Reduction of the pump power threshold in the nonlinear all-optical photonic crystal directional coupler switches

In this paper, the cross-phase modulation Kerr nonlinear effect is utilized to induce nonlinearity and a change in the coupling length by a high-intensity pump signal in the nonlinear photonic crystal directional coupler. We have analyzed and modified the nonlinear square-lattice directional couplers for the first time, to the best of our knowledge, and have improved the power consumption for inducing the nonlinear switching. We have shown that according to the slower group velocity and the increased confinement of the TM modes with lower frequency in the region with a higher dielectric constant in a square lattice, the required switching pump power of the structure is less than that of the same triangular structure.     

Design of compact narrowband optical filter on SOI using asymmetric directional coupler

A narrowband optical filter on Silicon-on-Insulator based on a directional coupler of continuous and periodic segmented waveguide is theoretically investigated. The study shows that the filtering action is better when a slot waveguide is considered as the continuous waveguide instead of a photonic nano-wire waveguide. A filter of extremely narrow bandwidth at any desired wavelength can be designed by optimizing the feature size of the different components of the device. Comparison of the footprint shows that the proposed filter is much more compact than other available filter. Typical value of proposed filter bandwidth can be as low as 2.5 pm with a footprint of 0.44 mm² which is much less than many available filters of similar bandwidth. In this study, the transfer matrix method has been used to find the effective refractive index and hence the propagation constant.     

Microwave nonlinear spin wave directional coupler

A nonlinear spin-wave directional coupler for the microwave range has been designed, constructed, and studied for the first time. Each of the four ports of the device can be used as the input or output for a microwave signal. The role of a control element in the coupler is played by a nonlinear spin wave shifter based on a thin ferromagnetic film. A distinctive feature of the proposed device is that an increase in the input power level leads to the signal switching from one to another output, which is caused by a power-dependent variation in the spin wave phase shift.     

Bidirectional optical coupler for plastic optical fibers

We have developed a low-loss bidirectional optical coupler for high-speed optical communication with plastic optical fibers (POFs). The coupler, which is fabricated by an injection molding method that uses poly (methyl methacrylate), has an antisymmetric tapered shape. We show that the coupler has low insertion and branching losses. The tapered shape of the receiving branch reduces beam diameter and increases detection efficiency coupling to a photodetector, whose area is smaller than that of the plastic optical fiber. The possibility of more than 15-m bidirectional transmission with a signaling bit rate up to 500 Mbits/s for simplex step-index POFs is demonstrated.     

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.     

Fast photonic switches based on GaAs nanostructures

Fast photonic switches, in which light controls light, can be created based on high-voltage GaAs nanostructures with a thin (nanodimensional) surface dielectric layer. The switches offer a high operation speed that ensures optical data recording and transmission at a rate of 10⁴-10⁵ cps, possess a large modulation amplitude, and can operate at a relatively low optical control signal power (I<1 W/cm²). These devices can be used in systems of optical data processing, optical computation systems, all-optical communication lines with optical addressing of informative signals, etc.     

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.     

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.     

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.     

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)     

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.     

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.     

Thermooptic coupler for multimode optical fibers

The detailed theory is presented together with design considerations and recent experimental results for an electronically controllable directional coupler for multimode optical fibers. The device, based on plastic-clad silica fiber, is of exceptionally simple construction and compact size and is highly reliable. A theoretical model of the coupler is described and compared with actual device performance. The coupling can be varied from -4 to -28 dB with an excess loss of <0.2 dB over the entire coupling range, in agreement with theory. Improvements in the present device design are also discussed.     

Iterative method for the design of a nonperiodic grating-assisted directional coupler

We propose and investigate the optimal design of a nonperiodic grating-assisted directional coupler by iterative methods using the beam propagation method. Computer simulations were carried out at wavelengths of 0.8, 1.3, and 1.5 mum, which are often used in optical communications and networking. We found that the complete power coupling lengths can be reduced considerably in comparison with those in the case of the periodic grating-assisted waveguides with the same set of parameters.