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However, long fiber loops reduce the free spectral range, leading to dense oscillation modes and requiring ultra-narrowband filters for single-mode operation, posing a long-standing trade-off between phase noise and mode purity. Here, we present a low-cost and scalable solution that breaks this trade-off. By inserting three cascaded zero-dispersion recirculating loops (ZD-RLs) into an amplified spontaneous emission (ASE)-based OEO, robust single-mode operation is achieved without using ultra-narrowband filters. The longest ZD-RL enhances the Q-factor of the microwave photonic filter (MPF) 4100 times, while the shorter loops increase the peak spacing within the MPF\u2019s passband to enable single-mode operation. This yields a sidemode suppression ratio of 72.7\u2009dB and phase noise of \u2013122\u2009dBc\u2009Hz\n                    <jats:sup>\u22121<\/jats:sup>\n                    at 10\u2009kHz offset at 10\u2009GHz, matching commercial generators. This work shows that even low-cost broadband ASE sources can generate high-quality microwave signals, bridging the gap between laboratory OEO demonstrations and practical signal generators.\n                  <\/jats:p>","DOI":"10.1038\/s41377-026-02413-3","type":"journal-article","created":{"date-parts":[[2026,9,16]],"date-time":"2026-09-16T02:15:57Z","timestamp":1789524957000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":1,"title":["Breaking the phase noise\u2013mode purity trade-off in long-loop OEOs via cascaded zero-dispersion recirculating loops"],"prefix":"10.1038","volume":"15","author":[{"given":"Zhuoran","family":"Wang","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Santiago","family":"Bernal","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"David V.","family":"Plant","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3760-4719","authenticated-orcid":false,"given":"Lawrence R.","family":"Chen","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,9,16]]},"reference":[{"key":"2413_CR1","doi-asserted-by":"publisher","first-page":"341","DOI":"10.1038\/nature13078","volume":"507","author":"P Gheli","year":"2014","unstructured":"Gheli, P. et al. 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