KTA-OPO for 1742 nm laser generation driven by a composite Nd:YVO4-based self-Raman laser.

In this work, a double-end diffusion bonded Nd:YVO4 self-Raman laser was designed to drive an intracavity, noncritically-phase-matched KTiOAsO4 (KTA) optical parametric oscillator (OPO). Both conversion efficiency and output power at 1.7 µm (the wavelength of the OPO signal field) were improved by effectively reducing the thermal lens effect and increasing the effective length of self-Raman medium. At an incident pump power of 15.4 W, the output power for 1742 nm output laser reached 2.16 W with a conversion efficiency of 14%, and the output having a pulse width of 10.5 ns and a pulse repetition frequency of 90 kHz. The competition between the OPO and cascaded Raman laser was observed when the incident pump power was above 12.4 W. The results highlight that in order to improve output power at 1742 nm, it is critical that both the cascaded, second-Stokes field at 1313 nm and the signal field generated at 1534 nm from the 1064 nm field driving the KTA-OPO be minimized, if not completely suppressed. This laser system combining the processes of stimulated Raman scattering and optical parametric oscillation for the generation of laser emission at 1742 nm may find significant application across a broad range of fields including biological engineering, laser therapy, optical coherence tomography and for the generation of mid-infrared laser wavelengths.

Decoupling bulk and surface characteristics with a bare tilted fiber Bragg grating.

The tilted fiber Bragg grating (TFBG) with dense comb-like resonances offers a promising fiber-optic sensing platform but could suffer from cross sensitivity dependent on bulk and surface environment. In this work, the decoupling of bulk and surface characteristics (indicated by bulk refractive index (RI) and surface-localized binding film) from each other is attained theoretically with a bare TFBG sensor. This is realized with the proposed decoupling approach based on differential spectral responses of cut-off mode resonance and mode dispersion represented as wavelength interval between P- and S-polarized resonances of the TFBG to the bulk RI and surface film thickness. The results demonstrate that with this method the sensing performance for decoupling bulk RI and surface film thickness is comparative to the cases in which either the bulk or surface environment of the TFBG sensor changes, with the bulk and surface sensitivities over 540 nm/RIU and 12 pm/nm, respectively.

Simultaneous generation and real-time observation of loosely bound solitons and noise-like pulses in a dispersion-managed fiber laser with net-normal dispersion.

We demonstrate the coexisting dynamics of loosely bound solitons and noise-like pulses (NLPs) in a passively mode-locked fiber laser with net-normal dispersion. The total pulse number of the single soliton bunch under the NLP operation regime almost increases linearly with increasing pump power, whereas the average pulse spacing decreases accordingly. Furthermore, pulse-to-pulse separation between adjacent soliton pulses in one soliton bunch keeps in the range of hundreds of picoseconds, which decreases from left to right with the change of time. Besides, the real-time observation has been performed by utilizing the time-stretch method, showing that each one of the loosely bound solitons on the NLP operation is actually composed of chaotic wave packets with random intensities. These findings obtained will facilitate the in-depth understanding of nonlinear pulse behaviors in ultrafast optics.

YVO4 cascaded Raman laser for five-visible-wavelength switchable emission.

We report switchable emission of visible light at five wavelengths generated in a $\text{YVO}_4$YVO4 cascaded Raman cavity driven by an acousto-optic ${Q}$Q-switched Nd:YAP laser at 1080 nm. The second-harmonic generation and sum-frequency generation of the fundamental, first-Stokes, and second-Stokes waves were realized based on the angle-tuned BBO crystal. The phase-match angle tuning span of a BBO crystal is only approximately 2.3° to allow the frequency mixing to produce five visible wavelengths on demand. At an absorbed pump power of 5.96 W and a pulse repetition frequency of 10 kHz, average output powers of 800, 340, 460, 190, and 326 mW were obtained for 539.9, 567.2, 597.4, 631.0, and 668.5 nm lasers, respectively. The results show that the cascaded Raman laser system with an angle-tuned BBO crystal can provide a convenient and efficient method to achieve wavelength-switchable visible emission for multi-wavelength laser applications.

Collisional dynamics in laser-induced plasmas: evidence for electron-impact excitation.

We have experimentally investigated the collisional dynamics in femtosecond-laser-induced plasmas and presented the evidence for electron-impact excitation through enhanced high-order harmonic (HH) generation. The measurements were carried out by using an elliptically polarized pump pulse to induce the underdense plasmas and by using a time-delayed linearly polarized probe pulse to drive the HH generation from the plasmas. We found that the rise time of this enhanced HH generation was insensitive to the ellipticity degree (ED) of pump pulse but sensitive to its laser intensity (LI). With further comparison between physical scenarios and qualitative analysis, we demonstrated that the atomic excitation causing the HH enhancement should be attributed to the electron-impact excitation, i.e., the excitation from the collision between neutral atoms and electrons during the lifetime of the underdense plasma.

Compact passively Q-switched RbTiOPO4 cascaded Raman operation.

Compact passively Q-switched RbTiOPO4 cascaded Raman laser operation has been investigated for the first time, to our knowledge. A Nd:YAG/Cr4+:YAG composite crystal was adopted for passively Q-switched fundamental laser generation. 440 mW multi-Stokes laser output with X(ZZ)X configuration according to the Porto notations was achieved under the incident pump power of 10 W. The measured spectra contained three wavelengths of 1149, 1165, and 1185 nm based on the Raman shift of 271 and 687 cm-1. Different from actively Q-switch cascaded Raman laser with Stokes wave from low order to high order oscillating in sequence, three intensity lines were output for the whole experiment in this passively Q-switched multi-Stokes laser. The output spectra could be varied and optimized based on the output coupler coating design. Output spectra not sensitive to incident pump power for passively Q-switched cascaded Raman laser have advantages in multiwavelength laser applications.

Efficient 1.7 µm light source based on KTA-OPO derived by Nd:YVO4 self-Raman laser.

An intra-cavity optical parametric oscillator (OPO) emitting at 1.7 µm derived by Nd:YVO4 self-Raman laser is demonstrated in this Letter, with a KTiOAsO4 (KTA) crystal used as nonlinear optical crystal. A laser diode end-pumped acousto-optic Q-switched Nd:YVO4 self-Raman laser at 1176 nm was employed as the pump source. At an incident pump power of 12.1 W and a pulse repetition frequency of 60 kHz, average output power up to 1.2 W signal light at 1742 nm was obtained, with diode-to-signal conversion efficiency of 10%. The pulse width was about 11 ns and spectral line width was less than 0.5 nm for the signal light. The results show that compact intra-cavity KTA-OPO derived by Nd:YVO4 self-Raman laser is an efficient method for 1.7 µm waveband laser generation, with potential applications in biological imaging, laser therapy, special materials processing, etc.

Comparison of 1.15 µm Nd:YAG\KTA Raman lasers with 234 and 671 cm-1 shifts.

Nd:YAG/KTA intra-cavity Raman lasers with two different Raman shifts are experimentally compared for eradiating around 1.15 µm. The different Raman processes are attributed to polarization dependent Raman gain of KTA crystal and nearly linear polarization of Nd:YAG fundamental light with Q-switch operation. Under an incident pump power of 10.2 W and a pulse repetition frequency of 15 kHz, 1.24 W output for the third-Stokes at 1150 nm in X(ZZ)X Raman configuration with the Raman shifts of 234 cm-1 and 1.21 W output for the first-Stokes at 1146 nm in X(YY)X Raman configuration with the Raman shifts of 671 cm-1were achieved by rotating the KTA crystal. The conversion efficiency in both Raman configurations were about 12% respect to the same incident pump power. The slight differences of spectra and pulses are investigated.

Compact self-cascaded KTA-OPO for 2.6 µm laser generation.

We reported a compact self-cascaded KTA-OPO source for 2.6 µm coherent light generation. The OPO is driven in a diode end-pumped and Q-switched Nd:YVO4 laser cavity. Two OPO processes occurred in the same KTA crystal with non-critical phase matching. At an incident diode pump power of 8.7 W and a pulse repetition frequency of 60 kHz, the OPO can generate a maximum average output power of 445 mW at 2.59 µm. The slope efficiency was about 12.7%, and the power fluctuation was less than 8%. Therefore, the self-cascade OPO based on KTA offers a promise scheme for the rugged and compact mid-infrared 2.6 µm laser generation.

Efficient laser operation based on transparent Nd:Lu2O3 ceramic fabricated by Spark Plasma Sintering.

Efficient laser operation of Nd:Lu2O3 ceramic fabricated by Spark Plasma Sintering (SPS) was demonstrated. Transparent Nd:Lu2O3 ceramic was successfully fabricated by Spark Plasma Sintering and its laser experiment was done. On the 4F3/2 to 4I11/2 transition, the obtained maximum output is 1.25W at the absorbed pump power of 4.15W with a slope efficiency of 38% and two spectral lines at 1076.7nm and 1080.8nm oscillated simultaneously. The slope efficiency of 38% is near two times higher than the previously demonstrated SPSed Nd:Lu2O3 ceramic lasers. On the 4F3/2 to 4I13/2 transition, the laser operated at the wavelength of 1359.7nm and the maximum output of 200mW was obtained at the absorbed pump power of 2.7W.

[Application of Extreme-Ultraviolet Ar Spectra Analysis in the Study of Divertor Impurity Screening in EAST Tokamak].

Divertor impurity injection on Tokamak is the most important means to achieve divertor impurity screening efficiency. In this paper, a fast-response extreme-ultraviolet (EUV) spectrometer is used to monitor the Ar emission lines during the EAST(Experimental Advanced Superconducting Tokamak)divertor Ar injection experiment. Based on the NIST(National Institute of Standards and Technology)atomic spectrum database, the emission lines from different ionized Ar ions in 2～50 nm wavelength range, e.g. Ar Ⅳ, Ar Ⅳ-Ⅺ and Ar ⅩⅣ-ⅩⅥ, are being identified. Ar ⅩⅥ 35.39 nm and Ar Ⅳ 44.22 nm with the ionization energy of 918.4 and 59.6 eV respectively are being monitored during the experiment with Ar puffing to observe the behavior of Ar impurities in different regions in plasmasimultaneously. The preliminary analysis on divertor impurity screening efficiency is carried outwith the time evolution of intensities of two Ar emission lines. The results of experiment puffing from the same gas puffing inlet (e. g. from lower outer target inlet) and withdifferent plasma configurations (e. g. lower single null, upper single null) show that the screening effect on the impurity injected from the divertor region is better thanfrom the main plasma region; the screening effect of lower divertor and particle pumping by internal cryopump installed in lower divertor is stronger than upper divertor.

Multi-order Stokes output based on intra-cavity KTiOAsO4 Raman crystal.

We report efficient multi-Stokes Raman output of a KTiOAsO4 (KTA) crystal driven by a laser diode end-pumped acousto-optic Q-switched Nd:YAG laser. The Raman outputs of two x-cut KTA crystals, one with a length of 20 mm and the other one of 25-mm, were experimentally compared. Under an incident pump power of 10.9 W, a maximum output power of 1.12 W with a pulse width of 7.6 ns and a pulse repetition frequency of 15 kHz were obtained. The conversion efficiency and slope efficiency with respect to the incident diode pump power were 10.3% and 15.2%, respectively. The laser output contains multiple Raman Stokes lines with different spectral strengths that varied with the pump power.

Efficient RTP-based OPO intracavity pumped by an acousto-optic Q-switched Nd:YVO4 laser.

This Letter describes an intracavity pumped optical parametric oscillator (OPO) based on noncritical phase matching RbTiOPO4 (RTP) crystal driven by a laser diode end-pumping acousto-optic Q-switched Nd:YVO4 laser. Simultaneous efficient signal light at 1.6 µm and idler light at 3.1 µm outputs were obtained. At an incident pump power of 10.5 W and a Q-switching pulse repetition frequency of 60 kHz, 1.42 W at center wavelength of about 1619 nm and 0.38 W at 3108 nm were achieved, with the diode to OPO total output conversion efficiency up to 17.1%. The pulse width is about 6.5 ns for the signal light corresponding to the fundamental light at 1064 nm of about 10 ns. The spectral widths of the signal and idler light are narrower than 0.5 and 1.0 nm. The result shows that the RTP crystal is an efficient crystal to generate eye-safe and mid-infrared lights by making full use of noncritical phase matching.

Laser operation of diode-pumped Er,Yb co-doped YAG ceramics at 1.6 µm.

1.6 µm eye-safe emission of a diode pumped Er,Yb co-doped YAG ceramic laser is firstly demonstrated. Operation of the ceramic laser under different ceramic sample lengths, co-doping concentrations and control temperatures were experimentally investigated. A maximum output power of 222 mW was achieved at an absorbed pump power of 8.1 W, corresponding to a conversion efficiency of 2.74%. Laser emission at 1.05 µm for transition of the Yb(3+) ions was also studied on the same ceramic samples. The results clearly show the existence of resonantly energy transfer from the Yb(3+) ions to Er(3+) ions.

Realization of thousand-second improved confinement plasma with Super I-mode in Tokamak EAST.

Mastering nuclear fusion, which is an abundant, safe, and environmentally competitive energy, is a great challenge for humanity. Tokamak represents one of the most promising paths toward controlled fusion. Obtaining a high-performance, steady-state, and long-pulse plasma regime remains a critical issue. Recently, a big breakthrough in steady-state operation was made on the Experimental Advanced Superconducting Tokamak (EAST). A steady-state plasma with a world-record pulse length of 1056 s was obtained, where the density and the divertor peak heat flux were well controlled, with no core impurity accumulation, and a new high-confinement and self-organizing regime (Super I-mode = I-mode + e-ITB) was discovered and demonstrated. These achievements contribute to the integration of fusion plasma technology and physics, which is essential to operate next-step devices.

Performance improvement of space-resolved extreme ultraviolet spectrometer by use of complementary metal-oxide semiconductor detectors at the Experimental Advanced Superconducting Tokamak.

Two pairs of space-resolved extreme ultraviolet (EUV) spectrometers working at 5-138 Å with different vertical observation ranges of -7 ≤ Z ≤ 19 and -18 ≤ Z ≤ 8 cm have been newly developed to observe the radial profile of impurity line emissions and to study the transport of high-Z impurity ions intrinsically existing in EAST tokamak plasmas. Both spectrometers are equipped with a complementary metal-oxide semiconductor (CMOS) detector (Andor Marana-X 4.2B-6, Oxford Instruments) with sensitive area of 13.3 × 13.3 mm2 and number of pixels equal to 2048 × 2048 (6.5 × 6.5 µm2/pixels). Compared to the currently operating space-resolved EUV spectrometers with a charge-coupled detector (CCD: 1024 × 255 pixels, 26 × 26 µm2) working at 30-520 Å, this spectrometer's performance was substantially improved by using the CMOS detector. First, the spectral resolution measured at full width at half maximum was improved in the whole wavelength range, e.g., Δλ1/2_CMOS = 0.092 Å and Δλ1/2_CCD = 0.124 Å at C VI 33.73 Å and Δλ1/2_CMOS = 0.104 Å and Δλ1/2_CCD = 0.228 Å at Mo XXXI 115.999 Å, thus enabling a more accurate analysis of spectra with complicated structure such as tungsten unresolved transition array in the range 45-65 Å. Second, the temporal resolution was largely improved due to the high-speed data acquisition system of the CMOS detector, e.g., Δt_CMOS = 15 ms/frame and Δt_CCD = 200 ms/frame at routine operation in the radial profile measurement. Third, signal saturation issues that occurred when using the old CCD sensor during impurity accumulation now disappeared entirely using the CMOS detector due to lower exposure time at high readout rates, which largely improved the observation performance in similar impurity burst events. The above-mentioned performance improvements of the space-resolved EUV spectrometer led to a rapid change in the W XXXIII (52.22 Å) radial profile during a single cycle of low-frequency sawtooth oscillation with fst = 5-6 Hz at a sufficient detector count rate.

Potential sodium D2 resonance radiation generated by intra-cavity SHG of a c-cut Nd:YVO4 self-Raman laser.

Intra-cavity frequency doubling with 589 nm emission from a compact c-cut Nd:YVO4 crystal self-Raman laser was investigated. A 15-cm-length LBO with non-critical phase-matching cut (θ = 90°, ϕ = 0°) was used for efficient second-harmonic generation. At a pump power of 16.2 W and a pulse repetition frequency of 40 kHz, output power up to 2.15 W was achieved with a pulse width of 16 ns and a conversion efficiency of 13.3% with respect to the diode pump power. The center wavelength was measured to be 589.17 nm with a Half-Maximum-Full-Width of 0.2 nm, which was well in accordance with the sodium D2 resonance radiation.

High-efficiency intracavity Nd:YVO4\KTA optical parametric oscillator with 3.6 W output power at 1.53 microm.

An efficient intracavity KTA optical parametric oscillator (OPO) driven by diode-end-pumped acousto-optical Q-switched Nd:YVO(4) laser is demonstrated. The mode mismatch between fundamental cavity and OPO cavity caused by the thermal lens effect in Nd:YVO(4) crystal as the pump power increased was studied. To lessen mode mismatch, a thermal lens-like cavity mirror made of plane BK7 glass induced by idler absorption was introduced into the OPO cavity. Under a diode pump power of 20 W, a maximum 1535nm light output power of 3.6 W was achieved at the pulse repetition rate of 60 kHz, corresponding to a diode-to-signal conversion efficiency of 18%. This is the highest efficiency reported for eye-safe laser based on intracavity OPO.

Efficient second harmonic generation of double-end diffusion-bonded Nd:YVO4 self-Raman laser producing 7.9 W yellow light.

A high power and efficient 588 nm yellow light is demonstrated through intracavity frequency doubling of an acousto-optic Q-switched self-frequency Raman laser. A 30-mm-length double-end diffusion-bonded Nd:YVO(4) crystal was utilized for efficient self-Raman laser operation by reducing the thermal effects and increasing the interaction length for the stimulated Raman scattering. A 15-mm-length LBO with non-critical phase matching (theta = 90 degrees, phi = 0 degrees) cut was adopted for efficient second-harmonic generation. The focus position of incident pump light and both the repetition rate and the duty cycle of the Q-switch have been optimized. At a repetition rate of 110 kHz and a duty cycle of 5%, the average power of 588 nm light is up to 7.93 W while the incident pump power is 26.5 W, corresponding to an overall diode-yellow conversion efficiency of 30% and a slope efficiency of 43%.

Yellow-light generation of 5.7 W by intracavity doubling self-Raman laser of YVO(4)/Nd:YVO(4) composite.

A high-power 588 nm light produced by an intracavity frequency-doubling acousto-optic Q-switched self-frequency Raman laser is reported. A 20-mm-long YVO(4)/Nd:YVO(4) composite crystal and a 15-mm-long LiB(3)O(5) (LBO) with noncritical phase-matching (theta=90 degrees , phi=0 degrees ) cut were adopted for efficient self-Raman laser operation and second-harmonic generation, respectively. By the optimizing the focus position of incident pump light and Q-switch repetition rate, yellow light with 5.7 W average power was generated under the pump power of 23.5 W, corresponding to the overall diode-yellow conversion efficiency of 24.2% and slope efficiency of 32%. The pulse width is about 16 ns, and the pulse energy is up to 95 microJ.