 at room temperature. The GaN/NbN/SiC epi-HBAR comb filter is capable of operating at high RF power levels, with linear and distortion-free performance seen up to at least 1 W of continuous wave (CW) power and up to at least 10 W of pulsed power. The compact epi-HBAR comb filters can be co-fabricated with GaN-based electronics and could potentially replace larger, off-chip or discrete-component comb filters. They can be used for spectrum sensing and as signal processing elements for remote sensing and pulsed radar.

A high-power monolithic continuous wave (CW) Yb doped fiber amplifier at 1064 nm has been demonstrated based on traditional large mode area step-index fiber, which generated 5.07 kW narrow line-width laser with near diffraction-limited beam quality. At the maximal output power, >35dB OSNR has been achieved with line-width being 370pm, and the measured beam quality is M2x = 1.252, M2y = 1.322. The influence of cooling temperature on SRS has been investigated in high power fiber lasers, which shows that the SRS at 4kW has been suppressed 24 dB by lowering the temperature from 20°C to 8°C. To the best of our knowledge, this is the highest narrow line-width laser power generated from the traditional large mode area step-index monolithic fiber amplifier.

In this paper, a systematic study of the relationship between nonlinear crystal length and intracavity second-harmonic generation (SHG) using MgO-doped periodically-poled lithium niobate (MgO:PPLN) is presented. The experimental results demonstrate a relationship between the maximum SHG power generated and the full-width at half maximum (FWHM) of the crystal’s temperature tuning curve to the length of the nonlinear optical crystal. It was shown that maximum SHG power increases rapidly with the increase of MgO:PPLN length, reaching a saturation length (~2 mm), which is much shorter than that predicted by the single-pass SHG theory. This saturation length of the MgO:PPLN crystal is almost independent on 808 nm pump power for typical powers used in continuous wave intracavity SHG lasers. In addition to this saturation effect, a broadening effect was also observed, the FWHM of the temperature tuning curve was shown to have a larger FWHM than that predicted by the single-pass SHG theory for MgO:PPLN shorter than the saturation length. This work has the benefit of allowing engineers to optimize nonlinear crystal length when developing intracavity SHG based diode-pumped solid state (DPSS) lasers.

We report the observation of photoluminescence emission from airborne gold, silver, and copper nanoparticles. A continuous wave 532 nm laser was employed for excitation. Photoluminescence from gold nanoparticles carried in a nitrogen gas flow was both spectrally resolved and directly imaged in situ using an intensified charge-coupled device camera. The simultaneously detected Raman signal from the nitrogen molecules enables quantitative estimation of the photoluminescence quantum yield of the gold nanoparticles. Photoluminescence from metal nanoparticles carried in a gas flow provides a potential tool for operando imaging of plasmonic metal nanoparticles in aerosol reactions., Graphical Abstract

The performance of a CMOS transmitter designed for planetary science in situ molecular sensing applications having an operational bandwidth of 180–190 GHz and peak output power of 0.6 mW (−2.22 dBm) is evaluated with a series of spectroscopic-based experimental trials. Continuous wave frequency modulated absorption schemes are exploited to probe the Doppler and sub-Doppler lineshape profiles of the water rotational transition at 183.310 GHz. These results demonstrate the tuning finesse and phase-noise characteristics of the integrated circuit embedded phase lock loop used to generate coherent mm-wave radiation are sufficient for high-precision molecular spectroscopy applications. A description of the pulse modulator designed into the CMOS circuitry allowing for implementation of sensitive emission-based Fourier transform detection schemes is provided with performance characterized for spectroscopically relevant pulse durations (40–500 ns). Results are accompanied by a spectral analysis of the transmitter pulse signal leakage, where the total isolation is measured to be 22 dB. The first emission-based molecular detections obtained with this source are presented demonstrating viability for this transmitter to be incorporated into future planned resonant cavity enhanced in situ molecular sensing systems.

Two-dimensional (2D) layered materials such as GaSe recently have emerged as novel nonlinear optical materials with exceptional properties. Although exhibiting large nonlinear susceptibilities, the nonlinear responses of 2D materials are generally limited by the short interaction lengths with light, thus further enhancement via resonant photonic nanostructures is highly desired for building high-efficiency nonlinear devices. Here, we demonstrate a giant second-harmonic generation (SHG) enhancement by coupling 2D GaSe flakes to silicon metasurfaces supporting quasi-bound states in the continuum (quasi-BICs) under continuous-wave (CW) operation. Taking advantage of both high-quality factors and large mode areas of quasi-BICs, SHG from a GaSe flake is uniformly enhanced by nearly 4 orders of magnitude, which is promising for high-power coherent light sources. Our work provides an effective approach for enhancing nonlinear optical processes in 2D materials within the framework of silicon photonics, which also brings second-order nonlinearity associated with 2D materials to silicon photonic devices.

An effective method of tuning the frequency of magnetron is studied in this article. The frequency-agile technique is based on changing the structure of the resonator by tuning elements. First, the working principle of this frequency-agile technique for changing an anode structure is proposed and simulation analysis suggested the good performance of this device. Next, a 3-D particle-in-cell simulation analysis is implemented to predict a frequency-adjustable magnetron with an output power of 2 kW, a mode separation of 50%, an efficiency of 67.7%, a bandwidth of 65 MHz, and a sinusoidal modulated signal. It was found in an experiment that our proposed prototype magnetron has a tunable bandwidth which meets with the simulation results. This effective method has a great potential for application in the frequency-agile magnetron.

Microscale lasers efficiently deliver coherent photons into small volumes for intracellular biosensors and all-photonic microprocessors. Such technologies have given rise to a compelling pursuit of ever-smaller and ever-more-efficient microlasers. Upconversion microlasers have great potential owing to their large anti-Stokes shifts but have lagged behind other microlasers due to their high pump power requirement for population inversion of multiphoton-excited states. Here, we demonstrate continuous-wave upconversion lasing at an ultralow lasing threshold (4.7 W cm−2) by adopting monolithic whispering-gallery-mode microspheres synthesized by laser-induced liquefaction of upconversion nanoparticles and subsequent rapid quenching (“liquid-quenching”). Liquid-quenching completely integrates upconversion nanoparticles to provide high pump-to-gain interaction with low intracavity losses for efficient lasing. Atomic-scale disorder in the liquid-quenched host matrix suppresses phonon-assisted energy back transfer to achieve efficient population inversion. Narrow laser lines were spectrally tuned by up to 3.56 nm by injection pump power and operation temperature adjustments. Our low-threshold, wavelength-tunable, and continuous-wave upconversion microlaser with a narrow linewidth represents the anti-Stokes-shift microlaser that is competitive against state-of-the-art Stokes-shift microlasers, which paves the way for high-resolution atomic spectroscopy, biomedical quantitative phase imaging, and high-speed optical communication via wavelength-division-multiplexing., Upconversion microlasers present a lot of advantages but also require high pumping powers. Here the authors present a high-performing microlaser based on anti-Stokes-shift in upconversion nanoparticles synthesized using a technique of liquid quenching.

We demonstrate the laser performance of a LD end-pumped Er:YSGG crystals with different doping concentrations comparatively. The experimental results show that 30 at. % Er:YSGG crystal with dimensions of 2 mm $\times2$ mm $\times8$ mm has the best laser output. In continuous-wave (CW) mode and pulse mode, the maximum output powers of 1.37 and 1.20 W are achieved, corresponding to slope efficiencies of 23.62% and 22.91%, respectively. To the best of our knowledge, compared with other Er $^{3+}$ -doped garnet laser crystals, we obtained a larger power and higher efficiency CW laser simultaneously for LD end-pumped Er:YSGG crystal. The beam quality factor M2 in x and y directions are determined to be 1.64/1.65, suggesting the Er:YSGG laser possesses a higher beam quality. All these results indicate that Er:YSGG is a promising mid-infrared gain medium for high efficiency and high power laser operation.

Greater than 190 °C continuous wave (CW) lasing is achieved from 905 nm high-efficiency vertical cavity surface-emitting lasers (VCSELs). The maximum power conversion efficiency (PCE) of 46.3% for ${4}~\mu \text{m}$ VCSELs was demonstrated under the ambient temperature of 30 °C. The maximum PCE decreases only 10% of its value from 30 °C to 90 °C, showing stable high-temperature characteristics. The thermal resistances and active region temperatures of the device versus ambient temperature are determined experimentally. Besides, size-dependent thermal characteristics are also analyzed by characterizing the spectral and spatial properties of devices with various apertures. It is found that the higher the ambient temperature is, the less the number of high-order modes is, and single fundamental mode operation can be realized in ${4}~\mu \text{m}$ VCSELs even at a relatively high temperature of 190 °C. The design and characterization of these high-temperature VCSEL devices might present the basis for future laser radar application.

Amorphous chalcogenide glasses are intrinsically metastable, highly photosensitive, and therefore exhibit numerous light-induced effects upon bandgap and sub-bandgap illumination. Depending on the pulse duration of the excitation laser, ChGs exhibit a series of light-induced effects spanning over femtosecond to seconds time domain. For continuous wave (CW) illumination, the effects are dominantly metastable in terms of photodarkening (PD) and photobleaching (PB) that take place via homopolar to heteropolar bond conversion. On the other hand, under nanosecond and ultrafast pulsed illumination, ChGs exhibit transient absorption (TA) that is instigated from the transient bonding rearrangements through self-trapped exciton recombination. In the first part of the review, we pay special attention to continuous wave light-induced PD and PB, while in the second part we will focus on the TA and controlling such effects via internal and external parameters, e.g., chemical composition, temperature, sample history, etc.

In this Article we explore the requirements for enabling high quality optically detected magnetic resonance (ODMR) spectroscopy in a conventional gradient force optical tweezers using nanodiamonds containing nitrogen-vacancy (NV-) centers. We find that modulation of the infrared (1064 nm) trapping laser during spectroscopic measurements substantially improves the ODMR contrast compared with continuous wave trapping. The work is significant as it allows trapping and quantum sensing protocols to be performed in conditions where signal contrast is substantially reduced. We demonstrate the utility of the technique by resolving NV- spin projections within the ODMR spectrum. Manipulating the orientation of the nanodiamond via the trapping laser polarization, we observe changes in spectral features. Theoretical modeling then allows us to infer the crystallographic orientation of the NV-. This is an essential capability for future magnetic sensing applications of optically trapped nanodiamonds.

This study presents an ultra-wideband receiver front-end, designed for a reconfigurable frequency modulated continuous wave radar in a 130 nm SiGe BiCMOS technology. A variety of innovative circuit components and design techniques were employed to achieve the ultra-wide bandwidth, low noise figure (NF), good linearity, and circuit ruggedness to high input power levels. The designed front-end is capable of achieving 1.5–40 GHz bandwidth, 30 dB conversion gain, a double sideband NF of 6–10.7 dB, input return loss better than 7.5 dB and an input referred 1 dB compression point of −23 dBm. The front-end withstands continuous wave power levels of at least 25 and 20 dBm at low band and high band inputs respectively. At 3 V supply voltage, the DC power consumption amounts to 302 mW when the low band is active and 352 mW for the high band case, whereas the total IC size is $3.08\, {\rm nm{^2}}$.

We propose and demonstrate an all-optical tunable polymer bottle micro-resonator fabricated by droplet dispensing method. A microfiber is used to excite whispering gallery mode (WGM) of the micro-resonator. Continuous wave (CW) laser is employed as pump light to tune the WGM. To illustrate tuning performance of the fabricated device, external and internal pump light methods are both employed. For external pump light method, the tuning efficiency is about 5.23 pm/mW. For internal pump light, tuning efficiency of 68.3 pm/mW is obtained. The experimental results reveal that the bottle micro-resonator (BMR) can achieve wavelength tuning more than one free spectral range (FSR), which indicates the WGM can be tuned within a transparent window covered the full wavelength range. In addition, the fabricated device has advantages of high tuning efficiency, small size, easy fabrication, and extremely low cost.

In this article, we propose a novel scheme for an optical frequency comb (OFC) generator which implements a gain switched (GS) semiconductor laser (SL) in the presence of external optical feedback (EOF). Numerical results demonstrate that with appropriate choices of the basic system parameters (i.e., the feedback coupling fraction, the laser bias current and the external cavity length), the linewidths in the OFCs generated by the GS SL with EOF can be significantly reduced below that of the free running continuous wave (CW) SL. Such a significant linewidth reduction can be described by a simple analytic expression presented in this article which is found to be in good agreement with the numerical results.

The synchronously periodic re-melting of molten pool was firstly introduced in additive manufacturing to promote the epitaxial growth of columnar structure using a novel quasi-continuous-wave (QCW) laser. The epitaxial growth of columnar structure was intensified and the single-crystal-like sample with highly oriented “zigzag” columnar grains was produced. The modified molten-pool geometry and the synchronously high-frequency re-melting of the molten pool contribute to the formation of single-crystal-like structure. This work reports a new route to promote the continuously epitaxial growth of dendrites for fabrication of single-crystal-like sample.

A time-resolved Raman spectrometer is demonstrated based on a 256\times 8 single-photon avalanche diodes fabricated in CMOS technology (CMOS SPAD) line sensor and a 573-nm fiber-coupled diamond Raman laser delivering pulses with duration below 100-ps full-width at half-maximum (FWHM). The collected backscattered light from the sample is dispersed on the line sensor using a custom volume holographic grating having 1800 lines/mm. Efficient fluorescence rejection in the Raman measurements is achieved due to a combination of time gating on sub-100-ps time scale and a 573-nm excitation wavelength. To demonstrate the performance of the spectrometer, fluorescent oil samples were measured. For organic sesame seed oil having a continuous wave (CW) mode fluorescence-to-Raman ratio of 10.5 and a fluorescence lifetime of 2.7 ns, a signal-to-distortion value of 76.2 was achieved. For roasted sesame seed oil having a CW mode fluorescence-to-Raman ratio of 82 and a fluorescence lifetime of 2.2 ns, a signal-to-distortion value of 28.2 was achieved. In both cases, the fluorescence-to-Raman ratio was reduced by a factor of 24-25 owing to time gating. For organic oil, spectral distortion was dominated by dark counts, while for the more fluorescent roasted oil, the main source of spectral distortion was timing skew of the sensor. With the presented postprocessing techniques, the level of distortion could be reduced by 88%-89% for both samples. Compared with common 532-nm excitation, approximately 73% lower fluorescence-to-Raman ratio was observed for 573-nm excitation when analyzing the organic sesame seed oil. publishedVersion

This article reports investigation of self-actuation in phase change material (PCM) germanium telluride (GeTe)-based radio frequency (RF) switches as a limitation to RF power handling. PCM switches are fabricated in-house with multiple design dimensions for evaluation and testing under various RF power levels. RF power handling capability and maximum dc current carrying capacity in the ON-state of the switches is studied for four different device layouts. Self-actuation in the OFF-state is reported at varying input RF power and dc voltage levels. A tradeoff between RF power handling capacity and device performance is discussed, establishing a correlation between design parameters and the power handling. The improved PCM switches exhibit a record high continuous wave (CW) RF power handling capability ranging from +35.5 to +39.2 dBm.

Resonant metasurfaces are an attractive platform for enhancing the non-linear optical processes, such as second harmonic generation (SHG), since they can generate very large local electromagnetic fields while relaxing the phase-matching requirements. Here, we take this platform a step closer to the practical applications by demonstrating visible range, continuous wave (CW) SHG. We do so by combining the attractive material properties of gallium phosphide with engineered, high quality-factor photonic modes enabled by bound states in the continuum. For the optimum case, we obtain efficiencies around 5e-5 % W$^{-1}$ when the system is pumped at 1200 nm wavelength with CW intensities of 1 kW/cm$^2$. Moreover, we measure external efficiencies as high as 0.1 % W$^{-1}$ with pump intensities of only 10 MW/cm$^2$ for pulsed irradiation. This efficiency is higher than the values previously reported for dielectric metasurfaces, but achieved here with pump intensities that are two orders of magnitude lower.

We present wide wavelength tuning of a gainswitched Er 3+ -doped ZrF 4 fiber laser in the 3.5 μm band for the first time to our knowledge. In this case, a dual-wavelength pumping (DWP) scheme is used including a continuous wave (CW) LD at 976 nm and two home-made Tm 3+ -doped fiber lasers (TDFLs) at 1981 nm operating in Q-switched and CW regimes, respectively. At the pump repetition rate of 50 kHz, stable gain-switched pulses can always be obtained within the whole tuning range of 3397.1~3690.2 nm (~293 nm) realized with a plane ruled grating. At a wavelength of 3450.5 nm, a maximum average power of 264.5 mW with a corresponding pulse energy of 5.29 μJ and width of 1.02 μs are achieved. The effects of pump power and laser wavelength on the output characteristics have also been investigated. The results indicate that such a gain-switched laser can be a versatile pulsed source for many applications.

It is shown that the use of a toothed nozzle array in the active medium generator (AMG) of continuous-wave chemical lasers (CWCLs) allows more efficient mixing of reagents to be achieved in the laser chamber than in an AMG with a slit nozzle array. The results of calculations of parameters of the active medium of a DF CWCL operating in the gain regime for the AMG with traditional and promising nozzle array designs are presented. A kinetic model of processes in the DF CWCL active medium is proposed, which allows obtaining a good agreement with the results of measurements of the small-signal gain.

In two waveguide heterostructures with quantum-well arrays of Hg0.892Cd0.108Te/Cd0.63Hg0.37Te with a thickness of 6.1 nm and Hg0.895Cd0.105Te/Cd0.66Hg0.34Te with a thickness of 7.4 nm, stimulated emission is first obtained at wavelengths of 10.3 and 14 μm under continuous optical excitation at 8 K. It is shown that, due to the presence of Cd in the quantum wells, the effect of fluctuations in the thickness of the quantum wells on the energy of interband transitions in it decreases, which can ultimately cause a significant decrease in the threshold intensity for stimulated emission.

Yttrium aluminium perovskite YAlO3 (YAP) crystal, doped with rare-earth ions, has been extensively studied as a diode-pumped laser host material. The wide interest to rare-earth ions doped YAP crystals is explained by its good thermal and mechanical properties, high natural birefringence, widely used Czochralski growth method. The aim of this work was to study the Yb3+:YAlO3 crystal as an active medium for high power mode-locked laser.Yb3+-doped perovskite-like aluminate crystals have unique spectroscopic and thermooptical properties that allowed using these crystals as an active medium of high power continuous wave (CW) and modelocked (ML) bulk lasers with diode pumping.growth method. The aim of this work was to study the Yb3+:YAlO3 crystal as an active medium for high power mode-locked laser.In our work spectroscopic properties of Yb:YAP crystal and laser characteristics in CW and ML regimes are investigated. Maximum output power of 4 W with optical-to-optical efficiency of 16.3 % and 140 fs pulse duration have been obtained for Yb:YAP E //c-polarization with 10 % output coupler transmittance. Tunability range as wide as 67 nm confirms high promise of using Yb:YAP crystal for lasers working in wide spectral range.

We have demonstrated a monolithic semiconductor passively mode-locked Bragg waveguide laser (PMBRL) based on a two quantum well active region. The lasers showed a threshold current of ~33 mA at a saturable absorber reverse bias of 0 V in continuous wave operation (CW), with emission wavelength of ~785 nm. A systematic study of a two section PMBRLs showed that these devices can generate pulse widths down to 211 fs at a 64 GHz repetition rate with a time-bandwidth product of 0.32. This range of pulse widths sets a new record for quantum well based devices.

Photothermal control has been widely used in the tuning of microelectronic devices due to its laser-assisted noncontact manipulation and cost-effective advantages. The Nd3+ heavily doped NaGdF4 nanocrystals (NCs) were proposed as a heat source at nanoscale, and its light-to-heat conversion efficiency was evaluated to be above 80%. Here, we obtained a whispering gallery mode lasing emission from the prepared polymer-coated silica microcavity under a 532 nm pulsed laser. The nanoheaters are combined with the microcavity to trigger the photothermal effect at the irradiation of a second 793 nm continuous wave (CW) laser, thereby realizing all-optical tuning of the resonance wavelength by changing the effective refractive index of the microcavity. By measuring the shift of lasing modes on exposure to CW laser with different power intensities, the maximum tuning range and sensitivity of 2 nm and 1.96 nm/(W·mm−2) were achieved as the power intensity increased from 0 to 1.02 W/mm2. Our work provides a method for developing a new generation of all-optical tunable lasers based on nanoscale photothermal effect.

A magnetic resonance spectrometer operating at several fixed frequencies in the millimeter range is designed on the basis of a new generation of microwave bridges and a cryogen-free magneto-optical cryogenic system. The spectrometer allows measurements of EPR in continuous wave and pulse (free-induction decay, FID and electron spin echo, ESE) modes, photo-EPR, and optically detected magnetic resonance (ODMR) in a wide range of temperatures (1.5–300 K) and magnetic fields (up to 7 T with the ability to invert the field). It is based on a line of unified microwave bridges with a powerful oscillator and a superheterodyne receiver. Currently, they operate in W and D bands (at 94 and 130 GHz, respectively), but the frequency bands can be extended. In addition to highly stable fixed-frequency oscillators with narrow spectrum (better than − 100 dBc at 10 kHz offset at 94 GHz), they have variable frequency oscillators, which simplify tuning of the microwave circuit with a resonator and allow operation with frequency modulation when using a non-resonant microwave system. In the latter, it is possible to study large samples and quickly change the operating frequency of the spectrometer, simply replacing the microwave bridge. For all frequencies, the spectrometer uses common hardware and original software. The performance of the spectrometer at 94 GHz and 130 GHz was tested in measurements of CW-EPR, ESE and ODMR of NV defects in diamond single crystals, Tb3+ and Ce3+ ions in yttrium aluminum garnet crystals, nitrogen donors, and V3+ ions in 6H-SiC.

Organic–inorganic lead halide quasi-two-dimensional (2D) perovskites are promising gain media for lasing applications because of their low cost, tunable colour, excellent stability and solution processability1–3. Optically pumped continuous-wave (CW) lasing is highly desired for practical applications in high-density integrated optoelectronics devices and constitutes a key step towards electrically pumped lasers4–6. However, CW lasing has not yet been realized at room temperature because of the ‘lasing death’ phenomenon (the abrupt termination of lasing under CW optical pumping), the cause of which remains unknown. Here we study lead halide-based quasi-2D perovskite films with different organic cations and observe that long-lived triplet excitons considerably impede population inversion during amplified spontaneous emission and optically pumped pulsed and CW lasing. Our results indicate that singlet–triplet exciton annihilation is a possible intrinsic mechanism causing lasing death. By using a distributed-feedback cavity with a high quality factor and applying triplet management strategies, we achieve stable green quasi-2D perovskite lasers under CW optical pumping in air at room temperature. We expect that our findings will pave the way to the realization of future current-injection perovskite lasers. Lead halide-based quasi-two-dimensional perovskite films with different organic cations are used to create stable green lasers under continuous-wave optical pumping in air at room temperature.

In this article, the design, characterization, and manufacture of a circuit for a ${W}$ -band, ultracompact continuous wave sheet beam klystron are described. This klystron, aiming to produce more than 1.5-kW output power, operates at a comparatively low voltage benefiting the device size and extending its application space. However, a lower operation voltage adversely affects the beam-gap interaction efficiency. Two multigap output cavities have been employed to achieve a higher beam interaction efficiency without introducing additional competing modes or shrinking the interval between the operation mode and competing mode. Inspired by the resynchronization technology, the second output cavity can be optimized for the spent beam from the first output cavity. Comprising a single cavity output structure, the circuit efficiency of this double output cavity circuit is increased from 2.37% to 4.96%. In this circuit, electromagnetic modeling and design have been optimized to avoid complicated structures which are constrained by current mechanical design and manufacturing techniques. The MAGIC simulation shows that, driven by a 1-W RF signal, this circuit can extract more than 1.7-kW output power from a 35-kV, 1-A beam. A prototype single output cavity circuit has been designed and manufactured to determine the required machining tolerances for the final double output cavity circuit.

The interband GaSb-based diode lasers emitting simultaneously in two narrow bands separated by either ~1.6 or ~3.3 THz were designed, fabricated and characterized. The device active region contained one asymmetric tunnel-coupled double quantum well with separation between two lowest electron subbands controlled by thickness of the tunnel barrier. The Y-branch 6th order distributed Bragg reflector devices have been fabricated with either deep or shallow etched ridge waveguides. The increase of the deeply etched ridge waveguide width from 10 to $20~\mu \text{m}$ improved laser threshold and efficiency thanks to reduction of the relative role of the sidewall defect recombination. Further improvement of the device performance parameter was achieved by shallow etching. The shallow etched lasers with stable dual-wavelength emission spectrum generated 100 mW of continuous wave output power at 20 °C.

The results of study of the influence of ionic conductivity on the optical absorption coefficient of lithium triborate crystals exposed to continuous wave high-power laser radiation at 1070 nm wavelength are presented.

Optical single sideband (OSSB) modulation is widely used in radio-over-fiber (RoF) system, nanophotonic phase noise filter, frequency-modulated continuous wave (FMCW) lidar and so on. We demonstrate a silicon dual-parallel Mach-Zehnder modulator (DP-MZM) with tunable optical power splitting ratio to generate OSSB signal with ultra-high sideband suppression ratio (SSR). To mitigate the SSR deterioration caused by the limited extinction ratios (ER) of the two sub-MZMs due to the process tolerance and the imbalance in their RF driving powers in practical RF links, we adjust the distribution of the incident optical power between the two sub-MZMs with a $1\times 2$ switch. With this technique, the SSR measured at 20 GHz is better than 39.1 dB. A theoretical analysis is also provided to explain how the unwanted sideband is suppressed by adjusting the optical power splitting ratio.

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).

Laser induced surface activation (LISA) technology requires refined selection of process variables to fabricate conductive microcircuits on a general polymer material. Among the process variables, laser mode is one of the crucial factors to make a reliable conductor pattern. Here we compare the continuous wave (CW) laser mode with the pulse wave (PW) laser mode through determination of the surface roughness and circuit accuracy. In the CW laser mode, the surface roughness is pronounced during the implementation of the conductive circuit, which results in uneven plating. In the PW laser mode, the surface is relatively smooth and uniform, and the formed conductive circuit layer has few defects with excellent adhesion to the polymer material. As a result of a change of laser mode from CW to PW, the value of Ra of the polymer material decreases from 0.6 m to 0.2 m; the value of Ra after the plating process decreases from 0.8 m to 0.4 m, and a tight bonding force between the polymer source material and the conductive copper plating layer is achieved. In conclusion, this study shows that the PW laser process yields an excellent conductive circuit on a polymeric material.

Growth of semipolar GaN laser diodes (LDs) on low-cost and large-size foreign substrates is crucial yet remains very challenging. In this study, we report the world’s first continuous-wave (CW) ele...

Continuous-wave (CW) operated laser is a key component for photonic chips. It has been highly desired to develop low-threshold, high-efficiency, long-term operational, low-cost and easily integratable CW laser. As excellent optical gain materials, quantum dots (QDs) have been intensively investigated for on-chip CW lasers owing to their high photoluminescence quantum yields, tunable emission wavelengths and easy integration. Base on the difference of preparation processes, QDs can be classified into epitaxial QDs and solution-processed colloidal QDs (CQDs). In this mini-review, we summarize the research progresses of epitaxial III–V semiconductor QD and solution-processed CQD-based CW lasers. The challenges associated with the realization of CQD CW lasers are discussed in detail. In particular, the emerging perovskite-based CW lasers are highlighted. Finally, a short perspective of QD-based CW lasers is presented.

In-fiber microcavities are usually fabricated by inva-sive methods that require costly setups and time-consuming processes. Inspired by the fiber fuse effect, we propose an efficient non-invasive method to fabricate in-fiber microcavities. The setup of the method was economic, using a watt-level continuous-wave laser. Spherical in-fiber microcavities of ~hundred-of-micron sizes could be fabricated and freely integrated in fibers. The fabrication rate was seconds-per-microcavity, being hundreds of times faster than previous methods. The microcavities could have ~1-micron-thin cavity walls and ~104 high quality factors as Fabry-Perot reso-nators. The method offers many unique features for microcavities and can be useful for large-scale fabrication. It can benefit many applications such as sensing and nonlinear frequency generation.

The method of molecular-beam epitaxy was used to fabricate an elastically balanced heterostructure of a quantum-cascade laser for the spectral range of 4.6 μm based on a heteropair of solid alloys, In0.67Ga0.33As/In0.36Al0.64As, and indium phosphide layers serving as waveguide cladding layers. An X-ray diffraction analysis demonstrated the high uniformity of the composition and thickness of layers in cascades of the heterostructure over the substrate area. Lasers with four cleaved facets show lasing at room temperature at a wavelength close to 4.6 μm with a comparatively low threshold current density of 1.1 kA/cm2.

We report the characterization of continuous-wave four-wave mixing (FWM) in SM1950 and SMF-28 silica fibers in the 2 μm wavelength band. Maximum conversion efficiencies of -21 dB and -25 dB are achived for SM1950 and SMF-28 fibers, respectively. In order to explore multichannel wavelength conversion at 2 μm, we characterize the wavelength conversion of a three-tone signal, which is generated by intensity modulation of a laser using a 10 GHz radio frequency (RF) signal. For all the three channels, we achieve a conversion efficiency of -25 dB using a 21.8 dBm pump. Recent demonstrations of efficient splicing of hollow-core fibers, which have been proposed for optical communications at 2 μm, with silica-based fibers paves the way for all-optical processing of multi-channel signals such as orthogonal frequency-division multiplexing (OFDM).

In this article, a millimeter-wave gyrotron using a metal photonic band gap (PBG) cavity as its RF interaction circuit is studied for its tunability. A PBG cavity operating in a TE7,2-like mode is designed and analyzed for its mode selective property at ~260 GHz. Particle-in-cell (PIC) simulation of the PBG gyrotron predicted a continuous wave (CW) RF output of ~121 W in the desired TE7,2-like mode with an axial mode number, $q = 1$ . Furthermore, the tunability of a PBG gyrotron is studied by varying the magnetic field, which, in turn, varies with the axial mode number of the desired operating mode. A broad continuous tunable bandwidth of ~1.5 GHz is obtained for more than ~1.5 W of the output power. The designed PBG cavity is thermally analyzed to study the structural deformation of metal rods and its effect on the performance of the tunable gyrotron.

We numerically demonstrate that noise properties of picosecond mid-infrared (mid-IR) supercontinuum (SC) in the anomalous region of a chalcogenide photonic crystal fiber (PCF) can be effectively controlled by a weak continuous wave (CW) trigger. Detailed simulation results show that in the mid-IR region, the characteristics of SC, including spectral bandwidth, temporal coherence, and signal-to-noise ratio (SNR), can be greatly enhanced by the CW trigger at selected wavelengths. The present study provides valuable insight into the CW-triggered mid-IR SC under the influence of modulation instability (MI) and stimulated Raman scattering (SRS).

The electrically-pumped terahertz quantum cascade laser (QCL) is characterized by high power emission, compact, broad frequency coverage, and so on, which shows abilities for frequency comb operations. Although free-running QCLs can work as frequency combs by designing the laser structure with small group velocity dispersions and/or inserting mirrors to compensate laser intrinsic dispersions, the ideal comb operation can only be obtained by firmly locking the repetition frequency and carrier frequency of a laser. In this work, we have reported a repetition frequency locking of a terahertz QCL emitting around 4.2 THz. When the 6-mm-long laser is operated in continuous wave mode without any locking techniques, the repetition frequency is measured to be ~6.15 GHz with a linewidth of hundred kilohertz. Once a phase lock loop (PLL) is applied to dynamically control the drive current of the QCL, we have demonstrated a successful repetition frequency locking of the laser with a signal to noise ratio of 80 dB. This technique can be employed for the frequency comb and dual-comb operations of terahertz QCLs for high-resolution applications.

As a bridge between crystal and fiber, the single crystal fiber (SCF) can combine the advantages of the two materials and show great potential in high-power lasers. In this paper, we select a Nd:YAG SCF as the gain medium in a diode-end-pumped linear cavity. The Nd:YAG SCF with the dimensions of ∅ = 1 mm × 50 mm was manufactured using the micro-pulling-down technique. At an incident pump power of 153.0 W, the maximum output power of the 1064 nm laser was 72.3 W. To the best of our knowledge, this is the highest power obtained in Nd:YAG SCF lasers. In addition, using a second-type phase-matched KTP crystal, the 10 W continuous wave green laser was obtained with an intracavity double-frequency structure.