Directional couplers

It is common to employ microwave power monitoring by means of a dual-directional coupler in the waveguide transmission system between the power tube and the useful load. Part of the coupled signals may be used for examination with spectmm analy2ers, frequency meters, and other microwave instmmentation for special purposes. Generally, this is not necessary in a practical appHcation. Many microwave measurement techniques have been described (59,60). AvailabiHty of components, plumbing, and instmmentation is weU described in trade journals. 

Fig. 1. Representative device configurations exploiting electrooptic second-order nonlinear optical materials are shown. Schematic representations are given for (a) a Mach-Zehnder interferometer, (b) a birefringent modulator, and (c) a directional coupler. In (b) the optical input to the birefringent modulator is polarized at 45 degrees and excites both transverse electric (TE) and transverse magnetic (TM) modes. The appHed voltage modulates the output polarization. Intensity modulation is achieved using polarizing components at the output.

Kreuwel H.J.M., Lambeck P.V., Beltman J.M.M., Popma T.J.A., Mode coupling in multilayered structures applied to a chemical sensor and a wavelength selective directional coupler, ProcECIO 87, Glasgow, UK, 1987,217-220. 

Fig. 2.3 By regarding a basic microring resonator as an optical circuit composed of a directional coupler and a curved waveguide (the cross hatched section), the characteristics of the microring resonator can be analyzed using simpler BPM code and transfer matrices instead of computation intensive FDTD simulation. Reprinted from Ref. 15 with permission. 2008 Institute of Electrical and Electronics Engineers...

Fig. 9.17 Silicon waveguide ring resonators employing (a) a directional coupler (DC), and (b) a multimode interference (MMI) coupler, as viewed from above using a scanning electron micro scope...

Direct hydration, of ethylene, 10 538 Direct hydrogenation, 6 827 Direct immunosensors, 14 154 Direct ingot (dingot) method, 25 409 Direct initiation, 14 270 Direct injection (DI) diesel engines, 12 421 Direct inlet injection, gas chromatography, 6 383, 415-416 Directional couplers, 17 446 Directional drilling techniques, in sulfur extraction, 23 572 Directive 89/107/EEC (EU), 12 36 Direct liquefaction, 6 827 Direct marketing, technical service personnel and, 24 343 Direct metal nitridation, 17 211-213 aerosol flow reactor, 17 211-212 Direct methanol fuel cells (DMFC),... 

Numerics has been used intensively in the field of integrated optics (10) since its early days, simply due to the fact that even the basic example of the slab waveguide requires the solution of a transcendental equation in order to calculate the propagation constants of the slab- guided modes. Of course, the focus was directed to analytical methods, primarily, as long as the power of a desktop computer did allow for a few coupled equations and special functions only e.g. to describe the nonlinear directional coupler in a coupled mode theory (CMT) picture. During the years, lots of analytic and semi-analytic approaches to solve the wave equation have been developed in order... 

What makes EIM susceptible to be mistaken are configurations with regions, where no guided mode can be determined. If more than an estimate of the propagation constant is required, e.g. in order to calculate the half-beat length of a buried channel waveguide directional coupler configuration from the mode indices of the symmetric and anti-symmetric supermodes, EIM should be superseded by a more accurate method. 

Figure 11. 4-port devices power splitter, relative phase shifter, directional coupler. 

The characteristics of those basic entities like power splitters, combiners, directional couplers etc. can be related to some basic waveguide properties, and the 4-port devices are described by 2 x 2 matrices, Eqs(28),... 

Experimental work and theoretical investigation show an important role of spatial chaos in optical fibers, directional couplers, and generally in all-optical switching devices [96/97]. 

The nonlinear directional coupler is potentially a useful device because it has four ports, two input and two output, and because the outputs can be manipulated with either one or two inputs. Optimally the two channels are identical, and the coupling occurs through field overlap between the two channels. As a result, when only one of the channels is excited with low powers at the input, the power oscillates between the two channels with a beat length Lb, just like what occurs in a pair of weakly coupled identical pendulii. As the input power is increased, a mismatch is induced in the wavevectors of the two channels, which decreases the rate of the power transfer with propagation distance. This leads to an increase in the effective beat length. There is a critical power associated with this device, for which an infinitely long, lossless NLDC acts as a 50 50 splitter, that is, Lb oo. 

Figure 7. A number of all-optical guided wave devices and their responses to increasing power. (a) Half beat length directional coupler. (b) One beat length directional coupler. (c) Distributed feedback grating relector. (d) Nonlinear Mach-Zehnder interferometer. (e) Nonlinear mode mixer. (f) Nonlinear X-switch. For nonlinear media (n2 0), the input power determines the output state.

At this time, many of the parameters listed above have not been measured for nonlinear organic materials. Measurement will be necessary for an assessment of the full potential of such materials. To date, only a nonlinear directional coupler has been implemented in a nonlinear polymer, (poly-4BCMU). For this device at 1060 nm, the nonlinear response was dominated by two-photon absorption, and some limited switching attributable to absorption changes was observed (27). More recent work at 1030 nm appeared to show switching attributable to refractive nonlinearities (28). Clearly there is much progress to be made. 

Evaporated PDA(12-8) film was used as a nonlinear optical medium in a layered guided wave directional coupler. The directional coupling phenomenon happens in two adjacent waveguide by periodical energy transfer. The theory of linear directional coupler was exactly established [11]. It can be reduced to coupled mode equations ... 

Figure 11. Power-dependent nonlinear coupling at a nonlinear directional coupler.

Figure 12. Schematica 1 structure of the nonlinear directional coupler.

Figure 4.14 Three different electrooptic waveguide devices [1] (a) Mach-Zehnder modulator with push-pull arrangement of the electrodes, (b) electrically controlled directional coupler, (c) coupler with phase-reversal electrodes.

Fig. 1 - Three common electro-optic device configurations Top Mach-Zehnder interferometer middle birefringent modulator bottom directional coupler...

If electro-optic modulators and directional couplers are to be driven by the electrical outputs from very large-scale integration (VLSI) semiconductor chips, then the required drive or VK voltages of EO devices must be less than 6 V (for... 

The forward and reflected electric power is measured in continuous wave (CW) mode before the exposures using a digital power meter (HP Model) and a dual directional coupler (Werlatone Model C1373). 

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