Efficient continuous-wave and short-pulse Ho3+-doped fluorozirconate glass all-fiber lasers operating in the visible spectral range

Nanosecond pulsed deep-red Raman laser based on the Nd:YLF dual-crystal configuration

A highly powerful nanosecond pulsed deep-red laser was demonstrated by intracavity second-harmonic generation of an actively Q-switched Nd:YLF dual-crystal-based KGW Raman laser in a critically phase-matched lithium triborate (LBO) crystal. The first-Stokes fields at 1461 and 1490 nm driven by the 1314 nm fundamental laser were firstly produced by accessing the Raman shifts of 768 and 901 cm-1 in the KGW crystal, respectively, and thereafter converted to the deep-red emission lines at 731 and 745 nm by finely tuning the phase-matching angle of the LBO crystal and carefully realigning the resonator. Integrating the benefits of the Nd:YLF dual-crystal configuration and the meticulously designed L-shaped resonator, this deep-red laser system delivered the maximum average output powers of 5.2 and 7.6 W with the optical power conversion efficiencies approaching 6.3% and 9.2% under the optimal pulse repetition frequency of 4 kHz, respectively. The pulse durations of 6.7 and 5.5 ns were acquired with the peak powers up to approximately 190 and 350 kW, respectively, and the resultant beam qualities were determined to be near-diffraction-limited with M2 ≈ 1.5.

High-repetition-rate and high-beam-quality all-solid-state nanosecond pulsed deep-red Raman laser

We report on a high-repetition-rate and high-beam-quality all-solid-state nanosecond pulsed deep-red laser source by intracavity second harmonic generation of the actively Q-switched Nd:YVO4/KGW Raman laser. The polarization of the 1342 nm fundamental laser was aligned with the Ng and Nm axes of KGW crystal for accessing the eye-safe Raman lasers at 1496 and 1526 nm, respectively. With the aid of the elaborately designed V-shaped resonator and the composite Nd:YVO4 crystal, excellent mode matching and good thermal diffusion have been confirmed. Under an optimal pulse repetition frequency of 25 kHz, the average output powers of the Raman lasers at 1496 and 1526 nm were measured to be 3.7 and 4.9 W with the superior beam quality factor of M2 = 1.2, respectively. Subsequently, by incorporating a bismuth borate (BIBO) crystal, the deep-red laser source was able to lase separately two different spectral lines at 748 and 763 nm, yielding the maximum average output powers of 2.5 and 3.2 W with the pulse durations of 15.6 and 11.3 ns, respectively. The resulting beam quality was determined to be near-diffraction-limited with M2 = 1.28.

Ultrafast true-green Ho:ZBLAN fiber laser inspired by the TD3 AI algorithm

Ultrafast lasers in the true-green spectrum, which are scarce due to the "green gap" in semiconductor materials, are in high demand for the surging field of biomedical photonics. One ideal candidate for efficient green lasing is Ho:ZBLAN fiber, as ZBLAN-hosted fibers have already reached picosecond dissipative soliton resonance (DSR) in the yellow. When attempting to push the DSR mode locking further into the green, traditional manual cavity tuning is faced with extreme difficulty, as the emission regime for these fiber lasers is so deeply concealed. Breakthroughs in artificial intelligence (AI), however, provide the opportunity to fulfill the task in a fully automated manner. This work, inspired by the emerging twin delayed deep deterministic policy gradient (TD3) algorithm, represents the first application, to the best of our knowledge, of the TD3 AI algorithm to generate picosecond emissions at the unprecedented true-green wavelength of ∼545 nm. The study thus extends the ongoing AI technique further into the ultrafast photonics region.

Efficient UV-visible emission enabled by 532 nm CW excitation in an Ho3+-doped ZBLAN fiber

Rare-earth-doped ZBLAN (ZrF₄-BaF₂-LaF₃-AlF₃-NaF) fibers have evolved to become promising candidates for efficient UV-visible emission because of their low phonon energy and low optical losses, as well as their well-defined absorption bands. We investigate the efficient emission of UV-visible light in a low-concentration (0.1 mol%) Ho³⁺-doped ZBLAN fiber excited by a 532 nm CW laser. In addition to the direct populating of the thermalized ⁵F₄+⁵S₂ levels by ground-state absorption, the upconversion processes responsible for UV-visible emission from the higher emitting levels, ³P₁+³D₃, ³K₇+⁵G₄, ⁵G₅, and ⁵F₃, of the Ho³⁺ ions are examined using excited-state absorption. The dependence of UV-visible fluorescence intensity on launched green pump power is experimentally determined, confirming the one-photon and two-photon characters of the observed processes. We theoretically investigate the excitation power dependence of the population density for nine Ho³⁺ levels based on a rate equation model. This qualitative model has shown a good agreement with the measured power dependence of UV-visible emission. Moreover, the emission cross-sections for blue, green, red, and deep-red light in the visible region are measured using the Füchtbauer-Ladenburg method and corroborated by McCumber theory, and the corresponding gain coefficients are derived. We propose an alternative approach to achieve efficient UV-visible emission in an Ho³⁺-doped ZBLAN fiber using a cost-effective, high-brightness 532 nm laser.

Trimodal Ratiometric Luminescent Thermometer Covering Three Near-Infrared Transparency Windows

Near-infrared (NIR) transparency windows have evoked considerable interest in biomedical thermal imaging owing to the superior tissue penetration and the high signal-to-noise ratio, allowing in vivo real-time temperature reading with nanometric spatial resolution. Here, we develop a multimode nonintrusive luminescent thermometer based on the Y₃Al₅O₁₂ (YAG):Cr³⁺/Ln³⁺ (Ln = Ho, Er, Yb) phosphor, which covers three NIR biological transparency windows, enabling cross-checking readings with high sensitivity and a high penetration depth. Utilizing the energy transfer between lanthanide ions and transition-metal ions, the Cr³⁺/Ln³⁺-activated upconversion emissions provide ideal signals for ratiometric luminescent thermometry of the NIR-I mode. The phonon-assisted downshifting emissions of Er³⁺/Ho³⁺ are used to construct the NIR-III/II mode, and the NIR-III mode is based on the thermal coupling between stark levels of ⁴I_13/2 (Er³⁺). Three independent modes show distinct thermometric performance in different NIR transparency windows and temperature ranges, and the combination of the three modes is conducive to obtain more accurate temperature readings in a broad temperature range, which paves the way toward versatile luminescent thermometers.

High-efficiency broadband tunable green laser operation of direct diode-pumped holmium-doped fiber

Green laser sources have become increasingly important for the application in scientific research and industry. Although several laser approaches have been investigated, the development of green lasers with the necessary efficiency and spectral characteristics required for practical deployment continues to attract immense interest. In this study, the efficient green laser operation of a Ho³⁺-doped fluoride fiber directly pumped by a commercial blue laser diode (LD) is experimentally investigated at various active fiber lengths. In the free-running laser, the slope efficiency was optimized up to 59.3% with 543.9 nm lasing, with respect to the launched pump power, using a 20-cm long active fiber. This is the maximum slope efficiency reported to date for a green fiber laser. A maximum output power of 376 mW at 543.5 nm was achieved by using a 17-cm long active fiber pumped at a maximum available launched pump power of 996 mW. Moreover, broadband tuning operation was demonstrated by employing a range of active fiber lengths, together with an intracavity bandpass filter. The operating wavelength was tunable from 536.3 nm to 549.3 nm. A maximum tuning power achieved was 118 mW at 543.4 nm for a 17-cm long active fiber. Moderate Ho³⁺-doped fiber length is shown to be effective in producing a high performance of a green fiber laser. The short-length of the active fiber considerably extends the green short wavelength operation due to limited reabsorption of the signal below 540 nm.

Oxychloride glass with low phonon energy as a choice for infrared laser materials

Recently, rare-earth-doped infrared (IR) luminescent glasses have drawn massive attention due to their potential applications in military, medical, and communications fields. In this Letter, we present a system of oxychloride Si-Ge-O-Cl glasses, suitable for rare-earth doping, which has been developed as a new, to the best of our knowledge, choice for IR luminescent materials. Raman spectra show a looser glass network because of the decreased phonon energy and density compared to the one in heavy-metal oxide glasses. The enhanced luminescence from the visible to the IR region has been obtained with a beneficial fluorescence decay time. The spectroscopy results indicate that the system of Si-Ge-O-Cl glasses may be a promising candidate for application in infrared laser materials with enhanced luminescence.

Few-layer bismuthene for robust ultrafast photonics in C-Band optical communications

The ultrafast photonics of different conventional two-dimensional (2D) materials have been studied intensively. Few-layer structure bismuthene has been reported as a new type of 2D material with high efficient electronics, strong mechanics and outstanding photonics properties. In this paper, a robust ultrafast pulse generation in communications-Band (C-Band) based on few-layer bismuthene has been reported. The characteristics and the ultrafast optical nonlinear properties of few-layer bismuthene have been investigated experimentally. The optical induced deposition method is employed to fabricate the saturable absorber based on bismuthene (BiSA). Most importantly, we also utilize BiSA for the ultrafast photonics, which demonstrates that a high-splitting-threshold robust ultrafast fiber laser with 1.3-ps pulse duration at 1531 nm has been obtained in the experiments. Even though we increase the pump power from the self-starting threshold (i.e. 86 mW) to 314 mW, the soliton pulse does not split. Moreover, the high-splitting-threshold laser operation can be achieved stably even if the lasers are exposed in air for at least half a year. It is demonstrated that the proposed bismuthene nonlinear components can be potentially applied to the optical communications with C-Band (i.e. 1530-1565 nm wavelength) to broaden the communications window.

Enhancing the luminescence performance in germanosilicate glasses controlled by ZnF2

Rare-earth-doped optical functional glasses have attracted great interest for their excellent luminous performance in the applications of optical communications and biomedical systems. To the best of our knowledge, it is demonstrated for the first time that more than seven times' enhancement of luminescence performance in the mid-infrared region (MIR) has been obtained in germanosilicate glasses controlled by ZnF₂. Larger absorption and emission cross sections of the Ho³⁺:  I₆5→I₇5 transition indicate that this kind of germanosilicate-zinc glass may provide high gain as a good medium for an efficient 2.85 μm laser system.

Tailoring optical nonlinearities of LiNbO3 crystals by plasmonic silver nanoparticles for broadband saturable absorbers

We report on the synthesis of plasmonic Ag nanoparticles (NPs) embedded in a LiNbO₃ crystal (AgNP:LN) by ion implantation and its application as an efficient broadband saturable absorber (SA) to realize Q-switched pulsed laser generation at both visible and near-infrared wavelength bands. The nonlinear optical response of AgNP:LN is considered as a synergistic effect between Ag NPs and LiNbO₃. We apply the AgNP:LN as visible-near-infrared broadband saturable absorbers (SAs) into Pr:LuLiF₄ bulk and Nd:YVO₄ waveguide laser cavity, achieving efficient passively Q-switched laser at 639 nm and 1064 nm, respectively. This work paves a new way to tailor the nonlinear optical response of LiNbO₃ crystals by using plasmonic Ag NPs, manifesting the significant potential as broadband SAs in the aspect of pulsed lasing.