Multimode Interference Device Research Articles

Ultra-broadband MMI power splitter from 1.26 to 1.67 μm with photonic bound states in the continuum

Photonic bound states in the continuum (BIC) have excellent effects in localizing light fields. Low propagation loss of TM mode has been achieved in different orders of BICs on an etchless lithium niobate (LiNbO3) platform. Herein, we simulatively demonstrate a 1 × 2 MMI (Multimode interference) power splitter based on waveguides constructed by low-refractive-index structure (ZEP-520A) on a high-refractive-index thin membrane (LiNbO3), which can optimize the bandwidth, insertion loss and other performance indicators of optical devices. Simulation results show that the working wavelength cover the entire communication band ranging from 1260 nm to 1675 nm, and the transmission efficiency is more than 48.7%, while the insertion loss is less than 0.01 dB covering a 415 nm bandwidth. The field distribution results verify that the designed MMI demonstrated high mode purity at the output, and the mode field is well localized in the LiNbO3 film, leading to a reduce of the dependence of the transmission efficiency originated from the quality of the sidewall of the device, and the device can evenly distribute the TM0 mode of the ultra-broadband incident light to the two output ports. Furthermore, the fabrication process of the MMI device can be immensely simplified by a one-step exposure method, which reduces the scattering loss and asymmetry caused by the fabrication.

A switchable multi-wavelength mode-locked fiber laser based on a multi-mode interference device

A switchable multi-wavelength, passively mode-locked fiber laser in the 1 μm regime is demonstrated in this paper. The key principles of working include the nonlinear polarization rotation (NPR) mechanism for the mode-locked pulse generation and a single mode-multimode-single mode (SMS) structure as the spectral filter. The SMS structure is coiled on the paddles of a polarization controller (PC). Stable mode-locking operation is achieved at the fundamental repetition rate of 7.97 MHz. The pulse duration is 429 ps at the pump level of 216 mW, with the dual-wavelength laser emitting at 1037.86 nm and 1045.72 nm. By properly tuning the paddles of PCs, multi-wavelength mode-locked laser spectra can be observed switching from triple- to sextuple-wavelength, with the emission bandwidth of 27.6 nm from 1035.6 nm to 1063.2 nm.

Refractometric Detection of Adulterated Milk Based on Multimode Interference Effects.

This paper reports on the refractometric detection of water-adulterated milk using an optical fiber sensor whose principle of operation is based on multimode interference (MMI). The device is manufactured in a simple way by splicing a segment of coreless multimode fiber (NC-MMF) between two single-mode fibers (SMFs); neither functionalization nor deposition of a sensing material is required. MMI takes place in the NC-MMF and, when fed with a broadband spectrum, a transmission peak appears at the output of the MMI device due to its inherent filter-like response, whose position depends on the effective refractive index (RI) of the medium surrounding the NC-MMF. Therefore, when the sensor is immersed in different milk–water mixtures, the peak wavelength shifts according to the RI of the mixture. In this way, adulterated milk can be detected from the wavelength shift of the transmission peak. The system was tested with two commercial brands of milk, and adulterations were clearly distinguished in both cases. In the range of interest, from no dilution up to 50% dilution, the sensor exhibits a linear response with a sensitivity of −0.04251 and −0.03291 nm/%, respectively, for the two samples tested. The measurement protocol is repeatable and allows for locating the peak wavelength within <0.34 nm over several repetitions using different samples with the same concentration. A thermal sensitivity of 0.85 nm/°C was obtained, which suggests that the temperature needs to be maintained as fixed during the measurements. The approach presented can be extended to other scenarios as a quality control tool in beverages for human consumption, showing the advantages of simple construction, high sensitivity, and the potential for real-time monitoring.

Exposed-core fiber multimode interference sensor

In this manuscript, we report on, to the best of our knowledge, the first experimental realization of a multimode interference device based on self-image phenomenon accomplished by using a microstructured-cladding exposed-core fiber set in a singlemode-multimode-singlemode (SMS) configuration. Its application as a refractive index sensor is also demonstrated. The specialty multimode fiber studied herein exhibits a holey microstructure that surrounds and defines the fiber core. The latter, in turn, interfaces the external fiber environment thanks to lateral access along the entire fiber length. To explore the device's sensing capabilities, numerical simulations have been carried out, and the system sensitivity and spectral characteristics were evaluated. The experimental results demonstrate a refractive index sensitivity of 890 nm/RIU in the refractive index range from 1.41 to 1.43 with a detection limit of 2.7 × 10−3 RIU. This novel SMS configuration has the potential to extend the capabilities of current multimode interference-based sensors by providing an additional path to the realization of refractive index monitoring in liquid samples.

3D integrated wavelength demultiplexer based on a square-core fiber and dual-layer arrayed waveguide gratings.

We present a 3D integrated wavelength demultiplexer using a square-core fiber (SCF) and matched dual-layer arrayed waveguide gratings (AWGs). The SCF works as a 3D fiber multimode interference device, which splits the input light into symmetric four spots. The spots are then coupled to a pitch-matched 4-waveguide network, each connecting an AWG. Interface waveguides are designed to improve the coupling efficiency between the SCF and the dual-layer chip. The four AWGs are designed with different central wavelengths and a large free spectral range (FSR) of 120 nm. To reach a small and uniform insertion loss among different channels, only the central channels of each AWG are used for demultiplexing. The device is fabricated on a polymer platform. The upper and lower layers of the chip are fabricated using the same photolithography mask but rotated 180° so that 4 different AWG designs can be mapped to a single chip. The measured transmission spectra of the four AWGs cover a bandwidth of 112 nm. The insertion loss variation is smaller than 1.4 dB. The designed device can find applications in fiber optic sensing, communication, and astronomy.

Compact acousto-optic multimode interference device in (Al,Ga)As.

Multimode interference (MMI) devices are key components in modern integrated photonic circuits. Here, we present acoustically tuned optical switches on an (Al,Ga)As platform that enable robust, compact and fast response systems improving on recently demonstrated technology. The device consists of a 2 × 2 MMI device fine-tuned in its center region by a focused surface acoustic wave (SAW) beam working in the low GHz range. In this way, we can tune the refractive index profile over a narrow modulation region and thus control the optical switching behaviour via the applied SAW intensity. Direct tuning of the MMI device avoids losses and phase errors inherent to arrayed waveguide based switches, while also reducing the dimensions of the photonic circuit.

Measuring the Thermo-Optic Coefficient of Liquids With Athermal Multimode Interference Devices

We demonstrate the use of thermally insensitive fiber optic multimode interference (MMI) devices to measure the thermo-optic coefficient (TOC) of liquids. Such athermal MMI devices are all-fiber, easy to fabricate and, in general, consist of a cascade of several multimode sections. Removing the inherent thermo-optical response of the MMI device is achieved passively in a material-based fashion. Starting from the general theory, we derive simple analytical expressions for the design of two-element MMI cascaded devices with arbitrary materials. Moreover, we verify that those expressions are valid irrespectively of the self-imaging operation. We designed and fabricated an athermal MMI device with a simple architecture consisting of a so-called no-core optical fiber (NCF), which is partly coated by PDMS, and we used it to measure the TOC of several standard liquids. Our results are in good agreement with the values of the TOC reported in the literature. By removing the thermal cross-sensitivity in our device, only one calibration measurement is necessary, which allows for straightforward information retrieval.

Arbitrary power-splitting-ratio achieved in 1×2 hybrid plasmonic multimode interference device by structure symmetry broken

The power-splitting-ratio (PSR) of the 1 × 2 multimode interference (MMI) device based on hybrid plasmonic waveguide is studied theoretically. Other than the finite and fixed PSR in conventional MMI, by rational breaking the symmetry of the structure, arbitrary PSR can be achieved, while the original advantages of MMI devices including large fabrication tolerance, wide operation bandwidth, and compact size (1 × 1.1–1.5 μm2) are well-kept. Besides, unlike the regular hybrid-plasmonic-waveguide based device suitable only to TM polarization, the power splitter design presented here could be also possible available to TE polarization incidence. Introducing tapered structure may further improve its performance on both TE and TM polarization cases.

Polymeric power splitters for multiplexing optical biosensors

Aiming efficient multiple simultaneous detection, two different designs of 1×N power splitters for single source feeding of an N-array biosensors operating at 1550 nm are here proposed and thoroughly compared and discussed. Taking into account the need for micro guiding structures, simpler and cheaper manufacturing processes, SU-8 polymer is adopted as the core material. Our first proposal is based on a cascade arranged directional couplers (DC) with a footprint of 4.2×0.42 mm2, uniform power division and losses of 0.54 dB. The second proposal is based on a multimode interference device (MMI) exhibiting a compact footprint of 1.25×0.23 mm2, highly uniform power division and significant low losses of 0.16 dB.

Laser-Induced Deposition of Carbon Nanotubes in Fiber Optic Tips of MMI Devices.

The integration of carbon nanotubes (CNTs) into optical fibers allows the application of their unique properties in robust and versatile devices. Here, we present a laser-induced technique to obtain the deposition of CNTs onto the fiber optics tips of multimode interference (MMI) devices. An MMI device is constructed by splicing a section of no-core fiber (NCF) to a single-mode fiber (SMF). The tip of the MMI device is immersed into a liquid solution of CNTs and laser light is launched into the MMI device. CNTs solutions using water and methanol as solvents were tested. In addition, the use of a polymer dispersant polyvinylpyrrolidone (PVP) in the CNTs solutions was also studied. We found that the laser-induced deposition of CNTs performed in water-based solutions generates non-uniform deposits. On the other hand, the laser-induced deposition performed with methanol solutions generates uniform deposits over the fiber tip when no PVP is used and deposition at the center of the fiber when PVP is present in the CNTs solution. The results show the crucial role of the solvent on the spatial features of the laser-induced deposition process. Finally, we register and study the reflection spectra of the as-fabricated CNTs deposited MMI devices.

Compact 2×2 parabolic multimode interference thermo–optic switches based on fluorinated photopolymer**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0402502), the National Natural Science Foundation of China (Grant Nos. 61575076, 61475061, 61605057, and 61675087), and the Jilin Provincial Industrial Innovation Special Fund Project, China (Grant No. 2016C019).

In this work, a dual-side parabolic structural (DSPS) multimode interference (MMI) thermo–optic (TO) waveguide switch is designed and fabricated by using novel low-loss fluorinated photopolymer materials. Comparing with the traditional dual-side linear structural (DSLS) MMI device, the effective length of the MMI coupling region proposed can be effectively reduced by 40%. The thermal stability of the waveguide material is analyzed, and the optical characteristics of the switching chip are simulated. The actual performances of the entire MMI switch are measured with an insertion loss of 7 dB, switching power of 15 mW and an extinction ratio of 28 dB. In contrast to the traditional MMI optical switch, the new type of parabolic structural MMI TO waveguide switch exhibits lower power consumption and larger extinction ratio. The compact fluorinated polymer MMI TO switches are suitable well for realizing miniaturization, high-properties, and lower cost of photonic integrated circuits.

Fiber Vector Bend Sensor Based on Multimode Interference and Image Tapping.

A grating-less fiber vector bend sensor is demonstrated using a standard single mode fiber spliced to a multimode fiber as a multimode interference device. The ring-shaped light intensity distribution at the end of the multimode fiber is subject to a vector transition in response to the fiber bend. Instead of comprehensive imaging processing for the analysis, the image can be tapped out by a seven-core fiber spliced to the other end of the multimode fiber. The seven-core fiber is further guided to seven single mode fibers via a commercial fan-out device. By comparing the relative light intensities received at the seven outputs, both the bend radius and its direction can be determined. Experiment has shown that a slight bend displacement of 10 µm over a 1.2-cm-long multimode fiber in the X direction (bend angle of 0.382°) causes a distinctive power imbalance of 4.6 dB between two chosen outputs (numbered C4 and C7). For the same displacement in the Y direction, the power ratio between the previous two outputs C4 and C7 remains constant, while the imbalance between another pair (C3 and C4) rises significantly to 7.0 dB.

Fiber-based three-dimensional multi-mode interference device as efficient power divider and vector curvature sensor

Three-dimensional multi-mode interference devices are demonstrated using a single-mode fiber (SMF) center-spliced to a section of polygon-shaped core multimode fiber (MMF). This simple structure can effectively generate well-localized self-focusing spots that match to the layout of a chosen multi-core fiber (MCF) as a launcher device. An optimized hexagon-core MMF can provide efficient coupling from a SMF to a 7-core MCF with an insertion loss of 0.6 dB and a power imbalance of 0.5 dB, while a square-core MMF can form a self-imaging pattern with symmetrically distributed 2 × 2, 3 × 3 or 4 × 4 spots. These spots can be directly received by a two-dimensional detector array. The device can work as a vector curvature sensor by comparing the relative power among the spots with a resolution of ∼0.1° over a 1.8 mm-long MMF.

Thermo-optic characteristic of DNA thin solid film and its application as a biocompatible optical fiber temperature sensor.

We report unique thermo-optical characteristics of DNA-Cetyl tri-methyl ammonium (DNA-CTMA) thin solid film with a large negative thermo-optical coefficient of -3.4×10-4/°C in the temperature range from 20°C to 70°C without any observable thermal hysteresis. By combining this thermo-optic DNA film and fiber optic multimode interference (MMI) device, we experimentally demonstrated a highly sensitive compact temperature sensor with a large spectral shift of 0.15 nm/°C. The fiber optic MMI device was a concatenated structure with single-mode fiber (SMF)-coreless silica fiber (CSF)-single mode fiber (SMF) and the DNA-CTMA film was deposited on the CSF. The spectral shifts of the device in experiments were compared with the beam propagation method, which showed a good agreement.

Reflection and Transmission Spectra of Multimode Interference Devices With Bragg Gratings

We numerically investigate the reflection and transmission spectra of multimode interference (MMI) devices incorporating uniform and sampled Bragg gratings with emphasis on their applications to an add-drop multiplexer (ADM). In numerical analysis, a rectangular index modulation is assumed as the grating and a 2×2 MMI device is treated to operate as a nearly two-mode interference (TMI) device, which is frequently used as an alternative to conventional directional couplers that have a disadvantage of large size. Although the first two guided modes are used in the TMI device, it is especially important to overlap their reflection bands. Therefore, a design method for attaining a sufficient overlap and some considerations are given. The spectral response of the TMI device with a uniform Bragg grating is analyzed by a combination of the mode-propagation method and the transfer-matrix method and discussed on the basis of the state of overlap. In the case of a sampled Bragg grating, it is found that although many modes are excited in the MMI section, multiple reflection bands (multi-channel) are created as expected. The treated device can be applied to single- and multi-channel ADMs.

Highly Sensitive Liquid Core Temperature Sensor Based on Multimode Interference Effects.

A novel fiber optic temperature sensor based on a liquid-core multimode interference device is demonstrated. The advantage of such structure is that the thermo-optic coefficient (TOC) of the liquid is at least one order of magnitude larger than that of silica and this, combined with the fact that the TOC of silica and the liquid have opposite signs, provides a liquid-core multimode fiber (MMF) highly sensitive to temperature. Since the refractive index of the liquid can be easily modified, this allows us to control the modal properties of the liquid-core MMF at will and the sensor sensitivity can be easily tuned by selecting the refractive index of the liquid in the core of the device. The maximum sensitivity measured in our experiments is 20 nm/°C in the low-temperature regime up to 60 °C. To the best of our knowledge, to date, this is the largest sensitivity reported for fiber-based MMI temperature sensors.

Thermally optimized variable optical attenuators on a polymer platform

Variable optical attenuators based on a 1&#xD7;1 multimode interference device (MMI) and a Mach&#x2013;Zehnder interferometer (MZI) have been designed, fabricated, and characterized. Thermo-optic simulations have been performed to find the optimal heater electrode layout in order to improve the heating efficiency, suppress thermal crosstalk, and enhance the dynamic attenuation range. Both types of attenuators feature low optical loss (&lt;0.7&#x2009;&#x2009;dB). The compact 1&#xD7;1 MMI provides a dynamic attenuation range of 35&#xA0;dB over the C band at 10.1&#xA0;mW heater power, while the MZI increases the dynamic range beyond 50&#xA0;dB at merely 1.8&#xA0;mW heater power.

Device-level characterization of the flow of light in integrated photonic circuits using ultrafast photomodulation spectroscopy

Advances in silicon photonics have resulted in rapidly increasing complexity of integrated circuits. New methods are desirable that allow direct characterization of individual optical components in-situ, without the need for additional fabrication steps or test structures. Here, we present a new device-level method for characterization of photonic chips based on a highly localized modulation in the device using pulsed laser excitation. Optical pumping perturbs the refractive index of silicon, providing a spatially and temporally localized modulation in the transmitted light enabling time- and frequency-resolved imaging. We demonstrate the versatility of this all-optical modulation technique in imaging and in quantitative characterization of a variety of properties of silicon photonic devices, ranging from group indices in waveguides, quality factors of a ring resonator to the mode structure of a multimode interference device. Ultrafast photomodulation spectroscopy provides important information on devices of complex design, and is easily applicable for testing on the device-level.

Design and Optimization of an Optofluidic Ring Resonator Based on Liquid-Core Hybrid ARROWs

In this paper, we present the design and analysis of an integrated optofluidic ring resonator based on liquid-core hybrid polymer-silicon antiresonant reflecting optical waveguide (h-ARROW). We perform a modal analysis of h-ARROW using the finite-difference method, in order to find the optimized optical configuration, which accomplishes single-mode operation and reduced attenuation losses. An accurate investigation of the bend sections is performed to preserve the single-mode behavior with reduced propagation losses. A hybrid liquid-core multimode interference (MMI) device is used as a coupling element in the ring layout, and three possible MMI configurations are simulated and compared. By properly designing and optimizing each optical element, we demonstrate, by simulations, the possibility to achieve a quality factor up to 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> with the extinction ratio of about 31 dB. Bulk and surface sensing performances of the device are also simulated and discussed.

An MMI-based wavelength combiner employing non-uniform refractive index distribution

A novel wavelength combiner using non-uniform refractive index distribution within a multimode interference device is proposed and simulated. The refractive index step creates separate localized modes with different effective refractive indices and two modes are strongly excited which form the basis of an interferometer. We applied the concept to 1.30/1.31 μm and 1.31/1.55 μm wavelength combiners on an InP substrate. The lengths of the devices are 1272 μm and 484 μm with simulated insertion losses of 0.6 dB and 0.67 dB respectively.