Taper Structure Research Articles

Mass-productive fabrication of a metal–insulator–metal plasmon waveguide with a linear taper for nanofocusing

The fabrication of a metal–insulator–metal plasmon waveguide with a linear taper is reported. Highly efficient nanofocusing of light with a Au–SiO2–Au waveguide with a three-dimensional taper had been demonstrated. However, conventional vertical taper structures were fabricated with a low-throughput process based on electron beam scanning. We propose an efficient, mass-productive fabrication process using a standard dry etching technique. A key improvement is the employment of a mixed gas of CHF3 and O2. By optimizing the gas composition and the cooling process of the substrate, a SiO2 vertical taper with an angle of 19°, which is very close to the optimum 20°, was successfully produced. At the tip section, an ultra-thin waveguide as thin as 5.6 nm, only one-third of the conventional demonstration, is reproducibly realized by the employment of an atomic layer deposition of Al2O3. Coupling efficiency as high as 72 % numerically demonstrated.

Highly Sensitive Temperature Fiber Sensor Based on Mach-Zehnder Interferometer

ABSTRACTWe propose a highly sensitive temperature fiber sensor based on a Mach-Zehnder Interferometer (MZI). The MZI is composed of a piece of no-core fiber (NCF), a single-mode fiber (SMF), and a fiber taper (FT) structure. To accomplish high sensitivity, the component was immersed in a material with high thermo-optic coefficient (TOC). The experimental results showed that a high temperature sensitivity of 1.56 nm/°C was achieved for surrounding material with a refractive index (RI) of 1.45. The presented sensor has numerous advantages, for instance, extremely high sensitivity, easy fabrication, simple structure, compact size, and in-line applications.

Experimental demonstration of a two-mode (de)multiplexer based on a taper-etched directional coupler.

We experimentally demonstrate a compact, low-cross talk and fabrication-tolerant two-mode (de)multiplexer on the silicon-on-insulator platform. The device consists of a silicon wire waveguide coupled to a taper-etched waveguide. The partially etched taper structure is used to relax fabrication tolerance and thus to ensure high mode-conversion efficiency. The device is 68 μm in length, with a TE0-to-TE1 mode conversion loss of better than -0.8 dB demonstrated over the C-band wavelengths. In addition, the device demonstrates a low TE0-to-TE0 through waveguide insertion loss of better than -1.3 dB with modal cross talk lower than -26 dB, over a 65 nm wavelength range. Finally, we have experimentally demonstrated that the device is tolerant of fabrication errors of up to 20 nm. Better than -6 dB TE0-TE1 conversion loss with a cross talk lower than -23 dB over a 55 nm bandwidth has been obtained with a fabrication tolerance as large as 40 nm.

A hybrid silicon evanescent quantum dot laser

We report the first demonstration of a hybrid silicon quantum dot (QD) laser, evanescently coupled to a silicon waveguide. InAs/GaAs QD laser structures with thin AlGaAs lower cladding layers were transferred by direct wafer bonding onto silicon waveguides defining cavities with adiabatic taper structures and distributed Bragg reflectors. The laser operates at temperatures up to 115 °C under pulsed current conditions, with a characteristic temperature T0 of 303 K near room temperature. Furthermore, by reducing the width of the GaAs/AlGaAs mesa down to 8 µm, continuous-wave operation is realized at 25 °C.

GaInAsP/silicon-on-insulator hybrid laser with ring-resonator-type reflector fabricated by N2 plasma-activated bonding

III–V/Si hybrid integration with direct bonding is an attractive method of realizing an electrophotonic convergence router with a small size and a low power consumption. Plasma-activated bonding (PAB) is an effective approach for reducing thermal stress during the bonding process because PAB achieves a high bonding strength with low-temperature annealing. This time, the fabrication of a GaInAsP/silicon-on-insulator (SOI) hybrid laser with Si ring-resonator-type reflectors was demonstrated by N2 PAB. By measuring the lasing spectra, we confirmed the reflective characteristics resulting from the cascaded Si ring resonators. We also investigated kink characteristics, which occur around the threshold current, of the current–light output (I–L) characteristics, and successfully approximated the kink characteristics by considering saturable absorption occurring at the III–V/Si taper tip. The taper structure was investigated in terms of a passive device as well as an active device, and a structure for eliminating saturable absorption was proposed.

The measurement of sucrose concentration by two-tapered all-fiber Mach–Zehnder interferometer employing different coupling structures and manufacture processes

The sucrose concentration measurement and characteristics of light coupling taper structure on sensitivity with various fabrication processes of taper structure for all-fiber Mach–Zehnder interferometer (AFMZI) are presented. Using fusion splicer with electrical discharge, the standard single-mode fiber is employed to be fabricated as conical coupling/decoupling taper structure. The basic two fabrication processes are designed as single fusion-stretching (SFS), multiple fusions without stretching (MF). The third advanced process is composed of SFS and multiple fusions without stretching processes, and called multiple fusions with single stretching (MFSS). Various types of coupling/decoupling taper structures were fabricated based on the three kinds of fabrication processes. The effects of geometry shape including taper waist, taper angle, and sensing length on sensing sensitivity of AFMZIs are estimated. The modifications of fiber core and cladding induced by thermal effect affect the refractive index distributions and shapes of taper structure. The effects of refractive index changes of fiber core and cladding on sensing sensitivity are also discussed. The AFMZI was tested by measuring aqueous sucrose solution of refractive index unit (RIU) from 1.333 to 1.420 RIU. The optical spectrums are measured by a spectrometer. The spectrum dip shifts and sensing sensitivity was measured and calculated, respectively. As shown in results, sensing sensitivities of AFMZIs of taper structure fabricated by MFSS and multiple fusions without stretching processing are generally higher than SFS. The reasons could be aimed on materials modification through thermal effect on blurring fiber core-cladding interface and proper taper angle of taper structure. The more homogeneous refractive index distribution on fiber core-cladding interface, the more detecting light power decoupled through core-cladding interface to interact with exterior environment and enhance the sensing sensitivity. Similarly, an appropriate taper angle also provides a better coupling/decoupling performance. The optimal sensitivities relative to low refractive index, high refractive index, and full refractive index range are 87.62, 133.55, and 104.20 nm/RIU, respectively, and the corresponding sensing length are 30, 50, and 30 mm, respectively, with taper angle of 25° and taper waist of 40 μm.

High Coupling Efficiency Silicon Waveguide to Metal–Insulator–Metal Waveguide Mode Converter

Conventional silicon optical waveguides can have efficient coupling to optical fibers using grating couplers, but there is no structure of comparable coupling efficiency, optical bandwidth, and small size for direct coupling from optical fibers to metal slot optical waveguides. In this paper, we propose two structures for efficient coupling from conventional silicon strip waveguide to metal–insulator–metal (MIM) waveguide. The previously reported approach of mode conversion using taper-funnel structures to gradually squeeze the strip mode into metal slot has a big difference in effective indices and a large mismatch between the modes, thus limiting their coupling efficiency. Here, we demonstrate that adding another silicon strip-slot mode converter before the silicon taper structure can improve the coupling efficiency. Furthermore, the reversed taper-funnel structure has lower metallic absorption loss, thus further increasing the coupling efficiency. We experimentally measured 3 dB/coupler loss for coupling to a 200-nm-wide MIM slot, and 4 dB/coupler in the case of coupling to an 80-nm-wide MIM slot at 1640-nm wavelength.

Structure and antireflection properties of SiNWs arrays form mc-Si wafer through Ag-catalyzed chemical etching

A simple and low cost MACE method was demonstrated for efficiently texturing commercial mc-Si wafer at room temperature. The effects of fabrication parameters (deposition time, HF concentration, H2O2 concentration, and etching time) on the morphology structure, antireflection property of textured mc-Si were carefully studied. The large scale SiNWs arrays with different structure can be obtained under various fabrication conditions. Meanwhile, the results indicate that the fabricate parameters have important effect on the reflectance of textured mc-Si sample in the order of etching time>deposition time>H2O2 concentration>HF concentration. The comprehensive research results indicate that it is more beneficial for the nanowire arrays with tapering structure and the length of 13μm to obtain excellent antireflection property. Under these optimization conditions, the textured mc-Si shows an outstanding anti-reflectance ability of ∼5.6%, which indicates that the Ag-catalysis etched mc-Si shows a huge potential application in high-efficiency polysilicon solar cells.

Polymer taper bridge for silicon waveguide to single mode waveguide coupling

Coupling of optical power from high-density silicon waveguides to silica optical fibers for signal routing can incur high losses and often requires complex end-face preparation/processing. Novel coupling device taper structures are proposed for low coupling loss between silicon photonic waveguides and single mode fibers are proposed and devices are fabricated and measured in terms of performance. Theoretical mode conversion models for waveguide tapers are derived for optimal device structure design and performance. Commercially viable vertical and multi-layer taper designs using polymer waveguide materials are proposed as innovative, cost-efficient, and mass-manufacturable optical coupling devices. The coupling efficiency for both designs is determined to evaluate optimal device dimensions and alignment tolerances with both silicon rib waveguides and silicon nanowire waveguides. Propagation loss as a function of waveguide roughness and metallic loss are determined and correlated to waveguide dimensions to obtain total insertion loss for the proposed taper designs. Multi-layer tapers on gold-sputtered substrates are fabricated through photolithography as proof-of-concept devices and evaluated for device loss optimization. Tapered waveguide coupling loss with Si WGs (2.74dB) was experimentally measured with high correlation to theoretical results.

Low-loss and high-Q Ta(2)O(5) based micro-ring resonator with inverse taper structure.

A low-loss and high-Q Ta(2)O(5) based micro-ring resonator is presented. The micro-ring resonator and channel waveguide with core area of the 700 by 400 nm(2) were fabricated on amorphous Ta(2)O(5) thin films prepared by reactive sputtering at 300°C and post annealing at 650°C for 3 hours. The Ta(2)O(5) micro-ring resonator with a diameter of 200 μm was coupled to the channel waveguide with a coupled Q up to 38,000 at a 0.9 μm coupling gap. By fitting the transmission spectrum of the resonator, the extracted loss coefficient inside the ring cavity and transmission coefficient of TE mode were 8.1dB/cm and 0.9923, leading to the estimated unloaded Q of higher than 44,000. In addition, based on the cut-back method, the propagation loss and the coupling loss of Ta(2)O(5) channel waveguide with an inverse taper were 1.5dB/cm and 3.2 dB, respectively. The proposed Ta(2)O(5) technology offers an unique alternative for fabricating high performance guided wave devices, and may well lead to novel applications in photonic integrated circuits.

Four-wave-mixing in the loss low submicrometer Ta₂O₅ channel waveguide.

A degenerate four-wave-mixing (FWM) operation in the Ta2O5 submicrometer channel waveguide has been successfully demonstrated. The propagation loss of 1.5 dB/cm and total insertion loss of 5.1 dB are realized in a 12.6 mm long waveguide with inverse taper structure. The wavelength and quadratic pumping power-dependent measurements on optical transmission confirm FWM performance and characterize the nonlinearity of waveguide. The conversion efficiency of -50 dB at coupled pump power of 40 mW is observed, suggesting that the nonlinear refractive index of Ta2O5 waveguide at 1550 nm is estimated to be 1×10(-14) cm2/W. Our primary results indicate that the Ta2O5 submicrometer channel waveguide has great potential in developing nonlinear waveguide applications.

Three dimensional polymer waveguide using hybrid lithography.

A three dimensional polymer waveguide with taper structure was demonstrated and fabricated by a reliable and effective hybrid lithography. The hybrid lithography consists of lithography to fabricate a polymer waveguide and gray scale lithography to fabricate a polymer taper structure. Laser ablation and shadow aluminum evaporation were designed for gray scale lithography. The length of the gray scale region ranging from 20 to 400 μm could be controlled by the laser power, the ablation speed, and the aluminum thickness. The slope angle was determined by the length of the gray scale region and the thickness of the photoresist. The waveguide taper structure could be transferred to the lower layer by the etching method. The taper structure can be used for integration of the waveguide with different dimensions.

Tunable Polarity in a III-V Nanowire by Droplet Wetting and Surface Energy Engineering.

Controllable axial switching of polarity in GaAs nanowires with minimal tapering and perfect twin-free ZB structure based on the fundamental understanding of nanowire growth and kinking mechanism is presented. The polarity of the bottom segment is confirmed to be (111)A by atomically resolved scanning transmission electron microscopy.

두 개의 팔을 가진 초광대역 자기상보형 시뉴어스 안테나

본 논문에서는 Cellular/GSM800, GSM900, ISM, GPS, DCS/GSM1800, PCS/GSM1900, WCDMA/UMTS/IMT2000, WiBro, WLAN, WiMax의 주파수 대역(0.824~5.85 GHz) 전체를 포함하는 초광대역 시뉴어스 안테나를 제안하였다. 제안한 시뉴어스 안테나는 두 개의 팔(arm)로 이루어져 있으며, 자기상보(self-complementary) 구조로 설계하여 주파수 무관 특성을 나타내도록 하였다. 안테나는 클로펜스타인 테이퍼(Klopfenstein taper) 구조의 광대역 발룬(balun)을 사용하여 <TEX>$50{\Omega}$</TEX>에 정합시켰다. 측정 결과, 제안한 안테나의 대역폭은 반사손실 -10 dB를 기준으로 할 때 0.76~6 GHz까지 5.24 GHz로, 목표 주파수 대역을 모두 포함하는 것을 확인하였다. 방사패턴은 E-plane과 H-plane 모두에서 전체 동작 주파수 대역에 걸쳐 거의 변화 없이 양방향성 브로드사이드 빔을 보였으며, 최대 이득은 2.32~6.01 dBi로 측정되었다. This paper presents a ultra-wideband sinuous antenna that operates in a whole frequency range (0.824~5.85 GHz) of Cellular/GSM-800, GSM900, ISM, GPS, DCS/GSM1800, PCS/GSM1900, WCDMA/UMTS/IMT2000, WiBro, WLAN and WiMax. The proposed antenna, which is composed of two sinuous arms, is designed as a self-complementary structure in order to have frequency-independent characteristics. It also uses a wideband balun of Klopfenstein taper structure to match to <TEX>$50{\Omega}$</TEX>. Experimental results show that the -10 dB return loss bandwidth of the proposed antenna is 5.24 GHz that ranges from 0.76 to 6 GHz, which covers all the frequency bands of the various wireless services. Within the entire operating frequency range, the measured radiation patterns in both E-plane and H-plane show nearly constant bidirectional broadside beams and the maximum antenna gain is measured to fall between 2.32~6.01 dBi.

Multimode waveguide spot size width converter for silicon photonics applications

We present the design of a parabolic multimode waveguide used as a spot size width converter. The designed structure allows reducing the input spot size width an order of magnitude using a very short structure with respect to linear or conventional tapers. An illustrative example of spot width reduction from 10 microns to a Si wire of 1 micron with a small length in the order of 17 microns and a coupling efficiency of ∼90% is presented. Using a linear taper structure with a taper length >100 μm, the coupled power is still less than that obtained by the parabolic multimode interference (PMMI) guide. This means that the PMMI offers a length reduction of more than 5× with respect to the linear taper to achieve the same spot reduction. © 2015 Society of Photo-Optical Instrumentation

Dual-parameter optical fiber sensor based on intermodal interferometer with an inline embedded fiber Bragg grating

A dual-parameter optical fiber sensor is proposed and demonstrated. It is based on an intermodal interferometer (IMI) with an inline embedded fiber Bragg grating (FBG). The IMI is formed by cascading a taper structure and a spherical-shaped structure through a segment of a single-mode fiber. Due to the different wavelength shifts of the IMI and FBG to temperature and liquid level, simultaneous measurement can be achieved. Experimental results indicate a good linear relation between the wavelength shift and external parameters (temperature and liquid level). The sensitivities of 0.066 nm/°C and −0.133 nm/mm are achieved experimentally for temperature and liquid level, respectively. The interesting properties of the sensor include good operation linearity, compact size, and high sensitivity.

Hybrid fiber interferometer for simultaneous measurement of displacement and temperature

A hybrid fiber interferometer sensing configuration for displacement and temperature measurements is proposed and experimentally demonstrated that is constructed by splicing a short section of polarization maintaining optical fiber to an end-cleaved single mode optical fiber with a tapering structure. The reflected spectrum changes with the variation of displacement and temperature. The sensing configuration uses the method of wavelength and intensity modulations for displacement and temperature measurements, respectively, to which the sensitivities are 0.01392 nm/μm, 0.0214 dBm/μm, 0.09136 nm/°C, and 0.15795 dBm/°C. Experimental results show that displacement and temperature can be measured simultaneously by demodulating the reflected spectrum.

A fiber laser sensor for liquid level and temperature based on two taper structures and fiber Bragg grating

A fiber laser sensor for simultaneous measurement of liquid level and temperature is proposed and demonstrated experimentally. The sensor is based on two taper structures and a fiber Bragg grating (FBG). The two taper structures form a novel fiber interferometer, which is fabricated by cascading two tapers in a section of single-mode fiber (SMF). The FBG and the interferometer serve as the filters of the laser cavity. Corresponding to the two filters, the laser outputs are stable dual-wavelength outputs, which have different characteristics to the liquid level and the temperature. The wavelength produced by the FBG is not sensitive to the liquid level. The temperature sensitivity of the wavelength produced by the FBG is 0.0123nm/°C. The wavelength produced by the interferometer is sensitive to the liquid level and the sensitivity is up to 0.2294nm/mm. The temperature sensitivity of the wavelength produced by the interferometer is 0.0648nm/°C. According to the different spectral responses of the liquid level and the temperature, simultaneous measurement can be realized. Furthermore, the proposed sensor has the advantages of less detection limit (DL), higher resolution and higher sensitivity compared to other optical fiber sensors.

High-Q and high-sensitivity width-modulated photonic crystal single nanobeam air-mode cavity for refractive index sensing

We propose a novel optical sensor based on a one-dimensional (1D) photonic crystal (PhC) single nanobeam air-mode cavity (SNAC). The performance of the device is investigated theoretically. By introducing a quadratically modulated width tapering structure, a waveguide-coupled 1D-PhC SNAC with a calculated high quality factor of 5.16×10(6) and an effective mode volume of V(eff)∼2.18(λ/n(si))(3) can be achieved. For the air mode mentioned above, the light field can be strongly localized inside the air region (low index) and overlaps sufficiently with the analytes. Thus, the suggested PhC SNAC can be used for high-sensitivity refractive index sensing with an estimated high sensitivity of 537.8 nm/RIU. To the best of our knowledge, this is the first PhC single nanobeam geometry that features both high Q-factors and high sensitivity, and is potentially an ideal platform for realizing ultracompact lab-on-a-chip applications with dense arrays of functionalized spots for multiplexed sensing.

Novel integration technique for silicon/III-V hybrid laser

Integrated semiconductor lasers on silicon are one of the most crucial devices to enable low-cost silicon photonic integrated circuits for high-bandwidth optic communications and interconnects. While optical amplifiers and lasers are typically realized in III-V waveguide structures, it is beneficial to have an integration approach which allows flexible and efficient coupling of light between III-V gain media and silicon waveguides. In this paper, we propose and demonstrate a novel fabrication technique and associated transition structure to realize integrated lasers without the constraints of other critical processing parameters such as the starting silicon layer thicknesses. This technique employs epitaxial growth of silicon in a pre-defined trench with taper structures. We fabricate and demonstrate a long-cavity hybrid laser with a narrow linewidth of 130 kHz and an output power of 1.5 mW using the proposed technique.