Raman Gain Spectrum Research Articles

Optimization of a wideband discrete Raman amplifier in a P2O5-doped optical fiber using multi-objective grey wolf algorithm

This paper demonstrates the use of the multi-objective grey wolf algorithm to optimize a discrete Raman amplifier (DRA) in a P2O5-doped optical fiber. Specifically, the multi-objective grey wolf algorithm is combined with the DRA to carry out a process that seeks to maximize the gain and minimize the ripple. The P2O5-doped optical fiber employed in this study has a Raman gain coefficient spectrum with multiple peaks with different frequency shifts. This allows them to be combined in more complex ways than optical fibers with a single peak in the Raman gain spectrum. Consequently, the gain curve produced with this fiber has the potential to be more adjustable even when fewer pumps are used. Thus, this paper explores this fact to perform, to the best of our knowledge, the first specialized optimization process reported in the scientific literature of a wideband discrete Raman amplifier in a P2O5-doped optical fiber. With a different gain profile of this fiber compared to those of traditional standard optical fibers, it was possible to design a wideband DRA, going from 1530 nm to 1675 nm, covering C+L+U bands, maintaining a ripple of up to 8 dB with a net gain of 14 dB using only 3 pumps. Moreover, this work demonstrates for the first time, through a comparative analysis, that the multi-objective grey wolf algorithm performs better than the standard and well-known non-dominated sorting genetic optimization algorithm to optimize a DRA in a P2O5-doped optical fiber. The proposed DRA is a feasible, low-cost, and simple alternative for building fiber amplifiers for future high-bandwidth and wideband wavelength division multiplexing (WDM) communication systems, network infrastructures such as data centers, undersea cables, 5G and beyond, and cutting-edge research applications.

Raman-assisted Kerr platicon and perfect soliton crystal formation in the cavity

In this paper, we investigate the soliton evolution dynamics influenced by the stimulated Raman scattering effect. The strong Raman gain ensures the breathing platicon and stable platicon formation at the cavity with normal quartic dispersion. The switching waves benefitted from the power transformation between pump mode and Raman gain spectrum could modulate the temporal waveform at the breathing platicon existence region. The oscillated characteristics of the temporal waveform mainly rely on the central frequency of the Raman gain spectrum. Meanwhile, at the anomalous quadratic dispersion regime, the Raman effects could capture the perfect soliton crystal formation which provides one novel method for the implementation of the harmonic mode-locking process. Our findings provide a viable way to demonstrate the cavity soliton dynamics and soliton-based applications.

Fiber Raman gain spectrum measurement over broad frequency shift range

Fiber Raman gain spectrum measurement over broad frequency shift range

Exact Solution of the Raman Response Function of Chalcogenide Fiber and Its Influence on the Mid-Infrared Supercontinuum

We fabricated a core-cladding Ge–Sb–Se glass fiber with a Ge12.5Sb15Se72.5 core and Ge15Sb10Se75 cladding, achieved a supercontinuum spectrum spanning from 2 μm to 9 μm by pumping the Ge–Sb–Se fiber with a core diameter of 11 μm using a femtosecond laser pump at 3.8 μm, and numerically simulated the supercontinuum generation using the generalized nonlinear Schrödinger equation. In particular, we investigate the effect of the different Raman response functions that were calculated using the traditional single Lorentzian model and a multiple vibrational mode model on the evolution of the supercontinuum by comparing the supercontinua obtained from simulation and experimental results. We demonstrate that the Raman response function generated by the multiple vibrational mode model captures the actual response behavior of the material, and the supercontinuum generated using this model has more accuracy. To the best of our knowledge, this is the first reported study on supercontinuum generation in Ge–Sb–Se fiber utilizing a Raman response function calculated using the multiple vibrational mode model. This significant advancement enables more accurate simulation of supercontinuum generation in fibers with a multi-peaked structured Raman gain spectrum and holds great potential for optimizing the performance of various mid-infrared supercontinuum sources.

Numerical Analysis of Dual-Wavelength Tungsten-Tellurite Fiber Raman Lasers with Controllable Mode Switching

Fiber laser sources in the spectral range near 1.7–1.8 μm are in highly demand for a lot of applications. We propose and theoretically investigate a dual-wavelength switchable Raman tungsten-tellurite fiber laser in the 1.7–1.8 µm range which can produce two stable modes at frequencies separated by ~7 THz with a pump at 1.55 µm. The Raman waves shifted by 19.8 THz (mode 1) and 27.5 THz (mode 2) from the pump frequency can be generated near two different maxima of the Raman gain spectrum (gain is higher at 19.8 THz and twice lower at 27.5 THz). We numerically simulate two-mode Raman lasing with allowance for energy transfer from the pump wave to modes 1 and 2, and from mode 1 to mode 2 due to inelastic Raman scattering. Diagrams of generation regimes depending on system parameters are constructed. We demonstrate controlled switching between two modes by changing the pump power. For the same intracavity losses for both Raman modes at relatively low pump powers, only mode 1 is generated. At medium pump power, generation occurs simultaneously in both modes. At relatively high pump power, only mode 2 is generated near the weaker maximum. This effect seems surprising, but a rigorous explanation with allowance for the nonlinear interaction between mode 1 and mode 2 is found. When losses for one of the modes change, switching of the generated regimes is also predicted.

Wavelength tunable Raman fiber laser based on Raman gain spectrum control

Wavelength tunable Raman fiber laser based on Raman gain spectrum control

Broadband tunable Raman fiber laser with monochromatic pump.

Raman fiber laser (RFL) has been widely adopted in astronomy, optical sensing, imaging, and communication due to its unique advantages of flexible wavelength and broadband gain spectrum. Conventional RFLs are generally based on silica fiber. Here, we demonstrate that the phosphosilicate fiber has a broader Raman gain spectrum as compared to the common silica fiber, making it a better choice for broadband Raman conversion. By using the phosphosilicate fiber as gain medium, we propose and build a tunable RFL, and compare its operation bandwidth with a silica fiber-based RFL. The silica fiber-based RFL can operate within the Raman shift range of 4.9 THz (9.8-14.7 THz), whereas in the phosphosilicate fiber-based RFL, efficient lasing is achieved over the Raman shift range of 13.7 THz (3.5-17.2 THz). The operation bandwidths of the two RFLs are also calculated theoretically. The simulation results agree well with experimental data, where the operation bandwidth of the phosphosilicate fiber-based RFL is more than twice of that of the silica fiber-based RFL. This work reveals the phosphosilicate fiber's unique advantage in broadband Raman conversion, which has great potential in increasing the reach and capacity of optical communication systems.

Pure quartic solitons in dispersion-engineered aluminum nitride micro-cavities.

Pure quartic soliton (PQS) is a new class of solitons demonstrated in recent years and provides innovations in nonlinear optics and its applications. Generating PQSs in micro-cavities offers a novel way to achieve coherent microcombs, presenting a promising application potential. Here we numerically investigate the PQS generation in a dispersion-engineered aluminum nitride (AlN) micro-cavity. To support PQS, a well-designed shallow-trench waveguide structure is adopted, which is feasible to be fabricated. The structure exhibits a dominant fourth-order dispersion reaching up to -5.35×10-3 ps4/km. PQSs can be generated in this AlN micro-cavity in the presence of all-order-dispersion and stimulated Raman scattering. Spectral recoil and soliton self-frequency shift are observed in the PQS spectrum. Furthermore, we find that due to the narrow Raman gain spectrum of crystalline AlN, the PQS evolves directly to chaos rather than turning into a breather. The threshold pump power with which the PQS turns into chaos is also theoretically calculated, which squares with the simulation results.

Modelling the Delayed Nonlinear Fiber Response in Coherent Optical Communications

Fiber nonlinearities, that lead to nonlinear signal interference (NLI), are typically regarded as an instantaneous material response with respect to the optical field. However, in addition to an instantaneous part, the nonlinear fiber response consists of a delayed contribution, referred to as the Raman response. The imaginary part of its Fourier transform, referred to as the Raman gain spectrum, leads to inter-channel stimulated Raman scattering (ISRS). ISRS is a nonlinear effect that redistributes optical power from high to lower frequencies during propagation. However, as the nonlinear fiber response is causal, the Raman spectrum obeys the Kramers-Kronig relations resulting in the real part of the complex valued Raman spectrum. While the impact of the imaginary part (i.e. ISRS) is well studied, the direct implications of its associated real part on the NLI are unexplored. In this work, a theory is proposed to analytically quantify the impact of the real Raman spectrum on the nonlinear interference power. Starting from a generalized Manakov equation, an extension of the ISRS Gaussian Noise (GN) model is derived to include the real Raman spectrum and, thus, to account for the complete nonlinear Raman response. Accurate integral expressions are derived and approximations in closed-form are proposed. Different formulations for the case of single -and dual polarized signals are derived and novel analytical approximations of the real Raman spectrum are proposed. Moreover, it is analytically shown that the real Raman spectrum scales the strength of the instantaneous nonlinear distortions depending on the frequency separation of the interacting frequencies. A simple functional form is derived to assess the scaling of the NLI strength. The proposed theory is validated by numerical simulations over C-and C+L band, using experimentally measured fiber data.

Anomalous slowdown of pump light in Raman fiber lasers.

Usually, the pump light in lasers should perform fast light owing to operating in the absorption band. In this study, we observe and demonstrate anomalous slowdown of the pump light in a Raman fiber laser. Experiments show that the pump light can be slowed down to sub-nanoseconds at a repetition rate of 50-500 MHz. Theoretical analysis shows that the hole-burning effect is formed at the Raman gain spectrum in the saturation regime, which imposes on the pump light by normal dispersion. Consequently, the pump light experiences an unusual slow light effect rather than the fast light effect after absorption. We believe it has promising potentials in the improvement of ultrashort pulse generation, and may have significant influence on improving the conversion efficiency in pulse-pumped laser systems.

Effective suppression of stimulated Raman scattering in direct laser diode pumped 5 kilowatt fiber amplifier using chirped and tilted fiber bragg gratings

We report here for the first time, to the best of our knowledge, a direct laser diodes pumped 5 kW 1080 nm fiber amplifier using chirped and tilted fiber Bragg gratings (CTFBGs) to suppress stimulated Raman scattering (SRS). With one or two CTFBGs, which are carefully designed and fabricated to match the Raman gain spectrum, inserted between the seed and the amplifier stage, effective SRS suppression is obtained. The maximum suppression ratio is about 20 dB with one CTFBG and 27.5 dB with two when the amplifier operates at 5 kW, with beam quality factor M2 of ∼1.8. By further improving the performance of CTFBGs and using more CTFBGs in a series, a better SRS suppression effect could be achieved, which is very useful for further power scaling of all-fiber laser systems.

Ultralow-quantum-defect Raman laser based on the boson peak in phosphosilicate fiber

Quantum defects (QDs) have always been a key factor of the thermal effect in high-power fiber lasers. Much research on low-QD fiber lasers has been reported in the past decades, but most of it is based on active fibers. Besides, Raman fiber lasers based on the stimulated Raman scattering effect in passive fiber are also becoming an important kind of high-power fiber laser for their unique advantages, such as their significantly broader wavelength-tuning range and being free of photon darkening. In this paper, we demonstrate an ultralow-QD Raman fiber laser based on phosphosilicate fiber. There is a strong boson peak located at a frequency shift of 3.65 THz in the Raman gain spectrum of the phosphosilicate fiber we employed. By utilizing this boson peak to provide Raman gain and adopting an amplified spontaneous emission source at 1066 nm as the pump source, 1080 nm Stokes light is generated, corresponding to a QD of 1.3%. The spectral purity at 1080 nm can be up to 96.03%, and the output power is 12.5 W, corresponding to a conversion efficiency of 67.2%. Moreover, by increasing the pump wavelength to 1072 nm, the QD is reduced to 0.74%, and the output power at 1080 nm is 10.7 W, with a spectral purity of 82.82%. To the best of our knowledge, this is the lowest QD ever reported for Raman fiber lasers. This work proposes a promising way of achieving high-power, high-efficiency Raman fiber lasers.

Generation of stable and breathing flat-top solitons via Raman assisted four wave mixing in microresonators

Flat-top-soliton (or platicon) dynamics in coherently pumped normal dispersion microresonators is important for both fundamental nonlinear physics and microcomb generation in the visible band. Here we numerically investigate the platicon generation that is initiated via Raman assisted four wave mixing instead of mode interaction. To show the possibility of generating coherent combs in the visible band, we design an aluminum nitride (AlN) microresonator with normal dispersion and investigate the comb generation dynamics in simulations. Stable platicon Kerr combs can be generated in this AlN microresonator using a 780-nm pump. Moreover, we also observe a breather platicon dynamics induced by the narrow Raman gain spectrum of crystalline AlN, which shows distinct dynamics from the dark soliton breathers reported in previous work that are dominated by Kerr effect. A phase diagram bearing the influence of the pump detuning and pump power on the breathing dynamics of the breather platicon is also computed. Furthermore, a transition to chaotic breathing is numerically observed.

Dual-wavelength random fiber laser incorporating micro-air cavity

A dual-wavelength random distributed feedback fiber laser with external micro-air cavity (MAC) is experimentally demonstrated. The structure that comprises of a fixed 200 μm air-gap distance between two flat-angle fiber-end facets behaves similarly to the role of reflectors. The generation of dual-wavelength lasing at 1552.5 and 1557.7 nm was achieved. The accomplishment is associated to the filtering effect of MAC superimposed with Raman gain spectrum. Equal peak power of approximately −12.7 dBm was attained at maximum 2.0 W pump power. In this case, the respective optical signal-to-noise ratios were 18.79 and 18.73 dB. Excellent stability of the spectral width was realized over 60 min observation time at room temperature, which denotes the practicality of this device in addition to the relatively simpler architecture.

Raman gain coefficient of Er3+ doped TeO2–Li2O–ZnO glasses

Tellurite glasses are good candidates for high-speed optical communications due to interesting properties and hence they present higher Raman cross section as compared to other glassy systems. In this work, tellurite glasses with 65TeO2-15Li2O-20ZnO (TLZ1) and 60TeO2-15Li2O-25ZnO (TLZ2) compositions and doped with different concentrations of Er2O3 (0.5; 1.0; and 2.0 mol%) were analyzed. The samples were synthetized by conventional melt-quenching method. The Raman gain coefficients of these glasses were obtained from Raman scattering experiments using 532 nm excitation. The Raman results presented a broad bandwidth from 225 to 1160 cm−1 that become broader with Er3+ concentration. Besides that, a slight difference on the 760 cm−1 band was observed with variation of the host composition. The Raman gain spectra presented a quenching effect for concentrations higher than 0.5 mol% and 1.0 mol% of Er2O3 for TLZ1 and TLZ2, respectively. This quenching have been attributed to the variation in TeOx units present in each composition. The Raman gain coefficient increased with Er3+ concentration and these values were two orders of magnitude higher than silicate glasses. Therefore, the materials studied present potential as new media for Raman amplifiers.

Simple method for estimating the fractional Raman contribution.

We propose a novel and simple method for estimating the fractional Raman contribution, fR, based on an analysis of a full model of modulation instability (MI) in waveguides. An analytical expression relating fR to the MI peak gain beyond the cutoff power is explicitly derived, allowing for an accurate estimation of fR from a single measurement of the Raman gain spectrum.

Dual-wavelength bidirectional pumped high-power Raman fiber laser

In this paper, we reported both the experimental demonstration and theoretical analysis of a Raman fiber laser based on a master oscillator–power amplifier configuration. The Raman fiber laser adopted the dual-wavelength bidirectional pumping configuration, utilizing 976 nm laser diodes and 1018 nm fiber lasers as the pump sources. A 60-m-long $25/400~\unicode[STIX]{x03BC}\text{m}$ ytterbium-doped fiber was used to convert the power from 1070 to 1124 nm, realizing a maximum power output of 3.7 kW with a 3 dB spectral width of 6.8 nm. Moreover, we developed a multi-frequency model taking into consideration the Raman gain spectrum and amplified spontaneous emission. The calculated spectral broadening of both the forward and backward laser was in good agreement with the experimental results. Finally, a 1.5 kW, 1183 nm second-order Raman fiber laser was further experimentally demonstrated by the addition of a 70-m-long germanium-doped passive fiber.

Multi-pumped tellurite-based Raman fiber amplifier based on Gaussian fitting of gain spectrum

The theoretical model and amplification principle of Raman fiber amplifier (RFA) are introduced. The gain curve and gain coefficient function which can accurately reflect the information of Raman gain spectrum (RGS) are obtained by the eighth-order Gaussian fitting of RGS of tellurite-based fiber. Combined with the technique of multi-pump, Raman power coupled wave differential equation is numerically solved by the fourth-order Runge–Kutta method. And Raman gain of each signal wavelength is obtained. According to the requirements of Dense Wavelength Division Multiplexing optical fiber transmission system for RFA, the designed tellurite-based Raman fiber amplifier (T-RFA) has improved the gain and bandwidth, and the smaller gain flatness is guaranteed. Finally, the average gain of the designed T-RFA obtained by Matlab numerical simulation is 25.518 dB, the maximum gain is 26.20 dB, the gain bandwidth is 56 nm and the gain flatness is 1.34 dB.

Ultra-flat wideband single-pump Raman-enhanced parametric amplification.

We experimentally optimize a single pump fiber optical parametric amplifier in terms of gain spectral bandwidth and gain variation (GV). We find that optimal performance is achieved with the pump tuned to the zero-dispersion wavelength of dispersion stable highly nonlinear fiber (HNLF). We demonstrate further improvement of parametric gain bandwidth and GV by decreasing the HNLF length. We discover that Raman and parametric gain spectra produced by the same pump may be merged together to enhance overall gain bandwidth, while keeping GV low. Consequently, we report an ultra-flat gain of 9.6 ± 0.5 dB over a range of 111 nm (12.8 THz) on one side of the pump. Additionally, we demonstrate amplification of a 60 Gbit/s QPSK signal tuned over a portion of the available bandwidth with OSNR penalty less than 1 dB for Q2 below 14 dB.

基于多模激光抽运的1.70 μm波段拉曼增益谱实验研究

基于多模激光抽运的1.70 μm波段拉曼增益谱实验研究