Index Multimode Fiber Research Articles

Generation of mode-locked square-shaped and chair-like pulse based on reverse saturable absorption effect of nonlinear multimode interference.

In this paper, the phenomenon of reverse saturable absorption of offset-spliced graded index multimode fibers (OS-GIMF) is revealed. And based on that, the stable square-shaped and chair-like mode-locked pulses are demonstrated with the maximum pulse energy of 0.14 µJ and 23.8 nJ respectively, while the OS-GIMF acts as a saturable absorber (SA) in fiber laser. By adjusting polarization controller (PC) and the pump power, square-shaped and chair-like pulse can be switched to each other. This multimode SA could sever as high power light source owing to its high damage threshold, compact structure and low cost.

C-band wavelength tunable mode-locking fiber laser based on CD-SMS structure.

A wavelength-tuning passively mode-locked all-fiber laser based on cascaded dual single mode fiber-graded index multimode fiber-single mode fiber (CD-SMS) structure is proposed in this paper. The CD-SMS structure geometry, where two SMSs were fused together, and each of SMS was coined into the paddles of polarization controller (PC). By adjusting the orientation angle of the paddles, magnitude of the nonlinear phase shifts of the light can be controlled to realize mode-locking and filtering. Experimentally, mode locking was obtained based on the SMS structure with pulse duration of 930fs at the central wavelength 1568nm and FWHM bandwidth of 2.9nm. The fundamental repetition rate obtained was 18.22MHz. By inserting another SMS into the ring cavity, and make-up CD-SMS, the five main peaks of mode-locked pulses were tuned in the range of 1533-1573nm was achieved by mechanically tuning the orientation angles of paddles of two PCs, the wavelength tuning range covers the whole C-band, reaching 40nm. However, the tunable range is limited by gain bandwidth of erbium ion. The wavelength-tuning mode locked fiber laser based on CD-SMS provides a way to a wide range of tunable ultrafast lasers.

Generation of wavelength-tunable and coherent dual-wavelength solitons in the C + L band by controlling the intracavity loss

A wavelength-tunable and dual-wavelength mode-locking operation is achieved in an Er-doped fiber laser using a hybrid no-core fiber graded index multimode fiber as the saturable absorber. In the tuning operation, continuously wavelength-tunable pulses with a tuning range of 46.7 nm, stable 3-dB bandwidth of around 5 nm, and pulse duration of ∼850 fs are obtained by increasing the intracavity loss of a variable optical attenuator. In the dual-wavelength operation, the two solitons at different wavelengths demonstrate the characteristics of mutual coherence. By increasing the intracavity loss, the spectral spacing can be tuned from 11 to 33.01 nm while maintaining the coherence of the solitons. Such coherent solitons have high potential for applications in dual-comb frequency and multicolor pulses in nonlinear microscopy.

All-fiber nonlinear optical switch based on polarization controller coiled SMF–GIMF–SMF for ultrashort pulse generation

A novel nonlinear optical switch based on a single mode fiber – graded index multimode fiber (GIMF) – single mode fiber (SMS) twined into polarization controller (PC) was proposed as a saturable absorber (SA), to generate ultrashort pulse in fiber laser. The SA device geometry, where a sandwiched GIMF section spliced to two standard SMF at both ends of GIMF, was coiled into a PC. By properly tuning the paddles of PC, mode-locking can be easily realized and the nonlinear saturable absorption characteristics of SMS-SA was changed. In the experiment, the modulation depth of the SMS-SA containing 0.6 m GIMF can be observed 9%, 10.5%, 21.7%, respectively. Based on SMS-SA mode-locking, the stable ultrafast pulse duration of 559 fs, with the FWHM bandwidth of 6.84 nm were generated. The fundamental repetition rate of 25 MHz and wavelength-tuning range of 14 nm can be obtained by adjusting the paddles. The SMS-SA excellent tunable nonlinear optical properties and all-fiber structure have strong potential application in the field of ultrafast photonics.

Mode-locked ytterbium-doped fiber laser based on offset-spliced graded index multimode fibers

We developed a passively mode-locked ytterbium-doped fiber laser based on offset-spliced graded index multimode fibers (OS-GIMFs), where OS-GIMFs play the role of saturable absorbers. The modulation depth of the saturable absorber is 15.28%, which significantly improves the performance of saturable absorber by adopting offset-spliced method. A stable mode-locked pulse train at 1040 nm with a 27.32 MHz repetition rate is generated when pump power reaches the threshold. The output pulse-width is a ∼11 ps and can be compressed to 980 fs by using a pair of gratings. The fiber laser is superior in terms of low cost, high damage threshold, and easy fabrication; thus, it has potential for application in ultrafast photonics and industrial processing.

Generation of pulse-width controllable dissipative solitons and bound solitons by using an all fiber saturable absorber.

A passively mode-locked fiber laser with controllable pulse width is demonstrated by use of an all-fiber saturable absorber based on a hybrid no-core fiber (NCF)-graded index multimode fiber (GIMF) structure incorporated into an Er-doped fiber ring cavity. Such a hybrid NCF-GIMF structure has a tunable intracavity filtering effect. As a result, the mode-locking operation is achieved with controllable pulse width and spectral bandwidth in the normal dispersion regime by only stretching the fiber device. Soliton pulses with the pulse width of 7.7-23ps are generated, and bound solitons with variable widths are also experimentally demonstrated. The results obtained reveal the versatility and flexibility of the NCF-GIMF structured device in controlling the pulse dynamics for different experimental requirements.

Self-starting mode-locked Tm-doped fiber laser using a hybrid structure of no core - graded index multimode fiber as the saturable absorber

We experimentally demonstrated a passively mode-locked all-fiber Tm laser using a no-core-graded-index multimode fiber structure as a saturable absorber based on the nonlinear multimode interference. Stable fundamentally mode-locking operation was obtained at a pump threshold of 180 mW. The output soliton pulses had a centre wavelength, spectral width, pulse duration, and repetition rate of 1895.7 nm, 3.3 nm, 1.25 ps, and 18.91 MHz, respectively. This is a simple, low-cost, stable, and convenient laser oscillator, with many potential applications in eye-safe ultrafast photonics.

Observation of controllable tightly and loosely bound solitons with an all-fiber saturable absorber

A hybrid no-core fiber (NCF)–graded index multimode fiber (GIMF) structure is used as a saturable absorber (SA) for mode-locked laser operation. Such an SA supports various types of soliton outputs. By changing the cavity parameters, not only the spatiotemporal mode-locking states with a stable single pulse but also tightly and loosely bound solitons are generated. Single 35.5 pJ solitons centered at 1568.5 nm have a 4 nm spectral full-width at half-maximum and an 818 fs temporal duration. Tightly bound soliton pairs with continuously tunable wavelength from 1567.48 nm to 1576.20 nm, featured with an ∼700 fs pulse train with a separation of 2.07 ps, have been observed by stretching the NCF-GIMF structured device. Meanwhile, several different pulse separations from 37.57 ps to 56.46 ps of loosely bound solitons have also been realized. The results provide help in understanding the nonlinear dynamics in fiber lasers.

Side polished graded index multimode fiber based refractive index sensor for biology measurement

A side polished graded index multimode fiber based refractive index sensor is proposed for use in biology measurements. The side polished graded index multimode fiber is fabricated using the wheel polishing method; that is, a part of the cladding and core of a graded index multimode fiber are removed to develop a leaking “window” called a side polished region. The optical power loss through this structure is sensitive to the refractive index of the analyte liquid that covers the leaking “window.” A highly linear correlation between the optical power loss and refractive index has been achieved. Additionally, we also investigated the influence of each structure parameter on the sensitivity and linearity. From simulation and experiment results, a fiber core of 50 μm and R of 10 μm are considered as the best choices of fiber structure with a sensitivity is 40.92 dB/RIU in the refractive index range of 1.300–1.450. A bio-culture solution of fetal bovine serum can be easily detected with this structure whose concentration gradient is two percent. It provides a simple and quick method to detect refractive indices in biology measurements from 1.300 to 1.450.

Research on the performance of a novel SPR sensor based on graded index multimode fiber

PurposeThis paper aims to propose and demonstrate a novel surface plasmon resonance (SPR)-sensing approach by using the fundamental mode beam based on a graded index multimode fiber (GIF). The proposed SPR sensor has high sensitivity and controllable working dynamic range, which expects to solve the two bottlenecks of fiber SPR sensor, including low sensitivity and the difficulty in multichannel detection.Design/methodology/approachThe low-order mode of the GIF to SPR sense, which keeps the sensitivity advantage of the single-mode fiber SPR sensor, is used. By using this new SPR sensor, the effect of light incident angle and gold film thickness on working dynamic range was studied. According to the study results, the smaller is the incident angle, the larger is the SPR working dynamic range and the longer is the resonance wavelength with a fixed gold film thickness; the larger is the gold film thickness, the longer is the resonance wavelength with a fixed grinding angle. After the parameter optimization, the sensitivity of these two parameter-adjusting methods reach 4,442 and 3031 nm/RIU.Originality/valueWhen the grinding angle of the GIF increases, the dynamic range of the resonance wavelength increases and has a redshift, sensitivity increases, and the resonance valley becomes more unobvious with a fixed gold film thickness. Similarly, when gold film thickness increases, the dynamic range of the resonance wavelength increases and has a redshift, sensitivity increases, and the resonance valley becomes more unobvious with a fixed grinding angle. These adjusting performance aforementioned lay the foundation for solving of the fiber-based SPR multichannel detection and increasing of the fiber-based SPR sensor sensitivity, which has a good application value.

3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam.

We proposed and experimentally demonstrated 3-dimensional dark traps for low refractive index bio-cells using a single optical fiber Bessel beam. The Bessel beam was produced by concatenating single-mode fiber and a step index multimode fiber, which was then focused by a high refractive index glass microsphere integrated on the fiber end facet. The focused Bessel beam provided two dark fields along the axial direction, where stable trapping of low refractive index bio-cells was realized in a high refractive index liquid bath. The all-fiber and seamlessly integrated structure of the proposed scheme can find ample potential as a micro-optical probe in in situ characterization and manipulation of multiple bio-cells with refractive indices lower than that of the liquid bath.

Ultrasonic imaging of seismic physical models using a fringe visibility enhanced fiber-optic Fabry-Perot interferometric sensor.

A fringe visibility enhanced fiber-optic Fabry-Perot interferometer based ultrasonic sensor is proposed and experimentally demonstrated for seismic physical model imaging. The sensor consists of a graded index multimode fiber collimator and a PTFE (polytetrafluoroethylene) diaphragm to form a Fabry-Perot interferometer. Owing to the increase of the sensor's spectral sideband slope and the smaller Young's modulus of the PTFE diaphragm, a high response to both continuous and pulsed ultrasound with a high SNR of 42.92 dB in 300 kHz is achieved when the spectral sideband filter technique is used to interrogate the sensor. The ultrasonic reconstructed images can clearly differentiate the shape of models with a high resolution.

Experimental and numerical study on refractive index sensors based on fibre Bragg gratings inscribed in multimode fibre

In this work is reported the experimental and numerical results of the refractive index response of etched fibre Bragg gratings written in a graded index multimode fibre. The responses of the modes coupled by the grating inscribed in a multimode fibre are compared with the mode coupled by a grating inscribed in single mode fibre. The results of this study show that the refractive index sensitivity and the dynamical range of etched fibre Bragg gratings written in multimode fibres are higher than the ones verified in single-mode fibres. The determination of oil-biodiesel blend concentrations are also compared as an example of practical applications. It is shown that a greater core diameter of the multimode fibre enables the Bragg gratings to exhibit enhanced sensitivity without requiring further fibre diameter reduction.

Seeded intermodal four-wave mixing in a highly multimode fiber

We experimentally and theoretically investigate the process of seeded intermodal four-wave mixing in a graded index multimode fiber, pumped in the normal dispersion regime. By using a fiber with a 100 micron core diameter, we generate a parametric sideband in the C band (1530-1565 nm), hence allowing the use of an Erbium-based laser to seed the mixing process. To limit nonlinear coupling between the pump and the seed to low-order fiber modes, the waist diameter of the pump beam is properly adjusted. We observe that the superimposed seed stimulates the generation of new spectral sidebands. A detailed characterization of the spectral and spatial properties of these sidebands shows good agreement with theoretical predictions from the phase-matching conditions. Interestingly, we demonstrate that both the second and the fourth-order dispersions must be included in the phase matching conditions to get better agreement with experimental measurements. Furthermore, temporal measurements performed with a fast photodiode reveal the generation of multiple pulse structures.

New method for calculating the coupling coefficient in graded index optical fibers

A simple method is proposed for determining the mode coupling coefficient D in graded index multimode optical fibers. It only requires observation of the output modal power distribution P(m, z) for one fiber length z as the Gaussian launching modal power distribution changes, with the Gaussian input light distribution centered along the graded index optical fiber axis (θ0=0) without radial offset (r0 = 0). A similar method we previously proposed for calculating the coupling coefficient D in a step-index multimode optical fibers where the output angular power distributions P(θ, z) for one fiber length z with the Gaussian input light distribution launched centrally along the step-index optical fiber axis (θ0=0) is needed to be known.

Side polished graded index multimode fiber based refractive index sensor for biology measurement

Side polished graded index multimode fiber based refractive index sensor for biology measurement

Theoretical modeling of a coupled plasmon waveguide resonance sensor based on multimode optical fiber

A coupled plasmon waveguide resonance (CPWR) sensor based on metal/dielectric-coated step index multimode optical fiber is proposed. Theoretical simulations using the four-layer Fresnel equations based on a bi-dimensional optical fiber model were implemented on four structures: Ag–ZnO, Au–ZnO, Ag–TiO2 and Au–TiO2. By controlling the thickness of dielectric layer, we managed to manipulate the CPWR resonance wavelengths. When a CPWR resonance dip is in the short wavelength region, it is insensitive to the change of surrounding refractive index (SRI) and can be used as a reference to improve the sensing accuracy of surface plasmon resonance (SPR) mode. With the increase of the thickness of the dielectric layer, the CPWR resonance dips shift to longer wavelength and the corresponding sensitivities increase. When the 1st CPWR resonance wavelength is near 1550 nm and SRI is around 1.333, the sensitivities of four structures reach 1360.61 nm/RIU, 1375.76 nm/RIU, 1048.48 nm/RIU and 1015.15 nm/RIU, respectively. The values are close to that of the conventional SPR optical fiber sensor while the spectral bandwidths of the optical fiber CPWR sensors are narrower.

Efficient strategy to increase higher order inter-modal stability of a step index multimode fiber.

We demonstrate a novel approach to enhance the mode stability through increased effective index difference (Δneff) between the higher-order modes (LP18, LP09 and LP19) of a multimode fiber. Fibers with large diameters have bigger effective mode areas (Aeff) and can be useful for high power lasers and amplifiers. However, a large mode area (LMA) results in an increased number of modes that can be more susceptible to mode coupling. The modal effective index difference (Δneff) strongly correlates with mode stability and this increases as the modal order (m) increases. We report here that the mode spacing between the higher order modes can be further enhanced by introducing doped concentric rings in the core. In our work, we have shown a more than 35% increase in the mode spacing between the higher order modes by optimizing the doping profile of a LMA fiber. The proposed design technique is also scalable and can be applied to improve the mode spacing between different higher order modes and their neighboring antisymmetric modes, as necessary.

Multiple Particles 3-D Trap Based on All-Fiber Bessel Optical Probe

We propose and demonstrate an all-fiber Bessel optical tweezers for multiple microparticles (yeast cells) three-dimensional (3-D) trap. To the best knowledge of us, it is the first time to achieve the 3-D stable noncontact multiple microparticles optical traps with long distance intervals by using a single all-fiber probe. The Bessel beam is produced by splicing coaxially a single-mode fiber and a step index multimode fiber. The convergence of the output Bessel beam is performed by molding the tip of the multimode fiber into a special semiellipsoid shape. The effective trapping range of the all-fiber probe is 0 to 60 μm, which is much longer than normal single fiber optical tweezers probes. The all-fiber Bessel optical probe is convenient to integrate and suitable for the lab on the chip. The structure of this fiber probe is simple, high precision, low cost, and small size, which provides new development for biological cells experiment and operation.

Design of a Wavelength-Tunable Optical Tweezer Using a Graded-Index Multimode Optical Fiber

A wavelength-tunable Optical Fiber Tweezer (OFT) based on a Graded Index Multimode Fiber (GIMF) with a flat endface is proposed. It is shown that the design can support a trapping position which is far from the tip of the GIMF compared with other common optical tweezing methods, hence reducing the possibility of a contact between the trapped particle and the fiber tip. Moreover, because of the wavelength dependence of the GIMF design parameters such the Numerical Aperture (NA), the trapping position can become wavelength-dependent. Therefore, the trapping position can be tuned over a long range using a common wave-length tunable laser. The proposed OFT differs from previous fiber-based demonstrations by using a flat-endface fiber making the fabrication and experiment quite easier than previously proposed tapered-endface OFTs.