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.
Spaceborne lidar (light detection and ranging) systems have a large potential to become powerful instruments in the field of atmospheric research. Obviously, they have to be in operation for about three years without any maintenance like readjusting. Furthermore, they have to withstand strong temperature cycles typically in the range of -30 to +50 °C as well as mechanical shocks and vibrations, especially during launch. Additionally, the avoidance of any organic material inside the laser box is required, particularly in UV lasers. For atmospheric research pulses of about several 10 mJ at repetition rates of several 10 Hz are required in many cases. Those parameters are typically addressed by DPSSL that comprise components like: laser crystals, nonlinear crystals in pockels cells, faraday isolators and frequency converters, passive fibers, diode lasers and of course a lot of mirrors and lenses. In particular, some components have strong requirements regarding their tilt stability that is often in the 10 μrad range. In most of the cases components and packages that are used for industrial lasers do not fulfil all those requirements. Thus, the packaging of all these key components has been developed to meet those specifications only making use of metal and ceramics beside the optical component itself. All joints between the optical component and the laser baseplate are soldered or screwed. No clamps or adhesives are used. Most of the critical properties like tilting after temperature cycling have been proven in several tests. Currently, these components are used to build up first prototypes for spaceborne systems.
In the field of atmospheric research lidar is a powerful technology to measure remotely different parameters like gas or aerosol concentrations, wind speed or temperature profiles. For global coverage, spaceborne systems are advantageous. To achieve highly accurate measurements over long distances high pulse energies are required. A Nd:YAG-MOPA system consisting of a stable oscillator and two subsequent InnoSlab-based amplifier stages was designed and built as a breadboard demonstrator. Overall, more than 500 mJ of pulse energy at 100 Hz pulse repetition frequency at about 30 ns pulse duration in single longitudinal mode were demonstrated. When seeded with 75 mJ pulses, the 2nd amplifier stage achieved an optical efficiency (pump energy to extracted energy) of more than 23 % at excellent beam quality. Recently, different MOPA systems comprising a single InnoSlab amplifier stage in the 100 mJ regime were designed and built for current and future airborne and spaceborne lidar missions. Amplification factors of about 10 at optical efficiencies of about 23 % were achieved. In order to address the 500 mJ regime the established InnoSlab design was scaled geometrically in a straight forward way. Hereby, the basic design properties like stored energy densities, fluences and thermal load densities were retained. The InnoSlab concept has demonstrated the potential to fulfill the strong requirements of spaceborne instruments concerning high efficiency at low optical loads, excellent beam quality at low system complexity. Therefore, it was chosen as baseline concept for the MERLIN mission, currently in phase B.
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.
For the CO2 and CH4 IPDA lidar CHARM-F two single frequency Nd:YAG based MOPA systems were developed. Both lasers are used for OPO/OPA-pumping in order to generate laser radiation at 1645 nm for CH4 detection and 1572 nm for CO2 detection. By the use of a Q-switched, injection seeded and actively length-stabilized oscillator and a one-stage INNOSLAB amplifier about 85 mJ pulse energy could be generated for the CH4 system. For the CO2 system the energy was boosted in second INNOSLAB-stage to about 150 mJ. Both lasers emit laser pulses of about 30 ns pulse duration at a repetition rate of 100 Hz.
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.
For future satellite based water vapor DIAL systems efficient and rugged sources preferably around 935 nm are required.
Especially for the WALES system (Water Vapour Lidar Experiment in Space) four wavelengths between 935.561 nm
and 935.906 nm (vac.) have to be addressed. A promising candidate for the direct generation within this spectral range is
a simple diode pumped setup based on compositionally tuned neodymium-doped mixed garnet crystals. Within the scope
of this work, novel Nd:(YxLu1-x)3Ga5O12-crystals (Nd:YLuGG) with different compositions (0≤x≤1) were investigated.
Beside the characterization of some relevant crystal properties laser experiments in quasi-continuous operation, Qswitched
operation and in single-longitudinal-mode operation were performed. By the R2-Z5-transition wavelengths
between 935.3 nm and 936.6 nm (vac.) can be addressed with different compositions x. At a repetition rate of 100 Hz
nearly 6 mJ were extracted in longitudinal multimode around 935.7 nm (vac.) from a Nd:(Y0.58Lu0.42)3Ga5O12-crystal.
The cavity was injection seeded and stabilized with the ramp-and-fire-method to obtain single frequency radiation. At
935.7 nm more than 4.7 mJ were generated. The laser could be tuned over a range of about ± 0.22 nm in single-longitudinal-
mode operation.
For future satellite based water vapour DIAL systems, efficient and rugged laser sources are required preferably around
935 nm. The quasi 4-level transition from R2 to Z5 in Nd:YGG is a promising candidate for its direct generation.
Q-switch operation at 100 Hz with pulse energies up to 7.7 mJ is reported as well as single frequency operation with an
injection seeded system stabilized by ramp-and-fire-method. The pulse energy of a 4.5 mJ oscillator was scaled to 32 mJ
with an InnoSlab-based amplifier at nearly diffraction limited beam quality of M2 < 1.4. Heterodyne measurements show
a line width of less than 28 MHz.
A resonator setup applying a double-sided diode end-pumped configuration and an electro-optical Q-switch for efficient
generation of 4 mJ pulses (< 60 ns fwhm) at 935 nm from Nd:YGG is presented, to our knowledge for the first time. The
optical-optical efficiency is 9 % (absorbed pump light to laser out). High quality crystals have been investigated,
showing high damage threshold, high efficiency and good optical properties permitting Q-switched mode of operation.
Experimental small signal gain data coincide with spectroscopic measurements. For vapour detection frequency stable
single mode operation is required. Injection seeding with a single frequency cw-signal has been successfully achieved.
Frequency control mechanisms are currently under investigation. The direct generation of 935 nm radiation simplifies
future LIDAR systems significantly compared to current approaches based on OPO, Raman or Ti:Sapphire technology.
Design and experimental characterization of a nonlinear optical converter module for the generation of widely tunable
UV radiation is presented. The module combines units for second, third and fourth harmonic generation of tunable
Ti:Sapphire lasers. A modified conversion scheme based on the combination of BIBO and BBO crystals reduces the
complexity of our former published UV setup - resulting in a significant increase of performance and long-term stability
of the system. Experimental characterization of the former and the improved UV setup are compared. The investigations
of the converter module are carried out with a widely tunable Ti:Sapphire laser with nanosecond pulses and a repetition
rate of 1 kHz. This laser provides a continuous tuning range of 690 nm to 1010 nm with pulse energies up to 2.0 mJ and
a spectral line width of less than 10 GHz resulting in an output power of the converter module of 1000 mW, 400 mW
and 200 mW respectively for the second, third and fourth harmonic generation. The new converter module is a decisive
step in the development of a hands-off solid-state laser system with a continuous tuning range from the UV to the NIR -
200 nm to 1000 nm.
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