Cascaded Multimode Interference Research Articles

Refractive index sensors based on cascaded multimode interference hetero-core optical fibers.

In this work, we demonstrate refractive index (RI) sensors based on the cascade of hetero-core structures using multimode and no-core optical fibers in sequence. The sensor device is engineered to have resonances at different wavelengths using different sensing region lengths. The device fabrication involves simple fiber cleaving and fusion splicing. For the experiments, the two sensor regions are exposed to liquids with different RIs. For the hetero-core fiber insertion length of 45mm, the transmission valley is centered at 1082.5nm with 15.1nm full width at half-maximum (FWHM) for an external medium RI of 1.3370. Additionally, it shifts 4.1nm towards longer wavelengths as the RI of the external medium increases to 1.3840. For the 30mm long hetero-core structure, the valley is centered at 1599.7nm with 23.3nm FWHM for an external medium RI of 1.3370, which shifts 7.4nm as the RI increases to 1.3840. The sensor sensitivities are up to -476d B/r e f r a c t i v e index unit (RIU) and 270nm/RIU. The resolution of the devices is estimated to be 2×10-5 R I U.

Wideband E00-E10 Silicon Mode Converter Based on 180 nm CMOS Technology

Mode division multiplexing (MDM) is a promising technology for the capacity enlargement of the optical transmission network. As a key element in the MDM system, the mode converter plays an important role in signal processing. In this work, a wideband E00-E10 silicon mode converter constructed by Y-branch and cascaded multimode interference coupler is demonstrated. The theoretical mode crosstalk is less than –29.2 dB within the wavelength range from 1540 nm to 1600 nm. By 180 nm standard CMOS fabrication, the tested mode conversion efficiency of 91.5% and the crosstalk of −10.3 dB can be obtained at 1575.9 nm. The 3 dB bandwidth is over 60 nm. The proposed E00-E10 silicon mode converter is applicable in mode multiplexing.

Robust Integrated Optical Unitary Converter Using Multiport Directional Couplers

An integrated optical unitary converter (OUC), which can realize arbitrary N × N unitary transformation on chip, is promising for widespread applications in various areas, such as optical communication, quantum information processing, and optical neural networks. Most of the integrated OUCs demonstrated to date comprise cascaded 2 × 2 Mach-Zehnder interferometers (MZIs) based on Reck's scheme or its variation, or cascaded multimode interference (MMI) couplers based on multi-plane light conversion (MPLC). However, these schemes are sensitive to fabrication errors, making it difficult to realize high-performance large-scale OUCs. In this work, we experimentally demonstrate a silicon photonic 4 × 4 OUC that is inherently robust to fabrication errors, based on a novel structure. The proposed structure consists of cascaded multiport directional couplers (DCs) and phase shifter arrays. In this scheme, the multiport DC assures unitary transformation with inherent robustness against fabrication errors. By optimizing all the phase shifters using a simulated annealing algorithm, we experimentally demonstrate reconfigurable 4-mode demultiplexing and error-free demultiplexing of 40-Gb/s non-return-to-zero (NRZ) signals.

Fluorinated photopolymer cascaded MMI-based integrated optical waveguide switching matrix with encoding functions.

In this work, thermo-optic tunable 4 × 4 cascaded multimode interference based integrated optical waveguide switching matrices are designed and fabricated using photopolymer lightwave circuits. The materials of the waveguide core and cladding are fluorinated epoxy-terminated copolycarbonate and polymethylmethacrylate, respectively. The driving power that controls matrices for binary encoding of different optical switching states are simulated and analyzed. The measured insertion loss of the actual chip is < 7.1 dB and the maximum crosstalk in adjacent channels is <-30 dB. The switching time is approximately 220 μs and the extinction ratio is obtained as 21.5 dB. This flexible encoding technique can be applied for achieving optical code-division multiple-access network coders.

Electromagnetically Induced Transparency (EIT) Like Transmission Based on 3 × 3 Cascaded Multimode Interference Resonators.

We propose a method for generating the electromagnetically induced transparency (EIT) like-transmission by using microring resonator based on cascaded 3 × 3 multimode interference (MMI) structures. Based on the Fano resonance unit created from a 3 × 3 MMI coupler with a feedback waveguide, two schemes of two coupled Fano resonator unit (FRU) are investigated to generate the EIT like transmission. The theoretical and numerical analysis based on the coupled mode theory and transfer matrix is used for the designs. Our proposed structure has advantages of compactness and ease of fabrication. We use silicon waveguide for the design of the whole device so it is compatible with the existing Complementary Metal-Oxide-Semiconductor (CMOS) circuitry foundry. The fabrication tolerance and design parameters are also investigated in this study.

Integrated Reconfigurable Unitary Optical Mode Converter Using MMI Couplers

Reconfigurable unitary optical mode converters that can convert multiple orthogonal spatial modes into different orthogonal modes without fundamental optical loss are important for mode-division-multiplexed (MDM) optical communication, imaging, and quantum computation. In this letter, we propose a novel integrated reconfigurable unitary optical mode converter, which consists of cascaded multimode interference couplers and phase shifter arrays. We numerically investigate its ability to realize arbitrary unitary mode conversion and demonstrate its applicability to mode unscrambling as well as mode switching in MDM transmission systems.

Optical mode switch based on multimode interference couplers

In this paper, we propose an optical mode switch based on two cascaded multimode interference (MMI) couplers. After a fundamental mode divided into two equal-power fundamental modes in the first MMI coupler, the thermo-optic effect is employed to modulate the phase of the two fundamental modes before directed to the next MMI for the purpose of mode switching. By adjusting the electric signals applied to the modulation arms, the proposed device can implement mode switching in three states: (a) one first-order and two fundamental modes simultaneously output, (b) one first-order mode output, and (c) two fundamental modes output. As a result, the simulated excess losses are −0.29 dB, −0.10 dB, and −0.63 dB, respectively.

Design and analysis of O–S–C triple band wavelength division demultiplexer using cascaded MMI couplers

Compact triple O–S–C band wavelength demultiplexer, which consists of series cascaded multimode interference (MMI) couplers has been carried out in this paper. The MMI coupler has been used to drop the wavelengths of 1510nm and 1550nm at bar port while the wavelength 1300nm into the cross port. Then another MMI coupler has been designed to separate the wavelength 1510nminto one port and wavelength 1550nm into another port. The triple wavelength demultiplexer function has been performed by choosing a suitable refractive index of the guiding region and geometrical parameters such as the width and length of MMI coupler. Numerical simulation with finite difference beam propagation method (BPM) has been utilized to design and optimize the operation of the proposed triple wavelength demultiplexer. The simulation results show that insertion losses of wavelength O, S and C, bands are 1.884dB, 1.452dB and 2.568dB, respectively, with isolations for each output waveguide ranging from 10dB to 28.72dB. The 3-dB bandwidth of insertion loss for 1300nm, 1510nm and 1550nm are 80nm, 20nm and 10nm, respectively.

Proposal for fabrication-tolerant SOI polarization splitter-rotator based on cascaded MMI couplers and an assisted bi-level taper.

A novel silicon-on-insulator (SOI) polarization splitter-rotator (PSR) with a large fabrication tolerance is proposed based on cascaded multimode interference (MMI) couplers and an assisted mode-evolution taper. The tapers are designed to adiabatically convert the input TM(0) mode into the TE(1) mode, which will output as the TE(0) mode after processed by the subsequent MMI mode converter, 90-degree phase shifter (PS) and MMI 3 dB coupler. The numerical simulation results show that the proposed device has a < 0.5 dB insertion loss with < -17 dB crosstalk in C optical communication band. Fabrication tolerance analysis is also performed with respect to the deviations of MMI coupler width, PS width, slab height and upper-cladding refractive index, showing that this device could work well even when affected by considerable fabrication errors. With such a robust performance with a large bandwidth, this device offers potential applications for CMOS-compatible polarization diversity, especially in the booming 100 Gb/s coherent optical communications based on silicon photonics technology.

Terahertz wave polarization beam splitter using a cascaded multimode interference structure.

A terahertz wave polarization beam splitter, based on two cascaded multimode interference structures with different widths, is designed and numerically demonstrated. The numerical calculation results show that the designed polarization beam splitter can split transverse-electric (TE) and transverse-magnetic (TM)-polarized terahertz waves into different propagation directions with high efficiency over a frequency range from 6.40 to 6.50 THz. This polarization beam splitter shows more than a 22.06 dB extinction ratio for TE-polarization and a 31.65 dB extinction ratio for TM-polarization. Using such a polarization beam splitter, the whole length of the polarization beam splitter is reduced to about 1/12 that of a conventional design. This enables the polarization beam splitter to be used in terahertz wave integrated circuit fields.

Design and experimental verification of all waveguide type triplexers using cascaded MMI couplers

A triplexer using three cascaded multimode interference (MMI) sections with different widths is designed, fabricated and characterized. The simulation results shows that with optimized cascaded MMI sections, a good triplexing function can be achieved. The designed triplexers were fabricated using ultraviolet lithography and inductively coupled plasma etching. The experimental results coincide well with the simulation.

The role of group index engineering in series-connected photonic crystal microcavities for high density sensor microarrays.

We experimentally demonstrate an efficient and robust method for series connection of photonic crystal microcavities that are coupled to photonic crystal waveguides in the slow light transmission regime. We demonstrate that group index taper engineering provides excellent optical impedance matching between the input and output strip waveguides and the photonic crystal waveguide, a nearly flat transmission over the entire guided mode spectrum and clear multi-resonance peaks corresponding to individual microcavities that are connected in series. Series connected photonic crystal microcavities are further multiplexed in parallel using cascaded multimode interference power splitters to generate a high density silicon nanophotonic microarray comprising 64 photonic crystal microcavity sensors, all of which are interrogated simultaneously at the same instant of time.

1×4 Power Splitter Based on Cascaded Multimode Interference Photonic Crystal Waveguide

Using of cascaded multimode interference waveguides in 2D photonic crystal power splitter are investigated by modeling and computation. Results show that with a five line defect waveguides and one pair of three line defect waveguides integrated, the total normalized transmission can be high as 94.9% with 30nm broad spectrum calculated and simulated by finite-difference time-domain method.

Shortened Polarization Beam Splitters With Two Cascaded Multimode Interference Sections

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This letter presents a design for shortening the length of a polarization beam splitter (PBS) by using two cascaded multimode interference (MMI) sections with different widths. By optimizing the MMI parameters, a short PBS of good performance is achieved. The total length of the PBS is reduced to about 1/28 of that of a conventional design. </para>

PLC-Based Novel Triplexer With a Simple Structure for Optical Transceiver Module Applications

A novel triplexer is introduced for optical communication applications in this letter. The triplexer consists of two cascaded multimode interference (MMI) couplers and can be fabricated by planar lightwave circuit technology. The concept of the extraneous self-imaging phenomenon, which is not generally mentioned in MMI theory, is applied to this triplexer to design a relatively simple and compact device. From the simulation results for wavelengths of 1310, 1490, and 1550 nm, the excess losses were determined to be 1.37, 0.63, and 0.79 dB, respectively, while the extinction ratios were 15.93, 24.28, and 18.44 dB, respectively.

Design and fabrication of an optical interleaver with cascaded multimode interference couplers

A lattice-form optical interleaver was designed and fabricated with a silicon-based silica waveguide. Cascaded multimode interference couplers were first employed in the lattice circuit and helped to relax the fabrication tolerance. The device shows good performance, the insertion loss is less than 2.25 dB, the passband ripple is less than 0.15 dB, the cross talk is less than -18.9 dB, and the 0.5 dB passband is more than 100 GHz.

Tolerance Analysis of Lattice-Form Optical Interleaver With Different Coupler Structures

A set of recursion equations is developed to synthesize a coherent two-port lattice-form optical delay-line circuit with different coupler structures. A fifth-order lattice-form optical interleaver is designed, and parameter tolerance is analyzed with crosstalk consideration. Results show that the phase tolerance of delay lines is six times the coupling-coefficient-angle tolerance of directional couplers. The coupling-coefficient-angle tolerance of directional couplers in an interleaver with stabilized couplers is improved by one order than that in an interleaver with simple directional couplers if global uniformity of the planar-lightwave-circuit process is supposed. The tolerance of an interleaver with cascaded multimode interference (MMI) couplers does not improve much, but an MMI coupler is more immune to process error than a directional coupler. Theoretical analysis meets the simulation results well. A stabilized coupler and a cascaded MMI coupler are both good substitutes for a simple directional coupler in lattice-form optical interleaver, and both help to reduce crosstalk

Novel approach to design high-performance large-port-count switches in low-index-contrast materials based on cascaded multimode interference couplers

Using a novel design approach, high-performance large-port-count switches based on cascaded multimode interference (MMI) couplers are shown to be feasible in low-index-contrast materials. This approach combines the transfer matrix method, optimization of the MMI dimensions, and mode propagation analysis (MPA) for calculation of phase shifts. 1/spl times/4, 1/spl times/8, 1/spl times/16, and 1/spl times/32 switches are designed, with insertion losses of 1 dB and crosstalk as low as -31.7 dB. Also, it is shown that this approach can balance the losses for different switch states.

Designing Optical Switches Based on Silica Multimode Interference Devices

A novel approach to design N×N silica-on-silicon optical switches based on phase-compensated and cascaded multimode interference structures is presented. Taking into consideration of weakly guiding features of silicaon-silicon waveguides, instead of using self-imaging theory, we apply a two-step design approach: In the first step the multimode sections are optimized for minimum loss and best uniformity. In the second step the phase shifting compensations are obtained by accurate calculation of relative phases of MMI sections. Using this novel approach we designed a 4×4 optical switch and achieved better device performance from our numerical simulation results. Compared with the configuration from the self-imaging theory, the results from our designing approach can reduce the device loss and crosstalk by 1dB and 8dB, respectively.

1×8 Cascaded Multimode Interference Splitter in Silicon-On-Insulator

A cascaded multimode interference 1×8 power splitter is proposed and fabricated in silicon-on-insulator material. The device consists of seven 1×2 power splitters arranged in a tree configuration. The cascaded splitter and its 1×2 splitter element have the power uniformity of approximately 1.5 dB and 0.3 dB, respectively.