Backward Pumping Scheme Research Articles

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.

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.

Random Laser Based on Ytterbium-Doped Fiber with a Bragg Grating Array as the Source of Continuous-Wave 976 nm Wavelength Radiation

A random narrow-linewidth lasing at a wavelength of 976 nm was obtained in an ytterbium-doped germanophosphosilicate fiber with an array of weakly reflecting fiber Bragg gratings (FBGs). A random laser cavity was formed by implementing the standard phase mask method of FBG inscription directly during the fiber drawing process. The UV radiation pulses of a KrF excimer laser (248 nm wavelength) synchronized with the fiber drawing speed were used to fabricate the in-fiber array of hundreds of similar FBGs. The developed laser’s slope efficiency in the backward-pumping scheme was measured as high as 33%. The stable continuous-wave operation mode of the laser was detected. The magnitude of the laser power fluctuations depends linearly on the cavity length. The random laser cavity modified with a single highlyreflected (90%) FBG demonstrates significantly better power stability and higher slope efficiency than the same one without an FBG.

Average-power (4.13 W) 59 fs mid-infrared pulses from a fluoride fiber laser system

We report a high-average-power mid-infrared ultrafast laser system consisting of a fluoride fiber mode-locked oscillator and a nonlinear amplifier. A backward pumping scheme was used in the amplifier to simultaneously realize pulse amplification and self-compression. The input signal polarization was demonstrated to play an important role in the self-compression process. Through the optimization of input polarization, a 4.13 W average-power 59 fs pulse at 2.8 µm was achieved, with an estimated pulse energy of 42.2 nJ and a peak power of 715 kW. To the best of our knowledge, this is the highest average-power pulse with sub-100-fs duration generated from a mid-infrared fiber laser system to date.

Experimental study of spectral broadening in kW-level narrow linewidth FBG-based fiber amplifiers under different pumping configurations

In this work, experimental investigations were presented on the variation of spectral broadening and optical efficiency in a kW-level narrow linewidth MOPA fiber laser under different pumping configurations. The results revealed that the backward-pumping scheme could reduce the spectral broadening and enhance the optical efficiency compared with the forward-pumping scheme. For a detailed study, the bidirectional pumping setup was employed. It could be seen that by removing the pump diodes from the forward direction and adding them to the backward direction one by one, the spectral bandwidth and the optical efficiency gradually decreased and increased, respectively. By coiling the gain fiber on a plate with a bending diameter of 9-13 cm, a multi-peak and unstable spectrum was observed. When this fiber was coiled on a cylinder with a fixed diameter of 10.5 cm, the spectral instability was removed and a single-peak spectrum was observed. For forward pumping only, the spectral bandwidth, the optical efficiency, and the maximum output power were measured to be 0.121 nm, 78.92%, and 822 W, respectively, while for backward pumping only, these values were measured to be 0.084 nm, 82.65%, and 860 W.

Analysis of gain and noise characteristics of O-band Bi-doped fiber amplifier under different pumping schemes

In this paper, the gain and noise characteristics of bismuth-doped fiber amplifiers in O-band (1260–1360 nm) under forward, backward and bidirectional pumping are comprehensively compared and optimized using numerical models. The validity of the model is verified. The results show that (1)the signal power is changed within a limited range does not have an effect on the noise figure. However, the noise figure will be worse while the power continues to increase, and the magnitude of the change is most pronounced under the backward pumping scheme. With an increase in signal power from − 25 dBm to − 10 dBm, the noise figure is relatively flat under all pumping schemes. While increasing from − 10–0 dBm, the noise figure under backward pumping increased by 0.9 dB; (2)Only if pump power is below a certain threshold, the gain obtained under bidirectional scheme is significantly higher than under forward and backward.With larger pump power, the gain obtained under all pumping schemes is almost the same. With pump power less than 1.2 W, the gain under bidirectional scheme is 0.9 dB higher than that under forward and backward schemes. With more than 1.2 W, the gain difference between bidirectional and forward and backward is only 0.2 dB. The analysis provides guidance for the selection and optimization of pumping structures in BDFA, thus helping to improve the performance of O-band transmission systems.

Raman Amplification Optimization in Short-Reach High Data Rate Coherent Transmission Systems.

We compared the transmission performances of 600 Gbit/s PM-64QAM WDM signals over 75.6 km of single-mode fibre (SMF) using EDFA, discrete Raman, hybrid Raman/EDFA, and first-order or second-order (dual-order) distributed Raman amplifiers. Our numerical simulations and experimental results showed that the simple first-order distributed Raman scheme with backward pumping delivered the best transmission performance among all the schemes, notably better than the expected second-order Raman scheme, which gave a flatter signal power variation along the fibre. Using the first-order backward Raman pumping scheme demonstrated a better balance between the ASE noise and fibre nonlinearity and gave an optimal transmission performance over a relatively short distance of 75 km SMF.

Photosensitive Yb-Doped Germanophosphosilicate Artificial Rayleigh Fibers as a Base of Random Lasers

Asingle-mode Yb-doped germanophosphosilicate fiber with ultra-low optical losses (less than 2 dB/km) was fabricated by means of the MCVD method utilizing an all-gas-phase deposition technique developed “in house”. The absorption and luminescent spectral properties of the fiber were thoroughly studied. The photosensitivity of the pristine (non-hydrogenated) fiber to 248 nm-laser radiation was confirmed by means of fiber Bragg grating (FBG) inscription directly during the drawing process. The random single-frequency lasing at the 1060-nm-wavelength obtained in the 21-m-long fiber with an array of weak FBG was reported. The developed laser slope efficiency in the backward-pumping scheme was measured as high as 32%.

Effects of gain distribution on self-similar amplification of picosecond pulses

The effects of gain distribution on self-similar amplification of picosecond pulses in a Yb-doped fiber laser system were studied by numerical simulation. Ultrashort laser pulses amplified in self-similar amplification theoretical model was established to analyze the impact of pump configuration, fiber length and total gain coefficient on the self-similar amplification evolution and laser output performance. Detailed numerical simulation reveals that the best self-similar amplification result can be found for different cases, where high-quality self-similar pulses with ~100 fs transform-limited pulse duration are obtained. It is demonstrated that the self-similar evolution speed in a forward-pumping scheme is faster than that in a backward-pumping scheme for a fixed seed pulse. Furthermore, the results indicate that for the self-similar amplifier with different fiber lengths and gain coefficients, the forward-pumping scheme shows better evolution results in lower seed energy and longer wavelength range, while the backward-pumping scheme is more suitable for the higher seed energy and shorter wavelength range.

Multi-wavelength Brillouin Raman random fiber laser with double Brillouin frequency shift

In this paper, a linear full-open-cavity Raman random fiber laser generating multi-wavelength Brillouin lines with double Brillouin frequency shift is proposed. The odd-order and even-order Stokes lights are respectively output from different end. Our contrastive experiments with forward and backward Brillouin pumping schemes reveal that the signal-to-noise ratio (SNR) of the laser output by backward pump is 24.5 dB, and the SNR of the laser output by forward pump is 19.5 dB under same Raman pump power of 831.8 mW. The SNR of backward-pumped laser output is higher and the wavelength number is more than that of forward-pumped case. But the Raman pump threshold for Brillouin Stokes light generation is higher for backward-pumped laser, which results in no stable Brillouin light generation at low Raman pump power. It is believed that the results not only can provide different ideas for the further optimization of random fiber laser structure, but also can help to further expand the application range of multi-wavelength Brillouin Raman lasers in optical communication systems and optical sensing fields.

Theoretical Analysis of Heat Distribution in Raman Fiber Lasers and Amplifiers Employing Pure Passive Fiber

In this paper, we study the thermal dissipation of Raman fiber laser and amplifier utilizing pure passive fiber as gain medium for the first time. Take into account the convective and conductive heat transferring process in the fiber, we consider the heat transferring and Raman conversion model based on the thermal conduction equations and the Raman coupled equations in the fiber. With the simulation of power distribution, the thermal profiles of Raman fiber laser are analyzed, including the transverse and longitudinal distributions of the heat load density, temperature, and thermal-induced refractive index change in the fiber. Meanwhile, the heat dissipation in multimode graded-index fiber and step-index fiber are also calculated and compared. The results show that the amplifier is superior to the resonator in heat alleviation, and the forward pumping scheme is also better to ease the thermal load than the backward and bidirectional pumping schemes, which have consult meaning for the suppression of thermal effects and the power scaling in Raman fiber lasers.

Wideband and flat gain series erbium doped fiber amplifier using hybrid active fiber with backward pumping distribution technique

Abstract A modern wideband and flat gain erbium-doped fiber amplifier (EDFA) is suggested and accomplished, by employing a recently fabricated hafnia-bismuth-erbium doped fiber (HB-EDF) and zirconia-erbium doped fiber (Zr-EDF) as a hybrid active fiber. The performance of the proposed EDFA is examined in both forward and backward pumping schemes, using 0.5 m long HB-EDF and 4 m long Zr-EDF in series structure to fulfill a wideband amplification that cover C- and L-telecommunication bands, respectively. At the optimum laser diodes powers, the backward pumping amplifier attained a gain flatness of 14.6 dB with the maximum gain variation of ±1.8 dB, throughout a wide bandwidth of 70 nm, that is from 1530 nm to 1600 nm. The noise figure fluctuates from 4.3 dB to 7.9 dB within the gain flatness band. Using the backward pumping distribution technique, the proposed amplifier demonstrates not only an efficient performance, but also a cost reduction since only one laser diode is utilized to pump two stages.

Dual-wavelength thulium/holmium-doped fiber laser generation in 2 μm region with high side-mode suppression ratio

We demonstrate a dual-wavelength laser with emission in the 2 μm wavelength region using a thulium/holmium co-doped fiber and a non-adiabatic tapered fiber as a comb filter. The configuration employed a backward pumping scheme and dual-wavelength laser was generated at 2023.53 and 2032.82 nm. The side-mode suppression ratio (SMSR) is more than 44 dB with overall good stability performance. The proposed configuration presents a simple yet practical scheme of generating 2 μm dual-wavelength laser with good SMSR. This investigation may offer a more efficient yet eye safe dual-wavelength related applications for end users.

2.19 kW narrow linewidth FBG-based MOPA configuration fiber laser.

In this paper, we demonstrated a monolithic fiber-Bragg-grating-based (FBG-based) master oscillator power amplification configuration fiber laser with a narrow linewidth at high-power level. Several approaches were implemented to reduce the seed laser linewidth and the magnification of spectrum broadening in order to achieve a narrow output linewidth. The narrow seed laser linewidth was obtained by restricting the reflection bandwidth of the FBG. To reduce the magnification of spectrum broadening, a backward pumping scheme was employed in the amplifier stage after its capacity to suppress laser spectrum broadening was preliminarily investigated experimentally. Further, by intentionally shortening the length of the active fiber in the amplifier and sharing the backward pumping power with the oscillator, the spectrum broadening was further inhibited without sacrificing optical efficiency. A maximum output power of 2.19 kW was achieved with a 3 dB spectrum bandwidth of only 86.5 pm. The beam quality at the maximum power was measured to be M2~1.46. No sign of transverse mode instability was shown during the experiments.

Performance optimization of hybrid optical amplifier for dense wavelength division multiplexed system

Today’s high capacity dense wavelength division multiplexed (DWDM) systems required the flattened gain spectra with large gain bandwidth. This required properly broadening the gain bandwidth of the optical amplifiers with proper amplification of the signals. In order to achieve this, a competent flat gain hybrid optical amplifier is demonstrated that comprised of a erbium doped fiber amplifier (EDFA) and Raman amplifier with multi-wavelength backward pumping scheme. Attempts were done in order to design a (E-R) hybrid optical amplifier (HOA) for 16 channels, 20 Gbps DWDM systems at reduced channel spacing of 0.2 nm and also compared with conventional amplifiers. The performance of the system is evaluated on the behalf of different parameters viz. gain, noise figure, output power, Optical Signal to Noise Ratio (OSNR). Numerous parameters of EDFA and Raman amplifiers are optimize to find the gain spectrum of E-R HOA. It is found that E-R HOA provides better gain uniformity with noise figure less than 5 dB. It is also found that for wavelength range 1553.13–1556.15 nm, a uniform gain of 38.85 dB is achieved for input power of “0” dB with noise figure variation of 0.29 dB without using any gain flattering scheme. The output power and OSNR of all the channels is measured at 0 dBm input power and found maximum power and SNR at 1556.15 nm channel frequency is 26.44 dBm and 40.71 dB respectively.

Analysis of Dual-Order Backward Pumping Schemes in Distributed Raman Amplification System

Abstract Backward pumping in fiber Raman amplifiers has been investigated in this paper in terms of on-off Raman gain, noise figure and optical signal-to-noise ratio. The results exhibit that with four first-order pumps and one second-order pump scheme can be employed to achieve 8.2 dB noise figure in 64 channel fiber optic communication system. It has also been reported that 2.65 dB gain ripple, 0.87 dB noise figure tilt and 2.02 dB OSNR tilt can be attained with the second-order pumping in fiber Raman amplifiers. The main advantage of the scheme is that only 50 mW second-order pump shows appreciable improvement in the system performance. It shows that further increase in first-order and second-order pump powers increase system noise implications.

Long-distance random fiber laser point sensing system incorporating active fiber.

In this paper, we investigate the benefits of introducing the active fiber into long-distance point-sensing system based on random fiber laser (RFL). In this scheme the active fiber is placed between two segments of single mode fiber (SMF) and backward RFL pumping scheme is used. Through the numerical analysis, the influence of active fiber location on the spectral and power performance for the RFL is carefully discussed. Compared with the scheme without active fiber, the lasing threshold is much lower and the optical signal to noise ratio (OSNR) of the lasing line can be much higher, and the location of the active fiber has significant flexibility. The RFL experimental results are well coincident with the theoretical analysis; also, the sensing performance of such a system is demonstrated.

Design and Performance Analysis of Raman Amplified XG-PON System

This paper describes the design considerations and performance analysis for an XG-PON Raman amplification scheme. The experimental results and theoretical calculations presented in this paper demonstrate the feasibility of Raman amplification in O-band access systems; thus, enhancing the performance of existing access installations while still preserving the passive nature of these networks. With the availability of cost-effective quantum dot laser diodes as high-power pumps, Raman amplification is proving to be an attractive candidate for access applications. We experimentally demonstrate that it is possible to achieve a total pump power of 759 mW using polarization multiplexed quantum dot laser diodes (QD-LDs), and using a backward pumping scheme we can support a purely passive XG-PON system with 50 km reach and 64 splits. With system level numerical simulations, we illustrate the use of forward or bi-directional pumping scheme to increase the splits further. We also confirm that the Raman pump output in the experiments, despite being below the single mode cut off for the standard SMF, was efficiently coupled into the trunk fibre using off-the-shelf components. We also show using numerical simulations that a flat gain spectrum can be obtained with less than 0.7 dB gain variation across the XG-PON upstream wavelength band (1260–1280 nm) using a multi-wavelength pumping scheme with just two pump wavelengths. From a detailed system analysis, we discuss the impact of broadband filtering (20 nm) at the receiver adopted by the XG-PON standards, and show that lower class (N1) receivers can be adopted for Raman gains higher than 16 dB.

Comparison of femtosecond pulse dynamics in a PCF amplifier with backward and forward pump schemes

A comparative study of femtosecond pulse dynamics in a photonic crystal fiber amplifier with backward and forward pump schemes is demonstrated experimentally and numerically. The output power, pulse duration and spectrum width of the amplified soliton pulse at different pump powers under the two pump schemes are discussed. The results show that the evolution of the soliton pulse shape in the temporal and spectral domains is also dependent on the direction of pump besides the output power. A narrower pulse duration and spectral width (i.e. smaller time-bandwidth-product), the much less nonlinearity accumulation (i.e. smaller B-Integral value) and a higher output power can be obtained in the backward pump scheme compared to that in the forward pump scheme. The physical origin can be explained by the interaction among group-velocity dispersion, self-phase modulation and nonuniform gain distribution along the fiber due to different end-pump directions. The results provide convincing support to choose the backward pump scheme rather than the forward pump scheme in soliton amplification region in a femtosecond fiber amplifier.

Reflectivity variation in asymmetric random distributed feedback Raman fiber laser

This paper demonstrates and discusses the effect of reflectivity on the intracavity power development and spectral profile of a 41.1 km asymmetric (half-opened cavity) random distributed feedback fiber laser with different pumping schemes. The laser cavity is confined by a fiber Bragg grating and the Rayleigh feedback amplified by Raman scattering effect that serves as virtual random distributed mirrors. The laser performance was observed by integrating a variety of power couplers while employing forward and backward pumping schemes. Forward pumping exhibits greater susceptibility to reflectivity variation compared to backward pumping. Meanwhile, higher reflectivity produced better threshold conditions but at the expense of lower saturation power. A power-saturated laser also manifested a broader spectrum than a laser conducted outside the saturation regime. These research findings will be beneficial in understanding the role of reflectivity and pumping configurations in enhancing asymmetric random distributed feedback fiber laser.