Μm Laser Pulses Research Articles

β-Ga2O3 Used as a Saturable Sbsorber to Realize Passively Q-Switched Laser Output

β-Ga2O3 crystals have attracted great attention in the fields of photonics and photoelectronics because of their ultrawide band gap and high thermal conductivity. Here, a pure β-Ga2O3 crystal was successfully grown by the optical floating zone (OFZ) method, and was used as a saturable absorber to realize a passively Q-switched all-solid-state 1 μm laser for the first time. By placing the as-grown β-Ga2O3 crystal into the resonator of the Nd:GYAP solid-state laser, Q-switched pulses at the center wavelength of 1080.4 nm are generated under a output coupling of 10%. The maximum output power is 191.5 mW, while the shortest pulse width is 606.54 ns, and the maximum repetition frequency is 344.06 kHz. The maximum pulse energy and peak power are 0.567 μJ and 0.93 W, respectively. Our experimental results show that the β-Ga2O3 crystal has great potential in the development of an all-solid-state 1 μm pulsed laser.

All-fiber wavelength-widely tunable multi-pulse gain-switched Tm3+-doped double-clad fiber laser

We report a wavelength-widely tunable multi-pulse gain-switched Tm3+-doped fiber laser. The laser has a continuous wavelength-tunable range of 105 nm in 2 μm band. The combination of Tm3+-doped double clad fiber and 793 nm pulsed pump source achieves large-energy pulse and multi-pulse operation. The single-pulse energy can reach 4.3 μJ and the multi-pulse energy can be 13.2 μJ. The effects of pump pulse energy, pulse repetition rate and pulse duration on multi-pulse generation are studied in detail. This easy-to-operate large-energy 2 μm pulsed fiber laser with adjustable pulse duration and repetition rate can provide a stable single-pulse or multi-pulse source in spectroscopy, medical surgery, material processing, etc.

Mid-infrared cascaded stimulated Raman scattering and flat supercontinuum generation in an As-S optical fiber pump at 2 µm.

We demonstrate broadband mid-infrared cascaded stimulated Raman scattering (SRS) and flat supercontinuum (SC) generation in a chalcogenide optical fiber made from As2S5 glass. By using a 2 µm nanosecond laser as the pump source, mid-infrared cascaded SRS up to six orders ranging from 2149 to 3425 nm was experimentally observed, and this all-fiber Raman laser operating at 3.43 µm was realized for the first time to our knowledge. By introducing a 2 µm femtosecond laser as the excited source, the broadband flat mid-infrared SC with the spectral range of ∼10dB (from ∼1030 to 3441 nm) was observed. Our results verify that the As2S5 optical fibers possess promising applications for tunable mid-infrared Raman fiber lasers and SC light sources pumped by 2 µm pulsed lasers.

Highly efficient optical fiber sensor for instantaneous measurement of elevated temperature in dental hard tissues irradiated with an Nd:YaG laser.

In this in vitro experiment, the effect of 1.064 µm pulsed laser on both enamel- and dentin-dental tissues has been investigated. A total of fifty-five dental hard tissue samples were exposed to Nd:YAG laser that possesses a pulse width of 9 ns and 850 mJ of total energy. An optical fiber sensor was put behind the samples to measure the temperature instantaneously. A novel, to the best of our knowledge, fiber sensor has been proposed and used to measure the heat generated in dental hard tissues instantaneously after the application of laser irradiation on the tissue surface. This optical sensor exhibits a fast response time of about 1 ms and high sensitivity with about 1.975 nm/°C. The findings of this study in decreasing the probability of pulpal necrosis structure while handling the tooth, whether for ablation, welding, or tooth resurfacing purposes, may establish standards for dentists and laser manufacturers (healthcare professionals) that should be followed.

Characterization of angularly resolved EUV emission from 2-µm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 µm-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 µm) pulse creates the EUV-emitting plasma. Irradiating 30- to 300 µm-diameter targets with 2 µm laser pulses, we find that the efficiency in creating EUV light around 13.5 nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2 µm drive laser intensity (0.6–1.8 × 1011 W cm−2) and pulse duration (3.7–7.4 ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5 nm and within the backward 2π hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 µm drive laser. With increasing target diameter, the emission in this 2% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1 µm-wavelength drive laser. Emission spectra, recorded in a 5.5–25.5 nm wavelength range, show significant self-absorption of light around 13.5 nm in the 1 µm case, while in the 2 µm case only an opacity-related broadening of the spectral feature at 13.5 nm is observed. This work demonstrates the enhanced capabilities and performance of 2 µm-driven plasmas produced from disk targets when compared to 1 µm-driven plasmas, providing strong motivation for the use of 2 µm lasers as drive lasers in future high-power sources of EUV light.

935 nm-diode-pumped passively Q-switched Er:Yb:Sr3Gd2(BO3)4 pulse laser at 1.5–1.6 μm

Eye-safe 1.5–1.6 μm pulse laser passively Q-switched by a Co2+:MgAl2O4 crystal was realized in a Z-cut, 1.8 mm-thick Er:Yb:Sr3Gd2(BO3)4 crystal end-pumped by a 935 nm laser diode. A 1534 nm pulse laser with energy of 9.1 μJ, repetition frequency of 15.6 kHz, and duration of 29 ns was obtained at an absorbed pump power of 2.5 W. Polarization of the output laser beam was measured to be linear and parallel to the Y optical indicatrix axis. Benefited from a broad and flat absorption band between 920 and 950 nm, the Er:Yb:Sr3Gd2(BO3)4 crystal may be a good gain medium for 1.5–1.6 μm laser pumped by a common commercial diode laser around 940 nm.

Parametric oscillation of acoustical phonon mode in magnetized doped III–V semiconductors

Using the hydrodynamic model of semiconductor plasmas and coupled mode theory of interacting waves, we develop a theoretical formulation followed by numerical analysis to study parametric oscillation of acoustical phonon mode in magnetized doped III–V semiconductors. The origin of nonlinear interaction is assumed to lie in effective second-order optical susceptibility ( $$\chi^{(2)}$$ ) arising due to nonlinear induced current density and acousto-optic polarization of the semiconductor medium. Expressions are obtained for threshold pump intensity ( $$I_{{0,{\text{th}}}}$$ ) and operational characteristics such as conversion efficiency ( $$\eta_{k}$$ ) and single pass signal power gain ( $$g_{{{\text{sp}}}}$$ ) of the proposed single resonant optical parametric oscillator. Numerical analysis is made for a representative n-InSb crystal irradiated by 10.6 µm pulsed CO2 laser. The effects of some important parameters including external magnetic field ( $$B_{0}$$ ), mirror reflectivity ( $$R$$ ), crystal length ( $$L$$ ), etc. on $$I_{{0,{\text{th}}}}$$ , $$\eta_{k}$$ and $$g_{{{\text{sp}}}}$$ of the single resonant optical parametric oscillator are analyzed in detail. The analysis establishes the technological potentiality of magnetized doped III–V semiconductors as the hosts for fabrication of highly efficient and widely tunable single-resonant optical parametric oscillators.

Application of Hollow-Core Photonic Crystal Fibers in Gas Raman Lasers Operating at 1.7 μm

A 1.7 μm pulsed laser plays an important role in bioimaging, gas detection, and so on. Fiber gas Raman lasers (FGRLs) based on hollow-core photonic crystal fibers (HC-PCFs) provide a novel and effective method for fiber lasers operating at 1.7 μm. Compared with traditional methods, FGRLs have more advantages in generating high-power 1.7 μm pulsed lasers. This paper reviews the studies of 1.7 μm FGRLs, briefly describes the principle and characteristics of HC-PCFs and gas-stimulated Raman scattering (SRS), and systematical characterizes 1.7 μm FGRLs in aspects of output spectral coverage, power-limiting factors, and a theoretical model. When the fiber length and pump power are constant, a relatively high gas pressure and appropriate pump peak power are the key to achieving high-power 1.7 μm Raman output. Furthermore, the development direction of 1.7 μm FGRLs is also explored.

Using Temporally Synthesized Laser Pulses to Enhance the Conversion Efficiency of Sn Plasmas for EUV Lithography

We have studied the laser pulse shape dependence of the conversion efficiency of λ = 1.03 μm laser pulse energy into 13.5 nm extreme ultraviolet (EUV) emission from a Sn laser-produced plasma. Laser pulses of arbitrary temporal shape ranging from hundreds of picoseconds to several nanoseconds were generated using a programmable pulse synthesizer based on a diode-pumped chirped pulse amplification Yb: YAG laser. Measurements show that the conversion efficiency favors the use of nearly square pulses of duration longer than 2 ns, in agreement with hydrodynamic/atomic physics simulations. A 35% increase in conversion efficiency was obtained when Q-switched pulses were substituted by square pulses of a similar duration. Experiments conducted irradiating a Sn target with a sequence of two time-delayed 250 ps pulses showed a 30 percent increase in the EUV yield respect to a single pulse of the same total energy when the pulse separation was optimum at 2.1 ns. This suggests that re-heating of the plasma with delayed laser pulses could be used to improve the EUV yield. The spectroscopic characterization of EUV emission and in-band EUV images that characterize the source size are also presented.

High-power, few-cycle 2 μm laser pulse generation based on soliton self-compression (Invited)

High-power, few-cycle 2 μm laser pulse generation based on soliton self-compression (Invited)

Highly Efficient Nanosecond 1.7 μm Fiber Gas Raman Laser by H2-Filled Hollow-Core Photonic Crystal Fibers

We report here a high-power, highly efficient, wavelength-tunable nanosecond pulsed 1.7 μm fiber laser based on hydrogen-filled hollow-core photonic crystal fibers (HC-PCFs) by rotational stimulated Raman scattering. When a 9-meter-long HC-PCF filled with 30 bar hydrogen is pumped by a homemade tunable 1.5 μm pulsed fiber amplifier, the maximum average Stokes power of 3.3 W at 1705 nm is obtained with a slope efficiency of 84%, and the slope efficiency achieves the highest recorded value for 1.7 μm pulsed fiber lasers. When the pump pulse repetition frequency is 1.3 MHz with a pulse width of approximately 15 ns, the average output power is higher than 3 W over the whole wavelength tunable range from 1693 nm to 1705 nm, and the slope efficiency is higher than 80%. A steady-state theoretical model is used to achieve the maximum Stokes power in hydrogen-filled HC-PCFs, and the simulation results accord well with the experiments. This work presents a new opportunity for highly efficient tunable pulsed fiber lasers at the 1.7 μm band.

In situ and tunable structuring of semiconductor-in-glass transparent composite.

SummarySemiconductor-in-glass composites are an exciting class of photonic materials for various fundamental applications. The significant challenge is the scalable elaboration of composite with the desirable combination of tunable structure, high semiconductor loading ratio, and excellent transparency. Here we report that the topological engineering strategy via hybridization of the glass network former enables to surmount the aforementioned challenge. It not only facilitates the in situ precipitation of (Ga2-xAlx)O3 domains with continuously tunable composition but also allows to simultaneously refine the grain size and enhance the crystallinity. In addition, the composites exhibit excellent transparency and can host various active dopants. We demonstrate the attractive broadband optical response of the composite and achieve the pulse laser operation in mid-infrared waveband. The findings are expected to provide a fundamental principle of in situ modification in hybrid system for generation of high-performance semiconductor-in-glass composites.

Multi-terawatt femtosecond 10 µm laser pulses by self-compression in a CO2 cell

We propose and numerically investigate a novel direct route to produce multi-terawatt femtosecond self-compressed 10 µm laser pulses suitable for the next generation relativistic laser-plasma studies including laser-wakefield acceleration at long wavelengths. The basic concept involves selecting an appropriate isotope of CO2 gas as a compression medium. This offers a dispersion/absorption landscape that is shifted in frequency relative to the driving CO2 laser used for 10 µm picosecond pulse generation. We show numerically that as a consequence of low losses and a broad anomalous dispersion window, a 3.5 ps duration pulse can be compressed to ∼300 fs while carrying ∼7 TW of peak power in less than 7 m. An interplay of self-phase modulation and anomalous dispersion leads to a ∼3.5 times compression factor, followed by the onset of filamentation near the cell exit to get below 300 fs duration.

Measurements of nonlinear absorption of intense 10 µm laser pulses in n-Ge, GaAs, and ZnSe.

We study the nonlinear absorption of high-power 10 µm radiation propagating through n-Ge, GaAs, and ZnSe materials widely used in the long-wave infrared range. Measurements are performed using both nanosecond and picosecond CO2 laser pulses providing intensities up to 100MW/cm2 and 5GW/cm2, respectively. Nonlinear absorption coefficients in optical quality n-Ge are found to be the same for both pulse durations. The nonlinear absorption coefficient of these materials scales inverse proportionally to their bandgap energies, indicating increased photoionization in the smaller bandgap materials.

Eye-safe 1.55 μm high-repetition-frequency self-Q-switched Er:Yb:Lu2Si2O7 pulse laser

An eye-safe 1.55 μm solid-state pulse laser with high repetition frequency is useful for some applications such as lidar. A high-repetition-frequency self-Q-switched 1.55 μm pulse laser was firstly demonstrated in an Er:Yb:Lu2Si2O7 crystal end-pumped by a continuous-wave 975 nm diode laser. The influences of various cavity parameters on the self-Q-switched pulse laser performance were investigated. At an absorbed pump power of 6.3 W and a pump beam diameter of 400 μm, a self-Q-switched 1.55 μm pulse laser with energy of 3.37 μJ, repetition frequency of 43 kHz, and duration of 1.04 μs was realized. When the pump beam diameter was changed to 100 μm, the repetition frequency and duration of the pulse laser increased to 230 kHz and decreased to 236 ns, respectively, while the pulse energy reduced to 1.22 μJ.

Pulse control in self-mode-locked 2.8 µm Er-doped fluoride fiber lasers

In this work, it had been demonstrated that the Er doped fluoride fiber laser emitting at 2.8 µm could accomplish the dark pulse based on self-mode-locked (SML). And the different distances of external feedback were also investigated in experiment. It was found that signal-noise ratio (SNR) of fundamental cavity repetition rate was decreased with increasing the distance of feedback. It was not only caused by increasing the length of cavity, but also influenced by the interaction between inter-cavity and external cavity. The fundamental cavity repetition rate of SML 2.8 µm pulsed laser could be controlled by adjusting distance of external feedback.

Continuously diode-pumped passively $Q$Q-switched eye-safe 1537 nm Er:Yb:Lu2Si2O7 pulse laser

It is still a technical challenge to obtain a 1.55 µm passively $Q$Q-switched ${{\rm Er}^{3 + }}/{{\rm Yb}^{3 + }}$Er3+/Yb3+ pulse laser with high energy and high repetition frequency, especially when it is end-pumped by a continuous-wave 0.98 µm diode laser. Benefitting from its long fluorescence lifetime of the $^4{{\rm I}_{13/2}}$4I13/2 upper laser level and high thermal conductivity, ${\rm Er}:{\rm Yb}:{{\rm Lu}_2}{{\rm Si}_2}{{\rm O}_7}$Er:Yb:Lu2Si2O7 crystal may be an excellent gain medium for realizing a high energy and high repetition frequency 1.55 µm pulse laser. At a continuous-wave absorbed pump power of 6.1 W, a 1537 nm pulse laser with energy of 45.5 µJ, repetition frequency of 1.32 kHz, and duration of 25 ns was first, to the best of our knowledge, realized in an ${\rm Er}:{\rm Yb}:{{\rm Lu}_2}{{\rm Si}_2}{{\rm O}_7}$Er:Yb:Lu2Si2O7 crystal.

Numerical simulation of PbSe quantum dots doped fiber ring laser with 1.7 µm wavelength

A lead selenide quantum dot-doped fiber ring laser operating at 1.7 µm wavelength is proposed and investigated by numerical simulation for the first time. Its temporal and spectral characteristics under different simulation parameters are presented. The fact that the obtained temporal profiles using different simulation parameters are largely different is observed and analyzed. These results offer us a new practical path to generate a 1.7 µm continuous-wave and pulsed laser for deep biomedical application.

High-peak-power 1.7 µm pulse generation by cascaded second- and third-order nonlinearity

We report an efficient and novel method for generating high-peak-power 1.7 µm laser pulses by cascaded optical parametric oscillation (OPO) and stimulated Raman scattering (SRS). The 1064 nm fundamental wave was first converted to 1535 nm by the KTA OPO, and further extended to 1.7 µm by a SRS process. The configuration of OPO + SRS can provide high-intensity pumping light for subsequent Raman conversion, and allows for better wavelength expansibility benefitting from the non-phase-matching requirement of SRS. Two types of Raman conversion using the low-frequency Raman shift in KY(WO4)2 and high-frequency Raman shift in YVO4 were further studied. Up to the 8th-order cascaded KY(WO4)2 Raman laser (KRL) using the high gain 87 cm−1 Raman mode and a YVO4 Raman laser (YRL) using the 890 cm−1 Raman mode emitting at 1.7 µm were realized, respectively. The output wavelengths at 1556, 1577, 1599, 1622, 1646, 1670, 1695, 1720 nm and the output wavelength at 1778 nm were observed in the KRL and YRL, respectively. The maximum total average output powers of 1.26 W and 1.05 W, minimum pulse widths of 8.4 and 24 ns and maximum pulse peak powers of 33.3 kW and 9.4 kW were obtained respectively from the KRL and YRL, enabling the 1.7 µm laser source to have practical applicability in medical imaging, industrial processing, and mid-infrared laser generation.

Two-dimensional tellurene nanosheets as saturable absorber of passively Q-switched Nd:YAG solid-state laser

For the first time, a group-VI single element nanomaterial was used as the optical saturable absorber (SA) to generate laser pulses. With two-dimensional (2D) tellurene as a passive Q-switch, 1.06 μm and 1.3 μm pulse laser operations were realized from a diode-pumped Nd:YAG crystal. The shortest pulse widths were 98 ns and 178 ns, and the highest peak powers were 2.68 W and 2.45 W, respectively. Our research determines that tellurene is an excellent SA material in the near-infrared region.