Ultracompact, low-loss directional couplers on InP based on self-imaging by multimode interference

Abstract

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...

Optical multi-mode interference devices based on self-imaging: principles and applications

“…In addition to the single images at distances given by (18), multiple images can be found as well. Let us first consider the images obtained half-way between the direct and mirrored image positions, i.e., at distances for v even (16) with p an odd integer.…”

“…Fig. 4 shows the schematic layout of the InGaAsP 2 x 2 multimode interference coupler reported in [18], [19]. The 8-pm wide multimode section supports 4 guided modes.…”

Ultracompact monolithic integration of balanced, polarization diversity photodetectors for coherent lightwave receivers

“…Thus, we expect 2.3 (5.2) dB total insertion loss for conversion of TE (TM) optical power in the fiber to detector photocurrents using AR-coated chips. This loss is comparable to that of a recently reported hybrid coherent receiver [131. Our MMI coupler's polarization dependence is much stronger than that of earlier devices [7] due to three factors: i) nonideal fabrication, resulting in fundamental mode loss (conversion to higher-order, substrate-leaky modes) for both polarizations, ii) reduced substrate leakage due to the very deep rib etch so that light from all modes reaches the vertical couplers, and iii) increased diffraction loss of higher-order TM modes in the vertical couplers due to the larger TM detector-to-MMI coupler separation. The multimode character of optical near-field patterns at coupler/taper outputs after cleaving away the vertical couplers supports this interpretation.…”

“…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).…”

“…Thus, this work shows that appreciable optical signal processing can be incorporated with photodetectors without compromising chip size or ease offabrication. Our compact optical design should also result in a broad spectral operating range ( = 70 nm) [7], [8]. We anticipate that our integration approach will prove widely applicable to advanced lightwave receivers for both wavelength-multiplexed (direct) and coherent detection.…”