Reduced reflections from multimode interference couplers

The passive components in the integrated optic chip applied by fiber optic gyroscope are analyzed and designed on silicon-on-insulator (SOI) wafers. These components include a polarizer and a multimode interference (MMI) coupler. The MMI coupler acts as the traditional Y-branch power splitter. The polarizer consists of two 3dB MMI couplers and two straight single mode waveguides between these two MMI couplers. These two straight waveguides have equal length but have different rib width. The mode propagation constants of TE or TM light are not same and the appropriate selection of the length of the straight waveguide will result in the splitting of TE and TM light. The scalar or semivectorial finite difference beam propagation method (FD-BPM) is used to design these photonic components. The mode parameters of straight waveguide with different rib width are analyzed by semivectorial FD-BPM and the reasonable length of straight waveguide was selected. The MMI coupler is designed by scalar FD-BPM and the length of multimode waveguide and the positions of the input or output ports are determined accurately. The curving waveguide is avoided completely, therefore the design is more reliable and the fabrication of the device is much easy.

An ultracompact and polarization-insensitive power splitter using a subwavelength-grating-based multimode interference (MMI) coupler on an SOI platform is proposed and analyzed in detail. By properly tailoring the structural parameters of the subwavelength gratings embedded in the center of the MMI coupler, the effective reflective indices for TE and TM modes supported by this MMI coupler can be engineered, leading to equal coupling lengths for the two polarizations and an efficient reduction in length for the used MMI coupler. As a result, an ultracompact polarization-insensitive power splitter can be realized. Moreover, to effectively minimize the loss, tapered waveguides are used, and two right angles are cut at both corners of the used MMI coupler. Results show that a footprint of ${2}.{2}\;\unicode{x00B5} {\rm m} \times {3}.{8}\;\unicode{x00B5} {\rm m}$2.2µm×3.8µm for the MMI region is achieved with an insertion loss of 0.07 dB for both TE and TM modes (polarization dependent loss ${\sim\;{0}\;{\rm dB} }$∼0dB) and a reflection loss of $ - {28}.{29}\;{\rm dB}$-28.29dB ($ - {31}.{25}\;{\rm dB}$-31.25dB) for TE mode at the wavelength of 1.55 µm. Insertion loss below 0.3 dB is obtained over the bandwidth of 200 nm, covering the C-band. In addition, fabrication tolerances to the structural parameters are analyzed, and the injected light propagating through the power splitter is also presented.

This paper shows mainly the performance study on tapered section of Multimode Interference (MMI) coupler having either parabolic or exponential taper section. The comparative study of the 1 × 2 MMI coupler with proposed structure, such as the normalized width variation, and variation of MMI length are investigated by the 2D finite difference beam propagation method (BPM). We have found that exponential tapered MMI coupler has advantageous compared to parabolic tapered MMI coupler. The relation between numbers of allowed modes with power transmission has been predicted along with pulse propagation through MMI coupler. At the output of proposed structure, S-bend waveguides used to provide sufficient separation between the two output waveguides. At the end of paper, compared the excess loss of proposed structure. Our results are unique, because, we have analyzed relatively a new structure compared to existing structure found in literature. Keywords: Linear waveguide, Parabolically tapered waveguide, Exponentially tapered waveguide, S-bend Arc waveguide

Unbalanced multimode interference (MMI) couplers are optical integrated components widely used in photonic integrated circuits. Couplers with two fixed coupling ratios: 85/15 and 15/85 are studied in this paper. Low-loss MMI couplers are designed and fabricated. Excitation of the first-order mode in access waveguides of MMI couplers can influence the excess losses and the coupling ratio of these components. The effect of the first-order mode excitation was simulated and confirmed by the experimental results, which are presented in this paper.

In this paper, a balanced optical fanout using 1/spl times/4 thin film metal-semiconductor-metal (MSM) photodetector array embedded in a photoimageable polymer multi-mode interference (MMI) coupler is fabricated, demonstrated, and compared to theoretical estimates. This is the first MMI coupler reported with an integrated photodetector array, and the first MMI coupler reported that has been fabricated with a photoimageable polymer. For optical signal distribution (e.g. optical clock distribution), balanced signal routing is an important optical function, and MMI couplers provide compact design, low splitting loss, and balanced output.

In this paper, the operation principles for optical switches based on two multimode interference (MMI) couplers are discussed in detail. Based on the analytical expressions of transfer matrix of the MMI couplers, the control mechanism and principles of the two-stage MMI coupler based switch are derived. The device can be controlled by proportionally adjusting the interstage variable phase shifter array. N different switching states can be realized on this device.

A novel compact silicon-on-insulator- (SOI-)based multimode interference (MMI) coupler with bilevel taper structures was designed. The MMI section and the S-bend sections of the input-output waveguides are deeply etched. The input-output waveguides connecting to single-mode fibers or other photonic light circuits are etched shallowly to yield single-mode operation. A bilevel taper is introduced in the transition region between the shallowly and deeply etched regions. It is predicted theoretically that this design will not only improve the quality of the self-imaging in the MMI section but will also make the structure compact. Both the excess loss and the nonuniformity of the MMI coupler are reduced. By use of a three-dimensional beam propagation method, the performance of a 1 x 4 MMI coupler based on a SOI is simulated as a numerical example of the novel design. The simulated nonuniformity and the excess loss are approximately 0.0285 and 0.2 dB, respectively.

A novel design of a bi-directional triplexer based on Bragg grating assisted multi-mode interference (MMI) coupler is proposed in this letter. The MMI coupler de/multiplexes the upload 1310nm channel and the download 1490nm channel, while the grating reflects the download 1550nm channel to the other download port. The bandwidths for 1310nm, 1490nm and 1550nm are about 110nm, 50nm, and 15nm, respectively.

An ultracompact multimode interference (MMI) coupler based on Si nanowire waveguides is optimized to achieve polarization-insensitive performances. According to numerical simulations, an optimal width of the MMI section to achieve a polarization-insensitive performance is obtained for a fixed thickness of the core layer. A polarization-insensitive 1times2 MMI coupler with a size of about 3.8 mum times15.3 mum is designed as an example. Numerical simulations show that this optimized MMI coupler has a good fabrication tolerance (about plusmn45 and plusmn330nm for the MMI width and length, respectively) and the 0.5-dB bandwidth is more than 60 nm for both polarizations

We design and experimentally demonstrate a dualmode 3 dB power splitter based on the self-imaging effect in a taper-assisted multimode interference (MMI) coupler. The quality reduction of the two mirrored half-power images of the input field due to accumulated high-order eigenmodes phase error will degrade the performance of dual-mode power splitter. We propose an eigenmode-selective-excitation method to suppress the excitation of high-order eigenmodes in the MMI. By optimizing the geometric parameters and injection position of the input taper waveguide, we realize power ratios of 95.9 % and 94.2 % for the lowest three and five eigenmodes when input TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> modes, respectively. The fabricated device occupies a footprint of 5 μm × 86.5 μm with 5.2 μm assisted tapers. The measured excess losses and crosstalks for both modes are less than 0.76 dB and -20.5 dB, on average, over the observable 60 nm bandwidth, respectively, which are consistent with the numerical simulations.

In this letter, a novel method to realize all-optical discrete sine transform (DST) and discrete cosine transform (DCT) is proposed. The approach requires only one multimode interference (MMI) coupler and several phase shifters at the input and output ports. By properly arranging the length of the MMI coupler and adjusting the phase shifters, the DST can be directly realized all-optically. Meanwhile, the DCT is related to the DST with a simple formula. By changing the input phase and relabeling the output ports, it can also be realized on the MMI structure. The proposed methodology has no limit on the number of the inputs.

Two types of uneven splitting-ratio 2×2 multi-mode interference (MMI) couplers based on silicon nanowires are designed, fabricated and characterized. The splitting ratios are 85:15 and 72:28, respectively. The devices have compact sizes and low excess losses. The footprints of the rectangular MMI region are only about 3μm×18μm and 3μm×14μm, and the minimum excess losses (ELs) are 1.30 dB and 0.82 dB. The measured splitting-ratios are consistent with the designed values. Based on their performance, these 2×2 MMI couplers are suitable candidates for the coupling section of microring resonators where a large resonance bandwidth is required for high speed signal processing. The uneven splitting capability also provides a convenient way to further optimize the Q factor and the bandwidth of the resonator.

Power splitting ratio couplers based on MMI structures with high bandwidth and large tolerance using silicon waveguides

An optical switch based on the multimode interference (MMI) coupler is demonstrated in this letter. The device is designed and fabricated in polymeric materials and the thermooptic (TO) effect of polymers, which has a negative TO coefficient, is employed to change the self-imaging effect of a side-heated MMI coupler and realize optical switching. The switching power is only 22 mW. The experimental results show that the crosstalk of the switch is about -20 dB at both the cross and bar states.

In this paper, an all-optical logic gate with multiple functions and asymmetric structure is theoretically designed and simulated using a silicon on insulator (SOI)-based multimode interference (MMI) coupler. By switching the optical signal to different input waveguide ports, the device can function as an OR, NOT, NAND, or NOR gate simultaneously or individually. Simulation results by using three-dimensional beam propagation method (3D-BPM) incorporated with effective index method (EIM) show that these logic gates can operate with low loss in a wide wavelength band as much as 100 nm. It is a kind of promising device for applications on the next-generation high-speed, compact logic circuits, ultrafast signal processing as well as future silicon photonics-based integrated circuits.