Fiber Raman Amplifier Research Articles

Modeling of Graded-Index Raman Fiber Amplifiers with Pump Depletion

Graded-index (GRIN) fibers are often used for making high-power Raman amplifiers. We employ numerical and semi-analytical techniques to model such amplifiers and include not only the signal’s amplification and pump’s depletion but also various nonlinear interactions between the signal and pump beams and the self-imaging effects within the GRIN fiber. We solve the coupled nonlinear equations of the pump and signal beams numerically. We also employ the variational technique to obtain simpler equations that can be solved much faster than the full model and still agree with it in most cases of practical interest. We discuss the evolution dynamics of the pump and signal beams, along with a novel process of energy exchange between the two beams because of self-imaging inside the GRIN fiber. The dependence of the signal’s amplification on various input parameters is analyzed in detail to optimize the device’s design and enhance the signal’s amplification for a given pump power and fiber length. Based on our analysis, we establish a resonant condition for the maximum energy transfer from the pump to the signal being amplified. We further show that the periodic self-imaging of the pump and signal beams inside a GRIN fiber leads to higher output powers compared to step-index fibers.

Numerical Investigation of Raman-Assisted Four-Wave Mixing in Tapered Fiber Raman Fiber Amplifier

The generation of unwanted higher-order Raman effects is the main factor restricting the power scaling of Raman fiber amplifiers (RFAs). This phenomenon arises from an interplay of physical processes, including stimulated Raman scattering (SRS), four-wave mixing (FWM), and the intricate temporal and spectral dynamics. Tapered fibers have demonstrated excellent nonlinear effects suppression characteristics due to the varying core diameter along the fiber, which is widely used in ytterbium-doped fiber lasers. In this paper, a comprehensive numerical investigation is conducted on the core-pumping tapered fiber RFAs considering Raman-assisted FWM. The higher-order Raman power in the tapered fiber is always kept at a low level, showing a weak Raman-assisted FWM effect. A numerical investigation is conducted to study the impact of the tapering ratio, the lengths of the thin part, tapered region, and thick part on the higher-order Raman threshold of RFAs. Furthermore, the impact of phase mismatch variations caused by changes in the seed wavelength, on the output signal power and nonlinear effects is analyzed. This paper presents, for the first time, a study on core-pumped RFAs using tapered fibers, providing a novel perspective on enhancing the power of RFAs.

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

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

Quantum-limited transmission distance of optically repeating systems using a hybrid EDFA/Raman-amplifier.

The transmission distance of optically repeating systems is ultimately bounded by quantum noises arising from attenuation and amplification. It is known that fiber Raman amplifiers (FRAs) can achieve longer quantum-limited distances than other amplification schemes owing to their distributed amplification characteristics. However, all-FRA systems are difficult to implement in practice. Therefore, a hybrid erbium-doped fiber amplifier (EDFA)/FRA system may be a reasonable solution for long-haul transmission. In this study, the quantum-limited transmission distance of hybrid EDFA/FRA systems is investigated. A formula for the quantum fluctuation variance during transmission was derived based on quantum mechanical treatment. Calculation results obtained with the derived formula quantitatively indicate that the quantum-limited transmission distance increases as the EDFA/FRA hybrid ratio varies from EDFAs alone to FRAs alone.

Repetition‐Rate and Wavelength Flexible Femtosecond Laser Pulse Generation

AbstractCompared to mode‐locked oscillators, gain‐switched diodes (GSD) have the key advantage of repetition‐rate agility. Yet, large pulse duration and poor coherence of the GSD pulses greatly limit their applications. Here, a GSD‐pumped Raman fiber amplifier is demonstrated, which can effectively generate femtosecond laser pulses with both repetition‐rate and wavelength agility. It is proved that both nonlinear optical gain and single‐frequency seed in the fiber amplifier play critical roles for improving the coherence of the generated laser pulses. In the experimental demonstration, pumped by 1065 nm GSD pulses, the nonlinear fiber amplifier can generate highly coherent 1121 nm femtosecond Raman pulses with up to 80.2% conversion efficiency. The Raman pulses can maintain high performance within the repetition‐rate tuning range from 1 to 150 MHz. The potential for generating 1178 nm femtosecond Raman pulses is also demonstrated with an optical conversion efficiency of 63.8%. This all‐fiber based femtosecond laser with both repetition‐rate and wavelength agility is a promising light source for applications such as nonlinear microscopy and micromachining.

Detailed scrutiny of FWM in holmium-doped fiber amplifier (HOFA) in WDM systems

Abstract Four wave mixing (FWM) in wavelength division multiplexed (WDM) systems is prominent performance degrading nonlinearity and limits the total capacity of the system. FWM is explored in different fiber optical amplifiers such as erbium-doped fiber amplifier (EDFA), Raman fiber amplifier (RFA), and also in semiconductor optical amplifier (SOA). Reported studies in HOFA revealed that nonlinear effects in these amplifiers are low; however, in present study, in this research article WDM-HOFA system has been simulated and studied FWM in detail at different HOFA parameters such as input power, length, core radius, doping radius, numerical aperture (NA), Ho ion density, and effects of co- and counter-pumping. Gain and noise figure (NF) are also investigated at aforementioned parameters, and performance is evaluated in terms of optical signal to noise ratio (OSNR) at 2,030 nm wavelength window. It is observed that least FWM obtained at 3 m Ho fiber length, 2 µm core radius and doping radius, 15.6 × 1023 m−3 Ho ion doping, 0.1 NA, −10 dB input power, 0.8 GHz channel spacing, and 4 WDM channels. However, better gain and NF comes at different values of these parameters, and as a result, there is trade-off between FWM and gain, NF.

High power cladding-pumped low quantum defect Raman fiber amplifier

Heat generated by the quantum defect (QD) in optically pumped lasers can result in detrimental effects such as mode instability, frequency noise, and even catastrophic damage. Previously, we demonstrated that boson-peak-based Raman fiber lasers have great potential in low QD laser generation. But their power scalability and heat load characteristics have yet to be investigated. Here, we demonstrate a boson-peak-based Raman fiber amplifier (RFA) with 815 W output power and a QD of 1.3%. The low heat generation characteristics of this low QD RFA are demonstrated. Both experimental and simulation results show that at this power level, the heat load of the low QD RFA is significantly lower than that of the conventional RFA with a QD of 4.8%. Thanks to its low heat generation characteristics, the proposed phosphosilicate-fiber-based low QD RFA provides an effective solution for the intractable thermal issue in optically pumped lasers, which is of significance in reducing the laser’s noise, improving the laser’s stability and safety, and solving the challenge of heat removing.

Sub-nanosecond Raman fiber amplifier based red-orange light source

We introduce a fully-integrated two-color sub-nanosecond fiber laser system that incorporates a backward-pumped polarization-maintaining (PM) Raman phosphosilicate fiber amplifier (RFA) followed by two fully-integrated fiber-coupled second harmonic generator (SHG) modules. The RFA is pumped by a continuous-wave (CW) Yb laser operating at 1116 nm. The pulsed signals are generated by gain-switched distributed feedback (DFB) laser diodes at 1178 nm and 1310 nm, respectively. The output pulsed DFB signals are independently or simultaneously amplified in the RFA. This amplification is achieved using both the broad SiO2 (∼13.2 THz) and relatively narrow P2O5 (39.9 THz) Stokes shifts. The laser system produces sub-nanosecond pulses at 589 and 655 nm, featuring repetition rates ranging from 40 to 100 MHz and an average power of up to 3 W (limited by the SHG crystal damage threshold) at each wavelength. The diffraction-limited output beams maintain an M2 value of < 1.05 across the entire range of output powers and repetition rates for both wavelengths.

Multi-Band ESCL Transmission Supported by Bismuth-Doped and Raman Fiber Amplification

Multi-Band ESCL Transmission Supported by Bismuth-Doped and Raman Fiber Amplification

Dense wavelength division multiplexing scheme based on effective distributed inline light fiber Raman amplifier configuration

Abstract This paper demonstrated the dense wavelength division multiplexing scheme based on effective distributed inline light fiber Raman amplifier configuration. Various forward/backward and bidirectional pumping power configurations are studied versus fiber reach. Output light signal power is demonstrated against fiber reach without Raman amplification technique. Output light signal power in the forward Raman amplification scheme is clarified with pumping power of both 500 mW and 700 mW in various fiber channel configurations. As well as output light signal power in the backward Raman amplification scheme with pumping power of both 500 mW and 700 mW in various fiber channel configurations. Amplification Raman gain parameter coefficient is demonstrated with various values of pumping power pump based on various fiber channel configurations. Backward amplification net parameter gain is studied for different single mode/true wave/freelight fibers channel configuration at different both pumping power values and fiber reach. As well as the forward amplification net parameter gain is clarified for different single mode/true wave/freelight fibers channel configuration at different both pumping power values and fiber reach.

Raman and erbium-doped fiber amplifiers hybrid bidirectional optical amplifier with low noise figure for fiber-optic radio frequency synchronization

Raman and erbium-doped fiber amplifiers hybrid bidirectional optical amplifier with low noise figure for fiber-optic radio frequency synchronization

Polarization dependent gain in Raman fiber amplifiers with multiple pumps.

In this paper, the polarization dependent gain (PDG) in Raman fiber amplifiers (RFAs) with multiple pumps is studied thoroughly. A comprehensive model, which takes the random polarization mode dispersion, the nonlinear coupling between the pumps, and the degree of polarization (DOP) of the pumps into account, is proposed. The complex nonlinear and random coupling inside the optical fiber is described by a set of nonlinear stochastic differential equations (SDEs), which enable co-simulation of the polarized part and the depolarized part of the multiple pumps. It is revealed that the average PDG and the PDG standard deviation are linearly proportional to the DOP of the pumps, which agrees with the single mode case. More importantly, when the pump wavelength is far away from the signal amplification range (pump-signal wavelength difference larger than 100 nm), its DOP still affects the PDG of the signal. Such a phenomenon is caused by the fact that the pumps interact with each other and the pump DOP could transfer among the pumps, which could enhance the PDG of the RFA. The findings in the work will have important implications for the design of the optical transmission systems with the multi-pump RFAs.

Optimized Design of Second-Order Tellurium-Based Fiber Raman Amplifier Based on Cooperative Search Algorithm

Optimized Design of Second-Order Tellurium-Based Fiber Raman Amplifier Based on Cooperative Search Algorithm

Effect of BaF2 and Y2O3 on the Raman scattering characteristics of fluorotellurite glasses

In this letter, we investigated the Raman scattering characteristics of a series of aTeO2-(90-a)BaF2-10Y2O3 (a = 85, 80, 75, 70, 65, 60, 55 mol%), bTeO2-(95-b)BaF2-5Y2O3 (b = 90, 85, 80, 75, 70, 65, 60, 55, 50 mol%) and cTeO2-(100-c)BaF2 (c = 95, 90, 85, 80, 75, 70, 65, 60 mol%) fluorotellurite glasses. With increasing the concentration of BaF2, the peak Raman gain coefficient at 785 cm−1 increased while the Raman gain bandwidth (full spectral width at half maximum of the Raman peaks around 785 cm−1) decreased, which was attributed to the increasing proportion of non-bridge oxygen bonds in the fluorotellurite glass systems. The same results were also observed for the case of the increasing of the concentration of Y2O3. In these samples, the 50TeO2-40BaF2-10Y2O3 glass has the largest Raman gain coefficient of 29.9 × 10−13 m/W, and the 95TeO2-5BaF2 glass has the widest Raman gain bandwidth of 7.35 THz for the pumping laser at 633 nm. Furthermore, the first-order Raman Stokes light peaked at ∼2373 nm was obtained by using fluorotellurite fiber based on the above glasses as Raman gain medium and a 2000nm picoseconds laser as pump light. Our results provide guidance for further improving the performance of Raman fiber lasers or amplifiers based on fluorotellurite fibers.

Efficient mid-infrared dispersive wave generation through soliton breakup and cascaded Raman amplification in an axially varying fluorotellurite fiber

In this paper, we demonstrate efficient mid-infrared dispersive wave (DW) generation through soliton breakup and cascaded Raman amplification in an axially varying fluorotellurite fiber. The input part of the fluorotellurite fiber has two zero-dispersion wavelengths and the remaining part has an all normal dispersion profile. The pump source is a femtosecond fiber laser with an operational wavelength of 1980nm, which is located at the anomalous dispersion region between two zero-dispersion wavelengths and close to the second zero-dispersion wavelength of the fluorotellurite fiber. As the pump light is launched into the fluorotellurite fiber, the pump light (or a higher-order soliton) experiences a temporal breakup and large pulse broadening, which enables nearly complete conversion of input solitonic radiation into resonant nonsolitonic radiation in the DW regime. Meanwhile, the generated DWs are improved by more than two orders of magnitude via cascaded Raman amplification in the fluorotellurite fiber, resulting in the generation of efficient mid-infrared DWs peaked at 2700 nm with an ultrahigh power division ratio of ∼ 85% and a compressible pulse width of ∼ 61 fs. Our work presents a way to realize ultrahigh-efficiency mid-infrared coherent light generation.

Ppb-level methane detection sensitivity based on a homemade Raman fiber amplifier and differential photoacoustic technology.

A high-power near-infrared wavelength-modulated differential photoacoustic spectroscopy sensor for parts-per-billion (ppb) level methane detection is reported by using a homemade Raman fiber optical amplifier. A commercial 1653.7nm continuous wave distributed feedback laser is employed as a seed source to excite a high light power of ∼550m W, which greatly improves sensor performance. Wavelength modulation spectroscopy and differential techniques are applied to further improve the signal-to-noise ratio of the photoacoustic signal. A 1σ minimum detection limit of ∼10p p b for methane detection is achieved with an integration time of 10s.

Transverse Mode Instability in Raman Fiber Amplifiers

Hybrid ytterbium-Raman fiber amplifiers have reached multi-kW levels in recent years, showing great promise for kW powers at new wavelengths. Many applications need diffraction-limited output which is limited by transverse mode instability (TMI). In this work, we extended our 3D amplifier model for studying TMI in ytterbium fiber amplifiers to include Raman gain and used this new model to study TMI in two Raman fiber amplifiers: a hybrid ytterbium-Raman fiber amplifier and a passive Raman fiber amplifier. Our previous study shows that gain saturation effect in ytterbium fiber amplifiers plays a significant role in suppressing TMI in kW ytterbium fiber amplifiers. This new study shows that in a Raman fiber amplifier in the absence of similar connection between local Raman gain and local Stokes wave, TMI is not much affected by seed power, fiber length or core diameter in the absence of mode-dependent loss. Mode-dependent loss is a key factor for increasing the TMI threshold in this case.

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.

Chaos synchronization of semiconductor lasers over 1040-km fiber relay transmission with hybrid amplification

Optical chaos communication and key distribution have been extensively demonstrated with high-speed advantage but only within the metropolitan-area network range of which the transmission distance is restricted to around 300 km. For secure-transmission requirement of the backbone fiber link, the critical threshold is to realize long-reach chaos synchronization. Here, we propose and demonstrate a scheme of long-reach chaos synchronization using fiber relay transmission with hybrid amplification of an erbium-doped fiber amplifier (EDFA) and a distributed fiber Raman amplifier (DFRA). Experiments and simulations show that the hybrid amplification extends the chaos-fidelity transmission distance thanks to that the low-noise DFRA suppresses the amplified spontaneous emission noise and self-phase modulation. Optimizations of the hybrid-relay conditions are studied, including launching power, gain ratio of DFRA to EDFA, single-span fiber length, and number of fiber span. A 1040-km chaos synchronization with a synchronization coefficient beyond 0.90 is experimentally achieved, which underlies the backbone network-oriented optical chaos communication and key distribution.

Quantum-noise-limited distance of coherent transmission using fiber Raman amplifiers

Quantum noise intrinsically restricts the ultimate transmission distance in fiber communication systems that use optical repeating amplifiers. This study theoretically investigates the quantum-noise-limited distance of optical coherent transmission using fiber Raman amplifiers. An analytical formula for the upper bound of the transmission distance is derived based on full quantum mechanics, considering the amplification noise and intrinsic fluctuation of a coherent state (or vacuum noise), and calculations are performed using the derived formula. The results quantitatively indicate that the Raman amplified systems potentially achieve considerably longer transmission distances, more than ten times, than a system with erbium-doped fiber amplifiers under ideal conditions.