Power scalability of diffraction-limited Yb-doped fiber amplifiers with consideration of radiation-induced attenuation

<sec>Yb-doped fiber amplifiers and their applications in radiation environments have become more and more attractive in recent years. However, the radiation effect will cause damage to the Yb-doped fibers, which can give negative effect on the output properties of Yb-doped fiber amplifiers. In this work, the influence of radiation effect on the transverse mode instability (TMI) of Yb-doped fiber amplifier is studied. TMI can couple the single light from the fundamental mode to high-order mode, thereby degenerating the beam quality of fiber amplifier. TMI is considered a key limitation of power up-scaling of fiber amplifiers.</sec><sec>In this work, the radiation effect on the TMI is studied theoretically, and a formula of TMI threshold is presented by taking the radiation-induced attenuation (RIA), the most important radiation effect for the TMI, into account. The formula is deduced by introducing the loss of signal light induced by RIA into the formerly reported TMI-threshold formula which can be obtained by the linear stability analysis of the numerical model studying the TMI. Then, the relationship between the TMI and radiation dose is also given with the help of Power-Law describing the relationship between the RIA and radiation dose.</sec><sec>With the formula, the variations of TMI threshold with the radiation dose and RIA are studied. It is found, as expected, that the TMI threshold decreases monotonically with the increase of RIA or radiation dose. Nevertheless, it is unexpectedly found that, to some extent, the gain coefficient of fiber amplifiers will also affect the radiation effect on TMI threshold. The results reveal that the increase of gain coefficient will lower the sensitivity of TMI threshold to the radiation dose. However, it is also implied that the gain coefficient cannot be too large because it can also make the TMI threshold lowered. Therefore, in order to maintain a high TMI threshold in a radiation environment, sufficient radiation resistance of Yb-doped fiber is essential.</sec><sec>Because the RIA can affect not only the TMI threshold but also the output power or efficiency of Yb-doped fiber amplifier, the comparison between two effects of RIA is also discussed. It is found that the threshold of TMI is more sensitive to the radiation than to the output power or efficiency (see the figure attached below), which means that the TMI can exist in the irradiated Yb-doped fiber amplifier, although the output power is reduced because of RIA. This result can be verified by the experimental observation reported formerly. As a result, TMI can become a key limitation to the output power of Yb-doped fiber amplifier in radiation environments. The relevant results can provide significant guidance for the applications of Yb-doped fiber amplifiers in radiation environments.</sec>

Spatio-temporal instability of fundamental mode in Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped large mode area polarization-maintaining fiber amplifiers was analyzed. Limitations of the amplifier output power and gain were investigated depending on the input signal parameters: spectral width, power level and polarization. The influence of backward reflection of the optical waves on the mode-instability threshold was also examined. The traveling electronic and thermal refractive index gratings following the optically-induced population gratings were found to provide the energy transfer from the main mode to the higher-order modes.

In this paper, a single-mode Q-switched 980 nm Yb-doped fiber amplifier were studied experimentally. In the experiment, we used a 980 nm acousto-optics (AO) Q-switched Yb-doped single-mode fiber laser as the seed source, which generated an average output power of 73 mW at 980 nm with the pulse width of 10 ns at repetition frequency of 16 kHz. Then, another single-mode Yb-doped fiber pumped by a cw 946 nm Nd:YAG laser composed the fiber amplifier. As a result, the 980 nm amplified light with 150 mW average output power was generated. Through frequency doubling the 980 nm laser with a BIBO crystal, an average output power of 8 mW of pulsed blue-green laser at 490 nm was generated.

Photodarkening and photobleaching processes affect the level of photodegradation of Yb-doped fibers. Characterization and modeling of each process is crucial to understand how to optimize the operating conditions of fiber amplifiers and lasers to obtain acceptable output power degradation. We sh ow that photobleaching is a ke y factor in the modeling and simulation of a 10-ns pulsed Yb-doped LMA fiber amplifier. Each parameter of the model was separately determined from induced excess loss measurements under selective pump and wavelength excitations. The model was used to simulate accurately the measured fiber amplifier degradation. Optimized fiber length and gain were calculated to improve the output power stability over time and increase the fiber lifetime. Furthermore, eight fibers have been fabricated with various Yb, Al, and P content using the MCVD process to optimize the core composition. The level of photodarkening in each fiber was evaluated by measuring separately rate coefficient and excess loss. It was found that all fibers followed a similar inversion-dependent rate while th e maximum excess loss was dependent on the ratios [Al]/[Yb] and [P]/[Yb]. The proposed model allows for rapid evaluation and optimization of fiber parameters and operation conditions to assist Yb-doped laser system design in ac hieving the desired performance with low photodegradation. Keywords: Fiber amplifier, large mode area, ytterbium, triple-clad, photodarkening, photobleaching, modeling, fiber characterization

Coherent combination is one of the most promising ways to realize high power laser output. A three- laser-beam coherent combination system based on adaptive optics (AO) technique has been set up in our laboratory. In this system, three 1064nm laser beams are placed side-by-side and compressed by two reflective mirrors. An active segmented deformable mirror (DM) is used to compensate the optical path difference (OPD) among three laser beams. The beams are overlapped onto a 2900Hz CCD camera to form an interference pattern while the peak intensity of the interference pattern is taken as the cost function to optimize by a stochastic parallel gradient descent (SPGD) algorithm. SPGD algorithm is realized on a RT-Linux dual-core industrial computer. A series of experiments have been accomplished and experimental results show that both static distorted aberrations in the beams and active distorted aberrations (which are brought in by a hot iron and the frequency is about 5Hz) can be compensated successfully when the gain coefficients and the perturbation amplitude of SPGD are chosed appropriately, thereby three beams can be well combined. For controlling the phase of fiber lasers, the phase characteristics of beams passing through Yb-doped dual-clad fiber amplifier are measured by means of investigating the interference pattern under different output power through experiments. The frequency of phase fluctuation is evaluated through analyzing the fluctuation of power within a 90um aperture of far-field focal spot. Experimental results show that the phase fluctuation frequencies of laser beam transmitted through fiber amplifier are mainly in the range of 100~1500Hz. As a result, to control the phase fluctuation of beams passing through fiber amplifier, the bandwidth of any potential phase control scheme must be greater than 1.5 kilohertz.

In this paper, we report on the development of a short-pulse dual-wavelength source consisting of mode-locked diode lasers and a single Yb-doped double-clad fiber amplifier. Two mode-locked external-cavity semiconductor oscillators operating at a repetition rate of 577 MHz with center wavelengths of 1040 nm and 1079 nm are synchronized, producing short pulses that are injected into a Yb-doped polarization-maintaining fiber for amplification. Numerical simulations are used to determine the optimal fiber length and seeding configuration for dual-wavelength amplification in the fiber. Each signal is amplified to an average power of 0.5 W with pulse durations of around 5 ps. Performance issues associated with two-signal amplification in Yb-doped fibers are discussed, as well as perspectives for increasing the wavelength separation of the seed lasers.

&lt;sec&gt;Power enhancement of single-frequency single-mode Yb-doped fiber amplifiers is significant for their applications. Considering their potential applications in radiation environments, the influence of radiation-induced attenuation (RIA) on the power enhancement of signal-frequency single-mode Yb-doped fiber amplifiers is studied in this work. A theoretical model for predicting the power limitation of single-frequency single-mode Yb-doped fiber amplifiers is proposed by considering the limitations of pump brightness, stimulated Brillion scattering (SBS), and transverse mode instability (TMI) on power, and taking RIA into account. It is revealed that RIA can not only greatly lower the power limit, but also make it more difficult to achieve power limitation. The analytic formula of power limit is deduced. It is found that the effect of RIA on the power limitation is mainly determined by the optimal length with no RIA. It is suggested that the reduction of power limitation caused by RIA can be weakened by shortening the optimum length of Yb-doped fiber.&lt;/sec&gt;&lt;sec&gt;The requirement of Yb-doped fiber for achieving certain target power is also discussed and the needed ranges of core diameter and fiber length are given analytically. It is found that the RIA will increase the difficulty in achieving the target power by limiting the option of Yb-doped fibers. In spite of that, it is also found that such an effect of RIA can be weakened by increasing the core absorption coefficient and pump brightness. Moreover, the numerical model and related formula can also reveal the influence of radiation dose by fitting the relationship between RIA and radiation dose through using the empirical expressions such as power law. They can provide significant guidance for designing and utilizing single-frequency single-mode Yb-doped fiber amplifiers in radiation environments.&lt;/sec&gt;

We report the generation of high peak power linearly polarized femtosecond optical vortex pulses based on parabolic pulse amplification within a large-mode-area few-mode Yb-doped fiber (YDF) amplifier system. A dispersion-managed passively mode-locked Yb-doped fiber oscillator with a Gaussian-shaped spectrum serves as the seed source, which evolves into parabolic shape in the subsequent preamplifiers. A q-plate-based mode converter, positioned before the power amplifier, is employed to convert the Gaussian-shaped beam into optical vortex beams with a topological charge of l = ±1. The resulting optical vortex beams are efficiently amplified to an average output power of ∼16.8 W at a repetition rate of 3.7 MHz, corresponding to a pulse energy of 4.5 µJ. The chirped output pulses are externally compressed to ∼200 fs, achieving a peak power of ∼12.7 MW. To the best of our knowledge, this represents the highest peak power achieved to date for optical vortex pulses generated from a conventional step-index fiber.

In this work, we present a monolithic single-frequency, single-mode and polarization maintaining Yb-doped fiber (YDF) amplifier delivering up to 6.9 W at 972 nm with a high efficiency of 53.6%. Core pumping at 915 nm and elevated temperature of 300 °C were applied to suppress the unwanted 977 nm and 1030 nm ASE in YDF, so as to improve the 972 nm laser efficiency. In addition, the amplifier was further used to generate a single-frequency 486 nm blue laser with 590 mW of output power by single-pass frequency doubling.

We used coupled-mode theory in an Yb-doped multimode fiber amplifier to compute the effects of gain saturation, nonlinear index, and fiber curvature on the evolution of the field. A positive nonlinear index results in power transfer to lower-order modes, usually the fundamental LP₁₀ mode, and for negative nonlinear index the reverse is predicted. The nonlinear interaction between modes breaks the core's cylindrical symmetry, resulting in recombination of degenerate LP mode pairs into super-modes: consisting of an expected in- and anti-phase pair, but also a quadrature of super-mode that reflects an increase of "information" capacity associated with nonlinearity. Convergence to all three super-modes was observed in our simulations, but the last more often. We also present observed evidence of mode phasing in experiments with two fiber amplifiers.

Summary form only given. The generation of ultrafast optical pulses that are used for many applications is increasingly based on Er:fiber lasers. The inherent advantages of this technology are compactness, stability, and turn-key operation. Tm- and Yb-doped fiber systems are promising candidates for reaching microjoule pulse energies [1,2,3].We present a setup exploiting the well-established Er:fiber technology that provides a femtosecond pulse train suitable for coherent seeding of both Yb: and Tm:amplifiers. A seed source generates pulses with a wavelength centered at 1550 nm. Four parallel amplification branches each provide 8 nJ pulse energy at a repetition rate of 40 MHz. This source implements also a passive phase-locking scheme [4]. The pulses are compressed in a silicon prism pair and then coupled into a highly nonlinear fiber (HNF). This scheme allows us to generate tailor-cut spectra with components spanning from 800 to 2300 nm that can be finely tuned by material insertion in the prism sequence [5]. The solitonic part of the spectrum is centered at 1970 nm and optimized to cover the entire gain bandwidth of Tm:silica. The dispersive part of a second HNF is designed to fit the gain maximum of Yb-doped fibers at a wavelength of 1030 nm. The benefit of Er:fiber seeding has been proven experimentally by confirming the full coherence of the spectral components generated in the HNFs [6].MHz and the transform limit for the pulse duration is 110 fs. The Er:fiber system acts as seed source for the parallel Yb: and Tm:fiber amplifiers. We reduce the repetition rate to 10 MHz via electro-optic modulators. To amplify the spectrum delivered by the HNF at a center wavelength of 1030 nm, we stretch the pulses with a grating pair to a temporal duration of 340 ps. The amplification occurs in a first Yb:fiber preamplifier pumped at a wavelength of 976 nm. The energy is then boosted in a 1.5 m long Yb-doped photonic crystal fiber (PCF) amplifier stage. The PCF double cladding structure enables pumping with high-power multimode laser diodes. At a pump power of 40 W, we measured pulse energies of 2.2 μJ. The spectrum after amplification has a bandwidth of 12 nm (FWHM) centered at 1032 nm (see Fig. 1.a). Recompression of these pulses leads to a pulse duration of 185 fs (see SHG FROG characterization in Fig. 1.b). The seed for the Tm:amplifier is stretched in 30 m of single-mode fiber directly spliced to the HNF. In a monolithic and truly single-mode Tm:fiber amplifier pumped at 796 nm, we demonstrate pulse energies of 250 nJ at a repetition rate of 10 MHz [6]. Fig. 1.c shows the pulse spectrum after amplification. It is centered at a wavelength of 1950 nm with a bandwidth of 50 nm. Both amplifiers are operating in a linear regime and there are no signs of a significant influence of nonlinear effects. The combination of high-energy, sub 200-fs pump pulses together with passive carrier-envelope phase stability of ultrabroadband seed pulses for parametric amplification paves the way towards extremely nonlinear optics and attosecond technology at unprecedented repetition rates and stability.

Tandem pumping has been proved as an efficient approach to realizing high power Yb-doped fiber lasers and amplifiers. Currently, the most widely used pump laser is operating at 1018 nm, where the relatively small absorption cross section of Yb-doped fiber inevitably leads to long fiber length for sufficient pump absorption. The long active fiber, however, would significantly lower the stimulated Raman scattering threshold, limiting further power scaling. Therefore, theoretical analysis is carried out with the aim of shortening the Yb-doped fiber length in the tandem pumping scheme by employing pump lasers with shorter wavelength, i.e. 1007 nm and 1010 nm in this work, and the simulation results indicate that higher overall efficiency and better signal-to-noise ratio (SNR) could be obtained in a high-power fiber amplifier with these shortwavelength pump lasers. Further simulation suggests that fiber lasers operating at 1007 nm with high efficiency and high SNR can be obtained by optimizing the cavity parameters.

A 3D spatially resolved coupled mode and perturbation analysis for the transverse mode instability threshold powers in Yb-doped fiber amplifiers is extended to include threshold power reduction induced by the pump laser relative intensity noise (RIN). A static intensity grating combines with the fluctuating pump intensity to generate an inversion standing wave. The forward component of the standing wave contributes to the nonlinear coupling between the fundamental mode and the higher-order mode (HOM), thereby reducing the threshold power. It is estimated that a pump RIN level <-150dB/Hz is required to achieve quantum limited performance. The model also quantifies the impact of unwanted laser power in the HOM on the threshold power reduction. The threshold power calculations as a function of RIN and laser power in the HOM are in qualitative agreement with recently published experimental measurements.

A space-time finite difference model is exercised to examine the performance of large mode area (LMA), high-power, Yb-doped pulsed fiber amplifiers. The simulations include forward and backward amplified spontaneous emission (ASE) and stimulated Raman scattering (SRS) and exhibit good agreement with experimental measurements. It is found that the dominant pulse energy limiting factor is the SRS, which is quantified by a Raman ratio R=0.05 equal to the ratio of the peak Stokes power to peak signal power. Interpulse power at the signal wavelength due to ASE generated by the preamplifier and incomplete extinction of the modulated seed laser diode contribute to the reduction in the pulse-background contrast. However, this is not the dominant pulse power limiting mechanism at moderate background levels, 35dB down from the pulse energy.

Photodarkening (PD) can degrade the output power and increase the thermal load in Yb-doped fiber lasers and amplifiers by increasing the propagation loss in the core [1,2]. PD can be attributed to color centers [3] and is reported to grow when the fiber is in use, over times from a few minutes to many hours (e.g., [2–4]). Typically, this is characterized in terms of a reduced output power of the operating device, but it is also possible to measure the transmission with non-resonant optical probes before, during, and after use. Although faster changes in the photodarkening are also possible [3], they have received little attention. Such fast changes can affect both the impact and the characterization of photodarkening.