Broad-band amplification using a novel amplifier topology

A new broad-band amplifier topology that provides a continuous gain across 75 nm (1528-1603 nm) of optical bandwidth using Er³⁺ -doped SiO₂ fiber is demonstrated. The noise figure is low and does not present any discontinuity across the entire amplification bandwidth. The topology has a buried 1550/1585-nm splitter to reduce input signal loss and to provide amplified spontaneous emission suppression. The topology has been optimized using hybrid fiber; Er³⁺-doped SiO₂ for the first stage and Er ³⁺-doped TeO₂ for the second stage, achieving 82 nm (1526-1618 nm) of optical bandwidth     

In0.53Ga0.47As MSM photodiodes withtransparent CTO Schottky contacts and digital superlattice grading

Metal-semiconductor-metal (MSM) photodiodes with an In_0.53 Ga_0.47As active region were investigated using a transparent cadmium tin oxide (CTO) layer for the interdigitated electrodes to improve the low responsivity of conventional MSM photodiodes with opaque electrodes. CTO is suitable as a Schottky contact, an optical window and an anti-reflection (AR) coating. The transparent contact prevents shadowing of the active layer by the top electrode, thus allowing greater collection of incident light. Responsivity of CTO-based MSM photodiodes with 1-μm finger widths and 2-μm finger spacings and without an AR coating between the electrodes was twice (0.62 A/W) that of a similar MSM photodiodes with Ti/Au electrodes (0.30 A/W). A thin 800 Å In_0.52Al_0.48 As layer was inserted below the electrodes to elevate the electrode Schottky barrier height. A digitally graded superlattice region (660 A) was also employed to reduce carrier trapping at the In _0.53Ga_0.47As/In_0.52Al_0.48As heterointerface which acts to degrade photodiode bandwidth. Bandwidth of opaque electrode MSMs was elevated nearly an order of magnitude over a previous MSM photodiode design with an abrupt heterointerface, whereas the bandwidth of transparent electrode MSM's only improved about five times, indicating resistive effects may be intervening     

Investigation of circularly polarized loop antennas with a parasitic element for bandwidth enhancement

It is demonstrated that the bandwidth of circular polarization (CP) can be significantly increased when one more parasitic loop is added inside the original loop. A single-loop antenna has only one minimum axial ratio (AR) point while the two-loop antenna can create two minimum AR points. An appropriate combination of the two minimum AR points results in a significant enhancement for the CP bandwidth. A comprehensive study of the new type of broad-band circularly polarized antennas is presented. Several loop configurations, including a circular loop, a rhombic loop, and a dual rhombic loop with a series feed and a parallel feed, are investigated. The AR (/spl les/2 dB) bandwidth of the circular-loop antenna with a parasitic circular loop is found to be 20%, more than three times the AR bandwidth of a single loop. For the rhombic-loop antenna with a parasitic rhombic loop, an AR bandwidth (AR/spl les/2dB) of more than 40% can be achieved by changing the rhombus vertex angle. The AR (/spl les/2 dB) bandwidths of the series-fed and parallel-fed dual rhombic-loop antennas with a parasitic element are 30% and 50%, respectively. A broad-band balun is incorporated into the series-fed dual rhombic-loop antenna for impedance matching. The broad-band CP performance of the loop antennas is verified by experimental results.     

Monolithic integration of vertical-cavity laser diodes and resonantphotodetectors with hybrid Si3N4-SiO2top Bragg mirrors

Top-emitting vertical-cavity surface-emitting lasers are monolithically integrated with resonance enhanced photodetectors (REPDs), utilizing dielectric Si₃N₄-SiO₂ top Bragg mirrors. High-efficiency (66%) REPDs with a narrow optical bandwidth of 1.25 nm were realized by adjusting the incoupling mirror's reflectivity. High-finesse REPDs with low QW-absorption, have smaller efficiencies (~10%), but extremely narrow optical bandwidths down to 0.196 nm. The high spectral resolution of these detectors leads to the appearance of higher-order transverse detector modes in the spectral response     

Technique for velocity-matched traveling-wave electroopticmodulator in AlGaAs/GaAs

A design for a velocity-matched traveling-wave directional-coupler intensity modulator in AlGaAs/GaAs is proposed. The structure utilizes a thin coating of Ta₂O₅ on the top of the modulator/electrode structure in order to achieve velocity matching between the optical wave and microwave signal. The film does not significantly affect the optical properties or voltage requirements of the modulator. The optical and RF characteristics of the modulator are analyzed using the effective-index and finite-difference methods. The optical bandwidth is calculated numerically, taking into account both the anticipated velocity mismatches due to fabrication tolerances and the calculated frequency-dependent microwave losses. The predicted small-signal bandwidth of a 3 mm long direction coupler biased at a null point is greater than 45 GHz, and exceeds 100 GHz (~50 GHz electrical bandwidth) when the coupler is biased in the linear region. This device is designed to operate at 830 nm with a maximum modulation voltage of 5 V. The figure of merit of the proposed device is therefore at least 10 GHz/V when an electrical bandwidth of 50 GHz is used     

Compact loaded slot ring antenna for circularly polarised radiation

A novel slot-ring circularly polarised antenna is reported. With sectorial slots loading, the axial ratio (AR) bandwidth is increased and the circumference of the slot-ring is also reduced to 0.5 lambda₀ (lambda₀ is the free-space wavelength at the centre frequency). The antenna features a 22.4% impedance bandwidth for |S ₁₁| les -10 dB, a 10.6% bandwidth for AR les 3 dB and stable radiation patterns over the whole operating band.     

A novel and effective PECVD SiO2/SiN antireflectioncoating for Si solar cells

It is shown that sequential plasma-enhanced chemical vapor deposition (PECVD) of SiN and SiO₂ can produce a very effective double-layer antireflection (AR) coating. This AR coating is compared with the frequently used and highly efficient MgF₂/ZnS double layer coating. The SiO₂/SiN coating improves the short-circuit current (J_SC) by 47%, open-circuit voltage (V_OC) by 3.7%, and efficiency (Eff) by 55% for silicon cells with oxide surface passivation. The counterpart MgF₂/ZnS coating gives similar but slightly smaller improvement in V_OC and Eff. However, if silicon cells do not have the oxide passivation, the PECVD SiO₂/SiN gives much greater improvement in the cell parameters, 57% in J_SC, 8% in V_OC, and 66% in efficiency, compared to the MgF₂/ZnS coating which improves J_SC by 50%, V_OC by 2%, and cell efficiency by 54%. This significant additional improvement results from the PECVD deposition-induced surface/defect passivation. The internal quantum efficiency (IQE) measurements showed that the PECVD SiO₂/SiN coating a absorbs fair amount of photons in the short-wavelength range (<500 nm); however, the improved surface/defect passivation more than compensates for the loss in J_SC and gives higher improvement in the cell efficiency compared to the MgF₂/ZnS coating     

基于半导体光放大器的非相干光源抗反膜的优化设计

对半导体光放大器(SOA)放大的自发发射(ASE)谱进行了实验和理论研究,并且分析了SOA端面反射率对ASE谱的谱宽以及平坦度的影响.结果表明,不恰当的抗反膜会严重减小输出光谱的带宽;而在采用具有宽带材料增益谱的有源区基础上,结合抗反膜的优化设计,则可以获得既宽又平坦的非相干光源.     

色散会聚透镜系统中的微米焦开关

为了研究宽带TEM22模厄米-高斯光束通过受光阑限制色散会聚透镜系统的微米焦开关现象,采用数值计算方法对TEM22模厄米-高斯光束的光强分布进行了分析,取得了传输轴上光强分布的数据.结果表明,带宽变化引起轴上TEM22模厄米-高斯光束两个光强极大的相对大小改变,结果导致光强主极大位置跃变;当相对带宽γ=0.231且菲涅...     

Measurement of facet reflectivity of antireflection-coatedelectroabsorption modulator using induced photocurrent

A new method to measure the facet reflectivity of an antireflection (AR)-coated electroabsorption (EA) modulator in the region of operating wavelengths is proposed. First, by measuring the induced photocurrent and reflectance simultaneously at the front facet of an EA waveguide, the cleaved facet reflectivity and propagation loss are determined. After coating the facet with AR, the residual reflectivity of AR-coated facet is obtained from the measured photocurrent spectra and the predetermined facet reflectivity. We demonstrate the reflectivity of a double-layer AR-coated EA modulator can be measured to be ~4×10^-4 at 1.55 μm for TE polarization by using the proposed technique     

High quantum efficiency and narrow absorption bandwidth of thewafer-fused resonant In0.53Ga0.47As photodetectors

We demonstrate greater than 90% quantum efficiency in an In_0.53Ga_0.47As photodetector with a thin (900 Å) absorbing layer. This was achieved by inserting the In_0.53 Ga_0.47As/InP epitaxial layer into a microcavity composed of a GaAs/AlAs quarter-wavelength stack (QWS) and a Si/SiO₂ dielectric mirror. The 900-Å-thick In_0.53 Ga_0.47As layer was wafer fused to a GaAs/AlAs mirror, having nearly 100% power reflectivity. A Si/SiO₂ dielectric mirror was subsequently deposited onto the wafer-fused photodiode to form an asymmetric Fabry-Perot cavity. The external quantum efficiency and absorption bandwidth for the wafer-fused RCE photodiodes were measured to be 94±3% and 14 nm, respectively. To our knowledge, these wafer-fused RCE photodetectors have the highest external quantum efficiency and narrowest absorption bandwidth ever reported on the long-wavelength resonant-cavity-enhanced photodetectors     

Lateral mode stabilization of diode lasers by means of apertured facet reflectors

We report the design, fabrication, and testing of an apertured facet reflector for a GaAs/GaAlAs room temperature CW diode laser. The reflector, composed of Al2O3, Si, Te, and Al2O3layers with the Te ablated by the laser beam itself at the mode center, has a higher reflectivity at its center compared to the unablated surrounding region. This nonuniform reflectivity acts to stabilize laterally the fundamental mode and substantially increase the "kink" level. Specifically, the "kink" level of a large optical cavity (LOC) heterostructure laser with an 8 μm wide stripe was increased from ≈ 10 mW before coating to above 30 mW by the addition of the apertured reflector.     

An ARROW optical wavelength filter: design and analysis

A novel ARROW (antiresonant reflecting optical waveguide)-based optical wavelength filter is proposed. The modal characteristics of the coupled ARROW structures is analyzed rigorously by the transverse resonance method. As an alternative configuration to the conventional directional coupler filter, the proposed device features large core size compatible with fiber and nearly periodic dependence of the coupling-length on the waveguide separation, which provides more flexibility for fabrication of the device. A design procedure for this type of coupled ARROW structure used for wavelength filtering is presented. The devices designed are simulated and verified by the beam propagation method. An ARROW wavelength filter based on SiO₂/TiO₂ has a FWHM bandwidth narrower than 7 Å     

宽带宽角圆极化贴片天线的实验研究

本文提出一种新的宽带宽角圆极化微带天线设计方法,在利用容性探针近耦合馈电的基础上,通过平衡馈电,使贴片天线的阻抗带宽(VSWR≤2)和宽角轴比带宽(45°圆锥空域内的AR≤3dB)达到20%以上.实验测试表明,利用该方法设计的贴片天线的带宽达到26.33%(VSWR、宽角AR同时满足要求),从而证实了该方法的正确性和实用性.     

On the Effects of an Antireflection Coating Impairment on the Sensitivity to Optical Feedback of AR/HR Semiconductor DFB Lasers

The sensitivity to optical feedback of 1.55- $mu{hbox {m}}$ antireflection (AR)/high-reflection (HR) DFB semiconductor lasers is presented in this paper. The onset of the coherence collapse, which is the most critical feedback regime for optical transmissions, is theoretically investigated with a stress on its dependence with facet phase effects (FPEs). Taking into account FPEs on both facets, the sensitivity to optical feedback is evaluated with respect to both the coupling strength coefficient and the feedback level. The first part of the paper shows that due to the HR-facet, a distribution up to several decibels on the coherence collapse thresholds is predicted over the whole DFB laser population. The second part concentrates on the coherence collapse dependence with respect to the AR coating. Calculations show an enhancement of the coherent collapse threshold distribution up to 10 dB due to the AR coating impairment. These simulations are of first importance for optical transmissions since they show that for AR coatings beyond $10^{-4}$ , the sensitivity to optical feedback of AR/HR DFB lasers is extremely difficult to evaluate from one laser to another. On the other hand, for AR coatings below $10^{-4}$ , all feedback performances are directly connected to the laser wavelength, and DFB lasers can be easily selected for high bit rate isolator-free transmission.      

Wide-band high-efficiency optical-to-electrical conversion stimulusprobe heads for testing large-signal responses of high-speed electronicdevices

Wide-band high-efficiency optical-to-electrical conversion stimulus probe heads have been developed for testing large-signal responses of high-speed electronic devices. Two types of such probes were demonstrated using a 1.55-μm 85-GHz-bandwidth waveguide p-i-n photodiode. The type-I probe employs a simple semirigid coaxial cable with a bias network for the electrical-signal transmission, resulting in a very low modal dispersion of <1.0 ps. The highest -3-dB bandwidth of 60 GHz was obtained for an output voltage of 230 mV _p-p, and was maintained beyond 50 GHz for output voltages of up to 400 mV_p-p. The type-II probe employs a broad-band InP high electron-mobility transistor distributed amplifier that boosts the electrical output signal amplitude over 1 V_p-p. The -3-dB bandwidth is 40 (35) GHz for output voltages up to 500 (1000) mV_p-p      

Application of indium-tin-oxide with improved transmittance at 1.3μm for MSM photodetectors

The optical transmittance of indium-tin oxide (ITO) at a wavelength of 1.3 μm has been improved by adding forming gas (H₂ /N₂) to the Ar sputtering gas. It is shown that the presence of H₂ in the plasma decreases the carrier concentration in ITO and increases the optical transmittance of a 320 nm-thick ITO film from 69.7% to 99.5%. The application of the high transmittance ITO to the fabrication of metal-semiconductor-metal (MSM) photodiodes on InAlAs/InGaAs heterostructures has resulted in an improvement of responsivity from 0.6 A/W to 0.76 A/W. This is double the responsivity of 0.39 A/W obtained for Ti/Au detectors. A 3-dB bandwidth of 6 GHz was obtained for the high transparency ITO device with 3 μm fingers and gaps and with an area of 50 μm×50 μm     

Epitaxial (Al,Ga)InP-oxide distributed Bragg reflectors for use invisible-wavelength optical devices

Epitaxially-grown distributed Bragg reflectors (DBR's) employing thermally oxidized AlAs as the low refractive index constituent and (Al,Ga)InP as the high index constituent are fabricated. The 4.5-pair Ga _0.5In_0.5 P-oxide and Al_0.5In_0.5 P-oxide DBR's exhibit high reflectivity (>90%) over a range of 635-967 nm and 470-676 nm, respectively. The (Al,Ga)InP-oxide DBR's are shown to require less material to produce high reflectivity and to have significantly wider bandwidth than all-semiconductor DBR's used in the visible spectrum     

Modeling of broad-band traveling-wave optical-intensity modulators

In this paper, an accurate simulation tool for the electrical and optical response of broad-band traveling-wave optical intensity modulators is presented, which takes into account multisectional electrical transmission lines. This model is applied to analyze a high-speed fully packaged LiNbO₃ Mach-Zehnder interferometer     

Performance of frequency-hopped NPCSMA for broad-band personalcommunication services (B-PCS) at millimeter waves in an urban mobileradio environment

A millimeter-wave (MMW) propagation model, based on multiray optical theory and the geometrical theory of diffraction (GTD), is used to statistically study the delay spread and coherence bandwidth (B_c ) for broad-band personal communication services (B-PCSs) in urban mobile radio environments. The results of the statistical model are compared with outdoor measurements near the street level with B_c varying over a range of frequencies (14-100 MHz). It is shown that the distribution of B_c depends on antenna heights, street layout, and frequency. The signal level statistics are evaluated and found to agree with the measurements. In general, these statistics are non-Rayleigh distributed. The additive thermal Gaussian noise (AGN) is calculated for such channels to evaluate the signal-to noise ratio (SNR). The performance of the medium-access control (MAC) sublayer is studied by first evaluating the probability of packet retransmission and then the performance of the FH-NPCSPMA protocol. The FH-NPCSMA protocol is the combination of frequency hopping spread spectrum (FH-SS) and nonpersistent carrier-sense multiple access (NPCSMA) that is proposed to improve the system throughput and average packet delay for both high traffic and multipath channels at MMW. The main advantage of the PH-NPCSMA protocol is to improve the performance over fading channels