OUSD (R&E) MODERNIZATION PRIORITY: Directed Energy TECHNOLOGY AREA(S): Sensors; Electronics OBJECTIVE: The objective of this project is to develop a bench-level source and turbulence simulator that can accurately simulate common laser guide star (LGS) beacons for LGS adaptive optics (AO) systems and test beds. Currently new LGS AO design concepts can only be tested in simulation or through expensive and man-power intensive on-sky testing. This project seeks to develop methods for accurately simulating an LGS beacon on an optics bench to enable rapid prototyping of new LGS AO technologies. The ideal bench source would accurately simulate sodium and Rayleigh beacons including atmospheric effects on the uplink and downlink propagation of the beacon, the temporal and spatial coherence properties of LGS beacons including beacon elongation, focus and angular anisoplanatism effects, user defined beacon jitter, and support multiple beacon constellations (including a mix of Rayleigh and sodium beacons, multiple Rayleigh, or multiple sodium beacons). In addition, the various parameters of the beacon being simulated should be reconfigurable (i.e. the user should be able to change the launch size of the beacon, beacon altitude, beacon elongation, number and type of beacons, different constellation configurations, side vs center vs full aperture launch, etc.). The final beacon and turbulence simulator must be fully characterized to ensure high confidence in the beacon parameters that are being simulated, provide a high degree of repeatability between experiments, and be easily configured to achieve desired beacon and turbulence parameters. The LGS beacon simulator must also support a broadband "natural" guidestar beacon to test the effectiveness of the LGS AO correction on a target of interest. In short, the objective is to create a bench-level source that accurately simulates LGS beacons in a repeatable and configurable atmospheric turbulence simulator to enable rapid and effective testing of LGS AO technology and dramatically speed up technology development. DESCRIPTION: The goal of the project is to create a perfectly representative LGS source and atmospheric turbulence simulator; however, practical considerations will inevitably lead to trade-offs between accurately simulating different beacon parameters and practical trade-offs to ensure predictable and repeatable beacon and turbulence parameters, enable easy reconfiguration, and meet cost or schedule constraints. Thus a detailed sensitivity analysis and requirements flow down will be a critical part of the project. The initial target LGS AO system is a 1-4m class telescope with a side-launched sodium or Rayleigh beacon. While the primary focus of the development effort is on simulating LGS beacons, the simulator must also include atmospheric turbulence to be effective. The turbulence simulator should include at least two Komologrov phase screens and be capable of generating atmospheric parameters covering a range of coherence lengths: threshold of 3-15 cm, objective of 1.5-20 cm; isoplanatic angle: threshold of 4 15 rad, objective of 2-20 rad; and greenwood frequency: threshold: 0-500 Hz, objective 0-1000 Hz. It is also highly desirable to have an option to remove turbulence effects for alignment and troubleshooting work. The sensitivity analysis should at a minimum consider the effects of beacon parameters on the most common LGS AO wavefront sensor (WFS), the Shack-Hartmann WFS (SHWFS), but ideally the sensitivity analysis would consider multiple WFS or be WFS agnostic to ensure future testing of new WFS concepts is also supported. For example, it's possible that the proposed method for simulating an elongated sodium beacon is not compatible with accurately simulating beacon jitter (i.e. jitter due to the laser launch telescope). At a minimum, the sensitivity analysis would be used to determine whether beacon jitter or beacon elongation has a more deleterious effect on the LGS AO performance of the baseline system and thus inform system development. Ideally, the sensitivity analysis would also include potential uses of the simulator for future development efforts. This additional analysis is more open ended and difficult, but a review of the current literature on LGS AO systems can be used to anticipate some possible use cases of a LGS source/turbulence simulator. A few examples include: laser tomography to mitigate focus anisoplanatism, uplink correction of the laser beacon, combining measurements from different beacon types (Rayleigh, Sodium, and/or natural), pulsed sodium systems to minimize beacon elongation, alternative WFS concepts (i.e. the Ingot WFS), etc. For example, when developing a method for generating multiple sodium beacons or multiple Rayleigh beacons, the effect of adding additional beacons on mitigating focus anisoplanatism can be estimated given the altitude of the beacons, the size of the telescope, and the turbulence profile. Thus a sensitivity analysis can be used to justify a limited number of beacons if necessary to meet system level trade-offs or budget constraints. In short, the initial stages of the project will focus not only on developing methods to simulate LGS beacons and atmospheric turbulence, but also the analysis necessary to make intelligent trade-offs in the final system design. The sensitivity analysis will also feed into requirements development and flow-down for development of techniques to simulate various aspects of the laser beacon. Requirements development should be done concurrently with the sensitivity analysis in an iterative process to identify limitations that may necessitate different designs or restrictions in capabilities. While most design and development work is anticipated to be based on computer simulation, any techniques that are key to the overall function and/or higher risk should be tested in a standalone hardware configuration. For example, the technique for generating an extended beacon is critical to the overall simulation of an LGS beacon and thus should be demonstrated with physical hardware. The demonstration can be at the component level and does not need to include the turbulence simulation or other aspects of the beacon simulation (jitter, Rayleigh scatter contamination, etc.). Once concepts are fully developed and critical concepts are demonstrated on an individual/component level, work will shift toward integration of the various components into a complete LGS source/turbulence simulator. The initial integration work will focus on simulating the most important LGS beacon parameters (as determined by the sensitivity analysis) in a turbulence simulator. The initial integrated design should be capable of simulating at least one beacon of each type (Rayleigh, Sodium, and a target or natural guidestar) with atmospheric turbulence effects (both uplink and downlink for LGS beacons), and demonstrate the capability to vary beacon and turbulence parameters with predictable and repeatable results. The design should include options for more advanced simulation scenarios, e.g. multiple beacons, uplink correction, etc., but initial testing will focus on thorough testing of basic system functionality and only move onto testing more complicated scenarios once basic functionality testing is successful. The integrated system should be small enough to fit on a standard optical bench (less than roughly 6'x8'x4' volume), have an optical output power of at least 1 W/cm^2 to the AO system using Class 3B lasers (objective: optical efficiency >1%), and have an output beam of ~1 in diameter. The initial system should be capable of operating over visible wavelengths but does not need to use laser sources that match standard Rayleigh and Sodium wavelengths. Ideally the system would be wavelength agnostic so that the end user could integrate any desired beacon wavelength, but at a minimum the system needs to simulate two different wavelengths (one for Rayleigh and one for Sodium, separated by at least 50 nm) and support a visible or near infrared broadband target source (>100 nm bandwidth). Once initial functionality has been demonstrated and tested, additional capabilities will be integrated and tested with the goal of identifying any system limitations or shortfalls that can be mitigated or resolved in later designs. If the initial integration stage is successful, further development will focus on optimizing the design for integration onto a LGS AO system that is capable of on-sky testing, either through modifications and redesign of the initial system or a completely new system design. The primary goal of integrating with a full-up LGS AO system would be to enable comparison testing between on-sky and bench results. Once again, analysis and testing would focus on identifying system limitations and shortfalls that can be used to improve future LGS ATS designs. Ultimately, the project should support the development of a robust LGS ATS capability that can be deployed onto multiple systems and be used to rapidly test new LGS AO technologies in support of Air Force and Space Force missions. The technology developed in this project can also be readily transitioned to support LGS AO systems on astronomical telescopes where a more realistic source/turbulence simulator could be very valuable for maximizing observation time. The sensitivity analysis will also feed into requirements development and flow-down for development of techniques to simulate various aspects of the laser beacon. Requirements development should be done concurrently with the sensitivity analysis in an iterative process to identify limitations that may necessitate different designs or restrictions in capabilities. While most design and development work is anticipated to be based on computer simulation, any techniques that are key to the overall function and/or higher risk should be tested in a standalone hardware configuration. For example, the technique for generating an extended beacon is critical to the overall simulation of an LGS beacon and thus should be demonstrated with physical hardware. The demonstration can be at the component level and does not need to include the turbulence simulation or other aspects of the beacon simulation (jitter, Rayleigh scatter contamination, etc.). Once concepts are fully developed and critical concepts are demonstrated on an individual/component level, work will shift toward integration of the various components into a complete LGS ATS system. The initial integration work will focus on simulating the most important LGS beacon parameters (as determined by the sensitivity analysis) in a unified LGS ATS. The initial integrated design should be capable of simulating at least one beacon of each type (Rayleigh, Sodium, and a target or natural guidestar) with atmospheric turbulence effects (both uplink and downlink for LGS beacons), and demonstrate the capability to vary beacon and turbulence parameters with predictable and repeatable results. The design should include options for more advanced simulation scenarios, e.g. multiple beacons, uplink correction, etc., but initial testing will focus on thorough testing of basic system functionality and only move onto testing more complicated scenarios once basic functionality testing is successful. The integrated system should be small enough to fit on a standard optical bench (less than roughly 6'x8'x4' volume), have an optical output power of at least 1 W/cm^2 to the AO system using Class 3B lasers (objective: optical efficiency >1%), and have an output beam of ~1 in diameter. The initial system should be capable of operating over visible wavelengths but does not need to use laser sources that match standard Rayleigh and Sodium wavelengths. Ideally the system would be wavelength agnostic so that the end user could integrate any desired beacon wavelength, but at a minimum the system needs to simulate two different wavelengths (one for Rayleigh and one for Sodium, separated by at least 50 nm) and support a visible or near infrared broadband target source (>100 nm bandwidth). Once initial functionality has been demonstrated and tested, additional capabilities will be integrated and tested with the goal of identifying any system limitations or shortfalls that can be mitigated or resolved in later designs. If the initial integration stage is successful, further development will focus on optimizing the design for integration onto a LGS AO system that is capable of on-sky testing. Either through modifications and redesign of the initial system or a completely new system design. The primary goal of integrating with a full-up LGS AO system would be to enable comparison testing between on-sky and bench results. Once again, analysis and testing would focus on identifying system limitations and shortfalls that can be used to improve future LGS ATS designs. Ultimately, the project should support the development of a robust LGS ATS capability that can be deployed onto multiple systems and be used to rapidly test new LGS AO technologies in support of Air Force and Space Force missions. The sensitivity analysis will also feed into requirements development and flow-down for development of techniques to simulate various aspects of the laser beacon. Requirements development should be done concurrently with the sensitivity analysis in an iterative process to identify limitations that may necessitate different designs or restrictions in capabilities. While most design and development work is anticipated to be based on computer simulation, any techniques that are key to the overall function and/or higher risk should be tested in a standalone hardware configuration. For example, the technique for generating an extended beacon is critical to the overall simulation of an LGS beacon and thus should be demonstrated with physical hardware. The demonstration can be at the component level and does not need to include the turbulence simulation or other aspects of the beacon simulation (jitter, Rayleigh scatter contamination, etc.). Once concepts are fully developed and critical concepts are demonstrated on an individual/component level, work will shift toward integration of the various components into a complete LGS source/turbulence simulator. The initial integration work will focus on simulating the most important LGS beacon parameters (as determined by the sensitivity analysis) in a turbulence simulator. The initial integrated design should be capable of simulating at least one beacon of each type (Rayleigh, Sodium, and a target or natural guidestar) with atmospheric turbulence effects (both uplink and downlink for LGS beacons), and demonstrate the capability to vary beacon and turbulence parameters with predictable and repeatable results. The design should include options for more advanced simulation scenarios, e.g. multiple beacons, uplink correction, etc., but initial testing will focus on thorough testing of basic system functionality and only move onto testing more complicated scenarios once basic functionality testing is successful. The integrated system should be small enough to fit on a standard optical bench (less than roughly 6'x8'x4' volume), have an optical output power of at least 1 W/cm^2 to the AO system using Class 3B lasers (objective: optical efficiency >1%), and have an output beam of ~1 in diameter. The initial system should be capable of operating over visible wavelengths but does not need to use laser sources that match standard Rayleigh and Sodium wavelengths. Ideally the system would be wavelength agnostic so that the end user could integrate any desired beacon wavelength, but at a minimum the system needs to simulate two different wavelengths (one for Rayleigh and one for Sodium, separated by at least 50 nm) and support a visible or near infrared broadband target source (>100 nm bandwidth). Once initial functionality has been demonstrated and tested, additional capabilities will be integrated and tested with the goal of identifying any system limitations or shortfalls that can be mitigated or resolved in later designs. If the initial integration stage is successful, further development will focus on optimizing the design for integration onto a LGS AO system that is capable of on-sky testing. Either through modifications and redesign of the initial system or a completely new system design. The primary goal of integrating with a full-up LGS AO system would be to enable comparison testing between on-sky and bench results. Once again, analysis and testing would focus on identifying system limitations and shortfalls that can be used to improve future LGS ATS designs. Ultimately, the project should support the development of a robust LGS ATS capability that can be deployed onto multiple systems and be used to rapidly test new LGS AO technologies in support of Air Force and Space Force missions. The technology developed in this project can also be readily transitioned to support LGS AO systems on astronomical telescopes where a more realistic source/turbulence simulator could be very valuable for maximizing observation time. The sensitivity analysis will also feed into requirements development and flow-down for development of techniques to simulate various aspects of the laser beacon. Requirements development should be done concurrently with the sensitivity analysis in an iterative process to identify limitations that may necessitate different designs or restrictions in capabilities. While most design and development work is anticipated to be based on computer simulation, any techniques that are key to the overall function and/or higher risk should be tested in a standalone hardware configuration. For example, the technique for generating an extended beacon is critical to the overall simulation of an LGS beacon and thus should be demonstrated with physical hardware. The demonstration can be at the component level and does not need to include the turbulence simulation or other aspects of the beacon simulation (jitter, Rayleigh scatter contamination, etc.). Once concepts are fully developed and critical concepts are demonstrated on an individual/component level, work will shift toward integration of the various components into a complete LGS ATS system. The initial integration work will focus on simulating the most important LGS beacon parameters (as determined by the sensitivity analysis) in a unified LGS ATS. The initial integrated design should be capable of simulating at least one beacon of each type (Rayleigh, Sodium, and a target or natural guidestar) with atmospheric turbulence effects (both uplink and downlink for LGS beacons), and demonstrate the capability to vary beacon and turbulence parameters with predictable and repeatable results. The design should include options for more advanced simulation scenarios, e.g. multiple beacons, uplink correction, etc., but initial testing will focus on thorough testing of basic system functionality and only move onto testing more complicated scenarios once basic functionality testing is successful. The integrated system should be small enough to fit on a standard optical bench (less than roughly 6'x8'x4' volume), have an optical output power of at least 1 W/cm^2 to the AO system using Class 3B lasers (objective: optical efficiency >1%), and have an output beam of ~1 in diameter. The initial system should be capable of operating over visible wavelengths but does not need to use laser sources that match standard Rayleigh and Sodium wavelengths. Ideally the system would be wavelength agnostic so that the end user could integrate any desired beacon wavelength, but at a minimum the system needs to simulate two different wavelengths (one for Rayleigh and one for Sodium, separated by at least 50 nm) and support a visible or near infrared broadband target source (>100 nm bandwidth). Once initial functionality has been demonstrated and tested, additional capabilities will be integrated and tested with the goal of identifying any system limitations or shortfalls that can be mitigated or resolved in later designs. If the initial integration stage is successful, further development will focus on optimizing the design for integration onto a LGS AO system that is capable of on-sky testing. Either through modifications and redesign of the initial system or a completely new system design. The primary goal of integrating with a full-up LGS AO system would be to enable comparison testing between on-sky and bench results. Once again, analysis and testing would focus on identifying system limitations and shortfalls that can be used to improve future LGS ATS designs. Ultimately, the project should support the development of a robust LGS ATS capability that can be deployed onto multiple systems and be used to rapidly test new LGS AO technologies in support of Air Force and Space Force missions. PHASE I: Phase I will consist of a sensitivity analysis to determine which properties of an LGS beacon are most relevant to an LGS AO system design on a 1-4m telescope. The sensitivity analysis will be critical to determining which areas to focus on for technical development and help resolve any potential trade-offs in future system design work. The sensitivity analysis will also feed into and be informed by requirements flow down and concept development, which will be based primarily on computer simulation but should include limited component level hardware design and testing for critical components to verify the adequacy of the technique in simulating a LGS beacon. Primary output of Phase One is a final report which covers the following topics: -Summary of the sensitivity analysis with key results showing which beacon parameters were found to be most important for an LGS beacon simulator -Detailed description of the preliminary design, highlighting key trade-offs and technical innovations in simulating an LGS beacon and the proposed method for integrating each component into a final integrated system -A top-level requirements flow-down and preliminary design of key components of the LGS source and turbulence simulator system, i.e. hardware generating extended beacons, phase wheels, laser sources, any active hardware (e.g. steering mirror, spatial light modulator, etc.), and any other custom or critical hardware -Summary of any component level testing, with comparison between test data and desired beacon/turbulence characteristics Primary output of Phase One is a final report which covers the following topics: -Summary of the sensitivity analysis with key results showing which beacon parameters were found to be most important for an LGS beacon simulator -Detailed description of the preliminary design, highlighting key trade-offs and technical innovations in simulating an LGS beacon and the proposed method for integrating each component into a final integrated system -A top-level requirements flow-down and preliminary design of key components of the LGS ATS system, i.e. hardware generating extended beacons, phase wheels, laser sources, any active hardware (e.g. steering mirror, spatial light modulator, etc.), and any other custom or critical hardware -Summary of any component level testing, with comparison between test data and desired beacon/turbulence characteristics PHASE II: Phase II will move on to the design and development of an integrated bench-level source, demonstrating not only accurate simulation of the key properties of a laser beacon, but also the ability to readily vary the beacon parameters with predictable and repeatable results. Testing and characterization of the setup will be completed to identify any shortfalls in the system setup. At a minimum testing of the integrated system should include a SHWFS in an open-loop configuration, ideally testing would be done with a full LGS AO system or test bed to demonstrate the effectiveness of the final design in its end-use case. The primary deliverable of Phase II will be an integrated LGS source and turbulence simulator system meeting the following requirements: -Fits within 6'x8'x4' (width, length, height) volume (smaller is preferred) -Includes at least two phase screens and is capable of simulating a range of atmospheric turbulence parameters: coherence length (threshold of 3-15 cm, objective of 1.5-20 cm); isoplanatic angle (threshold of 4 15 rad, objective of 2-20 rad); and greenwood frequency (threshold: 0-500 Hz, objective 0-1000 Hz), also includes option for removing turbulence from beam path -Simulates at least one Rayleigh beacon, one Sodium beacon and a broadband target source concurrently, Rayleigh and Sodium beacons must be in the visible band and at different wavelengths (>50 nm separation) and target source must be in the visible or near infrared band (400- 1000 nm) with >100 nm spectral bandwidth -System simulates uplink and downlink atmospheric effects on the laser beacons including focus and angular anisoplanatism effects (uplink and downlink can have different turbulence paths but should have the same turbulence statistics) -Predictable and repeatable laser beacon and turbulence parameters (user should be able to configure the system to achieve desired beacon and turbulence parameters with 1% -Simulate user-defined beacon jitter and include option for uplink correction (this can just be a place holder for an SLM or deformable mirror) A final report is also required covering: -Final system design, with detailed drawings (Zemax or equivalent optical design drawings, Solidworks or equivalent mechanical drawings, electrical design drawings, etc.) and specifications and data sheets for all key components (optical and electrical components) -Test results from basic functionality testing comparing measured results to predicted results and to desired results -Characterization results showing the accuracy and repeatability of varying system configuration parameters and any software required to generate system configurations from user defined beam/turbulence parameters -Lessons learned and recommendations for future system designs. PHASE III DUAL USE APPLICATIONS: If Phase II is successful, phase III will seek to further refine the initial design. The primary goal for phase III would be to adapt the phase II system for integration with an on-sky capable LGS AO system. This would either represent some modifications and redesign of the phase II system or a completely new design customized for optimal integration with the LGS AO system. Any redesign required would build on lessons learned from the phase II project. Once completed, testing would focus on comparing on-sky and bench level results to further validate the capabilities of the LGS source/turbulence simulator system. If the results of the on-sky comparison testing is favorable, the work would transition to designing and developing LGS source and turbulence simulator systems for specific Air Force and Space Force applications. The primary deliverable of Phase III will be an LGS source and turbulence simulator system integrated with an on-sky capable LGS AO system. The requirements of the system will be based on lessons learned from the phase II effort and the specific interface requirements of the LGS AO system. A final report will also be required at the conclusion of the Phase III effort which will focus primarily on the results of the comparison testing between on-sky and bench-level results but will also include all of the phase II final report topic areas. REFERENCES: Ruiyao Luo, Wenda Cui, Hongyan Wang, Wuming Wu, Quan Sun, Yu Ning, Xiaojun Xu, "Spatial light modulators based laser guide star simulator," Proc. SPIE 10173, Fourth International Symposium on Laser Interaction with Matter, 101731C (12 May 2017); doi: 10.1117/12.2267932; J. Huang, K. Wei, K. Jin, M. Li and Y. Zhang, "Controlling the Laser Guide Star power density distribution at Sodium layer by combining Pre-correction and Beam-shaping," Optics Communications, vol. 416, pp. 172-180, 2018.; R. Rampy, D. Gavel, S. Rochester and R. Holzlohner, "Toward optimization of pulsed sodium laser guide stars," Journal of the Optical Society of America B: Optical Physics, vol. 32, no. 12, pp. 2425-2434, 2015.; Roberto Ragazzoni, Davide Greggio, Valentina Viotto, Simone Di Filippo, Marco Dima, Jacopo Farinato, Maria Bergomi, Elisa Portaluri, Demetrio Magrin, Luca Marafatto, Federico Biondi, Elena Carolo, Simonetta Chinellato, Gabriele Umbriaco, Daniele Vassallo, "Extending the pyramid WFS to LGSs: the INGOT WFS," Proc. SPIE 10703, Adaptive Optics Systems VI, 107033Y (11 July 2018); doi: 10.1117/12.2313917; M. Lloyd-Hart, C. Baranec, N.M. Milton, T. Stalcup, M. Snyder, N. Putnam, and J.R.P. Angel, "First test of a wavefront sensing with a constellation of laser guide beacons," Astro. J., 634:679-686, 2005; Imelda A. De La Rue, Brent L. Ellerbroek, "Multiple guide stars to improve the performance of laser guide star adaptive optical systems," Proc. SPIE 3353, Adaptive Optical System Technologies, (11 September 1998); doi:10.1117/12.321723 KEYWORDS: Laser guide star; adaptive optics; atmospheric turbulence
