Bidirectional Pumping Scheme Research Articles

OSNR enhancement of Rayleigh-assisted Brillouin-Raman fiber laser via double pass configuration.

We propose and demonstrate a 10 GHz spacing multi-wavelength Brillouin-Raman fiber laser (MBRFL) with wide bandwidth and an outstanding optical signal-to-noise ratio (OSNR). This is achieved by utilizing loop mirror at one end of the laser cavity through a symmetrical bi-directional Raman pumping scheme. The setup is arranged in a double pass configuration by employing different lengths of dispersion compensating fibers (DCF). The attainment of MBRFL with outstanding performance necessitates the optimization of Raman pump power, Brillouin pump wavelength, and DCF length. By employing an 11 km DCF, when setting the Brillouin pump wavelength at 1531.4 nm, 504 Stoke lines are produced with a channel spacing of 0.08 nm. All the counted laser lines have less than a 1-dB peak amplitude variation and are spread across a 40.4 nm bandwidth that covers from 1531.4 to 1571.8 nm wavelength. In this case, the Raman pump power is fixed at 900 mW which results in average OSNR and Stokes peak amplitude level of 28 dB and -7 dBm respectively. To date, this is the simplest cavity design with the widest bandwidth and flattest spectrum together with outstanding OSNR attained in 10 GHz spacing multi-wavelength fiber lasers that incorporate a single low-power Raman pump unit.

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.

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.

A multi-wavelength Brillouin fiber laser of switchable frequency spacing based on a bi-directional ring cavity

A multi-wavelength Brillouin fiber laser (MW-BFL) of switchable frequency spacing is proposed and experimentally demonstrated. This MW-BFL comprises a ring cavity that includes a piece of 20-km-long single-mode fiber as the gain medium for Brillouin amplification. The switchability of wavelength spacing is implemented by adjusting the Brillouin pump (BP) coupling ratio into the ring cavity of a bi-directional pumping scheme, as the outputs of single and double Brillouin shifts are achieved with either a 50/50 coupler or a 99/1 coupler, respectively. A total of 11 lasing Stokes lines with single Brillouin shifts and 9 lasing Stokes lines with double Brillouin shifts have been observed when the BP power is set at 0 dBm. Both Brillouin lasers are wavelength-tunable, and the tuning ranges are 16 and 10 nm for the single- and double-spaced outputs, respectively.

Experimental demonstration of mode-matching and Sagnac effect in a millimeter-scale wedged resonator gyroscope

The highly efficient coupling of light from conventional optical components to optical mode volumes lies in the heart of chip-based micro-devices, which is determined by the mode-matching between propagation constants of fiber taper and the whispering-gallery-mode (WGM) of the resonator. Optical gyroscopes, typically realized as fiber-optic gyroscopes and ring-laser gyroscopes, have been the mainstay in diverse applications such as positioning and inertial sensing. Here, the mode-matching is theoretically analyzed and experimentally verified. We observe the Sagnac effect in a millimeter-scale wedged resonator gyroscope, which has attracted considerable attention and has been rapidly promoted in recent years. We demonstrate a bidirectional pump and probe scheme, which directly measures the frequency beat caused by the Sagnac effect. We establish the linear response between the detected beat frequency and the rotation velocity. The clockwise and counterclockwise rotation can also be distinguished according to the value of the frequency beat. The experimental results verify the feasibility of developing the gyroscope in a WGM resonator system and pave the way for future development.

First Demonstration of a Bidirectional Tandem-Pumped High-Brightness 8 kW Level Confined-Doped Fiber Amplifier

In this paper, we demonstrated a high-brightness bidirectional tandem-pumped confined-doped fiber amplifier with nearly 8 kW output power. The technical difficulties for realizing the bidirectional tandem-pumping scheme are comprehensively analyzed, according to which, the homemade high signal-isolation side-coupled pump and signal combiner as well as stable 1018 nm pump sources were developed and applied in the experimental study, successfully enabling the first demonstration of the bidirectional tandem pumping scheme. Moreover, benefiting from the confined-doped fiber design and the bidirectional tandem pumping technique, the transverse mode instability was successfully mitigated and the beam quality was well preserved during the power scaling process. Eventually, an output power of 7.88 kW was obtained with the beam quality factor of ∼1.97 and the optical-to-optical efficiency of ∼84.76%. This work provides a good reference for realizing large-mode-area fiber lasers with high output power and good beam quality by utilizing the confined-doped fiber design and bidirectional tandem-pumping technique.

Temperature characteristic of a gain fiber core in distributed side-coupled cladding-pumped fiber amplifiers with different pump schemes

In this paper, we study the temperature characteristic of a gain fiber core in distributed side-coupled cladding-pumped (DSCCP) fiber amplifiers by optical frequency domain reflectometry (OFDR). To the best of our knowledge, this is the first temperature distribution measurement of a fiber core in DSCCP fiber lasers. The temperature distributions are also firstly measured among forward, backward, and bidirectional pump schemes. In addition, the bidirectional pump scheme has lower temperature distribution than unidirectional pump schemes. The experimental fiber core temperature is in according with the numerical simulation, which was once introduced for the temperature study of DSCCP fiber amplifiers. By experimental measurement and theoretical modeling, it is found that the maximum core temperature is several tens of centimeters away from the injected end of the DSCCP fiber, such that it will distribute the heat load along the longitudinal direction and reduce the maximum thermal load with better thermal management advantages than ordinary double cladding fiber (DCF).

More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme.

We demonstrate the first all-fiber monolithic bidirectional tandem pumping amplifier, to the best of our knowledge, based on a 30/250 µm conventional ytterbium-doped double-clad fiber. By optimizing the bidirectional pumping power distribution, an output power of 6.22 kW is obtained with near single-mode beam quality (M2=1.53), and no transverse mode instability is observed. This work could provide an excellent reference for high-power, higher-brightness fiber lasers.

Bidirectional tandem-pumped high-brightness 6 kW level narrow-linewidth confined-doped fiber amplifier exploiting the side-coupled technique.

In this work, a bidirectional tandem-pumped high-power narrow-linewidth confined-doped ytterbium fiber amplifier is demonstrated based on side-coupled combiners. Benefiting from the large-mode-area design of the confined-doped fiber, the nonlinear effects, including stimulated Raman (SRS) and stimulated Brillouin scattering (SBS), are effectively suppressed. While the transverse mode instability (TMI) effect is also mitigated through the combination of confined-doped fiber design and the bidirectional tandem pumping scheme. As a result, narrow-linewidth fiber laser with 5.96 kW output power is obtained, the slope efficiency and the 3-dB linewidth of which are ∼81.7% and 0.42 nm, respectively. The beam quality is well maintained during the power scaling process, being around M2 = 1.6 before the TMI occurs, and is well kept (M2 = 2.0 at 5.96 kW) even after the onset of TMI. No SRS or SBS is observed at the maximum output power, and the signal-to-noise ratio reaches as high as ∼61.4 dB. To the best of our knowledge, this is the record power ever reported in narrow-linewidth fiber lasers. This work could provide a good reference for realizing high-power high-brightness narrow-linewidth fiber lasers.

2.1 μm, 1.52 μJ square-wave noise-like pulse from an all-fiber figure-of-9 Ho-doped oscillator and single-stage amplifier.

A 2.1 μm, high energy square-wave noise-like pulse (NLP) in an all-fiber Ho-doped fiber laser is proposed, which consists of an oscillator and a single-stage amplifier. In the figure-of-9 oscillator, mode-locking is achieved based on the nonlinear amplifying loop mirror, employing a long gain fiber to provide sufficient gain in 2.1 μm band and optimizing the cavity length to obtain maximum pulse energy output. With appropriate pump power and polarization state, the oscillator emits a 175.1 nJ square-wave NLP with center wavelength of 2102.2 nm and spike width of 540 fs. The 3-dB spectral width and pulse envelope width are 11.2 nm and 6.95 ns, respectively. The single-stage amplifier employs a bi-directional pump scheme. After amplification, 5.8 W NLP with a slope efficiency of 56.8% is obtained. The pulse energy of NLP is scaled to 1.52 μJ, which is the highest pulse energy of NLP at 2.1 μm to the best of our knowledge. The obtained high-energy square-wave NLP-fiber laser has great potential in mid-infrared laser generation.

Near-single-mode 2-kW fiber amplifier based on M-type ytterbium-doped fiber

High power fiber laser systems have attracted extensive attention due to compactness, good beam quality, efficient heat dissipation and high conversion efficiency. They are widely used in industrial processing, military, medical treatment and other fields. Over the past two decades, owing to the development of double cladding fiber and high-brightness laser diodes, the output power of fiber lasers has been greatly improved. Unfortunately, nonlinear effects (NLEs), such as stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS), restrict the further enhancement of the output power of fiber lasers. Apparently, increasing the core diameter is the most common way to suppress NLEs in the fiber, but this causes another limiting factor, i.e. mode instability (MI), resulting in the deterioration of the beam quality and in the limitation of the power scaling. Therefore, it is important and urgent to suppress the NLEs and MI simultaneously in fiber lasers. The M-type fiber, by designing refractive index profile, breaks through the stringent trade-off between mode area and numerical aperture (NA), so it possesses a larger mode area than the step index fiber, which helps to avoid NLEs and expand the power range. The M-type ytterbium doped double-clad fiber is fabricated by the modified chemical vapor deposition (MCVD) process with solution doping technology (SDT), the core/cladding diameter is 25/400 μm. The NA of high index ring and index dip in the core are 0.054 and 0.025, respectively. To test the performance of the M-type fiber during high-power operation, a 976 nm bidirectional pumped all-fiber amplifier is constructed. As a result, maximum output power of 2285 W is achieved with an optical-to-optical conversion efficiency of 66.5% under bidirectional pumping scheme, and the measured <i>M</i><sup> 2</sup> factor is 1.42, the central wavelength and 3 dB linewidth of output laser are 1080 nm and 3.01 nm, respectively. To the best of our knowledge, this is the highest output power in a continuous-wave fiber laser employing an M-type fiber at present. However, the MI effect is observed at the output power of 2252 W. The future work will focus on optimizing the structure of the M-type fiber to achieve a stabler higher-power and higher-efficiency laser output.

5-kW Monolithic Fibre Laser Oscillator with Ring Beam Output Employing Bidirectional-Pumping Scheme

5-kW Monolithic Fibre Laser Oscillator with Ring Beam Output Employing Bidirectional-Pumping Scheme

Inverse design of a Raman amplifier in frequency and distance domains using convolutional neural networks.

We present a convolutional neural network architecture for inverse Raman amplifier design. This model aims at finding the pump powers and wavelengths required for a target signal power evolution in both distance along the fiber and in frequency. Using the proposed framework, the prediction of the pump configuration required to achieve a target power profile is demonstrated numerically with high accuracy in C-band considering both counter-propagating and bidirectional pumping schemes. For a distributed Raman amplifier based on a 100 km single-mode fiber, a low mean set (0.51, 0.54, and 0.64 dB) and standard deviation set (0.62, 0.43, and 0.38 dB) of the maximum test error are obtained numerically employing two and three counter-, and four bidirectional propagating pumps, respectively.

5 kW monolithic fiber amplifier employing homemade spindle-shaped ytterbium-doped fiber.

We have demonstrated a 5 kW high-power monolithic fiber amplifier employing a homemade spindle-shaped ytterbium-doped fiber (YDF) based on the main oscillator power amplifier configuration. The YDF consists of a spindle-shaped core and cladding along the fiber length, with a core/cladding diameter of 27/410 µm at both ends and 39.5/600 µm in the middle. An output power of over 5 kW and beam quality of about 1.9 and an optical-to-optical conversion efficiency of 66.6% were achieved in the amplifier under a bidirectional pump scheme. While operating at the maximum power, the laser performance was evaluated, and the transverse mode instability and stimulated Raman scattering effects were well mitigated. To the best of our knowledge, this is the highest power demonstration in a continuous-wave fiber laser employing a tapered fiber. Further power scaling is promising by optimizing the structure of the YDF.

Bismuth doped fibre amplifier operating in E- and S- optical bands

Bismuth-doped fibre amplifiers offer an attractive solution for expanding the bandwidth of fibre-optic telecommunication systems beyond the current C-band (1530-1565 nm). We report a bismuth-doped fibre amplifier in the spectral range from 1370 to 1490 nm, with a maximum gain exceeding 31 dB, and a noise figure as low as 4.75 dB. The developed system is studied for forward, backward, and bi-directional pumping schemes and three different signal power levels. The forward pumping scheme demonstrates the best performance in terms of the achieved noise figure. The developed amplifier can be potentially used as an in-line amplifier with >20dB gain in the spectral band from 1405 to 1460 nm.

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.

Gain-forming effect on optical signal-to-noise ratio in Raman fiber amplifier

Abstract We have introduced the comprehensive optical-to-noise ratio (OSNR) to investigate simultaneously the effects of all three major linear noise sources in distributed Raman fiber amplifier (DRFA). The total noise power contains double Rayleigh backscattering (DRB) noise, amplified spontaneous emission (ASE) noise, and relative intensity noise (RIN). Also, we have introduced an approximated gain profile approach, which helps in evaluating the OSNR for higher-order pumping (HOP) configuration for backward, forward, and bidirectional DRFA schemes. Finally, by applying the dynamic and static full-numerical computations on a system of eight wavelength division multiplexing (WDM) channels (for amplifier lengths of 70, 140, and 210 km), we demonstrated that for RIN frequencies greater than 100 kHz, the higher-order bidirectional pumping scheme exceeds the backward schemes by 12 dB.

530 W all-fiber continuous-wave Tm-doped fiber laser

In the past decades, thulium-doped fiber lasers (TDFLs) operating in an eye-safe range have attracted considerable attention, for they have extensive applications such as LIDAR, free space communication, medical diagnostics and pumping source for holmium-doped fiber laser or optical parameter oscillator. In this paper, we report a high power all-fiberized TDFL based on main-oscillator power-amplifier (MOPA) configuration. The preform is fabricated by the modified chemical vapor deposition method combined with solution doping technique and drawn into a core/clad size of 25/400 μm. The numerical aperture of the TDF is 0.1. The concentration of Tm<sub>2</sub>O<sub>3</sub> and Al<sub>2</sub>O<sub>3</sub> are 2.6 wt% and 1.01 wt%, respectively, measured by an electron probe micro-analyzer. The cladding absorption is 3 dB/m at 793 nm measured by cut-back method. The oscillator consists of 8 m 25/400 TDF mentioned above and a pair of fiber Bragg gratings. The oscillator yields maximum power of 91 W with pump power of 202 W and a 3 dB spectral bandwidth as narrow as 75 pm. In the amplifier stage, the bi-directional pumping scheme is employed. The narrow linewidth seed with output power of 57 W is scaled to 530 W through one-stage amplification, corresponding to a slope efficiency of 50%. The central wavelength of the Tm-doped MOPA is 1980.89 nm and the linewidth is broadened to 0.11 nm at 530 W. The measured <i>M</i> <sup>2</sup> factor at 100 W is less than 1.3. Neither obvious amplified spontaneous emission nor non-linear effect is observed, and the output power is only limited by pump power. To the best of our knowledge, this is the highest output power of TDF at present.

A Raman-Pumped Dispersion and Nonlinearity Compensating Fiber For Fiber Optic Communications

An optical back propagation (OBP) technique using Raman pumped dispersion compensation fibers (DCF) is investigated to compensate for nonlinear impairments in WDM systems in real time. The proposed inline OBP module consists of an optical phase conjugator, amplifiers and a Raman pumped DCF. In order to suppress the nonlinear effects of the transmission fibers exactly, the power in the backpropagation fiber should increase exponentially with distance. This can be approximately achieved by using forward/backward Raman pumping of the dispersion compensating fiber (DCF). We introduce two configurations to realize the OBP. In this paper, we show that the OBP with forward/backward pumping provides 2.45 dB Q-factor gain compared to single-channel digital back propagation (DBP) when transmission distance is 1500 km for a WDM system with QAM-64. To minimize the variation of effective gain coefficient of the Raman pumped DCF as a function of distance, bidirectional pumping scheme which can provide the signal power profile closest to that required by the ideal OBP condition is proposed. The bidirectional pumping scheme provides a superior performance over forward/backward pumping and wideband DBP (i.e., DBP is applied on the entire WDM signal). Our numerical simulation results show that the bidirectional pumping scheme provides 7.6 dB and 5 dB advantage in Q-factor as compared to single-channel DBP and wideband DBP, respectively at a transmission distance of 5000 km. The maximum achievable reach of a long haul WDM system can be enhanced by $225\%$ using bidirectional pumping scheme as compared to wideband DBP.

A Comparative Analysis of Dual-Order Bidirectional Pumping Schemes in Optical Fiber Raman Amplification

Abstract In this paper, dual-order bidirectional pumping schemes of distributed fiber Raman amplifier are compared with standard first-order pumping in wavelength division multiplexed optical transmission systems. The novel comparison analysis is carried out in terms of Optical signal-to-noise ratio and Q-factor, on-off gain and noise figure by varying optical input power and fiber lengths. The results indicate that dual-order schemes present 0.02 dB higher OSNR and 5 dB higher Q-factor in comparison to first-order pumping when input optical power is varied from −4 to 5 dBm. Similarly, there is 4 dB higher on-off gain with dual order comparatively to first order when fiber length varied from 10 to 100 km. However, there is degradation in noise figure and Q-factor due to DRBS noise with dual-order pumping when fiber length from 10 to 100 km. Further, the signal power evolutions along fiber length show that there is 5 dBm improvement for 100 km fiber. The novelty of the work is that comparative analysis exhibits improvement in OSNR, on-off gain and Q-factor using dual-order bidirectional pumping.