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The requirement of amplification for wide-bandwidth, compensation of attenuation for long-distance, and high-speed communication generates a need for highly efficient amplifiers. There are several types of amplifiers, such as semiconductor and fiber amplifiers. However, Raman amplifiers have some benefits over others, such as being less expensive and having high-gain bandwidth products. A low noise figure and high and flat gain are advantages of second-order Raman amplifiers over first-order amplifiers. There are various ways to implement second-order Raman amplifiers by using dispersion compensating fibers (DCF) or single-mode fibers (SMF) of different configurations. In this paper, a novel hybrid amplifier has been proposed. This is a combination of an erbium-doped fiber amplifier (EDFA) and a Raman amplifier. Here, the Raman amplifier uses second-order Raman pumping for amplification, which is based on second-order stimulated Raman scattering (SO-SRS). Here, the Raman fiber is a single-mode fiber (SMF). Simulation has been done for output power, gain, and noise figure (NF). A comparison of first-order and second-order hybrid amplifiers has been done for the above parameters. Also, the proposed second-order hybrid amplifier has been compared to one of the previous versions. The results obtained show improvement in terms of output power, gain, and NF. The lowest NF obtained for the proposed amplifier is 4.42 dB at Raman length of 15 km and second order pump power of 450 mW. [ABSTRACT FROM AUTHOR]

It is hard to overstate the utility of lasers in modern technology. Optical-fiber-based lasers are of particular value thanks to their combination of small form factors, afforded by the coilability of the thin strands of fiber, and high beam-quality output. The optical fiber geometry also possesses a relatively high surface-area-to-volume ratio, rendering thermal management somewhat more straightforward than in other bulk laser types. Regardless, the generation of heat during the lasing process can still be problematic for a myriad of reasons, and conventional methods of thermal management do not comport with the potential compactness and elegance of fiber lasers as technological solutions. This Perspective summarizes recent advances in glass science and optical fiber engineering to support the provocative premise that heat generation in future laser systems can be entirely managed by a combination of fiber materials and novel laser physics. Letting the fiber manage heat itself would have significant impacts on enhancing system performance while greatly reducing size, weight, power-consumption, and cost. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering (SBS) is the major factor that limits the maximum optical fiber output power in narrow linewidth applications, which include important fields such as passive optical networks (PONs), high-power fiber amplifiers, and lasers. Great efforts have been dedicated to suppressing the SBS effect and increasing the maximum optical fiber output power. This paper focuses on key fiber-based techniques to suppress SBS. These techniques take advantages of the properties of optical fibers. We present how these properties (electric modes, acoustic modes, and material properties) could be utilized to suppress SBS. The fiber-based techniques are divided into transverse optical fiber design, longitudinal variant fiber design, and external perturbations (strain and temperature) on optical fibers. Transverse optical fiber design focuses on the mechanism electro-acoustic interaction. Large effective area fiber design and acoustic tailoring techniques have been discussed. Longitudinal variant fiber design considers the nonlinear SBS interaction along propagation distance, and various techniques related have been presented. External perturbations (strain and temperature) on optical fibers emphasize on how external static perturbations could modify the SBS effect. [ABSTRACT FROM AUTHOR]

We present a first-hand view of the early origins of femtosecond fiber technology built upon the foundations of ultrafast optics established in the sixties, seventies, and eighties. We discuss what considerations led to the establishment of early femtosecond oscillator designs and how they have been improved over the years. Another aspect of this article deals with the road to ever increasing pulse energies and average powers that can be achieved based on various amplification techniques. We go on to review the journey toward achieving greater precision enabled via the invention of frequency combs and the incorporation of frequency comb technology within the realm of fiber optics. The paper concludes with a brief overview of some of the most important fiber laser applications and provides a preview of what may lie ahead in the future. [ABSTRACT FROM AUTHOR]

In this work, compact praseodymium-doped fiber amplifiers (PDFAs) with single- and dual-pass configurations are designed and investigated in the O-band region based on theoretical simulation. The impact of doped fiber characteristics, input signal power and input pumping power on the gain and the noise figure of the proposed amplifiers are comprehensively considered and compared. At an input pumping power of 350 mW and optimum praseodymium-doped fiber (PDF) length, the obtained results show that dual-pass model recorded higher gain than the single-pass scheme. This occurs because of the dual propagation of the forwarded and amplified spontaneous emission (ASE) signal through the gain medium. Moreover, both proposed configurations exposed better amplification performance at an input signal power of −10 dB compared to the higher input signal power of 0 dB. The dual-pass regime shows higher amplification gain than the single-pass one of about 4 dB and a maximum power conversion efficiency (PCE) of 14%. Indeed, the PDF core radius and doping concentration are considered as vital factors. Decrement of doped-core radius leads to a notable gain increment and NF decay, whereas low doping concentrations show very small signal gain compared to the optimized value. [ABSTRACT FROM AUTHOR]

The spectrally flat supercontinuum generation in the wavelength range of 900–2400 nm is demonstrated in silica-based fibers of variable core diameter and dispersion. It is shown that, in comparison with standard optical fibers of the same length, supercontinuum spectra 200 nm wider can be realized in the samples under study. The significant difference between the spectral and temporal transformations of radiation depending on the direction of propagation is demonstrated in the researched fiber samples. [ABSTRACT FROM AUTHOR]

Octave-spanning supercontinuum conversion in three different rare-earth doped fiber amplifiers have been investigated. Using an erbium amplifier, it turned out to increase the output power to 445 mW with a spectral width of 1250 nm. For a thulium amplifier, an average output power of 390 mW and a spectral width of 569 nm was obtained. Additionally, for holmium, the average output power was 724 mW with a spectral width of 450 nm. For all cases, the output pulses envelope did not exceed 0.72 ns. [ABSTRACT FROM AUTHOR]

Supercontinuum (SC) generation that leads to the emission of broadband radiation has been extensively studied. In particular, SC sources encompassing the wavelength range of 2–5 μm have attracted considerable interest in the last decade, and a continuous increase in the output power and spectrum width has been observed. To enable broadband and high-power SC generation, suitable nonlinear media combined with appropriate pump sources must be used, maintaining the output as spectrally flat. This paper briefly reviews the current state-of-the-art SC sources restricted to those based on fluoroindate fibers, including systems pumped with femtosecond, picosecond, and nanosecond pulses. First, the concept of SC generation in optical fibers is briefly presented. This is followed by an examination of indium fluoride optical fibers, with an emphasis on their material and waveguide properties. Furthermore, the advances in SC generation in fluoroindate fibers, including the latest results on high-power (Watt-level) continuum generation adopting different pump schemes, are also explored. A record time-averaged output power of 11.8 W with a spectrum spanning from ~1.9 to 4.9 µm has been demonstrated, which is certainly not the power limit of this technology. Finally, potential future directions of research are discussed at the end of this paper. [ABSTRACT FROM AUTHOR]

Stimulated Brillouin scattering (SBS) is the major factor that limits the maximum optical fiber output power in narrow linewidth applications, which include important fields such as passive optical networks (PONs), high-power fiber amplifiers, and lasers. Great efforts have been dedicated to suppressing the SBS effect and increasing the maximum optical fiber output power. This paper focuses on key fiber-based techniques to suppress SBS. These techniques take advantages of the properties of optical fibers. We present how these properties (electric modes, acoustic modes, and material properties) could be utilized to suppress SBS. The fiber-based techniques are divided into transverse optical fiber design, longitudinal variant fiber design, and external perturbations (strain and temperature) on optical fibers. Transverse optical fiber design focuses on the mechanism electro-acoustic interaction. Large effective area fiber design and acoustic tailoring techniques have been discussed. Longitudinal variant fiber design considers the nonlinear SBS interaction along propagation distance, and various techniques related have been presented. External perturbations (strain and temperature) on optical fibers emphasize on how external static perturbations could modify the SBS effect.

We demonstrate here an environmentally stable and compact Er-doped fiber laser system capable of delivering sub-100-fs pulse duration with discontinuous selectable repetition rate from 10 to 70 MHz. This laser source employs a SESAM harmonic mode-locked soliton laser to facilitate tunable repetition rate. A single-mode-fiber (SMF) amplifier and a double-cladding-fiber (DCF) amplifier (both with double-pass configuration) bridged by a three-stage YVO4-based divider are used to manage the dispersion map and boost the seed pulses. With the help of ×8 replicas, the seed pulses are simultaneously amplified and compressed with 90% recombining efficiency, generating 4.60-nJ pulse energy at 10 MHz and 6.13-nJ pulse energy at 70 MHz. The main advantage of the proposed laser system is that when pulse repetition rate is switched on-demand, the pulse duration can be kept almost unchanged only by simply adjusting the pump powers of the fiber amplifiers, instead of changing any optical device (the spacing between gratings or the length of fibers).

Tetrahedral chromium (Cr4+)-ions is the main contributor to gain in the broadband single-mode Cr-doped crystalline core fibers (SCCFs). We have demonstrated a gross gain of 8-dB on a length of 27-cm for a 300-nm broadband SCCF using a thermal annealing technique. Gross gain and fiber length are the highest yet reported for the SCCF. The record gain is mainly due to both the longer fiber by aligned molten-zone growth and Cr4+-ions enhancement by thermal annealing. The enhancement mechanism of the Cr4+-ions in the SCCF is experimentally verified by the means of superconducting quantum interference device (SQUID) and electron probe X-ray micro-analysis (EPMA). As a practical application of the proposed 8-dB gain in the SCCFs in fiber transmissions, a 25-Gb/s error-floor free link is successfully demonstrated.

The spectrally flat supercontinuum generation in the wavelength range of 900–2400 nm is demonstrated in silica-based fibers of variable core diameter and dispersion. It is shown that, in comparison with standard optical fibers of the same length, supercontinuum spectra 200 nm wider can be realized in the samples under study. The significant difference between the spectral and temporal transformations of radiation depending on the direction of propagation is demonstrated in the researched fiber samples.

Octave-spanning supercontinuum conversion in three different rare-earth doped fiber amplifiers have been investigated. Using an erbium amplifier, it turned out to increase the output power to 445 mW with a spectral width of 1250 nm. For a thulium amplifier, an average output power of 390 mW and a spectral width of 569 nm was obtained. Additionally, for holmium, the average output power was 724 mW with a spectral width of 450 nm. For all cases, the output pulses envelope did not exceed 0.72 ns.

Supercontinuum (SC) generation that leads to the emission of broadband radiation has been extensively studied. In particular, SC sources encompassing the wavelength range of 2–5 μm have attracted considerable interest in the last decade, and a continuous increase in the output power and spectrum width has been observed. To enable broadband and high-power SC generation, suitable nonlinear media combined with appropriate pump sources must be used, maintaining the output as spectrally flat. This paper briefly reviews the current state-of-the-art SC sources restricted to those based on fluoroindate fibers, including systems pumped with femtosecond, picosecond, and nanosecond pulses. First, the concept of SC generation in optical fibers is briefly presented. This is followed by an examination of indium fluoride optical fibers, with an emphasis on their material and waveguide properties. Furthermore, the advances in SC generation in fluoroindate fibers, including the latest results on high-power (Watt-level) continuum generation adopting different pump schemes, are also explored. A record time-averaged output power of 11.8 W with a spectrum spanning from ~1.9 to 4.9 µm has been demonstrated, which is certainly not the power limit of this technology. Finally, potential future directions of research are discussed at the end of this paper.

The superfluorescent fiber source (SFS) with tunable optical spectrum has shown great application potential in the sensing, imaging, and spectral combination. Here, we demonstrate for the first time a 2-kW-level wavelength and linewidth tunable SFS. Based on a flexible filtered SFS seed and three stages of fiber amplifiers, the output power can be scaled from the milliwatt level to about 2 kW, with a wavelength tuning range of 1068–1092 nm and a linewidth tuning range of 2.5–9.7 nm. Moreover, a numerical simulation is conducted based on the generalized nonlinear Schrödinger equation, and the results reveal that the wavelength tuning range is limited by the decrease of seed power and the growth of amplified spontaneous emission, whereas the linewidth tuning range is determined by the gain competition and nonlinear Kerr effects. The developed wavelength and linewidth tunable SFS may be applied to scientific research and industrial processing. [ABSTRACT FROM AUTHOR]

In this work, we report the performance evaluation of Erbium/Ytterbium (Er3+/Yb3+) codoped fiber amplifier based on theoretical simulations. The proposed design achieved a broadband small signal gain of around 43.5 dB while incorporating a short length of 10 m codoped fiber at an optimized pump power of 100 mW. A minimum noise figure of 4.9 dB was observed at 1560 nm. [ABSTRACT FROM AUTHOR]

We demonstrate here an environmentally stable and compact Er-doped fiber laser system capable of delivering sub-100-fs pulse duration with discontinuous selectable repetition rate from 10 to 70 MHz. This laser source employs a SESAM harmonic mode-locked soliton laser to facilitate tunable repetition rate. A single-mode-fiber (SMF) amplifier and a double-cladding-fiber (DCF) amplifier (both with double-pass configuration) bridged by a three-stage YVO4-based divider are used to manage the dispersion map and boost the seed pulses. With the help of ×8 replicas, the seed pulses are simultaneously amplified and compressed with 90% recombining efficiency, generating 4.60-nJ pulse energy at 10 MHz and 6.13-nJ pulse energy at 70 MHz. The main advantage of the proposed laser system is that when pulse repetition rate is switched on-demand, the pulse duration can be kept almost unchanged only by simply adjusting the pump powers of the fiber amplifiers, instead of changing any optical device (the spacing between gratings or the length of fibers). [ABSTRACT FROM AUTHOR]

Tetrahedral chromium (Cr4+)-ions is the main contributor to gain in the broadband single-mode Cr-doped crystalline core fibers (SCCFs). We have demonstrated a gross gain of 8-dB on a length of 27-cm for a 300-nm broadband SCCF using a thermal annealing technique. Gross gain and fiber length are the highest yet reported for the SCCF. The record gain is mainly due to both the longer fiber by aligned molten-zone growth and Cr4+-ions enhancement by thermal annealing. The enhancement mechanism of the Cr4+-ions in the SCCF is experimentally verified by the means of superconducting quantum interference device (SQUID) and electron probe X-ray micro-analysis (EPMA). As a practical application of the proposed 8-dB gain in the SCCFs in fiber transmissions, a 25-Gb/s error-floor free link is successfully demonstrated. [ABSTRACT FROM AUTHOR]

An approach based on a comprehensive, unified model for fiber laser and amplifier systems is used in the design of high-power, high-energy systems. With guidance from the model, a chirally coupled-core Yb-doped fiber amplifier system is constructed. We use a two-stage system comprised of a 2.5-m long fiber preamplifier and a 3.0-m long fiber booster amplifier to illustrate the method. This compact system is capable of generating outputs of up to 1.2 mJ for 25-ns pulses at a repetition rate of 100 kHz, with average power of up to 121 W, and a slope efficiency of 82%.

The transient theory of stimulated Brillouin scattering (SBS) in optical fibers is used to investigate the effects of weak feedback of incident broadband laser fields exhibiting different spectral shapes. The proper boundary conditions are given for the forward- and backward-propagating electric fields inside the fiber with a finite amount of reflectivity at the fiber ends. When the linewidth of the laser approaches the Brillouin frequency shift, a weak reflection of the laser field overlaps with the SBS gain and can be amplified in the fiber, resulting in an apparent reduction in SBS threshold. It is shown that the spectral shape of the incident field and its overlap with the SBS gain spectrum affects the SBS signal. Numerical examples are presented for incident laser fields that (1) simulate a pulsed, multilongitudinal-mode laser and (2) are a constant amplitude with a sinusoidal phase modulation. For the pulsed field with mode spacing much less than the Brillouin linewidth, calculations show that an end reflection of 0.01% (-40 dB) of the laser field with a linewidth equal to 0.75 of the Brillouin frequency shift can reduce the effective SBS threshold by a factor of 1.5. When the incident laser field is constant amplitude and phase modulated with a sinusoid, calculations predict sharp and distinct enhancement of the reflected power when the ratio of the Brillouin frequency shift to the modulation frequency is an integer for end reflections as low as 0.001% (-50 dB). Experiments performed with a sinusoidal phase modulated signal confirm the theoretical predictions. These results have implications on the design of high-power fiber laser systems that utilize spectral broadening to suppress SBS. [ABSTRACT FROM AUTHOR]

We report a mid-infrared (MIR) spectral intensity-enhanced supercontinuum (SC) generation from a ZrF4-BaF 2-LaF3-AlF3-NaF fiber pumped by a nanosecond thulium-doped fiber laser. The spectral region covered from 1930 to 3500 nm with an average output power of 5.23 W. The average power ratio of the MIR SC for the wavelength > 2200 and > 2500 nm were 91.81% and 74.29% with respect to the total output power, respectively. The 1950-nm nanosecond seed pulses were generated from a long ring cavity based on semiconductor saturable absorber mode locker. The pulse repetition rate was 3.02 MHz with a tunable pulse duration of about 1.0 ns-3.9 ns. In addition, a 69.5-W short-wave infrared SC was obtained through a two-stage thulium-doped fiber amplifier with the spectrum extending from 1930 to 2500 nm. To the best of our knowledge, this is the first time such a spectral intensity-enhanced MIR SC directly pumped by a high-pulse-energy nanosecond thulium-doped fiber laser has been achieved.

In this paper, the properties of continuous-wave Yb-doped fiber amplifiers operating near 980 nm are investigated numerically. The effects of the seed light, active fiber configuration, pump power, pumping scheme and residual reflectivity of output port on performance of the 980-nm amplifier are revealed. It is found that besides the core-to-cladding ratio, both the seed power and pump power should large enough to realize the high output efficiency. It is also revealed that although the core-to-cladding ratio is important for the output power and efficiency, it only has a negligible effect on the optimum length of Yb-doped fiber. It is found that the optimum length is almost uniquely determined by the dopant concentration of Yb-doped fiber, and the plus of these two parameters are a constant almost unvaried with other parameters of Yb-doped fiber. With this result, a simple but effective method for estimating the optimum length of Yb-doped fiber is presented. Besides, it is also found that the residual reflectivity of output port should be no larger than 10 −5 to optimize the output property of the 980-nm fiber amplifier. We believe that the pertinent results can provide significant guidance for deeply understanding and designing the 980-nm Yb-doped fiber amplifier. [ABSTRACT FROM AUTHOR]

We report a 100-W-level, repetition rate and pulsewidth tunable, all-fiber, thulium-doped master oscillator power amplifier system. The master oscillator was actively Q-switched by a fiber-coupled acousto-optic modulator. The generated pulsewidth could be tuned from 1.16 μm to 46.3 ns by adjusting modulation repetition rate from 1 kHz to 2 MHz, and the pulse repetition rate (PRR) can also be tuned from 1 kHz to 2 MHz, correspondingly. The maximum output power could be boosted to > 110 W (slope efficiency of 55%) through three-stage thulium-doped fiber (TDF) amplifiers at PRR from 100 kHz to 2 MHz. The spectrum bandwidth was about 0.8 nm at the maximum output power without obvious indication of nonlinear effects. To the best of our knowledge, this is the first demonstration of > 110-W, all-fiber, actively Q-switched TDF laser, and it has realized the largest tuning ranges both with the pulsewidth and the PRR.

We present an improved design of a nonlinear amplifier loop mirror (NALM) mode-locked Er-fiber laser system with fiber amplifier and compressor contained in all polarization maintaining fiber architecture. Cooperated with a phase shifter in the NALM, the onset behaviors are investigated by optimizing the laser gain and net-cavity dispersion. As a result, the pulsation threshold is remarkably decreased and as short as 10-ms build-up time is achieved. In addition, with a single-stage bidirectional pumped Er-doped fiber amplifier, the self-started seed pulses of 2.6-mW average power, centered at 1576 nm, are nonlinearly compressed to 42-fs duration with single pulse energy of 2.2 nJ. Moreover, by appropriately optimizing the length of cavity gain and fiber in compressor, 28-fs duration with 3-nJ pulse energy is also achieved from a “morning call” pulse with 1567-nm central wavelength. [ABSTRACT FROM PUBLISHER]

The stimulated Raman scattering (SRS) in high-power continuous-wave (CW) fiber amplifier is investigated with the seeded Raman noise taken into account. It is revealed that the seeded Raman noise, as a portion of seed light, should enhance SRS in high-power CW fiber amplifier even when it is much weaker than the seeded signal light. The analytic formula for predicting the Raman threshold of bi-directional pumping fiber amplifier is given with the seeded Raman noise taken into account, and the predictions agree well with the numerical results. It is also revealed that the co-pumping amplifier suffer from the lowest Raman threshold, and the counter-pumping one is more beneficial to the SRS suppression than the bi-directional pumping one when the pump absorption of active fiber is larger than 8.7 dB. These results will provide guidance for the design of high-power CW fiber amplifier. [ABSTRACT FROM AUTHOR]

We report on investigation the potential of a 7 wt% (8.35 × 1020 Tm3+/cm3) doped silicate fibers for high-gain fiber amplifiers. Such a high ion concentration significantly reduces the required fiber length of high-power 2 μm fiber laser systems and allows the high-repetition rate operation in 2 μm mode-locked fiber lasers. To evaluate the feasibility of extracting high gain-per-unit-length from this gain medium, we measure several key material properties of the silicate fiber, including the absorption/emission cross-sections, upper-state lifetime, fiber background loss, and photodarkening resistance. We show through numerical simulations that a signal gain-per-unit-length of 3.78 dB/cm at 1950 nm can be achieved in a watt-level core-pumped Tm3+-doped silicate fiber amplifier. In addition, an 18-dB 2013-nm amplifier is demonstrated in a 50-cm 7 wt% Tm3+-doped double-clad silicate fiber. Finally, we experimentally confirm that the reported silicate host exhibits no observable photodarkening.

We demonstrate a coherent polarization beam combination of two-channel picosecond pulses based on a microcontroller-based phase-locking method. This simple and low-cost technique enables easy programming and automatic relocking of the relative phases between two fiber amplifiers. The total average power of the combined laser is 30.4 W, corresponding to a combining efficiency of 91%. The microcontroller-based phase-locking method provides a competitive solution for robust and programmable control of phases in the active coherent polarization beam combining system. [ABSTRACT FROM PUBLISHER]

We report efficient coherent beam combining of five kW-class fiber amplifiers seeded with pseudorandom phase-modulated light, using a 1 × 5 diffractive optical element (DOE). Each fiber amplifier channel was path length matched, actively polarized, and provided approximately 1.2 kW of near diffraction-limited output power (M2 < 1.1). A low-power sample of the combined beam after the DOE provided an error signal for active phase stabilization. After phase stabilization, the beams were coherently combined via the DOE. Notably, a total output power of ~5 kW was achieved with 82% combining efficiency and excellent beam quality (M2 < 1.1). The intrinsic DOE splitter loss was 5%. Additional losses due in part to nonideal polarization, amplified spontaneous emission content, uncorrelated wavefront errors, and fractional beam misalignments contributed to the efficiency reduction. Overall, multi-kW beam combining of pseudorandom-modulated fiber amplifiers was demonstrated for the first time. [ABSTRACT FROM AUTHOR]

We numerically and experimentally demonstrate the influences of gain fiber length, initial pulse center wavelength, duration, chirp, and temporal profile for efficient parabolic self-similar evolution, accounting for the strong gain shaping effect in fiber amplifiers under high gains. A spectrally resolved numerical model allowing for realistic descriptions of the variable wavelength-dependent gain is developed. The results predict the specific regions of the initial center wavelength and the duration for the self-similar evolution, which move toward a longer center wavelength and a broader duration range with the increasing gain fiber length. For short-length high-gain amplifiers, a proper negative initial chirp can not only resist the gain-shaping deformation and facilitate the self-similar evolution but also provide a broadband output. In addition, a triangular initial profile is found to be of minimum sensitivity to the detrimental gain shaping and optimum for efficient self-similar evolution. The experimental results confirm the numerical predictions. The studies of the fast parabolic pulse formation in short-length high-gain amplifiers presented here demonstrate the potential for performance scaling of femtosecond fiber amplifiers, few-cycle pulse sources, high-power frequency combs, and related applications. [ABSTRACT FROM PUBLISHER]

We report a mid-infrared (MIR) spectral intensity-enhanced supercontinuum (SC) generation from a ZrF4-BaF 2-LaF3-AlF3-NaF fiber pumped by a nanosecond thulium-doped fiber laser. The spectral region covered from 1930 to 3500 nm with an average output power of 5.23 W. The average power ratio of the MIR SC for the wavelength > 2200 and > 2500 nm were 91.81% and 74.29% with respect to the total output power, respectively. The 1950-nm nanosecond seed pulses were generated from a long ring cavity based on semiconductor saturable absorber mode locker. The pulse repetition rate was 3.02 MHz with a tunable pulse duration of about 1.0 ns–3.9 ns. In addition, a 69.5-W short-wave infrared SC was obtained through a two-stage thulium-doped fiber amplifier with the spectrum extending from 1930 to 2500 nm. To the best of our knowledge, this is the first time such a spectral intensity-enhanced MIR SC directly pumped by a high-pulse-energy nanosecond thulium-doped fiber laser has been achieved. [ABSTRACT FROM PUBLISHER]

We report the analysis of multimode pump absorption in double-clad rare-earth-doped fibers under realistic bending conditions. The finite-element beam-propagation method is used for the analysis. The fiber bending is approximated by the transformation of the refractive index profile of the fiber. Double-clad fibers of hexagonal, circular and stadium-like cross-sections are studied as examples. In addition, the double-clad waveguide structure of a two-fiber bundle is investigated. Simulations show that the bending effects cannot be neglected in double-clad fiber multimode pump propagation analysis and optimization. The reported rigorous numerical model opens new way to design double-clad fibers and to optimize the pump absorption efficiency. We show that high pump-absorption efficiency better than the ideal (ergodic) limit, can be achieved by simultaneous coiling and twisting of the double-clad fiber. [ABSTRACT FROM PUBLISHER]

In this paper, we suggest a novel model, based on input pump power and wave guidestructure, to calculate the Er-density profile in Erbium doped fiber amplifiers. This optimization is carried out for both SMF and DSF fibers. These optimized profiles have a Gaussian-like shape. Using the SMF optimized Er-density profile, high gain enhancement is obtained in a relatively short length of fibers. On the other hand, the DSF optimized profile shows small changes in the gain, which agrees with the previously report on other method of gain enhancement. This model is applicable to all active waveguides and any other dopant as well .

Within telecommunications, it is well established that optical preamplified receivers offer better sensitivity than linear-mode (LM) avalanche photodiodes (APDs) for direct-detection measurements. Long-range light detection and ranging (LiDAR), however, has predominantly made use of LM APDs for detection. This discrepancy arose due to the high coupling loss between the multimode return signals in LiDAR and single-mode optical preamplifiers. Recently, few-mode (FM) optical preamplifiers have been experimentally demonstrated, which solve this modal mismatch. We evaluate the applicability of FM preamplified receivers to long-range pulsed LiDAR in comparison with currently deployed LM APDs by considering the two receivers' noise, gain, and bandwidth characteristics. We show that FM preamplified receivers can offer an improvement in both receiver signal-to-noise ratio and bandwidth. [ABSTRACT FROM AUTHOR]