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 M2 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.
In the field of atmospheric research, lidar is a powerful technology that can measure gas or aerosol concentrations, wind speed, or temperature profiles remotely. To conduct such measurements globally, spaceborne systems are advantageous. Pulse energies in the 100-mJ range are required to achieve highly accurate, longitudinal resolved measurements. Measuring concentrations of specific gases, such as CH4 or CO2, requires output wavelengths in the IR-B, which can be addressed by optical-parametric frequency conversion. An OPO/OPA frequency conversion setup was designed and built as a demonstration module to address the 1.6-μm range. The pump laser is an Nd:YAG-MOPA system, consisting of a stable oscillator and two subsequent Innoslab-based amplifier stages that deliver up to 500 mJ of output pulse energy at 100 Hz repetition frequency. The OPO is inherited from the OPO design for the CH4 lidar instrument on the French–German climate satellite methane remote-sensing lidar mission (MERLIN). To address the 100-mJ regime, the OPO output beam is amplified in a subsequent multistage OPA. With potassium titanyl phosphate as nonlinear medium, the OPO/OPA delivered more than 100 mJ of output energy at 1645 nm from 450 mJ of the pump energy and a pump pulse duration of 30 ns. This corresponds to a quantum conversion efficiency of about 25%. In addition to demonstrating optical performance for future lidar systems, this laser will be part of a laser-induced damage thresholds test facility, which will be used to qualify optical components especially for the MERLIN.
In the field of atmospheric research, LIDAR is a powerful technology that can measure gas or aerosol concentrations, wind speed or temperature profiles remotely. To conduct such measurements globally, spaceborne systems are advantageous. Pulse energies in the 100 mJ range are required to achieve highly accurate, longitudinal resolved measurements. Measuring concentrations of specific gases, such as CH4 or CO2, requires output wavelengths in the IRB, which can be addressed by optical parametric frequency conversion.
An OPO/OPA frequency conversion setup was designed and built as a demonstration module to address the 1.6 μm range. The pump laser is an Nd:YAG-MOPA system, consisting of a stable oscillator and two subsequent Innoslab-based amplifier stages that deliver up to 500 mJ of output pulse energy at 100 Hz repetition frequency. The OPO is inherited from the OPO design for the CH4 lidar instrument on the French-German climate satellite MERLIN. In order to address the 100 mJ regime, the OPO output beam is amplified in a subsequent multistage OPA. With KTP as nonlinear medium, the OPO/OPA delivered more than 100 mJ of output energy at 1645 nm from 450 mJ of the pump energy and a pump pulse duration of 30 ns. This corresponds to a quantum conversion efficiency of about 25 %.
Besides demonstrating optical performance for future lidar systems, this laser will be part of a LIDT test facility, which will be used to qualify optical components especially for the MERLIN mission.
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.
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.
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.
We present a theoretical and experimental analysis of a pulsed 1645 nm optical parametric oscillator (OPO) to prove the
feasibility of such a device for a spaceborne laser transmitter in an integrated path differential absorption (IPDA) lidar
system. The investigation is part of the French-German satellite mission MERLIN (Methane Remote Sensing Lidar
Mission). As an effective greenhouse gas, methane plays an important role for the global climate.
The architecture of the OPO is based on a conceptual design developed by DLR, consisting of two KTA crystals in a
four-mirror-cavity. Using numerical simulations, we studied the performance of such a setup with KTP and investigated
means to optimize the optical design by increasing the efficiency of the OPO and decreasing the fluence on the optical
components. For the experimental testing of the OPO, we used the INNOSlab-based ESA pre-development model
ATLAS as pump laser at 1064 nm. The OPO obtained 9.2 mJ pulse energy at 1645 nm from 31.5 mJ of the pump and a
pump pulse duration of 42 ns. This corresponds to an optical/optical efficiency of 29%. After the pump pulse was
reduced to 24 ns, a similar OPO performance could be obtained by adapting the pump beam radius. In recent
experiments with optimized optical design the OPO obtained 12.5 mJ pulse energy at 1645 nm from 32.0 mJ of the
pump, corresponding to an optical/optical efficiency of 39%. Two different methods were applied to study the laser
damage thresholds of the optical elements used.
We present a theoretical and experimental analysis of a pulsed 1645 nm optical parametric oscillator (OPO) conducted to prove the feasibility of such a device for a spaceborne laser transmitter in an integrated path differential absorption (IPDA) lidar system. The investigation is part of the French-German satellite mission MERLIN (Methane Remote Sensing Lidar Mission). As an effective greenhouse gas, methane plays an important role for the global climate. The architecture of the OPO is based on a conceptual design developed by DLR, consisting of two KTA crystals in a four-mirror-cavity. One of the cavity mirrors is piezo-driven to provide single frequency operation of the OPO. Using numerical simulations, we studied the performance and alignment tolerances of such a setup with KTP and KTA and investigated means to optimize the optical design by increasing the efficiency and decreasing the fluence on the optical components. For the experimental testing of the OPO, we used the INNOSlab-based ESA pre-development model ATLAS as pump laser at 1064 nm. At a pulse frequency of 25 Hz this MOPA delivers a pump energy up to 45 mJ with a beam quality factor of about M² = 1.3. With KTP as nonlinear crystal the OPO obtained 9.2 mJ pulse energy at 1645 nm from 31.5 mJ of the pump and a pump pulse duration of 42 ns. This corresponds to an optical/optical efficiency of 29%. After the pump pulse was reduced to 24 ns a similar OPO performance could be obtained by adapting the pump beam radius.
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%.
We present a compact module, emitting nearly diffraction limited green laser light at 531 nm at an average output power
of more than 500 mW. As pump source for the second harmonic generation a DBR tapered laser with a total length of
6 mm was used. The RW section had a length of 2 mm including a 1 mm long passive DBR section. The devices were
mounted p-side up on a copper block. For this mounting scheme, the device reaches up to 7 W maximal output power. At
the power level of about 3.8 W used in the presented experiment, a wavelength of 1062.6 nm with a line-width below
0.02 nm (FWHM) was determined. More than 80% of the emitted power is originated within the central lobe of the beam
waist profile illustrating the nearly diffraction limited beam quality. Using a 30mm long MgO-doped periodically poled
LiNbO3 bulk crystal, the second harmonic wave is generated in a single-pass setup. Due to precise alignment and beam
shaping based on the results of numerical simulations and a properly temperature control of the PPLN crystal, a
maximum optical conversion efficiency of more than 14% (3.7%/W) was achieved. The fluctuation of the output power
is far below 1%.
Performance data of a gain switched Ti:Sapphire laser operating at a repetition rate of one kilohertz is presented. With
one single set of optics the tuning range of the laser exceeds 400 nm. The output power is 4.2 W at 800 nm with an
almost diffraction limited beam quality and a pulse duration of 8 ns. The laser control includes two operational modes:
Running the laser precisely at a certain wavelength or alternatively scanning the output wavelength continuously
throughout a chosen band. Thanks to its high pulse peak power, narrow line width and high beam quality, this laser is
well suited for frequency conversion.
Design, theoretical modeling, and experimental characterization of a widely tunable Ti:Sapphire laser with nanosecond pulses and high pulse peak power is presented. The laser provides a continuous tuning range of from 675 nm to 1025 nm with no exchange of optics required. At a pulse rate of one kilohertz it delivers pulse energies of up to 2.5 mJ, pulse durations of around 20 ns, a spectral bandwidth of 10GHz and an almost diffraction-limited beam quality of M2<1.2 with a smooth characteristic of these parameters over the full wavelength range. This clearly exceeds the performance data published so far with our previous designs. Effects, which tent to provoke spectral gaps in the past, are totally understood and definitely suppressed by a modified resonator design. The presentation contains a detailed description and discussion of performance determining design aspects, i.e. pump scheme and pump beam shaping, resonator design and the comparison of different tuning elements. As a main prerequisite of an appropriate resonator design thermal lensing in Ti:Sapphire crystals is discussed on the basis of experimental and theoretical results. This includes the wavelength dependency of the focal length, the astigmatism in end-pumped Ti:Sapphire crystals with Brewster-cut end faces, the influence of the pump-light distribution and different cooling schemes.
Generating the difference frequency of a frequency-doubled, widely tunable Ti:Al2O3 laser and a Nd:YAG laser provides tunable laser radiation in the visible spectrum range. The generated wavelength region closes the spectral gap between the fundamental and the second harmonic of the Ti:Sapphire laser. A prototype has being developed with a fully automated wavelength tuning, i.e. the wavelength tuning of the Ti:Sapphire laser, the angel tuning of the nonlinear crystals and the tuning of the temporal delay between the Ti:Sapphire and the Nd:YAG laser operate self-controlled. Design, theoretical modeling and experimental characterization of the
system are closely discussed. At a repetition rate of one kilohertz, the frequency-doubled Ti:Sapphire laser provides pulses of approximately 20 ns, a spectral line width of 20 GHz, a nearly diffraction limited beam quality and pulse energies of up to 850 μJ. The tuning range reaches from 340 nm to 510 nm. For the three wave
interaction process in a 8 mm long BBO crystal the Ti:Sapphire pulses (pump wave) are mixed with 3.5 mJ pulses of a Nd:YAG laser (signal wave). The generated idler wave has pulse energies of up to 280 μJ and pulse durations of approximately 10 ns in the spectral range between 510 nm and 680 nm. This yields to a conversion
efficiency of about 33% and a quantum conversion efficiency of more than 50%. To our knowledge, this clearly exceeds the values that has been obtained with comparable setups so far. Further increase of the efficiency is currently under investigation.
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