Development of an electrically tuneable Bragg grating filter in polymer optical fibre operating at 1.55 νm

We present a thorough study on the development of a polymer optical fibre-based tuneable filter utilizing an intra-core Bragg grating that is electrically tuneable, operating at 1.55 νm. The Bragg grating is made tuneable using a thin-film resistive heater deposited on the surface of the fibre. The polymer fibre was coated via the photochemical deposition of a Pd/Cu metallic layer with the procedure induced by VUV radiation at room temperature. The resulting device, when wavelength tuned via Joule heating, underwent a wavelength shift of 2 nm for a moderate input power of 160 mW, a wavelength to input power coefficient of -13.4 pm mW-1 and time constant of 1.7 s-1. A basic theoretical study verified that for this fibre type one can treat the device as a one-dimensional system. The model was extended to include the effect of input electrical power changes on the refractive index of the fibre and subsequently to changes in the Bragg wavelength of the grating, showing excellent agreement with the experimental measurements. © 2007 IOP Publishing Ltd.

Embedded multiplexed polymer optical fiber sensor for esophageal manometry

There is a growing interest for esophageal measurements which can provide important and reliable data when diagnosing the motor function of the sphincters and the esophageal body. Biocompatibility, sensing resolution and the comfort of the patient are key parameters for manometric sensing systems. A new sensing approach which could fulfill all these needs is presented in this paper consisting of an embedded polymer fiber sensor, based on multiplexed fiber Bragg gratings. A response to a radial pressure almost 6 times that of a comparable silica fiber based sensor is obtained.

Ultra small integrated optical fiber sensing system

This paper introduces a revolutionary way to interrogate optical fiber sensors based on fiber Bragg gratings (FBGs) and to integrate the necessary driving optoelectronic components with the sensor elements. Low-cost optoelectronic chips are used to interrogate the optical fibers, creating a portable dynamic sensing system as an alternative for the traditionally bulky and expensive fiber sensor interrogation units. The possibility to embed these laser and detector chips is demonstrated resulting in an ultra thin flexible optoelectronic package of only 40 µm, provided with an integrated planar fiber pigtail. The result is a fully embedded flexible sensing system with a thickness of only 1 mm, based on a single Vertical-Cavity Surface-Emitting Laser (VCSEL), fiber sensor and photodetector chip. Temperature, strain and electrodynamic shaking tests have been performed on our system, not limited to static read-out measurements but dynamically reconstructing full spectral information datasets.

High-frequency conductivity of optically excited charge carriers in hydrogenated nanocrystalline silicon investigated by spectroscopic femtosecond pump-probe reflectivity measurements

We report an investigation into the high-frequency conductivity of optically excited charge carriers far from equilibrium with the lattice. The investigated samples consist of hydrogenated nanocrystalline silicon films grown on a thin film of silicon oxide on top of a silicon substrate. For the investigation, we used an optical femtosecond pump-probe setup to measure the reflectance change of a probe beam. The pump beam ranged between 580 and 820nm, whereas the probe wavelength spanned 770 to 810nm. The pump fluence was fixed at 0.6mJ/cm2. We show that at a fixed delay time of 300fs, the conductivity of the excited electron-hole plasma is described well by a classical conductivity model of a hot charge carrier gas found at Maxwell-Boltzmann distribution, while Fermi-Dirac statics is not suitable. This is corroborated by values retrieved from pump-probe reflectance measurements of the conductivity and its dependence on the excitation wavelength and carrier temperature. The conductivity decreases monotonically as a function of the excitation wavelength, as expected for a nondegenerate charge carrier gas.

Optical injection-locking-based pump recovery for phase-sensitively amplified links

An injection-locking-based pump recovery system for phase-sensitively amplified links is proposed and studied experimentally. Measurements with 10 Gbaud DQPSK signals show penalty-free recovery of 0.8 GHz FWHM bandwidth pump with 63 dB overall amplification. © 2012 OSA.

An optical fiber Bragg grating tactile sensor

Tactile sensors are needed for many emerging robotic and telepresence applications such as keyhole surgery and robot operation in unstructured environments. We have proposed and demonstrated a tactile sensor consisting of a fibre Bragg grating embedded in a polymer "finger". When the sensor is placed in contact with a surface and translated tangentially across it measurements on the changes in the reflectivity spectrum of the grating provide a measurement of the spatial distribution of forces perpendicular to the surface and thus, through the elasticity of the polymer material, to the surface roughness. Using a sensor fabricated from a Poly Siloxane polymer (Methyl Vinyl Silicone rubber) spherical cap 50 mm in diameter, 6 mm deep with an embedded 10 mm long Bragg grating we have characterised the first and second moment of the grating spectral response when scanned across triangular and semicircular periodic structures both with a modulation depth of 1 mm and a period of 2 mm. The results clearly distinguish the periodicity of the surface structure and the differences between the two different surface profiles. For the triangular structure a central wavelength modulation of 4 pm is observed and includes a fourth harmonic component, the spectral width is modulated by 25 pm. Although crude in comparison to human senses these results clearly shown the potential of such a sensor for tactile imaging and we expect that with further development in optimising both the grating and polymer "finger" properties a much increased sensitivity and spatial resolution is achievable.

Strain and temperature sensitivity of a singlemode polymer optical fibre

We report experimental measurements of the strain and temperature sensitivity of the optical phase in a singlemode polymer optical fibre. These values were obtained by measuring optical path length change using a Mach-Zender interferometer.

A new non-contact optical device for ocular biometry

Background: A new commercially available device (IOLMaster, Zeiss Instruments) provides high resolution non-contact measurements of axial length (using partial coherent interferometry), anterior chamber depth, and corneal radius (using image analysis). The study evaluates the validity and repeatability of these measurements and compares the findings with those obtained from instrumentation currently used in clinical practice. Method: Measurements were taken on 52 subjects (104 eyes) aged 18-40 years with a range of mean spherical refractive error from +7.0 D to -9.50 D. IOLMaster measurements of anterior chamber depth and axial length were compared with A-scan applanation ultrasonography (Storz Omega) and those for corneal radius with a Javal-Schiötz keratometer (Topcon) and an EyeSys corneal videokeratoscope. Results: Axial length: the difference between IOLMaster and ultrasound measures was insignificant (0.02 (SD 0.32) mm, p = 0.47) with no bias across the range sampled (22.40-27.99 mm). Anterior chamber depth: significantly shorter depths than ultrasound were found with the IOLMaster (-0.06 (0.25) mm, p <0.02) with no bias across the range sampled (2.85-4.40 mm). Corneal radius: IOLMaster measurements matched more closely those of the keratometer than those of the videokeratoscope (mean difference -0.03 v -0.06 mm respectively), but were more variable (95% confidence 0.13 v 0.07 mm). The repeatability of all the above IOLMaster biometric measures was found to be of a high order with no significant bias across the measurement ranges sampled. Conclusions: The validity and repeatability of measurements provided by the IOLMaster will augment future studies in ocular biometry.

Poly(methyl methacrylate) model study of optical surface quality after excimer laser photorefractive keratectomy

Purpose: To evaluate lenses produced by excimer laser ablation of poly(methyl methacrylate) (PMMA) plates. Setting: University research laboratory. Methods: Two Nidek EC-5000 scanning-slit excimer laser systems were used to ablate plane-parallel plates of PMMA. The ablated lenses were examined by focimetry, interferometry, and mechanical surface profiling. Results: The spherical optical powers of the lenses matched the expected values, but the cylindrical powers were generally lower than intended. Interferometry revealed marked irregularity in the surface of negative corrections, which often had a positive “island” at their center. Positive corrections were generally smoother. These findings were supported by the results of mechanical profiling. Contrast sensitivity measurements carried out when observing through ablated lenses whose power had been neutralized with a suitable spectacle lens of opposite sign confirmed that the surface irregularities of the ablated lenses markedly reduced contrast sensitivity over a range of spatial frequencies. Conclusion: Improvements in beam delivery systems seem desirable.

Use of dual-grating sensors formed by different types of fiber Bragg gratings for simultaneous temperature and strain measurements

We report on a systematic investigation of the dependence of both temperature and strain sensitivities on the fiber Bragg grating type, including the well-known Type I, Type IIA, and a new type that we have designated Type IA, using both hydrogen-free and hydrogenated B/Ge codoped fibres. We have identified distinct sensitivity characteristics for each grating type, and we have used them to implement a novel dual-grating, dual-parameter sensor device. Three dual-grating sensing schemes with different combinations of grating type have been constructed and compared, and that of a Type IA-Type IIA combination exhibits the best performance, which is also superior to that of previously reported grating-based structures. The characteristics of the measurement errors in such dual-grating sensor systems is also presented in detail. © 2004 Optical Society of America.

Revealing statistical properties of quasi-CW fibre lasers in bandwidth-limited measurements

We introduce a general technique how to reveal in experiments of limited electrical bandwidth which is lower than the optical bandwidth of the optical signal under study, whether the statistical properties of the light source obey Gaussian distribution or mode correlations do exist. To do that one needs to perform measurements by decreasing the measurement bandwidth. We develop a simple model of bandwidth-limited measurements and predict universal laws how intensity probability density function and intensity auto-correlation function of ideal completely stochastic source of Gaussian statistics depend on limited measurement bandwidth and measurement noise level. Results of experimental investigation are in good agreement with model predictions. In particular, we reveal partial mode correlations in the radiation of quasi-CW Raman fibre laser.

Real-time heterodyned measurements of spatio-frequency dynamics in fibre lasers

We report high-resolution real-time measurements of spectrum evolution in a fibre. The proposed method combines optical heterodyning with a technique of spatio-temporal intensity measurements revealing fast spectral dynamics of cavity-based systems.

Continuous wave ultraviolet light-induced fiber Bragg gratings in few- And single-mode microstructured polymer optical fibers

We report observations and measurements of the inscription of fiber Bragg gratings (FBGs) in two different types of microstructured polymer optical fiber: few-mode and an endlessly single mode. Contrary to the FBG inscription in silica microstructured fiber, where high-energy laser pulses are a prerequisite, we have successfully used a low-power cw laser source operating at 325 nm to produce 1 cm long gratings with a reflection peak at 1570 nm. Peak reflectivities of more than 10% have been observed. © 2005 Optical Society of America.

Optical and structural ocular changes during incipient presbyopia

The primary aim of this thesis was to investigate the in vivo ocular morphological and contractile changes occurring within the accommodative apparatus prior to the onset of presbyopia, with particular reference to ciliary muscle changes with age and the origin of a myopic shift in refraction during incipient presbyopia. Commissioned semi-automated software proved capable of extracting accurate and repeatable measurements from crystalline lens and ciliary muscle Anterior Segment Optical Coherence Tomography (AS-OCT) images and reduced the subjectivity of AS-OCT image analysis. AS-OCT was utilised to document longitudinal changes in ciliary muscle morphology within an incipient presbyopic population (n=51). A significant antero-inwards shift of ciliary muscle mass was observed after 2.5 years. Furthermore, in a subgroup study (n=20), an accommodative antero-inwards movement of ciliary muscle mass was evident. After 2.5 years, the centripetal response of the ciliary muscle significantly attenuated during accommodation, whereas the antero-posterior mobility of the ciliary muscle remained invariant. Additionally, longitudinal measurement of ocular biometry revealed a significant increase in crystalline lens thickness and a corresponding decrease in anterior chamber depth after 2.5 years (n=51). Lenticular changes appear to be determinant of changes in refraction during incipient presbyopia. During accommodation, a significant increase in crystalline lens thickness and axial length was observed, whereas anterior chamber depth decreased (n=20). The change in ocular biometry per dioptre of accommodation exerted remained invariant after 2.5 years. Cross-sectional ocular biometric data were collected to quantify accommodative axial length changes from early adulthood to advanced presbyopia (n=72). Accommodative axial length elongation significantly attenuated during presbyopia, which was consistent with a significant increase in ocular rigidity during presbyopia. The studies presented in this thesis support the Helmholtz theory of accommodation and despite the reduction in centripetal ciliary muscle contractile response with age, primarily implicate lenticular changes in the development of presbyopia.

Visual performance and optical quality with soft lenses in keratoconus patients

Purpose: To assess visual performance and ocular aberrations in keratoconic patients using toric soft contact lenses (SCL), rigid-gas-permeable (RGP) contact lenses and spectacle lens correction. Methods: Twenty-two keratoconus patients (16 RGP lens wearers and six spectacle wearers) were fitted with toric SCL. Ocular aberrations were measured with and without the patient's habitual RGP lenses and with the SCL in place. In the spectacle wearers, aberrations were measured with and without the SCL. Visual performance (high- and low-contrast visual acuity) was evaluated with the patient's habitual correction and with the SCL. Results: In the RGP lens wearers both the habitual lenses and the toric SCL significantly reduced coma, trefoil, 3rd-order, 4th-order cylinder and higher-order root-mean-square (RMS) aberrations (p≤0.015). In the spectacle wearers the toric SCL significantly reduced coma, 3rd-order and higher-order RMS aberrations (p≤0.01). The patients' habitual RGP lenses gave better low-contrast acuity (p≤0.006) compared to the toric SCL; however, no significant difference was found between lens types for high-contrast acuity (p=0.10). In the spectacle wearers no significant differences in visual performance measurements were found between the patients' spectacles and the toric SCL (p≥0.06). Conclusion: The results show that RGP lenses provided superior visual performances and greater reduction of 3rd-order aberrations compared to toric SCL in this group of keratoconic patients. In the spectacle-wearing group, visual performance with the toric SCL was found to be comparable to that measured with spectacles. Nevertheless, with the exception of spherical aberration, the toric SCL were successful in significantly reducing uncorrected higher-order aberrations. Ophthalmic & Physiological Optics © 2012 The College of Optometrists.