Modal instabilities in high power fiber laser oscillators.

Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.

Highly efficient coupling of crystalline microresonators to integrated photonic waveguides.

Crystalline optical whispering gallery mode resonators made from alkaline earth fluorides can achieve exceptionally large optical finesse, and are used in a variety of applications, from frequency stabilization and narrow linewidth lasers, to low-noise microwave generation or soliton Kerr frequency combs. Here we demonstrate an efficient coupling method to resonators of these materials, which employs photonic integrated waveguides on a chip based on silicon nitride. By converting a mode from silicon nitride to a free-hanging silica waveguide on a silicon chip, coupling to a crystalline resonator is achieved with a high extinction, while preserving a quality factor exceeding 200 million. This compact, heterogeneous integration of ultra-high Q-factor crystalline resonators with photonic waveguides provides a proof of concept for wafer scale integration and robust, compact packaging for a wide range of applications.

Rare and Common Variants Conferring Risk of Tooth Agenesis.

We present association results from a large genome-wide association study of tooth agenesis (TA) as well as selective TA, including 1,944 subjects with congenitally missing teeth, excluding third molars, and 338,554 controls, all of European ancestry. We also tested the association of previously identified risk variants, for timing of tooth eruption and orofacial clefts, with TA. We report associations between TA and 9 novel risk variants. Five of these variants associate with selective TA, including a variant conferring risk of orofacial clefts. These results contribute to a deeper understanding of the genetic architecture of tooth development and disease. The few variants previously associated with TA were uncovered through candidate gene studies guided by mouse knockouts. Knowing the etiology and clinical features of TA is important for planning oral rehabilitation that often involves an interdisciplinary approach.

Integrated reconfigurable photonic filters based on interferometric fractional Hilbert transforms.

In this paper, we present integrated reconfigurable photonic filters using fractional Hilbert transformers (FrHTs) and optical phase tuning structure within the silica-on-silicon platform. The proposed structure, including grating-based FrHTs, an X-coupler, and a pair of thermal tuning filaments, is fabricated through the direct UV grating writing technique. The thermal tuning effect is realized by the controllable microheaters located on the two arms of the X-coupler. We investigate the 200 GHz maximum bandwidth photonic FrHTs based on apodized planar Bragg gratings, and analyze the reflection spectrum responses. Through device integration and thermal modulation, the device could operate as photonic notch filters with 5 GHz linewidth and controllable single sideband suppression filters with measured 12 dB suppression ratio. A 50 GHz instantaneous frequency measuring system using this device is also schematically proposed and analyzed with potential 3 dB measurement improvement. The device could be configured with these multiple functions according to need. The reconfigurable structure has great potential in ultrafast all-optical signal processing fields.

High power, compact, picosecond MOPA based on single trench fiber with single polarized diffraction-limited output.

We experimentally demonstrate an all-solid Yb-doped 30 µm core diameter single trench fiber. Measurements ensure a robust effective single-mode operation without the need of tight coiling as required for conventional fibers thanks to the ultralow NA (∼0.038) and resonant ring surrounding the core. All-solid and cylindrical design ensures the suitability for mass scale production with the added benefit of all-fiberized device structure. A compact master oscillator power amplifier (MOPA) has been built using this fiber delivering ∼23.5 ps pulses at 13.5 MHz repetition rate delivering up to ∼52 W of average output power corresponding to a pulse energy of ∼3.8 µJ and peak power of >160 kW, while maintaining ∼76% slope efficiency. The output beam exhibits a polarization extinction ratio of more than 15 dB and a M2 less than 1.15.

High-peak-power, high-energy, high-average-power pulsed fiber laser system with versatile pulse duration and shape.

We present a pulsed fiber laser system with average power up to 265 W, pulse energy up to 10.6 mJ, pulse duration adjustable in the range 500 ps-500 ns, repetition rate fully controllable from single-shot operation up to 1 MHz, and the ability to control peak power independently of pulse energy. The system has a compact, all-spliced construction. Such a versatile laser will have wide applications in materials processing.

Terahertz bandwidth photonic Hilbert transformers based on synthesized planar Bragg grating fabrication.

Terahertz bandwidth photonic Hilbert transformers are proposed and experimentally demonstrated. The integrated device is fabricated via a direct UV grating writing technique in a silica-on-silicon platform. The photonic Hilbert transformer operates at bandwidths of up to 2 THz (~16 nm) in the telecom band, a 10-fold greater bandwidth than any previously reported experimental approaches. Achieving this performance requires detailed knowledge of the system transfer function of the direct UV grating writing technique; this allows improved linearity and yields terahertz bandwidth Bragg gratings with improved spectral quality. By incorporating a flat-top reflector and Hilbert grating with a waveguide coupler, an ultrawideband all-optical single-sideband filter is demonstrated.

Ultra-wide detuning planar Bragg grating fabrication technique based on direct UV grating writing with electro-optic phase modulation.

A direct UV grating writing technique based on phase-controlled interferometry is proposed and demonstrated in a silica-on-silicon platform, with a wider wavelength detuning range than any previously reported UV writing technology. Electro-optic phase modulation of one beam in the interferometer is used to manipulate the fringe pattern and thus control the parameters of the Bragg gratings and waveguides. Various grating structures with refractive index apodization, phase shifts and index contrasts of up to 0.8 × 10(-3) have been demonstrated. The method offers significant time/energy efficiency as well as simplified optical layout and fabrication process. We have shown Bragg gratings can be made from 1200 nm to 1900 nm exclusively under software control and the maximum peak grating reflectivity only decreases by 3 dBover a 250 nm (~32 THz) bandwidth.

Phase controlled integrated interferometric single-sideband filter based on planar Bragg gratings implementing photonic Hilbert transform.

The monolithically integrated all-optical single-sideband (SSB) filter based on photonic Hilbert transform and planar Bragg gratings is proposed and experimentally demonstrated. An SSB suppression of 12 dB at 6 GHz and sideband switching are achieved via thermal tuning. An X-coupler, photonic Hilbert transformer, flat top reflector, and a micro heater are incorporated in a single silicon-on-silica substrate. The device can be thermally tuned by the micro heater on top of the channel waveguide. The device is fabricated using a combination of direct UV grating writing technology and photolithography.

Hollow-bottle optical microresonators.

Selective excitation of whispering-gallery and bottle modes in a robust hollow-bottle optical microresonator, fabricated from a silica microcapillary by a pressure-compensated, "soften-and-compress" method, is demonstrated. Characteristic resonance spectra of bottle modes were obtained by using a tapered fiber coupled at different locations along the hollow bottle. The spectral characteristics (Q-factor, excitation efficiency) are shown to have high tolerance to angular misalignment of the tapered fiber. In addition, introduction of localized losses on the outer surface of the resonator results in selective clean-up of the transmission spectrum and superior performance. A theoretical analysis of modal turning points and associated resonant wavelengths is used to explain the mechanism of mode-suppression and the resultant spectral cleaning.

Whispering gallery mode spectra of channel waveguide coupled microspheres.

Evanescent coupling of light from glass channel waveguides into the whispering gallery modes of glass microspheres of radius 15?m and 100?m is studied experimentally at wavelengths near 1550 nm. Fitting the positions, widths and heights of resonances in the experimental spectra to the characteristic equation for microsphere modes and to universal coupled microresonator theory, we establish sphere radius and index, identify mode numbers, and determine losses. The results provide detailed information for the design of optical devices incorporating microsphere resonators in planar lightwave circuits.

Adaptive pulse shape control in a diode-seeded nanosecond fiber MOPA system.

We demonstrate active shaping of the driving electrical pulses to a laser diode in order to compensate for the pulse shaping effects of gain saturation in an Yb doped fiber amplifier cascade and to allow the generation of user defined customized output pulse shapes. In particular we demonstrate the generation of square output pulses, which have the potential to significantly increase the maximum pulse energy extractable from an amplifier before the peak power reaches the threshold for SRS, and for high efficiency frequency conversion.

Superstrate index control of waveguide grating reflectivity.

Control of reflectivity of a relief grating in a high-index film overlaid on a monomode glass waveguide was achieved by adjustment of the superstrate refractive index with liquids. Weak gratings reflecting at 1531 nm were inscribed by UV laser ablation. The grating reflectivity was measured as the ratio of the transmission outside the reflection band, at 1536 nm, to that at the Bragg wavelength. Reflectivity with an air superstrate was <0.3 dB for both polarizations. In the TE polarization the grating strength increased to 20.5 dB after application of a liquid of index 1.45. In the TM polarization the strength increased to 27 dB with a superstrate index of 1.50. Good agreement was found with a theoretical model based on beam propagation.

Critical role for glycosphingolipids in Niemann-Pick disease type C.

Niemann-Pick type C (NPC) disease is a cholesterol lipidosis caused by mutations in NPC1 and NPC2 gene loci. Most human cases are caused by defects in NPC1, as are the spontaneously occurring NPC diseases in mice and cats. NPC1 protein possesses a sterol-sensing domain and has been localized to vesicles that are believed to facilitate the recycling of unesterified cholesterol from late endosomes/lysosomes to the ER and Golgi. In addition to accumulating cholesterol, NPC1-deficient cells also accumulate gangliosides and other glycosphingolipids (GSLs), and neuropathological abnormalities in NPC disease closely resemble those seen in primary gangliosidoses. These findings led us to hypothesize that NPC1 may also function in GSL homeostasis. To evaluate this possibility, we treated murine and feline NPC models with N-butyldeoxynojirimycin (NB-DNJ), an inhibitor of glucosylceramide synthase, a pivotal enzyme in the early GSL synthetic pathway. Treated animals showed delayed onset of neurological dysfunction, increased average life span (in mice), and reduced ganglioside accumulation and accompanying neuropathological changes. These results are consistent with our hypothesis and with GSLs being centrally involved in the pathogenesis of NPC disease, and they suggest that drugs inhibiting GSL synthesis could have a similar ameliorating effect on the human disorder.

Neurons in Niemann-Pick disease type C accumulate gangliosides as well as unesterified cholesterol and undergo dendritic and axonal alterations.

Niemann-Pick disease type C (NPC) is a lethal neurologic storage disorder of children most often caused by a defect in the protein NPC1. To better understand the disease we thoroughly characterized the cellular and morphological alterations occurring in murine, feline, and human NPC. Using immunocytochemistry and filipin histochemistry we show that both gangliosides and unesterified cholesterol are differentially stored in neurons of the cerebral cortex, cerebellum, and hippocampus, as well as in liver. Double fluorescence labeling revealed that GM2 ganglioside and unesterified cholesterol were partially co-localized in vesicular structures, and triple fluorescence labeling utilizing a LAMP-1 antibody identified many of these organelles as part of the late endosomal/lysosomal pathway. These observations, coupled with the proposed role of NPC1 in intracellular cholesterol movement, suggest that GM3 and GM2 gangliosides as well as unesterified cholesterol may be retrogradely cleared from late endosomes/lysosomes by an NPC1-dependent mechanism. Cellular consequences of the NPC metabolic defect as shown by parvalbumin immunocytochemistry and rapid Golgi staining, respectively, revealed characteristic axonal spheroids on GABAergic neurons and ectopic dendritogenesis that followed a species-specific gradient of: mouse < feline < human. These studies suggest that the homeostatic regulation of gangliosides and cholesterol in neurons is mediated by NPC1 and that perturbations in this mechanism cause a complex neuronal storage disorder.

Distributed-feedback fiber laser sensor for simultaneous strain and temperature measurements operating in the radio-frequency domain.

Radio-frequency (rf) beat frequencies between two longitudinal modes and two polarization modes of a birefringent dual-longitudinal-mode moiré distributed-feedback fiber laser are employed to measure strain and temperature simultaneously. Operating entirely in the rf domain, this approach potentially allows one to employ low-cost and precise rf measuring techniques. A strain-temperature cross sensitivity of the strain- and the thermo-optic coefficients, which can be neglected in wavelength-based grating sensors, has been observed. The achieved sensor accuracy was +/-15 microepsilon and +/-0.2 degrees C.

Gangliosides as modulators of dendritogenesis in normal and storage disease-affected pyramidal neurons.

Pyramidal cells initiate the formation of dendritic arbors in a prolific burst of neurite outgrowth during early cortical development. Although morphologically mature pyramidal neurons do not normally sprout additional primary dendrites, the discovery of ectopic dendritogenesis in neuronal storage diseases has revealed that these cells do retain this ability under appropriate stimulation. The capacity for renewal of dendritogenesis has been found to exhibit a species gradient with human > cat, dog, sheep > mouse. A consistent metabolic feature of ectopic dendrite-bearing pyramidal neurons is a heightened intracellular expression of GM2 ganglioside. Elevated expression of this same glycosphingolipid has also been found to correlate with normal dendritogenesis. Immature neurons in developing cat and ferret cortex exhibit high levels of GM2 ganglioside immunoreactivity coincident with normal dendritic sprouting and a similar relationship has now been shown for human cortical development. Ultrastructural studies of all three species revealed GM2 localized to vesicles in a manner consistent with Golgi synthesis and exocytic trafficking to the somatic-dendritic plasmalemma. We propose that GM2 ganglioside functions in glycosphingolipid-enriched microdomains (lipid rafts) in the plasmalemma to promote dendritic initiation through modulation of specific membrane proteins and/or their associated second messenger cascades.

Generation of a 40-GHz pulse stream by pulse multiplication with a sampled fiber Bragg grating.

A sinc-sampled fiber Bragg grating is used to achieve multiplication of the repetition rate of a pulse stream from 10 to 40 GHz. The spectral characteristics of the grating ensure that the resultant pulses-solitons of 3.4-ps width-have the same individual pulse characteristics as those of the input.

40-GHz pulse-train generation at 1.5 mum with a chirped fiber grating as a frequency multiplier.

Pulse-train multiplication based on the temporal Talbot effect in a linearly chirped fiber Bragg grating has been experimentally demonstrated. A 40-GHz repetition-rate, nearly transform-limited 10-ps duration optical pulse train at 1.533 mum has been obtained from a 2.5-GHz mode-locked Er- Yb:glass laser by use of a 100-cm-long linearly chirped apodized fiber grating.

Ferret pyramidal cell dendritogenesis: changes in morphology and ganglioside expression during cortical development.

Pyramidal cell ontogenesis and basilar dendritic differentiation were evaluated concomitantly with ganglioside expression and distribution in ferret cerebral cortex. Layer V neurons began basilar dendritogenesis on postnatal day 1 (P1) with a peak in dendritic arborization occurring at P21. Layer II/III neurons, in contrast, were in early stages of basilar dendritic differentiation at P14, resulting in a complex dendritic arbor at P28. High performance thin-layer chromatography showed numerous changes in ganglioside expression during cortical development, including a decline of GM2 in the mature cortex. The temporal expression and cellular distribution of GM2, GD2, GM1, GD3, and GM3 gangliosides in developing cerebral cortex were determined by immunocytochemistry. GM2 immunoreactivity (IR) was most prominent in layer V neurons between P1 and P21 and in layer II/III neurons between P14 and P28 with staining diminishing to near absent levels in the adult. GM2-IR appeared as punctate structures within the somatodendritic domain and by electron microscopy was shown to be membrane-bound vesicles often in close proximity to the plasmalemma. Expression of GM2, but not of other gangliosides studied, followed two well-documented developmental neurogenic gradients: ventrolateral to dorsomedial and radial (inside-first outside-last). Onset of significant GD2 expression in layer II/III and V pyramidal cells was delayed until P14 and persisted in adult neocortex. GD3 was localized most prominently to glial-like cells, whereas GM1 was primarily localized to white matter. The close temporal and spatial concordance of GM2-IR in cortical pyramidal neurons undergoing dendritogenesis is consistent with its proposed role as a modulator of dendritic differentiation.