Optical penetration models for practical prediction of femtosecond laser ablation of dental hard tissue.

OBJECTIVES: To develop and practically test high-precision femtosecond laser ablation models for dental hard tissue that are useful for detailed planning of automated laser dental restorative treatment. METHODS: Analytical models are proposed, derived, and demonstrated for practical calculation of ablation rates, ablation efficiency and ablated morphology of human dental enamel and dentin using femtosecond lasers. The models assume an effective optical attenuation coefficient for the irradiated material. To achieve ablation, it is necessary for the local energy density of the attenuated pulse in the hard tissue to surpass a predefined threshold that signifies the minimum energy density required for material ionization. A 1029 nm, 40 W carbide 275 fs laser was used to ablate sliced adult human teeth and generate the data necessary for testing the models. The volume of material removed, and the shape of the ablated channel were measured using optical profilometry. RESULTS: The models fit with the measured ablation efficiency curve against laser fluence for both enamel and dentin, correctly capturing the fluence for optimum ablation and the volume of ablated material per pulse. The detailed shapes of a 400-micrometer wide channel and a single-pulse width channel are accurately predicted using the superposition of the analytical result for a single pulse. CONCLUSIONS: The findings have value for planning automated dental restorative treatment using femtosecond lasers. The measurements and analysis give estimates of the optical properties of enamel and dentin irradiated with an infrared femtosecond laser at above-threshold fluence and the proposed models give insight into the physics of femtosecond laser processing of dental hard tissue.

Investigation of laser wavelength effect on the ablation of enamel and dentin using femtosecond laser pulses.

We investigated the effect of femtosecond (fs) laser ablation of enamel and dentin for different pulse wavelengths: infrared (1030 nm), green (515 nm), and ultra-violet (343 nm) and for different pulse separations to determine the optimal irradiation conditions for the precise removal of dental hard tissues with the absence of structural and compositional damage. The ablation rates and efficiencies were established for all three laser wavelengths for both enamel and dentin at room temperature without using any irrigation or cooling system, and the surfaces were assessed with optical and scanning electron microscopy, optical profilometry, and Raman spectroscopy. We demonstrated that 515 nm fs irradiation provides the highest rate and efficiency for ablation, followed by infrared. Finally, we explored the temperature variations inside the dental pulp during the laser procedures for all three wavelengths and showed that the maximum increase at the optimum conditions for both infrared and green irradiations was 5.5 °C, within the acceptable limit of temperature increase during conventional dental treatments. Ultra-violet irradiation significantly increased the internal temperature of the teeth, well above the acceptable limit, and caused severe damage to tooth structures. Thus, ultra-violet is not a compatible laser wavelength for femtosecond teeth ablation.

Femtosecond laser dentistry for precise and efficient cavity preparation in teeth.

High fluence focused femtosecond laser pulses were used to perform fast, high precision and minimally damaging cavity cutting of teeth at room temperature without using any irrigation or cooling system. The optimal ablation rates were established for both enamel and dentin, and the surfaces were assessed with optical and scanning electron microscopy, Raman spectroscopy and optical profilometry. No chemical change in the composition of enamel and dentin was observed. We explored temperature variations inside the dental pulp during the laser procedure and showed the maximum increase was 5.5°C, within the acceptable limit of temperature increase during conventional dental treatments.

Formation of nanochannels in sapphire with ultrashort Bessel pulses.

We explore, both by numerical simulations and experimentally, the flexibility in controlling Bessel beam parameters by re-imaging it into transparent material with a demagnifying collimator for the formation of high-aspect ratio nanochannels. Analysis of nanochannels produced by in-house precision-made axicon with 275 fs pulses in sapphire reveals the intensity threshold of ∼7.2 × 1013 W/cm2 required to create the cylindrical microexplosion. We estimate that the maximum applied pressure during the process was 1.5 TPa and that the resulting density of compressed sapphire in the nanochannel's shells are ∼1.19 ± 0.02 times higher than the pristine crystal, and higher than what was achieved before in spherical microexplosion with Gaussian pulses.

Real-time change of optical losses in chalcogenide waveguides induced by light illumination.

We prepared several GeGaSe waveguides with different chemical compositions and measured the change of optical losses induced by light illumination. Together with some experimental data in As2S3 and GeAsSe waveguides, the results showed that maximum change of the optical loss can be observed in the waveguides under bandgap light illumination. The chalcogenide waveguides with close to stoichiometric compositions have less homopolar bonds and less sub-bandgap states, and thus are preferential to have less photoinduced losses.

Designing absorbers for graphene based mid-infrared wide band waveguide photodetectors.

The mid-infrared (MIR) spectral region is of great importance in scientific and real-world applications ranging from detecting forming planets to identifying molecular species for industrial process control. Existing instrumentation to perform analyses is neither low cost nor compact, robust, or low power consumption, presenting opportunities for a planar integrated MIR sensing device to cost effectively detect and extract information on a widespread scale and in handheld devices. A key missing element in this vision is low cost waveguide photodetectors, which can cover the necessary wavelength range and are made with a wafer scale process. Graphene based detectors could fill this void. A parametric study is presented on broadband light absorption in graphene on waveguide devices of varied designs, index contrasts and dimensions. Generic design information is provided, and Genetic Annealing algorithms combined with Finite Element modal analysis provide a shortest design of 121 µm long that absorbs >90% of light from 1 to 10 µm, and a wide range of designs under 500 µm long. This shows for the first time that 2-D material based broadband waveguide MIR photodetectors could be viably integrated in MIR planar optics devices.

Greater than 50% inversion in Erbium doped Chalcogenide waveguides.

We report Er-doped Ge-Ga-Se films and waveguides deposited using co-thermal evaporation and patterned with plasma etching. Strong photoluminescence at 1.54 µm with intrinsic lifetime of 1 ms was obtained from deposited films with 1490 nm excitation. Erbium population inversion up to 50% was achieved, with a maximum of ~55% possible at saturation for the first time to the author's knowledge, approaching the theoretical maximum of 65%. Whilst gain was not achieved due to the presence of upconversion pumped photoinduced absorption, this nonetheless represents a further important step towards the realization of future chalcogenide Erbium doped waveguide amplifiers at 1550 nm and in the Mid-infrared.

Experimental demonstration of linearly polarized 2-10 µm supercontinuum generation in a chalcogenide rib waveguide.

This Letter reports the production of a supercontinuum extending from ≈2 µm to >10 µm generated using a chalcogenide buried rib waveguide pumped with 330 femtosecond pulses at 4.184 µm. This is, to the best of our knowledge, the broadest mid-infrared supercontinuum generated in any planar waveguide platform. Because the waveguide is birefringent, quasi-single-mode, and uses an optimized dispersion design, the supercontinuum is linearly polarized with an extinction ratio >100. Dual beam spectrophotometry is performed easily using this source.

High Q factor chalcogenide ring resonators for cavity-enhanced MIR spectroscopic sensing.

We report the characteristics of high Q factor chalcogenide ring resonators designed for sensing in the mid-infrared (MIR). The resonators consisted of an exposed Ge11.5As24Se64.5 core on a Ge11.5As24S64.5 bottom cladding and were fabricated in the racetrack coupled ring structure. Loaded Q factors at 5.2µm up to 58,000were obtained, corresponding to an intrinsic Q of 145,000 and a waveguide propagation loss of 0.84dB/cm.

980nm pumped erbium doped tellurium oxide planar rib waveguide laser and amplifier with gain in S, C and L band.

A thin film based erbium doped tellurium oxide (TeO2) waveguide amplifier producing gain from 1500nm to 1640nm when pumped at 980nm is demonstrated. At measured internal gains exceeding 14dB lasing due to end facet reflection set in producing the first tellurite waveguide laser. High gains were observed despite significant upconversion, whose impact appears to be mitigated to some extent by residual OH contamination. The device displayed no photosensitive effects from either the high pumping intensities used or the intracavity intensity at 1550nm.

1.8-10 µm mid-infrared supercontinuum generated in a step-index chalcogenide fiber using low peak pump power.

By pumping an 11-cm-long step-index chalcogenide fiber with ∼330 fs pulses at 4.0 µm from an optical parametric amplifier, mid-infrared supercontinuum generation spanning from ∼1.8 to ∼10 µm within a dynamic range of ±15 dB has been demonstrated at a relatively low power threshold of ∼3000 W.

Internal gain in Er-doped As2S3 chalcogenide planar waveguides.

Low-loss erbium-doped As2S3 planar waveguides are fabricated by cothermal evaporation and plasma etching. Internal gain in the telecommunications band is demonstrated for the first time in any chalcogenide glass and additionally in a thin film planar waveguide amplifier configuration.

Tunable wideband microwave photonic phase shifter using on-chip stimulated Brillouin scattering.

We present the first microwave photonic phase shifter using stimulated Brillouin scattering (SBS) on-chip. The unique ability of SBS to generate both narrowband gain and loss resonances allows us to achieve low ±1.5 dB amplitude fluctuations, which is a record for integrated devices, along with 240° continuously tunable phase shift. Contrary to previous SBS-based approaches, the phase shift tuning mechanism relies on tuning the power, not the frequency, of two SBS pumps, making it more suited to on-chip implementations. We finally demonstrate that SBS pump depletion leads to amplitude response fluctuations, as well as increasing the insertion loss of the phase shifter. Advantageously, shorter integrated platforms possess higher pump depletion thresholds compared to long fibers, thus offering greater potential for reducing the insertion loss.

Hybrid waveguide from As2S3 and Er-doped TeO2 for lossless nonlinear optics.

The fabrication and characterization of loss-compensated dispersion-engineered nonlinear As(2)S(3) on Er:TeO2 waveguides is reported for the first time, to the best of our knowledge. The hybrid waveguide is a strip loaded structure made from an Er-doped TeO2 slab and an etched As(2)S(3) strip. Almost complete loss compensation is demonstrated with 1480 nm pumping and a fully lossless waveguide with high nonlinear coefficient can be achieved with higher 1480 nm pump power.

Phase-sensitive amplification of light in a χ(3) photonic chip using a dispersion engineered chalcogenide ridge waveguide.

We report phase-sensitive amplification of light using χ((3)) parametric processes in a chalcogenide ridge waveguide. By spectrally slicing pump, signal and idler waves from a single pulsed source, we are able to observe 9.9 dB of on-chip phase-sensitive extinction with a signal-degenerate dual pump four-wave mixing architecture in good agreement with numerical simulations.

Automatic DGD and GVD compensation at 640 Gb/s based on scalar radio-frequency spectrum measurement.

We demonstrate what we believe to be the first real-time impairment-cancellation system for group-velocity dispersion (GVD) and differential group delay (DGD) for a 640 Gb/s single-channel signal. Simultaneous compensation of two independent parameters is demonstrated by feedback control of separate GVD and DGD compensators using an impairment monitor based on an integrated all-optical radio-frequency (RF) spectrum analyzer. We show that low-bandwidth measurement of only a single tone in the RF spectrum is sufficient for automatic compensation for multiple degrees of freedom using a multivariate optimization scheme.

Photonic-chip-based all-optical ultra-wideband pulse generation via XPM and birefringence in a chalcogenide waveguide.

We report a photonic-chip-based scheme for all-optical ultra-wideband (UWB) pulse generation using a novel all-optical differentiator that exploits cross-phase modulation and birefringence in an As2S3 chalcogenide rib waveguide. Polarity-switchable UWB monocycles and doublets were simultaneously obtained with single optical carrier operation. Moreover, transmission over 40-km fiber of the generated UWB doublets is demonstrated with good dispersion tolerance. These results indicate that the proposed approach has potential applications in multi-shape, multi-modulation and long-distance UWB-over-fiber communication systems.

Observation of Brillouin dynamic grating in a photonic chip.

We report demonstration of a Brillouin dynamic grating (BDG) in a photonic chip. A BDG was characterized in a 6.5 cm long chalcogenide (As(2)S(3)) rib waveguide using CW pumps in x polarization and read using a CW probe in y polarization. The measured reflectivity, on-off ratio, and 3 dB bandwidth (f(3 dB)) for the BDG were 0.4%, ~28 dB, and ~6 GHz, respectively.

Low-loss chalcogenide waveguides for chemical sensing in the mid-infrared.

We report the characteristics of low-loss chalcogenide waveguides for sensing in the mid-infrared (MIR). The waveguides consisted of a Ge11.5As24Se64.5 rib waveguide core with a 10nm fluoropolymer coating on a Ge11.5As24S64.5 bottom cladding and were fabricated by thermal evaporation, photolithography and ICP plasma etching. Over most of the functional group band from 1500 to 4000 cm⁻¹ the losses were < 1 dB/cm with a minimum of 0.3 dB/cm at 2000 cm⁻¹. The basic capabilities of these waveguides for spectroscopy were demonstrated by measuring the absorption spectrum of soluble Prussian blue in Dimethyl Sulphoxide.

Supercontinuum generation in the mid-infrared from a dispersion-engineered As2S3 glass rib waveguide.

We report the generation of a mid-infrared supercontinuum created by ≈7.5 ps duration pulses at 3260 nm passing through a dispersion engineered As(2)S(3) rib waveguide. The threshold for a 6.6 cm long waveguide was around 800 W and at 1700 W the spectrum extended from ≈2.9-4.2 µm and was limited on the long wavelength side by absorption in the cladding of this particular waveguide.