Multimode Fiber Amplifier Research Articles

Accurate modeling of ultrafast nonlinear pulse propagation in multimode gain fiber

The nonlinear propagation of picosecond or femtosecond optical pulses in multimode fiber amplifiers underlies a variety of intriguing physical phenomena as well as the potential for scaling sources of ultrashort pulses to higher powers. However, existing theoretical models of ultrashort-pulse amplification do not include some critical processes, and, as a result, they fail to capture basic features of experiments. We introduce a numerical model that combines steady-state rate equations with the unidirectional pulse propagation equation, incorporating dispersion, Kerr and Raman nonlinearities, and gain/loss-spectral effects in a mode-resolved treatment that is computationally efficient. This model allows investigation of spatiotemporal processes that are strongly affected by gain dynamics. Its capabilities are illustrated through examinations of amplification in few-mode gain fiber, multimode nonlinear amplification, and beam cleaning in a multimode fiber amplifier.

Efficient employment of Yb doped multi-mode fiber amplifiers based pumped laser with advanced light sources in high speed modulated optical communication systems

Abstract In this work, efficient employment of Yb doped multi-mode fiber amplifiers based pumped laser is simulated with advanced light sources in high speed modulated optical communication systems. The total lighted power through measured index multimode fiber based different base band modulation voltages dual drive MZM measured are clarified. The 3D graph of the mode field intensity distribution through measure index multimode fiber is simulated. Total electrical base ban power form through PIN photo-detector based various modulation voltages Dual drive MZM measured are demonstrated. The max Q form factor-min base band BER values through PIN photo-detector based different modulation voltages dual drive MZM measured are also simulated. The electrical base signal form noise ratio varies is simulated and clarified with modulation voltage based dual drive MZM measure varies. The max Q form base band Factor is demonstrated with modulation voltage based dual drive MZM measure varies. Total lighted base power form varies are clarified with modulation voltage based dual drive MZM measure varies. The different measured index multimode fiber characteristics based spatial VCSEL are also clarified and demonstrated.

Efficient Integrated Multimode Amplifiers for Scalable Long-Haul SDM Transmission

Scaling up cable capacity in a power- and cost-efficient manner is a goal of spatial-division multiplexing (SDM) in long-haul optical communication systems. Integration is a key to scaling SDM beyond the parallel single-mode fiber paradigm. Multicore or multimode fiber systems promise higher integration, but native multicore or multimode amplifiers pose challenges in efficient coupling of pump light into the amplifier, and in control of mode-dependent gain (MDG). We propose an integrated multimode amplifier design for signals in six spatial modes (12 spatial and polarization modes) that relies on excitation of only four pump spatial modes at 980 nm. Simulations of an exemplary amplifier predict a relatively flat gain above 10 dB, noise figure below 5 dB, and MDG standard deviation below 0.1 dB over the C-band (1530–1565 nm), with a power-conversion efficiency close to 30%. Key design features include a graded-index multimode fiber amplifier with optimized ring erbium doping profile, length and pump mode powers; and a cascade of wavelength- and mode-selective couplers that efficiently couples four pump diodes to four pump modes, while passing signal modes with minimal loss. Our design approach, which uses fewer pump diodes per signal mode than parallel single-mode systems, may provide an option for efficient, economical scaling of SDM long-haul systems.

Suppressing transverse mode instability through multimode excitation in a fiber amplifier

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, science, and defense. The power scaling for fiber amplifiers is currently limited by transverse mode instability. Most techniques for suppressing the instability are based on single- or few-mode fibers in order to output a clean collimated beam. Here, we study theoretically using a highly multimode fiber amplifier with many-mode excitation for efficient suppression of thermo-optical nonlinearity and instability. We find that the mismatch of characteristic length scales between temperature and optical intensity variations across the fiber generically leads to weaker thermo-optical coupling between fiber modes. Consequently, the transverse mode instability (TMI) threshold power increases linearly with the number of equally excited modes. When the frequency bandwidth of a coherent seed laser is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either the input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are required for fiber amplifiers in various applications.

High energy multi-mode nanosecond pulsed fiber amplifier based on the 940 nm LD pumping with effective suppression of the ASE and the SRS effect

We demonstrated, for the first time, an amplified spontaneous emission (ASE) and stimulated Raman scattering (SRS) free multi-mode nanosecond pulsed fiber amplifier based on the 940 nm LD pumping. The maximum pulse energy reached 15.3 mJ at the average output power of 306 W using a 100/400 Yb-doped extremely large mode area fiber. A near top-hat beam with M2 = 10.4 was achieved. This is the highest pulse energy generated from the 940 nm LD pumped multi-mode nanosecond pulsed fiber amplifier. In addition, comparative experiments based on the conventional 915 nm LD pumping and the 976 nm LD pumping were conducted. The results showed that the 915 nm LD pumping lead to intensive forward 1030 nm ASE, and the 976 nm LD pumping was limited by strong backward ASE power. These confirmed that the 940 nm LD pumping provided an effective method to suppress the ASE and the SRS effect in the high energy multi-mode nanosecond pulsed fiber amplifier.

Spatial beam narrowing in multimode graded-index fiber amplifiers: an analytic approach.

Doped and optically pumped graded-index (GRIN) fibers can be used to amplify an optical beam such that its spatial quality is improved at the output end of the fiber compared with that of the unamplified beam. We develop a simple model of the amplification process in such GRIN fiber amplifiers and show that the resulting equations can be solved analytically with suitable approximations. The solution shows that the width of the amplifying beam oscillates but also becomes narrower because of the radial dependence of the optical gain. The main advantage of our simplified approach is that it provides an analytic expression for the damping distance of beam-width oscillations that shows clearly the role played by various physical parameters.

Numerical analysis of beam self-cleaning in multimode fiber amplifiers

Recent experimental results have reported the observation of beam self-cleaning or, more generally, nonlinear beam reshaping in active multimode fibers. In this work, we present a numerical analysis of these processes by considering the ideal case of a diode-pumped signal amplifier made of a graded-index multimode fiber with uniform Yb doping. Simulations confirm that beam cleaning of the signal may take place even in amplifying fibers, in the absence of beam power conservation. Moreover, we show how the local signal intensity maxima, which are periodically generated by the self-imaging process, may influence the population inversion of the doping atoms, and locally saturate the amplifier gain.

Successive Interference Cancellation on Frequency-Selective Channels With Mode-Dependent Gain

In ultra-long-haul optical communication systems, polarization-dependent gain from single-mode semiconductor optical amplifiers or mode-dependent gain from multi-mode erbium-doped fiber amplifiers causes a channel’s achievable information rate (AIR) using linear minimum-mean-square error (MMSE) detection to become substantially lower than the capacity of optimal maximum likelihood detection owing to loss of orthogonality between modes. However, such capacity loss can be mitigated using techniques that retain reasonable complexity, such as successive interference cancellation (SIC). In wireless systems, multicarrier transmission with many subcarriers transforms the frequency-selective channel into a set of uncoupled memoryless subchannels, and it is possible to obtain good performance using SIC on each subchannel independently. Multicarrier transmission in optical fibers, however, is susceptible to performance loss caused by Kerr nonlinearity. Therefore, we study methods for applying SIC on frequency-selective optical fiber channels using single-carrier transmission. We compare the theoretical performance of MMSE and SIC equalizers on frequency-selective optical fiber channels operating at 64 Gbaud with transmission distances up to 15,000 km and evaluate the penalties of SIC implementation. We consider single-mode fiber links with random artificial polarization-mode dispersion inserted along the link to enhance frequency diversity, as well as multi-core fiber links supporting 14 spatial and polarization modes. We show that SIC can increase outage capacity by 19%, but the AIR increase is reduced by almost half if error correction coding is not performed across modes. We confirm that SIC can adapt to fast channel perturbations with only a 3% decrease in AIR.

Self-cleaning effect in an all-fiber spatiotemporal mode-locked laser based on graded-index multimode fiber

We demonstrate an all-fiber spatiotemporal mode-locked laser based on graded-index multimode fiber. Due to the high damage threshold of graded-index multimode fiber devices, the output single-pulse energy reach up to 11.67 nJ. The beam quality at mode-locked state is significantly improved compared with continuous wave state as a result of the self-cleaning effect. By amplifying the pulse in the multimode fiber amplifier, high power pulses with high beam quality can be obtained in the all-fiber system based on graded-index MMFs. We present an approach to get high single-pulse energy in the all-fiber multimode laser with a near single-mode output beam.

Spatial Beam Self-Cleaning in Tapered Yb-Doped GRIN Multimode Fiber With Decelerating Nonlinearity

We experimentally demonstrate spatial beam self-cleaning in an Yb-doped graded-index multimode fiber taper, both in passive and active configurations. The input laser beam at 1064 nm was injected for propagation from the small to the large core side of the taper, with laser diode pumping in a counterdirectional configuration. The Kerr effect permits to obtain high-beam quality amplification with no accompanying frequency conversions. As a result, our nonlinear taper amplifier may provide an important building block for multimode fiber lasers and amplifiers.

Bending diameter dependence of mode instabilities in multimode fiber amplifier

We systematically investigated the dependence of mode instability (MI) thresholds on bending diameters in ytterbium-doped fiber amplifiers. The MI thresholds of a fiber amplifier, which was based on a multimode ytterbium-doped fiber, were measured at various bending diameters. It is found that the correlation between MI thresholds and fiber bending diameters depends on the operation state of the active fiber. When the multimode active fiber is in the few-mode operation, the MI threshold is successfully raised from 717 W to 1084 W by increasing the fiber bending diameter from 11 cm to 60 cm. When the multimode active fiber operates in quasi-single mode, the MI threshold is raised from 717 W to 953 W by reducing the bending diameter from 11 cm to 8 cm. According to our experiments and previous reports, it is suggested that the influence of changing fiber bending diameter on MI might be closely related to the mode coupling effect.

Photonic lantern kW-class fiber amplifier.

Pump-limited kW-class operation in a multimode fiber amplifier using adaptive mode control and a photonic lantern front end was achieved. An array of three single-mode fiber inputs was used to adaptively inject the appropriate superposition of input modes in a three-mode gain fiber to achieve the desired mode at the output. Mode fluctuations at high power were compensated by adjusting the relative phase, amplitude, and polarization of the single-mode fiber inputs. The outlook for further power scaling and adaptive-optic compensation is described.

Shaping the light amplified in a multimode fiber

Propagation of light in multimode optical fibers usually gives a spatial and temporal randomization of the transmitted field similar to the propagation through scattering media. Randomization still applies when scattering or multimode propagation occurs in gain media. We demonstrate that appropriate structuration of the input beam wavefront can shape the light amplified by a rare-earth-doped multimode fiber. Profiling of the wavefront was achieved by a deformable mirror in combination with an iterative optimization process. We present experimental results and simulations showing the shaping of a single sharp spot at different places in the output cross-section of an ytterbium-doped fiber amplifier. Cleaning and narrowing of the amplifier far-field pattern was realized as well. Tailoring the wavefront to shape the amplified light can also serve to improve the effective gain. The shaping approach still works under gain saturation, showing the robustness of the method. Modeling and experiments attest that the shaping is effective even with a highly multimode fiber amplifier carrying up to 127 modes.

Azimuthal dependence of modal gain and its impact on gain equalization in multimode erbium-doped fiber amplifiers

Azimuthal-dependent modal gain for spatially degenerate signal modes in multimode erbium-doped fiber amplifiers (MEDFAs) has been numerically investigated by integrating the differential power rate equations dealing with multiple spatial modes. Results show that if the azimuthal mode numbers in signal modes are different from that in the pump modal, gain is nearly independent of the azimuthal orientations of the pump intensity pattern. High differential modal gain (DMG) between spatially degenerate signal modes is produced if signal and pump shares the same azimuthal mode number when modal gain varies periodically with the pump azimuthal angle. Such azimuthal dependence of modal gain is caused by the transverse azimuthal dependence of population inversion of erbium ions created by pump modes. Adjusting the power and azimuth of the pump mode has also been examined to reduce azimuthal-induced DMG and several optimal parameters for achieving gain equalization have been obtained for an MEDFA supporting the two lowest LP signal mode groups. Our results can be employed to equalize mode-dependent gain in MEDFAs.

Cascaded Four-Wave Mixing in Nonlinear Yb-Doped Fiber Amplifiers

We experimentally demonstrated high-power yellow and near-infrared laser emissions by cascaded four-wave mixing in a nonlinear Yb-doped multimode fiber amplifier, generating 2.0 W at 830 nm and 0.35 W at 594 nm. Moreover, multiple cascaded four-wave mixing processes which generated supercontinuum covering more than four octaves from 398 to 1700 nm was also investigated in Q-switched mode-locking regime. The group velocity matching for pump and parametric waves as well as multimode fiber amplification promoted the cascaded nonlinear frequency mixing processes.

Adaptive Modal Gain Equalization Techniques in Multi-Mode Erbium-Doped Fiber Amplifiers

We demonstrate two adaptive methods to equalize mode-dependent gain (MDG) in multi-mode erbium-doped fiber amplifiers (MM-EDFAs). The first method uses a spatial light modulator (SLM) in line with the amplifier pump beam to control the modal powers in the pump. The second method uses an SLM immediately after the MM-EDFA to directly control the modal powers in the signal. We compare the performance of the two methods applied to a MM-EDFA with a uniform erbium doping profile, supporting 12 signal modes in two polarizations. We show that root-mean-squared MDGs lower than 0.5 and 1 dB can be achieved in systems having frequency diversity orders of 1 and 100, respectively, while causing less than a 2.6-dB loss of mode-averaged gain.

High-power master-oscillator power-amplifier with optical vortex output

A high-power master-oscillator power-amplifier with optical vortex output is reported. The master oscillator for an optical vortex seed beam is a simple two-mirror Nd:YAG laser using a fiber-based pump beam conditioning scheme. The seed is amplified in a double-clad multimode fiber amplifier end-pumped by a high-power diode laser at 975nm yielding 10.7W of continuous-wave output at 1064nm in the first-order Laguerre–Gaussian beam with M2 ~ 2.11 for an absorbed pump power of 17.5W, corresponding to a slope efficiency of ~59 %. The ring-shaped intensity profile and the wave front handedness of the seed beam were well preserved in the fiber amplifier. The prospects of power scaling via this approach are discussed.

Pulse amplification characteristics of multimode fiber amplifier

Under small signal situation, the analytical expression for describing the output signal emitted from an Yb3+-doped fiber amplifier with a Gaussian input pulse signal is deduced, by full considering both fiber guided modes’ propagation constant and filling factor, which depend on the signal frequency. Pumped by continue wave (CW) source, the analytical expressions of the small signal gain and each guided mode's output power are obtained. Based on these analytical expressions, the various intensity distributions of output pulses amplified by the fiber amplifier with different Gaussian input pulse widths are analyzed.

Accurate modal gain control in a multimode erbium doped fiber amplifier incorporating ring doping and a simple LP_01 pump configuration

We experimentally validate a numerical model to study multimode erbium-doped fiber amplifiers (MM-EDFAs). Using this model, we demonstrate the improved performance achievable in a step index MM-EDFA incorporating a localized erbium doped ring and its potential for Space Division Multiplexed (SDM) transmission. Using a pure LP₀₁ pump beam, which greatly simplifies amplifier construction, accurate modal gain control can be achieved by carefully tuning the thickness of the ring-doped layer in the active fiber and the pump power. In particular, by optimizing the erbium-ring-doped structure and the length of active fiber used, over 20dB gain for both LP₀₁ and LP₁₁ signals with a maximum gain difference of around 2 dB across the C band are predicted for a pure LP₀₁ pump beam delivering 250 mW power at 980 nm.

Thermally induced mode coupling in rare-earth doped fiber amplifiers

We present a simple semianalytical model of thermally induced mode coupling in multimode rare-earth doped fiber amplifiers. The model predicts that power can be transferred from the fundamental mode to a higher-order mode when the operating power exceeds a certain threshold, and thus provides an explanation of recently reported mode instability in such fiber amplifiers under high average-power operation. We apply our model to a simple step-index fiber design, and investigate how the power threshold depends on various design parameters of the fiber.