High-power Fiber Laser Systems Research Articles

Less than 1% quantum defect fiber lasers via ytterbium-doped multicomponent fluorosilicate optical fiber.

Two ytterbium-doped fiber lasers exhibiting quantum defects of less than 1% are demonstrated, in which pumping at wavelengths of 976.6 and 981.0 nm yielded lasing at wavelengths of 985.7 and 989.8 nm, respectively. The multicomponent fluorosilicate active optical fiber, fabricated using the molten core method, has spectral characteristics similar to those of fluoride glasses, namely short average emission wavelength and long upper state lifetime. A best-case slope efficiency of 62.1% was obtained, matching the theoretical model very well. With further fiber and laser optimization, slope efficiencies approaching the quantum limit should ultimately be possible. A reduction in the quantum defect may offer significant mitigation of issues associated with fiber heating. As such, this work can serve as a possible direction for future scaling of high-power fiber laser systems.

Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold

A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of ~600W, which is a factor of 2 above the mode instability threshold.

High-efficiency (6 + 1) × 1 pump–signal combiner based on low-deformation and high-precision alignment fabrication

We demonstrate a new method for fabricating a (6 + 1) × 1 pump–signal combiner based on the reduction of signal fiber diameter by corrosion. This method avoids the mismatch loss of the splice between the signal fiber and the output fiber caused by the signal fiber taper processing. The optimum radius of the corroded signal fiber was calculated according to the analysis of the influence of the cladding thickness on the laser propagating in the fiber core. Besides, we also developed a two-step splicing method to complete the high-precision alignment between the signal fiber core and the output fiber core. A high-efficiency (6 + 1) × 1 pump–signal combiner was produced with an average pump power transmission efficiency of 98.0% and a signal power transmission efficiency of 97.7%, which is well suitable for application to high-power fiber laser system.

Mitigate transverse mode instability by linear inner-cladding fiber

Transverse mode instability (TMI) limits power scaling of fiber lasers. A semianalytical model is established to calculate the TMI threshold in high-power fiber laser systems of the multi-kW-class. A linear inner-cladding fiber can mitigate the TMI effect by smoothing the heat profile and decreasing the nonlinear coupling coefficient along the fiber. We investigate strong pump absorption of a linear inner-cladding fiber, which leads to shorter fiber length. Utilizing a 915-nm copumping scheme, the linear inner-cladding fiber can realize 10-kW output power in single-mode regime theoretically.

High-power single-frequency pulsed laser based on a Yb-doped large-pitch photonic crystal fiber

We have developed a high-power single-frequency pulsed fiber laser system utilizing a Yb-doped rod-type large-pitch photonic crystal fiber. The maximal peak power is 33 kW with a 0.4-mJ pulse energy and a 70-MHz linewidth, and the average power is 60 W at a 150-kHz repetition rate. At a higher repetition rate, the average power can be increased to 190 W, which is limited by the onset of transverse-mode instability in the large-pitch photonic crystal fiber. The output beam is linearly polarized and shows a good beam quality with a M2 value of 1.2.

2 kW pump-light-stripper-free distributed side-coupled cladding-pumped fiber oscillator

A 2 kW pump-light-stripper-free all-fiber distributed-pumping oscillator fabricated with the distributed side-coupled cladding-pumped Yb-doped fiber is demonstrated for the first time, to the best of our knowledge. An output power of 1969 W with a slope efficiency of 72.2% is obtained. By utilizing the final-section counter-pumping scheme, pure output spectra free from residual pump light are obtained without using any pump light stripper, which demonstrates that the pump light stripper is not indispensable for this configuration. Besides, no stimulated Raman scattering component is observed in the output spectra. The laser has the M2 factor ranging from 2.0 to 2.6. We believe that the pertinent results are helpful and valuable for designing high-power fiber laser systems.

Influence of quasi-periodic cladding on single mode behavior in a leakage channel fiber: Towards the enhancement of modal discrimination and low bending loss of the LP01 mode

A silica quasi-periodic clad leakage channel fiber (LCF) is proposed to enhance the mode filtering characteristics with large effective mode area (Aeff). In this work, we explore the fiber characteristics, namely, bending loss (BL), Aeff and differential loss for different micro-structured cladding LCF designs. Of the various LCFs, an octagonal core–octagonal quasi-periodic clad LCF provides a large loss ratio (>103) between fundamental mode (LP01 mode) and most competitive higher order mode (LP11 mode). Further, it exhibits a high BL of 66.48dB/m for LP11 mode by resonance mode filtering along with a bend induced modal discrimination at 1.05μm wavelength. The detailed numerical results reveal that the differential loss ratio goes high when the effective index of the second ring of eight sectional elements in a quasi-periodic cladding is scaled up and the Aeff increases when it is scaled down. Thus, the proposed octagonal core–octagonal quasi-periodic clad LCF exhibits a large mode area of 1161μm2, a low BL of 0.04dB/m for the LP01 mode and a high differential loss ratio of 1624 between LP11 and LP01 modes at a critical bending radius of 15cm with an air-filling ratio of 0.4. These interesting properties of the LCF could pave way for developing a compact high power fiber laser system.

Detection of microscopic defects in optical fiber coatings using angle-resolved skew rays.

Microscopic defects in optical fiber coatings can be an impending catastrophe for high-power fiber laser and telecommunications systems and are difficult to detect with conventional methods. We demonstrate a highly sensitive interrogation technique that can readily identify faults such as microscopic nicks, scrapes, low-quality recoatings, and internal defects in fibers and their coatings, based on skew ray excitation and angle-resolved analysis.

Narrowband widely tunable CW mid-infrared generator based on difference frequency generation in periodically poled KTP and KTA crystals

We demonstrate narrowband CW mid-infrared generators based on difference frequency generation in periodically poled KTP and KTA crystals. The crystals are used to nonlineary mix the beams from high-power narrowband fiber laser systems working at 1060- and 1550-nm spectral bands. We explore various techniques to control the linewidth of fiber lasers. The tunability achieved in a source based on nonlinear mixing in a KTA crystal exceeds the range of 3100–3620 nm.

Comparison of the threshold of thermal-induced mode instabilities in polarization-maintaining and non-polarization-maintaining active fibers

We present an investigation on the threshold of thermal-induced mode instability (MI) in ytterbium-doped polarization-maintaining and non-polarization-maintaining active fibers. By taking both electric fields along the two principal axes into consideration, we extend a previous steady-state theoretical model to analyze the polarization effect on MI. Based on the model, we analyzed whether maintaining the fiber’s polarization direction has an effect on the MI threshold, which showed that, for practical high power fiber laser systems, the MI threshold is nearly the same for polarization-maintaining and non-polarization-maintaining active fibers. Experimental validation of the theoretical analysis is presented with experimental results agreeing with the theoretical results, in which polarization-maintained and non-polarization-maintained fiber lasers with similar core/inner cladding diameters are employed.

Study and design of cladding power stripper for high power fiber laser systems

Heat handling has been a significant problem of the high power fiber laser systems as the output power increases rapidly. Cladding power stripper (CPS) which is used to deal with the unwanted optical power and light is required for higher cooling ability. So the methods of stripping the unwanted light attracted much attention recently, and the thermal effect is given. However, few investigations focus on the dissipation of the heat converted from the unwanted light. In this paper, an approach of active cooling for CPS is demonstrated. This is achieved by using microchannel cooling technology in heat sinking in CPS to improve the efficiency of heat exchange. In order to explain the mechanism of CPS the function of it and consistence of categories of the unwanted light are detailed firstly. Then microchannel heat sinking is proposed and verified by the heat exchange theory. At last, the design of the CPS with microchannel heat sinking is shown and following experiment is conducted. The final temperature of the device with 1000 W cladding power was demonstrated at last to verify the ability of heat distribution of the CPS component. This suggests that these CPSs can be used to stripe a thousand of watts of light in high power double cladding fiber lasers.

Theoretical study of the temperature distribution in high power gain fiber of gradient doping

Thermal effect in the gain fiber is one of the main factors which restrict the power improvement of high power fiber amplifiers. Previous studies have shown that the temperature distribution is closely related to the doping concentration along the gain fiber. In order to reduce the maximum temperature of the gain fiber, we propose to use doping concentration varying along the gain fiber as a method to disperse the thermal effect of the fiber laser and improve the laser output power. Based on the rate equation model and thermal conduction model, the thermal distributions and output powers of several different gradient doping gain fibers are simulated in the cases where the output powers are approximately the same. Our study shows that compared with the conventional constant doping gain fiber, linear doping of the rare earth ion along the gain fiber can reduce the maximum temperature of the gain fiber as well as the temperature of the fusion point greatly, thus improving the stabilities of the fusion point and the fiber laser amplifier. In the case of cosinoidal doping, the gain fiber can not only reduce the temperature of the fusion point but also make the temperature have a periodic distribution along the gain fiber, which can suppress the stimulated Brillouin scattering effect effectively. The exponential doping of the gain fiber can also reduce the maximum temperature and the temperature of the fusion point, which is beneficial to the further scaling of the fiber laser output power. At the same time, it can make the gain of the signal light have a uniform distribution along the gain fiber, which suppresses the mode instability effect and improves the output beam quality of the fiber laser. These conclusions also hold true when the pump power changes. Therefore, the gradient doping of the gain fiber proposed in this paper can optimize the temperature distribution along the fiber and improve the stability of the fusion point. Besides, it can improve the beam quality of the output laser and suppress the nonlinear effect and mode instability effect. The results indicate that the gradient doping of the gain fiber is an effective and feasible way to improve the output power of fiber amplifier. Last but not the least, it is possible to produce the gradient doping gain fiber by the laser heated pedestal growth method and the direct nanoparticle deposition technique. The investigation can present a reference for designing the gain fiber in high-power fiber laser systems.

Propagation of high-power fiber laser with high-order-mode content

Propagation properties of high-power fiber laser with high-order-mode (HOM) content are studied numerically for the first time to the best of our knowledge. The effect of HOM on the propagation property is evaluated by the power in the bucket (PIB) metric. It is shown that PIB is mainly dependent on HOM content rather than the relative phase between the fundamental mode and HOM. The PIB in vacuum is more than 80% when the power fraction of the HOM is controlled to be less than 50% at 5 km. The relative phase has an impact on the peak intensity position and concentration of the far-field intensity distribution. If an adaptive optics system is used to correct the peak intensity deviation, the results indicate that there exists a maximal value of PIB as relative phase increases. Such effect is weakened when propagating in turbulence. Compared to the laser beams without HOM, laser beams with HOM content are less influenced by the turbulence and can reduce average intensity fluctuation. The results may be useful in the design of a high-power fiber laser system.

Pump and signal power combiners for high-power fiber amplifiers applications

In this work we present our result on investigation of pump power combiner fabrication process in configuration (N+1)x1. Our fabrication method is based on tapering of fiber bundle consisting of 6 multimode fibers and one signal fiber. We present results on fabrication power combiners based on single mode fibers and large mode area fibers for the signal. Full Text: PDF References D. Noordegraaf et al., "All-fiber 7x1 signal combiner for incoherent laser beam combining", Proc. SPIE 7914 79142 (2011) CrossRef E. Snitzer, H. Po, F. Hakimi, R. Tumminelli, B. C. McCollum,"Double Clad Offset Core Nd Fiber Laser", Opt. Fiber Sensor Topical Meeting., New Orleans, LA, 1988. CrossRef Zervas, M.N., Codemard, C.A., "High Power Fiber Lasers: A Review," Selected Topics in Quantum Electronics, IEEE Journal of, vol.20, no.5, pp.219,241, Sept.-Oct. 2014 CrossRef F. Hakimi, H. Hakimi, "New side coupling method for double-clad fiber amplifiers", Lasers and Electro-Optics, 2001. CLEO '01. Technical Digest. Summaries of papers presented at the Conference on, pp.116, 11-11 May 2001. CrossRef J.P. Koplow, S.W. Moore, D.A.V. Kliner, "A new method for side pumping of double-clad fiber sources", IEEE Journal of Quantum Electronics, 39, 4, Apr 2003. CrossRef Ch. Li, D. Shen, J. Song, N. S. Kim, K. Ueda, "Absorption characteristics of high-power disc fiber laser by side pumping of laser diode", Lasers and Electro-Optics. CLEO/Pacific Rim '99, 3 (1999). CrossRef J. Swiderski, A. Zając, M. Skorczakowski, "Pulsed ytterbium-doped large mode area double-clad fiber amplifier in MOFPA configuration", Opto-Electronics Review, 15, 2, (June 2007). CrossRef Y. Jeong, et al., "40-Gb/s Widely Tunable Transceivers", IEEE Journal of Selected Topics In Quantum Electronics, 13, 3 (May/June 2007). CrossRef W. Shi, Q. Fang, X. Zhu, R. A. Norwood, N. Peyghambarian, "Fiber lasers and their applications", Applied Optics 53, 28 (2014). CrossRef D. J.Richardson, J. Nilsson, and W. A. Clarkson, "High power fiber lasers: current status and future perspectives", J. Opt. Soc. Am. B 27, 11 (2010). CrossRef F. Gonthier et al., "High-Power all-fiber components: The missing link for high power fiber lasers", Fiber Lasers: Technology, Systems and Applications, 266-276, (2004). CrossRef A. Braglia, A. Califano, Y. Liu and G. Perrone, "Architectures and components for high power CW fiber lasers", International Journal of Modern Physics B, 28, 12 (2014). CrossRef G. Sobon, P. Kaczmarek, D. Sliwinska, J. Sotor, K. Abramski, "High-Power Fiber-Based Femtosecond CPA System at 1560 nm", IEEE Journal of Selected Topics in Quantum Electronics, 20, 5 (2014). CrossRef D. J. DiGiovanni, and A. J. Stentz, "Tapered fiber bundles for coupling light into and out of cladding-pumped fiber devices", U.S. Patent 5,864,644 A, Jan. 26, (1999). DirectLink A. Braglia, A. Califano, Y. Liu and G. Perrone, "Architectures and components for high power CW fiber lasers", International Journal of Modern Physics B, 28, 12 (2014). CrossRef J. K. Kim et al., "Monolithic all-glass pump combiner scheme for high-power fiber laser systems", Optics Express, 18, 12, (24 May 2010). CrossRef B. Sévigny, P. Poirier, M. Faucher, "Pump combiner loss as a function of input numerical aperture power distribution", Proc. SPIE 7195, 719523 (2009) CrossRef A. Kosterin, V. Temyanko, M. Fallahi,M. Mansuripur, "Tapered fiber bundles for high power applications", Technical Digest. OFC/NFOEC 2 (2005) CrossRef P. Koška, Y. Baravets,P. Peterka, J. Bohata, M. Písařík, "Mode-field adapter for tapered-fiber-bundle signal and pump combiners", Applied Optics, 54, 4 (1 February 2015) CrossRef R. Wiley, B. Clark, "High-power fused assemblies enabled by advances in fiber-processing technologies", Proc. SPIE 7914, 79140F (2011). CrossRef

Adaptive fiber optics collimator based on flexible hinges.

In this manuscript, we present a new design for an adaptive fiber optics collimator (AFOC) based on flexible hinges by using piezoelectric stacks actuators for X-Y displacement. Different from traditional AFOC, the new structure is based on flexible hinges to drive the fiber end cap instead of naked fiber. We fabricated a real AFOC based on flexible hinges, and the end cap's deviation and resonance frequency of the device were measured. Experimental results show that this new AFOC can provide fast control of tip-tilt deviation of the laser beam emitting from the end cap. As a result, the fiber end cap can support much higher power than naked fiber, which makes the new structure ideal for tip-tilt controlling in a high-power fiber laser system.

High power fiber laser system for a high repetition rate laserwire

We present the development of a high power fiber laser system to investigate its suitability for use in a transverse electron beam profile monitor, i.e., a laserwire. A system capable of producing individual pulses up to $165.8\ifmmode\pm\else\textpm\fi{}0.4\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$ at 1036 nm with a full width at half maximum of $1.92\ifmmode\pm\else\textpm\fi{}0.12\text{ }\mathrm{ps}$ at 6.49 MHz is demonstrated using a master oscillator power amplifier design with a final amplification stage in a rod-type photonic crystal fiber. The pulses are produced in trains of 1 ms in a novel burst mode amplification scheme to match the bunch pattern of the charged particles in an accelerator. This method allows pulse energies up to an order of magnitude greater than the steady-state value of $17.0\ifmmode\pm\else\textpm\fi{}0.6\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$ to be achieved at the beginning of the burst with a demonstrated peak power of $25.8\ifmmode\pm\else\textpm\fi{}1.7\text{ }\mathrm{MW}$ after compression. The system is also shown to demonstrate excellent spatial quality with an ${M}^{2}=1.26\ifmmode\pm\else\textpm\fi{}0.01$ in both dimensions, which would allow nearly diffraction limited focusing to be achieved.

Triple-clad large-pitch fibers for compact high-power pulsed fiber laser systems

We present a novel ytterbium (Yb)-doped large-pitch fiber design with significantly increased pump absorption and higher energy storage/gain per unit length, which enables high-peak-power fiber laser systems with smaller footprints. Up to now index matching between core and surrounding material in microstructured fibers was achieved by co-doping the active core region with fluorine. Here we carry out the index matching by passively doping the cladding with germanium, thus raising its index of refraction. Hence, the fluorine in the core can be omitted, which leads to an effective increase of the core doping concentration, while detrimental effects such as photo-darkening and lifetime quenching are avoided by maintaining the bulk Yb concentration. Experiments and simulations show that a gain higher than 50 dB/m and an output average power higher than 100 W with excellent beam quality are feasible even with a fiber length of only 40 cm.

Coherent beam combination of fiber lasers with a strongly confined tapered self-imaging waveguide: theoretical modeling and simulation

Coherent beam combination (CBC) of fiber lasers based on self-imaging properties of a strongly confined tapered waveguide (SCTW) is studied systematically. Analytical formulas are derived for the positions, amplitudes, and phases of the N self-images at the output of a SCTW, which are important for quantitative analysis of waveguide-based CBC. The formulas are verified with numerical examples by a finite difference beam propagation method (FDBPM) and the errors of the analytical expressions are studied. This study shows that the analytical formulas agree well with the FDBPM simulation results when the taper angle is less than 1.4° and the phase distortion is less than λ/10. The relative errors increase as the taper angle increases. Based on the theoretical model and the FDBPM, we simulated the CBC of fiber laser array and compared the CBC based on the tapered waveguide with that based on the nontapered one. The effects of input beam number, aperture fill factor, and taper angle on the combination performance have been studied. The study reveals that a beam which has near-diffraction limited beam quality (M2≤1.41) and a single beam without side lobe in the far field can be achieved with tapered-waveguide-based CBC. It is shown that beam quality depends on input beam number, aperture fill factor, and taper angle. There exists a best fill factor which will increase as input beam number increases. The tolerance of the system on the fill factor and the taper angle is studied, which is 0.45<t<0.67 and θ<0.8°, respectively. The results may be useful for practical, high-power fiber laser systems.

Passive mitigation strategies for mode instabilities in high-power fiber laser systems

Mode instabilities have quickly become the most limiting effect when it comes to scaling the output average power of fiber laser systems. In consequence, there is an urgent need for effective strategies to mitigate it and, thus, to increase the power threshold at which it appears. Passive mitigation strategies can be classified into intrinsic, which are related to the fiber design, and extrinsic, which require a modification of the setup. In order to evaluate the impact of mitigation strategies, a means to calculate its power threshold and predict its behavior is required. In this paper we present a simple semi-analytic formula that is able to predict the changes of the mode instability threshold by analyzing the strength of the thermally-induced waveguide perturbations. Furthermore, we propose two passive mitigation strategies, one intrinsic and one extrinsic, that should lead to a significant increase of the power threshold of mode instabilities.

Large area photoconductive terahertz emitter for 1.55 μm excitation based on an InGaAs heterostructure

We present scalable large area terahertz (THz) emitters based on a nanoscale multilayer InGaAs/InAlAs heterostructure and a microstructured electrode pattern. The emitters are designed for pump lasers working at the telecommunication wavelength of 1.55 μm. Electric THz fields of more than 2.5 V cm−1 are reached with moderate pump powers of 80 mW, the corresponding spectrum extends up to 3 THz. The saturation characteristics have been investigated for different pump laser spot sizes. For small pump powers of less than 50 mW the emitted THz field is nearly independent of the spot size, for higher pump powers and small spot sizes a clear saturation of the generated THz pulse can be observed. Hence the use of scalable emitters is especially promising for high power fibre laser systems. The spectral content of the generated radiation is nearly independent of the parameters spot size, pump power, and bias voltage, which allows for stable operation in spectroscopic applications.