In this paper, we summarize the fundamental properties and review the latest developments in high power fiber lasers. The review is focused primarily on the most common fiber laser configurations and the associated cladding pumping issues. Special attention is placed on pump combination techniques and the parameters that affect the brightness enhancement observed in single-mode and multimode high power fiber lasers. The review includes the major limitations imposed by fiber nonlinearities and other parasitic effects, such as optical damage, transverse modal instabilities and photodarkening. Finally, the paper summarizes the power evolution in continuous-wave and pulsed ytterbium-doped fiber lasers and their impact on industrial applications.

High Power Fiber Lasers: A Review

We present a high power ultra-narrowband pulsed fiber amplifier at 2 μm. A single frequency fiber laser was modulated by a phase modulator and an intensity modulator to serve as the ultra-narrowband pulsed seed laser with a bandwidth of 307 MHz. The pulsed seed laser was amplified by a monolithic Tm-doped fiber master oscillator power amplifier (MOPA). The average output power reaches 105 W with a slope efficiency of 0.41. The output pulse train has a repetition rate of 1 MHz and a pulse width of 66 ns. The output power is limited by the onset of stimulated Brillouin scattering. Higher output power can be achieved by further broadening the linewidth or narrowing the pulse width to several nanoseconds. To the best of our knowledge, this is the first demonstration on a monolithic ultra-narrowband nanosecond pulsed MOPA at 2 μm with an average power exceeding 100 W.

105 W ultra-narrowband nanosecond pulsed laser at 2 μm based on monolithic Tm-doped fiber MOPA

This paper reports, for the first time to our best knowledge, a nearly diffraction-limited output in a Yb(3+)/Al(3+)/F(-) codoped double cladding silica fiber with a 50 μm core diameter and 0.02 core NA. The core glass with a diameter >6  mm was fabricated through solgel process combined with high temperature sintering. Laser performances of this fiber at different bend diameters were studied. The mean M2<1.1 in this 50 μm core diameter fiber was achieved at a bend diameter of 0.35 m. The core glass with the refractive index nearly equal to pure silica glass is suitable for the fiber design, such as large-mode-area photonic crystal fiber.

50 μm core diameter Yb³⁺Al³⁺/F⁻ codoped silica fiber with M²<1.1 beam quality.

High-peak-power transform-limited narrow-linewidth nanosecond all-fiber lasers are desired in a range of applications. However, their linewidths will be broadened by self-phase modulation (SPM). We propose a novel concept that generates transform-limited laser pulses by temporally shaping the pulse seed. The impact of the pulse shape on SPM-induced spectral broadening was studied numerically and experimentally. It was found theoretically that the square-shape pulsed laser is immune to SPM-induced spectral broadening. Based on this principle, we built a high-peak-power, linearly polarized, square-shape nanosecond all-fiber laser in a master oscillator power amplifier (MOPA) configuration. Stimulated Brillouin scattering (SBS) limited peak powers of 4.02 kW, 5.06 kW, 6.52 kW and 9.30 kW were obtained at pulse widths of 8 ns, 7 ns, 6 ns and 5 ns. Thanks to the square-shape pulsed seed, the linewidths at maximum peak power remained at 129.5 MHz, 137.6 MHz, 156.2 MHz and 200.1 MHz, respectively, close to the transform-limited values of 110.8 MHz, 126.6 MHz, 147.7 MHz and 177.3 MHz.

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High-peak-power temporally shaped nanosecond fiber laser immune to SPM-induced spectral broadening

We present a monolithic high power narrow-linewidth linearly polarized nanosecond all-fiber laser in a master oscillator power amplifier configuration. The pulsed seed is generated by modulating a single-frequency continuous wave laser at 1064 nm using an electro-optic intensity modulator. In order to suppress stimulated Brillouin scattering (SBS), the pulse width is set to be ~4 ns. When the repetition rate is set to be 20 MHz or 10 MHz, SBS has been observed in the main amplifier. The corresponding SBS thresholds are ~100 W and 120 W respectively. At the repetition rate of 5 MHz, the pulse width is compressed to ~3 ns and an average power of 136 W is obtained, corresponding to peak power of 8.5 kW. Further power scaling is limited by stimulated Raman scattering. At maximum output power, the beam quality (M2 factor) is ~1.1 and the polarization extinction ratio is >15.8 dB.

https://iopscience.iop.org/article/10.1088/2040-8978/18/1/015506/pdf

High power narrow-linewidth linearly polarized nanosecond all-fiber amplifier with near-diffraction-limited beam quality

The invention relates to a method for controlling array light beam co-target collimation of a target in a loop based on light spot centroid calculation. The coordinates of the centroid of a light spot on an imaging plane are obtained through a light spot centroid algorithm, a tilt control quantity which needs to be conducted on each light beam is obtained according to the quantitative relation between tilt control of a tilting mirror of the corresponding light beam and a light spot movement vector, and the centroid of the light spot is moved to a target spot of an image device through control over the tilting mirrors. Co-target collimation of all the light beams of the target in the loop is realized through sequential control cover the array light beams. There is no need to accurately describe or obtain a light path, and a control scheme is simple and convenient and easy to implement. The adopted centroid of the light spot serves as a performance evaluation function which is simple, convenient, high in real-time performance, and remarkable in effect, and consumes a short time. The method for controlling array light beam co-target collimation of the target in the loop based on light spot centroid calculation has extensive application prospects in satellite tracking, the directed energy technology and other fields.

Method for controlling array light beam co-target collimation of target in loop based on light spot centroid calculation

Numerical analysis on impact of temporal characteristics on stimulated Brillouin scattering threshold for nanosecond laser in an optical fiber

In this article, we analyze the factors that limit the output power increase for photonic crystal fiber lasers and set a theoretical model to calculate the maximum extractable power of ytterbium-doped photonic crystal fibers. Numerically, when the diameter of core is 76-\mu m and the pumping intensity is 0.21 W/(\mu m2×Sr), the output of pure silica and ytterbium-doped photonic crystal fiber lasers is 100-kW, considering the technology for the time being. The main limitations of power scaling are facet damage and thermal self-focusing. In case of the strict single-mode operation condition, the maximum extractable power is 100-kW when the numerical aperture is 0.05. Considering the strict single-frequency operation condition, the maximum extractable power of both pure silica and ytterbium-doped photonic crystal fiber lasers is 1.65-kW, where the main factor is stimulated Brillouin scattering effect. Compared with the previous results, the increase in the maximum extractable power depends on three parameters: the availability of high-brightness pump diodes, the endless single-mode characteristic of the photonic crystal fiber, and the high doping density which lead to efficient absorption coefficient of pumping light. Finally, we simulate numerical aperture that influences the maximum power of photonic crystal fiber lasers and compare the difference in maximum output power of photonic crystal fibers and double-cladding fiber lasers in single-frequency condition.

Analysis of the maximum extractable power of photonic crystal fiber lasers

The effects of spectral aberrations on the phase control system of coherent beam combining (CBC) of pulsed fiber lasers are analyzed. A typical system of CBC is calculated in detail and design guidelines are given.

Cheng Luo

Papers

Method for controlling array light beam co-target collimation of target in loop based on light spot centroid calculation

Numerical analysis on impact of temporal characteristics on stimulated Brillouin scattering threshold for nanosecond laser in an optical fiber

Analysis of the maximum extractable power of photonic crystal fiber lasers

Impact of spectral aberrations on the phase control system of coherently combined fiber lasers