mW Continuous Wave Research Articles

Results on the 1 MW CW 170 GHz gyrotron TH1509UA for ITER and DTT at the FALCON test stand

In this contribution, the tests of the pre-series gyrotron TH1509UA for the Divertor Tokamak Test facility (DTT) at the FALCON test facility are presented. This versatile test bed proves useful for testing continuous wave (CW) high-power gyrotrons, but also serves as a platform for testing components for the transmission line or the Upper Launcher of ITER and DTT. The gyrotron has demonstrated a power level of 1.02 MW at the gyrotron output window, corresponding to 980 kW at the output of the Matching Optics Unit (MOU) with a power variation during the pulse of < 2% after a stabilisation period. Additionally, an efficiency of 40% has been demonstrated during five consecutive 100 s pulses. Compared to the previous version, TH1509U, this gyrotron demonstrates the successful prevention of parasitic mode excitation over a wide range of parameters around the design operating point. The potential for even higher power performance has been shown in short pulses but not explored in long pulses yet, which instead focused on demonstrating compliance with the required specifications for the DTT project.

Mechanical overtone frequency combs

Mechanical frequency combs are poised to bring the applications and utility of optical frequency combs into the mechanical domain. So far, their main challenge has been strict requirements on drive frequencies and power, which complicate operation. We demonstrate a straightforward mechanism to create a frequency comb consisting of mechanical overtones (integer multiples) of a single eigenfrequency, by monolithically integrating a suspended dielectric membrane with a counter-propagating optical trap. The periodic optical field modulates the dielectrophoretic force on the membrane at the overtones of a membrane’s motion. These overtones share a fixed frequency and phase relation, and constitute a mechanical frequency comb. The periodic optical field also creates an optothermal parametric drive that requires no additional power or external frequency reference. This combination of effects results in an easy-to-use mechanical frequency comb platform that requires no precise alignment, no additional feedback or control electronics, and only uses a single, mW continuous wave laser beam. This highlights the overtone frequency comb as the straightforward future for applications in sensing, metrology and quantum acoustics.

High-Power, High-Efficiency Red Laser Diode Structures Grown on GaAs and GaAsP Metamorphic Superlattices

Three types of GaAsP metamorphic buffer layers, including linearly graded, step graded, and metamorphic superlattices, were compared for the purposes of virtual substrates for red laser diode heterostructures. Laser diodes were fabricated on GaAs substrates and relaxed GaAsP metamorphic superlattice virtual substrates. A laser diode structure with a tensile-strained quantum well on a standard miscut GaAs substrate achieved TM-polarized emission at a 638 nm wavelength with 45% peak power conversion efficiency (PCE) at a 880 mW continuous wave (CW) output power with T0 = 77 K and T1 = 266 K. An analogous laser diode structure with a compressively strained quantum well on the metamorphic superlattice emitted TE-polarized 639 nm light with 35.5% peak PCE at 880 mW CW with T0 = 90 K and T1 = 300 K.

A novel organic-inorganic ionic cocrystal - piperazine-1,4-diium tetrachloridocuprate(II) dihydrate delivering efficient optical limiting

The aqueous reaction of piperazine and copper chloride dihydrate yielded piperazine-1,4-diium tetrachloridocuprate(II) hydrate (PCu) cocrystal. Acid hydrolysis during pH optimization produced cationic piperazine-1,4-diium, which was neutralized by anionic tetrachloridocuprate(II) to form a stable ionic cocrystal. The PCu single crystal belongs to the space group C2/c of the monoclinic crystal system. High linear absorption with indirect bandgap and least possible crystal defects are coherently supported to the two-photon absorption assisted optical limiting phenomena in PCu cocrystal. The third order nonlinear refractive index (n2), absorption coefficient (β) and susceptibility (χ(3)) of title material are found to be (0.635±0.0058)×10-8 cm2W−1, (0.232±0.001)×10-4 cmW−1, (2.63±0.01)×10-6 esu respectively under continuous-wave laser (100 mW, 532 nm) excitation shows its suitability for nonlinear optical device applications. The variation in β measured at different nanosecond pulsed laser intensities confirmed the presence of sequential two-photon absorption. The noncovalent intermolecular charge transfer responsible for sequential two-photon absorption in PCu cocrystal is theoretically proved possible using DFT-UB3LYP/LANL2DZ+6-311G** level of quantum mechanical calculation. The onset optical limiting threshold values were measured to be (0.394±0.02)×1003 Wcm−2 and (1.62±0.02)×1012 Wm−2 for 100 mW continuous wave laser and 100 μJ nanosecond pulsed green laser (532 nm) irradiance respectively. Hence the PCu cocrystal can be a promising candidate for optofunctional applications.

Second harmonic generation in periodically poled MgO:LN crystal with 2 µm period created by e-beam irradiation

The periodical domain structure with period 2 μm was created in 1-mm-thick MgO doped lithium niobate single crystals by electron-beam irradiation. The second harmonic generation (SHG) around 370 nm was obtained during pumping the crystals by 100 mW continuous wave tunable Ti:Sapphire laser. The normalized efficiency was 0.05%/W. The dependence of the SHG power on the pump beam position has indicated that the periodical domain structure depth is about 300 μm. The obtained results have demonstrated that the electron-beam poling can be used for creation of the short pitch periodical domain structures for light frequency conversion.

Towards a 1.5 MW, 140 GHz gyrotron for the upgraded ECRH system at W7-X

• 1.5 MW gyrotron development for the ECRH system upgrade at the stellarator W7-X. • Numerically verified complete design of the 1.5 MW 140 GHz CW gyrotron. • Low-power tests of the gyrotron quasi-optical output coupler for theTE 28,10 mode. For the required upgrades of the Electron Cyclotron Resonance Heating system at the stellarator Wendelstein 7-X, the development of a 1.5 MW 140 GHz Continuous Wave (CW) prototype gyrotron has started. KIT has been responsible to deliver the scientific design of the tube (i.e. the electron optics design and the RF design), with contributions from NKUA and IPP. The prototype gyrotron has been ordered at the industrial partner, Thales, France, and is expected to be delivered in 2021. In parallel, a short-pulse pre-prototype gyrotron has been developed at KIT, to provide the means for a first experimental validation of the scientific design in ms pulses, prior to the construction of the CW prototype. This paper reports on the status of the 1.5 MW CW gyrotron development, focusing on the scientific design and its numerical and experimental validation.

Design verification of the gyrotron diamond output window for the upgrade of the ECRH system at W7-X

The 10 MW electron cyclotron resonance heating (ECRH) system at the stellarator Wendelstein 7-X (W7-X) currently relies on the successful operation of continuous wave (CW) 1 MW, 140 GHz gyrotrons which have chemical vapor deposition (CVD) diamond output windows cooled by the industrial silicon oil Dow Corning 200(R) 5 cSt. The window features a 1.8 mm thick diamond disk brazed to two copper cuffs with an aperture of 88 mm, which are then integrated in a steel housing. In the context of the upgrade of the ECRH system towards higher microwave power, this gyrotron design has been significantly advanced to fulfill the requirement of 1.5 MW CW operation, still at 140 GHz. A prototype of this new gyrotron is under development at Thales, France. This paper reports the computational fluid dynamics (CFD) conjugated heat transfer and structural analyses of the diamond window performed using the commercial code ANSYS V19.2 to investigate its performance at 1.5 MW operation. Furthermore, sensitivity studies were also carried out with respect to the absorbed power in the disk and the mm-wave beam radius at the window location. These analyses showed that the window design of the existing 1 MW gyrotrons still works quite well at higher power operation, thus verifying the performance of the window. Even in the worst case scenario of 1.5 kW absorbed power, the maximum temperature of 215 °C at the disk center can be safely accepted, being below the conservative limit of 250 °C for CVD diamond. In addition, the non-axial symmetric thermal gradients due to the geometry of the cooling channels lead to thermal stresses in the disk and the cuffs. However, they are much lower than the limits. The copper cuffs experience plasticity deformation in the region of the interface with the diamond disk up to a value of about 1.5 mm.

Photobiomodulation Therapy in the Management of Burning Mouth Syndrome: Morphological Variations in the Capillary Bed.

Burning mouth syndrome (BMS) is an idiopathic condition that manifests itself primarily with the onset of a burning sensation. The aim of this research was to perform photobiomodulation therapy (PBM) using a diode laser on the oral mucosa of BMS patients, followed by an objective evaluation of the morphological changes in the vascular bed underlying the mucosa using polarized light videocapillaroscopy. A group of 40 patients were included in the study. The patients were randomly divided into two groups (using simple randomization) as follows: 20 patients were assigned to the laser group and 20 patients were assigned to the placebo group. Each patient of the laser group received eight irradiations (with 4 Watt of power, wavelength 800 nm, energy 1200 Joules, irradiation time of 300 s, energy density 50 J/cm2, 60 mW continuous wave laser, and irradiance 180 mW/cm2), twice a week, blinded to the type of irradiation administered, for four consecutive weeks. The patients in the placebo group underwent the same sessions as the other patients, the only difference was the non-emission of the laser. An initial check of the vascular bed was performed with a polarized light videocapillaroscope. This was followed by treatment with a therapeutic diode laser and a subsequent check with a videocapillaroscope. We observed that in the group of patients who underwent laser therapy, there was a lasting improvement in symptoms. The capillary oral bed of patients in the placebo group did not show any statistically significant difference (p > 0.05). In the laser group we observed the following: in the buccal mucosa the diameter of the capillary had a reduction of 3 μm; in the upper lip mucosa, there was a reduction of 3 μm; in the lower lip mucosa, there was a reduction of 3 μm; and in the dorsal lingual surface, there was a reduction of 2 μm. An increase in capillary length was also obtained in all irradiated regions in the laser group patients (p < 0.05). PBM induces microcirculatory changes that are still present over a long period of time, such as an improvement in the clinical picture. The improvement in the symptoms has been correlated to the reduction of the capillary diameter. The placebo effect only led to a temporary improvement in symptoms that were unrelated to changes in the microcirculatory pattern.

Novel pyrazolate-bridged binuclear Ni(II), Cu(II) and Zn(II) complexes: Synthesis, X‐ray crystal structure and nonlinear optical studies

In this paper, a pyrazole based ligand (1) containing nitrogen and sulfur donors was prepared and used in the synthesis of the following new binuclear Ni(II), Cu(II), and Zn(II) complexes [Ni2(EtNCSPzMe2)2(µ-PzMe2)2] (2), [Cu2(EtNCSPzMe2)2(µ-PzMe2)2] (3) and [Zn2(EtNCSPzMe2)2(µ-PzMe2)2] (4). The ligand and complexes were characterized by spectroscopic and physico-chemical methods. The crystal structure of [Ni2(EtNCSPzMe2)2(µ-PzMe2)2] was determined by X-ray diffraction analysis. Crystal structure of the (2) describes a binuclear complex, in which nickel ions are bonded by N3S donor atoms set in distorted square planar coordination geometry and the dimethyl pyrazolate groups are placed as the bridge among the metal centers. The nonlinear optical responses of the prepared ligand and metal complexes were investigated under irradiation of 35 mW continuous wave He-Ne (632.8 nm) laser. The z-scan technique was applied for measurement the second-order refractive index (n2) of the synthesized samples. Results show that copper complex (3), demonstrates the highest value of the measured n2 (−10.18 × 10−7 mmW−1).

Low power all optical switching and implementation of universal logic gates using micro-bubbles in semiconductor nanocrystal solutions

We describe optical switching in solutions of semiconductor nanocrystals illuminated by a 404 nm continuous wave laser source, driven by the formation of a micro-bubble of solvent vapor in the solution. Low boiling solvents such as hexane show an oscillatory modulation of transmitted light intensity (period ∼4 s) while solvents with intermediate boiling points such as toluene give a stable switching response. An on/off ratio of 83% is observed in the transmitted pump beam. Using this, a pump beam (404 nm, 80 mW continuous wave) was shown to reversibly switch the state of a probe laser (630 nm, 5 mW continuous wave). This switch thus serves as an optical analog of an electronic transistor and demonstrates the potential for all optical switching of low power light beams. Further, all optical universal logic gates, NAND and NOR, were also demonstrated using the micro-bubble switch.

946 nm 25 mJ/3 ns Nd:YAG laser based on regenerative amplification of pulses

Regenerative amplification of 946 nm pulses which are sliced from 12 mW continuous wave laser diode is reported. Near-transform-limited 2.9 ns pulses with highly stable and smooth temporal shape were obtained with the energy of 25 mJ/50 Hz and M 2 < 1.2.

Third-order nonlinear optical properties of three chlorinated thienyl chalcones derivatives: synthesis, structural determination and Hirshfeld surface analysis

AbstractThree chlorinated theinyl chalcone derivatives, namely (E)-1-(5-chlorothiophen-2-yl)-3-(4-(methylthio)phenyl)prop-2-en-1-one (I), (E)-1-(5-chlorothiophen-2-yl)-3-(4-(dimethylamino)phenyl)prop-2-en-1-one (II) and (E)-1-(5-chlorothiophen-2-yl)-3-(2,3-dihydrobenzofuran-5-yl)prop-2-en-1-one (III), were synthesized and their crystal structures were determined by single-crystal X-ray diffraction analysis. CompoundsI,IIandIIIcrystallize in the monoclinic space groupsP 21/c(centrosymmetric),P 21/n(centrosymmetric) andPc(non-centrosymmetric), respectively. In all three compounds, the molecules are in a relatively planar conformation and adopt atransconfiguration with respect to the C=C double bond. The crystal packings are stabilized by weak hydrogen-bonds, π · · · π, C–H · · · π and C–Cl · · · π interactions. The intermolecular contacts and lattice energies were further analyzed by Hirshfeld surface analysis. The third-order nonlinear optical properties of these chalcone derivatives were investigated using the single beam Z-scan technique with a 5 mW continuous wave diode laser operating at 635 nm, where compoundIshowed the highest potential for optical application with its exclusive nonlinear refractive index and nonlinear susceptibility.

Continuous-wave Nd:GSGG/LBO green laser at 551.5 nm

We firstly obtain a continuous-wave (CW) Nd:GSGG green laser at 551.5 nm based on a well-designed concave-flat resonator and a LBO nonlinear crystal (θ = 90°, φ = 8.8°). The main peak lines at 936.4, 1061.2 and 1336.6 nm are suppressed completely. With a pump power of 6.25 W at 808.7 nm, 308 mW continuous wave (CW) laser at 551.5 nm is generated and the optical conversion efficiency is about 4.9%.

Fifth- and third- order nonlinear optical responses of olive oil blended with natural turmeric dye using z-scan technique

In this study, nonlinear optical responses of olive oil as a pure material and also blended with turmeric powder at different concentrations are demonstrated using z-scan technique. This material is found to be a novel natural nonlinear medium with large nonlinear refractive index. The modified Newton-Raphson method was used to solve the formula obtained from Gaussian Decomposition method for transmission of beam light in z-scan technique. It shows a negative third order nonlinear refractive index (n2) on the order of 10−7cm2/w under irradiation of maximum 50 mw continuous wave (CW) He-Ne laser at 632.8 nm wavelength. The third order nonlinear susceptibility (χ3) of olive oil was calculated on the order of 10−5 (esu). The fifth order nonlinear refractive index (n4) of this oil was calculated positive and on the order of 10−9cm4w2 for the first time according to the authors' knowledge. It is worth noting that the fifth order nonlinear susceptibility occurs at lower intensity with regards to other nonlinear materials. The thermal nonlinear refractive index of the olive oil is also calculated for pure olive oil and different concentrations of olive oil blended with turmeric.

A passively Q-switched Ho:SSO laser with a Cr2+:ZnSe saturable absorber

Represented here is a continuous-wave (CW) operation laser and a passively Q-switch (PQS) Ho:SSO microchip laser with a Cr2+:ZnSe saturable absorber (SA). An end-pumped Tm:YAP laser pumping was employed, which operated at room temperature. Up to our knowledge, it is the first time to report a PQS microchip Ho:SSO laser. A maximum 418 mW CW laser output was realized with an absorbed pump power of 2.5 W. The slope efficiency was 59.4%. In the PQS mode, the maximum output power was 332 mW, at the same time the slope efficiency was down to 57.1%. The maximum repetition frequency was 0.69 kHz. The maximum pulse energy of 481 μJ was realized corresponding to the pulse width of 103 ns. The pulsed characteristics were studied in this work. The output wavelength for different modes were also investigated.

High-power, high-spectral-purity GaSb-based laterally coupled distributed feedback lasers with metal gratings emitting at 2 μ m

We report on the fabrication of high-power, high-spectral-purity GaSb-based laterally coupled distributed feedback (LC-DFB) lasers emitting at 2 μm. Second-order Chromium-Bragg-gratings are fabricated alongside the ridge waveguide by lift off. Due to the introduction of gain coupling, the lasers exhibit a stable single mode operation [side-mode suppression ratio (SMSR) &amp;gt;40 dB] from 10 °C to 50 °C and the maximum SMSR is as high as 53 dB. At a heat-sink temperature of 10 °C, the lasers emit more than 40 mW continuous-wave in a single longitudinal mode. A high external quantum efficiency of 48% is obtained, resulting in a notable increase in power conversion efficiency peaking at 13%. The lasers achieve a comparable output power with that of the index-coupled LC-DFB lasers, while maintaining a better single mode performance. Thus, we prove the feasibility of the metal-grating LC-DFB structure to achieve high-power, frequency-stable semiconductor lasers through a simpler and much more convenient way.

Solving thermal issues in tensile-strained Ge microdisks.

We propose to use Ge-dielectric-metal stacking to allow one to address both thermal management with the metal as an efficient heat sink and tensile strain engineering with the buried dielectric as a stressor layer. This scheme is particularly useful for the development of Ge-based optical sources. We demonstrate experimentally the relevance of this approach by comparing the optical response of tensile-strained Ge microdisks with an Al heat sink or an oxide pedestal. Photoluminescence indicates a much reduced temperature rise in the microdisk (16 K with Al pedestal against 200 K with SiO2 pedestal under a 9 mW continuous wave optical pumping). An excellent agreement is found with finite element modeling of the temperature rise. This original stacking combining metal and dielectrics is promising for integrated photonics where thermal management is an issue.

Investigation on heat transfer analysis and its effect on a multi-mode, beam-wave interaction for a 140 GHz, MW-class gyrotron

The interaction cavity of a 140 GHz, 1 MW continuous wave gyrotron developed in UESTC will be loaded with a very large heat load in the inner surface during operation. In order to reduce the heat, the axial wedge grooves of the outside surface of the cavity are considered and employed as the heat radiation structure. Thermoanalysis and structural analysis were discussed in detail to obtain the effects of heat on the cavity. In thermoanalysis, the external coolant-flow rates ranging from 20 L/min to 50 L/min were considered, and the distribution of wall loading was loaded as the heat flux source. In structural analysis, the cavity's deformation caused by the loads of heat and pressure was calculated. Compared with a non-deformed cavity, the effects of deformation on the performance of a cavity were discussed. For a cold-cavity, the results show that the quality factor would be reduced by 72, 89, 99 and 171 at the flow rates of 50 L/min, 40 L/min, 30 L/min and 20 L/min, respectively. Correspondingly, the cold-cavity frequencies would be decreased by 0.13 GHz, 0.15 GHz, 0.19 GHz and 0.38 GHz, respectively. For a hot-cavity, the results demonstrate that the output port frequencies would be dropped down, but the offset would be gradually decreased with increasing coolant-flow rate. Meanwhile, the output powers would be reduced dramatically with decreasing coolant-flow rate. In addition, when the coolant-flow rate reaches 40 L/min, the output power and the frequency are just reduced by 30 kW and 0.151 GHz, respectively.

Development of over-MW gyrotrons for fusion at 14 GHz to sub-THz frequencies

Megawatt power gyrotrons are being developed for collaborative electron cyclotron heating (ECH) studies of advanced fusion devices and demonstration power plant (DEMO). (1) In the first experiment of a 300 GHz gyrotron, an output power above 0.5 MW in the TE32,18 single mode was achieved with a pulse width of 2 ms. This was the first observation of MW-scale oscillations in a DEMO-relevant gyrotron mode. It was also found that the reflection at the output window affected the determination of the oscillation mode. Furthermore, several single mode oscillations in the 226–254 GHz range were confirmed, which is important for the step-frequency tunable gyrotron in the sub-THz frequency range. (2) Based on the successful results of the 77 and 154 GHz large helical device (LHD) tubes, a new 154/116 GHz dual-frequency gyrotron with an output of over 1.5 MW is being designed. (3) A new record output of 1.38 MW was obtained using an existing 28 GHz gyrotron. A newly designed tube aimed at achieving a dual-frequency output power of 2 MW at 28 GHz (0.4 MW continuous wave) and 1 MW at 35 GHz was built. In the first experimental test, main mode oscillations were observed at the frequencies of 28.036 and 34.831 GHz with Gaussian-like output beams and output power of 1.27 and 0.48 MW, respectively. A total efficiency of 50% was achieved at 28 GHz operation.

A 220/247.5/275-GHz, 1.0-MW, Triple Frequency Regime Gyrotron

In this paper, a complete design methodology of a triple frequency gyrotron is presented, which can also be further applied to multifrequency operations. Frequencies for operation are selected as 220, 247.5, and 275 GHz for the localized and intense heating of magnetically confined plasmas (i.e., electron cyclotron heating and current drive) for future fusion reactors. A cautious selection procedure of the mode triplet is portrayed in accordance with the all possible physical and technical constraints. Typical cold cavity (beam absent) and single-mode (beam present) calculations are performed and presented with extended interaction structure (including an optimized nonlinear taper section). A triode type configuration is adopted for magnetron injection gun to produce the electron beam with desired characteristics as required for RF behavior. Time-dependent multimode calculations are presented with nonuniform magnetic field and beam parameters optimized by gun simulations. These rigorous calculations affirm proper working of the design with $\approx 1$ -MW continuous wave power for chosen mode triplet and efficiency $\approx 35$ %.