激光准直Cr原子束的实验研究

利用多普勒原理对Cr原子束进行横向准直.应用激光感生荧光技术稳定激光器的频率，把激光器的中心频率稳定在偏离Cr原子共振中心频率-5±0.26MHz的位置.根据理论计算出准直激光束的最小尺寸为13.7mm.根据实验数据选择合适的参数，实现利用多普勒原理横向准直Cr原子束，使原子束的横向分布缩小到原来的1/3. 关键词： 激光准直 激光感生荧光稳频 多普勒冷却     

Imaging and focusing of an atomic beam with a large period standing light wave

A novel atomic lens scheme is reported. A cylindrical lens potential was created by a large period ( 45 m) standing light wave perpendicular to a beam of metastable He atoms. The lens aperture (25 m) was centered in one antinode of the standing wave; the laser frequency was nearly resonant with the atomic transition 2³ S ₁–2³ P ₂ (=1.083 m) and the interaction time was significantly shorter than the spontaneous lifetime (100 ns) of the excited state. The thickness of the lens was given by the laser beam waist (40 m) in the direction of the atomic beam. Preliminary results are presented, where an atomic beam is focused down to a spot size of 4 m. Also, a microfabricated grating with a period of 8 m was imaged. We discuss the principle limitations of the spatial resolution of the lens given by spherical and chromatic aberrations as well as by diffraction. The fact that this lens is very thin offers new perspectives for deep focusing into the nm range.     

Measurement of isotope shifts,fine and hyperfine structure splittings of the lithium D lines

The lithium D lines were studied using a diode laser that was frequency modulated by an electro-optic modulator, to excite an atomic beam. The transmission of part of the laser beam through an etalon was monitored to correct for the nonlinearity of the laser scan. The results for the ^6,7Li 2 S _1/2 and 2 P _1/2 hyperfine splittings agree very well with the best existing data while those for the D1 isotope shift and ^6,7Li fine structure splittings disagree significantly from data obtained by a previous laser atomic beam experiment. Our result for the D1 isotope shift is very close to the latest value computed using Hylleraas variational theory. Received 8 April 2002 / Received in final form 26 June 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: wvw@yorku.ca     

Synthesis of DLC films by PLD from liquid target and dependence of film properties on the synthesis conditions

DLC (Diamond-like carbon films) were prepared by pulsed laser ablation of a liquid target at substrate temperatures from 18 to 600°C using 248 nm KrF excimer laser. The sp³ hybridization state carbon formation was additionally promoted by gaseous H₂O₂ flow through the reaction chamber and substrate excitation by the same laser beam. Deposited DLC films were characterised by Raman scattering spectroscopy and atomic force microscopy (AFM). Comparative AFM and Raman study shows that the increase in the content of sp³ type bonding in DLC is in correlation with the increase of the surface roughness of the samples prepared.     

Observations and analysis of resonant laser ablation of GaAs

Laser ablation of solid GaAs samples has been studied using one tunable pulsed dye laser. At relatively low laser power, enhancements of up to several hundred times have been observed in the yield of resonantly ionised Ga using laser wavelengths corresponding to the atomic transition 4² P _1/2-4² D _3/2. The influences of laser power and target geometry, on the ion yield and spectral profile, are discussed. It is argued that the resonant excitation and ionisation processes occur in the gas phase of the atoms ablated from the sample surface, and the observed asymmetric spectral profile results from laser-induced collisional processes, e.g., optical collisions, under conditions of relatively high atomic density in the interaction region. Potential applications are foreseen for resonant laser ablation in trace analysis.     

An atom faucet

We present a simple and efficient source of slow atoms. From a background vapour loaded magneto-optical trap (MOT), a thin laser beam extracts a continuous jet of cold rubidium atoms. The jet that is typical to leaking MOT systems is created without any optical parts placed inside the vacuum chamber. We also present a simple three dimensional numerical simulation of the atomic motion in the presence of these multiple saturating laser fields combined with the inhomogeneous magnetic field of the MOT. At a pressure of P _Rb87 = 10^-8 mbar and with a moderate laser power of 10 mW per beam, we generate a flux Φ = 1.3×10⁸ atoms/s with a mean velocity of 14 m/s and a divergence of 10 mrad. Received 13 January 2001     

High-power and efficiency continuous-wave operation of <Emphasis Type="Italic">a</Emphasis>-cut Ho:YAP laser at room temperature

We report the continuous-wave (CW) operation of a room-temperature a-cut Ho:YAP laser resonantly end-pumped by a diode-pumped Tm:YLF laser at 1.91 μm. A maximum CW output power of 14.6 W at 2118.7 nm for a-oriented Ho:YAP was obtained, corresponding to the slope efficiency of 69.35% and optical-to-optical conversion efficiency of 63.04% with respect to absorbed pump power. The laser operated at a single mode (TEM₀₀) with the beam quality factor of M ² ∼ 1.51.     

Sub-Doppler spectroscopy of atoms excited in the regime of Rabi oscillations in a thin gas cell

We carried out theoretical investigation about velocity-selective atomic excitation on long-lived (metastable) levels of an atomic vapour in a thin cell by a monochromatic laser beam, running in the normal direction. The regime of coherent Rabi oscillations is considered on the light-induced transition from a sublevel of the ground quantum term to a metastable atomic level. On the basis of density matrix equations for the two-level system, we analysed the atomic population density of the metastable level, when the sample is irradiated by resonant monochromatic laser beam with an annular cross-section versus atomic velocities and versus the detuning, the amplitude, and the geometry of the laser beam. It is shown that, in the centre of the annular region, it can be obtained a population distribution on the metastable level as a function of the laser detuning, characterized by a sharp narrow resonance profile, whose width is reduced with respect to the thermal Doppler width roughly by the ratio between the diameter of the irradiated region and the inner thickness of the cell. We suggest high-sensitive schemes, in order to detect these sub-Doppler resonances, by probing the population of the metastable state with a second laser beam, resonant with a transition leaving from the metastable level. The case of ¹S₀ → ³P₁ spin-forbidden transition of Ca is discussed in more detail     

Long wavelength vertical transition quantum cascade lasers operating CW at 110 K

The continuous wave (cw) operation of a quantum cascade laser at wavelengths ∼8 μm is reported. The structures, grown by molecular beam epitaxy in the AlInAs/GaInAs material system, are based on a vertical intersubband transition scheme and use a plasmon-enhanced waveguide geometry to reduce the losses and increase the confinement factor. The single mode optical power from one facet is 2 mW at a maximum operating temperature of 110 K. In pulsed operation the highest temperature is 210 K and the threshold shows a weak temperature dependence typical of this class of lasers, with aT_o=110 K.     

Multiwatt, tunable, diode-pumped CW Yb:GdCOB laser

We demonstrate that Yb-doped Ca₄GdO(BO₃)₃ (GdCOB) crystals are suitable for the development of high-power diode-pumped lasers emitting at around 1.04 μm. A 15%-doped Yb:GdCOB crystal was longitudinally pumped with a cw fiber-coupled diode emitting 10 W at 976 nm. While 5.2 W of diode power was absorbed, we obtained 3.2 W of 1043-μm laser light, with a beam quality factor M² equal to 3, and 2.5 W in a diffraction-limited beam. Furthermore, the laser is continuously tunable between 1018 and 1086 nm. Thermal effects have been investigated with a Shack–Hartmann wavefront analyser: although thermal lensing is not negligible, it does not affect the performance of the laser with the resonator design we used. Received: 1 August 2000 / Revised version: 18 September 2000 / Published online: 21 February 2001     

Polarized velocity selective spectroscopy of atomic rubidium using counterpropagating beams

A scheme to obtain dispersion-like profiles using polarized velocity selective spectroscopy is presented. A circularly polarized pump laser beam whose frequency is scanned, and a linearly polarized, probe beam locked to a resonant frequency in the atom cross at a rubidium absorption cell. The transmitted intensities of the probe beam, with mutually perpendicular polarization directions are detected as the frequency of the pump beam is scanned. The sum of these two signals gives absorption profiles, while the difference results in dispersion profiles. This scheme is tested in the D₂ manifold of atomic rubidium. Weaker cross-over lines are found to be present and the slopes of their dispersion profiles are found to be opposite to those of the atomic transitions. This allowed an unambiguous determination of the atomic lines in both ⁸⁵Rb and ⁸⁷Rb, something that is particularly useful for the identification of the repumping transition in neutral atom trapping experiments. The dispersion profiles obtained are also suitable for frequency locking to atomic transitions or cross-over lines in both isotopes.     

Antireflection structures written by excimer laser on CVD diamond

Two-dimensional antireflective periodical microstructures for the IR range are fabricated on the surface of CVD diamond films. These structures are created using an ArF excimer laser (λ=193 nm) and a direct writing scheme consisting of a beam collimator and a microscope objective to focus the beam onto the sample. Two different arrays are investigated. One has a spacing of 3 μm and is produced with single shots and the other one has a spacing of 4 μm and is produced with three shots per spot. The hole depth and shape are measured with an atomic force microscope (AFM). The optical transmittance and the scattering properties of the structure at 10.6 μm are reported for a CO₂ laser beam. With a spectrometer further transmission measurements in the range of 5 to 20 μm are performed. Received: 16 September 1999 / Accepted: 11 October 1999 / Published online: 24 March 2000     

Diode-laser-array end-pumped 14.3-W CW Nd:GdVO4 solid-state laser at 1.06 μm

A diode-laser-array end-pumped efficient CW Nd:GdVO₄ laser at 1.06 μm has been developed. A low-order-mode output power of 14.3 W was obtained at the maximum available pump power of 26 W, giving an optical conversion efficiency of 55% and an average slope efficiency of 62%. The laser output beam quality factor at full pump power was determined to be M²<1.8. It is also shown that only lightly doped Nd:GdVO₄ crystals are suitable for high-power end-pumped lasers. Received: 4 May 1999 / Published online: 29 July 1999     

Capillary discharge soft X-ray laslng in Ne-like Ar pumped by long current pulses

A capillary discharge soft X-ray laser operating at 46.9 nm on the transition 3p-3s (J = 0-1) of the Ne-like Ar has been realized by exciting the active medium with a long half-cycle duration current pulse of 140 ns. The current is produced by discharging a 10 nF water dielectric capacitor, initially charged to voltages lower than 200 kV by a six stage Marx generator, through a 15-cm long capillary channels. The laser amplification has been obtained by properly adjusting all the other experimental parameters. Utilizing a 3-mm in diameter Al₂O₃ capillary channel initially filled with 0.3 torr of Ar pressure, a laser beam, which has a 4-mrad divergence and a time duration of 1.3 ns, is characterized by a gain of 0.6±0.1 cm^-1. The stability of the plasma compression followed by the laser emission is verified. Received 13 December 2001     

Frequency chain towards the Ca intercombination line based on laser diodes: First step

Stable, narrow linewidth operation of red and 1.3 µm free-running laser diodes with external gratings in non-Littrow geometry is demonstrated. The resonance of the saturated fluorescence of an atomic beam with a contrast of 25% and a linewidth of 400 ± 50 kHz of the Ca intercombination line 4¹ S ₀–4³ P ₁ ( = 657 nm) is shown. A high-power (110 mW) single-mode external cavity laser diode at 1.3 µm is used for second-harmonic generation in a KTP crystal. The beat signal (signal to noise ratio about 25 dB) of 10 nW second-harmonic radiation at 1.3 µm and the radiation of a laser diode in the visible spectrum, as a step to realize a frequency chain, is observed.     

Linearly-polarized Tm-doped double-clad fiber laser

We report a linearly polarized Tm doped fiber laser. The fiber laser was set up by using a piece of polarization maintaining Tm doped double clad fiber of 5 m length as gain medium and a polarization beam splitter as a polarization selector. The fiber laser was pumped by a fiber pigtailed laser diode working at 790 nm with a maximum output power of 90 W. The linearly polarized Tm laser operated at wavelength around 2030 nm. A maximum output power up to 21.9 W was achieved when the pump power was 63.27 W with a threshold of 11.92 W, a slope efficiency of about 43.7%, and a polarization extinction ratio of 92.7% (11.37 dB). In addition to the blue fluorescence, we also observed the violet fluorescence under high pump power level. The up-conversion fluorescence was considered to be attributed to the ¹ G ₄ → ³ H ₆, and ¹ D ₂ → ³ F ₄ transitions of Tm ions, respectively.     

Surface evolution and laser damage resistance of CO2 laser irradiated area of fused silica

A 10.6 μm CO₂ laser has been reported to effectively mitigate the laser damage growth of fused silica. Two zones of the laser irradiated area are defined in this work: the distorted zone and the laser affected zone. The parameters of the two zones are studied at different CO₂ laser beam sizes, irradiation times, and powers by microscopy, profilometry, and photoelastic method. The results show that the diameter of laser affected zone is almost completely determined by the laser beam size and the distorted zone is associated with the mitigation range of CO₂ laser beam. The diameter and depth of the distorted zone increase as the laser power and irradiation time increase. The depth grows exponentially depending on the irradiation time. The maximum residual stress discrepancy is located near the boundary of the laser affected zone. The laser damage resistance test results show that the distorted zone and the laser affected zone have a better damage resistance than the original substrate.     

Formation of highly organised,periodic microstructures on steel surfaces upon pulsed laser irradiation

We present results on the growth of highly organised, reproducible, periodic microstructure arrays on a stainless steel substrate using multi-pulsed Nd:YAG (wavelength of 1064 nm, pulse duration of 7 ns, repetition rate of 25 kHz, beam quality factor of M ²∼1.5) laser irradiation in standard atmospheric environment (room temperature and normal pressure) with laser spot diameter of the target being ∼50 μm. The target surface was irradiated at laser fluence of ∼2.2 J/cm² and intensity of ∼0.31×10⁹ W/cm², resulting in the controllable generation of arrays of microstructures with average periods ranging from ∼30 to ∼70 μm, depending on the hatching overlap between the consecutive scans. The received tips of the structures were either below or at the level of the original substrate surface, depending on the experimental conditions. The peculiarity of our work is on the utilised approach for scanning the laser beam over the surface. A possible mechanism for the formation of the structures is proposed.     

High-power end-pumped CW Nd:GdVO4 laser formed with a flat–flat cavity

Formed with a flat–flat resonator, a diode-laser-array end-pumped CW Nd:GdVO₄ laser at 1.06 μm, capable of generating 8.6 W of TEM₀₀ output power with optical conversion efficiency of 43% and slope efficiency of 48%, has been developed. The laser beam was nearly diffraction limited, with the beam quality factor measured to be M²=1.22. Under the conditions of multi-mode operation, the laser was able to produce 11.2 W of low-order transverse mode radiation (M²<2) at the incident pump power of 22 W, giving an optical conversion efficiency of 51%, and a slope efficiency of 55%.     

26 mJ Total Output from a Gain-Switched Single-Mode Er3+-Doped Zblan Fiber Laser Operating at 2.8 $$ \mu $$m

We demonstrate a laser-diode-pumped gain-switched Er³⁺-doped ZrF₄-BaF₂-LaF₃-AlF₃-NaF (ZBLAN) fiber laser operating in a single transverse mode at 2.8 \( \mu \)m. The laser pulses produced offer high-pulse energies, with repetition rates ranging from 50 Hz to 10 kHz and a slope efficiency of approximately 14.3% with respect to the launched pump power. The average power at the 50 Hz repetition rate is 1.33 W, giving a maximum total output pulse energy of 26.6 mJ per pump pulse. The fiber laser operates in a single mode, with beam quality factor M² less than 1.2.