Diffraction-limited Divergence Research Articles

Surface grating VCSEL-integrated amplifier/beam scanner with high power and single mode operation

We propose and demonstrate a novel surface-grating vertical cavity surface emitting laser (VCSEL)-integrated amplifier/beam scanner. When the surface of the VCSEL section is periodically etched, a single slow-light mode which travels laterally into the amplifier section is selected due to the wavelength selectivity of the grating. The coupled slow light can be amplified with pumping the amplifier above the threshold. The far field angle of the amplified slow light can be continuously tuned through changing the pumping current at the VCSEL. We fabricated the integrated device with a 400 μm long VCSEL section and 1 mm long amplifier section, realizing in a single mode coupling power of over 7 mW and high output power of over 500 mW under pulsed operations of the amplifier. The continuous fan beam steering of 1.5° and a diffraction-limited narrow beam divergence of 0.06° are also achieved. We also fabricated device with extending the amplifier section length to 2 mm. A high single mode power of 1 W under pulsed operations is achieved, which is the record high power operation for single-mode VCSELs. With the performance of both high power and good beam quality, our new device shows great potential to be used as light source for LiDAR and other sensing applications.

Non-mechanical beam scanner based on VCSEL integrated amplifier with resonant wavelength detuning design

We demonstrate the non-mechanical beam steering and amplifier operation of a vertical cavity surface emitting laser (VCSEL) integrated Bragg reflector waveguide amplifier with a cut-off wavelength detuning design, which enables unidirectional lateral coupling, continuous electrical beam steering, and diffraction-limited divergence angle. We present the modeling of the proposed structure for unidirectional coupling between a seed single-mode VCSEL and slow-light amplifier. We also present the detailed operating characteristics including the near-field and far-field patterns, light/current characteristics, and lasing spectrum. The experimental measurements exhibit a single-mode output of over 8 mW under CW operation, a continuous beam steering range of 16°, and beam divergence below 0.1° as an optical beam scanner. The integrated amplifier length is as small as 0.9 mm, and thus we could expect much higher powers and higher resolution points by increasing the amplifier lengths.

Single-shot large field of view Fourier transform holography with a picosecond plasma-based soft X-ray laser.

It is challenging to obtain nanoscale resolution images in a single ultrafast shot because a large number of photons, greater than 1011, are required in a single pulse of the illuminating source. We demonstrate single-shot high resolution Fourier transform holography over a broad 7 µm diameter field of view with ∼ 5 ps temporal resolution. The experiment used a plasma-based soft X-ray laser operating at 18.9 nm wavelength with nearly full spatial coherence and close to diffraction-limited divergence implemented utilizing a dual-plasma amplifier scheme. A Fresnel zone plate with a central aperture is used to efficiently generate the object and reference beams. Rapid numerical reconstruction by a 2D Fourier transform allows for real-time imaging. A half-pitch spatial resolution of 62 nm was obtained. This single-shot nanoscale-resolution imaging technique will allow for real-time ultrafast imaging of dynamic phenomena in compact setups.

PRECISION MICROMACHINING OF METALS BY CUBR LASER

The ability to laser machine materials with high resolution and high throughput is critical in advanced manufacturing for a vast array of applications, from photovoltaic cells to bio-compatible micro-components. Copper bromide (CuBr) lasers with their excellent beam quality promised noticeable advantages and improvements in high precision and material processing at the microscale. The application of the CuBr laser as a precision tool for micromachining of different metals has been demonstrated. That good performance was a result of the combination of high power visible radiation, short pulses, and close to the diffraction-limited laser beam divergence with high-speed galvo scanner beam steering.

Combining and collimation of RGB laser beams with transmissive resonance domain diffractive optics

Resonance domain diffractive optical elements for combining RGB laser beams into a single collimated beam were designed, fabricated, and experimentally investigated. The input RGB beams were angular separated up to tens of degrees and set in a nearly Bragg arrangement for high diffraction efficiency. A single resonance domain diffractive lens delivered beam combining and collimation functions with reasonable residue divergence. The resonance domain diffraction grating delivered diffraction-limited residue divergence in combining the collimated RGB beams. Optical experiments with fiber-coupled RGB lasers and e-beam-fabricated beam combiners proved low residue beam divergence, a high polarization extinction ratio, and total measured diffraction efficiency of about 80%.

Tapered Quantum Cascade Laser Arrays Integrated with Talbot Cavities

Power scaling in broad area quantum cascade laser (QCL) usually leads to the deterioration of the beam quality with an emission of multiple lobes far-field pattern. In this letter, we demonstrate a tapered QCL array integrated with Talbot cavity at one side of the array. Fundamental supermode operation is achieved in the arrays with taper straight-end connected to the Talbot cavity. Lateral far-field of the fundamental supermode shows a near diffraction limited beam divergence of 2.7°. The output power of a five-element array is about three times as high as a single-ridge laser with an emission wavelength of around 4.8 μm. However, arrays with the taper-end connected to the Talbot cavity always show a high-order supermode operation whatever Talbot cavity length is.

On-chip platform for a phased array with minimal beam divergence and wide field-of-view.

Current silicon photonics phased arrays based on waveguide gratings enable beam steering with no moving parts. However, they suffer from a trade-off between beam divergence and field of view. Here, we show a platform based on silicon-nitride/silicon that achieves simultaneously minimal beam divergence and maximum field of view while maintaining performance that is robust to fabrication variations. In addition, in order to maximize the emission from the entire length of the grating, we design the grating's strength by varying its duty cycle (apodization) to emit uniformly. We fabricate a millimeter long grating emitter with diffraction-limited beam divergence of 0.089°.

Full control of far-field radiation via photonic integrated circuits decorated with plasmonic nanoantennas.

We theoretically develop a hybrid architecture consisting of photonic integrated circuit and plasmonic nanoantennas to fully control optical far-field radiation with unprecedented flexibility. By exploiting asymmetric and lateral excitation from silicon waveguides, single gold nanorod and cascaded nanorod pair can function as component radiation pixels, featured by full 2π phase coverage and nanoscale footprint. These radiation pixels allow us to design scalable on-chip devices in a wavefront engineering fashion. We numerically demonstrate beam collimation with 30° out of the incident plane and nearly diffraction limited divergence angle. We also present high-numerical-aperture (NA) beam focusing with NA ≈0.65 and vector beam generation (the radially-polarized mode) with the mode similarity greater than 44%. This concept and approach constitutes a designable optical platform, which might be a future bridge between integrated photonics and metasurface functionalities.

Superemission in vertically-aligned single-wall carbon nanotubes

Presently we used two samples of vertically aligned single-wall carbon nanotubes (VA SWCNTs) with parallelepiped geometry, sized 0.02cm×0.2cm×1.0cm and 0.2cm×0.2cm×1.0cm. We report absorption and emission properties of the VA SWCNTs, including strong anisotropy in both their absorption and emission spectra. We found that the emission spectra extend from the middle-IR range to the near-IR range, with such extended spectra being reported for the first time. Pumping the VA SWCNTs in the direction normal to their axis, superemission (SE) was observed in the direction along their axis. The SE band maximum is located at 7206±0.4cm−1. The energy and the power density of the superemission were estimated, along with the diffraction-limited divergence. At the pumping energy of 3mJ/pulse, the SE energy measured by the detector was 0.74mJ/pulse, corresponding to the total SE energy of 1.48mJ/pulse, with the energy density of 18.5mJcm−2/pulse and the SE power density of 1.2×105Wcm−2/pulse. We report that a bundle of VA SWCNTs is an emitter with a relatively small divergence, not exceeding 3.9×10−3rad. We developed a theoretical approach to explain such absorption and emission spectra. The developed theory is based on the earlier proposed SSH theory, which we extended to include the exchange interactions between the closest SWCNT neighbors. The developed theoretical ideas were implemented in a homemade FORTRAN code. This code was successfully used to calculate and reproduce the experimental spectra and to determine the SWCNT species that originate the respective absorption bands, with acceptable agreement between theory and experiment.

Superemission of Cr nanolayers

We continue our studies of metal nanolayers started earlier [1]. Presently, we investigate absorption, emission and superemission in Cr nanolayers. We provide experimental estimates of the energy density and power density of superemission of Cr nanolayers, along with estimates of the diffraction-limited light divergence. We report that the divergence of the superemission along the direction normal to the nanolayer corresponds to that of a point source, with the divergence angle of ca. 0.2rad. Additionally, we report surprising long-range transfer of the excitation energy in Cr metal films, at macroscopic distances of 1cm.

Longitudinal-transverse dynamics of super-Gaussian pulses in isotropic dielectrics

The set of equations governing the dynamics of the parameters of a super-Gaussian pulse propagating in an isotropic dielectric in regions of both normal and anomalous group velocity dispersion is obtained. It is demonstrated that stimulated Raman scattering can suppress the formation of a nonlinear focus. The expression for critical power of the signal for which self-focusing can still be compensated by diffraction-limited divergence is derived.

Ultimate efficiency of spectral beam combining by volume Bragg gratings

Spectral beam combining (SBC) by volume Bragg gratings (VBGs) recorded in photo-thermo-refractive (PTR) glass is a powerful tool for laser applications that require higher radiance than a single laser unit can achieve. The beam-combining factor (BCF) is introduced as a tool to compare various beam-combining methods and experiments. It describes the change of radiance provided by a beam-combining system but is not affected by the initial beam quality of the combined lasers. A method of optimization of VBGs providing the maximum efficiency of SBC has been described for an arbitrary number of beams. An experiment confirming the proposed modeling for a two-beam SBC system by a single VBG has demonstrated a total combined power of 301 W with a channel separation of 0.25 nm, combining efficiency of 97%, close to diffraction limited divergence with M(2)=1.18, BCF of 0.77, and spectral radiance of 770 TW/(sr·m(2)·nm), the highest to date for SBC.

RF-excited unstable-resonator planar CO2 laser on all-metal electrode—waveguide structure

The radiation characteristics of a planar CO2 laser excited by a diffusion-cooled rf discharge at a frequency of 40 MHz are studied. A single-mode cw lasing power of ≈50 W is achieved with an efficiency of ≈10% for a nearly diffraction-limited radiation divergence of 4—7 mrad. The spatial structure, output power, stability and laser radiation quality are studied as functions of longitudinal and angular alignments of the resonator mirror for two types of hybrid unstable-waveguide resonators of the laser. It is shown that for the resonator corresponding to the negative branch of the stability diagram, a misalignment of 0.02 rad of the mirrors leads to a 50% decrease in the output laser power, while its value for the positive branch resonator is about 100 times smaller. It is found that for the resonator corresponding to the negative branch, the sensitivity to the violation of confocal arrangement of the mirrors upon an increase in the resonator length is an order of magnitude higher. The dependence of the density of input rf power on the working gas pressure is studied experimentally in the interval 50—110 Torr. Power density values of 1—4 W cm-2 are obtained for normal discharge current density. These values are important for optimisation and scaling of the lasing characteristics of high-power planar CO2 lasers.

Compact neodymium phosphate glass laser emitting 100-J, 100-GW pulses for pumping a parametric amplifier of chirped pulses

A five-stage neodymium phosphate glass amplifier producing 1–1.5-ns radiation pulses of energy up to 110 J is described. The use of a multistage spatial filter based on an aperture line provides efficient extraction of the stored energy. The exit aperture filling factor is 0.65 and the output radiation divergence is equal to three times the diffraction-limited divergence. The energy efficiency of radiation conversion to the second harmonic is 60%. The amplifier is intended for pumping a chirped-pulse optical parametric amplifier.

Properties of self-focusing of elliptic beams

Stationary self-focusing of collimated elliptic Gaussian beams is studied numerically. It is shown that the critical self-focusing power increases with increasing the axial ratio of the initial elliptic cross section. Self-focusing is accompanied by oscillations of the beam transverse size caused by the competition between the diffraction-limited divergence and nonlinear compression. As the power of the beam with the axial ratio a/b≥3 increases, aberration self-focusing occurs, at which, after the formation and competition of several intensity maxima in the beam cross section, a global maximum appears at the beam centre, which forms a nonlinear focus. The approximation formula for the self-focusing distance is generalised to elliptic beams.

Demonstration of a Fully Spatial Coherent X-Ray Laser at 13.9 nm

We have recently reported the successful development of a fully coherent X-ray laser (XRL) at 13.9 nm by an oscillator-amplifier configuration with two targets. In the experiment, a seed XRL beam from the first target is injected into a plasma amplifier at the second target. The observed XRL beam has full spatial coherence and 0.2 mrad of nearly diffraction-limited divergence. In order to improve the output fluence, the amplification properties of the XRL beam have been investigated using various plasma lengths of the second amplifier target. The output energy has been improved by a factor of ten, increasing the length of the gain region to 10 mm, resulting in about 0.2 /spl mu/J of output energy.

Control of the radiation parameters of a copper vapour laser

The possibility of controlling the pulse shape and duration and the beam divergence of a copper vapour laser operating in the mode of double pump pulses, when the first pulse excites lasing in the active medium and the second amplifies it. It is shown that a change in the delay of the second pump pulse relative to the laser pulse initiated by the first pump pulse allows an efficient control of the laser-radiation characteristics. In this case, the coefficient of laser-radiation conversion into a beam with a diffraction-limited divergence may reach ~80 %.

X-ray laser beam with diffraction-limited divergence generated with two gain media.

We demonstrate an x-ray laser at a wavelength of 13.9 nm with a beam divergence of 0.2 mrad, which is 1.8 times the diffraction limit. The x-ray laser is generated with two gain media; the seed x-ray pulse from the first medium is amplified in the second medium. The effect of refraction on x-ray propagation is reduced by spatially and temporally controlling the injection of the seed x-ray to the second medium.

Investigation of an optical vortex beam with a leaky planar waveguide

First experimental investigation of an optical vortex beam by means of a multiple-beam interference device is reported. Launching of an optical vortex beam into a prism–film coupler (leaky-wave planar waveguide) gives high angular resolution due to the resonant excitation of a propagating leaky mode. Observation of the dark line in the cross-section of the reflected beam visualizes a narrow range of equal incident angles within an optical beam with diffraction-limited divergence. The expectations from optical ray approach and wave approach are compared with the experimental results.

New diode lasers with leaking emission in an optical cavity

A new type of a wide-aperture, high-power and efficient semiconductor laser with emission leaking from an active region and involved in lasing is proposed. The principle of its operation is described. Single-mode semiconductor lasers with output apertures at the optical facet of 5 × 6 μm, 7 × 7.5 μm, and 10 × 10 μm and diffraction-limited divergence angles of emission from 6.9 to 12° in the vertical plane and from 3.3 to 7.7° in the horizontal plane are created for the first time. A single-mode cw output power of 0.5 W is obtained at 980 nm in a single-frequency regime with diffraction-limited divergence angles in the horizontal and vertical planes equal to 5.7° and 12.3°, respectively. In the multimode regime, the output powers of 1.3 and 3.0 W were obtained with small divergence angles for ridge widths of 10 and 50 μm, respectively.