Electro‐optic polymer/titanium dioxide hybrid core ring resonator modulators

Compact electro‐optic (EO) modulators are desirable for a number of applications. In this study, a ring modulator has been fabricated in the titanium dioxide (TiO₂) core and EO polymer cladding waveguide structure. A 250‐nm thick TiO₂ core was utilized to minimize the ring radius down to 100 μm, to avoid using the top cladding between the EO polymer and the electrode, and to improve the poling efficiency. The resonance obtained by the ring modulator was observed to shift by 0.02 nm/V due to the enhanced in‐device EO coefficient of 105 pm/V. A modulation depth of 3 dB was observed at the frequency response function at 20 kHz using 2‐V V_p–p clock signal.     

Fiber optical communications devices

Both low attenuation silica optical fibers with peak transmission in the wavelength regions of 0.85 μm, and 1.05 μm, and improved lasers at both wavelengths are now available. In this review paper, the principal components for emission, modulation and detection are described. The characteristics of both semiconductor lasers, made of GaAs and related compounds, emitting at 0.85 μm or 1.05 μm and high neodymium-content lasers are discussed. For modulation, current modulation of GaAs lasers and external electro-optic modulation are considered. Concerning detection, the realisation of Si photodetectors suitable at 0.85 μm and the new photodetectors at 1.05 μm from Ga_1−x In _x As are reviewed.     

Photonic Electric-Field Sensor Utilizing an Asymmetric Ti:LiNbO3 Mach–Zehnder Interferometer with a Dipole Antenna

Abstract

The use of an asymmetric Ti:LiNbO₃ Mach–Zehnder interferometer with a dipole antenna to measure an electric-field strength is described in this article. The device has a small size of 46 × 7 × 1 mm and operates at a wavelength of 1.3 μm. The AC output characteristics show the modulation depth of ~75% at V_π voltage of ~5.3 V. The minimum detectable electric fields are ~0.28 V/m and ~0.646 V/m, corresponding to a dynamic range of about ~32 dB and ~26 dB at frequencies 20 MHz and 50 MHz, respectively. The sensors exhibit an almost linear response for the applied electric-field intensity from 0.298 V/m to 29.84 V/m.     

Interferometric optical isolator with Si guiding layer fabricated by wafer bonding

An interferometric optical isolator, with a Si guiding layer, employing a nonreciprocal phase shift was studied. The optical isolator was comprised of a magneto-optic waveguide with a magnetic garnet/Si/SiO₂ structure, which was fabricated by wafer bonding technique. The nonreciprocal phase shift in the magneto-optic waveguide with the Si guiding layer was calculated at a wavelength of 1.55 μm. Several kinds of layer structures in the magneto-optic waveguide were discussed.     

Design theory and experiment of acousto-optical tunable filter by use of flexural waves applied to thin optical fiber

Spectral response of acoustically induced microbending through thin optical fiber is discussed from mode-coupling of core and cladding modes. The thin fiber is analyzed in three-layered structure (core-cladding-air) to gain insights into acousto-optic modulation. We explained the dependence of core and/or cladding diameters on acoustic source parameters from numerical calculations. According to the calculations, we successfully fabricated all-optical tunable filter using this thin fiber that yields an efficient mode-coupling at flexural wave frequencies less than 1MHz. To increase the acousto-optic effect, we used a specially designed thin optical fiber (80 μm of cladding diameter) in the section where flexural wave is produced, and spliced both ends of the thin fiber to the tapered 125 μm fibers. The frequency and voltage tuning of fabricated filter is also confirmed by changing the driven frequency and applied voltage of the PZT, respectively. This result suggests a possibility of fiber-optic device application as all-optical tunable filter at 1.55 μm.     

1.55 μm AlGaInAs/InP polarization-insensitive optical amplifier with tensile strained wells grown by MOCVD

Polarization-insensitive AlGaInAs'InP semiconductor optical amplifier is realized at wavelength of 1.55 . The active layer consists of three tensile strained wells with strain of 0.40%. The amplifier is fabricated to ridge waveguide structure. The testing result shows the amplifiers have excellent polarization insensitivity (less than 0.8dB). The 1540 nm wavelength optical gain is 20 dB at the bias current of 200 mA.     

MOVPE Growth and Fabrication of 1.3μm High Power InGaAsP-InP Polarization-Insensitive Superluminescent Diodes with Complex Strained Quantum Wells

High power polarization-insensitive InGaAsP-InP multiple quantum well (MQW) superluminescent diodes (SLD's) emitting at 1.3μm were investigated. A combination of tensile strained and compressively strained quantum wells called complex strained MQW were used in a single active layer in order to obtain polarization insensitivity. Low-pressure metalorganic chemical vapor phase epitaxy was used for crystal growth. High resolution X-ray diffraction and photoluminescence spectra showed excellent crystal quality. The SLD's were fabricated to ridge waveguide structure with 7° tilted cavity, the two facets were coated with two layers anti-reflection TiO₂/SiO₂ films, residual facet reflectivity was found to be less than 0.04%. The SLD's exhibited a up to 18.8 mW optical output power and less than 1 dB polarization dependence of output power with a less than 0.5 dB optical spectra modulation at 250 mA.     

Structural and optical properties of Bi3.25Nd0.75Ti3O12 ferroelectric thin films

Ferroelectric Bi_3.25Nd_0.75Ti₃O₁₂ (BNT) thin films were grown on (111)Pt/Ti/SiO₂/Si substrates by a chemical solution method. The films were composed of large rod-like grains. XRD and Raman spectroscopy measurements showed they were polycrystalline perovskite structure with a good crystallinity. Pt/BNT/Pt capacitors had been fabricated and showed good ferroelectricity. The optical constants (n, k) of BNT thin films in the wavelength ranges of 0.2–1.7 μm and 2.5–11.4 μm were obtained by spectroscopic ellipsometry measurements. The dispersion of the refractive index in the interband transition region followed the single electronic oscillator model. The optical band gap was found to be about 3.61 eV. PACS 77.84.-s; 78.20.Ci; 77.80.-e     

Investigation of plasmonics resonance infrared bowtie metal antenna

An approximate resonance wavelength equation that varies with metal antenna structure size is developed to design a bowtie gold metal antenna working at near-infrared (IR) wavelength. Bowtie antenna structures with resonance wavelength of 1.06 μm, 1.55 μm and 10.6 μm are designed based on this equation. A finite-difference time domain (FDTD) algorithm with total field scattered field (TFSF) source simulation shows the resonance wavelength of the designed structures being precisely in agreement with the expected wavelengths from the equation. Planar integration of the metal bowtie antennas is discussed as well. Gold nanohole bowtie antenna arrays are fabricated and the near-field optical transmission properties of the nanohole array are investigated with a near-field scanning optical microscope (NSOM). Our experimental results verify the near-field optical transmission performance and further demonstrate that they are in agreement with the theoretical calculation results. The high enhancement efficiency and integration of the metal bowtie antennas open the possibility of a wide application in IR optoelectronics detection and imaging.     

Leakage current and capacitance characteristics of Si/SiO2/Si single-barrier varactor

We investigate, both experimentally and theoretically, current and capacitance (I–V/C–V) characteristics and the device performance of Si/SiO₂/Si single-barrier varactor diodes (SBVs). Two diodes were fabricated with different SiO₂ layer thicknesses using the state-of-the-art wafer bonding technique. The devices have very low leakage currents (about 5×10^-2 and 1.8×10^-2 mA/mm²) and intrinsic capacitance levels of typically 1.5 and 50 nF/mm² for diodes with 5-nm and 20-nm oxide layers, respectively. With the present device physical parameters (25-mm² device area, 760-μm modulation layer thickness and ≈10¹⁵-cm^-3 doping level), the estimated cut-off frequency is about 5×10⁷ Hz. With the physical parameters of the present existing III–V triplers, the cut-off frequency of our Si-based SBV can be as high as 0.5 THz. Received: 9 February 2001 / Accepted: 9 February 2001 / Published online: 23 March 2001     

1.53 μm photoluminescence from ErYb(DBM)<Subscript>3</Subscript>MA containing polymer

We investigated the optical properties of the ErYb(DBM)₃MA complexes and the ErYb(DBM)₃MA containing polymer. Absorption and photoluminescence spectra confirm that the presence of Yb³⁺ ions enhances luminescence efficiency of Er³⁺ ions. The full width at half maximum bandwidth (FWHM) is ∼80 nm wide around 1.53 μm wavelength. We also fabricated ErYb containing polymeric channel waveguides using reactive ion etching technique. As an input pump of 120 mW was used, a ∼1.53 μm spontaneous emission was obtained in a 4-mm-long waveguide. PACS 42.00.00; 42.70.Jk; 42.82.Et     

Properties of a Thin-Film Optical Waveguide Using Fluorinated Silicon Oxide and Organic Spin-on-Glass Films

A thin-film optical waveguide using a fluorinated silicon oxide (SiOF) as a core layer was investigated. An organic spin-on-glass (SOG) film was used for a cladding layer. The SiOF films were formed at 23°C by a liquid-phase deposition (LPD) technique using a supersaturated hydrofluosilicic acid (H₂SiF₆) aqueous solution. A thin-film optical waveguide structure for single mode was designed and fabricated, based on the dispersion properties of refractive indices for the LPD-SiOF and organic SOG films. The refractive indices at a wavelength of 632.8 nm were 1.430 and around 1.400 for the LPD-SiOF and organic SOG films, respectively. The thickness of LPD-SiOF films deposited was 1.18 μm. Thicknesses of cladding organic SOG films cured at 300 and 400°C were 1.28 and 1.31μm, respectively. The effective refractive indices for single mode were 1.4169 and 1.4158 at a wavelength of 632.8 nm for the cladding organic SOG films cured at 300 and 400°C, respectively, and differences between the measured and calculated incident angles were 0.84° and 1.29° for the cladding organic SOG films cured at these respective temperatures. A streak of guided-light was observed for the LPD-SiOF/SOG structure optical waveguide. The transmission loss was 7.6-7.9 dB/cm.     

Design and Fabrication of GaInAsP/InP VCSEL with Two a-Si/a-SiN x Bragg Reflectors

We report on the design and fabrication of a 1.55 μm wavelength Vertical Cavity Surface Emitting Lasers (VCSELs) which consists of two dielectric Bragg mirrors and a InGaAsP-based active region. The dielectric materials are amorphous silicon and amorphous silicon nitride. Layers of such materials have been deposited by magnetron sputtering and analyzed in order to determine their optical properties. A large refractive index difference of 1.9 is found between these materials. Distributed Bragg Reflectors (DBRs) based on these dielectric materials quarter wave layers have been studied by optical measurements and confronted to theoretical calculations based on the transfer matrix method. A maximum reflectivity of 99.5% at 1.55 μm and a large spectral bandwidth of 800 nm are reached with only four and a half periods of a-Si/a-SiN_x. The VCSEL was fabricated by metallic bonding process. This method allows to bond an InP-based active region as the gain medium on a Si substrate thanks to the formation of a Au–In alloy. This process is performed at a low temperature of 240°C without damaging the optical properties of the microcavity. This VCSEL has been characterized by an optical pumping experiment with a low and a high-density optical power and a laser emission has been obtained at room-temperature.     

Modeling and analysis of photoconductive detectors based on Hg<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te for free space optical communication

In this paper we report a theoretical analysis of a long wavelength photoconductive detector for characterizing and optimizing the device in respect of voltage responsivity, quantum efficiency, detectivity and noise equivalent power. The model has been applied to examine the potential of an n-type Hg_0.77Cd_0.23Te photoconductive detector for possible application in free space optical communication system operating at the atmospheric window near 9.6 μ m. In the present analysis we have taken into account all the major recombination mechanisms (e.g., Radiative, Auger, and Shockley-Read-Hall types) including the effect of surface recombination at the interfaces that shape the characteristics of photoconductor. The results obtained on the basis of our analysis reveal that in the absence of surface recombination the device exhibits a peak quantum efficiency of 90%, a maximum detectivity of 10⁸ MHz^1/2/W at 77 K, a 3 dB bandwidth of 117.86 MHz and noise voltage of 5.4 × 10⁻⁶ V/Hz^1/2. The sweep-out effect has been found to degrade the detectivity nearly by a factor of 10 at the same temperature and wavelength of operation. The estimated noise equivalent power of the photodetector is of the order of 10⁻⁹ W at 9.6 μm wavelength.     

Ultrabroad gain in an optical parametric generator with periodically poled KTiOPO4

An optical parametric gain bandwidth of 115 THz at full-width half maximum is generated from a picosecond Ti:sapphire pumped degenerate optical parametric generator. This ultrabroad bandwidth could be obtained by first identifying the wavelength where the nonlinear optical material has zero group-velocity dispersion (GVD). By pumping at half this wavelength the degenerate signal–idler pairs can accommodate ultrabroad bandwidths. The explanation for this is that the group velocities of the signal and the idlers are approximately matched and the GVD is small. However, in order to thoroughly investigate the degeneracy region around 1700 nm we fabricated several periodically poled KTiOPO₄ (PPKTP) crystals with different periods, and also one periodically poled RbTiOPO₄ (PPRTP). Both collinear and noncollinear configurations were employed for broadband parametric generation in this region. It was found that the optimum pump wavelength is in the region between 800 nm to 850 nm for PPKTP, and we could also conclude that a similar performance was found for PPRTP. This work will allow the design of optical parametric devices for generating few-cycle pulses in the spectral region between 1.1 μm and 3.8 μm. PACS 42.65.Re; 42.65.Ky; 42.65.-k     

Optical metamaterials at near and mid-IR range fabricated by nanoimprint lithography

Two types of optical metamaterials operating at near-IR and mid-IR frequencies, respectively, have been designed, fabricated by nanoimprint lithography (NIL), and characterized by laser spectroscopic ellipsometry. The structure for the near-IR range was a metal/dielectric/metal stack “fishnet” structure that demonstrated negative permittivity and permeability in the same frequency region and hence exhibited a negative refractive index at a wavelength near 1.7 μm. In the mid-IR range, the metamaterial was an ordered array of fourfold symmetric L-shaped resonators (LSRs) that showed both a dipole plasmon resonance resulting in negative permittivity and a magnetic resonance with negative permeability near wavelengths of 3.7 μm and 5.25 μm, respectively. The optical properties of both metamaterials are in agreement with theoretical predictions. This work demonstrates the feasibility of designing various optical negative-index metamaterials and fabricating them using the nanoimprint lithography as a low-cost, high-throughput fabrication approach. PACS 42.25.Bs; 81.16.Nd; 42.70.-a; 81.07.-b     

Thin-film poly-Si solar cells on glass substrate fabricated at low temperature

The performances of thin-film poly-Si solar cells with a thickness of less than 5 μm on a glass substrate have been investigated. The cell of glass/back reflector/n-i-p type Si/ITO is well characterized by the structure of naturally surface texture and enhanced absorption with a back reflector (STAR), where the active i-type poly-Si layer was fabricated by plasma chemical vapor deposition (CVD) at low temperature. The cell with a thickness of 2.0 μm demonstrated an intrinsic efficiency of 10.7% (aperture 10.1%), an open-circuit voltage of 0.539 V and a short-current density of 25.8 mA/cm² as independently confirmed by Japan Quality Assurance. No light-induced degradation is observed. The optical and transport properties of poly-Si cells are summarized. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 7 July 1999     

Potential for micromachined actuation of ultra-wide continuously tunable optoelectronic devices

Tailored scaling represents a principle of success that, both in nature and in technology, allows the effectiveness of physical effects to be enhanced. Mutation and selection in nature are imitated in technology, e.g. by model calculation and design. Proper scaling of dimensions in natural photonic crystals and our fabricated artificial 1D photonic crystals (DBRs, distributed Bragg reflectors) enable efficient diffractive interaction in a specific spectral range. For our optical microsystems we illustrate that tailored miniaturization may also increase the mechanical stability and the effectiveness of spectral tuning by thermal and electrostatic actuation, since the relative significance of the fundamental physical forces involved considerably changes with scaling. These basic physical principles are rigorously applied in micromachined 1.55-μm vertical-resonator-based devices. We modeled, implemented and characterized 1.55-μm micromachined optical filters and vertical-cavity surface-emitting laser devices capable of wide, monotonic and kink-free tuning by a single control parameter. Tuning is achieved by mechanical actuation of one or several air-gaps that are part of the vertical resonator including two ultra-highly reflective DBR mirrors of strong refractive index contrast: (i) Δn=2.17 for InP/air-gap DBRs (3.5 periods) using GaInAs sacrificial layers and (ii) Δn=0.5 for Si₃N₄/SiO₂ DBRs (12 periods) with a polymer sacrificial layer to implement the air-cavity. In semiconductor multiple air-gap filters, a continuous tuning of >8% of the absolute wavelength is obtained. Varying the reverse voltage (U=0–5 V) between the membranes (electrostatic actuation), a tuning range of >110 nm was obtained for a large number of devices. The correlation of the wavelength and the applied voltage is accurately reproducible without any hysteresis. In two filters, tuning of 127 and 130 nm was observed for about ΔU=7 V. The extremely wide tuning range and the very small voltage required are record values to the best of our knowledge. For thermally actuated dielectric filters based on polymer sacrificial layers, Δλ/ΔU=-7 nm/V is found. Received: 10 May 2002 / Published online: 8 August 2002     

Optical properties of one-dimensional photonic crystals fabricated by photo-electrochemical etching of silicon

The optical properties of one-dimensional photonic crystals (1D PCs), fabricated by electrochemical etching of periodic wall arrays on n-type (100) Si substrates, are investigated in this study. Several 1D PCs were fabricated with lattice periods varying from 4 to 7 μm and with trench depths in the range 160–210 μm. In-plane reflection spectra of the photonic structures at different depths were registered over a wide spectral range of 1.5–15 μm using Fourier Transform Infra-Red (FTIR) micro-spectroscopy. Some of the features observed in the reflection spectra of the structures investigated are believed to be as a result of interface roughness. A corrugated side-wall surface, an artifact of the fabrication technique, results in the degradation of optical reflection characteristics, principally mainly in the near IR spectral range, and the emergence of optical anisotropy. As a result of the periodicity, modulation of the reflection spectra, that is, the difference between the maxima and minima of the interference fringes, reached a value of 95% in the mid-infrared. The optical properties of the structures investigated indicate that they show promise for microphotonics applications.     

快速可调谐电光聚合物波导光栅

提出并设计了快速可调谐电光聚合物波导光栅.该波导光栅通过极化聚合物的线性电光效应 可实现谐振波长的纳秒级快速调谐，调谐灵敏度为61pm/V.研究了该波导光栅的反射谱和透 射谱特性与光栅周期,周期数,折射率调制函数及其调制大小的关系.讨论了波导光栅的材料 选择,制备工艺,快速可调谐性和偏振相关性.该波导光栅不仅克服了光纤光栅调谐速度慢和不利于大规模集成的不足，而且具有调谐灵敏度高,制备工艺与半导体工艺兼容和偏振无关 等优点. 关键词： 光通信器件 光波导 电光效应 聚合物 波导光栅 光纤光栅