Directed Energy Deposition (DED) is a metal additive manufacturing process capable of building and repairing large, complex metal parts from a wide range of alloys. Its flexibility makes it attractive for multiple industrial applications, e.g., in aerospace, automotive and biomedical fields. However, errors or defects introduced at any stage of the printing process can, if undetected, significantly impact the final result, rendering the printed part unusable. Potential in-situ correction methods of printing defects require fast and high-resolution on-the-fly 3D inspection inside the machine, but existing 3D monitoring methods often lack full 360° 3D coverage, require bulky setups, or are too slow for real-time layer-wise feedback. In this paper, we present a single-shot, multi-view polarized fringe projection profilometry (FPP) system designed for real-time in-situ 3D inspection during DED printing. Multiple camera-projector pairs are arranged around the deposition surface to measure depth from different viewpoints in single-shot, while cross-polarized image filtering suppresses specular reflections caused by varying surface reflectance across different alloys. The final 360° reconstruction is obtained via joint registration of the captured multi-view measurements. Our prototype has been deployed in a DED system and our first experiments demonstrate a depth precision better than δz < 60 μm on partially reflective and "shiny" metal surfaces, enabling accurate, layer-wise monitoring for closed-loop DED control.

We present a nanosecond-pulsed 655 nm laser source based on frequency-doubling a Raman-shifted fiber laser. At a repetition rate of 1.5 MHz, the source generates an average power of 3.3 W, corresponding to a pulse energy of 2.2 mu J, with a pulse duration of 1.8 ns. The fundamental Raman-shifted fiber laser operating at 1310 nm has a novel configuration where the first Raman shift is performed in an Yb-doped fiber amplifier and the second Raman shift is performed in a phosphosilicate fiber. Both Raman shifting stages are seeded with narrow linewidth CW signals, enabling the temporal properties of an amplified 1064 nm modulated laser diode to be transferred to narrow-band light at 1310 nm with very high conversion efficiency. The resulting micro-Joule-level, nanosecond pulses at 1310 nm are frequency-doubled to 655 nm in a double LBO crystal setup with a conversion efficiency of 51%. The multi-Watt, micro-Joule-level red pulses have near diffraction limited beam quality (M-2 <= 1.04), making this source ideally suited to biomedical imaging applications such as super-resolution and photoacoustic microscopy.

Confined by the Mid-Atlantic Ridge and the European continental shelf, the deep-sea acorn barnacle Bathylasma hirsutum (Hoek, 1883) lives in the northeast Atlantic deep sea, where it has been frequently reported in high current areas. Cemented to a solid substrate during its entire adult life, the species can only disperse by means of planktotrophic nauplius larvae. This study reports on the occurrence, ecology and genetic connectivity of B. hirsutum from four sites within the northeastern Iceland Basin and presents the first record of the species living affiliated with hydrothermal vent field on the Reykjanes Ridge axis. Vent-associated specimens were found to differ extrinsically from their naturally shaded conspecifics by a prominent brown-black shell precipitate. Energy Dispersive Spectroscopy revealed ferromanganese oxides to be the main component of these shell precipitates. Morphometric measurements of shell plates revealed specimens from the vent-associated habitat to be smaller compared to non-venting sites. Molecular species delimitation based on the mitochondrial COI and nuclear EF1 genetic markers aided species identification and revealed a low intraspecific genetic variability. Our findings suggest a pronounced genetic connectivity of B. hirsutum within the studied region and provide a first step towards a biogeographic study. As such, habitats of hydrothermal influence along the Mid-Atlantic Ridge are discussed as possible niches, as are deep-sea basins in the western Atlantic. In light of the reported affiliation with hydrothermal activity, we elaborate on the potential for the sister species Bathylasma corolliforme (Hoek, 1883) and Bathylasma chilenseAraya Araya & Newman, 2018 to utilise equivalent habitats in the Antarctic and Pacific Ocean, respectively. Our record of the unacquainted ecological niche occupation for B. hirsutum emphasises the need for further research on bathylasmatid acorn barnacles along the extensive Mid-Atlantic Ridge, where many biological communities remain to be discovered.

We report a single-cell level resolution (≤10 µm), laser desorption-based mass spectrometry imaging platform. An optical parametric amplifier is used to generate ∼100 ps, 200 nJ pulses at around 3 µm with a maximum repetition rate of 500 kHz. The pulses are tightly focussed on to fresh frozen animal tissue samples with a thickness of 10 µm. Small volumes of tissue are readily ablated by the laser and are subsequently chemically analyzed using a Rapid Evaporative Ionization Mass Spectrometry (REIMS) source installed on a time of flight mass analyzer. Raster scanning the samples through the laser focus enables the acquisition of mass spectrometry data which can be processed into images with pixel size 10 µm without oversampling, corresponding to cellular level resolution.

A real number is a rule that, when provided with a rational interval, answers Yes or No depending on if the real number ought to be considered to be in the given interval. Since the goal is to define the real numbers, this can only motivate the definition of which rules should be considered a real number. The rule must satisfy five properties and any rule that does so we call an oracle. Three of the properties ensure that we do not have multiple oracles representing the same real number. The other two properties ensure that the oracle does narrow down to a single real number. The most important property is the Separating property which ensures that if we divide a Yes interval into two parts, then one part is a Yes interval while the other is a No interval; the exception is if the division point, which is a rational number, happens to be the desired real number in which case both intervals are Yes intervals. We explore various examples and algorithms in using oracles in addition to establishing that the oracles do, in fact, form the field of real numbers. The concept of a Family of Overlapping, Notionally Shrinking Intervals is defined and found to be an essential tool in working with oracle arithmetic. Mediant approximations, which are related to continued fraction representations, naturally arise from an oracle perspective. We also compare and contrast with other common definitions of real numbers, such as Cauchy sequences and Dedekind cuts, in which the conclusion is that the oracle perspective is somewhat of a master map to the other definitions. We do an explicit example to contrast oracle arithmetic with decimal arithmetic and continued fraction arithmetic.

The early work of the Femtosecond Optics Group at Imperial College on optical soliton generation in fibres is reviewed that commenced with studies of ultrashort pulse generation using high order soliton compression for the production of pulses of 18 fs. Also described are seminal studies of soliton Raman generation using long pulse pumping where evolution from modulational instability led to continuum generation and tunable femtosecond generation. Early investigations of seeding modulational instability for enhanced soliton generation and spectral extension are described, as is the work on seeding of noise, soliton generation from noise bursts and soliton generation from amplified noise bursts. The first study of femtosecond soliton amplification in Er fibre amplifiers is described as well as synchronous Raman amplification of solitons. Various schemes reviewing our investigations of adiabatic soliton compression and amplification are included for the generation of pulsewidth and wavelength tunable soliton sources in fully fibre integrated geometries.

We demonstrate a new, to the best of our knowledge, method of generating mid-infrared pulses by difference frequency mixing the Stokes pulse generated by four-wave mixing in a photonic crystal fiber with the remaining pump pulse. The Stokes pulses generated by four-wave mixing are inherently overlapped temporally and spatially with the pump pulse at the output of the fiber. Focusing this output into a nonlinear crystal phase matched for difference frequency generation between the pump and Stokes pulses results in a simple method of generating mid-infrared pulses. With a pump source at 1.064 µm, and a photonic crystal fiber engineered to generate Stokes pulses at approximately 1.65 µm, we generate 160 mW of mid-infrared light at approximately 3 µm through difference frequency generation.

We review our development of wavelength tuneable, high average power, picosecond-pulse, mid-infrared sources in the three micron widow for proposed application in tissue ablation studies and with the objective of source simplification and exploring the potential of all-fiber integration. Initial systems were based on difference frequency generation (DFG) of two synchronous master oscillator power fiber amplifier (MOPFA) schemes. The generated idler was tuneable over the range 3.28–3.45 μ m, delivering greater than 3 W of average power, with a maximum pump to total DFG power conversion efficiency of 78 % . By simplifying the seed sources through synchronised in-line modulation of cw diode laser sources, more than 6 W was generated 3.31–3.48 μ m with similar efficiency and with near diffraction limited beam quality ( M^2=1.4 ). In an improved and significantly simplified experimental configuration a source emitting around 3 μ m was developed employing a novel χ^(3)/χ^(2) cascaded nonlinear conversion architecture. Picosecond pulses from a 1.064 μ m mode-locked Yb:fiber pump laser were used to generate 1.65 μ m signal pulses through χ^(3) based four-wave mixing in a polarisation preserving photonic crystal fiber (PCF). The output of the PCF was then directly focused into a periodically poled lithium niobate crystal, generating idler radiation around 3 μ m, with peak powers of ∼ 0.5 kW, via χ^(2) -based three-wave mixing between the pump and signal pulses.

A new definition of a real number is that it is a rule which says Yes or No based on whether the real number ought to be in a given rational interval. This is a teaser paper for formalizing, exploring, and generalizing this definition. The full exploration is given in the paper "Defining Real Numbers as Oracles".

Event-based structured light systems have recently been introduced as an exciting alternative to conventional frame-based triangulation systems for the 3D measurements of diffuse surfaces. Important benefits include the fast capture speed and the high dynamic range provided by the event camera - albeit at the cost of lower data quality. So far, both low-accuracy event-based as well as high-accuracy frame-based 3D imaging systems are tailored to a specific surface type, such as diffuse or specular, and can not be used for a broader class of object surfaces ("mixed reflectance scenes"). In this paper, we present a novel event-based structured light system that enables fast 3D imaging of mixed reflectance scenes with high accuracy. On the captured events, we use epipolar constraints that intrinsically enable decomposing the measured reflections into diffuse, two-bounce specular, and other multi-bounce reflections. The diffuse objects in the scene are reconstructed using triangulation. Eventually, the reconstructed diffuse scene parts are used as a "display" to evaluate the specular scene parts via deflectometry. This novel procedure allows us to use the entire scene as a virtual screen, using only a scanning laser and an event camera. The resulting system achieves fast and motion-robust (14Hz) reconstructions of mixed reflectance scenes with < 500 $\mu$m accuracy. Moreover, we introduce a "superfast" capture mode (250Hz) for the 3D measurement of diffuse scenes.

This publisher’s note contains corrections to Opt. Lett. 47, 5236 (2022)10.1364/OL.472780.

Mid infra-red generation in compact configurations providing diverse pulsewidth and linewidth formats with average powers at the watts level offers a wide range of applications opportunities ranging from remote sensing and polymer machining to medical diagnostics and imaging.Although very significant progress has been made with fluoride-based fibre lasers, large gaps exist in the achieved spectral coverage, while although successful supercontinuum has been reported throughout the near and mid infra-red, operational power levels at any given wavelength and pulse formats are somewhat restricted.There are a vast number of laser-based solutions for achieving mid infra-red generation [1], improved versatility can, however, be achieved using conventional master oscillator power fibre amplifier configurations in the infra-red to pump both parametric amplification and difference frequency generation in crystals to extend operation throughout the mid infra-red with wavelength tunability.Here we report the performance of three exemplar configurations with an objective of achieving integrated, turn-key operation of these sources and power scalability.For tunable generation in the region of 3 microns, where water and tissue exhibit high absorption, the simplest solution for pulse width and pulse repetition rate versatility is frequency difference generation of Yb and Er fibre MOPFAs in poled oxide crystals [2].In an initial demonstration, the Yb pump was fed from an actively mode locked external fibre cavity semiconductor laser, generating pulses of ~150 ps that were amplified to average powers of ~ 23 W in two stages, with a linewidth of 0.13 nm.The Er-MOPFA based signal was seeded from a tunable external cavity laser diode (1500nm-1580nm) that was modulated by a Mach Zehnder modulator driven by an electrical pulse generator at the common clock signal around 39.945 MHz.This produced pulses of ~400 ps and a linewidth of 0.03 nm at an average power of ~ 2 W.An optical delay line allowed temporal overlap of the pump and signal at the 40 mm long, MgO:PPLN poled crystal with a poling period of 29.98 m, which was mounted in a temperature controlled oven.The maximum focussed pump intensity was 28 MWcm -2 , well below the damage threshold of the crystal.Average idler powers of ~3.5W were recorded, tunable from 3.28 m -3.5 m, limited only by the tuning range of the Er signal, and with long term idler stability better than 0.4%.Shorter idler output pulse durations can be achieved by varying the time delay, hence temporal overlap between the pump and signal, however with an accompanying reduction in conversion efficiency.By optimising overlap and increasing pulse durations average idler powers in excess of 6W with linewidths of ~0.2 nm were achieved.Above 4 microns, oxide crystals exhibit increased absorption losses and the chalcopyrite crystals, such as ZnGeP2 (ZGP) are the preferred hosts for optical parametric generation in the range 4-7 m.One problem with ZGP is that pump wavelengths greater than 1.9 m are required to avoid absorption associated with the small bandgap energy of the material.On the other hand, CdSiP2 (CSP) with a bandgap energy of 2.45 eV permits pumping down to 1 m (where phase matching supports

We report a mid-infrared (MIR) source emitting at 3 μm, employing a novel χ(3)/χ(2) cascaded nonlinear conversion architecture. Picosecond pulses from a 1.064 μm mode-locked Yb:fiber pump laser are used to generate 1.65 μm signal pulses through χ(3) based four-wave mixing in photonic crystal fiber (PCF). The output of the PCF is then directly focused into a periodically poled lithium niobate crystal to generate idler radiation around 3 μm through χ(2) based three-wave mixing between the pump and signal pulses.

During RV MS Merian expedition MSM75, an international, multidisciplinary team explored the Reykjanes Ridge from June to August 2018. The first area of study, Steinahóll (150–350 m depth), was chosen based on previous seismic data indicating hydrothermal activity. The sampling strategy included ship- and AUV-mounted multibeam surveys, Remotely Operated Vehicle (ROV), Epibenthic Sledge (EBS), and van Veen grab (vV) deployments. Upon returning to Steinahóll during the final days of MSM75, hydrothermal vent sites were discovered using the ROV Phoca (Kiel, GEOMAR). Here we describe and name three new, distinct hydrothermal vent site vulnerable marine ecosystems (VMEs); Hafgufa, Stökkull, Lyngbakr. The hydrothermal vent sites consisted of multiple anhydrite chimneys with large quantities of bacterial mats visible. The largest of the three sites (Hafgufa) was mapped, and reconstructed in 3D. In total 23,310 individual biological specimens were sampled comprising 41 higher taxa. Unique fauna located in the hydrothermally venting areas included two putative new species of harpacticoid copepod (Tisbe sp. nov. and Amphiascus sp. nov.), as well as the sponge Lycopodina cupressiformis (Carter, 1874). Capitellidae Grube, 1862 and Dorvilleidae Chamberlin, 1919 families dominated hydrothermally influenced samples for polychaetes. Around the hydrothermally influenced sites we observed a notable lack of megafauna, with only a few species being present. While we observed hydrothermal associations, the overall species composition is very similar to that seen at other shallow water vent sites in the north of Iceland, such as the Mohns Ridge vent fields, particularly with peracarid crustaceans. We therefore conclude the community overall reflects the usual “background” fauna of Iceland rather than consisting of “vent endemic” communities as is observed in deeper vent systems, with a few opportunistic species capable of utilizing this specialist environment.

Open access dataset for figures in 'Optical parametric amplification seeded by four-wave mixing in photonic crystal fibres', accepted for publication in Nonlinear Frequency Generation and Conversion: Materials and Devices XXI, paper 11985-4, SPIE LASE Photonics West, 2022.

We demonstrate a nanosecond-pulsed 743 nm source by second harmonic generation of a cascaded phosphosilicate Raman fiber amplifier operating at 1485 nm. The source emits >1 W of 743 nm average power at a 5 MHz repetition rate.

We demonstrate a nanosecond pulsed 743 nm source through second-harmonic generation of a cascaded phosphosilicate Raman fiber amplifier system operating at 1485 nm. The amplifier is pumped by a 1240 nm phosphosilicate Raman fiber amplifier and seeded with a continuous-wave 1485 nm diode. This 1485 nm light is used for second-harmonic generation in periodically poled lithium niobate. Greater than 1 W of average power is generated at 743 nm with a corresponding pulse energy of 220 nJ at a repetition rate of 5 MHz. The source displays excellent beam quality (M x , y 2 ≤ 1.18) with ideal parameters for biomedical imaging applications.

We report a CdSiP2 (CSP) based seeded optical parametric generator (OPG), emitting sub-nanosecond duration, 3 MHz repetition rate, wavelength tunable mid-infrared (MIR) light at 4.2-4.6 μm. We generate up to 0.25 W at 4.2 μm with a total pump conversion efficiency of 42%. The OPG is pumped by a 1.24 μm Raman fiber amplifier system. This is the first demonstration of pumping CSP with a Raman fiber source in this region, and we show that Raman fiber sources in the near-infrared (NIR) are ideal pump sources for non-critically phasematched (NCPM) CSP devices. Pumping CSP at 1.24 μm permits the use of NCPM whilst decreasing the negative effects of both two-photon absorption and linear absorption losses, when compared to conventional 1 μm pumping. This offers a potential advantage for MIR power scaling of CSP parametric devices due to a reduced thermal load in the crystal from residual pump absorption. The OPG is seeded with a continuous-wave fiber supercontinuum source emitting radiation in the 1.7 μm region, to lower the threshold pump intensity required for efficient conversion. NCPM and temperature tuning of the crystal allow for simple wavelength tuning of the idler radiation. We report on laser damage induced by elevated crystal temperatures, which we propose is linked to the decrease in CSP bandgap energy with increasing temperature.

A new method of seeding X2 optical parametric converters with X3 fiber optical parametric sources is introduced. We demonstrate a tunable mid-infrared source operating at around 3 µm with the technique and discuss the potential of this architecture.

CdSiP2 (CSP) is a nonlinear optical semiconductor which can phasematch pump wavelengths throughout the near-infrared (NIR) to generate mid-infrared (MIR) light through parametric three-wave mixing. In this work, we investigate the unique combination of NIR Raman fiber amplifiers around 1.24 µm and non-critical phasematching in CSP, to demonstrate tunable sources in the 4-5 µm MIR region.