Multimode Interference Effect Research Articles

Generation of noise-like pulses in a linear-cavity Tm fiber mode-locked laser based on FMF saturable absorber

As a real saturable absorber (SA) based on the nonlinear multimode interference (NL-MMI) effect, the devices based on the few-mode fiber possess numerous exciting characteristics due to their all-fiber structure, straightforward manufacturing process. In this paper, by employing a SA with the SMF-FMF-SMF structure in a linear cavity, we demonstrate a stable mode-locked Tm-doped fiber laser. At the pump power of 1 W, a single noise-like pulse (NLP) with a pedestal pulse duration of 28 ps and coherent peak width of 0.77 ps is generated at a central wavelength of 1940 nm with a 3 dB bandwidth of 10.51 nm. The stable noise-like pulse can be sustained from the lasing pump threshold up to the maximum pump power of 2.2 W without experiencing pulse splitting. The experiment results in a peak average output power of 116 mW, along with a corresponding maximum pulse energy of 24.73 nJ at a repetition frequency of 4.69 MHz. The high stability of our fiber laser is confirmed by the signal-to-Noise Ratio (SNR) of 63 dB, and its enduring stability is additionally validated over an 8-hour period. The experimental findings suggest that the SMF-FMF-SMF structure has significant potential in the simple and robust all-fiber mode-locked lasers operating in the noise-like pulse regime.

Self-healing Bessel-Gaussian beam generation based on multimode interference effect

Two typed Bessel-Gaussian beams with good non-diffraction and self-healing properties have been generated by using tapered hollow tubes with double layers. The evolution process of light propagation in the tapered hollow tube cavity is studied theoretically. The theoretically analysis and simulation results show that the Bessel-Gaussian beam generation is due to the multimode interference. Two typed Bessel-Gaussian beams are generated by changing the inner layer thickness of such hollow tubes, which both experiences autofocusing at the output port and evolve into Bessel-Gaussian beams. The effect of single and multiple silica particles on their self-healing performance has also been analyzed, which has a good property for ultrafast laser micromachining.

Coupled topological edge and corner states in two-dimensional phononic heterostructures with nonsymmorphic symmetries

The exciting discovery of topological phononic states has aroused great interest in the field of acoustic wave control. However, conventional topological edge states and corner states localized at the interface and corner of the two-phase domain wall structures are limited by single channel transmission characteristics, which decreases the flexibility of designing multi-channel acoustic wave devices. Here, we propose a two-dimensional (2D) topological phononic heterostructure with nonsymmorphic symmetries to realize the multiple interface topological multimode interference effect based on the coupling of topological edge and corner states. Topological phase transitions are achieved by altering the rotation angle of the split-ring scatterers in a square lattice. The coupled edge states are generated by the coupling between the edge states of ordinary-topological-ordinary (OTO) interfaces. Moreover, the higher-order topology of the square phononic crystals (PCs) is characterized by nontrivial bulk polarization, the topological and coupled corner states splitting into two pairs appear in the square OTO bend structure owing to the nonsymmorphic PC lack of mirror symmetries. Finally, the topological robustness of the multimode interference effect of coupled edge and corner states against defects is demonstrated. Our results pave the way for guiding and trapping acoustic waves in topological nonsymmorphic heterostructures, whose multi-channel transmission capability can be employed for designing topological phononic filters, couplers and multiplexers.

Spectral Characteristics and Displacement Sensing of U-Shaped Single-Mode-Multimode-Single-Mode Fiber Structure.

The U-shaped fiber configuration represents the elementary form of micro-displacement sensing, characterized by its exceptional freedom and flexibility. The study proposes the U-shaped bent single-mode-multimode-single-mode (SMS) fiber structure that integrates the multimode interference (MMI) effect for enhanced mode dispersion and the Mach-Zönder interference (MZI) effect for spectral sensitivity improvement. The transmission spectral properties of the U-shaped SMS fiber structure with a bent radius over 1 cm are experimentally measured as the change in displacement varied within the range of 5 mm in this work. As the radius decreases, the spectrum shows redshift, which is related to the central wavelength of the peak or dips-a smaller wavelength results in a stronger redshift for the same displacement change. The average sensitivity of micro-displacement measurement within a range of 5 mm is 5.41 pm/μm, and the linearity is 99.62%. The maximum sensitivity of U-shaped SMS fiber structure is 34.46 pm/μm, with the minimum displacement change of approximately 5.804 nm. The transmission spectral properties of the U-shaped SMS fiber structure within the ranges of 50 μm, 500 μm, and 5 mm are experimentally measured in this work. This experiment observed a relatively uniform spectral drift pattern in a large range of micro-displacement sensing. The measurement range is limited by the limited spectral range of the light source and the discontinuous variation in the effective refractive index. This provides an experimental reference for further understanding the characteristics of U-shaped fiber structures and applying its application in micro-displacement sensing.

Nanosecond pulse generation mode-locked by an artificial saturable absorber based on SMF-MMF-SMF structure

We present a noteworthy demonstration of nanosecond pulse generation utilizing an all-fiber artificial saturable absorber (SA) structure, composed of single-mode multimode single-mode fiber (SMF-MMF-SMF), integrated into a passively mode-locked erbium-doped fiber laser (EDFL). The SMS-based SA effectively phase-locked the longitudinal modes within the elongated EDFL cavity, resulting in the production of mode-locked pulses operating at 1567 nm. Mode-locking is achieved through the Kerr effect of nonlinear multimode interference. The pulse repetition rate is synchronized with the fundamental frequency at 1.28 MHz. At a pump power of 269.1 mW, the laser achieves a remarkable peak average pulse energy of 6.0 nJ. Demonstrating outstanding stability, the laser operation exhibits a peak-to-pedestal extinction ratio of 70 dB for the fundamental frequency. We are confident in the substantial potential of this SMS structure, positioning it as a compelling candidate for integration into ultrafast fiber lasers, with promising prospects in the field.

Generation of spatiotemporal dark pulse or polarization domain wall solitons in a partial multimode mode-locked fiber laser

In this paper, we have experimentally investigated different operational states of a partial multimode Er-doped spatiotemporal mode-locked (STML) fiber laser. The STML is enabled by nonlinear multimode interference (NL-MMI) effect from the partial multimode cavity structure. It is found that by adjusting the multimode polarization controller (PC) and the pump power, the STML dark pulses, bright pulses, as well as the polarization domain wall solitons can all be obtained, alternatively. The fused-tapered multimode coupler is responsible for the dark pulses generation via the enhanced modal cross-phase coupling. It is believed that the present findings can offer an important insight into the spatiotemporal dynamics of the STML solitons.

Q-switched Erbium Doped Fiber Laser Generation Using the Kerr Effect of Multimode Interference

This work demonstrates a single mode fibre-step index multimode fibre-single mode fibre (SMS) structure as saturable absorber (SA) to generate Q-switched laser based on the Kerr-effect of multimode interference (MMI) in an Erbium-doped fibre laser (EDFL) cavity. A 156 mm length of step index multimode fibre (SIMF) was spliced directly into a ring cavity to modulate the cavity loss. By bending and applying proper pressure on the SIMF, steady Q-switched pulse was generated without strict length restriction of the SIMF. The Q-switched laser had a high repetition rate from 87 to 158 kHz, while the corresponding pulse width was low, ranging from 4.2 μs to 2.5 μs as the pump power increased from 115 to 342 mW. The signal-to-noise ratio (SNR) of the fundamental mode for the pulsed laser was about 67 dB at a central wavelength of 1530 nm. The experiment demonstrated that SMS structure can be used as stable SA to generate Q-switched laser for high power applications. It also indicated that even with large core diameter step index multimode fiber, SMS structure can be utilized to generate stable Q-switched pulsed laser with very simple cavity settings.

Single-cavity bidirectional Raman-assisted mode-locked fiber laser

Bidirectional mode-locked fiber lasers are promising dual-comb light sources. Stimulated Raman scattering (SRS) effect can significantly extend the wavelength range of mode-locked fiber lasers. In this work, we propose and demonstrate a single-cavity bidirectional Raman-assisted mode-locked fiber laser based on nonlinear multimode interference effect. By adjusting the polarization of the cavity, we obtain synchronous and asynchronous pulse trains in counterclockwise (CCW) direction and clockwise (CW) direction, as a consequence of changes in the dispersion and nonlinear effects. Due to the extension of the wavelength range of single-cavity bidirectional mode-locked fiber laser, we believe that this work will hold potential for future applications in dual-comb spectroscopy, asynchronous sampling and other areas.

An in-fiber sensor for simultaneous measurement of cholesterol concentration and temperature based on SPR and MMI

In this paper, we design an in-fiber two-parameter sensor with multimode fiber (MMF)-Au film coated hollow fiber (HCF)-MMF structure, which can simultaneously excite Surface Plasmon Resonance (SPR) effect and Multimode Interference (MMI) effect. A composite material of Au nanoparticles/β-cyclodextrin (AuNPs/β-CD) is deposited on the surface of the Au film coated HCF to realize highly-sensitive measurement of cholesterol concentration. Here, the AuNPs can not only enhance the measurement sensitivity of the SPR sensor, but also increase the numbers of combination sites of β-CD and cholesterol. Then, to solve the cross-sensitivity problem between temperature and cholesterol, the glycerin is selected as a temperature-sensitive material to fill into the inner channel of the HCF, making the MMI sensor sensitive to temperature, and finally realizing the simultaneous measurement of cholesterol concentration and temperature. The experimental results demonstrate that the wavelength shift of the SPR and the MMI are 12.7 nm and 7.9 nm, respectively, when the cholesterol concentration changes from 0 to 500 nM. The temperature sensitivity of the SPR and the MMI are −0.9 nm/°C and 2.64 nm/°C, respectively, in the temperature range of 30°C–46 °C. In addition, the sensor shows good recognition ability of cholesterol molecules in serum environment, with good stability, selectivity and repeatability, and has broad application prospects in the biomedical field.

High repetition frequency spatiotemporal mode-locked fiber laser based on cavity induced modulation instability

Intermodal dispersion and modal instability have been the primary challenges for achieving spatiotemporal mode-locked (STML) in multimode fiber. This paper utilizes the nonlinear multimode interference effect to redistribute the mode energy and suppress the intermodal dispersion. By building on this approach and combining it with cavity-induced modulation instability (CIMI), we experimentally demonstrate the STML pulse with a high repetition frequency of 1.116 GHz, corresponding to a harmonic order of 93. Our research not only significantly expands the application of spatiotemporal mode-locked lasers in high repetition frequency systems, but also provides valuable insights for further understanding the dynamics of spatiotemporal mode-locking.

Topological transport in heterostructure of valley photonic crystals.

We propose a heterogeneous structure, which are composed of two valley photonic crystals (VPCs) with opposite valley Chern numbers and air channel. With the increasing width of the air channel, valley-locked waveguide modes are found in topological bandgap by analyzing energy bands. Finite element method (FEM) simulation results show that the fundamental and high order modes are valley-locked, propagating unidirectionally under the excitation of chiral source, and possess higher flux compared to the valley-locked topological edge state in the domain wall. Besides, the immunity to backscattering in bend and couplers, and the robustness to random disorders are discussed in detail. We also investigate the one-way multimode interference (MMI) effect based on valley-locked waveguide modes, and design topological beam splitters. Our study provides a novel idea for topological transport with high flux, and more freedom to design valley-locked waveguide devices, including bends, couplers and splitters.

Thulium-doped multi-wavelength and mode-locked fiber laser based on multimode interference

We demonstrate a thulium-doped multi-wavelength mode-locked fiber laser with a single-mode-gradient refractive index multimode-single-mode fiber (SMF-GIMF-SMF) structure which acts as both a filter and a saturable absorber. Based on the multimode interference effect (MMI) filtering, wavelength-tunable operation with a tunable range of up to 15.69 nm can be obtained in the fiber laser. Moreover, tunable multi-wavelength continuous wave (CW) laser output of up to four wavelengths can be obtained by properly adjusting the pump power and polarization controller. Owing to mode interference and phase modulation, the SMS structure also plays a role as a saturable absorber (SA), which leads to the multi-wavelength output of the fiber laser accompanied by mode-locking. The state of dual-wavelength mode locking and the state of coexistence of continuous wave and mode-locking pulses at different wavelengths were observed in the laser. These results suggest that the all-fiber SMF-GIMF-SMF structure is a key device for fiber lasers to achieve multi-wavelength and mode-locked pulse output, which may find potential applications in nonlinear microscopy, biodegradable material processing, and terahertz spectroscopy.

Few-mode fiber based saturable absorber for ultra-fast fiber laser at 2 μm

The saturable absorption of the single mode-few-mode-single mode fiber (SMF-FMF-SMF) structure with respect to its passive mode-locking performance is still largely unexploited, although it is more conducive to the design and performance of the saturable absorber (SA) based on the nonlinear multimode interference effect. A series of FMF-based saturable absorbers with different FMF lengths are employed in passively mode-locked Tm fiber lasers for ultrafast pulse generations at 2 μm. The mode-locking performances using different FMF-based saturable absorbers were demonstrated. By comparing the saturable absorption characteristics and the respective mode-locking performances, we can conclude that it is easier to obtain the stable mode-locking output using saturable absorbers with the FMF lengths of 15.12 cm and 30.2 cm. The FMF saturable absorber with a 15.12 cm FMF has a large modulation depth of 40% and a saturation fluence of 108.23 μJ /cm2 which delivers ultrafast pulses with pulse width of 1.96 ps at 1878 nm. In addition, the harmonic mode-locking with stable and well-organized pulse trains could also be instantaneously achieved by increasing the pump power within the same cavity. Our results indicate that the SMF-FMF-SMF structure could operate as a SA while tuning its saturable absorption properties by changing the FMF length making it advantageous for mode locking fiber lasers.

Investigation of the transverse mode instability in an isolator-free quasi-unidirectional spatiotemporal self-mode locked fiber laser

A spatiotemporal (ST) self-mode locked (ML) Er-doped fiber laser based on the nonlinear multimode interference (NL-MMI) effect without an intracavity isolator has been proposed. Such a STML laser could operate in a quasi-unidirectional regime in both clockwise (CW) and counter-clockwise (CCW) directions. Different spatial dynamics of the STML solitons in the opposite directions are observed. For the STML solitons in the CCW direction, the beam profile is discretely distributed and the transverse mode instability (TMI) is observed. While for the STML solitons in the CW direction, the beam profile is relatively concentrated and can be partially controlled by adjusting the intracavity multimode (MM) fiber at the front-end of the MM coupler. These investigations would help to gain further understanding of the spatial and temporal dynamics and mechanism in the STML lasers.

U-fiber-based biosensor for temperature-compensated acetylcholine-specific measurement.

This paper presents a U-fiber-based biosensor to achieve temperature-compensated acetylcholine-specific measurement. The surface plasmon resonance (SPR) and multimode interference (MMI) effects are simultaneously realized in a U-shaped fiber structure for the first time, to the best of our knowledge. The experimental results show refractive index (RI) sensitivities of 3042 and 2958 nm/RIU and temperature sensitivities of -0.47 and -0.40 nm/°C for the MMI and SPR, which are greatly improved compared with the traditional structure. Simultaneously, a sensitivity matrix for detecting two parameters is introduced to solve the problem of temperature interference of biosensors based on RI changes. Label-free detection of acetylcholine (ACh) was achieved by immobilizing acetylcholinesterase (AChE) on optical fibers. The experimental results show that the sensor can realize the specific detection of acetylcholine and has good stability and selectivity, and the detection limit of the sensor is 30 nM. The sensor has the advantages of simple structure, high sensitivity, convenient operation, direct insertion into small spaces, temperature compensation, etc., which provide an important supplement to traditional fiber-optic SPR biosensors.

Vector dark–bright pulses from a ytterbium doped fiber laser mode-locked by nonlinear multimode interference

We report dark–bright pulse pair generation from the ytterbium doped fiber laser mode-locked by cascaded nonlinear multimode interference (MMI) effect under an all-normal dispersion configuration. The MMI is implemented in a step index single mode–multimode–single mode fiber structure. The combination of two MMI structures, acting like a saturable absorber, has been used for stable mode-locking operation in the resonator. By adjusting the polarization controller attached to one of the MMI structure, stable dark–bright pulses are observed at fundamental, second and third harmonic repetition rates. The optical spectrum of the dark–bright pulses is doubly peaked at 1036.6 and 1040.6 nm with a spectral width ∼0.7 nm. The width of the dark pulses varied from 72 to 50 ns and that of the bright pulses from 30 to 14 ns when the pump power is increased from 102 to 170 mW at the fundamental repetition rate mode-locking. The darkness of the dark and intensity of bright pulses increase with the pump power. Dark and bright pulses in the pair are resolved by placing a polarizing component confirming their vector nature.

Integrated label-free erbium-doped fiber laser biosensing system for detection of single cell Staphylococcus aureus

A critical challenge to realize ultra-high sensitivity with optical fiber interferometers for label free biosensing is to achieve high quality factors (Q-factor) in liquid. In this work a high Q-factor of 105, which significantly improves the detection resolution is described based on a structure of single mode -core-only -single mode fiber (SCS) with its multimode (or Mach-Zehnder) interference effect as a filter that is integrated into an erbium-doped fiber laser (EDFL) system for excitation. In the case study, the section of core-only fiber is functionalized with porcine immunoglobulin G (IgG) antibodies, which could selectively bind to bacterial pathogen of Staphylococcus aureus (S. aureus). The developed microfiber-based biosensing platform called SCS-based EDFL biosensors can effectively detect concentrations of S. aureus from 10 to 105 CFU/mL, with a responsivity of 0.426 nm wavelength shift in the measured spectrum for S. aureus concentration of 10 CFU/mL. The limit of detection (LoD) is estimated as 7.3 CFU/mL based on the measurement of S. aureus with minimum concentration of 10 CFU/mL. In addition, when a lower concentration of 1 CFU/mL is applied to the biosensor, a wavelength shift of 0.12 nm is observed in 10% of samples (1/10), indicating actual LoD of 1 CFU/mL for the proposed biosensor. Attributed to its good sensitivity, stability, reproducibility and specificity, the proposed EDFL based biosensing platform has great potentials for diagnostics.

Multi-Wavelength Spatiotemporal Mode-Locked Fiber Laser at 1.55 μm

We propose and demonstrate an effective and simple multi-wavelength spatiotemporal mode-locked (STML) laser. The laser introduces a multimode interference (MMI) effect using the core diameter mismatch of the fibers, thus enabling simultaneous mode-locking at multiple wavelengths. For the multi-wavelength mode-locking, each wavelength exists a matching pulse sequence, and different wavelengths can possess the same pulse state or different states. Meanwhile, at the dual-wavelength operation, by adjusting the polarization controllers, not only the central wavelength can be continuously adjusted from 1552.93 nm to 1572.64 nm, but also the wavelength interval can be adjusted from 2 nm to 19 nm, which benefits from the tunability of the MMI effect. Moreover, the spectral channels can be consecutively increased from 2 to 4. The obtained results provide a platform for understanding and studying the multi-wavelength STML properties. This flexible all-fiber laser has great potential for applications in the fields of optical signal processing, fiber sensing, and information coding.

Multimode interference in topological photonic heterostructure.

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.

Trace detection of cadmium (II) ions based on an air-hole-assisted multicore microstructured optical fiber

Cadmium ion (Cd2+) is one of the most crucial industrial pollutants that cause serious harm to the human body. For rapidly and sensitively determining the concentration of Cd2+ in a solution in real time, a highly selective Cd2+ sensor with high sensitivity is proposed based on a multimode interference effect in an air-hole-assisted multicore microstructured optical fiber (AA-MCF). Seven large air holes in an AA-MCF isolate of six fiber cores; one of them is cut open to expose two adjacent cores and form a microfluidic channel. It can be used as the region where the analyte liquid interacts with light. When the refractive index in this region varies, the multimode interference effect is influenced, so that the refractive index of the analyte can be quickly observed. Quantification is achieved by calculating the Cd2+ concentration through the refractive index. Theoretical and experimental results show that the sensitivity of this sensor made of an AA-MCF can reach 7.443 × 109 nm/(mol/L) and the experimental limit of detection is 6.0 × 10–12 mol/L. The proposed sensor exhibits high stability, specificity, and a compact structure, offering promising potential for biochemical sensing applications, such as environmental monitoring and disease diagnosis.