Laser Concept Research Articles

Terahertz Metasurface Quantum-Cascade VECSELs: Theory and Performance

Achieving both high power and high-quality beam pattern has been a longstanding challenge for terahertz (THz) quantum-cascade (QC) lasers largely due to their use of subwavelength metallic waveguides. Recently, the vertical-external-cavity surface-emitting laser (VECSEL) concept was demonstrated for the first time in the THz range and for a QC-laser. This is enabled by the development of an amplifying metasurface reflector capable of coupling incident free-space THz radiation to the QC-laser material such that it is amplified and reradiated. The THz metasurface QC-VECSEL initiates a new approach for making QC-lasers with high power and excellent beam pattern. Furthermore, the ability to engineer the electromagnetic phase, amplitude, and polarization response of the metasurface enables lasers with new functionality. This paper provides an overview of the fundamental theory, design considerations, and recent results for high-performance THz QC-VECSELs.

Narrow-line external cavity diode laser micro-packaging in the NIR and MIR spectral range

Narrow-linewidth tunable diode lasers are an important tool for spectroscopic instrumentation. Conventional external cavity diode lasers offer high output power and narrow linewidth. However, most external cavity diode lasers are designed as laboratory instrument and do not allow portability. In comparison, other commonly used lasers, like distributed feedback lasers (DFB) that are capable of driving a handheld device, are limited in power and show linewidths which are not sufficiently narrow for certain applications. We present new miniaturized types of tunable external cavity diode laser which overcome the drawbacks of conventional external cavity diode lasers and which preserve the advantages of this laser concept. Three different configurations are discussed in this article. The three types of miniaturized external cavity diode laser systems achieve power values of more than 50 mW within the 1.4 $$\mu$$ m water vapor absorption band with excellent side-mode suppression and linewidth below 100 kHz. Typical features outstand with respect to other type of laser systems which are of extended use such as DFB laser diodes. The higher output power and the lower linewidth will enable a higher sensitivity and resolution for a wide range of applications.

Photoepilation and Skin Photorejuvenation: An Update.

The effectiveness of intense pulsed light (IPL) and laser devices is widely accepted in aesthetic dermatology for unwanted hair removal and treatment of a variety of cutaneous conditions. Overall, most comparative trials have demonstrated similar effectiveness for IPL and laser devices. Literature studies alternatively favor the IPL and laser concepts, but the incidence of severe local pain and side effects were generally lower with IPL. IPL phototherapy, already established as a sound option in photoepilation and treatment of photoaging, hyperpigmentation and other skin conditions, is also considered first choice in the phototherapy of skin vascular malformations. When treating large areas, as often required in photoepilation and many aesthetic dermatology indications, IPL technologies show advantages over laser-based devices because of their high skin coverage rate. Compared to lasers, the wide range of selectable treatment settings, though a strong advantage of IPL, may also imply some more risk of local thermal side effects, but almost only in the hands of poorly trained operators. Overall, the strongest advantages of the IPL technologies are robust technology, versatility, lower purchase price, and the negligible risk of serious adverse effects in the hands of skilled and experienced operators.

Heat dissipation from a stationary brake disc, Part 2: CFD modelling and experimental validations

Following from the analytical modelling presented in Part 1, this paper details a comprehensive computational fluid dynamics modelling of the three-dimensional flow field around, and heat dissipation from, a stationary brake disc. Four commonly used turbulence models were compared and the shear stress turbulence model was found to be most suitable for these studies. Inferior cooling of the anti-coning disc type is well known but the core cause in static conditions was only now established. The air flow exiting the lower vane channels at the inner rotor diameter changes direction and flows axially over the hat region. This axial flow acts as a blocker to the higher vane inlets, drastically reducing convective cooling from the upper half of the disc. The complexity of disc stationary cooling is further caused by the change of flow patterns during disc cooling. The above axial flow effects slowly vanish as the disc temperatures reduce. Consequently, convective heat transfer coefficients are affected by both, the change in the flow pattern and decrease in air velocities due to reduced air buoyancy as the disc cools down. As in Part 1, the special thermal rig was used to validate the computational fluid dynamics results quantitatively and qualitatively. The former used numerous thermocouples positioned strategically around the brake disc, with the latter introducing the concept of laser generated light plane combined with a smoke generator to enable flow visualisation. Predicted average heat transfer coefficients using computational fluid dynamics correlate well with the experimental values, and even two-dimensional analytical values (as presented in Part 1) reasonably closely follow the trends. The results present an important step in establishing cooling characteristics related to the electric parking brake application in commercial vehicles, with future publications detailing heat transfer from the entire brake assembly.

Graphene actively Q-switched lasers

Graphene electro-optic modulators (GEOMs) are emerging as a viable alternative to conventional material-based modulators mainly due to their broadband and ultrafast performance. These GEOMs with combined advantages of small footprint and low energy consumption can potentially enable various high-performance applications that are not possible using conventional approaches. Here, we report the first actively Q-switched lasers with a GEOM. In contrast to the previously reported lasers that are passively modulated by two-dimensional layered material-based saturable absorbers, our actively modulated laser concept represents significant advantages, such as electrically tunable output parameters (e.g. output repetition rate, pulse duration and pulse energy) and electro-optical synchronization. Using a single GEOM, we generate broadband Q-switched pulses at ∼1.55 and 2 μm with output energies of up to 123 nJ. This indicates the broadband pulse generation capability of the graphene-based active devices, superior to widely used bulk material-based active modulation approaches. Our results demonstrate a simple and viable design towards broadband, high-repetition-rate, electrically modulated ultrafast lasers for various applications, such as telecommunications and spectroscopy.

Complex Active Optical Networks as a New Laser Concept.

Complex optical networks containing one or more gain sections are investigated, and the evidence of lasing action is reported; the emission spectrum reflects the topological disorder induced by the connections. A theoretical description compares well with the measurements, mapping the networks to directed graphs and showing the analogies with the problem of quantum chaos on graphs. We show that the interplay of chaotic diffusion and amplification leads to an emission statistic with characteristic heavy tails: for different topologies, an unprecedented experimental demonstration of Lévy statistics expected for random lasers is here provided for a continuous-wave pumped system. This result is also supported by a MonteCarlo simulation based on the ray random walk on the graph.

Femtosecond filament initiated, microwave heated cavity-free nitrogen laser in air

We present the results of numerical modeling of the igniter-heater concept for initiation of standoff, cavity free lasing action in the atmosphere when a femtosecond laser filament is used for plasma generation (igniter) and a microwave heater provides electron-collision pumping of electronic states in molecular nitrogen. By solving numerically the kinetic equation for the energy distribution function of electrons, generated in a femtosecond laser filament and heated by a microwave beam, we identify the conditions enabling single-pass, standoff UV-laser from molecular nitrogen in the atmosphere. The plasma density, the minimum amplitude of the microwave field, and the small-signal gain, necessary to achieve the lasing, are determined. We demonstrate that lasing build up time can be minimized and efficiency improved by using elliptically polarized laser pulses for filamentation. It is shown that realization of the filament-igniter, microwave-heater concept of the sky laser at low altitudes would require a microwave source of hundreds of kilowatt-megawatt power. The required microwave power can be reduced by several orders of magnitude when the igniter-heater scheme is used at the 10–30 km range of altitudes.

Discrete tuning concept for fiber-integrated lasers based on tailored FBG arrays and a theta cavity layout

We demonstrate a novel tuning concept for pulsed fiber-integrated lasers with a fiber Bragg grating (FBG) array as a discrete and tailored spectral filter, as well as a modified laser design. Based on a theta cavity layout, the structural delay lines originating from the FBG array are balanced, enabling a constant repetition rate and stable pulse properties over the full tuning range. The emission wavelength is electrically tuned with respect to the filter properties based on an adapted temporal gating scheme using an acousto-optic modulator. This concept has been investigated with an Yb-doped fiber laser, demonstrating excellent emission properties with high signal contrast (>35 dB) and narrow linewidth (<150 pm) over a tuning range of 25 nm.

Bosonic cascades of indirect excitons

Recently, the concept of the terahertz bosonic cascade laser (BCL) based on a parabolic quantum well (PQW) embedded in a microcavity was proposed. We refine this proposal by suggesting transitions between indirect exciton (IX) states as a source of terahertz emission. We explicitly propose a structure containing a narrow-square QW and a wide-parabolic QW for the realisation of a bosonic cascade. Advantages of this type of structures are in large dipole matrix elements for terahertz transitions and in long exciton radiative lifetimes which are crucial for realisation of threshold and quantum efficiency BCLs.

Investigation of Buckling Capacity of Metal Materials Manufactured by Laser 3D Printing

The buckling criterion for 3D printed metal materials is studied experimentally in this paper. Several metal columns are manufactured using a laser concept 3D printer using different sample orientations. The maximum length of the specimens is limited to 90mm. A standard buckling test is designed using pin-ended struts. The applied compressive force is measured versus the vertical displacement for all specimens. The critical force for each thickness and printing orientation is obtained by averaging the results obtained from five specimens for each case. It is observed that the buckling capacity of the 3D printed metal struts is strongly influenced by the printing direction. It can be concluded that, for these structures, buckling is a dominant behavior and the printing direction needs to be considered during the design process.

Terahertz difference-frequency quantum cascade laser sources on silicon

Terahertz quantum cascade laser sources based on intra-cavity frequency mixing are currently the only monolithic electrically pumped semiconductor devices that can operate in the 1–6 THz spectral range at room temperature. The introduction of the Cherenkov waveguide scheme in these devices grown on semi-insulating InP substrates enabled generation of tens of microwatts of average terahertz power output and wide spectral tunability. However, terahertz radiation outcoupling in these sources is still highly inefficient. Here we demonstrate that an application of the III–V-on-silicon hybrid laser concept to terahertz quantum cascade laser sources based on Cherenkov intra-cavity difference-frequency generation dramatically improves their output power and mid-infrared-to-terahertz conversion efficiency. The best-performing device transfer-printed on a float-zone high-resistivity silicon substrate produced 270 μW of peak power output at 3.5 THz at room temperature, a factor of 5 improvement over the best reference devices on a native semi-insulating InP substrate.

Coherent Interferometric Dual-Frequency Laser Radar for Precise Range/Doppler Measurement

A novel coherent interferometric dual frequency laser radar that merges the concept of laser and radio detection and ranging (ladar and radar, respectively), for both range and velocity measurement, is presented and experimentally demonstrated. The innovative architecture combines the broadband tunability of dual-wavelength optical sources, enabling a dynamic trade-off between precision and robustness in Doppler estimation, with the high stability of low frequency RF sources for the interferometric measure of the target range with extreme accuracy. The possibility to easily reconfigure the employed frequencies, allows to change the Doppler resolution, as well as the range ambiguity and precision, to dynamically adapt the system to reliably operate in different environments. Moreover, the coherent detection allows to enhance the signal to noise ratio reaching excellent performances also with low level of received power. The laboratory characterization provides an estimation of the system performances, in terms of resolution and sensitivity, as well as the indoor demonstration with targets of opportunity proves the effectiveness of the proposed architecture to operate in real scenarios.

Carbon nanotubes for ultrafast fibre lasers

Abstract Carbon nanotubes (CNTs) possess both remarkable optical properties and high potential for integration in various photonic devices. We overview, here, recent progress in CNT applications in fibre optics putting particular emphasis on fibre lasers. We discuss fabrication and characterisation of different CNTs, development of CNT-based saturable absorbers (CNT-SA), their integration and operation in fibre laser cavities putting emphasis on state-of-the-art fibre lasers, mode locked using CNT-SA. We discuss new design concepts of high-performance ultrafast operation fibre lasers covering ytterbium (Yb), bismuth (Bi), erbium (Er), thulium (Tm) and holmium (Ho)-doped fibre lasers.

Dipole-Nano-Laser Based Plasmonic Nano-Sensor for Sensing Photons and Bio-Chemical Analytes

We propose a new type of plasmonic nano-sensors that is based on the concept of the dipole nano laser (DNL). The DNL is made of a quantum dot (QD) coupled to a nano particle (NP). The very small size of the DNL renders the sensor one of the smallest proposed in literature. The system is embedded within a photonic band gap (PBG) material. The PBG material is shown to greatly narrow the DNL spectral width, which translates into a high and accurate sensibility for the proposed plasmonic nano sensor. The proposed system configuration can easily be adjusted to detect either photons or bio-chemical analytes.

Femtosecond Lasers over 100 Watts

AbstractThe concept of the world's first industrial femtosecond laser with high average power (&gt; 100 W), delivering 400 fs pulses that is presented here is flexible in pulse repetition rate and offers high pulse energy up to 300 μJ. The laser is designed for easy synchronization to polygon scanners and with optional burst mode. This laser enables higher throughput and productivity, coupled with superior quality processing. The availability of high power femtosecond lasers paves the way for further penetration of ultrafast laser sources into emerging industrial high quality laser processing.

Robustness analysis of a device concept for edge-emitting lasers based on strained germanium

We consider a device concept for edge-emitting lasers based on strained germanium microstrips. The device features an inhomogeneous tensile strain distribution generated by a SiN stressor deposited on top of the Ge microstrip. This geometry requires a lateral contact scheme and hence a full two-dimensional description. The two-dimensional simulations of the carrier transport and of the optical field, carried out in a cross section of the device orthogonal to the optical cavity, use microscopic calculations of the strained Ge material gain as an input. In this paper we study laser performance and robustness against Shockley–Read–Hall lifetime variations and device sensitivity to different strain distributions.

A New Concept in Picosecond Lasers

AbstractOver the last decade, mode‐locked laser sources in the ten picosecond pulse regime expanded into medical applications and high‐precision micro‐material machining. However, the complexity and the alignment sensitivity of picosecond lasers translated to higher cost and impeded a broader market entry. The goal of the iPLASE project was to develop a novel laser concept (Fig. 1) which is capable of generating sub‐10 picosecond pulses. To this end, we shortened the pulses of a novel amplified microchip laser using a pulse compressor. This article outlines the key achievements of the project.

Design concepts of monolithic metamorphic vertical-cavity surface-emitting lasers for the 1300–1550 nm spectral range

Possible design concepts for long-wavelength vertical-cavity surface-emitting lasers for the 1300–1550 nm spectral range on GaAs substrates are suggested. It is shown that a metamorphic GaAs–InGaAs heterostructure with a thin buffer layer providing rapid transition from the lattice constant of GaAs to that of InxGa1–xAs with an indium fraction of x < 0.3 can be formed by molecular-beam epitaxy. Analysis by transmission electron microscopy demonstrated the effective localization of mismatch dislocations in the thin buffer layer and full suppression of their penetration into the overlying InGaAs metamorphic layer.

InP-Based Vertical-Cavity Surface-Emitting Lasers With Type-II Quantum Wells

InP-based type-II quantum wells are a promising concept for long wavelength lasers beyond 2 μm. In this paper, recently developed vertical-cavity surface-emitting lasers for 2.5 μm with type-II quantum wells are introduced and their performance is discussed. These lasers demonstrate single-mode CW operation with an output power up to 400 μW and side-mode suppression ratio of 30 dB. The maximum temperature for CW operation is 10 °C. In order to further analyze their performance, a phenomenological model for the temperature dependences of threshold current densities is presented. Based on this model influence of heat sink temperature, current broadening and self-heating is quantified and conclusions are drawn for an improved design.

A switchable dual-wavelength fiber laser based on asymmetric fiber Bragg grating Fabry–Perot cavity with a SESAM

A switchable dual-wavelength fiber laser with an asymmetric fiber Bragg grating (FBG)–Fabry–Perot (FP) cavity based a semiconductor saturable absorber mirror (SESAM) is proposed and experimentally demonstrated. The proof of concept device consists of a FGB laser with an asymmetric FBG-FP cavity, a SESAM as mode loss modulator, and a intracavity FBG as wavelength selector by changing its operation temperature. The results demonstrate the new concept of dual-wavelength fiber laser based SESAM with asymmetric FBG-FP cavity and the technical feasibility.