Transfer matrix analysis of the unidirectional grating-assisted codirectional coupler

The unidirectional grating-assisted codirectional coupler (U-GACC) has recently been proposed. This unique structure permits irreversible coupling between orthogonal waveguide eigenmodes by means of simultaneous modulation of both the real and imaginary parts of the refractive index in the coupling region. Analysis of the U-GACC has until now relied on coupled mode theory, which can be restrictive in its application as a design tool. We analyze the U-GACC by the transfer matrix method, which demonstrates in a simple fashion why the device operates in a unidirectional manner. In addition, we show that for all practical designs, there is a limit to the minimum cross talk between outputs, a phenomenon that has not been previously identified.     

Multiple-wavelength focusing of surface plasmons with a nonperiodic nanoslit coupler

A novel type of multiple-wavelength focusing plasmonic coupler based on a nonperiodic nanoslit array is designed and experimentally demonstrated. An array of nanoslits patterned on a thin metal film is used to couple free-space light into surface plasmon polaritons (SPPs) and simultaneously focus different-wavelength SPPs into arbitrary predefined locations in the two-dimensional plane. We design and fabricate a compact triplexer on a glass substrate with an integrated silicon photodetector. The photocurrent spectra demonstrate that the incident light is effectively coupled to SPPs and routed into three different focal spots depending on the wavelength. The proposed scheme provides a simple method of building wavelength-division multiplexing and spectral filtering elements, integrated with other plasmonic and optoelectronic devices.     

A resonance method of measuring the reflection and transmission coefficients of a directional coupler

A method of determining the complex reflection and transmission coefficients is considered, based on measurements of the resonance characteristics of a short-circuited waveguide, excited through an inductive iris. The parameters of the coupler in the R2-54/3 type panoramic VSWR measuring instrument are measured. __________ Translated from Izmeritel’naya Tekhnika, No. 3, pp. 60–64, March, 2007.     

Iterative method for the design of a dual-phase-conjugation-mirror resonator with multiple apertures

Apertures have been used to select the low-order transverse modes in resonators. The additional diffraction losses result in a change in the transverse-mode structure, and the presence of apertures inside a resonator generally distorts the mode shape. The optimization of a multiple-aperture resonator demands an approach that differs from the conventional method in which the mode theory is used. We demonstrate an iterative design method to find optimal phase profiles for the reflector surfaces to build a resonator with multiple apertures to produce a lowest-order mode with much smaller diffraction loss and to satisfy the phase-conjugation conditions at the mirrors. The results are compared with conventional stable resonators, and we show that substantial improvement in round-trip loss and beam quality can also be obtained.     

Transmission function of a directional coupler with a saturable nonlinearity

It is shown that when saturation is taken into account, the transmission function of a directional coupler is more complicated than the Kerr function. Specifically, transmission characteristics with two critical points of self-switching exist. Pis’ma Zh. Tekh. Fiz. 25, 7–12 (September 12, 1999)     

Dynamic behaviour in a nonlinear directional coupler with feedback

Abstract

A new kind of resonator based on a nonlinear directional coupler with feedback is presented. The steady states are found and the map of the system is iterated, showing the existence of bistability, period doubling and chaos. The dependence of the dynamic features is discussed in terms of the parameters of the two different schemes of feedback studied in this work. A new device based on one of these schemes of feedback suitable as a routing unit is proposed.     

Optical switching in a symmetric three-waveguide nonlinear directional coupler

In this paper we study optical switching in a system of a three-waveguide nonlinear directional coupler. We consider a waveguide structure with three parallel waveguides in a linear, symmetric coupling configuration. We work in the case that the (absolute value of the) difference of the propagation constants of the outer and centre waveguides is much larger than the linear coupling constants. When one of the outer waveguides is initially excited, we show that for small values of the initial power the system behaves as a two-waveguide nonlinear directional coupler where only the two outer waveguides are involved in the switching. For larger values of the initial power, transfer of light between the initially excited waveguide and the centre waveguide is observed.     

Surface-acoustic-wave directional coupler for apodization of integrated acoustooptic filters

A SAW waveguide directional coupler that was used to provide a raised-cosine apodization of the SAW beam intensity, in order to achieve sidelobe suppression of an integrated acoustooptic filter, is demonstrated. The SAW guide coupler consisted of two closely spaced, evanescently coupled parallel acoustic waveguides. A single guided SAW mode was excited by a miniature transducer imbedded in a 100-mum-wide waveguide, and in excess of 99% of the SAW energy was transferred from the original waveguide to the coupled guide, and back, across a gap of 20 mum. The coupling length (for complete crossover) was 9.8 mm and depended exponentially on gap, as expected. This SAW coupler resulted in 10-dB sidelobe suppression compared to an unapodized acoustooptic filter.     

Temporal soliton switching in a rectangular nonlinear directional coupler

We address the theory of temporal soliton switching in a planar geometry directional coupler constructed from silica and doped silica glass and operating at the central wavelength of 1.55 mum, significant for erbium-doped amplification. We formulate the field in the coupler in terms of the supermodes of the total structure and take account of the two transverse dimensions of the rectangular channels. In the case of the weak coupling between channels consistent with elimination of pulse breakup, the effect of the fields in the outer corner regions of the channels results in a switching intensity that differs significantly from that derived from coupled-mode theory on the basis of a slab model of the coupler.     

A novel reflector-filter using a SAW waveguide directional coupler

The SAW waveguide technique was used to form a novel reflector-filter for the realization of compact IF filters in CDMA handsets. The reflector-filter design is an attractive technique to obtain characteristics with steep skirts in a short device length, because the SAW propagation path is folded and the frequency response is synthesized by utilizing both IDT and reflector responses. In this paper, we propose a new reflector-filter structure using a SAW waveguide directional coupler. For the implementation of the proposed reflector-filter, the key technology is the design of SAW waveguides. We have formed SAW waveguides using Al gratings loaded on the surface of the substrate. The pitch of the Al gratings has been chosen unequal to half the acoustical wavelength to avoid the occurrence of the gratings stopband at the filters passband position. Using the proposed reflector-filter structure, PCS-CDMA IF filters were fabricated on quartz substrates. The filter exhibited an insertion loss of 8.5 dB, a 5 dB bandwidth of 1.45 MHz, and a rejection of more than 33 dB at the center frequency +/-1.25 MHz with the package size of 4.8x9.1 mm (2). i.e., half the size of a conventional transversal filter was achieved using the proposed reflector-filter technique.     

Precise control of the center wavelength of a wavelength-selective single-mode-fiber-thin-film directional coupler

The coupled power and the center wavelength of a wavelength-selective single-mode-fiber-thin-film coupler are accurately determined. The center wavelength depends on the fiber-film spacing. The required remaining cladding thickness of the fiber is accurately obtained from the loss-calibration curve of the half-coupler. The thickness of the film is controlled by spinning speed to match the propagation constants of the fiber and the film. Coupling fiber was used for efficient coupling. Experiments were also carried out for a wavelength filter, and the results almost agree with theoretical values. Shifting of the center wavelength with the condition of fire polishing of the half-coupler is shown.     

Analysis of mode size transformation with a tapered directional coupler

A simple tapered directional coupler with a five-layered structure is employed for mode conversion between a single-order and higher-order modes. We investigate coupling from higher-order modes to a single-order field theoretically and experimentally. As a result, we confirm that the first two modes in the tapered waveguide are coupled with a single-order mode in another waveguide by computer simulation using the beam propagation method. Furthermore, we fabricated the actual device and observed the streak patterns of the first three modes.     

Multiscale finite element method for a locally nonperiodic heterogeneous medium

A generalization of the mathematical homogenization theory to account for locally nonperiodic solutions is presented. Such nonperiodicity may arise either due to the rapidly varying microstructure (e.g.: graded materials, microcracks) or because the macroscopic solution is not smooth and may have significant variation within a microstructure. In the portion of the problem domain where the material is formed by a spatial repetition of the base cell and the macroscopic solution is smooth, a double scale asymptotic expansion and solution periodicity are assumed, and consequently, mathematical homogenization theory is employed to uncouple the microscopic problem from the global solution. For the rest of the problem domain it is assumed that the periodic solution does not exist (cutouts, cracks, free edges in composites, etc.) and the approximation space is decomposed into macroscopic and microscopic fields. Compatibility between the two regions is explicitly enforced. The proposed method is applied to resolve the structure of the microscopic fields in the single ply composite plates with a centered hole and with a centered crack and in the [0/90] _s laminated plate. Numerical results are compared to the reference solution, an engineering global-local approach, and the direct extraction from the mathematical homogenization method.     

Efficient inverse scattering algorithm for the design of grating-assisted codirectional mode couplers

An efficient method for the design of optical devices based on codirectional grating-assisted mode coupling is presented. A low-complexity algorithm is developed to calculate the coupling function of a grating that accurately matches an arbitrarily given target spectral response. The method relies on the synthesis of the grating impulse response by means of an exact differential layer-peeling algorithm.     

Linearized electro-optic modulators based on a two-section Y-fed directional coupler

We experimentally demonstrate a linearized Y-fed directional coupler (DC) modulator based on an electro-optic (EO) polymer waveguide. The spurious free dynamic range of 119 dB/Hz2/3, which is 11 dB higher than that of the conventional Mach-Zehnder modulator, is achieved by introducing the reversed Δβ technique in the two-section Y-fed DC. The in-device EO coefficient (r33) of the fabricated device is as high as 79 pm/V in 1.55 μm wavelength, which is 88% of a single film r33 of LPD-80/APC.     

Optical fiber monitored by a directional coupler for delivering laser radiation in medical treatments

The safety of therapeutic laser treatments could be strongly improved by introducing real-time monitoring to the fiber delivery system. Since any fiber damage leads to a significant variation of the backscatter, its detection could be exploited to monitor the system, in particular the fiber trip. To assess this potential, the backscattered modal power distribution (BMPD) from flat, bulb, and damaged fiber tips were investigated. The BMPD detection was accomplished by using a conventional beam-splitter method and two directional couplers: a prism-fiber and a fiber-fiber lapped coupler. Unlike the conventional method, use of the couplers allows for the separate detection of backscattered and forward transmitted signals. Therefore variations in the backscatter that are due to only a change in the laser pulse can be determined. Moreover, the directional couplers, because of the amplification of high-index modes, allow for an increase in sensitivity of the method. This was particularly evident in our tests on fiber tips that had been dipped into water to obtain a better simulation of the real working conditions. Finally, the influence of the target on the BMPD was investigated as a function of its distance from the fiber tip. All the tests confirm that the target must be taken into account if the distance is <1 mm, but the monitoring system can also be used when the fiber tip works in contact with the tissue wall provided that one can ensure contact by pushing the tip against the target wall.