Flexible　optical　networks　(FONs)　are　used　to　handle　the　enormous　bandwidth　demands　and　significantly　improve　the　flexibility　and　efficiency　of　spectrum　resources.　This　flexibility　opens　the　door　to　strategies　that　can　optimize　the　allocation　of　spectrum　resources.　The　dynamic　setup　and　teardown　of　traffic　will　inevitably　fragment　these　resources　and　increase　the　network　blocking　probability.　Different　modulation　formats　can　be　configured　to　guarantee　efficient　spectrum　resource　allocation　by　taking　the　transmission　distance　into　account.　We　investigated　routing　and　fragmentation-avoiding　spectrum　allocation　for　the　unicast　service　over　FONs　with　the　constraints　of　spectrum　resource　and　transmission　distance.　To　alleviate　spectrum　fragmentation,　the　available　spectrum　adjacency　(ASA)　is　used　to　estimate　the　adjacency　among　available　spectrum　block　resources　on　routing　paths　or　links.　A　distance-adaptive　fragmentation-avoiding　spectrum　resource　allocation　(DA-FASA)　algorithm　based　on　ASA　and　genetic　operators　is　proposed　to　resolve　the　spectrum　fragmentation　problem　in　FONs.　The　DA-FASA　algorithm　defines　an　ASA　value　for　the　free　spectrum　blocks　on　each　routing　path　and　each　modulation　format　to　maximize　spectrum　availability,　which　effectively　reduces　the　spectrum　fragmentation　and　network　congestion.　Simulation　results　indicate　that　DA-FASA　exhibits　highly　efficient　performance　with　regard　to　the　bandwidth　blocking　probability　and　the　spectrum　utilization　ratio　under　different　network　scenarios,　compared　to　the　benchmark　algorithms.　(C)　2018　Society　of　Photo-Optical　Instrumentation　Engineers　(SPIE)