Overlay Waveguide Research Articles

Modal analysis and device considerations of thin high index dielectric overlay slab waveguides

The effect of adding a thin high index dielectric overlay layer onto a 3-layer slab waveguide demonstrates several interesting features that can be exploited in integrated optical device configurations. A simple modal analysis is employed to examine the behavior of guided light launched from a 3-layer waveguide structure then coupled and propagated in the 4-layer overlay region. Modal properties typically overlooked in conventional slab waveguides are made use of in the design and theoretical analysis of an MMI device and optical index of refraction sensor. The optical structure presented here can form the backdrop waveguide design for more complex and active devices.

High Sensitive Fiber Optic Temperature Sensor Based on a Side-polished Single-mode Fiber Coupled to a Tapered Multimode Overlay Waveguide

A high sensitivity fiber optic temperature sensor based on a side-polished fiber (SPF) coupled to a tapered multimode overlay waveguide (MMOW) is proposed and studied. Both tapered and non-tapered MMOW were considered to study the effect of tapering of MMOW on the characteristics of the device and to investigate the criticality of the uniformity of the multimode overlay waveguide over the SPF. Present study shows that tapering of the MMOW can be used to tune the desired wavelength range without any loss in the sensitivity. Sensitivity up to 9 nm/ $^{\circ}C$ within the temperature range of 25 to $100^{\circ}C$ can be achieved with the proposed sensor, almost 6 times higher compared even to state-of-the-art high-sensitivity grating-based fiber optic temperature sensors.

Seminumerical approach for modeling side-polished microstructured optical fiber-based devices

We developed a simple seminumerical method to model, get design parameters, and estimate performance of side-polished microstructured optical fiber (SPMOF)-based devices. As an example, we analyze a SPMOF directional coupler and show good agreement between the available experimental results and our theoretically estimated characteristics of such a coupler. To further demonstrate the utility of our design approach, we also propose a new wavelength filter design based on the use of a SPMOF half-coupler, which is loaded with an overlay planar waveguide and should find applications as a gain-flattening filter for erbium-doped fiber amplifiers or as a bandstop filter, in general. To show the applicability of our approach even to surface plasmonic components, we also propose a SPMOF polarizer design, which could be formed by coating the top polished surface of a SPMOF half-coupler with a metal layer.

Compact Bends for Achieving Higher Integration Densities for LiNbO$_{3}$ Waveguides

A new waveguide platform is demonstrated that allows the bend radii to be substantially decreased for titanium-diffused lithium-niobate (LiNbO3) waveguides using vertically integrated arsenic-trisulfide (As2S3) overlay waveguides. Power is transferred from a Ti-diffused waveguide into the overlay waveguide using tapers, guided by the As2S3 waveguide through the S-bend region and transferred back into another Ti-diffused waveguide. This structure also behaves like a polarization beam splitter. We present simulation results as well as measurements to show the feasibility of achieving low loss and reduced bend radii for electrooptic waveguides.

Design and realization of an all-fiber broadband tunable gain equalization filter for DWDM signals

Design and fabrication of a tunable gain equalization filter for dense wavelength division multiplexed (DWDM) signals through erbium doped fiber amplifiers (EDFA) is reported. It is based on a side-polished fiber (SPF) half-coupler block loaded with a displaceable tapered multimode overlay waveguide (MMOW). A simple and accurate normal mode analysis is employed to design this filtering device for its subsequent realization. Equalization of a typical EDFA gain spectrum in the C-band within +/-0.35 dB or even less in the presence of various ITU standard C-band DWDM signal channels is demonstrated under varied operating conditions like add/drop of signals. Tunability of the filter notch is achieved through displacement of the SPF relative to the MMOW.

측면 연마 광섬유를 이용한 가변 광 감쇠기

측면 연마 광섬유를 이용하여 가변 광 감쇠기를 실험적으로 구현하였다. 오버레이 물질로는 poly(vinylidene fluoride)와 poly (methyl methacrylate) 고분자 혼합체를 사용하였다. 혼합고분자의 온도를 변화시켜 오버레이의 굴절률을 조절하였고, 그로 인한 소산장 결합정도를 변화시켜 광섬유로부터 추출되는 광량을 조절하였다. 삽입손실은 0.1 ㏈ 이하였으며 30㏈이상의 감쇠를 실현하였다. The variable optical attenuator based on side polished fiber technique was experimentally demonstrated. Various combination mixtures of poly(vinylidene fluoride) and poly(methyl methacrylate) have been used as the overlay waveguide materials for the variable optical attenuator application. A controllable amount of optical power can be extracted via evanescent coupling by changing the temperature of the polymer mixture, thereby changing its refractive index. The VOA under test provides a maximum attenuation of 30 ㏈ and an insertion loss of 0.1 ㏈.

중간 결합층이 적층된 측면연마 광섬유 결합기의 해석 및 결합효율 개선

상부 도파로와 중간 결합층이 적층된 단일모드 광섬유를 기본구조로 하는 측면 연마 광섬유 결합기를 상호 광 결합이 일어나는 영역을 다층막 도파로로 등가화하여 결합 효율을 해석하였다. 다층막 구조에서 중간 결합층의 유무에 따른 반사도를 계산하여 상부 도파로의 굴절률의 함수로 도시하였고, 상부 도파로의 굴절률이 광섬유 코어의 굴절률보다 클 경우 중간 결합층의 조건에 의해서 광 결합이 증가되어짐을 보였다. 리튬나오베이트를 상부 도파로로 가지는 결합기의 실험결과를 톤해 이론적 해석의 타당함을 보였고 계산 결과와 일치함을 보였다. An in-line fiber coupler, based on side-polished single-mode fibers covered with an intermediate coupling layer and a planar waveguide, is analyzed by modeling the interaction region as an equivalent multi-layered planar waveguide. The reflectance for the multi-layered structure with and without buffer layer is illustrated as a function of the refractive index and thickness of the overlay waveguides. When the refractive index of the overlay waveguides is greater than that of the fiber core, the conditions for the intermediate coupling layer to increase the power coupling from the fiber to the overlays is also explained. Through the experimental results using a LiNbO<TEX>$_3$</TEX>planar waveguide, we show that the theoretical analysis is reasonable and in good agreement with the measured values.

측면 연마 광섬유와 결합된 상부 도파로의 클래딩을 이용한 편광 무의존 온도센서

본 논문에서는 측면연마 광섬유 결합기 구조에서 평면 도파로의 상부와 하부 클래딩의 열광학 효과를 이용하여 온도를 측정할 수 있는 편광 무의존 온도 센서를 제안하고 있다. 평면 도파로는 170<TEX>$\mu\textrm{m}$</TEX> 두께의 현미경용 덮게 유리를 적용함으로서 급격한 온도 변화에 대해 도파로의 변형이 발생하지 않고, 비대칭적 평면 도파로 구조로 인한 편광 의존 손실을 최소화하기 시킬 수 있는 장점을 가진다. 제작된 소자의 편광 의존 손실은 0.3dB 이하로 측정되었다. 피측정체의 온도는 센서의 결합 파장 이동으로부터 계측되어진다. 평면 도파로의 상부와 하부 클래딩의 열광학 계수에 따른 온도 센서의 온도 민감도를 측정하였고, 상ㆍ하부 클래딩의 열광학 계수 변화로 온도 민감도를 제어할 수 있음을 보였다. In this paper, we propose a polarization-insensitive temperature sensor using a thermo-optic effect of the upper and (or) lower cladding of a planar waveguide in contact with a side-polished fiber. A microscope cover glass with thickness of a 170 <TEX>${\mu}{\textrm}{m}$</TEX> is adopted as an overlay waveguide because this waveguide opposes sudden temperature change and ensures polarization-insensitive responses. The measured polarization-dependence loss is less than 0.3 dB. The temperature can be detected as a result of the shift in coupling wavelength of the sensor. We investigate the shift in coupling wavelength as a function of the temperature variation with respect to the different thermo-optic coefficients of lower and upper claddings. We also show that the temperature sensitivity of the device can be easily controlled by the thermo-optic coefficients of lower and upper claddings of the overlay waveguide.

측면 연마 광섬유 결합기를 이용한 파장분할 다중화 채널분리 필터

파장분할 다중화 시스템의 다중 채널분리 필터가 측면연마 광섬유 결합기를 이용한 빗살필터로 구현됨을 제안하였다 단일 모드 광섬유의 기본모드와 결합하는 상부 평면 도파로의 모드가 고차일수록 광 전력전달 효율은 낮아지는데, 두 도파로 사이 에 중간 결합층을 삽입함으로서 개선시킬 수 있고 최적화 조건에서 20dB 이상의 채널간 소멸비를 얻을 수 있음을 보였다. 200fm두께의 리튬나오베이트를 상부 평면 도파로로 적용한 경우 4m의 채널간격을 가지는 빗살필터 특성을 얻었으며, 중간 결합층의 두께가 1 Um일 때 굴절률이 1.52-1.53 범위에서 최대 광 전력전달이 일어남을 관측하였고 빔 전파 방법을 이용하여 계산한 결과와 거의 일치함을 보였다. Fiber-optic comb filters using a side-polished fiber coupler are proposed as multi-channel isolation filters on wavelength division multiplexing systems. We have demonstrated that the coupling efficiency between two waveguides is improved by the intermediate coupling layer in spite of the decrease of the optical power transfer between two waveguides due to the high-order modes of the overlay waveguide coupled with the side-polished single-mode fiber. When LiNbO<TEX>$_3$</TEX>with a 200-<TEX>${\mu}{\textrm}{m}$</TEX>-thickness was applied as a planar-overlay-waveguide, the comb filtering characteristics with a 4 nm-channel-spacing were achieved and the maximum power coupling occurred at the 1-<TEX>${\mu}{\textrm}{m}$</TEX>-thickness and the refractive index in range 1.52 to 1.53 of an intermediate coupling layer. If the intermediate coupling layer is optimized, an extinction ratio with more than 20 dB can be obtained. These experimental results are in good agreement with the BPM simulation.

Thermooptically tunable side-polished fiber comb filter and its application

Tunable side-polished fiber comb filters based on the thermooptic effect of a polymer layer inserted between a side-polished fiber and an overlay waveguide are presented. The thermal electrodes formed on a polymer layer control the refractive index of one and its index variation induces the shift in coupling wavelength by the phase change of an evanescent field at the overlay waveguide boundaries. The tuning rate as a -0.017 nm/mW and the response time with a 50 ms/nm have been achieved. The authors have also demonstrated that this comb filter makes it possible for the tunable erbium-doped fiber ring laser to be used as a lasing-wavelength selective element. The tunable range of this laser is obtained about 17 nm (1545-1562 nm).

Polarization Insensitive Fiber-Optic Refractometer based on a Side-polished Fiber Coupler with a Thick Glass Waveguide

Side-polished single-mode fiber covered with a thick planar-glass-waveguide is studied as a polarization-insensitive fiber-optic refractometer The properties of the device are very sensitive to the index variation of an upper cladding on top of the overlay waveguide. We investigate the spectral response, the polarization properties, and the relationship between the shift in coupling wavelength and the index variation of an upper cladding layer Index resolutions of the order of 10-4 have been achieved.

Optical fiber sensor for water detection using a side-polished fiber coupler with a planar glass-overlay-waveguide

For monitoring the presence of water at the splicing points of an optical fiber, the water detection sensors based on a side-polished fiber coupler with a commercial microscope-cover-glass are proposed. The sensors are very sensitive to the variation of an upper cladding layer’s index on the overlay waveguide. When the sensors detect water, the coupling wavelengths shift to longer wavelength at about 2 nm than that of an air cladding. The variation of an optical loss as a function of the wavelength shift is monitored using an optical time-domain refractometry to detect the installed positions and the immersion locations of the sensors. The advantages of our sensor for these purposes are discussed in.

Power coupling between fiber and multimode slab composite waveguide

We discuss power coupling in devices composed of a fiber with partly or completely removed cladding and multimode slab overlay waveguide of any complexity. A fiber mode excites with a high efficiency only one mode of the composite structure in the wide range of system parameters. But only when the effective refractive index of this mode almost equals to the one of the initial fiber mode the modal value of electric field is high enough to produce effective interactions with active and passive elements in the region of the overlay waveguide. In the narrow range of system parameters last two modes are excited with a nearly equal efficiency providing a regular directional coupler. The power-coupling coefficient essentially depends on the thickness of remaining fiber cladding and an optimal value of this cladding is about 1.5–2 μm.

Tunable in-line fiber optic comb filter using a side-polished single-mode fiber coupler with LiNbO 3 overlay and intermediate coupling layer

Using a side-polished single-mode fiber covered with a polished LiNbO 3 overlay and an intermediate coupling layer, tunable fiber-optic comb filters are demonstrated. The device behaviors based on the modal properties of the fiber and the planar LiNbO 3 waveguide are analyzed by two dimensional beam propagation methods (2-D BPM) and discussed the role of an intermediate coupling layer in terms of coupling efficiency. We also show that the thermo-optic effects of this layer can be utilized to tune the comb filter. When the polished x-cut LiNbO 3 with 200 μm thickness is used as a multimode overlay waveguide, the comb output spectra with free spectral range of 4 nm are measured in 1550 nm wavelength range. The tuning rate as a function of the refractive index of an intermediate coupling layer, Δλ/ Δn b , is about −0.129 nm/−0.001. The experimental results are in good agreement with the calculated results.

Fiber-optic chemical sensing with Langmuir–Blodgett overlay waveguides

Fiber-optic chemical sensing has been demonstrated with a side-polished single-mode optical fiber, evanescently coupled to chemically sensitive Langmuir-Blodgett (LB) overlay waveguides. The sensors exhibit a channel-dropping response centered on a wavelength that is dependent on the thickness and the refractive index of the overlay waveguide. It has been shown that pH-sensitive organic dyes proved to be suitable materials for the formation of an overlay waveguide whereas LB deposition provides the required thickness control. A theoretical model of the sensor response, based on the Kramers-Kronig relations and phase matching of the guided modes within the optical fiber and overlay waveguide, shows good agreement with experimental results.

Coupled-mode description of fibre-to-two-layered planar waveguide coupler

A coupled-mode model is presented, which describes the behaviour of a complex waveguiding system consisting of a single-mode optical fibre evanescent field coupled to a planar waveguide which consists of two layers (overlay waveguide and superstrate layer). The real two dimensions of the waveguide cross-sections are taken into account, as well as the transverse electric or transverse polarization of the planar waveguide modes. This model provides a proper quantitative description of the coupler, revealing the influence of the superstrate layer onto the light transfer into the coupler. Some possible applications of such an in-line fibre device are considered.

Refractometers and tunable components based on side-polished fibers with multimode overlay waveguides: role of the superstrate

A normal mode analysis is applied to the investigation of the role of the superstrate in terms of its refractive index and thickness in estimating the performance of fiber refractometers and the tunability of fiber components based on evanescent coupling of a side-polished fiber to a multimode overlay waveguide. The model is shown to yield a complete transfer function and information about all the major characteristics of such in-line fiber devices. The results closely match the trends of the reported experiments.

Modelling a Fiber Half-block with Multimode Overlay Waveguide

We report a simple numerical analysis to model the performance of in-line fiber optic components which rely on selective coupling between a polished single-mode fiber half-block and a highly multimode overlay waveguide. Important device performance characteristics of these devices are describable in terms of throughput power as a function of various waveguide parameters. Our model is based on treating the fiber as an equivalent planar waveguide (EPG) and obtaining the normal modes of the resultant multilayer structure through a matrix formalism. Throughput power exiting the fiber is obtained by calculating the overlap between the EPG mode and the normal modes. The agreement between our model and the reported experimental results on in-line fiber modulators is shown to be excellent. This model should, therefore, be very useful in estimating the performance of a host of such in-line fiber components.

A Tunable Fiber-Optic Bandpass Filter Based on Polished Fiber to Planar Waveguide Coupling Techniques

A novel "continuous-fiber" optical bandpass filter is reported whose operation relies upon the interaction between a single-mode "cutoff" fibre and a high-index, planar, multimode overlay waveguide. The fiber overlay device structure exhibits a comb-type intensity versus wavelength bandpass filter response function. Experimental investigations of both oil and solid-state over-lay devices are reported, with passband linewidths down to <4 nm and passband spacing between 45 and 256 nm, depending on overlay index and thickness. Rejection ratios (off-resonance) of >20 dB and on-resonance insertio losses of 0.5 dB were recorded. Tuning of the passband over a maximum range of 53 nm was demonstrated by varying the overlay waveguide superstrate index. Additionally, with materials such as LiNbO3 electro-optic tuning is possible and preliminary experimental results of active tuning are reported. A tuning figure of 100 V/nm was observed. The device shows potential for application in fiber laser wave-length selection/tuning and WDM schemes.

Tunable in-line fiber-optic bandpass filter

An in-line fiber-optic bandpass wavelength filter has been demonstrated based on a single-mode fiber, side polished into the core, in evanescent contact with a high-index multimode overlay waveguide. Passband linewidths down to 5.2 nm were observed, while spacing between transmission peaks was seen to range from 45 to 256 nm for variation of the overlay waveguide parameters. Additionally, rejection ratios of >20 dB and insertion losses as low as 0.5 dB were observed.