Ultrafast laser inscribed waveguides in tailored fluoride glasses: an enabling technology for mid-infrared integrated photonics devices.

Zirconium fluoride (ZBLAN) glass, the standard material used in fiber-based mid-infrared photonics, has been re-designed to enable the fabrication of high index-contrast low-loss waveguides via femtosecond laser direct writing. We demonstrate that in contrast to pure ZBLAN, a positive index change of close to 10-2 can be induced in hybrid zirconium/hafnium (Z/HBLAN) glasses during ultrafast laser inscription and show that this can be explained by an electron cloud distortion effect that is driven by the existence of two glass formers with contrasting polarizability. High numerical aperture (NA) type-I waveguides that support a well confined 3.1 µm wavelength mode with a mode-field diameter (MFD) as small as 12 µm have successfully been fabricated. These findings open the door for the fabrication of mid-infrared integrated photonic devices that can readily be pigtailed to existing ZBLAN fibers.

Novel concept for visible and near infrared spectro-interferometry: laser-written layered arrayed waveguide gratings.

With the growing complexity of astronomical instruments devoted to interferometry, such as MATISSE (a 4 telescope beam combiner) or FIRST (a 9 sub-apertures beam combiner), and the rebirth of space projects such as LIFE (a mid-infrared interferometer), integrated optics devices can be an interesting and complementary approach for beam combination of a large number of apertures. Moreover, one of the approaches for beam combination is pairwise combination of the inputs (either from individual telescopes or from aperture masking on a single telescope), which scales as N(N-1)/2 for an N input system. Astrophotonics devices are attractive to reduce mass and system complexity, while achieving all the beam combination in a single chip, even for a high number of inputs. The aim of this work is to develop a compact photonic device for astronomical applications and demonstrate a proof-of-concept of a spectro-interferometer. In this paper ultrafast laser inscription is used to fabricate three arrayed waveguide gratings (AWGs) stacked vertically. This arrangement enables spectral dispersion and interferometry to be measured simultaneously. Individual AWGs were designed for operation at 633 nm, and demonstrated at 633nm and 830nm. A scan between 790 and 830nm was also achieved to study the wavelength behavior of the AWG. Using a segmented mirror, light at 633nm or 830nm was injected simultaneously into three AWGs layered 40 µm apart, showing analogous behavior for all three layers and no unexpected crosstalk. Finally the three outputs were vertically combined to obtain interference fringes, showing the feasibility of spectro-interferometry and opening the way for compact astrophotonic devices devoted to phase closure studies, used in astronomy to reduce the effect of atmospheric turbulence.

Femtosecond laser written arrayed waveguide gratings with integrated photonic lanterns.

We demonstrate for the first time functional arrayed waveguide gratings (AWGs) fabricated using the femtosecond laser direct-write technique. This fabrication technique is a mask-less alternative to lithography enabling design flexibility and rapid prototyping. It is ideal for customized small scale production for new applications. The devices were demonstrated in the visible region at 632.8 nm with a measured free spectral range (FSR) of 22.2 nm, and 1.35 nm resolution. To highlight the advantages of using a 3-dimensional fabrication technique, a 3-port photonic lantern was integrated with an AWG in a single monolithic chip. Integration of this type is not feasible with lithography-based AWG fabrication and can increase the functionality of AWGs for sensing applications.

Modal noise in an integrated photonic lantern fed diffraction-limited spectrograph.

In an attempt to develop a streamlined astrophotonic instrument, we demonstrate the realization of an all-photonic device capable of both multimode to single mode conversion and spectral dispersion on an 8-m class telescope with efficient coupling. The device was a monolithic photonic spectrograph which combined an integrated photonic lantern and an efficient arrayed waveguide grating device. During on-sky testing, we discovered a previously unreported type of noise that made spectral extraction and calibration extremely difficult. The source of the noise was traced to a wavelength-dependent loss mechanism between the feed fiber's multimode near-field pattern and the modal acceptance profile of the integrated photonic lantern. Extensive modeling of the photonic components replicates the wavelength-dependent loss, and demonstrates an identical effect on the final spectral output. We outline that this could be mitigated by directly injecting into the integrated photonic lantern.

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.

Spectral narrowing of Yb:YAG waveguide lasers through hybrid integration with ultrafast laser written Bragg gratings.

Laser written waveguides in crystalline materials can be used to make highly efficient, high gain lasers. The bi-directional emission from such lasers however is typically broadband with poor spectral control. Hybridizing a tapered, mode matched laser written Bragg grating with a broadband Yb:YAG crystalline waveguide laser, we demonstrate single longitudinal mode output from one end of the device. Careful control of the grating characteristics led to laser thresholds below 90 mW, slope efficiencies greater than 42% and output powers greater than 20 mW.

Towards femtosecond laser written arrayed waveguide gratings.

The fabrication of arrayed waveguide gratings (AWGs) using the femtosecond laser direct-write technique is investigated. We successfully demonstrate the fabrication of large planar waveguides that act as 2D free propagation zones. These slabs were found to have a highly uniform refractive index with a standard deviation of 1.97% relative to the total index contrast. The incorporation of low loss linear adiabatic tapers resulted in an increase of transmission by 90%. Strategies for manufacturing integrated laser written AWGs using continuous contouring to avoid lossy defects are discussed and demonstrated.

Widely tunable short-infrared thulium and holmium doped fluorozirconate waveguide chip lasers.

We report widely tunable (≈ 260 nm) Tm(3+) and Ho(3+) doped fluorozirconate (ZBLAN) glass waveguide extended cavity lasers with close to diffraction limited beam quality (M(2) ≈ 1.3). The waveguides are based on ultrafast laser inscribed depressed claddings. A Ti:sapphire laser pumped Tm(3+)-doped chip laser continuously tunes from 1725 nm to 1975 nm, and a Tm(3+)-sensitized Tm(3+):Ho(3+) chip laser displays tuning across both ions evidenced by a red enhanced tuning range of 1810 to 2053 nm. We also demonstrate a compact 790 nm diode laser pumped Tm(3+)-doped chip laser which tunes from 1750 nm to 1998 nm at a 14% incident slope efficiency, and a beam quality of M(2) ≈ 1.2 for a large mode-area waveguide with 70 µm core diameter.

Competition of Faraday rotation and birefringence in femtosecond laser direct written waveguides in magneto-optical glass.

We consider the process of Faraday rotation in femtosecond laser direct-write waveguides. The birefringence commonly associated with such waveguides may be expected to impact the observable Faraday rotation. Here, we theoretically calculate and experimentally verify the competition between Faraday rotation and birefringence in two waveguides created by laser writing in a commercial magneto-optic glass. The magnetic field applied to induce Faraday rotation is nonuniform, and as a result, we find that the two effects can be clearly separated and used to accurately determine even weak birefringence. The birefringence in the waveguides was determined to be on the scale of Δn = 10(-6) to 10(-5). The reduction in Faraday rotation caused by birefringence of order Δn = 10(-6) was moderate and we obtained approximately 9° rotation in an 11 mm waveguide. In contrast, for birefringence of order 10(-5), a significant reduction in the polarization azimuth change was found and only 6° rotation was observed.

Versatile large-mode-area femtosecond laser-written Tm:ZBLAN glass chip lasers.

We report performance characteristics of a thulium doped ZBLAN waveguide laser that supports the largest fundamental modes reported in a rare-earth doped planar waveguide laser (to the best of our knowledge). The high mode quality of waveguides up to 45 um diameter (~1075 µm(2) mode-field area) is validated by a measured beam quality of M(2)~1.1 ± 0.1. Benefits of these large mode-areas are demonstrated by achieving 1.9 kW peak-power output Q-switched pulses. The 1.89 µm free-running cw laser produces 205 mW and achieves a 67% internal slope efficiency corresponding to a quantum efficiency of 161%. The 9 mm long planar chip developed for concept demonstration is rapidly fabricated by single-step optical processing, contains 15 depressed-cladding waveguides, and can operate in semi-monolithic or external cavity laser configurations.

2.1 µm waveguide laser fabricated by femtosecond laser direct-writing in Ho3+, Tm3+:ZBLAN glass.

We report the first Ho3+ doped waveguide laser, which was realized by femtosecond direct-writing of a depressed cladding structure into ZBLAN glass. Tm3+ sensitizing allows the 9 mm long Ho3+ gain medium to be conveniently pumped at 790 nm, achieving an optical-to-optical slope efficiency of 20% and a threshold of 20 mW. The potentially widely tunable laser produces up to 76 mW at 2052 nm and also operates at shorter wavelengths near 1880 nm and 1978 nm for certain cavity configurations.

Non-specific internalization of laser ablated pure gold nanoparticles in pancreatic tumor cell.

We investigate the intracellular uptake of 7.3 nm, 21.2 nm and 31.3 nm average size pure colloidal gold nanoparticles synthesized using femtosecond laser ablation technique in pure water. Dark-field imaging, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) was used to assess the uptake of these pure gold nanoparticles in the pancreatic tumor cell line. We show that these ligand-free gold nanoparticles are non-toxic to these cells. The nanoparticles and cell images indicated that unmodified gold nanoparticles interacted with the cells, despite negative surface charge on both the cells and the nanoparticles. We also demonstrate that the uptake of the gold nanoparticles is size-dependent.

Fifty percent internal slope efficiency femtosecond direct-written Tm³âº:ZBLAN waveguide laser.

We report a 790 nm pumped, Tm³âº doped ZBLAN glass buried waveguide laser that produces 47 mW at 1880 nm, with a 50% internal slope efficiency and an M² of 1.7. The waveguide cladding is defined by two overlapping rings created by femtosecond direct-writing of the glass, which results in the formation of a tubular depressed-index-cladding structure, and the laser resonator is defined by external dielectric mirrors. This is, to the best of our knowledge, the most efficient laser created in a glass host via femtosecond waveguide writing.

Annealing dynamics of waveguide Bragg gratings: evidence of femtosecond laser induced colour centres.

There is still significant speculation regarding the nature of femtosecond laser induced index change in bulk glasses with colour centre formation and densification the main candidates. In the work presented here, we fabricated waveguide Bragg gratings in doped and undoped phosphate glasses and use these as a diagnostic for monitoring subtle changes in the induced refractive index during photo- and thermal annealing experiments. Reductions in grating strengths during such experiments were attributed to the annihilation of colour centres.

High aspect ratio taper-free microchannel fabrication using femtosecond Bessel beams.

We present a systematic study of femtosecond laser microchannel machining in glass using nondiffracting Bessel beams. In particular, our results identify a source and focusing parameter working window where high aspect ratio taper-free microchannels can be reproducibly produced without sample translation. With appropriate source parameters, we machine channels of 2 microm diameter and with aspect ratios up to 40. We propose the filamentation stability of the Bessel beam propagation as the critical factor underlying the controlled and reproducible results that have been obtained.

Single photon emission from diamond nanocrystals in an opal photonic crystal.

We present the first optical measurement of a single nitrogen-vacancy (NV) center in a three-dimensional photonic crystal. The photonic crystal, fabricated by self-assembly of polystyrene microspheres, exhibits a photonic stopband that overlaps the NV photoluminescence spectrum. A modified emission spectrum and photon antibunching were measured from the NV centers. Time-resolved fluorescence measurements revealed a 30% increase in the source lifetime. Encapsulation of single NV centers in a three-dimensional photonic crystal is a step towards controlling emission properties of a single photon source.

Study of the influence of femtosecond laser polarisation on direct writing of waveguides.

Optical devices were fabricated in fused silica using the femtosecond direct write technique. We found that the transmission of light through directly written waveguides, whether straight or curved, can be increased by writing waveguides using circularly rather than linearly polarised radiation.

Slit beam shaping method for femtosecond laser direct-write fabrication of symmetric waveguides in bulk glasses.

We report both theoretical and experimental results of a slit beam shaping configuration for fabricating photonic waveguides by use of femtosecond laser pulses. Most importantly we show the method supports focusing objectives with a long depth of field and allows the direct-writing of microstructures with circular cross-sections whilst employing a perpendicular writing scheme. We applied this technique to write low loss (0.39 dB/cm), single mode waveguides in phosphate glass.

Characterizing output beams for lasers that use high-magnification unstable resonators.

Laser beams generated from high-magnification on-axis unstable resonators by use of hard-edged optics typically have a doughnut-shaped distribution in the near field (i.e., a flat-top profile with a hole in the middle for an axially coupled beam). We derive analytical expressions describing this distribution by using the flattened Gaussian beams concept. The superposition of two flattened Gaussian beams whose flatness and steepness of edges are controlled by defined parameters (i.e., the beam width and the order) is used to analyze the output beam intensity along the propagation axis. Finally, experimental measurements of beam propagation from a copper-vapor laser fitted with a high-magnification unstable resonator show excellent agreement with theoretical predictions.

High-average-power (15-W) 255-nm source based on second-harmonic generation of a copper laser master oscillator power amplifier system in cesium lithium borate.

We have generated 15 W of UV (255-nm) radiation with an optical conversion efficiency of 28% by frequency doubling the 510.6-nm output of a high-beam-quality, high-power copper laser system in cesium borate lithium (CLBO). We found that the superior performance of CLBO relative to beta-barium borate is attributable largely to the small UV absorption and wide temperature acceptance bandwidth of CLBO, which reduces thermal dephasing during high-power UV generation.