Bend-Free Optical Power Transfer Using Photonic Crystal Waveguide Arrays

We report the study of 2-D photonic-crystal waveguide arrays (PCWA) composed of $N$ identical waveguides coupled evanescently with each other. The coupling properties of the waveguide modes are investigated using coupled-mode theory and finite-difference time domain method. One straightforward application of such an analysis is to route input power from a central waveguide to side waveguides. As a result, appropriate designs of PCWAs may permit the realization of efficient, compact and novel devices. For instance, we show that power dividers, switchers, and Mach–Zehnder interferometers can be feasible using $N =3$ channels. On the other hand, $N =5$ waveguides can divide the input power by 1/4 at a distance of approximately 37.2 $ mu{hbox {m}} $. Waveguide bends and Y-type junctions are used heavily for power transfer but they are prone to scattering losses; hence, lowering the transmission efficiency. They can be eliminated by means of PCWAs in the design of optical power distribution through photonic circuits.      

Enhanced Transmission in a Fiber-Coupled Au Stripe Waveguide System

We experimentally demonstrated the enhanced transmission in a fiber-coupled Au stripe waveguide system using a linearly tapered (LT) structure at a telecommunication wavelength of 1.55 $mu{hbox {m}}$. The LT structure consists of two 100- $mu{hbox {m}}$-long tapered regions connecting various widths of input and output waveguides with a waist region. The lowest insertion loss of the 1-cm-long LT-Au stripe waveguide is $sim$4.3 dB, when it has 6-$mu{hbox {m}}$ -wide input and output waveguides and a 4- $mu{hbox {m}}$-wide waist waveguide. The insertion loss is reduced by $sim$ 2 dB compared to the 4-$mu{hbox {m}}$-wide and 1-cm-long straight Au stripe waveguide, which is achieved by decreasing the coupling loss. The losses of the LT region, which has a tapered angle of less than 0.3$^{circ}$ between the input–output waveguides and the waist waveguide, are smaller than 0.4 dB. We showed that the insertion loss of the Au stripe waveguide can be reduced by introducing the LT structure, which can also provide efficient mode conversion.      

Photonic Crystal Slab Waveguides Based on Antiresonant Reflecting Optical Waveguide Structures

A novel two-dimensional photonic crystal slab waveguide based on an antiresonant reflecting optical waveguide (ARROW) structure is proposed and designed. Lightwaves propagating in this waveguide are confined by antiresonance reflection vertically and the photonic band gap laterally. In order to obtain the characteristics of the ARROW-based photonic crystal waveguides, the three-dimensional finite-difference time-domain simulations are performed. With a lateral adiabatic taper, a coupling efficiency of 80.3% from a single-mode fiber to the ARROW-based photonic crystal waveguide of a single-line defect is obtained. In addition, propagation losses less than 10 dB/mm and bend losses of 0.23 and 0.39 dB/bend for the designed 60$^{circ}$ and 120$^{circ}$ bends are achieved at an operating wavelength of $1.55~mu{hbox {m}}$.      

Waveguide Interconnection in Silica-Based Planar Lightwave Circuit Using Femtosecond Laser

The interconnection of waveguides inside a silica- based planar lightwave circuit (PLC) is demonstrated both two- and three-dimensionally by using a femtosecond laser. The waveguides written with a femtosecond laser can be successfully connected to waveguides inside a PLC with low loss. Unlike previous work on the direct writing of 2-D and 3-D waveguides in bulk glass, the waveguide must be written a few tens of micrometers beneath the surface of a PLC composed of multilayers of different glasses. To realize a low-loss waveguide, we studied the sensitivity difference for femtosecond pulses in each layer in detail and developed a multiple-scanning technique with a femtosecond laser for writing waveguides. In addition, we investigated a mode-field (MF) diameter control technique to allow us to achieve the low-loss interconnection of PLC and written waveguides. We also studied particular problems caused by nonlinear optical effects, such as the positional displacement of written waveguides from the focal point. As a result, we achieved a low-loss waveguide with almost the same MF diameter as a PLC waveguide and performed the first demonstration of interconnection between PLC waveguides. The excess losses at 1550 nm were 1.4 and 2.3 dB for 2-D and 3-D interconnection, respectively.      

Silicon Photonic Wire Filter Using Asymmetric Sidewall Long-Period Waveguide Grating in a Two-Mode Waveguide

We demonstrate a silicon photonic wire filter using asymmetric sidewall long-period waveguide gratings for the first time. The proposed device consists of single-mode waveguide sections, a two-mode section with corrugated gratings, and taper sections to connect them. The operation of this device is based on the codirectional coupling between two core modes. By adopting a high refractive index contrast waveguide, the period and depth of grating are given as 4.44 m and 5 nm, respectively. Thus, the total length of long-period grating is significantly reduced to 260 m. The measured maximum attenuation at the center wavelength is about 13 dB. The bandwidth of the transmission dip is 15 nm. Finally, issues on the design and the performance of our device are discussed.     

AlGaAs-based two-dimensional photonic crystal slab with defectwaveguides for planar lightwave circuit applications

An AlGaAs-based near-infrared 2-D photonic crystal (PC) with an air-bridge structure featuring defect waveguides has been developed. For the sample without defect waveguides, measurements of the optical transmission characteristics in the wavelength range from 850 nm to 1100 nm showed a deep attenuation due to a bandgap with 30-35 dB attenuation and transmittance of nearly 100% for the guided modes. Optical propagation properties of defect waveguides were obtained by two methods: measurements of transmission spectra and plan-view observations of the optical beam trace along the waveguide with an infrared-vidicon camera. 3-D finite-difference time-domain simulations for the band structure and transmission spectra in the air-bridge slab with and without defect waveguides have revealed the appearance of four defect propagation modes specific to the defect waveguide, between two slab modes for the defect-free photonic crystal slab. These defect modes were experimentally identified in the measured transmission spectra     

3D-Printed Broadband Dielectric Tube Terahertz Waveguide with Anti-Reflection Structure

We demonstrate broadband, low loss, and close-to-zero dispersion guidance of terahertz (THz) radiation in a dielectric tube with an anti-reflection structure (AR-tube waveguide) in the frequency range from 0.2 to 1.0 THz. The anti-reflection structure (ARS) consists of close-packed cones in a hexagonal lattice arranged on the outer surface of the tube cladding. The feature size of the ARS is in the order of the wavelength between 0.2 and 1.0 THz. The waveguides are fabricated with the versatile and cost efficient 3D-printing method. Terahertz time-domain spectroscopy (THz-TDS) measurements as well as 3D finite-difference time-domain simulations (FDTD) are performed to extensively characterize the AR-tube waveguides. Spectrograms, attenuation spectra, effective phase refractive indices, and the group-velocity dispersion parameters β ₂ of the AR-tube waveguides are presented. Both the experimental and numerical results confirm the extended bandwidth and smaller group-velocity dispersion of the AR-tube waveguide compared to a low loss plain dielectric tube THz waveguide. The AR-tube waveguide prototypes show an attenuation spectrum close to the theoretical limit given by the infinite cladding tube waveguide.     

Dispersion compensation with photonic crystal line-defect waveguides

A concept for dispersion compensation in transmission is proposed, based on modes anti-crossing in photonic crystal (PC) line-defect waveguides. Quasi-constant positive and negative dispersion is possible in order of 100 ps/nm/mm on the bandwidth of 100 GHz. An adiabatic taper is proposed for efficient coupling into PC structure.     

Flatband Slow Light in Asymmetric Line-Defect Photonic Crystal Waveguide Featuring Low Group Velocity and Dispersion

In this paper, an asymmetric photonic crystal (PC) waveguide is proposed for slow light transmission. A row of air holes is removed to form a line-defect waveguide, and the lateral symmetry of the waveguide is broken by shifting the holes in the PC cladding on one side along the waveguide axis. Two structural parameters are carefully adjusted: the amount of shift compared with the array of holes in the cladding on the other side, and the radius of the holes closest to the waveguide core in the shifted PC cladding. In the asymmetric waveguide, it is possible to obtain flat band modes with low group velocity (c/50) and low dispersion (on the order of 10⁴ ps²/km) over a signal bandwidth of 40 GHz. The delay-bandwidth product (DBP) of the proposed slow-light device is analyzed and compared with the DBP of the PC waveguides reported in literatures. We find that our structure yields a significant increase in DBP, and improves the effective bandwidth in which we can obtain slow modes with both low group velocity and vanishing dispersion.     

Combined-Phase-Shift Waveguide Polarizer

A waveguide polarizer exploiting two different phase shift phenomena is presented in this letter. Iris-type discontinuities are in fact introduced in a waveguide structure having different propagation constants for the two principal polarizations. In this way, the required 90 $^circ$ differential phase shift is obtained combining the iris phase shift with the waveguide one. Several operative conditions arise from the combination of the two contributions. This approach has been used to design a broadband waveguide polarizer for the $C$-band antenna feed system of the Sardinia Radio Telescope with ${-}$ 40 dB reflection coefficients and a ${-}$35 dB cross polarization level in a 30% bandwidth.      

Proposal for a Grating Waveguide Serving as Both a Polarization Splitter and an Efficient Coupler for Silicon-on-Insulator Nanophotonic Circuits

A grating waveguide is introduced and designed to serve as both a polarization splitter and an efficient vertical coupler between a fiber and a silicon-on-insulator (SOI) nanophotonic waveguide. Through this grating waveguide, the light from the fiber can be efficiently coupled into an SOI chip where the two orthogonally polarized waves are separated to travel towards the opposite directions along the waveguide. According to our simulations, the optimized structure can give a high coupling efficiency of about 50% for both polarizations, as well as a large bandwidth of over 70 nm and a very low polarization crosstalk below ${-}$22 dB at the output ports.      

硅基槽型微环谐振器及其调谐特性研究

仿真和实验研究了含槽型(slot)光波导的反馈波导型微环谐振器的特性,将槽型光波导集成到Si基微环谐振器中,丰富Si基光波导的功能,为新型光电子器件的实现提供途径。通过锥形波导结构实现从传统波导到槽型波导的模式转换,减小传输损耗,采用时域有限差分法(FDTD)研究了光功率的分布和模式转换过程。结果显示,光功率逐渐转移到锥形结构两侧的槽型波导中并最终形成槽型波导中的传输模式,通过优化锥形结构能实现较高的模式转换效率,可以达到90%以上。采用电子束刻写技术和等离子刻蚀技术制备了反馈波导型槽型微环谐振器。实验显示,锥形波导能够实现模式的转换,光传输过程良好。通过在槽型波导中填充电光聚合物来改变槽型光波导的折射率,测量结果显示,传输谱谐振峰发生了明显移动,移动幅度达到5.6nm,器件具备很好的可调谐性。     

High Quality-Factor 1-D-Suspended Photonic Crystal/Photonic Wire Silicon Waveguide Micro-Cavities

We have successfully fabricated and characterized suspended one-dimensional (1-D) photonic crystal/photonic wire (PhC/PhW) waveguide micro-cavities based on silicon-on-insulator (SOI). Our experiments have shown an enhancement of the resonance ${Q}$ -factor from 18 700 to approximately 24 000, with normalized optical transmission of 70%, after removing the silica cladding underneath the silicon waveguide. We have also demonstrated that, for this condition, the resonance peak wavelength can be controlled by varying the length of the micro-cavity. These results were obtained by removing the silica cladding below the silicon waveguide to produce a “hanging” wire waveguide. The three-dimensional (3-D) finite-difference time domain (FDTD) simulation approach used shows good agreement with measured results.      

Dielectric Resonator in a Waveguide Below Cutoff

A structure is considered consisting of a waveguide 2 below cutoff, connected to waveguides above cutoff, 1 and 3, by means of suitable junctions. A dielectric resonator is introduced in waveguide 2. Its effect on the transmission curve of the structure is evaluated in the limit epsilon/sub r/spl rarr/spl infin/. Theoretical results are compared with experiment.     

High-Speed (60 GHz) and Low-Voltage-Driving Electroabsorption Modulator Using Two-Consecutive-Steps Selective-Undercut-Wet-Etching Waveguide

Based on a novel structure of waveguide, a broadband electroabsorption modulator (EAM) with low driving voltage and high extinction ratio has been demonstrated in this letter. The waveguide of InGaAsP-InP p-i-n layer structure is fabricated by two consecutive steps of selective undercut-wet-etching: 1)HCl : H₃PO₄ on p-InP (p- layer), and 2)H₃PO₄ : H₂O₂ : H₂O on InGaAsP (active region), showing a wide ridge with a narrow undercut active region. Low capacitance and low cladding impedance can thus be simultaneously attained in such waveguides, leading to low microwave loss and high-speed electrooptical (EO) response. A ridge as wide as 8 mum with a 3-mum- wide active region and a 450-nm gap height in the undercut portion has been fabricated. A 350- mum -long waveguide of EAM is designed, revealing a high extinction ratio of > 30 dB (D.C.) and a modulation efficiency of > 20 dB/V (D.C.) with polarization-insensitive operation at a wavelength of 1550 nm. As high as 60 GHz of a 3-dB bandwidth is measured in the high-speed EO conversion. Calculations by an equivalent circuit model are quite fitted with the measurement, revealing that broadband performance is mainly attributed to the low microwave propagation loss in such waveguides.     

Etched Waveguide Grating Variable 1 $,times,$2 Splitter/Combiner and Waveguide Coupler

We propose and test a silicon waveguide grating which serves dual functions: as a 1$,times,$ 2 variable integrated beam splitter/combiner and as an out-of plane diffractive element for coupling light between a single-mode optical fiber and a 500-nm-wide silicon-on-insulator waveguide. An integrated Mach–Zehnder interferometer made with this novel functional element had over 20-dB extinction ratio. The splitting ratio can be tuned by changing the launch position of the optical fiber. The grating coupler had over 36% coupling efficiency and a 1-dB spectral bandwidth of 37 nm.      

Two-Dimensional Ferroelectric Photonic Crystal Waveguides: Simulation, Fabrication, and Optical Characterization

The optical properties of two dimensional photonic crystal (PhC) waveguides were investigated using ferroelectric barium titanate (BTO) thin films as the optical medium. The photonic band structure was calculated using a 2-D finite difference time domain (FDTD) method; a broad band gap is observed that results from the high refractive index contrast. The simulated transmission spectra indicate the stop band of PhC is mainly determined by three parameters: lattice constant, refractive index contrast, and waveguide mode order. From transmission measurements the PhC with a lattice constant ${a}=420$ nm shows a strong light dispersion and the other PhC with ${a}=450$ nm shows a 120-nm broad stop band. Strong localization of visible light within the PhC cavities is demonstrated from the light scattering images. The observed strong light confinement and its spatial intensity profile due to resonance agree with the calculated profiles. From polarized optical microscopy we discovered the scattered light wavelength was highly sensitive to magnitude of the lattice constant. The optical scattering properties indicate BTO PhC can potentially serve as micrometer size electro-optically tunable switches and color filters.      

CMOS-Compatible All-Si High-Speed Waveguide Photodiodes With High Responsivity in Near-Infrared Communication Band

Submicrometer silicon photodiode waveguides, fabricated on silicon-on-insulator substrates, have photoresponse from <1270 to 1740 nm (0.8 AW^-1 at 1550 nm) and a 3-dB bandwidth of 10 to 20 GHz. The p-i-n photodiode waveguide consists of an intrinsic waveguide 500times250 nm where the optical mode is confined and two thin, 50-nm-thick, doped Si wings that extend 5 mum out from either side of the waveguide. The Si wings, which are doped one p-type and the other n-type, make electric contact to the waveguide with minimal effect on the optical mode. The edges of the wings are metalized to increase electrical conductivity. Ion implantation of Si⁺ 1times10 ¹³ cm^-2 at 190 keV into the waveguide increases the optical absorption from 2-3 dBmiddotcm^-1 to 200-100 dBmiddotcm^-1 and causes the generation of a photocurrent when the waveguide is illuminated with subbandgap radiation. The diodes are not damaged by annealing to 450 degC for 15 s or 300 degC for 15 min. The photoresponse and thermal stability is believed due to an oxygen stabilized divacancy complex formed during ion implantation     

Transmission Bandwidth of a Lens Waveguide with a Curved Axis

In a lens waveguide with a curved axis, light beams appreciably different in frequency split and take different paths due to the effect of chromatic aberration of lens. The deviation from a main beam can grow cumulatively through the curved lens waveguide. This effect knits permissible bandwidth to be transmitted without loss of beams. For bends having tilts and offsets at the connections to the straight waveguides, the deviation of the light beam from the guide axis due to the effect of chromatic aberration is derived. With random circular bends the bandwidth is inversely proportional to the square root of the number of bends and proportional to the average radius of curvature. For a normal design of the curved waveguide the allowable bandwidth is expected to be sufficiently broad for signal transmission, but it is narrow in the sense of optical frequency.     

Loss Reduction of Waveguide Crossings by Wavefront Matching Method and Their Application to Integrated Optical Circuits

This paper describes a waveguide crossing design that uses the wavefront matching (WFM) method. The WFM method enables us to design waveguide crossings with lower loss than simple crossings composed of two straight waveguides, without any crosstalk degradation. We report the procedure for designing waveguide crossings based on the WFM method and some experimental results. For experimental confirmation, we made a waveguide crossing test circuit using silica-based planar lightwave circuit (PLC) technology. By comparing the results obtained with samples constructed with different $Delta$ waveguides, we show that our design method is very efficient for higher $Delta$ waveguides suitable for high-density integration. In addition, we describe an example application of our designed crossings to an integrated PLC device, namely a wavelength multiplexer with a variable optical attenuator. We show that waveguide crossings designed by the WFM method are useful for improving the loss characteristic of highly integrated PLC devices.