Diffraction-limited Beam Quality Research Articles

Comprehensive Theoretical Study of Mode Instability in High-Power Fiber Lasers by Employing a Universal Model and Its Implications

We present comprehensive theoretical study of mode instability with a universal model for the first time, which includes quantitative study on the various influence factors and mitigation methods. With large numerical results, the influence of various fiber and system parameters has been studied to summarize mitigation methods. The mode instability threshold behavior of multiwavelength pumping configurations has been studied for the first time, and the influence of pump wavelength shifting in the tandem pumping technique and amplitude modulation have also been given for the first time. The mitigation methods are discussed with review of the latest development in experimental results. The drawbacks and possible overcome measures are also discussed. By properly combining the mitigation methods, an approach to achieve beyond 10-kW diode pumped fiber laser with near diffraction limited beam quality has been proposed, which provides a creative way to overcome the mode instability limitation on diode pumped fiber lasers, and demonstrates the effectiveness of the mitigation methods.

Narrow-Strip Single-Longitudinal-Mode Laser Based on Periodic Anodes Defined by i-Line Lithography

A low-cost regrowth-free narrow-strip single-longitudinal-mode diode laser, based on periodic anodes defined only by i-line lithography is proposed. With the periodic anodes separated by surface insulated grooves acting as a 44th-order gain grating, a high gain contrast is achieved without introducing effective index coupling. The CW output power of the device without facet coatings is up to 100.9 mW/facet at 980.63 nm in single-longitudinal-mode operation with a single facet having a power conversion efficiency of up to 18.6% and a slope efficiency of 0.53 W/A. The highest side-mode suppression ratio is 45 dB with a linewidth below 889 MHz, at 350 mA. The beam parameter product, M ${\,}^{2}$ , in the slow axis is 1.85, achieving a near diffraction-limited beam quality. A novel method for easy surface gratings fabrication is first presented, wherein, 4-μm-width periodic surface grooves, acting as waveguide ridges, are defined once by i-line lithography only without regrowth, nanoscale gratings fabrication, or facet coating. Owing to the low-cost fabrication methods and good performances, our single-longitudinal-mode devices can be used for widespread practical applications.

Highly efficient nanosecond 560 nm source by SHG of a combined Yb-Raman fiber amplifier.

We demonstrate a nanosecond 560 nm pulse source based on frequency-doubling the output of a combined Yb-Raman fiber amplifier, achieving a pulse energy of 2.0 µJ with a conversion efficiency of 32% from the 976 nm pump light. By introducing a continuous-wave 1120 nm signal before the cladding pumped amplifier of a pulsed Yb:fiber master oscillator power amplifier system operating at 1064 nm, efficient conversion to 1120 nm occurs within the fiber amplifier due to stimulated Raman scattering. The output of the combined Yb-Raman amplifier is frequency-doubled to 560 nm using a periodically poled lithium tantalate crystal with a conversion efficiency of 47%, resulting in an average power of 3.0 W at a repetition rate of 1.5 MHz. The 560 nm pulse duration of 1.7 ns and the near diffraction-limited beam quality (M2≤1.18) make this source ideally suited to biomedical imaging applications such as optical-resolution photoacoustic microscopy and stimulated emission depletion microscopy.

Quasi-static mode instability in few-mode fiber amplifier

One of the most outstanding limitations in the evolution of the power scaling of fiber laser with near diffraction limited beam quality has been the mode instability since it was found in 2010. For a long time, researchers have focused on the dynamic mode instability (DMI) theoretically and experimentally, and it was not until 2016 that a new analytical model called quasi-static mode instability (QSMI) was proposed. Unlike DMI, because of the one-way energy transfer characteristic on a specific time scale, QSMI will show no apparent fluctuations with respect to the time domain traces. In this paper, based on a counter-pump few-mode fiber amplifier schematic system, the output power, beam quality and time traces of the amplifier under changing seed laser power are measured to investigate its mode instability effect. The ytterbium-doped fiber of the amplifier has a core diameter of 25 μm and inner cladding diameter of 400 μm, which can support 4-5 modes to be transmitted in the amplifier. The experimental results reveal that QSMI happens in the few-mode fiber amplifier. Taking 234 W seed power for example, it is found that when the output power reaches 2030 W, the optical-to-optical efficiency begins to fell from 86% to 32%, and at the same time the M2 value has an abrupt degradation from 2.2 to 2.8, which indicates that MI happens. On the other hand, it can be seen from the time traces of the output laser that there exist no rapid fluctuations, and the Fourier analysis shows no sign of DMI characteristic frequency components either. Quoting the definition of drifting ratio σ, when the output power is 2030 W under 234 W seed power, it is only 4%, and thus verifying that it is QSMI instead of DMI. The experiment also indicates that increasing the seed power has an effective influence on enhancing the mode instability power. When the seed power is raised from 86 W to 528 W, the corresponding threshold power is increased from 1560 W to 3090 W. And for 528 W seed power, when the output laser surpasses 3000 W, the optical-to-optical efficiency does not decline as fast as other relatively low seed power. To sum up, the mode instability effect represents a kind of quasi-static property in these large core diameter few-mode fiber amplifiers, which needs further studying.

3.05 kW monolithic fiber laser oscillator with simultaneous optimizations of stimulated Raman scattering and transverse mode instability

We report a high power monolithic ytterbium-doped fiber laser oscillator with an output power of 3.05 kW, which is achieved by simultaneous optimizations of the stimulated Raman scattering (SRS) and transverse mode instability (TMI). The optimizations of the SRS are designed and utilized in the construction of the fiber laser oscillator, while the TMI threshold is optimized with the study of the dependence of TMI threshold on the pump distribution. In the fiber laser oscillator, the TMI threshold is enhanced by ∼30% when the counter-pump scheme is employed instead of the co-pump scheme. By applying bidirectional-pump scheme and appropriately distributing the pump power, the TMI threshold is further enhanced and the monolithic fiber laser oscillator achieves an output power of 3.05 kW with near diffraction limited beam quality.

5 kW Near-Diffraction-Limited and 8 kW High-Brightness Monolithic Continuous Wave Fiber Lasers Directly Pumped by Laser Diodes

Tandem pumping technique are traditionally adopted to develop >3-kW continuous-wave (cw) Yb3+-doped fiber lasers, which are usually pumped by other fiber lasers at shorter wavelengths (1018 nm e.g.,). Fiber lasers directly pumped by laser diodes have higher wall-plug efficiency and are more compact. Here we report two high brightness monolithic cw fiber laser sources at 1080 nm. Both lasers consist of a cw fiber laser oscillator and one laser-diode pumped double cladding fiber amplifier in the master oscillator-power amplifier configuration. One laser, using 30-μm-core Yb3+-doped fiber as the gain medium, can produce >5-kW average laser power with near diffraction-limited beam quality (M2 8-kW average laser power with high beam quality (M2: ∼ 4). The slope efficiency of the fiber amplifier with respect to the launched pump power reach 83%. To the best of our knowledge, this is the first detailed report for >5-kW near-diffraction-limited and >8-kW high-brightness monolithic fiber lasers directly pumped by laser diodes.

High Average Power Second-Harmonic Generation of a CW Erbium Fiber MOPA

We report the generation of 28 W of 780 nm radiation with near diffraction limited beam quality (M2 ≤ 1.15) by frequency-doubling a continuous-wave erbium fiber master oscillator power amplifier system in a periodically poled lithium niobate crystal. The second-harmonic generation conversion efficiency reached 45% with no roll-off observed, suggesting that further power scaling should be possible with higher fundamental pump powers. The generated second-harmonic had a 3 dB spectral bandwidth of 0.10 nm. The presented architecture represents a simple and effective route to generating high-brightness radiation around 780 nm.

Analysis of Fundamental-Mode Beam Transport in Highly Multimode Fibers

In view of the growing demand for fiber beam delivery in manufacturing systems with close to diffraction-limited beam quality, we present an analysis of the propagation of a nearly diffraction-limited laser beam through 5 to 10 m long multimode step-index fibers with a numerical aperture of 0.111 and core diameters of up to 80 $\mu$ m. The influence of the launching conditions are investigated theoretically and experimentally by varying the beam size of the input beam. At a core diameter as high as 80 $\mu$ m, corresponding to a mode field area of 2800 $\mu$ m2, and an optimum launching condition, the transmitted M2 was measured to be around 1.3. It is shown that bending of the fibers down to radii as small as 2 cm barely influences the beam quality factor of the transmitted beam for all the tested fibers as the M2 was always found to be smaller than 1.5. The threshold of stimulated Raman scattering was measured to be around 60 kW for a fiber loosely placed on the table ( $\text{r}_{\text{bend}}$ $\geq$ 25 cm) with a total length of 10 m and a core diameter of 80 $\mu$ m.

Multi-pass-cell-based nonlinear pulse compression to 115 fs at 7.5 µJ pulse energy and 300 W average power.

We demonstrate nonlinear pulse compression by multi-pass cell spectral broadening (MPCSB) from 860 fs to 115 fs with compressed pulse energy of 7.5 µJ, average power of 300 W and close to diffraction-limited beam quality. The transmission of the compression unit is >90%. The results show that this recently introduced compression scheme for peak powers above the threshold for catastrophic self-focusing can be scaled to smaller pulse energies and can achieve a larger compression factor than previously reported. Good homogeneity of the spectral broadening across the beam profile is verified, which distinguishes MPCSB among other bulk compression schemes.

175 fs-long pulses from a high-power single-mode Er-doped fiber laser at 1550 nm

Development of Er-doped ultrafast lasers have lagged behind the corresponding developments in Yb- and Tm-doped lasers, in particular, fiber lasers. Various applications benefit from operation at a central wavelength of 1.5 μm and its second harmonic, including emerging applications such as 3D processing of silicon and 3D printing based on two-photon polymerization. We report a simple, robust fiber master oscillator power amplifier operating at 1.55 μm, implementing chirp pulse amplification using single-mode fibers for diffraction-limited beam quality. The laser generates 80 nJ pulses at a repetition rate of 43 MHz, corresponding to an average power of 3.5 W, which can be compressed down to 175 fs. The generation of short pulses was achieved using a design which is guided by numerical simulations of pulse propagation and amplification and manages to overturn gain narrowing with self-phase modulation, without invoking excessive Raman scattering processes. The seed source for the two-stage amplifier is a dispersion-managed passively mode-locked oscillator, which generates a ∼40 nm-wide spectrum and 1.7-ps linearly chirped pulses.

Single mode 4.3 kW output power from a diode-pumped Yb-doped fiber amplifier.

We investigate the average power scaling of two diode-pumped Yb-doped fiber amplifiers emitting a diffraction-limited beam. The first fiber under investigation with a core diameter of 30 µm was able to amplify a 10 W narrow linewidth seed laser up to 2.8 kW average output power before the onset of transverse mode instabilities (TMI). A further power scaling was achieved using a second fiber with a smaller core size (23µm), which allowed for a narrow linewidth output power of 3.5 kW limited by stimulated Brillouin scattering (SBS). We mitigated SBS using a spectral broadening mechanism, which allowed us to further increase the output power to 4.3 kW only limited by the available pump power. Up to this power level, a high slope efficiency of 90% with diffraction-limited beam quality and without any sign of TMI or stimulated Raman scattering for a spectral dynamic range of higher than -80 dB was obtained.

Generation of 180 W average green power from a frequency-doubled picosecond rod fiber amplifier.

We report on the generation of 180 W average green power from a frequency-doubled picosecond rod fiber amplifier. In an Yb-doped fiber master-oscillator-power-amplifier system, 2.3-ps 704 MHz pulses are first amplified in small-core fibers and then in large-mode-area rod fibers to produce 270 W average infrared power with a high polarization extinction ratio and diffraction-limited beam quality. By carrying out frequency doubling in a lithium triborate (LBO) crystal, 180 W average green power is generated. To the best of our knowledge, this is the highest average green power achieved in fiber-based laser systems.

High average power parametric wavelength conversion at 3.31-3.48 m in MgO:PPLN.

We present results of high average power mid-infrared (mid-IR) generation employing synchronized nanosecond pulsed ytterbium and erbium fiber amplifier systems using periodically poled lithium niobate. We generate greater than 6 W of mid-IR radiation tunable in wavelength between 3.31-3.48 μm, at power conversion efficiencies exceeding 75%, with near diffraction limited beam quality (M2 = 1.4). Numerical modeling is used to verify the experimental results in differing pump depletion regimes.

Active beam cleanup of pulsed slab laser based on 59-unit adaptive optics system

Beam quality is one of the key technical targets in the high-power laser system, and it can be dramatically improved by using adaptive optics. In this paper, a research on the near field distribution of the pulsed slab laser has been carried out. A Shack Hartmann wavefront sensor was used to measure the aberrations, and the wavefront reconstruction was accomplished afterwards. Because of the special near field distribution, the diffraction limited factor β has been revised for better describing beam quality. The theory model of the correction of aberrations with deformable mirror (DM) was introduced. Based on the theory model, the numerical analysis of the correction ability of DM with 59 actuators and correction of measured wavefront have been carried out, the result showed that the peak value of far field was improved 3.2 times, and the beam quality was increased 3 times. An experimental setup for beam cleanup of pulsed laser has been established. A demonstrated experiment was carried out on this platform. The result of the experiment indicated that the output beam quality of the pulsed slab laser can be validly improved by beam cleanup using adaptive optics system with 59-actuator DM. Specifically, the peak value of far field has been raised by 3 times, and the factor β has been increased by 3 times as well, which was mostly in accordance with the numerical analysis. The conclusion of this paper offered reference for the achievement of near diffraction limited beam quality in the future.

Experimental demonstration of transverse mode instability enhancement by a counter-pumped scheme in a 2 kW all-fiberized laser

Transverse mode instability (TMI) has become the major limitation for power scaling of fiber lasers with nearly diffraction-limited beam quality. Compared with a co-pumped fiber laser, a counter-pumped fiber laser reveals TMI threshold enhancement through a semi-analytical model calculation. We demonstrated a 2 kW high-power counter-pumped all-fiberized laser without observation of TMI. Compared with the co-pumped scheme, the TMI threshold is enhanced at least 50% in counter-pumped scheme, moreover, stimulated Raman scattering and four-wave mixing are suppressed simultaneously.

Angled cavity photonic crystal lasers with asymmetrical high-order surface gratings

980 nm angled cavity photonic crystal (PC) laser diodes with asymmetrical high-order surface gratings (aHSGs) are proposed and fabricated. The one-dimensional PC structure in the epitaxy is used to expand the fundamental transverse mode and reduce the vertical divergence. An angled cavity with aHSGs is fabricated to achieve a nearly diffraction-limited beam quality and narrow spectral width. Experimentally, a continuous-wave output of 0.85 W/facet, a low divergence of 1.5 × 10.6°, and a narrow spectral width of 0.07 nm are achieved. The lateral beam quality is superior with an of 1.96.

Raman-shifted wavelength-selectable pulsed fiber laser with high repetition rate and high pulse energy in the visible.

A high-pulse-energy, diffraction-limited, wavelength-selectable, visible source, based on Raman frequency shifting of a frequency-doubled Yb-doped fiber laser, has been studied. The relative length-scaling laws of Raman gain and self-phase modulation push the design towards short fiber lengths with large core size. It is experimentally demonstrated that the Raman clean-up effect in a graded-index multi-mode fiber is not sufficient to obtain diffraction-limited beam quality in the short fiber length. Thus, a large-core photonic crystal fiber is used to maintain diffraction-limited performance and output pulse energies of ~1 μJ, at a 1-MHz repetition rate and 1.3-ns pulse-width are successfully achieved. This step-tunable visible source should find applications in photoacoustic microscopy.

Efficient high-harmonic generation from a stable and compact ultrafast Yb-fiber laser producing 100\xa0\u03bcJ, 350\xa0fs pulses based on bendable photonic crystal fiber

The development of an Yb3+-fiber-based chirped-pulse amplification system and the performance in the generation of extreme ultraviolet (EUV) radiation by high-harmonic generation is reported. The fiber laser produced 100 μJ, 350 fs output pulses with diffraction-limited beam quality at a repetition rate of 16.7 kHz. The system used commercial single-mode, polarization maintaining fiber technology. This included a 40 μm core, easily packaged, bendable final amplifier fiber in order to enable a compact system, to reduce cost, and provide reliable and environmentally stable long-term performance. The system enabled the generation of 0.4 μW of EUV at wavelengths between 27 and 80 nm with a peak at ~45 nm using xenon gas. The EUV flux of ~1011 photons per second for a driving field power of 1.67 W represents state-of-the-art generation efficiency for single-fiber amplifier CPA systems, corresponding to a maximum calculated energy conversion efficiency of 2.4 × 10−7 from the infrared to the EUV. The potential for high average power operation at increased repetition rates and further suggested technical improvements are discussed. Future applications could include coherent diffractive imaging in the EUV, and high-harmonic spectroscopy.

Nonlinear pulse compression in a multi-pass cell.

We demonstrate a scheme for nonlinear pulse compression at high average powers based on self-phase modulation in a multi-pass cell using fused silica as the nonlinear medium. The scheme is suitable for compression of pulses with peak powers exceeding the threshold for critical self-focusing. At >400 W of input power, the pulses of a Yb:YAG-Innoslab laser system (10MHz repetition rate, 850fs pulse duration) are spectrally broadened from 1.6 to >13.5 nm bandwidth while maintaining almost diffraction-limited beam quality. The chirp is removed with a dispersive mirror compressor, and pulse durations of 170fs at an output power of 375W are achieved. The compression unit reaches an overall transmission of >90%.

High power TEM00 picosecond output based on a Nd:GdVO4 discrete path Innoslab amplifier.

We propose a technique for eliminating self-lasing and suppressing amplified spontaneous emission in a partially pumped slab amplifier with a discrete path configuration. High gain character and homogeneous gain distribution were well preserved with the proposed scheme. Based on the Nd:GdVO4 crystal, a 99 W, 12.4 ps TEM00 laser output was achieved with a 42% extraction efficiency. The diffraction limited beam quality remained undisturbed after amplification with Mx2=1.09 and My2=1.07 in the orthogonal directions.