Microstructured waveguides in z-cut LiNbO3 by high-repetition rate direct femtosecond laser inscription

We report on the operational parameters that are required to fabricate buried, microstructured waveguides in a z-cut lithium niobate crystal by the method of direct femtosecond laser inscription using a highrepetition-rate, chirped-pulse oscillator system. Refractive index contrasts as high as −0.0127 have been achieved for individual modification tracks. The results pave the way for developing microstructured WGs with low-loss operation across a wide spectral range, extending into the mid-infrared region up to the end of the transparency range of the host material.

Design and fabrication of micro-structured waveguides in lithium niobate

We review our recent work on the numerical design and optimisation of buried, micro-structured waveguides (WGs) that can be formed in a lithium niobate (LiNbO3) crystal by the method of direct femtosecond laser inscription. We also report on the possibility of fabricating such WGs using a high-repetition-rate, chirped-pulse oscillator system. Refractive index contrasts as high as -0.0127 have been achieved for individual modification tracks. The results pave the way for developing micro-structured WGs with low-loss operation across a wide spectral range, extending into the mid-infrared region up to the end of the transparency range of the host material. © 2014 IEEE.

Control of the properties of micro-structured waveguides in LiNbO3 fabricated by direct femtosecond laser inscription

This paper reports on buried waveguides fabricated in lithium niobate (LN) by the method of direct femtosecond (fs) laser inscription. 5% MgO doped LiNbO3 was chosen as the host material because of its high quality and damage threshold, as well as relatively low cost. Direct fs inscription by astigmatically shaped beam in crystals usually produces multiple 'smooth' tracks (with reduced refractive index), which encircle the light guiding 'core', thus creating a depressed cladding WG. A high-repetition rate fs laser system was used for inscription at a depth of approximately 500 μm. Using numerical modelling, it was demonstrated that the properties of fs-written WGs can be controlled by the WG geometry. Buried, depressed-cladding WGs in LN host with circular cross-section were also demonstrated. Combining control over the WG dispersion with quasi-phase matching will allow various ultralow-pump-power, highly-efficient, nonlinear light-guiding devices - all in an integrated optics format.

Refractometer based on a liquid core FBG device

A 1.2X500μm slot was engraved across a fiber Bragg grating (FBG) using femtosecond laser patterning and chemical etching. liquid core FBGs were constructed and their sensitivity to refractive index of up to 10-6/pm was measured.

Biochemical sensor based on a novel all-fibre cavity ring down spectroscopy technique incorporating a tilted fibre Bragg grating

A novel all-fibre cavity ring down spectroscopy technique is demonstrated where a tilted fibre Bragg grating in the cavity provides sensitivity to surrounding refractive index. A decay time of 450ns was attained when sensing water.

Plasma assisted femtosecond laser inscription in dielectrics

Principles of the femtosecond fabrication of the optoelectronic components in glass are explained and illustrated by examples of the in-bulk writing. The results of the experimental investigation of the dependence of the induced index change on the pulse energy and the numerical modelling of the corresponding laser-glass interaction are presented. The distribution of the plasma density is simulated that may bridge the gap between the models of the pulse propagation and the induced permanent refractive index change. © 2006 American Institute of Physics.

Respiratory monitoring using fibre long period grating sensors

We demonstrate the use of a series of in-line fibre long period grating curvature sensors on a garment, used to monitor the thoracic and abdominal volumetric tidal movements of a human subject. These results are used to obtain volumetric tidal changes of the human torso showing reasonable agreement with a spirometer used simultaneously to record the volume at the mouth during breathing. The curvature sensors are based upon long period gratings written in a progressive three layered fibre that are insensitive to refractive index changes. The sensor platform consists of the long period grating laid upon a carbon fibre ribbon, which is encapsulated in a low temperature curing silicone rubber.

The application of a long period grating array for respiratory function monitoring

In this paper, multiplexed sensor network capable of monitoring the shape changes of the torso for respiratory function monitoring is developed. As a demonstration, LPGs written into refractive index insensitive, progressive three layered fibre are embedded into supporting material is then placed on a resuscitation training manikin simulating respiration. A derivative spectroscopy interrogation technique is implemented and the bend sensitivity of the LPGs is used to reconstruct the shape of the manikin's torso. © 2003 IEEE.

Optical biochemical sensors based on long-period fibre gratings UV-inscribed in D-fibre with enhanced sensitivity by HF etching process

We report a new concept of biochemical sensor device based on long-period grating structures UV-inscribed in D-fiber. The surrounding-medium refractive index sensitivity of the devices has been enhanced significantly by a hydrofluoric acid etching process. The devices have been used to measure the sugar concentrations showing clearly an encoding relation between the chemical concentration and the grating spectral response, demonstrating their capability for potential biochemical sensing applications.

Supermode-noise suppression using a nonlinear Fabry–Pérot filter in a harmonically mode-locked fiber ring laser

A simple efficient method for stabilizing a harmonically mode-locked fiber ring laser is proposed. In this method, a linear optical filter and a nonlinear Fabry–Pérot filter in which the refractive index is optical intensity dependent are located in the laser cavity. The linear filter is used to select a fixed lasing wavelength, and the Fabry–Pérot filter introduces a negative all-optical feedback mechanism that is able to suppress pulse-to-pulse amplitude fluctuations in the laser cavity. The scheme was experimentally demonstrated using a fiber Bragg grating as the linear filter and a laser diode biased below threshold as the nonlinear Fabry–Pérot, and stable harmonically mode-locked pulses with a supermode noise suppression ratio >55 dB were obtained.

A high sensitivity long period grating Mach-Zehnder refractometer

A refractive index sensing system has been demonstrated, which is based upon an in-line fibre long period grating Mach-Zehnder interferometer with a heterodyne interrogation technique. This sensing system has comparable accuracy to laboratory-based techniques used in industry such as high performance liquid chromatography and UV spectroscopy. The advantage of this system is that measurements can be made in-situ for applications in continuous process control. Compared to other refractive index sensing schemes using LPGs, this approach has two main advantages. Firstly, the system relies on a simple optical interrogation system and therefore has the real potential for being low cost, and secondly, so far as we are aware it provides the highest refractive index resolution reported for any fibre LPG device.

Detection of organic aromatic compounds in paraffin by a long-period fiber grating optical sensor with optimized sensitivity

A long-period grating (LPG) sensor is used to detect small variations in the concentration of an organic aromatic compound (xylene) in a paraffin (heptane) solution. A new design procedure is adopted and demonstrated to maximize the sensitivity of LPG (wavelength shift for a change in the surrounding refractive index, (dλ/dn3)) for a given application. The detection method adopted is comparable to the standard technique used in industry (high performance liquid chromatograph and UV spectroscopy) which has a relative accuracy between ∼±0.5% and 5%. The minimum detectable change in volumetric concentration is 0.04% in a binary fluid with the detection system presented. This change of concentration relates to a change in refractive index of Δn ∼ 6 × 10-5. © 2001 Elsevier Science B.V.

Electrografting of poly (carbazole-co-acrylamide) onto highly oriented pyroltic graphite. A cyclovoltammetric, atomic force microscopic and ellipsometric study

The electrocopolymerization of carbazole and acrylamide on highly oriented pyrolytic graphite (HOPG) from ACN solutions via cyclovoltammetry (CV) was studied in order to evaluate the possibility to deposit uniform and thin but pinhole-free and still reactive coatings onto graphite-like substrates. The morphology of the coatings was investigated using atomic force microscopy and the coating thicknesses and optical parameters were measured using ellipsometry. It was found that under the chosen conditions thin (coating thickness hf>180 nm) and relatively smooth (root mean square surface roughness RMS<150 nm) P(Cz-co-AAm)-coatings exhibiting a uniform globuoidal morphology can be deposited onto graphite. From a certain coating thickness (hf>50 nm) no pinholes could be detected. It was found that the thickness of the deposited coatings increases almost linearly with increasing number of CV-cycles while keeping all other experimental parameters (scan rate and comonomer concentration ratio) constant. No influence of the comonomer concentration ratio on the film thickness and coating appearance could be observed, however, at quite low initial concentrations. However, the CV-scanning rate has quite a significant influence on the thickness of the deposited coatings. Higher scan rates (100 mV/s) result in thin (hf≈22 nm) coatings whereas at lower scan rates (<50 mV/s) coatings with thicknesses of approximately 50 nm were obtained. The optical coating parameters (the refractive index n and extinction coefficient k) seem to be independent of the deposition parameters and therefore averaged values of n̄=1.54±0.03 and k̄=0.08±0.03 were obtained.

Polymer optical fiber grating as water activity sensor

Controlling the water content within a product has long been required in the chemical processing, agriculture, food storage, paper manufacturing, semiconductor, pharmaceutical and fuel industries. The limitations of water content measurement as an indicator of safety and quality are attributed to differences in the strength with which water associates with other components in the product. Water activity indicates how tightly water is "bound," structurally or chemically, in products. Water absorption introduces changes in the volume and refractive index of poly(methyl methacrylate) PMMA. Therefore for a grating made in PMMA based optical fiber, its wavelength is an indicator of water absorption and PMMA thus can be used as a water activity sensor. In this work we have investigated the performance of a PMMA based optical fiber grating as a water activity sensor in sugar solution, saline solution and Jet A-1 aviation fuel. Samples of sugar solution with sugar concentration from 0 to 8%, saline solution with concentration from 0 to 22%, and dried (10ppm), ambient (39ppm) and wet (68ppm) aviation fuels were used in experiments. The corresponding water activities are measured as 1.0 to 0.99 for sugar solution, 1.0 to 0.86 for saline solution, and 0.15, 0.57 and 1.0 for the aviation fuel samples. The water content in the measured samples ranges from 100% (pure water) to 10 ppm (dried aviation fuel). The PMMA based optical fiber grating exhibits good sensitivity and consistent response, and Bragg wavelength shifts as large as 3.4 nm when the sensor is transferred from dry fuel to wet fuel. © 2014 Copyright SPIE.

Graphene-assisted microfiber for optical-power-based temperature sensor

Combined the large evanescent field of microfiber with the high thermal conductivity of graphene, a sensitive all-fiber temperature sensor based on graphene-assisted micro fiber is proposed and experimentally demonstrated. Microfiber can be easily attached with graphene due to the electrostatic 6 force, resulting in an effective interaction between graphene and the evanescent field of microfiber. The change of the ambient temperature has a great influence on the conductivity of graphene, leading to the variation of the effective refractive index of microfiber. Consequently, the optical power transmission will be changed. The temperature sensitivity of 0.1018 dB/°C in the heating process and 0.1052 dB/°C in the cooling process as well as a high resolution of 0.0098 °C is obtained in the experiment. The scheme may have great potential in sensing fields owing to the advantages of high sensitivity, compact size, and low cost.