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Raman fiber amplifier experimental setup. (a) Implementation of the amplifier; (b) cross-section through the input fiber bundles and the fusion point of the pump and signal combiner (PSC). BQA, beam quality analyzer; OPM, optical power meter; OSA, optical spectrum analyzer.
Source publication
We report a 2 kW all-fiberized Raman fiber amplifier with efficient brightness enhancement based on the graded-index fiber. The maximum power output reaches up to 2.034 kW centered at 1130 nm, with a conversion efficiency of 79.35% with respect to the injected pump power. To the best of our knowledge, this is the highest conversion efficiency obtai...
Contexts in source publication
Context 1
... described in Figure 1(a), the RFA comprises a master oscillator as the seed laser and combined fiber lasers as the pump for the amplifier. The oscillator is an RFL at 1130 nm pumped by a YDFL at 1080 nm. ...
Context 2
... output fiber of the PSC has core and cladding diameters of 62.5 µm and 125 µm, respectively, and the numerical aperture (NA) of the core is 0.275. The cross-section of the input bundles and the fusion point are shown schematically in Figure 1(b). The conventional PSC, which has been widely applied in cladding-pumped fiber lasers and amplifiers, utilizes the pump and signal laser beams injected separately into the cladding and core of double-clad fiber [36][37][38] . ...
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Citations
... These advantages are of great significance for solving the aforementioned problems of the active fiber scheme [19,20]. In recent years, based on either oscillator or amplifier structure, remarkable achievements have been witnessed in RFLs pumped by YDFLs typically, whose output power has reached up to multiple kilowatts at 1120nm [21][22][23] and 1130nm [24][25][26]. It proves the feasibility of Raman gain to generate high-power laser. ...
... However, due to the linear increase in Stokes beam power, the value of BE is gradually improved from 7.7 to 11.3. In the above process, as more multi-wavelength pumps are launched to the fiber, the power density in the fiber core increases gradually, the quantum defect leads to the generation and accumulation of massive waste heat in the fiber core, especially with the power scaling beyond kilowatt level [21][22][23][24][25][26]. The temperature of the fiber core increases rapidly that could result in a series of nonlinear effects on fiber system, such as the thermal lens effect and additional noise which eventually leads to the degradation of the output beam quality. ...
Raman fiber lasers (RFLs), relying primarily on stimulated Raman scattering (SRS) and directly pumped by laser diodes (LDs), offer a valuable avenue for exploring nonlinear photonics, molecular vibrational imaging, and passive photonic devices. However, the obstacles of the poor pump brightness and higher-order Stokes generation limit the further power scaling of those lasers. Herein, to improve the pump brightness and suppress higher-order Stokes, fifteen wavelength-combined multimode LDs are used as pump sources for cladding-pumped triple-clad fiber. The output power reached 1082W at the wavelength of 1020nm. The overall slope efficiency is 81.9% and the optical-optical conversion efficiency is 49.4%. To the best of our knowledge, this is the highest power output for RFLs directly pumped by multimode LDs. The fusion of spectral combination multi-wavelength diodes pumping technology and the use of multi-cladding fiber may provide a promising way of achieving high-power special wavelengths fiber sources.
... In recent years, graded-index (GRIN) multimode fibers have been used as Raman amplifiers, mainly motivated by two properties of such fibers. First, a larger core diameter allows for higher output powers exceeding kilowatt levels [4][5][6][7]. Second, the spatial quality of the amplified beam is improved through a phenomenon known as the Ramaninduced beam cleanup [8,9]. ...
... We are interested in finding an approximate solution of Eqs. (5) and (6), obtained with some reasonable assumptions. In practice, the pump beam is much more intense than the signal beam at the input end of the fiber. ...
... In a 2020 experiment [5], only a 20-m-long piece of GRIN fiber was employed to suppress the onset of the second-order Stokes inside the amplifier. The fiber has a single cladding of 125 µm diameter, and its core had a diameter of 62.5 µm. ...
A semi-analytic model of the amplification process is presented for Raman amplifiers made with graded-index multimode fibers. When the pump beam remains much more intense than the signal being amplified, it evolves in a self-similar fashion and recovers its initial width periodically. Using this feature, the width of the amplified signal is found to satisfy an equation whose form is similar to that of a damped harmonic oscillator. We use this equation to discuss the spatial beam narrowing occurring inside a Raman amplifier. In addition to oscillating with a period ${\sim}{1}\;\text{mm}$ ∼ 1 mm , the beam also narrows down during its amplification inside a graded-index fiber on a length scale ${\sim}{1}\;\text{m}$ ∼ 1 m . The main advantage of our simplified approach is that it provides an analytic expression for the damping distance of width oscillations that shows clearly the role played by various physical parameters.
... Ultrafast fiber lasers with high average power and high pulse energy are in great demand in industrial material processing as it can reduce thermal diffusion of processing area and improve spatial resolution of nanofabrication, for instance, hard material processing, glass cutting, high-precision micromachining [1][2][3][4][5][6][7][8][9]. Generally, ultrafast fiber oscillator with nJ-scale pulse energy and mW-scale average power is unable to meet application requirements, so further power amplification is required. ...
A high-average-power, high-pulse-energy picosecond chirped pulse amplification (CPA) laser system based on an extra-large-mode-area (XLMA) triple-clad fiber (TCF) was demonstrated. The ultrashort pulses, generated from all-fiber mode-locked oscillator, stretched and then were pre-amplified to 10 W through a series of fiber power amplifiers. Subsequently, the average output power was amplified to 620 W corresponding to a pulse energy of 0.62 mJ via XLMA TCF. Additionally, the amplified pulses were compressed to a pulse duration of 7.6 ps with an average power of 423 W and a compression efficiency of 68.2%. The ultrashort laser is a promising light source for application of water-guided laser processing, albeit with a beam quality factor of 20 and 21 along two orthogonal axes.
... As a result, the RRFL has found numerous applications, such as remote sensing [4]- [6], communication [7], [8], and speckle-free imaging [9], [10]. Moreover, the Raman gain for generating a laser is wavelength agile [11], [12] and free of photo-darkening effect [13], allowing the RRFL to be a promising candidate on high-power laser generation at special spectral bands [14]- [18]. ...
Random distributed-feedback Raman fiber lasers (RRFLs) are promising contenders in the applications of remote sensing, biotechnology, imaging, thanks to the unique properties on low coherence and wavelength agility. Despite recent advances on RRFL, the lack of brightness enhancement (BE) releasing high requirement on pump brightness remains a significant technological challenge for further power scaling. To this extent, we propose in this paper the first-ever cladding-pumped (CP) RRFL with BE to our best knowledge. The triple-clad purely passive fiber with designed cladding-to-core area ratio is utilized to suppress the generation of higher-order Stokes light. Pumped by two combined Yb-doped fiber lasers at 1080 nm, signal laser with maximum output power of 1 kW at 1132 nm is achieved with an optical-to-optical conversion efficiency of 92% (ratio between signal and absorbed pump power, which is 74% when vs. launched power). Moreover, the modal dynamics indicates the strong mode coupling induced by the complex interaction among the distributed gain, feedback, and other nonlinear effects, leading to the modal instability in the CP RRFL. Even so, this demonstration paves the way for powerful RRFLs beyond kilowatt with BE, which is of interest for diversified potential applications as diverse as industry and security.
... In addition, RFL may operate without conventional cavity, via Rayleigh backscattering forming random distributed feedback along a singlemode passive fiber, see [2] and citation therein. Another direction which attracts lots of attention deals with the great potential in achieving brightness enhancement (BE): highbrightness signal laser at low-brightness pumping, in particular, in commercially available multimode GRIN fibers, either in laser [3,4] or in amplifier [5] configurations. In the GRIN-fiber RFL, linear cavity consisting of two bulk mirrors [3] which may be replaced by fiber Bragg gratings (FBGs) inscribed directly in the fiber core near its ends thus providing all-fiber configuration [4], or half-open cavity with one FBG and random distributed feedback (DFB) via Rayleigh backscattering along the GRIN fiber [6,7] are utilized for the multimode Raman laser operation with brightness enhancement. ...
... In the GRIN-fiber RFL, linear cavity consisting of two bulk mirrors [3] which may be replaced by fiber Bragg gratings (FBGs) inscribed directly in the fiber core near its ends thus providing all-fiber configuration [4], or half-open cavity with one FBG and random distributed feedback (DFB) via Rayleigh backscattering along the GRIN fiber [6,7] are utilized for the multimode Raman laser operation with brightness enhancement. As a pump, either combined multimode LDs [3,4,6] or high power Yb-doped fiber lasers (YDFL) [5,7] are used, with sufficiently higher power in the latter case. ...
Raman lasing in multimode GRIN fibers is accompanied by sufficient improvement of the output beam quality in comparison with that for pump radiation, which offers opportunities to wavelength-agile fiber lasers of new type. Here we compare power scaling and brightness enhancement capabilities of Raman laser based on multimode GRIN-fiber of 62.5/125 um core/cladding diameters pumped by ∼700 W multimode source with beam quality M ² ∼10, performed in two different cavity configurations: 1) linear cavity based on two fiber Bragg gratings and 2) half-open cavity with one FBG and random distributed feedback via Rayleigh backscattering along the GRIN fiber.
... The laser driver is mainly composed of a front end, pre-amplification, main amplification transmission, energy system, measurement control, and range system. The energy level, pulse width, focal spot shape, repetition frequency, positioning accuracy, and stability of the laser driver are the key to the success of an ICF fusion experiment [3][4][5][6][7][8]. ...
Based on the lens of spatial filter parallel auxiliary assembly and collimation method, aiming at the unloading and locking problem of the filter lens after collimating, a lens parallel adjustable axial locking structure was proposed. A special elastic chuck structure was designed to realize clamping axial friction locking; each branch realized interference fit, and the adjustable structure of “line-line-line” contact realized equivalent fixation through spring pre-tightening. Compared with radial locking, the abrupt error of the axial locking lens in the unloading process of a collimating auxiliary machine was small, and the posture state maintenance and structural response stability were strong. The established homogeneous transformation matrix (HTM) and lens global error showed that the pose torsion roll error of the radial locking structure was significantly suppressed. The relative stability experiment showed that the angular drift error of the lens axial locking structure was ± 0.2 µrad. The design effectively improved the unloading anti-mutation characteristics and stability of the filter lens, which provided an experimental basis for the application of intelligent collimation assisted assembly and adjustment scheme in the upgrading of a high-power laser device.
... Up to hundreds watt-level high-power RFL in all-fiber format have been demonstrated by several independent groups [28][29][30][31] . By adopting a main-oscillator-power-amplifier structure, up to 3 kW RFL has been demonstrated [32,33] . Usually, the high-power fiber lasers aforementioned have QDs of about 4~5% based on the 13.2 THz frequency shift. ...
The quantum defect (QD) is an important issue that demands prompt attention in high-power fiber lasers. A large QD may aggravate the thermal load in the laser, which would impact the frequency, amplitude noise and mode stability, and threaten the security of the high-power laser system. Here, we propose and demonstrate a cladding-pumped Raman fiber laser (RFL) with QD of less than 1%. Using the Raman gain of the boson peak in a phosphorus-doped fiber to enable the cladding pump, the QD is reduced to as low as 0.78% with a 23.7 W output power. To our knowledge, this is the lowest QD ever reported in a cladding-pumped RFL. Furthermore, the output power can be scaled to 47.7 W with a QD of 1.29%. This work not only offers a preliminary platform for the realization of high-power low-QD fiber lasers, but also proves the great potential of low-QD fiber lasers in power scaling. © The Author(s), 2022. Published by Cambridge University Press in association with Chinese Laser Press
... Meanwhile, the output powers of OPOs and non-diamond Raman lasers are yet to approach even hundreds of watts; restricted by thermal lensing in the gain materials. The only other technology competitive enough to pursue such power gains is the fiber-based Raman lasers [98,[101][102][103][104][166][167][168][169][170][171][172][173], as evident from the figure. ...
... Meanwhile, the output powers of OPOs and non-diamond Raman lasers are yet to approach even hundreds of watts; restricted by thermal lensing in the gain materials. The only other technology competitive enough to pursue such power gains is the fiber-based Raman lasers [98,[101][102][103][104][166][167][168][169][170][171][172][173], as evident from the figure. As already discussed, power enhancement also brings in an additional requirement of good beam quality. ...
... The brightness achieved by a DRL is more than three orders of magnitude than realized by a non-diamond Raman laser. On the other hand, Raman fiber lasers have displayed BE similar to DRLs [102,103,169,[173][174][175][176][177]. However, in most of these cases, the thermal load in fibers with elevating output powers have prevented the beam quality to reach to near diffraction-limited output. ...
Optical parametric oscillators (OPOs) and Raman lasers are two nonlinear-based laser technologies that extend the spectral range of conventional inversion lasers. Power and brightness scaling of lasers are significant for many applications in industry, medicine, and defense. Considerable advances have been made to enhance the power and brightness of inversion lasers. However, research around the power scaling of nonlinear lasers is lacking. This paper reviews the development and progress of output power of continuous-wave (CW) crystalline OPOs and Raman lasers. We further evaluate the power scalability of these two laser technologies by analyzing the cavity architectures and gain materials used in these lasers. This paper also discusses why diamond Raman lasers (DRLs) show tremendous potential as a single laser source for generating exceedingly high output powers and high brightness.
... spatio-spectral filtering, which compensate for randomization. It should be noted that in FBG-free kW-level Raman amplifiers 25 , the Kerr effect alone should play a significant role in the observed beam cleanup. Remarkably, the CW Stokes power in RFLs is about two orders of magnitude lower than that in amplifiers 25 , as well as the typical power level which is required for Kerr-cleaning of sub-nanosecond pulses in ~ 10 m of GIF 18,29 . ...
... It should be noted that in FBG-free kW-level Raman amplifiers 25 , the Kerr effect alone should play a significant role in the observed beam cleanup. Remarkably, the CW Stokes power in RFLs is about two orders of magnitude lower than that in amplifiers 25 , as well as the typical power level which is required for Kerr-cleaning of sub-nanosecond pulses in ~ 10 m of GIF 18,29 . This means that in a km-long fbre cavity the Kerr self-cleaning effect may already enter into play at tens of Watts power levels. ...
... This opens the way to high-power fibre lasers at wavelengths ≤ 980 nm, with frequency doubling into the blue range. The significant power scaling capabilities 25,26 and inherent tunability of LD-pumped RFL may lead to widespread use in environmental, cultural heritage, biomedical and material processing applications. Nonlinear optical control of spatio-spectral beam parameters will provide a new platform of wavelengthagile high-power LD-pumped light sources. ...
Multimode fibres provide a promising platform for boosting the capacity of fibre links and the output power of fibre lasers. The complex spatiotemporal dynamics of multimode beams may be controlled in spatial and temporal domains via the interplay of nonlinear, dispersive and dissipative effects. Raman nonlinearity induces beam cleanup in long graded-index fibres within a laser cavity, even for CW Stokes beams pumped by highly-multimode laser diodes (LDs). This leads to a breakthrough approach for wavelength-agile high-power lasers. However, current understanding of Raman beam cleanup is restricted to a small-signal gain regime, being not applicable to describing realistic laser operation. We solved this challenge by experimentally and theoretically studying pump-to-Stokes beam conversion in a graded-index fibre cavity. We show that random mode coupling, intracavity filtering and Kerr self-cleaning all play a decisive role for the spatio-spectral control of CW Stokes beams. Whereas the depleted LD pump radiation remains insensitive to them.
... Although it was pumped by four 350-W single-mode YDFLs, the authors claim that the YDFLs are incoherently combined through the combiner into the cladding, degrading the beam quality factor (M 2 ) to 14.7, which is as bad as a diode laser. A similar setup but a graded-index (GRIN) fibre Raman amplifier reached 2 kW in a demonstration by another group [25]. Accompanied with the recent development on spectrally combined bright diode laser for material processing [26], this might become another new standard way to build multi-kW lasers in the future. ...
Fibre Raman lasers are an alternative to high power rare-earth-doped fibre lasers with benefits such as wavelength versatility and resilience to photodarkening. However, stimulated Raman scattering is a weak nonlinear process that requires high pump intensities, which are difficult to reach with high power multimode diode lasers, which are favoured for pumping. Nevertheless, research has shown that power-scaling of fibre Raman lasers pumped directly by such diode laser is possible. Whereas high-power diodelaser-pumped fibre Raman lasers have been investigated with only a single wavelength (narrow-bandwidth) pumping thus far, this study aims to further power-scale diode laserpumped fibre Raman laser by using wide-span multi-wavelength pumping. First, I examined two-wavelength (976 and 950 nm) pumped fibre Raman laser. Two high-power diode lasers are spectrally combined and launched into graded-index fibre. Although the separation between the two wavelengths was far from ideal (neither smaller than the Raman linewidth nor comparable to the Raman shift), the laser emitted 23 W at a single wavelength. This indicates that the separation among pump wavelengths is not so critical. Next, another three high power diode lasers at 969, 940, and 915 nm were spectrally added to the pump, expanding the total pump bandwidth to 683 cm-1 (915 – 976 nm). Three different fibres (silica-core step-index fibre, graded-index fibre with high germanium-doping, and step-index fibre with high germanium-doping) were tested. All three cases led to high-power single-wavelength output, reaching 32, 67, and 101 W, respectively. Also, this showed the advantages of germanium-doping. (e.g., higher Raman gain and pump launching efficiency) Lastly, numerical simulations showed that a pump bandwidth of up to ~1000 cm-1 can be used to pump a fibre Raman laser with an efficiency of more than 70%, for a clad-to-core area ratio up to ~7. Overall, the results show that multi-wavelength pumping of fibre Raman laser allow for efficient high power lasers. Thus, the basic concept of a multi-wavelength-pumped fibre Raman laser that may evolve into a new type of kW-laser is established in this thesis.
