Ytterbium-doped Fiber Research Articles

An actively Q-switched multi-wavelength ytterbium fiber laser with cascaded polarization-maintaining Bragg gratings

In this paper, we investigate the multi-wavelength lasing characteristics of an actively Q-switched ytterbium-doped fiber laser with two cascaded polarization maintaining fiber Bragg gratings (PM-FBGs). By properly adjusting the polarization states in the cavity, a total of 15 wavelength combinations, including single-, dual-, tri-, and quad-wavelength emission states, have been achieved. Stable Q-switched pulses with repetition rates ranging from 10 to 28 kHz are obtained. The influence of repetition rate and pump power upon the pulse width are also investigated. The high spectral stability in lasing operation indicates effectiveness of the proposed cascaded PM-FBGs in actively Q-switched multi-wavelength fiber laser configurations.

Crucible-free fabrication of ZrB2–SiC composite ceramics with ultra-fine eutectic microstructure by laser surface zone-melting: The selection of lasers

Here we report the crucible-free fabrication of ZrB2–SiC composite ceramics by laser surface zone-melting (LSZM, a rapid solidification method) using Nd: YAG laser, ytterbium-doped fiber laser, and CO2 laser, respectively. Based on the comparative investigation of the thickness and the microstructure of the laser processed zone, this work highlights that the CO2 laser is the most suitable laser resource for the fabrication of highly dense and large-scale ZrB2–SiC composite ceramics with the eutectic colonies consisting of maze-shaped eutectic microstructure by LSZM. The high temperature gradient up to ∼4 × 105 K/m using CO2 laser extremely reduces the phase size down to submicron-scale, which contributes to the high relative density (∼99%), Vickers hardness (24.18 GPa) and fracture toughness (7.4 MPa m1/2) of the ZrB2–SiC composite ceramics. This work demonstrates for the first time that LSZM is a promising method for the rapid fabrication of dense ZrB2 based ultra-high temperature bulk ceramics.

Random Laser Based on Ytterbium-Doped Fiber with a Bragg Grating Array as the Source of Continuous-Wave 976 nm Wavelength Radiation

A random narrow-linewidth lasing at a wavelength of 976 nm was obtained in an ytterbium-doped germanophosphosilicate fiber with an array of weakly reflecting fiber Bragg gratings (FBGs). A random laser cavity was formed by implementing the standard phase mask method of FBG inscription directly during the fiber drawing process. The UV radiation pulses of a KrF excimer laser (248 nm wavelength) synchronized with the fiber drawing speed were used to fabricate the in-fiber array of hundreds of similar FBGs. The developed laser’s slope efficiency in the backward-pumping scheme was measured as high as 33%. The stable continuous-wave operation mode of the laser was detected. The magnitude of the laser power fluctuations depends linearly on the cavity length. The random laser cavity modified with a single highlyreflected (90%) FBG demonstrates significantly better power stability and higher slope efficiency than the same one without an FBG.

The Gain performance of Ytterbium Doped Fiber Amplifier

Ytterbium doping provides high capacity for producing large gain per unit length, and versatility in pump optimization for a given gain length .The paper presents gain characteristic of Ytterbium doped Fiber Amplifier in terms of amplifier length, Yb ion density, pump power and signal power at pump wavelength λp =910 nm and signal wave length λs =975 nm s. A maximum gain of (39, 37 and 33dB) obtained for pump powers (150, 100 and 50 mw). The gain occurs over short length (tenth of centimeters) of the amplifier and increase with increasing Yb ion density. It begins to be moderately saturated when ion concentration exceeds 1.3 e+025. After certain pump power gain values become saturated while it decreases rapidly by increasing signal power. As a result of laboratories unavailability in the region the layout done by using Optisystem 8.0 software simulation (Optical Amplifier and Communication System Design Software)

Kilowatt-Level Supercontinuum Generation in Random Raman Fiber Laser Oscillator With Full-Open Cavity

High-power supercontinuum (SC) sources are important for remote sensing, homeland defense, and scientific research. Despite recent advances in power scaling of the SC generation in optical fibers, the power record of SC fiber laser sources is still much lower than conventional high-power fiber lasers. Here we demonstrate a method for kilowatt-level SC generation based on the random Raman fiber laser (RRFL) with a full-open cavity. Theoretically, a model including cascaded Raman effect, modulation instability, and Rayleigh scattering is established to qualitatively highlight the dynamics of the kilowatt-level SC generation. Experimentally, injecting the amplified ytterbium-doped fiber laser and Raman fiber laser into the RRFL oscillator formed by a piece of long passive fiber, 3-kW-level SC spanning from 925 to 1862 nm is achieved. The experimental results suggest that the full-open-cavity RRFL is a promising approach for kilowatt-level SC generation and the theoretical model demonstrates the importance of Rayleigh backscattering during the SC generation.

On the initiation of fiber fuse damage in high-power ytterbium-doped fiber lasers

Fiber fuse effect can occur spontaneously and propagate along optical fibers to cause widespread damage; it threatens all applications involving optical fibers. This paper presents two results. First, it establishes that the initiation of fiber fuse (IFF) in silica fibers is caused by virtual-defect-induced absorption. Critical temperatures and critical optical powers for IFF are simulated for the first time using a 3D solid-state heat transfer model with heat source generated by the virtual-defect-induced absorption. In this method, formation energies of the virtual defects can be uniquely determined, which offers critical information on the chemical reasons for fiber fuse. Second, this paper offers a method to evaluate operating temperatures of fiber lasers. General analytical solutions of the operating temperatures along gain fibers are deduced. Results of 976-nm laser-diode-pumped and 1018-nm tandem-pumped ytterbium-doped fiber (YDF) amplifiers using 10/130-μm YDFs are calculated. Potential limits caused by fiber fuse are discussed.

Observation of four self-sweeping regimes in a single-mode bi-directional ytterbium-doped fiber ring laser.

In this work, for the first time, four self-sweeping regimes in a single-mode bi-directional ytterbium-doped fiber ring laser are observed by adjusting the polarization controller (PC): normal self-sweeping, reverse self-sweeping, mixed state, and wavelength stop state. In addition, regulating the PC can artificially selectively make the laser operate in normal self-sweeping or reverse self-sweeping within a certain pump power range, and their self-sweeping characteristics (e.g., sweeping rate, sweeping range, etc.) and intensity dynamics are investigated in detail, respectively. In conclusion, we can flexibly regulate the sweeping direction and sweeping characteristics of the bi-directional self-sweeping fiber ring laser in a simple approach by adjusting the PC, which is potentially valuable for its practical application.

First Demonstration and Comparison of 5 kW Monolithic Fiber Laser Oscillator Pumped by 915 nm and 981 nm LDs

Fiber laser performances including transverse mode instability (TMI), stimulated Raman scattering (SRS) and optical-to-optical efficiency are in connection with the pump wavelength. Here we studied the output characteristics of a 5-kW ytterbium-doped fiber laser oscillator pumped with two different pump sources, i.e., 915 nm and 981 nm laser diodes (LDs). The output characteristics of fiber laser oscillators pumped by 915 nm and 981 nm have been compared strictly and directly with the same structure in a high-power situation. Experimental results show that both pump wavelengths can scale the power up to more than 5 kW by suppressing the TMI effect. While in the case of pumping by the 981 nm LDs, the laser oscillator has an optical-to-optical efficiency of 87%, which is 13% higher than that of the 915 nm pumped scheme. In addition, due to the higher backward pumping ratio and lower total pump power, the laser oscillator has a better SRS suppression ratio when pumped at 981 nm. Thus, it reveals a great potential to balance the limitations of TMI and SRS for scaling up to an even higher output while pumping at 981 nm. All the devices of the oscillator are commercial, and it will be helpful for the commercialization of high-power fiber laser oscillators.

175 W average power from a single-core rod fiber-based chirped-pulse-amplification system.

We report on a fiber-based chirped-pulse-amplification laser system with bulk transmission grating compression to a pulse duration of 357 fs, average power of 175 W, and pulse energy of 233μ J. The compressed pulse train has a beam quality factor M2 of 1.21. The power amplifier is based on a state-of-the-art single-mode photonic crystal rod-type ytterbium-doped fiber operating at 248 W of average power and a repetition rate of 750 kHz. The long-term stability of the laser system has been tested continuously for more than 4000 hours and shows no sign of transverse mode instability.

Power-dependent effective reflection of fiber Bragg grating as output coupler of Ytterbium-doped fiber laser

In this paper, we discuss the effective reflection of a fiber Bragg grating and its dependence on laser power when it is used as an output coupler of an ytterbium-doped fiber laser (here the effective reflection is considered to be a part of intracavity laser power reflected by the grating back to the laser cavity). We propose and discuss an experimental technique based on spectral and power analysis and derive simple formulae that permits one to obtain the intra-cavity power and the grating effective reflection. We show that, due to spectral broadening, the effective reflection dramatically decreases with increasing laser power, the effect precisely fitted using the derived formulae describing this quantity. The experimental part of the present study is based on an analysis of the operational regime of a long-wavelength ytterbium-doped fiber laser.

Effect of the seed power spectral density on spectral broadening in kW-level narrow-linewidth fiber amplifiers

Narrow-linewidth high-power fiber amplifiers are demanded in the spectral beam combining technique for power scaling. This article has experimentally studied the seed power spectral density (PSD) effect on the spectral broadening in all-fiber high-power narrow-linewidth fiber amplifier based on fiber Bragg grating (FBG) structure. It was found that the spectral bandwidth of the amplifier output reduces by increasing the PSD of the seed source. During the experiments, a ytterbium-doped fiber (YDF) amplifier was injected by three seed sources with the PSDs of 1.27 W/pm, 1.5 W/pm and 1.8 W/pm to scale up the seed power. After amplification, the resultant 3-dB spectral bandwidth of the amplifier was measured to be 88 pm, 61 pm, and 41 pm, respectively, at kW-level power. Meanwhile, we developed a 990 W fiber amplifier with a 3-dB spectral bandwidth of 41 pm; the narrowest spectrum achieved using a simple FBG-based master oscillator power amplifier configuration at this power level.

Performance Enhancement of Ytterbium-Doped Fiber Amplifier Employing a Dual-Stage In-Band Asymmetrical Pumping

The performance of doped fiber amplifiers can be enhanced significantly with the help of multi-stage pumping technique provided that various critical parameters of pumps including their optical power and wavelength are optimized. We report the performance enhancement of a ytterbium doped fiber amplifier (YDFA) for a 1.02–1.08 m spectral region with an optimized design based on a novel dual-stage in-band asymmetrical pumping scheme. By accurately adjusting the optical power and wavelength of pumps in both the stages, a record peak gain of around 62.5 dB and output power of 4.5 W are achieved for a signal wavelength of 1.0329 m at an optimized length of Ytterbium-doped silica fiber and optimized doping concentration of Yb. Moreover, a minimum noise figure (NF) of 4 dB is observed for a signal wavelength of 1.0329 m at the optimized parameters. Similarly, the effect of using high and low pump powers at the first and the second stage, respectively, on NF of the amplifier is also investigated at different values of signal powers. It is observed that the value of NF increases significantly by using high pump power at the first stage and low pump power at the second stage.

Low-temperature properties of the ytterbium-doped fiber laser cooled by the thermoelectric cooler

The low-temperature property of ytterbium-doped fiber lasers is quite important in various applications. A fiber laser operating at 1018 nm is fabricated and its gain fiber is cooled to −30℃ by the temperature control system enabled on the thermoelectric coolers. The cooling effect of the system is better than that of the water-cooling system. The temperature of fiber core is obtained by an in-situ temperature measurement system based on the optical frequency domain reflectometry. Experimental results show that the output power decreases about 5.7 % while the temperature is adjusted from 20℃ to −30℃. Meanwhile, the central wavelength and the linewidth change little during the temperature tunning.

Controlled generation of mode-switchable nanosecond pulsed vector vortex beams from a Q-switched fiber laser.

We reported and demonstrated a ring Q-switched Ytterbium-doped fiber laser that can generate mode-switchable nanosecond pulsed vector vortex beams between two different orders. In the spatial optical path of the fiber laser, several cascaded Q-plates, divided into two Q-plate groups, are applied for intracavity mode conversion between LP01 mode and vector vortex beams. In one Q-plate group, two quarter-wave plates are inserted to achieve the addition and subtraction of the order of Q-plates. By tuning the polarization state in the cavity, mode-switchable vector vortex beams (VVBs), including cylindrical vector beams (CVBs), elliptically polarized cylindrical vector beams (EPCVBs), and vortex beams, of two different orders can be generated on demand. The experimental results show that by using the group of 1st and 3rd orders Q-plates, the 2nd and 4th orders mode-switchable VVBs (vortex beams with topological charges of ±2, ±4, CVBs and EPCVBs of 2nd- and 4th-order) can be obtained from the fiber laser. The slope efficiency, pulse width, and repetition rate are 33.4%, 360 ns, and 241kHz respectively. To the best of our knowledge, this is the first time to realize the direct generation of mode-switchable VVBs on the arbitrary position of the higher-order Poincaré sphere between two different orders from a fiber laser. This work lays the foundation for the flexible generation of arbitrary modes of VVBs with multiple different orders in the laser cavity.

Generation of arbitrary higher-order Poincaré sphere beam from a ring fiber laser with cascaded Q-plates

In this paper, vector vortex beams at arbitrary position on the higher-order Poincaré (HOP) sphere were directly generated by using a ring Ytterbium-doped fiber laser with cascaded Q-plates in the laser cavity. Here, two identical Q-plates were placed in cascade and utilized to achieve the intracavitary mode transformation between the LP01 mode and vector vortex modes. The change of the polarization state in the cavity was analyzed by using the Jones matrix, and the polarization distribution of arbitrary HOP sphere beam generated by this fiber laser was verified by the Stokes vector, theoretically. Then, it was confirmed experimentally that arbitrary HOP sphere beam (e.g., cylindrical vector beams, elliptically polarized cylindrical vector beams, and circularly polarized vortex beams) can be manipulated by changing the linear, elliptically, and circular polarization state in the cavity.

Bound noise-like pulse generation from Yb-doped passively mode-locked fiber lasers

We demonstrated two different types of bound noise-like pulses (NLPs) in ytterbium-doped mode-locked fiber lasers using the nonlinear polarization rotation technique. In the anomalous dispersion area, there were multiple spikes in the intensity autocorrelation (IAC) trace. In the normal dispersion area, the spike presented only in the center the of IAC trace. The results of the experiment show that the bound state NLPs may have partial coherence.

More than 6 kW near single-mode fiber amplifier based on a bidirectional tandem pumping scheme.

We demonstrate the first all-fiber monolithic bidirectional tandem pumping amplifier, to the best of our knowledge, based on a 30/250 µm conventional ytterbium-doped double-clad fiber. By optimizing the bidirectional pumping power distribution, an output power of 6.22 kW is obtained with near single-mode beam quality (M2=1.53), and no transverse mode instability is observed. This work could provide an excellent reference for high-power, higher-brightness fiber lasers.

Experimental Study on Transverse Mode Instability Characteristics of Few-Mode Fiber Laser Amplifier Under Different Bending Conditions

We studied the transverse mode instability (TMI) characteristics of few-mode fiber laser amplifier employing ytterbium-doped fiber (YDF) with core/cladding diameter of 30/400 μm under different bending diameters. The gain fiber is coiled in ellipse shape with minimum bending diameter changed from 9 cm, 10 cm, 11 cm to 12 cm in the bending part, and the experimental results show an anomaly characteristic of TMI. The TMI threshold improved from 772 W, 1149 W and 1458 W to 1641 W when the bending diameter of YDF increase from 9 cm, 10 cm, 11 cm to 12 cm, accompanied by the deterioration of beam quality from 1.60, 1.67, 1.87 to 1.95. This is mainly because tightly bending in few-mode fiber will promote the overlap between modes and lead to mode coupling, which may trigger TMI. Besides, the bending losses of HOMs decrease while bending diameter increasing, so that there is higher output efficiency and deteriorated beam quality when bending diameter increase. Although the phenomenon is not agreed with common TMI with near single-mode fiber laser, the result will be helpful for the design and power scaling of few-mode fiber lasers.

Versatility of Stimulated Raman Scattering Based Power Combining of Multiple Fiber Lasers

We demonstrate the versatility of our recently demonstrated technique of Stimulated Raman Scattering induced simultaneous wavelength conversion and power combining of multiple Ytterbium (Yb) doped fiber lasers in terms of number of input pump sources and the wavelength spacing between them. We experimentally show that this technique works when the Raman gain coefficient between the input chosen pump sources is 1) negligible, i.e., when pump sources are separated in wavelength by as small as 5nm, 2) approximately half of the peak value, and 3) more than half of the peak value. Our experiments also show that the technique works for more than two Ytterbium-doped fiber lasers. Experimental results were shown for power combining of 1074nm and 1079nm lasers, 1088nm and 1117nm lasers, and 1074nm, 1088nm and 1117nm lasers with >80% of the power in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.5~\mu \text{m}$ </tex-math></inline-formula> band in each case. Also, through simulations, we elucidate the reasons for asymmetric Raman gain that is responsible for power combing and show power combining of 6 pump sources, with arbitrary wavelength spacing showing the versatility of this technique.

Synchronously pumped mode-locked ultrafast ytterbium-doped fiber laser

Synchronously pumped mode-locking, which avoids the usage of saturable absorbers or intracavity modulators, is a promising technique to generate ultrashort pulses for industrial applications. However, because of the lack of all-fiber single-mode ultrafast pump sources, it is difficult to apply this technique in ytterbium-doped fiber lasers (YDFLs). To the best of our knowledge, this study has demonstrated a synchronously pumped mode-locked ultrafast YDFL for the first time. Based on a home-built 976-nm pulsed fiber laser as the pump, it generates dissipative solitons with a pulse duration of 31.3 ps and a repetition frequency of ∼ 27.95 MHz. The laser is developed with an all-polarization-maintaining architecture; therefore, it exhibits excellent environmental stability and self-starting capacity. Furthermore, we have also conducted experimental work for investigating the mechanism of this mode-locking technique. The results indicate that instead of the gain modulation effect, the pulsed pump-induced cross-phase modulation effect plays a dominant role in pulse shaping. This study demonstrates a potential strategy for realizing high-performance ultrafast fiber sources in the spectral regions without efficient modulators or saturable absorbers.