Bruno Labranche

Material micromachining using a pulsed fiber laser platform with fine temporal nanosecond pulse shaping capability

We report on recent advances in laser material processing using a novel pulsed fiber laser platform providing pulse shape agility at the nanosecond time scale and at high repetition rates. The pulse shapes can be programmed with a time resolution of 2.5 ns and with an amplitude resolution of 10 bits. Depending on the desired laser performances, the pulses are generated either by directly modulating the drive current of a seed laser diode or by modulating the output of a seed laser diode operated in CW with electro-optic modulators. The pulses are amplified in an amplifier chain in a MOPA configuration. Advanced polarization maintaining LMA fiber designs enable output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M2< 1.1) and narrow line widths suitable for efficient frequency conversion. Micro-milling experiments were carried out with stainless steel, in which processing microstructures of a few tens of microns in size usually represents a challenge, and aluminum, whose thermal conductivity is about 20 times higher than stainless steel. The results obtained with two metals having very different thermal properties using different pulse shapes with durations varying between 3 ns and 80 ns demonstrate the benefits of using lasers offering flexible pulse durations and controllable pulse intensity profiles for rapidly optimizing a process in different applications while using the same laser with respect to conventional methods based on pulsed laser with fixed pulse shapes. Numerous applications are envisioned in a near future, like the micromachining of multi-layered structures, in particular when working with the harmonics of the laser.

Diode-pumped-cw and quasi-cw Nd:GGG(Ca,Mg,Zr) laser

A Nd:GGG(Ca,Mg,Zr) and a Nd:YAG laser were compared when end-pumped by a continuous-wave laser diode. With a 30-mW single-stripe laser diode, 2.4 mW and 4.6 mW of output power were obtained for the Nd:GGG(Ca,Mg,Zr) and the Nd:YAG laser, respectively. With a 1-watt laser-diode array, the output power increased to 223 mW for the Nd:GGG(Ca,Mg,Zr) and 297 mW for the Nd:YAG laser. The Nd:GGG(Ca,Mg,Zr) was also pumped by a 19-watt quasi-continuous-wave laser-diode bar. Side and end pumping were compared. With the appropriate optics the end-pumped geometry was more efficient and gave more power, 47% slope efficiency with 4.3 watts of output power, compared to side-pumped geometry, 16% slope efficiency with 2.6 watts.

Micro-milling process improvement using an agile pulse-shaping fiber laser

We demonstrate the usefulness of INOs pulse-shaping fiber laser platform to rapidly develop complex laser micromachining processes. The versatility of such laser sources allows for straightforward control of the emitting energy envelop on the nanosecond timescale to create multi-amplitude level pulses and/or multi-pulse regimes. The pulses are amplified in an amplifier chain in a MOPA configuration that delivers output energy per pulse up to 60 μJ at 1064 nm at a repetition rate of 200 kHz with excellent beam quality (M2 < 1.1) and narrow line widths suitable for efficient frequency conversion. Also, their pulse-on-demand and pulse-to-pulse shape selection capability at high repetition rates makes those agile laser sources suitable for the implementation of high-throughput complex laser processing. Micro-milling experiments were carried out on two metals, aluminum and stainless steel, having very different thermal properties. For aluminum, our results show that the material removal efficiency depends strongly on the pulse shape, especially near the ablation threshold, and can be maximized to develop efficient laser micro-milling processes. But, the material removal efficiency is not always correlated with a good surface quality. However, the roughness of the milled surface can be improved by removing a few layers of material using another type of pulse shape. The agility of INOs fiber laser enables the implementation of a fast laser process including two steps employing different pulse characteristics for maximizing the material removal rate and obtaining a good surface quality at the same time. A comparison of material removal efficiency with stainless steel, well known to be difficult to mill on the micron scale, is also presented.

Modeling of an active TV system for surveillance operations

Night vision capability has become an indispensable tool for military and civilian surveillance operations. Low-light- level television (LLLTV) and Forward-Looking-IR (FLIR) devices have long been used for these applications. Nevertheless, both have their shortcomings when the identification of the target is essential for the success of the mission. LLLTV cannot provide god image resolution in ultra low-light level conditions and is very sensitivity to parasitic light. FLIR system have poor resolution when the temperature difference contrast conditions are not met.

Modified NDIR technique for HF monitoring in an industrial environment

Over the years, various optical techniques have been developed to provide in-situ real time HF monitoring of aluminum smelters and stacks. The most popular approaches have used either laser based absorption techniques or various forms of non-dispersive infrared spectroscopy (NDIR). The very long path lengths encountered in roof vents would seem to favor laser-based approaches or very narrow spectral sources in order to avoid interference from the water absorption background. However, in practice, it is possible to use a special NDIR technique using 3-IR filters. In-situ result show that these monitors can measure HF concentrations over a very large dynamic range, require essentially no periodic calibration and have very low maintenance cost. The choice of the central wavelengths and width of the filters is critical and was made possible with the help of a sophisticated atmospheric propagation computer model (FASCODE). Recent developments in solid state technology was also beneficial since the drift-prone PbS detectors could be replaced with much more stable InAs detectors. The performance of this technology is illustrated with the results of a 2-probe instrument monitoring a total distance of close to 1 km with simultaneous and independent information for each half hall. Thanks to the use of infrared fiber cables, the principal instrument module is conveniently located on the ground floor.

Use of fluoride fibers for NDIR monitoring of hydrogen fluoride in aluminum smelters

Monitors specifically designed to measure hydrogen fluoride (HF) have been tested and evaluated over a period of 4 years in a very demanding industrial environment. These instruments use a non-dispersive IR spectroscopy (NDIR) technique and IR optical fibers as light guides between the central unit and the remote measurement points. In this application, the use of optical fiber provides two great advantages: 1) The central unit can be left in a control room where the conditions are much less difficult and, 2) the capability of the central unit to process the optical information coming from two probe heads significantly reduces the overall costs by measurement points. Since all- fiber components are not common for these fibers, special optical components were designed and built. We have found that the limits of this, and probably most other similar techniques, have more to do with long term stability than short-term sensitivity. In that respect, special attention to the fiber cable assembly and mounting is critical and some examples and mounting is critical and some examples will illustrate this point. Results of test for monitors use din roof top vents of aluminum smelters are given.

Rugged approach for hydrogen fluoride monitoring in aluminium smelters

A real time monitor, specifically designed to measure hydrogen fluoride (HF) concentration at the exit of the air purification system of aluminum smelters, has been tested and evaluated over a full year. The system has been designed to be rugged, with a low operating cost and easy to install and maintain in operation. These objectives have been achieved using a cheap halogen lamp as a light source, a simple micromotor as a light modulator, a pair of interference filters as optical analyzing elements and optical fibers as light guides between the central unit and the remote measurement points. In this application, the use of optical fiber provides two great advantages. Firstly, measurements in very demanding conditions become possible since the central unit, which has the task to make the entire optical electronic and digital processing, can be left in a control room where the conditions are much less difficult. Secondly, the capability of the central unit to process the optical information coming from two probe heads significantly reduces the overall costs by measurement points. The detection sensitivity limit achieved is 0.1 mg/m3 with normal stack diameter. The accuracy is around 5% depending on the care devoted to the calibration process. The response time can be adjusted over a large range but is typically set at 10 seconds.

Fiber laser pumped tunable Cr2+:ZnSe laser

A 2.4 μm room-temperature continuous-wave Cr2+:ZnSe laser pumped by a Thulium Fiber Laser is experimentally investigated. The laser delivers up to 720 mW of output power with absorbed power slope efficiency of 52%. The laser is tunable from 2294 to 2678 nm. Gain-switching is also demonstrated by using a Pulsed Erbium Fiber Laser pump source. A maximum pulse energy of 0.36 μJ is achieved with an absorbed threshold of 0.28 μJ and an absorbed power slope efficiency of 41%.

Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber

Nonlinear compression for generation of high energy ultrashort pulses using an Yb-doped large mode area tapered fiber is reported. Single-stage amplifier gain larger than 43 dB is achieved, with energy of seed pulses (35 ps, 200 kHz) boosted up to 50 μJ at the amplifier output. Spectral broadening induced by self-phase modulation is shown to take place advantageously along the larger end of the counter-pumped active tapered fiber, where the mode area scales beyond 1000 μm2. Pulse durations as short as 1 ps and peak powers exceeding 16 MW are demonstrated thereafter using a chirped volume Bragg grating as a dispersive compressor. Efficient suppression of higher-order modes in the large mode area tapered fiber yields diffraction-limited output (M2 < 1.2) for optimal pulse compression.

Yb-doped large mode area tapered fiber with depressed cladding and dopant confinement

A polarization-maintaining Yb-doped large mode area fiber with depressed-index inner cladding layer and confinement of rare-earth dopants has been drawn as a long tapered fiber. The larger end features a core/clad diameter of 56/400 μm and core NA ~ 0.07, thus leading to an effective mode area over 1000 μm2. The fiber was tested up to 100 W average power, with near diffraction-limited output as the beam quality M2 was measured < 1.2. As effective single-mode guidance is enforced in the first section due to enhanced bending loss, subsequent adiabatic transition of the mode field in the taper section preserves single-mode amplification towards the larger end of the fiber.