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In this work, a narrow-linewidth polarization-maintaining (PM) all-fiber amplifier with near-diffraction-limited beam quality and record output power is presented. First, a 4.45-kW PM fiber amplifier with a 3-dB linewidth of 0.08 nm and root mean square (rms) linewidth of 0.22 nm is achieved based on optimized phase modulation. However, the sideband of the spectrum broadens significantly during the amplification process, which is mainly caused by the additional intensity variation of the injected signal. Meanwhile, an up to 5.04-kW linearly polarized fiber laser with a relatively stable spectral bandwidth is achieved by effectively suppressing spectral broadening. At the maximum output power, the rms linewidth is 0.2 nm, the beam quality factor M2 is less than 1.3, the polarization extinction ratio (PER) is 16.5 dB, and the signal-to-noise ratio (SNR) is approximately 53 dB. The further power scaling of the amplifier is mainly limited by the pump power. To the best of our knowledge, this is the maximum output power of a narrow linewidth linearly polarized fiber amplifier to date.

As a major limitation for power scaling of high power narrow linewidth fiber master oscillator power amplifiers (MOPAs), Stimulated Brillouin Scattering (SBS) induced self-pulsing in polarization maintaining (PM) fiber amplifiers is well characterized and analyzed in this paper by comparing different white noise signal (WNS) phase-modulated modes in experiments. It is found that the self-pulsing effect is not observed in the PM-amplifier with single-frequency laser seed injection, and cascaded WNS modulation provides superior self-pulsing suppression than single WNS modulation with similar output linewidth. Moreover, the experimental results indicate that the self-pulsing threshold can hardly be predicted only by the output linewidth or the defined SBS threshold in a WNS phase modulated fiber amplifier system. As self-pulsing is originated from the spectral spikes in WNS modulated system, we theoretically analyzed characteristics of these spikes in different phase-modulation modes. It indicates the spectral peak intensity can be reduced by cascaded modulation, for which self-pulsing can be suppressed. The theoretical predictions agree well with the experimental results. At the same time, in order to suppress the mode instability effect, a plum blossom shaped bending mode selection device is used in this high-power narrow linewidth fiber amplifier system. Finally, a 32 GHz cascaded WNSs modulated, over than 2.5 kW linearly polarized all-fiber amplifier with a slope efficiency of 86.7% is demonstrated. The polarization extinction ratio (PER) is measured larger than 14 dB and the beam quality factor M2 maintains lower than 1.3 in the power scaling process.

In this paper, we construct a narrow-linewidth amplifier based on polarization-maintaining (PM) fiber, which can be injected by a fiber oscillator seed or white noise signal phase-modulated seed. The stimulated Brillouin scattering (SBS), SRS, spectral broadening, self-pulsing properties of the fiber amplifier when its’ injected signal is 0° or 45° relative to the slow axes of the PM fiber are studied. It is indicated that when the injected signal is launched at 45° relative to the slow axes, the nonlinear effects are significantly suppressed compared with the case of the injected signal is launched parallel to the slow axis. The spectral broadening rate is reduced from 0.14 pm W−1 to 0.0747 pm W−1, the ratio of Raman to laser peak intensity is increased from 20 dB to 30 dB, the SBS threshold is increased by >24%, and the self-pulsing threshold is increased by 28.7%. It is worth noting that the nonlinear properties corresponding to the injected polarization state in our experiment are quite different from some earlier works.

A high-beam-quality and high-energy long-pulse laser has been developed with VRM(variable reflectivity mirror)coupling and spherical aberration compensation. The thermal lens and the thermal birefringence are compensated for by the 10 mm DL rod side-pumped laser module. The output mirror employs VRM coupling with R=35% in the center, which presents a energy of 1.04 J at a pulse width of 195 μs and a repetition rate of 400 Hz with a beam quality of M2≈8.4. The spherical aberration compensation improves the beam quality M2 factor to 2.15; the power stability (RMS) is 0.4% at 5 min, and the optical-optical efficiency is 19.7%.

Continuous fiber laser with ultra-high power and narrow linewidth is one of the key devices in the field of high-precision industrial processing, beam combining, and nonlinear frequency conversion. Under the premise of ensuring the signal quality, continuously increasing the output power is the focus of high-power narrow-linewidth fiber lasers. Driven by the white noise or pseudo-random binary sequence (PRBS), using cascaded phase modulations to broaden the spectrum of the seed source to suppress the stimulated Brillouin scattering (SBS) effect in the master oscillator power amplifier (MOPA) structure is an effective solution to increase the output power. However, this type of optical spectrum needs to be optimized, and the randomness of the driving signal causes a self-pulsing effect, which limits the further increase of the output power. In this paper, the influence of the frequency interval and randomness of the driving signal on the SBS effect in the laser system is analyzed. The modulated spectral type can be simply adjusted through changing the bit rate and inversion probability. Combining with high-order phase modulation, an approximate rectangular spectral broadening of the seed source with a tunable bandwidth up to 30 GHz is achieved. Compared with the cascaded white noise case, the output power of this scheme is increased by 600 W under the extended bandwidth of 27 GHz. It is fully verified that the seed source spectrum with high in-band flatness and low randomness can effectively suppress the SBS effect in the fiber laser and increase the output power.

In this work, the effect of the temporal instability on stimulated Raman scattering (SRS) characteristics in a high power Yb-doped fiber amplifier is studied experimentally and theoretically. It is found that the SRS intensity is strongly influenced by the type of the seed source. When the power amplifier is seeded by a normal fiber oscillator laser, the SRS threshold is much lower than the value estimated by Smith threshold formula. The maximum output power in the experiment is 1910 W, which is limited by SRS effect. While the power amplifier is seeded by a phase modulated single frequency laser, the maximum output power is 2400 W without observation of any SRS. Such phenomenon can be explained by the different time-domain characteristics of the two different types of seed sources. On the one hand, the detailed measurements are performed experimentally. On the other hand, a numerical model is proposed. Both the experiment and the model reveal that strong intensity fluctuations exist in fiber amplifier seeded by a fiber oscillator laser. These intensity fluctuations lead to a much higher SRS output. With these considerations in mind, the model predictions are in good qualitative agreement with the experimental data.

In this paper, we propose and experimentally demonstrate a 2.4-kW narrow-linewidth amplifier seeded by a random fiber laser (RFL) with Yb-doped fiber gain. The high power lasing is realized by employing a master oscillator power amplifier (MOPA) configuration, a full width at half-maximum linewidth of 0.23 nm and a beam quality of M 2 = 1.28 are obtained at the maximum output. Theoretical models for the spectral evolution in high-power fiber amplifiers seeded by RFL are also proposed. It is found that spectral broadening exists in high-power MOPA using RFL as the seed. Meanwhile, spectral broadening can be further suppressed by employing active fiber with shorter length or larger core diameter. The theoretical predictions are in qualitative agreements with the experimental results.

In this paper, based on a linear polarized, white noise signal (WNS) phase modulated all fiber amplifier, the self-pulsing characteristics of polarization maintaining (PM) amplifier at the different phase modulation parameters with the same linewidth are analyzed experimentally. It is demonstrated that the self-pulsing threshold is closely related to the details of the modulation spectrum affected by the frequency and the signal power of the WNS, and it is near impossible to calculate by the linewidth. Besides, by suppressing the self-pulsing effect, we obtain an output of 2009 W with a linewidth of 22 GHz. The polarization extinction ratio (PER) is larger than 15 dB, and the M2 is lower than 1.2.

The dependence of the spectral broadening characteristics on the number of longitudinal modes is studied experimentally in a bi-directional pumping fiber amplifier. It is found that the linewidth of the amplifier decreases with reducing the longitudinal mode number of the seed source. In this letter, we present a 68 pm-linewidth kilowatt-level fiber laser using a simple master oscillator power amplifier structure. Meanwhile, we theoretically analyze the spectral broadening characteristics of the seeds with different longitudinal mode numbers. The conclusions are in agreement with the experimental results. This provides a simple and efficient method for obtaining the high power narrow linewidth fiber laser used in spectral beam combining.

We present an investigation on the spectrum broadening in continuous-wave, sub-nanometer linewidth high power fiber amplifiers caused by the multiple four-wave mixing (FWM) process. The spectrum broadening employing two different types of narrow linewidth seeds, including the multi-longitudinal-mode seed and the broadened single frequency seed generated by high speed phase modulation, is studied. It is shown both theoretically and experimentally that the multi-longitudinal-mode seed experiences serious spectrum broadening induced by the FWM among various longitudinal modes, while the modulated seed can maintain the spectrum profile during the amplifying process even with some noise fluctuation. The different broadening results are mainly caused by the random phase distribution of the multiple waves. It is further explained by an exact solution of the degenerate FWM with three waves. The theoretical predictions on the spectrum and power dependence of the output laser linewidth are in quantitative agreement with the experimental results up to kilowatt.

In this paper, we demonstrate a high-power, narrow-linewidth, polarization-maintaining fiber amplifier with near-diffraction-limited beam quality. By optimizing the phase modulation signal, a nearly top-hat-shaped spectrum was generated for self-pulsing suppressing. That results in doubling the self-pulsing threshold we got from conventional white noise signal phase modulation with the same optical linewidth. Based on an optimized signal and a high power, polarization-maintaining, counter-pumped fiber amplifier, we obtain a 3.25 kW narrow-linewidth linearly polarized laser output with a linewidth of ∼ 20 G H z , the polarization extinction ratio is about 15 dB, and the M 2 is less than 1.22 at the maximum output power. To the best of our knowledge, this is the first demonstration of a narrow-linewidth, linear polarization, all-fiber amplifier with 3.25 kW laser output.

We report on the hybrid beam combination of five unpolarized fiber lasers with the output power of 5.02 kW. Each fiber laser provides the output with average power of 1.1 kW and full width half maximum (FWHM) linewidth of 0.3 nm. The five beams with different wavelength are first split into two orthogonal linear polarizations, spectrally combined by dual multilayer dielectric (MLD) diffraction gratings in turn, finally combined into a single beam using a polarization beam combiner. This hybrid system features a combining efficiency of 91.2%, the beam quality of M2=2.96×1.52, and excellent prospects for significant power scaling.

In this paper, we demonstrate 47 GHz linear polarized fiber amplifier injected by a simple fiber oscillator laser seed source with narrow linewidth and near diffraction-limited beam quality. Output powers of 419 W, 778 W, and 1107 W are achieved with 3dB linewidth of 26.5 GHz, 41 GHz, and 47 GHz, respectively. The M2 is 1.25 in the x-direction and 1.23 in the y-direction at the maximum laser power, respectively. The measured PER is large than 96% during the power scaling process. However, The SRS is observed when the laser power is 1107, the SNR is about 47dB, which means that the SRS effect has become a serious limitation for further power scaling of such PM-amplifier seeded by the fiber oscillator laser seed source.

In this work, we realize a high-power, narrow linewidth, linearly polarized laser output based on a compact all-fiber polarization maintaining (PM) master oscillator power amplifier. The seed is a fiber oscillator laser (FOL) with few-longitudinal-mode for spectral broadening suppression. The spectral broadening characteristics in the FOL seed and in the PM-amplifier with different pumping schemes are studied experimentally and theoretically. The effect of different types of fibers in the amplifier on spectral broadening is also analysed theoretically. Finally, we demonstrate a narrow linewidth linearly polarized all-fiber amplifier operating at the maximum output power of 3.08 kW with a 3 dB linewidth of 0.2 nm. The polarization extinction ratio is measured to be larger than 93.5% and the M2 maintains lower than 1.45 in the power scaling process. To the best of our knowledge, this is the highest demonstrated output power for narrow linewidth linearly polarized all-fiber lasers with the charactristics of low cost and a compact structure.

In this paper, we demonstrate a high-power, narrow-linewidth, near-diffraction-limited all-fiber laser based on a one-stage high power amplifier configuration. The seed source is a fiber oscillator with a short cavity length in order to suppress the spectral broadening effect, the linewidth is 0.027 nm at the laser power of 7.3 W. Then only one-stage high-power amplifier is used to scale up the seed power. It is found this simple laser structure can obtain an output power of 2.4 kW with a linewidth of 0.24 nm. Meanwhile, the center wavelength only shifts 0.025 nm during the amplification process, which is about 0.1 times of the linewidth. The output laser at maximum power has a near-diffraction-limited beam quality, the M 2 is 1.27 in the x-direction and 1.35 in the y -direction, respectively. Besides, for the first time, we discover the beam fluctuations (different from the mode instability) with a frequency of a few Hz, this new phenomenon limits the further improvement of output power.

A high energy, high beam quality short-pulse diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with two amplifier stages is demonstrated. The two-rod birefringence compensation was used as beam quality controlling methods, which presents a short-pulse energy of 40 mJ with a beam quality value of M2 = 1.2 at a repetition rate of 400Hz. The MOPA system delivers a short-pulse energy of 712.5 mJ with a pulse width of 12.4 ns.The method of spherical aberration compensation is improved the beam quality, a M2 factor of 2.3 and an optical-to-optical efficiency of 27.7% is obtained at the maximum laser out power.The laser obtained 1.4J out energy with polarization integration.

The behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier with different local heat load is studied theoretically and experimentally. A theoretical model is constructed, and the effects of different laser parameters on the local thermal load of the gain fiber are analyzed theoretically, such as pump direction, pump linewidth, thermal conductivity of cooling medium. An experimental structure is described. The effects of different local heat load on the MI threshold when the fiber amplifier have the same total or average heat load is studied. The theoretical and experimental results reveal that the MI threshold can be estimated by the local heat load of the gain fiber.

We demonstrate a high-efficiency, high-average-power, CW master oscillator power amplifier based on a conduction-cooled, end-pumped Yb:YAG slab architecture at room temperature (RT). Firstly, the CW amplification property is theoretically analyzed based on the kinetics model for Yb:YAG. To realize high-efficiency laser amplification extraction for RT Yb:YAG, not only intense pump but also a high-power seed laser is of great importance. Experimentally, a composite Yb:YAG crystal slab with three doped and two un-doped segments symmetrically is employed as the gain medium, which is end-pumped by two high-power, 940-nm diode lasers. A high-power, narrow-spectral-width, 1030-nm fiber seed laser then double passes the composite slab to realize efficient power amplification. For 0.8-kW seed input, maximum output power of 3.54 kW is obtained at 6.7 kW of pump power, with the optical conversion efficiency of 41% and the highest slope efficiency of 59%. To the best of our knowledge, this is the highest power and efficiency reported for Yb:YAG lasing at RT except thin-disk lasers.

In this work, we study the characteristics of self-pulsing in a polarization-maintained fiber amplifier operated with different linewidths based on white noise source phase modulation. It indicates that the self-pulsing is almost simultaneous with the stimulated Brillouin scattering process, and its threshold is increasing near-linearly with the linewidth. By optimizing the laser structure, the threshold of self-pulsing increases by a factor of 1.5. We demonstrate a high-power linear polarization and all-fiberized amplifier with narrow linewidth and near-diffraction-limited beam quality. The output power scales to 1.5 kW with the pumping efficiency of 83%. The full width at half-maximum linewidth was measured to be 13 GHz. The polarization extinction ratio was larger than 13 dB. The beam quality M2 was about 1.14 at the maximum laser power.

We demonstrate the suppression of stimulated Raman scattering (SRS) in a kilowatt fiber amplifier by reducing the temporal fluctuations of the seed oscillator, which is achieved by simply inserting a long piece of transmission fiber between the seed source and amplifier. We experimentally investigate the dependence of the amount of Raman light in the amplifier on the seed oscillator’s transmission fiber length. We also numerically analyze the temporal features of the corresponding seed sources and amplifiers. Experiments and simulations indicate that long transmission fibers will flatten temporal fluctuations of the seed laser, and thereby the SRS effect in the amplifier can be mitigated. The difference of the SRS suppression mechanisms between the method proposed here and the conventional method employing spectrally wider fiber Bragg grating is also presented.

The effect of the pump source and the gain fiber on the output properties of ytterbium doped fiber laser (YDFL) work near 980 nm are analyzed theoretically. Base on this analysis, we design a high power all fiberized ytterbium doped laser operating near 980 nm with a core/cladding of 80/130 μm. A 980 nm YDFL experimental setup was constructed, 16.7W 980 nm fiber laser was achieved with an optical-optical conversion efficiency of 32.4%.

A spectral beam combination(SBC) system with dual Multi-Layer Dielectric(MLD) grating dispersion compensation configuration, which can combine multiple fiber lasers into a common aperture beam with high beam quality and relax the linewidth requirement of the individual fiber laser, has become gradually one of the most important technique route of fiber laser beam combining. The basic principle and the key technologies of the SBC system with dual-MLD-grating configuration were introduced and analyzed briefly. The recent progress of the main key technologies about high power beam combinable narrow linewidth fiber laser, high power high efficiency short wavelength narrow linewidth fiber laser, large dispersion high efficiency MLD grating and high integration dense beam combining were reviewed. The recent progress on the key technologies of SBC with dual-MLD-configuration in Institute of Applied Electronics, China Academy of Engineering Physics, were also presented. The potential of the SBC with dual-MLD-configuration was prospected.

For powers exceeding a threshold the spectral broadening in fiber amplifiers becomes a significant challenge for the development of high power narrow bandwidth fiber lasers. In this letter, we show that the spectral broadening can be partly caused by four-wave mixing(FWM) process in which the power of the central wavelength would transfer to the side ones. A practical FWM induced spectral broadening theory has been derived from the early works. A numerical model of fiber amplifier has been established and FWM process has been added to the model. During the simulation process, we find that when a 10 GHz, several watts narrow bandwidth laser is seeded into a few modes fiber laser amplifier, the FWM induced spectral broadening effect might continually increase the FWHM of the spectra of the continuum laser to 100 GHz within the amplification process to several hundred watts which has been convinced by our experiments. Some other results have also been analyzed in this paper to complete the four-wave mixing induced spectral broadening theory in fiber amplifiers.

Tapering will raise the signal loss in an end-pumped (N+1)×1 type combiner. In this paper, the Thermally Expanded Core (TEC) technique is used in the signal loss optimization experiment with the tapering ratio of the pump combiner is 0.6. The experimental results indicate that the coupling efficiency of the 1.55μm signal light increases from 81.1% to 86.6%, after being heated 10 minutes at the homo-waist region of the tapered signal fiber with an 8mm wide hydroxygen flame. Detail analysis shows that the TEC technique can both reduce the loss of the LP01 mode and the LP11 mode in the signal fiber.

We experimentally investigate the behavior of the mode instability (MI) threshold in the double cladding Yb-doped fiber amplifier when the amplifier is pumped by broad linewidth laser diodes and narrow linewidth laser diodes respectively. It is found that the MI threshold increases by 26% when the amplifier is pumped by the broad linewidth laser diodes. Experiment results show that the MI threshold is affected by the local heat load rather than the average or the total heat load. The calculation shows that the local heat deposit actually plays the key role to stimulate the MI behaviour. At the MI threshold position in the fiber, the local heat deposit also changes dramatically. The effect of the thermal conductivity on the MI threshold is also studied. Our investigation shows that the MI threshold increases from 1269 W to 1950 W when the thermal conductivity of the fiber amplifier is increased from 0.3 W/(m·K) to 5 W/(m·K). Through optimizing the pump linewidth and the cooling efficiency of the gain fiber, the MI threshold is doubled in our experiment.