{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,27]],"date-time":"2026-02-27T04:14:50Z","timestamp":1772165690158,"version":"3.50.1"},"reference-count":31,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2023,4,2]],"date-time":"2023-04-02T00:00:00Z","timestamp":1680393600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"CENTRO 2020 programme","award":["CENTRO-01-0145- FEDER-022141"],"award-info":[{"award-number":["CENTRO-01-0145- FEDER-022141"]}]},{"name":"CENTRO 2020 programme","award":["101008280"],"award-info":[{"award-number":["101008280"]}]},{"name":"CENTRO 2020 programme","award":["PTDC\/EEI-TEL\/2697\/2021"],"award-info":[{"award-number":["PTDC\/EEI-TEL\/2697\/2021"]}]},{"name":"CENTRO 2020 programme","award":["UIDB\/50008\/2020-UIDP\/50008\/2020"],"award-info":[{"award-number":["UIDB\/50008\/2020-UIDP\/50008\/2020"]}]},{"name":"CENTRO 2020 programme","award":["2020.07521.BD"],"award-info":[{"award-number":["2020.07521.BD"]}]},{"name":"CENTRO 2020 programme","award":["2022.07168.CEECIND"],"award-info":[{"award-number":["2022.07168.CEECIND"]}]},{"name":"CENTRO 2020 programme","award":["100010434"],"award-info":[{"award-number":["100010434"]}]},{"name":"CENTRO 2020 programme","award":["LCF\/BQ\/PR20\/11770015"],"award-info":[{"award-number":["LCF\/BQ\/PR20\/11770015"]}]},{"name":"MSCA RISE programme","award":["CENTRO-01-0145- FEDER-022141"],"award-info":[{"award-number":["CENTRO-01-0145- FEDER-022141"]}]},{"name":"MSCA RISE programme","award":["101008280"],"award-info":[{"award-number":["101008280"]}]},{"name":"MSCA RISE programme","award":["PTDC\/EEI-TEL\/2697\/2021"],"award-info":[{"award-number":["PTDC\/EEI-TEL\/2697\/2021"]}]},{"name":"MSCA RISE programme","award":["UIDB\/50008\/2020-UIDP\/50008\/2020"],"award-info":[{"award-number":["UIDB\/50008\/2020-UIDP\/50008\/2020"]}]},{"name":"MSCA RISE programme","award":["2020.07521.BD"],"award-info":[{"award-number":["2020.07521.BD"]}]},{"name":"MSCA RISE programme","award":["2022.07168.CEECIND"],"award-info":[{"award-number":["2022.07168.CEECIND"]}]},{"name":"MSCA RISE programme","award":["100010434"],"award-info":[{"award-number":["100010434"]}]},{"name":"MSCA RISE programme","award":["LCF\/BQ\/PR20\/11770015"],"award-info":[{"award-number":["LCF\/BQ\/PR20\/11770015"]}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["CENTRO-01-0145- FEDER-022141"],"award-info":[{"award-number":["CENTRO-01-0145- FEDER-022141"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["101008280"],"award-info":[{"award-number":["101008280"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["PTDC\/EEI-TEL\/2697\/2021"],"award-info":[{"award-number":["PTDC\/EEI-TEL\/2697\/2021"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["UIDB\/50008\/2020-UIDP\/50008\/2020"],"award-info":[{"award-number":["UIDB\/50008\/2020-UIDP\/50008\/2020"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["2020.07521.BD"],"award-info":[{"award-number":["2020.07521.BD"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["2022.07168.CEECIND"],"award-info":[{"award-number":["2022.07168.CEECIND"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["100010434"],"award-info":[{"award-number":["100010434"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT\/MCTES","doi-asserted-by":"publisher","award":["LCF\/BQ\/PR20\/11770015"],"award-info":[{"award-number":["LCF\/BQ\/PR20\/11770015"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["CENTRO-01-0145- FEDER-022141"],"award-info":[{"award-number":["CENTRO-01-0145- FEDER-022141"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["101008280"],"award-info":[{"award-number":["101008280"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["PTDC\/EEI-TEL\/2697\/2021"],"award-info":[{"award-number":["PTDC\/EEI-TEL\/2697\/2021"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["UIDB\/50008\/2020-UIDP\/50008\/2020"],"award-info":[{"award-number":["UIDB\/50008\/2020-UIDP\/50008\/2020"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["2020.07521.BD"],"award-info":[{"award-number":["2020.07521.BD"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["2022.07168.CEECIND"],"award-info":[{"award-number":["2022.07168.CEECIND"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["100010434"],"award-info":[{"award-number":["100010434"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100001871","name":"FCT","doi-asserted-by":"publisher","award":["LCF\/BQ\/PR20\/11770015"],"award-info":[{"award-number":["LCF\/BQ\/PR20\/11770015"]}],"id":[{"id":"10.13039\/501100001871","id-type":"DOI","asserted-by":"publisher"}]},{"name":"\u201cla Caixa\u201d Foundation","award":["CENTRO-01-0145- FEDER-022141"],"award-info":[{"award-number":["CENTRO-01-0145- FEDER-022141"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["101008280"],"award-info":[{"award-number":["101008280"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["PTDC\/EEI-TEL\/2697\/2021"],"award-info":[{"award-number":["PTDC\/EEI-TEL\/2697\/2021"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["UIDB\/50008\/2020-UIDP\/50008\/2020"],"award-info":[{"award-number":["UIDB\/50008\/2020-UIDP\/50008\/2020"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["2020.07521.BD"],"award-info":[{"award-number":["2020.07521.BD"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["2022.07168.CEECIND"],"award-info":[{"award-number":["2022.07168.CEECIND"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["100010434"],"award-info":[{"award-number":["100010434"]}]},{"name":"\u201cla Caixa\u201d Foundation","award":["LCF\/BQ\/PR20\/11770015"],"award-info":[{"award-number":["LCF\/BQ\/PR20\/11770015"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Photonics"],"abstract":"<jats:p>The continuous growth of Internet data traffic is pushing the current radio-frequency wireless technologies up to their physical limits. To overcome the upcoming bandwidth bottleneck, Free-Space Optics (FSO) is currently deemed as a key breakthrough toward next-generation ultra-high-capacity wireless links. Despite its numerous advantages, FSO also entails several particular challenges regarding the mitigation of the stochastic impairments induced by turbulence and the strict alignment requirements. One of the main issues of FSO communication systems is the mitigation of pointing errors and angle-of-arrival (AoA) fluctuations, which arise from misalignments induced by atmospheric turbulence and vibrations at the transmitting and receiving stations. A common approach to mitigate the impact of pointing errors is the use of an acquisition, tracking and pointing (ATP) system on one or both ends of the FSO link. In this paper, we present a characterization of the pointing errors and the AoA impact on the power budget of the FSO link to quantify the misalignment impairments. Afterwards, we experimentally demonstrate an FSO link with an ATP mechanism at both ends, managed by a control plane that enables the continuous and accurate alignment of the FSO link. To increase the misalignment tolerance, the ATP mechanism comprises two stages: the first one is based on a spatial diversity method provided by a quadrant detector, while the second stage maximizes the optical received power. Lastly, the impact of the beam misalignment on the achievable information rate of a coherent optical wireless system is theoretically addressed and characterized.<\/jats:p>","DOI":"10.3390\/photonics10040394","type":"journal-article","created":{"date-parts":[[2023,4,3]],"date-time":"2023-04-03T02:32:27Z","timestamp":1680489147000},"page":"394","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":19,"title":["Requirements and Solutions for Robust Beam Alignment in Fiber-Coupled Free-Space Optical Systems"],"prefix":"10.3390","volume":"10","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-2850-0045","authenticated-orcid":false,"given":"Manuel M.","family":"Freitas","sequence":"first","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Universidade de Aveiro, Campus Universit\u00e1rio de Santiago, 3810-193 Aveiro, Portugal"}]},{"given":"Marco A.","family":"Fernandes","sequence":"additional","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Universidade de Aveiro, Campus Universit\u00e1rio de Santiago, 3810-193 Aveiro, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4664-9238","authenticated-orcid":false,"given":"Paulo P.","family":"Monteiro","sequence":"additional","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Universidade de Aveiro, Campus Universit\u00e1rio de Santiago, 3810-193 Aveiro, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6404-8431","authenticated-orcid":false,"given":"Fernando P.","family":"Guiomar","sequence":"additional","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Universidade de Aveiro, Campus Universit\u00e1rio de Santiago, 3810-193 Aveiro, Portugal"}]},{"given":"Gil M.","family":"Fernandes","sequence":"additional","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Universidade de Aveiro, Campus Universit\u00e1rio de Santiago, 3810-193 Aveiro, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2023,4,2]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1639","DOI":"10.1109\/JSAC.2009.091213","article-title":"1.28 terabit\/s (32 \u00d7 40 Gbit\/s) WDM transmission system for free space optical communications","volume":"27","author":"Ciaramella","year":"2009","journal-title":"IEEE J. Sel. Areas Commun."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"7357506","DOI":"10.1109\/JPHOT.2022.3193112","article-title":"Self-Alignment FSOC System With Miniaturized Structure for Small Mobile Platform","volume":"14","author":"Sun","year":"2022","journal-title":"IEEE Photonics J."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"1226","DOI":"10.1109\/LPT.2022.3210024","article-title":"Beam Steering and Divergence Control Using Variable Focus Liquid Lenses for WDM FSO Communications","volume":"34","author":"Mai","year":"2022","journal-title":"IEEE Photonics Technol. Lett."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Fernandes, M.A., Monteiro, P.P., and Guiomar, F.P. (2021, January 13\u201316). 400G MIMO-FSO Transmission with Enhanced Reliability Enabled by Joint LDPC Coding. Proceedings of the 2021 European Conference on Optical Communication (ECOC), Bordeaux, France.","DOI":"10.1109\/ECOC52684.2021.9605896"},{"key":"ref_5","unstructured":"Takahashi, H., Ishimura, S., Nishimura, K., and Tsuritani, T. (2022, January 18\u201322). Highly-Tolerant Free-Space Parallel Optical Wireless Communication Links with Signal-to-Signal SNR Difference Compensation. Proceedings of the 2022 European Conference on Optical Communication (ECOC), Basel, Switzerland."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"20136","DOI":"10.1364\/OE.426551","article-title":"Adaptive Optical Beam Alignment and Link Protection Switching for 5G-over-FSO","volume":"29","author":"Fernandes","year":"2021","journal-title":"Opt. Express"},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Guiomar, F.P., Lorences-Riesgo, A., Ranzal, D., Rocco, F., Sousa, A.N., Carena, A., Teixeira, A.L., Medeiros, M.C.R., and Monteiro, P.P. (2019, January 22\u201326). High-capacity and rain-resilient Free-Space Optics link enabled by time-adaptive probabilistic shaping. Proceedings of the 45th European Conference on Optical Communication (ECOC 2019), Dublin, Ireland.","DOI":"10.1049\/cp.2019.0864"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"6529","DOI":"10.1109\/JLT.2020.3012737","article-title":"Adaptive Probabilistic Shaped Modulation for High-Capacity Free-Space Optical Links","volume":"38","author":"Guiomar","year":"2020","journal-title":"J. Light. Technol."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"1104","DOI":"10.1109\/COMST.2018.2804323","article-title":"A Survey on Acquisition, Tracking, and Pointing Mechanisms for Mobile Free-Space Optical Communications","volume":"20","author":"Kaymak","year":"2018","journal-title":"IEEE Commun. Surv. Tutor."},{"key":"ref_10","doi-asserted-by":"crossref","unstructured":"Mai, V., and Kim, H. (2022, January 26\u201328). Variable Focus Lens-Based Beam Steering and Divergence Control for WDM Free-Space Optical Communication. Proceedings of the 2022 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA.","DOI":"10.1364\/OFC.2022.M1C.6"},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Al Akkoumi, M., Refai, H., and Sluss, J. (2008, January 28\u201330). A tracking system for mobile FSO. Proceedings of the SPIE\u2014The International Society for Optical Engineering, San Jose, CA, USA.","DOI":"10.1117\/12.763875"},{"key":"ref_12","doi-asserted-by":"crossref","unstructured":"Al-Akkoumi, M.K., Refai, H.H., and Sluss, J.J. (2008, January 19\u201324). A tracking system for mobile FSO. Proceedings of the SPIE LASE, San Jose, CA, USA.","DOI":"10.1117\/12.763875"},{"key":"ref_13","unstructured":"Pascual, A., Rivera, C., Ines, J., and Collado, M. (2021, January 17\u201319). Gimbal mechanism design based on piezoelectric motors. Proceedings of the ACTUATOR, International Conference and Exhibition on New Actuator Systems and Applications, Online."},{"key":"ref_14","unstructured":"Optotune, F.S.M. (2023, February 06). Available online: https:\/\/www.optotune.com\/fast-steering-mirrors."},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Lahari, S.A., Raj, A., and Soumya, S. (2021, January 30\u201331). Control of fast steering mirror for accurate beam positioning in FSO communication system. Proceedings of the 2021 International Conference on System, Computation, Automation and Networking (ICSCAN), Puducherry, India.","DOI":"10.1109\/ICSCAN53069.2021.9526537"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"128","DOI":"10.1016\/j.precisioneng.2021.09.003","article-title":"Design and precision tracking control of a high-bandwidth fast steering mirror for laser beam machining","volume":"73","author":"Zhong","year":"2022","journal-title":"Precis. Eng."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Yu, Z., Lihao, W., Yang, W., Yonggui, Z., Yichen, L., and Zhenyu, W. (2023, February 06). Control Algorithm of a Novel MEMS Fast Steering Mirror for High Precision Beam Pointing. Available online: https:\/\/ssrn.com\/abstract=4281452.","DOI":"10.2139\/ssrn.4281452"},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"355","DOI":"10.1007\/s10297-005-0033-5","article-title":"Free-space laser communications with adaptive optics: Atmospheric compensation experiments","volume":"1","author":"Weyrauch","year":"2010","journal-title":"J. Opt. Fiber Commun. Rep."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"376","DOI":"10.1364\/OPTICA.1.000376","article-title":"Adaptive-optics-based simultaneous pre- and post-turbulence compensation of multiple orbital-angular-momentum beams in a bidirectional free-space optical link","volume":"1","author":"Ren","year":"2014","journal-title":"Optica"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"1453","DOI":"10.1364\/AO.54.001453","article-title":"Coherent free space optics communications over the maritime atmosphere with use of adaptive optics for beam wavefront correction","volume":"54","author":"Li","year":"2015","journal-title":"Appl. Opt."},{"key":"ref_21","first-page":"1289","article-title":"HAPS based FSO Links Performance Analysis and Improvement with Adaptive Optics Correction","volume":"21","author":"Ata","year":"2022","journal-title":"IEEE Trans. Wirel. Commun."},{"key":"ref_22","unstructured":"Ho, T.H. (2007). Pointing, Acquisition, and Tracking Systems for Free-Space Optical Communication Links, University of Maryland."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Wu, J., Ke, X., Yang, Y., Liang, J., and Liu, M. (2022). Correction of Distorted Wavefront Using Dual Liquid Crystal Spatial Light Modulators. Photonics, 9.","DOI":"10.3390\/photonics9060426"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"1508","DOI":"10.1364\/OE.10.001508","article-title":"Demonstration of new technology MEMS and liquid crystal adaptive optics on bright astronomical objects and satellites","volume":"10","author":"Dayton","year":"2002","journal-title":"Opt. Express"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"2017","DOI":"10.1109\/JLT.2022.3221610","article-title":"Orbital Angular Momentum Encoded Beam Tracking and Wavefront Sensing","volume":"41","author":"Grunwald","year":"2022","journal-title":"J. Light. Technol."},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"19687","DOI":"10.1038\/s41598-019-55670-1","article-title":"A space division multiplexed free-space-optical communication system that can auto-locate and fully self align with a remote transceiver","volume":"9","author":"Abadi","year":"2019","journal-title":"Sci. Rep."},{"key":"ref_27","unstructured":"Ruder, S. (2016). An overview of gradient descent optimization algorithms. arXiv."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1109\/TIT.1979.1055985","article-title":"On the Shannon capacity of a graph","volume":"25","author":"Lovasz","year":"1979","journal-title":"IEEE Trans. Inf. Theory"},{"key":"ref_29","doi-asserted-by":"crossref","unstructured":"Guiomar, F.P., Fernandes, M.A., Nascimento, J.L., and Monteiro, P.P. (2021, January 13\u201316). 400G+ Wireless Transmission via Free-Space Optics. Proceedings of the 2021 European Conference on Optical Communication (ECOC), Bordeaux, France.","DOI":"10.1109\/ECOC52684.2021.9606136"},{"key":"ref_30","unstructured":"Gruen, R., and Kirkland, R. (2023, March 21). Available online: https:\/\/www.corning.com\/data-center\/worldwide\/en\/home\/knowledge-center\/data-center-evolution-overcome-cabling-challenges.html."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Nascimento, J.L., Fernandes, M.A., Guiomar, F.P., and Monteiro, P.P. (2021, January 11\u201312). Experimental Analysis of the Impact of Indoor Turbulence on FSO for Intra-Datacenter Communications. Proceedings of the 2021 Telecoms Conference (ConfTELE), Leiria, Portugal.","DOI":"10.1109\/ConfTELE50222.2021.9435555"}],"container-title":["Photonics"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2304-6732\/10\/4\/394\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T19:08:30Z","timestamp":1760123310000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2304-6732\/10\/4\/394"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2023,4,2]]},"references-count":31,"journal-issue":{"issue":"4","published-online":{"date-parts":[[2023,4]]}},"alternative-id":["photonics10040394"],"URL":"https:\/\/doi.org\/10.3390\/photonics10040394","relation":{},"ISSN":["2304-6732"],"issn-type":[{"value":"2304-6732","type":"electronic"}],"subject":[],"published":{"date-parts":[[2023,4,2]]}}}