Laserstrahlquellen auf Basis eines neuartigen Neodym-dotierten Mischgranats für Wasserdampf-DIAL-Systeme bei 935 nm

For remote sensing of certain trace gases in the atmosphere like water vapor using the Differential Absorption LIDAR (DIAL) technique pulsed single frequency laser sources are required which can be tuned to certain absorption lines of that gas. A special set of four wavelengths between 935.561 and 935.906 nm is beneficial for spaceborne measurements of atmospheric water vapor density distributions according to the studies for the WALES (Water Vapor Lidar Experiment in Space) mission. Today, usually broadband laser media like Ti:Sapphire or frequency converted lasers are used in order to address these wavelengths and to fulfill all remaining beam requirements. In both cases usually frequency doubled and maybe diode pumped Nd:YAG-lasers at 532 nm are used as pump source. Thus both concepts offer only low electro-optical efficiencies and lead to comparatively complex setups, which is not suitable for space applications. Alternatively, specific laser media can be grown that offers direct diode pumping and whose emission wavelengths can be tuned to the application wavelength by the crystal composition. Hereby, generally a more compact and efficient setup is possible. In the context of this work Nd:(Y_{x}Lu_{1-x})_{3}Ga_{5}O_{12} crystals (with 0<=x<=1) were used for the first time in order to address the above-mentioned wavelengths. Thereby, especially the crystal properties relevant for the design of an appropriate laser source are analyzed. This comprises the absorption and emission cross sections, the lifetimes of the upper laser level, the changes of the refraction index with temperature and the heat conductivities for different crystal compositions. This data is used in design studies in order to identify advantageous configurations for stable oscillators and Innoslab-amplifiers. Oscillators based on different crystal compositions are investigated experimentally. Though a composition which emits at 935.7 nm vac. and which allows the addressing of all relevant WALES wavelengths is found. Furthermore, a Nd:YGG-based demonstrator emitting at 935.3 nm in single frequency mode, consisting of an oscillator and an amplifier stage, is implemented and analyzed. Herewith, a pulse energy of 30.5 mJ is obtained at a pulse repetition frequency of 100 Hz, a pulse duration of 52.5 ns, a beam quality of M2<1.4 and a spectral purity of <99.996%. This system is successfully employed at the DLR-IPA for ground based measurements of atmospheric water vapor densities. The maximal optical-to-optical efficiencies (pump light to laser light) are 10% and 3.2% for the oscillator and amplifier, respectively. This first attempt to generate light with that new laser material already comes up to the optical efficiencies and exceeds the spectral quality achieved by the above mentioned state of the art concepts. Moreover, the high efficiencies achieved in oscillators show the potential of these novel laser sources, especially for airborne and spaceborne water-vapor DIAL systems. Here, high efficiencies are a prerequisite for mission feasibility

Laserstrahlquellen auf Basis eines neuartigen Neodym-dotierten Mischgranats für Wasserdampf-DIAL-Systeme bei 935 nm

For remote sensing of certain trace gases in the atmosphere like water vapor using the Differential Absorption LIDAR (DIAL) technique pulsed single frequency laser sources are required which can be tuned to certain absorption lines of that gas. A special set of four wavelengths between 935.561 and 935.906 nm is beneficial for spaceborne measurements of atmospheric water vapor density distributions according to the studies for the WALES (Water Vapor Lidar Experiment in Space) mission. Today, usually broadband laser media like Ti:Sapphire or frequency converted lasers are used in order to address these wavelengths and to fulfill all remaining beam requirements. In both cases usually frequency doubled and maybe diode pumped Nd:YAG-lasers at 532 nm are used as pump source. Thus both concepts offer only low electro-optical efficiencies and lead to comparatively complex setups, which is not suitable for space applications. Alternatively, specific laser media can be grown that offers direct diode pumping and whose emission wavelengths can be tuned to the application wavelength by the crystal composition. Hereby, generally a more compact and efficient setup is possible. In the context of this work Nd:(Y_{x}Lu_{1-x})_{3}Ga_{5}O_{12} crystals (with 0<=x<=1) were used for the first time in order to address the above-mentioned wavelengths. Thereby, especially the crystal properties relevant for the design of an appropriate laser source are analyzed. This comprises the absorption and emission cross sections, the lifetimes of the upper laser level, the changes of the refraction index with temperature and the heat conductivities for different crystal compositions. This data is used in design studies in order to identify advantageous configurations for stable oscillators and Innoslab-amplifiers. Oscillators based on different crystal compositions are investigated experimentally. Though a composition which emits at 935.7 nm vac. and which allows the addressing of all relevant WALES wavelengths is found. Furthermore, a Nd:YGG-based demonstrator emitting at 935.3 nm in single frequency mode, consisting of an oscillator and an amplifier stage, is implemented and analyzed. Herewith, a pulse energy of 30.5 mJ is obtained at a pulse repetition frequency of 100 Hz, a pulse duration of 52.5 ns, a beam quality of M2<1.4 and a spectral purity of <99.996%. This system is successfully employed at the DLR-IPA for ground based measurements of atmospheric water vapor densities. The maximal optical-to-optical efficiencies (pump light to laser light) are 10% and 3.2% for the oscillator and amplifier, respectively. This first attempt to generate light with that new laser material already comes up to the optical efficiencies and exceeds the spectral quality achieved by the above mentioned state of the art concepts. Moreover, the high efficiencies achieved in oscillators show the potential of these novel laser sources, especially for airborne and spaceborne water-vapor DIAL systems. Here, high efficiencies are a prerequisite for mission feasibility

Water Vapour Differential Absorption Lidar measurements at 935 nm using a Nd:YGG laser

A novel diode-pumped, single-frequency laser system emitting at 935 nm was developed to serve as the transmitter for water differential absorption lidar measurements. This laser uses Nd:YGG (Y3Ga5O12) as the active medium. The system was directly diode-pumped at 806 nm and was built up in a master-oscillator power-amplifier configuration. It consists of a stable resonator in rod geometry and employs as the amplifier a stable-unstable hybrid resonator in an end-pumped slab design. Single frequency operation is achieved by injection seeding. The range of continuously tunable single-frequency radiation extends to ~0.4 nm centered around 935.31 nm. More than 30 mJ of pulse energy at 100 Hz repetition rate with a beam quality (M2) of better than 1.4 and Q-switched pulse duration of 52 ns in single frequency mode were generated. Since water vapor DIAL demands for stringent requirements for the spectral properties those were carefully investigated. Values of the spectral purity of >99.995% were determined using long-pass absorption measurements in the atmosphere exceeding the requirements by a large margin. Finally, first time water vapor DIAL measurements were performed using a Nd:YGG laser. The reported results show much promise of these directly pumped lasers at 935 nm for future space-borne and airborne water vapor lidar systems

Diode pumped Single-Frequency-Nd:YGG-MOPA for water vapor DIAL measurements: design, setup and performance

A diode-pumped Q-switched and injection-seeded single-frequency-laser, generating tunable laser radiation at 935 nm, is presented. Using Nd:YGG (Y3Ga5O12) as the active medium, the laser that was developed to serve as a transmitter for water vapor lidar measurements. The configuration consists of a stable resonator in rod geometry that is injection seeded by a narrowband diode laser and stabilized by the ramp-and-fire technique. Energy scaling was done in a power amplifier in slab geometry. Both oscillator and amplifier crystal were diode pumped at 806 nm. More than 30 mJ pulse energy at 100 Hz repetition rate with a beam propagation factor of M2 < 1.4 and pulse duration of 52 ns in single frequency mode were generated. To our knowledge this is the first direct generation of 935 nm Q-switched pulses from Nd:YGG suitable for water vapor measurements. The reported results show great promise of this laser in applications where high efficiency and reduced complexity is indispensable such as for spaceborne or airborne water vapor lidar instruments

CHARM-F: The Airborne Integral Path Differential Absorption Lidar for Simultaneous Measurements of Atmospheric CO2 and CH4

CHARM-F (CO2 and CH4 Atmospheric Remote Monitoring – Flugzeug) is DLR’s airborne Integral Path Differential Absorption lidar for simultaneous measurements of the column weighted-average dry-air mixing ratios of atmospheric carbon dioxide and methane, designed to be flown on board DLR’s new High-Altitude, LOng-range research aircraft, HALO. After recalling the context of the project, the measurement principle and the technological challenges, we report on the design of the instrument

Water Vapor Differential Absorption Lidar Measurements using a Diode-Pumped All-Solid-State Laser at 935 nm

A diode-pumped, single-frequency laser system emitting at 935 nm has recently been developed to serve as the transmitter for water vapor differential absorption lidar (DIAL) measurements. This laser uses Nd:YGG (Y3Ga5O12) as the active medium and emits radiation directly at 935 nm without the need of additional frequency conversion processes. The system was diode-pumped at 806nm and was built up in a master-oscillator power-amplifier configuration. It generates more than 30 mJ of pulse energy at 100 Hz repetition rate with a beam quality (M2) of better than 1.4. Since water vapor DIAL demands for stringent requirements of the spectral properties those were carefully investigated in the scope of this paper. Single frequency operation is achieved by injection seeding and active length control of the oscillator cavity. The range of continuously tunable single-frequency radiation extends to ~0.4 nm centered around 935.31 nm. Values of the spectral purity of >99.996% were determined using long-pass absorption measurements in the atmosphere exceeding the requirements by a large margin. Finally, for the first time water vapor DIAL measurements were performed using a Nd:YGG laser. The reported results show much promise of these directly pumped lasers at 935 nm for future spaceborne but also airborne water vapor lidar systems

Feasibility and performance study for a space-borne 1645nm OPO for French-German satellite mission MERLIN

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

Development and First Results of a new Near-IR Airborne Greenhouse Gas Lidar

An airborne lidar system has been developed to measure the two most important anthropogenic greenhouse gases, carbon dioxide and methane. The instrumental setup and first results onboard the German research aircraft HALO are discussed