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An Nd:YAG-MOPA system consisting of a stable oscillator and two subsequent Innoslab-based amplifier stages has been designed and built as a technology demonstrator for future lidar applications. Overall, the authors demonstrate that it generates more than 500 mJ of pulse energy at a 1064 nm wavelength and 100 Hz pulse repetition frequency at about 30 ns pulse duration in the single longitudinal mode. Seeded with 75 mJ pulses, the second amplifier stage achieved an optical efficiency (extracted energy to pump energy) of more than 23% while preserving excellent beam quality. To address the 500 mJ regime while retaining the basic system properties, an established Innoslab design was scaled geometrically.
© 2017 Optical Society of America
1. INTRODUCTION
In the field of atmospheric research, laser-based active optical instruments are powerful devices that can measure several very important climatic parameters, such as gas or aerosol densities, wind speed, or temperature profiles.
One of the key components of such systems is the laser source, which usually has to be tailored to the specific application with respect to energetic, temporal, and spectral properties, as well as the transversal beam quality.
Several lidar applications require pulse energies in the 100 mJ range at specific wavelength and driver lasers in the 0.5 J class [1]. Currently, Innoslab-based sources in airborne or spaceborne lidar systems have pulse energies of 10 to about 150 mJ at repetition rates of about 10–100 Hz. High electro-optical conversion efficiency and very good beam quality have been demonstrated. Both are crucial requirements for spaceborne laser systems. While the scaling of the repetition rate and average power of Innoslab-based laser sources has been demonstrated in the past decade in different setups, the scaling of pulse energies beyond 150 mJ was left open [2].
The laser wavelength is a key parameter in atmospheric research. As the required wavelength does not usually overlap with the typical emission wavelengths of preferred high-power laser media, such as 1064 nm in Nd:YAG, the laser wavelength has to be adapted in subsequent nonlinear converters. Alternatively, the laser wavelength can be tuned by appropriate crystal material design, which is often called compositional tuning [3].
Based on the available Innoslab amplifier platform at about 100 mJ pulse energy and repetition rates of about 100 Hz [2,4,5], we investigated the straightforward energy scaling of this setup to the 500 mJ level both numerically and experimentally. The design, setup, and performance of this setup will be described here in detail.
By demonstrating such a laser source, we have made an important step toward enabling future lidar missions, such as ADM-Aeolus Follow-On or a MERLIN-Follow-On-system [4,6]. The presented setup is compatible to the FULAS platform for space-based lasers [7,8].
2. DESIGN
For the development of the high-energy amplifier stage, a set of design rules was defined. One important design driver is the heritage from existing, well-established laser sources and designs in the field of airborne and spaceborne lidar systems. As described in Ref. [2], nearly the same architecture has been used for the lidar systems A2D2G and CHARM-F. A2D2G is designed to measure wind profiles and is based on the Doppler shift of backscattered photons at 355 nm of the frequency-tripled pump source at 1064 nm [4]. CHARM-F is used to measure the column-averaged volume mixing ratios of and through integrated path differential absorption (IPDA) [9]. For both gases, two separated laser sources were developed and based on a similar laser architecture and design [5]. All these designs were based on a laboratory model designed, assembled, and tested at ILT within the scope of an ESA (European Space Agency) pre-development study for the ATLAS laser source on ATLID, a spaceborne atmospheric backscatter lidar instrument [4].
All systems consist of a master oscillator power amplifier (MOPA) arrangement operating at a 100 Hz repetition rate and comprising a stable single-frequency oscillator with about 8 mJ of pulse energy at nearly diffraction-limited beam quality and about 30–40 ns pulse duration. This beam is amplified in a single seven-pass Innoslab amplifier stage to about 80–90 mJ, while most of the residual beam properties are retained. For all amplifier stages, Nd:YAG slab crystals are used and have an optical aperture of , a length of 20 mm, and a Nd-doping level of 0.7 at. %. The slabs are pumped with a pump line height of about 1.8 mm by commercially available passively cooled fast-axis collimated vertical diode laser stacks (8 bars) at 808 nm with subsequent beam-shaping optics. The thermal lens inside the laser crystal in the fast-axis direction with respect to the diode orientation has a focal length of about 3 m at the working point at a 100 Hz repetition rate, according to the calculations. The geometrical length of one amplifier pass for all systems is 100 mm. A geometrical magnification was applied to the beam. The peak fluences were in the range of for all passes. The same design is also used for CHARM-F [5,9]. A downscaled version is used for the MERLIN laser [6,10], where less than 40 mJ of pump energy at the 1064 nm wavelength are required.
The goal for this design study was to show that the 500 mJ level can be reached with one subsequent booster stage based on the same technology by applying simple scaling rules. One boundary condition was that the same passively cooled eight-bar pump stacks of the first Innoslab stage can be used for the 500 mJ stage. For the combination of pump stacks, polarization coupling was not implemented yet in order to enable a fully redundant pump unit without design changes. This might be relevant for future spaceborne systems.
One design rule was to keep power densities in the crystal media in a similar range. In case of the first amplifier, the 8 mJ of the oscillator was amplified to about 90 mJ. For the following considerations concerning the pump geometry, it is assumed that the 500 mJ amplifier is seeded with a beam in the 100 mJ class.
To come to the 500 mJ level, a 100 mJ pulse has to be amplified by factor of 5. For the second amplifier, the pump energy and the cross-sectional area of the first amplifier had to be scaled by this factor. All crystal dimensions and the pump line were scaled by a factor of and the Nd-doping by . This ensures that the focal length of the thermal lens and the thermal power densities at the cooling interfaces remain constant. The number of pump diode stacks was scaled from 1 to 4 for both crystal faces. This is feasible, as the pump diodes are operated at a very moderate working point in the first amplifier stage. The same diode stacks mentioned above were arranged geometrically in a simple array. The brilliance of this arrangement could be increased easily by more sophisticated geometrical stacking, but pump brilliance is not a crucial design driver in this case.
The scaling rules for the shaping of the laser beam are derived differently. The incoming pulse energy is about a factor of 10 higher than for the first stage (80–90 mJ versus 8 mJ). As the pump height and, therefore, the beam shape in the vertical direction are only scaled by a factor of , the beam has to be broadened by factor of about in order to keep the energy flux constant on the optical facet. Furthermore, the amplification () is lower than that in the first amplifier stage ().
The above-mentioned values are given here in order to understand the general scaling rules which were applied here. To find an arrangement that offers high overlap efficiencies at moderate energy fluxes, numerical simulations were performed additionally (see Fig. 1). Here, an incoming pulse energy of 80 mJ and a pump energy of 2.5 J were assumed. The precise design parameters of the amplifier are given later.
Fig. 1. Optical energy density distribution for a seed energy of 80 mJ and a pump energy of 2.5 J on the incoupling mirror (first row), laser crystal (second row), and outcoupling mirror (third row) within the second amplifier stage, generated by a detailed simulation based on wave optics, including amplification and diffraction effects.
Within the second amplifier, the beam passes only five instead of seven times through the crystal. For the horizontal direction, a plane–plane mirror configuration was chosen with a magnification of 1.0 instead of 1.4. In this case, the incoming beam has to be shaped geometrically in order to ensure a proper broadening of about 1.4 per crystal pass caused by inherent divergence.
Further pulse energy scaling is generally feasible applying the same scaling rules. But this would require even larger laser crystals and appropriate packages, which might become technical limitations. The 500 mJ amplifier as described here could also be scaled in pulse energy by stronger pumping. In this case, parasitic lasing of the amplifier arrangement might become the first technical limitation. Scaling the repetition rate of this arrangement would at first lead to stronger thermal lenses, which can be compensated for by the appropriate mirror curvature. The duty cycle of the diode laser stacks could be doubled according to the specifications.
3. SETUP
The oscillator and the first amplifier stage are built on a baseplate adopted from the CHARM-F system and manufactured from a solid aluminum block. The section on the bottom side where the optical parametric oscillators (OPOs) are integrated in the CHARM-F system has been constructed with honeycombs instead. On the optical sides, certain ribs and borders allow for high stiffness. The entire plate is mounted isostatically at three points in order to minimize mechanical deformations.
The second amplifier stage is built up on a breadboard with off-the-shelf optomechanics and completed with some customized mechanics for the pump optics. The functional groups of the beam generation chain and a photo of the complete system are depicted in Fig. 2.
Fig. 2. Functional groups of the beam generation chain (top) and photo of the whole laser system (bottom).
The output pulses of the MOPA front end are amplified to 500 mJ pulse energy in a single Innoslab booster stage. The MOPA front end consists of a seeded oscillator and a single Innoslab amplifier stage, described in detail in Ref. [5]. The spectral, temporal, and main spatial beam properties are generated in the low-energy -switched and injection-seeded Nd:YAG-based oscillator. The pulse energy of 7.5 mJ from the oscillator is amplified in the subsequent Innoslab amplifier stage to the 75 mJ level.
A. Oscillator
A photo of the oscillator section with the beam path and a scheme of it are given in Fig. 3. The most relevant oscillator properties are listed in Table 1.
Fig. 3. Photo of the oscillator section with sketched-in beam path (top), and scheme of the oscillator with outcoupling mirror (OC), folding mirrors (FMs), quarter-wave plates (QWPs), pumping mirrors (PMs), crystal (CRY), Pockels cell (PC), thin-film polarizer (TFP), and end mirror (EM) (bottom).
Table 1. Summary of the Design Parameters of the Oscillator
The laser-active medium is a slab-shaped Nd:YAG crystal, which is longitudinally pumped from both end faces. For pumping, two fiber-coupled diode laser modules are used, each emitting at the 808 nm wavelength. The appropriate output facets of the fibers (dia. 600 μm, NA 0.22) are imaged with a two-lens setup into the crystal. The pump light and laser light are separated by a dichroic mirror on both sides of the crystal. For cooling, the laser crystal is soldered into a copper heat sink. For reasons of compactness, the optical path is folded one time between the outcoupling mirror and the laser crystal to guarantee the desired pulse duration. Active high-quality switching is done with a setup comprising a thin film polarizer, a Pockels cell, and a quarter-wave plate. The polarizer is used as a mirror. The end mirror is mounted on a piezo actuator for cavity length control.
A spectrally narrowband cw-laser signal of about 10 mW from a commercially available fiber laser source is transversally mode matched and coupled through the polarizer into the cavity. To ensure a spectrally stable emission, the fiber laser is protected from any noise signals coming from the oscillator by a Faraday isolator, which offers an attenuation larger than 30 dB. In order to generate single-frequency pulses, the oscillator cavity length has to be in resonance with the cw-seed signal. For this, the Ramp&Fire technique is applied, where the transmission of the seed signal through the cavity is measured with a photodiode behind an extracavity folding mirror. This occurs during the pumping phase while the cavity length is ramped. The occurrence of interference peaks is detected by an electronic board, which triggers the Pockels cell driver [11]. Two quarter-wave plates around the crystal generate a twisted mode, thus preventing a spatial hole burning in the crystal with the standing wave of a single-longitudinal mode [12]. The oscillator laser pulses are transversally mode matched to the subsequent first Innoslab amplifier stage, and an isolator inhibits feedback from the amplifier into the oscillator.
B. Innoslab Amplifier Stages
Both of the Innoslab amplifier stages, shown in Figs. 4 and 5, consist of an Nd:YAG crystal pumped longitudinally from both ends and a mirror configuration to fold the input beam several times through the crystal. The mirrors also shape the amplified beam.
Fig. 4. Photo of the first amplifier section with the beam path sketched in orange (top) and scheme of the first amplifier (bottom).
Fig. 5. Detailed photo of the amplifier resonator of the second amplifier stage with the beam path sketch in red (top) and the scheme of the second amplifier (bottom).
The main design parameters of both Innoslab amplifier stages are summarized in Table 2.
Table 2. Summary of the Design Parameters of the Two Innoslab Amplifier Stages
The crystals of the first and second amplifiers have a slab geometries with widths of 18 and 40 mm, heights of 4 and 9 mm, and lengths of 20 and 45 mm, respectively. In addition, they have Nd-doping levels of 0.7% and 0.3%, respectively. Both large surfaces of the crystals are soldered to a copper heat sink for effective cooling. The widths of the slab crystals are aligned with the slow axes of the pump diodes. With this configuration, a unidimensional thermal lens is generated perpendicular to the large cooling surfaces.
Two plane dichroic pump mirrors separate the pump light at 808 nm and the laser light at 1064 nm. Two end mirrors are used for the beam shaping of the folded laser beam. The radii of the spherical mirrors of the first amplifier are 478 mm (convex) and (concave), and the radii of the convex cylindrical mirrors of the second amplifier are 3735 and 5660 mm. Together with the cylindrical thermal lenses of the laser crystals, these configurations are stable in the fast axis (with respect to the diode bars of the pump, perpendicular to the plane of the laser breadboard). Its eigenmodes are geometrically matched in the beam diameter to the respective full heights of the pump volume. For the second amplifier, the mirror configuration forms a plane–plane resonator in the slow axis.
Within the amplifier resonators, which have lengths of 102 and 196 mm, the input beams coming from the oscillator and the first amplifier pass the crystal seven and five times, respectively. These values are amplified by a factor of up to 12.
While the beam diameter in the fast axis is constantly matched to the full pump height, the beam is broadened in the slow axis by the magnification of the mirrors and its inherent divergence. Therefore, the intensity distributions of the outgoing beams have an elliptical shape, where the small axes are parallel to the fast axes of the pump diode bars. As the broadening factor is matched to the single-pass amplification, the fluence and, consequently, the amplification properties are kept nearly constant for all crystal passes.
Between the two amplifiers, the beam is transversally shaped in both directions by two cylindrical telescopes to guarantee mode matching in the second amplifier.
The pump configuration for the two Innoslab stages is based on the first amplifier stage of both MOPAs of the CHARM-F system. The beam profile of the pump light in the laser crystal is line shaped, with a top-hat distribution in the slow axis. In this axis, the laser crystal is fully pumped. In the perpendicular fast axis, the pump distribution is Gaussian, and the light only partially pumps the crystal.
The pump light in both amplifier stages is emitted at 808 nm by the same passively cooled vertical diode stacks, each of which contains eight fast-axis-collimated diode bars. In the first amplifier, there is one stack for each crystal side, while for the second amplifier, there are four stacks for each crystal side arranged in a simple array. For the combination of pump stacks, the polarization as one degree of freedom was left unused in order to integrate a fully redundant pump unit without design changes. This might be important for future spaceborne systems.
The spatial intensity distribution is homogenized in the slow axis in a light mixing slab in order to compensate for inhomogeneous distributions, for example, in case of emitter failure [13,14]. The fast axis is focused into the crystal, and the homogenized slow axis distribution is imaged into the crystal. The measured pump-light distribution for the second amplifier is shown in Fig. 6. The pump profiles for the two amplifier stages have heights of 1.8 and 3.8 mm and widths of 18.2 and 40.2 mm, respectively. The two pump sides provide maximum combined pulse energies of 420 and 1950 mJ, respectively, at a repetition rate of 100 Hz when the crystals are pumped at a center wavelength of 808 nm.
Fig. 6. Beam profile of the pump line in the crystal of the second amplifier with a top-heat distribution in the slow axis and a Gaussian distribution in the fast axis.
4. PERFORMANCE
Table 3 lists the main performance parameters for the oscillator and the two subsequent Innoslab amplifier stages. Please note that, after a full characterization, the oscillator and first amplifier are operated at a moderate working point with 8.2 and 75 mJ pulse energies (with 200 μs pump pulse duration) for the seeding of the second amplifier to ensure long-term durability.
Table 3. Summary of the Output Parameters of the Three Stages
A. Oscillator
In the oscillator, pulses with about 8 mJ pulse energy and 30 ns pulse duration are generated with an optical-to-optical efficiency (pulse energy/pump pulse energy) of about 31% at a nearly diffraction-limited beam quality (). Figure 7 shows the pulse energy, efficiency, and pulse duration depending on the pump pulse energy (from the fiber of the pump modules) for different pump pulse durations.
Fig. 7. Pulse energy (left axis), efficiency (output pulse energy to pump-pulse energy from the pump fibers, right axis) and pulse duration (inset), depending on the pump-pulse energy for different pump-pulse durations.
B. First Amplifier Stage
The first amplifier delivers pulse energies up to 105 mJ with an optical-to-optical efficiency up to 23% and thereby typical values of more than 20% for the efficiencies [4]. The pulse duration of 31 ns is nearly unchanged, and the beam quality () is only slightly reduced compared to the oscillator.
In Fig. 8, the pulse energy and efficiency (extracted pulse energy/pump pulse energy) are shown, as they depend on the pump pulse energy (behind all pump optics, excluding the pump mirrors) for different seed energies.
Fig. 8. Measured output pulse energy (left axis) and extraction efficiency (extracted pulse energy to pump-pulse energy behind all pump optics excluding the pump mirrors, right axis) of the first amplifier as a function of the pump energy (with 200 μs pump pulse duration) for different seed energies.
C. Second Amplifier Stage
The second amplifier delivers pulse energies up to 525 mJ with an optical-to-optical efficiency of up to 23%. The pulse duration is nearly unchanged at 31 ns. The output beam is shaped in both directions by two cylindrical telescopes to obtain a symmetric, spherical output beam. The beam quality (; see Fig. 9) is only slightly reduced when compared to the first amplifier stage.
Figure 10 shows the pulse energy and efficiency (extracted pulse energy/pump pulse energy) depending on the seed pulse.
Fig. 10. Measured pulse energy (left axis) and efficiency (right axis) after the second amplifier as a function of the seed energy into the second amplifier at a pump-pulse energy of 1900 mJ and pump-pulse duration of 200 μs.
5. SUMMARY
For the first time, an energy-scaled Innoslab amplifier has been demonstrated in the 500 mJ regime on the breadboard level. The optical design was based on a well-established configuration that was developed for space applications. It was shown that advantageous properties of the Innoslab concept, such as high efficiency and beam quality, could be retained. The scaled amplifier is fully compatible to the FULAS platform and thus enables us to extend the parameter range of next-generation lidar sources.
Beside the function as a technology demonstrator, this setup is part of an LIDT test facility [15] that was developed and is being operated by ILT for the qualification of optical components, especially for current space missions. At present, the conversion to 1645 nm with OPO/optical parametric amplifier (OPA) stages has been integrated into the setup. Here, the conversion to the 100 mJ regime has already been demonstrated [16] and is based on the optical parametric amplification of the output of an OPO similar to the MERLIN OPO [17].
The LIDT test facility will be operated at both 1064 and 1645 nm, especially for the MERLIN mission. For tests at further wavelengths, such as 532 or 355 nm, the beam coming from the booster can be converted with tailored conversion stages.
Funding
Deutsches Zentrum für Luft- und Raumfahrt (DLR) (50EE1228); Bundesministerium für Bildung und Forschung (BMBF).
Acknowledgment
We thank Jörg Luttmann for the detailed work on the optical design for the oscillator and the first amplifier, Raphael Kasemann for the mechanical engineering of the overall system, Gerd Kochem for the mechanical engineering of the pump optic of the second amplifier and its setup and characterization, Jürgen Klein for the electrical engineering of the overall system, Matthias Winzen for the soldering of the laser crystals, and Marco Hoefer for the helpful discussions.
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