CW Regime Research Articles

Ti-6Al-4V Cutting by 100W Fibre Laser in Both CW and Modulated Regime

Titanium and its alloys are nowadays widely used in many sectors: in the medical field (orthopedic and dental ones), in the architectural field, in the chemical plants field and in aeronautic [1]. In this last field it is more and more used both for its contribution to make lightweight and time durable structures and for its compatibility with new materials, first of all Carbon Fiber Reinforced Plastics (CFRP). Cutting of titanium sheets is one of the primary requirements in the fabrication of most of the components. Laser cutting offers several advantages over conventional cutting methods. It includes narrow kerf width (minimum material lost), straight cut edges, low roughness of cut surfaces, minimum metallurgical and surface distortions, easy integration with computer numerically controlled (CNC) machines for cutting complex profiles and importantly non-contact nature of the process (suitable for cutting in hostile environments and in areas with limited access) [2]. However, due to very limited literature available on laser cutting of titanium, it is very difficult to predict the cut surface quality and optimum process parameters for laser cutting, especially when dross-free cuts are required. Laser cutting of titanium and titanium alloys needs to be carried out with an inert gas, this due to the high reactivity of the titanium with the oxygen at high temperatures [3]. However when the available power is limited, as in the present case, the use of a reactive gas (air) can help to achieve cutting speed value reasonable for industrial applications. The aim of this work is to study the cutting of Ti-6Al-4V rolled sheets 1 mm in thickness, by means of a 100 W fibre laser, (SPI-Red Power) working at wavelength  = 1090 nm. The maximum cutting speed were measured in both CW and pulsed regime at different mean power and different duration. Furthermore, the kerf geometry and the heat affected zone (HAZ) were studied decreasing the cutting speed from the maximum to the 80 % of this values. The results obtained showed that both the power and the cutting speed influence the cutting kerf geometry and HAZ. In particular the synergy of power and speed, resulting roughly into the heat input, seems to rule the whole cutting process.

Efficient infrared (≈1.9–2.0 μm) laser operation in color-defect-free Tm:NaGd(MoO4)2 crystal

Color-defect-free 5 at.% Tm:NaGd(MoO4)2 crystals have been grown in a Na2MoO4/Na2Mo2O7 flux. Using a hemispherical optical cavity and pumping at λ = 794.5 nm with a Ti-sapphire laser, up to 850 mW of output power at λ ≈ 1900 nm was obtained at 300 K with an output coupler transmission of 8%. In the cw regime, the slope efficiency versus absorbed power was η = 45% and the pump power laser threshold was ≈180 mW. The laser was tunable from 1875 to 1975 nm and the emission had a FWHM bandwidth ≈20 nm, indicating the potential for ultrashort laser pulse generation.

Effect of annealing on the structural and nonlinear optical properties of ZnO thin films under cw regime

We report on the studies of the effects of annealing on the structural and third-order nonlinear optical properties of ZnO thin films deposited on quartz substrates by the RF magnetron sputtering technique. The films were annealed in the temperature range 400–1000 °C. The third-order nonlinear optical studies were carried out using the z-scan technique under continuous wave (cw) He–Ne laser irradiation at 633 nm wavelength. The effects of annealing on the structural properties were examined using x-ray diffraction and atomic force microscopy (AFM). The (0 0 2) preferred orientation increased with increase in annealing temperature up to 800 °C. The crystalline phases of SiO2 were observed at higher annealing temperatures. The appearance of an extraneous phase was confirmed by AFM images and optical transmittance spectra. The samples exhibited nonlinear absorption and nonlinear refraction under the experimental conditions. The negative sign of the nonlinear refractive index n2 indicated that the films exhibit self-defocusing property due to thermal nonlinearity. The nonlinear refractive index n2, the nonlinear absorption coefficient βeff and the third-order optical susceptibility χ(3) were found to be of the highest orders. The estimated value of third-order optical susceptibility χ(3) was of the order of 10−3 esu. Multiple diffraction rings were observed when the samples were exposed to the laser beam. The appearance of rings was due to the refractive index change and thermal lensing. With increase in laser intensity, the variations of the self-diffraction ring patterns were studied experimentally. The films also exhibited strong optical limiting properties under cw laser excitation, and reverse saturable absorption was the dominant process leading to the observed nonlinear behaviour.

1 mJ Passively Q-switched Nd:YAB laser using a V:YAG crystal

We describe the design and operation of a diode-pumped compact and efficient Nd:YAB laser operating at 1338 nm. We use a passive Q-switch V:YAG crystal to achieve 1 mJ Q-switched pulses at the laser output. The laser elaborated provides an average power of 1.5 W in the CW regime and 1 W in Qswitched mode with an optical efficiency of 15 and 10%, respectively. A 65 mm plano-concave cavity is formed with an output in the TEM00 mode. At a pulse width of 15 ns, the achieved peak power is 66 kW.

χ(3) and optical power limiting measurements of Polyaniline and its derivative Poly (o-toluidine) under CW regime

We present the synthesis and results on the third-order optical nonlinearity and optical limiting properties of Polyaniline and Poly (o-toluidine) studied in N,N-Dimethyl Formamide solution using a single-beam z-scan technique. Experiments were performed using continuous wave He–Ne laser operating at 633nm wavelength as an excitation source. The samples exhibit absorption and refraction nonlinearities. The negative sign of the nonlinear refractive index n2 indicates that both materials exhibit self-defocusing optical nonlinearity. The nonlinear refractive index n2, the nonlinear absorption coefficient βeff and the third-order optical susceptibility χ(3) were determined and found to be of the order of 10−7cm2/W, 10−2cm/W and 10−7esu respectively. The FTIR and UV–visible characterization spectra confirm the expected molecular structure of the polymers. Multiple diffraction rings were observed when the samples were exposed to laser beam. The appearance of rings is due to the refractive index change and thermal lensing. The polymer samples with different concentrations exhibits strong optical power limiting under continuous wave laser at the experimental wavelength. Reverse saturable absorption is the dominating mechanism for the observed absorption nonlinearities.

Design of RF power coupler for superconducting cavities

A new power coupler has been designed and is being prototyped by Argonne National Laboratory (ANL) for use with any of the ANL proposed superconducting (SC) half- or quarter-wave cavities for SARAF [1] and Project-X [2]. The 50 Ohm coaxial capacitive coupler is required to operate in the CW regime with up to 15 kW of forward power and under any condition for the reflected power. A key feature is a moveable copper plated stainless steel bellows which will permit up to 3 cm of axial stroke and adjustment of the external quality factor by roughly one order of magnitude in the range of 105 to 106. The mechanical and vacuum design includes two ceramic windows, one operating at room temperature and another at 70 Kelvin. The two window design allows the portion of the coupler assembled onto the SC cavity in the clean room to be compact and readily cleanable. Other design features include thermal intercepts to provide a large margin for RF heating and a mechanical guide assembly to operate cold and under vacuum with high reliability.

Passively Q-switched microchip Er, Yb:YAl_3(BO_3)_4diode-pumped laser

We report, for the first time to our knowledge, a diode-pumped cw and passively Q-switched microchip Er, Yb:YAl(3)(BO(3))(4) laser. A maximal output power of 800 mW at 1602 nm in the cw regime was obtained at an absorbed pump power of 7.7 W. By using Co(2+):MgAl(2)O(4) as a saturable absorber, a TEM(00)-mode Q-switched average output power of 315 mW was demonstrated at 1522 nm, with pulse duration of 5 ns and pulse energy of 5.25 μJ at a repetition rate of 60 kHz.

Single-frequency hybrid laser with an output power up to 3 W at a wavelength of 1064 nm

A high-power single-frequency laser with an output power of 2.5 W in the cw regime at a wavelength of 1064 nm has been developed using a hybrid scheme based on a master singlefrequency semiconductor laser (wavelength 1064 nm, lasing linewidth less than 3 MHz) and a two-cascade fibre amplifier pumped by high-power laser diodes. At pump powers of 4.8 W in the first cascade and 6.8 W in the second cascade the total gain is about 100.

Two-frequency vanadate lasers with mutually parallel and orthogonal polarisations of radiation

Luminescent and lasing properties of α-cut Nd : YVO4, Nd : GdVO4, Nd:Gd1-xYxVO4, and Nd:Sc1-xYxVO4 vanadate crystals are experimentally studied for the p- and s-polarisations of the emission at the 4F3/2 — 4F11/2 transition. Two-frequency lasing was obtained with mutually orthogonal and parallel polarisations of radiation in the cw, Q-switching, and mode-locking regimes. A scheme of a laser — amplifier of two-frequency pulses is realised.

High-power efficient cw and pulsed lasers based on bulk Yb : KYW crystals with end diode pumping

End-diode-pumped lasers based on one and two Yb : KYW crystals operating in cw and Q-switched regimes, as well as in the regime of mode-locking, are studied. The single-crystal laser generated stable ultrashort (shorter than 100 fs) laser pulses at wavelengths of 1035 and 1043 nm with an average power exceeding 1 W. The average output power of the two-crystal laser exceeded 18 W in the cw regime and 16 W in the Q-switched regime with a slope efficiency exceeding 30 %.

Extended ABCD matrix formalism for the description of femtosecond diffraction patterns; application to femtosecond digital in-line holography with anamorphic optical systems

We present a new model to predict diffraction patterns of femtosecond pulses through complex optical systems. The model is based on the extension of an ABCD matrix formalism combined with generalized Huygens-Fresnel transforms (already used in the CW regime) to the femtosecond regime. The model is tested to describe femtosecond digital in-line holography experiments realized in situ through a cylindrical Plexiglas pipe. The model allows us to establish analytical relations that link the holographic reconstruction process to the experimental parameters of the pipe and of the incident beam itself. Simulations and experimental results are in good concordance. Femtosecond digital in-line holography is shown to allow significant coherent noise reduction, and this model will be particularly efficient to describe a wide range of optical geometries. More generally, the model developed can be easily used in any experiment where the knowledge of the precise evolution of femtosecond transverse patterns is required.

Tm3+-based waveguide lasers in monoclinic double tungstates

We review our recent results on the fabrication and characterization of Tm3+-based waveguide lasers on monoclinic potassium double tungstates. We demonstrated laser oscillation in the CW regime, at ∼1.84μm, in slab and channel waveguides. In addition, using such guiding layers, we have demonstrated passive Q-switching in a slab waveguide laser.

Optimization of end-pumped, actively Q-switched quasi-III-level lasers

The new model of end-pumped quasi-III-level laser considering transient pumping processes, ground-state-depletion and up-conversion effects was developed. The model consists of two parts: pumping stage and Q-switched part, which can be separated in a case of active Q-switching regime. For pumping stage the semi-analytical model was developed, enabling the calculations for final occupation of upper laser level for given pump power and duration, spatial profile of pump beam, length and dopant level of gain medium. For quasi-stationary inversion, the optimization procedure of Q-switching regime based on Lagrange multiplier technique was developed. The new approach for optimization of CW regime of quasi-three-level lasers was developed to optimize the Q-switched lasers operating with high repetition rates. Both methods of optimizations enable calculation of optimal absorbance of gain medium and output losses for given pump rate.

130 mW average power, 46 nJ pulse energy, 102 ps pulse duration from an Er^3+ fiber oscillator passively mode locked by a resonant saturable absorber mirror

An Er(3+) fiber laser passively mode locked by a resonant saturable absorber mirror achieves more than 130 mW average power at 1560 nm from a Fabry-Perot cavity. The pulsed regime is self-starting from the CW regime without any Q-switch transition. The output pulse has a duration of 10.2 ps and can be extracavity dechirped with 42% efficiency down to 614 fs, which represents 1.2 times the Fourier limit imposed by the spectrum. To date, this corresponds to the highest averaged power directly extracted at such a wavelength from a fiber laser mode locked with a saturable absorber mirror.

Comparative study of homoepitaxial single crystal diamond growth at continuous and pulsed mode of MPACVD reactor operation

Homoepitaxial growth of single crystal diamond by microwave plasma chemical vapor deposition in pulsed regime of a 2.45 GHz MPACVD reactor operation at pulse repetition rates of 150 and 250 Hz was investigated. The high quality CVD diamond layers were deposited in the H 2–CH 4 gas mixture containing 4% and 8% of methane, gas pressures of 250 and 260 Torr and substrate temperature of 900 °C without any nitrogen addition. The (100) HPHT single crystal diamond seeds 2.5 × 2.5 × 0.3 mm (type Ib) were used as substrates. At pulse repetition rate 150 Hz the high quality single crystal diamond was grown with growth rate of 22 μm/h. The comparison of the single crystal diamond growth rates in CW and pulsed wave regimes of MPACVD reactor operation at microwave power density 200 W/cm 3 was made. It was found that at equal power density, the growth rate in pulsed wave regime was higher than in CW regime. Differences in single crystal diamond growth for two sets of experiments (with continuous and pulsed wave regimes) were explained.

$Q$-Switched DFB Fiber Laser Based on a Shape Memory Alloy Optical Fiber Phase Modulator

A detailed analysis of the behavior of a DFB fiber laser based on a high-speed tunable intra-cavity phase-shifter is presented. The phase-shifter made of a specially designed shape memory alloy is inducing a birefringence, thus allowing the laser to operate on either x or y polarization mode in the CW regime or in the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -switched regime. The spectral characteristics of the laser are analyzed in detail for both regimes and the cavity properties are derived from the relaxation oscillation frequency.

High-power pulsed diode-pumped Er:ZBLAN fiber laser

We report on the operation and performance of a gain-switched Er:ZBLAN fiber laser based on an active pulsed diode pump system. The produced laser pulses offer high peak powers while retaining the high average powers and efficiency of the cw regime. The measured pulse duration was about 300 ns and nearly independent of the pump repetition frequency. The maximum obtained 68 W of peak power is the highest reported, to our knowledge, for diode-pumped Er:ZBLAN fiber lasers, and the 2 W of average power at the repetition frequency of 100 kHz is 2 orders of magnitude higher than previously reported average power in a pulsed regime. The obtained slope efficiency was 34%.

Hybrid microspheres for nonlinear Kerr switching devices

Electronic Kerr effect in a polyfluorene derivative is used to reversibly switch near infrared probe beam resonantly coupled to a hybrid polymer-silica microspherical resonator. NIR pumping at 780 nm in pulsed laser regime is used for non-linear switching of the WGM resonances that shift as much as 2 GHz for 50 mW of average pump power, compared to a shift of 250 MHz for the same average pump power at CW regime. The absence of temporal drift and the magnitude of this shift confirm the Kerr nature of the switching, ruling out thermooptical effects.

Beat-note locking in dual-polarization lasers submitted to frequency-shifted optical feedback

We derive a delay-differential equation model that describes continuous-wave (cw) or passively Q-switched (PQS) two-frequency solid-state lasers submitted to frequency-shifted feedback (FSF). The study focuses on the locking of the beat note between the two free-running laser frequencies to a reference external frequency. The locking domain is obtained analytically in the cw regime. The PQS regime is treated by adding a saturable absorber population in the model equations. In this case, numerical simulations permit us to evaluate a locking range that is smaller than in the cw case. We find good agreement between the theoretical predictions and experiments carried out with a cw diode-pumped dual-polarization Nd:YAG laser as well as with previously published experimental results obtained with cw Er:Yb:glass [ Opt. Lett. 32, 1099 (2007)] and PQS Nd:YAG [ Opt. Lett. 33, 2524 (2008)] lasers. Applications of the FSF locking technique include the lidar-radar technique, for which a highly coherent beat note is required.

Nonlinear optical response of liquid crystalline azo‐dendrimer in picosecond and CW regimes

AbstractThe nonlinear optical response of the liquid crystalline multiarm star‐shaped azodendrimer was investigated in picosecond pulse and CW regimes at 532 nm. The polymer exhibited large nonlinear refractive coefficient in two regimes (n2 = −2.88 × 10−13 cm2/W and −1.1 × 10−10 cm2/W under picoseconds pulse excitation, whereas n2 = −1.4 × 10−6 cm2/W and n2 = −8.8 × 10−5 cm2/W under CW laser excitation in solution and film, respectively). The mechanism accounting for the process of nonlinear refraction was discussed. The value of photoinduced birefringence in the polymer film was also measured (Δn ∼ 10−3) under CW laser excitation at 532 nm. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011