Multimode interference effect in plasmonic subwavelength waveguides and an ultra-compact power splitter

Fiber laser sensor based on multimode interference (MMI) effect can achieve high sensitivity with lower temperaturestrain cross sensitivity by designing optimally the MMI structure. An erbium-doped fiber laser for temperature, stress and refractive sensing based on single mode fiber spliced a short section of no-core fiber was proposed in this paper. The optical field characteristics of this MMI structure have been analyzed, indicating the spectral filtering effect existed. An erbium-doped fiber laser with the MMI structure inserted as the sensor element, the enhanced sensing sensitivity is achieved on the sensing experiments of ambient temperature, mechanical stress and liquid refractive index. The sensitivity of this MMI-based fiber laser sensor is up to 13.24 pm/°C with the measured range from 30 °C to 220 °C, 1.29pm/μe from 0 to 7776.05μe, 180.21nm/RIU from 1.34 to 1.42, respectively.

We report on the generation of dual-wavelength noise-like pulse (NLP) from a passively mode-locked all-fiber laser based on multimode interference (MMI) effect. The theory to evaluate and design transmission spectrum of MMI filter is analyzed. A homemade MMI filter was employed in an Er-doped fiber ring laser with NPE configuration and dual-wavelength NLP at 1530 and 1600 nm was obtained with 3-dB bandwidth of 18.1 and 41.9 nm, respectively. The output had a signal-to-noise ratio higher than 35 dB and can achieve self-started operation.

Nano-scale power splitters based on Si hybrid plasmonic waveguides are designed by utilizing the multimode interference (MMI) effect as well as Y-branch structure. A three-dimensional finite-difference time-domain method is used for simulating the light propagation and optimizing the structural parameters. The designed 1 × 2 50:50 MMI power splitter has a nano-scale size of only 650 nm × 530 nm. The designed Y-branch power splitter is also very small, i.e., about 900 nm × 600 nm. The fabrication tolerance is also analyzed and it is shown that the tolerance of the waveguide width is much larger than±50 nm. The power splitter has a very broad band of over 500 nm. In order to achieve a variable power splitting ratio, a 2×2 two-mode interference coupler and an asymmetric Y-branch are used and the corresponding power splitting ratio can be tuned in the range of 97.1%:2.9%-1.7%:98.3% and 84%:16%-16%:84%, respectively. Finally a 1×4 power splitter with a device footprint of 1.9 μm × 2.6 μm is also presented using cascaded Y-branches.

In this Letter, topological photonic heterostructures, which are composed of finite-size photonic crystals with different topological phases, are proposed. The coupled topological edge states (CTESs), which originate from the coupling between topological edge states, are found. By using the finite element method, the multimode interference effect of CTESs is predicted and investigated. Paired and symmetrical interferences are discussed, and the respective imaging positions are calculated. In addition, the multimode interference effect is topologically protected when introducing disorders. As examples of application, frequency and power splitters of topological edge states based on the multimode interference effect are designed and demonstrated numerically. Our findings pave a new, to the best of our knowledge, way of designing topological photonic integrated circuit applications such as filters, couplers, multiplexers, and so on.

We report on a novel tuning mechanism to fabricate an all-fiber tunable laser based on multimode interference (MMI) effects. It is well known that the wavelength response of MMI devices exhibits a linear dependence when the length of the multimode fiber (MMF) section. Therefore, tuning in the MMI filter is achieved using a ferrule (capillary tube of 127 μm diameter) filled with a liquid with a higher refractive index than that of the ferrule, which creates a variable liquid MMF. This liquid MMF is used to increase the effective length of the MMI filter and tuning takes place. Using this simple scheme, a tuning range of 30 nm was easily achieved, with very small insertion losses. The filter was tested within a typical Erbium doped fiber (EDF) ring laser cavity, and a tunable EDF laser covering the full C-band was demonstrated. The advantage of our laser is of course the simplicity of the tunable MMI filter, which results in an inexpensive tunable fiber laser.

The study of quantum cascade lasers (QCL) in the terahertz frequency region by the use of THz time-domain spectroscopy has gained a lot of interest within the last few years [1]. Good physical insight was obtained for THz QCLs employing a single plasmon waveguide (surface plasmon waveguide) [2,3]. In comparison to that the double plasmon or metal-metal waveguide QCL uses a metal/semiconductor/metal structure as a waveguide. THz QCLs with this type of waveguide are showing the highest operating temperatures so far [4]. The metal-metal THz QCLs confine the whole mode in the active region of subwavelength size. In order to investigate metal-metal THz QCLs by THz pulses an efficient method of THz pulse launching needs to be employed. Several methods for coupling of free space THz pulses into the subwavelength waveguide, such as horn antenna, gratings or lenses can be used. A further method is a waveguide emitter [5,6] which benefits from the near infrared excitation beam focus which is much smaller than the corresponding THz focus.

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.

Tunable dual-wavelength terahertz wave power splitter

We experimentally demonstrate a fiber laser refractometer based on the use of a multimode interference (MMI) fiber structure. The MMI filter is constructed with an uncladded fiber segment which acts as sensing element to determine the refractive index (RI) of liquid solutions. The laser emission, generated at the 2 μm waveband, is wavelength shifted in linear proportion to the RI of the liquid. The proposed fiber laser refractometer exhibits high sensibility of 937.81 nm/RIU.

A compact integrated 980/1550-nm wavelength multi/demultiplexing coupler based on multimode interference effects is demonstrated. The device is useful for integration with rare-earth doped waveguide amplifiers and lasers. It is shown that the device performance can be optimized with an appropriate choice of the coupler width. The coupler is fabricated in a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -SiON deeply-etched rib waveguide system. Theoretical and experimental results for the demultiplexer configuration are presented. At an optimum coupler length of 458 μm, contrast of about 18 dB, and on-chip insertion loss of 0.5 dB at both wavelengths are achieved.

A compact and all-fiber erbium-doped tunable fiber laser is presented. This can provide an optical power in excess of +3.85 dBm with a tunability of 17 nm within the region of 1558.33-1575.32 nm. The tuning mechanism is based on the multimode interference effect.

An all-fiber continuous and discrete liquid level sensor using Multimode Interference (MMI) effects is demonstrated. The sensing range and also multiplexed operation can be easily controlled by just changing the length of the MMI sensor.

In this paper, we have introduced a surface plasmonic two-mode interference (SPTMI) coupler having a silicon core, GaAsInP side cladding, and silver top and bottom cladding as an optical power splitter. A wide range of tunability from the 50∶50 splitting ratio to 1∶99 is achieved by refractive index modulation of the GaAsInP cladding with application of varying optical pulse power. The coupling length of the SPTMI-based splitter is ∼11.5 times less than that of a previously reported optical power splitter based on multimode waveguide holograms. The proposed optical power splitter has potential in development of large-scale integrated circuits due to its compactness and high fabrication tolerance.

Mode instability (MI) has become one of the main restrictions for further power scaling of large mode area (LMA) fiber systems. A periodical large-small-core (PLSC) filter in a strongly pumped LMA fiber amplifier based on multimode interference and self-imaging effect is proposed to suppress high order modes and improve the beam quality. An investigation on the modeling of light propagation within this filter structure is presented. Transverse mode competition and individual transverse mode power distribution in a strongly pumped fiber amplifier are theoretically analyzed. By employing this PLSC mode filter in the 30/400 strongly pumped LMA fiber amplifier, the percentage of the fundamental mode rises from 27.8% (without filter) to 95.6%, and the percentage of the high order modes correspondingly reduces from 72.2% (without filter) to 4.4%. By the modal power decomposition, the optical field pattern in free space is established and the M2 parameter of beam quality decreases dramatically from 2.24 to 1.12 (0 relative phase) and from 3.01 to 1.28 (π/2 relative phase). This study provides a new method to achieve high beam quality in LMA fiber amplifiers and this filter would be extended to larger mode area fiber amplifiers.

A polarisation-insensitive planar 980/1550 nm wavelength (de)multiplexing coupler based on multimode interference (MMI) effects is presented. The device is useful for integration with rare-earth doped waveguide amplifiers and lasers. The waveguide parameters and, in particular, the coupler width are chosen to optimise the device performance and obtain polarisation-insensitivity.