Suppression of Raman gain in single-transverse-mode dual-hole-assisted fiber

2 × 4 kW near-single-mode laser output assisted by an optimized bidirectional oscillating-amplifying integrated fiber laser configuration

Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) can achieve the two-ports laser amplification based on a single cavity, showcasing a promising prospect. In order to improve both the laser power and beam quality, we first simulate and optimize the stimulated Raman scattering (SRS) effect in the B-OAIFL. The simulation results show the SRS effect can be suppressed by optimizing the diameter as well as the length of the active fiber at different locations. With the guidance of theoretical and experimental analysis for the combined suppression of SRS and transverse mode instability (TMI), a near-single-mode B-OAIFL with 2 × 4 kW was demonstrated. Based on this foundation, we further devoted ourselves to the pursuit of the optimization of the structure and performance. The necessity of the configuration of side pump, which was initially introduced for its exceptional performance in stabilizing temporal chaos, was reevaluated in detail. With its negative impacts on efficiency improvement and SRS suppression were analyzed and verified, we removed this configuration and finally demonstrated a more simplified design with superior performance. A total bidirectional output of 8105 W was achieved, with an O-O efficiency of 79.6% and a near-single-mode beam quality of M A 2∼1.36,M B 2∼1.63. No signs of TMI were observed, and the signal-to-SRS suppression ratio was over 38 dB. The results still demonstrate a promising potential for power scaling based on this configuration and parameters.

Theoretical study of graded-index fiber for beam quality improvement and SRS suppression

A confined-doped graded-index fiber model was established with reference to Nufern's 42C ytterbium-doped fiber. The core NA of the fiber is ∼0.06; the doping/core/cladding diameter is 30/42/250 µm; and the gain ion doping distribution is double parabolic. Based on a forward tandem-pumped amplifier structure, the numerical simulation was carried out on the physics characters of the fiber model. The fourth-order Runge-Kutta method was used to solve the boundary value problem during the numerical simulation. The differences between the graded-index fiber (GIF) and the step-index fiber (SIF) were studied theoretically, and the results illuminated that the beam quality of the GIF was better than the SIF. The stimulated Raman scattering (SRS) power in the GIF was lower than that in the SIF. These results show that the GIF has great potential for increasing the laser output power while maintaining good beam quality and can provide theoretical guidance and reference to experimental research of the high-power fiber laser.

Demonstration of 3kW × 2 ports bidirectional output oscillating-amplifying integrated fiber laser employing chirped and tilted fiber Bragg gratings for co-SRS suppression

Bidirectional output oscillating-amplifying integrated fiber laser (B-OAIFL) is a newly developed configuration with many advantages like compactness and good reliability. In this work, a B-OAIFL with a low time-stabilized threshold was constructed by employing a pair of side pump/signal combiner in the oscillating section, which demonstrates smooth temporal characteristics with no pulse detected by the photodetector at the output power level of only a few of tens Watts. We investigated the effect of side pumping on the Raman Stokes light and verified its contribution to mitigating the temporal-chaos-induced stimulated Raman scattering (SRS). The phenomenon of co-SRS caused by the mutual excitation of backward Stokes light from two amplifying sections under bidirectional pumping was first reported and studied. A pair of chirped and tilted fiber Bragg gratings (CTFBGs) were applied between the oscillating and amplifying sections to suppress the co-SRS, and the effect of the number of CTFBGs on the suppression of co-SRS was studied in detail experimentally. Finally, we successfully suppressed the co-SRS, and achieved a 3kW × 2 ports laser output, with a near-single-mode beam quality of M A 2∼1.3,M B 2∼1.4. In contrast, without the use of CTFBG, only a 2 kW-level output was obtained from each port, limited by co-SRS (with an SRS suppression ratio of less than 15 dB). The maximum output power of end A and end B is 3133 W and 3213 W, with the SRS suppression ratio of about 27.6 dB and 28.1 dB, respectively. No TMI features were observed under bidirectional pumping. The results demonstrate a significant potential for further power scaling based on this configuration. To the best of our knowledge, it is the highest output power achieved based on the B-OAIFL.

Femtosecond laser fabrication of chirped and tilted fiber Bragg gratings for stimulated Raman scattering suppression in kilowatt-level fiber lasers

Chirped and tilted fiber Bragg gratings (CTFBGs) are important all-fiber filtering components in high-power fiber lasers for stimulated Raman scattering (SRS) suppression. The fabrication of CTFBGs in large-mode-area double-cladding fibers (LMA-DCFs) by femtosecond (fs) laser is reported for the first time to the best of our knowledge. The chirped and tilted grating structure is obtained by scanning the fiber obliquely and moving the fs-laser beam relative to the chirped phase mask at the same time. By this method, the CTFBGs with different chirp rates, grating lengths, and tilted angles are fabricated, and the maximum rejection depth and bandwidth are ∼25 dB and ∼12 nm, respectively. To test the performance of the fabricated CTFBGs, one is inserted between the seed laser and the amplifier stage of a 2.7 kW fiber amplifier, and an SRS suppression ratio of ∼4 dB is achieved with no reduction in laser efficiency and degradation in beam quality. This work provides a highly fast and flexible method to fabricate large-core CTFBGs, which is of great significance to the development of high-power fiber laser systems.

Towards Ultimate High-Power Scaling: Coherent Beam Combining of Fiber Lasers

Fiber laser technology has been demonstrated as a versatile and reliable approach to laser source manufacturing with a wide range of applicability in various fields ranging from science to industry. The power/energy scaling of single-fiber laser systems has faced several fundamental limitations. To overcome them and to boost the power/energy level even further, combining the output powers of multiple lasers has become the primary approach. Among various combining techniques, the coherent beam combining of fiber amplification channels is the most promising approach, instrumenting ultra-high-power/energy lasers with near-diffraction-limited beam quality. This paper provides a comprehensive review of the progress of coherent beam combining for both continuous-wave and ultrafast fiber lasers. The concept of coherent beam combining from basic notions to specific details of methods, requirements, and challenges is discussed, along with reporting some practical architectures for both continuous and ultrafast fiber lasers.

SRS suppression in multi-kW fiber lasers with a multiplexed CTFBG

In this paper, we present experimental studies on newly developed multiplexed chirped tilted fiber Bragg gratings (MCTFBGs) for stimulated Raman scattering (SRS) suppression for high-power fiber laser systems. The MCTFBG device is composed of five continuous segments of chirped tilted fiber Bragg gratings (CTFBGs), which are inscribed into the large-mode-area (LMA, 25/400μm) fibers. The SRS suppression capability of the MCTFBG device has been successfully demonstrated with a master oscillator power amplification (MOPA) fiber laser system at the output level of 3.4 KW. The experimental observation thus indicates the MCTFBG's excellent SRS suppression capability at a very high power level (∼15 dB under >3 kW) and high thermal handling capacity (∼1.48℃/kw). Our work thus provides a key development of essential fiber grating components that can effectively suppress the SRS suppression at a very high power level.

A novel high-power all-fiberized flexible spectral filter for high power linearly-polarized Raman fiber laser

Power scaling of linearly polarized Raman fiber laser (LPRFL), which has wide application potentials, is mainly limited by the generation of high-order Stokes light. In this paper, we propose a novel flexible spectral filter with all-fiberized configuration and high-power handling. Combining with the polarization-dependence of Raman gain, the filter could be used to efficiently suppress high-order Stokes light in LPRFL and thus help further power scaling. The filter is fabricated by two 45° cross-splice of three pieces of polarization maintaining (PM) passive fibers. The bandwidth and central wavelength of transmission spectrum of the spectral filter could be flexibly tuned through changing the length and temperature of the cross-spliced fiber. The insertion loss of the filter fabricated in the lab is measured to be as low as 0.07 dB. The filter is employed in a LPRFL, and the maximum output power of the LPRFL is increased by 48.7%.

Spectral shaping for suppressing stimulated-Raman-scattering in a fiber laser

We report on the effects of spectral shaping of the output coupler fiber Bragg grating (OC-FBG) in a Yb-doped fiber laser on the laser emission spectrum for the purpose of inhabiting stimulated Raman scattering (SRS). The lasers with four different OC-FBGs were built and characterized. We found that the laser with a multiple reflection peak chirped-moiré OC-FBG produced a broad laser emission linewidth, which, in turn, led to about 100 times lower the SRS emission as compared with the laser with a 0.19 nm Gaussian-shaped OC-FBG. A nearly flat-top laser output spectrum was obtained from the lasers with a triangle-shaped and chirped-moiré OC-FBG, respectively.

Propagation-length independent SRS threshold in chirally-coupled-core fibers

Both analytical study and numerical simulations show that the propagation-length independent Stimulated Raman Scattering (SRS) threshold can be achieved by Stokes wave suppression in optical fibers. We propose a specific design based on Chirally-Coupled-Core (CCC) fibers with spectrally-tailored wavelength-selective transmission to suppress the Stokes wave of Raman scattering. Fibers with length-independent nonlinearity threshold could be particularly advantageous for high power lasers and fiber beam delivery for material processing applications.

Solid-core photonic bandgap fibers for cladding-pumped Raman amplification

Cladding-pumped solid-core photonic bandgap Raman fiber amplifiers are analyzed theoretically as possible candidates for power scaling. An example fiber design with a mode field diameter of 46 µm and a cladding diameter of 250 µm is calculated to exhibit 0.12 dB/km of confinement loss at the first Stokes wavelength and >10 dB/m at the second Stokes wavelength for a pump wavelength of 1.567 µm while maintaining low loss and large mode area in realistic coiling configurations. A Raman amplifier based on this fiber with 85 kW of output power, 78% optical conversion efficiency, a maximum heat load of 130 W/m, and a length of 235 m is simulated.

Suppression of stimulated Raman scattering employing long period gratings in double-clad fiber amplifiers

We report on the suppression of stimulated Raman scattering (SRS) in a double-clad fiber amplifier using long-period gratings (LPGs). The LPGs, fabricated with a CO(2) laser, achieve SRS suppression by coupling the Stokes wavelength from the active core into the cladding. With only three LPGs inserted into a fiber pulse amplifier, the extractable Raman-free output power was nearly doubled. A numerical simulation of the setup shows good agreement with the experimental results.

Modeling the inhibition of stimulated Raman scattering in passive and active fibers by lumped spectral filters in high power fiber laser systems

We report on the simulation of stimulated Raman scattering inhibition by lumped spectral filters both in passive optical transport fibers and in fiber amplifiers. The paper includes a detailed theoretical study that reveals the parameters that have the strongest influence on the suppression of the Raman scattering, such as the filter distribution and the insertion losses at the signal wavelength. This study provides guidelines for the use of spectral filtering elements, such as long period gratings, for Raman scattering inhibition in real-world high power fiber amplifiers.

Distributed suppression of stimulated Raman scattering in an Yb-doped filter-fiber amplifier

Suppression of stimulated Raman scattering (SRS) is demonstrated in a cladding-pumped fiber amplifier. The Yb-doped amplifier fiber design incorporates a high-index ring that resonantly couples SRS wavelengths out of the gain material, thus filtering the gain. Modeling shows that fiber asymmetry plays an important role in the filtering spectrum.