Ultrashort pulse lasers with pulse durations < 1 ps make it possible to cold process a wide range of materials, while introducing virtually no heat into the workpiece. Industrial ultrashort pulse lasers are currently mainly limited to the wavelength range around 1 μm and below. With optical parametric frequency conversion, however, the addressable wavelength can be extended to the IRB range (1.5 to 3.0 μm). Based on a commercially available laser emitting at a wavelength of 1030 nm, the system presented here generates laser light at a wavelength of 2.06 μm in a two-stage process. First, in an optical parametric generator (OPG), part of the pump power is converted into the degenerated signal and idler field (2.06 μm). In an optical parametric amplifier (OPA), this field is further amplified by the remaining pump power. An optional seeding with a narrow-band diode laser can be used to influence the output bandwidth in a targeted manner. An output power of 18.5 W was generated from approximately 80 W input power. At a pulse repetition rate of 800 kHz, this corresponds to a pulse energy of approximately 23 μJ. Moreover, a beam quality M<sup>2</sup> of 1.8 and 2.0 in horizontal and vertical direction was achieved. The pulse duration at 2 μm at this operating point is about 600 fs at a pump pulse duration of 900 fs. At an operating point with optimized power, a maximum output power of about 28 W, corresponding to about 35 μJ of pulse energy, was generated. The overall conversion efficiency at this working point was more than 35 percent.
We present a laser source providing up to 18 W and 1.5 mJ at a wavelength of 3 μm. The output is generated by frequency conversion of randomly polarized multimode radiation at 1064 nm of an Nd:YAG laser in a two-stage conversion setup. The frequency converter comprises an optical parametric oscillator and a subsequent optical parametric amplifier using PPLN as nonlinear medium in both stages. To implement fiber-based beam delivery for materials processing, we coupled the output at 3 μm to a multimode ZrF<sub>4</sub>-fiber. This source was then used to remove epoxy resin from the surface of CFRP samples.
The spectral stability of a previously reported Ho:YLF single frequency pulsed laser oscillator emitting at 2051 nm is drastically improved by utilizing a narrow linewidth Optically Pumped Semiconductor Laser (OPSL) as a seed for the oscillator. The oscillator is pumped by a dedicated gain-switched Tm:YLF laser at 1890 nm. The ramp-and-fire method is employed for generating single frequency emission. The heterodyne technique is used to analyze the spectral properties. The laser is designed to meet a part of the specifications for future airborne or space borne LIDAR detection of CO<sub>2</sub>. Seeding with a DFB diode and with an OPSL are compared. With OPSL seeding an Allan deviation of the centroid of the spectral distribution of 38 kHz and 517 kHz over 10 seconds and 60 milliseconds of sampling time for single pulses is achieved. The spectral width is approximately 30 MHz. The oscillator emits 2 mJ pulse energy with 50 Hz pulse repetition frequency (PRF) and 20 ns pulse duration. The optical to optical efficiency of the Ho:YLF oscillator is 10 % and the beam quality is diffraction limited. To our knowledge this is the best spectral stability demonstrated to date for a Ho:YLF laser with millijoule pulse energy and nanosecond pulse duration.
Based on established short pulse lasers with an output wavelength around 1 μm optical parametric frequency converters open up the spectral range between 1.4 and 4.0 μm for the first time in a power range of interest to laser material processing. The systems can be flexibly adapted as regards wavelength, pulse parameters and spectral properties to the requirements of various applications.<p> </p> We will discuss technical implementation and characterization of different optical parametric generators (OPG) based on periodically poled Lithium Niobate (PPLN) to show the parameter flexibility of this approach as well as current technical limits. Actual design examples will address output wavelengths between 1.6 μm and 3.4 μm with output powers ranging from several watts to tens of watts. The pulse parameters of these lasers range from a pulse duration of 9 ps with a repetition rate of 86 MHz to 1.5 ns and 100 kHz.<p> </p> The spectral bandwidth of the OPG examined can be very large. In particular, spectral bandwidths of about 100 nm are measured at the degenerated point, where the output wavelength is equal to twice the pump wavelength. Even beyond this point, a spectrum of typically a few tens of nanometers width generally accompanies a large conversion efficiency (>50 %). For applications that require a narrower spectrum, the OPG can be operated in a seeded mode, where only a few milliwatts of power from a continuously emitting laser diode are sufficient to seed a pulsed high power OPG efficiently and reduce the bandwidth to few nanometers.
Laser radiation of 3 μm wavelength was generated by frequency conversion of an industrial IR laser and applied in the context of CFRP bonding pre-treatment. Reinforced and non-reinforced epoxy resins were treated with this radiation varying the relevant parameters such as laser power or treatment time. The interaction between laser radiation of 3012 nm and 1064 nm wavelength and matrix resin was analyzed mechanically (e.g. ablation depth), optically (such as fiber exposure) and chemically (e.g. contamination removal). The results gathered show that, even with the small achievable pulse fluences, a sufficient treatment of the specimens and a sensitive removing of the contaminated layers are possible.
Extensive studies on frequency doubling with ppSLT crystals are presented. This includes a detailed discussion on
design aspects and theoretical modeling predictions as well as experimental studies comparing the performance of
ppSLT crystals from different providers with and without MgO doping. Experimental analyses of their acceptance
parameters and crystal homogeneity are conducted with a pulsed microchip laser with low peak (6 kW) and low average
power (50 mW) resulting in a maximum conversion efficiency of up to 80 % for high quality MgO doped crystals. Based
on these results a compact converter module with fiber coupling is designed and tested with the radiation from the
microchip laser and a fiber laser source in comparison. The fiber laser provides an average power of about 1 W. Even at
this - still very moderate - power level a significant efficiency drop can be observed. Despite the advantage of higher
pulse peak (25 kW) power from the fiber laser source, careful design adaptations of the converter are required even to
preserve a conversion efficiency beyond 50%.
In this work the reduction of conversion efficiency due to spectral bandwidth of fiber laser radiation is investigated.
Subsequently, compensation optics to correct the spectral phase mismatching inside the nonlinear crystal is dimensioned
and tested. For the experimental study a laboratory fiber laser setup is used consisting of a seed diode and a three stage
fiber amplifier. The laser delivers an average output power of up to 100 W at 1 MHz. Even below the Raman threshold
the output is far away from Fourier limit, providing a nearly Lorentzian spectral shape and a temporal pulse width of
800 ps. As the bandwidth increases nearly linearly with the pump power of the third amplifier stage, this parameter could
be controlled for the experiments.
All conversion experiments are conducted with a moderate load of the nonlinear crystals, i.e. intensity less than
150 MW/cm2. Without compensation of the spectral phase mismatch, a maximum conversion efficiency of 15 % is
attained for a Type I configuration with a 20mm long LBO crystal. Using the compensation setup 27 W of green light are
obtained from 60 W infrared light at a bandwidth of 4.7 nm. Therefore the efficiency rises to 44% at the same load.