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This study investigated the perforation time of polyamide 6.6 using fiber lasers at two different wavelengths: 1070 and 1943 nm. The novelty of this research lies in the comparison of perforation times at equivalent laser irradiances on the polymer sample with two different colors of polyamide 6.6: natural and black. The results revealed that, at comparable irradiance levels and beam diameters, the 1943 nm laser source perforated the polyamide 6.6 sample faster than the 1070 nm laser source. The difference in perforation time was found to be significantly higher for natural-colored polyamide 6.6 compared to black-colored polyamide 6.6. These findings suggest that, for material processing of polyamide 6.6, especially in terms of perforation, the use of 2 μm laser sources should be privileged over 1 μm laser sources. [ABSTRACT FROM AUTHOR]

We present investigations on output power limitations of near-diffraction-limited compact Ho 3 + :YAG laser resonators employing a homogeneous or segmented laser crystal. An approach for designing a segmented crystal is presented. Maximum output powers of 57.6 W and 51.9 W are reached with the homogeneous and segmented crystal, respectively, resulting in pulse energies of 1.14 mJ and 1.04 mJ at a repetition rate of 50 kHz in Q-switched operation. Interferometric experiments are conducted to derive the radial temperature profile for both crystals. Simulations based on a split-step beam propagation method are used to model the longitudinal temperature gradient in both crystals. [ABSTRACT FROM AUTHOR]

In most applications of laser technology and optics the beam quality, the ability to focus a laser beam and the achievement of a good optical resolution play an important role. The compensation of distortions using adaptive optics requires fast wavefront measurement. Classical wavefront analysis schemes use matrix operations, which show a nonlinear computation time dependence with matrix size, making it difficult to achieve high-speed control loops at a high resolution. A novel wavefront sensor system is presented using a massively parallel k-nearest neighbor classifier chip in an embedded hardware setup. Our miniaturized sensor is able to detect one optical distortion within about 80 μs allowing its use for high-speed adaptive optics applications. [ABSTRACT FROM AUTHOR]

Abstract: Er3+ doped Y3Al5O12 (Er:YAG) single crystal fibers, with a diameter of 800μm and a length up to 80cm, were grown by the micro-pulling-down technique. The structural and optical characterization of these fibers shows a perfect single crystalline structure and a very high optical quality suitable for laser operation. With 6cm long samples, we demonstrated a continuous wave laser delivering an output power of 950mW for a pump power of 36W coming from fiber-coupled laser diodes emitting at 1532nm. To our knowledge, this is the first laser ever achieved with directly grown Er:YAG single crystal fibers. [Copyright &y& Elsevier]

The present work aims to establish a correlation between the characteristics of YAG and Er:YAG commercial powders produced by two different synthesis routes and sintered ceramic microstructures and their optical aspect by taking into account the influence of pressure applied during the Spark Plasma Sintering (SPS) process. Physical and chemical characteristics of the powders were compared using various techniques such as SEM, XRD, laser diffraction and chemical analyses. Their behaviours were evaluated through a rheological study, compressibility tests and dilatometry cycles using SPS. This paper pinpoints the most important powder features which influence the optical quality of YAG and Er:YAG ceramics. The optical quality is mainly affected by the porosity, related to powder characteristics that affect particle rearrangement, densification and grain growth. The applied pressure induces microstructural heterogeneities depending on the starting material used and resulting in core-shell aspects of sintered ceramics. [ABSTRACT FROM AUTHOR]

We report on the wavelength dependence of the orientation of the optic axes in monoclinic crystals using undoped and neodymium-doped KGd(WO) (KGW) samples. Measurements have been performed over a wide wavelength range from 420 up to 1,580 nm with more than 70 measurement points. This provides the widest measurement range of the optic axis variation ever reported in literature for KGW crystals. With such a measurement, which can be performed in principle in all biaxial crystals, the refractive indices values can be checked with a precision down to 10. [ABSTRACT FROM AUTHOR]

Abstract: YAG nanopowders with an average grain size of 30nm have been successfully synthesized by the co-precipitation method using nitrates with precipitant of ammonium hydrogen carbonate. The influence of precipitation conditions such as pH, aging time and calcination temperature on the formation of secondary phases has been studied. The accurate control of pH value at every stage of precipitation process is crucial to avoid the presence of YAM (Yttrium Aluminium Monoclinic, Y4Al2O9) and yttrium oxide (Y2O3) after calcination. The reaction mechanisms have been investigated using different techniques such as infrared spectroscopy, x-ray diffraction and thermal analyses. The YAG phase is formed around 1050°C passing through an intermediate phase called YAP (Yttrium Aluminium Perovskite, YAlO3). Local chemical heterogeneities are responsible for the deviation of the Y:Al ratio and the formation of YAP during heat treatment. [Copyright &y& Elsevier]

We investigate the temperature dependence of an in-band core-pumped thulium-doped fiber laser with a low SWaP (size, weight, and power) architecture. The temperature investigation is carried out both experimentally and numerically by a simulation model. We demonstrate experimentally that the investigated setup is resistant for temperatures till 353 K. In addition, we explain the observed behavior by considering the temperature depended spectroscopic parameters of thulium-doped silica fibers. Finally, a numerical investigation is carried out for higher temperatures up to 573 K and higher output powers up to 12 W as well as for different wavelengths and show that the considered fiber lasers works still efficient at these temperature ranges. We show the reliability of the considered thulium-doped fiber laser architecture for applications in harsh environment.

We present an efficient Q-switched dual-crystal Ho 3+ :YAG laser resonator achieving an output power of 56 W at an incident pump power of 100.7 W with a slope efficiency of 64.4%. The setup was pump power limited. Notably, by carefully tuning multiple etalons placed within the cavity, we achieved a single-line spectral emission at 2122 nm. This deviates from the typical emission wavelengths of 2090 nm or 2096 nm for Ho 3+ :YAG at comparable output powers in Q-switched operation. The laser exhibited very good beam quality, approaching diffraction-limited performance with an excellent M 2 - < 1.2. At the maximum output power, a pulse FWHM of 100 ns was measured.

We present current results of a novel, to the best of our knowledge, type of setup for quantum ghost imaging based on asynchronous single photon timing using single photon avalanche diode (SPAD) detectors, first presented in [Appl. Opt.60, F66 (2021)APOPAI0003-693510.1364/AO.423634]. The scheme enables photon pairing without fixed delays and, thus, overcomes some limitations of the widely used heralded setups for quantum ghost imaging [Nat. Commun.6, 5913 (2015)NCAOBW2041-172310.1038/ncomms6913]. It especially allows three-dimensional (3D) imaging by direct time of flight methods, the first demonstration of which will be shown here. To our knowledge, it is also the first demonstration of 3D quantum ghost imaging at all.

A continuous-wave crossed-Porro prism Ho 3+ :YAG laser is presented and compared with a corresponding mirror resonator. A maximum output power of 30.7 W is reached with a slope efficiency of 67.4 % with respect to the absorbed pump power. The laser output beam shows a very good beam quality of better than M 2  < 1.2 which clearly surpasses that of the mirror resonator. In terms of alignment sensitivity, the crossed-Porro prism resonator is superior to the mirror resonator due to the retro-reflective nature of the prisms in the axis around the apex.

A Tm 3+ :Ho 3+ -codoped free-space single-oscillator fiber laser is under investigation with special focus on the power scalability of emission wavelengths from 2.1 µm to 2.2 µm. Using a tunable diffraction grating, a 200-nm tunable laser source is built. Laser output powers above 10 W are delivered from 1990 nm up to 2190 nm, demonstrating the range for stable high-power laser operation. By replacing the diffraction grating by a highly reflective, narrow linewidth volume Bragg grating, power scaling is performed at 2.1 µm and is even enabled at a wavelength of 2.2 µm. Using a volume Bragg grating (VBG) at 2.1 µm, a slope efficiency of 49% is measured with an output power of 262 W. Using another VBG with a center wavelength of 2.2 µm, the fiber laser delivers a record power of 77 W with a slope efficiency of 29%.

We present first results of a novel type of setup for quantum ghost imaging based on asynchronous single photon timing using single photon avalanche diode (SPAD) detectors. This scheme enables photon pairing with arbitrary path length difference and does, therefore, obviate the dependence on optical delay lines of current quantum ghost imaging setups [Nat. Commun.6, 5913 (2015)NCAOBW2041-172310.1038/ncomms6913]. It is also, to our knowledge, the first quantum ghost imaging setup to allow three-dimensional imaging.

This focus issue offers a glimpse into the breadth and depth of research in optics and image processing at the Fraunhofer Institute of Optronics, System Technologies and Image Exploitation (abbreviated Fraunhofer IOSB). The institute belongs to the Fraunhofer Society, which is Europe's largest organization for applied research. The society comprises 72 institutes spread throughout Germany as well as subsidiaries in several European countries, and the USA, UK, Chile and Singapore, each focusing on different fields of applied science. It is named after the Bavarian physicist Joseph von Fraunhofer (1787-1826), the inventor of the spectrometer, optical lens manufacturer and an entrepreneur, who exemplified the goals of the society, i.e.: a focus on research excellence, innovation, and commitment to market and customer-oriented research.

A high-pulse-energy mid-infrared light source is presented, based on a zinc-germanium-phosphide optical parametric oscillator (ZGP OPO) pumped by an actively $Q$-switched high-pulse-energy ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ laser. The ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ pump laser source is capable of generating a pulse energy of 15 mJ from a single ${{\rm Ho}^{3 +}}{:}{\rm YAG}$ rod at room temperature at a pulse repetition frequency (PRF) of 700 Hz. A maximum power of 20.1 W at a central wavelength of 2090 nm can be obtained in continuous operation, with a slope efficiency of 45.1%. A good beam quality with an ${{\rm M}^2}$ better than 1.3 was achieved in $Q$-switched operation. The presented laser architecture was used as a suitable pump source for a ZGP-based OPO. Operated at a PRF of 2 kHz and pumped with a pulse energy of 8 mJ, a low conversion threshold of 1.5 W and a maximum total output power of 6.3 W could be obtained in a linear ZGP-based OPO. At maximum power, the peak power of the generated mid-infrared radiation exceeded 120 kW, while the beam quality was affected by the strong gain lens building inside the nonlinear material as a consequence of the high-energy pump pulses.

A single-frequency polarization-maintaining holmium-doped fiber master oscillator power amplifier operating at signal wavelengths of $2054\;{\rm nm}$ and $2090\;{\rm nm}$ is presented. The two-stage setup delivers up to $240\;{\rm W}$ peak power and $6.7\;\unicode{x00B5} {\rm J}$ pulse energy for a pulse width of $30.2\;{\rm ns}$ at a repetition rate of $100\;{\rm kHz}$. The first amplifier stage is designed by simulation, tailored for high gain at the signal wavelength range, favoring amplification at $2090\;{\rm nm}$. The design is discussed, and the measured values are compared with the simulation. The second stage is investigated regarding the efficiency for co- and counter-pumping. Stimulated Brillouin scattering was found to be the limiting factor for pulse peak power scaling in the second stage. The measured output pulse shapes are discussed and compared to pulse shapes derived with the Frantz-Nodvik model.

A mid-infrared ${{\rm ZnGeP}&#95;2}$ optical parametric oscillator pumped by a ${{\rm Tm}^{3 +}}$-doped fiber laser is reported, providing pulse energies of $230 \;\unicode{x00B5}{\rm J}$, pulse widths of 40 ns, and peak powers of ${\sim}6\;{\rm kW} $ with excellent efficiency and beam quality. The pump source is an actively $Q$-switched single oscillator optimized to generate high pulse energies.

Adaptive optics systems are used to compensate for distortions of the wavefront of light induced by turbulence in the atmosphere. Shack-Hartmann wavefront sensors are used to measure this wavefront distortion before correction. However, in turbulence conditions where strong scintillation (intensity fluctuation) is present, these sensors show considerably worse performance. This is partly because the lenslet arrays of the sensor are designed without regard to scintillation and are not adaptable to changes in turbulence strength. Therefore, we have developed an adaptable Shack-Hartmann wavefront sensor that can flexibly exchange its lenslet array by relying on diffractive lenses displayed on a spatial light modulator instead of utilizing a physical microlens array. This paper presents the principle of the sensor, the design of a deterministic turbulence simulation test-bed, and an analysis how different lenslet arrays perform in scintillation conditions. Our experiments with different turbulence conditions showed that it is advantageous to increase the lenslet size when scintillation is present. The residual phase variance for an array with 24 lenslets was up to 71% lower than for a 112 lenslet array. This shows that the measurement error of focal spots has a strong influence on the performance of a Shack-Hartmann wavefront sensor and that in many cases it makes sense to increase the lenslet size. With our adaptable wavefront sensor such changes in lenslet configurations can be done very quickly and flexibly.

A bidirectional 793 nm diode-pumped actively Q-switched Tm 3+ , Ho 3+ -codoped silica polarization-maintaining (PM) double-clad (DC) fiber laser is reported. With this fiber laser, 55 W of average output power with 100 ns pulse width at 200 kHz repetition rate and 2.09 µm wavelength is obtained. The pump power injection with end-caps fusion-spliced on fiber tips provides good power stability (< 1.1%) and beam quality factors (M 2 < 1.7). The fiber laser output beam polarization factor is 97.5%. At 55 W, no thermal-induced damage is observed on any optical element, and power scaling of the laser is only pump-power-limited in the range of the total available pump power (180 W).

Divergence compensation, optimization of the optical-to-optical efficiency, and high beam quality of signal and idler beams of a high-energy mid-infrared ZnGeP 2 (ZGP) optical parametric oscillator (OPO) have been demonstrated by use of a Galilean telescope inside the nonplanar fractional-image-rotation enhancement (FIRE) ring resonator. With a small variation of the distance between the lenses of the telescope, the divergences of signal and idler beams could be adjusted. Up to 36 mJ of mid-infrared pulse energy in the 3-5 µm wavelength range is obtained with 92 mJ of pump energy on crystal. The beam quality factors M 2 are < 1.5 for the resonant signal beam and the non-resonant idler beam, respectively. Actually, this is an improvement of the beam quality by a factor 3 for the signal and ~2.7 for the idler beam compared without using a telescope inside the FIRE ring resonator.

The beam quality of a high-energy mid-infrared ZnGeP 2 (ZGP) optical parametric oscillator has been improved by use of a negative lens inside the nonplanar fractional-image-rotation enhancement (FIRE) resonator compensating for the thermal lensing and gain guiding effects. With an f=-0.9  m CaF 2 lens, the beam quality is 1.5 and ∼2 for the resonant signal beam and the non-resonant idler beam, respectively. Up to 33 mJ of mid-infrared pulse energy in the 3-5 μm wavelength range is obtained with 94 mJ of pump energy on the crystal. This is an improvement of the beam quality by a factor of 3 for the signal beam and ∼2 for the idler beam compared without using a lens inside the FIRE resonator.

Optical parametric generation is demonstrated in orientation-patterned gallium arsenide, pumped by a novel single-oscillator simultaneously Q-switched and mode-locked thulium-doped fiber laser, downconverting the pump radiation into the mid-infrared wavelength regime. The maximum output energy reached is greater than 2.0 μJ per pump pulse.

A single-oscillator actively Q-switched mode-locked (QML) thulium-doped silica fiber laser is presented and used to pump a ZrF 4 -BaF 2 -LaF 3 -AlF 3 -NaF (ZBLAN) fiber for mid-infrared (mid-IR) supercontinuum (SC) generation. The fiber laser provided high-peak-power levels directly from the oscillator delivering single mode-locked pulse energies up to 48 μJ, being 2-4 orders of magnitude higher than conventional continuous wave mode-locked lasers. By pumping a ZBLAN fiber specially designed for high-output-power SC generation, 7.8 W have been achieved in all spectral bands with a spectrum extending to 4.2 μm.

A high-energy mid-infrared ZnGeP 2 (ZGP) optical parametric oscillator (OPO) based on the nonplanar fractional-image-rotation enhancement resonator pumped by a 2.05 μm Ho 3+ :YLF laser is presented. Up to 120 mJ pulse energy in a rotationally symmetric beam is generated in the 3-5 μm wavelength range at 1 Hz repetition rate. Slope efficiencies of up to 78% are achieved with respect to the pump pulse energy incident onto the ZGP crystal. The OPO pulses have a duration close to 15 ns, corresponding to a maximum peak power of 8 MW. A measurement of M 2 dependence on pump beam diameter is presented.

Background: Hybrid tumours of the salivary glands are rare neoplasms. They are composed of at least two different tumour entities located in the same topographic area and account for only 0.1% of all salivary gland tumours. The most common component is an adenoid cystic carcinoma. There are several possible forms of hybrid tumours, which are most commonly located in the parotid gland., Case Report: We report on a 59-year-old female, who presented with a lesion of the caruncula of the left sublingual gland. The biopsy showed an adenoid cystic carcinoma in combination with a salivary duct carcinoma. Treatment consisted of tumour resection, bilateral selective neck dissection and adjuvant radiotherapy. Histopathologically, at least 30% of the tumour mass was composed of a salivary duct carcinoma and 70% of an adenoid cystic carcinoma. At 58 months after treatment, the patient is alive without evidence of recurrent disease., Conclusion: To our knowledge, the presented case is the first description of a hybrid tumour of the sublingual gland. Furthermore, the post-therapeutic course is encouraging, as hybrid tumours of the salivary glands usually have a poor prognosis.

First results on a diode-pumped multikilowatt-class Er(3+):YAG solid-state heat-capacity laser (SSHCL) are reported. The laser achieves an output power of 4650 W and output energies in excess of 440 J. A moderate crystal temperature increase due to crystal heating of 56.7 K/s is measured at 11.3 kW of pump power and the temperature-related power drop is determined to 8.8 W/K. The presented work is believed to be the first multi-kilowatt-class resonantly diode-pumped Er(3+):YAG laser.

The aim of this study was to investigate the contact angles of six different type 3 dental impression materials (DIMs). Contact angles (n = 3) were continuously determined for 20 seconds after placing 4 MuL of deionized water onto a thin layer (50 Mum) of freshly mixed DIM (polyvinyl siloxane, C-silicone, polyether, and hybrid DIMs). Data were subjected to parametric statistics (P < .05). Contact angles ranged from 76.1 +/- 5.6 degrees to 119.5 +/- 1.8 degrees at 0 seconds and from 11.7 +/- 1.6 degrees to 74.7 +/- 1.1 degrees at 5 seconds after droplet deposition. The siloxanes showed significantly higher contact angles at 0 and 5 seconds compared to all other DIMs tested (P < .05). The newly introduced DIMs with a polyether backbone showed promising results regarding their hydrophilicity.

The transient wavelength performance of high-power InP laser diode stacks with internal Bragg gratings is studied experimentally. Because of the wavelength selective nature of the internal grating, which is decoupled from the actual gain profile dynamics of the laser diode, the wavelength drift with respect to temperature and current is greatly reduced and steady-state conditions are reached after a short settling time of less than 50 ms.

A diode-pumped Q-switched Tm(3+),Ho(3+)-codoped double-clad silica fiber laser is reported providing average powers of up to 12.3 W at 2072.7 nm and stable pulse widths of only 45 ns and repetition rates in the range 10-100 kHz. Instabilities in pulse shape occur at nonoptimum cavity alignment and were found to be linked to the mechanism responsible for self mode locking.

A diode-pumped Q-switched Tm(3+)-doped double-clad silica fiber laser is reported providing average powers of up to 30 W at pulse widths of only 41 ns and repetition rates in the range of 10-125 kHz. Up to 270 microJ pulse energy was produced. Amplified spontaneous emission (ASE) buildup limits the maximum peak power, and the pump power and average output power at the point of ASE induced clamping were found to depend linearly on the repetition rate.