Abstract-This paper presents an overview of integrated optics outline of the modal propagation analysis (MPA), which will be used later to describe image formation by general and restricted multimode interference (Sections IV and V, we conclude by comparing the properties of MMI devices with those of more conventional routing and coupling devices.

Abstract-The self-imaging property of a homogeneous multimoded planar optical waveguide has been applied in the design of passive planar monomode optical couplers based on multimode interference (MMI). Based on these designs, 3 dB and cross couplers were fabricated in Si02/A120~/Si02 channel waveguides on Si substrates. Theoretical predictions and experimental results at 1.52-pm wavelength are presented which demonstrate that MMI couplers offer high performance: on-chip excess loss better than 0.5 dB, high reproducibility, low polarization dependence and small device size.

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Absh-nct-We report on the reflection properties of multimode interference (MMI) devices: we distinguish between reflection back into the input waveguides and internal resonance modes due to the occurrence of simultaneous self-images. Because of self-imaging, reflection CBn be extremely efficient, even in the case of MMI devices with optimized transmission. This conclusion is confirmed by the observed spectral behavior of InP-based ring lasers incorporating MMI 3dB couplers and MMI power splitters. Several techniques are proposed to minimize the influence of these reflections.

We report extremely compact (494~pm-long 3 dB splitters, including input/output bends), polarization-insensitive, zero-gap directional couplers on InP with .a highly multimode interference region that are based on the self-imaging effect. We measured cross-state extinctions better than 28 dB and on-chip insertion losses of 0.5 dB/coupler plus 1 dB/ cm guide propagation loss at 1523 nm wavelength.Directional couplers are key components in integrated optoelectronics, being used in power dividers, modulators and switches, wavelength (de) multiplexers, and polarization splitters. Their major drawback for monolithic integration is their large size, typically several millimeters or more, due to large coupling lengths and due to the size of the branching network that separates the access waveguides. ' The zero-gap two-mode interference coupler has a much shorter coupling length than the conventional synchronous directional coupler, but it cannot be used in conjunction with deeply etched waveguides that are suitable for very compact monomode bends.' In particular, the abrupt branching guide-to-coupler transitions resulting from the large index differences required for compact bends can degrade coupler crosstalk.3 Here we demonstrate that high extinction and compact size can be achieved by combining deeply etched monomode guides for compact bends with zero-gap, multimode interference (MMI) couplers. Such couplers have previously been proposed for easing the fabrication of zero-gap couplers by eliminating the need for well-defined Y junctions;4 here we show that MM1 couplers are suitable for high extinction (typically 28 dB ) , polarization-insensitive, compact (submillimeter) devices with low on-chip insertion loss, The self-imaging effect which occurs in the multimode interference region of our coupler is crucial to our design (Fig. 1). Such self-imaging has been reported by Ulrich': in a multimode waveguide of length L = (p/q) 3L, where L, is the beat length for the two lowest-order modes, the input field is imaged onto the output when (p/q) = 2,4, . . . . A reverse image is produced at the output for (p/q) = 1,3, . . . and likewise a linear combination of the image and its reverse occur when (p/q) = l/2,3/2, . . . . At these lengths, a super-resonance occurs in which all excited modes interfere constructively. Consequently, the coupler operates independently of its excitation, thus permitting the use of deeply etched waveguides for the input and output branching network. These waveguides combine monomode operation with short radii of curvature. The independence of excitation also allows the input waveguides to be *)AT&T Bell Laboratories; 600 Mountain Avenue, Murray Hill, NJ 07974. "Department of Electrical Engineering, Delft University of Technology, 2600 GA Delft, The Netherlands. 'ILawrence Livermore National Laboratory, Livermore, CA 94550.spaced far apart (we employed a 2.4 pm separation between the access waveguides). Increasing the distance. between the access waveguides reduces problems, as associated with the...

A fast, accurate, inexpensive, and nondestructive method to determine the density of very porous ceramics has been found. This method is an extension of the water-displacement technique with three weighings and the use of glycerol instead of water. For small (1 .I3 g ) , 29% porosity B,C pellets, the precision is 0.002 glcm' and the accuracy is better than 0.006 g/cm' compared with the geometric density. [

Multimode interference (MMI) 3 dB couplers and MMI power combiners have been integrated with compact InP/InGaAsP ring lasers (R=150 μm). Radiative loss from the curves is small, and MMI 3 dB couplers are shown to be more efficient than conventional Y junctions. In addition, we demonstrate improved efficiency by combining counterpropagating beams in a single output by means of a MMI combiner. Single mode spectral behavior, with 35 dB side mode suppression, has been measured and is explained as resulting from coupled cavities.

Abstract-We monolithically integrate an optical front-end on InP for balanced, polarization-diversity coherent lightwave reception which is only 1 3 mm long. Low on-chip insertion loss (<4.5 dB) and balanced photoresponse (1.05:l or better) are achieved at 1.5 pm wavelength using straightforward, regrowth-free fabrication. Low capacitance photodetectors ( I 0.15 pF) are employed for high bandwidth operation.ONOLITHIC integration of optical waveguide de-M vices with other optoelectronics enhances on-chip functionality, and improves packaging cost and reliability by minimizing hybrid optical interconnections. A target application for such integration has been coherent lightwave receiver front-ends, combining photodetectors with 3 dB waveguide couplers for balanced operation [11-[6] and polarization-splitting optics for polarization-insensitive reception using diversity architectures [5], [6]. Compatibility with high 111-V materials' cost, however, requires high-yield integration processes and compact device size. The large size, typically several millimeters, of waveguide couplers with associated input-output branching is a serious obstacle to cost-effective monolithic integration. We previously reported alternative approaches for ultra-compact 3 dB couplers [7] and polarization-selective detectors [8]. Here we integrate both device types on a single InP chip to realize an ultracompact (1.3 x 0.4 mm'), balanced, polarization-diversity photodetector which is easy to fabricate (no epitaxial regrowth, minimal photolithography) and exhibits low detector capacitance (< 0.15 pF).Our device (Fig. 1) consists of two single-mode input rib waveguides for photosignal and local oscillator (LO) inputs, a multi-mode interference [9] (MMI)