In this study, we compare and illustrate how weak end-facet reflections affect high-power fiber oscillators and fiber amplifiers' stimulated Raman scattering (SRS) threshold. The simulation results reveal that the enhancement of weak end-facet reflections could decrease the SRS threshold of high-power fiber lasers significantly, especially for fiber amplifiers employing phase-modulated single-frequency lasers as seed lasers. Further comparisons point out that weakening reflections at either the input or output end-facet provides an effective approach to improve the SRS threshold of high-power fiber lasers. In addition, weak end-facet reflections could even invalidate the strategy of suppressing the SRS effect through filtering out the Raman component in seed laser. The theoretical approach and findings may serve as a useful guide for designing high-power fiber laser systems with SRS limitations.
Coherent beam combination of fiber lasers is considered to be a promising technology to obtain high-brightness laser. In tiled-aperture coherent beam combination, in order to obtain the best combination effect, a high fill factor is required for the beam array output from the fiber collimators. Thus, the collimator array must be closely arranged and each single-mode Gaussian beam must be truncated, which brings great difficulties to design a fiber collimator. We propose a scheme of fiber collimator based on rod lens, which compares to the fiber collimator with the thin lens, has the following advantages. Firstly, rod lenses for beam collimation can be arranged in an array without a frame to get a high fill factor of fiber collimators. Secondly, the single-mode Gaussian beam from fiber can be truncated in any proportion to get a good result of coherent beam combination. Thirdly, almost all of the laser energy output from a fiber include the stray light with the large NA is emitted into space outside the collimator by the rod lens. Fourth, the structure of fiber collimator with the rod lens is simple, which makes it easy to build a large number of fiber lasers array. In this paper, we have developed a rod lens collimator and carried out the preliminary experiments, the results show that the rod lens collimator can emit a collimated beam with the good beam quality, and all stray light with the large NA in the collimator can be exported to space.
Coherent Beam Combining (CBC) can scale the overall output power while maintaining high beam quality. In recent years, this technique has also been employed to generate the high-power Optical Vortex Beam (OVB). In this work, we designed a coherent phased array, and an Optical Vortex Beam Array (OVBA) was generated via the CBC technique. Numerical simulation was studied, and an experimental setup was set up. The results showed that the OVBA could be generated in the far field. Meanwhile, the phase detection results indicated that the OVBA included several phase singularities. This work can present a new idea for manipulating the high power structured optical fields.
The tandem pumping technique has been proven effective in power scaling of fiber lasers, which enabled the most powerful high-brightness fiber lasers to date. However, further performance improvement of tandem-pumped fiber lasers is challenged by the contradiction between nonlinearity suppression and good beam quality maintaining. To address these challenges, researchers from worldwide devoted to expediting the development of high-power tandem-pumped fiber lasers, and a series of new results and new applications emerged in recent years. This paper will highlight the recent advances in the enabling techniques of high-brightness tandem-pumped fiber lasers and the new applications of tandem pumping since the first review paper on this topic in 2017. A brief outlook on the further development of tandem pumping technique is also given.
High mode purity of high-power fiber lasers is strongly required in 3-dimensional printing, high brightness beam combing, and high accuracy material processing. Mode decomposition (MD) is an effective technique for diagnosing the mode composition of a high-power fiber laser. In particular, dynamic mode evolution could be commonly generated during power scaling process due to thermal and nonlinear effects. Consequently, the fast and accurate mode decomposition (MD) method is strongly required. The non-iterative fast mode decomposition based on matrix operation method is theoretically a promising technique to achieve ultra-fast MD with high accuracy. However, this technique of realizing MD is mainly limited by the noise of the light-spot image in a practical system. In this report, the effect of the image noise on the noniterative fast mode decomposition technique is carefully investigated. Simulation results show that the decomposition accuracy decreases as the intensity of noise increases. Nevertheless, the effect of image noise on the non-iterative fast mode decomposition method depends not only on the noise intensity but also on the coefficient matrix condition number of the matrix equations. Furthermore, the higher the condition number of coefficient matrix of linear equations is, the more influence of image noise on the non-iterative fast MD accuracy. The results presented could give instructive reference for further optimizing the non-iterative fast mode decomposition technique used in practical high-power fiber lasers.
High power narrow-linewidth fiber lasers have been highly desired for various applications. In this paper, we presented new considerations of high power narrow-linewidth fiber amplifiers operating within 1030~1060 nm by using newgeometry active fiber, and the power-breakthrough operating at 1050 nm was achieved. We also reported our recent achievements on power scaling of high-power narrow-linewidth fiber lasers which operates within 1060~1080 nm based on system-optimized step-index and confined-doped active fibers, including linear-polarized and stochastic-polarized ones. Meanwhile, our new progress on special designed active fibers assisted high power fiber amplifiers with sub-GHz to within 1.5 GHz were demonstrated. Besides, the basic considerations of end-reflection on the SBS and SRS effects will be discussed, which provide a new insight for the suppression of SBS and SRS effects.
In this paper, an innovation coherent beam combining (CBC) architecture to generate the structured light beams array was proposed and experimented. The simulation and experimental results reveal that the optical vortex beams array (OVBA) with multi-modes can be generated effectively in the far field. The OVBA is composed of multiple sub-OVB in the intensity distribution. Furthermore, the number of OVBs can be modulated by changing the fill factor of the laser array in the near field. In particular, the performance of a OVBA copier was observed, which may deepen the understanding of creating the structured light fields by CBC technique. The experiment results were in excellent agreement with the simulation results. This work could provide valuable and practical reference on generation and manipulation of high power structured light beams
An all-fiber high-power Mamyshev oscillator (MO) with only one amplification stage was experimentally demonstrated. The achieved maximum output power was 3.4 W with 77 nJ pulse energy and could be compressed to ~100 fs. By adjusting the pump power, the phenomenon of harmonic mode locking is observed in the experiment, and the highest 5th order harmonic can be achieved, which corresponds to the repetition rate of 44.1 MHz. This compact MO ultrafast laser could operate stably several hours and the power fluctuation within 5 h was less than 0.12%. Such a high power ultrafast laser oscillator could apply a promising source for advanced fabrication, biomedical imaging, micromachining and other practical applications.
KEYWORDS: Education and training, Photonics, Digital signal processing, Electronics engineering, Machine learning, Laser applications, Computer science, Atmospheric optics, Signal processing, Signal detection
We will show that investigation of coherent beam combining provide good multidisciplinary education and training environment for graduate students. Knowledge including optics and photonics, electronics engineering, computer science, and so on, can be learned and mastered.
An all fiber-based Mamyshev oscillators (MO) is experimentally demonstrated to achieve high energy pulse output. In this high energy MO system, the maximum single pulse energy of 153 nJ was achieved with 1.5 W average power. The pulse width could be externally compressed to < 100 fs by a pair of diffraction gratings. Considering the insertion loss of gratings, the maximum peak power was >1 MW. Meanwhile, this system is verified to have good long-term stability and can run several hours stably. This is the highest record in pulse energy from the all fiber-based ultrafast laser oscillator with picosecond/femtosecond pulse duration, to the best of our knowledge. Such a high energy oscillator could apply a promising source for laser micromachining, advanced fabrication, biomedical imaging, and other practical applications.
In this report, we will introduce our recent advances in developing the deep-learning-based coherent fiber laser array systems for power scaling and spatial light structuring. Our motivation is to construct a deep-learning network for estimating the thermal and environmental induced phase errors, and further compensate the phase errors by the phase control servo with the assistance of the network outputs. Technical progresses in terms of the network optimization, two-stage control scheme, and optical field information acquisition will be covered. Moreover, the prospects and challenges towards the future implementation of intelligent control for CBC systems will be discussed.
High power, narrow linewidth all-fiber amplifiers are under intensive investigation in recent years. In this paper, the research status of high power, narrow linewidth all-fiber amplifiers (including those operate at single frequency regime) is briefly summarized. Then the recent progress in our research group is introduced, including more than 500 Watt level single frequency fiber amplifier, more than 4 kW linearly-polarized narrow linewidth all-fiber amplifier and more than 6 kW narrow linewidth all-fiber amplifier. Performance exploring of the operating spectrum property is also discussed.
A gain-managed nonlinear (GMN) tapered fiber amplifier was experimentally demonstrated. The achieved single pulse energy was 707 nJ and the compressed pulse duration was 67 fs with 10 MW peak power at 1054 nm central wavelength.
In this report, by comparing the RIN and beam pointing error of the signal laser at different output powers, the impact of the modal degeneration on both the spacial and temporal noise properties of high-power single-frequency fiber amplifiers will be demonstrated. The new finding reveal that obvious mode-related enhancement of the RIN could occur well below the conventional transverse mode instability (TMI) threshold of the fiber amplifier while the mode-related enhancement of the beam pointing error occurs near the TMI threshold. This work could provide a new insight for obtaining high-power, high spacial stability and low-noise single-frequency fiber lasers.
A novel phase modulation technique, calling multi-phase coded signal (MPCS) modulation, is demonstrated for stimulated Brillouin scattering (SBS) effect managing in high power narrow-linewidth polarization-maintained all-fiber amplifier. Combining with laser gain competition where two signal lasers operating at 1045 nm and 1064 nm are applied, the output power of 1064 nm laser could be boosted to 1023 W with FWHM linewidth of 4.6 GHz. The polarization extinction ratio (PER) is 13.3 dB and the output beam quality is measured to be M2=1.11. This work could provide a feasible method for linewidth controlling in high power narrow linewidth fiber amplifier.
In this letter, a two-stage phase control technique is proposed to increase the control bandwidth of the target-in-the-loop (TIL) system. In this technique, the first stage phase control is enabled by LiNbO3 phase modulator to compensate the phase noises in the fiber amplifiers, and the second stage phase control is enabled by the liquid crystal (LC) to compensate the phase noises induced by the atmospheric turbulence. We built a TIL coherent beam combining system with 3-channel coherent fiber lasers over a 40 m atmospheric propagation path. In our experiment, the stochastic parallel gradient descent (SPGD) algorithm was employed for phase control. When the phase control system was in the close loop, the performance of laser beam projection was significantly improved, and the phase locking bandwidth for transmitter side phase distortions reached 1 kHz. This method can be used for applications such as energy transmission and free-space optical communication.
Yb-Raman fiber amplifier is an effective solution to achieving high-power narrow linewidth fiber laser operating at 1100-1200 nm region, in which the selection of the pump source at around 1064 nm has a significant effect on the output power and spectral properties. In this paper, we report an Yb-Raman fiber amplifier pumped by a random fiber laser. Hundred-watt level output power with 3 dB spectral linewidth of only 0.23 nm is obtained by optimizing the linewidth of the random fiber laser. The results could provide a practical reference for the systematic design of the narrow linewidth Yb-Raman fiber amplifier.
In this study, we design and fabricate a novel type of active fiber——double-tapered double-clad fiber (DT-DCF). Based on this self-developed DT-DCF, we have constructed an all-fiberized fiber amplifier that is operating under a continuous-wave (CW) regime at 1080 nm wavelength. The maximum output power of the system reaches 4 kW, which, to the best of our knowledge, is the highest output power of tapered fiber-based laser systems. The amplifier exhibits near-single-mode beam quality (M2=1.33) at the highest output power with a slope efficiency of 83%. Our result successfully verifies the potential of power scalability of DT-DCF, and the performance of our system can be further enhanced by fiber design optimization.
Recently, the use of Yb-Raman fiber amplifier has led to a breakthrough in the power of narrow linewidth laser at 1120 nm. In this paper, the effects of seed filtering on the output laser characteristics in a narrow linewidth Yb-Raman fiber amplifier are studied. The experimental results indicate that the seed filtering can not only suppress the ASE noise in the output laser, but also have an influence on the evolution of the sideband noise of the signal spectra. This work can contribute to the understanding of narrow linewidth Yb-Raman fiber amplifiers and can provide a reference for the systematic design.
In this paper, a two-stage phase control method was proposed to increase the control bandwidth of the target-in-the-loop coherent beam combining (CBC) system. Firstly, the principle of the target-in-the-loop CBC system based on two-stage phase control was introduced. In order to verify the feasibility of two-stage phase control technology, then a 7-channel fiber laser array beam combining system was established. The experimental research showed that when the phase noise in the fibers and on the transmission path from the collimators was controlled by two phase controllers respectively, the laser array coherently combined in the far field stably. The power in bucket was 11.4% in the close-loop, which was 70.1% of the theoretical value. The normalized mean voltage detected by the photoelectric detector increased from 0.107 to 0.648, with an increase of 6.1 times. This experiment initially verified the feasibility of the two-stage phase control method, which will be helpful for the control bandwidth increasing in the target-in-the-loop CBC system.
Transceiver collimator is an advanced component in high power fiber lasers long distance transmission. Here, based on fiber weak taper technology, an all-fiber laser emitting and target detecting transceiver collimator is proposed. Meanwhile, a (18+1)-channel optical fiber bundle detection experiment was succeed build.With a home-made signal processing circuit, the on-line dynamic tracking image display was achieved, and the target detection and transceiver collimator was experimental demonstrated.
A novel method for active coherent beam combining by Particle Swarm Optimization (PSO) algorithm is demonstrated in this paper. The principle of this method is introduced, and its advantages are presented in detail. In the simulation, 37 fiber lasers are coherently combined by employing stochastic parallel gradient descent algorithm with 91 steps, and then, the combined beams are combined by PSO algorithm with only 30 steps. And the result shows that the more laser elements, the more remarkable PSO algorithm is in the CBC system. Because of the high control ability of PSO algorithm in coherent beam combining combined with traditional algorithm, it is scalable to phase-locking system in a large number of fiber lasers.
The pulse shape from the pulsed amplifier always distorts because of gain saturation effect, an effective method is active control the output pulse shape by reshaping the pulse shape of the laser seed. We demonstrated a new method for active temporal pulse shape control of fiber amplifier with adaptive proportional control. We numerically researched three proportional control methods, including static proportional control, adaptive proportional control and piecewise adaptive proportional control. The results show that proportional control can generate arbitrary temporal pulse shape with high accuracy and less iterations even if parameters of the fiber amplifier are unknown.
We proposed a design of coherent fiber-optics-array collimator (CFAC) which is mainly composed of a single unitary collimating lens and prism. The CFAC system can be regarded as a “sub-aperture” of the whole fibers array in the tiledaperture scheme to expand combining channels efficiently due to its simple and compact structure. Then, we setup an experiment to verify the feasibility of the CFAC system with seven fiber lasers arranged in two dimensions and the CBC in 1064nm wavelength using single-frequency dithering algorithm is successfully achieved. By careful calculation, the residual errors among the laser beams are suppressed below λ/20 through an active-piston-phase control.
Stimulated Brillouin scattering (SBS) suppression in high power, narrow linewidth and all polarization-maintained amplifiers with near diffraction limited (NDL) beam quality by rectangular optical spectrum is demonstrated. Rectangular spectrum is generated by using cascaded phase modulations. In the preliminary experiment, output power of 509 W with spectral linewidth of ~ 3 GHz is obtained. At maximal output power, the polarization extinction ratio (PER) is measured to be ~ 14 dB and the beam quality (M2 factor) is M2 ~1.2. The technique presented give useful reference to control the spectral linewidth in high brightness fiber amplifiers. By combining with other linewidth controlling techniques, several kilowatt-level output power with spectral linewidth of < 10 GHz could be expected.
Stimulated Brillouin scattering (SBS) management in kilowatt level, high brightness, narrow spectral width, and all polarization-maintained amplifiers by using laser gain competition is demonstrated. This method can increase the SBS threshold of the fiber amplifier by more than twice without changing the linewidth. By further incorporating phase modulation technique, the SBS threshold is scaled to be as high as 32.4 times compared with single frequency amplification. Output power of 1082 W is achieved with polarization extinction ratio (PER) of ~ 14 dB, linewidth of <7.6 GHz and beam quality (M2 factor) of ~M2 =1.14 in a preliminary validation experiment. This kind of fiber source has great potential in the applications of high power coherent beam combining (CBC) or spectral beam combining (SBC) systems.
In this work, we establish a unified temporal-spatial model to study the temporal-spectral dynamics of Ytterbium-doped fiber lasers (YDFLs). Different from the previously reported theories, this model is capable of obtaining the temporal property of YDFLs from the relaxation oscillation region to the relative stable region in the time window of millisecond scale with the time resolution of sub-nanosecond scale. The simulation results reveal that the three temporal instabilities, i.e. sustained self-pulsing (SSP), self-mode locking (SML) and turbulence-like pulsing (TLP), coexist in a multilongitudinal mode YDFL. When the output power evolution of the YDFL is observed in different time scales, different temporal instability phenomena dominate in the observation. Along with the increase of pump power, a single regular SML pulse would gradually break up into irregular TLP pulses and the life of the SSP pulse envelop would decrease. Besides, the spectral evolution property of the YDFLs at different pump powers are given and significant spectral broadening is observed.
Thermal effect of the gain fiber is one of the main factors which limit the power improvement of high power fiber lasers. Using traditional temperature measurement methods, we can only obtain the surface temperature of fiber while the core temperature cannot be detected. In this paper, the temperature of the gain fiber core in all-fiber laser is measured by optical frequency domain reflectometry(OFDR). The fiber laser is operating at 1080nm and the wavelength of detecting light emitted by OFDR is approximately 1550nm. The different wavelength of the two lasers has ensured the proper operating of fiber laser without effecting the operating lasers at 1080nm. The maximum output power of the fiber laser is 58.95W and results show that OFDR is a practical way to monitor the temperature of fiber laser system.
In this manuscript, we demonstrate an all-fiberized, single-frequency and polarization-maintained (PM) amplifiers with wavelength tuned from 1065 nm to 1090 nm. The ASE is suppressed by a signal to noise ratio of higher than 27 dB, and each wavelengths can be amplified to be 250 W output power. The stimulated Brillouin scattering (SBS) effect in such high power amplifiers is suppressed by employing a high dopant fiber (10 dB/m). The polarization extinction ratio (PER) of the amplifier is over 20 dB at the maximum output power. It should be noted that although the experiments are conducted at the wavelength from 1065 nm to 1090 nm with a step of 5 nm, the wavelength can also be continuously tuned.
In the pulsed fiber amplifiers with repetition frequency of several tens kHz, amplified spontaneous emission (ASE) is easy to build up because of the low repetition frequency and weak pulse signal. The ASE rises the difficulty to amplify the weak pulse signal effectively. We have demonstrated an all-fiber preamplifier stage structure to amplify a 40 kHz, 10 ns bandwidth (FWHM) weak pulse signal (299 μW) with center wavelength of 1062 nm. Compared synchronous pulse pump with continuous wave(CW) pump, the results indicate that synchronous pulse pump shows the better capability of increasing the output power than CW pump. In the condition of the same pump power, the output power of synchronous pulse pump is twice as high as CW pump. In order to suppress ASE, a longer gain fiber is utilized to reabsorb the ASE in which the wavelength is shorter than 1062nm. We amplified weak pulse signal via 0.8 m and 2.1 m gain fiber in synchronous pulse pump experiments respectively, and more ASE in the output spectra are observed in the 0.8 m gain fiber system. Due to the weaker ASE and consequent capability of higher pump power, the 2.1 m gain fiber is capable to achieve higher output power than shorter fiber. The output power of 2.1 m gain fiber case is limited by pump power.
Fiber lasers are now attracting more and more research interest due to their advantages in efficiency, beam quality and flexible operation. Up to now, most of the high power fiber lasers have random distributed polarization state. Linearlypolarized (LP) fiber lasers, which could find wide application potential in coherent detection, coherent/spectral beam combining, nonlinear frequency conversion, have been a research focus in recent years. In this paper, we will present a general review on the achievements of various kinds of high power linear-polarized fiber laser and its application. The recent progress in our group, including power scaling by using power amplifier with different mechanism, high power linearly polarized fiber laser with diversified properties, and various applications of high power linear-polarized fiber laser, are summarized. We have achieved 100 Watt level random distributed feedback fiber laser, kilowatt level continuous-wave (CW) all-fiber polarization-maintained fiber amplifier, 600 watt level average power picosecond polarization-maintained fiber amplifier and 300 watt level average power femtosecond polarization-maintained fiber amplifier. In addition, high power linearly polarized fiber lasers have been successfully applied in 5 kilowatt level coherent beam combining, structured light field and ultrasonic generation.
In this work, the spectral broadening and the stimulated Raman scattering (SRS) properties of fiber oscillators with different temporal and spectral characteristics were analyzed theoretically. We propose a spectral model for the amplification process in fiber amplifiers and a qualitative model for fiber oscillators through the superposition of multi-longitudinal modes. It is revealed that there are there are clearly linear and nonlinear broadening regions along with the power scaling in the MOPA structure, while the growth rate and the SRS threshold are all closely related to the temporal and spectral characteristics of the fiber oscillator. The preliminary analysis of the simulation results shows that both the noise floor and the temporal fluctuations in fiber oscillators will significantly impact the SRS threshold of the MOPA structure, while only the temporal fluctuations in fiber oscillators would impact the spectral broadening properties. The developed model represents a powerful tool for the optimization of a fiber oscillator through its structural parameters.
A high power single-frequency fiber amplifier with linear polarization is demonstrated based on the master oscillator power amplifier configuration, consisting of a single-frequency seed laser at 1064.1 nm and three-stage amplifiers. To suppress stimulated Brillouin scattering, a short piece of polarization-maintaining active fiber with large core diameter of 25 μm and high pump absorption coefficient is adopted in the main amplifier. Besides, step-distributed longitudinal strain is intentionally imposed on the active fiber to broaden the effective SBS gain spectrum and correspondingly improve the SBS threshold. As a result, a pump-limited single-frequency output of 414 W is obtained with no sign of SBS and mode instability, experimentally showing that the SBS threshold is improved by at least two times through introducing extra strain.. The slope efficiency of the main amplifier is about 80 %. The polarization degree is better than 98 % at all the power levels. To the best of our knowledge, this is the highest output power of single-frequency polarization-maintained fiber amplifier based on all-fiber structure.
We report on the high power amplification of narrow-linewidth polarization-maintained (PM) laser in all-fiber PM MOPA configuration, which can operate with linewidth around 2GHz at above 1kW power level. Pump-limited maximum narrow-linewidth output power is 1.4kW without SBS effect, and the linearly-polarization single-mode output power was limited to ~550W by mode instabilities (MI). The beam quality of the fiber amplifier (M2 ) was measured under different power, which degrades gradually from ~1.4 to ~2.2 after the onset of MI. The polarization extinction rate is measured to be about 90% before the onset of MI and reduce dramatically after MI sets in. The experimental results are analyzed based on a novel semi-analytical model, which has taken the effects of gain saturation into account. The theoretical results agree well with the experimental results. Mitigating MI by coiling the gain fiber has been analyzed and demonstrated numerically. It shows that, by tight coiling of the gain fiber to a radius of 4cm, the MI threshold can increase to 2.5 times higher than that without coiling or loose coiling, which means that the amplifier have the potential to achieve MI-free 1.4kW output power.
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