Flat-top beams exhibit symmetrical intensity and steep edge in cross-section with wide application. Especially, orbital angular momentum (OAM) endows it with the potential to achieve uniform angular momentum transfer or phase encoding. In this Letter, we observed a flat-top beam carrying +ℏ OAM by intracavity incoherent superposition in an X-cavity Tm: YAG ceramic laser. The corresponding M2 factors are 2.24 and 2.08 with a typical output power of 470 mW around ~2011.2 nm. Simultaneously, a radially polarized vortex beam (RPVB) with +ℏ OAM was observed. Further simulations and experiments revealed that a flat-top beam is formed by the incoherent superposition of RPVB and a Gaussian beam. Using a homemade M-Z interferometer, we observed vortex stripes' chirality reversal before and after the focal plane of the convex lens. To the best of our knowledge, this is the first observation of a flat-top beam with OAM at 2 µm wavelength.

We report on the power scaling and high-efficiency operation of a Ho:Lu2O3 ceramic laser at 2.1 μm in-band pumped by a high-power, narrow-linewidth Tm-doped fiber laser at 1942 nm. The 0.5 at. % Ho3+-doped Ho:Lu2O3 ceramic was fabricated in-house through a vacuum sintering plus hot isostatic pressing (HIPing) process, showing a relatively low scattering loss of <2.1% cm−1. A record output power of 10.2 W at 2121 nm was achieved with 45 W of absorbed pump power, corresponding to a slope efficiency of 27.5%. The beam quality factors (M2) at the maximum output were measured to be 1.34 and 1.28 in the x- and y-directions, respectively. The laser power and efficiency were enhanced by two orders of magnitude and nearly 30-fold compared to the previous results, which confirms the significant power scaling potential of in-band pumped Ho:Lu2O3 ceramic lasers for hundred-watt-level operation upon improved quality of the ceramic with a more homogeneous and denser microstructure.

In this study, we demonstrate a high-power and widely-tunable Tm-doped all-fiber master oscillator power amplifier (MOPA) in a core- and tandem-pumping configuration. The ground-state bleaching effect of Tm-doped fiber tandem-pumped at 1943nm was investigated. The master oscillator is a Tm-doped fiber ring laser incorporating a tunable bandpass filter to realize a narrow linewidth and wavelength-tunable operation. The MOPA delivered 103 W of output power at 1980nm, with >83 W maintained over a 60 nm tuning range from 1960 to 2020nm, achieving a high slope efficiency of ∼ 80%. The laser exhibited a spectral linewidth of ∼0.15 nm and a high optical signal-to-noise ratio (OSNR) of >42 dB, attributed to the effective suppression of amplified spontaneous emission (ASE) by tandem pumping. The power stability (RMS) at a scale of ∼10 min was measured to be approximately 0.75%. A diffraction-limited beam quality factor M2 of ∼1.18 (horizontal) and 1.16 (vertical) was measured at the maximum laser output. The laser power is pump-limited without the onset of parasitic oscillation or thermal-induced transverse mode instability (TMI) effect, even at the maximum power level. This represents the first demonstration, to the best of our knowledge, of an all-fiber-integrated, wavelength-tunable Tm-doped fiber laser reaching the 100-watt level under efficient tandem pumping.

In this paper, we demonstrated a high-efficiency, high-energy, and widely tunable mid-infrared optical vortex parametric oscillator based on a ZnGeP2 (ZGP) crystal, pumped by a 2.05 mu m first-order vortex laser operating at a repetition rate of 1 kHz. The oscillator generated a signal beam that consistently carried orbital angular momentum (OAM) across a continuous tuning range from 3.57 to 4.02 mu m, while the corresponding idler beam remained OAM-free and exhibited a Gaussian-like intensity profile. At a pump pulse energy of 6.4 mJ, the signal vortex beam achieved a maximum output energy of 2.7 mJ at a central wavelength of 3.86 mu m, corresponding to an optical-to-optical conversion efficiency (OOCE) exceeding 42 %, which clearly demonstrated efficient OAM transfer from the pump to the signal beam. At maximum output, the signal vortex beam exhibited beam quality factors M-2 of 2.7 and 2.6 in the x and y directions, respectively.

This paper reports 2 μm intracavity vector vortex pattern generation in a diode-pumped isotropic Tm:Y2O3 ceramic laser with an X-folded cavity configuration. By employing off-axis non-collinear hybrid pumping technology and control of the aperture, it demonstrates the direct generation and switching of four distinct polarized vortex beams-linearly polarized, elliptically polarized, anti-radially polarized and anti-azimuthally polarized-with a central wavelength of ∼2050 nm and carrying orbital angular momentum (OAM) of ±1ℏ. The corresponding M2 factors are measured between 2.03 and 2.3, while maintaining high stability (minimum RMS of 0.47%), high purity (maximum SNR of 16.7 dB), and typical output power up to 242 mW. Additionally, various vortex arrays with different numbers of vortices having OAM of ±1ℏ are generated, and their basis vector synthesis and orbital angular momentum are theoretically simulated and experimentally verified. To the best of our knowledge, this represents the first laser that directly generates vector vortex beams using isotropic ceramics at the 2 μm wavelength. As such, it establishes a paradigm for structured light field manipulation based on ceramic gain media in the mid-infrared spectral region.

We present a pump-enhanced monolithic optical parametric oscillator (OPO) capable of W-level single-frequency mid-infrared (MIR) output and optical vortex beam generation. Utilizing a Gaussian pump and a pump-enhanced cavity design, the monolithic OPO achieved a maximum idler output power of 1.4 W and a tuning range of 3016–3200 nm. The OPO maintained single-frequency operation with a linewidth of 218 kHz at W-level output. Due to thermal effects, the OPO had an RMS power stability of 0.7% over one hour and long-term mode-hop-free performance. When pumped with an optical vortex, orbital angular momentum could be effectively transferred to the idler or signal beam, achieving maximum idler or signal vortex powers of 0.6 W or 1.8 W, respectively. Additionally, idler vortex generation was demonstrated across the entire tuning range.

The development of multidimensional lasers, particularly 2 µm vector vortex pulsed beams, has garnered significant interest for applications in laser medicine and optical communication. This paper reports the successful achievement of mode-locked azimuthally polarized vector beams (APVB) and radially polarized vector beams (RPVB) in a c-cut Tm:CaYAlO4 crystal cavity. Utilizing a semiconductor saturable absorber mirror (SESAM), the laser produced output powers of 110 mW (APVB) and 98 mW (RPVB), with pulse durations of 41.2 ps and 49.7 ps, and vortex topological charges of -1 and +1, respectively. This represents the first stable mode-locked states of the cylindrical vector vortex beam in a 2 µm solid-state laser.

The negative dispersion of silica fibers near 2 mu m wavelength leads to formations of attractive soliton-patterns in Thulium-doped mode-locked fiber lasers (TDMLFL), including single-solitons(SS), bound-solitons(BS), multisolitons(MS), soliton molecules(SM), as well as noise-like pulses(NLP). However, the current manual or physically controlled methods cannot accurately identify and quickly adjust the diverse solitons. Here, we successfully realized the fine identification and automatic searching of continuous waves, Q-switching, noise-like pulses, multi-solitons, and single-solitons by constructing a genetic algorithm based self-tuning pump power and time- spectrum feedback agent in a TDMLFL. The searched SS have a duration of 1.269 ps, a central wavelength of 1966 nm and a typical Kelly-sideband spectrum. The minimum consuming time of globally finding a singlesoliton is-40 mins, and the corresponding recovery-time is-2 mins. To the best of our knowledge, this is the first time that an intelligent searching and recognition of single soliton in 2 mu m TDMLFL and also the first report of soliton-patterns fully intelligent identification and searching without prior parameters in soliton mode locked fiber lasers.

We demonstrate a stable and compact 2 µm mode-locked Ho-doped fiber oscillator incorporating an intracavity S-waveplate for the direct generation of cylindrical vector beams. The oscillator employs a figure-9 nonlinear amplifying loop mirror (NALM) configuration entirely composed of polarization-maintaining fibers, ensuring environmentally robust and self-starting mode locking. The output mode-locked pulses exhibit a central wavelength of 2090.8 nm, a repetition rate of 33.8 MHz, and a spectral bandwidth of 6.1 nm. After inserting a tunable bandpass filter, a continuous wavelength tuning range from 2024 nm to 2046 nm is achieved while maintaining single-pulse operation and a polarization extinction ratio over 98%. This work presents a simple and efficient approach to generating high-purity vectorial ultrashort pulses in the 2 µm region, providing a promising platform for high-precision micro- and nanofabrication of polymers and transparent materials.

The spatiotemporal mode-locking (STML) method of multimode fiber lasers has become a key to breaking through the energy and mode bottleneck of conventional ultrafast single-mode fiber lasers in 2 mu m range. However, the complex spatiotemporal dynamics of typical soliton wave-breaking caused by the large negative dispersion (typically of -70 ps(2)/km) and nonlinear phase shift has seriously hindered the generation and control of STML in this wavelength range. Here, we demonstrate an intelligent strategy of 2 mu m STML by integrating a nonlinear polarization rotation (NPR) based hybrid fiber structure of single-mode, few-mode and multimode fibers. Two typical STML states including noise-like pulses (NLP) and solitons were searched, with central wavelengths of 2007 nm and 1993 nm, pulse durations of 447 fs and similar to 596 fs. The transverse modes of typical NLP STML exhibit a uniform Gaussian-like distribution, with the M-2 factors of similar or equal to 5.5 (the 13.5 % beam width for broad energy distribution) and similar or equal to 3 (the 50 % beam width for core energy distribution). This is smaller than the theoretical M-2 value of 8.6 for the multimode fiber, which differs from the complex mode distribution with multiple energy centers in the continuous-wave(CW) state. This further indicates the modulation effect of different transverse modes caused by the typical spatiotemporal nonlinearity of NPR mode-locking in hybrid fibers. The average searching time for STML states is similar to 4.7 mins during 10 test samples. To our knowledge, this is the first time that 2 mu m STML has been achieved based NPR and intelligent control, providing an innovative strategy for development of 2 mu m ultrafast lasers with multidimensional control and large energy.

In this letter, we present a high-power, narrow-linewidth single-frequency Tm-doped all-fiber master oscillator power amplifier (MOPA) based on a distributed cladding-pumping scheme (DCPS). A theoretical investigation was conducted on the dynamic behavior of stimulated Brillouin scattering (SBS) along the cascaded hybrid active fiber in the DCPS-enabled Tm-fiber amplifier. A ring-cavity configuration was employed to construct a single-frequency seed source with a segment of unpumped holmium-doped fiber as an ultranarrow bandwidth filter, producing a low-noise single-longitudinal-mode (SLM) lasing operation at 1998.13 nm with a spectral linewidth of ∼8.5 kHz and an optical signal-to-noise ratio (OSNR) of > 65 dB. Through distributed counter-pumping of the Tm-fiber amplifier, the single-frequency seed was successfully power-scaled to 425 W at a 9 kHz spectral bandwidth with a slope efficiency of 53%. A high OSNR was maintained at >59 dB at the full output power level with a diffraction-limited beam quality factor M2 of less than 1.20. The measurement of the relative intensity noise (RIN) demonstrated excellent low-noise properties over the entire laser power range. To the best of our knowledge, this work represents the highest power level ever achieved from a monolithic single-frequency Tm-fiber laser in the 2 µm spectral region.

In this paper, we explored the different performances of the evolution of polarization control parameters under algorithm optimization in one-dimensional, two-dimensional, and three-dimensional parameter space. We found that the genetic algorithm (GA) operation has a typical converging trend as the epoch increasing of groups in ultra-fast pulse generation. The corresponding deviations were calculated to be 10.8%, 11.2%, and 5.0%, respectively, which is different from the traversal algorithm operation. The typical noise-like pulse state was effectively automatically searched at different length cavities, with center wavelengths of 1973, 1988.7, and 2002.7 nm, repetition rate of 9.4, 10.4, and 5.4 MHz, and typical spike duration of similar to 224 fs. Our research shows the effectiveness of the GA in searching and controlling the ultrashort pulse output of a 2-mu m thulium-doped fiber laser.

In the past few years, annular structured beams have been extensively studied due to their unique “doughnut” structure and characteristics such as phase and polarization vortices. Especially in the 2 µm wavelength range, they have shown promising applications in fields such as novel laser communication, optical processing, and quantum information processing. In this Letter, we observed basis vector patterns with orthogonality and completeness by finely cavity-mode tailoring with end-mirror space position in a Tm:CaYAlO4 laser. Multiple annular structured beams including azimuthally, linearly, and radially polarized beams (APB, LPB, and RPB) operated at a Q-switched mode-locking (QML) state with a typical output power of ∼18 mW around 1962 nm. Further numerical simulation proved that the multiple annular structured beams are the coherent superposition of different Hermitian Gaussian modes. Using a self-made M–Z interferometer, we have demonstrated that the obtained multiple annular beams have a vortex phase with orbital angular momentum (OAM) of l = ±1. To the best of our knowledge, this is the first observation of vector and scalar annular vortex beams in the 2 µm solid-state laser.

In this paper, we report on a high-power and widely tunable thulium-doped fiber laser (TDFL) based on a monolithic master oscillator power amplifier (MOPA) system. The master oscillator is a Tm fiber ring laser incorporating a tunable bandpass filter to realize narrow linewidth and wavelength tunable operation. The MOPA generated 1010 W ∼1039 W of output power over a tuning range of 107 nm from 1943 to 2050nm with slope efficiencies of more than 51% and spectra linewidth of ∼0.5 nm. Power stability (RMS) in ∼10 min scale is measured to be ∼0.52%. A diffraction-limited beam quality factor M2 of ∼1.18 is measured at 920 W of laser output. Output power is pump-limited without the onset of parasitic oscillation or amplified spontaneous emission (ASE) even at the maximum power level. This is the first demonstration, to the best of our knowledge, on an all-fiber integrated wavelength-tunable TDFL at 2 µm with output power exceeding 1 kW.

We report on a high-power Ho:Y2O3 ceramic laser at 2.1 µm with controllable output beam profile ranging from LG01 donut, flat-top to TEM00 mode using a simple two-mirror resonator. In-band pumped at 1943nm using a Tm fiber laser beam shaped via a coupling optics comprising a capillary fiber and lens-combination to achieve distributed pump absorption in Ho:Y2O3 and hence selective excitation of the target mode, the laser yields 29.7 W of LG01 donut, 28.0 W of crater-like, 27.7 W of flat-top and 33.5 W of TEM00 mode output for absorbed pump power of 53.5 W, 56.2 W, 57.3 W and 58.2 W, respectively, corresponding to a slope efficiency of 58.5%, 54.3%, 53.8% and 61.2%. This is, to the best of our knowledge, the first demonstration of laser generation with continuously tunable output intensity profile at ∼2 µm wavelength region.

We report on power scaling and efficient operation of a Ho:Y2O3 ceramic laser at 2.1 µm in-band pumped with an incoherently beam combined high power and narrow-linewidth Tm fiber source at 1931.2 nm. The 0.5 at.% Ho3+ doped Ho:Y2O3 ceramic is fabricated in-house with scattering loss of < 0.25% cm-1. Up to 210.5 W of continuous-wave output power has been generated at 2117 nm for 366 W absorbed pump power shaped with a one-dimensional top-hat profile, corresponding to a slope efficiency of 60.0% with respect to the absorbed pump power. A slope efficiency of 67.5% has been demonstrated with 160 W of output power using a circular beam pump configuration. Results presented in this work verify the superior power scaling capability of a Ho:Y2O3 ceramic laser with high efficiency at ∼2.1 µm.

We report transmitting 2-μm laser power of 63 W through a 200-μm-core As 2 S 3 fiber, under 120 W incident power. For the first time, a flexible chalcogenide fiber show the capability of delivering 100W-level 2-5μm laser.

Mid-infrared self-cascaded noncritically-phase-matched (NCPM) KTP optical parametric oscillator (OPO) was developed, generating multiple wavelengths at 2.95, 3.29 and 3.37 mu m. The polarization state of 2.95 mu m is orthogonal to that of the other two wavelengths. Under 1064 nm intracavity pumping, the first NCPM KTP-OPO can generate signal and idler waves at 1.57 and 3.29 mu m, respectively. Then the 1.57 mu m is further used as pump light for the second NCPM OPO in the same KTP, generating signal and idler waves at 2.95 and 3.37 mu m. Broadband spectral feedback condition is established between the OPO cavity mirrors, enabling the two NCPM OPOs to operate simultaneously in the same KTP and OPO resonator. With regard to 2.95 mu m, the achieved maximum average output power and minimum pulse width are respectively 1.18 W and 5 ns, corresponding to a pulse peak power of 56.9 kW with 4.2 kHz repetition rate. As for the dual-wavelength at 3.29 and 3.37 mu m, the maximum total average output power is 0.22 W. Our work provides an effective scheme to obtain multiple wavelength laser output in 3 mu m region with orthogonal polarization state.

Objective Midinfrared lasers emitting in the 2-3 mu m range are a popular research topic in laser science and applications. Presently, the majority Tm-doped laser research is focused on the F-3(4) -> H-3(6) transition, with the laser wavelength ranging from 1.8 to 2.1 mu m. Tm3+ ions have multiple potential laser transition processes owing to their rich energy level structure. Exploring the potential laser transition process of activating ions outside of the conventional waveband is an important step toward directly obtaining a new midinfrared wavelength. The H-3(4) -> H-3(5) transition in Tm3+ ions at 2. 3 mu m has attracted considerable attention lately. The following are the advantages of achieving a 2.3 mu m laser operation in a Tm-doped laser medium. Tm 3 ' ions absorb strongly at similar to 800 nm, which is the emission wavelength of commercial AIGaAs laser diodes (LDs) . Then, a low-cost LD-pumped all-solid-state 2.3 mu m Tm-doped laser can be designed. Furthermore, the H-3(4) -> H-3(5), transition is a four-level structure that can support room temperature laser operation with a negligible reabsorption effect. The 2. 3 mu m region is in the weak absorption zone of water, and the laser operation is less affected by the surrounding air humidity. Compared with the scheme of using Cr2+-doped II-IV media to realize 2.3 mu m laser, the preparation of Tm-doped laser materials is more mature and reliable, and the types of the matrix are more abundant (such as Tm: YAG, Tm: YAP, Tm: YLF) , which provides more possibilities for finding Tm-doped laser materials with excellent spectral characteristics, high thermal conductivity, high optical transmittance, and stable physicochemical properties. However, the population trapping effect of the F-3(4) level limits the power scaling of a 2. 3 mu m Tm-doped laser on the transition of H-3(4) -> H-3(5). On one hand, Tm3+ ions (H-3(4) + 3H6 -> F-3(4)+ F-3(4)) have a strong cross-relaxation process. On the other hand, the long lifetime of F-3(4), level will result in a large accumulation of Tm3+ ions at the F-3(4) level. This will reduce the population at the H-3(4) level which is essential for the 2. 3 mu m laser transition. Methods for effectively depopulating the F-3(4) level must be investigated. Methods This paper validates the scheme of 785 and 1470 nm dual-wavelength pumped 2.3 mu m Tm-doped laser. The pumping at 785 nm corresponds to the ground state absorption (GSA) of the H-3(6) -> H-3(4) transition. The pumping at 1470 nm corresponds to the excited state absorption (ESA) of the F-3(4) -> H-3(4), transition. Tm3+ ions are pumped to the H-3(4) level by the H-3(6) -> H-3(4), transition to populate the upper laser level. The Tm3+ ions that have accumulated on the F-3(4) level are then accurately transferred to the H-3(4) level via the F-3(4) -> H-3(4), transition. This effectively increases the Tm3+ ions at the H-3(4) level (Fig. 1) . The laser medium used in the experiment is Tm: YAP crystal, which was chosen based on the following criteria. The YAP crystal has high thermal conductivity and low phonon energy from the perspective of the matrix, which is conducive to achieving high output power. Because of the anisotropy of the Tm : YAP crystal structure, the absorption, and emission spectrums are anisotropic. The H-3(6) -> H-3(4) absorption spectrum of Tm: YAP crystal covers 770-810 nm waveband, which is suitable for using high-power AlGaAs LD as the pump source. The fluorescence emission spectrum of Tm : YAP for H-3(4) -> H-3(5) transition can cover 2.25-2.5 mu m, which can support multiple lasing wavelengths. The GSA pump source is a fiber-coupled, 785 nm LD with a core diameter of 400 mu m and a numerical aperture of 0.22. The ESA pump source of 1470-nm LD has a core diameter of 200 mu m and a numerical aperture of 0.11. The alpha-cut TmYAP crystal has a doping concentration ( atomic number fraction) of 2% and a size of 4 x 4 x 8 mm(3). Its two light-passing faces are antireflective coated at 780-810 and 2250-2500 nm. The laser crystal is wrapped in indium foil and installed in a copper block cooled using water at 15 degrees C. Results and Discussions The effectiveness of using GSA and ESA dual-wavelength pumping to increase the output power of a 2.3 mu m Tm-doped laser has been experimentally validated. In GSA and ESA dual-wavelength pumped a-cut TmYAP crystal, a maximum laser output power of 2.28 W is obtained with a T = 1.5% output coupler at dual-wavelengths of 2274 and 2383 nm, an increase of 65.2% compared with the case of single-wavelength pumping scheme (Fig. 5). With a T = 2.8% output coupler, 942 mW at 2383 nm laser is obtained, an increase of 84.3% compared with the case of the single-wavelength pumping scheme (Fig. 6). A specially coated output mirror with a high transmittance at 2200-2350 nm and transmittance of 0.5 yo at 2400-2500 is used to further realize the 2446 nm laser operation. The maximum output power is 1.62 W, an increase of 48.6 Yo compared with the case of the single-wavelength pumping scheme ( Fig. 5 ). Further, when 1470 nm pump light is introduced, the laser oscillation thresholds for incident 785 nm pump power are greatly reduced from 18.9 to 9.9 W, 29.9 to 23.4 W, and 12.1 to 6.6 W for OC1 ( T =1.5 yo ) , OC2 ( T = 2.8 degrees A) and OC3 ( T = 0.5%) , respectively (Fig. 6) . These results indicate that 1470 nm ESA pumping can accurately depopulate the F-3(4), level while effectively increasing the populations at the H-3(4) level. Conclusions The GSA and ESA dual-wavelength pumped Tm: YAP laser emitting in the 2.3 mu m region have been realized successfully. The Tm: YAP crystal has a maximum output power of 2.28 W at 2.3 mu m. To the best our knowledge, this is the highest CW output power obtained in a 2.3 mu m Tm-doped solid-state laser. Making full use of the broadband emission spectrum of the H-3(4)-> H-3(5) transition of the Tm: YAP crystal, a watt-level LD-pumped 2.5 mu m Tm: YAP laser is achieved using the dual-wavelength pumping scheme. Our results show that GSA and ESA dual-wavelength pumping is an effective technical means to achieve a high power output of 2.3 mu m Tm-doped laser.