High-repetition-rate femtosecond-laser micromachining of low-loss optical-lattice-like waveguides in lithium niobate

A series of waveguides were inscribed in lithium niobate by tightly focused femtosecond-laser pulses of 11-MHz repetition rate and 790-nm wavelength. To establish the inscription conditions for optimal low-loss waveguides, within each sample we varied laser pulse energy, speed and direction of translation stage movement, and focus depth of the beam. We deployed two new approaches to enhance the inscription results: 1) increase of the pulse energy with increasing focus depth inside the material to compensate for the corresponding decrease of refractive-index modification, and 2) decrease of the laser energy for the modification tracks closer to the waveguide’s core region to reduce scattering losses due to high-laser-energy driven non-uniformities. All waveguides had an optical-lattice-like hexagonal packing geometry with track-spacing of 9.9 μm (optimized for effective suppression of high-order modes). Each structure comprised 84 single-scan Type-II-modification tracks, aligned with the crystalline X-axis of lithium niobate. After heat treatment at 350 °C for 3 hours, the lowest propagation loss of less than (0.4±0.1) dB/cm and (3.5±0.3) dB/cm for the ordinary and extraordinary light polarization states, respectively, were achieved at the 1550- nm wavelength. These low-attenuation waveguides were obtained with the inscription energy varying between 50.6 nJ and 53.6 nJ and the translation speed of 10 mm/s. The corresponding refractive-index contrast of individual tracks was (–1.55±0.04)×10-3 . The waveguides also showed low attenuation in the visible and near-infrared portion of the spectrum (532 nm to 1456 nm). Our results offer promising means for the development of low-loss waveguides with preserved-nonlinearity and high thermal stability

High-repetition-rate femtosecond-laser inscription of low-loss thermally stable waveguides in lithium niobate

Optical-lattice-like WGs were fabricated in z-cut LiNbO3 by HRR pulse laser. Low propagation loss was observed in both orthogonal polarizations in the visible and near-IR spectrum. Single-mode guiding was maintained after high-temperature annealing

Femtosecond laser inscription of optical waveguide-based devices on lithium niobate

This work explored the use of high-repetition-rate femtosecond laser pulses for the direct writing of optical waveguide-based devices in a sample of z-cut lithium niobate crystal. The effects of inscribing parameters including pulse energy, writing speed, writing direction and focus depth on the optical and physical properties of a laser-induced straight structure were revealed, and systematically investigated for the optimal regime of low-loss waveguide fabrication. Also, the impacts of optical aberrations due to refractive index mismatch and birefringent astigmatism on the laser focus were numerically studied and discussed with the results obtained from the experiments. To test the thermal-dependent characteristics of an inscribed sample, a series of heat treatments in a temperature range between 250 ºC and 950 ºC was applied. It was found that the heats up to the temperature of 500 ºC enhanced the overall refractive index contrast of a Type-II laser modification, and also reduced the residual stress. For the temperatures greater than 500 ºC, the annealing resulted in the deterioration of an inscribed structure. In the part of straight waveguide fabrication, an optical-lattice-like geometry which consisted of multiple damaged tracks arranged in a multi-layer hexagonal packing was implemented for writing the depressed-cladding waveguide with various sets of inscribing parameters. To address the laser-focusing issue stemmed from the spherical aberration effects, the pulse-energy variation schemes were applied – resulting in more circular and symmetric optical mode-fields. The lowest propagation losses of (0.4 ± 0.1) dB/cm and (3.5 ± 0.2) dB/cm for TE and TM polarised light, respectively, at 1550 nm were achieved after thermally annealed at 350 ºC for 3 hours. In addition, the waveguides were found to be thermally stable and showed the low-loss guiding up to the temperature of 700 ºC. The fundamental guiding mode was observed over a wide range of spectral from 500 nm to 1550 nm. Our laser inscription technique was also applied to fabricate the s-bend structures and power splitters which were based on the multi-mode interference. The computer simulation in COMSOL was used to optimize the interference pattern inside the waveguides such that the high intensity transmission could be obtained. The lowest insertion losses in TE mode of (4.23 ± 0.14) dB and (4.31 ± 0.20) dB for the two-output and three-output splitters, respectively, at 1550 nm were measured after the sample was annealed at 250 ºC for 3 hours. Besides, the integration of s-bends and the splitter to allow the wider separation (293.7 μm) between two splitter’s output branches was demonstrated. The insertion loss of this structure was found to be (4.96 ± 0.17) dB, and the splitting ratio of 0.48:0.52 was achieved for the TE propagation mode

Low-loss optical-lattice-like waveguides in lithium niobate by high-repetition-rate femtosecond laser inscription

Tightly focused femtosecond laser pulses of 11-MHz repetition rate and 790-nm wavelength were used to fabricate a set of waveguides in a lithium niobate (LiNbO3) crystal. To establish the inscription conditions for optimal low-loss waveguides, within each sample we varied the laser pulse energy, the speed and direction of translation stage movement, and the focus depth of the laser beam. We deployed two new approaches to enhance the inscription results: 1) increase of the laser pulse energy with increasing focus depth inside the material to compensate for the corresponding decrease of the refractive-index contrast between exposed and unexposed areas, and 2) decrease of the laser energy for the modification tracks closer to the waveguide’s core region to reduce the scattering losses due to high-laser-energy driven waveguide’s non-uniformities. All waveguides had an optical-lattice-like hexagonal packing geometry with a track spacing of 10 μm (optimised for effective suppression of high-order modes). Each structure comprised 84 single-scan Type-II-modification tracks, aligned with the crystalline X-axis of the LiNbO3 wafer. As observed through the optical microscope, the diameters of core and outer cladding of the waveguide were approximately 24 μm and 95 μm, respectively. After thermal annealing at 623K for 3 hours, the minimum attenuation coefficients of (0.4±0.1)dB/cm and (3.5±0.3)dB/cm for the ordinary and extraordinary light polarisation states, respectively, were achieved at the 1550-nm wavelength. These low-attenuation waveguides were obtained with an inscription energy varying between 50.6 nJ and 53.6 nJ and a translation speed of 10 mm/s. The corresponding refractive-index contrast of individual tracks was −(1.55±0.04)×10-3. The waveguides also showed low attenuation in the visible and near-infrared portion of the spectrum (from 532 nm to 1456 nm). Our results offer promising means for the development of low-loss, preserved-nonlinearity waveguides that are suitable for several applications, including telecommunications, nonlinear integrated optics and quantum optics

Differences in the subunit interface residues of alternatively spliced glutathione transferases affects catalytic and structural functions

GSTs (glutathione transferases) are multifunctional widespread enzymes. Currently there are 13 identified classes within this family. Previously most structural characterization has been reported for mammalian Alpha, Mu and Pi class GSTs. In the present study we characterize two enzymes from the insect-specific Delta class, adGSTD3-3 and adGSTD4-4. These two proteins are alternatively spliced products from the same gene and have very similar tertiary structures. Several major contributions to the dimer interface area can be separated into three regions: conserved electrostatic interactions in region 1, hydrophobic interactions in region 2 and an ionic network in region 3. The four amino acid side chains studied in region 1 interact with each other as a planar rectangle. These interactions are highly conserved among the GST classes, Delta, Sigma and Theta. The hydrophobic residues in region 2 are not only subunit interface residues but also active site residues. Overall these three regions provide important contributions to stabilization and folding of the protein. In addition, decreases in yield as well as catalytic activity changes, suggest that the mutations in these regions can disrupt the active site conformation which decreases binding affinity, alters kinetic constants and alters substrate specificity. Several of these residues have only a slight effect on the initial folding of each subunit but have more influence on the dimerization process as well as impacting upon appropriate active site conformation. The results also suggest that even splicing products from the same gene may have specific features in the subunit interface area that would preclude heterodimerization

Characterization of the impact of alternative splicing on protein dynamics: The cases of glutathione S-transferase and ectodysplasin-A isoforms

Recent studies have shown how alternative splicing (AS), the process by which eukaryotic genes express more than one product, affects protein sequence and structure. However, little information is available on the impact of AS on protein dynamics, a property fundamental for protein function. In this work, we have addressed this issue using molecular dynamics simulations of the isoforms of two model proteins: glutathione S-transferase and ectodysplasin-A. We have found that AS does not have a noticeable impact on global or local structure fluctuations. We have also found that, quite interestingly, AS has a significant effect on the coupling between key structural elements such as surface cavities. Our results provide the first atom-level view of the impact of AS on protein dynamics, as far as we know. They can contribute to refine our present view of the relationship between AS and protein disorder and, more importantly, they reveal how AS may modify structural dynamic couplings in proteins. © 2012 Wiley Periodicals, Inc.Grant sponsor: Spanish Ministerio de Educacio´n y Ciencia; Grant number: BIO2006-15557; Grant sponsor: Spanish Ministerio de Ciencia e Innovación; Grant number: BFU2009-11527; Grant sponsor: Consolider E-Science; Grant number: BIO2009-10964; Grant sponsor: Consejo Superior de Investigaciones Científicas (CSIC); Grant number: 200420E578; Grant sponsor: Spanish Red de Supercomputación; Grant numbers: BCV-2008-1-0012, BCV-2009-1-0003; Grant sponsor: Ministerio de Sanidad y Consumo, Fondo de Investigación Sanitaria, Spain; Grant number: CD08/00241; Grant sponsor: European Union (Scalalife project), Fundación Marcelino Botin.Peer Reviewe