High resolution TCSPC imaging of diffuse light with a one-dimensional SPAD array scanning system.

We report a time-correlated single-photon counting (TCSPC) imaging system based on a line-scanning architecture. The system benefits from the high fill-factor, active area, and large dimension of an advanced CMOS single-photon avalanche diode (SPAD) array line-sensor. A two-dimensional image is constructed using a moving mirror to scan the line-sensor field-of-view (FOV) across the target, to enable the efficient acquisition of a two-dimensional 0.26 Mpixel TCSPC image. We demonstrate the capabilities of the system for TCSPC imaging and locating objects obscured in scattering media - specifically to locate a series of discrete point sources of light along an optical fibre submerged in a highly scattering solution. We demonstrate that by selectively imaging using early arriving photons which have undergone less scattering than later arriving photons, our TCSPC imaging system is able to locate the position of discrete point sources of light than a non-time-resolved imaging system.

Focus issue introduction: recent advances in astrophotonics.

The first Optics Express Focus Issue on Astrophotonics was published in 2009. Since that time the field has experienced substantial growth and pathfinder astrophotonic systems are routinely being tested on large telescopes around the globe. This second Focus Issue on Astrophotonics is intended to capture some of these significant developments and convey a sense of future directions for the field.

pH sensing through a single optical fibre using SERS and CMOS SPAD line arrays.

Full exploitation of fibre Raman probes has been limited by the obstruction of weak Raman signals by background fluorescence of the sample and the intrinsic Raman signal of the delivery fibre. Here we utilised functionalised gold nanoshells (NS) to take advantage of the surface-enhanced Raman spectroscopy (SERS) effect to enhance the pH responsive spectrum of 4-mercaptobenzoic acid (MBA). However, the fibre background is still dominant. Using the photon arrival time-resolving capability of a CMOS single-photon avalanche diode (SPAD) based line sensor, we recover the SERS spectrum without a fibre background in a 10 s measurement. In this manner, pH sensing through a multimode fibre at a low excitation power that is safe for future in vivo applications, with short acquisition times (10 or 60 s), is demonstrated. A measurement precision of ± 0.07 pH units is thus achieved.

Ultrafast laser inscription of a 121-waveguide fan-out for astrophotonics.

Using ultrafast laser inscription, we report the fabrication of a prototype three-dimensional 121-waveguide fan-out device capable of reformatting the output of a 120-core multicore fiber (MCF) into a one-dimensional linear array. When used in conjunction with an actual MCF, we demonstrate that the reformatting function using this prototype would result in an overall through put loss of ≈7.0 dB. However, if perfect coupling from the MCF into the fan-out could be achieved, the reformatting function would result in an overall loss of only ≈1.7 dB. With adequate development, similar devices could efficiently reformat the output of so-called "photonic lanterns" fabricated using highly multicore fibers.

Compact, highly efficient ytterbium doped bismuthate glass waveguide laser.

Laser slope efficiencies close to the quantum defect limit and in excess of 78% have been obtained from an ultrafast laser inscribed buried channel waveguide fabricated in a ytterbium-doped bismuthate glass. The simultaneous achievement of low propagation losses and preservation of the fluorescence properties of ytterbium ions is the basis of the outstanding laser performance.

Ultrafast laser inscription of an integrated photonic lantern.

We used ultrafast laser inscription to fabricate three-dimensional integrated optical transitions that efficiently couple light from a multimode waveguide to a two-dimensional array of single mode waveguides and back. Although the entire device has an average insertion loss of 5.7 dB at 1539 nm, only ≈0.7 dB is due to mode coupling losses. Based on an analysis which is presented in the paper, we expect that our device should convert a multimode input into an array of single modes with a loss of ≈2.0 dB, assuming the input coupling losses are zero. Such devices have applications in astrophotonics and remote sensing.

Strain field manipulation in ultrafast laser inscribed BiB3O6 optical waveguides for nonlinear applications.

A novel technique was used to control the spatial overlap of the orthogonal linearly polarized waveguide modes in ultrafast laser inscribed BiB(3)O(6) waveguides. We report that the strain fields induced by the expansion of material in the laser focus can be considered independently in the design of "type II" waveguides guiding orthogonal linearly polarized light. The waveguide with the optimal mode overlap was used for type I birefringently phase-matched second-harmonic generation of a continuous wave laser source at 1047 nm.

Lasing action at around 1.9 µm from an ultrafast laser inscribed Tm-doped glass waveguide.

We report optical guiding and laser action at around 1.9 µm in a Tm³âº-doped fluorogermanate glass waveguide fabricated using ultrafast laser inscription. A monolithic laser cavity was constructed by directly butt coupling dielectric mirrors to each facet of the 6.0 mm long Tm³âº-doped waveguide. When the waveguide was pumped by a Ti:sapphire laser tuned to 791 nm, laser oscillation was achieved at around 1.91 µm. This waveguide laser exhibited a maximum slope efficiency of 6% and a maximum output power of 32 mW when pumped with 620 mW of incident laser power.

Ultrafast laser inscription of near-infrared waveguides in polycrystalline ZnSe.

We report the successful fabrication of a low-loss near-IR waveguide in polycrystalline ZnSe using ultrafast laser inscription. The waveguide, which was inscribed using the multiscan fabrication technique, supported a well-confined mode at 1.55 µm. Propagation losses were characterized at 1.55 µm using the Fabry-Perot technique and found to be 1.07 dB · cm(-1) ± 0.03 dB · cm(-1).

Dynamics of picosecond laser ablation for surgical treatment of colorectal cancer.

Endoluminal surgery for the treatment of colorectal neoplasia is typically carried out using electrocautery tools which imply limited precision and the risk of harm through collateral thermal damage to the adjacent healthy tissue. As a potential alternative, we present the successful colonic epithelial laser ablation by means of picosecond laser pulses. Laser ablation studies performed in ex-vivo colon tissue result in cavities with comparable thickness to early stage colorectal cancers. The corresponding histology sections exhibit only minimal collateral damage to the surrounding tissue and the depth of the ablation can be controlled precisely by means of the pulse energy. High-speed imaging has been used for the first time to visualize picosecond laser ablation of cancerous tissue in a clinically relevant model. This information was correlated with histopathology and optical surface profilometry revealing the dynamic nature of the laser tissue interaction and the need for temporal or spatial separation of pulses for optimum efficacy with regards to tissue removal. Overall, the application of picosecond laser pulses to ablate endoluminal bowel lesions demonstrates significantly improved precision and reduced thermal damage to the adjacent tissue in comparison to conventional procedures and hence will enable more precise surgical treatment of cancers.

Ultrafast laser inscribed Yb:KGd(WO4)2 and Yb:KY(WO4)2 channel waveguide lasers.

We demonstrate laser action in diode-pumped microchip monolithic cavity channel waveguides of Yb:KGd(WO(4))(2) and Yb:KY(WO(4))(2) that were fabricated by ultrafast laser writing. The maximum output power achieved was 18.6 mW with a threshold of approximately 100 mW from an Yb:KGd(WO(4))(2) waveguide laser operating at 1023 nm. The propagation losses for this waveguide structure were measured to be 1.9 dBcm(-1).

Shaping ultrafast laser inscribed optical waveguides using a deformable mirror.

We use a two-dimensional deformable mirror to shape the spatial profile of an ultrafast laser beam that is then used to inscribe structures in a soda-lime silica glass slide. By doing so we demonstrate that it is possible to control the asymmetry of the cross section of ultrafast laser inscribed optical waveguides via the curvature of the deformable mirror. When tested using 1.55 mum light, the optimum waveguide exhibited coupling losses of approximately 0.2 dB/facet to Corning SMF-28 single mode fiber and propagation losses of approximately 1.5 dB.cm(-1). This technique promises the possibility of combining rapid processing speeds with the ability to vary the waveguide cross section along its length.

Ultrafast-laser inscription of a three dimensional fan-out device for multicore fiber coupling applications.

A fan-out device has been fabricated using ultrafast-laser waveguide-inscription that enables each core of a multicore optical fiber (MCF) to be addressed by a single mode fiber held in a fiber V-groove array (FVA). By utilizing the unique three-dimensional fabrication capability of this technique we demonstrate coupling between an FVA consisting of a one-dimensional array of fibers and an MCF consisting of a two-dimensional array of cores. When coupled to all cores of the MCF simultaneously, the average insertion loss per core was 5.0 dB in the 1.55 mum spectral region. Furthermore, the fan-out exhibited low cross-talk and low polarization dependent loss.

Frequency-doubling in femtosecond laser inscribed periodically-poled potassium titanyl phosphate waveguides.

Frequency doubling has been achieved in femtosecond-laser-inscribed single-mode waveguides written in two periodically-poled potassium titanyl phosphate crystals. A conversion efficiency of 0.22 %W(1) was obtained for first-order quasi-phase matching at 980 nm and an efficiency of 0.02 %W(-1) for third-order quasi-phase matching at 800 nm.

Multiple rare earth emissions in a multicore tellurite fiber with a single pump wavelength.

A three-core tellurite glass fiber having different combinations of rare earth oxide dopants in each core has been fabricated using shaped die-extrusion. Three cores, doped with Ho(3+)-Tm(3+)-Yb(3+), Er(3+)-Ce(3+), and Tm(3+)-Yb(3+) respectively, exhibited visible upconversion (blue, green and red) and infrared emissions at 1.4 microm, 1.5 microm, 1.9 microm and 2.05 microm when pumped at a wavelength of 980 nm. The prospects for multi-band amplifiers and lasers are discussed.

Ballistic and snake photon imaging for locating optical endomicroscopy fibres.

We demonstrate determination of the location of the distal-end of a fibre-optic device deep in tissue through the imaging of ballistic and snake photons using a time resolved single-photon detector array. The fibre was imaged with centimetre resolution, within clinically relevant settings and models. This technique can overcome the limitations imposed by tissue scattering in optically determining the in vivo location of fibre-optic medical instruments.

Endoscopic sensing of alveolar pH.

Previously unobtainable measurements of alveolar pH were obtained using an endoscope-deployable optrode. The pH sensing was achieved using functionalized gold nanoshell sensors and surface enhanced Raman spectroscopy (SERS). The optrode consisted of an asymmetric dual-core optical fiber designed for spatially separating the optical pump delivery and signal collection, in order to circumvent the unwanted Raman signal generated within the fiber. Using this approach, we demonstrate a ~100-fold increase in SERS signal-to-fiber background ratio, and demonstrate multiple site pH sensing with a measurement accuracy of ± 0.07 pH units in the respiratory acini of an ex vivo ovine lung model. We also demonstrate that alveolar pH changes in response to ventilation.

Femtosecond laser inscription of optical waveguides in Bismuth ion doped glass.

We report on the fabrication of high quality embedded channel waveguides inside Bi-doped silicate glass using femtosecond waveguide inscription. Waveguides are fabricated using both single and multi-scan fabrication techniques. Refractive index modifications of up to Deltan = 4.3 x 10(-3) are observed, allowing the fabrication of waveguides nearly mode-matched to telecom fibers. When optically pumped at 980 and 810 nm broadband fluorescence emission centered at 1.3 microm with a FWHM of up to 500 nm is detected.