High-power Fiber Amplifiers Research Articles

Theoretical Study on Transverse Mode Instability in Raman Fiber Amplifiers Considering Mode Excitation.

Raman fiber lasers (RFLs), which are based on the stimulated Raman scattering effect, generate laser beams and offer distinct advantages such as flexibility in wavelength, low quantum defects, and absence from photo-darkening. However, as the power of the RFLs increases, heat generation emerges as a critical constraint on further power scaling. This escalating thermal load might result in transverse mode instability (TMI), thereby posing a significant challenge to the development of RFLs. In this work, a static model of the TMI effect in a high-power Raman fiber amplifier based on stimulated thermal Rayleigh scattering is established considering higher-order mode excitation. The variations of TMI threshold power with different seed power levels, fundamental mode purities, higher-order mode losses, and fiber lengths are investigated, while a TMI threshold formula with fundamental mode pumping is derived. This work will enrich the theoretical model of TMI and extend its application scope in TMI mitigation strategies, providing guidance for understanding and suppressing TMI in the RFLs.

Generation of vectorial vortex beams by a coherently combined laser array

With the superpositions of spin and orbital angular momentum, vectorial vortex beams (VVBs) have attracted great attention in recent years. Many approaches have been developed to generate such beams, but high-power output is not supported. Here, we report the coherent beam combining (CBC) for generating VVBs that are compatible with high-power fiber amplifiers. The laser array is composed of two discrete vortex arrays with left-handed and right-handed circular polarization states. The phase and polarization of each beamlet are manipulated simultaneously. Experimentally, the phase noise in the amplifiers is compensated by the all-fiber internal phase control feedback structure. Quarter-wave plates are employed as polarization state converters. When the system is in the closed loop, a distinct and stable VVB can be obtained in the far-field. This work could provide a significant reference for the future implementation of high-power structured beams.

Modeling of High-Power Graded-Index Fiber Amplifiers

Graded-index fibers have been used in recent years to make high-power fiber lasers and amplifiers. Such fibers exhibit self-imaging, a phenomenon in which any optical beam periodically reproduces its original shape in undoped fibers (no gain). In this work, we employed analytic and numerical techniques to study how self-imaging affects the evolution of a signal beam inside a nonlinear graded-index fiber amplifier, doped with a rare-earth element and pumped optically to provide gain all along its length. We also exploited the variational technique to reduce the computing time and to provide physical insights into the amplification process. We compared the variational and fully numerical results for the two pumping schemes (clad pumping and edge pumping) commonly used for high-power fiber amplifiers and show that the variational results are reliable in most cases of practical interest. The stability of the signal beam undergoing amplification is examined numerically by launching a noisy Gaussian beam at the input end of the amplifier. Our results show that the quality of the amplified beam should improve in the case of edge pumping when a narrower pump beam provides an optical gain that varies considerably in the radial direction of the fiber. Such an improvement does not occur for the clad pumping scheme, for which the use of a relatively wide pump beam results in a nearly uniform gain all along the fiber.

Super broadband light source: a promising master oscillator source for high brightness and high-power fiber amplifiers.

Stimulated Raman scattering (SRS) and transverse mode instability (TMI) effects are crucial limitation factors for further power scaling of high-power fiber amplifiers with near-diffraction-limited beam quality. It is an important research direction to carry out laser system optimization from the perspective of seed construction. In this work, we experimentally investigated the impact of utilizing different seed lasers on the SRS and TMI characteristics in high power ytterbium-doped fiber amplifiers. Both the phase modulated single frequency fiber laser (PMSFL), fiber oscillator laser (FOL), and superfluorescent fiber source (SFS) as a typical type of temporally stable broadband light source (SBLS) are employed as seed lasers. The experimental results indicated that the SRS intensity in the fiber amplifier is influenced by the properties of the seed laser source. Notably, the SRS threshold of the SBLS could be comparable to that of a low-noise PMSFL source. There is a significant increase in TMI threshold when employing SBLS as a seed laser in high-power fiber amplifiers, which is 3.86times that of the injected seed laser with a 3dB narrow linewidth of 0.74nm. Considering the combined threshold characteristics of SRS and TMI, it can be found that SBLS is a promising master oscillator source for high brightness and high-power fiber amplifiers. Our work could provide a good reference for the selection of a master oscillator source, and we believe that it is an important direction to go beyond the power limitation of high-power fiber lasers by constructing a light source with broader spectral linewidth and temporal stability.

5 kW power-level 1050 nm narrow-linewidth fiber amplifier enabled by biconical-tapered active fiber.

Effective wavelength extension is vital in the applications of high-power narrow-linewidth fiber lasers. In this work, we demonstrate a 5-kW power-level narrow-linewidth fiber amplifier at 1050 nm utilizing a homemade biconical-tapered Yb-doped fiber (BT-YDF). Up to ∼4.96 kW fiber laser is achieved with a 3 dB linewidth of ∼0.54 nm and a beam quality factor of Mx 2 = 1.46, My 2 = 1.6. The experimental comparisons reveal that BT-YDF has the advantages of improving a stimulated Raman scattering threshold and balancing transverse mode instability suppression in the fiber amplifier. This work could provide a good reference for extending the operating wavelength of high-power fiber amplifiers.

Evaluation of the impact of weak end feedback on the SBS threshold in high-power narrow-linewidth fiber amplifiers

In this work, we quantitatively investigate the SBS threshold in high-power narrow-linewidth fiber amplifiers seeded with phase-modulated single-frequency lasers in presence of weak end feedback. The impacts of the end feedback and spectral linewidths on the SBS threshold are demonstrated in detail through comparative experiments and numerical simulations, respectively. In the experiment, we have pointed out a practical method to estimate the end reflectivity in high-power fiber amplifiers. Based on this estimation, the SBS threshold characters of the high-power narrow-linewidth fiber amplifiers with different end reflectivity and seed linewidths are investigated. By reducing the end reflectivity, a 2.85 times SBS threshold enhancement has been achieved at the most susceptible linewidth (16.8 GHz). Furthermore, we propose a spectral evolution model to investigate the SBS threshold in high-power narrow-linewidth fiber amplifiers, which is even capable for calculating SBS thresholds of the systems with tens of GHz linewidth while weak end reflection is considered. The simulation results demonstrate that end reflection will obviously affect the SBS threshold when the linewidth of the seed laser is broadened beyond 5 GHz, especially for the spectral linewidth of seed lasers nearing the Brillouin frequency shift. Besides, when the end reflectivity is set to be stronger than -65 dB, the SBS threshold performs a tendency to decline and then rise with the growth of seed linewidth. The experiment and simulation results provide a new optimization sight for the SBS effect suppression in high-power narrow-linewidth fiber amplifiers.

E-band telecom-compatible 40 dB gain high-power bismuth-doped fiber amplifier with record power conversion efficiency

Multi-band transmission is one of the key practical solutions to cope with the continuously growing demand on the capacity of optical communication networks without changing the huge existing fiber base. However, ultra-broadband communication requires the development of novel power efficient optical amplifiers operating beyond C- and L-bands, and this is a major research and technical challenge comparable to the introduction of the seminal erbium-doped fiber amplifiers that dramatically changed the optical communication sector. There are several types of optical fibers operating beyond C- and L-bands that can be used for the development of such amplifiers, specifically the fibers doped with neodymium, praseodymium, thulium, and bismuth. However, among these, Bi-doped fibers are of special interest as the most promising amplification medium because, unlike the others, different Bi-associated active centers allow amplification in an enormous band of overall width of 700 nm (1100–1800 nm). Such spectral coverage can be obtained by using different host materials, such as aluminosilicate, phosphosilicate, silica, and germanosilicate glasses. Here, we report a novel Bi-doped fiber amplifier with record characteristics for E-band amplification, including the highest power conversion efficiency among telecom-compatible E-band amplifiers reported to date. This bismuth-doped fiber amplifier (BDFA) features a maximum gain of 39.8 dB and a minimal noise figure of 4.6 dB enabled by 173 m Bi-doped fiber length. The maximum achieved power conversion efficiency of 38% is higher than that of L-band Er-doped fiber amplifiers. This performance demonstrates the high potential of BDFA for becoming the amplifier of choice in modern multi-band optical communication networks.

Theoretical study of filtered and amplified PRBS phase modulation for SBS suppression in a 2.5 kW, 10 GHz monolithic fiber amplifier

We report a theoretical and experimental study on stimulated Brillouin scattering (SBS) suppression in a monolithic fiber amplifier with filtered and amplified pseudo-random binary sequence (PRBS) phase modulation. Theoretically, we use a time-dependent three-wave coupled nonlinear system considering both active fiber and passive fiber to describe the acoustic phonon, laser, and Stokes characteristics in a fiber amplifier. The SBS threshold power after filtered PRBS phase modulation is numerically evaluated to obtain the optimal parameters, and the time-averaged distributions of the counter-pump power, laser power, and Stokes power at different positions along the fiber length of the fiber system are simulated. Also, we established a four-stage fiber amplifier system to verify our theory. The configuration of the fiber amplifier system includes a filtered and amplified PRBS phase-modulated single-frequency fiber laser, a three-stage pre-amplifier, and a counter-pumping main stage, subsequently. 2.5 kW output power with an FWHM linewidth of 9.63 GHz is accomplished by a domestic ytterbium-doped double-clad fiber with core/cladding diameters of 20.2/400 µm. The reflectivity of the main stage is 0.049‰ at the maximum output power, which indicates the proposed architecture is under the SBS threshold. The experiments verify the accuracy of the theoretical model, which provides a reliable reference for evaluating the SBS suppression capability of the high-power narrow-linewidth fiber amplifier phase modulated by the filtered and amplified PRBS signal.

Raman Oscillation Induced by Weak Internal Reflections of Fiber Devices in High-Power Fiber Amplifiers

Raman Oscillation Induced by Weak Internal Reflections of Fiber Devices in High-Power Fiber Amplifiers

Design of three-level Nd-doped laser fiber based on anti-resonant structure

900-nm Nd-doped fiber laser can find widespread applications including biomedical diagnosis, laser detection, and spectral analysis. The four-level gain competition of Nd<sup>3+</sup> around 1060 nm severely constrains the laser power scaling of the 900-nm three-level Nd-doped fiber laser. In this work, we propose a large-mode-area Nd-doped double-layer solid-core anti-resonant fiber with a core diameter of 27 μm for generating a high-power 900-nm laser based on the resonant and anti-resonant conditions of anti-resonant fiber. The transmission properties and mode profiles of the designed fiber are analyzed theoretically by using the full-vector finite-element method combined with an optimized mesh size. By introducing the double-layer anti-resonant elements into the active fiber and optimizing the fiber structure parameters and refractive index distribution, the high-order modes are well coupled with cladding modes. Finally, the designed fiber exhibits a confinement loss below 0.1 dB/m for fundamental mode and the confinement losses of all high-order modes are greater than 10 dB/m in 880–913 nm band. More importantly, around 1060 nm, the confinement losses of all modes can reach up to 100 dB/m, which enables the designed Nd-doped fiber to effectively suppress parasitic lasing and even amplified spontaneous emission. The Nd-doped solid-core anti-resonant fiber proposed in this study shows broad application prospects in the fields of 900-nm high-power fiber laser and amplifier. The developed chemical vapor deposition process combined with stack-and-draw technology can be adopted for the fabrication of the designed fiber. In order to ensure the optical performance of the anti-resonant fiber, it is necessary to accurately control the thickness of all anti-resonant tubes, the glass composition of the active core and background area in actual fabrication.

Brightness enhancement on a narrow-linewidth fiber Bragg grating-based master oscillator power amplification fiber laser

Abstract In this paper, we have experimentally demonstrated a high-power and high-brightness narrow-linewidth fiber amplifier seeded by an optimized fiber oscillator. In order to improve the temporal stability, the fiber oscillator consists of a composite fiber Bragg grating-based cavity with an external feedback structure. By optimizing the forward and backward pumping ratio, the nonlinear effects and stimulated Raman scattering-induced mode distortion of the fiber amplifier are suppressed comprehensively, accompanied with the simultaneous improvement of beam quality and output power. The laser brightness is enhanced further by raising the threshold of transverse mode instability by approximately 1.0 kW by coiling the gain fiber with a novel curvature shape. Finally, a 6 kW narrow-linewidth laser is achieved with beam quality (M2) of approximately 1.4. The laser brightness doubled compared to the results before optimization. To the best of our knowledge, it is the highest brightness narrow-linewidth fiber laser based on a one-stage master oscillator power amplification structure.

Suppression effect of auxiliary laser on stimulated Raman scattering effect of high-power Yb-doped fiber laser amplifier

<sec>A novel technique to suppress the stimulated Raman scattering (SRS) effect in high-power ytterbium-doped fiber amplifier is proposed and theoretically investigated by introducing an auxiliary laser to manipulate the gain distribution in the amplifier.</sec><sec>By injecting an auxiliary laser with shorter wavelength than the signal into the amplifier, the auxiliary laser, owing to its larger stimulated emission cross-section, initially extracts a significant portion of the laser gain. At this point, the gain of the longer-wavelength signal laser is suppressed to a certain extent. As the pump power is depleted in the rear segment of the gain fiber, the amplified auxiliary laser, which has larger absorption cross-section than the signal, is gradually absorbed by the active fiber and transfers its power to the signal laser. This process enhances the gain of the long-wavelength signal laser, enabling it to be rapidly amplified at the end of the amplifier. Compared with the amplification of the singular signal laser, the introduction of an extra auxiliary laser shifts the high-gain region of the signal laser to the rear portion of the amplifier, thereby reducing the effective length and alleviating the interaction strength between the signal laser and Stokes wave, in order to obtain a higher SRS threshold.</sec><sec>The SRS threshold of a 20 μm/400 μm fiber amplifier is investigated by using numerical simulation under different wavelengths of the auxiliary laser and different power ratios of the signal laser to auxiliary laser. The results indicate that incorporating an auxiliary laser with an appropriate wavelength and power level can significantly reduce the interaction strength between the signal and Stokes wave, thereby enhancing the SRS threshold of the amplifier efficiently. Specifically, in a 1080 nm fiber amplifier utilizing a 20 μm/400 μm ytterbium-doped large mode area fiber, if the total power of the 1080 nm signal and 1040 nm auxiliary laser is set to 200 W, while with a power ratio of 1:25, the SRS threshold increasing from 3.14 kW (singular signal laser) to 8.42 kW can be anticipated. Moreover, based on the auxiliary laser amplification technique that suppresses the SRS effect, the output power enhancement of fiber lasers with the structure of master oscillator power amplifier (MOPA) is also analyzed. This technical solution is relatively straightforward to implement and can be seamlessly integrated with other techniques aimed at reducing the SRS effect, which is promising to promote further power scaling of all-fiber amplifier.</sec>

Self-Phase Modulation-Induced Instability of High-Power Narrow-Linewidth Fiber Amplifiers

In this study, we investigated the effect of self-phase modulation (SPM)-induced modulation instability (MI) on the spectral purity of high-power narrow-linewidth fiber amplifiers and established a spectral evolution model for SPM-induced MI in those amplifiers. The spectral evolution process of MI under different laser powers was simulated and analyzed. The results show that, at low power (100 W), SPM can cause a dynamic change in the spectral sideband secondary peak and the spectral wingspan. An increase in laser power led to the cascade effect of MI, forming a zigzag secondary sideband with a larger spectral width and causing the spectral main peak and spectral broadening to split. Experiments based on the fiber Bragg grating (FBG) of oscillating seed sources were carried out on high-power narrow-linewidth laser amplifiers, and the above spectral evolution phenomenon was observed. The experimental results indicate that the spectral evolution model based on SPM-induced MI can effectively explain the dynamic change in the spectral secondary peak, spectral wingspan and zigzag broadening phenomenon in the power amplification process of narrow-linewidth lasers.

Nonlinear spectral compression in high-power narrow-linewidth polarization maintaining fiber amplifiers for SBS suppression.

In this paper, we propose a method for narrowing the spectrum in high-power narrow-linewidth polarization-maintaining (PM) fiber amplifiers and investigate its potential for suppressing the stimulated Brillouin scattering (SBS). In this method, in addition to common phase modulation to suppress SBS, precisely designed amplitude modulation is induced to generate self-phase modulation in a high-power PM fiber amplifier. In this co-modulation way, the spectrum can be gradually compressed along the fiber. Compared to phase modulation alone or fiber-Bragg-gratings (FBGs) based narrow-linewidth fiber oscillator schemes, in which the spectrum remains the same or broadens, this scheme can achieve a higher SBS threshold for the same output spectral linewidth. Experiments on a ∼ 3 kW peak power quasi-continuous wave (QCW) fiber amplifier show that the co-modulation scheme can compress the spectrum from 0.25 nm to 0.084 nm as output peak power increases from 13 W to 3.2 kW and enhances the SBS threshold by ∼1.7 times compared to traditional FBGs-based fiber oscillator schemes, and by ∼1.4 times compared to common phase modulation schemes. This co-modulation scheme has the potential for mitigating SBS in high-power fiber amplifiers.

Modeling of a secondharmonic spectrum in passive phase demodulation.

Spectral compression by passive phase demodulation provides an effective way to obtain a high-power single-frequency second harmonic (SH) laser. In this method, a single-frequency laser is broadened by (0, π) binary phase modulation for stimulated Brillouin scattering suppression in a high-power fiber amplifier and compressed to single frequency after frequency doubling. The effectiveness of compressing is determined by the properties of the phase modulation system, including the modulation depth, frequency response of modulation system, and modulation signal noise. A numerical model is developed to simulate the influence of these factors on the SH spectrum. The simulation results reproduce the experimental observation well, including the reduction of the compression rate at higher-frequency phase modulation, emergence of spectral sidebands and pedestal.

Selection principle of seed power in high-power narrow linewidth fiber amplifier seeded by a FBGs-based fiber oscillator.

Here, we have experimentally demonstrated the selection principle of the seed power in a narrow linewidth fiber amplifier seeded by fiber oscillator based on a pair of fiber Bragg gratings. During the study on the selection of seed power, the spectral instability of the amplifier is found when a low power seed with bad temporal characteristics is amplified. This phenomenon is thoroughly analyzed from seed itself and the influence of the amplifier. Increasing the seed power or isolating the backward light of amplifier could effectively eliminate the spectral instability. Based on this point, we optimize the seed power and utilize a band pass filter circulator to isolate the backward light and filter the Raman noise. Finally, a 4.2 kW narrow linewidth output power is achieved with signal to noise ratio of 35 dB, which has exceeded the value under the highest output power reported in this type of narrow linewidth fiber amplifiers. This work provides a solution for high power and high signal to noise ratio narrow-linewidth fiber amplifiers seeded by FBGs-based fiber oscillator.

Fiber-Based Techniques to Suppress Stimulated Brillouin Scattering

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.

Polarization-maintaining large-mode-area solid-core anti-resonant fiber for high-power fiber lasers

A polarization-maintaining large-mode-area solid-core anti-resonant fiber is investigated in this work with double-layer anti-resonant elements for improved capability on fundamental mode guidance and higher order mode suppression, and structural and material asymmetry on inner-layer cladding tubes for high birefringence. Mode coupling between fundamental core mode in one polarization direction and mode on two high-index inner cladding tubes is used to enhance the fiber birefringence under large core diameter. Eventually, an optimized design of polarization-maintaining large-mode-area solid-core anti-resonant fiber with the currently largest mode field area of 3026 μm2 and birefringence of 1.20 × 10−5 can be anticipated with the confinement losses for all the higher order modes exceeding 10 dB/m. Moreover, confinement loss below 0.1 dB/m can still be obtained for the fundamental core mode in both polarization direction even under the mode-coupling effect. Active polarization-maintaining anti-resonant fiber is analyzed and the refractive index of the central rare-earth-doped region is optimized for the first time for both well guided signal and pump laser. Large pump absorption coefficient can thus be anticipated from the multimode laser core-pumped active large-mode-area fiber, which facilitates short active fiber length needed for sufficient laser gain. The fiber proposed here offers great potential for the development of high-performance fiber laser sources including high-power single-frequency fiber amplifiers and high-peak-power ultrashort pulsed fiber amplifiers.

Over 10-kW fiber laser spectral beam combination based on dichromatic mirrors

Owing to the advantages of good beam quality, high conversion efficiency, convenient thermal management and compact structure, high power fiber lasers have been widely desired in industrial processing. So far, the output power has been strictly limited by the nonlinear effects and transverse mode instability. In order to break through the power limitation, here we experimentally demonstrate a power boosting technology called spectral beam combination based on dichromatic mirrors. Firstly, high power fiber amplifiers with different central wavelengths are established for the spectral combination. Secondly, utilizing a dual-beam combined system and two homemade high power fiber amplifiers, an output power of 10 kW is achieved, with a remarkable combination efficiency of 98.3% and a beam quality of <i>M</i> <sup>2</sup>～1.33. Secondly, using a three-beam combined system, an output power of 13.5 kW is obtained with a combination efficiency of 96.8% and beam quality of <i>M</i> <sup>2</sup>～1.61. By increasing the number of input beams and their output power as well, we believe that a higher output power can be achieved based on dichromatic mirror spectral beam combination.

Review of High Average Power and High Beam Quality LD-Pumped Ytterbium-Doped Fiber Laser Oscillators and Amplifiers

Review of High Average Power and High Beam Quality LD-Pumped Ytterbium-Doped Fiber Laser Oscillators and Amplifiers