B S O I [ Laser, Quantum Technology & Active Sensor Systems ] r e f o h n u a r F www.iosb.fraunhofer.de ISSN 1616-8240

Editorial Prof. Dr. rer. nat. habil. Marc Eichhorn Dr. rer. nat. Helge Bürsing Dear friends of Fraunhofer IOSB, quantum technology is often seen as a difficult, not intuitively understandable scientific subject – even for scientists. Nevertheless, it will more than before influence and shape our living in the 21st century. Over decades quantum computers were predicted and their possible game-changing capabilities were advertised – now first systems are a reality. The laser, a quantum technology first experimentally realized by Maiman in 1960, extends its applications to more and more wavelengths with increasing power levels and thus enables many new fields of application and new capabilities. Profiting from these evolutions, new active optro- nic sensor systems emerge that combine lasers, optics, opto-electronic sensors and image exploitation and pave the way for new methods to make objects better visible and measu- rable to humans and machines. In this visIT issue we want to shed some light onto the topics of lasers, photonic quantum technology and related active optronic sensor systems by giving you an insight into the con- tributions of Fraunhofer IOSB to these future key technologies in civil and defense applica- tions. The focus of this visIT hereby lies mainly on the near-to-mid infrared spectral range. We start our overview with quantum ghost imaging, a technology creating images from entangled photons that actually never have seen the object. As the object is illuminated by single photons, it is predicted that such imaging can be realized without any possibility of being detected. Yet more classical, we then discuss latest results on 3D gated viewing, which uses direct laser illumination, and current work on an active compressive sensing camera. As last example for optronic systems, we show that they can be used to speed up maintenance and safety tasks in wind parks using laser Doppler vibrometry on rotor blades of wind turbines. All these applications rely on specific laser sources. Thus, laser source research is a prerequi- site for future novel applications. Especially in the defense sector – but more and more also for civilian applications – access to critical components of laser and optronic systems and a technological sovereignty therein is of utmost importance. Therefore, Fraunhofer IOSB is performing research on critical components, focusing on crystals for lasers and non-linear converters and fiber components for high-power fiber lasers in the short- and mid-wave infrared spectral range in order to cover the critical parts of the whole value chain of IR solid-state and fiber laser sources. Based on the extended capabilities offered through such components, novel laser sources become possible. We finally give an insight into our high- power solid-state and fiber laser sources research and development in the short-wave and mid-wave infrared spectral range. We wish you an enjoyable and informative reading of this issue. Fraunhofer IOSB, October 2022 Prof. Dr. rer. nat. habil. Marc Eichhorn Dr. rer. nat. Helge Bürsing Dr. Christelle Kieleck vis IT 3 Laser, Quantum Technology & Active Sensor Systems Dr. Christelle Kieleck

Entangled photon pairs are created through Fraunhofer IMS. A novel image reconstruc- spontaneous parametric down-conversion (SPDC) of photons from a pump laser within tion scheme had to be developed and was recently filed as a patent [5]. a nonlinear crystal that is customized to support the efficient generation of broad- band entangled photon pairs in user-defined spectral ranges. Fraunhofer IOSB has a custom-designed quantum light source for the emission of entangled photon pairs with To date, the system still has a low TRL and serves as a technology demonstrator, but the improved detector technology currently already available will significantly increase the system's performance, and further im- 1550 nm and 550 nm (Fig. 2). The SPDC provements are already on the horizon. source further emits photons randomly in New technological challenges will have to space and time and is thereby inherently be solved, like transferring and real-time hardened against accidental detection by processing of large amounts of data an external observer and hostile jamming and miniaturization of the system. Each attacks [2]. Only the observer is able to se- parate the returning quantum signal from ambient noise since only they have access technological upgrade will increase the performance and broaden the field of pos- sible applications over the coming years. to additional exclusive information from entangled partner photons for filtering. The advantages of QGI are well established scientifically since its discovery in 1995 by Pittman et al. [3]. Fraunhofer IOSB de- veloped the first asynchronous version of that setup to enable 3D imaging of remote Initial application target areas are appli- cations with disruptive quantum benefits of QGI over traditional systems, such as non-detectable active imaging and range- finding systems. Later, QGI will compete with established traditional systems that have to minimize illumination intensity, targets without any moving parts [4]. This whether for reasons of phototoxicity, was only possible following recent advan- safety for the eyes, or energy consumption. ces in single-photon detection technology with state-of-the-art detectors provided by Fig. 3: Quantum Ghost Imaging of an object with only one photon per pixel on average Figure 2: Quantum Light source (center) and compact single-photon detector (left) [1] D. Walter, C. Pitsch, G. Paunescu, P. Lutz- mann, »Quantum ghost imaging for remote sensing,« Proc. SPIE 11134, Quantum Commu- nications and Quantum Imaging XVII (2019); doi: 10.1117/12.2529268 [2] D. Walter, C. Pitsch, G. Paunescu, P. Lutz- mann, »Detection and jamming resistance of quantum ghost imaging for remote sensing,« Proc. SPIE 11160, Electro-Optical Remote Sensing XIII (2019); doi: 10.1117/12.2532379 [3] T. B. Pittman, Y. H. Shih, D. V. Strekalov, and A. V. Sergienko, »Optical imaging by means of two-photon quantum entanglement,« Phys. Rev. A 52, R3429–R3432 (1995). [4] Pitsch et al. »Quantum ghost imaging using asynchronous detection,« Applied Optics Vol. 60, Issue 22, pp. F66-F70 (2021). [5] D. Walter, C. Pitsch, »System und Verfahren zur Bildrekonstruktion unter Verwendung eines optischen Aufbaus zur aktiven Beleuchtung mit einzelnen Photonen«, Deutsche Patentanmeldung 102020203150.9 (2020). vis IT 5 Laser, Quantum Technology & Active Sensor Systems

REAL-TIME GENERATION OF 3D DATA WITH HIGH to generate 3D information is to move the The brightness of each cube corresponds gate through the scenery by increasing the to the image intensity of an object, which camera delay time from frame to frame. would be located at the respective distance. The resulting tomographic sequence can Figure 3 shows the reset and signal level ga- then be used for 3D reconstruction. The ted viewing images of an example scenario drawback of this approach is the long mea- with four persons. Here, the difference in surement duration and the required time for the gate lengths can be clearly seen. data post-processing. In the following, a By comparing the pixel gray values in the more sophisticated method with real-time falling slope of the reset level image with capability for the generation of 3D data with the pixel gray values in the plateau of the a gated viewing system is presented. signal level image, relative 3D data within The sensitivity within the gate as a function this range can be calculated. Finally, with of the distance to the gated viewing system, i.e. the gate profile, has a trapezoidal shape in a first approximation. That is because the gate profile is mathematically the convolution of the laser pulse function and the detector the knowledge of the gate parameters, absolute 3D data as shown in Figure 4 is obtained. CONCLUSION gain function. Therefore, the gate function The presented technique of CDS-based ge- consists of a rising and a falling slope with a neration of 3D data works in real-time because plateau in between. the two gated viewing images are simulta- By correlated double sampling (CDS) whose neously captured from one single laser pulse. principle is explained in Figure 1, two gated- viewing images with gate profiles of different lengths are simultaneously generated from one single laser pulse. In Figure 2, generic gate profiles for the reset and signal level are depicted. Further, four cubes with different degrees of brightness are shown as overlays onto each gate profile where the plateau of the signal level over- laps with the falling slope of the reset level. Therefore, in contrast to a system with a scanning concept, this approach is ex- tremely suited for dynamic scenarios with moving objects. The range accuracy can be estimated to be in the centimeters range. Figure 4: Color-coded range image (red-blue = 83-86 m) reconstructed from the two shown gated viewing images. The four persons can be clearly distinguished in terms of their distance. Depending on the system design, ranges up to several kilometers are possible. Figure 3: Example of a pair of gated viewing images for the reset (left) and signal level (right). [1] B. Göhler, P. Lutzmann: »Review on short- wavelength infrared laser gated-viewing at Fraunhofer IOSB«, Opt. Eng. 56(3), 031203 (2016) [2] B. Göhler, P. Lutzmann: »Super-resolution depth information from a short-wave infrared laser gated-viewing system by using correla- ted double sampling«, Proc. SPIE Vol. 10434, 104340M (2017) [3] B. Göhler, P. Lutzmann: »Extending the 3D range of a short-wave infrared laser gated-view- ing system capable of correlated double samp- ling«, Proc. SPIE Vol. 10796, 107960D (2018) [4.] B. Göhler, P. Lutzmann: »Range-intensity profile of a SWIR laser gated-viewing system for varying temporal response of in-pixel amplification«, Proc. SPIE Vol. 11160, 1116003 (2019) vis IT 7 Laser, Quantum Technology & Active Sensor Systems

Figure 2. Experimental setup used for active CS in sensor-side approach. a beam modulated by the complementary patterns. Each reflection is collected and recorded by a single element (bucket) de- tector. The test objects (acorns) are illumi- nated by a pulsed fiber laser at a center wavelength of 1.55 µm. The trigger output signal of the DMD controls the laser emis- sion and synchronizes the data acquisition with the laser pulse. Typically, the image is reconstructed using the signal from one detector only. Either the signal modulated by the applied patterns, or the signal modulated by the complementary patterns may be used for the image recon- struction. The simultaneous recording of both signals has important benefits. The quality of the reconstructed image is im- proved if the difference between them is used for the reconstruction. The variations of the ambient light and the illumination light intensity may be corrected using the signals. Simultaneous image acquisition in two different spectral ranges is possible if suitable detectors and filters are inserted. There are different possibilities to define the modulation patterns. Figure 3 shows an ex- CS imaging has the potential to be imple- mented in active imaging systems and to ample of image reconstructions using binary reduce their complexity and cost. The method patterns based on Hadamard matrices at can be adapted for very low light levels using different levels of compression. single-photon detectors. Furthermore, the distance for active systems is most likely en- hanced and atmospheric perturbations might have a smaller effect in some situations. This is of particular interest in defense applica- tions [1]. [1] Du Bosq, T. et al, »An overview of joint activities on computational imaging and compressive sensing systems by NATO SET- 232,« Proc. SPIE 10669, 10669OH (2018); doi: 10.1117/12.2307852 [2] Paunescu, G., Lutzmann, P., Wegner, D., »Compressive sensing for active imaging in SWIR spectral range,« Proc. SPIE 10796, 107960A (2018); doi: 10.1117/12.2325377 [3] Paunescu, G., Wegner, D., Lutzmann, P., »SWIR imaging through strong atmospheric tur- bulence: comparing a focal-plane-array camera and a compressive sensor,« Proc. SPIE 11160, 111600B (2019); doi: 10.1117/12.2533093 vis IT 9 Laser, Quantum Technology & Active Sensor Systems Figure 3. Image reconstruction with different levels of compression M/N [%] using the sensor- side CS approach. N = 256 x 256 is the number of pixels in the reconstructed image and M<N is the number of patterns used for the reconstruc- tion. The target is illuminated by laser pulses at 1.55 µm.

ROTOR BLADES OF WIND TURBINES cessing and tracking system. This allows sine-like varying Doppler shift between ƒ Evaluation of the structural condition of a the laser to follow moving objects and, e.g. -20 MHz and +20 MHz. The evalua- WTG – lifetime extension or repowering? in particular, enables measurement on tion of the actual vibration signal is thus ƒ Detection of new hidden damage the rotating rotor blades of wind turbines similarly complex as listening to a radio through periodic measurements during operation. station whose transmission frequency ƒ Localization and analysis of altered vibra- 3. The signal acquisition and processing of the complete frequency band. ƒ Identification and quantification of sound the vibrometer determines and compen- emission during operation. constantly wanders back and forth over tion behavior sates for the additional Doppler shift that APPLICATION occurs due to the object's own motion. It Measurements on rotor blades in operation Since access to offshore wind turbines exceeds the frequency shift of the vibra- tions to be measured mostly by several orders of magnitude (powers of ten) and have already been successfully demons- trated under field conditions. It enables a variety of potential future applications, is therefore called »macro Doppler shift«. which are being explored together with When viewed from an oblique angle to the rotor plane, the rotor blades move the project partner, Nawrocki Alpin: ƒ Highly spatially resolved vibration data for for maintenance work is associated with considerable effort, distant measurement methods are of even greater interest here than for onshore turbines. With the deve- lopment of a sufficiently precise stabilized platform to compensate for ship or wave with respect to the measuring device at a validation of simulation models motion, the tracking and vibrometry system speed of up to several times 10 m/s. This ƒ Structural and aerodynamic optimization could be used in this environment in the relative movement causes a continuously of rotor blades future. The project is supported by the Federal Ministry for Economic Affairs and Energy based on a decision by the German Bundes- tag, reference number 0325287A. Figure 2: Example of a vibration spectrum of a rotating rotor blade acquired by the laser Doppler vibrometry system. Weitere Informationen über das Projekt finden Sie hier: https://www.iosb.fraunhofer.de/en/projects- and-products/distant-vibration-measurement- wind-turbines.html 11 vis IT Laser, Quantum Technology & Active Sensor Systems

Figure 2: As-grown crystal boules, drilled boule after extraction and extracted laser rods. Figure 3: X-Ray diffractometer for qualitative measurements conditions are being studied to reach and ensure the highest quality of the crystals. The optical components are cut out from and severely reduce the efficiency of the laser sources. laser crystals as well as the exploration of improved processing methods for crystal the as-grown crystal boules and polished At Fraunhofer IOSB we are investigating using dedicated processes to improve the special laser crystals in a modern laboratory optical damage threshold. environment. To reduce the thermal lensing effect, laser crystals with special dopants growth. The equipment at Fraunhofer IOSB comprises also specific measurement tech- nologies enabling comprehensive analysis of crystals (figure 3). X-ray and spectro- scopic investigation, interferometry, and NOVEL LASER CRYSTALS and geometries are being studied in specially the analysis of the laser-induced damage For the generation of laser radiation, crystals designed laser sources. In this context, we threshold of optical components (at 2 µm from the class of the garnets are currently investigated in particular (figure 2). These laser crystals are rare earth-doped oxide crystals. The performance of laser sources are investigating the promising approach and at 3–5 µm) are also available for joint of developing laser crystals with gradually research projects. graded doping levels. is limited by the physical properties of laser crystals among other limiting factors. At high power, the laser crystal heats up, and temperature differences within the crystal RESEARCH SERVICES FOR CUSTOMERS A lot of scientific knowledge and expertise from the field of crystal growth will also be made available to Fraunhofer IOSB custo- result in undesirable effects like thermal mers and partners as part of joint research lensing, depolarization and aberrations projects in the future. This includes the re- that negatively affect the beam quality search and development of new NLO and 13 vis IT Laser, Quantum Technology & Active Sensor Systems

TE LASER SOURCES IN THE SHORT-WAVE AND MID-WAVE Figure 2: part of the pulsed Ho3+:YAG laser setup. Figure 3: Dependence of pulse energy, pulse duration, and average output power on the pulse repetition rate. [3] A test rig is available at Fraunhofer IOSB optical parametric oscillators (OPO) in the for qualifying the laser-induced damage MWIR and LWIR spectral ranges as well as threshold (LIDT) of SWIR laser components average output power exceeding 10 W (see figure 3) and a pulse power of up to 1.2 MW, while preserving a very good beam quality [3]. Until recently, pulse energies of this and optics. The developed laser technology order could only be realized at lower repe- tition rates or were restricted to more com- plex laser architectures including amplifier stages. enables the characterization of the LIDT up to high intensity and fluence values, directly at the wavelength of interest. Due to the lack of suitable laser sources, manufacturers of SWIR laser optics currently often carry EXPERTISE TO BENEFIT OUR out tests at other wavelengths and then, CUSTOMERS on this basis, only can estimate the damage To qualify laser radiation, the research group threshold in the SWIR range. To do so, they uses the newest and outstanding measuring have to make assumptions which often equipment, such as infrared cameras, wave- represent an unacceptable uncertainty [4], front sensors and optical spectrum analyzers covering the relevant spectral range from SWIR to LWIR. rendering research and optimization of optical damage thresholds difficult. In con- trast, our new capabilities both support the development of optics at IOSB and will also In accordance with the Fraunhofer model, be available for our partners and customers the technological skills and competences as part of joint research projects, aiming at resulting from fundamental research activi- ties flow into the realization of demanding customer projects. The expertise we gained the improvement of production processes and in particular the refinement of optical components. enables us to work on challenging R&D projects aiming at novel solid-state lasers for selected applications in the 1.5–2.1 µm and 3–12 µm spectral range. Similarly, customer- and partner-specific solutions can be realized in joint research projects, in particular in the fields of LIDAR, laser sources for tissue ablation in medicine. In order to meet the specific requirements, the laser parameters such as wavelength, pulse duration, and repetition rate can be tuned across a wide range. [1] Poprawe, Reinhart: Lasertechnik für die Ferti- gung: Grundlagen, Perspektiven und Beispiele für den innovativen Ingenieur. Berlin Heidelberg New York: Springer-Verlag, 2006. ISBN 978-3-540- 26435-4. p. 1-526 [2] Kohnen, Thomas: Refraktive Chirurgie. Berlin Heidelberg New York: Springer-Verlag, 2011. ISBN 978-3-642-05406-8. p. 1-284 [3] Michael Griesbeck, Hendrik Büker, Madeleine Eitner, Katharina Goth, Peter Braesicke, Marc Eichhorn, and Christelle Kieleck, »Mid-infrared optical parametric oscillator pumped by a high- pulse-energy, Q-switched Ho3+:YAG laser,« Appl. Opt. 60, F21-F26 (2021) [4] Schaaf, Peter: Laser Processing of Materials: Fundamentals, Applications and Developments. Berlin Heidelberg: Springer Science & Business Media, 2010.ISBN 978-3-642-13281-0. p. 1-234 15 vis IT Laser, Quantum Technology & Active Sensor Systems

COMPETENCES IOSB provides cutting-edge knowledge to address research and development needs in high-power fiber lasers in the SWIR (short- wave infrared) and MWIR (mid-wave infrared) spectral range. Among those are capabi- lities to take a laser from concept, to an experimental level and up to a demonstra- tor packaged ready to emit. Research and development focuses on high-power fiber lasers emitting in the 1.5 µm and 1.9-2.1 µm ranges and beyond, with an emphasis on defense applications. As often several ways exist to achieve cer- tain output parameters by using different fiber components, pumping schemes, rare earth-doped fibers, and operating regimes, IOSB uses several in-house developed si- mulation tools to investigate and optimize laser designs prior to realization. We also design our own optimized fiber components like fiber endcaps, fiber clad- ding-mode strippers, signal pump combiners, etc., that allow for integrating specific func- tions or increased damage thresholds for power-scaled lasers. The creation of our new state-of-the-art research facility for de- velopment and manufacturing of specialty fibers and their corresponding preforms at our new Oberkochen site, will enable the access to novel fibers for lasers with unique properties well suited for specific applications in the SWIR and MWIR spectral range. Furthermore, using the latest developed high-power/energy fiber lasers, we perform laser damage testing on fibers and optical (fiber) components in the context of our own laser research. In common research projects, these capabilities are also made available to our partners and clients in re- search and development of optical compo- nents and coatings. Figure 3: View into a new high-power fiber laser lab RECENT ACHIEVEMENTS Lately, we have developed a pulsed 2 μm fiber laser source with single-frequency ultra-narrow linewidth operating with nano- second pulses [1] (figure 1). The structure relies on a master oscillator power amp- lifier (MOPA) with an adjustable output pulse shape, flexible repetition rates and peak powers in excess of 10 kW. Another fiber laser architecture that we are investi- gating is based on Q-switching [2] (figure 2). Compared to a MOPA, this topology is more compact. However, the pulse shape and duration is less flexible and determined by the dynamics of the laser, close to a Gaus- sian pulse shape. We have successfully demonstrated significant pulse energy sca- ling using both configurations for pumping an OPO to generate wavelengths in the 3-5 μm range. Other applications for these lasers include LIDAR or supercontinuum generation. With continuous wave fiber lasers, we have achieved record values in power scaling at long wavelengths in silica fibers [3]. The laser configuration was based on an active fiber surrounded by some free-space optical components. Up to 145 W (resp. 262 W) of output power were extracted at 2.2 µm (resp. 2.1 µm). In addition, we have investi- gated power scaling of an all-fiber configu- ration based only on a seed laser and one amplifier. Using this simple MOPA structure, we demonstrated 937 W of output power [4]. The emission properties of this laser are well suited for applications such as material/ metal processing and HEL applications. Figure 3 shows typical pumping equipment for kW-class 2 µm fiber lasers. [1] Lorenz D., et al., »Three-stage MOPA 2 µm fiber laser for ZGP OPO pumping,« SPIE procee- dings vol. 11985, paper 119850H (2022). https:// doi.org/10.1117/12.2610238 [2] Schneider J., et al., »High pulse energy ZnGeP2 OPO directly pumped by a Q-switched Tm3+-doped single-oscillator fiber laser,« Optics Letters, 46(9), 2139 (2021). https://doi.org/10.1364 ol.422702 [3] Forster P., et al., »High-power continuous- wave Tm3+:Ho3+-codoped fiber laser operation from 2.1 μm to 2.2 μm,« Optics Letters, 33(10), 1044–1046 (2022). https://doi.org/10.1364/ OL.458921 [4] Romano C. et al., »937 W Thulium:silica fiber MOPA operating at 2036 nm,« Europhoton proceedings (2022). 17 vis IT Laser, Quantum Technology & Active Sensor Systems

Figure 3: Fiber end caps spliced onto fibers Figure 4: Cross-sectional view of an end face pump combiner with polarization-maintaining central signal fiber. EXAMPLE OF TAILORED FIBER END CAPS AND PUMP COMBINERS FOR HIGH- POWER FIBER LASERS In the development of high-power fiber lasers, the increased thermal and optical stress on fiber end faces poses a particular challenge. The high power densities at optical interfaces between air and glass lead to thermal damage or laser-induced ablation of fiber end faces. Splicing fiber end caps to fibers can provide a solution to this issue. range. Figure 3 shows fiber end caps which have already been spliced onto fibers. The aperture and length of a fiber end cap de- pend on the laser output power and the optical characteristics of the output fiber. fiber-integrated pump combiners, the launched pump power can be maximized depending solely on the availability of the fiber-coupled pump sources and their brightness. Power scaling in fiber lasers requires an efficient concept for incoupling the pump radiation. Using free-space optics, highly There are two common designs for pump combiners: side and end face pump combiners. One of the research topics at complex optical systems would be required Fraunhofer IOSB is the fabrication of end due to the etendue of the pump sources. The required complexity of such optics ul- timately technologically limits the coupled Fiber end caps are substrates often made pump radiation and, thus, the laser output from fused silica with an aperture of se- power. Pump combiners reduce the com- veral millimeters. The base surface on the output side has an anti-reflective coating to prevent back reflection of the emerging laser beam. Splicing a fiber end cap to the fiber increases the beam size of the laser beam diverging inside the glass substrate (Figure 2). This reduces the power density of the laser beam below the damage thres- plexity of coupling optics to a minimum so that a maximum laser output power can be achieved. Pump combiners bundle pump radiation from several multimode fibers into one active fiber. This allows the pump sources to be spatially separated from each other, hold of the air-glass interface while keeping resulting in better heat dissipation, which the laser output power constant. Therefore, fiber end caps enable the realization of high- power fiber lasers in the multi-kilowatt in turn leads to an increase in durability of the entire laser system. Furthermore, by eliminating coupling optics through face pump combiners, where several pump fibers and a signal fiber are fused. Figure 4 shows a cross-sectional view of an end face pump combiner. The signal fiber is in the center and is enclosed by the six pump fibers. Future work at Fraunhofer IOSB aims to further develop and optimize new processes that fully control the fabrication of fibers and fiber components. In common research projects, IOSB capabilities are also made available to our partners and clients to in- vestigate and enhance fiber components capabilities. 19 vis IT Laser, Quantum Technology & Active Sensor Systems