Gaussian Output Beam Research Articles

High-Efficiency Orbital Angular Momentum Beams Multiplexing System With Compact Shaper and Transformation Optics

In recent years, mode-division multiplexing (MDM) has been intensively proposed to cope with the information capacity crisis. However, how to multiplex a large number of OAM modes efficiently with low insertion loss and broad bandwidth is still a crucial issue. To solve this problem, we design a multiplexing system in which we make two functional blocks including a compactable shaper and a pair of free-form refractive optical elements. In one block, a self-designed compactable shaper is used to turn the Gaussian beam output from a single mode fiber into the elongated beams with tilted plane wave so that it can meet the phase requirement for the subsequently transformation. In the other block, a pair of free-form refractive optical elements is reversely used to multiplex multi modes based on coordinate transformations. Based on these structures, OAM multiple modes multiplexing experiment has been successfully performed in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C+L</i> band and the insertion losses of all modes are less than 1.6 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dB</i> . Furthermore, in the limited number of modes, the energy insertion loss does not increase with the increase of the number of OAM modes. The scheme offers a great potential for the integration in the next platform of MDM.

Quasi-optical mode converter for a high power TE8,3-mode gyrotron

We successfully developed a Gyrotron HS-III (28-GHz/400-kW/5-s) for electron cyclotron resonance heating in a spherical tokamak EXL-50, which operated in the TE8,3 mode. An internal quasi-optical-mode converter was designed to separate the spent electron beam from the outgoing power and convert the operating mode to a fundamental Gaussian wave beam. The particle swarm optimization algorithm was used to reduce diffraction loss from the launcher in such a low-frequency band. This enabled us to optimize the perturbation distribution of the launcher and beam waist of the target function to obtain the largest possible scalar correlation coefficient. The launcher was followed by three mirrors: one quasi-parabolic mirror and two phase-correcting mirrors. These were also optimized based on the scalar diffraction theory and Katsenelenbaum–Semenov algorithm to allow us to change the beam direction and increase the purity of the Gaussian output beam. Simulation results revealed a transmission efficiency of 96% and vector Gaussian mode purity of 99.8% at the window plane.

Wavelength-Dependent Kerr Beam Self-Cleaning in Spatiotemporal Mode-Locked Multimode Fiber Laser

We report a spatiotemporal mode-locked multimode fiber laser, which relies on spatial beam self-cleaning via the nonlinear Kerr effect to attain high pulse energy with near Gaussian output beam shape. Single and multiple pulses with switchable central wavelengths have been obtained. Moreover, it is found that the beam self-cleaning behavior observed at different wavelengths has distinct threshold. The experimental results are valuable to the dynamic characteristics of spatiotemporal mode-locked multimode fiber laser, making them much more accessible for applications.

Squared Focal Intensity Distributions for Applications in Laser Material Processing.

Tailored intensity profiles within the focal spot of the laser beam offer great potential for a well-defined control of the interaction process between laser radiation and material, and thus for improving the processing results. The present paper discusses a novel refractive beam-shaping element that provides different squared intensity distributions converted from the Gaussian output beam of the utilized femtosecond (fs) laser. Using the examples of surface structuring of stainless-steel on the micro- and nano-scale, the suitability of the beam-shaping element for fs-laser material processing with a conventional f-Theta lens is demonstrated. In this context, it was shown that the experimental structuring results are in good agreement with beam profile measurements and numerical simulations of the beam-shaping unit. In addition, the experimental results reveal the improvement of laser processing in terms of a significantly reduced processing time during surface nano-structuring and the possibility to control the ablation geometry during the fabrication of micro-channels.

High-power single mode GaSb-based 2 μm superluminescent diode with double-pass gain

We report a broadband superluminescent diode operating around a 2 μm wavelength, optimized for high-power broadband operation. The high power operation is achieved by using a GaInSb/AlGaAsSb heterostructure positioned in a ridge waveguide with a J-shaped layout to form a double-pass geometry. To avoid lasing at high current while enabling high gain, a cavity suppression element is used. This combination allows demonstration of an output power as high as 120 mW for continuous-wave (CW) operation at room temperature, with a spectral full width at half maximum of about 43 nm. The maximum power spectral density was measured to be 1.8 mW/nm, which is about a fourfold increase compared to the state-of-the-art results for this wavelength range. To avoid heating, the diode was also driven with low duty-cycle current pulses; in this case, a peak power of more than 300 mW was achieved without any sign of roll-over (power was limited by the current injected). For CW operation, the central emission wavelength could be tuned by current injection between 1900 nm at 200 mA and 2027 nm at 2000 mA. Devices produce a Gaussian output beam that is suitable for coupling to single mode waveguides.

Adaptation of a Dual-Frequency 104/140 GHz Gyrotron for Operation at 175 GHz

A dual-frequency gyrotron capable of operation in the TE28,7 cavity interaction mode at 140 GHz, or in the TE22,5 mode at 104 GHz, has been developed for use in electron cyclotron heating in the W7-X stellarator at IPP Greifswald. The gyrotron incorporates an internal converter design that has been numerically optimized to convert either of the two operating modes into a high-quality Gaussian output beam. During short-pulse factory testing, the gyrotron produced 900 kW at 140 GHz, and 520 kW at 104 GHz. After delivery to IPP, the gyrotron was conditioned to long-pulse operation at 140 GHz, demonstrating 30-minute pulses at several power levels up to 811 kW, and producing ten consecutive ten-minute pulses at 811 kW as well. After long-pulse capabilities were demonstrated at 140 GHz, IPP requested an analysis of the feasibility of operating the gyrotron (without internal modification) in an additional mode with a frequency near 175 GHz. Several potential interaction modes were evaluated to determine the required operating parameters for excitation of these modes, and to assess the expected interaction efficiency, output power, internal diffraction losses, and output beam quality. The most promising modes appear to be the TE33,9 (173 GHz) and the TE34,9 (176 GHz), which should generate 400-500 kW of RF in a suitable magnet capable of producing the necessary 7.1 T field required for operation at these higher frequencies. Because the existing gyrotron’s internal converter was not optimized for these modes, however, internal losses are expected to be higher than usual (up to 7%), and the output beam pattern will require external phase-correction in order to produce a Gaussian beam. A feasibility analysis for such external phase correction has been performed, demonstrating that a high-quality beam can be recovered using numerically synthesized external phase-correcting mirror surfaces.

Apodized fully-etched surface grating coupler using subwavelength structure for standard silicon-on-insulator waveguide

We design and demonstrate the fully-etched apodized grating couplers based on the silicon-on-insulator (SOI) platform using subwavelength structure for both transverse electric (TE) and transverse magnetic (TM) modes operation. The subwavelength grating (SWG) is used to engineer the refractive index using second-order effective medium theory (EMT). The whole designing procedure is given in details, especially a feasible and programmable method is developed to precisely manipulate the coupling strength of each grating cycle. A perfect Gaussian output beam can be synthesized for the TE mode operation, achieving a field overlap up to 98.3% with the Gaussian fiber mode. The simulated peak coupling losses are −4.63 and −2.99 dB for the TE mode and the TM mode, respectively, which are comparable with conventional shallowly etched grating couplers, realizing a fabrication simplification without performance penalty. The measured peak coupling loss is −7.6 dB for the TE mode coupler with a 1 dB bandwidth of 45 nm, and −6.1 dB for the TM mode coupler with a 1 dB bandwidth of 34 nm.

Er3+Active Yb3+Ce3+Co-Doped Fluorozirconate Guided-Wave Chip Lasers

We report efficient 1.55-μm laser operation in a waveguide chip laser femtosecond-laser inscribed in bulk fluorozirconate glass. The ternary doped glass is composed of erbium as the active laser ion, ytterbium sensitizer, and cerium to enhance the branching ratio from the pump to the erbium upper laser state. We have achieved slope efficiencies up to 25%, with a low 100-mW threshold. To meet diverse application requirements, we also report the wide tunability of 1507-1593 nm, and large single-transverse-mode Gaussian beam-output from the 55 μm diameter waveguides.

System Development and Performance Testing of a W-Band Gyrotron

A high-power W-band gyrotron has been designed and performance tested in Korea, with an output power in the range of tens of kilowatts. The gyrotron consists of a diode-type electron gun operating at 40 kV, a TE6,2 mode interaction cavity, and a mode converter for producing a highly Gaussian output mode beam. Presented here are the detailed component design procedure and the experimental results of the gyrotron’s performance evaluation. A maximum power of 62 kW was achieved with an efficiency of 22 %, and a highly Gaussian output beam was observed. The gyrotron’s output beam is analyzed, and its transmission through an oversized waveguide is discussed. This gyrotron is the first gyrotron developed in Korea with high power greater than 10 kW and high frequency greater than 90 GHz.

Development of the Multifrequency Gyrotron FU CW GV with Gaussian Beam Output

Gyrotron FU CW GV has been developed as a multifrequency gyrotron for operation over the frequency range from 162 to 265 GHz at frequencies separated by steps of approximately 10 GHz. The oscillation modes were selected; the radii of the caustic surfaces for the electromagnetic waves of the modes had similar values in the waveguide, and it was therefore expected that these modes would be converted into Gaussian beams by a mode converter. In reality, more than ten modes oscillated and the Gaussian-like beams were radiated. A double-disk window with variable spacing maintains the transmittance through the window at a high level over a wide range of frequencies. Using this window, output powers of more than 1 kW were observed for almost all the expected modes.

Demonstration of ultra-low NA rare-earth doped step index fiber for applications in high power fiber lasers

In this paper, we report the mode area scaling of a rare-earth doped step index fiber by using low numerical aperture. Numerical simulations show the possibility of achieving an effective area of ~700 um² (including bend induced effective area reduction) at a bend diameter of 32 cm from a 35 μm core fiber with a numerical aperture of 0.038. An effective single mode operation is ensured following the criterion of the fundamental mode loss to be lower than 0.1 dB/m while ensuring the higher order modes loss to be higher than 10 dB/m at a wavelength of 1060 nm. Our optimized modified chemical vapor deposition process in conjunction with solution doping process allows fabrication of an Yb-doped step index fiber having an ultra-low numerical aperture of ~0.038. Experimental results confirm a Gaussian output beam from a 35 μm core fiber validating our simulation results. Fiber shows an excellent laser efficiency of ~81%and aM² less than 1.1.

Walk-off reduction, using an external optical plate and Bessel–Gaussian interaction

To reduce the walk-off angle of the extraordinary third-harmonic ultraviolet wave at 355 nm generated by type II KTiOPO4 and type I β-BaB2O4 optical crystals, and the Gaussian output beam of a Q-switched Nd:YAG laser, a simple theoretical model was developed based on a rotatable BK7 plate of variable thickness. By rotating the plate up to 35° along the beam direction, we reduced the walk-off angle up to ∼ 13%. The same phenomenon is predicted by the model, confirming the performance of the model. It is found that, due to the walk-off effect, the intensity profile of the third-harmonic generation beam is slightly degraded. To compensate for the observed phenomena and further reduce the walk-off, we used a combination of a convex lens and an axicon to transform the beam profile of the interacting fundamental and second-harmonic generation waves to the zero-order Bessel–Gaussian form. As a result, the walk-off is decreased to ∼48.81 mrad, providing ∼30% relative reduction. By using the same BK7 plate rotated up to 35° along the third-harmonic beam direction, the walk-off angle is further reduced to 38.9 mrad. Moreover, it is observed that the beam profile of the emerged Bessel–Gaussian third-harmonic generation beam remains unchanged with no degradation.

High efficiency single-mode-multimode-single-mode fiber laser with diffraction-limited beam output.

We designed and tested an all-fiber, high efficiency Yb-doped laser operating at 1088nm with a single-mode-multimode-single-mode (SMS) structure. A larger-mode-area gain fiber of 1.5m length, with 20/130μm core/cladding diameters was used to increase the absorption, and a diffraction-limited Gaussian output beam was obtained from the single-mode output fiber. Using a 976nm laser diode as the pump source, the laser generated an output power up to 38.5W with a slope efficiency of 70%. The output beam qualities, with and without SMS structure, were compared and showed that the fiber laser with the SMS structure can achieve high gain, short fiber length, and excellent beam quality.

OPCPA pump laser based on a regenerative amplifier with volume Bragg grating spectral filtering

A novel Nd:YVO4−based regenerative amplifier system operating in the picosecond regime featuring a volume Bragg grating (VBG) as an intracavity spectral narrowing element is described. This compact amplifier provides pulses with duration of ∼85 ps operating at repetition rates ranging from single shot to 10 kHz. The VBG is used to passively tailor the pulse duration and achieve transform-limited pulses with 50 pm FWHM of spectral linewidth. A Gaussian output beam profile is obtained from the amplifier at all repetition rates. The intracavity VBG also guarantees a high spectral purity by efficiently preventing the build-up of out-of-band ASE. The spectral, spatial and temporal properties of this amplifier make it highly suitable for OPCPA pumping applications.

Gyrotrons for magnetic fusion applications at 110 GHz and 170 GHz

Two megawatt-class gyrotrons at frequencies of 110 GHz and 170 GHz have recently been fabricated at CPI. The 110 GHz gyrotron is designed to produce 1.2 MW of output power for 10-second pulses, and will be used for electron cyclotron heating and current drive on the DIII-D tokamak at General Atomics. This gyrotron has completed factory testing and has been delivered to General Atomics for installation and additional testing. The 170 GHz gyrotron, though specified as a 500 kW CW system, has been designed with the goal of generating up to 1 MW CW. Oak Ridge National Laboratory will use this gyrotron in ITER ECH transmission line testing. This gyrotron has been fabricated and is awaiting factory testing, Design features of each gyrotron are described, and test data for the 110 GHz gyrotron are presented. 1 Design and testing of a 1.2 MW, 110 GHz, 10-second gyrotron CPI's VGT-8115 gyrotron is specified to generate 1.2 MW of output power at 110 GHz, for pulse lengths up to 10 seconds. The gyrotron design is based on that of an earlier prototype (1), but incorporates several improvements, including a larger collector formed from a strengthened copper alloy, as well as a numerically-optimized dimpled-wall launcher and phase-correcting mirror system. The gyrotron operates in the magnetic field produced by a 43 kG superconducting magnet system (SCM) with three separately controllable axial coils (two for generating the desired field in the gyrotron cavity region, and one for adjusting the field at the cathode location) and two orthogonal sets of transverse field coils to ensure that the electron beam is centered in the cavity. The SCM cryostat maintains the necessary temperatures for the superconducting coils using a sealed refrigeration system that does not consume liquid cryogens. The gyrotron employs a single-anode magnetron injection gun to generate an annular electron beam. The electron beam interacts with the TE22,6,1 mode of the cylindrical interaction cavity, and this mode is converted to a Gaussian output beam using the optimized launcher and mirror system. The Gaussian beam passes through an edge- cooled chemical vapor deposition (CVD) diamond disc window. The depressed collector dissipates the residual power in the electron beam by using a combination of iron shielding to expand the size of the annular beam and active sweeping with a large collector coil to vary the beam strike point and lower the time-averaged power-density. A schematic of the gyrotron layout is shown in Figure 1.

Experimental Results on a 1.5 MW, 110 GHz Gyrotron with a Smooth Mirror Mode Converter

We present an internal mode converter (IMC) design for a 1.5 MW, 110 GHz gyrotron operating in the TE22,6 mode. The launcher, designed using the codes Surf3d and LOT, converts the cavity waveguide mode into a nearly pure Gaussian beam. The Gaussian beam output from the launcher is shaped by a series of 4 smooth, curved mirrors to provide a circular output beam with a flat phase front at the gyrotron window. By employing smooth mirrors rather than mirrors with phase correcting surfaces, such an IMC is less sensitive to alignment issues and can more reliably operate with high efficiency. The IMC performance was verified by both cold test and hot test experiments. Beam pattern measurements in each case were in good agreement with theoretical predictions. The output beam was of high quality with calculations showing that the Gaussian Beam content was 95.8 ± 0.5% in both hot and cold test.

2.2-MW Record Power of the 170-GHz European Preprototype Coaxial-Cavity Gyrotron for ITER

A 2-MW continuous-wave (CW) 170-GHz coaxial-cavity gyrotron for electron cyclotron heating and current drive in the International Thermonuclear Experimental Reactor (ITER) is under development within the European Gyrotron Consortium (EGYC1), a cooperation between European research institutions. To support the development of the industrial prototype of a CW gyrotron, a short-pulse tube (preprototype) is used at KIT Karlsruhe (former FZK) for experimental verification of the design of critical components, like the electron gun, beam tunnel, cavity, and quasi-optical RF output coupler. Significant progress has been achieved recently. In particular, RF output power of up to 2.2 MW with 30% output efficiency has been obtained in single-mode operation at 170 GHz. Furthermore, a new RF output system has been designed, with an efficient conversion of the generated RF power into a Gaussian RF output beam. The results have been successful, yielding a Gaussian mode content ~96%.

Plasma structures observed in gas breakdown using a 1.5 MW, 110 GHz pulsed gyrotron

Regular two-dimensional plasma filamentary arrays have been observed in gas breakdown experiments using a pulsed 1.5 MW, 110 GHz gyrotron. The gyrotron Gaussian output beam is focused to an intensity of up to 4 MW/cm2. The plasma filaments develop in an array with a spacing of about one quarter wavelength, elongated in the electric field direction. The array was imaged using photodiodes, a slow camera, which captures the entire breakdown event, and a fast camera with a 6 ns window. These diagnostics demonstrate the sequential development of the array propagating back toward the source. Gases studied included air, nitrogen, SF6, and helium at various pressures. A discrete plasma array structure is observed at high pressure, while a diffuse plasma is observed at lower pressure. The propagation speed of the ionization front for air and nitrogen at atmospheric pressure for 3 MW/cm2 was found to be of the order of 10 km/s.

Laser direct exposure of photodefinable polymer masks using shaped-beam optics

In this paper, we explore laser direct-write photolithography and ablation processes to finely pattern polymer resists, enabling the creation of densely patterned circuit substrates. The exposure dynamics are first modeled followed by presentation of data. For the exposure experiments, a tripled-YAG ultraviolet (UV) laser drilling system with highly attenuated output was used to directly expose a photodefinable resist followed by chemical developing. To improve the exposure process, special beam-shaping optics were utilized which convert the normal Gaussian beam output to a "top hat" flat exposure, ideal for even exposure of the photoresist across the beam aperture. To further improve the exposure process, a square-shaped aperture was used to compensate for the spatial average fluence variation across the scanned imaged line. For the ablation experiments, the square-shaped beam was used at higher fluence to directly ablate the resist without subsequent developing. Laser direct patterning is presented as an alternative to mask-imaging photolithography to produce fine-line traces (50 /spl mu/m or smaller) for flex circuits, circuit boards, and other interconnect substrates.

Iodine spectrometer based on a 633 nm transmission-grating diode laser

We report a new iodine spectrometer for absolute frequency measurements of various transitions of molecular iodine in the 633 nm region. To obtain good frequency accuracy, the laser beams used in spectroscopy have been optimized for spatial and spectral quality. This is achieved by injection locking a microlensed diode laser with nearly Gaussian output beam to an external-cavity diode laser. The external-cavity laser has been designed for enhanced frequency stability and reproducibility by minimizing the effects of ambient pressure and temperature, and by utilizing transmission-grating geometry for compact and robust structure. The high stability combined with the invariant output beam pointing makes the laser design adaptable also for other purposes of length metrology and atom optics.