High-power linearly-polarized tunable Raman fiber laser

In this study, we demonstrate an all-fiber high-power linearly-polarized tunable Raman fiber laser system. An in- house high-power tunable fiber laser was employed as the pump source. A fiber loop mirror （FLM） serving as a high reflectivity mirror and a flat-cut endface serving as an output coupler were adopted to provide broadband feedback. A piece of 59-m commercial passive fiber was used as the Raman gain medium. The Raman laser had a 27.6 nm tuning range from 1112 nm to 1139.6 nm and a maximum output power of 125.3 W, which corresponds to a conversion efficiency of 79.4%. The polarization extinction ratio （PER） at all operational wavelengths was measured to be over 21 dB. To the best of our knowledge, this is the first report on a hundred-watt level linearly-polarized tunable Raman fiber laser.

High power tunable Raman fiber laser at 1.2μm waveband

Development of a high power fiber laser at special waveband,which is difficult to achieve by conventional rare-earth-doped fibers,is a significant challenge.One of the most common methods for achieving lasing at special wavelength is Raman conversion.Phosphorus-doped fiber(PDF),due to the phosphorus-related large frequency shift Raman peak at 40 THz,is a great choice for large frequency shift Raman conversion.Here,by adopting 150 m large mode area triple-clad PDF as Raman gain medium,and a novel wavelength-selective feedback mechanism to suppress the silica-related Raman emission,we build a high power cladding-pumped Raman fiber laser at 1.2μm waveband.A Raman signal with power up to 735.8 W at 1252.7 nm is obtained.To the best of our knowledge,this is the highest output power ever reported for fiber lasers at 1.2μm waveband.Moreover,by tuning the wavelength of the pump source,a tunable Raman output of more than 450 W over a wavelength range of 1240.6–1252.7 nm is demonstrated.This work proves PDF’s advantage in high power large frequency shift Raman conversion with a cladding pump scheme,thus providing a good solution for a high power laser source at special waveband.

High-power single-frequency fiber amplifiers:progress and challenge[Invited]

Unlike conventional continuous-wave lasers with wide spectra,the amplification of single-frequency lasers in optical fibers is much more difficult owing to the ultra-high power spectral density induced nonlinear stimulated Brillouin scattering effect.Nevertheless,over the past two decades much effort has been devoted to improving the power scaling and performance of high-power single-frequency fiber amplifiers.These amplifiers are mostly driven by applications,such as high precision detection and metrology,and have benefited from the long coherence length,low noise,and excellent beam quality of this type of laser source.In this paper,we review the overall development of high-power single-frequency fiber amplifiers by focusing on its progress and challenges,specifically,the strategies for circumventing the stimulated Brillouin scattering and transverse mode instability effects that,at present,are the major limiting factors of the power scaling of the single-frequency fiber amplifiers.These factors are also thoroughly discussed in terms of free-space and all-fiber coupled architecture.In addition,we also examine the noise properties of single-frequency fiber amplifiers,along with corresponding noise reducing schemes.Finally,we briefly envision the future development of high-power single-frequency fiber amplifiers.

2kW random fiber laser based on hybrid Yb-Raman gain [Invited]

High-power operation is one of the most important research topics surrounding random fiber lasers(RDFLs).Here we optimized the cavity structure and proposed a new scheme based on hybrid gain to address the issue of high-power backward light in traditional kilowatt-level RDFLs.Consequently,a record power of 1972 W was achieved while the maximum backward leaked power only reached 0.12 W.The conversion efficiency relative to the laser diode pump power was 68.4%,and the highest spectral purity of the random lasing reached 98.1%.This work may provide a reference for high-power RDFLs,Raman fiber lasers,and long-wavelength Yb-doped fiber lasers.

Multi-wavelength random fiber laser with a spectral-flexible characteristic

In past decades,multi-wavelength lasers have attracted much attention due to their wide applications in many fields.In this paper,we demonstrate a multi-wavelength random fiber laser with customizable spectra enabled by an acousto–optic tunable filter.The operating wavelength range can be tuned from 1114.5 to 1132.5 nm with a maximal output power of 5.55 W,and spectral channel tuning can also be realized with a maximal number of five.The effect of gain competition and the interaction between Raman gain and insertion loss are also analyzed.Furthermore,the output spectra can be ordered by radiating appropriate radio frequency signals to the acousto–optic tunable filter.This work may provide a reference for agile shape spectrum generation and promote multi-wavelength random fiber laser practicability in sensing,telecommunications,and precise spectroscopy.

Near-diffraction-limited linearly polarized narrow-linewidth random fiber laser with record kilowatt output

In this paper, we propose and experimentally investigate a linearly polarized narrow-linewidth random fiber laser(RFL) operating at 1080 nm and boost the output power to kilowatt level with near-diffraction-limited beam quality using a master oscillation power amplifier. The RFL based on a half-opened cavity, which is composed of a linearly polarized narrow-linewidth fiber Bragg grating and a 500 m piece of polarization-maintained Ge-doped fiber, generates a 0.71 W seed laser with an 88 pm full width at half-maximum(FWHM) linewidth and a 22.5 dB polarization extinction ratio(PER) for power scaling. A two-stage fiber amplifier enhances the seed laser to the maximal 1.01 k W with a PER value of 17 dB and a beam quality of M_x^2=1.15 and M_y^2=1.13. No stimulated Brillouin scattering effect is observed at the ultimate power level, and the FWHM linewidth of the amplified random laser broadens linearly as a function of the output power with a coefficient of about 0.1237 pm∕W.To the best of our knowledge, this is the first demonstration of a linearly polarized narrow-linewidth RFL with even kilowatt-level near-diffraction-limited output, and further performance scaling is ongoing.

kW-level,narrow-linewidth linearly polarized fiber laser with excellent beam quality through compact one-stage amplification scheme

In this manuscript, we demonstrate high-power, narrow-linewidth linearly polarized fiber laser with excellent beam quality through compact one-stage amplification scheme. By employing a single-mode–multimode–single-mode structure seed laser, a linearly polarized Yb-doped fiber laser with narrow linewidth and high output power is achieved.This laser, when used as a master oscillator, can be capable of suppressing the ASE in the process of power amplification.Thus, only one-stage amplification structure is used to scale up the laser power, and linearly polarized output with a polarization extinction ration of 14 d B, a narrow linewidth of 0.3 nm and an output power of 1018 W are achieved.Moreover, due to the good beam quality of seed laser and the well-designed amplifier stage, the beam quality of the output laser is near-diffraction-limited with M_x^2 ~1.18 and M_y^2 ~ 1.24 at the maximum power, and without mode instability occurring.

Pump scheme optimization of an incoherently pumped high-power random fiber laser

Optical signal-to-noise ratio(OSNR) is one of the most significant parameters for the performance characterization of random fiber lasers(RFLs) and their application potentiality in sensing and telecommunication. An effective way to improve the OSNR of RFLs is pump scheme optimization, for example, employing a temporally stable source as the pump. In this paper, the output performance of an incoherently pumped RFL dependence on the pump bandwidth has been investigated both in experiment and theory. It is found that a high-OSNR RFL can be achieved with broadband amplified spontaneous emission(ASE) source pumping, and a relatively broad pump bandwidth can also help suppress the spectral broadening while maintaining an ultra-high spectral purity.By optimizing the pump bandwidth to ~10 nm, maximum OSNR of ~39 dB(corresponding to a spectral purity of ~99.96%) with more than 99 W output power can be obtained. Moreover, for the pump bandwidth of 0.6–40 nm, the spectral purity can reach as high as >99% with the pump power ranging from ~85 to ~117 W.In addition, with the aid of theoretical simulation based on a modified power balance model, we find that the increment of pump bandwidth can decrease the effective Raman gain coefficient, further influencing the gain characteristics, nonlinear effects, and eventually the output performance. This work provides new insight into the influence of the pump characteristics on the output performance of incoherently pumped RFLs.

High Power Linearly Polarized Raman Fiber Laser With Stable Temporal Output

We demonstrate a high power linearly polarized Raman fiber laser(RFL)pumped by an amplified spontaneous emission(ASE)source.Temporal-stable operation of RFL could be ensured owing to the employment of ASE,which mitigates the inherent intensity noise compared with the classic scheme adopting laser oscillator as pump source.In this experiment,the RFL has up to 119.5W output power,with central wavelength of 1129.2nm,and full width at half maximum(FWHM)linewidth of about 4.18nm.The polarization extinction ratio(PER)of the Raman laser is about 23dB.Moreover,this laser has excellent long-term and short-term stabilities in terms of the output power and time domain.

In-band pumping avenue based high power superfluorescent fiber source with record power and near-diffraction-limited beam quality

High power superfluorescent fiber sources(SFSs), which could find wide applications in many fields such as middle infrared laser generation, Raman fiber laser pumping and spectral beam combination, have experienced a flourishing time in recent years for its unique properties, such as short coherence length and high temporal stability. The challenge for performance scalability of powerful SFS mainly lies on the physical issues including parasitic laser oscillation and modal instability(MI). In this contribution, by employing in-band pumping avenue and high-order transverse-mode management, we explore a high power SFS with record power, near-diffraction-limited beam quality and spectral manipulation flexibility. An ultimate output power of 3.14 kW can be obtained with high temporal stability and a beam quality of M^2= 1.59 for the amplified light. Furthermore, the dynamics of spectral evolutions, including redshifting of central wavelength and unsymmetrical broadening in spectral wings, of the main amplifier with different seed linewidths are investigated contrastively. Benefiting from the unique high pump brightness and high MI threshold of in-band pumping scheme, the demonstrated system also manifests promising performance scaling potential.

Powerful Narrow Linewidth Random Fiber Laser

在这篇论文，我们表明狭窄的线宽随机纤维激光，它采用悦耳的泵激光为效率优化，一个狭窄乐队的纤维布拉格栅栏( FBG )和构造一个一半开的洞的单个模式纤维的节选择操作波长，并且分开泵光输入和随机的放射激光输出的一个传播者。光谱线宽在下面到 42.31， GHz 通过由 FBG 过滤被完成。8.97 W 泵光什么时候在优化波长集中了 1036.5 nm 被发射进一半开的洞，与 2.15 W 的最大的产量力量放射激光的 1081.4 nm 随机被完成，它比以前的报导结果更强大。

Vortex random fiber laser with controllable orbital angular momentum mode

In this paper, we propose and experimentally demonstrate a vortex random fiber laser(RFL) with a controllable orbital angular momentum(OAM) mode. The topological charge of the vortex RFL can range from-50 to 50 with nearly watt-level output power. A triangular toroidal interferometer is constructed to verify the spiral phase structure of the generated vortex random laser with a special coherence property. Vortex RFLs with fractional topological charge are also performed in this work. As the first demonstration of a vortex RFL with a controllable OAM mode(to the best of our knowledge), this work may not only offer a valuable reference on temporal modulation of a vortex beam and optical field control of an RFL but also provide a potential vortex laser source for applications in imaging, sensing, and communication.

2-kW-level superfluorescent fiber source with flexible wavelength and linewidth tunable characteristics

The superfluorescent fiber source(SFS)with tunable optical spectrum has shown great application potential in the sensing,imaging,and spectral combination.Here,we demonstrate for the first time a 2-kW-level wavelength and linewidth tunable SFS.Based on a flexible filtered SFS seed and three stages of fiber amplifiers,the output power can be scaled from the milliwatt level to about 2 kW,with a wavelength tuning range of 1068–1092 nm and a linewidth tuning range of 2.5–9.7 nm.Moreover,a numerical simulation is conducted based on the generalized nonlinear Schr?dinger equation,and the results reveal that the wavelength tuning range is limited by the decrease of seed power and the growth of amplified spontaneous emission,whereas the linewidth tuning range is determined by the gain competition and nonlinear Kerr effects.The developed wavelength and linewidth tunable SFS may be applied to scientific research and industrial processing.

Optical rogue wave in random fiber laser

The famous demonstration of optical rogue waves(RWs),a powerful tool to reveal the fundamental physics in different laser scenarios,opened a flourishing time for temporal statistics.Random fiber laser(RFL)has likewise attracted wide attention due to its great potential in multidisciplinary demonstrations and promising applications.However,owing to the distinctive cavity-free structure,it is a scientific challenge to achieve temporal localized RWs in RFLs,whose feedback arises from multiple scattering in disordered medium.Here,we report the exploration of RW in the highly skewed,transient intensity of an incoherently pumped RFL for the first time,to our knowledge,and unfold the involved kinetics successfully.The corresponding frequency domain measurements demonstrate that the RW event arises from a crucial sustained stimulated Brillouin scattering process with intrinsic stochastic nature.This investigation highlights a novel path to fully understanding the complex physics,such as photon propagation and localization,in disordered media.

Ultralow-quantum-defect Raman laser based on the boson peak in phosphosilicate fiber

Quantum defects(QDs)have always been a key factor of the thermal effect in high-power fiber lasers.Much research on low-QD fiber lasers has been reported in the past decades,but most of it is based on active fibers.Besides,Raman fiber lasers based on the stimulated Raman scattering effect in passive fiber are also becoming an important kind of high-power fiber laser for their unique advantages,such as their significantly broader wavelength-tuning range and being free of photon darkening.In this paper,we demonstrate an ultralow-QD Raman fiber laser based on phosphosilicate fiber.There is a strong boson peak located at a frequency shift of 3.65 THz in the Raman gain spectrum of the phosphosilicate fiber we employed.By utilizing this boson peak to provide Raman gain and adopting an amplified spontaneous emission source at 1066 nm as the pump source,1080 nm Stokes light is generated,corresponding to a QD of 1.3%.The spectral purity at 1080 nm can be up to 96.03%,and the output power is 12.5 W,corresponding to a conversion efficiency of 67.2%.Moreover,by increasing the pump wavelength to 1072 nm,the QD is reduced to 0.74%,and the output power at 1080 nm is 10.7 W,with a spectral purity of 82.82%.To the best of our knowledge,this is the lowest QD ever reported for Raman fiber lasers.This work proposes a promising way of achieving high-power,high-efficiency Raman fiber lasers.

Passively spatiotemporal gain-modulation-induced stable pulsing operation of a random fiber laser

Unlike a traditional fiber laser with a defined resonant cavity, a random fiber laser(RFL), whose operation is based on distributed feedback and gain via Rayleigh scattering(RS) and stimulated Raman scattering in a long passive fiber, has fundamental scientific challenges in pulsing operation for its remarkable cavity-free feature. For the time being, stable pulsed RFL utilizing a passive method has not been reported. Here, we propose and experimentally realize the passive spatiotemporal gain-modulation-induced stable pulsing operation of counterpumped RFL. Thanks to the good temporal stability of an employed pumping amplified spontaneous emission source and the superiority of this pulse generation scheme, a stable and regular pulse train can be obtained.Furthermore, the pump hysteresis and bistability phenomena with the generation of high-order Stokes light is presented, and the dynamics of pulsing operation is discussed after the theoretical investigation of the counterpumped RFL. This work extends our comprehension of temporal property of RFL and paves an effective novel avenue for the exploration of pulsed RFL with structural simplicity, low cost, and stable output.

Cladding-pumped Raman fiber laser with 0.78%quantum defect enabled by phosphorus-doped fiber

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.

Hundred-watt-level linearly polarized tunable Raman random fiber laser

A high power linearly polarized tunable Raman random fiber laser（RFL） was studied theoretically and experimentally. The parameters required for the system design were obtained through numerical simulation, based on which a hundred-watt-level linearly polarized tunable RFL was successfully demonstrated. The central wavelength can be continuously tuned from 1113.76 to 1137.44 nm, and the output power exceeds 100 W for all of the lasing wavelengths with the polarization extinction ratio（PER） exceeding 20 d B at the maximum output power.Besides, the linewidth, spectral evolution, and temporal dynamics of a specified wavelength（1124.72 nm） were investigated in detail. Moreover, the theoretical results and the experimental results fit well. To the best of our knowledge, this is the first time for a hundred-watt-level linearly polarized tunable RFL ever reported.

Toward high-power nonlinear fiber amplifier

Stimulated Raman scattering(SRS) effect is considered to be one of the main obstacles for power scaling in general-type fiber lasers. Different from previous techniques that aim at suppressing SRS, nonlinear fiber amplifier(NFA), which manipulates and employs the SRS for power scaling in rare-earth-doped fiber, is under intensive research in recent years.In this paper, the authors will present an all-round study on this new kind of high-power fiber amplifier. A theoretical model is proposed based on the rate equation and amplified spontaneous emission(ASE), with random noise taken into account. By numerical solving of the theoretical model, the power scaling potential, heat analysis and advantages in suppressing the undesired backscattering light are quantificationally analyzed for the first time. Then two different types of high-power NFAs are demonstrated individually. Firstly, a laser diode pumped NFA has reached kilowatt output power,and the results agree well with theoretical predictions. Secondly, a tandem-pumped NFA is proposed for the first time and validated experimentally, in which 1.5 kW output power has been achieved. The authors also briefly discuss several new issues relating to the complex nonlinear dynamics that occur in high-power NFAs, which might be interesting topics for future endeavors.

Highly twisted M-line of a vortex beam due to the coupling of ultrahigh-order modes

In modern optics, particular interest is devoted to the phase singularities that yield complicated and twisted phase structures by photons carrying optical angular momentum.In this paper, the traditional M-line method is applied to a vortex beam(VB) by a symmetric metal cladding waveguide chip, which can host numerous oscillating guided modes via free space coupling.These ultrahigh-order modes(UOMs) result in high angular resolution due to the high finesse of the resonant chip.Experiments show that the reflected pattern of a VB can be divided into a series of inner and outer rings, whilst both of them are highly distorted by the M-lines due to the UOMs’ leakage.Taking the distribution of the energy flux into account, a simple ray-optics-based model is proposed to simulate the reflected pattern by calculating the local incident angle over the cross section of the beam.The theoretical simulations fit well with the experimental results, and the proposed scheme may enable new applications in imaging and sensing of complicated phase structures.