Step-index Multimode Fiber Research Articles

Power over fiber using a multimode optical power with a core diameter of 50 µm

We report on the properties of the Power over Fiber (PoF) transmission link using a High-Power Laser Source operating at 976 nm and using three types of optical fiber with a core diameter of 50 µm. Two step-index profile multimode optical fibers and one fiber with a gradient index were used for optical power transmission. Optical light was converted to electricity using commercially available Photovoltaic Power Convertors (PPCs) with a maximal optical input power of 1.5 W and experimental PPCs with a maximal optical input power of 4.0 W. We experimentally proved optical power transmission up to a distance of 300 m. In the case of the commercially available working PPC and using the gradient index fiber we achieved a result of 0.534 W of electrical power and using the experimental PPC we achieved 0.645 W. In the case of the step-index optical fiber, the result was 1.3 W.

Optical solitons in multimode fibers: recent advances

Optical solitons in multimode fibers were predicted 40 years ago and extensively investigated theoretically. Transmission experiments in nonlinear multimode fibers have gained renewed interest, motivated by their potential to extend the capacity of long-distance transmission systems; only in the last few years, new experiments have revealed unexpected properties of optical solitons propagating in graded-index and step-index multimode fibers, partially re-writing the existing theory. Here we provide an overview of the recent experimental, numerical, and theoretical studies that revealed those new properties. It will be shown that multimode fiber solitons form with specific pulse width and energy dependent on the wavelength, and that they naturally evolve toward fundamental-mode Raman solitons. New soliton fission mechanisms, governed by the modal dispersion, will be explained. Possible applications in space-division multiplexed systems will be discussed. A recent thermodynamic approach to soliton condensation will be described.

Advantages of internal reference in holographic shaping ps supercontinuum pulses through multimode optical fibers

The use of wavefront shaping has found extensive application to develop ultra-thin endoscopic techniques based on multimode optical fibers (MMF), leveraging on the ability to control modal interference at the fiber’s distal end. Although several techniques have been developed to achieve MMF-based laser-scanning imaging, the use of short laser pulses is still a challenging application. This is due to the intrinsic delay and temporal broadening introduced by the fiber itself, which requires additional compensation optics on the reference beam during the calibration procedure. Here we combine the use of a supercontinuum laser and an internal reference-based wavefront shaping system to produce focused spot scanning in multiple planes at the output of a step-index multimode fiber, without the requirement of a delay line or pulse pre-compensation. We benchmarked the performances of internal vs external reference during calibration, finding that the use of an internal reference grants better focusing efficiency. The system was characterized at different wavelengths, showcasing the wavelength resiliency of the different parameters. Lastly, the scanning of focal planes beyond the fiber facet was achieved by exploiting the chromato-axial memory effect.

Spectral-temporal-spatial customization via modulating multimodal nonlinear pulse propagation

Multimode fibers (MMFs) are gaining renewed interest for nonlinear effects due to their high-dimensional spatiotemporal nonlinear dynamics and scalability for high power. High-brightness MMF sources with effective control of the nonlinear processes would offer possibilities in many areas from high-power fiber lasers, to bioimaging and chemical sensing, and to intriguing physics phenomena. Here we present a simple yet effective way of controlling nonlinear effects at high peak power levels. This is achieved by leveraging not only the spatial but also the temporal degrees of freedom during multimodal nonlinear pulse propagation in step-index MMFs, using a programmable fiber shaper that introduces time-dependent disorders. We achieve high tunability in MMF output fields, resulting in a broadband high-peak-power source. Its potential as a nonlinear imaging source is further demonstrated through widely tunable two-photon and three-photon microscopy. These demonstrations provide possibilities for technology advances in nonlinear optics, bioimaging, spectroscopy, optical computing, and material processing.

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.

All-fiber Q-switched erbium-doped fiber laser generation with SMF-SIMF-SMF structure as artificial saturable absorber

A passively Q-switched Erbium-doped fiber laser (EDFL) was successfully demonstrated using SMF (single-mode fiber)-SIMF (step-index multimode fiber)-SMF or SMS structure as a saturable absorber (SA). The SMS structure operates on the Kerr-effect of multimode interference (MMI). To modulate the cavity loss, a 158 mm length of SIMF was fusion spliced into a ring cavity. By properly adjusting the in-line polarization controller (PC) that holds the SIMF, we were able to generate steady Q-switched pulses. As the input pump power increased from 115 to 343 mW, the repetition rate of the Q-switched laser increased from 87 to 158 kHz, while the corresponding pulse width decreased from 4.2 µs to 2.5 µs. The signal-to-noise ratio (SNR) of the fundamental mode for the pulsed laser was around 72 dB at a central wavelength of 1530 nm. Our results indicate that the SMS structure can be used as an SA to generate stable Q-switched EDFL with very simple cavity settings.

An in-line cascaded eight-cores-fiber sensor for 25 kHz to 300 kHz ultrasound detection

In this work, we proposed and demonstrate an in-line cascaded eight-cores-fiber (ECF) sensor and tested the feasibility for wide range of ultrasound detection. The sensor is fabricated by directly cascaded two single mode fibers (SMF), two short step-index multimode fibers (MMF)and an ECF in the middle, and the ECF structure can effectively enhance the intensity of the mode-interference between core mode and higher-order modes. Thus, by simulating the transmission modes in ECF and analyzing transmission spectrum of sensors with different ECF lengths, an ultrasound sensor with the ECF length of 2.5 cm can be obtained. We constructed the ultrasound detection experimental system, and the experimental results show that the sensor can detect ultrasound signals with frequency from 25 kHz to 300 kHz, and have a high signal-to-noise ratio (maximal value of 76 dB). The ultrasound sensing sensitivity is calculated as 2.86 mV/kPa. The proposed sensor has compact structure and large measurement range, which shows the good potential in high sensitivity ultrasound detection.

Theoretical and experimental investigation of the steady-state power distribution in multimode step-index plastic optical fibers

In this work we have examined theoretically and experimentally the influence of mode coupling on steady-state distribution (SSD) in step-index plastic optical fiber (SI POF). Numerical and analytical solution of the PFE under steady-state conditions are compared to our experimental results. A numerical solution of the power flow equation (PFE) has been obtained by the explicit finite difference method (EFDM). We demonstrated that SSD in the SI POF is achieved at length of 140 m, which is much shorter length compared to the case with silica fibers. The reported results appear to be particularly important since none of the main optical properties of an optical fiber (attenuation, bandwidth) can be accurately determined until the transmitted optical power reaches its SSD, due to strong influence of mode coupling phenomena that occur during the transmission process. These results are important for employment of the SI POFs as a part of communication or sensory systems.

Tunable-wavelength broadband spectrum dissipative soliton mode-locked fiber laser

The tunable-wavelength broadband spectrum dissipative soliton (DS) is demonstrated in Yb-doped mode-locked laser using an all-fiber structure saturable absorber (SA), that is, graded index multimode fiber (GIMF)-step index multimode fiber (SIMF)-GIMF with 4.70% modulation depth and 4.55 mW saturation power. Increasing pump power alters intra-cavity balance between loss and gain and nonlinear and dispersion, causing wavelength adjustment and spectrum broadening. The center wavelength shifts towards shorter wavelength with rang of 1037.77–1024.83 nm, corresponding 3-dB bandwidth widens from 6.10 to 9.23 nm and pulse width narrows from 23.80 to 13.38 ps. The signal-to-noise ratio (SNR) is 71.2 dB at the repetition rate of 14.63 MHz. The GIMF-SIMF-GIMF structure has a huge potential to serve as both SA and filter component for the tunable-wavelength broadband spectrum Yb-doped mode-locked fiber laser (MLFL), which is favorable for developing the field of nonlinear microscopy and material processing.

Newly Developed Method for Liquid Thin Film Thickness Measurement Using Optical Fiber–Based Reflective Probe

A new ray-tracing–based calibration method for an Optical-fiber-based Reflective Probe (ORP) was developed. This technique enables thickness measurement in micrometers in wavy thin liquid film flow, which is simpler and quicker than other liquid film measurements. First, the relationship between the film thickness and ORP signal was calculated through the ray-tracing simulator. The signal trend showed a steep rate of change within a few-hundred-micron thicknesses, thanks to the emission nature of the step index multimode fiber. The ray-tracing–based calibration was established using the calculated relationship. Second, the calibration method was validated under quiescent conditions. The calibrated ORP measured the thickness and then was compared to visualization. Good agreement was confirmed between the two results at a maximum difference of 20% under 1000 μm in thickness. Finally, thickness measurement for the wavy thin film flow was performed. Airflow (jG = 40 to 75 m/s) was introduced into the rectangle test section, and a small amount of tap water (Q = 30 to 90 mL/min) was injected into the channel plate. The difference in the measured thickness between ORP and high-speed visualization was around 20%. The effectiveness of the new calibration method and ORP measurement including its uncertainty will be discussed.

Modeling optical emission from a highly multi-mode step-index fiber via ray tracing and the limitations: publisher's note.

This publisher's note amends the author listing of Appl. Opt.60, 10885 (2021)APOPAI0003-693510.1364/AO.434229.

Investigation of mode coupling in strained and unstrained multimode step-index POFs using the Langevin equation

The Langevin equation (LE) is used to evaluate mode coupling in multimode step-index polymer optical fiber (SI POF) that is both unstrained and strained. The numerical solution of the LE matches the numerical solution of the power flow equation (PFE). Strain-induced mode coupling is noticeably stronger in strained fiber than in unstrained fiber of the same types. Therefore, compared to similar lengths for unstrained fibers, the coupling length of the equilibrium mode distribution (EMD) is attained and the length of fiber required to produce a steady-state distribution (SSD) are both much shorter for strained fibers. We have demonstrated that the mode coupling in strained and unstrained multimode SI POFs that comes from the random perturbations (RPs) of the fiber can be successfully treated by the LE. The study's findings can be used to improve communication and sensory systems that use multimode SI POFs under different bending circumstances. Additionally, it is crucial to be able to compute the modal distribution of the SI POFs used in the optical fiber sensory system at a specific length and under various bending scenarios.

Corrosion sensor based on surface plasmon resonance effect of core-offset splicing fiber

A corrosion monitoring sensor is proposed based on the surface plasmon resonance (SPR) effect of core-offset splicing fiber. Using the arc fusion splicing method, this sensor is fabricated by cascading together a single-mode fiber (SMF), a graded-index multimode fiber (GI-MMF), and a step-index multimode fiber (SI-MMF). The SMF and GI-MMF are core offset spliced to enhance evanescent field by coupling the transmission light into the cladding. The outer layer of GI-MMF is coated with 50 nm silver film, which can effectively excite the SPR effect and detect corrosion. The sensor has good reproducibility, and shows a high sensitivity for early corrosion in atmospheric weak acid pollutants, which can be applied to early metal corrosion monitoring and weak acid atmospheric corrosion evaluation.

Tapered fiber optic refractive index sensor using speckle pattern imaging

In this study, a low-cost and compact refractive index (RI) sensor based on speckle pattern imaging has been proposed and demonstrated numerically and experimentally. We have proven that speckle pattern images recorded with a CCD camera placed at the end of the tapered optical fiber (TOF) are highly sensitive to the RI of the medium to be measured. The effects of the waist diameter and length of the tapered region, which is the sensing part of the fiber, on sensitivity were investigated using the beam propagation method (BPM). It has been seen that the numerical results have compatible with the experimental results. Tapered fiber sensor was fabricated by electric arc discharge technique using step index multimode fiber (SI-MMF) with core and cladding diameters of 105 and 125 μm. The sensor sensitivity increases as the waist diameter decreases and the waist length increases. The sensor, which waist diameter is reduced to 10 μm, is sensitive to RI changes of the order of 10-4. It has been clearly seen that the proposed sensor based on speckle pattern imaging is highly sensitive to RI change.

Investigation of space division multiplexing in multimode step-index silica photonic crystal fibers

The feasible distance is presented for space division multiplexed (SDM) transmission along multimode silica step-index photonic crystal fiber (SI PCF) by solving the time-independent power flow equation (TI PFE). These distances for two and three spatially multiplexed channels were determined to depend on mode coupling, fiber structural parameters, and launch beam width in order to keep crosstalk in two- and three-channel modulation to a maximum of 20% of the peak signal strength. We found that the length of the fiber at which an SDM can be realized increases with the size of the air-holes in the cladding (higher NA). When a wide launch excites more guiding modes, these lengths become shorter. Such knowledge is valuable for the use of multimode silica SI PCFs in communications.

Experimental Observation of the Spontaneous Emission of a Space–Time Wavepacket in a Multimode Optical Fiber

We provide a complete analysis, from theory to experiment, of the spontaneous emergence of a discretized conical wave of X-type (i.e., a localized 2D+1 space-time wavepacket) when an intense ultrashort pulse nonlinearly propagates in a multimode fiber. In particular, we reveal that this spatiotemporal phenomenon corresponds to broadband intermodal dispersive wave emission from an unsteady localized wave structure formed during nonlinear propagation. Theoretical phase-matching predictions are experimentally and numerically confirmed in a commercially available step-index multimode fiber. Our results provide a general understanding of phase-matched radiations emitted by nonlinear waves in multidimensional dispersive optical system.

Quantum bit error rate timing jitter dependency on multi-mode fibers.

The full time-jitter response of a single-photon detector can make a significant contribution to the quantum bit error rate (QBER) of high repetition rate quantum key distribution (QKD) implementations. Although there have been studies into understanding the contribution for single-mode optical fiber coupled single-photon detectors, the contribution of larger, multimode core diameters to the QBER have not been explored in detail. With the growing importance of free-space QKD, which typically use multimode fibers to reduce coupling loss, it is vitally important to understand how the multimode fiber coupling will impact the total QBER. This work studies the impact of the time-jitter contribution to QBER when coupling a commercial off-the-shelf silicon single-photon avalanche diode with various multimode fibers while simulating operating at 1 GHz with empirical measurements taken at 1 MHz repetition rate. It was found that step-index multimode fibers can significantly increase the QBER, while graded-index fibers can provide an QBER contribution similar to a single-mode fiber. The results highlight that there is a significant benefit in using graded index multimode fibers for a free-space QKD receiver, particularly for high repetition rate applications.

Laguerre-Gaussian Expansion of Step-Index Multimode Fiber Beams in Free-Space

We show that the complex conjugate of the overlap integrals between the fiber end-facet field and the Laguerre-Gaussian modes provides the expansion coefficients for the step-index multimode fiber beams. We also show that the fractional power in different azimuthal mode orders remains unchanged from fiber to free-space propagation.

A high-sensitivity high-resolution intravascular photoacoustic catheter through mode cleaning in a graded-index fiber

Intravascular photoacoustic imaging has been developed to evaluate the possibility of plaque rupture in atherosclerosis by high spatial resolution imaging of lipid. However, the detection sensitivity and spatial resolution are compromised by the poor focusing caused by a multimode fiber. In this work, we report an intravascular photoacoustic catheter with mode self-cleaning in a graded-index fiber to improve the beam quality and the sensitivity for lipid detection. Compared with the higher-order modes in a step-index multimode fiber, the lower-order modes generated by the self-cleaning effect in the graded-index fiber greatly enhanced the photoacoustic spatial resolution and detection sensitivity. The dominant ringing artifact caused by laser absorption of the ultrasound transducer was further reduced by using stripe suppression. A lipid plaque mimicking phantom was imaged for evaluation. Lipid particles with a small diameter of 75.7 µm were clearly observed.

Full-beam spatial coherence measurements of supercontinuum generation in multimode fibers

We study experimentally the spatial coherence across the full spatial beam profile of supercontinuum light generated in both in graded-index and step-index multimode fibers. We observe a decrease of the coherence area with an increase of the injected pump power. Numerical simulations based on linearly polarized modes and mode coupling theory are in good agreement with our experiments and indicate that the decrease of coherence area is strongly related to a change in the modal amplitude distribution.