Intermodal phase and group velocity matching for frequency tripling and doubly phase matched cascaded wave mixing in high index contrast double-clad optical fibers.

Frequency conversion in glass optical fibers requires both phase and group velocity matching between the pump and the higher harmonic when working with short pulses. We show that phase and group velocities can be matched simultaneously for third order nonlinear processes, by considering that the third harmonic propagates in the higher order azimuthally symmetric LP03-mode. Moreover, the pump and frequency tripled signals can form an intermodal two-color pump to trigger a cascaded wave mixing process, which generates the second harmonic LP01-mode. This opens avenues for second harmonic generation without need for a second order nonlinearity in the optical fiber.

Mass-manufacturable scintillation-based optical fiber dosimeters for brachytherapy.

Scintillation-based fiber dosimeters are a powerful tool for minimally invasive localized real-time monitoring of the dose rate during Low Dose Rate (LDR) and High Dose Rate (HDR) brachytherapy (BT). This paper presents the design, fabrication, and characterization of such dosimeters, consisting of scintillating sensor tips attached to polymer optical fiber (POF). The sensor tips consist of inorganic scintillators, i.e. Gd2O2S:Tb for LDR-BT, and Y2O3:Eu+4YVO4:Eu for HDR-BT, dispersed in a polymer host. The shape and size of the tips are optimized using non-sequential ray tracing simulations towards maximizing the collection and coupling of the scintillation signal into the POF. They are then manufactured by means of a custom moulding process implemented on a commercial hot embossing machine, paving the way towards series production. Dosimetry experiments in water phantoms show that both the HDR-BT and LDR-BT sensors feature good consistency in the magnitude of the average photon count rate and that the photon count rate signal is not significantly affected by variations in sensor tip composition and geometry. Whilst individual calibration remains necessary, the proposed dosimeters show great potential for in-vivo dosimetry for brachytherapy.

Simultaneous modal phase and group velocity matching in microstructured optical fibers for second harmonic generation with ultrashort pulses.

Optical fibers provide a favorable medium for nonlinear optical processes owing to the small mode field size and concurrently high optical intensity combined with the extended interaction lengths. Second harmonic generation (SHG) is one of those processes that has been demonstrated in silica glass optical fibers. Since silica is centrosymmetric, generating SHG in an optical fiber requires poling of the glass. In addition and when one wants to use ultrashort pulses for SHG, achieving both phase and group velocity matching is crucial. Although fibers that feature either modal phase velocity or group velocity matching for SHG have been reported, the possibility of simultaneous modal phase and group velocity matching was never reported before. In this paper we address this challenge, and for the first time to our knowledge, we show that it is feasible to do so with silica microstructured optical fibers featuring at least one ring of air holes in the cladding and a heavily Germanium doped core (above 25 mol.%) by exploiting the LP01(ω) and LP02(2ω) modes at 1.06 µm pump and 0.53 µm second harmonic wavelengths. This finding can greatly impact applications requiring waveguide based SHG generation with ultrashort pulses, including microscopy, material characterization and nonlinear imaging.

3D direct laser writing of microstructured optical fiber tapers on single-mode fibers for mode-field conversion.

We present a design and fabrication approach for 3D printed polymer microstructured optical fiber tapers on standard single-mode glass fibers for efficient and compact mode-field conversion. This paves the way towards complex functionalized fiber tips for various applications, like sensors and beam shaping components, currently limited by the mode-field size and distribution of standard optical fibers. In this paper, we demonstrate the potential of mode-field converting tapers for relaxing the misalignment tolerance in fiber-to-fiber connections and maximizing the coupling efficiency in fiber-to-chip connections. We demonstrate a mode-field diameter expansion ratio of 1.7 and reduction ratio of 3 and show that our microstructured tapers achieve a comparable performance in coupling efficiency as their step-index counterparts, while providing greater robustness.

IR femtosecond pulsed laser-based fiber Bragg grating inscription in a photonic crystal fiber using a phase mask and a short focal length lens.

Fiber Bragg grating inscription with infrared femtosecond pulsed lasers in photonic crystal fiber is far from being trivial due to the presence of air holes in the cladding region and the non-linear nature of the absorption process inducing the required refractive index changes. We have studied this problem numerically and experimentally for a phase mask-based writing setup equipped with short focal length cylindrical lenses, which are often used for through-coating and high temperature stable grating writing. We have shown that for a cylindrical lens with a focal length f of 10 mm, the hexagonal lattice PCF needs to be translated away from the beam waist position by around 15 µm to efficiently deliver the energy to the core region. We have also investigated the importance of the PCF's angular orientation and we have shown that for some optimal positions the same behavior is observed for cylindrical lenses with different focal lengths. Finally, we have succeeded in writing a 4 dB strong grating in a photonic crystal fiber with a 1030 nm femtosecond pulsed laser in around 4 seconds, using an acylindrical lens with f = 10 mm.

Anomalous transparency in photonic crystals and its application to point-by-point grating inscription in photonic crystal fibers.

It is common belief that photonic crystals behave similarly to isotropic and transparent media only when their feature sizes are much smaller than the wavelength of light. Here, we counter that belief and we report on photonic crystals that are transparent for anomalously high normalized frequencies up to 0.9, where the crystal's feature sizes are comparable with the free space wavelength. Using traditional photonic band theory, we demonstrate that the isofrequency curves can be circular in the region above the first stop band for triangular lattice photonic crystals. In addition, by simulating how efficiently a tightly focused Gaussian beam propagates through the photonic crystal slab, we judge on the photonic crystal's transparency rather than on isotropy only. Using this approach, we identified a wide range of photonic crystal parameters that provide anomalous transparency. Our findings indicate the possibility to scale up the features of photonic crystals and to extend their operational wavelength range for applications including optical cloaking and graded index guiding. We applied our result in the domain of femtosecond laser micromachining, by demonstrating what we believe to be the first point-by-point grating inscribed in a multi-ring photonic crystal fiber.

Reflective liquid crystal hybrid beam-steerer.

We report on efficient optical beam-steering using a hot-embossed reflective blazed grating in combination with liquid crystal. A numerical simulation of the electrical switching characteristics of the liquid crystal is performed and the results are used in an FDTD optical simulator to analyze the beam deflection. The corresponding experiment on the realized device is performed and is found to be in good agreement. Beam deflection angles of 4.4° upon perpendicular incidence are found with low applied voltages of 3.4 V. By tilting the device with respect to the incoming optical beam it can be electronically switched such that the beam undergoes either total internal reflection or reflection with a tunable angle.

Numerical modeling of femtosecond laser inscribed IR gratings in photonic crystal fibers.

During grating inscription in photonic crystal fibers (PCFs) the intensity of the inscribing laser beam is non-uniformly distributed over the core region due to the interaction with the air holes in the fiber's microstructure. In this paper we model and study the non-uniformity of the index modification and its influence on the grating reflection spectra, taking into account the non-linear nature of the index change. For femtosecond laser inscription pulses at 800 nm, we show that the intensity redistribution in the PCF core region can result in Type II index changes even if the peak intensity of the incident beam is well below the corresponding threshold. Our coupled mode analysis reveals that the non-uniform nature of the index change can seriously affect the reflectivity of the grating due to a limited overlap of the guided mode with the transverse index modulation profile for almost all angular orientations of the PCFs with respect to the inscription beam. We also evaluate the influence of PCF tapering and we found that for the considered PCF a significant increase in the induced index change and reflectivity is observed only for taper diameters below 40 µm.

Geometrical study of a hexagonal lattice photonic crystal fiber for efficient femtosecond laser grating inscription.

We have studied transverse propagation of femtosecond pulse duration laser light through the microstructure of hexagonal lattice photonic crystal fibers. Our results provide insight in the role of the microstructure on the amount of optical power that reaches the core of the PCF, which is of particular importance for grating inscription applications. We developed a dedicated approach based on commercial FDTD software and defined a figure of merit, the transverse coupling efficiency, to evaluate the coupling process. We analyzed the propagation of femtosecond laser pulses to the core of a wide range of PCFs and studied the influence of the PCF orientation angle, the air hole pitch and air hole radius on the energy reaching the core. We have found that the transverse coupling efficiency can benefit from a dedicated design of the microstructured cladding and an accurate fiber orientation. We designed a dedicated PCF microstructure that enhances transverse coupling to the core at a wavelength of 800 nm.