Index Multimode Fiber Research Articles

Low-complexity turbulence resilience enabled by a multi-mode bi-directional transceiver

Free-space optical (FSO) communication systems are acutely affected by the pointing issues and distortions that result from mechanical instability and environmental factors such as turbulence. These distortions have generally prevented single-mode bi-directional systems from being deployed without adaptive optics due to high optical losses. We investigate and compare the performance of both step and graded index multi-mode fibers for bi-directional communications over an emulated 400 m FSO channel. We propose that OM5 graded index fiber will simultaneously provide a near Gaussian optical transmission mode and a factor of greater than 5 increase in the field of view compared to single-mode fiber. We demonstrate that OM5 can support an error-free throughput of 10 Gbps for low-turbulence (D/r0 = 3) and 9.1 Gbps for high-turbulence (D/r0 = 9) using commercial bi-directional small form-factor pluggable (SFP+) transceivers with no adaptive optical components.

Narrow bandwidth mode-locked fiber laser with the GIMF-based saturable absorber

We have demonstrated the realization of the nearly transform-limited conventional solitons (CSs) with narrow spectral bandwidth in an Er-doped linear-cavity fiber laser, which is mode-locked by the graded index multimode fiber (GIMF)-based saturable absorber (SA). To our knowledge, this is the first report of the GIMF-based SA in the narrow bandwidth soliton generation. Simultaneously, the wavelength tunability of 0.48 nm and controllable Kelly sidebands for the narrow bandwidth solitons can be realized by the polarization controller (PC). Thereby, the minimum spectral bandwidth of the CSs reaches to be 0.1 nm, accompanied by the pulse duration of 20.8 ps and pulse energy of 1.07 nJ. Our work provides a compact and low-cost alternative for the narrow bandwidth solitons and offers a promising platform in the fields such as spectroscopy, quantum optics and other fields.

Tunable mode-locked holmium-doped fiber laser based on GIMF-SIMF-GIMF structure at 2.09 µm

In this study, a structure of graded-index multimode fiber-step index multimode fiber-graded index multimode fiber (GIMF-SIMF-GIMF) was used as a saturable absorber (SA) to generate mode-locking in a holmium-doped fiber laser (HDFL). The HDFL was pumped with a compact in-house thulium-doped fiber laser (TDFL) operating at 1943 nm with a maximum output power of 2.1 W. Mode-locked pulses with a pulse duration of 1.61 ps were successfully generated at the center wavelength of 2091.1 nm. The pulse had a repetition rate of 12.79 MHz with a bit of fluctuation of 2.09 %, indicating stable pulses. The mode-locked pulses were also tunable with a range from 2086.5 nm to 2091.1 nm by stretching the GIMF part of the SA. This first report demonstrated mode-locking using nonlinear multimode interference (NL-MMI) using GIMF-SIMF-GIMF structure as a saturable absorber in HDFL operating near 2.1 µm wavelength.

Q-switched temperature fiber sensing device based on graded index and D-shaped multimode fiber

In the paper, a Q-switched temperature sensing device is proposed and verified, which consists of a modulator based on a large core multimode fiber and a graded index multimode optical fiber (LMF-GIMF), and a sensing unit formed by sandwiching a D-shaped multimode fiber (MMF) between two single mode fibers (SMF). Utilizing the modulator to transform continuous light into pulsed laser, a stable Er3+-doped Q-switched laser with a repetition frequency of 23.04 kHz and a modulation depth of 7.47 % has been achieved. Under its pumping, the SMF-MMF-SMF sensing structure performed thermal detection with a sensitivity of −81.5 pm/℃ as temperature varies in the range of 25 °C to 65 °C. The temperature sensing device based on Q-switching can be used as an independent pulse laser to realize the switch between the light source and the sensing device. At the same time, the sensing device can provide real-time feedback according to the change of the central wavelength, which allows for immediate adjustments in the operational state, enhancing the flexibility and responsiveness. In addition, the characteristics of the sensor based on Q-switched laser can reduce thermal damage and extends the service life of the sensor. As far as we know, this is the first report on Q-switched temperature sensing based on the LMF-GIMF modulator. The proposed sensing device has the advantages of high signal-to-noise ratio, low operating threshold, minimal thermal damage, and serves as an independent Q-switched laser source, which can be applied to pulse laser measurement with high precision and time resolution in the future.

Optical fiber sensor for curvature and temperature simultaneous measurement based on an anti-resonance mechanism

Based on an anti-resonance mechanism, we propose and experimentally demonstrate an optical fiber sensor for simultaneous measurement of curvature and temperature. The sensor structure consists of a hollow fiber and a twisted graded index multimode fiber spliced between two single-mode fibers. By using the twisted graded index multimode fiber to change the energy distribution of the mode field, not only improve the coupling efficiency of the sensor but also the sensitivity of the curvature measurement. The experimental results show that the intensity of the resonance peak based on the anti-resonance mechanism decreases with the increase of curvature, while the resonance peak wavelength is insensitive to curvature. In addition, due to the thermal expansion and thermo-optic effect of the fiber, the wavelength of the resonance peak is red-shifted with increasing temperature, and the intensity of the resonance peak is insensitive to temperature. Therefore, the intensity and wavelength signals of the resonance peak are monitored separately, and the simultaneous measurement of displacement and temperature is realized. The maximum sensitivity of the proposed curvature sensor can reach −2.84 dB/m−1, the dynamic range of curvature is 0 m−1 to 20.18 m−1, and the temperature sensitivity of 20 pm/∘C is realized. The proposed sensor has a compact structure, simple preparation, high coupling efficiency, and good repeatability, making it an ideal choice for simultaneous measurement of curvature and temperature in structural monitoring, mechanical manufacturing, and other fields.

High speed broadband light spatial laser system with hybrid linear/parabolic/measured indexed multimode optical fibers performance signature

Abstract This work clarified various multimode fiber configurations with spatial continuous wave lasers based different coding configurations for high speed fiber systems. The optimum lighted power form after linear index multimode fibers versus spectral wavelength is clarified. The optimum lighted form power after parabolic index multimode fibers versus spectral wavelength is demonstrated. The max lighted form power after measured index multimode fibers versus spectral wavelength is illustrated. Optimum lighted form signal power variations after linear index multimode fibers versus time are demonstrated. The max optimum form signal band form power after parabolic index multimode fibers versus time is indicated. The max optimum lighted signal power after measured index multimode fibers versus time is clarified. Total lighted signal power form after linear index multimode fibers is demonstrated. The total lighted signal power form after parabolic index multimode fibers is clarified. Total lighted power signal form after measured index multimode fibers is clarified. Max optimum signal electrical power form amplitude after PIN photodetector based various index multimode fibers is simulated and studied. Total lighted base form power against MMF length for different proposed optical multimode fibers is studied. Signal base band per noise form ratio against MMF length for different base proposed band optical multimode fibers is demonstrated. Total signal form electrical band power against MMF length for different proposed optical multimode fibers is observed. Max Q base band factor form against MMF length for different base band proposed lighted multimode fibers is clarified.

Wavelength-tunable quasi-unidirectional spatiotemporal mode-locked solitons in an isolator-free Er-doped fiber laser

In this letter, we have demonstrated the realization of the wavelength-tunable spatiotemporal mode-locked (STML) operation in an Er-doped partial multimode fiber laser without the intracavity isolator. The laser adopts an all-fiber structure and the saturable absorption effect is attributed to nonlinear multimode interference (NL-MMI) from the single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) design. The STML laser can operate in the quasi-unidirectional regime with an emission strength difference of larger than 30 dB for the two counter-propagating lasers. And the switching between the dominating operating directions in the clockwise (CW) or counter-clockwise (CCW) directions can be easily achieved by adjusting the MM polarization controller (PC). Both STML conventional solitons (CSs) and the STML soliton molecules are obtained with a similar tunable wavelength from ∼1.56 μm to ∼1.58 μm for the opposite operating directions. Furthermore, the spatial dynamics of the STML soliton are also investigated in detail. The experimental results are of great help for us to explore nonlinear spatiotemporal dynamics and provide potential applications in fields of all-optical signal processing, fiber sensing and information coding.

Diverse mode operation fiber laser mode-locked by nonlinear multimode interference

We present an all-fiber passively mode-locked (ML) laser with a nonlinear multimode interference (NLMI)-based saturable absorber (SA) capable of generating five pulse modes. The SA consists of two centrally aligned graded index multimode fiber (GIMF) with different diameters (105-50 µm) and features a widely adjustable transmission with saturable/reverse-saturable absorption. Based on this, dissipative soliton (DS), Q-switched rectangular pulse (QRP), dissipative soliton resonance (DSR), noise-like pulse (NLP) and bright-dark pulse pairs (BDP) are observed at three dispersions without additional filter. The DS has a pulse energy, bandwidth and duration of up to 1.15 nJ, 17.98 nm and ∼2.78 ps. The achievable pulse duration and energy of DSR and NLP are 5.21, 48.06 ns and 4.53, 5.12 nJ, respectively. Furthermore, it is demonstrated that the BDP is superimposed by a chair-case pulse (CP) and a rectangular pulse (RP) belonging to orthogonal polarization states. The versatility, flexibility, simplicity and energy scalability of the large-core hybrid GIMF-SA, make it interesting and highly attractive in ultrafast photonics.

Domain-wall dark pulse generation with SMF-GIMF-SMF structure as artificial saturable absorber

We experimentally demonstrated the generation of domain-wall dark pulse in an Erbium-doped fiber laser using the combination of a 10 cm graded index multimode fiber sandwiched by single mode fibers as artificial saturable absorber. The interaction of phase difference in grade index multimode fiber allowed the stable dual-wavelength oscillation in the cavity. The dual-wavelength centered at 1567.2 nm and 1569.4 nm produces the topological defect in temporal domain and achieved a dark pulse formation with repetition rate of 21.5 MHz. The highest average pulse energy is calculated as 769.6 pJ with pulse width of 5 ns. Throughout the operating pump power range, the average pulse energy and output power increase linearly, with R2 of 0.9999 and achieved the laser efficiency of 9.33%. From the measurement in frequency domain, the signal-to-noise ratio is measured as 49 dB. As compared to reported DW dark pulse works, the proposed structure only required a short length of multimode fiber, which allowed the domain-wall dark pulse to achieve higher pulse repetition rate. The venture of domain wall dark pulse is potentially to pave the foundation toward sustainable industrial growth.

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.

976-nm pulse-type switchable all-fiber mode-locked laser based upon nonlinear multimode interference

A 976-nm pulse-type switchable mode-locked fiber laser is experimentally demonstrated, for the first time to our knowledge, with a single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure-based saturable absorber (SA). The GIMF is bent into a given state to strengthen its effect on establishing mode-locking. By adjusting the intra-cavity polarization controller, the switching between dissipative solitons (DSs) and noise-like pulses (NLPs) can be achieved. Meanwhile, we also realize a second harmonic mode-locking (HML) in the same laser cavity. The laser provides a solution to realize 976 nm mode-locking operation with a simple and low-cost configuration and can serve as a potential tool for various applications.

Mode‐Locked Fiber Lasers Based on Graded Index Multimode Fiber

Mode‐Locked Fiber Lasers Based on Graded Index Multimode Fiber

The influence of refractive index profile on supercontinuum beams self-cleaning in graded index multimode fiber

We propose a theoretical scheme to improve the fundamental mode energy proportion of the supercontinuum (SC) generation in graded-index multimode fiber (GRIN-MMF). With this scheme, we conduct numerical investigations into the influence of various core refractive index profile parameters on beam-self cleaning of fundamental mode, establishing an optimal parameter range (α=2.0∼2.06). Within this range, the energy proportion of the fundamental mode gradually increases from 32% to 65% as the propagation distance. Additionally, our simulation provides the initial demonstration that the supercontinuum (SC) broadening is correlated with the distribution of core refractive indices within the graded-index multimode fiber.

Multimode parabolic index fiber integrated with spatiotemporal vertical cavity surface emitting laser sources for optical fiber system improvement

Abstract This study has demonstrated the multimode parabolic index fibers integrated with spatiotemporal VCSEL sources for the optical fiber system improvement. Average radial intensity is clarified against fiber radius variations with 0.1 % refractive difference index step. The Max base band Q factor form is demonstrated against the fiber refractive index step variations. The Max base band Q factor form variations are demonstrated against VCSEL diode bias current variations. The Max base band Q factor form variations are clarified against the fiber core radius variations. The Max base band Q factor form is simulated against the fiber cladding thickness variations. Multimode fiber calculation report at the wavelength 1550 nm for the light source (Pol. X = VCSEL LP [0,1]) is reported. The Max lighted base optical band form power is clarified versus time after parabolic index multimode fiber with 0.1 % refractive difference index step. The Max optical lighted base band power form is demonstrated against wavelength after parabolic index multimode fiber with 0.1 % refractive difference index step. The Max base Q band form factor is simulated after spatial APD photodetector receiver with 0.1 % refractive difference index step. The fiber base band mode form intensity in x and d directions is demonstrated within the fiber with 0.1 % refractive difference index step. The fiber base band encircled flux form is studied and simulated versus fiber radius variations with 0.1 % refractive difference index step.

Efficient employment of Yb doped multi-mode fiber amplifiers based pumped laser with advanced light sources in high speed modulated optical communication systems

Abstract In this work, efficient employment of Yb doped multi-mode fiber amplifiers based pumped laser is simulated with advanced light sources in high speed modulated optical communication systems. The total lighted power through measured index multimode fiber based different base band modulation voltages dual drive MZM measured are clarified. The 3D graph of the mode field intensity distribution through measure index multimode fiber is simulated. Total electrical base ban power form through PIN photo-detector based various modulation voltages Dual drive MZM measured are demonstrated. The max Q form factor-min base band BER values through PIN photo-detector based different modulation voltages dual drive MZM measured are also simulated. The electrical base signal form noise ratio varies is simulated and clarified with modulation voltage based dual drive MZM measure varies. The max Q form base band Factor is demonstrated with modulation voltage based dual drive MZM measure varies. Total lighted base power form varies are clarified with modulation voltage based dual drive MZM measure varies. The different measured index multimode fiber characteristics based spatial VCSEL are also clarified and demonstrated.

Characterization of the ultrafast fiber laser mode-locked by nonlinear multimode interference

A passively mode-locked fiber laser with switchable output states is demonstrated by using an all-fiber saturable absorber (SA) based on a hybrid single mode fiber-graded index multimode fiber-single mode fiber-graded index multimode fiber-single mode fiber (SMSMS) incorporated into a ring cavity. We investigate the vectorial characteristics of the output soliton in order to analyze the influence of polarization dependence of the SMSMS structure on solitons generation. Due to the appearance of vector solitons, it is proved that nonlinear multimode interference (NL-MMI) plays a dominant role in solitons generation but not nonlinear polarization rotation (NPR). Meanwhile, abundant mode-locked states are achieved in the laser, such as fundamental soliton, higher-order harmonics, bound twin-pulse solitons, and quasi-harmonic bound states. This configuration provides a useful platform for investigating ultrafast soliton dynamics.

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.

Non-Hermitian Mode Cleaning in Periodically Modulated Multimode Fibers.

We show that the simultaneous modulation of the propagation constant and of the gain/loss coefficient along the graded index multimode fibers results in unidirectional coupling among the modes, which leads to either the enhancement or the reduction of the excitation of higher order transverse modes, depending on the modulation parameters. In the latter case, effective mode cleaning is predicted, ideally resulting in single-mode spatially coherent output. The effect is semi-analytically predicted on a simplified Gaussian beam approximation and numerically proven by solving the wave propagation equation introducing the non-Hermitian modulated potential.

A novel active continuous-wave FLRD strain sensor based on frequency-shifted interferometry

ABSTRACT An active continuous-wave fiber loop ringdown strain sensor based on frequency-shifted interferometry is presented and experimentally investigated. By measuring the decay rate of the continuous light in the ringdown loop (RDL), the sensor achieves strain measurement without optical pulsation and rapid detection. A bidirectional erbium-doped fiber amplifier is embedded in the loop to form an active fiber RDL, to compensate the inherent loop loss. The strain sensor head was built by splicing a segment of normal graded index multimode fiber (MMF) into the RDL composed of single-mode fiber. In this configuration, the core-cladding mode conservation and insertion loss of the MMF changes with the strain applied, making it possible to detect different levels of strains. The proposed device was tested with the strains in the range from 0 to 2.25 mϵ, which exhibits a linear response with a sensitivity of 0.29 km-1·mϵ-1, with a measurement resolution of 22 mϵ.