Backward Pumping Research Articles

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.

Investigation of Bi-Directionally, Dual-Wavelength Pumped Extended L-Band EDFAs

We present numerical and experimental analysis of bi-directionally, dual-wavelength pumped extended L-band erbium-doped fiber amplifiers (EDFAs) that use a standard 1480 nm laser diode as the forward pump source and a high-power C-band light source as the backward pump source. When using a ~1535 nm backward pump, efficient amplification can be obtained over the extended L-band with no penalty to the noise figure. Experimentally, a bi-directionally pumped L-band EDFA achieves a wideband 20 dB-gain, covering 1570–1619 nm, with a noise figure lower than 5.6 dB. With either 1530.7 nm or 1537.8 nm as the backward pump wavelength, and 1480 nm as the forward pump wavelength, the power conversion efficiency is 40% higher compared to a configuration using 1480 nm pumping for both directions. To our best knowledge, it is the first time that C-band light has been used as the backward pump source for extended bandwidth of L-band EDFAs operating beyond 1610 nm.

Asymmetric Second-Harmonic Generation with High Efficiency from a Non-chiral Hybrid Bilayer Complementary Metasurface

Asymmetric optical responses of photonic systems for forward and backward incidences are of great importance for practical applications. In this work, we theoretically demonstrate second-harmonic generation (SHG) with high contrast of harmonic generation efficiency for forward/backward pumping in non-chiral nanostructures. Hybrid metasurfaces consisting of dielectric nanodisks and their complementary plasmonic nanostructures exhibit strongly asymmetric and highly efficient SHG. Numerical results show that the SHG efficiency of 0.57 × 10−3% can be obtained for forward pumping with such a metasurface made of lithium niobate and gold. This value of SHG efficiency is 53 times higher than the case of backward pumping, revealing strongly asymmetric SHG.

Orbital Angular Momentum Optical Amplifier Based on PbS-Doped Ring-Core Fiber

A unique orbital angular momentum (OAM) optical amplifier based on ring-core fiber doped with lead sulfide (PbS) is realized for the first time in this study. The PbS-doped ring-core fiber is fabricated by using the method of modified chemical vapor deposition. The cross-sectional microscope image of the fiber is obtained, and the difference in the refractive index between the ring core and the cladding of the fiber, measured by a three-dimensional refractive-index tester, reached 1%. The content of sulfur and lead in the ring core of the PbS-doped ring-core fiber is confirmed by energy spectrum analysis. The first order OAM optical amplifier based on PbS-doped ring-core fiber is built. The experiment confirms that the air-hole PbS-doped ring-core fiber can support the OAM |l|=1 modes. In addition, 2.3 dB of on–off gain is obtained for the OAM |l|=1 mode at 1550 nm. Its fluorescence spectrum ranges from 1100 to 1600 nm using the backward pumping method.

1535–1620 nm Widely Tunable Watt-Level Single-Mode Er:Yb Codoped All-Fiber MOPA

We report a continuous-wave 1535–1620 nm widely tunable single-mode Er:Yb codoped cladding-pumped all-fiber laser source based on the MOPA scheme. The output power ranged from 0.785 W (at 1615 nm) to 2.007 W (at 1565 nm), which is the highest output power from a C+L tunable all-fiber laser source. The 3 dB spectral width was less than 150 pm at all wavelengths. No Yb emission was observed. We proposed a mid-way backward pumping configuration in which the pump light was injected between two gain fibers and propagated backward to broaden the effective wavelength range of the fiber amplifier. Experiments verified the capability of our configuration to improve the suppression ratio of C-band amplified spontaneous emissions from 13 dB to 35 dB when the signal laser was operating at 1620 nm.

Intelligent gain flattening in wavelength and space domain for FMF Raman amplification by machine learning based inverse design

We propose a machine learning based approach to design few-mode DRAs by using neural networks to optimize the pump wavelengths, powers and mode content in order to obtain flat gain spectrum with low mode-dependent gain (MDG). Based on the proposed intelligent inverse design method, amplification optimization for the random fiber laser based two-mode DRA can be achieved with gain flatness of 1.0 dB and MDG of 0.6 dB at 14.5 dB on-off gain level. For backward pumping four-mode DRA, gain flatness of 0.46 dB and MDG of 0.3 dB can be achieved at 12.5 dB on-off gain.

A few mode EDFA with different pumping schemes: performance evaluation

The performance of few-mode (FM) erbium doped fiber amplifiers (EDFAs) with different pumping configurations is investigated and evaluated. An improvement in both gain and noise figure is achieved with a bidirectional core pumping FM-EDFA using unequal pump power with pump wavelengths of 980 nm and 1480 nm for forward and backward pumping, respectively. Moreover, the bidirectional pumping FM-EDFA is studied in case of equal pumping wavelength at 980 nm and different pump powers. Then, the system performance for the case of 6-spatial modes is tested for backward core pumping at 980 nm. The backward core pumping achieved a high modal gain with a low noise figure compared to bidirectional core pumping. Meanwhile, we report a cladding-pumped 6-modes EDFA with higher modal gain ~ 35 dB over the C-band, zero differential modal gain and low noise figure using both backward and bidirectional pumping.

Gain and noise figure performance of an EDFA pumped at 980 nm or 1480 nm for DOFSs

Erbium-doped fiber amplifier (EDFA) is widely recognized as one of the most important optical amplifiers in distributed optical fiber sensors which has been able to compensate the loss of the optical power. The 980 nm and 1480 nm are the most efficient pumps for amplifying the 1550 nm signal in EDFA. This paper presented an investigation for influences of pump power, signal power and EDF length on the gain and noise figure in an EDFA which pumped by 980 nm and 1480 nm pumps. It is also a comparison for each of the created gain and noise figure at the forward, backward, and bi-directional pumping configurations, separately. The effects of Rayleigh scattering, temperature, homogeneous interactions between Erbium ions and the amplified spontaneous emission are considered. As a result, Bi-directional pumping shows the highest gain value, and forward pumping presents the lowest noise figure for each pump. Moreover, backward pumping creates the highest noise figure and forward pumping generated the lowest gain value. The gain of 980 nm pump is higher than 1480 nm up to 5-m EDF length and for longer lengths, it becomes vice versa. The 980 nm pump caused a lower noise figure than 1480 nm for EDF length up to 10 m and it becomes vice versa for longer lengths.

Relative intensity noise comparison of fiber laser and amplified spontaneous emission sources

The performance of Raman fiber laser and amplifier largely depends on the temporal stability of the pump sources. Here the relative intensity noise of three different sources are experimentally investigated and compared, which are fiber laser oscillator, master oscillator power amplifier, and amplified spontaneous emission. The amplified spontaneous emission source exhibits the lowest intensity noise. The fiber laser oscillator and amplifier have inevitable relaxation oscillation and longitudinal mode beating, which results in higher intensity noise and frequency dependent features. The results firmly explain the recent demonstrations in the performance improvement with amplified spontaneous emission as pump sources and the beneficiation of backward pumping for single frequency fiber amplifier.

Experimental and numerical study on the thermal control strategy for a gas foil bearing enhanced with thermoelectric modules

The authors report the method for temperature gradient control in gas foil bearings with the use of thermoelectric modules. The paper presents a prototype installation consisting of a gas foil bearing with integrated 36 thermoelectric modules, an electric spindle, measurement and control system. The results of a preliminary qualitative and quantitative assessment of the investigated temperature gradient control are shown, making use of the carried out numerical simulations. Steady state thermal analyses were performed for the defined control currents assigned to the Peltier modules. The metamodeling technique improved the search for a suboptimal choice on the control currents values in terms of temperature gradient control efficiency. Consideration of simultaneous forward and backward heat pumping directions for the selected thermoelectric modules improved temperature gradient reduction by additional 10%.Moreover, the paper is complemented with the results of an experimental test performed for a prototype installation of a gas foil bearing. A straightforward adaptive algorithm, dedicated to control thermal parameters of the bearing’s foil, was successfully tested. The control algorithm allowed for a significant temperature gradient reduction, however considerable oscillations in the temperature profile were observed. In order to eliminate these oscillations, a PID controller was implemented into the algorithm. Consequently, the magnitude of the temperature oscillations was reduced by 60%.

Compact, high repetition rate, 4.2 MW peak power, 1925 nm, thulium-doped fiber chirped-pulse amplification system with dissipative soliton seed laser.

We report the demonstration of a high average- and peak-power, 1925 nm, thulium-fiber based chirped pulse amplification (CPA) system. A compact, dissipative soliton thulium-fiber, mode-locked seed produced pre-chirped pulses with 25 ps duration, 45 mW output power and repetition rate of 15.7 MHz. After stretching to 105 ps in 83 m of normal dispersion fiber, the pulses were amplified in a core-pumped pre-amplifier and a cladding pumped power amplifier to average output powers of 28 W and 30 W with forward and backward pumping, respectively, with the output power limited only by the available pump power. After a pair of fused silica transmission gratings with an efficiency of 71%, the amplified pulses were re-compressed to 297 fs yielding pulses with a peak power of 4.2 MW (backward pumped) and a pulse energy of 1.27µJ.

SOA/EDFA/RAMAN optical amplifier for DWDM systems at the edge of L & U wavelength bands

Abstract This paper presents an enhanced hybrid SOA/EDFA/RAMAN optical amplifier design for Dense Wavelength Division Multiplexed (DWDM) Systems at the edge of L & U wavelength bands. A semiconductor optical amplifier is used as a booster in conjunction with the other two kinds of optical amplifiers (EDFA and RAMAN) to achieve better amplification performance. Forty DWDM channels at 10 Gbps in the frequency range 185–185.975 THz spaced by 25 GHz are transmitted over 200 km of standard telecommunication fibre. The hybrid amplifier performance is characterized in terms of maximum gain achieved, gain flatness and amplifier noise figure (NF). A comparison between forward pumping and backward pumping of the RAMAN amplifier is considered for different lengths of the RAMAN fibre. The maximum achieved gain of the hybrid optical amplifier is over 30 dB with 1 dB gain flatness using 30 km of fibre for the RAMAN amplifier with total forward pumping power of 1650 mW and total backward pumping power of 730 mW. However, the hybrid amplifier has a gain of 26 dB and gain flatness better than 0.8 dB when using 25 km of fibre for the RAMAN amplifier with combined total forward pumping power of 1270 mW and total backward pumping power of 850 mW. The amplifier noise figure obtained is less than 5.7 dB for all DWDM channels.

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 Analysis of Distributed Raman Amplifier Using Different Pumping Methods in DWDM Communication Systems

Abstract The objective of this work is to investigate and compare the impact of forward, backward and bidirectional pumping methods in repeater less distributed Raman fiber amplifier system and to analyze its benefits in long haul optical communication links. The effect of different pump wavelengths and pump powers are also analyzed. By adjusting the forward pump power as well as backward pump powers carefully, the transmission performance can be improved. The pump power ratio is adjusted to 20% so that the degradation of signal due to non-linear effect is reduced. The distributed gain and pump penetration depth for different pump power ratios are also examined. A comparison of different pumping methods reveals that bidirectional pumping gives the optimum performance in terms of higher gain and lower noise figure when compared to single pumps and unidirectional pumping. Low pump power ratios lead to better transmission performance than higher pump ratios due to lesser non-linear effects. The simulation results indicate that signal power evolution decreases with the increase in length of the fiber. It is also found that the pump power is more relevant during the first 50 km of the distributed fiber line and it decreases as it goes beyond 100 km. By choosing the optimum pumping methods and number of pumps, Raman Amplifiers can be used in almost all places where optical amplifiers are required.

Ultra-Low-Noise Mode-Division Multiplexed WDM Transmission Over 100-km FMF Based on a Second-Order Few-Mode Raman Amplifier

We demonstrate, for the first time, an ultra-low-noise mode-division multiplexed transmission with eight wavelength-division multiplexed channels over a 100-km few-mode fiber based on a second-order few-mode Raman amplifier. To minimize the mode dependent gain, two backward depolarized pumps at the wavelengths of 1455 nm (75 mW) and 1360 nm (1890 mW) are equally launched both into the LP11a and LP11b modes. The Raman on–off gains of the LP01 and LP11 modes are both above 8 dB and the mode dependent gain is less than 0.7 dB. An amplified spontaneous emission noise improvement of 1.8 dB is achieved in the second-order few-mode Raman amplifier, with respect to the first-order Raman amplification. The effective noise figure of the second-order few-mode Raman amplifier is about –3 dB, which is ∼1.4 dB lower than that in the first-order pumping case. In the transmission experiments, a single-mode transmission over LP01 mode and a mode-division multiplexed transmission over LP11a and LP11b modes are performed separately. The bit-error-rates for all the transmission channels are significantly improved due to the low-noise figure amplification of the second-order few-mode Raman amplifier.

High pumping-power fiber combiner for double-cladding fiber lasers and amplifiers

A high pumping-power fiber combiner for backward pumping configurations is fabricated and demonstrated by manufacturing process refinement. The pump power handling capability of every pump fiber can extend to 600 W, corresponding to the average pump coupling efficiency of 94.83%. Totally, 2.67-kW output power with the beam quality factor M2 of 1.41 was obtained, using this combiner in the fiber amplifier experimental setup. In addition, the temperature of the splicing region was less than 50.0°C in the designed combiner under the action of circulating cooling water. The experimental results prove that the designed combiner is a promising integrated all-fiber device for multikilowatt continuous-wave fiber laser with excellent beam quality.

Effect of Raman amplification on the modulation instability threshold

A numerical simulation model for investigating modulation instability in a Raman-assisted fiber-optics system is presented and used. The modulation instability (MI) spectrum and its threshold are numerically analyzed based on this theoretical model. The different MI behaviors between forward and backward Raman pumping are studied in detail, respectively. The MI threshold in the case of forward Raman pumping is lower than that in the case of backward Raman pumping. Experiments are conducted based on the theoretical analysis, and the results agree well with the numerical calculations.