Output Beam Divergence Research Articles

Low-loss tunable beam collimator and expander assembly with no moving parts using anengineered diffuser and varifocal lenses.

In this paper, we present a novel design for a tunable beam collimator. A variable collimator assists in achieving an adaptive size of an output collimated beam. Alternatively, it can also provide an adjustable output beam divergence angle for a noncollimated beam output. Tunable collimators are highly desirable for various applications in testing, engineering, and measurements. Such devices are also useful in providing tunable illumination of samples or targets in microscopes and emulating different target distances for characterizing the performance of camera systems in laboratory settings. The proposed collimator has two distinct advantages: it is light-efficient compared with pinhole-based collimator designs, and it delivers a large range of output beam sizes without involving the mechanical motion of bulk components. These attributes are achieved via the use of an engineered diffuser (in the place of a pinhole) and a pair of large aperture tunable focus lenses, which deliver a tunable magnification to the output collimated beam. In laboratory experiments, we achieve an optical transmission efficiency of 90% for the proposed tunable collimator.

Design of an all hollow fiber‐coupled middle infrared laser heterodyne radiometer (LHR)

AbstractAn integrated optics method, using hollow fibers which easily ensures optimal beam alignment of the local oscillator (LO) and incoherent solar radiation and promotes high photomixing efficiency, is proposed for a compact, rugged, and high‐efficiency all hollow fiber‐coupled photomixing assembly in the mid‐infrared (IR). The method is adopted for the design of an all‐hollow fiber‐coupled mid‐IR laser heterodyne radiometer (LHR) using an 8 µm external‐cavity quantum cascade laser as LO. Coupling efficiency, attenuation coefficients, total‐loss transmission, and output beam divergence of the HE11 mode in hollow fibers for the all hollow fiber‐coupled mid‐IR LHR are discussed in detail and are shown to highly depend on the launch conditions. Meanwhile, the selection of input and output coupling lenses that are associated with the hollow fibers is also studied for achieving the theoretical loss of the HE1m mode in hollow fibers and high heterodyne performance. It shows that using a hollow fiber for photomixing supersedes conventional spatially separated optics.

Continuous-wave and pulsed operation of Ho:YLF laser end-pumped by Tm:fiber laser

Continuous-wave and pulsed operation of a Ho:YLF laser was demonstrated. A Tm:fiber laser with 70 W output power was used for the pump source. In this experimental setup, the end-pumped gain medium with 0.5% and 1% Ho concentration in the optical resonators were used. In the continuous-wave regime, 14.2 and 12.8 W linearly polarized power with a concave-concave resonator for 1.0% and 0.5% Ho dopes were extracted, respectively. These amounts correspond to slope efficiencies of 51.9% and 71.3% and optical-to-optical efficiencies of 39.4% and 54.7% for absorbed pump power, respectively. To avoid optical damage, a U-shaped concave-concave resonator was installed, and the resulting spot size in the resonator was about 1.31 mm. The spot size changes about 10% along the total physical length of the resonator. In the pulsed operation regime, for 0.5% Ho concentration with 20 ms pump pulse duration and 10 Hz repetition rates, the free-running output energy was 172 mJ with 16 ms pulse duration. In addition, the absorbed pump energy was about 485 mJ, corresponding to the slope and the optical-to-optical efficiencies of 14.8% and 12.3%, respectively. Also, 27 mJ pulse energy with 29.7 ns duration was achieved for Q-switched operation. Finally, the angle of output beam divergence was measured at approximately 1.1 mrad, and the laser emission wavelength was 2.0568 µm with 0.0065 µm line width.

Specific features of radiation flux formation in diode-pumped lasers and amplifiers with active elements made of Nd : YAG ceramics

The spatial, temporal, and spectral characteristics of the output radiation of diode-pumped laser systems and optical amplifiers with active elements made of Nd : YAG ceramics are studied. It is shown that the divergence of radiation in the subthreshold regime has the form of concentric nested cones emerging from the active element (AE) faces at angles of 2 – 4 °. A model explaining the experimentally observed phenomenon is proposed based on the mechanism of scattering of the generated beam from the granular structure of the optical ceramics. Analysis of the divergence pattern makes it possible to estimate the grain size and the optical quality of ceramics. The output beam divergence in the lasing regime is determined mainly by the cavity parameters and does not exceed several milliradians. The scattering events in this case serve as sources of additional optical losses. Luminescence enhancement and parasitic oscillations in the AE volume restrict the possibility of using Nd : YAG ceramics for fabricating AEs of high-power optical amplifiers.

A high-throughput assembly of beam-shaping channel-cut monochromators for laboratory high-resolution X-ray diffraction and small-angle X-ray scattering experiments

A four-bounce monochromator assembly composed of Ge(111) and Ge(220) monolithic channel-cut monochromators with V-shaped channels in a quasi-dispersive configuration is presented. The assembly provides an optimal design in terms of the highest transmittance and photon flux density per detector pixel while maintaining high beam collimation. A monochromator assembly optimized for the highest recorded intensity per detector pixel of a linear detector placed 2.5 m behind the assembly was realized and tested by high-resolution X-ray diffraction and small-angle X-ray scattering measurements using a microfocus X-ray source. Conventional symmetric and asymmetric Ge(220) Bartels monochromators were similarly tested and the results were compared. The new assembly provides a transmittance that is an order of magnitude higher and 2.5 times higher than those provided by the symmetric and asymmetric Bartels monochromators, respectively, while the output beam divergence is twice that of the asymmetric Bartels monochromator. These results demonstrate the advantage of the proposed monochromator assembly in cases where the resolution can be partially sacrificed in favour of higher transmittance while still maintaining high beam collimation. Weakly scattering samples such as nanostructures are an example. A general advantage of the new monochromator is a significant reduction in the exposure time required to collect usable experimental data. A comparison of the theoretical and experimental results also reveals the current limitations of the technology of polishing hard-to-reach surfaces in X-ray crystal optics.

High‐energy high‐power near‐diffraction‐limited 1064 and 532 nm picosecond Nd:YAG laser

The authors describe a diode-pumped Nd:YAG laser that produces 920 mJ output energy pulses with a pulse duration of 76 ps at 200 Hz pulse repetition rate. Second-harmonic generation with a pulse energy of 730 mJ and pulse duration of 63 ps was obtained by using LiB3O5 crystal with II-type phase matching. The output beam divergence was about 1.5 × DL (the diffraction limit) and 1.9 × DL for 1064 and 532 nm wavelengths, respectively.

Multipass amplifiers with self-compensation of the thermal lens.

We present an architecture for a multipass amplifier based on a succession of optical Fourier transforms and short propagations that shows a superior stability for variations of the thermal lens compared to state-of-the-art 4f-based amplifiers. We found that the proposed multipass amplifier is robust to variations of the active medium dioptric power. The superiority of the proposed architecture is demonstrated by analyzing the variations of the size and divergence of the output beam in the form of a Taylor expansion around the design value for variations of the thermal lens in the active medium. The dependence of the output beam divergence and size is investigated also for variations of the number of passes, for aperture effects in the active medium, and as a function of the size of the beam on the active medium. This architecture makes efficient use of the transverse beam filtering inherent in the active medium to deliver a beam with excellent quality (TEM00).

0.43 J/100 ps Nd:YAG laser with adaptive compensation of thermally induced lens.

We demonstrate a diode-pumped master oscillator power amplifier Nd:YAG 1064nm laser with sub-joule-level output energy of 0.43J, high pulse repetition rate of 200Hz, and short pulse duration of 100ps. The 5mJ master oscillator consisted of a 100nJ/100ps Nd:YVO4 microchip laser and end-pumped Nd:YAG regenerative amplifier. The two-pass output amplifier was based on two ∅10×140 mm Nd:YAG laser rods. We apply an adaptive compensator with an analyzer based on an astigmatic optical system and a quadrant photodiode to compensate for low-order wave-front non-stationary thermal distortions in a high-energy laser amplifier. The adaptive compensator demonstrates high sensitivity to curvature changes of λ/100, and it provides output beam divergence near 1.5×DL in 100ps 0.43 J×200 Hz of operation mode.

Thermal oxidation-resistant GeO2 ATR hollow waveguide based on NiCr capillary tube and its thermal effects

Transmission characteristics of a metallic tube GeO2 ATR hollow waveguide degenerate due to metal thermal oxidation caused by transmission, coupling and misalignment losses. NiCr has intrinsic resistance to thermal oxidation and is often used in thermocouple heater. Thermogravimetric analysis shows that NiCr substrate tube material suffers no thermal oxidation with temperature up to 1100 °C. Thus, NiCr capillary tube is investigated for construction of a thermal oxidation-resistant GeO2 ATR hollow fiber. A NiCr capillary tube GeO2 ATR hollow fiber was fabricated based on liquid phase deposition. The fiber sample has a transmission attenuation of 0.25 dB/m when transmitting a 6-W CO2 laser. Sample temperature distributions that originated from transmission loss and coupling loss are theoretically simulated and practically measured. Results confirm that the simulation can properly tell the influence of laser power losses on the fiber temperature and is further used to predict the maximum input power of the ATR fibers. To illustrate the thermal oxidation resistance of NiCr ATR hollow fiber, NiCr-based and stainless steel (SUS)-based ATR hollow fibers are directly irradiated by 6 W CO2 laser. The SUS fiber input end became darkened. No obvious changes happened on NiCr fiber. Output beam profile and divergence angle are also taken to support the analysis. The NiCr capillary tube GeO2 ATR hollow fiber is a good candidate for construction of durable GeO2 ATR hollow fiber in high power applications.

Solution‐Processed Low Threshold Vertical Cavity Surface Emitting Lasers from All‐Inorganic Perovskite Nanocrystals

Recently, newly engineered all‐inorganic cesium lead halide perovskite nanocrystals (IPNCs) (CsPbX3, X = Cl, Br, I) are discovered to possess superior optical gain properties appealing for solution‐processed cost‐effective lasers. Yet, the potential of such materials has not been exploited for practical laser devices, rendering the prospect as laser media elusive. Herein, the challenging but practically desirable vertical cavity surface emitting lasers (VCSELs) based on the CsPbX3 IPNCs, featuring low threshold (9 µJ cm−2), directional output (beam divergence of ≈3.6°), and favorable stability, are realized for the first time. Notably, the lasing wavelength can be tuned across the red, green, and blue region maintaining comparable thresholds, which is promising in developing single‐source‐pumped full‐color visible lasers. It is fully demonstrated that the characteristics of the VCSELs can be versatilely engineered by independent adjustment of the cavity and solution‐processable nanocrystals. The results unambiguously reveal the feasibility of the emerging CsPbX3 IPNCs as practical laser media and represent a significant leap toward CsPbX3 IPNC‐based laser devices.

Investigation of self-filtering unstable resonator for soft x-ray lasers

In this paper, it is proposed that the self-filtering unstable resonator (SFUR) is suitable for soft x-ray lasers which have active medium with a short-gain-lifetime. In order to evaluate the idea, we have analyzed the self-filtering unstable resonator in two dimensions with Ne-like Fe soft x-ray active medium for transitions 2p53p→2p53s and 3d94d→3d94p at 25.5nm. The role of field-limiting aperture in mode propagation has been shown. Moreover, beam quality factor M2 has been calculated and output mode behavior studied and compared with a plane-parallel (PP) resonator of equal length. The calculations indicate that the M2 factor in SFUR resonator is smaller than PP resonator and therefore output beam divergence is lower and the mode quality is much better. It is expected that a high quality beam of soft x-ray laser with brightness of the order 1013W/cm2sr and energy more than 10nJ may be achievable with a properly designed diffraction-filtered unstable resonator.

Double-pass pumped extracavity optical parametric oscillator with passive optical feedback suppression

We demonstrate a simple way of building a compact eye-safe laser source based on a double-pass pumped extracavity optical parametric oscillator (EOPO) without an optical Faraday isolator. A Nd:KGW pump laser (PL) and the KTP EOPO located close to the PL are designed with stable cavities, while the 100% end system mirrors form an unstable cavity with sufficiently high threshold at 1067 nm. At the 6.6 J pump of the Nd:KGW laser, the EOPO output energy obtained at 1577 nm is 14.7 mJ, corresponding to the conversion efficiency of 33% with respect to the fundamental. The pulse width is 7 ns and the output beam divergence is 3 times diffraction limit.

Adjustable hollow-cone output x-ray beam from an ellipsoidal monocapillary with a pinhole and a beam stop.

A combined shading system (CSS) consisting of a beam stop and a pinhole is proposed to be used between an ellipsoidal monocapillary (EM) and a conventional laboratory x-ray source to obtain an adjustable hollow-cone output beam for different experiments with no need for changing the EM. The CSS can change the incident x-ray beam on the EM by adjusting the position of the beam stop and the pinhole, with the corresponding change of the output beam of the EM. In this study, the adjustable hollow-cone output x-ray beam of an 80-mm-long EM with a CSS was studied in detail with a laboratory Cu x-ray generator with a focal spot diameter of 50 μm. The adjustable range of the focal spot size of the EM was from 8.6 to 58.7 μm. The adjustable range of the gain of the focal spot of the EM was from 0 to 1350. The beam divergence of the hollow-cone output beam of the EM ranged from 6 to 16.75 mrad. The illumination angle of the hollow-cone output beam of the EM ranged from 0 to 5.95 mrad. In addition, the potential application of the proposed adjusting method in testing the performance of the EM is briefly discussed.

As_2S_3–silica double-nanospike waveguide for mid-infrared supercontinuum generation

A double-nanospike As2S3-silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2 dB at 1550 nm in the case of free-space in-coupling, which is ∼7 dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.

Improved Monoblock laser brightness using external reflector

The Monoblock laser has become the laser of choice in long-range, eye-safe laser range finders. It is eye-safe with emission at 1570nm, high pulse energy, simple construction, and high efficiency when pumped by a laser-diode stack. Although the output beam divergence of a typical Monoblock with a 3 mm×3 mm cross section is relatively large (10-12mrad), it can be reduced to <1 mrad using a telescope with large magnification. In this paper we present a simple and compact technique for achieving significant reduction in the Monoblock beam divergence using a partial reflector that is placed a short distance from the optical parametric oscillator (OPO). Using a 38mm long Monoblock with a 10mm long potassium titanyl phosphate OPO, we achieved a beam divergence of <4 mrad, corresponding to a >2.5× reduction from the unmodified laser. Performance using this technique with various feedback and etalon spacings is presented.

Low-Loss Hollow Waveguide Fibers for Mid-Infrared Quantum Cascade Laser Sensing Applications

We report on single mode optical transmission of hollow core glass waveguides (HWG) coupled with an external cavity mid-IR quantum cascade lasers (QCLs). The QCL mode results perfectly matched to the hybrid HE11 waveguide mode and the higher losses TE-like modes have efficiently suppressed by the deposited inner dielectric coating. Optical losses down to 0.44 dB/m and output beam divergence of ∼5 mrad were measured. Using a HGW fiber with internal core size of 300 μm we obtained single mode laser transmission at 10.54 μm and successful employed it in a quartz enhanced photoacoustic gas sensor setup.

Harmonic mode-locking in quantum dot lasers with tunnel-coupled waveguides

Passive mode-locking has been studied in the lasers with tunnel-coupled waveguides with the active region based on InAs/InGaAs quantum dots emitting in the range of 1.3–1.31 μm. Specific optical mode formation in this composite waveguide reduces the output beam divergence and raises the temperature stability of the lasing wavelength. The reduced optical confinement factor facilitates obtaining passive modelocking regimes. High response rate of the saturable absorber associated with the carrier escape time shorter than 20 ps allows generating a third harmonic with the frequency as high as 46.1 GHz.

Beam Properties of Visible Proton-Implanted Photonic Crystal VCSELs

We investigate the optical properties of proton-implanted photonic crystal (PhC) vertical-cavity surface-emitting lasers (VCSELs) emitting in the visible spectrum. The fabricated lasers have a threshold current of 1.3 mA and single-mode output power greater than 1 mW at room temperature. The incorporation of a PhC into the top facet of a proton-implanted VCSEL results in a stable single-fundamental-mode operation with a side-mode suppression ratio larger than 30 dB and a constant beam divergence independent of injection current level or ambient temperature. Using an index-step optical fiber model, we compare the effects of different hole etching depths to variations in output beam divergence. By varying the design and etching depth of the hole pattern, the lasers can either be optimized for low beam divergence or low threshold currents. The controllable refractive index guidance effect from the PhC allows for precise engineering of the optical properties of these visible VCSELs for consumer and imaging applications.

Resonant-Phonon Terahertz Quantum-Cascade Lasers and Video-Rate Terahertz Imaging

We review the development of terahertz quantum-cascade lasers (QCLs) that can be uniquely qualified based on a resonant-phonon depopulation scheme. Record performances in terms of operating temperature and optical power output are reported. The best temperature performance is achieved in the metal-metal (MM) waveguides, which provide near-unity mode confinement and low waveguiding loss at terahertz (THz) frequencies even for cavities with subwavelength dimensions. A pulsed operation up to a heat-sink temperature of 169 K at v ~ 2.7 THz and a continuous-wave (CW) operation up to 117 K at v ~ 3 THz are demonstrated with a five-level design that has a two-well injector region. Some of the key temperature degradation mechanisms for this design are discussed. For operation at lower frequencies (v < 2 THz), a one-well injector design is developed that reduces intersubband absorption losses in the injector region. A QCL operating at v = 1.59 THz (lambda = 188.5 mum) up to a heat-sink temperature of 71 K in cw mode is demonstrated with that design. To obtain high-power output and low beam divergence from the MM waveguides, a lens-coupled scheme is demonstrated. A peak power output of 145 mW, a beam width of 4.8deg, and a maximum lasing temperature of 160 K are obtained from a 4.1 THz QCL in this configuration. In the latter part of the paper, we report on the demonstration of video-rate (20 frames/s) terahertz imaging with QCLs as the source for illumination and a 320 times 240 element room-temperature microbolometer focal plane array as the detector. The QCLs for the imaging system are processed into semiinsulating surface-plasmon waveguides, and are operated in a cryogen-free thermomechanical cooler in quasi-CW mode at a heat-sink temperature of ~30 K. Real-time imaging in transmission mode is demonstrated at a standoff distance of 25 m with a v ~ 4.9 THz QCL in this setup.

Electroabsorption-Modulated DFB Laser Integrated With Dual-Core Spot-Size Converters

A 1.55-mum single shallow ridge electroabsorption- modulated distributed feedback laser that is monolithically integrated with a buried-ridge-stripe dual-core spot-size converter (SSC) at the input and output ports was fabricated by combining selective area growth, quantum-well intermixing, and dual-core integration techniques simultaneously. These devices exhibit a threshold current of 34 mA, a side mode suppression ratio of 38.0 dB, a 3-dB modulation bandwidth of 11.0 GHz, and a modulator extinction ratio of 25.0 dB dc. The output beam divergence angles of the SSC in the horizontal and vertical directions are as small as 7.3deg times 18deg, respectively, resulting in 3.2-dB coupling loss with a cleaved single-mode optical fiber.