Efficient delivery of 60 J pulse energy of long pulse Nd:YAG laser through 200 μm core diameter optical fibre

Most of today’s industrial Nd:YAG lasers use fibre-optic beam delivery. In such lasers, fibre core diameter is an important consideration in deploying a beam delivery system. Using a smaller core diameter fibre allows higher irradiances at focus position, less degradation of beam quality, and a larger stand-off distance. In this work, we have put efforts to efficiently deliver the laser output of ‘ceramic reflector’-based long pulse Nd:YAG laser through a 200 μm core diameter optical fibre and successfully delivered up to 60 J of pulse energy with 90% transmission efficiency, using a GRADIUM (axial gradient) plano-convex lens to sharply focus down the beam on the end face of the optical fibre and fibre end faces have been cleaved to achieve higher surface damage thresholds.     

Tuning the high-order harmonic lines of a Nd:Glass laser for soft X-ray laser seeding

We report here more than 50% coverage of the XUV spectral range between 18 nm and 35 nm by tuning the high-order harmonics generated by a fixed frequency Nd:Glass laser system. The tuning range achieved is suitable to seed Ni-like Y, Zr and Mo soft X-ray lasers.     

Nd:YAG waveguide laser with 1.3 W output power, fabricated by direct femtosecond laser writing

Tracks of modified material were written with femtosecond-laser pulses in neodymium-doped YAG crystals. Due to a stress-induced change of the refractive index, waveguiding beside the tracks and between two adjacent tracks with a distance of approximately 25 μm was observed. Loss measurements resulted in guiding losses of about 1.6 dB/cm for the double track waveguide. Spectroscopic investigations of the ⁴ F _3/2→⁴ F _11/2 transmission lines of the neodymium ions, which are close to the modified region, revealed a small stress-induced red shift of the lines. Laser oscillation of single-track waveguides and double-track waveguides was demonstrated with Ti:Sapphire laser pumping at a wavelength of 808 nm. Best laser performance with about 1.3 W output power at 2.25 W launched pump power was achieved using a double-track waveguide with a separation of 27 μm at an outcoupling transmission of 95%.     

Fabrication of integrated microchip for optical sensing by femtosecond laser direct writing of Foturan glass

By the one-continuous fabrication procedure of hollow microstructures using femtosecond (fs) laser direct writing followed by thermal treatment, successive chemical wet etching and additional annealing, three-dimensional integration of microoptics with microfluidics, i.e., a planoconvex microlens with a microfluidic chamber, in a single Foturan glass chip was achieved. Further integration of an optical waveguide was performed through internal refractive index modification by fs laser direct writing after the fabrication of the microlens and the microchamber. An “all-in-one” microchip that is highly effective for on-chip photonic biosensing can be manufactured by the present technique with easy assembly of each microcomponent and without any cumbersome processes for stacking and joining substrates. Experimental demonstration of photonic biosensing using the integrated microchip has revealed that fluorescence analysis and absorption measurement of liquid samples can be performed with efficiencies enhanced by factors of 8 and 3, respectively.     

High-energy PPMgLN optical parametric oscillator pumped by a 1.064 μm E-O Q-Switched Nd:YAG laser

A high-energy PPMgLN optical parametric oscillator (OPO) pumped by a E-O Q-switched Nd:YAG laser working at 1.064 μm was successfully illustrated. A maximum output pulse energy of 3.4 mJ was obtained with a pump threshold of 1.5 mJ and a slope efficiency of 30% around room temperature. The OPO output signal and idler wavelength were 1552 and 3384 nm, respectively. The damage to the input surface of PPMgLN crystal was carefully observed with a damage threshold of 4.6 J/cm².     

The analysis of the transient temperature distribution of double-slab Nd:YAG laser medium

A novel double-slab Nd:YAG laser, which uses face-pumped slab medium cooled by liquid with different temperatures on both sides, is proposed. The thermal distortion of wavefront caused by the non-uniform temperature distribution in the laser gain media can be self-compensated. According to the method of operation, the models of the temperature distribution and stress are presented, and the analytic solutions for the model are derived. Furthermore, the numerical simulations with pulse pumping energy of 10 J and repetition frequencies of 500 and 1000 Hz are calculated respectively for Nd:YAG laser medium. The simulation results show that the temperature gradient remains the approximative linearity, and the heat stress is within the extreme range. Then the absorption coefficient is also discussed. The result indicates that the doping concentration cannot be too large for the high repetition frequency laser. It has been proved that the high repetition frequency, high laser beam quality, and high average outp     

Diode-pumped laser with Yb:YAG single-crystal fiber grown by the micro-pulling down technique

Laser emission obtained from an Yb:YAG single-crystal fiber directly grown by the micro-pulling down technique is demonstrated for the first time. We achieved 11.2 W of continuous wave (CW) output power at 1031 nm for 55 W of incident pump power at 940 nm. In the Q-switched regime, we obtained pulses as short as 17 ns, for an average power of 2.3 W at 2 kHz corresponding to an energy of 1.15  mJ. In both cases, the M ² factor was 2.5. This single-crystal fiber showed performance similar to a standard rod elaborated by the Czochralski method. The potential of Yb³⁺-doped single-crystal fibers is presented for scalable high-average and high-peak-power laser systems.     

Study of the optical bistability in the laser oscillation of Yb:GdVO4 crystal

We have studied the effect of optical bistability exhibited in the laser oscillation of Yb:GdVO₄ crystal, revealing the complexities arising from the coexistence and switching of σ and π polarization states characteristic of this crystal. In terms of absorbed pump power, the range for bistable operation can be in excess of 1 W, while the output power generated at the up-threshold can reach as high as 0.71 W. The studies also show significant influence of the Yb concentration, the crystal thickness, the output coupling, and the resonator configuration on the bistable laser operation.     

Suppressing effect of the laser extraction on heat generation in Nd:YAG

The suppressing effect of the laser extraction on heat generation in Nd:YAG was investigated. The extraction efficiency could be deduced from the slope efficiency, and heat generation in Nd:YAG could be obtained with the heat model developed, which was verified by the previous experiment. The essential reasons were given to explain the change trend of heat generation under the condition of laser extraction.     

High-efficiency optical compression of Ti:sapphire laser pulses at 800 nm using a silver-coated hollow fiber

A 50 cm silver coated hollow fiber with inner diameter of 250 μm and filled with argon has been used to compress optical pulses from a Ti:sapphire laser at 800 nm. Input pulses with energy of 250 μJ and duration of 110 fs were used and compressed pulses with energy of 220 μJ and duration of 20 fs were generated by using a prism compressor. Numerical and experimental results are compared. There is good agreement between the measured beam diameters of the hollow-fiber output pulse and the calculated values obtained from propagation of the HE₁₁ mode into free space. For comparison, a similar uncoated fused-silica hollow fiber was also used to obtain 20 fs compressed pulses with an energy of 190 μJ. Received: 7 September 2002 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. Fax: +1-780/492-1811, E-mail: mohebbi@ee.ualberta.ca     

Energy reduction:a technique for seed-injection locking of single-axial-mode Q-switched Nd:YAG laser

A new technique for seed-injection locking of single-axial-mode (SAM) Q-switched Nd:YAG laser is re- ported.The technique called energy reduction (ER) is utilized when SAM operation is locked to design its feedback scheme.This method ensures long-term 100% seed-injection locking performance and the pulsed SAM output energy can reach as high as 200 mJ.Both temporal and spatial interferometric experiments have been executed to confirm the SAM oscillation.     

Analysis of the nonlinear absorption mechanisms in ablation of transparent materials by high-intensity and ultrashort laser pulses

The mechanisms of nonlinear absorption in transparent materials under irradiation with ultrashort laser pulses are considered theoretically. Nitride semiconductor, sapphire and others transparent dielectrics were investigated. The ablation threshold for these materials is within multi-TW/cm² range. The model was used based on the tunneling absorption under the irradiation by high-intensity ultrashort pulses in terms of the theory of ionization of solid in a field of strong electromagnetic wave. The effect of the energy gap of material on the threshold of laser ablation was adequately explained.     

Continuous adjustment of group delay by tuning the argument of coupling coefficient in microring coupled-resonator optical waveguides

We derive the dispersion relation of coupled-resonator optical waveguides (CROWs) without approximation. And, we use the exact dispersion relation of CROWs established to calculate the group delay of microring CROWs and obtain a result similar to the experimental result reported by Poon et al. Further, through numerical simulation with the parameters used to simulate the experimental result, we found that the output group delay of microring CROWs could be adjusted continuously by changing the argument of the coupling coefficient θ resulting from the shift of the dispersion band. But, the adjustment of output group delay was not linear and meticulous control of θ could lead to a more favorable adjustment of the output group delay. The continuous adjustment of group delay is of great significance for applications of microring CROWs in delay lines and optical buffers of future all-optical communication systems.     

Travelling-wave 1.57-µm optical parametric oscillator driven by a pulsed multimode Nd3+:YAG laser

The generation properties of an optical parametric oscillator (OPO) with a three-mirror ring resonator have been investigated under conditions of a pulsed pump by multimode radiation from a Nd³⁺:YAG laser. KTP crystals were used as the OPO nonlinear medium. At pump energies up to 100 mJ, decreasing the diameter d of the Nd³⁺:YAG laser beam causes a decrease in OPO radiation divergence and an increase in the generation efficiency at λ = 1.571 μm despite a decrease in the differential efficiency. At d = 2.25 mm and a KTP crystal total length of 40 mm, the efficiency of the pump conversion to the signal-wave pulse reaches 36.5%. Based on the traveling-wave OPO, a compact, highly effective, eye-safe laser source radiating pulses of up to 35 mJ in energy, 11 nsec in duration, and 10 Hz in repetition rate at electrical pump energy of ≤ 8 J is developed. At the 86.5% level of the total pulse energy, the source-beam divergence does not exceed seven diffraction limits. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 4, pp. 516–523, July–August, 2008.     

Simultaneous compression of the pulsewidth and pulse train of a passively mode-locked Nd:YAG laser

Simultaneous compression of the pulsewidth and the pulse train of a passivelymode-locked Nd:YAG laser has been achieved by using a plano-convex unstable resonatorwith a nonlinear Sagnac ring interferometer.A single pulse energy of >30mJ and apulsewidth of ≤10ps have been obtained.Using this system,the LAGEOS and ETALONsatellites laser ranging have been performed successfully.     

Effects of Nd:YAG (532 nm) laser radiation on ‘clean’ cotton

The use of pulsed laser radiation in order to remove small particles from a substrate has gained a growing interest in the last decade, finding applications in several fields ranging from the microcircuits industry to cultural heritage restoration and conservation. The application of such a technique requires the knowledge of the correct laser irradiation parameters to be used in order to obtain a desired result avoiding substrate damage. In this paper we have studied the effect of frequency-doubled (532 nm) Nd:YAG laser radiation on clean cotton samples. We have observed that yellowing is present even at low fluences. This suggests that less invasive laser assisted particle removal techniques, some of which have already been proposed by different authors, must be considered. PACS 42.62.-b; 61.80.-x     

High repetition rate,Q-switched Ho:YAG laser resonantly pumped by a 20 W linearly polarized Tm: fiber laser

We describe an efficient, low-threshold, continuous-wave (CW) and Q-switched operation of a Ho:YAG laser resonantly, single-pass pumped by a 20 W linearly polarized narrow line width Tm: fiber laser at the wavelength of 1,908 nm. At room temperature for an output coupler of 30 % transmission, a maximum continuous-wave output power of 13.3 W for 18.9 W of absorbed pump power was achieved, corresponding to a slope efficiency of 73 %. In a quasi continuous-wave pumping regime, for several output couplers slope efficiencies of almost 82 % were observed. For a Q-switched operation, a Brewster-cut acousto-optic modulator was used. In a CW pumping regime, the pulse repetition frequency (PRF) was changed from 4 to 15 kHz. Under a Q-switched operation, the maximum output power of 12.25 W in relation to 15 kHz PRF was obtained; however, the maximum peak power of almost 250 kW at the PRF of 4 kHz was demonstrated. In the best case, for 4 kHz PRF, pulse energies of 2.18 mJ with a 8.8 ns FWHM pulse width (one of the shortest pulse durations observed in holmium-doped Q-switched lasers) were achieved. The laser operated at the wavelength of 2,090.23 nm with the FWHM line width of 0.95 nm. The beam quality factor of M ² was measured to be below 1.42 in both X and Y axis.     

Pulse-series 1.57 μm optical parametric oscillator pumped by a Q-switched Nd:YAG laser with a variable reflectivity mirror

A method of generating 1.57 μm pulse-series eye-safe laser based on optical parametric oscillator (OPO) was developed. The 1.57 μm pulse-series OPO was pumped by a Q-switched Nd:YAG laser with a variable reflectivity unstable resonator. Every pulse-series includes three micro-pulses, and the pulse repetition rates, pulse numbers and pulse intervals were adjustable. The maximum output energy of the 3-micro-pulse-series was 274 mJ. The optical-optical efficiency of the OPO was about 40%.