High-power Fiber Laser Research Articles

Investigation of photodarkening in tandem-pumped Yb-doped fibers

Investigation of photodarkening (PD) in Yb-doped fibers tandem-pumped at 1018 nm is reported. For a homemade Yb-doped aluminosilicate double-clad fiber (YADF), the transmitted power of a 633 nm probe beam is reduced by 2.4% over 2 hours for the tandem pumping configuration at 1018 nm, which is significantly smaller than 33.3% for a laser diode (LD) pumping at 976 nm. A tandem-pumped Yb fiber amplifier also shows a much smaller decrease in the amplified output power over time than a LD-pumped Yb fiber amplifier. Based on fluorescence spectra of the YADF, we can not only associate PD of the YADF to intrinsic oxygen deficiency centers or Tm3+ impurities but also confirm the impact of the excited Yb3+ ion density on PD. The benefits of the tandem pumping in a high-power Yb fiber laser system will be discussed.

Suppressing the amplified spontaneous emission in the high-power 1018-nm monolithic fiber laser by decreasing the feedback from the inner reflections

We showed that decreasing the feedback that comes from the inner reflections of counter-propagating light could suppress the amplified spontaneous emission (ASE) in the output of 1018-nm fiber lasers. With a simulation model to solve the power spectrum of multiple signals, we calculated the values of the feedback required for aimed power scaling. We realized low feedback by modifying the counter-propagating side of the setup, which included a home-made cladding light stripper on a no-core fiber that was angle-cleaved. With that, we demonstrated a 1018-nm monolithic fiber laser producing 506.4 W with 1 .58 - M 2 beam quality, 84.9% optical–optical efficiency, and 148-pm linewidth. The maximum intensity of the ASE noises was > 50 d B lower than that of the 1018-nm signal, which could allow further power scaling with safety and stability.

High-power erbium-doped fiber laser with a carbon nanotubes-doped sol-gel glass mode-locker

• In this study, for the first time, a carbon nanotube-doped sol-gel glass composite absorber is fabricated using a low-temperature sol-gel method. • The main component of the sol-gel glass is compact amorphous inorganic silicon-dioxide, which has high laser-damage threshold. Generally, broadband absorbers has much non-saturable loss and low laser damage threshold, which limits the average output power of the fiber lasers mode locked by broadband absorbers. Exceptions are the absorbers in evanescent wave mode locking, in which way only weak laser passes through the absorber section. However, evanescent wave mode locking method needs to change the original structure of the fiber. Most of the commercial fiber-lasers adopt face-to-face passive mode-locking. Therefore, it is important to increase the damage threshold of the broadband absorbers. In this study, for the first time, a single-walled carbon nanotube (SWNT)-doped sol-gel glass composite absorber is fabricated using a low-temperature sol-gel method. The main component of the sol-gel glass is compact amorphous inorganic silicon-dioxide, which has high laser-damage threshold. The SWNT-doped sol-gel glass is integrated into the erbium-doped fiber (EDF) laser and mode-locking occurs at 1559 nm. The shortest pulse duration is 456 fs, and the maximum average output power is 110.65 mW. Compared to most EDF lasers based on broadband absorbers, the average output power is one of the highest values. The results indicate that encapsulating absorber in the inorganic sol-gel matrix is a promising method for fabricating high power ultrafast fiber laser type absorbers.

3 W tunable 1.65 µm fiber gas Raman laser in D2-filled hollow-core photonic crystal fibers

We demonstrate here a multi-watt tunable 1.65 µm fiber laser based on deuterium-filled hollow-core photonic crystal fibers (HC-PCFs) by stimulated Raman scattering (SRS). Owing to the fiber transmission properties of the HC-PCFs, pure rotational Stokes wave at 1.65 µm can be generated when pumped with a tunable 1.5 µm erbium-doped fiber amplifier (EDFA). When pumped at 1540 nm, with a 20-meter-long HC-PCF filled with 16 bar deuterium, a record average Stokes power of ∼2.91 W at 1645 nm is achieved, which is about 3.64 times higher than the similar experiment previously reported, and the corresponding power conversion efficiency is about 58%. Over the whole Stokes tunable range of 1645 to 1656 nm, the average output power is >2.5 W and the conversion efficiencies are >54%. A numerical model is also established to analyze the rotational SRS process in deuterium-filled HC-PCFs, the simulation results are in good agreement with experiments. This work opens a new opportunity for high-power tunable fiber lasers operating in the 1.7 μm band.

Analytical formulation of a high-power Yb-doped double-cladding fiber laser

A detailed formalism to achieve an analytical solution of a lossy high power Yb-doped silica fiber laser is introduced. The solutions for the lossless fiber laser is initially attained in detail. Next, the solution for the lossy fiber laser is obtained based on the lossless fiber laser solution. To examine the solutions for both the lossless and lossy fiber laser, two sets of values are compared with the exact numerical solutions, and the results are in a good agreement. Furthermore, steps and procedures for achieving the final solutions are explained clearly and precisely.

Underwater laser cutting of stainless steel up to 100 mm thick for dismantling application in nuclear power plants

For dismantling nuclear power plants, an underwater cutting study was conducted using a high power fiber laser. With a laser power of 9 kW, cutting tests were performed on stainless steel with a thickness of 50–100 mm. For this study, a waterproof cutting head consisting of a focusing element with a long focal length of 600 mm and a supersonic nozzle was used for high cutting efficiency. For thicknesses of 70 mm or less, it was possible to cut only with a single supersonic nozzle, while for a thickness of 80 mm or more, an initially pierced specimen or a double nozzle was required for successful cutting. Through these techniques, we were able to perform the cutting underwater up to a thickness of 100 mm. Furthermore, the kerf widths were as small as 1–2 mm, the amount of secondary waste will likely be small, which will reduce waste disposal costs.

Influence of Bragg reflection of chirped tilted fiber Bragg grating on Raman suppression in high-power tandem pumping fiber amplifiers.

The key remaining technological challenge to the realization of further power scaling for high-power fiber laser systems is overcoming the stimulated Raman scattering (SRS) effect. In past years, chirped and tilted fiber Bragg gratings (CTFBGs) have been demonstrated to be a simple and effective way to suppress SRS in high-power fiber amplifiers. However, the weak reflection at the Bragg wavelength could be strongly amplified, which not only limits the power and efficiency but also degrades the beam quality. We report here, for the first time to the best of our knowledge, the influence of the residual Bragg reflection of CTFBGs on SRS suppression in high-power fiber laser systems. Two groups of CTFBGs with different Bragg reflection wavelengths are fabricated and used for the comparison experiments. Test results show that the CTFBGs of longer Bragg wavelengths have a better suppression effect, in particular, at a higher power level. By further moving the Bragg wavelength of a CTFBG out of the Raman gain spectral range, a better suppression effect and a promotion in laser efficiency could be achieved, which is very useful for further power scaling.

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.

Building-up and suppression of cladding mode coupling in fiber Bragg gratings

The building-up and suppression of cladding-mode coupling in UV exposure fiber Bragg gratings (FBGs) is investigated experimentally and theoretically. Experimental results reveal that fundamental mode can simultaneously couple to cladding modes from different groups in the un-tilted FBGs as the transmission spectrum takes on multilevel. Theoretical results indicate that due to the non-uniform refractive index profile on the transverse plane of the FBGs, core-guided fundamental mode not only couples to axis-symmetrical cladding modes LP0n, but also transfers to circular-symmetrical cladding modes like LP1n if they distribute in the core region. Based on the symmetry property of the coupled cladding modes, the ‘ring-by-ring’ inscription is theoretically proposed to suppress the cladding mode coupling for the first time, which plays a significant role in high-power fiber lasers research and application.

High power microsecond fiber laser at 1.5 μm.

In this work, we demonstrate a single frequency, high power fiber-laser system, operating at 1550 nm, generating controllable rectangular-shape μs pulses. In order to control the amplified spontaneous emission content, and overcome the undesirable pulse steepening during the amplification, a new method with two seed sources operating at 1550 nm and 1560 nm are used in this system. The output power is about 35 W in CW mode, and the peak power is around 32 W in the pulsed mode. The repetition rate of the system is tunable between 50 Hz to 10 kHz, and the pulse duration is adjustable from 10 μs to 100 μs, with all on the fly electronically configurable design. The system demonstrates excellent long and short time stability, as well as spectral and spatial beam quality.

Experimental Investigations on TMI and IM-FWM in Distributed Side-Pumped Fiber Amplifier

The inter-modal four-wave mixing (IM-FWM) and transversal mode instabilities (TMI) are experimentally studied in high-power distributed side-pumped fiber amplifiers. To the best of our knowledge, we have made the first demonstration of TMI and IM-FWM which can be suppressed simultaneously by increasing the bending diameter. Besides, the experimental results show that the counter-pumping scheme is beneficial to suppress both of IM-FWM and TMI, when comparing with co-pumping scheme. Furthermore, these two effects cannot be observed when the M2 factor is lower than 1.3, which can give a condition to estimate whether these two effects should be considered or not. The pertinent study can provide some guidance for understanding TMI and IM-FWM in the high-power fiber laser and amplifier.

Heat dissipation model and temperature distribution of Yb-doped double-clad fiber in the composite system

Abstract To address the thermal effect of 3000 W Yb-doped double-clad fiber (YDCF), a double-layer substrate is designed to encapsulate a high power fiber laser. A thermal resistance model is proposed where the contact between the layers is ideal, and the thermal conductivity and thickness, as well as the position of the additional layer, are considered. The axial temperature of the fiber is the lowest, and the temperature uniformity the best, in an Hm-Al system. In addition, the thermal resistance model is also established where the contact between the layers is nonideal. The influence of the interfacial heat transfer coefficient and the thickness of the isotropic and orthotropic layers on the temperature distribution of the fiber are considered. The average temperature of the fiber is the lowest, and the temperature uniformity the best, in an Iso-Ortho system. Numerical simulations are used to verify the two thermal resistance models. The results show that the trend of the change in the analytical value is consistent with that of the simulation value. These thermal resistance models are reasonable and accurate and can be used to analyze more complex fiber laser packaging structures.

Highly Efficient Fiber Cladding Light Stripper Fabricated by Chemical Mask Etching Method

We presented the chemical mask etching method to fabricate an all-fiber cladding light stripper (CLS). The morphology and density of the micron-sized microstructures generated on the fiber surface could be flexibly controlled in fabrication process, which was beneficial to the temperature performance and high-power operation capacity of the device. Based on the method, a theoretical model was developed to optimize the structure of the CLS. Initial experimental processing parameters were also obtained to realize uniform scattering and high attenuation coefficient. An all-fiber cladding light stripper with a high attenuation coefficient of 42 dB was achieved when the stripped cladding light was 1.117 kW. The temperature rising rate was only 0.011 °C/ input-Watt under the condition of air cooling, thanks to all-fiber structure and the uniform scattering design. The average stripped power density reached 9.19 × 106 W/m2 within a 135 mm long CLS. The CLS fabricated by this technique has great application in the high power fiber laser system.

Thermal defect characterization and heat conduction modeling during fiber laser cutting carbon fiber reinforced polymer laminates

High-power fiber laser has been proven to be feasible for cutting carbon fiber reinforced polymers with several advantages including noncontact force, high efficiency and flexibility, while the characteristics of thermal damage and heat conduction in materials are not yet fully understood. Continuous-wave fiber laser was applied in this work to cut 2.0-mm-thick carbon fiber reinforced polymer laminates with different layup configurations. The influence of processing parameters including laser power and cutting speed on thermal damage was investigated. The characteristics of various thermal defects on different positions of machined surface were analyzed using high-resolution SEM and mathematical models. Interestingly, swollen fibers were observed and they connected together to form irregular swollen masses. According to further analysis on the initial heat distribution, it showed that cutting speed was the main factor affecting heat accumulation. In addition, modified heat conduction model was developed to analyze heat transfer within unidirectional carbon fiber reinforced polymer laminates in comparison with experimental results, which can be applied to predict heat affect zone during high-power fiber laser cutting composite materials.

Tin selenide for high-power and ultrafast Q-switched fiber laser

Tin selenide (SnSe), as one of transition metal chalcogenides (TMCs), has aroused much attention due to the unique property of thickness-dependent. Here, the property of the nonlinear saturable absorption for SnSe is investigated. Using the SnSe saturable absorber (SA), the fiber laser is realized. The SnSe film is synthesized by the chemical vapor deposition method. In addition, a typical two-arm transmission measurement is used to investigate the power-dependent nonlinearity of the SnSe SA, and its modulation depth is 17.89%. Moreover, the pulse duration is 624.6 ns at 1561.8 nm. The output power is up to 31.6 mW. This is not only the first exploration of the application of SnSe in the field of optical nonlinearity, but also reveals its potential and great prospects in high-power and ultrafast Q-switched fiber laser.

Fabrication of a side pump combiner and realization of a 2 kW single mode all-fiber laser oscillator

In this paper, we investigate a (2 + 1)×1 side pump combiner with high power handling capability and high beam quality numerically and experimentally. Through theoretical research, we have found how the combiner’s parameters influence its pump coupling efficiency and maximum load power. According to the numerical analysis, we fabricate a side pump combiner, which consists of two pump fibers (220/242 µm, NA = 0.22) and a signal fiber (20/400 µm, NA = 0.06/0.46). The coupling efficiency of the side pump combiner is 97%, which is tested under 2 kW input pump power. Using two side pump combiners and four high power 976 nm laser diodes, a bi-directionally pumped fiber laser oscillator is constructed and tested. The fiber laser oscillator’s maximum output power is 2150 W when injecting 3070 W pump power, corresponding to a slope efficiency of ∼72%. No stimulated Raman scattering or transverse mode instability was observed during the operation test. Thanks to the advantage of maintaining high beam quality of the home-made side pump combiner, this all-fiber laser oscillator achieves single mode laser output (M2=1.05) even when the output power increases to more than 2 kW. By further improving the fabrication technique and using larger core fibers, output laser power could be greatly increased, which is very important for high-power fiber lasers.

A bend-resistant double-trench fiber with leakage channels for HOMs suppression

A double trench fiber with parabolic-profile core (PPDTF) and leakage channels (LC-PPDTF) is proposed in this paper. Due to the leakage channels, the loss of the higher-order modes (HOMs) is enhanced dramatically while the fundamental mode is hardly affected. High loss ratio (>400) of the higher-order modes to the fundamental mode is achieved in a wide range of bend radius and bend orientation at wavelength 1.06μm. The highest loss ratio reaches 2211, which is 62% higher than that of ordinary PPDTF without leakage channels. A large mode area of more than 450 μm2 hardly varies with the bend radius is realized. The fiber with a 100 μm core diameter and a mode area over 1284 μm2 is also presented. This fiber design shows great potential in compact high-power fiber lasers and amplifiers.

Fabrication of superimposed fiber Bragg gratings and applications in fiber laser oscillators

A novel dual-use superimposed fiber Bragg grating (FBG) consisting of a normal FBG operating at 1.55 μm and a chirped FBG (CFBG) operating at 1.08 μm is fabricated and applied in a fiber laser oscillator. The 1.08 μm CFBG is used as a reflector of the fiber oscillator and the FBG is used as an in-core temperature on-line monitor of the laser simultaneously. According to the temperature characteristics of the FBG, the real-time inner temperature can be obtained by detecting the shift of the reflection spectrum. Experimental results have proved the effectiveness of the proposed temperature monitor, which can give the temperature inside a fiber laser comparing to the normal surface measurement techniques such as infrared (IR) camera. This paper provides a simple method to detect the local in-core temperature of fiber lasers, which is useful for investigations of high-power fiber lasers system.

Transverse mode instability

This work presents a review on the effect of transverse mode instability in high-power fiber laser systems and the corresponding investigations led worldwide over the past decade. This paper includes a description of the experimental observations and the physical origin of this effect, as well as some of the proposed mitigation strategies.

A modified leaky multi-trench fiber with managing chromatic dispersion

A new scheme of modified multi-trench fiber (MTF) with chromatic dispersion management is proposed. In this structure, the low refractive-index trenches are broken along the x and y axes by four gaps. The rod-arrays, composed of high Ge-doped rods, are arranged symmetrically in the horizontal gaps. The numerical investigations indicate that the mode area of 1051μm2 can be achieved in effective single-mode (SM) operation at 1.064μm wavelength. The high-order modes (HOMs) are well suppressed. Moreover, through optimizing the key parameters of rod-arrays, the zero chromatic dispersion is controllable and can be maintained at 1.064μm wavelength. We hope that the modified MTF can be further applied to high power fiber lasers and amplifiers.