Saturable Absorber For Passive Q-switching Research Articles

The impact of yttrium aluminum garnet stoichiometry deviation on the conversion efficiency of tetravalent chromium ions

In this work, the conversion efficiency of Cr4+ ions and the optical properties of ceramics based on chromium-doped yttrium aluminum garnet were investigated. Increasing the conversion efficiency of Cr3+ to Cr4+ opens up broad prospects for using YAG ceramics as saturable absorbers for passive Q-switching of Nd- and Yb-solid-state lasers. The aim of this work was to study the effect of magnesium oxide concentration on the conversion efficiency of Cr3+ to Cr4+ under conditions of Al3+ (4.8 mol.%) and Y3+ (2.9 mol.%) cations excess in the garnet structure as well as the stoichiometric ratio Y3+/Al3+ = 3/5. The possibility of changing the concentration of Cr4+ ions in both the octahedral and tetrahedral positions due to the formation of magnesium substitution defects when the composition deviates from the yttrium-aluminum garnet stoichiometry has been studied. Chromium-doped transparent optical ceramics based on yttrium-aluminum garnet with different ratios of Y3+/Al3+ cations was obtained by the two-stage coprecipitation method. Magnesium oxide was used as a sintering additive and charge compensator for converting chromium ions from the trivalent to the tetravalent state in concentrations of 0 to 0.2 wt.%. It was found that aluminum excess in the yttrium-aluminum garnet matrix leads to a decrease in the Cr3+ to Cr4+ conversion efficiency. A deviation of stoichiometry towards an excess of yttrium leads to a decrease of air annealing temperature for oxidizing chromium ions to the tetravalent state and their transition to the tetrahedral position. However, the samples optical transmittance with an excess of yttrium was lower than in the cases of an excess of aluminum and stoichiometry. It was found that with an increase in the vacuum sintering temperature from 1780 to 1820 °C in the samples with an excess of aluminum, an increase in the concentration of Cr4+ ions in the tetrahedral position occurs, as evidenced by higher absorption intensity at a wavelength of 1030 nm. For the cases of stoichiometry and an excess of yttrium in garnet, this effect is less pronounced. Samples of optical ceramics of yttrium-aluminum garnet with a light transmission of more than 75 % in the visible and near-IR ranges with pronounced absorption bands of chromium have been obtained. The results of the work can be applied to the creation of passive switches for solid-state lasers.

MXene/Graphene Oxide Heterojunction as a Saturable Absorber for Passively Q-Switched Solid-State Pulse Lasers.

Owing to their unique characteristics, two-dimensional (2-D) materials and their complexes have become very attractive in photoelectric applications. Two-dimensional heterojunctions, as novel 2-D complex materials, have drawn much attention in recent years. Herein, we propose a 2-D heterojunction composed of MXene (Ti2CTx) materials and graphene oxide (GO), and apply it to an Nd:YAG solid-state laser as a saturable absorber (SA) for passive Q-switching. Our results suggest that a nano-heterojunction between MXene and GO was achieved based on morphological characterization, and the advantages of a broadband response, higher stability in GO, and strong interaction with light waves in MXene could be combined. In the passively Q-switched laser study, the single-pulse energy was measured to be approximately 0.79 µJ when the pump power was 3.72 W, and the corresponding peak power was approximately 7.25 W. In addition, the generation of a stable ultrashort pulse down to 109 ns was demonstrated, which is the narrowest pulse among Q-switched solid-state lasers using a 2-D heterojunction SA. Our work indicates that the MXene–GO nano-heterojunction could operate as a promising SA for ultrafast systems with ultrahigh pulse energy and ultranarrow pulse duration. We believe that this work opens up a new approach to designing 2-D heterojunctions and provides insight into the formation of new 2-D materials with desirable photonic properties.

Fabrication of Highly Transparent Co:MgAl2O4Ceramic Saturable Absorber for Passive Q-switching in 1.5μm

Fabrication of Highly Transparent Co:MgAl₂O₄Ceramic Saturable Absorber for Passive Q-switching in 1.5μm

Transition-metal-doped saturable absorbers for passive Q-switching of visible lasers

We experimentally investigate crystals doped with tetravalent chromium or divalent cobalt as saturable absorbers for passive Q-switching of visible solid-state lasers. The recovery time of the ground-state and excited-state absorption cross sections are determined by pump-probe and Z-scan measurements, respectively. We provide saturation intensities, useful wavelength ranges of the investigated materials, and advices to realize passive Q-switching of visible lasers using these crystals as saturable absorbers.

Graphitic Carbon Nitride Q-Switched Tm:CNNGG Laser at 2 Micrometer Wavelength Region

The Q-switching performance of graphitic carbon nitride (g-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) based on saturated absorption is demonstrated in 2 micrometer wavelength region for the first time. The g-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> saturable absorbers, processed by thermal oxidation etching, are employed for passive Q-switching in a Tm:CNNGG laser. With a 5 hours long thermal oxidation etched g-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> saturable absorber, the Q-switched Tm:CNNGG laser generates a shortest pulse duration of 132 ns with a repetition frequency of 16 kHz and a peak power of 98.5 W. The experimental results indicate that g-C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> is an excellent saturable absorber for passive Q-switching in 2 micrometer wavelength region.

Hybrid organic small molecules as a saturable absorber for passive Q-switching in erbium-doped fiber laser

This work demonstrates a Q-switched fiber laser by utilizing hybrid organic small molecules (HOSM) based on Tris-(8-hydroxyquinoline) aluminum (Alq3) and N,N′-Di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as a saturable absorber. The HOSM was embedded into poly(vinyl alcohol) and inserted into a fiber laser cavity to achieve pulsing at 1560.1 nm. The pulse repetition rate was tuned from 66.3 kHz to 109 kHz whereas the pulse width duration decreased from 6.3 µs to 2.2 µs as the laser diode power gradually increased from 56 to 262 mW. Then a tunable bandpass filter (TBF) was used to produce tunable wavelength operation. As the TBF was tuned, the wavelength of the Q-switched laser shifted continuously from 1519.6 nm to 1562.8 nm. The results show that HOSM could be an efficient, easy to fabricate, and inexpensive saturable absorber for generating single and tunable wavelength Q-switched fiber laser.

Bismuth functionalized GaAs as saturable absorber for passive Q-switching at 1.34 μm

First principles simulations showed bismuth doped GaAs could modify the electronic band structure. Hereby, Bi:GaAs exhibited excellent nonlinear absorption properties at 1.3 μm with a modulation depth of 6.2%, larger than that of pure GaAs. Subsequently, the passively Q-switched Nd:YVO4 lasers were demonstrated at 1.34 μm with Bi:GaAs and GaAs the saturable absorbers. Stable pulses with the shortest duration of 64 ns at a repetition rate of 138 kHz were generated with Bi:GaAs saturable absorber. In the meantime, we also observed Bi ion doping accelerates the free-carrier recovery, indicating Bi:GaAs excellent candidant for 1.3 μm pulse generation.

Ti2AlC-based saturable absorber for passive Q-switching of a fiber laser

We report the feasibility of a MAX-phase material for implementation as a saturable absorber. Our saturable absorber was fabricated as a composite of Ti2AlC particles and polyvinyl alcohol (PVA) using a sandwich-structured fiber-ferrule platform. The saturation intensity and modulation depth of the prepared SA were measured at ∼31.5 MW/cm2 and ∼6.3%, respectively. Using the Ti2AlC/PVA composite-based SA within an erbium-doped fiber laser ring cavity, stable Q-switched pulses were readily obtained at a wavelength of 1.56 µm. This experimental demonstration unveils the potential of micrometer-sized MAX-phase particles for implementation as low-cost, practical, saturable absorbers.

Application of few-layer WSe2/Sapphire in passively Q-switched solid-state lasers as saturable absorber and output coupler

We demonstrate that it is possible for a few-layer WSe2/Sapphire to act in specific high-gain solid-state lasers as both saturable absorber for passive Q-switching and as etalon mirror for output coupling. A passively Q-switched Yb:LuPO4 microchip laser formed in this manner is realized. With 11.5 W of pump power absorbed, a maximum output power of 1.8 W is produced at a pulse repetition rate of 1.2 MHz with a slope efficiency of 25%; the resulting pulse energy, duration, and peak power are, respectively, 1.5 μJ, 30 ns, and 50 W. Our work provides a simple way of making compact, reliable, high-repetition-rate pulsed microchip or miniature solid-state lasers based on 2D saturable absorbers.

Few-layer Bi2Te3: an effective 2D saturable absorber for passive Q-switching of compact solid-state lasers in the 1-μm region.

An experimental investigation was carried out to evaluate the potential of few-layer Bi2Te3 topological insulator in use as a saturable absorber for passive Q-switching of compact solid-state lasers in the 1-μm spectral region. By incorporating a sapphire-based few-layer Bi2Te3 sample into a Yb:LuPO4 laser that was formed with a 4-mm plane-parallel resonator, we realized efficient, high-power, high-repetition-rate pulsed laser operation. Depending on the output coupling utilized, single- or dual-wavelength laser action could be achieved. A maximum output power of 5.02 W at 1014.5 nm was produced at a pulse repetition rate of 1.67 MHz, with an optical-to-optical efficiency of 41% and a slope efficiency of 54%; while operating at 1004.9/1012.7 nm, the pulsed laser could produce an output power of 3.94 W at 1.38 MHz, with a pulse duration being as short as 34 ns. The largest pulse energy and highest peak power achieved were 3.0 μJ and 85.3 W. The results demonstrated in our experiment reveal the great potential of the few-layer Bi2Te3 topological insulator in the development of pulsed compact solid-state lasers in the 1-μm region.

MoS_2 saturable absorber for passive Q-switching of Yb and Tm microchip lasers

A multi-layer MoS2-based saturable absorber (SA) enables Q-switched operation of Yb and Tm microchip lasers. The saturation intensity of MoS2-SA is determined to be about ~0.5 MW/cm2. The Q-switched Tm laser generated 1.27 W at 1929 nm with a slope efficiency of 43% and conversion efficiency with respect to the continuous-wave mode as high as 81%. The pulse characteristics were 175 ns / 7.5 µJ corresponding to a repetition rate of 170 kHz. With the Q-switched Yb laser, 220 ns / 0.5 µJ pulses were achieved at 1030 nm. We show that using MoS2-SA at ~2 μm is advantageous as compared to multi-layer graphene. We also prove that MoS2-SA is more efficient at ~2 μm compared to ~1 μm due to a lower saturation intensity and higher fraction of the saturable loss.

Passive Q-switching laser performance of Yb:Gd3Ga5O12 garnet crystal

We report on the passive Q-switching laser performance of the Yb:Gd3Ga5O12 garnet crystal. With a Cr4+:YAG crystal used as saturable absorber for passive Q-switching, an average output power of 5.31 W was produced at a pulse repetition rate of 62.5 kHz, the optical-to-optical and slope efficiencies being, respectively, 45% and 61%. Laser pulses of 140 μJ in energy and 5.8 ns in duration were also obtained at 22.2 kHz, with a corresponding peak power amounting to 24.1 kW.

Diode-pumped Nd:YVO4 laser passively Q-switched with graphene oxide spin coated on ITO substrate

Two graphene oxide saturable absorbers (GO-SAs) spin coated on indium tin oxide (ITO) substrate have been prepared and their transmissivity spectra have also been measured. The performance of the diode-pumped passively Q-switched Nd:YVO4 laser with GO-SA has been investigated for the first time. The shortest pulse width of 113 ns and the highest peak power of 4.3 W are obtained at the pump power of 5.04 W. The output laser characteristics have demonstrated that graphene oxide is a promising saturable absorber for passive Q-switching.

Compact Passively Q-Switched Yb:YAG Multipass Ring Lasers

An efficient and compact continuous-wave Yb:YAG laser with a slope efficiency of 50.3% with respect to the absorbed power was demonstrated using a multipass ring cavity (MPRC). Large cavity length ranges can be implemented using MPRCs with the same optical elements but different beam configurations. When Cr:YAG was used as the saturable absorber for passive Q-switching, various pulse widths and repetition rates were obtained. When a two-mirror MPRC, which is a compact ring laser cavity was used, the spatial hole-burning effect was reduced, and the spontaneous noise from the gain medium was not directly incident on the saturable absorber, causing less perturbation of the population difference of the saturable absorber than that of a linear cavity. This indicates that MPRCs can reduce the time jitter problem of a passively Q-switched laser. In this study, a Yb:YAG MPRC laser with a 33 ns pulse width is demonstrated, and the timing jitter of the repetition period was restrained to around 11%. The obtained experimental results are in good agreement with simulation results.

연속 발진 다이오드 레이저로 여기된 수동형 Q-스위치 Nd:YVO4레이저의 출력 펄스 안정화

Cr:YAG 결정을 포화흡수체로 사용하여 수동형 Q-스위치시킨 다이오드 레이저 여기 Nd:<TEX>$YVO_4$</TEX> 레이저를 제작하였다. Cr:YAG 결정의 한 면에 여기 빛 파장(808 nm)에 대해 고반사 코팅을 한 후, Nd:<TEX>$YVO_4$</TEX> 결정과 접촉시켜 공진기를 구성하였다. Cr:YAG의 한 면에 여기 다이오드 레이저를 반사시키는 코팅을 입혀 공진기의 광변환 효율을 높임과 동시에, 여기빔에 의한 포화흡수체의 표백현상을 방지하여 안정된 출력이 나오게 하였다. 레이저 이득 매질 및 포화흡수체의 온도는 열전 냉각기와 냉각수를 사용하여 안정화 시켰다. 온도안정화를 시킨 상태에서 발진되는 펄스의 첨두 출력 요동(peak to peak)은 4%였다. 다이오드 레이저의 출력이 1 W일 때, 출력 펄스의 반복률은 평균 9 KHz 였으며, 최소 펄스폭은 7.11 ns, 최고 출력은 16.27 mW였다. A Cr:YAG crystal was used as a saturable absorber for passive Q-switching of a Nd:<TEX>$YVO_4$</TEX> laser which was pumped by a 1-W continuous wave laser diode. The first surface of the Cr:YAG was high-reflection coated for the pump wavelength. The high-reflection coating improved the absorption efficiency of the pump beam in the Nd:<TEX>$YVO_4$</TEX> through double pass absorption. It also prevented pump beam induced partial bleaching of the Cr:YAG. The peak-to-peak pulse fluctuation of passively Q-switched laser output was approximately 4 %. The minimum pulse-width was measured to be 7.11 ns. Also, the average pulse repetition rate was 9 kHz and the maximum output power was 16.27 mW.

Mode-locked diode-pumped vanadate lasers operated with PbS quantum dots

The use of glasses doped with PbS nanocrystals as intracavity saturable absorbers for passive Q-switching and mode locking of c-cut Nd:Gd0.7Y0.3VO4, Nd:YVO4, and Nd:GdVO4 lasers is investigated. Q-switching yields pulses as short as 35 ns with an average output power of 435 mW at a repetition rate of 6–12 kHz at a pump power of 5–6 W. Mode locking through a combination of PbS nanocrystals and a Kerr lens results in 1.4 ps long pulses with an average output power of 255 mW at a repetition rate of 100 MHz.

Dual-polarization and dual-wavelength diode-pumped laser operation from a birefringent Yb3+-doped GdAl3(BO3)4 nonlinear crystal

From two positions of the concave output mirror of the laser cavity, diode-pumped ordinary or extraordinary polarization lasing of an Yb3+ -doped GdAl3(BO3)(4) nonlinear birefringent crystal is obtained. Translating the mirror at an intermediate position, simultaneous ordinary and extraordinary polarization lasing at 1046 (2.5 nm FWHM) and 1040 nm (2.2 nm FWHM), respectively, is obtained with up to 53% slope efficiency. Using a YAG:Cr4+ saturable absorber for passive Q-switching the cavity, ordinary and extraordinary pulses are obtained with an energy as high as 236 mu J and with about 30 ns duration. The two pulses of orthogonal polarizations can be separated by up to a few hundred ns, depending on the experimental conditions. The simultaneity of the pulses is observed but is difficult to maintain. A model constituted by two coupled equations reproduces reasonably well the experimental data. Repetition rates up to 3 kHz have been explored, corresponding to 4.65 W absorbed pump power.

Saturable absorbers for passive Q-switching of erbium lasers emitting in the region of 3 μm

The phototropic properties of Fe:ZnSe, Co:ZnSe, and Co:ZnS single crystals have been investigated. It is shown that these crystals can be used to advantage as the saturable absorbers in solid-state erbium lasers emitting in the region of the 3-μm range. The absorption cross sections of the ground states of the Co2+ ion in the ZnSe (σGSA = 11·10−20 cm2) and ZnS (σGSA = 5.6·10−20 cm2) crystals and of the Fe2+ ion in the ZnSe (σGSA = 50·10−20 cm2) crystal at λ = 2.79 μm were determined experimentally. It has been established that the above-indicated crystals in the excited state absorb light weakly. The use of these crystals as passive Q switches made it possible to realize, for the first time, the regime of Q-switching of a Cr,Er:YSGG laser emitting at a wavelength of 2.79 μm. Single pulses with an energy of 60 mJ and a duration of 170 nsec were obtained in the regime of passive Q-switching.