Higher-order Transverse Modes Research Articles

Adjoint Algorithm Design of Selective Mode Reflecting Metastructure for BAL Applications.

Broad-area lasers (BALs) have found applications in a variety of crucial fields on account of their high output power and high energy transfer efficiency. However, they suffer from poor spatial beam quality due to multi-mode behavior along the waveguide transverse direction. In this paper, we propose a novel metasurface waveguide structure acting as a transverse mode selective back-reflector for BALs. In order to effectively inverse design such a structure, a digital adjoint algorithm is introduced to adapt the considerably large design area and the high degree of freedom. As a proof of the concept, a device structure with a design area of 40 × 20 μm2 is investigated. The simulation results exhibit high fundamental mode reflection (above 90%), while higher-order transverse mode reflections are suppressed below 0.2%. This is, to our knowledge, the largest device structure designed based on the inverse method. We exploited such a device and the method and further investigated the device's robustness and feasibility of the inverse method. The results are elaborately discussed.

Mode-Locked Operation of High-Order Transverse Modes in a Vertical-External-Cavity Surface-Emitting Laser.

Understanding the mechanism of mode-locking in a laser with high-order transverse mode is important for achieving an ultrashort pulses train under more complicated conditions. So far, mode-locking with high-order transverse mode has not been reported in other lasers except the multimode fiber laser. This paper demonstrates robust mode-locking with high-order transverse mode in a Kerr-lens mode-locked vertical-external-cavity surface-emitting laser for the first time, to the best of our knowledge. While the longitudinal modes are locked, continuous mode-locking accompanied by high-order transverse mode up to TEM40 is observed. The threshold of the mode-locking is only a little bigger than that of the lasing. After the laser oscillation is built up, the mode-locked pulse train can be obtained almost immediately and maintained until the thermal rollover of the laser. Output powers of 717 mW under fundamental mode and 666 mW under high-order transverse mode are achieved with a 4.3 ps pulse duration and 1.1 GHz pulses repetition rate, and some phenomenological explanations to the related characteristics of the mode-locked operation of high-order transverse mode in the vertical-external-cavity surface-emitting laser are proposed.

Progress and challenges of the ECH transmission line design for DTT

The design of the Transmission Line (TL) as a part of the Electron Cyclotron Heating (ECH) system for Divertor Tokamak Test facility (DTT) is approaching the conceptual design maturity. With an ECH system of 16 MW installed for the first phase and with a total of 32 gyrotrons (170 GHz, ≥ 1 MW, 100 s) the TL design is undertaking the challenge of an evacuated Multi-Beam TL (MBTL) concept to deliver the large number of beam lines from the gyrotron hall to the torus hall buildings. The system is organized in 4 clusters, each of them including 8 beamlines. The routing consists of single-beam TL section used to connect the gyrotron output to a beam-combiner mirror unit for each cluster, a common MBTL running in a suspended corridor reaching the Tokamak building and a beam-splitter mirror unit to connect to the ex-vessel optics and launchers sections located in the equatorial and upper ports of one sector, for a total of 4 DTT sectors. The TL mirrors will be actively water cooled to cope with the heat load in long pulses due to the high power incident radiation, with the possibility to include advanced concepts for the cooling design compatible with additive manufacturing technology. The characteristics of the system and its components are presented, showing both the progress of the adopted solutions and the current design. Since the main challenge of this TL is to maintain the overall losses below 15%, in this paper we present the expected ohmic and spillover losses, including beam coupling simulations evaluating losses given by high order Transverse Electro-Magnetic modes (i.e. aberrations). We describe how the effects have been estimated with electromagnetic simulations and how losses could be mitigated, since TL efficiency could significantly drop due to the presence of non-idealities, like the deformations of mirrors surface ascribed to the microwaves heat loads and possible misalignments and aberrations effects occurring along the line.

Use of periodically slotted SiO2 in a layered surface acoustic wave structure for transverse mode suppression

This work demonstrates the applicability of periodically slotted SiO2 (PSS) structures (PSSs) for controlling the urface acoustic wave (SAW) slowness shape and eliminating the transverse modes in layered SAW structures represented by the Incredible High Performance SAW. The use of PSS comprising SiO2 blocks and air gaps provides larger acoustic anisotropy for slowness shape manipulation. The effect of PSSs on transverse mode suppression is revealed. The slowness curves and frequency responses with different structural parameters of PSS are calculated by FEM analysis. Then, an electrode is designed for further spurious resonance suppression. A hammer-head configuration and Si3N4 overlay are applied. It is shown that higher-order transverse modes in the passband were well suppressed without reduction of the electromechanical coupling factor.

Enhanced standing-wave acoustic levitation using high-order transverse modes in phased array ultrasonic cavities

Airborne acoustic trapping by ultrasonic phased arrays has seen great advances in recent years, and yet the manipulation of objects with different shapes and sizes or heavy particles remains challenging. Here, we demonstrate that the manipulation capabilities of a standing-wave acoustic levitator can be extended by introducing intracavity high-order transverse (HOT) modes in the azimuthal direction, enabling the simultaneous trapping of several objects within a wide range of shapes and sizes with positional and rotational stability, including objects with sizes larger than one wavelength and weights in the scale of millinewtons. The conditions to generate different HOT modes are theoretically analyzed and experimentally implemented. We numerically calculate the pressure distributions, exhibiting good qualitative agreement with the experimental pressure distributions obtained with schlieren images. In addition, we calculate the acoustic force field for several examples of HOT modes and different particle sizes, which leads to a qualitative understanding of the experimental observations.

Switching of several self-mode-locking states in an optically pumped semiconductor disk laser

Self-mode-locking (SML) of fundamental TEM00 mode and high-order transverse mode were demonstrated in an optically pumped semiconductor disk laser and these states can be switched by simply adjusting the elements of the laser. The repetition rate of the pulse train was 1 GHz, using a straight cavity. Furthermore, we measured the pulse durations and found that the pulse duration of SML pulse with higher-order transverse mode is shorter than that of fundamental mode at the same pump power. To the best of our knowledge, this is the first experiment to obtain two SML states in a single laser. By using the method of changing the cavity length and adjusting the angle of the output coupler, the repetition rate can be continuously tuned from 3 GHz to 3.2 GHz.

High-peak-power optically pumped passively mode-locked semiconductor laser with minimal components.

A high-peak-power sub-500-fs mode-locked optically pumped semiconductor laser is innovatively developed with only three components of a semiconductor gain chip, a semiconductor saturable absorber, and a focusing lens. The developed laser near the threshold pump power of 3.9 W can be operated with stable fundamental mode locking. The laser output can be naturally turned into the stable harmonic mode locking (HML) with the order gradually changing from 2nd to 8th by increasing the pump power from 4.0 W to 5.0 W. Due to the onset of the high-order transverse modes, the order of HML is fixed at 8th for a pump power greater than 5.0 W. For the HML with order less than 8th, the overall peak power and pulse width in the HML are approximately 0.36 kW and 550 fs, respectively. In the operation of 8th-order HML, the minimum pulse width and maximum peak power can reach 480 fs and 0.95 kW, respectively.

Single-Mode-VCSEL With a Ring-Shaped Self-Aligned Recessed Metal Mode Filter

This article presents a single-mode vertical-cavity surface-emitting laser (VCSEL) using a novel ring-shaped self-aligned recessed metal (SARM) mode filter on the emitting window as a means of transverse mode control. The SARM structure acts as a spatial filter to suppress the higher-order transverse modes through selective recessed-etching of the top mirror and the self-aligned deposition of the p-type contact metals for cavity tuning and blocking the higher-order modes. The SARM mode filter could be a potentially high-yield alternative technique for the mass production of single-mode VCSELs, demonstrating a single-mode emission with 1.55-mW power and a 30.7-dB side mode suppression ratio.

High-power distributed feedback lasers based on InP corrugated sidewalls at λ∼2 μm

We report a high-power single-mode InP-based 2 μm distributed feedback (DFB) laser with a second-order buried grating and corrugated sidewalls. A second-order semiconductor grating is used for in-plane feedback and vertical out-coupling. The corrugated sidewalls are used to eliminate higher-order transverse modes. For the DFB laser with a 2 mm long cavity and 15 μm wide ridge, the maximum continuous-wave edge-emitting and surface-emitting single-mode powers at 300 K are up to 81 and 42 mW, respectively. A single-lobed far-field radiation pattern with a low divergence angle of approximately 8.6° is achieved by a device with a ridge width of 15 μm. The single-longitudinal-mode emission wavelength of the fabricated laser can be adjusted from 2003.8 nm at 288 K to 2006.9 nm at 313 K without any mode hopping. Robust single-mode emission with a side-mode suppression ratio of 30 dB is achieved under all injection currents and temperature conditions.

Single-mode quasi PT-symmetric laser with high power emission

Large-area lasers are practical for generating high output powers. However, this often comes at the expense of lower beam quality due to the introduction of higher-order modes. Here, we experimentally demonstrate a new type of electrically pumped, large-area edge-emitting lasers that exhibit a high power emission (∼0.4 W) and a high-quality beam (M2∼1.25). These favorable operational characteristics are enabled by establishing a quasi PT-symmetry between the second-order mode of a large area two-mode laser cavity and that of a single-mode auxiliary partner cavity, i.e., by implementing a partial isospectrality between the two coupled cavities. This in turn enlarges the effective volume of the higher-order modes. As a result, a selective pump applied via current injection into the main laser cavity can provide a stronger modal gain to the fundamental mode, and thus lead to lasing in the single mode regime after filtering out higher order transverse modes. The reported experimental results confirm this intuitive picture and are in good agreement with both theoretical and numerical analysis. Above all, the employed material platform and fabrication process are compatible with the industrial standards of semiconductor lasers. This work provides the first clear demonstration, beyond previous proof-of-concept studies, of the utility of PT-symmetry in building laser geometries with enhanced performance and, at the same time, useful output power levels and emission characteristics.

2.11 GHz all-fiber harmonic mode-locked laser based on highly nonlinear hybrid multimode interference saturable absorber

This paper proposes a harmonic mode-locked fiber laser based on a novel highly nonlinear hybrid multimode interference saturable absorber (SA). The hybrid multimode interference SA is integrated by tapered graded-index multimode fiber (TA-GIMF) and Bi2Te3. The numerical simulation results show that TA-GIMF can improve the stability of nonlinear multimode interference mode-locking by filtering higher-order transverse modes. In addition, the larger evanescent-wave fields generated by TA-GIMF are more beneficial to the light–matter interaction. By depositing Bi2Te3 on TA-GIMF, the nonlinear saturation absorption properties and nonlinear coefficients in the laser cavity are further improved. Based on the hybrid multimode interference SA, the repetition rate of the fiber laser reached 2.11 GHz, corresponding to the 30th harmonic of the fundamental repetition rate of 70.41 MHz. Our results are the highest repetition rate generated based on the NL-MMI effect and provide novel insights into implementing highly integrated, high repetition-rate mode-locked fiber lasers.

High-power femtosecond vortices generated from a Kerr-lens mode-locked solid-state Hermite-Gaussian oscillator.

We report the generation of high-order transverse modes from a Kerr-lens mode-locked femtosecond laser. Two different orders of Hermite-Gaussian modes were realized by non-collinear pumping, which were converted into the corresponding Laguerre-Gaussian vortex modes using a cylindrical lens mode converter. The mode-locked vortex beams, with an average power of 1.4 W and 0.8 W, contained pulses as short as 126 fs and 170 fs at the first and second Hermite-Gaussian mode orders, respectively. This work demonstrates the possibility of developing Kerr-lens mode-locked bulk lasers with various pure high-order modes and paves the way for generating ultrashort vortex beams.

High-power density, single plasmon, terahertz quantum cascade lasers via transverse mode control

Terahertz (THz) quantum cascade lasers (QCLs) have been shown to emit peak powers greater than 1 W from a single facet in a single plasmon geometry. However, this is typically achieved by increasing the laser ridge width, resulting in higher-order transverse modes, limiting the achievable power density. Here, we control and fully suppress these modes through thin metallic side-absorbers, showing laser action solely on the fundamental transverse mode operation without sacrificing high THz peak powers. This leads to enhanced power densities and electric fields of up to 1.8 kW/cm2 and 1.17 kV/cm, respectively, opening up the possibility of applying THz QCLs as pump sources for investigations of nonlinear THz physical phenomena.

Nanoscale imaging of super-high-frequency microelectromechanical resonators with femtometer sensitivity

Implementing microelectromechanical system (MEMS) resonators calls for detailed microscopic understanding of the devices, such as energy dissipation channels, spurious modes, and imperfections from microfabrication. Here, we report the nanoscale imaging of a freestanding super-high-frequency (3 – 30 GHz) lateral overtone bulk acoustic resonator with unprecedented spatial resolution and displacement sensitivity. Using transmission-mode microwave impedance microscopy, we have visualized mode profiles of individual overtones and analyzed higher-order transverse spurious modes and anchor loss. The integrated TMIM signals are in good agreement with the stored mechanical energy in the resonator. Quantitative analysis with finite-element modeling shows that the noise floor is equivalent to an in-plane displacement of 10 fm/√Hz at room temperatures, which can be further improved under cryogenic environments. Our work contributes to the design and characterization of MEMS resonators with better performance for telecommunication, sensing, and quantum information science applications.

Enhancement of slope efficiency of a dually modulated photonic-crystal surface-emitting laser over a wide range of emission angles by introducing a backside reflector

Dually modulated photonic-crystal surface-emitting lasers (DM-PCSELs) are a new type of semiconductor laser that enable on-chip, mechanical-free, high-power, high-beam-quality 2D beam scanning over a wide field of view. These lasers are attracting attention for application in light detection and ranging, and the improvement of their slope efficiency is desired for this application. Thus far, the highest experimentally demonstrated slope efficiency is approximately 0.4 W/A at wavelengths of around 940 nm, which was limited by the fact that roughly half of the laser light was emitted toward and absorbed by the backside electrode. In this work, in order to improve the slope efficiency of DM-PCSELs, we utilize the light emitted toward the backside of the device by introducing a distributed Bragg reflector (DBR) as a backside reflector. In consideration of this laser’s ability to emit beams over a wide field of view, we design laser structures that facilitate a commensurate enhancement of efficiency at various emission angles. Next, we discuss the effect of the DBR on the confinement of transverse modes in thickness direction as well as the suppression of higher-order transverse modes. Then, we analyze the emission characteristics of a DM-PCSEL with a DBR and calculate that the theoretical slope efficiency is enhanced to 0.7–0.8 W/A, which is twice that of the device without a DBR, over a wide range of emission angles from 0° to 30°. We then fabricate the devices and experimentally demonstrate the emission of a single-lobed beam with a high slope efficiency of 0.7–0.8 W/A.

Design and fabrication of 940 nm vertical cavity surface emitting laser single-emitter device

As a key part of vertical cavity surface emitting laser (VCSEL), active region will seriously affect the threshold and efficiency of the device. To obtain the appropriate laser wavelength and material gain, the design of In<sub>0.18</sub>Ga<sub>0.82</sub>As strain compensated quantum well is optimized. The relationship between the lasing wavelength of multiple quantum wells (MQWs) and the thickness is calculated. Considering the influence between the active region temperature and the lasing wavelength, the thickness of the quantum well is chosen as 6 nm, and the quantum barrier thickness is chosen as 8 nm, corresponding to the lasing wavelength of 929 nm. The material gain characteristics of the MQWs at different temperatures are simulated by Rsoft. The material gain exceeds 3300/cm at 300 K, and the temperature drift coefficient of the peak wavelength is 0.3 nm/K. In this work, Al<sub>0.09</sub>Ga<sub>0.91</sub>As and A<sub>l0.89</sub>Ga<sub>0.11</sub>As are chosen as the high- and the low-refractive index material of distributed Bragg reflector (DBR), and 20 nm graded layer is inserted between two types of materials. The influence of the graded layer thickness of DBR on the valence band barrier and reflection spectrum are calculated and analyzed. The increase of graded layer thickness can lead the band barrier peak and the reflection spectrum bandwidth to decrease. The reflection spectrum and phase spectrum of P-DBR and N-DBR are calculated by the transmission matrix mode (TMM): the reflectance of DBR is over 99% and the phase shift is zero at 940 nm. The optical field distribution of the whole VCSEL structure is simulated, in which the standing wave peak overlaps with the active region, and the maximum gain can be obtained. Using the finite element method (FEM), the effect of oxidation confined layer on the injection current is simulated. The current in the active region is effectively limited to the position corresponding to the oxidation confined hole, and its current density is stronger and more uniform. The optical field distributions in different modes of photonic crystal-vertical cavity surface emitting laser (PC-VCSEL) are simulated, and different modes have different resonant wavelengths. The values of quality factor Q in different modes of VCSEL and PC-VCSEL are calculated, Q of the fundamental mode is higher than that of higher transverse mode. It is demonstrated that the photonic crystal air hole structure can realize the output of basic transverse mode by increasing the loss of high order transverse mode. The VCSEL and PC-VCSEL with oxidation hole size of 22 μm are successfully fabricated, in which the photonic crystal period is 5 μm, the air pore diameter is 2.5 μm, and the etching depth is 2 μm. Under continuous current test, the maximum slope efficiency of VCSEL is 0.66 mW/mA, the output power is 9.3 mW at 22 mA, and the lasing wavelength is 948.64 nm at 20 mA injection current. Multiple wavelengths and large spectrum width are observed in the spectrum of VSCEL, which is an obvious multi-transverse mode. The maximum fundamental transverse mode output of PC-VCSEL reaches 2.55 mW, the side mode suppression ratio (SMSR) is more than 25 dB, and the spectrum width is less than 0.2 nm, indicating that the photonic crystal air hole has a strong control effect on the transverse mode, and the laser wavelength is 946.4 nm at 17 mA.

Generation of continuous-variable high-dimensional entanglement with three degrees of freedom and multiplexing quantum dense coding

High-dimensional entanglement is a critical foundation for the growing demand for information capacity to implement the high-capacity quantum task. Here, we report continuous-variable high-dimensional entanglement with three degrees of freedom (frequency, polarization, and orbital angular momentum) directly generated with a single type-II optical parametric oscillator (OPO) cavity. By compensating both for dispersion in frequency modes and astigmatism in higher-order transverse modes, the OPO is capable of oscillating simultaneously and outputting thousands of entanglement pairs. The three degrees of freedom high-dimensional entanglement are verified simultaneously possessing frequency comb, spin, and orbital angular momentum entanglement via 14 pairs of Hermite–Gaussian mode correlations measurement. Then, the “space-frequency” multiplexing quantum dense coding communication is also demonstrated by using the entanglement resource. It shows the great superiority of high-dimensional entanglement in implementing the high-capacity quantum task. Apart from an increased channel capacity, it is possible to conduct deterministic high-dimensional quantum protocols, quantum imaging, and especially quantum computing.

Exploring the effect of transverse patterns on the temporal characteristics in self-mode-locked Nd: YVO[formula omitted] lasers

We investigate the influence of transverse mode distribution on the self-mode-locked (SML) states by employing off-axis pumping in an Nd:YVO4 laser with a linear concave-plano cavity. The higher-order transverse modes in the cavity are excited by adjusting the off-axis pumping displacement, and the dependence of the time characteristics of the SML laser on the transverse modes is analyzed. When the laser cavity length is 50 mm, and the off-axis displacement is adjusted offsets in the radial x and y directions to be (0,0) μ m, (45,58) μ m, and (80,0) μ m, the laser is operated in the longitudinal SML state with a repetition rate of 2.72 GHz, the amplitude modulation mode-locked pulse with a modulation frequency of 0.68 GHz and a repetition rate of 2.72 GHz, and the total-self-mode-locked with a repetition rate of 0.68 GHz, respectively. The corresponding transverse mode patterns are TEM00, TEM00+TEM01+TEM10 superposition, and TEM00+TEM10+TEM20 superposition. Finally, numerical simulations reconstruct the experimental mode-locked pulse and the corresponding transverse mode patterns based on the theory of coherent superposition of laser modes.

Minimizing the influence of higher-order transverse modes on the precision of cavity ring-down measurements

In cavity ring-down (CRD) measurements, the excitation of higher-order transverse modes and their random occurrence together with the fundamental mode inside the ring-down cavity (RDC) inevitably induce measurement errors of the decay time constant, which lead to a reduction in the measurement precision of the CRD instrument. To analyze and minimize the influence of higher-order transverse mode excitations on the CRD measurement precision, in this paper we report on the experimental observations of the impact of mode matching lens, aperture inside the cavity and trigger threshold on the excitation probability of fundamental and higher-order transverse mode excitations. The theoretical analysis and experimental results showed that when the optical misalignment is negligibly small, the occurrence probability of higher-order transverse mode excitations can be effectively suppressed by using a single lens between the laser and RDC to match preferably the fundamental model of the laser beam into the RDC, or inserting an aperture of an appropriate diameter inside the RDC, or using a high trigger threshold to record only CRD events with high amplitudes, or the combination of these approaches. For example, when the mode-matching lens is located at the optimal experimental position, and combined with the 2.2 mm diameter aperture, a 100% suppression of the excitation of high-order transverse modes can be achieved. Our results demonstrated that these methods effectively eliminated the decay time constant determination errors caused by the higher-order transverse mode excitations and therefore improved the precision of CRD experiments for high reflectance measurements. In practice the combination of mode-matching and appropriate trigger threshold setting is preferable for highly precise and yet fast CRD measurements.

Spatial rogue waves in the actively Q-switched Nd:YAG laser under low Kerr nonlinearity.

We report the generation of spatial rogue waves in an actively Q-switched Nd:YAG laser. We observed that spatial rogue waves can emerge when the laser operates in a low-power regime below the self-focusing limit, while the lasing takes place at a large number of high-order transverse modes. These results suggest that the main mechanism leading to rogue waves generation is the modal overlap while large Kerr nonlinearity in the cavity is not a necessary factor in the process of spatial rogue waves formation. We have also investigated the spatio-temporal and coherence properties of the observed rogue waves.