Near thermal noise limit, 5W single frequency fiber laser base on the ring cavity configuration.

In this study, we present an ultralow noise single-frequency fiber laser operating at 1550 nm, utilizing a traveling-wave ring cavity configuration. The frequency noise of the laser approaches the thermal noise limit, achieving a white noise level of 0.025 Hz2/Hz, resulting in an instantaneous linewidth of 0.08 Hz. After amplification, the output power reaches 4.94 W while maintaining the same low white noise level as the laser oscillator. The integration linewidths of the laser oscillator and amplifier are 221 Hz and 665 Hz, respectively, with both exhibiting relative intensity noises that approach the quantum shot noise limit. To the best of our knowledge, this work shows the lowest frequency noise combined with relatively high power for this type of ring cavity fiber laser.

Observation of 2†µm multiple annular structured vortex pulsed beams by cavity-mode tailoring.

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.

Widely-tunable all-fiber Tm doped MOPA with > 1†kW of output power.

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.

High power Ho:Y2O3 ceramic laser with controllable output intensity profile at 2.1 µm.

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.

2†µm cylindrical vector beam generation from a c-cut Tm:CaYAlO4 crystal resonator.

Different from the traditional ideal column symmetry cavities, we directly generated the cylindrical vector pulsed beams in the folded six-mirror cavity by employing a c-cut Tm:CaYAlO4 (Tm:CYA) crystal and SESAM. By adjusting the distance between the curved cavity mirror (M4) and the SESAM, both the radially polarized beam and azimuthally polarized beam are generated around 1962 nm and the two vectorial modes can be freely switched in the resonator. Further increased the pump power to 7 W, the stable radially polarized Q-switched mode-locked (QML) cylindrical vector beams were also obtained with an output power of 55 mW, the sub-pulse repetition rate of 120.42 MHz, pulse duration of ∼0.5 ns and the beam quality factor M2 of ∼2.9. To our knowledge, this is the first report of radially and azimuthally polarized beams in the 2 µm wavelength solid-state resonator.

Laser frequency stabilization based on Fano resonance in a microcylinder cavity.

We investigate the application of Fano resonance in microcylinder cavities for laser frequency stabilization. By combining Fano resonance and the differential subtraction method, we successfully reproduce the error signal of the traditional Pound-Drever-Hall (PDH) technique. The frequency noise of the laser, when locked to both microsphere and microcylinder cavities, approaches the thermal noise limit. The microcylinder cavity, with a high Q factor of ∼108, benefiting from its large mode volume, exhibits a significant reduction in frequency noise by one order of magnitude compared with a microsphere in the frequency range of 0.1 to 10 kHz, achieving a minimum noise of ∼2.25 Hz2/Hz at 10 kHz. As this approach eliminates the need for additional electronic circuits typically used in the PDH technique, it holds promise as a cost-effective and reliable solution for laser frequency stabilization.

High power and efficient operation of a Ho:Y2O3 ceramic laser with over 210 W of output power at 2.1 µm.

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.

Flexible chalcogenide glass large-core multimode fibers for hundred-watt-level mid-infrared 2-5 µm laser transmission.

The rapidly-developed high-power mid-infrared 2-5 µm laser technology requires a compact, flexible low-loss glass fiber for power delivery or laser generation. With the broadest bandwidth of low-loss transmission window in mid-infrared region amongst all mid-infrared glass fibers, chalcogenide glass fiber is the best candidate covering the whole 2-5 µm range. Multi-hundred-watt high-power delivery for 5.4-µm CO laser was previously demonstrated in a multimode chalcogenide fiber with a 1-mm-diameter large core, at the cost of giving up one of the most desirable fiber advantages, the flexibility. Indeed, chalcogenide glass fibers with decent flexibility have never exhibited hundred-watt-level power transmitting capability in the 2-5 µm range. In this paper, we have experimentally demonstrated 100-watt-level power transmission in multimode As2S3 chalcogenide fibers, using a customized high-power 2-µm thulium doped silica fiber laser source. With effective forced cooling, the multimode As2S3 fiber with 200 µm core diameter can resist incident laser power of 120 W and deliver transmitted power of 63 W. Nano-sized scattering center related laser damage mechanism and the cylindrical heat transfer model have been proposed to explain the high-power damage process of chalcogenide glass fibers. The calculation is in good agreement with the experiments. It is promising to further enhance the transmitted power above 100 W in flexible chalcogenide glass large-core fibers.

SESAM mode-locked Tm:Y2O3 ceramic laser.

We demonstrate a widely tunable and passively mode-locked Tm:Y2O3 ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y2O3 laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y2O3 transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

High-power 1640†nm Er:Y2O3 ceramic laser at room temperature.

We report on the high-power operation of an Er:Y2O3 ceramic laser at approximately 1.6 µm using a low-scattering-loss, 0.25 at. % Er3+-doped ceramic sample fabricated in-house via a co-precipitation process. The laser is in-band pumped by an Er, Yb fiber laser at 1535.6 nm and generates 10.2 W of continuous-wave (CW) output power at 1640.4 nm with a slope efficiency of 25% with respect to the absorbed pump power. To the best of our knowledge, this is the first demonstration of an approximately 1.6 µm Er:Y2O3 laser at room temperature. The prospects for further scaling of the output power and lasing efficiency via low Er3+ doping and reduced energy-transfer upconversion are discussed.

Single longitudinal mode lasing near the exceptional point in a fiber laser using a tunable isolator.

Parity time symmetry breaking was obtained in a specially designed fiber ring laser with a homemade tunable isolator in the cavity. The dynamic evolution of the cavity eigenmodes around the exceptional point (EP) was further experimentally studied. We showed that operating the laser near the EP can facilitate single longitudinal mode lasing. A single-frequency fiber laser with a linewidth of 163 Hz was first, to the best of our knowledge, demonstrated near the EP of the cavity without using any filter with a narrow bandwidth.

Narrow linewidth self-injection locked fiber laser based on a crystalline resonator in add-drop configuration.

Strong and narrow spectral feedback plays a key role in self-injection locking (SIL) single-frequency lasers, especially in the stabilization of single longitudinal mode (SLM) operation of a multi-mode laser. Here, we report on a narrow linewidth SIL fiber laser that adopts a fiber add-drop configuration composing of two tapered fibers and a high-Q MgF2 crystalline whispering-gallery-mode resonator. The feedback from the drop port could be controlled and optimized for stable SLM lasing of multi-mode fiber lasers. A stable single-frequency fiber laser with white frequency noise as low as ∼0.4 Hz2/Hz, corresponding to an instantaneous linewidth of ∼1.26 Hz, is demonstrated. Compact, controllable, and all-fiber configuration in this work to achieve an ultra-narrow linewidth laser will attract interest in many applications.

Direct generation of ultrafast vortex beam from a Tm:CaYAlO4 oscillator featuring pattern matching of a folded-cavity resonator.

Optical vortices, beams with spiral wavefronts and screw phase dislocations have been explored in applications in optical manipulation, quantum optics, and the next generation of optical communications. In traditional methods, optical vortices are generated using space light modulators or spiral phase plates, which would sharply decrease the integration of optical systems. Different from previous transverse mode conversion outside the cavity, here we experimentally demonstrate a direct generation of ultrafast vortex beam from a Tm:CaYAlO4 oscillator by pattern matching of a six-mirror-folded-cavity resonator. By accurately adjusted the angle of the end mirror and the distance L between the M4 and the SESAMs to control the beam diameter of laser incidence on the gain medium in the sagittal and tangential planes, a stable 2 µm ultrafast vortex laser emission of annular Laguerre-Gaussian (LG) mode was obtained with a maximum output power of 327 mW and pulse duration of 2.1 ps. A simple YAG crystal plate was used as handedness selector and a homemade Mach-Zehnder (MZ) interferometer has verified the vortical property of the LG01 mode. By furtherly controlling the cavity mode pattern matching, other stable transverse-mode operations for TEM00, high-order Hermite-Gaussian (HG) transverse mode and doughnut-shaped beams were successfully realized. This work provides a flexible and reliable way to generate mid-infrared ultrafast vortex beams and is of special significance for applications in the areas of molecular spectroscopy and organic material processing amongst others.

Sub-five-optical-cycle pulse generation from a Kerr-lens mode-locked Yb:CaYAlO4 laser.

Direct generation of ultrashort few-optical-cycle pulses in various wavelength regions has attracted great attention in recent decades. In this paper, generation of less than five-optical-cycle pulses from a Kerr-lens mode-locked ${\rm Yb}{:}{\rm CaYAlO}_4$ laser is demonstrated. Pumped by a 976 nm fiber laser, stable near-Fourier-transform-limited ultrashort soliton pulses centered around 1080 nm with a repetition rate of ${\sim}{113.7}\;{\rm MHz}$ were obtained. The obtained pulses have a pulse duration as short as 17 fs if a ${{\rm Sech}^2}$-shaped pulse profile is assumed, corresponding to about 4.68 optical cycles. To the best of our knowledge, this is the shortest pulse directly generated from mode-locked rare-earth-doped solid-state oscillators.

High-peak-power 786†nm and 452†nm lasers based on 1064†nm intracavity-driven cascaded nonlinear optical frequency conversion.

We demonstrated high-peak-power 786 nm and 452 nm lasers based on 1064 nm intracavity-driven cascaded nonlinear optical frequency conversion (CNOFC). The 1064 nm fundamental wave generated from the LD-side-pumped Nd:YAG was first intracavity converted to 1572 nm by a noncritically phase-matched KTP OPO. Then a LBO-based second harmonic generation of 1572 nm was served as cascaded process to produce 786 nm laser radiation. The maximum average output power at 786 nm was 1.34 W, corresponding to a pulse peak power of 14.2 kW with 11.2 ns pulse width and 8 kHz pulse repetition rate. Furthermore, a third stage of sum frequency mixing between 786 nm and 1064 nm was designed to achieve the blue emission at 452 nm. The 452 nm blue laser delivers 263 mW, 6.2 ns pulses with a peak power of 5.3 kW, paving the way for achieving high-peak-power blue lasers.

Period doubling eigenstates in a fiber laser mode-locked by nonlinear polarization rotation.

Due to the weak birefringence of single mode fibers, solitons generated in fiber lasers are indeed vector pulses and exhibit periodic parameter change including polarization evolution even when there is a polarizer inside the cavity. Period doubling eigenstates of solitons generated in a fiber laser mode-locked by the nonlinear polarization rotation, i.e., period doubling of polarization components of the soliton, are numerically explored in detail. We found that, apart from the synchronous evolution between the two polarization components, there exists asynchronous development depending on the detailed operation conditions. In addition, period doubling of one polarization component together with period-one of another polarization component can be achieved. When the period tripling window is obtained, much complexed dynamics on the two polarization components could be observed.

Few-moded ultralarge mode area chalcogenide photonic crystal fiber for mid-infrared high power applications.

We demonstrate a novel few-moded ultralarge mode area chalcogenide glass photonic crystal fiber for mid-infrared high power applications. The numerical simulation indicates that the fiber has ultralarge mode areas of ∼10500 µm2 and ∼12000 µm2 for the fundamental mode LP01 and the lowest higher-order mode LP11, respectively. Dual-moded operation is confirmed experimentally at 2 µm, in good agreement with the numerical simulation. By selectively launching technique, low bending loss of 0.7 dB/m, equivalent to 0.55 dB/turn, has been observed in the fiber with a small bending radius of ∼12 cm, indicating excellent bending resistance of the few-moded fiber with such a large mode area. The fiber has been demonstrated to sustain an incident power density up to 150 kW/cm2 under 2-µm CW laser irradiation, showing the potential of the fiber for high-power applications in mid-infrared.

High-energy 2 µm pulsed vortex beam excitation from a Q-switched Tm:LuYAG laser.

We report on the first, to the best of our knowledge, direct generation of pulsed vortex beams at 2 µm from a ${ Q}$Q-switched Tm:LuYAG laser. High-energy Laguerre-Gaussian (${{\rm LG}_{0,l}}$LG0,l) pulsed laser beams with well-defined handedness are selectively excited through spatially matched pump gain distribution and asymmetric cavity loss without using any intracavity handedness-selective optical elements. Pulse energies of 1.48 mJ for the ${{\rm LG}_{0, + 1}}$LG0,+1 mode and 1.51 mJ for the ${{\rm LG}_{0, - 1}}$LG0,-1 mode, respectively, are achieved at a repetition rate of 500 Hz. The pulsed laser beams with helical wavefronts are potentially useful for studying orbital angular momentum transformation dynamics, generation of mid-IR vortex beams, and nanostructuring of organic materials.

SESAM mode-locked Tm:LuYO3 ceramic laser generating 54-fs pulses at 2048 nm.

Mode-locked laser operation near 2.05 µm based on a mixed sesquioxide Tm:LuYO3 ceramic is demonstrated. Continuous-wave and wavelength-tunable operation is also investigated. Employing a GaSb-based semiconductor saturable absorber mirror as a saturable absorber, a maximum average output power of 133 mW is obtained for a pulse duration of 59 fs. Pulses as short as 54 fs, i.e., eight optical cycles are generated at a repetition rate of ∼78MHz with an average output power of 51 mW. To the best of our knowledge, this result represents the shortest pulse duration ever achieved from Tm-based solid-state mode-locked lasers.

Gain-switched laser diode seeded TDFA with 409 W picosecond pulses and 142 W spectrally flat supercontinuum output.

We report on a high-power picosecond all-fiber Tm-doped fiber amplifier (TDFA) seeded by a gain-switched laser diode (LD) in the 2 µm spectral range. A total average output power of 409 W (304 ps) has been generated at 320 MHz of repetition rate with 10 dB bandwidth of ~48 nm centered at 1970 nm. Over 140 W of spectrally flat supercontinuum (SC) output has been produced at 40 MHz of repetition rate with optimized fiber length. The 10 dB spectral bandwidth was 615 nm, ranging from 1965 to 2580 nm. The prospects for further power scaling of LD seeded ~2 µm picosecond all-fiber sources are discussed.