Evanescent Field Coupling Research Articles

Tunable photonic crystal fiber coupler based on a side-polishing technique.

A tunable photonic crystal fiber (PCF) coupler, which couples part of the optical power in one PCF with that in another PCF, has been made by side polishing. We fabricated the PCF coupler by mating two side-polished PCFs. We achieved evanescent field coupling between the core modes of the two PCFs by using side polishing to bring the cores close to each other. By adjusting the mating angle between the two side-polished PCFs we obtained as much as 90% tunability in the coupling ratio. The spectrum of the coupling ratio was almost flat, with small ripples, over a 400-nm wavelength range.

Controllable in-line UV sensor using a side-polished fiber coupler with photofunctional polymer

A novel ultraviolet (UV) sensor using evanescent field coupling between single-mode-polished fiber and photochromic dye-dispersed polymer waveguide was demonstrated. It was found that resonance wavelength shifts occurred due to variations in the refractive index of the photochromic dye-dispersed polymer waveguide relative to UV intensities. A spiroxazine dye was chosen as the overlay material because of its photochromic isomerization induced by UV irradiation. The wavelength responses of these sensors by UV exposure power were measured 0.18 and 0.25 nm/mW when UV was exposed for 5 and 10 s, respectively. The recovery time for the sensor was independent of UV exposure power, and 80% recovery time was shown 90 s.

광기능성 폴리머의 소산장 결합을 이용한 측면연마 광섬유형 자외선 센서

측면연마된 광섬유와 광기능성 색소가 분산된 평면도파로 결합기를 이용한 새로운 형태의 자외선 센서를 제작하여 그 특성을 측정하였다. 자외선 조사에 의해 유효굴절률이 변화하는 광변색성 색소가 분산된 폴리머를 평면 도파로 재료로 사용하였다. 박막 광도파로는 광변색성 색소인 스파이록사진을 사용하였다. 자외선 조사시간을 20초, 30초, 40초로 증가시켰을 경우 센서의 파장응답은 1.44nm/mW, 1.64nm/mW, 1.78nm/mW였고 1550nm 파장에서 센서의 90% 복귀시간은 100초로 나타났다. In this paper, we investigated a novel UV sensor using evanescent field coupling between the side polished fiber and photo-functional polymer waveguide. It was found that resonant wavelength shifts occur due to variation in the refractive index of polymer planar waveguide for its photo-functional properties on exposed UV. Spiroxazine (photochromic dye) was used as the planar waveguide. The resonant wavelength responses were exhibited at 1.44 nm/mW, 1.64 nm/mW, and 1.78 nm/mW when UV irradiations were exposed for 20 seconds, 30 seconds, and 40 seconds, respectively. The recovery time of sensor was independent of UV exposure power and 90% recovery time was 100 seconds.

High-sensitivity temperature sensor using a side-polished single-mode fiber covered with the polymer planar waveguide

A temperature sensor based on evanescent field coupling between a single-mode side-polished fiber and a polymer planar waveguide was demonstrated. Due to thermooptic effects of a polymer planar waveguide, the resonance wavelength of the device is very sensitive to ambient temperature. Two kinds of polymers were used as planar waveguide materials. Fabricated sensors have a sensitivity of -0.37 nm//spl deg/C and -0.71 nm//spl deg/C, respectively. We also measured the thermooptic coefficient of employed polymer approximately.

Development of an evanescent-field fibre optic sensor for Escherichia coli O157:H7

An intensity-modulated fibre optic sensor was developed for Escherichia coli O157:H7. The interaction between the whole natural bacteria and the guided lightwave was carried out by means of evanescent-field coupling. A correlation between optical response and the current number of bacteria was achieved. The device sensitivity had been calibrated for initial number of bacteria ( N 0) from 10–800. The sensor sensitivity was 0.016 (±0.001) dB/h/ N 0. The sensing mechanism starts together with the log phase leading the present sensor response to be five to ten times faster than conventional bacteriological techniques.

High-sensitivity temperature sensor based on overlay on side-polished fibers

A temperature sensor based on overlay on side-polished fibers is presented. It works on the basis of the evanescent field coupling between a single-mode side-polished optical fiber and a dielectric waveguide overlay. The spectrum of a white light beam transmitted through the former system exhibit sharp and deep regularly spaced resonances whose central wavelenths are linearly shifted with temperature. It has a wavelength sensitivity of /spl sim/1 nm//spl deg/C, which is one order of magnitude better than that obtained with fiber Bragg sensors.

InGaAsP/InP Laterally Coupled Distributed Feedback Ridge Laser

A 1.3-µm-wavelength InGaAsP/InP laterally coupled distributed feedback ridge laser with improved structure and characteristics is reported in this paper. For easier fabrication of the first order grating with 198 nm period on and beside the ridge waveguide side-walls, two-step etching has been used to form a narrow ridge waveguide with a special shape. The grating is patterned by electron beam lithography and deeply transferred also by two-step etching, in order to enhance the lateral evanescent field coupling. A stable CW laser oscillation with a low threshold current of 17 mA at room temperature and a high side mode suppression ratio of 45 dB for a laterally coupled distributed feedback ridge laser has been achieved. An external slope efficiency of 0.25 W/A and an output power of about 25 mW are also available in our most recent devices with anti-reflection and high reflection facet coatings. The optical transmission of a Gbit/s order modulated signal has also been tested using our preliminary laser modules through a 1 km optical fiber.

Interesting effect of optical Kerr nonlinearity in expanding single-mode regime of optical fibers using dispersion-flattened profiles

An interesting effect of optical Kerr nonlinearity is reported from theoretical investigation on the fundamental and first higher mode cutoff frequencies in single-mode fibers doped with highly nonlinear materials like CdS x Se 1− x with W-type dispersion-flattened profiles. It is observed that whereas the single-mode regime decreases absolutely for dispersion-shifted profiles due to reduction of the first higher order mode cutoff frequency as is evident from earlier studies, it increases for W-type profiles for such nonlinearity taking its effect on the nonzero fundamental mode cutoff frequency into consideration. Thus, one can go further down below the fundamental mode cutoff frequency while using W-fibers in single mode nonlinear fiber devices involving evanescent field coupling in the low V region.

A Novel and Accurate Approximation for the Fundamental Mode in Singlemode Fibers having Arbitrary Index Profiles

A novel and extremely accurate near Gaussian approximation for the fundamental mode in single-mode optical fibers having arbitrary index profile is proposed and verified.The spot sizes calculated from present function matches exactly with the analytical and exact numerical results for step and parabolic index fibers respectively over a practical range of values of normalised frequency V. For low V region, where Gaussian approximation is not faithful, it will be suitable to predict accurately the propagation characteristics of devices like directional coupler involving evanescent field coupling.

In-Line Fiber Polarization Selector and Intensity Modulator

Liquid-crystal-based fiber components that exploit the high birefringence and electrooptic effect of liquid crystals, are versatile. A single-mode fiber enclosed with planar-aligned liquid crystals is reported on in this paper. The fiber was etched nearly to its core. The output light was exercised through the evanescent field coupling between the fiber and surrounding liquid crystals. To achieve an effective access to the evanescent field, the etch depth was studied first. When the fiber was etched to 10-μm diameter, obvious light leakage occurred. Once the electric field was switched on, the polarization selection and intensity modulation were examined. A theoretical description of the optical characteristics were also given.

Variational analysis of single‐mode graded‐index fibers based on a novel approximation of the fundamental modal field

Gautam DasS. N. SarkarUniversity of CalcuttaDepartment of Electronic ScienceCalcutta 700 009, IndiaE-mail: sns@cucc.ernet.inAbstract. A novel and extremely accurate near Gaussian approxima-tion for the fundamental mode in single-mode fibers having arbitrary in-dex profiles is proposed and verified by the variational method. The spotsize calculated from our approximation matches closely the analyticaland exact numerical results for step and parabolic index fibers over alarge range of values of the normalized frequency V. For the low Vregion where the Gaussian approximation is not accurate, our results aresuitable to accurately predict the propagation characteristics of devicessuch as directional couplers involving evanescent field coupling.

Evanescent field coupling between a single-mode fiber and a planar waveguide with absorption or gain

A coupled-mode model is presented, which is capable of describing the light transfer between a single-mode fiber and a planar waveguide (PWG), taking into account the TE or the TM polarization of the PWG modes and an absorption or a gain into the PWG or the cover medium. Applying this model to the practical case of a coupler of limited length (both with distributed and fixed coupling strength) I show that the initially strongly asymmetric resonance curve of coupling broadens, becoming more and more symmetric as the absorption increases. When the PWG and/or the cover medium possesses a gain, a reverse resonance (with an amplified fiber power) appears next to the resonance minimum of the fiber power.

Evanescent field coupling of dipole to a surface: configurational resonance at long distances

Light excited dipole interacts with nearby surfaces and, at certain dipole-surface distances, configurational resonances happen. The evanescent field coupling can be separately calculated and the configurational resonances due to the evanescent coupling may happen at long dipole-surface distances. The numerical results of field enhancement for silver particle-surface system are provided and discussed. In the light of the theory, we also predict that selective (from the homogeneous field) detection of the evanescent field may be realizable, especially at these resonance distances.

Estimating Performance of Fiber Optic Modulators/Switches with Multimode Electrooptic Overlay/Interlay Waveguide

We report here a simple and accurate semi-numerical approach to estimating the device characteristic parameters of in-line fiber optic modulators and switches based on evanescent field coupling between a side-polished single-mode fiber and a multi moded electrooptic waveguide as overlay/interlay. In this model the side polished fiber is replaced by an equivalent planar waveguide, thus reducing the basic device structure of our study to a 1-D multilayer, which can be investigated by using a matrix method and simple overlap integrals. Our model inherently takes into account the polarization sensitivity of such devices due to the birefringence of the electrooptic multimode waveguide. Our results are in close agreement with reported experiments on such devices with respect to important device characteristics.

Optical transmission of a "Continuous fiber" component comprising a multilayer structure with the thin metal film

The evanescent field coupling between a single-mode fiber and a multilayer planar waveguide which includes a thin metal film has been investigated theoretically and experimentally. The shift and spreading phenomena for the optical transmission resonances (dips) of the TM-mode depending on the superstrate refractive index have been observed and explained. The role of losses in the resonance weakening and spreading phenomena is clarified. Possibilities for tuning the TM-mode resonance independently of the TE-resonance, and even to have coincidence of the transmission dips for both TE- and TM-modes are discussed.

Near-field optical beam induced current measurements on heterostructures

We report near-field optical beam induced current (NOBIC) measurements on semiconductor quantum well (QW) structures. A subwavelength fiber tip is coupled with a tunable laser source and scanned over a sample surface. The induced photocurrent reveals the compositional profile of quantum structures. Semiconductor QW structures were designed and fabricated by molecular beam epitaxy (MBE) to study the wavelength dependence and resolution capability of NOBIC. We demonstrated that the resolution of this technique strongly depends on the aperture size. For aperture sizes that allow for coupling of evanescent fields from the tip into the semiconductor as propagating fields, the resolution strongly depends on the excitation wavelength due to the variation of the optical penetration depth. For smaller apertures, the optical field remains evanescent in the semiconductor and resolution is essentially independent of the wavelength.

Thin Films of Ferroelectrics Made by Pulsed Laser Deposition for Optoelectronic Applications

ABSTRACTWe have deposited ferroelectric thin films by pulsed laser deposition to be used as an inline optoelectronic modulator utilizing evanescent field coupling to a side-polished optical fiber. In order to satisfy waveguiding conditions, amorphous quartz substrates (n= 1.447) were necessary. Ferroelectric films of (Pb0.91La0.90)(Zr0.65,Ti0.35)O3, Pb0.80La0.20TiO3 (PLZT), and Sr0.5Ba0.5TiO3 were deposited on quartz substrates and thin Bi4Ti3O12 buffer layers were explored as a chemical barrier and to improve crystallographic texturing. The morphology of thick films (˜3 µm) of PLZT was not acceptable for optical applications, whereas the Sr0.5Ba0.5TiO3 films were very smooth, but had some cracking. The Sr0.5Ba0.5TiO3 films were optically determined to be uniform and of sufficient quality for use in fabricating in-line fiber optic modulators.

On the Properties of Surface Plasmon Polarization Beam Splitters

Abstract The transmission and cross-coupling characteristics of a surface-plasmon-based polarization beam splitter with multimode interlay between two polished optical fibre blocks are presented. Through the continuous monitoring of the characteristics as a function of interlay refractive index and wavelength it was found that the device is symmetrical in terms of optical power coupling in the cross direction and asymmetrical in terms of transmission, with respect to input launch direction. The fibre fixed in the metal-coated block has a transverse electric (TE) mode transmission which is independent of the refractive index of interlay. The transmission of the other (uncovered) block shows resonance-type interaction for both TE and transverse magnetic modes owing to evanescent field coupling to the multimode interlay.

Monolithic integration of a waveguide InGaAs/InP pin photodiode with a locally ion implanted JFET for receiver OEIC applications

The device fabrication for a monolithic integration of a waveguide (WG), a photodiode (PD) and a junction field-effect transistor (JFET) in the InGaAs/InP material system is described. In an optimised WG/PD layer sequence, grown by metal organic vapour-phase epitaxy (MOVPE), JFETs have been realised using local Si and Be ion implantations. The JFETs (1.5 μm × 200 μm) have a maximum transconductance of 160 mS/ mm and a cutoff frequency of 12 GHz. The PDs are vertically coupled to the InGaAsP WG by evanescent field coupling. They show a low dark current of 3 nA and a 3 dB bandwidth of 5 GHz at −10 V bias. With the presented layer structure, a transimpedance receiver OEIC with monolithically integrated WG has been realised. The receiver sensitivity is −27.3 dBm at a BER of 10−9 for 400 Mbit/s data rate.

Analysis of the evanescent field coupling between a fiber and a planar waveguide using the explicit finite difference method with a nonuniform mesh

We investigate an optical waveguide system consisting of an unclad fiber core suspended at a constant distance parallel to the surface of a planar waveguide. The coupling and propagation of light in the combined system is studied using the three-dimensional explicit finite difference beam propagation method with a nonuniform mesh configuration. The power loss in the fiber and the field distribution in the waveguide are studied as a function of various parameters, such as index changes, index profile, and propagation distance, for the combined system.