Multimode Interference Theory Research Articles

Proposal of mosaic-based 2 × 2 3-dB couplers for prospective on-chip communication in mid-infrared wavelengths around 2.1 µm

We propose two types of mosaic-based 2 × 2 3-dB couplers for wavelengths around 2.1 µm. For type I, the proposed device provides excess loss of < 0.22 dB and imbalance of < ± 0 . 07 d B over the wavelength range from 2.06 to 2.14 µm. Compared with the standard multimode-interference-based 2 × 2 3-dB coupler, the imbalance is improved by ∼ 0.4 d B . For type II, coupling length in the multimode-interference region is about three times shorter than the one based on multimode-interference theory. It provides excess loss of < 0.49 d B and imbalance of < ± 0 . 23 d B over the wavelength range from 2.06 to 2.14 µm. Both proposed devices show high fabrication tolerance to hole diameter variation. Especially for type II, the fabrication-tolerant direct binary search was effective to obtain such a structure. These mosaic-based 2 × 2 3-dB couplers will play an important role in highly integrated photonic circuits.

Angle sensor for humidity-insensitive angle measurement based on multimode interference

A multimode interference (MMI) fiber optics angle sensor based on a cyclic transparent amorphous fluoropolymer (CYTOP) multimode fiber is reported Two silica single-mode fibers (SMFs) are glued to a segment of CYTOP fiber to form a single-mode-multimode-single-mode structure that produces an MMI spectrum. The centers of the SMF and the CYTOP fiber are aligned using the translation stage of the fiber fusion splicer in manual mode. The angle sensing mechanism based on MMI theory is investigated and experimentally verified. A large measurement range of 110° and a linear sensitivity of 36.2 pm/deg is demonstrated. The angle measurement is humidity-insensitive due to the weak moisture absorption of the CYTOP fiber and the design of sensor packaging.

Design Study of Broadband and Ultrahigh-Resolution Imaging Spectrometer Using Snapshot Multimode Interference in Fiber Bundles

Imaging spectrometry plays a significant role in various scientific realms. Although imaging spectrometers based on different schemes have been proposed, the pursuit of compact and high-performance devices is still ongoing. A compact broadband and ultrahigh-resolution imaging spectrometer (CBURIS) is presented, which comprises a microlens array, multiple fiber bundles, a microscope, and a two-dimensional detector array. The principle of the device is to spatially sample and integrate the field information via the front microlens array and then further process with the fiber bundles and imaging system based on the multimode interference theory. From both the theoretical and numerical analysis, this CBURIS design is a superior concept that not only achieves a 0.17° spatial resolution and ultrahigh spectral resolution (resolving power exceeds 2.58 × 106 at 1.55 µm) from the visible to mid-infrared region but also has the advantages of snapshot measurement, thermal stability, and a compact footprint compared with most existing imaging spectrometers.

Multimode interferometer-based torsion sensor employing perfluorinated polymer optical fiber.

This paper reports a torsion sensor based on the multimode interference theory. The sensor is fabricated by sandwiching a section of perfluorinated polymer optical fiber (POF) between two silica single mode fibers to construct a single-mode-multimode-single-mode (SMS) structure. The perfluorinated POF is easily connected to the optical fiber via the precise alignment of ceramic ferrules and ceramic mating sleeve. With the considerable flexibility and deformability of the perfluorinated POF, the proposed sensor is especially suitable for torsion measurement. Experimental results show that a wavelength sensitivity of 106.762 pm/(rad/m) and an intensity sensitivity of 0.165 dBm/(rad/m) are obtained within a large torsion rate of -100∼100 rad/m.

Compact On-Chip Optical Components Based on Multimode Interference Design Using High-Contrast Grating Hollow-Core Waveguides

Low-loss high-contrast grating (HCG) hollow-core waveguide (HCW) is shown to be a promising candidate for building a variety of compact multimode interference (MMI) based optical components. First, we show that HCG-HCWs demonstrate self-imaging property essential for designing MMI components. Furthermore, strong confinement of light in the waveguide core and low-index guiding leads to compact optical components. Simulation results show extremely compact 2 × 2 coupler with a length of 26 μm (16 λ), footprint (length × width) of 85 μm2 (35 λ2) and a 3-dB splitter with a length of 3.6 μm (2.4 λ), and a footprint of 12 μm 2 (5 λ2). Simulation results agree well with the predictions of MMI theory and the design can be easily extended to realize 1 × N splitters and N × N couplers. As a proof of concept, a 1 × 4 splitter with equal power splitting ratio and low insertion loss is designed. In addition, an ultracompact 2 × 2 optical switch based on HCG-HCW is proposed and simulated. The two waveguide channels are isolated by a single layer of HCG. By a small change of the refractive index of this HCG, light can switch between the channels. By proper optimization of HCG parameters, only higher order modes are excited in the MMI region resulting in an ultracompact optical switch with a switching length of 60 μm, ∼25× smaller than conventional 2 × 2 MMI coupler.

Self-imaging of orbital angular momentum (OAM) modes in rectangular multimode interference waveguides.

We study the propagation of orbital angular momentum (OAM) modes in rectangular multimode waveguides. Due to the multimode interference effect, an OAM mode input forms self-images at certain propagation distances. As OAM modes can be decomposed as the superposition of a pair of quarter-wave phase-shifted even and odd modes, their symmetry properties lead to two different self-imaging categories - forming the OAM-maintaining and the field-splitting self-images. We analyze these phenomena using multimode interference theory, and establish the rules governing the OAM-maintaining self-imaging, which allows the multi-mode interference waveguides to be used as OAM mode splitters and couplers.

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.

1 $\times$ 2 Wavelength Multiplexer With High Transmittances Using Extraneous Self-Imaging Phenomenon

This paper introduces a wavelength multiplexer (MUX) using the extraneous self-imaging (Ex_SI) phenomenon, which is not mentioned in multimode-interference theory. The Ex_SI phenomenon in silica-based multimode waveguides was experimentally studied. Then, using data for the Ex_SI phenomenon, the wavelength MUX for the wavelengths of 1310 and 1550 nm was developed. The optimum length of the multimode waveguide, with a width of 18 mum, was confirmed as a 3670- mum wavelength MUX. For the wavelengths of 1310 and 1550 nm, the excess losses were determined to be -0.4 and -0.45 dB, respectively, while the extinction ratios were 16.9 and 19.7 dB, respectively

Extraneous self-imaging phenomenon with weak-guiding condition

We discuss a self-imaging phenomenon in a multimode interference (MMI) coupler. From experiment, different self-images, which are undefined in MMI theory, are observed. These undefined self-images are named "extraneous self-images" (Ex_SIs) for convenience. To estimate the applicability of the Ex_SIs, the characteristics of both the single self-image (0 dB self-image, 0 dB SI), which is defined in MMI theory, and the Ex_SI are compared and analyzed through simulation and experiment. The results show that the Ex_SI has an imaging period that is the same as the 0 dB SI and that the excess loss and the extinction ratio of the Ex_SI improve more than that of the 0 dB SI as the imaging period increases.

Experimental study of the self‐imaging phenomenon with weak‐guiding condition

AbstractThis paper investigates a self‐imaging phenomenon in a multimode interference (MMI) coupler. In the experiment, different self‐images which are undefined in MMI theory are observed. This undefined self‐image is named the “pseudo‐self‐image ” (PSI) as a matter of convenience. The characteristics of both the single self‐image (0‐dB self‐image: 0‐dB SI), which is defined in MMI theory, and the PSI are compared and analyzed through simulation and experimentation to estimate the applicability of the PSI. Results show that the PSI has an imaging period the same as the 0‐dB SI, and the excess loss and the extinction ratio of the PSI improve more than those of the 0‐dB SI as the imaging period increases. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48:2354–2357, 2006;Published online in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/mop.21942