Compact multimode interference coupler with tapered waveguide geometry

In this paper, a novel MMI coupler, based on general interference, with tapered waveguide geometry has been proposed for reduction of coupling length. The coupling characteristics and power imbalance of the proposed structure are compared with conventional MMI structures by using a mathematical model based on sinusoidal modes. It is seen that the beat length for tapered MMI coupler with angle of taper ∼1.05° is reduced by ∼24% of that of conventional MMI coupler and the coupling characteristics obtained with the mathematical model, match well with those obtained by more sophisticated BPM computer aided design software. The power imbalance for tapered 3 dB MMI coupler is more sensitive to the wavelength than that for conventional 3 dB MMI coupler and variation of power imbalance with fabrication tolerance for both the MMI coupler is almost same.     

Compact 3-dB tapered multimode interference coupler in silicon-on-insulator

We have fabricated a compact 3-dB multimode interference coupler with a large silicon-on-insulator cross section. To reduce the length of the usual symmetric interference multimode interference coupler, we propose using a parabolically tapered structure. The length of the device is 398microm . The device has a uniformity of 0.28 dB.     

Micro-displacement sensor with multimode fused coupler and concave mirror

Fiber optic displacement sensor is demonstrated using a multimode fused coupler probe and a concave mirror target. It is observed that a focal length of the concave mirror make a significant influence to the displacement response, which illustrates three linear slopes with the second slope peaks at the position that is in vicinity of two times of the focal length. The highest sensitivity of 0.56 mV/mm is obtained with an infinity focal length and it is decreased as the focal length of the concave mirror decreases. The widest linear range of approximately 3.3 mm is observed in the third slope region with focal length of 6.0 mm. The highest linear range obtained with the use of 50:50 coupler.     

Analysis of multiple reflections in hybrid photonic crystal multimode interference coupler

In this paper the analysis of multiple reflections in photonic crystal (PhC) multimode interference (MMI) couplers using eigen-mode expansion method is presented. The analysis is conducted on a hybrid PhC structure which consisted of 1-D PhC multimode waveguide sandwiched between 2-D PhC input/output waveguides. In PhC multimode waveguide, where the mechanism of wave confinement is not due to total internal reflection but due to photonic bandgap properties, the reflectivity at 2-D PhC facet wall would be very large for all the guided modes in the waveguide when ever the image formed due to MMI effect does not coincides with the output access waveguide.     

Large core, multimode, chalcogenide glass fibre coupler by side-polishing

A novel method for obtaining a multimode optical coupler in chalcogenide glass fibre is described. The fibres used had a large $\text{ Ge }_{17}\text{ As }_{18}\text{ Se }_{65}$ Ge 17 As 18 Se 65 core comprising about 74 % of the total cross-sectional area with an optical cladding of $\text{ Ge }_{17}\text{ As }_{18}\text{ Se }_{62}\text{ S }_{3}$ Ge 17 As 18 Se 62 S 3 . A single arc-shaped U-groove was formed in each of two silicate glass blocks. A length of fibre was embedded into each groove and glued in place. The centre of each fibre length was subsequently side-polished. Finally two such embedded fibres were glued together in close contact to form the coupler, whilst monitoring the coupling ratio at the output.     

Multiport N x N multimode air-clad holey fiber coupler for high-power combiner and splitter

We report 2 x 2 and 4 x 4 multimode air-clad holey fiber couplers fabricated by a novel fusion and tapering technique. The devices showed excellent port-to-port coupling uniformity over a wide spectral range of 800-1650 nm. The 4 x 4 coupler showed a low insertion loss of 9.9 dB and excess loss of 3.9 dB at 1310 nm. Because of its unique air cladding and the high numerical aperture of the holey fiber structure, the device showed strong potential in high-power applications.     

Slow-wave directional coupler and its applications

The directional coupler and the traveling-wave directional filter made of coupled slow-wave microstrip lines are proposed and their characteristics discussed. These structures exhibit interesting characteristics not readily available in conventional transmission lines.This work is supported by U.S. Army Research Office contract DAAG29-84-K-0076.     

Simple design of optical fiber displacement sensor using a multimode fiber coupler

A simple design sensor is demonstrated using a fabricated multimode plastic fiber coupler in conjunction with reflective intensity modulation technique. The performances of this sensor are investigated for different light sources. This sensor uses only one fiber for sending and receiving the light and therefore only the back slope exists. The sensor shows the highest performance with the use of yellow light source, which has the highest intensity and the smallest beam divergence. The sensitivity, linear range, resolution and dynamic range of the sensor are obtained at 0.0001 mV/μm, 1500 μm, 70 μm, and 5.0 mm, respectively. The simplicity of the design, high degree of sensitivity, dynamic range and the low cost of the fabrication make it suitable for real field applications.     

Conception of a 90-GHz metamaterial-based coupler for astrophysical applications

Designed for astrophysical applications, this coupler must be as small as possible to be integrated in a low temperature superconducting detection chain. By using metamaterials, dimensions can be reduced and performances enhanced compared with a conventional coupler. The novelty of the work presented in this paper is a coupler that operates at millimeter waves (W band) and cryogenic temperatures (4 K) for astrophysics applications and more precisely for the detection of the cosmological microwave background radiation.     

Design of a wavelength demultiplexer based on a parabolically tapered multimode interference coupler

A compact wavelength demultiplexer is designed for the operation at 1.30 and 1.55 μm wavelengths using the three-dimensional semi-vectorial beam propagation method. The parabolically tapered multimode interference (MMI) coupler based on the deep-etched SiO₂/SiON rib waveguide is introduced into the present demultiplexer for reducing the length. The numerical results show that a MMI section of 330.0 μm in length, which is only 76% length of a straight MMI coupler, is achieved with the contrasts of 42.3 and 39.2 dB in quasi-TE mode, and 38.4 and 37.8 dB in quasi-TM mode at wavelengths 1.30 and 1.55 μm, respectively and the insertion losses below 0.2 dB. The modified finite difference scheme is applied to approximate the resulting equations along the transverse directions, in which the discontinuities of the derivatives of magnetic field components H_y and H_x along the vertical and horizontal interfaces, respectively, are involved.     

Design and fabrication of the star coupler based on SOI material

A 1×25 star coupler is designed through calculation and beam propagation method (BPM) simulation. Improvement methods are focused on the design of the tapered waveguides in the device, improving the uniformity of the output light power of the star coupler. Utilizing the conventional Si process technology, the device is fabricated based on silicon-on-insulator (SOI) material. The test result shows that the star coupler has a perfect function of power splitting.     

An ultra‐compact multimode interference coupler with a subwavelength grating slot

Multimode interference couplers (MMIs) are fundamental building blocks in photonic integrated circuits. Here it is experimentally demonstrated, for the first time, a two‐fold length reduction in an MMI coupler without any penalty on device performance. The design is based on a slotted 2 × 2 MMI fabricated on a commercial silicon‐on‐insulator (SOI) substrate. The slot is implemented with a subwavelength grating (SWG) comprising holes fully etched down to the oxide cladding, thereby allowing single etch step fabrication. The device has been designed using an in‐house tool based on the Fourier Eigenmode Expansion Method. It has a footprint of only 3.5 μm x 23 μm and it exhibits a measured extinction ratio better than 15 dB within the full C‐band (1530 nm‒1570 nm). SWG engineered slots thus offer excellent perspectives for the practical realization of MMIs couplers with substantially reduced footprint yet with outstanding performance.     

Semi-vectorial analysis of a compact wavelength demultiplexer based on the tapered multimode interference coupler

Based on a parabolically tapered multimode interference (MMI) coupler with a deep-etched SiO₂/SiON rib waveguide, a compact wavelength demultiplexer operating at 1.30 and 1.55\mum wavelengths is proposed and analysed by using three-dimensional semi-vectorial finite-difference beam propagation method (3D-SV-FD-BPM). The results show that a MMI section of 330.0\mum in length, which is only 76% length of a straight MMI coupler, is achieved with the contrasts of 42.3 and 39.2dB in quasi-TE mode, and 38.4 and 37.8dB in quasi-TM mode at wavelengths 1.30 and 1.55\mum, respectively, and the insertion losses below 0.2dB at both wavelengths and in both polarization states. The alternating direction implicit algorithm with the Crank--Nicholson scheme is applied to the discretization of the 3D-SV-FD-BPM formulation along the longitudinal direction. Moreover, a modified FD scheme is constructed to approximate the resulting equations along the transverse directions, in which the discontinuities of the derivatives of magnetic field components H_y and H_x along the vertical and horizontal interfaces, respectively, are involved.     

Simulations of refractive index variation in a multimode interference coupler: Application to gas sensing

An optical sensor based on multimode interference couplers structures has been studied. We have introduced the optical parameters of materials used in existing optical integrated circuits and a sensitive material characterised in previous studies. The variation of optical properties such as the refractive index of the covering sensitive material leads to a modification of waveguiding conditions in the multimode interference coupler. A gas sensor can be developed by measuring the variation of light intensity at the output of the multimode interference coupler structure under gas exposure. Simulations have been performed in order to optimise the output light intensity variations to increase sensitivity. We have shown that these structures are more sensitive than Mach-Zehnder interferometers, and that the relation between dimensions and sensitivity of the multimode interference coupler is not trivial.     

Performance comparison between plastic-based fiber bundle and multimode fused coupler as probes in displacement sensors

Fiber optic displacement sensors with bundled and fused coupler fiber probes are being investigated. The effects of axial displacements on the detected output voltages are also investigated for fiber bundle probe sensor. The resolutions obtained are 0.9 and 4.0 μm, for the front and back slopes, respectively. A comparison is made with a commercial plastic based multimode fused coupler which gives a better range of 2.0 mm. This is longer than that achieved by the fiber bundle probe which is only 0.4 mm and has a lower resolution of 34 μm for the case of 50:50 coupling ratio.     

Schemes of a fiber-optic multiplexing sensor array based on a 3 X 3 star coupler

A Michelson-based fiber-optic low-coherence interferometric quasi-distributed sensing system permitting absolute length measurement in a sensor array is proposed. The main part of the sensing system is a fiberoptic 3 x 3 star coupler. The architecture of fiber-optic sensors can be easily realized as a linear sensor array, twin sensor arrays, or a loop sensor array. The proposed sensing scheme will be useful for the measurement of strain distribution. An important application could be deformation sensing in smart structures. A six-sensor array is demonstrated experimentally.     

彩色融合技术在军事中的应用

采用基于l空间的色彩传递理论实现实时的红外与可见光彩色融合技术,并介绍了自行研制的红外与微光彩色融合系统的组成和工作流程。介绍了彩色融合技术在军事中的典型应用:夜间狙击手探测和伪装车辆探测,并给出了实验图像。设计了一种图像中目标探测性的主观评价方法,并对两组实验图像进行了评价。实验结果表明,彩色融合技术能够大大提高军事目标的探测性。     

Infrared and millimeter-wave sensors for military special operations and law enforcement applications

We consider the application of infrared and millimeter-wave sensors, developed for the most part during the Cold War, to the solution of problems encountered by military special operations units and law enforcement personnel. These problems include detection of weapons concealed beneath clothing, through-the-wall surveillance, and wide-area surveillance under poor lighting conditions. Key sensors used in these applications are infrared cameras, millimeter-wave passive and active cameras, and millimeter-wave real-aperture and holographic radars. This paper discusses each type of sensor, describes its operation, and gives an example of its output, except in those cases where the device is early in its development phase and thus no outputs are available. All of these sensors form images, but the images are of varying quality. We conclude with a brief discussion of methods of using multiple sensors to improve performance.N. C. Currie and R. W. McMillan are permanently employed by the Georgia Institute of Technology, Georgia Tech Research Institute. They are working at Rome Laboratory supported by the Air Force Office of Scientific Research University Resident Research Program.     

Detailed investigation of self-imaging in multimode photonic crystal waveguides for applications in power and polarization beam splitters

Properties of the self-imaging effect based on multimode interference (MMI) in multimode photonic crystal waveguides (PCWs) are investigated and analyzed in detail. By combining photonic band gap (PBG) and total internal reflection (TIR) effects in PCWs, self-imaging phenomena are achieved for both TE and TM polarizations. To observe the images reproduced by this self-imaging phenomenon, finite-difference time-domain (FDTD) simulations are performed on a multi-mode PCW. From these numerical simulations the reproduced images are identified and their positions along the propagation axis are described. We also, present the design and simulation of novel polarization-insensitive power splitter and polarization splitter. The simulation results show that the optimized devices have excellent transmission efficiencies as well as wide operating frequency bandwidths and small structure size. So, the proposed devices are promising and may play an important role in high-density photonic integrated circuits in the future.