A　novel　narrow　bandwidth　spectral　beam　combining　structure　of　cm　bar　with　a　short　external　resonator　based　on　rhombic　prism　combinations　is　proposed　and　demonstrated.　A　pair　of　rhombic　prism　combinations　are　employed　between　the　LDA　and　the　transform　lens　in　the　external　resonator,　which　consists　of　a　rectangular　quartz　plate　in　the　middle　and　two　quartz　rhombic　prisms　on　either　side　in　opposite　orientations　in　each　combination.　The　beam　emitted　from　a　standard　cm　bar　is　segmented　and　rearranged,　and　spectral　beam　combining　is　carried　out　along　the　slow　axis.　A　spectral　bandwidth　of　about　5.46　nm　is　obtained　by　employing　a　transform　lens　with　a　short　focal　length　of　150　mm,　which　is　only　about　1/3　of　that　of　the　conventional　spectral　beam　combining,　and　a　narrow　spectral　bandwidth　of　1.14　nm　is　obtained　when　a　5x　de-magnifying　telescope　is　employed　in　the　above　external　resonator,　the　distance　between　LDA　and　diffraction　grating　is　500　mm　while　that　between　LDA　and　output　coupler　is　650　mm.　This　novel　structure　provides　a　compact　external　resonator　scheme　for　narrow　bandwidth　spectral　beam　combining.　©　2024　Elsevier　B.V.