Innovated single flush on-line solid-phase extraction in bead injection format for flow programming-based determination of creatinine in human urine.

Reaction-based assays are commonly automated and miniaturized via flow analysis. However, aggressive reagents can affect or destroy even the chemically resistant manifold during long-term use. Using on-line solid-phase extraction (SPE) can eliminate this drawback and allow for high reproducibility and further advanced automation, as presented in this work. Determination of creatinine in human urine, an important clinical marker, by sequential injection analysis was achieved using bead injection on-line SPE with specific UV spectrophotometric detection, providing the necessary sensitivity and selectivity of the method for bioanalysis. The automated SPE column packing and disposal, calibration, and fast measurement highlighted the improvements in our approach. Variable sample volumes and a single working standard solution eliminated matrix effects, broadened the calibration range, and accelerated the quantification. Our method comprised an injection of 20 µL of 100 × times diluted urine with aqueous acetic acid solution pH 2.4, sorption of creatinine in a strong cation exchanger SPE column, washing out urine matrix with 50% aqueous acetonitrile, and elution of creatinine with 1% ammonium hydroxide. The SPE step was accelerated by a single flush of the column when the eluent/matrix wash/sample/standard zones sequence was created in the pump holding coil, and then the sequence of the zones was flushed into the column at once. The whole process was continually spectrophotometrically detected at 235 nm, subtracted from the signal at 270 nm. A single run duration was less than 3.5 min. Method relative standard deviation was <5.0% (n = 6). A calibration range was linear within the range of 0.02-0.30 µg creatinine (R > 0.999), covering 1.0-15.0 mmol/L creatinine in urine. The standard addition method used two different volumes of a single working standard solution for quantification. Results proved the effectiveness of our improvements in the flow manifold, bead injection, and automated quantification. The accuracy of our method was comparable to the routine enzymatic assay of real urine samples in a clinical laboratory.

An integrated optical modulator operating at cryogenic temperatures.

Photonic integrated circuits (PICs) operating at cryogenic temperatures are fundamental building blocks required to achieve scalable quantum computing and cryogenic computing technologies1,2. Silicon PICs have matured for room-temperature applications, but their cryogenic performance is limited by the absence of efficient low-temperature electro-optic modulation. Here we demonstrate electro-optic switching and modulation from room temperature down to 4 K by using the Pockels effect in integrated barium titanate (BaTiO3) devices3. We investigate the temperature dependence of the nonlinear optical properties of BaTiO3, showing an effective Pockels coefficient of 200 pm V-1 at 4 K. The fabricated devices show an electro-optic bandwidth of 30 GHz, ultralow-power tuning that is 109 times more efficient than thermal tuning, and high-speed data modulation at 20 Gbps. Our results demonstrate a missing component for cryogenic PICs, removing major roadblocks for the realization of cryogenic-compatible systems in the field of quantum computing, supercomputing and sensing, and for interfacing those systems with instrumentation at room temperature.

Attenuation Coefficients of Tubular Conduits for Liquid Phase Absorbance Measurement: Shot Noise Limited Optimum Path Length.

We trace the history of liquid core waveguides (LCWs, also called liquid core optical fibers) and the role Teflon AF (TAF) has played in their development. We show that, in any shot noise limited situation, the optimum signal-to-noise ratio (S/N) occurs at a path length of 1/αa{ln[1 + 2(αa/αb)]}, approximately 2/αb under most conditions, αa and αb being the light attenuation coefficient due to the analyte and the background, respectively. The analysis shows that LCW length should be selected depending on the applicable αb value. An overly long LCW may exhibit a lower signal-to-noise ratio. Water-filled TAF-clad fused-silica (FS) tubes show the lowest attenuation across the wavelength range. Nevertheless, except at λ ≥ 600 nm, the observed αb values far exceed those reported for pure water: it appears that both impurities in the water and waveguide losses are involved. In examining the attenuation in various water-filled tubes, we find that the transmission of air-surrounded FS tubes is second only to TAF-clad FS tubes and is better than that of TAF tubes or externally mirrored FS tubes. Surprisingly, except for a window centered at ∼250 nm, light transmission in a water-filled poly(tetrafluoroethylene) (PTFE) tube is worse than in poly(ether ether ketone) (PEEK) tubing. Light transmission in PTFE tubes improves with increasing wall thickness.

Flow injection programmed to function in batch mode is used to determine molar absorptivity and to investigate the phosphomolybdenum blue method.

The ultimate goal of flow-based analytical techniques is to automate serial assays of a target analyte. However, when developing any reagent-based assay, the underlying chemistry has to be investigated and understood a step, which is almost always the most challenging component of the optimization effort. The difficulty lies in that almost all reagent-based assays were initially developed and optimized in a batch mode, with the aim to perform assays manually, within a time frame of up to 15 min, while flow injection techniques are designed to monitor concentration gradients at times prior to reaching chemical equilibria and while performing up to two assays per minute. This work resolves this discrepancy by using programming Flow Injection (pFI) that operates in a batch mode within a time frame of 1 min or less, with the aim of optimizing an assay under the same conditions and using the same instrument in which the assay will be performed. This novel concept is verified by determining a molar absorptivity of Fe(II) ferrozine complex and by comparing it with literature data. Next, the pFI-batch technique was used to investigate and optimize the phosphate assay, based on formation of phosphomolybdenum blue, with the aim of maximizing sensitivity and improving the limit of detection of this widely used method.

Quantum Logic with Cavity Photons From Single Atoms.

We demonstrate quantum logic using narrow linewidth photons that are produced with an a priori nonprobabilistic scheme from a single ^{87}Rb atom strongly coupled to a high-finesse cavity. We use a controlled-not gate integrated into a photonic chip to entangle these photons, and we observe nonclassical correlations between photon detection events separated by periods exceeding the travel time across the chip by 3 orders of magnitude. This enables quantum technology that will use the properties of both narrow-band single photon sources and integrated quantum photonics.

Engineering integrated photonics for heralded quantum gates.

Scaling up linear-optics quantum computing will require multi-photon gates which are compact, phase-stable, exhibit excellent quantum interference, and have success heralded by the detection of ancillary photons. We investigate the design, fabrication and characterisation of the optimal known gate scheme which meets these requirements: the Knill controlled-Z gate, implemented in integrated laser-written waveguide arrays. We show device performance to be less sensitive to phase variations in the circuit than to small deviations in the coupler reflectivity, which are expected given the tolerance values of the fabrication method. The mode fidelity is also shown to be less sensitive to reflectivity and phase errors than the process fidelity. Our best device achieves a fidelity of 0.931 ± 0.001 with the ideal 4 × 4 unitary circuit and a process fidelity of 0.680 ± 0.005 with the ideal computational-basis process.

QUANTUM OPTICS. Universal linear optics.

Linear optics underpins fundamental tests of quantum mechanics and quantum technologies. We demonstrate a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of that circuit. Our six-mode universal system consists of a cascade of 15 Mach-Zehnder interferometers with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters, input of up to six photons, and their measurement with a 12-single-photon detector system. We programmed this system to implement heralded quantum logic and entangling gates, boson sampling with verification tests, and six-dimensional complex Hadamards. We implemented 100 Haar random unitaries with an average fidelity of 0.999 ± 0.001. Our system can be rapidly reprogrammed to implement these and any other linear optical protocol, pointing the way to applications across fundamental science and quantum technologies.

Theoretical modeling and experiments on a DBR waveguide laser fabricated by the femtosecond laser direct-write technique.

We present a model for a Yb-doped distributed Bragg reflector (DBR) waveguide laser fabricated in phosphate glass using the femtosecond laser direct-write technique. The model gives emphasis to transverse integrals to investigate the energy distribution in a homogenously doped glass, which is an important feature of femtosecond laser inscribed waveguide lasers (WGLs). The model was validated with experiments comparing a DBR WGL and a fiber laser, and then used to study the influence of distributed rare earth dopants on the performance of such lasers. Approximately 15% of the pump power was absorbed by the doped "cladding" in the femtosecond laser inscribed Yb doped WGL case with the length of 9.8 mm. Finally, we used the model to determine the parameters that optimize the laser output such as the waveguide length, output coupler reflectivity and refractive index contrast.

Non-classical interference in integrated 3D multiports.

We demonstrate three and four input multiports in a three dimensional glass platform, fabricated using the femtosecond laser direct-write technique. Hong-Ou-Mandel (HOM) interference is observed and a full quantum characterization is performed, obtaining two photon correlation matrices for all combinations of input and output ports. For the 3-port case, the quantum visibilities are accurately predicted solely from measurement of the classical coupling ratios.

Cladding mode coupling in highly localized fiber Bragg gratings II: complete vectorial analysis.

Highly localized fiber Bragg gratings can be inscribed point-by-point with focused ultrashort pulses. The transverse localization of the resonant grating causes strong coupling to cladding modes of high azimuthal and radial order. In this paper, we show how the reflected cladding modes can be fully analyzed, taking their vectorial nature, orientation and degeneracies into account. The observed modes' polarization and intensity distributions are directly tied to the dispersive properties and show abrupt transitions in nature, strongly correlated with changes in the coupling strengths.

Optimizing the net reflectivity of point-by-point fiber Bragg gratings: the role of scattering loss.

We present an experimental and theoretical analysis of the influence of scattering losses on the net reflectivity of fiber Bragg gratings inscribed with a femtosecond laser and the point-by-point technique. We demonstrate that the ratio of the coupling strength coefficient to the scattering loss coefficient varies significantly with the inscribing laser pulse energy, and highlight that an optimal pulse-energy range exists for achieving high-reflectivity gratings. These results are critical for exploiting high power fiber laser opportunities based on point-by-point gratings.

Ultrafast laser-written dual-wavelength waveguide laser.

We report the performance of a dual-wavelength waveguide laser based on a phase-modulated sampled-grating architecture fabricated using the femtosecond laser direct-write technique. The waveguide laser was written in Yb-doped phosphate glass and had a narrow linewidth (<10 pm), high signal-to-noise ratio (>60 dB), 5 mW output power per channel, and wavelength separation of 10 nm.

Slow-light enhanced correlated photon pair generation in a silicon photonic crystal waveguide.

We report the generation of correlated photon pairs in the telecom C-band at room temperature from a dispersion-engineered silicon photonic crystal waveguide. The spontaneous four-wave mixing process producing the photon pairs is enhanced by slow-light propagation enabling an active device length of less than 100 µm. With a coincidence to accidental ratio of 12.8 at a pair generation rate of 0.006 per pulse, this ultracompact photon pair source paves the way toward scalable quantum information processing realized on-chip.

Point-by-point inscription of apodized fiber Bragg gratings.

We demonstrate apodized fiber Bragg gratings (FBGs) inscribed with a point-by-point (PbP) technique. We tailor the grating phase and coupling amplitude through precise control over the longitudinal and transverse positions of each laser-inscribed modification. This method of apodization is facilitated by the highly localized, high-contrast modifications generated by focused IR femtosecond laser inscription. Our technique provides a simple method for the design and implementation of PbP FBGs with complex apodization profiles.

Linearly polarized fiber laser using a point-by-point Bragg grating in a single-polarization photonic bandgap fiber.

We present a narrow-linewidth, linearly polarized neodymium-doped fiber laser that incorporates a point-by-point Bragg grating inscribed into the core of a single-polarization all-solid photonic bandgap fiber. The Bragg grating was written within a single-polarization wavelength band of the fiber; thus, the Bragg reflection was polarized. This all-fiber laser produced 7.2 W, linearly polarized output with 25 pm FWHM and 19.6 dB polarization extinction ratio.

Control of light transmission in laser-written phase-shifted Bragg grating couplers.

We demonstrate the fabrication by direct laser writing and the operation of a directional coupler containing Bragg gratings in each waveguide. We achieve high-precision control over the longitudinal shift between the gratings, which feature first-order Bragg resonance at telecommunication wavelengths. We observe fundamental differences between light transmission characteristics in couplers with unshifted and shifted gratings in agreement with theoretical predictions.

Three-dimensional imaging of direct-written photonic structures.

Third-harmonic generation microscopy has been used to analyze the morphology of photonic structures created using the femtosecond laser direct-write technique. Three-dimensional waveguide arrays and waveguide-Bragg gratings written in fused-silica and doped phosphate glass were investigated. A sensorless adaptive-optical system was used to correct the optical aberrations occurring in the sample and microscope system, which had a lateral resolution of less than 500 nm. This nondestructive testing method creates volume reconstructions of photonic devices and reveals details invisible to other linear microscopy and index profilometry techniques.

Cladding mode coupling in highly localized fiber Bragg gratings: modal properties and transmission spectra.

The spectral characteristics of a fiber Bragg grating (FBG) with a transversely inhomogeneous refractive index profile, differs considerably from that of a transversely uniform one. Transmission spectra of inhomogeneous and asymmetric FBGs that have been inscribed with focused ultrashort pulses with the so-called point-by-point technique are investigated. The cladding mode resonances of such FBGs can span a full octave in the spectrum and are very pronounced (deeper than 20dB). Using a coupled-mode approach, we compute the strength of resonant coupling and find that coupling into cladding modes of higher azimuthal order is very sensitive to the position of the modification in the core. Exploiting these properties allows precise control of such reflections and may lead to many new sensing applications.

Point-by-point written fiber-Bragg gratings and their application in complex grating designs.

The point-by-point technique of fabricating fibre-Bragg gratings using an ultrafast laser enables complete control of the position of each index modification that comprises the grating. By tailoring the local phase, amplitude and spacing of the grating's refractive index modulations it is possible to create gratings with complex transmission and reflection spectra. We report a series of grating structures that were realized by exploiting these flexibilities. Such structures include gratings with controlled bandwidth, and amplitude- and phase-modulated sampled (or superstructured) gratings. A model based on coupled-mode theory provides important insights into the manufacture of such gratings. Our approach offers a quick and easy method of producing complex, non-uniform grating structures in both fibres and other mono-mode waveguiding structures.

All-optical, actively Q-switched fiber laser.

All-fiber lasers offer increased robustness and simplicity over other fiber laser systems. Current active Q-switching techniques for all-fiber lasers rely on electro-mechanical transducers to strain-tune an intra-cavity fiber-Bragg grating, which adds complexity and can lead to vibrational sensitivity. An all-optical technique for achieving active Q-switched operation is a more elegant approach and would maintain the inherent robustness and simplicity of an all-fiber laser system. In this work, we studied the optical tuning of a fiber-Bragg grating by resonant optical pumping and optimized it for application to active systems. We incorporated an optically-tunable fiber-Bragg grating into a fiber laser and demonstrated active Q-switching at 35 kHz with this all-optical, all-fiber laser system. We highlight the potential to operate at >300 kHz with the current embodiment. To our knowledge, this is the first demonstration of an optically-driven active Q-switch in a fiber laser. Further potential to operate at MHz frequencies is discussed.