Backward Pumping Research Articles

The Simulation of Mode Control for a Photonic Lantern Adaptive Amplifier

A photonic lantern is a low-loss device that connects a single multimode waveguide to multiple single-mode waveguides and can enhance the beam quality of a fiber laser by adaptively controlling the optical parameters (amplitude, phase, polarization) at the input. In this work, we combined the gains and losses of individual modes within the fiber amplifier and introduced a mode content parameter at the amplifier’s output as an evaluation function to simulate mode control effects. Mode competition within the gain fiber can degrade the control effect of the fundamental mode and lead to it taking a longer time for the control to converge. Optimal parameters, such as the gain fiber length and pumping method, were identified to improve control effectiveness. Specifically, an optimal gain fiber length of 8 m was determined, and backward pumping was found to achieve higher pumping efficiency and better control results. The system demonstrated significant power amplification potential and could stabilize mode control under different pumping powers ranging from 50 W to 5 kW. In conclusion, our research demonstrates that an adaptive fiber amplifier based on a photonic lantern can achieve a stable, high-power, large-mode-field, near-fundamental-mode output from the gain fiber. Although mode competition within the gain fiber can degrade the control effect of the fundamental mode and cause the control to take a longer time to converge, these aspects should be further studied to improve the control’s effectiveness. These findings contribute to the development of advanced simulation models that guide high-power mode control experiments and deepen our understanding of physical processes in science and technology.

Optimization of nanosecond laser drilling strategy on CFRP hole quality

The drilling and cutting of carbon fiber-reinforced epoxy resin matrix composite (CFRP) structural parts is a prerequisite for one-off moulding and assembly connections. However, the thermal ablation effect observed during nanosecond laser hole-making of CFRP results in significant accuracy errors and thermal damage defects in the quality of the holes obtained from the process. To enhance the quality of laser-drilling CFRP holes, a spiral drilling path was employed in this work. The influence of diverse drilling methodologies, encompassing the trajectory of the laser beam, the spacing between scans, and the direction of the suction system's pumping, on the quality of the holes was examined. The impact of these techniques on the precision and integrity of the holes was assessed in terms of their dimensions, the quality factor, the width of the heat-affected zone (HAZ), and the prevalence of microscopic defects. The results demonstrated that when the drilling strategy involves moving the laser beam from the outside to the inside (Scheme I), a scanning spacing of 20 μm, and backward pumping, the optimal micro-hole accuracy and surface morphology, as well as minimal thermal damage defects can be achieved. This study provides a reference for further optimization of the nanosecond laser drilling process.

Near-unity asymmetric transmission of linearly polarized light with high Q-factor based on the coherent coupling of dual resonances in bilayer dielectric chiral metasurfaces

This report shows that near-unity asymmetric transmission (AT) of linearly polarized light with high Q-factor can be realized in bilayer dielectric chiral metasurfaces, the unit cell of which consists of silicon slab with rectangular holes on its both sides which are arranged perpendicularly to each other. The preceding work has revealed that the theoretical upper limit of the AT is given as the amplitude reflection coefficient of background system without nanostructure, when a single resonance mainly contributes. If double resonances with overlapping spectral shapes concurrently contribute to the AT, however, their coherent coupling enables breaking the above upper limit and near-unity AT can be obtained. As one of the contributing resonances in the metasurface is quasi-BIC, the resultant AT has also high Q-factor in addition to the near-unity value. Thus, the presented photonic system exhibits highly efficient asymmetric third-harmonic generation behavior, which has the efficiency ratio between forward and backward pumping in the orders from several hundreds to thousands. High-Q near-unity AT shown in this work will find practical applications for direction-dependent nonlinear frequency conversion, information processing, optical sensing, and polarization multiplexing.

Simultaneous realization of high gain and low DMG of four-mode EDFA under bidirectional hybrid-mode pump.

We theoretically and experimentally verify that, the bidirectional hybrid-mode pumping scheme can address the optimization problem of trade-off between high gain and low differential modal gain (DMG) of four-mode erbium-doped fiber amplifier (4M-EDFA), in comparison with traditional both forward and backward hybrid-mode pumping scheme. It is noticed that, when the total pump power is fixed, the bidirectional hybrid-mode pumping scheme can not only achieve higher gain, but also suppress DMG due to different overlap integrals for the forward and backward pumping schemes. The bidirectional hybrid-mode pumped 4M-EDFA is developed with the forward pumping at LP02 mode and the backward pumping at LP21 mode, under a pump power ratio of 30%:70%. Thus, we can achieve an average gain of up to 21.16 dB and a low DMG of 0.43 dB at 1550 nm, and an average gain of up to 20.64 dB with a DMG of less than 1.6 dB over the C-band. In particular, the bidirectional hybrid-mode pumping scheme allows us to tailor the gain characteristics of the few-mode erbium-doped fiber amplifiers (FM-EDFAs), by adjusting the power ratio between forward and backward pumps.

All-fiber amplification of highly chirped dissipative solitons around a 1.3 micron using stimulated Raman scattering

An all-fiber amplification of highly chirped dissipative solitons (DSs) by stimulated Raman scattering in a standard passive fiber with continuous-wave pumping is demonstrated for the first time to our knowledge. DSs with a duration of 20 ps and a repetition rate of 15.6 MHz at a wavelength of 1275 nm are amplified by a pump wave at 1205 nm. On–off Raman gain dependence on the amplifier length and pump power, as well as the pumping configuration, are experimentally studied. Uniform amplification has been achieved with a net gain of 10 dB resulting in a pulse energy of 13 nJ at backward pumping. Further Raman amplification is limited by emerging the next Stokes component. The output pulses are compressed by a factor of 50 down to a duration of 400 fs. As a result, the peak power reached the level of 9 kW. The demonstrated scheme can be a simple and robust alternative to the widely used parametric amplification of chirped pulses outside the dopant amplification band, and the resulting pulses can be used in multiphoton microscopy and other applications.

Performance Enhancement of Er–Yb: Co-doped Waveguide Amplifier Employing Backward Pumping in the Presence of Energy Transfer Upconversion

Performance Enhancement of Er–Yb: Co-doped Waveguide Amplifier Employing Backward Pumping in the Presence of Energy Transfer Upconversion

Experimental evaluation on OSNR and DMG performance of cascade FM-EDFA systems by equivalent input spectrum method

An all-fiber few-mode erbium-doped fiber amplifier (FM-EDFA) is constructed by the integration of few-mode isolator wavelength division multiplexers (FM-IWDMs), which act as the multiplexing or demultiplexing function between signal and pump light. The gain performance of the FM-EDFAs under forward, bidirectional and backward pumping cases is investigated by experiment. For LP01, LP11e and LP11o-mode signals, the minimum differential mode gain (DMG) of 0.14 dB can be achieved by forward LP11e and LP11o-mode pumping. We propose the equivalent input spectrum method to experimentally simulate the cascade FM-EDFA systems. For the three-mode amplification, the optical signal-to-noise ratio (OSNR) and overall DMG after 8-stage transmission are respectively 13.89 dB and 1.59 dB, which may theoretically support 1000 km-long transmission of 100 Gb/s DP-QPSK signal. A cascade amplification system composed of six-mode EDFA with the DMG of 1.35 dB is also considered to verify the feasibility of the equivalent input spectrum method.

30.7 Tb/s Transmission over 400 km using 40 WDM channels of DP-16 QAM and Raman amplification without inline repeaters

Forty wavelength-division multiplexing (WDM) channels of 96 Gbaud (768 Gb/s) were transmitted over 400 km of SMF using only a Raman amplifier, which was accomplished through numerical simulations. Each WDM channel comprised eight electrical subcarriers of single sideband Nyquist-pulse-shaping. 16 quadrature amplitude modulation and dual polarization modulation were applied to each subcarrier channel and so one data symbol carried eight bits. The roll-off factor in the optical subcarrier spectrum was investigated from 2% to 11%, and the change in roll-off was analyzed according to the subcarrier channel in a single WDM channel. The spectral linewidth of the light source in the transmitter was changed up to 10 MHz, and the effect on the performance was examined for a coherent transmission system with constellation analysis. The optical input power per WDM channel was optimized and the wavelength spacing between WDM channels were analyzed using nonlinear effects from adjacent WDM channels. A transmission experiment for total 30.7 Tb/s over 400 km was performed using the obtained optimal values. BER evaluation was achieved by DSP algorithms including Kramers–Kronig detection and Nyquist filtering. All 40 WDM channels exhibited sufficient transmission performance using only the Raman backward pumping of the four pump wavelengths. The proposed system can be applied to unrepeatered metro‐networks or data-center interconnects.

Experimental comparison study of the performance of reflector-based O-band BDFA under different pumping schemes

A bismuth-doped fiber amplifier (BDFA) operating in O-band with a double-pass structure in combination with a reflector is experimentally compared in detail under three pumping schemes. Different pump powers and signal input powers are used to investigate the effect on gain and noise figure (NF). The results show that not all wavelengths in the O-band but only wavelengths in the 60 nm range (1290 nm-1350 nm) can be improved with a double-pass, where the improvement is most prominent at 1320 nm; The gain improvement can only be obtained with a double-pass when the signal is input with low-power (approximately below −7 dBm). Among the three pumping schemes, the gain enhancement is most prominent with bidirectional pumping. A maximum gain of 31 dB was achieved at 1320 nm for −20 dBm input signal in double-pass with the pump power of 500 mW under bidirectional pumping scheme, which is an improvement of nearly 25.4% compared to single-pass. In addition, compared to forward and backward pumping also improved by 5.7% and 23.8%. This work demonstrates that the performance of O-band BDFAs can be effectively enhanced by reflectors, which have positive practical significance and reference value.

Pumping scheme for ultra-wideband WDM transmission using distributed Raman amplification

This paper describes the pumping schemes for ultra-wideband (UWB) WDM transmission using only distributed Raman amplification. By using numerical analysis based on the Gaussian noise model, we evaluate the amplification performances for all-Raman UWB-WDM transmission using higher-order pumping and/or forward pumping in addition to conventional 1st-order backward pumping. Moreover, we discuss the effect of forward Raman gain in 1st-order bidirectional pumping configuration, and clarify that the forward Raman gain should be carefully optimized taking account of both noise performance and nonlinear degradation.

Backward pumped distributed Raman amplifier: enhanced gain

The backward Raman amplifier (RA) can considered as one of the best solutions for optical communication, especially in Wavelength Division Multiplexing technology. They reduce the nonlinear effects, have low noise figure and a wide frequency range. The work in this paper aims to reduce the attenuation of optical signal due to its propagation optical fiber and increase both amplifier gain and output signal power. Two backward Raman models are proposed. Proposal one model consists of two cascaded RAs and the other (proposal two) consists of three cascaded RAs. Three backward pump power levels 200, 400, and 600 mW are used to simulate the models with the three types of fibers: single-mode fiber, Truewave, and Freelight, at an amplifier length of 100 km. Proposal two achieves a maximum gain of 31 dB at 600 mW pump power 600 mW using Truewave optical fiber, with 27.7 dBm maximum output signal power. This proposal is evaluated showing 11.15% gain enhancement and 200 mW saved power when compared to previously published work.

Bidirectional Cladding-Pumped Multicore EDFA and its Optimization for Energy Efficiency by Tuning Pump Condition

This work investigates the feasibility of improving the energy efficiency of a cladding-pumped multicore (MC) EDFA by utilizing bidirectional (BD) pumping. We combined a conventional forward-cladding-pumped MC-EDFA with backward cladding pumping and tuned the pump-gain characteristics and gain shape by controlling the optical pump power ratio of its forward and backward pumps. Since EDFAs have wavelength dependence, output power equalization by each wavelength channel is extremely important for amplifying the wavelength division multiplexing (WDM) signal. The standard method of current equalization is to adjust the attenuation by the output power of each wavelength channel. The main issue with the current standard equalization is the indispensable optical power loss caused the reduced optical amplification efficiency of the EDFA. Our previous work demonstrated that the optical power loss due to equalization can be reduced by controlling the optical power ratio of the forward and backward pumps. This leads to improved energy efficiency of the signal amplification and equalization of both the WDM and spatial division multiplexing (SDM). <p xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In this paper, we report our detailed investigation into the tuning amplification characteristics of the BD-MC-EDFA. Our main focus was the gain, noise figure (NF), and gain shape of a 7-core BD-MC-EDFA prototype when forward and backward optical pump power was changed. Under 8 W of total optical pump power, the percentage of 60% forward cladding optical pump power was found to be optimal for efficiency considering the minimum output optical power loss due to equalization. This condition translates into a 1.7% pump conversion efficiency (PCE) improvement, which corresponds to a 24% reduction of the total optical pump power compared to conventional MC-EDFA with only forward cladding pumping. Although the NF deteriorated by 1.0 dB in wavelength average in the 4.5-THz signal bandwidth in exchange for the efficiency improvement, degradation of the 34.2-GBaud polarization multiplexed 16 quadrature amplitude modulation (PM-16QAM) signal amplified by the BD-MC-EDFA operated under the optimized condition was negligible.

Fiber Raman Amplifier Based on Improved Whale Algorithm

In this paper, an improved whale algorithm to optimize pump power and wavelengths of second-order fiber Raman amplifier (FRA) has been proposed. Adopted by the strategies of adaptive gradients and reverse learning, the normal whale algorithm is greatly improved. An optimized FRA is designed based on the six-wavelength backward pump structure. Compared by optimizing both wavelength and power with optimizing only power, the gain flatness curves of the two algorithms, the optimal combination scheme, and the corresponding noise characteristics are got. A 6-pumped FRA has been designed in which the error of on-off gain is 0.08% and the ripple is only 0.18dB.

Femtosecond laser fabrication of large-core fiber Bragg gratings for high-power fiber oscillators

In this paper, a fs-laser phase mask inscription system based on a galvanometer scanning strategy is designed and set up for the fabrication of large-core fiber Bragg gratings (FBGs). Based on this setup, a homogeneous cross-sectional refractive index modulation can be achieved in the core of a large-mode-area fiber, and a pair of FBGs are fabricated in fibers with a core diameter of 30 µm. To investigate the performance of the fabricated FBGs, a high power all-fiber oscillator is built using a pure backward pumping structure. The FBGs work well, and the maximum output power of 7920 W is achieved with an optical–optical conversion efficiency of 77.3%. To the best of our knowledge, this is the highest power of all-fiber oscillators based on fs-written FBGs. This work provides a flexible, stable, and economic scanning strategy for large-core FBG inscription and exhibits excellent performance for high power fiber lasers.

High-energy 2.8-μm Ultrashort Pulses Generation in an Er:ZBLAN Fiber Amplifier

Compact, high-energy ultrashort pulsed fiber lasers in the 2.8 μm wavelength have attracted extensive interest in many scientific and industrial applications. Here, we demonstrated the generation of μJ-level ultrashort pulses from a quasi-all-fiberized Er:ZBLAN amplifier employing a 2.8-μm frequency-shifted soliton as a seed laser. With backward pumping, the pulse energy was enhanced to 0.282 μJ when the average output power was amplified to 1.023 W. Mechanisms for the generation and amplification of ultrashort pulses were theoretically revealed by numerical simulation, and further cascaded indium fluoride (InF3) fiber to obtain a 4.26-μm redshift soliton numerically. This compact fiber amplification system consisting of frequency-shifted-based seed pulses and fluoride fiber amplifiers will be of practical interest in applications.

Raman-amplification-assisted twisted light multiplexing transmission over large-core fiber

Twisted light-carrying orbital angular momentum (OAM) with a helical phase front and doughnut intensity profile has been widely investigated in fiber-optic communications. To facilitate long-distance OAM mode multiplexing transmission, an efficient OAM (de)multiplexer and a suitable inline OAM amplifier are highly desired. Here, we demonstrate three OAM modes and 22 wavelength channels multiplexing transmission with an OAM distributed Raman amplifier (DRA) over 110-km large-core fiber assisted by a homemade all-fiber OAM (de)multiplexer. To characterize the performance of the OAM DRA, two pump configurations including the first-order bidirectional pumping and second-order backward pumping are proposed and demonstrated. The obtained results indicate the successful implementation of the long-distance OAM mode multiplexing transmission assisted by the DRA and an all-fiber OAM (de)multiplexer with favorable performance. These demonstrations may open up new perspectives for ultra-long-haul capacity scaling fiber-optic communications employing OAM modes.

Flexible Raman Amplifier Optimization Based on Machine Learning-Aided Physical Stimulated Raman Scattering Model

The problem of Raman amplifier optimization is studied. A differentiable interpolation function is obtained for the Raman gain coefficient using machine learning (ML), which allows for the gradient descent optimization of forward-propagating Raman pumps. Both the frequency and power of an arbitrary number of pumps in a forward pumping configuration are then optimized for an arbitrary data channel load and span length. The forward propagation model is combined with an experimentally-trained ML model of a backward-pumping Raman amplifier to jointly optimize the frequency and power of the forward amplifier's pumps and the powers of the backward amplifier's pumps. The joint forward and backward amplifier optimization is demonstrated for an unrepeatered transmission of 250 km. A gain flatness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$&lt; $</tex-math></inline-formula> 1 dB over 4 THz is achieved. The optimized amplifiers are validated using a numerical simulator.

Generation of ultrafast laser with 11 MW peak power from a gain-managed nonlinear tapered fiber amplifier

A gain-managed nonlinear (GMN) tapered fiber amplifier with a monolithic configuration is experimentally demonstrated. Both the forward and backward pumping scheme were investigated, and the results indicate that the latter is advantageous in suppressing the unwanted stimulated Raman scattering and the resulting mode distortion while boosting the pulse energy with a shortest pulse width of sub-50 fs. Specifically, a maximum single pulse energy of 707 nJ and a peak power of 11 MW with a fundamental Gaussian distribution beam profile were respectively realized without temporally stretching the seed pulse. In addition, the backward pumped amplifier was further examined through changing its seeding power, and it is found that a relatively low input power is preferable for obtaining high performance laser pulses with shorter duration while without compromise on the output single pulse energy. It is believed that the combination of the GMN amplification and the large mode area tapered fiber provides a promising scheme for realizing high energy ultrafast lasers with a monolithic and concise structure.

The effect of fiber length and signal power on the flattened gain Raman amplifier

Raman Fiber Amplifiers offer an adjustable gain spectrum in terms of band width ,gain window and gain spectrum shape ,thus the composite Raman gain can be very flat over a wide band In this work an investigation on Raman gain with backward pumping is introduced using twelve pump lasers with optimum pump power to produce gain flattened for 94 nm band width from 1520 to 1614 nm that contains 100 equal spaced channels, with again ripple of 1.28dB which contain the C-and L-band. Over 25 Km span. Then the effect of fiber length on the gain flatting were introduced and the gain saturation have been analyzed.

First Demonstration and Comparison of 5 kW Monolithic Fiber Laser Oscillator Pumped by 915 nm and 981 nm LDs

Fiber laser performances including transverse mode instability (TMI), stimulated Raman scattering (SRS) and optical-to-optical efficiency are in connection with the pump wavelength. Here we studied the output characteristics of a 5-kW ytterbium-doped fiber laser oscillator pumped with two different pump sources, i.e., 915 nm and 981 nm laser diodes (LDs). The output characteristics of fiber laser oscillators pumped by 915 nm and 981 nm have been compared strictly and directly with the same structure in a high-power situation. Experimental results show that both pump wavelengths can scale the power up to more than 5 kW by suppressing the TMI effect. While in the case of pumping by the 981 nm LDs, the laser oscillator has an optical-to-optical efficiency of 87%, which is 13% higher than that of the 915 nm pumped scheme. In addition, due to the higher backward pumping ratio and lower total pump power, the laser oscillator has a better SRS suppression ratio when pumped at 981 nm. Thus, it reveals a great potential to balance the limitations of TMI and SRS for scaling up to an even higher output while pumping at 981 nm. All the devices of the oscillator are commercial, and it will be helpful for the commercialization of high-power fiber laser oscillators.