Soft X-ray imaging of thick carbon-based materials using the normal incidence multilayer optics

The high transparency of carbon-containing materials in the spectral region of “carbon window” (λ ～ 4.5–5 nm) introduces new opportunities for various soft X-ray microscopy applications. The development of efficient multilayer coated X-ray optics operating at the wavelengths of about 4.5 nm has stimulated a series of our imaging experiments to study thick biological and synthetic objects. Our experimental set-up consisted of a laser plasma X-ray source generated with the 2nd harmonics of Nd–glass laser, scandium-based thin-film filters, Co/C multilayer mirror and X-ray film UF-4. All soft X-ray images were produced with a single nanosecond exposure and demonstrated appropriate absorption contrast and detector-limited spatial resolution. A special attention was paid to the 3D imaging of thick low-density foam materials to be used in design of laser fusion targets.     

Collisional excitation soft X-ray laser at 23.6 nm in a laser-produced cylindrical target

We have tested soft X-ray lasing in neon-like germanium with cylindrical targets where wave guiding and plasma confinement may affect lasing. An intense soft X-ray laser beam of 0.05 MW peak power and a narrow beam divergence (8 mrad) was produced at 23.6 nm with a 4 cm long straight cylindrical target of 0.72 mm inner diameter. Bending the cylindrical target to form a toroidal shape increased the lasing intensity by a factor of 3 accompanied with reduction of the beam divergence from 8 to 6 mrad.     

SODIUM-LIKE RECOMBINATION X-RAY LASER

Experimental studies of soft X-ray laser for the sodium-likerecombination scheme conducted at the LF12 Laser Facility of SIOM(Shanghai Instituteof Optics and Fine Mechanics)are reported.Soft X-ray amplification was observed insodium-like copper ions by line-focused laser irradiation of slab targets.Based on thespatially-resolved measurements,the spatial distribution of the laser gain for the transition6g—4f(72.22(?))of sodium-like copper ions was obtained,and the maximums of gaincoefflcients and gain-length products at 550μm from target surface were 2.0 cm~(-1) and 3. 6,respectively.The evolution of spectrally discriminated spatial uniformity of line-shapedplasmas as lasing gain media produced by uniform laser illumination in line-focus isalso investigated.It is found that under this experimental condition the line-shapedplasma is very uniform after laser heating.     

利用皮秒脉冲激光驱动瞬态X射线激光

报道了利用皮秒激光驱动产生瞬态类镍银X射线激光的实验结果.采用一路脉冲宽度为数百皮秒的激光作为预脉冲，配合另一路皮秒激光作为主脉冲联合驱动平面靶，获得了一定强度的类镍银X射线激光输出，输出能量约为5—10nJ. 关键词： 瞬态X射线激光 长短脉冲联合驱动 皮秒脉冲激光     

Capillary discharge tabletop soft X-ray lasers reach new wavelengths and applications

For many years, researchers have envisioned the development of compact high repetition rate tabletop soft X-ray lasers that could be routinely used in application in numerous disciplines. With demonstrated average powers of several mW and mJ-level pulse energy at 46.9 nm, capillary discharge-pumped lasers are the first compact lasers to reach this goal. In this paper we summarize the development status of high repetition rate tabletop soft X-ray lasers based on capillary discharge excitation, and give examples of their successful use in several applications. Results of the use of a capillary discharge pumped 46.9 nm laser in dense plasma interferometry, soft X-ray reflectometry for the determination of optical constants, and laser ablation are described. The observation of lasing at 53 nm line in Ne-like Cl with output pulse energy up to 10 μJ is also reported.     

低泵浦能量类氖锗等离子体软X射线激光实验

提出并采用具有一定时间间隔的双束脉冲加热锗薄膜靶方法，在泵浦激光能量１００Ｊ，靶长１０ｍｍ的实验条件下，观察到波长１９．６１ｎｍ和２３．６３ｎｍ两条类氖锗３ｓ－３ｐ激光跃迁线。１９．６１ｎｍ（Ｊ＝０－１）的激光跃迁线发射强度较２３．６３ｎｍ（Ｊ＝２－１）激光线的强度大，其发散角较后者小。     

Demonstration of a 100 Hz repetition rate gain-saturated diode-pumped table-top soft x-ray laser

We demonstrate the operation of a gain-saturated table-top soft x-ray laser at 100?Hz repetition rate. The laser generates an average power of 0.15?mW at λ=18.9 nm, the highest laser power reported to date from a sub-20-nm wavelength compact source. Picosecond laser pulses of 1.5?μJ energy were produced at λ=18.9 nm by amplification in a Mo plasma created by tailoring the temporal intensity profile of single pump pulses with 1?J energy produced by a diode-pumped chirped pulse amplification Yb:YAG laser. Lasing was also obtained in the 13.9?nm line of Ni-like Ag. These results increase by an order of magnitude the repetition rate of plasma-based soft x-ray lasers opening the path to milliwatt average power table-top lasers at sub-20?nm wavelengths.     

Optimization of nickel-like collisional soft-X-ray lasers at low pump power

We report on the experimental demonstration of saturated X-ray laser output from collisionally pumped Ni-like Pd, Ag, and Sn at wavelengths of 14.7, 14.0, and 12.0 nm, respectively. Using a 100 ps drive pulse irradiation and a `twin-0.25%' prepulse configuration on flat slab targets, gain-length products >16 were obtained for the three elements. The drive power required for saturation was only 250 GW in a 100 ps pulse (25 J), corresponding to an irradiance of 10 TW/cm².     

Characterization of the 46.9-nm soft X-ray laser beam from a capillary discharge

Intense lasing had been obtained from argon plasma in the soft X-ray region from a capillary discharge plasma system. Different diagnostics have been used to characterize the lasing properties by recording the temporal, spatial, and spectral profiles of the emission. The divergence measurement indicates that the soft X-ray laser beam has good directionality with a divergence of 3.5 mrad. The spectrum of the laser beam measured using a transmission grating showed intense lasing line at 46.9 nm. Diffraction orders as high as 10th orders were observed. The temporal profile recorded with a vacuum diode showed a distinct laser peak with a pulse width ~1.2 ns (FWHM). In addition, the coherence of the X-ray laser beam was also confirmed from the high-contrast interference fringes (visibility ~85 %) recorded using double slits.     

Role of the wall ablation in the operation of a 46.9 nmAr capillary discharge soft x‐ray laser

A capillary discharge pumped soft x‐ray laser operating at 46.9 nm on the 3p–3s transition of the Ne‐like Ar has been realized by pumping the active medium with a relatively slow current pulse (dI/dt ≈ 6 · 10¹¹ A/s). In order to study the role of the ablation in the production of the laser effect, the intensity of the amplified 46.9 nm line has been investigated using the same pumping current pulses in the plastic (polyacetal) and ceramic (Al₂O₃). We showed that the ablation of the capillary walls is unfavorable both for the compression and stability of the plasma and consequently for the soft x‐ray laser production. The amplification and lasing effects are observed only in the ceramic channel. The measurements of the line intensity at 46.9 nm showed the lasing with a gain‐length product of ≈ 9, a laser pulse energy of ≈ 5 μJ, a pulse duration of 1.3 ns and a beam divergence of ≈ 3.5 mrad. In addition, effect of the scaling of the time of lasing with the initial plasma diameter was demonstrated experimentally and compared with a one‐dimensional MHD model.     

Investigation of the pump wavelength influence on pulsed laser pumped Alexandrite lasers

Recent theoretical modelling and experimental results have shown that excess lattice phonon energy created dur ing the non-radiative energy transfer from the ⁴T₂ pump manifold to the ²E storage level in Alexandrite when pumped with wavelengths shorter than ∼645 nm causes chaotic lasing output. Shorter pump wavelengths have also been associated with increased non-radiative energy decay and reduced laser efficiency. We report studies of fluorescence emission spectra of Alexandrite illuminated at a range of wavelengths from green to red, which demonstrate reduced fluorescence yield for shorter pump wavelengths at elevated crystal temperatures. Investigations of pulsed laser pumping of Alexandrite over the same spectral range demonstrated reduced pump threshold energy for longer pump wavelengths. High repetition rate pulsed pumping of Alexandrite at 532, 578 and 671 nm showed stable and efficient laser performance was only achieved for red pumping at 671 nm. These results support the theoretical model and demonstrate the potential for scalable, red laser pumped, all-solid-state Alexandrite lasers.PACS 42.60.Lh; 42.60.Mi     

PERFORMANCE COMPARISON OF THE NEON-LIKE CHROMIUM SOFT X-RAY LASING DRIVEN BY A DOUBLE LASER PULSE WITH DIFFERENT DURATION

A comparison has been made of performance of the neon-like chromium soft X-ray lasing at 28.5 nm driven by a double 900 ps pulse at 6 TW/cm², with that driven by a double 200 ps pulse at similar irradiance. The double 200 ps pulse has been found to be much more efficient to drive X-ray lasing with higher intensity.     

Tunable double-clad ytterbium-doped fiber laser based on a double-pass Mach-Zehnder interferometer

We have demonstrated an adjustable double-clad Yb³⁺-doped fiber laser using a double-pass Mach-Zehnder interferometer. The laser is adjustable over a range of 40 nm from 1064 nm to 1104 nm. By adjusting the state of the polarization controller, which is placed in the double-pass Mach-Zehnder interferometer, we obtained central lasing wavelengths that can be accurately tuned with controllable spacing between different tunable wavelengths. The laser has a side mode suppression ratio of 42 dB, the 3 dB spectral width is less than 0.2 nm, and the slope efficiencies at 1068 nm, 1082 nm and 1098 nm are 23%, 32% and 26%, respectively. In addition, we have experimentally observed tunable multi-wavelengths lasing output.     

Stable multiwavelength fiber ring laser with equalized power spectrum based on a semiconductor optical amplifier

We experimentally demonstrated a new structure of a multiwavelength semiconductor optical amplifier (SOA) ring laser based on a fiber Sagnac loop filter that can generate up to 25 stable output lasing wavelengths at room temperature. By varying the length of a polarization-maintaining (PM) fiber within the Sagnac loop filter, the wavelength spacing between the output lasing wavelengths can be changed to a desired value. By tuning a polarization controller (PC) within the Sagnac loop filter, stable multiwavelength 1550-nm operation with up to 17 lasing lines within 3 dB power level variation and with a wavelength spacing of ∼0.8 nm was achieved. The optical signal-to-noise ratios (OSNRs) of all the lasing wavelengths are greater than 40 dB.     

Collisionally pumped hybrid soft X-ray laser in Ne-like sulphur

We describe an experiment demonstrating XUV amplification following collisional excitation in a capillary discharge plasma irradiated by a picosecond IR laser pulse. Guiding and temporally resolved transmission of the pump laser beam are also demonstrated and analysed. The short pump laser pulse heated rapidly the electrons producing amplification in the 3p¹S₀–3s¹P₁ transition of Ne-like sulphur at 60.84 nm. The estimated gain–length product was equal to 6.8, while the beam divergence reached 2.5 mrad for 30 mm capillary. This new, hybridly pumped collisional soft X-ray laser with the transient gain offers a new way towards efficient table-top XUV sources.     

The Spectrum of Ne-Like Ar and the 4d-4p Transition of Kr7+ at 45 nm in Capillary Discharge Experiment

Capillary discharge soft X-ray laser, which is always called “table-top soft X-ray laser,” is one of the effective methods for miniaturization. The method is deemed to have experimental facility with practicable value. With capillary discharge mechanism, Ne-like Ar at 46.9 nm soft X-ray laser has been obtained, and the experimental results and the analyses are reported in this paper. Experimentally, a current pulse with the amplitude of 20 kA was used to excite three different gases: Ar, Kr, and N₂. The signals were detected by an X-ray diode (XRD), and the emission spectra were monitored with a Rowland spectrograph. The J = 0–1 line and one 3d-3p transition of Ne-like Ar were detected, with wavelengths of 46.9 nm and 48.5 nm, respectively. Adjusting the pure Ar to the appropriate pressure, the authors found that the J = 0–1 line gains amplification, while 3d-3p does not. When the Ar pressure was 26 Pa, the J = 0–1 line completely dominated the spectrum. N₂ and Kr were used as the materials of capillary discharge respectively. The spike signal was obtained by XRD, then the spectrum was detected by Rowland spectrograph, and the spectrum of Kr⁷⁺ was found.     

A review of Ni-like X-ray laser experiments undertaken using the VULCAN laser at the Rutherford Appleton Laboratory

Recent progress using the VULCAN laser at the Rutherford Appleton Laboratory to pump X-ray lasing in nickel-like ions is reviewed. Double pulse pumping with ∼100 ps pulses has been shown to produce significantly greater X-ray laser output than single pulses of duration 0.1–1 ns. With double pulse pumping, the main pumping pulse interacts with a pre-formed plasma created by a pre-pulse. The efficiency of lasing increases as there is a reduced effect of refraction of the X-ray laser beam due to smaller density gradients and larger gain volumes, which enable propagation of the X-ray laser beam along the full length of the target. The record shortest wavelength saturated laser at 5.9 nm has been achieved in Ni-like dysprosium using double pulse pumping of 75 ps duration from the VULCAN laser. A variant of the double pulse pumping using a single ∼100 ps laser pulse and a superimposed short ∼1 ps pulse has been found to further increase the efficiency of lasing by reducing the effects of over-ionisation during the gain period. The record shortest wavelength saturated laser pumped by a short ∼1 ps pulse has been achieved in Ni-like samarium using the VULCAN laser operating in chirped pulse amplified (CPA) mode. Ni-like samarium lases at 7.3 nm.     

LD端面抽运Nd:YAG 1319/1338nm双波长激光器研究

从LD端面抽运固体激光器的激光阈值公式出发,建立了双波长激光同时振荡的阈值条件,理论计算了腔镜对于两个波长的透过率关系,实现了LD端面抽运Nd:YAG 1319nm/1338nm双波长激光连续和准连续输出.双波长激光连续输出功率可达6W,斜效率为30%;准连续输出功率在重复频率50kHz时可达4.75W,斜效率为24.73%,脉冲宽度为55.05ns;腔内插入布儒斯特片,在重复频率为50kHz时,双波长激光准连续线偏振输出功率可达2.22W,不稳定性小于0.52%,M² 关键词： 端泵Nd:YAG激光器 1319nm/1338nm双波长 声光调Q 太赫兹波     

LD端面抽运Nd:YAG 1319nm/1338nm双波长激光器研究

从LD端面抽运固体激光器的激光阈值公式出发，建立了双波长激光同时振荡的阈值条件，理论计算了腔镜对于两个波长的透过率关系，实现了LD端面抽运Nd:YAG 1319nm/1338nm双波长激光连续和准连续输出.双波长激光连续输出功率可达6W，斜效率为30%；准连续输出功率在重复频率50kHz时可达4.75W，斜效率为24.73%，脉冲宽度为55.05ns；腔内插入布儒斯特片，在重复频率为50kHz时，双波长激光准连续线偏振输出功率可达2.22W，不稳定性小于0.52%，M²     

Development of a saturated transient X-ray laser at 13.9 nm in LSAI

We summarise the progress achieved at LSAI in the understanding and optimisation of the transient collisional pumping of X-ray lasers using an ultra-short sub-ps heating pulse. The effect of travelling-wave irradiation on the lasing signal on the 4d¹S₀→4p¹P₁ Ni-like Ag line at 13.9 nm is studied in detail. Under specific irradiation conditions strong laser emission is also obtained on another spectral line at 16.05 nm, which is identified as the 4f¹P₁–4d¹P₁ transition in Ni-like Ag.