High-beam-quality, 5.1 J, 108 Hz diode-pumped Nd:YAG rod oscillator-amplifier laser system

A high efficiency, high beam quality diode-pumped Nd:YAG master oscillator power-amplifier (MOPA) laser with six amplifier stages is demonstrated. The oscillator with two-rod birefringence compensation was designed as a thermally determined near hemispherical resonator, which presents a pulse energy of 223 mJ with a beam quality value of M² = 1.29 at a repetition rate of 108 Hz. The MOPA system delivers a pulse energy of 5.1 J with a pulse width of 230 μs, a M² factor of 3.6 and an optical-to-optical efficiency of 38.5%. To the best of our knowledge, this is the highest pulse energy for a diode-pumped Nd:YAG rod laser operation with a high beam quality and a pulse width of hundreds of microseconds at a repetition rate of over 100 Hz.     

Q-switched Nd lasers pumped directly into the F3/2 emitting level

The influence of the direct pumping into the ⁴F_3/2 emitting level on the output characteristics of continuous-wave (CW) pumped, passively or actively (acoustooptic, AO) Q-switched Nd lasers is discussed. In case of passive Q-switching by Cr⁴⁺:YAG saturable absorber (SA) crystal, the change of pumping wavelength from 0.81 μm into the highly-absorbing ⁴F_5/2 level to 0.88 μm into the ⁴F_3/2 level of Nd does not modify the energy of the Q-switch pulse, but increases the pulse repetition rate and the laser average power for the same absorbed pump power. This is demonstrated with 0.81 and 0.88 μm CW laser diode-pumped Nd:YAG and Nd-vanadate lasers with average output power in the watt-level range at 1.06 μm. The effect is explained by the control of passive Q-switching by the intracavity photon flux that is influenced by the pump wavelength and by the initial transmission of the SA crystal. On the other hand, it is discussed and experimentally proved that due to the possibility to control externally the frequency of switching, in case of the AO Q-switched Nd laser the change of the pump wavelength from 0.81 to 0.88 μm increases the pulse energy for a fixed frequency, leading to a corresponding increase of the average laser power.     

Multiwavelength Q-switched CO2 laser with continuous discharge

A low-pressure (20 mbar) CO₂ laser allows to extract pulses at several selected wavelengths simultaneously from the same active medium. We demonstrated this, using an industrial laser modified by a Q-switch and a resonator with two branches. In one branch the wavelengths are spatially separated, whereas in the other they oscillate in one common transverse mode. We designed a multi-wavelength resonator which requires a single additional reflector compared to usual laser cavities. It provided tunable oscillation at six wavelength simultaneously.     

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     

Effect of the spiral phase element on the radial-polarization (0, 1) LG beam

Radially-polarized beams can be strongly amplified without significant birefringent-induced aberrations. However, radially-polarized beam is a high-order beam, and therefore has to be transformed into a fundamental Gaussian beam for reduction the beam-propagation factor M². In effort to transform the radially-polarized beam to a nearly-Gaussian beam, we consider effect of a spiral phase element (SPE) on the Laguerre-Gaussian (LG) (0, 1)^∗ beam with radial polarization, and compare this with the case when the input beam is a LG (0, 1)^∗ beam with spiral phase and uniform or random polarization. The LG (0, 1)^∗ beam with radial polarization, despite its identity in intensity profile to the beam with spiral phase, has distinctly different properties when interacting with the SPE. With the SPE and spatial filter, we transformed the radially-polarized (0, 1)^∗ mode with M² = 2.8 to a nearly-Gaussian beam with M² = 1.7. Measured transformation efficiency was 50%, and the beam brightness P/(M²)² was practically unchanged. The SPE affects polarization state of the radially-polarized beam, leading to appearance of spin angular momentum in the beam center at the far-field.     

High efficiency TEM00 mode, diode-pumped, conduction-cooled, Nd:YAG zig-zag slab laser

We report a compact, conduction-cooled, highly efficient, continuous wave (CW) Nd:YAG slab laser in diode-side-pumped geometry. To achieve high efficiency, a novel laser head for Nd:YAG slab has been developed. For an absorbed pump power of 27.6 W, maximum output power of 10.4 W in multimode and 8.2 W in near-diffraction-limited beam quality has been obtained. Slope and optical-to-optical conversion efficiencies are 45.3% and 37.7% in multimode with beam quality factors (M²) in x and y directions equal to 32 and 8, respectively. TEM₀₀ mode operation was achieved in a hybrid resonator with slope and optical-to-optical conversion efficiencies of 43.2% and 29.7%, respectively. Beam quality factors in x and y directions are ?1.5 and ?1.6 for the whole output power range. The laser radiation was linearly polarized and polarization contrast ratios are >1200:1 in the multimode and 1800:1 in the TEM₀₀ mode operation. In passive Q-switching with Cr⁴⁺:YAG crystal of 68% initial transmission, 18 ns pulsewidth has been achieved with an average power of 2 W at a repetition rate of 16 kHz.     

Performance of a zoom homogenizer for reshaping coherent laser beams

We have reshaped the TEM₀₀ beam emitted by a diode-pumped Nd-Yag laser by means of an optical homogenizer with zoom. The laser beam, first enlarged up to a size of (100 × 37.5) mm², is homogenized and resized to a final dimension continuously adjustable from (130 × 4.5) mm² up to (130 × 52) mm². We measured the plateau uniformity, the root mean square fluctuation and the edge steepness of the beam according to the ISO standard definitions, showing a poor reliability of the above parameter values due to the definitions themselves, and propose an amendment to overcome this problem. We obtained a very sharp edge steepness, but the spatial coherence of the laser beam put a lower limit to the high-frequency intensity fluctuations on the plateau region of the homogenized beam. Finally, we discuss the optimum homogenizer design for spatially coherent laser beams, including the depth of focus issue.     

Frequency splitting phenomenon of dual transverse modes in a Nd:YAG laser

We observed frequency splitting phenomenon of dual transverse modes (TEM_00q and TEM_01q) in a Nd:YAG microchip standing wave laser utilizing intracavity stress birefringence effects. Four resonance frequencies (ν_00qe, ν_00qo, ν_01qe, and ν_01qo, respectively) were produced and tuned by changing the diametral compression force applied on the laser crystal. The transverse mode frequency spacing for the same longitudinal mode number (Δν_trans) was 1.16 GHz, and the magnitude of frequency splitting (Δν) ranged from 0 MHz to 1.16 GHz. Based on this phenomenon, a four-mode differential standing wave laser, whose signal characteristics were a little like those of a four-mode differential travelling wave laser gyro however with a much larger frequency splitting range, was produced. The theoretical analysis is in good agreement with the experimental results. This phenomenon not only can be used to make lasers with two or more frequency differences, but also can be used to make high-resolution self-sensing laser sensors (e.g. laser force sensors and laser accelerometers).     

Q-switched and continuous-wave mode-locking of a diode-pumped Nd:Gd0.64Y0.36VO4−Cr4+:YAG laser

We report on a diode-pumped passively mode-locked Nd:Gd_0.64Y_0.36VO₄ laser with a Cr⁴⁺:YAG saturable absorber. Q-switched mode locking (QML) with 90% modulation depth was obtained. The peak power of the mode-locked pulse near the maximum of the Q-switched envelope was estimated to be about 1.7 MW at the pump power of 12 W. Besides QML, continuous-wave mode locking was also experimentally realized, for the first time to our knowledge, in the laser under a strong intracavity pulse energy fluence. The mode-locked pulse width is about 2.96 ps at a repetition rate of 161.3 MHz.     

Convergence of general beams into Gaussian intensity profiles after propagation in turbulent atmosphere

It is shown that a general shaped laser beam will eventually approach a Gaussian average intensity profile after propagation in turbulent atmosphere. In our formulation, source field at the exit plane of the laser is taken as the product of arbitrary functions of source transverse coordinates with Gaussian exponential modulations. Following the expansion of the arbitrary functions in terms of Hermite polynomials, the average receiver intensity expression is derived using the extended Huygens-Fresnel principle and the conditions for the intensity profile to assume a Gaussian shape are stated. The results are illustrated by simulating various source field distributions.     

Self-Q-switched and mode-locked Nd,Cr:YAG laser with 6.52-W average output power

Simultaneous self-Q-switched and mode-locked have been demonstrated in a diode-pumped Nd,Cr:YAG laser. For the first time as we know, almost 100% modulation depth has been achieved at an intracavity intensity of 5.6 × 10⁵ W/cm². The maximum average output power of 6.52 W corresponding to a slope efficiency of 30% is obtained at 1064 nm. The laser produces high-quality pulses in a TEM₀₀-mode at the pump power of 16.5 W. The pulse duration of the mode-locked pulses is about 600 ps with 136 MHz repetition rate.     

A global design of high power Nd-Yb co-doped fiber lasers

A global optimization method - niche hybrid genetic algorithm (NHGA) based on fitness sharing and elite replacement is applied to optimize Nd³⁺-Yb³⁺ co-doped fiber lasers (NYDFLs) for obtaining maximum signal output power. With a objective function and different pumping powers, five critical parameters (the fiber length, L; the proportion of pump power for pumping Nd³⁺, η; Nd³⁺ and Yb³⁺ concentrations, N_Nd and N_Yb and output mirror reflectivity, R_out) of the given NYDFLs are optimized by solving the rate and power propagation equations. Results show that dividing equally the input pump power among 808 nm (Nd³⁺) and 940 nm (Yb³⁺) is not an optimal choice and the pump power of Nd³⁺ ions should be kept around 10-13.78% of the total pump power. Three optimal schemes are obtained by NHGA and the highest slope efficiency of the laser is able to reach 80.1%.     

Production of intensities of ∼ 1019 W/cm2 by a table-top KrF laser

We report on the generation of intensities of 10¹⁹ W/cm^–2 by focusing the output beam of a table-top hybrid dye-excimer laser system operating at 248 nm. The laser system uses a pulsed dye laser and a single, commercially available excimer gain module. Considerations and optical arrangements for the optimization of the phase-front and the beam homogeneity of ultraviolet excimer amplifiers are presented.Prof. F.P. Schäfer on the occasion of his 65th birthday.     

Frequency stabilization of an internal mirror HeNe laser at 543.5 nm to I2-Saturated absorptions

An internal mirror green HeNe (GRENE) laser at 543.5 nm has been frequency stabilized to I₂-Doppler free absorption signals with a frequency stability of few parts in 10¹⁰. The saturated absorption signals were detected using doubly-differential saturated absorption which eliminates the Doppler background and allows first-derivative locking methods. Two absorption lines R(12)26-0 and R(106)28-0 of ¹²⁷I² were investigated and the hyperfine structure was resolved. Random polarization flips of the GRENE were suppressed by applying a transverse magnetic field to the laser tube near the anode.Present address: Physikalisch-Technische Bundesanstalt, Bundesallee 100, D-3300 Braunschweig, Fed. Rep. Germany     

High-efficiency continuous-wave and Q-switched diode-end-pumped multi-wavelength Nd:YAG lasers

We demonstrate high-efficiency diode-end-pumped multi-wavelength Nd:YAG lasers for continuous-wave and Q-switched operation. For the continuous-wave case, the Nd:YAG laser oscillates at 1.06 and 1.3 μm simultaneously; the maximum output power of 2.0 W (M² = 1.3) and 3.6 W (M² = 1.8) have been achieved at the incident pump power of 20.3 W, with the respective average slope efficiencies of 12.0% and 21.4%, for the lines of 1.06 and 1.3 μm, respectively. For the Q-switched operation, we achieve the average output power of 1.3 W (M² = 2.7) at 1.06 μm and 2.0 W (M² = 3.0) at 1.3 μm with the corresponding peak power of 10.2 and 4.2 kW under an incident pump power of 20.3 W.     

X-ray laser experiments using double foil nickel targets

New target geometries for collisional excitation X-ray laser experiments (in nickel) were proposed, analyzed, and experimentally studied on the Glass Development Laser (GDL). Experiments using a short line focus lens with new target geometries showed general agreement with predictions. The new geometries are designed to yield a higher gain and reduced refraction due to: (1) a higher plasma density, (2) a wider lateral density profile, and (3) a concave lateral density profile. These new geometries were: (1) two parallel exploding (thin) foils, irradiated from one side only, (2) two ablating (thick) foils, one of which is irradiated on its inner face, and (3) an exploding foil in front of an ablating foil, irradiated by a single laser beam incident on the thin foil. New experiments with a long line focus are in progress. The intensity ratio of Ne-like and F-like Ni lines is used to deduce the temperature, and these results together with the absolute intensity, yield the density profile. The results show improvement achieved with the new target geometries: the density is higher (leading to a higher gain), and the concave density profile results in collimation rather than divergence of the X-ray laser beam. Theoretical developments included: (1) prediction of high gain on new type transitions in neon like ions involving the excitation of an inner (2s) electron, and (2) development of a ray tracing code for an amplifying medium of varying (e.g., collimating) lateral density profile. Results using this code are shown for concave lateral density profiles.     

Homogeneous focusing with a transient soft X-ray laser for irradiation experiments

We report the work done on a transient soft X-ray laser (SXRL) beam to deliver a proper extreme UV irradiation source for applications. The same optical tool was first demonstrated on a quasi stationnary state (QSS) soft X-Ray laser at the PALS Institute in Prague. The problem set by the transient soft X-Ray laser developed by the LIXAM at the LULI installation in Palaiseau is more crucial, first because the beam spatial profile is more irregular secondly because high repetition rate soft X-ray laser facilities in the future are based on this SXRL type. The spots obtained show a 20 micron average diameter and a rather homogeneous and smooth profile that make them a realistic irradiation source to interact with targets requiring relatively high fluence (near 1 J/cm²) or intensity (near 10¹¹ W/cm²) in the extreme UV domain.     

Specialty Yb fiber amplifier for microchip Nd laser: Towards ∼1-mJ/1-ns output at kHz-range repetition rate

We demonstrate and optimize, for a mJ/ns release at the wavelength 1.064 μm, the operation of a compact laser system designed in the form of a hybrid, active-passive, Q-switched Nd³⁺:YAG/Cr⁴⁺:YAG microchip laser seeding an Yb-doped specialty multi-port fiber amplifier. As the result of the amplifier optimization, ∼1 mJ, ∼1 ns, almost single-mode pulses at a 1-10-kHz repetition rate are achieved, given by a gain factor of ∼19 dB for an 11-μJ input from the microchip laser. Meanwhile, a lower pulse energy, ∼120 μJ, but a much higher gain (∼25 dB) are eligible for the less powerful (0.35 μJ) input pulses.     

Mode profiles in resonators with a near 90° roof-top mirror

The transverse modes of a laser resonator comprising a near 90° roof-top mirror and a feedback mirror are studied theoretically. We relate this configuration to an almost plane roof-top resonator and apply the Fox and Li technique to compute the lowest-order symmetric and antisymmetric modes. The effects of perturbations to the vertex angle of the roof-top mirror, misalignments of the feedback mirror and curved feedback mirrors are examined.     

Diode-pumped Tm:KY(WO4)2 laser passively Q-switched with PbS-doped glass

Glass doped with PbS quantum dots is presented as a saturable absorber (SA) for a passive Q-switching of a diode-pumped 1.9 μm Tm:KYW laser. Output pulses with energy of 44 μJ at a repetition rate of 2.5 kHz with an average output power of 110 mW were obtained. The Q-switching conversion efficiency was 33%. The absorption saturation intensity of the glass doped with PbS quantum dots with a mean radius of 5.2 nm at a wavelength of 2 μm was measured to be 1.5 MW/cm².