Two dimensional gradient-index beam shapers fabricated using ultrafast laser inscription.

In this paper gradient-index beam shapers are fabricated using the ultrafast laser inscription method. This method enables the fabrication of two-dimensional refractive index profiles inside silica glass, resulting in highly robust and compact beam shapers. The magnitude of this refractive index change can be tailored by adjusting the laser pulse energy, enabling arbitrary two-dimensional refractive index profiles to be manufactured. The process is then demonstrated by fabricating planar waveguides with quadratic index profiles that predictably resize Gaussian beams. Then a more complex two-dimensional refractive index profile is fabricated to transform an input Gaussian beam into a super-Gaussian (flat-top) beam.

Quantitative morphology of femtosecond laser-written point-by-point optical fiber Bragg gratings.

We investigate the morphology of femtosecond laser, single pulse-inscribed, point-by-point (PbP) fiber Bragg gratings. Direct measurement of a PbP grating's refractive index profile was carried out with micro-reflectivity analysis. PbP gratings were imaged at sub-micrometer scale with scanning electron microscopy, Raman and photoluminescence studies were performed to probe the structural and electronic changes. Comparison of results from different characterisation techniques suggests that the creation of an increased refractive index region around the micro-void is due to contributions from both densification and the formation of highly polarizable non-bridging oxygen bonds.

Building hybridized 28-baseline pupil-remapping photonic interferometers for future high-resolution imaging.

One key advantage of single-mode photonic technologies for interferometric use is their ability to easily scale to an ever-increasing number of inputs without a major increase in the overall device size, compared to traditional bulk optics. This is particularly important for the upcoming extremely large telescope (ELT) generation of telescopes currently under construction. We demonstrate the fabrication and characterization of a hybridized photonic interferometer, with eight simultaneous inputs, forming 28 baselines, which is the largest amount to date, to the best of our knowledge. Using different photonic fabrication technologies, we combine a 3D pupil remapper with a planar eight-port ABCD pairwise beam combiner, along with the injection optics necessary for telescope use, into a single integrated monolithic device. We successfully realized a combined device called Dragonfly, which demonstrates a raw instrumental closure-phase stability down to 0.9° over $8\pi$ phase piston error, relating to a detection contrast of ${\sim}6.5 \times {10^{- 4}}$ on an adaptive-optics-corrected 8 m telescope. This prototype successfully demonstrates advanced hybridization and packaging techniques necessary for on-sky use for high-contrast detection at small inner working angles, ideally complementing what can currently be achieved using coronagraphs.

Achromatic photonic tricouplers for application in nulling interferometry.

Integrated-optic components are being increasingly used in astrophysics, mainly where accuracy and precision are paramount. One such emerging technology is nulling interferometry that targets high contrast and high angular resolution. Two of the most critical limitations encountered by nullers are rapid phase fluctuations in the incoming light causing instability in the interference and chromaticity of the directional couplers that prevent a deep broadband interferometric null. We explore the use of a tricoupler designed by ultrafast laser inscription that solves both issues. Simulations of a tricoupler, incorporated into a nuller, result in an order of a magnitude improvement in null depth.

Scalable photonic-based nulling interferometry with the dispersed multi-baseline GLINT instrument.

Characterisation of exoplanets is key to understanding their formation, composition and potential for life. Nulling interferometry, combined with extreme adaptive optics, is among the most promising techniques to advance this goal. We present an integrated-optic nuller whose design is directly scalable to future science-ready interferometric nullers: the Guided-Light Interferometric Nulling Technology, deployed at the Subaru Telescope. It combines four beams and delivers spatial and spectral information. We demonstrate the capability of the instrument, achieving a null depth better than 10-3 with a precision of 10-4 for all baselines, in laboratory conditions with simulated seeing applied. On sky, the instrument delivered angular diameter measurements of stars that were 2.5 times smaller than the diffraction limit of the telescope. These successes pave the way for future design enhancements: scaling to more baselines, improved photonic component and handling low-order atmospheric aberration within the instrument, all of which will contribute to enhance sensitivity and precision.

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability.

Unlike other crystals, the counter intuitive response of bismuth germanate crystals ([Formula: see text], BGO) to form localized high refractive index contrast waveguides upon ultrafast laser irradiation is explained for the first time. While the waveguide formation is a result of a stoichiometric reorganization of germanium and oxygen, the origin of positive index stems from the formation of highly polarisable non-bridging oxygen complexes. Micro-reflectivity measurements revealed a record-high positive refractive index contrast of [Formula: see text]. The currently accepted view that index changes [Formula: see text] could be brought about only by engaging heavy metal elements is strongly challenged by this report. The combination of a nearly perfect step-index profile, record-high refractive index contrast, easily tunable waveguide dimensions, and the intrinsic high optical non-linearity, electro-optic activity and optical transparency up to [Formula: see text] of BGO make these waveguides a highly attractive platform for compact 3D integrated optics.

Channel Waveguides in Lithium Niobate and Lithium Tantalate.

Low-loss photonic waveguides in lithium niobate offer versatile functionality as nonlinear frequency converters, switches, and modulators for integrated optics. Combining the flexibility of laser processing with liquid phase epitaxy we have fabricated and characterized lithium niobate channel waveguides on lithium niobate and lithium tantalate. We used liquid phase epitaxy with K2O flux on laser-machined lithium niobate and lithium tantalate substrates. The laser-driven rapid-prototyping technique can be programmed to give machined features of various sizes, and liquid phase epitaxy produces high quality single-crystal, lithium niobate channels. The surface roughness of the lithium niobate channels on a lithium tantalate substrate was measured to be 90 nm. The lithium niobate channel waveguides exhibit propagation losses of 0.26 ± 0.04 dB/mm at a wavelength of 633 nm. Second harmonic generation at 980 nm was demonstrated using the channel waveguides, indicating that these waveguides retain their nonlinear optical properties.

Characterizing concrete corrosion below sewer tidal levels at chemically dosed locations.

Unexplainable concrete softening below the water line has been observed by Sydney Water in their gravity sewer network, some of which is subjected to corrosion control methods using chemical ferrous chloride (FeCl2) dosing of the wastewater. We applied a combination of physical and chemical tools to determine the properties of the top 20 mm of concrete cores recovered from sewer pipes. These techniques consist of neutron tomographic imaging, scanning electron microscopy, hardness mapping, and pH profiling. Concrete cores were collected from roof (crown), tidal (wall) and below flow regions of gravity sewer pipes of Sydney Water's wastewater system from locations that received no treatment as well as locations dosed with FeCl2. All samples showed a degree of softening of the surface exposed to the sewerage with an associated depletion in calcium concentration and reduced pH in the same regions.

Boson band mapping: revealing ultrafast laser induced structural modifications in chalcogenide glass.

The formation of femtosecond laser direct-written waveguides in gallium lanthanum sulfide (GLS) chalcogenide glass with a peak index contrast of Δnmax=0.023 and an average positive refractive index change of Δnwaveguide=0.0049 is explained for the first time, to the best of our knowledge. Evidence of structural change and ion migration is presented using Raman spectroscopy and electron probe microanalysis (EPMA), respectively. Raman microscopy reveals a frequency shift and a change in full-width at half maximum variation of the symmetric vibration of the GaS4 tetrahedra. The boson band is successfully used to identify and understand the material densification profile in a high refractive index glass waveguide. EPMA provides evidence of ion migration due to sulfur, where the observation of an anion (S2-) migration causing material modification is reported for the first time. These results will enable optimization of future mid-infrared and nonlinear integrated optical devices in GLS glass based on femtosecond laser written waveguides.

Revisiting ultrafast laser inscribed waveguide formation in commercial alkali-free borosilicate glasses.

Alkali-free borosilicate glasses are one of the most used dielectric platforms for ultrafast laser inscribed integrated photonics. Femtosecond laser written waveguides in commercial Corning Eagle 2000, Corning Eagle XG and Schott AF32 glasses were analyzed. They were studied in depth to disclose the dynamics of waveguide formation. We believe that the findings presented in this paper will help bridge one of the major and important gaps in understanding the ultrafast light-matter interaction with alkali-free boroaluminosilicate glass. It was found that the waveguides are formed mainly due to structural and elemental reorganization upon laser inscription. Aluminum along with alkaline earth metals were found to be responsible for the densification and silicon being the exchanging element to form a rarefied zone. Strong affinity towards alkaline earth elements to form the densified zone for waveguides written with high feed rate (>200 mm/min) were identified and explained. Finally we propose a plausible solution to form positive refractive index change waveguides in different glasses based on current and previous reports.

Towards a photonic mid-infrared nulling interferometer in chalcogenide glass.

Nulling interferometry enables astronomers to advance beyond the resolving power of ground-based telescopes with the goal of directly detecting exo-planets. By diminishing the overwhelming emission of the host star through destructive interference, radiation from young companions can be observed. The atmospheric transmission window centered around 4 µm wavelength is of particular interest because it has a favorable contrast between star and planet as well as a reduced atmospheric disturbance. For robustness and high stability, it is desirable to employ integrated devices based on optical waveguide technology. Their development is hindered at this wavelength range due to the lack of suitable host materials and compatible fabrication techniques to create low-loss photonic devices. This paper details our work on femtosecond laser direct-written optical waveguides and key components for an on-chip nulling interferometer inside gallium lanthanum sulphur glass. By combining cumulative heating fabrication with the multiscan technique, single-mode optical waveguides with propagation losses as low as 0.22 ± 0.02 dB/cm at 4 µm and polarization-dependent losses of < 0.1 dB/cm were realized. Furthermore, S-bends with negligible bending loss and broadband Y-splitters with 50/50 power division across a 600 nm wavelength window (3.6 - 4.2 µm) and low losses of < 0.5 dB are demonstrated. Directional couplers with an equal splitting ratio complement these main building blocks to create a future compact nulling interferometer with a total projected intrinsic loss of < 1 dB, a value that is sufficient to perform future on-sky experiments in relatively short observation runs on ground-based telescopes.

Beam shaping of a broad-area laser diode using 3D integrated optics.

Typical high power broad-area semiconductor lasers exhibit a highly astigmatic beam profile. However, many applications require a homogenous and circular symmetric beam. Thus coupling into circular multimode optical fibers is often employed. The strip-like astigmatic output of the diode laser underfills the circular multimode fiber, thus a decrease in beam quality occurs after fiber coupling due to mode mixing inside the optical fiber. This Letter presents a 3D integrated optics approach to shape the output of a broad-area laser diode. Ultrafast laser inscription is utilized to create a pair of photonic lanterns connected back to back inside a glass chip that captures and shapes the output of a commercial 976 nm wavelength broad-area laser diode with 95 µm emitter width. Compared to coupling to a 105 µm diameter, 0.15 numerical aperture step-index multimode fiber, the photonic chip-based approach results in a 13× higher beam quality and 7× greater brightness.

Ultrafast laser inscription in ZBLAN integrated optics chips for mid-IR beam combination in astronomical interferometry.

Astronomical interferometry is a unique technique that allows observation with angular resolutions on the milliarcsec scale by combining the light of several apertures hundreds of meters apart. The PIONIER and GRAVITY instruments at the Very Large Telescope Interferometer have demonstrated that silica-based integrated optics (IO) provide a small-scale and highly stable solution for the interferometric beam combination process. Yet, important science cases such as exoplanet hunting or the spectroscopic characterization of exoplanetary atmospheres are favorable for observation in the mid-IR, namely the atmospheric windows L and L' band (3-4 µm), a wavelength range that is not covered by conventional silica-based IO. Here, we propose laser-inscribed IO 2×2 couplers in ZBLAN and experimentally assess the critical properties of the component for broadband mid-IR interferometry. We measure the splitting ratio over the 2.5 to 5.0 µm range and find excellent broadband contrast over the L (3.1-3.6 µm) and L' (3.6 - 4.0 µm) bands. Furthermore, we quantify the dispersion properties of the coupler and find a phase variation as low as 0.02 rad across the L and L' band, respectively. By optimizing the NA of our injection beam, we measured a very high total throughput of 58% over the L band including Fresnel reflection and coupling losses. We also compare our findings to recent advances in mid-IR IO in GLS and discuss its advantages and disadvantages for the implementation in future mid-IR interferometers.

Monolithic mode-selective few-mode multicore fiber multiplexers.

With the capacity limits of standard single-mode optical fiber fast approaching, new technologies such as space-division multiplexing are required to avoid an Internet capacity crunch. Few-mode multicore fiber (FM-MCF) could allow for a two orders of magnitude increase in capacity by using the individual spatial modes in the different cores as unique data channels. We report the realization of a monolithic mode-selective few-mode multicore fiber multiplexer capable of addressing the individual modes of such a fiber. These compact multiplexers operate across the S + C + L telecommunications bands and were inscribed into a photonic chip using ultrafast laser inscription. They allow for the simultaneous multiplexing of the LP01, LP11a and LP11b modes of all cores in a 3-mode, 4-core fiber with excellent mode extinction ratios and low insertion losses. The devices are scalable to more modes and cores and therefore could represent an enabling technology for practical ultra-high capacity dense space-division multiplexing.

2 W single-longitudinal-mode Yb:YAG distributed-feedback waveguide laser.

Single longitudinal mode (SLM) lasers are important tools for many scientific and commercial applications. SLM operation can be achieved in distributed-feedback (DFB) lasers based on Bragg structures. Semiconductor waveguide DFB lasers are well-established devices, but their output power is limited to a few hundred milliwatts. DFB lasers have also been demonstrated in dielectric waveguides. However, in this case the output power was even lower. Here we present the first monolithic Yb:YAG DFB laser. The waveguide and the DFB structure were fabricated in the volume of an Yb:YAG crystal by ultrafast laser inscription. The DFB laser delivered 2 W of output power at a slope efficiency of 61% in SLM operation under pumping with an optically pumped semiconductor laser. This power level outperforms previously demonstrated dielectric DFB waveguide lasers by nearly an order of magnitude. Our approach paves the way for compact, robust, and highly efficient high-power SLM laser sources.

Compact integrated actively Q-switched waveguide laser.

A miniaturized deformed helix ferroelectric liquid crystal transducer cell was used in combination with a femtosecond laser inscribed active waveguide to realize a compact actively Q-switched laser source. The liquid crystal cell was controlled by a low-voltage frequency generator and laser pulse durations below 40 ns were demonstrated at repetition rates ranging from 0.1 kHz to 20 kHz and a maximum slope efficiency of up to 22%. This novel, integrated and low-cost laser source is a promising tool for a broad range of applications such as trace gas sensing, LIDAR, and nonlinear optics. To the best of our knowledge, this is the first demonstration of an actively Q-switched glass waveguide laser that has a user-variable repetition rate and can be fully integrated.

Digital waveguide adiabatic passage part 2: experiment.

Using a femtosecond laser writing technique, we fabricate and characterise three-waveguide digital adiabatic passage devices, with the central waveguide digitised into five discrete waveguidelets. Strongly asymmetric behaviour was observed, devices operated with high fidelity in the counter-intuitive scheme while strongly suppressing transmission in the intuitive. The low differential loss of the digital adiabatic passage designs potentially offers additional functionality for adiabatic passage based devices. These devices operate with a high contrast (>90%) over a 60 nm bandwidth, centered at ∼ 823 nm.

Reconfigurable SDM Switching Using Novel Silicon Photonic Integrated Circuit.

Space division multiplexing using multicore fibers is becoming a more and more promising technology. In space-division multiplexing fiber network, the reconfigurable switch is one of the most critical components in network nodes. In this paper we for the first time demonstrate reconfigurable space-division multiplexing switching using silicon photonic integrated circuit, which is fabricated on a novel silicon-on-insulator platform with buried Al mirror. The silicon photonic integrated circuit is composed of a 7 × 7 switch and low loss grating coupler array based multicore fiber couplers. Thanks to the Al mirror, grating couplers with ultra-low coupling loss with optical multicore fibers is achieved. The lowest total insertion loss of the silicon integrated circuit is as low as 4.5 dB, with low crosstalk lower than -30 dB. Excellent performances in terms of low insertion loss and low crosstalk are obtained for the whole C-band. 1 Tb/s/core transmission over a 2-km 7-core fiber and space-division multiplexing switching is demonstrated successfully. Bit error rate performance below 10-9 is obtained for all spatial channels with low power penalty. The proposed design can be easily upgraded to reconfigurable optical add/drop multiplexer capable of switching several multicore fibers.

On-chip generation of heralded photon-number states.

Beyond the use of genuine monolithic integrated optical platforms, we report here a hybrid strategy enabling on-chip generation of configurable heralded two-photon states. More specifically, we combine two different fabrication techniques, i.e., non-linear waveguides on lithium niobate for efficient photon-pair generation and femtosecond-laser-direct-written waveguides on glass for photon manipulation. Through real-time device manipulation capabilities, a variety of path-coded heralded two-photon states can be produced, ranging from product to entangled states. Those states are engineered with high levels of purity, assessed by fidelities of 99.5 ± 8% and 95.0 ± 8%, respectively, obtained via quantum interferometric measurements. Our strategy therefore stands as a milestone for further exploiting entanglement-based protocols, relying on engineered quantum states, and enabled by scalable and compatible photonic circuits.

In vitro neuronal depolarization and increased synaptic activity induced by infrared neural stimulation.

Neuronal responses to infrared neural stimulation (INS) are explored at the single cell level using patch-clamp electrophysiology. We examined membrane and synaptic responses of solitary tract neurons recorded in acute slices prepared from the Sprague-Dawley rat. Neurons were stimulated using a compact 1890 nm waveguide laser with light delivered to a small target area, comparable to the size of a single cell, via a single-mode fiber. We show that infrared radiation increased spontaneous synaptic event frequency, and evoked steady-state currents and neuronal depolarization. The magnitude of the responses was proportional to laser output.